Basil Tikoff. Department of Geology and Geophysics. University ..... granitoids [e.g., Gapais, 1989; Paterson et al., 1989]. indicates that emplacement of the Mono ...
TECTONICS, VOL.16,NO.3,PAGES442-459, JUNE1997
Transpressionalshearingand strike-slip partitioning in the Late
Cretaceous
Sierra
Nevada
magmatic arc, California Basil Tikoff Department of GeologyandGeophysics. University of Minnesota, Minneapohs
MicheldeSaintBlanquat Laboratoire P6trophysique et Tectonique, CNRS,Universit6 PaulSabatier, Toulouse, France Abstract. The RosyFinchshearzone(RFSZ)represents an example of synmagmatie,strike-slip tectonicsin the east
central Sierra Nevada and provides information on tmnspressional tectonicswxthmmagmaticarcs. The RFSZ (1-
of gramticrocksandbecause magmatic fabricsmaybe overprinted by laterintrusions and/ordeformations. Despite thesepotentialproblems,tectonicinterpretatlons havebeen offeredfor many Mesozoicarcs, includingthe CoastBelt
4 km wide and 80 km long)is contained in LateCretaceous [Crawford et al., 1987;Rusmore and Woodworth, 1991],north granitoids (92-83 Ma) of the Mono Pass Intrusive Suite. Cascades[BtowJl and Walker, 1993], Idaho Batholith Dextral strike-slipdeformationis indicatedby a continuous [Manduca et al., 19931, SierraNevada[Tobtsch et al, 19951, band of orthogneiss, mylonitic,and cataclastm deformation, andAndes[Megard, 1987] characterizedby subverticalfoliationswith subhorizontal TheSierraNevada bathohth hasbeenextensively mapped linearions.The styleof dextralshearing in theRosyFinch (seeBateman[1992]for overview)andtherel'ore provides an shear varies along strike, from wide zones of ductile excellent setting to study the dynamics of batholtth•c deformationin the youngestplutonsto narrow zonesof processes (Figure 1). The CretaceousSierra Nevadabatholith cataclastic deformation m the olderplutons.Deformation of waseraplaced between125 and83 Ma [Sternet al., 1981]and the youngestgranitoldsts synmagmatlc, as constrained by represents a minimum granitmd volume of ~4 x 105km 3 bothfieldobservations andisotopic studies.Dextralshearing [Tobischet al., 1995]. The batholithlies alongthe western
ccurs concurrently incontemporaneous lntmsive Suites to
ae north and south (Tuolumneand Mount Whitney) suggestingstrike-slipmovementalongthe axis of active
edgeof the NorthAmerican eraton,evidenced by preserved
Paleozoic stratigraphy andhigh87Sr/ 86Srratios within the
granitoids alongits eastern edge[Bateman, 1992] In general, theoldestCretaceous plutonsareIntruded in western partof the are acted concurrentlywith strike-slipmovement, thebatholith, andmagmatism movedeastward at anaverage
plutonIsm. Field studiesalsosuggestthatcontraction across
consistentwith a transpressional setting for the Late
rate of ~2.7 mm/yr [Chen and Moore, 1982; Stern et al,
Cretaceous SierraNevadamagmatic arc. Comparison of the 198l]. Intrusion of plutonsapparently occurred at deeper fietd datawith strainmodelsof obliqueplateconvergence crustaldepthson theeasternpartof the batholith(~5 kbar) suggests that the RFSZ is a preserved ductilesignature of thanin the westernpart(~1-2 kbar) [Agueand Brimhall, transcurrentmotion of strike-slip partitioningwithin a transpressmnal orogenic system. Recent plate
19881.Theaverage timefor coolingof plutonsfrom~700øC to ~500øC, based on thedifference in closure temperature for reconstructions indicate a switch from simstral to dextral Pb tn zirconandAr tn hornblende, increases from~10m.y.m obliqueconvergence at ~95 Ma, whichusconsistent with the the western partof thebatholithto 1-3 m.y. for theeastern timingconstraints on thedextralmovement on theRFSZ. part[Renne et al., 1993;Tobisch et al., 1995]. Theincreased rateof cooling,especially combined with the geobaromemc Introduction data,maybe a resultof rapidexhumation duringtheLate Cretaceous [eg., Renne et al., 1993], as proposed for the The exactnatureof the kinematics withinmagmatic arcs Sierra Nevada [Pickett andSaleeby, 1993]. has been the source of debate •or several decades. southern Thebatholith hasbeensubdivided, basedlargelyonU/Pb Interpretations includeextensmnal[e.g., Tobischet al., dating of zircons. This study focuses on the youngest 1995,Grocottet al., 1994],transtensional [e,g.,Ward,1995; plutonism, which occurred between 92 and 83 Ma and PetfordcmdAtherton,1992],andtranspressional settings to theCathedral Rangeintrusive epoch[Ei,ernden [e.g.,TtkoffandTeyssier, 1992,1994b, HuttonandReavy, corresponds and Klstler, 1970; Stern et al., 1981]. The depthof 1992], The structuralrecordof tectonism in ancient,eroded of the plutons •ntruded duringthe Cathedral magmaticarcs is partialat best,becauseof the moltennature crystallization Rangeintrusiveepochis uppercrustal. Geobarometry on hornblende of theCretaceous plutons of theeast-centralSierra Copyright 1997byllteAnlerican Geophy,,Ical Umon Nevadasuggests depths of crystallization as shallowas4 km [AgueandBrimhall,1988]. Geobarometry on hornblende Papernumber97TC00720 withinthe Mount Whitneyand Paradiseplutonsof the 0278-7407/97/97TC-0072051200 Whitney Intrusive Suiteindicate crystallization depths of 1-2 442
'IlKOFFANDSAINTBLANQUAT:SIERRA NEVADAMAGMATICARC
443
kbar [Hirt,i989]. Sedimentary deposition in theMinarets In general. deeper crustal exposures of LateCretaceous Caldera of the RitterRangependant at 100 Ma [Fiskeand plutons arefoundsouthward withintheSierraNevada batholith Tobiscb, 1978]suggests thattheplutons, andthustheshear [Agtteand Brimhall,1988;Saleeby,1990]. In theLake tone,wereat a relativelyshallow(< 4 kin) depth. The lsabellaregionsouthof theWhitneyIntrusive Suite(Figure
plagmclase compositions in theEvolution Basinalaskite l), theestimated depths of crystallization are2 to 4 kbar suggest thatit washypersolvus, consistent withshallow [Aguea•d Bri.tball.1988;Saleeby.1990].Thisis the eraplacement [Bateman, 1992]. location of theproto-Kern Canyon fault[Busby-Spera and 121 ø
120 ø ,
i
california
40ø "'•;•.•..•'•' '" San Francisco
•.-.% 119 ø •.•,-'--• ..... •a.•
'•/,•.•0
'
•.-•.._-:•::.,m,•.' ..:,..'0,•.•. .........'................ •
e: :
'•1•,' : •
ß
...•,
37 ø..!i •..:..,. 'r• .;i • ...... : • •
•
YoungestIntrusiveSuites 36........
•.• Sretace( • • Jurassic •
•
Triassic
wall-rocks
35.......... ,..•.... ß
•
..............
San Andrea sfault
100 km ,
Figure1. Mapof the92-83Ma Intrusive Suites in theCretaceous SienaNevada batholith IStenetal., 1981],whichwereint•d• during theCathedral Range Intrusive epoch.Theouterplutous of eachlnt•sive Suite aregenerally moremarie andolder.Theoverail trend ofthese Intrusive Suites isN25øW, pa•ilcltothe Crelaceous batholith. M•ificd fromBateman11992]andHirt [ 1989].
444
TIKOFFANDSAINTBLANQUAT: SIERRA NEVADAMAGMATICARC
Saleeby, 1990], a dextral shearzone that was active coeval
with deformation withintheRosyFinchshearzone.
Rosy Finch shear zone (RFSZ)
TheRFSZ extends through theplutons oftheMono Pass
In thispaper, wepresent evidence of dextral transpression Intrusive Suite, including theMono Creek granite (86Ma)[B.
duringthe youngestInagmatism (-92-83 Ma} in the Sierra
Carl, personal communication, 1996],LakeEdison granodioritc (88_+1 Ma)[Tobisch etal., 1995], theLamarck recordedin the 80 km longRosyFinchshearzone(RFSZ). granodiorite (91.9Ma}[Coleman etat.,1995], andTurret Peak Thisshear zonewasactive atrelatively shallow crustal depths granite (undated). TheRFSZalsoextends farther south into (I-2 kbars)and from -88 to 80 Ma, synchronous with theEvolution Basin alaskite (undated) (Figure 2}. The plutons eraplacement of theyoungest plutonsof theCretaceous Sierra of theMonoPassIntrusive Suitearesimilar in ageand Nevada magmaticarc. Dextml deformationwithin the arc is
Nevadabatholith.TheRFSZis partof thelargerSierraCrest
shearzoue system[Tikoff, 1994;Greeneand Schweickert, 1995], which recordsdextral deformationfor over 300 km
alongthe axisof activeplutonism of theSierraNevadaarc.
