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SUPERIMPOSED LARAMIDE AND MIDDLE TERTIARY

DEFORMATIONS IN THE NORTHERN SANTA

RITA MOUNTAINS, PIMA COUNTY,

ARIZONA

By

James J. Hardy, Jr.

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A thesis submitted to the Faculty and Board of Trustees

of the Colorado School of Mines in partial fulfillment of

the requirements for the degree of Doctor of Philosophy

(Geology and Geological Engineering) .

Golden, Colorado

Signed:

Signed:

Golden, Colorado

Date b f~'7 /1'77-,

Dr. Eric Nelson

Dr. Roger S att Professor and H~ad, Department of Geology and Geological Engineering

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ROCKS OF THE NORTHERN SANTA RITA MOUNTAINS

Precambrian Continental Granodiorite

The Continental Granodiorite (Drewes, 1968) is a

coarse-grained, highly porphyritic granodiorite to quartz

monzonite that underlies a large part of the western side of

the northern Santa Rita Mountains (Plate 1). In general,

the rock is poorly exposed and typically weathers to gruss,

recessed slopes or low knobby outcrops in all but the

steepest canyons and gullies. This characteristic is

probably due to the highly fractured nature of the rock.

In the study area, the Continental Granodiorite is

composed of brown to light gray biotite granodiorite or

biotite quartz monzonite that contains potassium feldspar

phenocrysts as much as 5 cm in diameter. The rock typically

contains between 10% and 20% biotite (locally

recrystallized) and/or chlorite mixed with biotite in a

coarse-grained groundmass of quartz and potassium feldspar.

Drewes (1976) notes that some local outcrops of Continental

Granodiorite south of the study area near Box Canyon,

contain a few percent hornblende although none was noted in

this study. Modal analyses presented by Drewes (1976) show

the rock to contain an average of 24% quartz, 42%

plagioclase, 19% microcline and 12% biotite.

Next to faults, the Continental Granodiorite is

typically very highly fractured and stained with hematite,

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jarosite and geothite to an orange-reddish brown color

(Figure 12). Narrow veins and veinlets coated with films of

chlorite and epidote are present locally in the vicinity of

Laramide stocks and intrusions. Also within these zones,

biotite is typically recrystallized.

Isotopic age data on the Continental Granodiorite are

sparse. The only dates obtained from the granodiorite

within the study region have all been highly disturbed (57

Ma and 163 Ma) (Marvin and others, 1973). However, the rock

is texturally and compositionally similar to the Rincon

Valley Granodiorite exposed in the southeastern Rincon

Mountains. These rocks have whole rock and K-Ar (biotite)

ages of 1410 ±50 Ma and 1415 ±50 Ma, respectively (Marvin

and Cole, 1978). The Continental Granodiorite is probably

of a similar age.

Paleozoic Strata

Paleozoic strata in the northern Santa Rita Mountains

are highly deformed. Parts of the Paleozoic section have

been metamorphosed and deformed to calc-silicate tectonites,

coarse-grained marble and other lower-grade metamorphic

rocks. The rocks, in places, have been isoclinally folded,

penetratively deformed, attenuated, thickened, and many

times are present in complex fault-bounded blocks. As such,

it is difficult to determine true stratigraphic thickness

for many of the formations. Thus, the thickness values

listed in the descriptions of the following units should be

46

:::-:::.::~::.·:::::·.7:::::.:c.===:=======::~.:=:::::-.::::::::::::::-::::::.c;::;-::;;:.: .......................... ,

Figure 12: Precambrian Continental Granodiorite immediately below the east side of the Helvetia klippe. Shears that dip from upper left to lower right of photo dip approximately 40° west.

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thought of as "structural" thicknesses that may not equal

the original stratigraphic thickness of the units.

47

Descriptions of nearby undeformed Paleozoic strata are

given to acquaint the reader with possible original

straiigraphic thickness of the northern Santa Rita Mountains

section before deformation.

Bolsa Quartzite (Middle Cambrian)

For reference, the type locality of the Bolsa Quartzite

was defined by Ransome (1904) in Bolsa Canyon west of

Bisbee, Arizona. At that locality, the Bolsa is

approximately 430 feet thick. In the Whetstone Mountains

approximately 20 miles east of the study area, an undeformed

section of Bolsa Quartzite measures 451 feet thick (Tyrrell,

1957; Wrucke ahd Armstrong, 1987).

The Bolsa Quartzite in the study area consists of

between 80 and approximately 1,000 feet of gray to white

orthoquartzite, arkosic sandstone and pebble conglomerate.

The basal part of the Bolsa consists of poorly sorted

arkosic sandstone and a distinctive chert or quartz pebble

conglomerate that rests in both depositional and fault

contact with the underlying Continental Granodiorite. The

formation becomes generally finer grained up section and is

characterized by abundant cross stratification in places.

The extreme thickness variations are due to tectonic

thickening and thinning during faulting and penetrative

deformation.

48

In the study area, exposures of the resistant Bolsa

Quartzite typically form prominent ledges and outcrops that

locally emphasize the trace of faults. Exposures of steeply

east-dipping to slightly overturned Bolsa underlie the

distinctive and resistant north-trending "Backbone Ridge"

that extends between Weigles Butte and Harts Butte in the

south-central part of the study area (Figure la; Plate 1).

Abrigo Formation (Upper and Middle Cambrian)

The Abrigo Formation was named by Ransome (1904) for

exposures of thin-bedded, cherty limestones in Abrigo Canyon

near Bisbee, Arizona. In the northern Santa Rita Mountains,

the Abrigo Formation is in both depositional and fault

contact with the underlying Bolsa Quartzite and overlying

Martin Formation. As such, the thickness varies between 150

and 1,150 feet. Undeformed sections of the Abrigo Formation

in the nearby Whetstone Mountains have a maximum thickness

of 866 feet (Creasy, 1967; Wrucke and Armstrong, 1987).

Exposures of the Abrigo Formation in the study area are

poor and the unit typically forms recesses, embayments and

slopes. However, the general outcrop pattern follows that

of the Bolsa Quartzite,typically forming the north-trending

slopes immediately above distinctive Bolsa outcrops that

define Backbone Ridge. The best exposures occur in the

southern part of section 11; T18S, RISE near the Specialty

Minerals, Inc. marble quarry (Figure 13 and Figure la). At

this locality, the Abrigo Formation consists of light tan to

49

Figure 13: Bold outcrops of Escabrosa Limestone (marble) near Specialty Minerals, Inc. quarry. Photo looks north toward the Santa Catalina Mountains and the Tucson basin. Dark outcrops to the left are steeply dipping (near vertical) Bolsa Quartzite. Low area in the middle ground is underlain by Abrigo Formation.

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gray, thinly laminated limestone and siltstone interbeds.

These strata are locally metamorphosed to low grade calc­

silicate hornfels characterized by the presence of sparse

amounts of actinolite and tremolite.

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Locally, in areas such as the west side of Peach Knob

and in the lowEOr/part of the section at the Specialty

Minerals, Inc. quarry, rocks of the Abrigo Formation have

been converted to limonite and hematite-rich gossans. The

zones are characterized by dark to bright red, frothy crusts

of boxworks typical of iron oxides after sulfides.

Martin Formation (Upper and Middle Devonian)

The Martin Formation consists of light gray to dark

blue gray, thin- to thick-bedded dolomite, limestone and tan

sandstone and shale. The dolomite horizons commonly form

ledge and cliff topography, whereas clastic units usually

underlie slopes. Sandstone and shale horizons appear to be

more common within the upper parts of the formation.

Exposures of the Martin Formation commonly follow those

of the underlying Abrigo Formation. However, the contact

between the units is rarely well exposed. Structurally

controlled thickness of the Martin Formation varies between

120 and 640 feet. This compares with a stratigraphic

thickness of 311 feet for an undeformed section in the

Whetstone Mountains (Creasy, 1967; Wrucke and Armstrong,

1987). The type locality on Mount Martin near Bisbee has a

thickness of 340 feet (Ransome, 1904).

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Escabrosa Limestone (Early Mississippian)

The Escabrosa Limestone typically consists of bright

white to light gray, thick- to massive-bedded, coarse­

grained marble. This is especially true in the northern

part of the area where the unit forms massive marble cliffs

with as much as 1,000 feet of vertical relief (Figure 13).

Within the Specialty Minerals, Inc. quarry, the marble is

exceptionally pure with only rare dark hornfels interbeds

and occasional remnants of fossil horn corals. Elsewhere in

the study area, the Escabrosa Limestone is typically

somewhat less coarse-grained and contains more dark

interbeds.

Skarn mineralization is not widely developed in the

Escabrosa Limestone. The pure nature of the Escabrosa

Limestone typically precluded the formation of any

significant zones of tactite alteration except in local

zones near the Rosemont copper deposit. At Rosemont,

tactite is generally restricted to veins and fault zones

that host assemblages of garnet or forsterite or an

assemblage containing diopside-wollastonite-vesuivianite

(McNew, 1981). Intensity of the metamorphism in the

Escabrosa Limestone at Rosemont decreases to the south so

that eventually limestones are only weakly metamorphosed,

characterized only by the destruction of fossil material.

The Escabrosa Limestone is everywhere separated from

the underlying Martin Formation by a fault. On the west

side of Peach Knob and in the Cooper World mine area (see

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Plate 1), the entire Escabrosa section has been faulted out.

The section is typically between 200 and 440 feet thick,

although it is as much as 1,400 feet thick where the section

has obviously been thickened by folding or ductile flow. In

the Whetstone Mountains, the Escabrosa Limestone is 742 feet

thick (Creasy, 1967; Wrucke and Armstrong, 1987). The type

section on Escabrosa Ridge near Bisbee is estimated to be

750 feet thick (Ransome, 1904).

Horquilla Limestone (Late and Middle Pennsylvanian)

The Horquilla Limestone consists of medium gray to pale

yellow, mottled, thin- to medium-bedded-, fine to coarsely ~------" '''-'"

crystalline limestone, silty limesto1e, and dolomite with

shale interbeds. Thinly laminated to nodular, brown to

black chert zones are quite common giving the unit a

distinctive appearance. The basal part of the unit contains

dark gray to black mudstone/shale horizons. This horizon

makes an excellent marker in the lowermost Horquilla

Limestone. It was used to define the base of the Horquilla

Limestone and thus, distinguish the Horquilla from the upper

parts of the Escabrosa Limestone. Tactite alteration is

common in the Horquilla Limestone and it is a major host of

copper mineralization in the Rosemont orebody (McNew, 1981).

In the southern end of the district near the Rosemont

deposit, McNew (1981) has carefully described the tactite

alteration assemblages associated with porphyry copper­

related mineralization. The mineral assemblages in the

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Horquilla Limestone near the middle of the district consist

of complete replacement by garnet (andradite) and lesser

diopside that was later overprinted by quartz and calcite

veins. Selective replacement of silty limestone horizons

becomes common south of the center of the district. In this

zone, McNew (1981) describes an assemblage of diopside,

wollastonite, and vesuvianite that generally corresponds to

zones of copper sulfide mineralization. Tactite alteration

in the extreme southern end of the district is characterized

by diopside, tremolite, potassium feldspar and epidote

replacement of silty-units.

The Horquilla Limestone is mostly in depositional

contact with the underlying Escabrosa Limestone and

overlying Earp Formation. Thus, the Horquilla Limestone has

the most consistent thickness of any of the Paleozoic units.

Where in stratigraphic contact with units both above and

below, the Horquilla Limestone varies between 1,060 and

1,400 feet in thickness. However, the unit is as thin as

200 feet in some fault-bounded slices. The Horquilla

Limestone in the Whetstone Mountains is 1,226 feet thick

(Creasy, 1967; Wrucke and Armstrong, 1987). Cooper (in

Gilluly, 1954) described a complete section of Horquilla

Limestone in the Gunison Hills near Dragoon as being 1,595

feet thick.

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Earp Formation Late Pennsylvanian and Early Permian)

The Earp Formation consists of a sequence of reddish

brown to bright orange, thinly bedded sandstone, siltstone

and shale interlayered with lesser amounts of gray

limestone, dolomite, and conglomerate. The distinctive

color makes the Earp Formation easy to recognize in the

field even in areas of poor exposure.

54

The Earp Formation is only exposed at two localities

near the center of the map area. The unit is in

depositional contact with the Horquilla Limestone but is in

fault contact with overlying units. Both exposures of the

Earp Formation are truncated along strike by faults.

Thickness of the Earp Formation as exposed is approximately

1,200 feet. However, the Earp Formation at both locations

is repeated by isoclinal, recumbent folding. In the

Whetstone Mountains, Tyrrell (1957) and Creasy (1967)

measured Earp Formation sections of 578 feet and 801 feet,

respectively.

Epitaph Formation (Early Permian)

The Epitaph Formation consists of a series of

interbedded light to medium gray limestone, dolomitic

siltstone, mudstone and marl. The upper part of the

formation is dominated by somewhat cherty limestone and

dolomite beds. The lower two-thirds of the formation

consists mostly of clastic strata with interbedded gypsum

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deposits. On a regional scale, the Epitaph Formation can be

broken into three members (Wrucke and Armstrong, 1987).

However, Drewes (1972) in his work in the northern Santa

Rita Mountains divided the Epitaph into an upper carbonate

member and a lower clastic-dominated member. This two part

distinction was followed in thi~ study. The lower clastic­

dominated part of the sequence comprises the bulk of the

Epitaph Formation in the study area.

Exposures of the Epitaph Formation are limited to the

middle part of the study region where it is exposed in fault

bounded blocks. Here, the boundaries of the Epitaph

Formation with other units are everywhere a fault; and,

where seen, the contact between the Epitaph and the

overlying Scherrer Formation is depositional (Plate 1).

within these fault bounded blocks, the lower part of the

Epitaph Formation has a thickness of approximately 800 feet,

whereas the upper member is 80 to 200 feet thick. The

Epitaph Formation in the Whetstone Mountains has been

measured by Tyrrell (1957) as 973 feet and by Creasy (1967)

as 1,198 feet.

Scherrer Formation (Early Permian)

Strata of the Scherrer Formation can be subdivided into

lower, middle and upper members. The lower member is

composed of white to light gray, fine to very fine-grained

quartzite that locally contains weak cross stratification.

These quartzites are typically recrystallized. Quartzites

of the lower member are overlain by a thin (<100 feet)

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interval of gray dolomite that lies below an upper member

composed of white to light brown sandstone. Poor exposures

make it difficult to separate the various members in all

parts of the study area. Thus, in many places the different

members are mapped together as one unit.

The most complete exposures of Scherrer Formation

strata are those within a north to northwest-trending fault­

bounded wedge immediately north of the Broad Top Butte

quartz latite stock. Here, the Scherrer Formation is

approximately 800-850 feet thick. Other exposures of the

Scherrer Formation are present along the east side of

Backbone Ridge south of Gunsight Pass and within the

Sycamore Canyon fold-thrust belt. Highly attenuated strata

in the Sycamore Canyon exposures vary between 100 and 600

feet thick. A narrow, 120 foot thick, fault-bounded sliver

of Scherrer Formation also occurs in the Helvetia klippe.

Undeformed exposures of Scherrer Formation in the Whetstone

Mountains have been measured as 560 feet (Creasy, 1967) and

470 feet (Tyrrell, 1957).

Concha Limestone Permian

The Concha Limestone is a distinctive, very dark gray,

medium- to thick-bedded, cliff-forming limestone

characterized by abundant brown to tan chert nodules. The

color and cliffy nature of Concha exposures make the

formation an excellent marker unit in areas of complex

deformation. Near intrusions the unit is commonly

recrystallized or converted to iron-rich gossan and skarn

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along zones 10 to 50 feet wide. The Concha is typically

quite fossiliferous throughout southern Arizona, but no

identifiable fossils were seen in the study area.

57

Exposures of Concha Limestone form the crest of

Backbone Ridge north of Gunsight Pass and lower slopes of

the ridge to the south (Figure 2). A relatively undeformed

exposure of Concha Limestone near Sycamore Spring (south

half of section 12, T18S, R15E) is in stratigraphic contact

with both strata above and below, and is approximately 600

feet thick. Deformed, or fault-bounded sequences of Concha

Limestone vary between 100 and almost 900 feet thick.

Thickness of the Concha Limestone in the Whetstone Mountains

is 557 feet (Tyrrell, 1957).

Rainvalley Formation (Permian)

The Rainvalley Formation is comprised of a sequence of

light gray to blue-gray limestone, dolomite and sandstone

that typically is in depositional contact with the

underlying Concha Limestone and overlying Mesozoic strata.

