TECTONICS, VOL. 14, NO. 4, PAGES 945-961, AUGUST 1995

The Gem Lake shear zone: Cretaceous dextral transpression in the Northern pendant, eastern ,

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 . 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 . 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 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 characterizedby a northweststriking the Saddlebag Lake pendant, is consideredto be correlative slaty cleavage, which is generally parallel to transposed with the Ordovician Palmetto Formation of the Roberts bedding and axial-planar to tight mesoscopic folds. This Mountains allochthon [ Schweickertand Lahren, 1987; Greene northwest trending structural fabric is developed throughout et al., 1989; Greene, 1995]. the NRP and is associatedwith prominent thrust faults which GREENE AND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA 947

. .

Sa d d I e b a g La k"'e'";::!;;;i;?i?i•ii•ii:;11 •!;:;•::---: Dana pendant ..... Meadows" M o n o L a k e

ß ß

... ;Northern Ritter ::•: pendant

Ritter Range:• pe nd a nt :;':'i:?i::!iiiiiili:ii•::.::: :-..

-:i;:.!:i!-Parker......

37 ø

50' ..

...... :/:.i-:-::i Kuna Crest segment

.....: . - -.:..

... (•) lensesof ...... -.. uPz marble...... -. -

•,'1,Gemshear Lakezone '".... :•:•'".... ß , , ::• alluviumQuaternary ....ßß ß i:;i.i.i...... ß , ß :•-] granodioriteof -:: Blacktop, "'...... KunaCrest (91 Ma) ß ' ' Peak .:'"

i.• Canyongraniteof (210Lee Ma)Vining ...... ß , , •'•":......

• sequenceSawmillCanyon(J) ':::.':::. '...... :':'"':"...... ::""::'::.i"

•;• Koipsequence (Tr-J) ':: segmentGemLake Gem !;• Danasequence (Tr)

PalmettoFormation (O) N I o I • calcareoussiliceous subunitsandcalc- 0• kmI

Figure 2. Generalizedgeologic map of the NorthernRitter Rangependant, showing the Gem Lake and Kuna Crest segmentsof the Gem Lake shearzone (modified from Greene [ 1995]).

imbricate Lower Jurassicrocks of the Koip and Sawmill southeastside of the NRP is interpretedto result primarily Canyonsequences. These structures are cut by Late Cretaceous from distributed deformation related to the Gem Lake shear plutons[Schweickert and Lahren, 1987, 1993a, b] and indicate zone. that a significant phase of Jurassicor Early Cretaceous Northwest trending structural fabric that is present in contractional deformation affected the entire NRP. pendantsthroughout the central and southernSierra Nevada Thereare no stretchinglineations associated with regional [Noklebergand Kistler, 1980; Nokleberg,1983] was formerly deformationin the Paleozoic rocks. Moderate to poorly interpretedto be predominantlyJurassic in age and relatedto developed stretching lineation in Mesozoic rocks on the the Nevadan orogeny [Nokleberg and Kistler, 1980; 948 GREENE AND SCHWEICKERT: GEM LAKE SHEARZONE, SIERRANEVADA

Schweickert et al., 1984]. However, in some areas a northwest trending structural fabric is present in rocks of Early and middle Cretaceousage [Tobischet al., 1986; Saleeby et al., 1990], suggesting that some of the northwest trending structuralfabric in the NRP may be as young as Cretaceous. Mylonitic foliation associatedwith the Gem Lake shear zone is subparallel to, and generally merges with, the northwest trending structural fabric. A prominent stretching lineation is present on foliation planes within the shear zone. Structural relations in the NRP suggestthat Late Jurassicor Early Cretaceous regional contractional deformation, which resulted in thrust faulting and the third-generation contractional fabric, progressively evolved into dextral transpressional deformation in the middle Cretaceous, possiblywith reactivationof the older contractionalfabric.

Transpressional Deformation in the Gem Lake Shear Zone

The Gem Lake shearzone is a high-strainzone developed by a combination of dextral shearing and subvertical stretchingassociated with horizontalshortening. As described finite in detail below, horizontal exposureswithin the shear zone strain ellipsoid show dextral strike-slip shear indicators, including X asymmetric porphyroclasts, S-C fabric, rotated veins, and asymmetricminor folds. In contrast,steeply dipping surfaces Figure 3. Model for dextral transpressionaldeformation in commonly show evidenceof subverticalstretching, including the Gem Lake shear zone. Full arrows indicate horizontal symmetrically extended porphyroclastsand well-developed, contraction, resulting in vertical extension within the shear steeply plunging stretching lineation. zone and development of extension lineation visible on A conceptual model for this style of deformation is vertical surfaces. Half arrows indicate synchronousdextral illustrated in Figure 3. A ductilely deforming dextral shear shearing, resulting in dextral strike-slip displacement and zone is located between two unstrained blocks. Horizontal developmentof dextral shear indicators visible on horizontal shortening perpendicular to the shear zone boundaries surfaces(after Sandersonand Marchini [ 1984]). (indicatedby full arrows) resultsin vertical stretchingof the rocks within the shearzone and the developmentof a steeply plunging stretching lineation. Synchronousdextral simple SuperiorProvince of Canadaand by Hudlestonet al. [1988] in shearparallel to the shearzone boundaries(indicated by half the Vermilion District of northern Minnesota. arrows) results in dextral displacementacross the shear zone, as indicated by asymmetricporphyroclasts and other dextral strike-slip shear indicators. Note that in this model the The Gem Lake Shear Zone in the stretching lineation tracks the direction of maximum finite Northern Ritter Range Pendant stretching(x axis in Figure 3), not the displacementdirection. The Gem Lake shearzone as defined in this paperconsists This combination of horizontal simple shear and of three segments:the Gem Lake segment, the Kuna Crest subverticalextension is characteristicof transpressionalshear segment,and the Ritter Range segment(Figure 4). The simple zonesin which the stretchingdirection is perpendicularto the shear and pure shear componentsof deformationare variably sheardirection [Sanderson and Marchini, 1984; Simpsonand partitionedwithin and betweenthese segments. The Gem Lake De Paor, 1993; Robin and Cruden, 1994]. Modeling studies and Ritter Range segmentsare dominatedby dextral simple [Fossen and Tikoff, 1993; Fossenet al., 1994] have shown shear with subordinate vertical stretching; the Kuna Crest that with increasing finite strain in a transpressionalshear segment exhibits dominant subvertical stretching with zone, the stretching lineation will migrate to a subvertical subordinate dextral simple shear. The following sections orientation, reflecting the orientation of the maximum finite describethe Gem Lake and Kuna Crest segmentsseparately and stretchingaxis. This result holds true even for transpressional summarize evidence that these segmentsare componentsof shear zones in which the simple shear component of the sameshear zone. The Ritter Range segmentis describedin deformation (and the amount of strike-slip displacement)is a later section. large relative to the pure shear component [Tikoff and Teyssier, 1993b; Fossenet al., 1994]. Transpressionalshear Gem Lake Segment zones with coexisting horizontal strike-slip shear indicators and vertical stretchinglineation have also been describedby The Gem Lake segmentis the primary strand of the Gem Robin and Cruden [1994] in the Larder Lake Break in the Lake shear zone. It is a northweststriking, steeply southwest GRF_•NEAND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA 949

Saddlebag pendant .•-• withCretaceous major plutongranitic boundariesrocks, ..::..::.•Triassic and Jurassic Figure2: -• granitic rocks NorthernRitter I Mesozoicvolcanic and Rangependant,,) sedimentary rocks Kuna Crest segment • Paleozoic Gem Lake miogeoclinal rocks

segment Paleozoic eugeoclinal rocks GemLake shear zone % RosyFinch shear zone Ritter Range pendant pendant Ritter Range Mt.Morrison segment ! I 0 km 10

Rosy Finch ...... shear zone

...... 119 ...... 119 ø .... 118o

Figure 4. Generalizedgeologic map of partof theeast-central Sierra Nevada, showing the Gem Lake, Kuna Crest,and Ritter Range segments of theGem Lake shear zone (modified from Bateman [ 1992]).

