GSA Bulletin: Roberts Mountains allochthon and the western

ABSTRACT

The truncated southwestern edge of theRoberts Mountains allochthon is exposed inthe Northern Ritter Range pendant in theeastern Sierra Nevada, structurally overlyingparautochthonous rocks of the Cordilleranmiogeocline. The Northern Ritter Range pen-dant exposes units that have the same strati-graphic affinities and structural relationshipsas rocks of the Antler orogenic belt in Nevada.

Paleozoic metasedimentary rocks exposedin the pendant consist of two major units: (1) astructurally complex, disrupted chert andargillite unit interpreted to be correlative withthe Roberts Mountains allochthon; and (2) astratigraphically coherent siliceous and cal-careous unit, the Rush Creek sequence, that isinterpreted to be part of a transitional outershelf and slope assemblage of the lower Paleo-zoic Cordilleran miogeocline. In the NorthernRitter Range pendant, the Roberts Mountainsallochthon structurally overlies the RushCreek sequence along a north-striking, steeplydipping fault zone that may be a preservedremnant of the Roberts Mountains thrust,which in north-central Nevada emplaced theallochthon over outer shelf and slope strata ofthe Cordilleran miogeocline during the LateDevonianÐEarly Mississippian Antler orogeny.

These stratigraphic and structural belts aretruncated on the southwest side of the pen-dant by the Gem Lake shear zone, a north-west-trending dextral strike-slip fault associ-ated with the Cretaceous Sierra Nevadabatholith. The Northern Ritter Range pen-

dant thus defines both the southwestern limitof the Antler orogenic belt and the western-most exposures of parautochthonous miogeo-clinal rocks in the central Cordillera.

INTRODUCTION

Anumber of roof pendants and wall rock septaof Paleozoic metasedimentary rocks are pre-served within the Mesozoic Sierra Nevadabatholith in the eastern Sierra Nevada of Califor-nia (Fig. 1). These pendants contain an importantrecord of stratigraphic and structural relationsalong the Paleozoic Cordilleran continental mar-gin, relations that elsewhere in eastern Californiahave been obscured by voluminous Mesozoicplutonism or buried by Cenozoic strata.

Numerous hypotheses for the late Paleozoicand early Mesozoic tectonic evolution of thisregion have been proposed, including accretionof allochthonous terranes (Nokleberg, 1983),sinistral truncation of the Paleozoic continentalmargin and southward offset of Paleozoic rocks(e.g., Davis et al., 1978; Stone and Stevens, 1988;Walker, 1988), and east-vergent contractional de-formation superimposed on an original promon-tory in the continental margin (Dickinson, 1981;Oldow, 1984). Subsequent Cretaceous dextraldisplacement of Paleozoic rocks was proposedby Schweickert and Lahren (1990), Stevens et al.(1992), and Kistler (1993), among others. Impor-tant to all these models is the geographic extentof autochthonous or parautochthonous miogeo-clinal rocks of the Cordilleran continental mar-gin, and the location and nature of the boundarybetween these miogeoclinal rocks and allochtho-nous eugeoclinal rocks to the north and west.

The focus of this paper is the Northern RitterRange pendant, which is located in the east-central

Sierra Nevada (Fig. 2) at the eastern edge ofYosemite National Park. The pendant consists of a3Ð5-km-wide septum of Paleozoic metasedimen-tary rocks and Mesozoic metavolcanic rocks thatextends 20 km northwest from the larger RitterRange pendant (Kistler, 1966a, 1966b; Huber andRinehart, 1965; Greene, 1995). Strata in theNorthern Ritter Range pendant generally strikenorthwest and dip steeply to the southwest (Fig. 3).All pre-Tertiary rocks in the Northern Ritter Rangependant have been metamorphosed under green-schist to epidote-amphibolite facies conditions.

In this paper we describe the western limit ofparautochthonous miogeoclinal rocks of theCordilleran continental margin in the NorthernRitter Range pendant, and the fault contact thatseparates them from lower Paleozoic rocks thatwe correlate with the Roberts Mountainsallochthon. Similar structural relations occur innorth-central Nevada, where basinal rocks of theRoberts Mountains allochthon were emplacedover outer shelf and slope strata of the Cordilleranmiogeocline during the Late DevonianÐEarlyMississippian Antler orogeny. We interpret thejuxtaposition of basinal rocks and miogeoclinalrocks in the Northern Ritter Range pendant to in-dicate the southwestern limit of the Antler oro-genic belt in the east-central Sierra Nevada.

PALEOZOIC STRATIGRAPHY

Previous studies grouped all Paleozoicmetasedimentary rocks in the Northern RitterRange pendant into one generalized unit termedthe Lewis sequence (Kistler, 1966a, 1966b),which was thought to be late Paleozoic in age(Kistler and Nokleberg, 1979). New and moredetailed mapping by Strobel (1986) and Greene(1995) allows division of the Lewis sequence

1294

Roberts Mountains allochthon and the western margin of the Cordilleranmiogeocline in the Northern Ritter Range pendant, eastern Sierra Nevada,California

David C. Greene* Department of Geology and Geography, Denison University, Granville, Ohio 43023

Richard A. Schweickert Department of Geological Sciences, University of Nevada, Reno, Nevada 89557

Calvin H. Stevens Department of Geology, San Jose State University, San Jose, California 95192

GSA Bulletin, October 1997; v. 109; no. 10; p. 1294Ð1305; 10 figures; 2 tables.

