TECTONICS, VOL. 14, NO. 4, PAGES 945-961, AUGUST 1995 The Gem Lake shear zone: Cretaceous dextral transpression in the Northern Ritter Range pendant, eastern Sierra Nevada, California David C. Greene and Richard A. Schweickert Departmentof GeologicalSciences, University of Nevada,Reno Abstract. The Gem Lake shear zone is a northweststriking, transpressionalshear zone exposedin the Northern Ritter steeply dipping, dextral transpressionalshear zone that Range pendantin the east-centralSierra Nevada (Figure 1). providesthe first direct evidencefor dextral deformationin Late Mesozoic dextral strike-slip faults have previously been wall-rock pendantsin the central part of the Sierra Nevada proposedin the centraland eastern Sierra Nevada by a number batholith. The Gem Lake shear zone is a minimum of 30 km in of workers [e.g., Nokleberg, 1983; Saleeby et al., 1986; lengthand extendsat least from the northend of the Northern Lahren et al., 1990; Tikoff and Teyssier,1992; Stevenset al., Ritter Rangependant to the southeastedge of the Ritter Range 1992; Kistler, 1993]. However, the location, extent, and pendant.The amountof displacementon the zone is uncertain, regional tectonic significance of these faults have been but matching fault slivers of Pennsylvanian(?)marble in the controversial[e.g., Schweickertand Lahren, 1991; Saleeby Northern Ritter Range pendantto similar exposuresnorth of and Busby, 1993] and remain an important unresolved the Mount Morrison pendant suggestsa minimum dextral question.The Gem Lake shearzone providesthe first direct offset of 20 km. The Gem Lake shearzone was active in early field evidence for a dextral shear zone in the wall-rock Late Cretaceous time, from before 91 Ma (the age of the pendantsof the centralSierra Nevadaand along with other syntectonicgranodiorite of Kuna Crest) to at least 80 Ma proposedfaults suggeststhat middleCretaceous deformation (40Ar/39Arage of syndeformationalmica from the shearzone in the Sierra Nevada may have been characterized by in the Ritter Range pendant).Deformation in the Gem Lake distributeddextral transpressionon anastomosing,northwest shearzone is characterizedby combineddextral simple shear trending shearzones of regionalextent. and subvertical stretching,which are variably partitioned in In this paperwe first describethe Gem Lake shearzone in anastomosinghigh-strain zones. In the shear zone at Gem the NorthernRitter Rangependant, with emphasison its field Lake, predominantly dextral deformation is indicated by expression,structural style, and deformationalhistory. We porphyroclastasymmetries, S-C fabric, and asymmetric then discusspossible regional continuations of the Gem Lake crenulations; a component of subvertical stretching is shear zone and the tectonic implications of a dextral indicated by a moderately to steeply plunging stretching transpressionalshear zone system in the eastern Sierra lineation. In a segmentof the shear zone at Kuna Crest, a Nevada. In this paper we emphasizeevidence for dextral strongly developed stretching lineation indicates transpressionin the wall-rockpendants of the SierraNevada predominantly subvertical stretching, with a lesser batholith. Tikoff and Teyssier[1992, 1993a] and Tikoff and componentof dextralstrike slip. The Gem Lake shearzone is Greene [1994] have describedevidence for similar dextral consideredto be part of a proposedregional system of shear transpressionin the Late Cretaceous granitoids of the zones in the eastern Sierra Nevada, the Sierra Crest shear zone batholith itself. We considerthat transpressionaldeformation system.This dextral transpressionalsystem was active prior in the wall rocks and the batholith are related and suggestthat to, and synchronouswith, intrusionof the Late Cretaceous the Gem Lake shearzone is part of a proposedSierra Crest Sierra Nevada batholith; deformation ceased shortly after shearzone system[Tikoff, 1994; Tikoffand Greene, 1994], pluton emplacement.The Sierra Crest shear zone system defined as an anastomosingsystem of dextral shear zones includespossibly related shear zones as far northas Saddlebag exposedlocally throughout the easternSierra Nevada (Figure Lakependant and at leastas far southas Oak Creek pendant and 1). the Owens Valley, indicatinga possiblestrike length greater than 150 km. Geologic Setting The Northern Ritter Range pendant(NRP) is located in the Introduction east-central Sierra Nevada, at the eastern edge of Yosemite National Park. The NRP is one of a number of roof pendants In this paperwe describeevidence for, andthe components and wall-rock septapreserved within Mesozoicgranitic rocks of, the Gem Lake shear zone, a northweststriking, dextral of the Sierra Nevada batholith. It consistsof a narrow septum of Paleozoic metasedimentary rocks and Mesozoic metavolcanic rocks that extends 20 km northwest from the Copyfight1995 by the AmericanGeophysical Union. larger Ritter Range pendant [Kistler, 1966a, b; Huberand Papernumber 95TC01509. Rinehart, 1965]. Strata in the NRP generally strike northwest, 0278-7407/95/95TC-01509510.00 dip steeplyto the southwest,and are youngerto the southwest. 