dipping,high-strain zone that generallyforms the contact cleavedmylonites occur in very fine-grainedsiliceous tuffs, S- between Paleozoic metasedimentaryrocks and Mesozoic C fabrics in fine- to medium-grainedtuffs, asymmetric metavolcanicrocks. The Gem Lake segmentextends the crenulation cleavage in phyllonites and phyllosilicate-rich lengthof theNRP, from the north end of ParkerPass valley to schists, and asymmetric pressure shadows and winged thesouth end of theNRP, 2 km southof GemLake (Figure 2). porphyroclastsystems in medium- to coarse-grainedlapilli This high-strainzone is especiallywell-developed in the tuffs and tuff breccias.Quartz veins are commonlysheared and vicinity of Gem Lake, whereit was describedby Strobel boudinaged, with asymmetric pressure shadows around [1986]. individual boudins.All thesefeatures have asymmetriesthat The GemLake segment is characterizedby anastomosingindicate dextral strike-slip displacement.A significant zonesof myloniticfoliation striking N30-60W and dipping componentof verticalstretching is evidencedby a moderately steeply to the southwest(Figure 5), generallyparallel to northwest striking regional foliation. Foliation in the shear Gem Lake shear zone zoneis distinguishedfrom the regionalfoliation by its more N N intensedevelopment, including thinly spacedcleavage and poles to concentrationsof phyllosilicateson shearplanes. In addition, • lineationfoliation ee foliation in the shearzone is associatedwith asymmetric porphyroclasts and other indicators of strike-slip deformation,and a prominentdown dip extensionlineation is z• z• p. ß I present on foliation surfaces. Extension lineation is less well developedin metavolcanicrocks away from the shearzone and not presentin metasedimentaryrocks outsidethe shearzone. # •eN=57 Locally well-developedslaty cleavageand aligned,flattened •--...._w...... ••N=19 .L...•N=44 volcanicclasts indicate a significantcomponent of flattening deformationparallel to mylonitic foliation. However, it has GemLake segment KurtaCrest segment not been possibleto distinguishflattening associated with Figure 5. Equalarea stereonets showing the orientationsof middle Cretaceoustranspression from that associatedwith foliation and lineation in the Gem Lake and Kuna Crest previous regional deformation. segmentsof the Gem Lake shear zone. The great circles Mylonitic foliation is most prominent in volcanic and representaverage orientation of foliation: strikingN40W and volcaniclasticrocks of the Koip sequence.The developmentof dipping80øSW in theGem Lake segment, striking N10W and deformationalfeatures depends in part on lithology:highly dipping80øNE in the KunaCrest segment. 950 GREENEAND SCHWEICKERT:GEM LAKE SHEARZONE, SIERRANEVADA

to steeply plunging stretching lineation which is visible on the trace of the shearzone is mostly obscuredby alluvium, but steeply dipping surfaces. scattered outcrops of Paleozoic and Mesozoic rocks on the At Gem Lake, the shear zone consists of a kilometer-wide sides of Parker Pass valley contain mylonitic foliation typical zone of highly strained metavolcanic and metasedimentary of the shear zone. rocks. Particularly good exposuresof the shear zone occur in Conditions of deformation. The metavolcanic rocks lapilli tuffs of the Koip sequencealong the northeasternshore generally contain plagioclase, quartz, K-feldspar, biotite, of Gem Lake. To the eastin the RushCreek sequence,the shear chlorite, sericite, calcite, and epidote. A complex zone is manifestedby progressivebreakup and overprintingof metamorphic and alteration history has been documentedin a preexistingnortheast trending structuralgrain by northwest the adjacent Ritter Range pendantby Hanson et al. [1993]. trending mylonitic foliation (Figure 6) [Strobel, 1986]. The Within the shear zone, quartz porphyroclasts in silicic shear zone continues 2 km southeast of Gem Lake to the metavolcanic rocks may show indistinct core-and-mantle southernend of the NRP, where it is overlappedby Tertiary structure,and undulatoryextinction and subgrainsare evident basalt. in porphyroclast cores. Elongate quartz porphyroclastsare In the Palmetto Formation northwest of Gem Lake, the boudinaged, whereas feldspar porphyroclasts generally shear zone is characterized by anastomosing domains of display extensional fractures and microfaults. These mylonitic chert and siliceous argillite and by intense slaty microstructures suggest deformation under subgreenschist cleavage developedin more argillaceousunits. A network of facies conditions, near the lower limit of quartz crystal orange-weathering,siliceous, mylonitic domains separatedby plasticity [Simpsonand De Paor, 1991]. less-deformed country rocks is well exposed on benches Dextral shear indicators. In the Gem Lake segment, northeast of Alger Lakes (Figure 2). The more localized dextral strike-slip deformationis indicated by porphyroclast character of shear deformation within the Palmetto Formation asymmetries,S-C fabric, rotatedveins, and asymmetricminor may be a result of strain softening within these mylonitic folds. Dextral shear indicators were observed at Gem Lake, domains. Gem Pass,Alger Lakes, , southeastof Parker Pass, The shear zone bifurcatesnorthwest of Alger Lakes (Figure and at the northern end of Kuna Crest. At Gem Lake, 2): one splay continues northwest across Mesozoic rocks asymmetric winged inclusions [Hanmer and Passchier,1991] toward the Kuna Crest segmentat Helen Lake, and the other are prominent in crystal lithic tuffs and tuff breccias of the splay strikes north-northwestand follows the Paleozoic/Me- Koip sequence(Figure 7). Pressure-shadowwings with dextral sozoic contactthrough Parker Pass.Northwest of Parker Pass, asymmetry, delineated by recrystallizedcalcite, sericite, and quartz, are well developed around feldspar and quartz porphyroclasts (Figure 7a). Sericite concentrationson the :::• graniteCanyonof(210 Lee Me)Vining high-pressurecomers of porphyroclastsform quarterstructures i:• Koip sequence (Tr and Jr) "•76..,,./ • [Hanmer and Passchier,1991], also indicatingdextral shear (Figure 7b). RushCreek sequence (O-D) ,,-',•----'.-•::•. I Many porphyroclastsystems have a compoundhistory, in • chertandsiliceous argill'te which an inclusion with a sigma-typepressure shadow that • calc-silicaterocks originally formed with dextral asymmetryhas beenrotated in a clockwise direction (Figures 7a and 7b). These rotated • marbleandquartzite ...•'.•?-,'••••• inclusions are similar to delta-type porphyroclastsystems • trail • •'••x• •'" •'•:::•: describedby Passchief and Simpson [1986]. However, the wings of the inclusionsdescribed here consist of a quartz- •e • %0•' •q __ calcite-sericite mineral assemblagethat precipitated in the pressureshadow, rather than material tectonicallyeroded from •,•,•:::••o'%• • , _.•- .•/- •_• {::::::::::- the rotating grain as in delta-type systems. These porphyroclast systems may indicate changing conditions during shear deformation,with relatively higher temperatures and/or slower strain rates (favoring recrystallization) alternating with lower temperaturesand/or faster strain rates (favoring porphyroclast rotation) [Hanmer and Passchier, 19911. Northwestof Gem Lake at Gem Pass(Figure 2), $-C fabric is well developed in fine-grained crystal tuff and sericite schist. Shear planes (C planes) striking N20-50W and dipping 80ø L•ke • , meters,,, 5•0 $W are discrete, planar to anastomosing,and defined by segregations of quartz and phyllosilicates. $chistosity ($ 1119o19' planes), defined by pressure-solutionsurfaces and sericite, is Figure 6. Generalizedgeologic map of the Gem Lake shear axial planar to crenulations with a dextral asymmetry. zone at Gem Lake, showing the breakup and overprintingof Syntheticshear bands (C' planesof Berth• et al. [1979]) with northeasttrending structural grain in the Rush Creek sequence dextral offset are prominentin many sectionsand commonly by the northwesttrending Gem Lake shearzone (modified from contain syndeformationalcalcite with curved grainsand grain Strobel [1986]). shapepreferred orientation indicating dextral shear. GREENEAND SCHWEICKERT:GEM LAKE SHEAR ZONE, SIERRA NEVADA 951

Figure•7. Deformation features in crystal-lithic lapilli tuff, Gem Lake segment, northeast shore of Gem Lake. (a) Winged lithic and porphyroclastsystems indicating dextral shear. Small porphyroclastin lower right shows clockwise-rotated pressure shadows. (b) Photomicrographof clockwise-rotated plagioclase porphyroclast.Field of view is 2 mm. Figures7a and 7b are orientedhorizontally, with top of photo to the southwest.(c) Steeply plunging lineation on foliation plane. Figure 7c is orientated vertically, parallel to foliation. (d) Photomicrograph of plagioclase porphyroclasts stretched vertically, parallel to steeply plunginglineation, with elongatecalcite filling pressureshadow. Field of view is 2 mm. Figure 7d is oriented parallel to lineation and perpendicularto foliation. 952 GREENEAND SCHWEICKERT:GEM LAKE SHEARZONE, SIERRA NEVADA

Figure 7. (continued)