*E-mail: [email protected]

(here abandoned) into three separate units: (1) achert and argillite unit correlated with part of theRoberts Mountains allochthon (Schweickert andLahren, 1987; Greene et al., 1989); (2) asiliceous and calcareous unit named the RushCreek sequence (Strobel, 1986), correlated withOrdovician through Devonian continental slopeand rise facies of the Cordilleran miogeocline(Stevens and Greene, 1994; 1995); and (3) rarefault slices of upper Paleozoic marble correlatedwith the Pennsylvanian Mount Baldwin Marble

and indicating local development of a late Pale-ozoic carbonate platform (Greene and Dutro,1991; Stevens and Greene, 1994; Greene andSchweickert, 1995).

Chert-Argillite Unit

The chert-argillite unit in the Northern RitterRange pendant is structurally complex and con-sists predominantly of black chert that grades intosiliceous argillite and locally to slate (Fig. 4).

These rocks are thin bedded, dark rusty orangeweathering, black to dark gray, and locally pyritic.Light grayÐweathering phosphatic lenses, thinstringers, and nodules are locally abundant in thechert (Greene, 1995). Discontinuous, 1Ð2-m-thicklenses of sandy, highly cleaved marble are presentat a few locations, and thin-bedded quartz siltstoneis locally abundant in the northern part of the pen-dant. The chert-argillite unit commonly appearswell bedded in outcrop, but individual layers arediscontinuous on a scale of 1Ð5 m (Fig. 5A). Lo-cally, disrupted layering, intraclast breccia beds,and chaotic units suggest that significant syndepo-sitional deformation has occurred (Fig. 5B).

Very fine grained, thin-bedded to finely lami-nated, calc-silicate hornfels is present locally inthe chert-argillite unit. The calc-silicate rocksform isolated lenses, 2Ð100 m thick and as longas 900 m, that are folded and deformed with theenclosing chert and argillite. The calc-silicaterocks are more competent than surroundingrocks, and the discontinuous nature of the inter-calated lenses suggests large-scale boudinageand tectonic disruption of internal stratigraphy.

The predominance of chert and argillite in thisunit indicates that deposition occurred in a basi-nal environment. The presence of continentallyderived sandy limestone and quartz siltstone sug-gests that the chert-argillite unit was deposited ina continental margin basin probably adjacent toNorth America, as has been shown by Finney etal. (1993) for similar rocks of the Roberts Moun-tains allochthon in central Nevada.

Correlation and Age. No fossils have beenrecovered from the chert-argillite unit in theNorthern Ritter Range pendant. The unit is, how-ever, continuous northward from the NorthernRitter Range pendant into the Saddlebag Lakependant (Kistler, 1966a; Greene, 1995), where itwas correlated by Schweickert and Lahren(1987) with an Ordovician chert and shale unit inthe Candelaria Hills in west-central Nevada(Fig. 6). In this section we discuss the evidencesupporting this correlation and its tectonostrati-graphic implications.

In the Saddlebag Lake pendant, the chert-argillite unit consists of highly deformed blackchert and siliceous argillite, interbedded siltstoneand calc-silicate hornfels, and minor lenses ofquartzite, basalt, and limestone (Schweickert andLahren, 1987; 1993). The unit there is uncon-formably overlain by as much as 30 m of lithicsandstone and chert-pebble conglomerate con-taining limestone clasts with Permian conodonts,which are assigned to the Diablo Formation(Schweickert and Lahren, 1987). These rocks areoverlain by as much as 950 m of thinly beddedsiltstone and fine sandstone locally containingchert and shale clasts and chert-pebble to chert-cobble conglomerate. These rocks are assigned to

NORTHERN RITTER RANGE PENDANT, EASTERN SIERRA NEVADA

Geological Society of America Bulletin, October 1997 1295

Figure 1. Generalized tectonic map showing the location of the Northern Ritter Rangependant and elements of the Antler orogenic belt from central Nevada to the Mojave Desert.Box indicates area of Figure 6. KFÑKern Canyon fault; MKÑMineral King pendant; SL +SMÑSnow Lake and Sachse Monument pendants. Modified from Jennings (1977), Albersand Stewart (1972), and Kleinhampl and Ziony (1985); additional data are from Walker(1988), Dunne and Suczek (1991), and Schweickert and Lahren (1991).

the Candelaria Formation (Schweickert andLahren, 1987).

In the Candelaria Hills, the stratigraphicallylowest unit consists of thin-bedded, dark gray toblack chert interbedded with dark gray siliceousargillite and laminated phyllitic shale, and struc-turally interleaved with sandy limestone and cal-careous quartz sandstone (Stanley et al., 1977;Stewart, 1979). Locally abundant graptolites inthe phyllitic shale indicate an Ordovician age(Ross, 1961; Stewart, 1979), whereas the cal-careous quartz sandstone contains Devonian con-odonts (Stanley et al., 1977; Stewart, 1979). Thechert and shale unit is unconformably overlain bythe Permian Diablo Formation, which consists ofas much as 25 m of thinly interbedded, medium-to coarse-grained sandstone and chert-pebbleconglomerate (Speed et al., 1977). The DiabloFormation is overlain by the Lower Triassic Can-delaria Formation, consisting of more than 975 mof green to brown shale and sandy shale, massivebrown sandstone, and interbedded sandstone andchert-pebble conglomerate (Ross, 1961; Albersand Stewart, 1972).

Correlation of the Paleozoic succession in theCandelaria Hills with that in the SaddlebagLake and Northern Ritter Range pendants is fur-ther strengthened by notable similarities in theirstructural geometries. Both areas consist ofstructurally disrupted sections of predominantlychert and argillite unconformably overlain byrelatively intact upper Paleozoic strata. Two ma-jor generations of mid-Paleozoic structureswere recognized in lower Paleozoic rocks in theCandelaria Hills by Oldow (1984). First-generation, tight to isoclinal folds with well-developed axial planar cleavage are overprintedand reoriented by second-generation folds withpredominantly northeast-striking, upright axialsurfaces. These structures are similar to those

GREENE ET AL.

