04 Nourse 1994 Sonora Metamorphic Core Complex

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TECTONICS, VOL. 13, NO. 5, PAGES 1161-1182, OCTOBER 1994

Tertiary metamorphic core complexes n Sonora,

northwestern Mexico

Jonathan A. Nourse

Department f GeologicalSciences,CaliforniaStatePolytechnicUniversity, Pomona

Thomas H. Anderson

Department f Geologyand PlanetarySciences, niversityof Pittsburgh,Pennsylvania

Leon T. Silver

Divisionof Earth and PlanetarySciences,California nstituteof Technology,Pasadena

Abstract. Several ranges encompassingmore than 35,000

km 2 of Sonora, Mexico, contain distinctly lineated and

foliated granitic and metamorphic rocks that constitute the

lower plates of metamorphic core complexes. Penetrative

deformation is characterized by gently dipping mylonitic

foliation acrosswhich northeast rending stretching ineation

is everywhere developed. Prominent northwest trending

fractures, dikes, and normal faults are orthogonal to the

lineation. Most kinematic ndicators n lower plate mylonitic

rocksrecord op-to-the-southwestenseof shear. Upper plate

stratigraphicsequences nclude Mesozoic supracrustal ocks,

Tertiary volcanic and sedimentary rocks, and allochthonous

Precambrianbasement. Tilted blocks of upper plate strata

generally overlie the mylonites along gently dipping

detachmentaults. PreviouslypublishedU-Pb and K-At ages

from lower plate granitic orthogneisses,pper plate volcanic

sequences, and crosscutting dikes constrain the time of

mylonitic deformationand detachment aulting in severalof

these areas o late Oligocene-earlyMiocene. Partitioning of

extensionalstrain in Sonora was influenced by pre-Tertiary

crustalstructure.The belt of core complexes eveloped cross

two contrastingblocks of continental crust separatedby the

N60W striking Mojave-Sonora megashear. Portions of the

southern Papago block (northeast of the megashear)

consisting f Jurassicmagmaticarc rocks and Upper Jurassic-

Cretaceous siliciclastic and carbonate strata resting upon a

concealed, ectonically fragmentedPrecambrianbasementwere

especially susceptible to crustal attenuation. Some core

complexesof the southern Papago block occur within zones

trending northwest that may coincide with Late Jurassic

lineaments. In the Caborca block (southwest of the

megashear), ore complex-relatedocks and structures ave not

been identifiedwhere surfaceexposures f Middle Proterozoic

basement nd overlying Upper Proterozoic-Paleozoiclatform

strata are common. However, extensional mylonitic fabrics

are locally developed long he marginsof a Tertiary two-mica

granite batholith. Core complexes on both sides of the

Copyright 1994 by the AmericanGeophysical nion.

Paper number93TC03324.

0278-7407/94/93TC-03324510.00

megashear ppear o be preferentiallydevelopedwhere Tertiary

graniteshave intruded egionsof crustwith basement isrupted

by pre-Tertiary structures. Sonorancore complexespreserve

an extensional ectonichistory comparablewith that described

from core complexes arther north in the United States and

Canadian Cordillera. The timing of mid crustal extension n

Sonora (25-18 Ma) is contemporaneouswith the timing of

core complex development n Arizona, Nevada, and Utah.

Extension occurred later in these areas than in the Pacific

Northwest-British Columbia region but earlier than in the

Mojave Desert-Death Valley region. Middle Tertiary

mylonitic fabrics of similar style and orientation have not

been recognized farther south in Mexico. The southern

terminus of the mid-Tertiary Cordilleran core complex belt

appears o be in Sonora.

Introduction

Purpose and Objectives

Extensive egionsof central and northernSonora Figure 1)

are underlain by gneissesand schistswith mylonitic fabrics.

Much of this crystalline ock, once assumedo be Precambrian

in age, is now known to have acquired ts penetrative abric

during Tertiary time [Anderson et al., 1980; Silver and

Anderson, 1984]. Deformational style at all scales is

dominated by extensional structures similar to those

documentedn numerousmetamorphic ore complexesof the

United Statesand CanadianCordillera [Crittendon et a/..,1980,

and references therein]. Since the reconnaissance studies

reported by Davis et al. [1981] and Silver and Anderson

[1984], exposures f Tertiary gneissand schist n Sonorahave

been interpreted as lower plates of metamorphic core

complexes,remnants of normal slip, midcrustal shear zones.