Rotated veins and asymmetricfolds of mylonitic foliation plunging stretching lineation. There is no consistent with a dextral displacementsense are locally prominentat overprinting relationship between structuresrelated to strike both mesoscopicand microscopicscales, particularly in the slip and those related to stretching. Gem Lake area. Hinge lines of asymmetricfolds plunge Extension fractures and pressureshadows associatedwith steeply to the northwest,parallel to the stretchinglineation. the stretching lineation contain the same calcite-sericite- Stretching lineation. A prominent, .northwest quartz mineral assemblage as the S-C fabric and the plunging stretching lineation in the Gem Lake segment asymmetric porphyroclast systems associated with dextral (Figure 5) is evidenton steeplysouthwest dipping foliation shearing.In addition, feldspar and quartz porphyroclastsshow planes (Figure 7c). It is defined by elongateaggregates of similar brittle to brittle-ductile conditions of deformation in phyllosilicates,extended lithic clasts,and extendedquartz and both stretching-parallel and strike-slip-parallel sections. We feldspar porphyroclasts. interpret these relations to indicate that dextral shearingand At Gem Lake, lithic clastsare fracturedand symmetrically subvertical stretchingin the Gem Lake segmentwere coeval. extended in the lineation direction, and the extension fractures The Gem Lake segmentis thus a transpressionalshear zone in are filled by calcite. Lineation-parallel thin sections show which the stretching direction is perpendicular to the slip boudinagedand brittlely extendedporphyroclasts (Figure 7d), direction. which generally have symmetrical pressure shadows containing calcite, sericite, and quartz. This stretching Kuna Crest Segment lineation indicates symmetrical stretching in a subvertical direction,parallel to the rotationaxis for dextralsimple shear The Kuna Crest segment is characterized by a zone of deformation. prominently lineated rocks, exposed from the north end of Summary. In the Gem Lake segment,dextral strike-slip Kuna Crest to the vicinity of Blacktop Peak (Figure 2). deformationis indicatedby porphyroclastasymmetries, S-C Although structures in the Kuna Crest segment indicate fabric, rotated veins, and asymmetricminor folds observedon dominantly subvertical stretching, subordinate dextral horizontal surfaces.Subvertical stretching is evidencedby structures are also present, and the Kuna Crest segment is symmetricallyextended porphyroclasts and by steeply consideredto be a segmentof the Gem Lake shearzone. GREENEAND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA 953

Stretching lineation in the Kuna Crest segment is best recrystallization[e.g., Simpsonand DePaor, 1991;Hirth and developedin siliceouswall rocks at or near the contactwith Tullis, 1992]. The higher temperatureof deformationof this the of Kuna Crest; lineation is less well dike as comparedto the surroundingcountry rock indicates developedin volcanicrocks of the Koip sequenceand is rarely that late deformation on the Kuna Crest segmentwas coeval visible in the granodiorite itself. Thus, although the Kuna with intrusionof dikes of the granodioriteof Kuna Crest Crest segmentis spatiallyassociated with the granodioriteof Lineation and foliation. Bedding and foliation Kuna Crest, the intensity of deformationhas been influenced orientations in the Kuna Crest segment, and throughout the significantlyby wall-rock lithology. At the north end of Kuna Kuna Crest area, strike N10E to N20W and dip steeply to the Crest, the zone of most intense deformation follows the east (Figure 5). This contrastswith the rest of the NRP, in contact between the Palmetto Formation and the Sawmill which the structural fabric dips predominantly to the Canyon sequence, rather than the granodiorite/wall-rock southwest. As in the main Gem Lake shear zone, foliation is contact.Lineation is progressivelyless well developedin the subparallel to and an intensification of regional structural Palmetto Formation to the east, but the decrease in fabric. Lineation plungessteeply to the south. deformation intensity is gradual, and lineation can be Stretching lineation associated with the Kuna Crest recognizedlocally as far eastas ParkerPass valley. segmentis particularly well exposednortheast of Kuna Lake, Conditions of deformation. Stretching in the Kuna where quartz siltstoneof the Palmetto Formation, a Jurassic(?) Crest segment occurred at relatively low temperatures, as quartz porphyry dike (Figure 8), and a dike from the indicatedby the limitedelongation of .quartzporphyroclasts in granodiorite of Kuna Crest are all moderately to poorly rocks with a strongly deformed matrix. For example, a quartz foliated but strongly lineated. Stretchinglineation is defined porphyry dike involved in the shear zone has a strongly by elongateaggregates of phyllosilicatesand by segregations developed lineation defined by streaks of muscovite and of quartz and feldspar.Foliation is predominantlydefined by biotite on foliation planes. However, large quartz phenocrysts compositionaland grain-sizedifferences and consistsof relict (5-10 mm in diameter) in the porphyry are only slightly layering accentuatedby subsequentmetamorphic segregation. extended (axial ratios of 2:1). Steeply plunging symmetrical West of Blacktop Peak, the granodioriteof Kuna Crest pressure-shadowwings around phenocrystsand foliation- displays a well-developed, northwest striking, solid-state parallel lattice-preferredorientation of quartz in the matrix foliation subparallelto the pluton margin (Figure 9). The suggestpredominantly pure sheardeformation. foliation is defined by highly elongate quartz and feldspar In contrast, a dike of the granodioriteof Kuna Crest that grains and biotite folia wrappingaround feldspar grains. Rare intrudes the shear zone northeast of Kuna Lake (Figure 2) K-feldspar porphyroclasts have sigma-type asymmetry recordsa significantlyhigher temperatureof deformation.In indicating dextral shearing. A locally developed stretching this dike, quartz forms elongate, multigrain ribbons, and K- lineation plunges steeply to the southeast, parallel to feldspar porphyroclastsconsist of highly elongateaggregates stretchinglineation in the adjacentSawmill Canyonsequence. of subgrainsshowing evidenceof grain-boundarymigration Foliation is intenseonly within a few metersof the wall-rock

Figure 8. Extensionlineation on subverticalfoliation surfacesin quartz porphyrydike, Kuna Crest. 954 GREENE AND SCHWEICKERT: GEM I.AKE SHEAR ZONE, SIERRA NEVADA

the Kuna Crest segment subhorizontal shortening and subverticalstretching were the dominantdeformation.

Relation of Kuna Crest segment to granodiorite of Kuna Crest. Complex and locally contradictory relations between intrusion of the granodioriteof Kuna Crest and deformation in the shear zone indicate that intrusion of the granodioriteat 91 Ma (U/Pb zircon age [Stern et al., 1981]) was coeval with deformation in the Gem Lake shear zone. The main mass of the granodiorite adjacent to the Kuna Crest segment displays a variably developed solid-state foliation parallel to the wall-rock contact but generally does not contain a prominent stretching lineation. Contact relations between deformed rocks of the shear zone and granodiorite vary along strike. South of the Kuna Crest, strongly lineated wall rocks locally form within only slightly deformed granodiorite, and undeformed late pegmatite veins cut highly deformed wall rocks, indicating deformationprior to intrusion. However, near Helen Lake the granodiorite/wall-rock contact is a complex zone of interleaved, deformed granodiorite and foliated wall rock suggesting synintrusional deformation. Pegmatite veins in the wall rock at Helen Lake are boudinaged vertically, indicating synintrusionalto postintrusionvertical stretching. West of Blacktop Peak, the granodioritecontains rare, steeply plunging folds delineatedby felsite veins. Well-developed,• steeply dipping, solid-state foliation in the granodiorite (describedabove) is axial-planar to these folds. Subhorizontal felsite dike swarms are locally prominent in this area, also demonstrating synemplacementto early postemplacement subvertical stretching.

Comparison of Kuna Crest Segment Figure 9. Solid-state foliation in granodiorite of Kuna With Gem Lake Segment Crest approximately2 m from contactwith wall rocks.View is northwest at a subvertical surface, located in cirque west- Both the Gem Lake and Kuna Crest segmentsare dextral northwest of Blacktop Peak. transpressional shear zones. Based on the relative developmentof small-scalestructures, dextral strike slip was dominantin the Gem Lake segment,and subverticalstretching was dominant in the Kuna Crest segment.Both shear zones exhibit a similar structural style, involving distributed contact and decreases to a poorly developed magmatic(?) deformation which reactivated an older regional structural fabric. foliation within a few hundredmeters. Kistler [1966a] mapped this area as the Cretaceous "sheared granodiorite of Koip Both the Gem Lake and Kuna Crest segmentswere active Crest"; subsequently, Bateman [1992] included it in the before and during emplacementof Late Cretaceousplutons. Jurassic"sheared of Koip Crest and the South Fork of The Kuna Crest segment,as discussedabove, was activeduring Bishop Creek." However, outside the shear zone these rocks and after emplacementof the granodioriteof Kuna Crestat 91 are petrographically identical to the granodiorite of Kuna Ma. The Gem Lake segment does not intersect Cretaceous Crest. They are here consideredto be Cretaceousgranodiorite granitoids,but 40Ar/39Ardating of syntectonicamphibole to of Kuna Crest that has undergone localized solid-state the southeastin the Ritter Range segment of the shear zone deformation associatedwith the Kuna Crest segment of the (Figure4) indicatesdeformation at 85 Ma [Sharpet al., 1993], ßGem Lake shear zone. probably synchronouswith postemplacementdeformation of Dextral deformation. A component of subhorizontal the granodioriteof Kuna Crest. dextral simple shear within the Kuna Crest segment is The Gem Lake and Kuna Crest segments display an indicated by rare asymmetricfolds, rotated tensiongashes, and anastomosingmap pattern(Figure 2). A splayof the Gem Lake locally developed S-C fabrics with shear planes parallel to segmenttrends obliquely into the Kuna Crestsegment south of regional foliation. Dextral shear indicators were observedat Helen Lake, and at the north end of Kuna Crest the Gem Lake more than nine locations,primarily along the northern part of and Kuna Crest segmentsappear to merge.This anastomosing the Kuna Crest, near Helen Lake, and west of Blacktop Peak. map pattern, combinedwith the similaritiesin deformation, However, structures indicative of dextral shear are much less structuralstyle, and timing, indicatesthat the Kuna Crestand prominent than in the Gem Lake segment, indicating that in Gem Lake segmentsare parts of the sameshear zone. GREENEAND SCHWEICKERT:GEM LAKE SHEAR ZONE, SIE• NEVADA 955