1296 Geological Society of America Bulletin, October 1997

Figure 2. Generalized geologic map of the Northern Ritter Range pendant. Line AÐA′indicates location of the cross section shown in Figure 3. Simplified from Greene (1995).

Figure 3. Diagrammatic cross section ofthe Northern Ritter Range pendant. Paleo-zoic metasedimentary rocks of the RushCreek sequence are structurally overlain bythe chert-argillite unit, which is faultedagainst Mesozoic metavolcanic rocks of theKoip sequence. The location of section line isshown in Figure 2.

recognized in the Northern Ritter Range pen-dant, which include first-generation tight to iso-clinal folds and upright second-generation foldsinterpreted to have been predominantly north-east trending prior to reorientation during third-generation folding. These structures are describedin detail in a later section.

In summary, significant similarities in lithol-ogy, structural style, and stratigraphic successionbetween the Candelaria Hills in west-centralNevada and the Saddlebag Lake and NorthernRitter Range pendants in the eastern SierraNevada support correlation of the Paleozoic suc-cessions in these three areas.

Tectonostratigraphic Nomenclature. Thechert and shale unit in the Candelaria Hills wasoriginally assigned to the Palmetto formation byFerguson et al. (1954), and that name was usedby Schweickert and Lahren (1987, 1993) andGreene (1995) for the correlative rocks in theSaddlebag Lake and Northern Ritter Range pen-dants. Other workers, however (e.g., Ross, 1961;Stewart and Poole, 1974; Stewart, 1980; Stevensand Greene, unpub. data), have noted marked dif-ferences between the chert and shale unit in theCandelaria Hills and the Palmetto Formation inthe Palmetto Mountains (Fig. 6). In the PalmettoMountains, rocks mapped as Palmetto Formationlocally appear to depositionally overlie theCambrian Emigrant Formation (Buckley, 1971;Albers and Stewart, 1972; Stewart, 1980). Thoserocks closely resemble siliceous and calcareousrocks of the Ordovician Convict Lake Formationin the eastern Sierra Nevada, which we interpretas deeper water equivalents of autochthonous Or-

dovician shelf carbonates to the south (Stevensand Greene, 1995; unpub. data). The chert andshale unit in the Candelaria Hills, in contrast, isstructurally complex and contains structurallyinterleaved Ordovician and Devonian strata withrare interstratified mafic volcanic rocks (Stewart,1980). Such assemblages in north-centralNevada are generally considered to be part of theRoberts Mountains allochthon, and correlativewith units such as the Vinini Formation (Stewart,1980; Oldow, 1984; Finney and Perry, 1991).

We therefore support the correlation of blackchert and siliceous argillite in the SaddlebagLake and Northern Ritter Range pendants withthe chert and shale unit in the Candelaria Hills(Schweickert and Lahren, 1987, 1993), and weinterpret both these areas as exposures of theRoberts Mountains allochthon. Because of theambiguities in previous usage, we prefer not touse the term Palmetto Formation and insteadrefer to the chert and argillite in the NorthernRitter Range pendant informally as the Òchert-argillite unit.Ó

In summary, eugeoclinal rocks of the RobertsMountains allochthon are exposed in the Cande-laria Hills, in the Saddlebag Lake pendant, and inthe Northern Ritter Range pendant. The RobertsMountains allochthon thus extends from centralNevada as far southwest as the Northern RitterRange pendant on the eastern edge of the SierraNevada, implying that, as in central Nevada,structurally underlying parautochthonous rocksof the Cordilleran miogeocline should lie to thesoutheast, either within or southeast of the North-ern Ritter Range pendant.

Rush Creek Sequence

The Rush Creek sequence was informally de-fined by Strobel (1986) to include a stratified suc-cession of siliceous, calc-silicate and calcareousmetasedimentary rocks exposed north of AgnewLake (Fig. 4). The Rush Creek sequence has sincebeen expanded to include similar rocks exposedfor 9 km along the southeast edge of the NorthernRitter Range pendant (Fig. 2) (Greene, 1995).

The Rush Creek sequence includes five majorrock types: calc-silicate hornfels, siliceousargillite, bedded chert, sandy marble, and cal-careous quartzite (Table 1). These are interbed-ded on both meter scale and 10Ð100 meter scale,giving exposures of the Rush Creek sequence adistinctive banded appearance (Fig. 7). The rocksgenerally are fine grained and thin bedded. Raresedimentary structures, developed primarily inthe calc-silicate hornfels, include fine plane lam-inations, cross laminations, graded beds, andsmall scour and fill structures. No syndeposi-tional deformational features were observed,whereas in the chert-argillite unit such featuresare relatively common. Calcareous rocks makeup about 50% of the Rush Creek sequence, incontrast to the chert-argillite unit, in which cal-careous rocks compose less than 5% of the sec-tion. Although the overall sequence is substan-tially deformed by thrust faults and tightmesoscopic to macroscopic folds (Fig. 2), indi-vidual strata in the Rush Creek sequence can betraced for as much as 1 km.

Correlation and Age. No age-diagnostic fos-sils have yet been discovered in the Rush Creek



Figure 4. Diagrammaticlithologic columns of thechert-argillite unit and theRush Creek sequence,showing lithology, struc-tural relations, and pro-posed correlations. Litho-logic column of the RushCreek sequence is from thearea north of Agnew Lakeand is modified fromStrobel (1986). Numbers inparentheses are the unitdesignations of Strobel(1986). Lithologic columnof the chert-argillite unit isdiagrammatic, and is notto scale.

sequence. Nineteen chert and marble samplesprocessed for conodonts yielded only oneunidentifiable fragment (J. Repetski, 1991, per-sonal commun.), and rare trace fossils are nondi-agnostic. The Rush Creek sequence is here con-sidered to be Ordovician through Devonian inage, on the basis of lithologic correlation with a

distinctive stratigraphic succession of lower Pa-leozoic rocks that is present in many of the roofpendants in the eastern Sierra Nevada (Rinehartand Ross, 1964; Greene and Stevens, 1994;Stevens and Greene, 1994, 1995; Wise, 1996).