A primary objectiveof this paper s to document he lithologic

and structural elationshipswithin these core complexes. A

second objective is to analyze possible influences of pre-

Tertiary crustalstructureon the geographic istributionof core

complexes n Sonora. Why are the core complexes eveloped

in some areas but not in others?

Occurrences f Tertiary mylonite n Sonora Figure 1) form a

north-northwest rending belt extending from the latitude of

1161


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1162

NOURSEET AL.' TERTIARY CORECOMPLEXES, ONORA,MEXICO

(#13)

U/PbSample ocation; ge n Ma

K/Ar Sample Location; Age in Ma

Detachmenault

Middleertiary

Mylonitic Fabric

Late Mesozoic-Early Cenozoic

Regional Metamorphic Fabric

All Other Bedrock

USA

:MEXICO

Caborca

.'ABORCA

1120

Magdalena

} aAna

(#10)'

Figure,,_ '23-;

o

I'iure 11 '

I

I

I

I

KI

o

.

Hermosi o mM

36-32

(#1

58

Figure1. Mapof northernndcentral onorahowinghedistributionf Tertiarymetamorphicore

complexesnd ublishedsotopicges eatingn he iming f extension.gesampleocationsrekeyed

to descriptionsndcitationsn Table1. Pre-Tertiarytratigraphicelationshipsn the Caborcalock,

southernapagolock, ndPinal lock resummarizedn Figure .


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NOURSEET AL.: TERTIARYCORECOMPLEXES, ONORA,MEXICO

1163

Hermosilloo the nternationalorder.Theyareunevenly

distributed n both sidesof a hypothetical ate Jurassic

transformault known as the Mojave-Sonora egashear

[Silverand Anderson, 974;Anderson nd Silver,1979]. In

terms of areal exposure, he core complexesare less

conspicuouso the southwestof the megashear. We

hypothesizehatpositioningf Tertiary ranitesndspatially

associated ylonite oneswas nfluenced y pre-Tertiary

compositionalheterogeneities nd mechanicaldiscontinuities

in the Sonoran crust.

In the next section,we summarizehe pre-Tertiary

stratigraphyof three crustal blocks in northern and central

Sonora,wo of whichhostcorecomplexes.Evidenceor a

middleTertiary age of fabric developments tabulatedrom

publishedork. Additionally, e describeieldrelationships

and structural atternsrom eight geographicallyistinct

lower plate mylonite domains. In distinguishinghe

extensionalmylonitic abrics rom preextensionalabrics,we

are able to constrain he geometryand kinematics f several

Tertiary hear ones.Detachmentaults ndupper late ocks,

wherepresent, re brieflydescribed.Finally,we pointout

likely pre-Tertiary tructuralontrols n the partitioningf

midcrustal extension across Sonora.

Previous Work on Tertiary Extension in Sonora

Subhorizontaloliations nd ineationsn gneissicranite

bodiesof Sonorawere first mapped n rangesnorth of

MagdalenandSanta na Salas, 968].Apparent-Pbages

from irconn theSierraGuacomearanodioriteneissFigure

1 and Table 1) indicatea crystallizationge of 78+3 Ma

Table . PublishedeochronologicalataBearingn heAgeof Extensionn Sonoranore omplexes