petrologle relationships totheTuolumne andMount Whitney Intrusive Suites [Stern etaL,1981; Hirt,1989; Bateman,
1992].However, theplutons oftheMono Pass Intrusive Suite showanen6chelon arrangement of elongate plutons rather
We tentatively associate thedevelopment of theRFSZto a than thenested pattern oftheTuolumne andMount Whitney change in theangleofplateconvergence of theFarallon plate, Intrusive Suites [e.g.,Bateman, 1992; TikoffandTevssier. from obliquelysinistral to obliquely dextral,at -95 Ma 19921. offshore of Mesozoic California [KelleyandEngebretson, TheRFSZwasfirstrecognized bySherlock andHamilton 1994;D.C.Engebretson, personal communication, 1995]. [1958]asa "flaser gneiss" withintheMonoCreek granite and
•••r Range Pendant
vv •v v
'x.•x,',•.'.,,. •. '• -•.:.•
Round Valley
/r'"-'-"--• '• '• '•• ••:• "--•:':'""':'••••'•••'•• Peak
.......
.a.i,e
F•6URE 6 •'::"•"•*-•;-•• Lake Edison •• '• ---•-:--'•••.•...."'• Granodiorite
•f __Granite '., :• :---•' a.a.,..e
10'kin Mr.Goddard Pendant
Figure 2.•e mapped extent ofthe dextral Rosy Finch shear zone (RFSZ) asitpasses from the Gem Lake she,it zone through the Mono Pass Intrusive Suite, the Evolution Basin alaskite, and adjacent roof pendants. The Mono Pass Intrusive Suite consists or,from NEtoSW, IheRound Valley Peak Mono Creek, Lake Edison, Lalnarck, ,lad Turret Peak plutons. DelBrmation isductile, except within the Lan;arck and Turret Peak pintons. Foliation within the plutons outside the shear zone isgenerally NWoriented and subvertical. Fold symbols designate folded [oliation intheLamarck granodiorite.
TIKOFF AND SAINT BLANQUAT: SIERRANEVADA MAGMATIC ARC
445
Cath•r.
.
North •ono
m•
•
Cr•k
C•k
Figure 3. Stereonetsof shearbands(squares)and linearion(circles)orientationswithin RFSZ suggesting predominantly strike-slip movement. Linearions, defined by sLretchedminerals in shear plane. are dominantlysubhorizontalthroughoutshearzone.
waspartiallymappedby Lock,roodand Lydon[1975] within theMono Creek granite,the Lake Edisongmnodiorite.and alongthe contactof the Lamarekand Turret Peak plutons.
alaskite(Figare2). The dextralshearzoneis at least80 km long, 1-4 km wide, and generallycontainssteeplydipping foliationwith shallowlyplunginglinearion(Figure3). The
Fromthe MonoCreekgraniteto withinthe EvolutionBasin alaskite. the RFSZ is continuous over 80 km (Figure2). The details of thefieldobservations aregivenin theAppendix. In summary,the RFSZ runsthroughthe plutonsof the MonoPassIntrusiveSuite and into the EvolutionBasin
styleof dcxtralshearing changes alongstrike,depending on the age and composition of the pluton. The Mono Creek
granite,theyoungest plutonof the IntrusiveSuite,displays ductilebehavior (Figure4). The LakeEdisonplutondisplays predominately ductilebehavior, although thereareindications
Figure 4. SmLctures withinthe RFSZin theMonoCreekgranite,adjacent to southedgeol MountMorrison pendant.(4a) S-C fabricsdevelopedin a orthogneiss.Largeporphyroclasts are imbricated,locally recrystallizcd, anddeflectfoliation.(4b) Plastically deformed quartzin a mylonitc.Bottomof photograph is 6.5 mm acrossin cross-polarized light
446
TIKOFFANDSAINTBLANQUAT: SIERRA NEVADAMAGMATICARC
a)
b)
'-' •-',•,•-'.,;r,•.',•..•. :•-•i, .-•1"•',-.; ,• . •;,..:,•,.....o ,
•,,.,
..
•,
•,,
. •
:,,.,..,. ,,
Figure 5. Deformation of LakeEdisongranodiorite in RFSZ. (Sa) Elongatedmafic enclaveand welldeveloped foliation. (5b) Thin shearbanddeveloped betweenfeldsparcrystal,indicatingslightlylower temperature shearingthan in the Mona Creekgranite. Bottomof photographis 3.3 mm acrossin crosspolarized light.
of shear localizationand lower temperaturedefommlion (Figures 5 and 6}. Farther south, the older Lamarck granodiorite and Turret Peak granite record more brittle
sampled 184sitescovering thepluton;ishomogeaeously a• possible(Figure 11).
deformation,includinga largecataclastic zonewhichoffsets Analysis the contactof theseplutons(Figure7). Microstructurally, Microstructural bothplutonsshowpervasive fracturing of minerals(Figures8 The microstructures we haveobservedusingthin-section and 9), althoughthe Turret Peakgranitebehavesin a more analysiscouldbe subdivided into threetypes(FigureIll:
ductilemanner,because of its higherquartzcontent(Figure
dcfornmtion in themagmatic state(M), high-temperature solid state(HT), andmediumto low-temperature solidstate(MLT). difficultto recognize in thefieldandis presently onlyknown Thedetailsof thisstudyarepresented by SaintBlanquat and 10). Dextral shearingin the Evolution Basin alaskite is
to occuradjacentto the Lamarckcontact.