Regional Characteristics of the the Ritter Range segment has yielded ages of 85 Ma Gem Lake Shear Zone (hornblende) and 81 Ma (biotite and sericite) [Sharp et al., 1993]. The Gem Lake shear zone includes the Gem Lake segment, At , on the southeast edge of the Ritter the Kuna Crest segment, the Ritter Range segment, and Range pendant (Figure 4), lenses of foliated crinoidal marble adjacentsplays (Figure 4). The Gem Lake shearzone probably are imbricated with Mesozoic metavolcanic rocks. These correlates directly with nearby shear zones to the north and marble lenses contain chert nodules with prominent pressure- south and generally with a much larger set of similar-trending shadow wings indicating dextral shear (Figure 10) and are shear zones collectively called the Sierra Crest shear zone identical to fossiliferous marble lenses exposed in the Gem system [Tikoff, 1994; Tikoff and Greene, 1994]. The Lake shear zone northwestof Alger Lakes (describedbelow). following sectionsdetail the extent, displacement,and timing Dextral transpressionaldeformation related to the Gem Lake of deformation associated with the Gem Lake shear zone. shear zone can thus be traced as far south as Minaret Summit Subsequentsections describe the connectionsand correlations on the southeastedge of the Ritter Range pendant. between the Gem Lake shear zone and the proposedSierra Crest shear zone system, contrast this system with other Amount of Displacement proposed shear zones in the central and southern Sierra The magnitude of dextral displacementon the Gem Lake Nevada, and discuss the relation of shear zone deformation to shear zone is difficult to constrainbecause displacement was pluton emplacement. predominantly parallel to the regional trends of structures. However, an estimateof displacementis provided by isolated Northward Continuation lenses of fossiliferous marble imbricated with the Palmetto North of Kuna Crest, a prominentsolid-state foliation that Formation in the shear zone northwest of Alger Lakes (Figure is on strike with the Gem Lake shear zone is presentin both 2) [Greene, 1995]. These lenses of marble contain a late the Half Dome Granodiorite and the granodiorite of Kuna Paleozoic shallow-water fauna indicating a general age range Crest. A steeply southeastplunging extension lineation is from Late Mississippian through mid-Permian (J.T. Dutro, well developedonly near the granodiorite/wall-rockcontact. written communication, 1988) and conodont fragments Quartz ribbonsparallel to elongatefeldspar and mafic minerals compatible with a Carboniferous age (J. Repetski, oral indicate predominantlysolid-state deformation. communication, 1991). The contact between these deformed granitoids and wall Structurally imbricated lenses of crinoid-bearing marble rocksof the Koip sequenceis well exposed1 km southwestof that are very similar to those at Alger Lakes are also exposed Gaylot Peak (Figure 4), where both the granodioriteof Kuna at Minaret Summit [Huber and Rinehart, 1965], where they Crest and the wall rocks are highly deformed. Protomylonite contain mylonitic foliation and chert nodules with prominent with local S-C fabric is developed in the granodiorite, and dextral asymmetry(Figure 10). The same distinctivecrinoid- complex deformationfeatures including ductilely deformed bearing marble is exposedat Mammoth Rock (Figure 4), at pseudotachylite(?)layers are presentin the Koip sequence. Pappas prospect (3 km east of Mammoth Rock), and in the Five kilometers northwestof Gaylot Peak at Sawmill Canyon, Mount Morrison pendant[Rinehart and Ross, 1964]. All these a prominent downdip lineation is developed in the Jurassic exposureshave been correlatedwith the PennsylvanianMount wall rocks of the Sawmill Canyon sequence[Schweickert and Baldwin Marble in the Mount Morrison pendant, which Lahren, 1993a]. These relations suggestthat the Gem Lake consistsof a distinctive, dark gray marble containing crinoid columnals and chert nodules [Rinehart and Ross, 1964; Huber shear zone may continueto the north for some distancealong the west edge of the SaddlebagLake pendant,involving both and Rinehart, 1965; Kistler and Nokleberg, 1979; Greeneand the Koip and Sawmill Canyon sequencesand Late Cretaceous Dutro, 1991]. granitoids of the Tuolumne Intrusive Suite [Tikoff, 1994; Correlation of the upper Paleozoic marble lenses at Alger Tikoff and Greene, 1994]. Lakes with thoseat Minaret Summit suggestsa minimum of 20 km of dextral displacement.If the marble lenses at Minaret Summit were displacedfrom the Mount Baldwin Marble in the Ritter Range Segment Mount Morrison pendant, there is a possibility of at least 7 South of the NRP, the trace of the Gem Lake shear zone is km of additional displacementbetween Minaret Summit and concealed beneath Tertiary volcanic cover, but 5 km along the north end of the Mount Morrison pendant (Figure 4). strike to the southeastin the Ritter Range pendant(Figure 4), Because the isolated lenses of marble lie entirely within the anastomosingdomains of highly sheared rock occur in the shear zone, the extent of the marble lensesprobably does not basalpart of the Koip sequenceand along the Paleozoic/Meso- reflect the total displacementacross the shear zone, and the zoic contact. The Ritter Range segment contains orange- above estimates are therefore minimum offsets. weathering, siliceous mylonitic rocks with sigma-type An alternative possibility is that the Pennsylvanian(?) inclusion and porphyroclast systems indicating dextral marble lenses at Alger Lakes are depositionalremnants of a shearing. A locally well-developed S-C fabric, with shear more widespread upper Paleozoic Antler overlap sequence. planes striking N40-60W and dipping vertically, also shows This alternative is unlikely, however, for the following dextral shearing.Stretching lineation and strain measurements reasons:(1) the marble lensesoccur only within the Gem Lake in the younger volcanic section to the west indicate strong shear zone, in associationwith widespreadevidence of dextral flatteningdeformation and vertical stretching[Tobisch et al., shearing; (2) the marble lenses occur only as rare, isolated 1977]. The 40Ar/39Ardating of synkinematicminerals within slices structurally imbricated with surroundingunits; and (3) 956 GREENEAND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA

Figure10. Ductileflow around chert inclusion in marble,Ritter Range segment, Minaret Summit. Photo is orientedhorizontally, with top to thesouthwest. Light-colored wings attached to upperright and lower left of inclusionare composed of white calcite initially formed in thepressure shadows of the inclusion during dextral shearing[Hanmer and Passchier,1991 ]. the marble lensesare not associatedwith other units of the 4øAr/39Arages of 85 Ma for syntectonicamphibole and 80 Ma upperPaleozoic section (in contrastto all knownparautoch- for syntectonicbiotite and white mica. thonousexposures of theMount Baldwin Marble). However, if South of Minaret Summit, the Gem Lake shear zone is the marblelenses are in fact locallyderived remnants, then probably continuouswith the Rosy Finch shear zone dextral displacementon the Gem Lake shearzone could be much less. (discussedin detailbelow). The RosyFinch shear zone is exposed in the Lake Edison Granodiorite, which has a concordantU/Pb age of 88+1Ma [Tobischet al., 1995]•and in Timing of Displacement the MonoCreek Granite, which has yielded a discordantU/Pb Althoughthe exacttiming of displacementon the Gem ageof 88 Ma anda concordantU/Pb age of 76 Ma [ Sternet al., Lake shearzone is unknown,available age constraints 1981]. Displacementon the Rosy Finch shear zone is indicatethat the Gem Lake shear zone was active in earlyLate consideredto be synmagmaticto early postmagmatic [Tikoff Cretaceoustime, from before 91 Ma to possiblyas young as and Teyssier,1992; Tobischet al., 1995],and Tobischet al. 76 Ma. Field relationsin the NRP (discussedabove) [1995] reportedAr/Ar agesof 87 Ma on metamorphic demonstratethat the Gem Lake shear zone was active during hornblendeand 83 Ma on metamorphicbiotite from within the intrusionof thegranodiorite of Kuna Crest at 91 Ma. However, shearzone in theLake Edison pluton. Thus late displacement the metamorphic rocks in the NRP show evidence of on a continuousGem Lake-Rosy Finch shear zone is younger significantlymore shear deformation than the granodiorite. than 88 Ma and could be as young as 76 Ma. Dextral Ductileshear fabrics, asymmetric folds and porphyroclasts, displacement on theRosy Finch shear zone is probablyin the andextension lineations are all prominentin metamorphicrange of 1.2-8 km (B. Tikoff, personalcommunication, rocksin theGem Lake shear zone but only locally present in 1994), whichis substantiallyless than the minimumof 20 km the granodiorite.In addition,inclusions of protomylonitic of displacementsuggested here for the Gem Lake shearzone. wall rocksoccur in muchless deformed granodiorite near the Thisis consistentwith theinterpretation that displacement on granite/wall rock contact on Kuna Crest. These observations the GemLake shear zone in theNRP beganprior to intrusion indicate that initial deformationin the Gem Lake shearzone of theLate Cretaceous plutons in whichthe Rosy Finch shear precededintrusion of the granodiorite. zone is exposed. North of the NRP, local solid-state foliation that is on In summary,available age dataindicate that the Gem Lake strikewith the GemLake shearzone is presentin the Half shearzone was active from before 91 Ma to possiblyas young DomeGranodiorite, which has an agebetween 88 and86 Ma as 76 Ma. Thesedata do not constrainthe beginningof [Bateman,1992]. In metavolcanicrocks in theRitter Range deformationin the shearzone, which is only knownto be segmentof the shear zone, Sharp et al. [1993] obtained youngerthan 201 Ma. Metavolcanicrocks of thisage underlie GREENEAND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA 957 the Sawmill Canyon sequence [Schweickert et al., 1994], have had significant displacementprior to intrusion of Late which is the youngestmetamorphic unit involved in the shear Cretaceousgranitoids. zone. South of the Rosy Finch shear zone, northwest trending However, at least two lines of evidence suggest that zones of shearedgranitoids and metavolcanicrocks as young displacement on the shear zone probably occurred as 131 Ma (U/Pb age) [Tobisch et al., 1986] have been predominantlyin early Late Cretaceoustime. reportedin the Mount Goddardpendant (Figure 11) [Bateman, 1. A minimum dextral displacementof 20 km is suggested 1965, 1992; Bateman and Moore, 1965; Moore, 1978; here for the Gem Lake shearzone. At an averageslip rate of 5 Tobisch et al., 1986; Saleeby et al., 1990]. Although the mm/year, this much displacementcould be achievedin 4 m.y. sense of displacement in these sheared rocks is presently and twice this displacementin less than 10 m.y. Thus the age unknown, significant subvertical stretching consistent with range of dated deformationin the Gem Lake shear zone (>91 that observedin the Gem Lake shearzone has been reportedby Ma to 80 Ma) is more than sufficient to account for the Tobisch et al. [1986]. suggesteddisplacement. Northwest trending zones of sheared granitoids have also 2. Reconstructionsof Cretaceousplate convergencevectors been mapped north of the Oak Creek pendant (Figure 11) between North America and Farallon plates [Engebretsonet [Moore, 1963; Saleebyet al., 1990] and are separatedfrom the al., 1985; Glazner, 1991; Tobisch et al., 1995] indicate that a Mount Goddardpendant only by the Late CretaceousCartridge large dextral tangential componentof convergenceacross the Passpluton [Moore, 1963]. Penetrativeductile deformationin plate boundarydeveloped only after about 100 Ma. Oblique the Oak Creek pendantitself [Longiaru, 1987; Saleebyet al., convergencein modern plate boundary settingsis known to 1990] suggeststhat the zone of shearing could conceivably result in strain partitioningand strike-slip deformationwithin continue through the Oak Creek pendantand into the Owens the arc [e.g., Jarrard, 1986; Cashmanet al., 1992; McCaffrey, Valley. Deformationin the Oak Creek pendantand associated 1992; Teyssier et al., 1995], and it seems reasonable to shearedgranitoids has been datedat between110 and 105 Ma interpret the dextral transpressionalGem Lake shear zone in (U/Pb) [Saleebyet al., 1990]. the Sierra Nevada magmaticarc as a manifestationof post-100 The degree of continuity of these various shearzones and Ma dextral oblique convergence. sheared granitoids is unknown. Nevertheless, a nearly Tobisch et al. [1995] have recently suggested that a continuoussystem of middle Cretaceousshear zones can be transition from predominantly contractional deformation to traced from the SaddlebagLake pendant to at least the Oak dextral transpressionaldeformation occurred in the eastern Creek pendant,a distancegreater than 150 km (Figure 11). A Sierra Nevada at around 90 Ma, based on dating of dextral transpressionalshear zone systemof this extentwould contractional shear zones in the west-central Sierra Nevada and have important implications for middle Cretaceoustectonics the age of deformation in the Rosy Finch shear zone. We and pluton emplacementin the Sierra Nevada. suggest that this transition may have occurred somewhat We propose to call this system of distributed dextral earlier, in the range of 110-100 Ma, based on the evidence shearing the Sierra Crest shear zone system [Tikoff, 1994; presentedhere for significantactivity on the Gem Lake shear Tikoff and Greene, 1994]. This is intendedas a collectiveterm zone prior to 91 Ma. for locally defined, but possibly related, middle Cretaceous dextral shear zones in the easternSierra Nevada including, but not limited to, the Gem Lake, Rosy Finch, and associated Regional Tectonic Implications shear zones discussed above.

Correlation of the Gem Lake Shear Zone Other Proposed Shear Zones in the Central and South of Minaret Summit, the Gem Lake shear zone is Southern Sierra Nevada concealed by Quaternary volcanic rocks of , but shearedgranite and wall rocks associatedwith The Gem Lake shear zone and its possible continuations the dextral Rosy Finch shear zone have been reportedalong provide direct field evidencefor a dextral shearzone systemof the northeastedge of the Mono Creek pluton (Figure 11), 10 regional extent in the central and eastern Sierra Nevada. As km to the southeastof Minaret Summit [Tikoff and Teyssier, noted earlier, similar dextral shear zones have been suggested 1992; Tikoff, 1994]. by a numberof workersbased on other lines of evidence. The Rosy Finch shear zone is 3-4 km wide through the IBB 2 and IBB 3. Two intrabatholithic dextral faults, central part of the Mono Creek pluton, displays consistent termed IntrabatholithicBreaks 2 and 3 (IBB 2 and IBB 3) by dextral shearindicators throughout, and is synemplacementto Kistler [1990, 1993] and the Axial and Eastern Batholithic early postemplacement[Tikoff and Teyssier, 1992]. The shear Breaks(AIB and EIB) by Saleebyand Busby[1993], have been zone continuessoutheastward through the Lake Edison pluton proposedon the basisof apparentdextral stepsin the trace of [Lockwood and Lydon, 1975], the Lamarck pluton, and into the Sri=0.706isopleth (Sri = initialratio 87Sr/86Sr). The Gem the northernpart of the Evolution Basin pluton (Figure 11) (B. Lake and associated shear zones lie between the traces of these Tikoff, personal communication, 1994). The Rosy Finch proposed intrabatholithic faults (Figure 12). The proposed shear zone developed to accommodate active dextral Mojave-Snow Lake fault coincideswith IBB 2 along part of its deformation as these Late Cretaceousplutons intruded and length. cooled. The Rosy Finch shear zone as presently exposed The precise locations of these intrabatholithic breaks are probably recordsthe last incrementsof dextral deformationon not well constrained by published initial strontium data an older shear zone that may, like the Gem Lake shear zone, points, and Kistler's [1993] proposedtrace of IBB 3 conflicts 958 GREENE AND SCHWEICKERT: GEM LAKE SHEAR ZONE, SIERRA NEVADA

! 118ø130' I pendant NorthernRitter ...... Range pendant SaddlebagLake .. Figure2'

Gem Lake shear zone

Mr.