The basal part of the Rainvalley Formation is composed of

approximately 10-15 feet of medium-grained, well-sorted

quartz sandstone. This basal section is overlain by

interstratified limestone and dolomite horizons that

commonly form low, knobby, rounded outcrops.

Several exposures of the Rainvalley Formation occur in

the north-central part of the study area. For the most

part, these exposures are in depositional contact with

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Concha strata below and Early Cretaceous Willow Canyon

Formation strata above. These Rainvalley intervals have

thicknesses that vary between 700 and 1,000 feet. This is

consider~bly thicker than similar sections in the Whetstone

Mountains where the Rainvalley is only 187 feet thick

(Tyrrell, 1957). However, considering complex early to

middle Mesozoic tectonism in southeastern Arizona during the

interval between Concha and Willow Canyon deposition,

irregular thicknesses due to erosion are to be expected.

MESOZOIC STRATA

Jurassic(?) Rocks of Sycamore Canyon

A lithologically distinct sequence of variably foliated

and metamorphosed strata is exposed in the core of a large,

southeast-plunging syncline in Sycamore Canyon (secs. 2 and

12; T18S, RISE) north of the Specialty Minerals, Inc. quarry

(Plate 1). The lower part of the sequence rests in sheared

depositional contact with either Permian Rainvalley

Formation or Concha Limestone. Along this contact the

Paleozoic units are typically converted to tectonite. The

Sycamore Canyon strata consist of an interbedded sequence of

clastic and volcanic rocks. Generally, these strata are

composed of moderately metamorphosed to slightly

recrystallized arkosic sandstone, volcanic-clast

conglomerate and siltstone. These strata are intercalated

with rhyodacitic and crystal-lithic tuffs and with a

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conspicuous 15-30(?) foot-thick, bright white quartz arenite

horizon.

Drewes (1971) assigned these rocks to parts of the

Jurassic Gardiner Canyon Formation and Canelo Hills

Volcanics. However, because these rocks occur within a

structurally isolated zone and because they have no

counterparts elsewhere in the area this age assignment is

tentative at best. Nevertheless, these strata appear to be

quite similar to parts of the Jurassic strata described by

Riggs (1991) in the Cobre Ridge area of the Pajarito

Mountains, southwest of the Santa Rita Mountains. At Cobre

Ridge, eolian quartz arenite and other quartz-rich

sandstones are interbedded with tuffs, debris flow breccias

and conglomerates. In fact, a particularly distinctive

characteristic of Early to Middle Jurassic strata in

southern Arizona is the association of eolian quartz arenite

with volcanic rocks (Riggs and Busby Spera, 1991). A

Jurassic age assignment is, therefore, given the Mesozoic

strata in Sycamore Canyon although correlation with specific

formations has not been made.

Glance Conglomerate Latest Jurassic-Early Cretaceous)

Within the northern Santa Rita Mountains, rocks of the

Glance Conglomerate are exposed mainly along.the eastern

side of Backbone Ridge (Figure 2; Plate 1). North and west

of Rosemont the Glance is exposed only in small windows

through the overlying Willow Canyon Formation. However,

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south of Rosemont near Sycamore Canyon (sec. 36; TI8S, RISE

and sec. 1; TI9S, RISE), along the southern margin of the

study area, a thick sequence of Glance Conglomerate is

exposed that is typified by several radically different

facies. In the northern Santa Rita Mountains, strata of the

Glance Conglomerate unconformably overlie upper Paleozoic

rocks, Cambrian Bolsa Quartzite, or Precambrian Continental

Granodiorite.

In Sycamore Canyon, where Glance Conglomerate overlies

Precambrian granitic rocks, Bolsa Quartzite and Abrigo

Formation, it is composed of a poorly sorted, clast­

supported, pebble and cobble conglomerate. Where on

basement, the clasts are exclusively composed of angular to

subrounded porphyritic granodiorite or broken feldspar

grains (1-2 cm in diameter) in a coarse-grained arkosic

matrix. Glance Conglomerate that rests on Bolsa Quartzite

and Abrigo Formation is still dominated by granitic clasts

although quartzite clasts comprise approximately 25-30% of

the clast material. Thickness of the Glance Conglomerate at

this locality varies from 5 feet to approximately 600 feet.

Approximately 800 feet east of the Sycamore Canyon

exposures, Glance Conglomerate rests on the Horquilla

Limestone above a sheared depositional contact. Here, the

Glance Conglomerate is composed of a distinctive

limestone/marble clast conglomerate. Angular clasts and

boulders within this sequence range from <1 inch to as much

as 6 feet in diameter and rest in a sandy red arkosic

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matrix. Smaller clasts near the bottom of the section are

locally stretched.

The limestone-clast conglomerate grades upward into a

mixed-clast conglomerate composed of subangular clasts of

limestone, quartzite and granite (Bilodeau and others,

1987). Dark gray, aphanitic to vesicular, epidote-rich

andesite flows are present within this mixed clast zone.

Drewes (1971) originally mapped these as a Tertiary

intrusion, but locally concordant contacts and angular

andesite cobbles in overlying conglomerates caused Bilodeau

(1979) to suggest that these are lava flows that locally

filled channels during Glance deposition. Total thickness

of Glance Conglomerate, including volcanic units and both

conglomerate facies, is between 1,000 and 1,300 feet

(Bilodeau, 1979; Bilodeau and others, 1987).

Willow Canyon Formation (Late Cretaceous)

Rocks of the Willow Canyon Formation are extensively

exposed throughout the eastern part of the study area and

along Sycamore Canyon northeast of the Specialty Minerals,

Inc. quarry. Strata of the Willow Canyon Formation are

generally conformable with the underlying Glance

Conglomerate and overlying Apache Canyon Formation.

However, locally, such as along the Deer Springs fault, in

the Sycamore Canyon fault zone, or within the Helvetia

klippe, rocks of the Willow Canyon Formation are faulted

against a variety of Paleozoic units (Plate 1).

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Stratigraphy of the Willow Canyon Formation is

generally consistent throughout the study area, consisting

of a sequence of pale red to pale olive arkosic sandstone,

lenticular conglomerate, pebbly sandstone and siltstone.

These strata are thought to be distal to alluvial plain

facies of the slightly older or time equivalent Glance

Conglomerate (Sumpter, 1986).

62

In general, lower parts of the Willow Canyon Formation

are composed mostly of coarse-grained sandstone, lithic

sandstone and lenticular conglomerate horizons. Clasts

within the conglomerates are composed mostly of subrounded

clasts of sandstone, granite, quartzite and volcanic rock in

a sandy matrix. Thick, dark, aphanitic andesite flows and

agglomerates are present near the base of the formation in

several places, most notably in Sycamore Canyon and west of

Rosemont Junction. The upper two-thirds of·the formation

contains fewer conglomerate horizons and is dominated by

fine- to coarse-grained sandstones. Fine-grained sandstone

and siltstone are the predominate strata near the top of the

Willow Canyon Formation.

In most localities, rocks of the Willow Canyon

Formation are unaltered. The exceptions occur near stocks

and dikes of quartz latite porphyry that are generally

associated with ore grade mineralization in the district. A

particularly good example occurs where Willow Canyon strata

are intruded by quartz latite porphyry associated with

mineralization at the Rosemont orebody (sec. 36; T18S, RISE)

in the southern part of the district. Here, the rocks are

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intensely fractured and highly altered. Potassic alteration

is accompanied by distinctive blebs of epidote and garnet

along with bright orange iron oxide staining after

disseminated sulfide minerals, mainly pyrite.

Drewes (1971) estimates the Willow Canyon Formation in

the Santa Rita Mountains to be approximately 2,200 feet

thick. However, thicknesses based upon cross sections

indicate that the Willow Canyon may be as much as 4,000 feet

thick where folded into southeast-plunging folds northeast

of Sycamore Canyon. In the type area in the Whetstone

Mountains, Tyrrell (1957) describes sections 426 feet and

574 feet thick, whereas Archibald (1982) measured a section

about 70 feet thick.

Apache Canyon Formation (Early Cretaceous)

Approximately 1,500 to 2,000 feet (Drewes, 1971) of

siltstone, mudstone and subordinate sandstone of the Apache

Canyon Formation conformably overlie rocks of the Willow

Canyon Formation. The best exposures of Apache Canyon

Formation are immediately northeast of Sycamore Canyon and

west of Rosemont Junction where large-scale folds create a

sinuous belt of outcrop.

Fissile to thin-bedded pale red to olive green

siltstone and mudstone dominate the Apache Canyon Formation.

Subordinate horizons of gray limestone, sandstone and thin

conglomerate intervals are also present within the sequence.

To the east, in the Whetstone Mountains, the formation is

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composed predominately of limestone with lesser amounts of

clastic strata up section (Archibald, 1987). The Apache

Canyon Formation is laterally equivalent to the Willow

Canyon Formation and represents lacustrine depositional

environments (Archibald, 1987).

Shellenberger Canyon Formation (Early Cretaceous)

64

Rocks of the Shellenberger Canyon Formation were mapped

only in reconnaissance fashion along the northeastern margin

of the study area. The formation is approximately 1,500

feet thick (Drewes, 1971) and lies conformably above the

Apache Canyon Formation near Sycamore Canyon and is

unconformably overlain by rocks of the Laramide age Salero

Formation. However, a large north-trending fault through

the eastern end of Sycamore Canyon places Shellenberger

Canyon Formation against younger Cretaceous strata as well.

The Shellenberger Canyon Formation is composed

principally of brownish gray sandstone and arkose that are

intercalated within a sequence of black mudstone, limestone

and siltstone. Limestone horizons are typically thinly

bedded and rarely more than 3 feet thick (Drewes, 1971).

According to Archibald (1987), the sand facies of the

Shellenberger Canyon Formation represent channel sands

within a fluviodeltaic system prograding into a lacustrine

setting.

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LATE CRETACEOUS-EARLY TERTIARY (LARAMIDE) ROCKS

Salero Formation

65

Rocks of the Salero Formation form a heterogeneous

assemblage of east to southeast-dipping dacite to rhyodacite

flows, welded tuff, coarse fanglomerate, tuff, and exotic

rubble breccia. The Salero Formation is exposed across much

of the eastern part of the study area and either overlies or

is faulted against Early Cretaceous strata of the

Shellenberger Canyon Formation (Plate 1). Other extensive

exposures of the Salero Formation are located south of the

study area in the central and southern parts of the Santa

Rita Mountains and are described by Drewes (1971).

The Salero Formation in the northern Santa Rita

Mountains was broken into three broadly recognizable

stratigraphic units by Finnell (1970). These were followed

in the course of this mapping. The basal part of the Salero

Formation consists of various andesitic to dacitic flows,

flow breccias, tuffs and conglomerates. However, the most

distinctive unit is a coarse rubble breccia characterized by

exotic blocks and fragments of sedimentary and volcanic

rocks up to 3 feet in diameter. Most of the sedimentary

blocks resemble strata of the Apache Canyon, Willow Canyon

and Shellenberger Canyon formations and typically form a

jumble of angular exotic fragments with little fine-grained

matrix. Drewes (1971) notes the presence of exotic blocks

up to 1,000 feet long within Salero Formation in the

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66

southern part of the range, but no evidence was seen of

blocks that size within the study area. Finnell (1970)

estimated this lower member to be 2,000 feet thick.

The lower member is overlain by a thick rhyodacite

welded tuff. In the southern part of the Santa Rita

Mountains, Drewes (1968) reports a K-Ar date of 72.5 ±2.2 Ma

(biotite) from the welded tuff member of the Salero

Formation. Because of similar stratigraphic positions, it

is assumed that the welded tuff unit in the northern Santa

Rita Mountains is of a similar age. Finnell (1970) measured

this ignimbrite as about 1,500 feet thick.

The upper part of the Salero Formation is composed of

interbedded conglomerate, coarse sedimentary breccias,

coarse-grained arkosic sandstone and rare rhyodacite flows.

This unit is approximately 2,700 feet thick (Finnell, 1970).

In general, the Salero Formation is characterized by

basal andesites that are overlain by exotic block breccias,

welded tuffs and mixed sedimentary-volcanic horizons,

respectively. Lipman and Sawyer (1985) interpreted the

thick sequences of Salero Formation as caldera fill. In

this model, the different volcanic and sedimentary rocks

reflect progressively deeper parts of the caldera system

that varies from andesite near the base to welded tuff and

sedimentary rocks farther up section.

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Broad Top Butte Quartz Latite Porphyry Intrusions (Paleocene)

67

Small quartz latite porphyry stocks, dikes and plugs

(Greaterville intrusions of Drewes, 1972) are closely

associated with copper mineralization in the northern Santa

Rita Mountains. These widely scattered intrusions are

exposed in the following areas: 1) along and east of the

crest of Backbone Ridge at Rosemont and Gunsight Pass; 2)

within the upper plate of the Helvetia klippe; and, 3) in

Sycamore Canyon where quartz latite porphyry intrudes the

northwest-trending fault zone (see Figure 2 for locations)

The intrusions have a rather distinctive appearance

characterized by high fracture density, numerous equant vugs

and a light gray color (Figure 14). Phenocrysts are

typically composed of either doubly terminated quartz or

elongate to euhedral, light pink orthoclase that vary

between 2 and 13 mm in diameter. Scattered biotite

phenocrysts are variably altered to chlorite, and locally,

feldspars have been converted partly to sericite and/or

kaolinite. The fine-grained groundmass is composed mostly

of quartz, plagioclase and orthoclase but is commonly

altered to sericite and clay and is locally silicified.

McNew (1981) describes alteration in'the quartz latite

porphyry intrusions at Rosemont as consisting of quartz­

muscovite ±K-feldspar ±chlorite ±calcite and quartz ±K­

feldspar ±phlogopite ±chlorite.

68

Figure 14: Quartz latite porphyry outcrop immediately above low-angle normal fault, east side of Helvetia klippe. Fractures trend north-northwest.

!

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Finely disseminated or quartz vein-related pyrite,

chalcopyrite, bornite, malachite, galena and sphalerite(?)

are present throughout the ore-related intrusions at

Rosemont, Gunsight Pass and the Helvetia klippe. In

addition, quite distinct, gem quality hematite, probably

after chalcopyrite - chalcocite, is present as large clots

within the intrusions at Rosemont. However, these minerals

rarely comprise more than 1-2% of the rock by volume. Local

areas of garnet- and epidote-bearing endoskarn are present

near contacts with carbonate rocks at Gunsight Pass and

Rosemont; and, iron oxides and epidote are common on

fracture surfaces. Northeast-trending joints (veins) of

probable Laramide age (c.f., Rehrig and Heidrick, 1972,

1976; Heidrick and Titley, 1982) in the Broad Top Butte

stock are locally filled with secondary biotite and are

commonly characterized by narrow argillic alteration

selvages. Northwest-trending joints, which may be of middle

Tertiary age, are unmineralized.

The intensity of the alteration within the quartz

latite intrusions is such that in many places the rock is

converted to a mass of light green sericite that surround

large quartz phenocrysts. These intrusions, although now

-modally quartz latites, were probably originally either

granodiorite or quartz monzonite in composition (Kirwin,

Lowell, and Gahlen, personal communication, 1996). These

intrusions probably underwent intense potassic alteration

that was followed by biotite and feldspar destructive

quartz-sericite alteration that has left the rock with its

current modal composition. Such a theory could explain the

1

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uncommon association of quartz latite intrusions and

porphyry copper mineralization. Instead, a more typical

association of intermediate composition intrusions and

porphyry copper mineralization would be the case in the

Rosemont-Helvetia district.

70

If the Helvetia quartz latite intrusions were, indeed,

originally of intermediate composition it is possible that

they may be related to the unaltered granodiorite intrusions

exposed in the western part of the district. In such a

model, the mineral-related intrusions would be small, high­

level plugs or stocks that came from the larger granodiorite

bodies. Age data which are discussed below also seem to be

consistent with such a hypothesis. The data suggest that

unaltered granodiorite stocks are slightly younger (55-53

Ma) than the quartz latite intrusions (55-59 Ma). If these

dates represent cooling ages, it is not unreasonable to

expect the deeper granodiorite intrusions to have cooled

more slowly than the quartz latite plugs that were emplaced

at higher levels of the crust.