Map Lithology LocalGeologic Dating Interpreted

Location Setting Technique Age,Ma

1 Sierra gneissic

Mazatan bio granite

2 Puerto del foliated 2-

S o 1 mica granite

3 Sierra foliated bio

Guacomea granodiorite

4 northern 2-mica granite

Aconchi

batholith

5 northern hornblende

Aconchi andesite

batholith

6 northeast of basaltic

Aconchi andesite

batholith

7 Tubutama latite

basin

8 northern 2-mica

Sierrade a granite

Madera

9 Sierra la alkalic

Ventana latite

10 Magdalena basaltic

basin andesite

11 southern

Sierra de

la Madera

12 southern

Baboquiveri

Mountains

13 southern

Baboquiveri

Mountains

14 Cerro

Carnero

rhyolite

microdiorite

rhyolite

porphyry

biotite schist

muscovite

schist

foliated biotite

granodiorite

lower plate,

Mazatan complex

isolated lower

plate exposure

lower plate, Magdalena-

Madera complex

lower plate, Aconchi

complex

dikes intrude lower

plate granite, Aconchi

complex

flow in Baucurit

Formation,upper

plate, Aconchi complex

flow in upperplate

sedimentary equence,

Tubutamacomplex

lower plate, Magdalena-

Madera complex

upper plate, Magdalena-

Madera complex

upper plate, Magdalena-

Madera complex

dike intrudes lower

plate mylonitic granite

and upper plate

conglomerate

dikes intrude lower

plate mylonitic

granite

intrudes microdiorite

dikes of site 12

U-Pb zircon 58+2

U-Pb zircon 57+2

U-Pb zircon 78+3

K-Ar plag 36.5 +0.8

K-Ar musc 3 6.0+0.7

K-Ar bio 32.0+-0.7

K-Ar whole 28.3 +0.7

rock 26.7+ 0.6

(2 samples)

K-Ar whole 21.7+0.4

rock


rock

K-Ar bio 33.2+0.7

(2 samples) 26.3 +0.6


rock to

(5 samples) 22.7+0.5

K-Ar whole 22.7+0.4

rock to

(3 samples) 21.6+1.0

K-Ar whole 19.8+0.4

rock

K-Ar whole 24+2

rock 22.4+0.6

(2 samples)

U-Pb zircon 24+2

roof rocks of Carnero K-Ar bio 16.9+0.6

pluton, lower plate, K-Ar musc 16.2+0.5

Carnero omplex (2 samples) 16.1+0.5

lower plate, Carnero K-Ar bio 14.8+0.5

complex

Reference

Anderson t.al. [ 1980]

Anderson t al. [1980]

Anderson t al. [1980]

Damon et al. [1983a, b]



Damon et al. [1983a]

Damon et al. [1983a]

Roldan-Quintana 1979]

Gomez-Caballero et al.

[1981]

Gilmont [1978]

Damon, pers. comm.,

[1990]

Miranda-Gasca and

de Jong, 1992

Miranda-Gasca and

de Jong [1992]

Miranda-Gasca and

de Jong [1992]

Goodwin and Haxel

[1990]

Goodwin and Haxel

[1990]

Hayarea et al. [ 1984]

Hayama et al. [ 1984]

Hayarea et al. [1984]

Hayama et al. [ 1984]


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1164 NOURSE ET AL.: TERTIARY CORE COMPLEXES, SONORA, MEXICO

[Andersonet al., 1980]. This age demonstratedhe occurrence

of post-Precambrian tectonism in the Santa Ana region.

Additional geochronologicalwork nearby and in other parts of

Sonora led to the identification of several more granitic

orthogneisses Figure 1) whose penetrative mylonitic fabrics

formed during Tertiary time [Andersonet al., 1980; Silver and

Anderson, 1984; Roldan-Quintana, 1991].

One common problem in recent studies Davis et al., 1981;

Nourse, 1989, 1990; De Jong et al., 1988; Jacques-Ayala et

al., 1990; G. B. Haxel and T. H. Anderson,unpublisheddata,

1981) has been to distinguishmetamorphicand deformational

fabrics of Tertiary age from older fabrics which themselves

may be polygenetic. Three episodes of ductile fabric

development may have occurred in the region since Late

Jurassic time, with the relative importance of each event a

matter of debate. As described later, Tertiary extensional

structures in Sonora may be recognized by a regionally

consistent orientation and style.

The metamorphic core complexes shown in Figure 1 include

all known outcrops of mylonite derived from Tertiary

granitoids and nearby country rocks. These layered rocks are

commonly overlain in low-angle fault contact by essentially

unmetamorphosedmiddle Tertiary or older rocks. Figure 1

distinguishesareas with Tertiary extensional mylonitic fabric

from areas with Late Jurassicor Laramide metamorphic abric.

K-Ar ages rom someupper plate volcanic ocksand from dikes

which crosscut detachment zones (Table 1) place an early

Miocene upper limit on the timing of detachment aulting.

From recent to middle Miocene, tectonic activity in Sonora

has been dominated by postdetachment "Basin and Range

style" block faulting. Most ranges of the region are bounded

by steep, north striking normal faults. Adjacent valleys are

filled with late Cenozoic detritus (Figures 3 and 11).

Northwest and northeast rends exist but are less pronounced.