Tikoff [1997]. In brief, magmatic(M) textureis characterized
by large and equantquartzgrains,poorly alignedmarie
Evidencefor Synmagmatic Shearing
minerals and feldspar phenocrysts,and lack of features
in orderto constrain the timingof shearing relativeto the
eraplacementof the plutons, we have completeda microstructural and anisotropy of magneticsusceptibility studyof theMonaCreekgranite. TheMonaCreekgranite was chosen because it is theyoungest plutonlhatisal'l•cledbythe RFSZ andshouldhaveno overprinting relations.We have
indicative of solid-state deformation. The lit solid-state deformationtexture is difficult to differentiatein the field from
themagmatic texture,although it involves a slightdecrease in thequartzgrainsizeanda morepronounced shape-preferred orientation of K-feldsparmcgacrysts.The MLT solid-state deformationtextureis an orthogneissic textureeasily
..
Figure 6. MapofBear Creek traverse where the3 kmwide RFSZ crosses theMona Creek granite
{KMCG)/Lake Edison granodiorite (KLEG)contact. Arrows represent orientation of subvertical shear planes, and thenumbers indicate theplunge oflinearion within theshear planes. Theboxed numbers indicate thehorizontal finitestrain ellipse (XZ section) calculated frommarieenclaves. Sinistral offseton
brittle/ductile fractures isshown bydashed lines, although deformation isincreasingly brittle toward the Lake Edison pluton and iacreasiagly ductile toward theMona Creek pluton. Thenumbers adjaccat tothe faultsindicate thesinistral offsetof thepluton coiltact in meters.
TIKOFF AND SAINT BLANQUAT: S1ERRANEVADA MAGMATIC ARC
447
I LakeEdison] Granodiorite ]
"
•
IMono Creek
IGranodiorite L..... k II • x• '••111 litit" '•lllt I' IGranite llll I t''
.. •1'•!'•' / Lamarck ' , I• • t•I IIt t • ! ,•lt•,l',•,
Sinistral
Faults
au zone) Porphyritic .•.•11'•ll G ....diorite ,','• '•I • / •i I T....,Peak • ••.
,lll•l I•
I Granite I • • •11• ',,ll[i •
,
•
2km
•.De•ral •'• 'Mylonittc/ CataclasticZone
Figure 7. Map of RFSZ in centralareaof Mono PassIntosire Suite. Del•rmationbecomesincreasingly brittle in the Lamarck pluton. and the Lamarck•urret Peak pluton is offset approximately1.2 km alongthe RFSZ in the West Pinnaclcscreek, the only placewherethe RFSZ offsetsplutoncontacts. recognizablein tile field. It is characterizedby a significant reductionin grain size, developmentof shearbands,pressure shadows on sidesof feldsparsmegacrysts, andbrokenfeldspar megacrysts. The distributionof the microstructures directlycomparesto themappedpatternof the RFSZ (Figures 2 and II). MLT textures are foundeverywherein the RFSZ. HT microstructures are found immediatelyadjacentto the RFSZ. on both sides •hen theshearzonerunsthroughthe middleof the pluton. Far fromtheRFSZ,the magmatictexturesare preserved. [lowever, the outcropscharacterizedby MLT deformationwithin the
deformation. For example, large K-feldspars commonly contain cracks thrilled by quartz and/or plagioclase, interpretedas resultingfrom brittle deformationat the grain scalein the presence of residualmelt [Bouchez et al.. 1992]. Thus the microstructural analysissupportsthe model of solid-statedeformationsuperimposed on a higher-temperature deformation,as originally determinedby the porphyroclast analysis[Tikoffand Teyssier,1994a]. The latestdeformation
evidently took place in low-temperature, solid-state conditions,as evidencedby the choritizationof the marie phasesandthequartzmicrostructures. This dataalsosuggests RFSZalsoindicateprevioushightemperature and/ormagmatic that the •nag•natic to high-temperature, solid-state
Figure 8. Structureswithin the Lam;•rckgranodiorite within the RFSZ. (Sa) Pervasivclyductilcly deformedLamarckfrom PiuteCreektraverse,indicated by elongatemaficcnclaves.(8b) Photomicrograph
showing brecciawith ultramylonite makingup theelasticfragments, suggesting ductilepriorto brittle deformation. Bottomof photograph is 3.3 •nmacross in planepolarized light.