Age Determinations

As part of this study, a sample was collected from the

previously undated quartz latite that intrudes the

northwest-trending Sycamore Canyon fault zone (see Plate 1;

Figure 2). Unlike the South Johnson Ranch quartz monzonite

(Drewes, 1971) stock immediately to the southwest (Figure

2), this intrusion has no foliation, lineation or

-I I

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71

cataclastic fabric. Argon-Argon analyses of biotite and

chlorite after biotite from this intrusion give a plateau at

58.8 ±1.4 Ma (Table 1; Figure 15). These data suggest that

the crystallization age of this intrusion is approximately

59 Ma.

TABLE 1

SAMPLE: 95-6-27-4

ROCK TYPE: Quartz Latite Porphyry MINERAL: Biotite, chlorite after biotite, -80/+270 mesh

TEMP (36Ar/4°Ar ) (39Ar/4°Ar) 39Ar Cum. 4°Ar*% Age(Ma)

900°C 2.19E-04 4.66E-02 15.66 93.46 80.14 ±24.4

1000 °c 1.19E-04 6.54E-02 41. 21 96.41 59.18 ±3.01

1200 °c 3.05E-04 6.37E-02 69.69 90.92 57.35 ±2.48

1400 °c 5.69E-05 6.62E-02 98.97 98.23 59.55 ±1. 97

1800 °c 2.24E-03 2.89E-02 100.00 33.67 46.99 ±83.2

Argon-Argon age determination by 5-step incremental heating of Helvetia quartz latite porphyry intrusion. * = measured quantity. See Figure 18c for sample locations.

This Ar-Ar age determination is slightly older than

those obtained by Marvin (1973) for other quartz latite

porphyry intrusions in the area (Table 2), but is still

within analytical error of the other data. Thus, age

relationships between the various quartz latite intrusions

~i __ '--'- ~ ali 81%]

140

126 l-

112 t-

98 I-

,..-... 84 Cd

6 70

.. 1

.. Q) Ol)

< 56

42 to

28 l-

14 i-

I I

10 20

'·ta ~-";'ll>' ~ ~;;:2~ ~ ~ c_

95-6-27-4 811188 Biotite

Plateau: 58.8 ± l.4Ma (points 2 to 4)

83.3% of 39 Ar

I

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30 40 50 60 70

39Ar%

4

5

I

80

, ~-~

I

90

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II

II

•

..

100

~ Figure 15: Ar-Ar3~sotopic data for biotite from sample 95-6-27-4, plateau diagram ( Ar% vs. Age) for quartz latitle prorhyry.

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suggest that a single intrusive event occurred at about 59

Ma in the late Paleocene.

TABLE 2

POTASSIUM-ARGON AGE DETERMINATIONS

ROCK TYPE: Helvetia Quartz latite porphyry MINERAL: Biotite * = measured

SAMPLE # K% Ar*%

66D1612 . 7.3 95

66D1472 6.0 90

66D185 6.6 94

67D245 7.0 87

ROCK TYPE: Helvetia Granodiorite MINERAL: Biotite * = measured

'.

SAMPLE # K% Ar*%

66D51 5.9 88

66D51 5.9 90

From Marvin and

Helvetia Quartz Monzonite Intrusions (Paleocene)

AGE (Ma)

55.20 ±2.0

57.0 ±2.3

57.10 ±2.1

57.60 ±2.10

AGE (Ma)

53.3 ±2.0

54.8 ±2.0

others, 1973

73

Several quartz monzonite to granodiorite stocks

underlie a large part of the western edge of the study area.

Intrusions such as the Huerfano and Helvetia stocks (Drewes,

1 I

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74

1971) intrude only Precambrian Continental Granodiorite;

however, the South Johnson Ranch and Shamrod stocks of

Drewes (1971) also intrude a variety of Paleozoic strata

(Figure 2). In outcrop, the rocks typically weather to low,

smooth and rounded knobs and easily decompose to grus and

residual boulders.

With the exception of the South Johnson Ranch stock,

rocks of the Helvetia intrusions are composed mostly of

light gray to white, coarse- to medium-grained, massive

granodiorite and quartz monzonite (Figure 16). Small aplite

masses are scattered throughout these intrusions but were

not mapped separately. The rock is composed primarily of

quartz, plagioclase, potassium feldspar, biotite and rare

hornblende. Unlike the nearby quartz latite intrusions,

these rocks are typically unaltered and only rarely show

weak argillization of feldspars and chloritization of

biotite.

Modal analyses done by Drewes (1971) show typical

granodiorite compositions with plagioclase (39% to 48%)

dominant over potassium feldspar (19% tQ 29%). Like the

mineral-related quartz latite intrusions, the Helvetia

stocks are late ~aleocene in ~ge; Argon-Argon analyses done

as part of this study and K-Ar age determinations on biotite

by Marvin and others (1973) yielded dates of between 40 Ma

and 55 Ma (Table 2). These age determinations and

implications are discussed more below.

75

Figure 16: Exposure of Helvetia quartz monzonite below the west side of the Helvetia klippe.

.i

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76

The South Johnson Ranch stock (Figure 2) differs from

the other Helvetia intrusions in that it intrudes the

northwest-trending Sycamore Canyon fault zone, contains

local tectonite zones, and has been potassically altered.

Discrete zones (up to 50 feet wide) within the granodiorite

have been converted to medium-grained mylonitic augen gneiss

characterized by a N60W-trending, 60-80NE-dipping foliation

with moderately northwest-plunging mineral lineations. In

outcrop, the tectonite is light gray in color with a

hypidiomorphic, equigranular texture. The foliation is

defined by thin, elongate quartz grains, aligned potassium

feldspar augen, cataclastically deformed feldspar and

locally, by alignment of biotite and muscovite or sericite

grains (Figure 17). Fine-grained muscovite typically occurs

as local pods or blebs along foliation surfaces.

The mylonitic fabric has overprinted a weakly

potassically altered, sericite-bearing quartz monzonite­

granodiorite that is highly fractured and moderately iron-

stained. Potassic alteration is evident in relatively fresh

exposures by the presence of light salmon-colored, equant

feldspar and thick books of secondary biotite. Spots of

iron oxide after pyrite and rare disseminated pyrite and

copper carbonate minerals further attest to the altered and

mineralized character of this rock.

Figure 17: Mylonitic quartz monzonite from South Johnson Ranch stock. Sample is approximately 6 inches wide.

77

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78

Age Determinations

Ar-Ar isotopic analyses of this intrusion show it to

have had a complex thermal history. Analysis of muscovite

taken from mylonitic gneiss zones within the South Johnson

Ranch stock yields a plateau age at 51.9 ±0.2 Ma using 7 of

ten steps (90.4% of 39Ar) (Table 3; Figures 18a and 18b).

TABLE 3

SAMPLE: 95-6-27-1 ROCK TYPE: Gneissic to mylonitic quartz

monzonite/granodiorite MINERAL: Muscovite, -80/+200 Mesh, purity >98%

TEMP

1. 27E-03 5.75E-02 5.06 62.56

2.40E-04 7.59E-02 9.51 92.80

1.76E-04 7.66E-02 15.76 94.70

2.07E-04 7.57E-02 25.31 93.80

1. 65E-04 7.56E-02 42.49 95.03

1.60E-04 7.57E-02 64.66 95.19

1. 58E-04 7.44E-02 83.08 95.22

1.19E-04 7.42E-02 95.47 96.39

1.50E-04 7.17E-02 99.68 95.49

6.26E-04 6.46E-02 100.00 81. 42

AGE (Ma)

44.29±2.63

49.70±1.88

50. 23±1. 33

50. 38±1. 06

51. 05±0. 83

51.09±0.72

51.96±0.36

52.74±0.42

54.04±0.37

51. 20±14. 2

Argon-Argon analysis of muscovite from South Johnson Ranch quartz monzonite intrusion. * = measured quantity. See Figure 18c for location of sample.

. ~ ~ ~ ~""",:",,:,,-j

70

66

63

59

~ 56

6 52 ~ < 49

45

42

38

35

.. --~-'

95-6-27-1 Ml1187

Muscovite Plateau: Increments 2. to 8

51.9 ± .2 Ma 90.4% of released 39 Ar

i:rnm 1iB\

121 3 1 41 5 6 7 8 19

30 40 50

39Ar%

60 70

10-'"

80 90 100

Figure 18a: Ar-Ar isotopic3~ata for muscovite from sample 95-627-1, plateau diagram ( Ar% vs. Age), mylonitic quartz monzonite. ~

~

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Rl5EIR16E ----':""""--r*,

1.1 !" t91 5

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Ridge

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EXPLANATION

[£ill Alluvium

@IE] Gravel

~ Pantano Fm.

[f!!] Welded Tuff

~ Laramide Quartz latite

~ Laramide granitic rocks

[Bill Laramide volcanic rocks

[]I) Shellenberger Canyon Fm.

~ Apache Canyon Fm.

~ WiliowCanyonFm.

[§J Glance Conglomerate

~ Paleozoic Strata

~ Precambrian granite

~ ~

Oblique-slip, reverse fault

~ High-angle fault

'-. '-.

Normal fault

'-.

//

//

Jg /// //

:;

Folds

-/ /

&/ /

/

1 mile

N

~

Figure 18b: Map showing Ar-Ar sample site locations. CD

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81

This age is slightly younger than K-Ar ages obtained by

Marvin (1973) for other granodiorite intrusions in the area

(Table 2). However, analysis of potassium feldspar from

relatively undeformed zones within the stock yielded no

definitive plateau with data scatter well outside of

experimental errors (Table 4; Figure 19).

TABLE 4

SAMPLE: 95-6-27-1

ROCK TYPE: Gneissic to mylonitic quartz monzonite

MINERAL: K-feldspar, -80/+200 Mesh, purity >95%

TEMP 39Ax Cum AGE (Ma)

1. 19E-03 5.96E-02 4.53 64.81 44.29±0.44

7.66E-04 1.14E-01 14.80 77.24 27.76±0.69

2.14E-04 1.34E-01 27.98 93.51 28.56±0.42

1. 98E-04 1.25E-01 38.71 93.99 30.68±0.36

4.52E-04 1.11E-01 46.21 86.51 31. 87±0. 44

7.22E-04 9.40E-02 54.26 78.56 34.14±0.49

8.11E-04 8.03E-02 75.12 75.96 38.58±0.43

9.82E-04 7.40E-04 98.97 70.90 39.08±0.37

1.22E-04 7.05E-02 99.69 63.99 37.05±2.19

6.33E-04 6.57E-02 100.00 81. 23 50.24±14.3

Argon-Argon analysis of potassium feldspar from tectoni zed parts of South Johnson Ranch stock. * =

measured quantity. See Figure 18b for location of sample.

L,~_,_,

'C? e <U

-<

\:"'-'--

80

73 t-66

59 I-

52 l-

45 11

38 ~

31 ~

24 I-

17 F-

lO

-,---" IBII ~L:)

95-6-27-' Fll187 K-Feldspar Release Spectrum No Plateau

-..

L, 6 5

2 3

I I I _1 I I

10 20 30 40 50 60 39Ar%

~ L-_'_

10-'

7 8

9/

I I I

70 80 90 100

Figure 19: Ar-Ar isotopic data for potassium feldspar from sample 95-6-27-1, plateau diagram (39 Ar vs. Age), mylonitic quartz monsonite.

CD N

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83

The difference between the tightly constrained ages

obtained from the muscovite and potassium feldspar could be

explained in a number of different ways. The most simple

explanation may be due to the differences in the closure

temperatures of the different minerals. Because muscovite

has a much higher closure temperature than potassium

feldspar (325°C vs. ~230°C), the differences may represent a

simple cooling gradient in which the muscovite reached

closure temperatures several million years prior to that of

the potassium feldspar. In this scenario, the South Johnson

Ranch stock would have been intruded into the Sycamore

Canyon fault zone during the last stages of movement within

that zone at about 52 Ma and is therefore, late syn­

kinematic in nature.

The other possibility to explain the disparate ages is

that the feldspar was originally 40 Ma or older (possibly

even about 51 Ma - step 10) and was heated, deformed and

partially recrystallized at about 28 Ma. This

interpretation, although consistent with the data, does not

fit particularly well with the iqferred geologic history of

the region that is discussed in other sections of this

dissertation. In particular, there is no evidence of any

deformational and/or thermal event in the northern Santa

Rita Mountains that might have occurred at this time.

Middle Tertiary deformation in the area is constrained to a

period post-Pantano deposition (see Tertiary Rocks section

below), or post-middle Miocene time, about 5-10 Ma later

than the inferred age of the potassium feldspar.

I

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84

TERTIARY ROCKS

Pantano Formation (Oligocene to Miocene)

Rocks of the Pantano Formation depositionally overlie

units of the upper Salero Formation along the extreme

eastern boundary of the study area. As do all strata within

the eastern part of the study area, rocks of the Pantano

Formation dip moderately east or southeast.

The Pantano Formation consists of an interbedded

sequence of conglomerate, conglomeratic sandstone, and

various horizons of matrix to clast supported conglomerate

(Figure 20). There are very abrupt transitions from medium­

to fine-grained arkosic sandstone into conglomerate lenses 6

inches to 2 feet thick. Clasts typically vary from pebble

size (0.25 inch diameter) to rare clasts of up to 8 inches.

However, most conglomerate sequences contain clasts less

than 1 inch in size. Clast composition is comprised of a

heterogeneous mixture of sandstone, argillite, aphanitic

volcanic rock, lithic tuff, granite and reworked tuffaceous

conglomerate.

The type area for the Pantano Formation is located a

few miles north of the study area, near Cienega Gap where

the name is applied to approximately 5,000 feet of tilted

mid-Tertiary strata. The top of the Pantano Formation is

not exposed in the study area, but Finnell (1970) estimates

, \ \

85

Figure 20: Exposure of east-southeast-dipping Oligocene­Miocene Pantano Formation. Strata dip approximately 30° at this locality.

- )

,1 1

IJ

the formation to be approximately 6,300 feet thick. The

exact age of the Pantano Formation is poorly constrained.

However, Dickinson (1991) reports that a basal ignimbrite

within the sequence at Cienega Gap yields a K-Ar age on

biotite of 34.4 ±0.8 Ma. Dickinson further notes that a

86

lava flow near the middle of the formation has a K-Ar whole

rock age of 24.9 ±2.6 Ma. Thus, it appears that Pantano

deposition began near the middle Oligocene and continued

through at least middle Miocene time. This age control is

critical, as tilted Pantano Formation strata are an

important criteria in determining the age of Tertiary

faulting in the northern Santa Rita Mountains.

.

. ~.:.; ".''OJ

I

STRUCTURAL GEOLOGY

Changes to Previous Mapping

Previous mapping in the northern Santa Rita Mountains

by Harold Drewes of the U.S. Geological Survey (Drewes,

87

1972) defined the stratigraphic and structural framework of

the region. The mapping of Drewes (1972) was the basis from

which the Geologic Map of the Rosemont-Helvetia Mining

District (Plate 1) was made. The distribution of map units

presented on Plate 1 of this study are almost identical to

those defined by Drewes (1972). However, the work presented

on Plate 1 of this study differs greatly from Drewes (1972)

in the interpretation of the contacts between many units.

Drewes (1972) apparently chose to place faults between

many stratigraphic units to explain abnormal thickness

variations of Paleozoic or Mesozoic strata. In using this

concept to define faults, Drewes did not recognize the

extremely ductile manner in which much of the Paleozoic

section deformed by either attenuation, folding or

transposition of bedding. As such, Drewes was forced to use

numerous bedding parallel faults to "remove" strata but yet . .

keep a normal stratigraphic order in which entire formations

were typically thinned, but rarely completely removed.

Recognition that the Paleozoic section, in particular,

was deformed by highly ductile processes does not require

the use of nearly as many faults as shown in Drewes (1972)

1

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88

and yet easily explains the normal stratigraphic order that

is characteristic of the study area. Thus, any contact that

lacked discernible fault planes, fault rocks, or

deformational fabric and that did not disrupt the normal

stratigraphic order by omitting or repeating formations (or

distinctive mappable horizons) were mapped as normal

depositional contacts during the course of this study.