Geologic Framework

Pre-Tertiary Crustal Structure of Sonora

In Sonora, as in adjacentparts of Arizona and other parts of

the Basin and Range province, Cenozoic extensional ectonic

processes were imposed upon continental crust of

heterogeneousstructure and composition (Figures 1 and 2).

Pre-Jurassiccrust in central and northern Sonora s composed

of two distinct Middle Proterozoic crystalline provinces

overlain by Upper Proterozoic and/or Paleozoic and Early

Mesozoic platform strata. The provinces underlain by

Precambrian crust (Caborca block in the southwest and Pinal

block in the northeast) are separatedby a belt of Jurassic

plutonic, volcanic, and sedimentary ocks which we refer to as

the southernPapago block. All three blocks are intruded by

Late Cretaceousplutons and batholiths [Andersonand Silver,

1974, 1977; Silver and Chappell, 1988]. However, only

portions of the Caborca block and the southernPapago block

are hosts to Tertiary metamorphiccore complexes Figure 1).

Pre-Tertiary stratigraphic relationships within the three

blocks are summarized n Figure 2.

Pinal block. Precambrian basement that extends into

Sonora from southern Arizona is composedof 1.68 Ga Pinal

Schist and younger Mid-Proterozoic intrusives, overlain by

Upper Proterozoic-Paleozoic platform strata [Cooper and

Silver, 1964; Anderson and Silver, 1977; Davis, 1980, and

references therein]. Along an irregular contact near the

international border the Precambrian and Paleozoic rocks are

overlain and intrudedby Jurassicvolcanic and plutonic ocks.

Southwest of a line between Nogales and Cananea, the

basement or the Jurassic equences not exposed.

Southern Papago block. Jurassic rocks form a

northwest trending belt that separates the two Precambrian

provinces. The distinctive lithologic assemblage, which

includesvolcanic, plutonic, and sedimentary ocks, was named

the "Papago terrane" where mapped in south-centralArizona

[Haxel et al., 1984]. We adopt the name "southernPapago

block" for the continuation of similar lithologies in Sonora.

The Jurassic rocks compose part of a continental margin

magmatic arc [Andersonand Silver, 1978; Tosdal et al., 1988].

Portions of this arc in Sonora and southern Arizona were

broken by northwest rending faults during Late Jurassic ime,

creating elongate basins that filled with syntectonic

conglomerate and younger Lower Cretaceous marine strata

[Titley, 1976; Bilodeau et al., 1987, Kitz and Anderson, 1988;

Nourse, 1990; Jacques-Ayala t al., 1990]. One of these aults,

the Mojave-Sonoramegashear, orms the abrupt southwestern

boundary of the southernPapago block [Andersonand Silver,

1979]. Jurassicstrata adjacent to the megashear re highly

deformed by thrustsand folds.

Caborca block. The secondPrecambrianprovince, which

lies southwest of the megashear, includes widespread

exposures of Mid-Proterozoic granite and gneiss

unconformably overlain by a thick Upper Proterozoic-

Phanerozoic supracrustalsequence. Postulated correlation of

these rocks to a stratigraphic section preserved n the Inyo

Mountains of eastern California provided one of the early

arguments for large scale sinistral displacement on the

megashear Silver and Anderson, 1974]. Most of the basement

exposed south of Caborca appears to be structurally

autochthonous. Near the megashear, however, mapped

exposures f basementmay overlie Mesozoic rocks along low-

angle faults [Anderson et al., 1984; Rodriguez-Castaneda,

1986].

Early-Middle Tertiary Magmatism and

Sedimentation in Sonora and Southern Arizona

A record of latest Cretaceous-earlyTertiary (Laramide)

orogenesisand peraluminousmagmatism analogous o that

described n the Papago errane [Haxel et al., 1984] may be

preservedwithin portions of the southernPapago block of

Sonora. Two-mica graniteof known or suspected aleogene

age intrudes egionally metamorphosed nd deformed ocks n

several localities [Silver and Anderson, 1984; Nourse, 1989].

Fabrics in these metamorphic rocks record different

deformational conditions and structural styles than do the

younger mylonitic fabrics. Ongoing debate centers on

whether the age of the older fabrics is Late Jurassic or

Laramide. Structures f both agesare very likely preserved

locally.