448
TIKOFF AND SAINT BLANQUAT: SIERRA NEVADA MAGMATIC ARC rapidinsightintothebulkinternalstructure of a pluton and allowsthe definition of planarandlinearfabricswhich difficultto recognize ia the field.Numerous previous studic• havedemonstrated a closeparallelism between theellipsoid of magnetic susceptibility andgrainshapefabrics[e.g.,Ellwood andWhitney, 1980;Bottcheg etal., 1990;Crttdetl andLauneau. 1994]. The correspondence in our studyof theAMS dataand thevisiblefabricwithintheMonoCreekplutonindicates the utilityof AMS for analysis of bothsolid-state fabrics andthe macroscopically moresubtlemagmaticfabrics. The details 0f methodologyand resultsfor the Mono Creek granite availableelsewhere [SaintBlanqttatand Tikoff,1997],andwe will concentratehere only on application of the results.
Because of the ferromagnetic (i.e., magnetite bearing) nature of the Mono Creek granite, the AMS fabric is principally controlled by the shape anisotropy of the
b)
magnetitegrains and the anisotropyof magnetitegrain distribution[e.g., Hargraveset al., 1991; Grdgoireet 1995]. The shapeof the magneticsusceptibilityellipsoidis oblate for 85% of the sites, indicating a planar rock fabric.
The magneticfabricof the Mono Creekgraniteis characterized by a sigmoidalmap patternof lineationand foliation•Figure
I l) [SaintBlanquatet al., 1993;SaintBlanquatandTiko•, 1997]. AMS resultsgenerallyshow a E-W magneticlineati0n near the northwestand southeastportionsof the pluton,which rotates into a NW-SE
orientation toward the center of the
pluton and eventually rotates into a NNW-SSE orientation within the RFSZ. The same sigmoidalpatternis observedfor the foliation (Figure I I), suggesting dcxtral shearingand passiverotationof the magneticfoliationand lineationduring shearingalong the RFSZ. The E-W magneticlineatiouin the northwestand southeastportionsof the plutonsis thoughtto
representthe initial orientationbefore solid-stateshearing.
. , .
•
"-.
_
The magnetic lineations are moderately to steeplyplunging in the northwestand southeastparts of the pluton and within the northeastern "bulge" of the pluton. The magnetic lineations within the RFSZ are subparallel (_+ 10ø) to the shallowly-plunging field lineations. In order to investigate the timing of shearing versus eraplacementlbr tile Mono Creek granite, we must compare the AMS and microstructural study (Figure l l). The microstructural study is a strongargumentfor the syntectunic
eraplacement of the Mono Creek granite,and the gradation of microstructures indicatesa relationshipbetweenmagmatic microstructures
Figure 9. Field photographs of structureswithin Torret Peak pluton. (9a) Dextral asymmet• of t•lds foxing in gneissicfabric of Turret Peak granite,2 m on its western margin. (9b) Ribbonquartzand S-C fabricgradingimo cataclasticdeformationwith visibleshearbands. Figures9b and 9c are from meter-scalemylontic zone within Piutc Creek traverse. Bottomsof both photographs are 2.6 mm acrossin planc-•larizcd light.
outside
the
shear
zone
and
solid-state
microstructures withinthe RFSZ. The continuityof horizonlal magnetic foliations and lineations across the RFSZ, from magmatic through high-temperature to lower-temperature
deformation,suggeststhis patternis a result of dextral shearing which started in the unsolidified magma during eraplacement and continuedthroughcrystallization.
Other Evidence for Syntectonic Eraplacement
deformation oe½llrred overa widershearzone,suggesting localizationo1'strainwith increasingsolid-statedefomlation and decreasingtemperature.This interpretationis further supportedby the AMS analysis(seebelow). Fabric From AMS Analysis
AMS (anisotropy of magneticsusceptibility) providesa
Solid-statedeformationfollowing continuously from magnlatic deformation, a commonobservation in syntectonic
granitoids [e.g., Gapais, 1989;Patersonet al., 1989]. indicates thatemplacement of theMonoCreekgranite and sheari•gare contemporaneous. Gapais[1089]showed from mechanical and thermal constraints that shallow level
449
TIKOFF AND SAINT BLANQUAT: SIERRANEVADA MAGMATIC ARC
Kdf
N
I",,•
Figure 10.
26•_/
._o "• 10
•
16
•-•-J... 'r %•-,,
I
Map of PiutcCreektraversewherethe RFSZ occursin IheTurretPeak(KTP) and Lamarck
plutons (KLG).Symbols areIhesame asinFigure 6,except forQuaternary alluvium (Qal) and rectavolcanic wall rocksof the Mount Goddardpendant(KJr). Mylonite/eataclastic deformationis concentrated in West Pinnaclescreek, shownin dark gray, althoughdextralshearingis foundacrossthe entirewidlh of the Turret Peak pluton.