Late Jurassic-Early Cretaceous Faulting

Evidence of Late Jurassic-Early Cretaceous tectonism

occurs south of the Rosemont copper deposit at the southern

end of Backbone Ridge where two different facies of the

Glance Conglomerate overlie Paleozoic strata and Precambrian

Continental Granodiorite (Plate 1). The significance of

these depositional relationships lies in the abrupt

termination of the Paleozoic section that underlies Backbone

Ridge. In this locality, a southward-thickening(?) wedge of

both limestone and granite clast facies of Glance

Conglomerate overlies various Paleozoic units and

Precambrian basement. This contact appears to be mostly

depositional in nature. Locally, however, the contact is

sheared and characterized by a weak foliation in some of the

carbonate rocks below the contact. Local zones of stretch

pebble tectonites are also present in Glance Conglomerate

above the contact. Thus, there appears to be evidence of

some movement along this contact, but stratigraphic

relationships suggest that total amount of transport was

minimal.

I J

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89

This abrupt termination of the thick Paleozoic section

and the exposure of Precambrian basement to erosion during

Glance deposition is difficult to explain without the

presence of a Late Jurassic-Early Cretaceous (Bisbee-age)

normal fault. The most likely location for such a fault

would be about 1 mile south of the southern margin of the

study area in Box Canyon (Figure 2). Only rare exposures of

Paleozoic strata or Glance Conglomerate are present anywhere

south of Box Canyon until south of the Sawmill Canyon fault

zone (Plate 2) .

This relationship between Glance Conglomerate,

Paleozoic strata and Precambrian basement suggests the

presence of a northeast-side-down normal fault in the Box

Canyon vicinity. As shown in Figure 21, such a fault would

explain the occurrence of Glance Conglomerate and the

preservation of Paleozoic strata north of Box Canyon on the

down thrown block. The thick accumulation of similar strata

south of the Sawmill Canyon fault zone could also be

preserved in the model proposed here. Conversely, such a

fault would also explain the general lack of Paleozoic rocks

and the thin exposures of Glance Conglomerate and other

lower Cretaceous strata between Box Canyon and the Sawmill

Canyon fault zone. In this area, the Paleozoic strata would

have been removed by erosion on the up-thrown side of the

fault (horst) and Early Cretaceous rocks were not commonly

deposited.

I · ... 1

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r,/' (

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Figure 21. preservation faults.

N

Schematic diagram depicting a method for strata by Bisbee-age normal of Paleozoic

90

the

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91

Laramide Deformation

Paleozoic, Jurassic and Early Cretaceous strata in the

northern Santa Rita Mountains are characterized by a

consistent set of northwest-trending folds that are

spatially associated with north- and northwest-trending

reverse faults. These compressional structures appear to be

related to northeast-southwest oriented compression

associated with Late Cretaceous-Early Tertiary Laramide

deformation. Laramide-age structures have been rotated

approximately 40-50 degrees down to the east-southeast.

This rotation is about a north to N30E-trending axis of

rotation. Davis (1979) first recognized that this rotation

was related to post-folding, eastward tilting of the

Cretaceous section. Evidence for this event is given later

in this dissertation, but to describe the true geometry of

Laramide structures it was necessary to remove the younger

rotation. An axis of rotation of N10E with 45° of counter­

clockwise (top-to-the-west) rotation was chosen based upon

strike and dip of Oligocene-Miocene Pantano Formation and

preliminary paleomagnetic data. The rationale and evidence

for this particular axis is discussed in the section on

middle Tertiary deformation.

Structures that are related to Laramide compression

consist of both large and small-scale folds, an east-facing

monocline, thrust or reverse faults, and prominent joint

sets in various plutonic rocks.

I ~

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I l

92

Sycamore Canyon Fault Zone

Conspicuous, megascopic folds in Paleozoic and lower

Cretaceous strata are confined mainly to rocks in the

northern part of the study area. In particular, the most

clearly defined zone of Laramide folding is associated with

a reverse fault that is exposed within and north of Sycamore

Canyon (secs. 1,2,11,12; T18S, R15E; Plate 1 and Figure 2).

At this locality, Paleozoic, Jurassic, and Early Cretaceous

strata are folded about northwest-trending axes, intruded by

late synkinematic Laramide granitic and quartz latite stocks

and are characterized by the development of tectonite

fabrics such as stretch pebble conglomerates, foliation,

lineation and intra-folial folds.

The Sycamore Canyon fault zone is approximately 1 mile

wide, trends N45-60W and is bounded on the southwest and

northeast by moderate to steeply southwest- dipping faults.

Rocks within the zone are variably converted to marble

tectonite and mylonitic augen gneiss; however, large areas

within the zone contain no discernible deformational fabric.

A series of broad, open, generally upright folds in Late

Cretaceous Willow Canyon and Apache Canyon strata occur

immediately northeast of Sycamore Canyon. These folds trend

S40-75E and plunge between 15° and 56° southeast. Within

the fault zone is an upright, to steeply northeast-inclined,

asymmetric syncline that occurs in Paleozoic and Jurassic

strata between the two bounding faults (see Plate 1; Figures

22 and 23); this fold plunges 56° S48E.

'----- '-'-' -' '-'--

B SW

5000 J 4600

4200 -.j

3800

3400 --I

3000 --I

2600 --I

~ i.:..-.::.-.:.....:...<

Tgr

400ft L 400ft .

Q

~

IDC'J 8b

Dips I dirac/ly

toward viewer

\

----

I

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/ I I 8b / / ,T A

I / Dips / 8a I diroctly

I toward / viower

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'nr -B' NE

t ~OO 4600

Qt ~ 4200 /

\

~\ '- ,/

//1 , \ / , ,/ _.f-- 3800

Tgr \ \ \ \ \ '---- /",-'/l \ I. I- 3400

I'\. " "'- '<! '- - ' - ,/ A" " .-77 I- 3000

I,'~ ~ - _--/>1 /" I- 2600

Figure 23. on Plate 1. section.

Cross section B - B' through Sycamore Canyon fault zone. Units as T is toward, A is away from view. See Plate 1 for location of

\..0 .4

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F ...... , ... ".! f ':~ 1 -

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95

Removal of middle Tertiary rotation indicates that,

during the Laramide, the Sycamore Canyon fault zone had a

trend that varied from N30W to N40W with a general southwest

dip between 30° and 80°. Currently the zone typically

trends between N45-65W with dips of 50-75° southwest

although there are local steep northeast dips as well (Plate

1). Table 5 is a comparison of fold attitudes in Early

Cretaceous strata before and after removal of middle

Tertiary tilting. This comparison shows removal of middle

Tertiary rotation restores these folds to subhorizontal

attitudes that are consistent with that expected in a high­

level compressional tectonic environment. In general,

removal of middle Tertiary rotation changed fold trends to

only slightly more northwest-trends.

TABLE 5

AXIAL TRENDS OF MEGASCOPIC FOLDS IN SYCAMORE CANYON

FOLD-FAULT ZONE, NORTHEAST OF MARBLE QUARRY

POST-TILTING FORMATION BEFORE TILTING

52° S50E Kw 7° S56E

46° S53E Kw 0° S69E

.56° S75E Kw 11° S69E

50° S44E Kw 5° S50E

56° S48E Pz/Js 12,° S53E

Moderate plunging fold axes have been restored to subhorizontal attitudes by rotation about a horizontal, NI0E axis with 45° of counter-clockwise rotation.

. I

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Offset of Paleozoic and Mesozoic strata indicate that

there may have been some component of strike-slip movement

along the zone during Laramide time. Reconstruction of

Permian Scherrer Formation and Concha Limestone across the

96

trace of the fault zone suggest that there may be as much as

9,500 feet of left-lateral displacement along the zone (See

Plate 1). However, because Paleozoic and Mesozoic strata

are so highly folded it is not clear that such inferred

offset is valid. In particular, highly attenuated exposures

of Scherrer Formation occur along the northwestern end of

Sycamore Canyon and do not appear to be significantly out of

place with respect to Scherrer Formation on the other side

of the Sycamore Canyon fault. The clear offset of the

Paleozoic-Mesozoic unconformity does, however, argue for

strike-slip displacement along Sycamore Canyon.

The folds in Cretaceous strata north of Sycamore Canyon

are typically subparallel to the trace of the Sycamore

Canyon fault zone (Plate 1). Such an orientation suggests

that they are not strictly a product of left-lateral

displacement within the zone. Rather, the Sycamore Canyon

fault zone probably formed as top-to-the-northwest, left­

lateral, oblique-slip fault in response to northeast­

directed Laramide compression.

Slickenline Data

Mine workings in the Specialty Mineral, Inc. marble

quarry exposed numerous slickenside surfaces in the marble.

Slickenline data collected from the more prominent surfaces

I I

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97

are shown in Figure 24a. These lineations have mean plunge

and trend of 81° S46W. Counter-clockwise rotation of these

data about the NI0E, 45° top-to-the-west axis to account for

middle Tertiary faulting moves the data to a mean plunge and

trend of 50° S69W (Figure 24b). This direction is

incompatible with the orientation of the Sycamore Canyon

fault zone during Laramide time. A 50° S69E azimuth would

require the fault zone to dip northeast in Laramide time,

not southwest as described in the previous paragraphs.

Thus, this fabric is probably related to post-Laramide (mid­

Tertiary(?)) movement unrelated to Laramide deformation.

Tectonites

Jurassic strata, Paleozoic carbonate rocks and Laramide

granitic rocks within the Sycamore Canyon fault zone have

all been variably converted to tectonites. Where

tectoni zed, these rocks are characterized by the development

of steeply northeast- and southwest-dipping, northwest-

trending foliation (Plate 1). In the Jurassic strata and

adjacent Paleozoic carbonate rocks, phyllitic foliations are

roughly parallel to bedding and generally parallel to the

trend of the Sycamore Canyon fault zone.

Two different Laramide intrusions invade the Sycamore

Canyon fault zone (Plate 1). Both intrusions form

northwest-trending, elongate bodies that parallel the trace

of the fault zone. One intrusion, a quartz latite porphyry

similar to skarn-related stocks to the south, is

holocrystalline with no evidence of deformation.

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J I

98

N

o

o

o o

o

~ D\Jo

o

o o

n = 40 C.1. = 5-10-15%

Figure 24a: Lower Hemisphere, equal-area projection of slickenline data, Specialty Minerals quarry (Sycamore Canyon fault zone). Mean trend and plunge is 81° S46W. Contours in 5, 10, and 15% of total data points per 1% area of net.

,J

1

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99

N

o

o o

o o o

o o

o

o o o

o

n = 40 C.1. = 5-10-15%

Figure 24b: Lower hemisphere, equal-area projection of slickenline data (Specialty Minerals quarry) rotated to account for middle Tertiary tilting. Data set is the same as in Figure 24a. Rotation moves trend and plunge from southwest to southeast azimuth. Contours in 5, 10, and 15% of total data points per 1% area of net.

1

". " , ..... j

100

However, the South Johnson Ranch stock, at the northwestern

end of Sycamore Canyon, differs from the first intrusion as

it contains local zones of tectonite fabric even though it

is of a similar age (Paleocene).

The South Johnson Ranch stock is characterized by

discrete tectonite zones <1 foot to about 50 feet in width.

Tectonite zones are most common along the margin of the

intrusion and are typically composed of gneissic to

mylonitic augen gneiss characterized by fine-grained

aggregates of muscovite on foliation planes (see Figure 17).

The foliation is locally accompanied by moderately northwest

plunging, mineral lineations.

Argon-Argon age determinations were done on both

muscovite and potassium feldspar from this deformed rock.

This work was done in order to constrain the timing of

tectonite formation and better define the age of movement

within the Sycamore Canyon fault zone. The results of the

analyses on this rock are difficult to interpret. Muscovite

yields a plateau age at 51.9 ±0.2 Ma, whereas the potassium

feldspar separate gives no plateau with data scatter well

outside experimental errors. A best fit interpretation

using an inversion isoc~ron yields a~ age of about 28 Ma.

The plateau age given by the muscovite is interpreted to

indicate that the muscovite crystallized at about 52 Ma.

This muscovite is probably metamorphic in origin as

muscovite is not present in unaltered or undeformed rocks

elsewhere within the study area. Therefore, the muscovite

"1

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101

is probably related to deformational events that occurred at

about 52 Ma.

East-facing Monocline/Santa Rita Thrust

To the south, the Sycamore Canyon fault zone merges

with, or intersects, an east-facing, north-trending

monocline and the associated east-vergent Santa Rita thrust

or reverse fault (Plate 1 and Figure 2). The presence and

original configuration of these structures is difficult to

discern because they have been affected by rotation related

to middle Tertiary extensional faults. The Santa Rita

thrust fault probably merged with, or intersected, the

Sycamore Canyon fault zone in some manner, but younger

extensional faults have obscured the exact nature of that

relationship.

The trace of the Santa Rita thrust can be seen on Plate

1 as the north-trending, west-dipping fault contact between

steeply east-dipping to slightly overturned Paleozoic strata

and southeast-dipping rocks of upright Paleozoic strata and

rocks of the Cretaceous Willow Canyon Formation (Figures 25

and 26). Removal of approximately 30° to 50° of middle

~ertiary eastward tilting indicates that the configuration

of the fault during Laramide time was that of a steeply

west-dipping reverse fault that placed Paleozoic strata over

Early Cretaceous rocks.

Middle Tertiary rotation that has resulted in the

current structural configuration of the Santa Rita thrust

I

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C Backbone

West Ridge ,J,

8a 6000

Cb

5600 P8

5200

4800

Paleozoic

4400 ~strata ?

400 ft

4000 400 ft

....... .......

....... -:Y .......

....... .......

102

c· East

6000

5600

5200

4800

4400

4000

Figure 25: Cross section C - C' across Santa Rita Reverse fault. Units as on Plate 1. See Plate 1 for location of section.

'.~-I ,.

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104

fault and monocline exposed large areas of Precambrian

basement west of Backbone Ridge (Plate 1; Figure 2). This

basement terrane would have comprised the Precambrian core

of a north-trending basement-cored uplift whose eastern

boundary was (is) defined by the Santa Rita thrust fault and

attendant monocline or fault propagation fold. Such north

to north-northwest-trending monoclines and basement-cored

uplifts were proposed by Davis (1979) as the principal

method of Laramide contraction in southeastern Arizona.

Structural restoration of the Santa Rita thrust suggests a

method of deformation in the northern Santa Rita Mountains

that is consistent with that proposed by the Davis (1979)

model.

Structural Control of Intrusions

The location of skarn-related quartz latite intrusions

at the Rosemont deposit and Broad Top Butte suggest that

their emplacement was controlled by the Santa Rita thrust

fault (Plate 1; Figure 2). Drilling data collected by

Asarco, Inc. shows the quartz latite intrusions associated

with the Rosemont orebody to be rootless dikes and small

rootless stocks. However, the amount and intensity of skarn

at Rosemont suggests the occurrence of a much larger stock

in the area during the Laramide that has now either been

removed by faulting or erosion.

The Broad Top Butte stock at Gunsight Pass occurs

immediately adjacent to the trace of the Santa Rita fault.

-I

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105

The location of this intrusion suggests that its emplacement

was controlled by the fault. However, neither of these

intrusions contain any tectonite fabric or other evidence of

fault or shear zone-related deformation. Thus, the

emplacement of these intrusions at 59 Ma was probably after

movement along the Santa Rita thrust has ceased.

Joint Patterns in Laramide Intrusions

Two joint sets are present within Laramide intrusions

of the Rosemont-Helvetia district (Figure 27). These

directions are nearly orthogonal and trend N37E and N32W.

Joints in the Broad Top Butte pluton are best characterized

as long, through-going, moderate to steeply-dipping planar

surfaces that cut through the rock at regularly spaced

intervals of between 1 and 10 inches. Many of the

northeast-trending joints (veins) are filled with copper

sulfide minerals or pyrite, iron oxides after pyrite and

chalcopyrite, epidote, and rarely secondary(?) biotite.

Some, but not all, joints are rimed by narrow argillic

alteration selvages.

In the quartz latite intrusions at both Rosemont and

within the Helvetia klippe, fracture density varies greatly.

At these localities adjacent to copper skarn mineralization,

the intrusions typically exhibit potassic alteration that

has been variably overprinted by intense quartz-sericite

alteration. In such areas, fractures can occur as regularly

spaced, through-going systems similar to those in the Broad

Top Butte stock (see Figure 14), or more commonly, in zones

I )

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106

N

+

~6 c=> +

GP + 0+ tJ 0

o {I. + +

0

+

+

o

EXPlANA TION

o East-side above n = 68 f1 Brood Top Butte stock C.I. = 2-4-6-8%

+ Joints in Kw

Figure 27: Lower hemisphere, equal-area projection of poles to joints in Laramide intrusions. Contours of all data combined. Contours in 2, 4, 6, and 8% of total data points per 1% area.

\·.·.·····:1 ;.