In the cliffs north of Agnew Lake (Fig. 7),Strobel (1986) described a section of the Rush

Creek sequence consisting of nine units with anaggregate thickness of more than 600 m (Fig. 4).The lower part of this section consists of thin-bedded, green and white banded calc-silicatehornfels. The middle part of the section consistsof interbedded black chert, siliceous argillite andmarble, overlain by thin-bedded, green andwhite banded calc-silicate hornfels. These bedsare overlain by a distinctive 65-m-thick unit ofcalcareous sandstone and sandy marble thatforms a prominent white marker horizon in cliffexposures (Fig. 7), and is overlain by blackphosphatic chert and slate. The upper part of thesection consists of thin-bedded, green and whitebanded calc-silicate hornfels overlain bysiliceous argillite.

The middle part of this stratigraphic sectionbears a marked resemblance to the Ordovicianto Devonian stratigraphic sequence in theMount Morrison pendant. The chert, argillite,and marble probably are correlative with theConvict Lake Formation, the green and whitebanded calc-silicate hornfels with the AspenMeadow formation, the calcareous quartz sand-stone with the Mount Morrison Sandstone, andthe overlying black chert and slate with theSquares Tunnel Formation (Fig. 4).

We interpret the calc-silicate hornfels at thebase of the section in the Rush Creek sequenceand the calc-silicate hornfels and siliceousargillite units in the upper part of the section asthrust repetitions of Convict Lake and AspenMeadow formations, as indicated in Figures 4and 7. The thrust faults, however, are generallycryptic in outcrop, and direct evidence of thrustfaulting was recognized only in a cliff face ex-posing black chert truncated and structurallyoverlain by calc-silicate hornfels (Fig. 7).

In summary, the Rush Creek sequence is inter-preted to be correlative with Ordovician throughDevonian strata in the Mount Morrison pendant,and with similar rocks exposed in pendants bothnorth and south of the Northern Ritter Rangependant. These Ordovician through Devonianstrata are deeper water equivalents of slope andplatform facies rocks in the Inyo Mountains(Stevens and Greene, 1994, 1995). We interpretthe Rush Creek sequence as part of the transi-tional, outer continental shelf and slope assem-blage of the Cordilleran miogeocline.

Upper Paleozoic Limestone

Isolated lenses of gray marble and silicifiedmarble are exposed 1.5 km northwest of AlgerLakes (Fig. 2) (Brook et al., 1979; Greene, 1995).We interpret these lenses to be fault slices withinthe Gem Lake shear zone (Greene and Dutro,1991; Greene and Schweickert, 1995). Fossilifer-ous, silicified marble is exposed as subcrop and

GREENE ET AL.


Figure 5. (A) Tight folds and dismembered bedding in quartz siltstone of the chert-argilliteunit. (B) Disaggregated quartz-rich laminae in a siliceous argillite matrix suggest chaotic softsediment deformation; chert-argillite unit, Parker Pass. Knife is 9 cm in length.

A

B

talus within a 300-m-long, northwest-striking zonethat appears to be structurally imbricated with thechert-argillite unit. The silicified marble is locallybrecciated, but the contained fossils are otherwiseundeformed; there is no evidence of cleavage orductile deformation. This contrasts sharply withthe surrounding chert and argillite, which arehighly cleaved, tightly folded, and contain evi-dence of at least three phases of penetrative, duc-tile deformation (described in a following section).

Lenses of dark gray, coarsely recrystallized,variably silicified crinoidal marble 20 to 100 m inlength and containing locally abundant blackchert nodules are exposed a few hundred metersto the west. These lenses of marble are imbri-cated with chert and siliceous argillite in a zoneof intense shearing and alteration along the con-tact between the chert-argillite unit and overlyingmetavolcanic rocks of the Late Triassic to EarlyJurassic Koip sequence.

Correlation and Age. The fossil assemblagein the marble lenses is dominated by large crinoidcolumnals, and contains productoid and spirifer-

oid brachiopods, possibly including the generaDictyoclostus and Martinia. The assemblage alsoincludes tabulate corals and stenoporoid bry-ozoans. Identifications are tentative, but the fos-sils indicate a general age range of Late Missis-sippian through mid-Permian (J. T. Dutro, 1988,written commun.).

Isolated fault blocks of fossiliferous marblevery similar to those at Alger Lakes are exposedat Minaret Summit and at Mammoth Rock, 20km and 28 km south of Alger Lakes, respec-tively (Fig. 6). The fossiliferous marble at theselocations has been correlated with the Pennsyl-vanian Mount Baldwin Marble in the MountMorrison pendant (Rinehart and Ross, 1964;Huber and Rinehart, 1965), which is a dark graymarble characterized by large crinoid columnals,nodular chert, and a macrofossil assemblage in-cluding productoid brachiopods similar to Dicty-oclostus (Rinehart and Ross, 1964). The MountBaldwin Marble was deposited in a relativelyshallow water carbonate platform environmentand indicates localized uplift probably associ-

ated with the Antler orogeny (Stevens andGreene, 1994; unpub. mapping).

We have proposed (Greene and Dutro, 1991;Greene and Schweickert, 1995) that the marblelenses in the Northern Ritter Range pendant arecorrelative with the Mount Baldwin Marble, andwere displaced northwestward from the vicinityof the Mount Morrison pendant in middle Creta-ceous time by the dextral Gem Lake shear zone.These marble lenses do not date the chert-argilliteunit or the Rush Creek sequence, as suggestedpreviously (Kistler and Nokleberg, 1979).