Ritter Range

37 ø 37 ø 30' 30' Rosy•in shear zone

Mr. pendant

.. :.:: :.. -:.::...... ---:•:•:;•.•.:...... ::.:::;-•:::.i:;:::::::::::::::::::::::::::::::

%•,•Shear zones faults and undifferentiated sheared ':'"'--:•Tertiary and Quaternaryrocks

I'/'::1withCretaceous major plutongraniticboundaries rocks, :::...:...•Triassic andJurassic graniticrocks shearedgramtold •;• Mesozoicvolcanic and sedimentaryrocks • Paleozoicmiogeoclinal rocks

'½•.?:•i Paleozoiceugeoclinal rocks kmi 20I I I 118 ø

Figure 11. The Gem Lake shearzone and possiblecontinuations in the easternSierra Nevada that comprise the proposedSierra Crest shearzone system.Names of selectedLate Cretaceousplutons are shown;ages are as follows: Kuna Crest, 91 Ma U/Pb; Mono Creek, 88 Ma, 76 Ma U/Pb [Stern et al., 1981]; Lake Edison, 88+1 Ma U/Pb[Tobisch et al., 1995];Lamarck, 90 Ma U/Pb[Stern et al., 1981];Evolution Basin, 85, 80, and79 Ma K/Ar [Everndenand Kistler, 1970]. Figure 11 is modifiedfrom Bateman [ 1992], Saleebyet al. [ 1990], and B. Tikoff (unpublishedmapping, 1994); trace of IBB 3 is from Kistler [1993]. with known field relations in the Saddlebag Lake pendant developed along the axis of the Sierra Nevada in middle [Schweickert and Lahren, 1993a, b] and the Mount Morrison Cretaceous time. The Gem Lake shear zone and associated pendant [ Greene et al., 1995]. Evidence for correlation of structures of the Sierra Crest shear zone system may be Triassic granites acrossthe proposedtrace of IBB 3 [Kistler, preservedremnants of suchdeformation. The northwardoffset 1993] is equivocal at best, and there is no documented of the Sri=0.706 isopleth could in part reflect this distributed evidenceof dextral shearingalong the proposedfault trace. An deformation, rather than being the result of large offset on two alternative location for IBB 3 would be the Sierra Crest shear discretefault traces as presentlyproposed [Kistler, 1993]. zone system. Published initial strontium data points [Kistler, Mojave-Snow Lake fault. The Mojave-Snow Lake 1993] are compatible with this suggestion,although the 90 fault (Figure 12), with up to 400 km of dextral displacement, km of dextral displacementpostulated by Kistler [1993] for has been proposedon the basis of stratigraphiccorrelation of IBB 3 is substantiallylarger than the displacementwe infer for miogeoclinal rocks and Jurassicintrusive rocks in the Snow the Gem Lake shear zone. Lake pendantwith rocks of the Mojave Desert [Lahren and Possibly, a broad zone of distributeddextral deformation, Schweickert, 1989; Lahren et al., 1990; Schweickert and involving numerous anastomosingstrike-slip shear zones, Lahren, 1990]. The trace of this proposedfault is near the axis GREENE AND SCHWEICKERT: GEM LAKE SHEAR ZONE, S[E• NEVADA 959

119 ø granitic plutons that are interpreted to have intruded and obscured the trace of the fault). These timing constraints suggest that displacement on the Mojave-Snow Lake fault occurredbefore the last phaseof deformationin the Gem Lake shear zone. Possibly, an early phase of displacementon the Gem Lake shearzone was synchronouswith displacementon the Mojave-Snow Lake fault, and the Gem Lake shearzone was proposedSierra Crest• later reactivated during intrusion of the Late Cretaceous Mojave-Snow Lake shear zone system ) fault (=IBB 2) plutons. In this case, the Gem Lake shear zone might have been a subsidiarystrand of a Mojave-SnowLake fault system, 370 analogous to the various strands of the present-day San Andreas fault system [Greene and Dutro, 1991]. Other possibilities are (1) the Mojave-Snow Lake and Gem Lake shearzones do not overlap in time and are unrelatedstructures, and (2) displacementon the Mojave-SnowLake fault may have possible northern been younger than is presentlybelieved. continuation of the proto-Kern Canyon Proto-Kern Canyon shear zone. The proto-Kern 0 km 0 ...... shear Canyon shear zone (Figure 12) is a north trending, dextral

360 36o shear zone exposed for more than 130 km in the southern :• Mesozoic Sierra Nevada [Busby-Spera and Saleeby, 1990; Saleeby, gr a ni t ic ro c k s iii:ii??...... 1992]. The shear zone was active between about 105 Ma and Mesozoicvolcani'• and '' 't"•%¾'-•' ' ' sedimentaryrocks 83 Ma, based on U/Pb dating of synkinematic and Paleozoic :::::::::::::::::::::'•::: postkinematic intrusive rocks [Busby-Spera and Saleeby, mlogeocl•nal' ' rocks 1990]. Sheared granitoids that are probably associatedwith a Paleozoic -•-::[-;'"•*'• •••'••:.W[ sh•r zone northward continuationof the proto-Kern Canyon shear zone eugeoclinalrocks:•??'•?..:••• are separatedfrom similar shearedrocks in and adjacentto the ...... :.:?:•.::•.::••::.•:[:[::•f::•:•.?•.::• ...... :.•:•:•::::•::•:•...... ß. :•:: ß :.•;..:.....?:.;.:::•;:::•::;•9•:•[.•:•::•::::•- Oak Creek pendantonly by the and Paradise plutons (83-86 Ma U/Pb) [Moore, 1981; Chen and Moore, 1982; Moore and Sisson, 1985]. This suggeststhe possibility of a connectionbetween the shearzones prior to emplacement of the Mount Whitney intrusive suite, as originally proposed Figure 12. Proposeddextral shearzones in the centraland by Schweickert [1981]. Tikoff and Greene [1994] reported southernSierra Nevada. Abbreviations are: BC, BoydenCave dextral shearing in the Mount Whitney intrusive suite and pendant;MG, Mount Goddardpendant; MK, Mineral King suggested that the proto-Kern Canyon shear zone be pendant;MM, Mount Morrison pendant;NR, Northern Ritter considereda part of the Sierra Crest shearzone system. Range pendant;OC, Oak Creek pendant;RR, Ritter Range Relation to Pluton Emplacement pendant;SL, SaddlebagLake pendant;SN, SnowLake pendant. Figure 12 is modified from Jennings [1977], with data from Timing constraints discussed above indicate that Schweickertand Labten [ 1991], Kistler [ 1993], and Saleeby displacementon the GemLake shearzone was synchronous and Busby [1993]. with intrusionof Late Cretaceousplutons of the Tuolumne IntrusiveSuite (which includes the granodioriteof KunaCrest and the Half Dome Granodiorite)and the John Muir Intrusive Suite (which includes the Lake Edison Granodiorite and the Mono Creek Granite). Plutons of these Late Cretaceous of the SierraNevada batholith, approximately coincident with intrusive suitesappear to have intrudedthe active Gem Lake the position of IBB 2 [Kistler, 1993]. and Rosy Finch shearzones and to have been subsequently A dextral shear zone of middle Cretaceousage in the deformed in the shear zones. Mineral King pendant[Saleeby and Busby,1993] and sheared The Gem Lake shearzone thus provides direct field evidence granitoids of middle Cretaceousage in the Boyden Cave for dextral transpressionaldeformation in the central eastern pendant [Saleeby et al., 1990] occur near or along the Sierra Nevada during the intrusion of Late Cretaceous proposedtrace of the Mojave-Snow Lake fault. Steeply granitoidsand strengthensthe casefor a geneticlink between plunging stretchinglineations of probablemiddle Cretaceous dextralfaulting and emplacementof Late Cretaceousplutons, age that are presentin these and many other pendantsin the as proposedby Glazner [1991], Saleeby [1991], and Tikoff southernSierra Nevada [Saleeby et al., 1990; Saleebyand and Teyssier[1992, 1993a],among others. Busby, 1993] are consistent with transpressional Evidence for major dextral transpressionaldeformation is deformation. locallyrecorded in wall-rockpendants as describedhere (e.g., Lahren et al [1990] concludedthat displacementon the the Gem Lake shear zone), whereas only the minor Mojave-Snow Lake fault occurredafter 148 Ma (the age of postemplacementphase of this deformationis recordedin the dikes in the Snow Lake pendant that are correlated with the plutons themselves(e.g., the Rosy Finch shear zone). We Independencedike swarm)and before about 110 Ma (the ageof speculatethat emplacementand cooling of Late Cretaceous 960 GREENE AND SCHWEICKERT: GEM LAKE SHEARZONE, SIERRANEVADA

plutons may have substantiallyaltered the rheology of the adjacentto the Mount Whitney intrusivesuite. While thereis Sierra Nevada block, welded the shear zones, and possibly as yet insufficientdata to demonstrateconclusively that all of resulted in the transfer of dextral displacementfrom the arc to this deformationis directly related, the evidenceis consistent forearc or back arc regions. with the hypothesisthat the middle CretaceousSierra Nevada was characterized by distributed dextral transpressional deformation, with numerous active shear zones that are now Conclusions only locally preservedin metamorphicpendants and older In the Gem Lake shear zone, dextral shearing and plutonsbordering the Late Cretaceousbatholith. subvertical stretching associated with horizontal shortening Acknowledgments.Field work for thisstudy was supported in partby a are directly related and resulted from dextral transpressional Sigma Xi Grant-in-Aid of Researchand by a National Science deformation.Widespread evidence exists for dextral strike-slip Foundationgrant to R.A. Schweickert.Discussions with S. Nitchman,B. and/or contractional deformation with subvertical stretching Tikoff, and O.T. Tobischwere very helpful in developingthe ideas during middle Cretaceous time in the central and southern presentedhere. R.W. Kistler assistedwith relocatingthe fossil Sierra Nevada. Other examples include the Rosy Finch shear occurrencenear Alger Lakes, and J.T. Dutro and J.E. Repetski zone, the proto-Kern Canyon shearzone, shearedrocks in the examinedfossils from that locality.Helpful reviewswere providedby SaddlebagLake, Mount Goddard, Oak Creek, Mineral King, D.M. Herring,B. Tikoff, P.H. Cashman,J.H. Trexler,R.B. Miller, and and Boyden Cave pendants, and sheared granitoids in and an anonymousreviewer.