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107

of very high fracture density characterized by anastomozing

fractures and fracture cleavage with abundant iron oxides

(Figure 28).

Based upon regional criteria the northeasterly trending

set is most likely related to Laramide tectonism. The

northwest trending set, which is restricted mainly to rocks

within the Helvetia klippe is, however, probably related to

emplacement of the Helvetia klippe in middle Tertiary time.

The implication of this joint pattern are discussed more

completely in the section on Tertiary deformation.

East-northeast-trending (N75E ±200) sets of mineralized

joints, veins and dikes are a common occurrence in Laramide

plutons of southwestern North America (Rehrig and Heidrick,

1972; Heidrick and Titley, 1982). When rotated 45° counter­

clockwise (top-to-the-west) about a horizontal N10E axis to

account for middle Tertiary deformation; the northeast­

trending fractures are reoriented to an azimuth of S84E, or

roughly an east-west orientation.

Rehrig and Heidrick (1972) and Heidrick and Titley

(1982) note that the pervasive, orthogonal, east-northeast­

and west-northwest-oriented fracture sets in Laramide

plutons are regional in scope and not restricted to either

specific plutons or structural domains. Because of this

regionally consistent pattern, both Rehrig and Heidrick

(1972) and Heidrick and Titley (1982) infer that these

patterns could only have resulted from a long-lived, fixed­

stress field that was in place in southwestern North

, ....•••..••••.•••••••••.••••••.. ,;,;; .•• ;.;,;, ;;,;,;;,0;,0;;;';;;';;'; ====

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108

Figure 28: Intensely fractured quartz latite porphyry near Rosemont copper deposit.

1 )

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109

America during Laramide time. It is this regional tectonic

stress field that controlled the orientation of fractures

and subsequently, dikes that formed during the emplacement,

crystallization, and cooling history of Laramide intrusions

(Heidrick and Titley, 1982).

The basic premise proposed by these workers is that the

prominent east-northeast-trending fracture, vein and dike

patterns reflect the orientation of the axis of maximum

regional compression during a period of Laramide deformation

in southwestern North America (Figure 29). More

specifically, these workers believe differential vertical

uplift of north-northwest-trending basement uplifts occurred

during a period of weak east-northeast-directed compression.

Under such stress conditions, the axis of regional

compression was consistently oriented within a vertical

plane oriented approximately N75E (Rehrig and Heidrick,

1972). The east-northeast-trending fracture pattern, thus,

represents the mean tectonic stress field at the time of

crystallization of mineralized Laramide plutons at about 60

Ma. This orientation was approximately N75E ±20o,

subhorizontal and subparallel to the resulting fracture

patterns (Heidrick and Titley, 1982). This regional stress

field is also roughly normal to the trend of the basement­

cored uplift and fault propagation fold in the northern

Santa Rita Mountains.

1

I

~~w~ TOTAL (A-X)

10429

110

EXPLANATION OF PORPHYRY COPPER DEPOSITS

* •

Major Intrusion - Wall Rock Porphyry

Minor Intrusion - Wall Rock Porphyry Significant Fault-vein or Manto Type

N

t

Figure 29: Laramide volcano-tectonic framework of southwestern New Mexico and southern Arizona shows the distribution of porphyry copper deposits, summation of strike rosettes, derived inter-mineral paleostrain field, position of inferred composite Andean arc orogen, and several transverse east-northeast-trending porphyry breaks. The northwest-trending linear discontinuities (1-6) are from Titley (1976) and include the Comobabi-Nogales discontinuity (1), the Sawmill Canyon fault zone (2), the Bisbee discontinuity (6). Figure from Heidrick and Titley, 1982.

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111

TERTIARY DEFORMATION

Middle Tertiary deformation in the northern Santa Rita

Mountains has affected the attitude and geometry of all

Laramide structures. It was necessary to remove the effects

of this younger deformation on Laramide age faults and folds

in order to obtain the correct attitude of such structures

during Laramide time. Regional relationships,

reinterpretation of detailed map relations by Drewes (1972),

and reconnaissance mapping during this study indicate that

the entire northern Santa Rita Mountains north of the

Sawmill Canyon fault zone have been tilted 40° to 50° to the

east-southeast at some time following Laramide deformation.

However, this interpretation is complicated by the fact that

only a few faults within the study area are clearly of

middle Tertiary age. Much of the middle Tertiary tilting

history must be inferred from evidence other than exposed

faults, although several clear examples of post-Laramide

faults do exist.

The most compelling evidence for middle Tertiary

tilting and rotation comes from examination of the

Oligocene-Miocene Pantano Formation in the eastern part of

the study area (Plate 1). Here, Pantano Formation strata

unconformably overlie Late Cretaceous rocks of the Salero

Formation and dip 25° to 40° east-southeast. Underlying

rocks of the Salero Formation dip east-southeast between 40°

and 90°. These relationships indicate that rocks of the

Salero Formation, as well as, underlying Early Cretaceous

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112

and Paleozoic strata dipped shallow to moderately east­

southeast prior to Pantano Formation deposition. Such pre­

existing dips of Salero and older strata are a result of

Laramide deformation associated with formation of the Santa

Rita thrust fault and monocline. Nevertheless, the 25° to

40° dips in Pantano strata indicate that the area was tilted

at some point after middle Miocene time. As noted earlier,

Dickinson (1991) reports a 24.9 ±2.6 Ma age from lava flows

near the middle of the Pantano Formation at Cienega Gap

north of the study area. Such a date suggests that Pantano

strata in the northern Santa Rita Mountains were tilted

post-25 Ma.

In the model described above, tilting of the Pantano

Formation and older rocks occurred above a shallow, west­

dipping low-angle normal fault (Figures 25 and 26) that is

not exposed within the map area. This fault, or perhaps a

splay from this fault, appears to surface approximately 1.5

miles southeast of the study area in the southern Empire

Mountains. Finnell (1970) shows complex fault relationships

that juxtapose rocks of the Pantano Formation, Salero

Formation, Early Cretaceous strata, and Paleozoic rocks.

Although complex in nature, the juxtaposition of Early

Cretaceous and Middle Tertiary str~ta indicates. several

thousand feet of displacement (i.e., Pantano Formation

juxtaposed against Early Cretaceous Apache Canyon

Formation) .

Rocks within the Helvetia klippe lie above a moderate

to shallow, west-dipping low-angle normal fault (Figure 26).

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113

This fault dips 10° to 40° west and juxtaposes Paleozoic and

Early Cretaceous strata and Laramide quartz latite porphyry

above Precambrian crystalline rocks and Laramide quartz

monzonite. This fault has been previously described as a

Laramide thrust fault (Creasey and Quick, 1955; Drewes,

1972) based upon structural relationships that place

Paleozoic and Mesozoic strata over Laramide granodiorite and

quartz monzonite and upon the interpretation of certain key

fault contacts.

Drewes (1972) interpreted the contact between Laramide

quartz latite porphyry and Precambrian Continental

Granodiorite along the eastern margin of the Helvetia klippe

(Plate 1) as intrusive. Re-examination of this contact

during this study indicates that it is a moderately west­

dipping fault characterized by the presence of chloritic

breccia and protomylonite (Figure 30a, 30b, 30c). Such

rocks are typical along many other mid-Tertiary low-angle

faults in southwestern North America (c.f., Crittenden and

others, 1980). Drill holes collared in quartz latite

porphyry (Table 6; Figure 26) of the Helvetia klippe

encounter a low-angle fault characterized by intense

shearing and chlorite breccia at depths of 255 to 320 feet

(Asarco, In.c. drill logs). Below the fault, Precambrian

Continental Granodiorite is encountered. Together, these

data provide evidence that the quartz latite porphyry in the

Helvetia klippe occurs entirely within the upper plate of

this fault and does not intrude the fault as shown by Drewes

(1972). This evidence indicates that movement of the

Helvetia klippe last occurred at some point after 53 Ma,

114

/

Figure 30a: East side of Helvetia klippe looking south. West-dipping fault contact between Paleozoic strata (light color) on right and Precambrian Continental Granodiorite to the left. Fault dips about 40° west.

115

Figure 30b: Low-angle, west-dipping fault contact between light-colored quartz latite porphyry (right) and Precambrian Continental Granodiorite on the left. Dark-colored rock in the mid ground (near backpack) is a zone of chlorite breccia and protomylonite formed along trace of the fault.

Figure 30c: Close-up view of chlorite breccia and protomylonite shown in figure 30b.

116

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117

the age of the Laramide granodiorite and quartz monzonite in

the lower plate of the fault.

TABLE 6

Asarco drill hole data - Helvetia klippe

Hole Bedrock Fault Depth TD Depth/Formation

G-7 Tql 329.5 ft 353 ft/Precambrian

G-I0 Pen 255.5 ft 282 ft/Precambrian

A-757 Tql 315 ft 389 ft/Precambrian

A-7l1 Ph 522 ft 626 ft/Precambrian

L-24 Ph 490 ft 585 ft/Precambrian

L-26 Om 390 ft 495 ft/Precambrian

Fault depth and TD are vertical depth from surface. All holes are vertical. Tql is Laramide Broad Top Butte quartz latite porphyry; Pcn is Permian Concha Formation; Ph is Pennsylvanian Horquilla Limestone; Dm is Devonian Martin Formation. Data courtesy of Asarco, Inc. See Figure 26 for location of holes.

Consistent moderate to steep east to southeast plunge

of Laramide folds in Paleozoic and Early Cretaceous strata

also argue for tilting in post-Laramide time. As noted by

Davis (1979) and confirmed in this study, folds in the

northern Santa Rita Mountains plunge east-southeast (Figure

31). Moderately to steeply plunging Laramide age folds in

southern and southeastern Arizona are unusual and restricted

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118

N

r:~ V

n 16 C.1. = 5-10-15-20%

Figure 31: Lower hemisphere, equal-area projection of trend and plunge of small-scale fold axes in Paleozoic and Mesozoic strata. Mean trend and plunge is 51° S58E. Contours in 5, 10, 15, and 20% of total data per 1% area.

·1

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. 1

n I I J fl ,J U

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119

almost exclusively to the northern Santa Rita and adjacent

parts of the Empire Mountains (Davis, 1979).

Subhorizontal fold axes are typical in regions of

classic Laramide and Sevier age compressional tectonism in

southwestern North America. This is particularly true in

Arizona where Late Cretaceous-Early Tertiary compressional

deformation is widespread (see figure 3 of Davis, 1979).

Krantz (1989) pointed out that characteristic differences

between major regional structures of Arizona define

structural provinces that correspond to the major

physiographic provinces of the state the Colorado Plateau

of northern Arizona, the Transition Zone of central Arizona,

and the mountain ranges of southeastern Arizona and

southwestern Arizona. Each of these regions experienced

quite different and varied episodes of Laramide deformation .

However, the one salient characteristic common to all of

these regions is the subhorizontal nature of fold axes

associated with Laramide deformation. Thus, moderately to

steeply plunging folds in the northern Santa Rita Mountains

are most easily explained by some mechanism other than that

associated with Laramide compressional deformation. Tilting

by low-angle normal faults is the simplest method by which

to rotate these folds. Not only is rotation along low-angle

normal faults mechanically compatible with observed fold

attitudes, the magnitude of rotation is also compatible with

dips of Pantano Formation strata that constrain the amount

of tilting (i.e., 40°-50°).

·.1 , I

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120

Preliminary paleomagnetic data were obtained from

several sites within Laramide age plutons in the northern

Santa Rita Mountains. These samples were taken to

independently test the amount of rotation that has occurred

within the Rosemont-Helvetia porphyry system since

emplacement in Eocene time. This test can be done by

comparing the characteristic remnant magnetization direction

of the particular samples with that expected for the Early

Tertiary geomagnetic field direction during the Eocene.

More than 50 oriented samples were collected as part of

this study. These samples were gathered from nine widely

spaced sites in the Rosemont-Helvetia district. However,

because of the altered nature of many of the exposures, site

means obtained from only four of these locations (Figure 32)

showed clear demagnetizations. These samples were collected

from variably altered exposures of quartz latite porphyry

(Sites HP004A, HP004B), unaltered Laramide(?) andesite dikes

and Laramide granodiorite (Site HP002), and unaltered

andesite dikes (Site HP0034) and rhyolite dikes (Site

HP0030) of probable middle Tertiary age. In each case, care

was taken to collect samples from relatively unaltered, hard

exposures deemed to be reasonably free of the surface

effects of weathering.

Samples were treated to both thermal and alternating­

field (AF) demagnetization. Samples exposed to thermal

demagnetization were progressively heated to temperatures

above 640°C, whereas samples treated by AF demagnetization

:_,,-,-j ~ ~

I I I I I I I I I

T I

.~' -"--,' ~

Tgr

PCc

"'Ji~ ~

Rl5f ImE ------,..-,

HP0350

HPO HP0371HP03

~

11 !~ ,~ I s,_-----

.. ,c'l]llf; .. -'-.~"--'

EXPLANATION

~ Alluvium

@f9J Gravel

C!!PJ Pantano Fm,

[j!ij] Welded Tuff

[!S!J Laramide Quartz latite

~ Laramide granitic rocks

[}!YJ Laramide volcanic rocks

~ Shellenberger Canyon Fm,

~ Apache Canyon Fm,

[]!J Willow Canyon Fm,

[!!] Glance Conglomerate

em Paleoloic Strata

[ffi] Precambrian granite

~ ~

Oblique-slip, reverse fault

~ High-angle fault

Normal fault -..... -.....

-.....

--)..--:.;

Folds

-/ /

&/ /

/

1 mile

N

~ Figure 32: Location map showing sample sites from which paleomagnetic data was obtained. i--'

N i--'

J

' .•.. ~] l' ;

I ~, .. ; .... ~ l:"

I

I n . ~ I!

~J

J , I

j ,. J

I .J

J

were taken to a maximum magnetic field of HAF of 800 Oe

(Figures 33 and 34) .

122

Mean site directions for data collected from Laramide

intrusions (HP002 and HP004A, HP004B), are shown on an

equal-area plot in Figure 35, and indicate westerly

declinations and moderate inclinations. These directions

are significantly different than the expected Eocene

direction for southern Arizona (Table 7) and suggest

eastward tilting of approximately 45° about a N4E-trending

axis. These data are in remarkably good agreement with

geological criteria discussed above that suggest east-,

southeast tilting of between 30° and 50° about a north­

trending axis of rotation.

Data for sites HP030 and HP034 show little or no

evidence of the rotation common to site HP002 and HP004. An

equal-area plot of these data (Figure 36) shows that they

fall very close to the expected Eocene paleomagnetic

direction and, thus, require little or no tilting to align

with the Eocene direction. However, both sites also fall

very close to the expected Miocene paleopole which has a

slightly more shallow inclination and a more northerly

declination. Such agreement with Miocene directions is

expected because these two sites appear to be in post­

Laramide dikes of probable middle Tertiary age.

Unfortunately, these rocks have not been isotopically dated

and their exact age remains in doubt. These dikes are

either quartz-sanidine rhyolite porphyry or andesite

porphyry and are unlike any known Laramide rocks in the

I J

r[ .. J

I

( 1 J

~j

, 1 ,1

····1 _.'

123

Up,N HPOO2Al

4

3

2

1

W 2 6 7

E

1

2

3 1 X 1O-3A1m

Down,S

Figure 33: Illustration of progressive thermal demagnetization of one sample from site HP002 showing stable reversed polarity direction for temperatures above 300°C. Filled circles = horizontal projection; open circles = vertical projection.

I

I

I .. J

HP004A2

NRM Up,N 7

6

5

100

200 4

300 400 3

.500 2

-3 1 X 10 Aim

1 8

2 1

W E 1

2

NRM 3 012 ·Do\-vn, S

Figure 34: Illustration of alternating-field demagnetization of one sample from site HP004. Stable normal polarity direction observed for fields >200°C. Symbols as in Figure 33.

124

") " I .. 1

I

:/ ~ : l ",I

I I

.. I

I ' ... j

! 1

.J

w-

Axis of Tilt = 4°

~. Angle of Tilt = 45°

Expected Eocene Direction

+

s

125

E

Figure 35: Equal area projection depicting the difference between expected and observed Eocene paleomagnetic directions for rotated Laramide intrusions. Observed direction is overall mean of results from sites HP002 and HP004 combined. Mean direction is: Inclination= 30.6°; Declination= 316.3°; a95= 8.3°. Expected Eocene direction of sampling location is: Inclination= 52°; Declination= 252°; a95= 3°. Tilt of 45° about a horizontal axis of azimuth N4E brings expected direction to observed direction.