STRUCTURE

Chert-Argillite Unit

Paleozoic Structures. Contrasting styles ofPaleozoic deformation help distinguish the chert-argillite unit from the structurally underlyingRush Creek sequence (Table 2). Paleozoic struc-tures typical of the chert-argillite unit includelocally abundant syndepositional deformational



Figure 6. Generalized geologic map of the Antler orogenic belt from central Nevada to the Sierra Nevada. Abbreviations: CFÑCoaldale fault;CHÑCandelaria Hills; EFÑExcelsior fault; FFÑFish Lake Valley fault; GL-RFszÑGem LakeÐRosy Finch shear zone; LCÑLog Cabin Mine pen-dant; MMÑMount Morrison pendant; MSÑMinaret Summit; MRÑMammoth Rock; NRÑNorthern Ritter Range pendant; OVÑOwens Valley;PMÑPalmetto Mountains; RMTÑRoberts Mountains thrust; RRÑRitter Range pendant; SLÑSaddlebag Lake pendant; TNÑTonopah;WIÑWhite-Inyo Mountains. Paleozoic eugeoclinal rocks may be present locally as klippes in and northwest of the Palmetto Mountains (e.g., McKee,1968b). The dextral fault in Owens Valley is based on reconstruction of Devonian submarine fan facies offset between the White-Inyo Mountains andthe Mount Morrison pendant (Stevens et al., 1995; Stevens and Greene, unpub. data). Figure is modified from mapping of Albers and Stewart (1972),Kleinhampl and Ziony (1985), and Bateman (1992); additional data are from Stewart (1985) and Schweickert and Lahren (1990).

features (D0) (Fig. 5B); intrafolial isoclinal foldsand bedding-parallel cleavage (D1); and tight,northwest-plunging folds with variably orientedaxial surfaces and foliation (D2) (Fig. 8).

Slaty cleavage (S1) is characteristic of D1 de-formation and is well developed in slates andargillaceous interbeds in chert, where it typicallyparallels bedding. Dark streaks and laminae inthe hinges of F1 folds in chert suggest the localdevelopment of axial surface S1 spaced cleavagein the chert layers. Folds that are definitely as-signable to F1 are rare, and are typically rootless,intrafolial, isoclinal folds of decimeter scale.Bedding in the chert-argillite unit generally isdismembered and discontinuous on a scale of 1to 5 m (Fig. 5), which we interpret to result pri-marily from bedding transposition during D1 de-formation. In the Rush Creek sequence, in con-trast, D1 structures were not observed, andindividual layers can commonly be traced fortens to many hundreds of meters.

Second-generation structures (D2) in the chert-argillite unit consist predominantly of uncommontight folds of bedding and S1 foliation. Most F2folds are 1Ð2 m in amplitude, have northwest-trending, steeply plunging hinge lines, and vari-ably striking, steeply dipping hinge surfaces (Fig.8). Axial surface cleavage (S2) generally consistsof poorly developed spaced cleavage in the hingezones of F2 folds. F2 folds are generally larger inwave length than F1 folds, and are close to tightrather than isoclinal. It is probable that large map-scale F2 folds are also present in the chert-argilliteunit in the Northern Ritter Range pendant, as re-ported by Brook (1977) in the Saddlebag Lakependant. However, the lack of coherent strati-graphic markers in the chert-argillite unit makesidentification of such structures difficult, andnone was observed in this study.

D1 and D2 structures are middle to late Paleo-zoic in age, because they are not present in either

the lower Koip sequence (222 Ma U/Pb age,Schweickert and Lahren, 1987) or the PermianDiablo Formation, which overlies the chert-argillite unit to the north in the Saddlebag Lakependant (Schweickert and Lahren, 1987). D1

structures are restricted to the chert-argillite unitand must therefore have formed prior to the faultjuxtaposition of the unit with the Rush Creek se-quence. These structures may represent preem-placement deformation within the Roberts Moun-tains allochthon, as suggested by Oldow (1984)for similar structures in the Candelaria Hills. TheD2 structures, which are also present in the RushCreek sequence, are interpreted to have formedduring the Late DevonianÐEarly MississippianAntler orogeny, following previous interpreta-tions of correlative structures in the SaddlebagLake pendant by Brook (1977) and Schweickertand Lahren (1987, 1993). Whereas age con-straints on these structures are permissive of de-velopment as late as the middle Permian, the onlyknown orogenic episode in the region during thisperiod is the mid-Paleozoic Antler orogeny.

Mesozoic Structures. The most prominentstructural fabric in the chert-argillite unit formedduring an important phase of mid-Mesozoicdeformation (D3), and is characterized by well-developed cleavage striking N30¡Ð60¡W, and bysteeply northwest-plunging, tight, mesoscopicfolds (Fig. 8). These structures are presentthroughout the Northern Ritter Range pendant inrocks as young as Early Jurassic (201 Ma U/Pbage; Schweickert et al., 1994), and are cut by plu-tons of the Late Cretaceous Tuolumne IntrusiveSuite (91 Ma U/Pb age; Stern et al., 1981). D3structures developed after juxtaposition of thechert-argillite unit and the Rush Creek sequence,and commonly overprint and obscure D1 and D2structures. We interpret the variable orientation ofD2 structures in the chert-argillite unit (Fig. 8) tobe a result of reorientation by the northwest-trend-

ing D3 structures, which are more strongly devel-oped in this unit than in the Rush Creek sequence.