!otlalionpoles
IIn•atlons
10 km
..
...... .:%. !:.•:•.
:•'•;•:½"•:.. 'j•....• magma, .....
•
limi• ofthe Roay •nch ah•r •ne 1 le045'
Fisure 11. Summary mapof microslructu•l Icxtures andmagnetic foliations [romAMSs•udyin IheMono
Creekgranite.Medium-Low tem•rature(MLT)deforrnation co•spondsto Ihcmapped RFSZ,flanked by areaof High-temperature (tiT) audMagmalie (M) deformation. Themagnetic lblialions aredominanlly verticalthroughout the plulon. TheRFSZdeQccts themagnetic l•diation Irajectories in a dcxtralmanner. The geomclric, structural, and Icmporal continuity between the magmatic andsolid-state delbrmatioa slrongly supports a 0•odclof shearing duringeraplacement of theMouoCreekgranite.
450
TIKOFFANDSAINTBLANQUAT: $1ERRA NEVADAMAGMATICARC
syntectonic plutonsarethebestcandidates for producing wtde, large-scaleorthogneiss sheets(i.e., shearzones),as seenin the RFSZ.
Shearing m theCathedral Peakgranodionte isdescribed asan orthogue•ss, andthestyleof deformation is extremely s•mllar todeformation in theMonoCreekgranite.Preliminary AMS
Othercompellingevidence for syntectonic eraplacement is
andstructural analyses suggest syntectonic emplacement of theCathedral Peakgranodiorite [Davis,1996].AlongtheNE Edison granodiorite. The lack of suchoffset means that sideof the Half Domegranodiorite, dextraldeformation •s displacement waseither(1) accommodated ductilelywhilethe bothductile andlocalized in brittle-ductile shear zones.These granitoidswerestill partiallymoltenand/or(2) tectonicroom datasuggest thattheRFSZ,GemLakeshear zone,andCascade was createdfor the Mono Creek granite. For example, Lakeshearzoneare partof a thoroughgoing, dextralshear the lack of discrete offset between the Mono Creek and Lake
infilling a tectonicopening,suchas a large-scale, pull-apart zonesystem(Figure 12), structure,does not reqmreoffset of the contactbetweenthe
opening(theMonoCreekgranite)andtheadjacent wallrock (LakeEdisongranodiofite) [e.g.,TikoffandTeyssier, 1992]. Geochronologtcal evidence alsosupports the simultaneity It Is suspected thatthe RFSZcontinues through the Basinplutonandinto the WhitneyIntrusive State of plutonism and deformation in the RFSZ,conducted along Evolutmn the BearCreektraverse(Figure7) [Tobstch et al., 1995].U/Pb
Reconnaissance fieldworkin thisregionhasindicated small-
z•rcon analysis yielded anage of88+ 1,40Ar/ 39Aranalysisscaledextralshearzonesin small plutons(Cotterand Pass plutons) between theMonoPass andWhitney onhornblende displayed aplateau at87.1 +0.2Ma,and40Ar/ Cartridge Intrusive Suites.TheseshearzonesshowN-NNW oriented 39Atplateau ages from biotite yielded ages of83.7 + 0.4and 83.1 + 0.3 Ma. On the basisof thesedata and microstructural
shear bands, dextral shear-sense indicators, andshallowly plunginglinearions. Corroborating evidencefor N-NW
work, it was hypothesized that deformation alongthe RFSZ oriented dextralshearing includes documentation of themore initiatedduringor immediately aftereraplacement of theLake obvioussmall-scale, brittle to brittle-ductile,simstralfaults Edisongranodiorite astheplutoncooledthrough 650ø-700øC [Moore, 1963;Lockwoodand Moore, 1979J. TheseE-W
[Tobisch et al, 1995] Deformation continued through biotite
faults forma conjugate settothedextral sheanng, closure(83 Ma), given the involvementof bratitc in the oriented observed. Th•ssame typeof conjugate, brittlefaulting shearing.Consequently, theseresultsrequirethatdeformation where is foundadjacentto the Rosy Finch shearzone in theLake occurredduring eraplacement of the youngerMono Creek granite.
Geochronological evidence •s also available from
Edisonplutonand deformation is essentially synchronous withmovement ontheRFSZ[e.g.,Martelet al., 1988].
Farthersouth,ductileshearingis found in the northern deformation of theRitterRangependant, immediately northof section of theParadise andMountWhitneyplutons, 20 km the MonoCreekgranite(Figure2). Sharpet al. [1993]dated both strike-slipshearzonesrelatedto the Gem Lake shear
northof themapped extentof theproto-Kern Canyon fault (Figure12) S-C structures, porphyroclast tails,andfolded
zone [Greene and Schweickert,1995] and cleavage aplite dikesall •ndicatea dextral senseof shearwith a 200N developmentat 85-80 Ma using the Ar/Ar methodon muscovite.Sinistralfaultsthat crosscut theshearzonein the
LakeEdison granodiorite aredated at 79-75Ma [Segall etal., 1990].These results areinternally consistent andsuggest that deformation occurredbetween88 and79 Ma in theRFSZ.