-I

'"I -}

1 .J

I

I

I I

126

TABLE 7

Mean site directions for Laramide and middle Tertiary

intrusions, Rosemont-Helvetia district, Pima County, AZ.

Site (n)

HP002 ( 6)

HP004 (5)

Average

HP003 (4 )

HP034 (5 )

Eocene Direction

I

-39.84

21. 22 -'

30.60

43.52

55.04

52.00

D alpha-95 k R

121. 45 5.2 117.70 5.96

328.64 12.0 41. 69 4.90

316.30 8.3 20.60

350.75 5.7 258.56 3.99

350.25 3.5 484.41 4.99

352.00 3.0

Notes: I and 0 = inclination and declination of paleomagnetic directions (in degrees), uncorrected for structural tilt. n = number of samples from each locality, k = precision parameter (Fisher, 1953). Alpha-95 = radius of 95% confidence. R = vector sum of n unit vectors. Average = average mean paleomagnetic directions for sites HP002 and HP004.

II

, I .. I

LJ

I

I I

I J

w

•

N

HP030

t /

+

S

HP030, HP034: Site Means

Expected Eocene direction (1=52, D=352, olpha-95=3)

127

E

Figure 36: Equal area projection depicting the paleomagnetic directions for unrotated mid-Tertiary(?) intrusions from sites HP030 and HP034. Observed site means fall near expected Miocene direction suggesting that intrusions are post-rotation.

) , i

~I )

I

I \.. j

I

. ) ,. ". ,.

. j

128

study area. Because of their distinctly different

composition, these rocks are not believed to be Laramide in

age. The fact that they show no paleomagnetic evidence for

tilting also suggests that they are younger than the middle

Tertiary rotational event.

I . )

I

. I J

129

TECTONIC INTERPRETATION OF THE NORTHERN SANTA RITA MOUNTAINS

The complex tectonic history of the northern Santa Rita

Mountains spans the period of Late Precambrian(?) to middle

Tertiary time. Phanerozoic deformational events include

several distinct episodes. These are: 1) a Late Triassic to

Middle Jurassic period of magmatic arc activity; 2) latest

Jurassic to Early Cretaceous normal faulting or

transtensional extension in back-arc settings; 3) Late

Cretaceous to Early Tertiary (Laramide) compressional

deformation and arc-related magmatism; and 4) middle

Tertiary low-angle extensional faulting.

These varied deformational events have a common link,

however, in that the location and style of subsequent

structures (at least through Laramide time) were influenced

by the pre-existing structural patterns. This concept is

fundamental to the understanding of the tectonic development

of the southern Cordillera in this part of Arizona. It is

particularly important to realize the role that prominent

northwest- to west-northwest-striking faults had in the

control of Jurassic and Early Cretaceous sedimentation and

subsequently in the localization of some Laramide structures

and intrusions in southern Arizona .

Early to Middle Jurassic Period

The most prominent and long-lived structure in the

northern Santa Rita Mountains is the Sawmill Canyon fault

I ~'i .. ' *:.; c· L~.::.

I

I I

J

130

zone. Evidence cited by other workers and summarized

earlier indicates that the Sawmill Canyon fault zone was the

northwestern margin of a long arc graben depression that

characterized the Late Triassic and Early to Middle Jurassic

magmatic arcs of southwestern North America between about

220 and 180 Ma.

Figure 37 shows that the northeastern boundary of the

Jurassic arc graben depression in southeastern Arizona can

be traced confidently (as a projection of the Sawmill Canyon

fault zone) from the Huachuca Mountains through the Canelo

Hills into the Santa Rita Mountains. Northwest of the Santa

Rita Mountains, the trace of this boundary is less clearly

defined by prominent faults. However, it can be seen as the

boundary between Jurassic and other rocks that generally

falls along the projected trace of Sawmill Canyon fault

zone. The regional relationships defined by this work

support the hypothesis of Busby-Spera (1988) that the

Sawmill Canyon fault zone was the fundamental control on the

northern extent of Jurassic volcanic and sedimentary rocks

in southern Arizona.

The stratigraphic, structural and geochronologic

relationships s?uth of the S?wmill Canyon fault zone are

compatible with syndepositional normal faults that localized

Early Jurassic sedimentation (Riggs and Busby-Spera, 1990).

The 15,000 feet of sedimentary and volcanic rocks of the

Mount Wrightson Formation deposited south of the Sawmill

Canyon fault zone within a time span of approximately 10 Ma

at about 200 Ma are evidence of a major structural

~ ,,_C_,_, ~

32'-

Santa Rosa Mountains

'-<1~~0~~ 8 0110'<1'"

•

.. • ~r]

• Marana

•

-:~ Mountains ~

n~ " 0(/\1\ "

"

".

111·

I

~

Santa Catalina

MountaIns

III Tucson

SawmIll Canyon Faull Zone

Figure through

37: Map showing the projected trace southern Arizona.

of

I~:>~ l1li

the Sawmill

• Benson

Tombstona

N

t 10 I

Mil"

~<j

Canyon fault zone

-32'

f-' W f-'

r .J

-!

1 .. j

1 •• 1

132

depression. However, there is little evidence to suggest

that there was any widespread deposition of Jurassic rocks

north of the Sawmill Canyon fault zone. Such rates and

patterns of deposition could only be accommodated by a

rapidly subsiding depression that was quickly being filled

by volcanic rocks and sedimentary strata (Figure 38). Very

little volcanic strata escaped the bounds of the arc-graben

depression to be deposited north of the Sawmill Canyon fault

zone because of the rapid subsidence and occurrence of low­

standing volcanic complexes and calderas.

Late Jurassic to Early Cretaceous Period

The Late Jurassic through Early Cretaceous

deformational history is basically a continuation of

extensional faulting seen in Early and Middle Jurassic time.

Similar to the earlier parts of the Jurassic, northwest- to

west-northwest-striking faults and associated basins were

still the major structural elements. However, arc volcanism

had waned and the area was part of an extensive back arc

region inboard of the Early Cretaceous magmatic arc.

Late Jurassic and Early Cretaceous normal faults

associated with the Bisbee basin commonly reactivated pre­

existing Early and Middle Jurassic northwest-striking

structures . In particular, latest Jurassic and Early

Cretaceous strata of the Glance Conglomerate and slightly

younger Bisbee Group strata (Willow Canyon, Apache Canyon,

and Shellenberger Canyon formations) were deposited in

", \

\ !

,1

U

I

.)

.J

I ;J

sw I~ Arc

Graben

NE

~ I~ Craton ~I /

/Eolian quartz sand/

/ Jurassic volcanic

rocks

p£ + pz

rocks

~ Sawmill Canyon fault Zone

133

Figure 38: Tectonic model of the Jurassic arc-graben depression, southern and southeastern Arizona. Figure depicts the formation of low-standing volcanic centers and caldera complexes within the confines of the graben. Volcanic rocks are intercalated with eolian sandstone deposits and sedimentary breccias derived from fault scarps. See text for further discussion of model. Modified from Busby-Spera, 1988, figure 2.

, j

U

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-.' ~

134

alluvial fan settings within northwest-trending grabens and

half grabens adjacent to these faults (Bilodeau, 1979;

Klute, 1987). Several examples of this relationship occurs

in the Santa Rita Mountains along the Sawmill Canyon fault

zone and other northwest-striking faults (see Figure 6).

Laramide Period

Evidence of a complex sequence of Laramide

deformational and magmatic events is recorded in the

northern Santa Rita Mountains. These events are

interrelated pulses of compressional deformation, volcanism

and intrusion and are depicted graphically on Figure 39.

Although these events probably overlapped in time, they will

be discussed separately to clarify the particular features

of the different phases of Laramide deformation. The

discussion that follows divides Laramide events in the Santa

Rita Mountains into early, middle and late phases.

Early Laramide Events (80-70 Ma)

Salero Formation

Rocks of the Salero Formation in Santa Rita Mountains

have been interpreted by Lipman and Sawyer (1985) to be

remnants of large dissected ash-flow calderas. As such,

these rocks similar to other exposures of ash-flows and

exotic-block breccias throughout southeastern Arizona

(Figure 40).

1_-. ~

I , 1

-J

I I

:1 I

J

.\

135

80Ma 70 60 50 40 Ma

Salero Deposition ?-.

Inversion +-

+-Sycamore Cany~ f~lt~on!? __

Basement Cored Uplift

Santa Rita fault

He velia Granodiorite ?_

Quartz latite -I-- EARLY ---t---- MIDDLE ----1--- LATE ---;

Figure 39: Sequence of Laramide deformational events in the northern Santa Rita Mountains. Division of Early, Middle and Late events as discussed in text ..

1

1

]

I ~ l' l"",

I

I FI' f /;.

'"".] : ~ , .

t j

"" "J

111°

33°

32°

Porphyry Copper Deposit

o laramide Volcanic rocks

o miles

30 I

110 0

N

Figure 40: Exposures of Salero Formation and similar rocks (Laramide) in southern Arizona. Modified from Lipman and Sawyer, 1985.

136

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1 !

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I

I~ • I r~li .~

; j L.~

137

In the southern Santa Rita Mountains, Lipman and Sawyer

(1985) interpreted the thick sequence of andesite, exotic­

block breccia, and welded tuff (72 Ma) to represent caldera

fill. In this model, the various rock types represent

progressively deeper parts of the caldera wall. This

contact is characterized by a high angle of intersection

between Salero rocks and the caldera-wall (e.g., a buttress

unconformity). These workers also interpret several

granitic intrusions along the northern contact between

Laramide volcanic rocks and pre-Cretaceous strata to be ring

intrusions.

In the study area, however, the Lipman and Sawyer model

of Salero Formation deposition is more difficult to apply.

The same general stratigraphic sequence occurs in the

northern Santa Rita Mountains as to the south, but the

contact with underlying lower Cretaceous strata appears to

be a steeply southeast-dipping, low angle unconformity (see

Plate 1) not the high angle, buttress unconformity described

by Lipman and Sawyer. Nonetheless, rocks of the Salero

Formation represent deposition of similar stratigraphic

sequences that appear to record a similar volcanic history;

caldera formation as envisioned by Lipman and Sawyer (1985)

or, perhaps, synorogenic volcanism and deposition related to

some as yet, undefined orogenic event(s).

This study has defined no particularly compelling

evidence to support either a caldera or synorogenic

deposition hypothesis in the northern Santa Rita Mountains.

The unconformable nature of the contact between Salero

. I

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I ~ l.-:-r

I

I I ... ' ... -.•... ) r L

. . I ~

138

Formation and underlying lower Cretaceous rocks argues

against the caldera model. The low angle unconformity that

characterizes the contact between rocks of the Salero

Formation and Early Cretaceous strata suggest that

Cretaceous rocks were not involved in any significant

deformation before deposition of Salero strata. Salero

rocks also show no evidence of decreasing dip upsection that

might suggest an origin as synorogenic growth strata shed

into a basin adjacent to the uplift. Thus, the origin of

the Salero Formation in the northern Santa Rita Mountains

remains enigmatic. The Salero Formation does, however,

record a period of intense volcanism and deformation(?) that

pre-dates major Laramide fold and fault events.

Middle Laramide Events (70-60 Ma)

This phase of Laramide deformation in the Santa Rita

Mountains is characterized by the development of northeast­

and east-vergent thrust faults, reverse faults and folds.

Plate motions in the Pacific Ocean during this time period

are typically directed approximately N30 o -40 o E (Atwater,

1989). Such plate motion vectors are consistent with the

development of both northwest-trending folds and reverse

faults during this time period. Atwater (1989) describes

absolute plate motions that change from approximately N30 0 E

in Early Laramide time (75 Ma) to more east-northeast­

trending vectors (N60 0 E) during latter stages of Laramide

deformation at about 60 Ma.

1

J I

I

. I k_l

J

139

Inversion of Jurassic intra-arc basin

Thrust fault geometries and the configuration of

Jurassic and Early Cretaceous strata across the Sawmill

Canyon fault zone (Figure 41) are typical of structures

developed during tectonic inversion of extensional basins

(i.e., reactivation of extensional fault systems during

later compression) (c.f., McClay and Buchanan, 1992; Cooper

and Williams, 1993; McClay, 1993). Such tectonic features

have not been described previously in this part of southern

Arizona although Davis (1996, personal communication) has

recently proposed the existence of such structures in the

Huachuca Mountains southeast of the study area. However,

recent work (Powell and Williams, 1993; McClay and others,

1993) has recognized the presence of these structures in

several parts of Montana and British Columbia.

Structural and stratigraphic prerequisites defined by

Cooper and others (1993) to be common to inverted tectonic

terranes are evident along the trace of the Sawmill Canyon

fault zone. These prerequisites are: 1) development of a

basin actively controlled by faults such that a synrift or

passive in fill stratigraphic sequence can be recognized; and

2) a change in the regional stress system resulting in the

extensive re-use of the pre-existing fault system, with

uplift affecting the hanging wall rather than the footwall.

These prerequisites are expressed quite well in the Santa

Rita Mountains where they include: 1) a thick sequence of

synrift Jurassic strata southwest of the Sawmill Canyon

1 i

I [1

I

'j .i

ssw

Mt. Wrightston

, , " " , , ' "

Sawmill Canyon Fault Zone

Explanation

Ks = Ft. Criltenden Formation IUb = Bisbee Group Strata IUs = Bathtub and Temporal Formations Jvs = Mf. Wrightston Formation Ps = Paleozoic Strata Yg = Precambrian granite

NNE

7000 ft.

6000

5000

4000 -.. 3000

2000 ......

1000

5 miles

Figure 41: zone.

Cross section through Sawmill Canyon fault

140

I ~ .. , .,'

I

141

fault zone that are currently elevated a minimum of 6,500

feet relative to regional marker horizons such as lower

Cretaceous strata north of the fault; and 2) a change in the

regional stress system from tensional in the Jurassic and

Early Cretaceous to compression during Laramide time. This

change could allow reactivation of the pre-existing

extensional fault system.

Early Laramide compression was generally oriented in

northeast-southwest direction. Thus, northwest-striking,

steeply southwest-dipping faults would have been in an

optimal. configuration, roughly perpendicular to compressive

stress, to be reactivated as steeply dipping, northeast-

vergent thrust or reverse faults (Figure 42). Steeply

dipping faults are efficient at producing structural

elevation but can accommodate only minor amounts of

shortening, therefore any significant amount of shortening

must be accomplished in part by development of footwall

shortcut faults, hanging wall by~pass faults or folding of

hanging wall strata.

As shown in Figure 41, both reverse and hanging wall

by-pass faults are present along the trace of the Sawmill

Canyon fault zone. Foldi~g of hanging ~all strata and

development of a hanging wall by-pass fault are especially

evident in Figure 41 where Jurassic strata dip inward into

the thrust zone. This model is characteristic of simple

inversion of a half graben where the inversion fold in the

hanging wall tends to be an asymmetric monocline facing the

footwall (Cooper and others, 1993). However, the northward

I

"1 L. '.