Laramideorogenesisn southern rizonawas ollowedby the

accumulationof predominantly conglomeratic ower(?) to

middle Tertiary sedimentsn broadcontinental asins Eberly

and Stanley, 1978]. Upward in the Cenozoic sequence,


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NOURSE ET AL.' TERTIARY CORE COMPLEXES,SONORA,MEXICO 1165

CABORCA BLOCK; SOUTHERN PAPAGO

BLOCK:

PINAL BLOCK

LATE

CRETACEOUS

EARLY

CRETACEOUS

;i.,ATE

JURASSIC

MIDDLE

JURASSIC

ARLY

JURASSIC

TRIASSIC

PALEOZOIC

i

LATE

PROTEROZOIC

MIDDLE

PROTEROZOIC

Laalmaormation:4.j

ndesirelowsnd .

Bisbeeroup'

Ceo deOroFomation:

limestonend udstone'

Glance

Unconformi

(generally

not xsed)

Anfimonioormation

dstone, silmtone,

lesmne,umne;

ttoniccontacti

-- Carca blk ment

mm d fme-

pffo s .

Unconformity

layergnes,chis

d mphilim, ind

' by 1.7Ga,1.4Ga, d

1.1Ga nite

andesfielows ndbrecctjs

'.- ', ', ..':

0 Glance onelomerate:

,.0 yntectonicuvialongloneat:

.,/'x'Unconformity'v'

.V)IX V

\ Jurassicagmaticrc:

Crhyoliteorphyrylowsnd

.Anterstratifiedith uartzrenite

q and olcaniclasficonglomerate.'

) ly intrudedygranite

orphyryills

(Pre-Jurassic

Basement

Conceded)

sandstone, udstone,':

.:.rhyoliteuff,

Bisbee.roup'.'

GlanceonlomerateC

o - --o 0 '--

,muncon]ormtty

_0

Exposed)

,,,,,,

rgrained

feldspathiclaffo--------

?Vnonorm*it'"f'l: h

Pinal Schist,

intrudedy .([5

a, .4Gaand

1.1Ga ra' ite

Figure 2. Simplified stratigraphic olumnsshowingcontrasting re-Tertiarygeology n three crustal

blocks f northern ndcentralSonora.Patternsor rockunitscorrespondo pre-Tertiary eologymappedn

Figures and11. Stratigraphicomenclaturendageconstraintsrecompiledromstudies y Taliaferro

[1933], Valentine, 1936; Cooperand Silver [1964], Salas [1968], Silver and Anderson 1974], Anderson

and Silver 1974, 1977, 1978, 1979, 1981], Corona [1979],Davis [1980], Gonzales-Leon1980], Stewart

et al. [1984],Montano,1984;Haxel et al. [1984],Bilodeau, t al. [1987],Silver and Chappell 1988],

Tosdalet al. [1988], Gonzales-Leon nd Jacques-Ayala1988],Kitz and Anderson 988],Nourse [1989,

1990],Jacques-Ayala t al. [1990].

volcanic ocks with K-Ar agesbetween30 and 25 Ma become

progressivelymore abundant [Damon and Mauger, 1966;

Shafiqullahet al., 1980]. In Sonora,similar sedimentary/

volcanic sequences ave been described outhof Magdalena

[Nourse, 1989;Miranda-Gascaand De Jong, 1992], in the

vicinity of Tubutama [Frye, 1975; Gomez-Caballeroet al.,

1981], andnear the marginsof the Aconchibatholith Roldan-

Quintana, 1979].

Timing of Ductile Deformation In Metamorphic

Core Complexes of Sonora and Arizona

Controversy as commonlyaccompanied iscussionsf the

timingof ductiledeformationn Cordilleranmetamorphicore

complexes.The issue s complicated y two problems. (1)

Many of the ductile abricsare polygenetic.2) Intrusions sed

to date the fabrics have commonlyexperienced omplex

thermalhistories.Amongsomecorecomplexesn Sonora nd

southern rizona,studyof K-At, Rb-Sr,andU-Th-Pb sotopic

systemshas facilitated geochronologicalnterpretationof

lower plate gneisses Shakel et al., 1977; Anderson et al.,

1980; Shafiqullah et al., 1980; Wright et al., 1981, 1986;

Silver and Anderson, 1984; Reynolds et al., 1986, 1988;

Gehrelsand Smith, 1991; Goodwinand Haxel, 1990]. Table 1

and Figure 1 summarizeexisting geochronological ata and

sample locations from Sonora that bear on the timing of

metamorphic ore complexdevelopment.