945 946 G• AND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRANEVADA California]Nevada 118o Small structural slices of fossiliferous marble of late I I I Paleozoic (Pennsylvanian?)age [Brook et al., 1979; Greene and Dutro, 1991; Greene, 1995] are interleaved with the Palmetto Formation northwest of Alger Lakes (Figure 2). .'•. These fault slices are correlative with the Pennsylvanian N Mount Baldwin Marble, which is exposed in the Mount I Morrison pendant30 km to the southeast[Greene and Dutro, Northern 1991]. Ritter Range The Paleozoic metasedimentaryunits are structurally and , pendant unconformably overlain to the southwest by the Upper Triassicto Lower JurassicKoip sequence[Huber and Rinehart, 1965; Kistler, 1966a, b; Schweickert and Lahren, 1987, rancisco 1993b]. The Koip sequenceis a varied assemblageof volcanic and subordinatevolcanogenic sedimentary rocks. It consists Sierra Cre s t shear zone predominantly of lapilli tuffs and tuff breccias and includes ]•oja Ve -SnowLake system volcanic flows, hypabyssal intrusions, and lenses of 2) volcanogenic sandstone.Composition of the volcanic rocks ranges from basalt to rhyolite but is predominantlydacitic to rhyodacitic [Huber and Rinehart, 1965]. The baseof the Koip 360 sequencein the SaddlebagLake pendanthas been dated at 222 - Ma (U/Pb [Schweickert and Lahren, 1987]), and an ash-flow tuff from the top of the sequencehas yielded a U/Pb age of 201 Ma [Schweickert et al., 1994]. ":•i'. graniticMesozoic rocks Gatlock Fault The Koip sequence is overlain by a Lower Jurassic • Westernmetamorphic belt volcanosedimentaryunit informally referred to as the Sawmill 340 Canyon sequencefor a well-exposedsection in the Saddlebag ............•...•,•Antlerorogenic belt Los Angeles Lake pendant[Greene, 1995]. The Sawmill Canyonsequence is ::'"":•i•Paleozoic exposedin a thin strip along the west edge of the NRP, where miogeoclinal belt it is composedof thinly layered volcanogenicsandstones and 118o 122o I calcsilicate rocks, now metamorphosed predominantly to Figure 1. Generalizedregional map showingthe locationof biotite schist. the Northern Ritter Range pendant in relation to the Sierra The NRP is borderedon the eastby the Late Triassic granite Nevada and the proposed Sierra Crest shear zone system. of Lee Vining Canyon (210 Ma U/Pb) [Chen and Moore, Modified from Jennings [1977], with data from Schweickert 1982]. On the west the NRP is borderedby the Late Cretaceous and Labten [ 1990] and Saleebyand Busby[ 1993]. granodioriteof Kuna Crest (91 Ma U/Pb) [Stern et al., 1981], which is part of the outerzone of the TuolumneIntrusive Suite [Bateman, 1992]. A geologicmap of the NRP (Figure 2) thus representsan oblique crosssection through the pendant.All pre-Tertiary rocksin the NRP havebeen metamorphosed under greenschist Structure of the NRP to epidoteamphibolite facies conditions; however, the prefix Multiple generationsof structuresin the NRP record various meta- has been omitted in subsequentdescriptions where the contractionaland transpressionalregimes of middle Paleozoic protolith is readily discernible. to late Mesozoic age. Two generations of middle or late Paleozoicstructures are recognizedin the NRP [ Greene, 1995]. Stratigraphy of the NRP Theseinclude (1) isoclinalfolds and transposedbedding in the The structurally lowest unit in the NRP is the lower PalmettoFormation that are generallynorthwest trending as a PaleozoicRush Creek sequence[Strobel, 1986; Greene, 1995], result of reorientationby third-generationstructures; and (2) which consists of siliceous and calc-silicate hornfels, chert, northeast trending mesoscopicfolds and map-scale nappe marble,and quartzite.The RushCreek sequence is interpreted folds that are best developedin the Rush Creek sequence.One as a transitional facies of the Paleozoic Cordilleran or both of these sets of structuresprobably formed during the miogeocline [Strobel, 1986; Greene et al., 1989; Greene, Antler orogeny, and a southwesternextension of the Roberts 1995]. Mountains thrust has been proposedto separatethe Palmetto The structurally overlying Palmetto Formation consists Formation and Rush Creek sequencein the NRP [Greene et aL, predominantly of highly disrupted and deformed chert and 1989; Greene, 1995]. siliceousargillite, with local calc-silicatehomfels and quartz A third generation of structuresthat formed in Jurassicor siltstone. This unit, together with exposuresto the north in Early Cretaceoustime is