I

,J

142

sw Late Jurassic Time NE

Intra-arc basin

/, ..... -/,'-- .... "-,.." -,' .... -" /-,1 /-,1 /-\.1'-

- I ~ - .... ,.. - - - I: - ... I' - - -Fl8- ..... I' - - - I ~ - ..... ,. - - - I ~ - .... I' - -

I' ,1/ ,I" ,1-,... ,1/ ,I ,t'/ ,1'/-,1'/_,,./_,/'_ //-\.//_,/ .... -,//_,//_~

Sawmill Canyon fault zone

-1-/-1- .... -1-/-1-,

1 ... - ,1 .... - ,1,..- ,1".- ..... ,1'/-,1'/-,1'/-,//­,//- .... //_,//_,//-

,../,_ .... '_1 .... /,_,'_' ... /,p-€:-' ,..../,_,'_' /' ,1/_,11'_" /_,1 /-,1

""--/'--/'--/"\.--/' -/_1_/_1_/_1_

,,1,,- .... 1/- -

Late Early Cretaceous Time

... / - .... I' .... - ..... I' ... '_ ,'" I' ,..'_ , ... ,-F/_,//_,//_,//_,I'/_

"/_"/_'1/_"/_" ,-/,,--,..,,--/,,-- - I' .... '- I_/' _ - ........... I _ /PE:;.-- ........ I_/' - ... •

. - .... - ,

/"-/, .... -" -,1/-,,/-,1/-, ,,'---1"--1''---'" 1-/-1-/-1-/-1-

,"/- /'-': ... - ... '--'.,: ... - ",'-'<"/- ... , ,//_,//_,//_,-//-

/ ... I - .... '\. - I /" I - ,-"" _ I r;t - '-'- _ I ... "" I - ,-"" _ I I'

,,/_,1/_,,/_,1/_, ""--/'--1"--1"--1" 1-/-1-/-1-/-1-1'-1-

- ,11'- ,1/- ,1/- ,11'-' //,/1' ,1'1' ,1'/_,// //_,1'/_,//_,//_,//­_"/_"1'_,'/_'1/_"

'-1"'-/"-"-"-/"-/ /-,,1/-,1

- - ./ " -

Middle Laramide Time

o 0

- -1.-1'_1_/ ... 1_/_1_

/ / - ,,' _ I ,/ ".- ,,'_ I ..: /p£'_ I ,I' / - / '_ ' -

/1'_,1'1'_,//_,1'; "/_"1'_"

/ I .... - I' I 1 , I I' - ... I

,--/'--1"­-1'-1-1'-1-1' ,1/-,,1/-,1

//_,/1'_,1'1'_

/ / - , / / - , / " - , _,1/_'1/_"1'

1'1 I / ' _ - " 1'1 I I' ' _ - p€;" /' _ - , / I I

,--/'--",--",,,­- / - , __ ,. - I _ / - •

current _erosion

surface

Figure 42: Schematic model for the tectonic inversion of the Jurassic intra-arc basin in Laramide time. JRv Jurassic volcanic rocks, PZ Paleozoic strata, PC Precambrian basement, Ks Cretacous strata.

. I

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~ 6

I .-.1 ~

I

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J

143

dipping strata could also be related to formation of an

initial rollover anticline during normal faulting during the

Jurassic.

Evidence for tectonic inversion of the Jurassic intra-

arc basin along the Sawmill Canyon fault zone seems

conclusive, but there is little direct evidence to define

the exact timing of this deformation. Regional evidence

indicates the general involvement of Jurassic and lower

Cretaceous strata in faulting but offers no clues as to

timing of deformation during the Laramide. It is suspected,

however, that the inversion structures formed early in

Laramide time when compressive stresses were oriented

northeasterly. Evidence cited by Rehrig and Heidrick (1972)

and Heidrick and Titley (1982) indicates that Laramide

compression was oriented more east-northeasterly during the

latter stages of Laramide deformation in southeastern

Arizona, a direction not as compatible with inversion as

northeast-directed compression. Furthermore, reconstruction

of plate motions by Atwater (1989) showing northeast­

directed subduction between 71 and 65 Ma lend further

support to the argument that inversion occurred during this

time period.

Development of the Basement-cored uplift

Two domains of folds and reverse faults are present in

the northern Santa Rita Mountains. Taken together, the

Sycamore Canyon fault zone and the Santa Rita fault define

II l.:

I

I I

I

J j

1

J

144

the northern and eastern parts, respectively, of a north­

trending basement-cored uplift (Figure 43). Although partly

obscured by the effects of middle Tertiary normal faulting,

the Sycamore Canyon fault zone and the Santa Rita reverse

fault merge so that their intersection form the northeast

corner of this structure. The roughly N500W strike of the

Sycamore Canyon fault zone and preservation of Jurassic

strata within the zone suggest that this structure may have

had a pre-Laramide history similar to other northwest­

striking zones in the region.

Development of the basement-cored uplift suggests that

movement along the Sycamore Canyon fault zone and Santa Rita

reverse fault were roughly synchronous. However, the two

zones are characterized by very different structural styles.

The Sycamore Canyon fault zone is characterized by

development of northwest-trending folds and moderate to

steeply dipping left-lateral, oblique slip reverse faults.

The Santa Rita fault zone, however, is typified by an upper

plate homocline of east-dipping Paleozoic strata thrust over

lower Cretaceous rocks. The Sycamore Canyon fault zone also

differs from the Santa Rita fault by the presence of open

and generally upright folds in Early Cretaceous strata

immediately north of the fault, local zones of tectonite in

Paleozoic and Jurassic(?) rocks, and late synkinematic

intrusions.

The mean orientation of folds in the Sycamore Canyon

fault zone is N49°W, a direction compatible with

northeastward-directed compressive stress in the Laramide.

I

I

1 i

I

I T 18 S

T 19 S

SYCAMORE CANYON FAULT ZONE

N

I 0

mile

~ ~

1

BASEMENT - CORED UPLIFT

Monocline

Folds

Thrust I Reverse fault

R15E I R16E

(\ SANTA RITA REVERSE FAULT

~.-\ \ ,T. itt i 19

R15E R~6E

145

Figure 43: Map showing the relative position of Laramide tectonic features within the northern Santa Rita Mountains at approximately 60 Ma.

I

-1 ,

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I I

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I

i I

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146

Offset of Paleozoic and Mesozoic strata across the trace of

the fault zone do, however, indicate the possibility of up

to 9,500 feet of left lateral displacement across the zone

(Figure 44). Because folds northeast of the fault zone are

roughly subparallel to the trace of the fault they probably

did not form in a pure strike-slip environment. Instead,

the Sycamore Canyon fault zone is inferred to have developed

as a top-to-the-northeast, oblique-slip reverse fault.

Northeast-directed Laramide compressive stress was oriented

in such a manner as to impart both left-lateral and reverse

slip displacement on northwest-trending faults.

The timing of movement along the Sycamore Canyon fault

zone can be reasonably well-defined. Two different

intrusions intrude the zone and were dated by Ar-Ar methods

in this study. One of these intrusions, the South Johnson

Ranch granodiorite, has been locally converted to augen

gneiss, whereas the other intrusion located approximately

0.25 miles to the east has not been deformed. Muscovite

taken from the gneissic parts of the South Johnson Ranch

stock yielded a plateau age of 51.9 ±0.2 Ma. The other,

unnamed, intrusion, an undeformed quartz latite porphyry,

yielded a plateau age of 58.8 ±1.4 Ma from biotite.

These data are interpreted to indicate that deformation

within the Sycamore Canyon fault zone was largely over by

about 59 Ma. However, some later, but only local, movement

occurred at about 52 Ma with the deformation of the South

Johnson Ranch stock. The thermal event associated with this

intrusion was not of wide enough extent to reset the cooling

: __ .. _.~ .. i

I

South r s Ps v 's John~on

Ranch tfJ\ Zoo.

I Stock

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\

\ I 1

1

1

I I I

T \

Tgr

pec

-

1J 1"_ r9\ 5 1 _____ _

*41J

RJ5E IRJ61

~ U~ RI!B

EXPLANATION

~ Alluvium

@jj] Gravel

~ Pantano Fm.

r::I;ill Welded Tuff

[!ill Laramide Quartz latite

~ Laramide gr"nillc rocks

~ Laramide volcanic rocks

~ Shellenberger Canyon Fm.

[}!] Apache Canyon Fm.

[]!] Willow Canyon Fm.

C§] Glance Conglomerate

[§] Paleozoic Strata

[ili] Precambrian granite

~ Oblique-slip, reverse fault "-­~

High-angle fault

~ Normal fault .......

"- .......

,...."'" ,...."'"

Cfg ,....,....,....

,....?

:;

Folds

-/ /

&/ /

/

1 mile

N

~ Figure 44: Possible offset of Early Cretaceous strata along trace of Sycamore Canyon Fault zone. f-'

~

-.J

'1

I

I I

.. .".' .... j

j

148

ages in the adjacent quartz latite porphyry, but may have

been responsible for formation of widespread marble skarn In

this area.

Figure 45 is a forward model depicting the development

of the Santa Rita fault zone. The model shows the geometric

development of steeply west-dipping, north-trending reverse

fault that cuts Precambrian basement and forms a fault

propagation fold in overlying Paleozoic and Mesozoic strata.

The model is drawn using the concepts of Narr and Suppe

(1994) for the kinematic evolution of basement-involved

structures.

The model depicts the evolution of the Santa Rita

reverse fault from a period at about 70 Ma through full

development at about 60 Ma. The slight angular unconformity

between the Salero Formation and underlying Early Cretaceous

strata along with the obvious involvement of Salero rocks in

the deformation suggest that the fold did not begin to

develop until post-Salero time, or about 70 Ma. Progressive

shortening initiates the formation of a fault-propagation

fold over the evolving basement fault until a period at

about 60 Ma just before the onset of calc-alkaline magmatism

and related mineralization.

The forward model is consistent with field observations

and cross sections presented earlier in this paper.

However, the model is somewhat speculative because only a

small part, of what must have been a large structure, still

exists.

I

···.1

I

149

w E

Ks

Pu Ps

~~*-1:%"~, ~ : .u·'",;.,,~>:;;f$k,;/ '<_~~,:,_;({:~;;%~.q:: <:>:U.'f"':;'f;i.~~ ~'>:.,..: J£",,«~,,r~r,,.";,""i)!, Pe A