Geochronologicalomparison f lower and upperplatesof

core complexes rom Sonoraand Arizona indicates hat the two

regionssharedsimilar historiesof thermalactivity and fabric

development uringMesozoicandTertiary ime. Many of the

corecomplexesecordone or more periodsof Mesozoicductile

deformation.Relatedmetamorphicabricsmay be intrudedby

postkinematicwo-micagraniteof Paleogene ge [Anderson t

al., 1980; Haxel et al., 1984]. Mylonitic fabrics and brittle

structureselated o detachmentaulting are late and overprint

granite and volcanic strata. Some of the granite bodies hat

recordmylonitic deformation ield early Miocene U-Pb ages

[Reynolds et al., 1986; Wright et al., 1986; Gehrels and

Smith, 1991]. Thus ductile deformation in many areas

persisted to the beginning of Miocene time. Widespread,


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1166 NOURSE ET AL.: TERTIARY CORE COMPLEXES, SONORA, MEXICO

nearly coeval K-At ages show that the culminatingprocesses

of lower plate cooling, coupled with tectonic unroofing of

upper plate, occurred hroughoutmost of the region between

about25 and 18 Ma [Silver and Anderson,1984; Reynoldset

al., 1988; Table 1].

Tertiary Gneissand Schist n Sonora

Rocks n Sonora hat record ductile deformationof Tertiary

age may be recognized by the remarkably systematic

orientation of northeasterly trending stretching lineation

developed upon subhorizontal oliation. This distinctive

mylonitic fabric is known to be imposed upon Paleocene

granite Anderson t al., 1980] and s therefore ertiary n age.

The fabric commonly extends nto the country rocks adjacent

to dated plutons and although considerable variation in

intensity exists, consistently oriented structural elements

have been mapped and correlated in rocks far from dated

samples. Outcropsof Tertiary gneissand schist orm a north-

northwest rending belt extending rom Mazatan through he

border town of Sasabe [Anderson et al., 1980; Figure 1].

Penetratively deformed rocks are not exposed continuously

within this band but occur as domainssegmented y sharpor

transitional boundaries.

Exposuresof Tertiary mylonite in northern and central

Sonora are subdividedbelow into eight geographicdomains.

Five of thesedomainsoccur within or near the southwest art

of the southern Papago block (Figure 3). Portions of the

Magdalena-Maderadomain and the Jarillas-Potrero-Tortuga

domainwere mappedby Salas [1968],Andersonet al. [1980],

andNourse [1989]. Data from the Tubutama-Mesquital omain

were collectedduringreconnaissance ork by G. B. Haxel and

T. H. Anderson (unpublished data, 1981) and Nourse

(unpublisheddata, 1987). Various parts of the Pozo Verde

domain have been mapped by Davis [1980], Davis et al.

[ 1981], Haxel et al. [ 1982], and Goodwin and Haxel [ 1990].

Portionsof the Carnero domain have been studiedby T. H.

Anderson nd L. T. Silver (unpublished ata, 1979), Hayama et

al. [1984], and Jacques-Ayala et al. [1990]. The three

remainingTertiary mylonite domainsare exposedsouthwest f

the megashearwithin the Caborca block (Figure 11). Our

descriptionof the Aconchi domain draws upon the work of

Roldan-Quintana [1991]. Field reconnaissancen the Puerto

del Sol and Mazatan domainswas carriedout by T. Anderson

(unpublisheddata, 1979), and complimentscontemporaneous

work suchas thatby Peabody 1979].

Core ComplexesDeveloped n the Southern

Papago Block

General Character and Boundaries

The widest part of the Tertiary core complexbelt is in the

southernPapagoblock between he Imuris lineamentand the

Mojave-Sonora megashear (Figure 3). Within this region,

Tertiary gneiss and schist have formed from predominantly

supracrustalocksof Jurassic nd Early Cretaceous ge ntruded

by Late Cretaceous nd youngerplutons Anderson nd Silver,

1979; Anderson et al., 1980; Montano, 1984; Nourse, 1989,

1990]. Away from the outcropsof mylonitic rocks, volcanic

and volcaniclasticJurassicstrata are commonly folded along

west and northwest rendingaxes,and lineationdirectionsmay

differ from the regionally consistentnortheast trend of the

Tertiary mylonites [Corona, 1979; Nourse, 1989].