References

Bateman,P.C., Geologicmap of the Blackcap Fossen, H., B. Tikoff, and C. Teyssier, Jennings,C.W., Geologic map of California, Mountain quadrangle, Fresno County, Modeling transpressionaland transtensional scale 1:750,000, Cahf Geol. Data Map 2, California, scale 1:62,500, U.S. Geol. Surv. deformation, Nor. Geol. Tidsskr., 74, 134- Calif. Div. of Mines and Geol., Sacramento, Map, GQ-428, 1965. 145, 1994. 1977. Bateman,P.C., Plutonismin the centralpart of Glazner, A.F., Plutonism,oblique subduction, Kistler, R.W., Geologic map of the Mono the Sierra Nevada Batholith, Cahfornia, U.S. and continentalgrowth: An example from Craters quadrangle,Mono and Tuolumne Geol. Surv.Prof Pap., 1483, 186 pp., 1992. the Mesozoic of California, Geology, 19, Counties, California, scale 1:62,500, U.S. Bateman,P.C., and J.G. Moore, Geologicmap 784-786, 1991. Geol. Surv. Map, GQ-462, 1966a. of the Mount Goddard quadrangle,Fresno Greene, D.C., The stratigraphy,structure, and Kistler, R.W., Structureand metamorphismin and Inyo Counties, California, scale regional tectonic significance of the the Mono Craters Quadrangle, Sierra 1:62,500, U.S. Geol. Surv. Map, GQ-429, Northern Ritter Range pendant, eastern Nevada, California, U.S. Geol. Surv. Bull., 1965. Sierra Nevada, California, Ph.D. 1221-E, El-E53, 1966b. Berth6, D., P. Choukroune, and D. Gapais, dissertation,Univ. of Nevada, Reno, 1995. Kistler, R.W., Two different lithospheretypes Orientations pr6f•rentielles du quartz et Greene, D.C., and J.T. Dutro, Stratigraphic in the Sierra Nevada, California, in The orthogneissificationprogressive en r6gime affinity and structuralimplications of late Nature and Origin of CordiIIeran cisaillant: L'exemple du cisaillementsud- Paleozoic fossils in the Ritter Range Magmatism, edited by J. L. Anderson, armoricain, Bull. Mindral. , 102, 265-272, pendant,eastern Sierra Nevada, California, Mem. Geol. Soc. Am., 174, 271-281, 1990. 1979. Geol. Soc. Am. Abs. Programs, 23, 30, Kistler, R.W., Mesozoic intrabatholithic Brook, C.A., M. Gordon, M.J. Mackey, and 1991. faulting, Sierra Nevada, California, in G.F. Chetelat, Fossiliferous Upper Greene, D.C., R.A. Schweickert, and R.J. Mesozoic Paleogeography of the Western Paleozoic rocks and their setting in the Strobel, Possible westward continuation of -II, editedby G. Dunn, and K. Ritter Range and Saddlebag Lake roof the Roberts Mountains thrust in the McDougall, pp. 247-261, Pac. Sect. Soc. of pendants, central Sierra Nevada, Northern Ritter Range pendant (NRP), Econ. Paleontol.and Mineral., Los Angeles, California, Geol. Soc.Am. Abstr. Programs, eastern Sierra Nevada, California, Geol. Calif., 1993. 11, 71, 1979. Soc.Am. Abstr.Programs, 21, 86, 1989. Kistler, R.W., and W.J. Nokleberg, Busby-Spera,C.J., and J.B. Saleeby,Intra-arc Greene, D.C., J.M. Wise, and C.H. Stevens, Carboniferous rocks of the eastern Sierra strike-slipfault exposedat batholithiclevels The Laurel-Convict Fault: A Triassic left- Nevada, in The Mississippian and in the southern Sierra Nevada, California, lateral strike-slipfault in the Mt. Morrison PennsyIvanian (Carboniferous) Systemin Geology,18, 255-259, 1990. pendant,eastern Sierra Nevada, CA, Geol. the United States--California, Oregon, and Cashman, S.M., H.M. Kelsey, C.F. Erdman, Soc.Am. Abstr.Programs, 27, 22, 1995. Washington, U.S. Geol. Surv. Prof. Pap., H.N.C. Cutten, and K.R. Berryman, Strain Hanmer, S., and C. Passchier, Shear-sense 111 O-C, CC-21-26, 1979. partitioningbetween structural domains in indicators:A review, Pap. Geol. Surv. Can., Lahren, M.M., and R.A. Schweickert, the forearc of the Hikurangi subduction 90-17, 72 pp., 1991. Proterozoic and Lower Cambrian zone, New Zealand, Tectonics, 11, 242- Hanson,R.B., S.S. Sorensen,M.D. Barton,and miogeoclinalrocks of Snow Lake pendant, 257, 1992. R.S. Fiske, Long-term evolution of fluid- Yosemite-Emigrant Wilderness, Sierra Chen, J.H., and J.G. Moore, Uranium-lead rock interactions in magmatic arcs: Nevada, California: Evidence for major isotopic ages from the Sierra Nevada Evidence from the Ritter Range pendant, Early Cretaceous dextral translation, batholith, California, J. Geophys.Res., 87, Sierra Nevada, California, and numerical Geology,17, 156-160, 1989. 4761-4784, 1982. modeling,J. Petrol.,34, 23-62, 1993. Lahren, M.M., R.A. Schweickert, M.M. Engebretson,D.C., A. Cox, and R.G. Gordon, Mattinson, and J.D. Walker, Evidence of Relative motions between oceanic and Hirth, G., and J. Tullis, Dislocation creep regimes in quartz aggregates,J. Struct. uppermostProterozoic to Lower Cambrian continentalplates in the Pacific Basin,Spec. Geol., 14, 145-159, 1992. miogeoclinal rocks and the Mojave-Snow Pap.Geol. Soc. Am., 206, 59 pp., 1985. Lake fault: Snow Lake pendant, central Evernden, J.F., and R.W. Kistler, Chronology Huber, N.K., and C.D. Rinehart,Geologic map Sierra Nevada, California, Tectonics, 9, qf emplacementqf Mesozoic bathoIithic of the Devils Postpile quadrangle,Sierra 1585-1608, 1990. complexes in California and western Nevada, California, scale 1:62,500, U.S. Lockwood, J.P., and P.A. Lydon, Geologicmap Nevada, U.S. Geol. Surv. Prqf. Pap., 623, Geol. Surv.Map, GQ-437, 1965. of the Mount Abbot quadrangle,central 42 pp., 1970. Hudleston, P.J., D. Schultz-Ela, and D.L. Sierra Nevada, California, scale 1:62,500, Fossen, H., and B. Tikoff, The deformation Southwick, Transpressionin an Archean U.S. Geol. Surv.Map, GQ-1155, 1975. matrix for simultaneoussimple shearing, greenstonebelt, northernMinnesota, Can. Longiaru,S., Tectonicevolution of Oak Creek pure shearing,and volumechange, and its J. Earth Sci., 25, 1060-1068, 1988. volcanic roof pendant, eastern Sierra application to transpression-transtension Jarrard, R.D., Relations among subduction Nevada, California, Ph.D. dissertation,205 tectonics, J. Struct. Geol., 15, 413-422, parameters, Rev. Geophys.,24, 217-234, pp., Univ. of Calif., SantaCruz, 1987. 1993. 1986. McCaffrey, R., Oblique plate convergence, GREENEAND SCHWEICKERT:GEM LAKE SHEARZONE, SIERRA NEVADA 961