lPu - Om

"""'" I / _... I :-~ '- 1-/ -=- pC _ ... I " _" I " _

_ - " f "_ - ;Y I "_ - '" I "_ - " I ... _ - ;' 1 "_ - I' I 70 Ma

Ks

Pu B

Ks

c

Ks

Pu D " 1/,_'" _ )~~~~~~~~p!s~~.pe I /

lPu

~~~~Me '-- ," ,-~ ~ I' Cu I' "- / - .... / - .... ,." _ ... " /'_ '\. /'

r ... / I - / ... _ " ... / I - I' ~€!, " • - " ... _ " ... / I - / '\. _ "'\. " I - ,,"" _ ./ ... "

60Ma

Figure 45: Forward model of the Kinematic development of the Santa Rita Monocline between 70 and 60 Ma. Units as follows: PG= Precambrian granite; Gu=Cambrian rocks; Dm=Martin Formation; Me=Escabrosa Limestone; ~u=

Pennsylvanian strata; Pe=Earp Formation; Ps=lower Permian rocks; Pu=Upper Permian rocks; Ks=Cretaceous strata.

] I

I . , ~ I

150

Late Laramide Events (60-50 Mal

The late Laramide orogeny in the northern Santa Rita

Mountains is defined as a period of calc-alkaline magmatism.

It is during this period that the area was intruded by

numerous quartz monzonite-granodiorite and quartz latite

porphyry intrusions with the development of attendant copper

mineralization. There is no evidence of any widespread

contractional deformation in the area post-60 Ma. East­

northeast to east-trending joints are the dominant structure

in these rocks. Only the South Johnson Ranch stock shows

any sign of a ductile deformation fabric; and, several other

intrusions that intruded along Laramide age faults are

undeformed. This intrusive event is responsible for the

skarn, marble and copper occurrences in the area .

The age determinations on the various stocks provide a

background upon which to build a model for Laramide

intrusion. Before this study, all dates in the area had

been done by K-Ar methods on biotite. These data apparently

defined two periods of Laramide intrusion - one associated

with copper mineralization and quartz latite porphyry at

about 57 to 55 Ma, and another at 55-53 Ma related to the

mostly unaltered quartz monzonite plutons.

Argon-Argon age determinations made during this study,

and recognition that the region has been rotated by middle

Tertiary low-angle normal faults, allows for another

hypothesis. The Ar-Ar data indicate that the quartz latite

porphyry intrusions are somewhat older (59 Mal than

. I )

~ ~

~ I

j

151

previously recognized, suggesting that the K-Ar dates

probably represent cooling ages. Given this model, it is

likely that the 55 to 53 Ma dates obtained from the quartz

monzonite also are cooling ages. It is, therefore, possible

that the quartz latite porphyry intrusions represent high­

level apotheoses that rose off of the deeper quartz

monzonite pluton. The high-level stocks cooled more quickly

than the deeper granodiorite and were altered unlike the

deeper level.intrusions (Figure 46). This model eliminates

the need for two separate intrusion events of different

compositions.

Middle Tertiary

Middle Tertiary deformation in the northern Santa Rita

Mountains is characterized by widespread tilting and

rotation in an extensional regime. These events contrast

sharply with contractional deformation typical of the

Laramide. This period was a time of large calderas and

related ignimbrites. It was also a period of widespread

cataclastic metamorphism and deformation associated with

development of metamorphic core complex terranes.

Top-to-the-east Rotation, Southeast tilting

Middle Tertiary extension in the northern Santa Rita

Mountains was accommodated by 25° to 50° top-to-the-east or

southeast rotation and resulted in southeastward tilting of

Paleozoic through middle Tertiary strata. Rotation occurred

I

I

I XXXXxxleXxx)r

X x x ~x x x ~x x x X?:x x.}:'-:x X?: "_'WX,,,_li'x ... _lt

Figure 46: intrusions

Skarn

Quartz latite Intrusions

Schematic model in the northern

V v"v " 'I \

//_,// ... ,,- .... ,-/"­

- " I ;' --/,--/

152

/_"' /_, /' ;,1,-",,,-,.1

,\1 ;_,1 /_,1 /-,."-,, ,--/'--" .... --/'--/ .... .,. -1'-1-/-1-/-1-/-'-/-

- - - - ... '_ I

-"'-/1 .... -,..,,-,,1\'-,.1 /_, 1/_, 1/-,. /_, 1/

-/"--/'--""--/'-'-/-1-/-1-/-1-/­- ,1,.- ,1,.- ,1,..- ,I /-,.I'/-,r/-,;,/-- ... " /' - .... ... /' - ... ... /' - ... /_",,_'1/_,1/-

Granodiorite Intrusion

Precambrian Continental Granodiorite

for Santa

the emplacement Rita Mountains.

of Laramide No scale.

I ,

'11

.'··.1 '1 ~ 1

I I

'I ,. : .. J

153

about a north to north to N300E axis as determined by strike

and dip of Pantano Formation rocks and preliminary

paleomagnetic data. This extension was accommodated ~y at

least two shallow, west-dipping low-angle normal faults and

several synthetic high-angle normal faults. The

hypothetical main, or master fault, appearing to have

accommodated most of the extension within the region is not

exposed. Rather, it is inferred from the necessity to have

such a fault at depth to accommodate eastward tilting of

Pantano and older strata, and to rotate Laramide fold axes

to moderate southeast plunges. This scenario is shown in

Figure 47 and depicts the rotation of the basement-cored

uplift in a top-to-the-east manner.

A progressive, schematic reconstruction of middle

Tertiary deformation within the Helvetia klippe is shown in

Figure 48. Although not balanced, this reconstruction does

demonstrate that the Helvetia klippe has been moved

significantly to the west along originally moderate to steep

(70°) west dipping normal faults. The restoration

reconstructs rocks in the klippe to positions close to

counterparts in the lower plate.

The master listric or detachment fault is envisioned as

a southward continuation of, or splay from, the Catalina

detachment fault bordering the southwestern flank of the

Santa Catalina-Rincon metamorphic core complex near Tucson

(Figure 49). Examination of regional maps (c.f., Drewes,

1980; Dickinson, 1991; and Plate 2, this study) indicates

-, 'I

I

I

'I

,.J

WEST

Ks

Pu s

A

EAST

Ks

Pu

/ - , , .... -,...,-,.."- .... ' /-,1/-'\.1/-,1

""--/' __ I''\._

60 MA

154

West East

B

Figure 47: Santa Rita

.... - ..... "-/1 -,./-,1/-,,/ -/,--" .... --/,--1-/-/-/-1-/-1 -,1,...-,,1,..-,1,.

,./ .... /- - ... ',./ ... /-P£' .... ",/ /_- .... ',./..-'1/_ .... '/_, /_, ,-,..,-/,,-.,...,,-,., _,1/_" /_,1 /_,1"-,.,--/,--/,--",--1-/_1_/_1_/_1_/_1 - ,1,...- ,1 .... - ,1 .... - ,',... //-, .... /-,/ /-, ... /-, .... //_,//_,//_,//_,//­"/_'1/_'1/_,,/_,

PRESENT

.... -/1'- .... "-/"-,...1'-,.1 /1 1'" -,1/_" /_" ;_,1 ,,_,I /_,1/_,,1'_ ,--/,--/,-- --/ ,.,--/'--/'--

,_- ,. - - ,_- ,. - , __ I' - , __ ,. _ I - ,. _ I _ .... - I - P'£.- I

,I .... ,1/ ,1/ ,I,... ,1,...-,1,.-,1/-,11' ,. /- ," /- ," /- ,/ /- ,/ /_ ,1'''- ,I' /- ,; /- ,/ ,. .... - .... ,. ,. - .... ,. ,. - .... ,. ,. - .... ,. ,. - .... ... .... - .... ... ,. - .... ,. ,. - ... ,. .... I 1'_' I /_'"' /_, I /_, I 1'_' I /_, 1/_' I /_,

""-/1 .... -,.1,-,..1 ,...,,-,..."-/1 ,..,- .... t\.

POST-MIOCENE

Schematic forward model of rotation of the during middle Tertiary reverse fault

deformation. Units as in Figure 45 Pantano Formation depicted as Tsp.

Oligocene-Miocene

I

I

I

155

West East Pr

Om

Tgr

1000' L 1000'

Figure 48a: Current structural configuration of the Helvetia klippe. Modified from cross-section D-D' (Figure 26) .

Om

1000' L 1000'

ql

Pr

Tgr

Figure 48b: Removal of slip along steep, west-dipping normal fault.

J I ~ H :., .. i

I

J

J

1000' L 1000'

156

Figure 48c: Counter-clockwise rotation of Helvetia klippe about north-trending axis to account for slip along unexposed fault at depth.

, I

L.I

i .

·'·'··'··1

1000' L 1000'

Figure 48d: Restoration of slip along splay of Helvetia fault, placing Devonian Martin Formation adjacent to similar rocks in lower plate.

157

'I .~ •. J

I . I

.... 1

.] , )

I

Pr-----'"' ......

1000' L 1000'

Figure 48e: Restoration of Paleozoic strata along west side of Helvetia klippe to a position near Paleozoic strata in lower plate. Reconstruction also restores Laramide granitic rocks across Helvetia fault.

158

"', j I

I

I

·····1 ":'

,.J

"j

J

\ Tucson

(ololino Delochmenl Foull

Yg

/

/ ! Q Horthern

SonIa Rilo Mlns TIll Domain

Sawmill Conyon

Q

Explanation Normalloolt

low-angle NOlmolfault

Strike· slip fault

o I

Whetstone Mountains

I

N

~ mnes

5 I

159

Figur~ 49: Schema~ic diagram showing the extent of northern Santa Rita Mountains tilt domain. Yg=Prec~mbrian

granite; Ys=Precambrian strata; PS=Paleozoic sedimentary rocks; Jv=Jurassic volcanic rocks; Jvs=Jurassic volcanic and sedimentary rocks; KJb=Bisbee Group strata; KJs= Cretaceous Jurassic rocks; Kg=Glance conglomerate; Ks= Cretaceous strata; TKv=Laramide volcanic rocks; Tki= Laramide intrusions; Tso=Tertiary sedimentary rocks; Tsm= Oligocene-Miocene Pantano Formation; Q=Quarternary sedimentary rocks.

-I I

I !

I I

I

160

Pantano Formation strata, as well as Paleozoic and Mesozoic

units have the same general east to southeast dipping

structural configuration from the southern end of the Rincon

Mountains into the Rosemont-Helvetia district. Furthermore,

folds in Early Cretaceous strata as far south as the Sawmill

Canyon fault zone plunge moderately east-southeast (see

Drewes, 1972, plate 3). However, Mesozoic rocks south of

the Sawmill Canyon fault zone show no evidence of this

eastward tilting, and typically dip moderately to the north.

These structural relationships suggest that the

northern end of the Santa Rita Mountains, possibly as far

south as the Sawmill Canyon fault zone, are allocthonous

above this inferred, west-dipping detachment surface (Figure

49) . In this model, the Sawmill Canyon fault zone would

have accommodated left slip movement in middle Tertiary time

having acted as a large tear fault or accommodation zone

separating rotated and extended rocks north of the zone from

non-extended rocks to the south.

Another low-angle, 10° to 40°, west-dipping fault

surface is, however, exposed within the study area. This

fault underlies rocks of the Helvetia klippe and juxtaposes

Paleozoic and Early Cretaceous strata and Laramide quartz

monzonite porphyry above Precambrian crystalline rocks and

Laramide quartz monzonite (Plate 1, Figure 2). This fault

has previously been mapped as a Laramide thrust fault

(Creasy and Quick, 1955; Drewes, 1972) based upon structural

relationships that place Paleozoic and Mesozoic strata above

Laramide rocks. The presence of chloritic breccia and

I

I

I

I i

161

protomylonite, rocks characteristic of mid-Tertiary low­

angle normal faults (Crittenden and others, 1980), and the

general younger-on-older (Laramide on Precambrian) nature of

this contact suggest, however, that it is a low-angle normal

fault of mid-Tertiary age.

This configuration of normal faults has segmented the

northern Santa Rita Mountains into a lower, middle and upper

plate. The lower plate is not exposed in the Santa Rita

Mountains, but lies at depth below the master detachment

surface shown in Figure 47 and is composed of rocks of

unknown character and type but perhaps analogous to those

exposed in the lower plate of the Catalina metamorphic core

complex. Rocks above the master detachment and below the

Helvetia klippe form the middle plate and are composed of

Precambrian crystalline rocks through Tertiary Pantano

Formation. Rocks of the middle plate are extensively

exposed in the northern Santa Rita Mountains. Precambrian,

Paleozoic, Mesozoic and Laramide rocks of the Helvetia

klippe form the upper plate.

Local structural relationships and sparse kinematic

indicators such as slickensides and chattermarks indicate

that movement along the Helvetia klippe was in a roughly

east to west direction. Kinematic indicators suggest

movement along a S800W vector, whereas, structural

reconstructions and geometric relationships indi~ate a mean

movement vector of N80oW. Reconstruction of such east to

west movement places rocks of the Helvetia klippe adjacent

to the Broad Top Butte quartz latite intrusion (see Figure

I ".:J

162

48). This indicates that the intrusion within the Helvetia

klippe is a displaced fragment of the Broad Top Butte stock

and that the Peach-Elgin copper deposit along the west edge

of the klippe (see Figure 2) was once located approximately

2 miles to the east of its present position along the trace

of the Santa Rita fault and about 1 mile north of the

Rosemont deposit. This reconstruction yields a minimum of

122% of mid-Tertiary extension. Complete restoration of

middle Tertiary extension across the study area to a

horizontal datum (i.e., rotation of Pantano Formation back

to horizontal) suggests total extension of approximately

150-160% since middle Tertiary time.

~ J

I

I

.. 1

1 . i

.. 1

REGIONAL CONSIDERATIONS AND IMPLICATIONS

The new structural interpretations presented in this

study have redefined the structural style within the area

and defined two distinct periods of deformation. Placing

these data and interpretations into a regional context

provides insight into not only the regional geodynamic

development of southeastern Arizona in the Laramide, but

also in middle Tertiary time as well. The following are

some salient observations that bear on the tectonic

development of southeastern Arizona.

163

The terrane of basement-cored uplifts first described

by Davis (1979) for parts of southeastern Arizona continues

as far west as the Santa Rita Mountains. However, the Santa

Rita Mountains appear to mark the westernmost extent of this

style of deformation in southern Arizona. The western edge

of the Santa Rita Mountains and the southern parts of the

Sierrita Mountains host the westernmost outcrops of

relatively unmetamorphosed Paleozoic strata in southern and

southeastern Arizona. Furthermore, the last vestiges of

undeformed and relatively autocthonous Precambrian

crystalline rocks are exposed here as well (see Plate 2) .

The importance of these Precambrian and Paleozoic rocks

become fundamental considering that they lie adjacent to a

terrane characterized by Late Cretaceous-Early Tertiary

(Laramide) high-grade metamorphism and deformation, thrust

plates characterized by upper-plate Cretaceous crystalline

-1

D I

rOCj

.. -

" I',

I

164

rocks, and a lack of Paleozoic and/or Precambrian rocks. In

fact, except as small fensters or tectonic slices, no

Paleozoic or Precambrian rocks crop out anywhere southeast

of the Sierrita Mountains until in the Caborca region of

Sonora, Mexico (Figure 3). This intervening 20,000 km2

"Papago terrane" of Haxel and others (1984) has a Mesozoic

geologic history that is significantly different than

adjacent parts of southern and southeastern Arizona (Figure

50) .

The Papago terrane is characterized by extensive areas

of Jurassic volcanic and granitic rocks; rifting or thinning

of the Proterozoic and Paleozoic craton prior to, or

contemporaneous with, the onset of arc magmatism in the

Jurassic; and, widespread dominance of thrust faults,

regional dynamothermal metamorphism, and peraluminous

granite plutonism during Laramide compressional deformation

(Haxel and others, 1984; Goodwin and Haxel, 1990; Tosdal,and

others, 1990). Haxel and others (1984) suggest tens of

kilometers of horizontal crustal shortening that culminated

between 80 and 58 Ma.

Deformation in the Papago terrane is almost completely

synchronous with the tectonic inversion and basement-cored

uplift events farther to the east. However, the general

lack of Paleozoic and Mesozoic supracrustal rocks between

the Papago terrane and the western Santa Rita Mountains

makes it difficult to link the two deformational styles.

Perhaps the simplest interpretation of the Papago terrane is

that it is a more deeply eroded part of the Laramide orogen

L.: __ '-- ~-; ~ ~Yj

111 0

Marana.

320

-

. -.

Figure 50: Regional development of southeastern Arizona betewwn 70 and

;fIIII ~ '-':'---":-.

III Tucson

Laramide 60 Ma.

I

structures

~

-.:rv

Miles

• Benson

110·

Laramide Inversion Structure

Boundary of Basement-cored Uplift

10

()~ \ \ Mor.goon

~-

in southern and

-320

I---' 0\ (Jl

I

I I

;J

166

than is seen in most parts of southeastern Arizona. This

might be explained, in part, by the occurrence of more

prolonged and extensive inversion of the Jurassic intra-arc

basin in this area during Laramide time. In this situation,

inversion may have proceeded to a point where deeper parts

of the orogen were emplaced at structurally and

topographically higher positions than elsewhere along the

zone. This idea is given some credibility because the

northeastern margin of the Papago terrane generally

coincides with the northern boundary (i.e., the trace of the

Sawmill Canyon fault zone) of the Jurassic basin (Figure

50) . However, this explanation does not easily explain the

abrupt eastern termination of the Papago terrane west of the

Santa Rita Mountains. This north-trending, diffuse

termination zone corresponds to the Santa Cruz River basin

and may suggest some underlying fundamental structural

control that is not seen at the surface.

Documentation of middle Tertiary extension in the

northern Santa Rita Mountains points out a rather

problematic situation regarding directions of extension in

this time period. Figure 51 is a schematic drawing that

shows the various directions of extension in southern and

western Arizona. Extension in the Catalina-Rincon

metamorphic core complex is directed approximately S50oW,

whereas extension in the northern Santa Rita Mountains is

west-directed. These directions are distinctly different

than those in the Papago terrane and Sierrita Mountains

where extension is typically north to north-northwest-

I

i

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._. j Lb

.1

I i

C.-.·l

,.i··l ( ! . ~-

cJ

'J

;'"-, \ 5.11,'<. Oligocene - Miocene tilt Domain

Unrotated terrane

Regional Breakaway

~ Transfer fault or Accommodation Zone

@ Metamorphic core complex

*

Beming of S( shear

Porphyry copper deposit

167

COLORADO PLATEAU

/ Inferred extension direction from porphyry copper deposil

,..,-....... -.. Sanla Cruz River basin boundary zone o SludyArea

N

I o 30

~

Figure 51: Middle Tertiary tilt domains and extension directions of the southern Cordillera. Modified from Wilkins and Heidrick, 1995.

n,

",C, r ,

~

, ,I

r~

i i " ,:1

I I J

J

directed (N13°W) (Goodwin and Haxel, 1990; Wilkins and

Heidrick, 1995).

Both regional and local relationships appear to

indicate that movement within these different extensional

168

domains was roughly synchronous, occurring in late Oligocene

and Miocene time. It is unclear as to how these domains are

related geodynamically and how regional crustal extension

can accommodate such radically opposed, yet synchronous,

extension directions. Nevertheless, one thing does appear

to be required in this model - a major north-trending

terrane boundary or accommodation zone that basically

coincides with the Santa Cruz River basin (Figure 51) which

is also the physiographic boundary between the Mountain and

Desert subprovinces of the Basin and Range. Thus, the

north- to north-northwest-trending zone that separates the

Papago terrane from the rest of southeastern Arizona also

separates domains of southwest to west-directed extension

from areas of north-directed extension. The characteristics

of that boundary, however, remain equivocal.

, I

J

-"I

. I

I

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J

_I

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169

CONCLUSIONS

The Laramide orogeny in the northern Santa Rita

Mountains can be broken into three, inter-related periods of

deformation and intrusion. The period between about 70 and

60 Ma is characterized by the development of a suite of

structures that includes northwest-trending folds, and

inverted Jurassic intra-arc basin south of the Sawmill

Canyon fault zone and a north to north-northwest-trending

basement-cored uplift. The formation of these structures

was largely controlled by the location of pre-existing,

Jurassic and older, northwest-trending normal faults. These

normal faults were reactivated as either reverse faults or

left-lateral, oblique-slip reverse faults in Laramide time.

Reactivation of the pre-existing normal faults in

Laramide time was largely responsible for the location and

style of compressional deformation that characterizes

Laramide structures in the northern Santa Rita Mountains.

Northeast-directed compression acting on steeply dipping,

northwest-trending faults focused deformation along zones

such as the Sawmill Canyon and the Sycamore Canyon fault

zones. Within and along these zones, compressional

deformation was characterized by northeast-directed reverse

faults, basin inversion south of Sawmill Canyon and

formation of northwest-trending folds in Sycamore Canyon.

As Laramide compression rotated to more east-northeast

orientation the Sycamore Canyon fault zone accommodated

left-lateral, oblique-slip reverse movement. Furthermore,

I ·····1

·1

! I

, i ..•

170

this zone acted as the northern boundary of a north to

north-northwest-trending basement-cored uplift that was

characterized by an east-facing monocline or fault

propagation fold that developed in association with the

north-trending Santa Rita reverse fault. Intrusion of

granodiorite to quartz monzonite stocks with attendant

copper mineralization at about 60 Ma occurred largely after

this deformational event.

Northeast-dir-ected Laramide compression also was

responsible for tectonic inversion of the Jurassic basin

that marked the trace of the Jurassic arc through this area.

Basin inversion during the Laramide reactivated the

northwest-trending Sawmill Canyon fault zone. This fault,

which through Jurassic to at least Middle Cretaceous time,

appears to have accommodated south-side-down movement. In

the Laramide, these faults were reactivated as steeply

southwest-dipping reverse faults that juxtaposed Jurassic

and Early Cretaceous strata against Precambrian crystalline

rocks and lower Cretaceous strata. This basin inversion

event is responsible for the current topographic and

structural position of Jurassic rocks south of the Sawmill

Canyon fault zone which appear to be elevated a minimum of

6,500 feet relat~ve to rocks north of the fault. This

inversion structure may continue to the northwest along the

projection of the Sawmill Canyon fault zone and may demark

the northeastern boundary of the Papago terrane of Haxel and

others (1984). Inverted basins have not been previously

recognized in the southern Cordillera of southwestern North

America.

~I

I

1

I ',' J '" j

I

]

171

The presence and style of Laramide structures in the

northern Santa Rita Mountains was partly concealed by

subsequent middle Tertiary extension and rotation which

tilted the basement-cored uplift and attendant structures

approximately 30 0 -50 0 to the east and southeast. This

extensional event turned the originally steeply west-dipping

Santa Rita reverse fault into a shallow «20 0) west-dipping

thrust fault and rotated originally subhorizontal fold axes

to moderate southeast plunges. Total middle Tertiary

extension was approximately 150 to 160%. Previous work in

the area had failed to recognized the presence and

importance of the extensional faults.

West-directed middle Tertiary extension may continue as

far south as the Sawmill Canyon fault zone. The low angle

normal fault inferred to underlie the northern Santa Rita

Mountains may be a continuation of the Catalina fault system

that rims the Santa Catalina-Rincon metamorphic core complex

to the north. The regional occurrence of eastward dips in

Oligocene-Miocene strata of the Pantano Formation and

southeast plunges of folded Paleozoic and Lower Cretaceous

strata, indicate a domain of tilted rocks continues south

from the Rincon Mountains into the northern Santa Rita

Mountains and possibly as far south as the Sawmill Canyon

fault zone. Rocks south of the Sawmill Canyon fault zone

are largely northeast-dipping with subhorizontal fold axes

and therefore, unaffected by middle Tertiary tilting along

the proposed detachment fault.