The Imuris lineament,which boundscore complexesof the

southern Papago block, separatesunmetamorphosed rust to

the northeast rom ductilely extendedcrust to the southwest.

This northwest rending ineament s the most prominentof a

seriesof fractures hat segment angesbetweenNogales and

Caborca Figures 1 and 3). It separates ierraCibuta from the

Sierras Guacomea, Jojoba, and Magdalena to the southwest.

Jurassic volcanic strata composing Sierra Cibuta lack

mylonitic foliation and stretching ineation. At the lineament

a slight increase in metamorphism is evident in the

recrystallizationof the groundmass f lithologically similar

quartz porphyry. Lineation and foliation parallel to the

regional Tertiary orientation ntensify to the southwestand

become penetrative within 5 km of the lineament. Similar

relationshipsmay be observed o the northwest n the vicinity

of E1 Correo.

Southeast f Imuris, granitic ocksof the northernSierrade

la Madera displaya very weak Tertiary fabric where ntersected

by the Imuris lineament. Within a few kilometers to the

southwest, well-developed mylonitic foliation is

characteristic. Northwest trending mafic dikes are also

commonnear the lineament. These dikes intrude the Tertiary

fabric and appear to mark a localized region of brittle

extension.

If the Imuris lineament s projected orthwestwardcrosshe

international border, it intersects the southern Baboquivari

Mountainsof Arizona (Figure 3). Within a 10-km-widezone,

swarmsof northwest rendingmicrodioriteand rhyolite dikes

intrudemetamorphosedurassic ocks and weakly to strongly

foliated Paleocene(?) wo-mica granite [Hazel et al., 1982;

Goodwin and Hazel, 1990]. As in the Sierra Madera, these

dikes strike orthogonal to mylonitic stretching lineation

observedn the granite Goodwinand Hazel, 1990]. They also

mark the approximate position of a northwest trending

mylonitic front.

Two isolatedexposures f Tertiary gneissand schistoccur

near the Mojave-Sonora megashear Figure 3). At Cerro

Carnero, characteristic Tertiary mylonitic fabric is

superimposedon granites that have intruded Jurassic and

Cretaceous supracrustal rocks with Late Jurassic and/or

Laramide structures. At Cerro Prieto, stronglyrecrystallized

marble, quartzite, and diorite display locally well-developed

stretchingineationsof the Tertiary trend.

Throughouthe southern apago lock, ndividualexposures

of Tertiary mylonite are bounded y either ow-angleor high-

angle normal faults. Where exposed, he structurallyhighest

portions of mylonitic sections terminate abruptly against

unmetamorphosedower Cretaceous r Tertiary rocks. Brittle

detachmentaults mark the boundary etween ower and upper

plates. The detachment aults in turn are broken by steep

normal faults which may strike north, northwest,or northeast

(Figure 3).

Magdalena-Madera

This region is named for the prominentTertiary mylonite

zone and associated detachment fault exposed in Sierra

Magdalenaand Sierra de la Madera (Figures3-5). Pre-middle


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NOURSE ET AL.: TERTIARY CORE COMPLEXES, SONORA, MEXICO 1167

11145 111030 111Ol5

0 IOkm

31030

IPozo

M^ ' SYMBOLS t

.........:&........:......:..

'-iiiii'-'..-iStrongiddleertiaryyloniticabric

Detachmentault

ilighngleault

MexicanFederal llghway

Well maintainedconnecting oute

. IMURI$ LINEAMENT

EXPLANATION11o 045'

.__ Post-middleioceneedimentaryndolcanictrata

Low... ddleertiaoedimentaryndolcanictrata

.-..JUpper... c-...er ret......ds...

reaire, and carbonate

l ..... c.u-r,.,r..,'r,'nd.'o,...

Upperroterozoic-Paltzoiclatformtrata

Middlerolerozicryslallineasement

> /.

+ + ++

+ 4- - +-+

4- + 4' 4' +

+ + - +

+ 4- + +

Santa Teresa i

"

/..- '7

31Ol5 '

XA L

>

Documents

04 Nourse 1994 Sonora Metamorphic Core Complex