Rush Creek Sequence

Paleozoic Structure. Paleozoic structures inthe Rush Creek sequence are characterized bytight, southwest- and northeast-trending meso-scopic folds with moderately plunging hinge lines(Figs. 8 and 9); map-scale, tight to isoclinal foldswith extensive overturned limbs referred to hereas Ònappe foldsÓ (Figs. 2 and 3); and major thrustimbrication (Fig. 7) (Greene, 1995). These struc-tures are increasingly overprinted in northern andwestern areas by northwest-trending mid-Mesozoic structures. Axial surface foliation asso-ciated with D2 folds consists of a locally well-developed, finely spaced, disjunctive cleavagefilled by white quartz veins. Small-scale isoclinalfolds and bedding-parallel cleavage equivalent toD1 structures in the chert-argillite unit were notobserved in the Rush Creek sequence. The oldeststructures recognized are major nappe folds, in-cluding a northeast-trending anticline north ofAgnew Lake, and large folds exposed on thesouth and east flanks of Mount Wood (Fig. 2).Nappe folds on the east side of the Northern RitterRange pendant are north- to northeast-trending,but to the west are refolded into a northwest trendby later mid-Mesozoic folds. These large nappefolds cannot be traced continuously; the map pat-tern is inferred from exposed folds, bedding atti-tudes, and extrapolation of units across cover.More small thrust faults imbricate the section thancan be resolved at this map scale, and the actualmap pattern is more complex than shown.

Nappe folds and associated northeast-trendingstructures are cut by the 210 Ma granite of LeeVining Canyon, indicating that these structuresare Triassic or older. We interpret these structuresto be mid- to late Paleozoic in age, probably cor-

GREENE ET AL.


TABLE 1. DESCRIPTIONS OF LITHOLOGIC UNITS IN THE RUSH CREEK SEQUENCE

Lithology Description Mineralogy Protolith

Calcsilicate Off-white to tan when weathering, gray-green when fresh, Quartz, tremolite, Variablyhornfels commonly banded in outcrop, very fine-grained to feldspar, argillaceous and(35%) microcrystalline, very thin- to thin-bedded, finely clinozoisite, calcareous

laminatedin part, varies from dominantly siliceous to calcite, sericite, chert and quartzdominantly calcareous. opaques siltstone

Siliceous Red-brown rusty when weathering, dark gray to black Quartz, mica, Siliceous shaleargillite when fresh, very fine-grained, very thin- to thin-bedded, feldspar,(30%) laminated in part, varies from dominantly siliceous to opaques

dominantly argillaceous.Metachert Dark orange rusty when weathering, black to dark gray Quartz, mica, Chert to

(20%) when fresh, very fine-grained to cryptocrystalline, very opaques argillaceousthin- to thin-bedded, locally contains white weathering chertphosphatic streaks and nodules.

Marble Light to dark gray when weathering, dark gray when fresh, Calcite, quartz, Variably(10%) fine- to medium-grained, medium- to thick-bedded, hornblende, carbonaceous

commonly contains fine to medium detrital quartz grains, opaques and sandylocally contains abundant black chert nodules. marble

Quartzite White to light gray when weathering, light gray when fresh, Quartz, feldspar, Quartz arenite,(5%) thin-bedded to massive, with medium- to coarse-grained, calcite feldspathic

well-rounded detrital quartz grains in a varied calcareous and/ormatrix grading to sandy marble. calcareous in

part

relative with D2 structures in the chert-argilliteunit and possibly related to thrust emplacementof the Roberts Mountains allochthon during theLate DevonianÐEarly Mississippian Antlerorogeny. Alternatively, some or all of these struc-

tures could be related to a period of Late Triassiccontractional deformation recognized in theSaddlebag Lake pendant (Schweickert andLahren, 1987, 1993) and the Log Cabin Minependant (Stevens and Greene, unpub. mapping).

This is considered unlikely, however, becausestructures known to be associated with Triassicdeformation trend north to northwest (e.g.,Schweickert et al., 1988; Schweickert andLahren, 1993), whereas the early structures in theRush Creek sequence trend predominantly north-east (Fig. 8) (Strobel, 1986; Greene, 1995).

Mesozoic Structure. Mid-Mesozoic struc-tures in the Rush Creek sequence are similar to,but less prominent than, those in the chert-argillite unit (Fig. 8), and are characterized bynorthwest-striking, steeply southwest-dippingfoliation with rare, steeply northwest-plungingtight folds. These structures become progres-sively less well developed to the east in theRush Creek sequence, where northeast-trendingPaleozoic structures predominate (Strobel, 1986;Greene, 1995).

In the southern part of the pendant a right-lateral ductile shear zone of Cretaceous age, theGem Lake shear zone (Strobel, 1986; Greene andDutro, 1991; Greene and Schweickert, 1995), hascut out the chert-argillite unit and forms the con-tact between the Rush Creek sequence andMesozoic volcanic rocks of the Koip sequence(Fig. 2). Mesozoic deformation in the RushCreek sequence increases in intensity toward thisshear zone, resulting in increasing foliation de-velopment and a loss of stratigraphic continuity.

POSSIBLE ROBERTS MOUNTAINSTHRUST

The Rush Creek sequence is structurally over-lain by the chert-argillite unit along a north- tonorthwest-striking, steeply west-dipping faultcontact (Figs. 2 and 3). The fault trace is 5 km inlength, and extends from Gem Pass to the northface of Mount Wood, in part following the loca-tion of a fault originally mapped by Kistler(1966a). Southeast of Alger Lakes, the faultmerges with the Cretaceous Gem Lake shearzone, which has overprinted and obscured theearlier fault trace. North of Mount Wood, thefault is cut by the granite of Lee Vining Canyon(210 Ma U/Pb zircon age; Chen and Moore,1982), indicating that the fault is no younger thanLate Triassic time.