Sierra CrestShearZoneSystem
plunging linearion on C surfaces (Figure3). Discrete dextral
offsetIs alsoseenin thenorthern partof theParadise pluton [Moore, 1981]. PreliminaryAMS data from uncleformed
MountWhitneysamples showsubhorizontal magnenc lineat•ons andsteeply dippingmagnetic foliation Thew•dth of the shearzone,orthogneiss deformation, and the AMS analysis •s suggestive of a close relation between
emplacement andshearing of theMountWhitneygramte. Theproto-Kern Canyon faultis another majorstrike-shp Thel•osyFinchshear zoneis in structural continuity w•th shearzone/fault system thatwasdemonstrably acuveat ~85 theGemLakeshear zone(Figure 12)[Tikoffand Greene, 1994; Ma dunngthelastpulsesof magmatism in the SierranArc Greeneand Schweickert, 1995]. The GemLakeshearzoneis a [Busby-Spera and Saleeby,1990]. Thisdeformarran zone NNW-striking, vertically-oriented shearzone that affects exhibits dextral, ductile motion andhasshallowly plunging Paleozoic andMesozom strata of thenorthern RitterRange mineralhneations.The shearzoneis continuous for over130 North
of the RFSZ
pendant andthegranodiorite of KurtaCrest The shearzoneis
kin,frointheParadise granodmfite (Figure l) to theGarlock fault,andmayberelated to thrust motion involving theRand the zoneis described as dextraltranspresslonal, anda schist [Saleebv etal., 1993].Preliminary estimates of dextral minimum of 20 kmof right-lateral offsetis inferred [Greene offset along theshear zone areabout 40km[Busby-Spera and and Schweickert,1995], Saleeby, 1990].It is strongly suggested, frombothtemporal Deformation in theGemLakeshearzone•s, in turn,in andspatialrelations, thattheRosyFinchandproto-Kern structural continuity withdextralshearing in theCascade Lake Canyon arepartofthesame shear zone system [Tikoff, 1994] shearzone(Figure12) [Davis,1996;Tikoffand Greene, In hghtof theincreasing evidence for majorstfike-shp 1997]. TheNNWstoking, steeply d•pping Cascade Lake motionthroughout theSierraNevadaduringtheemplacement shear zone affects theHalfDome andCathe4ral Peak plutons of of the92-83Maplutons, the"Sierra Crest shear zonesystem" theTuolumne Intrusive Suite.Ductile de'xtral shearing and hasbeensuggested asa nametodescribe theentiresystem of shallowly plunging lineations occur forover10kmalong the dextral,arcparallelshearing thatoccurs in theLateCretaceous NE sideof theTuolmnne Intrusive Suite(Ftgures 3 and12). Sierra Nevada [T•koff,1994;Greene andSchwetcke•t, 1995]. inferredto beactivefrom91 to 80 Ma. Deformation within
TIKOFFANDSAINTBLANQUAT:SIERRA NEVADAMAGMATICARC
451
•-•• c[sz1181,oo' w
38ø00 'N'-"••i•'
,j• •, .,,/'•TUOLUMNE INTRUSIVE SUITE
p i
s• MOUNT
WHITNEY
INTRUSIVE
SUITE
30 km
•KC
Figure 12. Dexlral shearingalong Ihe SierraCrestshearzone system,includingthe Tuolumne,Mono Pass,andWhitneyIntrusiveSuites. PlutonsincludeCP, Cathedral Peak;HD, Half Dome;KC, KunaCrest; RVG, RoundValleyPeak;MC, MonoCreek;LE, LakeEdison;L, Lamarck; TP, TurretPeak;EB, Evolution Basin;MW. MountWhitney;P, Paradise; andSL, Sugarloaf. Shearzonesinclude CLSZ,Cascade Lake;GLSZ, GemLake;andRFSZ,RosyFinch. CircledagesgiveknownU/Pbageson zirconfromplutons.Blackarrows andmappatternindicateknowndextralshearing. Theseshearzones do not necessarilyconnect;it is possihle thatthey may be orientedin an en 6chelonsystem,and/or varioussegmentsmay have been active at different times during dextral shearing within the arc. Although the continuityof the Sierra Crest shearzone systemis not known, it is clear that dextral offset accompanied the latest magmatismin the Sierran arc. Further, the dextral shearing
plutonismwithin the SierraNevadamagmaticarc. The Sri =
0.706line(isoplcth of 87Sr/86Sr initialratio)is offset by several arc-parallel,NW trendingfaults (interbatholithic breaks 2 and 3) within the Sierran block, amountingto -200
km of dextraldisplacement duringtheEarlyto LateCretaceous [Kistler, 1990]. Lahren et al. [1990] suggesteda 400 km dextrnl offset across the Sierra Nevada on the basis of
occursonly along the axis of arc magmatismduring
stratigraphic correlation, although they inferred that
eraplacement of the 92-83 Ma plutons. Usingdextralshearing
movement
occurred before intrusion
of the Cretaceous
in tile Tuolumne Intrusive series as the northern extent and the
batholith. However,given the evidencefor dextrnlshearing, proto-Kern Canyonfault as the southernextent[Bttsbv-Spent theseoffsetspartiallyoccurredduringthe youngestplutonism in the Cretaceous batholith. and Saleeby,1990], the SierraCrest shearzone systemis a
310 km long,dextralintrabatholith shearzonesystemin the SierraNevadaarc that wasactiveduringeraplacement of the
Evidence for Transarc Contraction
granitoids.