slip vectors, and forearc deformation, J. Sanderson, D.J., and W.R.D. Marchini, Newell, and D.L. Peck, Isotopic U-Pb Ages Geophys.Res., 97, 8905-8915, 1992. Transpression,J. Struct.Geol., 6, 449-458, of Zirconfrom the Granitoi& of the Central Moore, J.G., Geology qf the Mount Pinchot 1984. Sierra Nevada,California, U.S. Geol. Surv. quadrangle, southern Sierra Nevada, Schweickert, R.A., Tectonic evolution of the Prof. Pap., 1185, 1981. California, U.S. Geol. Surv. Bull., 1130, Sierra Nevada Range, in The Geotectonic Stevens, C.H., P. Stone, and R.W. Kistler, A 1963. Developmentqf California, editedby W.G. speculativereconstruction of the middle Moore, J.G., Geologicmap of the Marion Peak Ernst, pp. 87-131, Prentice-Hall, Paleozoic continental margin of quadrangle, Fresno County, California, EnglewoodCliffs, N.J., 1981. southwestern North America, Tectonics, 11, scale 1:62,500, U.S. Geol. Surv. Map, GQ- Schweickert, R.A., and M.M. Lahren, 405-419, 1992. 1399, 1978. Continuation of the Antler and Sonoma Strobel,R.J., Stratigraphyand structureof the Moore, J.G., Geologic map of the Mount orogenicbelts to the easternSierra Nevada, Paleozoic rocks in the Rush Creek Whitney quadrangle, Inyo and Tulare California, and late Triassic thrusting in a drainage, northernRitter Range pendant, Counties, California, scale 1:62,500, U.S. compressionalarc, Geology, 15, 270-273, California, M.S. thesis, 123 pp., Univ. of Geol. Surv. Map, GQ-1545, 1981. 1987. Nevada, Reno, 1986. Moore, J.G., and T.W. Sisson,Geologic map of Schweickert, R.A., and M.M. Lahren, Teyssier, C., B. Tikoff, and M. Markley, the quadrangle,Tulare County, Speculativereconstruction of a major early Oblique plate motion and continental California, scale 1:62,500, U.S. Geol. Surv. Cretaceous(?) dextral fault zone in the tectonics,Geology, 23, 447-450, 1995. Map, GQ-1584, 1985. Sierra Nevada: Implications for Paleozoic Tikoff, B., Transpression:Strain theory and Nokleberg, W.J., Wallrocks qf the central and Mesozoic orogenesis in the western application to the emplacement and Sierra Nevada batholith, Cah.'fornia: A United States, Tectonics, 9, 1609-1629, deformation of granite, Sierra Nevada, collage of accreted tectono-stratigraphic 1990. California, Ph.D. dissertation,277 pp., Univ. terranes, U.S. Geol. Surv. Prof. Pap., 1255, Schweickert, R.A., and M.M. Lahren, Age and of Minn., Minneapolis,1994. 1983. tectonicsignificance of metamorphicrocks Tikoff, B., and D. Greene, Transpressional Nokleberg, W.J., and R.W. Kistler, Paleozoic along the axis of the Sierra Nevada deformation within the Sierra Crest shear and Mesozoic deformations in the central batholith:a criticalreappraisal, in Paleozoic zone system, Sierra Nevada, California Sierra Nevada, California, U.S. Geol. Surv. Paleogeography o.f the Western United (92-80 Ma): Vertical and horizontal Prof. Pap., 1145, 1-24, 1980. States-H, edited by J.D. Cooper and C.H. stretchinglineations within a single shear Passchier,C.W., and C. Simpson,Porphyroclast Stevens, pp. 653-676, Pac. Sect. Soc. of zone, Geol. Soc. Am. Abstr. Programs, 26, systemsas kinematic indicators,J. Struct. Econ. Paleontol.and Mineral., Los Angeles, 7, 385, 1994. Geol., 8, 831-844, 1986. Calif., 1991. Tikoff, B., and C. Teyssier, Crustal-scale,en Rinehart, C.D., and D.C. Ross, Geology and Schweickert, R.A., and M.M. Lahren, echelon "P-shear" tensional bridges: A mineral deposits of the Mount Morrison Structuraldevelopment of SaddlebagLake possiblesolution to the batholithicroom quadrangle,Sierra Nevada, Cahfornia, U.S. pendant,eastern Sierra Nevada, California: problem,Geology, 20, 927-930, 1992. Geol.Surv. Prof. Pap., 385, 106 pp., 1964. Implicationsfor crustal evolution, Geol. Tikoff, B., and C. Teyssier, Highly Robin, P.F., and A.R. Cruden, Strain and Soc.Am. Abstr. Programs,25, 144, 1993a. transpressivetectonics and emplacementof vorticity patterns in ideally ductile Schweickert, R.A., and M.M. Lahren, granitoidsbetween en echelonP-fractures: transpressionzones, J. Struct. Geol., 16, Tectonics of the east-central Sierra East-central Sierra Nevada batholith, 447-466, 1994. Nevada--Saddlebag Lake and Northern California, Geol. Soc.Am. Abstr. Programs, 25, 5, 155, 1993a. Saleeby,J., The CretaceousSierra Nevada- A Ritter Rangependants, in CrustalEvolution transtitchingbatholithic belt, Geol. Soc.Am. qf the Great Basin and Sierra Nevada, Tikoff, B., and C. Teyssier, Strain models of Abstr. Programs,23, 94, 1991. edited by M.M. Lahren, J.H. Trexler, Jr., transpression-transtensionand implications and C. Spinosa,pp. 313-351, Univ. of Nev., for finite versus instantaneous structures in Saleeby,J., Structureof the northernsegment Reno, 1993b. orogens, Geol. Soc. Am. Abstr. Programs, of the proto-Kern Canyon fault zone Schweickert, R.A., N.L. Bogen, G.H. Girty, 25, A-281, 1993b. (PKCFZ), southern Sierra Nevada, R.E. Hanson, and C. Merguerian, Timing Tobisch, O.T., R.S. Fiske, S. Sacks, and D. California, Geol. Soc.Am. Abstr. Programs, and structural expressionof the Nevadan Taniguchi, Strain in metamorphosed 24, 80, 1992. orogeny,Sierra Nevada, California, Geol. volcaniclastic rocks and its bearing on the Saleeby,J.B., and C. Busby,Paleogeographic Soc. Am. Bul., 95, 967-979, 1984. evolutionof orogenicbelts, Geol. Soc.Am. and tectonic setting of axial and western Schweickert, R.A., M.M. Lahren, and J.D. Bul., 87, 1411-1420, 1977. metamorphic flamework rocks of the Walker, New age and structuralconstraints Tobisch, O.T., J.B. Saleeby, and R.S. Fiske, southern Sierra Nevada, California, in on volcanism, thrusting, and penetrative Structural history of continentalvolcanic Mesozoic Paleogeographyo.f the Western deformation of rocks in Saddlebag Lake arc rocks, eastern Sierra Nevada, United States-H, edited by G. Dunn and K. pendant, Yosemite National Park, California: A case for extensional tectonics, McDougall,pp. 197-226, Pac. Sect.Soc. of California, Geol. Soc.Am. Abstr. Programs, Tectonics, 5, 65-94, 1986. Econ. Paleontol.and Mineral., Los Angeles, 26, 89, 1994. Tobisch, O.T., J.B. Saleeby, P.R. Renne, B. Calif., 1993. Sharp, W.D., O.T. Tobisch,and P.R. Renne, McNulty, and W. Tong, Variations in Saleeby,J., et al., Centennialcontinent/ocean Late Cretaceous (85-80 Ma), syn-arc deformationfields duringdevelopment of a transect # 10, C-2 central California cleavage development in metamorphic large-volumemagmatic arc, central Sierra offshore to Colorado Plateau, scale rocks of the Ritter Range, central Sierra Nevada, California, Geol. Soc. Am. Bull., 1:500,000, Geol. Soc. o.f Am., Boulder, Nevada, California, Geol. Soc. Am. Abstr. 107, 148-166, 1995. Colo., 1986. PrrLœrams,25, 145, 1993. Saleeby, J.B., R.W. Kistler, S. Longiaru, J.G. Simpson,C., and D.G. De Paor, De.formation D.C. Greene and R.A. Schweickert, Moore, and W.J. Nokleberg, Middle and Kinematicsof High Strain Zones, Short Department of Geological Sciences, Cretaceous silicic metavolcanic rocks in the CourseNotes, 116 pp., Geol. Soc. of Am., University of Nevada, Reno, NV 89557. (e- Kings Canyon area, central Sierra Nevada, Boulder, Colo., 1991. mail: dgreene@ mines.unr.edu) California, in The Nature and Origin of Simpson, C., and D.G. De Paor, Strain and Cordilleran Magmatism, edited by J. L. kinematic analysisin general shear zones, Anderson, Mere. Geol. Soc. Am., 174, 251- J. Struct. Geol. 15, 1-20, 1993. (ReceivedJune 9, 1994;revised May 10, 1995; 270, 1990. Stern, T.W., P.C. Bateman,B.A. Morgan, M.F. acceptedMay 14, 1995.)