The fault trace is most clearly expressed atmap scale as an abrupt truncation of northeast-striking marble and calc-silicate beds of the RushCreek sequence against northwest-striking fabricin the chert-argillite unit. The fault trace, thoughpoorly exposed, can be followed down the south-west face of Mount Wood (Fig. 10), where it ismarked by a 25Ð50-m-wide zone of disruptedbedding and hydrothermal alteration. Rockswithin this zone form disconnected lenses withanomalous orientations. Structures are ambigu-ous and no indicators of slip direction were ob-



A

B

Figure 7. Photo (A) and line drawing (B) of the Rush Creek sequence north of Agnew Lake,viewed looking west from the town of June Lake. The prominent white band in the face is cal-careous quartz sandstone correlated with the Mount Morrison Sandstone (mm). Dark unitabove is chert and slate of the Squares Tunnel Formation (sq), separated from calc-silicate horn-fels of the Aspen Meadow formation (am) on the right skyline by thrust faults. clÑConvict LakeFormation, TrlvÑgranite of Lee Vining Canyon, QtÑQuaternary talus.

served. At its northern end, the fault trace is par-allel to northeast-trending bedding, mesoscopicfolds, and major nappe folds in the Rush Creeksequence (Fig. 2). However, the fault truncatesnappe folds to the south, indicating that the ex-posed fault postdates the earliest phase of foldingin the Rush Creek sequence.

Correlation and Age. This fault contact in theNorthern Ritter Range pendant juxtaposes thehighly deformed, basinal chert-argillite unit of theRoberts Mountains allochthon against less-deformed siliceous, calc-silicate, and calcareousouter continental shelf and slope rocks of theRush Creek sequence. Such juxtaposition is thedefining characteristic of the Roberts Mountainsthrust in north-central Nevada (Roberts et al.,1958), and we provisionally interpret this faultcontact in the Northern Ritter Range pendant tobe a remnant of the Roberts Mountains thrust,which developed during the Late DevonianÐEarlyMississippian Antler orogeny.

Mesozoic reactivation of the Roberts Moun-tains thrust is a common feature of the Antler oro-genic belt, and Mesozoic faults are generally dif-ficult to distinguish from the Paleozoic thrust(e.g., Ketner and Smith, 1982; Speed and Sleep,1982). Available age constraints on this fault inthe Northern Ritter Range pendant are permissiveof displacement as young as Late Triassic time. Itis thus possible that the exposed thrust fault in theNorthern Ritter Range pendant was active duringthe Triassic, synchronous with the Lundy Canyonthrust and other Triassic thrust faults in the Sad-dlebag Lake pendant. The Lundy Canyon thrustwas interpreted by Schweickert and Lahren

GREENE ET AL.


TABLE 2. STRUCTURES IN THE NORTHERN RITTER RANGE PENDANT

Generation Observed in Structures Orientation of Agefoliation and (and age constraints)

hinge surfaces

D5 All units Spaced cleavage, rare open Strike N30°–70°E, Jurassic or Earlyfolds dip vertical Cretaceous

(post-201 Ma,pre-91 Ma)

D4 All units Thinly spaced cleavage, rare Strike N50°–90°W, Jurassic or Earlyopen folds dip vertical Cretaceous

(post-201 Ma,pre-91 Ma)

D3 All units Tight mesoscopic folds, well Strike N30°–60°W, Jurassic or Earlydeveloped cleavage, map-scale steeply Cretaceousfolds, thrust faults southwest-dipping (post-201 Ma,

pre-91 Ma)D2 Rush Creek Tight mesoscopic folds and local Strike N5°W–N60°E Syn- or post-Devonian,

sequence disjunctive cleavage, map-scale (avg. N30°E), pre-Permiannappe folds and thrust faults steeply dipping

D2 Chert-argillite unit Uncommon tight folds, poorly Strike variable, Syn- or post-Devonian,developed cleavage steeply dipping pre-Permian

D1 Chert-argillite unit Bedding-parallel cleavage, rare Reoriented by Mid-Paleozoic,intrafolial isoclinal folds later deformation possibly Devonian

(post-Ordovician,pre-Permian)

Figure 8. Equal area stereonets showing the orientation of mesoscopic structures in the chert-argillite unit and the Rush Creek sequence. Great circles indicate synoptic orientation of planarstructures. (A) Late Paleozoic (D2) structures in the chert-argillite unit, in part reoriented by D3deformation. (B) Mesozoic (D3) structures in the chert-argillite unit. (C) Late Paleozoic structuresin the Rush Creek sequence, probably equivalent to D2 in the chert-argillite unit. (D) Mesozoicstructures in the Rush Creek sequence, equivalent to D3 in the chert-argillite unit. Some data inthe Rush Creek sequence are from Strobel (1986).

(1987, 1993) to be east vergent and to have a min-imum displacement of about 10 km. The thrustduplicates Upper Triassic metavolcanic rocks andis cut by the 219 Ma Mt. Olsen pluton, indicatingthat it developed in the Late Triassic during thevolcanic episode represented by the lower part ofthe Koip sequence. However, the development ofcontractional structures (D2) in the NorthernRitter Range pendant that are not present in over-lying Permian and Early Triassic units indicatesthat important late Paleozoic contractional defor-mation occurred in the Northern Ritter Rangependant, which is most reasonably interpreted tohave resulted from the Antler orogeny (Noklebergand Kistler, 1980; Schweickert and Lahren, 1987;Greene, 1995).

DISCUSSION: REGIONAL EXTENT OFTHE ANTLER OROGENIC BELT

The Roberts Mountains thrust forms the lead-ing edge of the Antler orogenic belt, which ex-tends in a well-defined north-south belt fromnorth-central Nevada to near Tonopah in west-central Nevada (Fig. 6). Southwest of Tonopah,the Antler orogenic belt apparently bendsabruptly to the west, and is much less well de-

fined. Many workers have proposed post-Antlerstructural complications in this region, includingapproximately 110 km of dextral displacementon the Excelsior and Coaldale faults (Stewart,1985), 50 km of dextral displacement on the FishLake Valley fault (McKee, 1968b), large Ceno-zoic extension in the region between the Pal-metto Mountains and the Candelaria Hills (Stew-art and Diamond, 1990; Oldow et al., 1994), and65 km of dextral displacement on a cryptic faultin the Owens Valley (Stevens et al., 1995).Restoration of these proposed displacements re-duces the abrupt westward bend in the Antlerorogenic belt, and would result in the NorthernRitter Range pendant being originally locatedsubstantially closer to previously defined parts ofthe orogenic belt (Stevens and Greene, unpub.data). The recognition of the southwestern limitof the Antler orogenic belt in the Northern RitterRange pendant provides an important westernanchor point for any attempt to reconstruct thissection of the orogenic belt.

Eugeoclinal rocks that are probably relatedto the Antler orogenic belt are also exposed inthe Sachse Monument pendant (Lahren andSchweickert, 1994), which is located about 50km northwest of the Northern Ritter Range

pendant (Fig. 1). Lahren and Schweickert(1994) interpreted these rocks to structurallyoverlie miogeoclinal rocks exposed in the adja-cent Snow Lake pendant, in a structural rela-tionship similar to that in the Northern RitterRange pendant. Both the Sachse Monumentand Snow Lake pendants, however, probablyoriginated in the present Mojave Desert region,and were transported to their present locationby Cretaceous dextral displacement on the pro-posed MojaveÐSnow Lake fault (Lahren andSchweickert, 1989, 1994). These rocks aretherefore unrelated to the Paleozoic continentalmargin in the central Sierra Nevada region.Aside from these highly allochthonous expo-sures, eugeoclinal rocks of the Antler orogenicbelt do not occur west of the Northern RitterRange pendant. Instead, rocks of eugeoclinalaffinities are exposed more than 150 km to thesoutheast, in the Mineral King and KernPlateau pendants in the southern Sierra Nevada(Fig. 1) (Dunne and Suczek, 1991; Schweickertand Lahren, 1991).

These eugeoclinal rocks may have been offsetsoutheastward from the main Antler orogenic beltby major sinistral strike-slip faulting that is pro-posed to have truncated the southwest-trendingCordilleran continental margin during late Paleo-zoic time (e.g., Davis et al., 1978; Stone andStevens, 1988; Walker, 1988; Stevens et al.,1992). The presence of eugeoclinal rocks of theAntler orogenic belt in the Northern Ritter Rangeand Saddlebag Lake pendants constrains the loca-tion of the proposed sinistral truncation to be westof the Northern Ritter Range pendant, as noted bySchweickert and Lahren (1990, 1993). Alterna-tively, Dickinson (1981) proposed that the presenttrace of the Antler orogenic belt is the result of anoriginal bend in the Cordilleran continental mar-gin, suggesting that eugeoclinal rocks in thesouthern Sierra Nevada are not significantly dis-placed from their original position. In this case,the absence of eugeoclinal rocks between theNorthern Ritter Range pendant and the southernSierra Nevada would be the result of lack ofpreservation or removal by subsequent tectonism.

The Antler orogenic belt is currently trun-cated on the west side of the Northern RitterRange pendant by the dextral Gem LakeÐRosyFinch shear zone (Tikoff and Teyssier, 1992;Greene and Schweickert, 1995), which was ac-tive in middle to Late Cretaceous time. Therecognition of large dextral displacements alongthe axis of the Sierra Nevada batholith duringCretaceous time (e.g., Lahren and Schweickert,1989; Kistler, 1993; Greene and Schweickert,1995) suggests the possibility that the presentgap between exposures of eugeoclinal rocks inthe southern Sierra Nevada and the NorthernRitter Range pendant may be at least in part a



Figure 9. F2 fold ex-posed in vertical cliff ofcalc-silicate hornfels, RushCreek sequence, south faceof Mount Wood. Hammeris 32 cm in length.

result of Mesozoic dextral truncation, and im-plies that displaced fragments of the Antler oro-genic belt may also be present north of theNorthern Ritter Range pendant, as suggested forthe eugeoclinal rocks in the Sachse Monumentpendant by Lahren and Schweickert (1994).

ACKNOWLEDGMENTS

This research was supported in part by Na-tional Science Foundation grants EAR-92-18174to Stevens and EAR-87-07312 to Schweickert,and by a Sigma Xi Grant-in-Aid of Research to

Greene. R. J. Strobel gave a helpful introductionto the Rush Creek area. R. W. Kistler provided as-sistance in relocating the fossil locality near AlgerLakes, and J. T. Dutro examined fossils from thatlocality. J. E. Repetski processed samples fromboth the chert-argillite unit and the Rush Creeksequence in an unfortunately unsuccessful searchfor conodonts. Helpful reviews were provided byM. Brandon, G. C. Dunne, P. H. Cashman, D. M.Herring, R. B. Miller, and J. H. Trexler.

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Figure 10. Photo (A) and line drawing (B) of the southwest face of Mount Wood, viewed look-ing east from Alger Lakes. Well-bedded calc-silicate hornfels, siliceous argillite, and marble ofthe Rush Creek sequence are juxtaposed against the dark-weathering chert-argillite unit to thenorthwest and along the base of the cliffs by the proposed Roberts Mountains thrust. Viewshows an oblique section through structure.

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GSA Bulletin: Roberts Mountains allochthon and the western