Isotopic and stratigraphic correlations also suggest
intrabatholilh, strike-slipdisplacement duringthe youngest
There is abundant evidence within the Sierra Nevada arc for
localized
transarc contraction,
simultaneous with dcxtral
452
TIKOFFANDSAINTBLANQUAT:SIERRANEVADA MAGMATIC ARC
shearing, along the active axis of magmatismduring consistent witha contractional or transpresslonal deformation emplacement of the92-83Ma plutons In the centralSierra duringplutonism. Nevada,theKaiserPeak,QuartzMountain,andBenchCanyon Thepattern of fabricdevelopment withintheGemLake shearzonesall showdowndip lineations andreversesense-of- shear zonealsosupports theideaof regional transpresslonal
shearindicators[McNulty,1995;Tobisch et al., 1995]. The
deformatzon [Greene andSchwezckert, 1995].In particular,
KaiserPeakand QuartzMountainshearzoneswere activeas late as 90 Ma, and the BenchCanyonshearzone may have
downdip llneatlonscombinedwith dextral shear-sense indicators and flatteningfabricsare predictedby strain modehng of transpressional zones[e.g.,Fossen et al, 1994, TikoffandTeyssier, 1994b]andareobserved m theGemLake
beenactiveepisodically from 90 to 78 Ma [Tobischet al., 1995]. Extensionaldeformationis recordedduring the same
time period(~90 Ma) in the Courtfightshearzone,although deformationis clearlyrelatedto emplacement of the Mount
shear zone.
Given granodiorite [Tobtschet al., 1993]. Fieldwork on the metavolcanic rocksof the RitterRangebetweenthe Tuolumne and Mono PassIntrusiveSuitesdocuments theemplacement of the late-stage plutons during a compressionalevent, approximately95 _ 7 Ma [Tobischand Fzske,1982] More recent geochronologlcalwork [Sharp et al., 1993] suggests that the last episodeof slaty-cleavage development, with an associateddown dip lineation,occurredat -85 Ma, concurrent with eraplacementof the Mono Creek and CathedralPeak plutons. The plutons themselves also show evidence for
foliat•on of theLamarek pluton,withthefolddefined bythe orlentat•on of flattened mafieenclaves (Ftgure2) [Bateman,
Otherevidence of synmagmatlc contractzon zsthe"folded' 1992]. While thisfeatureis somewhat unusual,it forms an anticline/syncline pairthat continuous for 25 km withfold hingesnearlyparallelto the traceof the pluton(F•gure21 Further, the axial plane is vertzealand NW oriented.
subparallel to foliatton in adjacent plutons.If thisfolding the plutonis causedby contractzonal deformation, ratherthan resultingfrom magmatic flow/emplacement,it musthave
occurred simultaneously withtheemplacement of theyounger granites,-90 to 83 Ma
contractional deformation. In the central Sierra Nevada, the
Mono Creek,Lake Edison,andTurretPeakplutonsoutsideof
Discussion
the extent of the RFSZ containa subvertical,~N40øW-onented
Elongate, en 6chelon Plutons parallelto the elongation of the individualplutonsandpasses Displacements wzthlnthe plutohiesectionof a magmatic continuouslyfrom one pluton to another. This subvertical arc may leave httle structural signature because foliat•onis partzcularly noticeableevenin the bulbouspluton magmatic and/or solid-statefoliatlon The foliation zs sub-
"ends,"suggestinga regionalcontractionalcomponent[Saint Blanquat and Tikoff, 1997]. NW trending, subvertical fohatlon zs also seen within the Cathedral Peak and Half Dome
plutonsof the TuolumneIntrusiveSuite[Bateman,1992]and
in the Paradiseand MountWhitneyplutonsof the Mount WhitneyIntrusiveSuite[Moore, 1981] Thereare large-scale, subhorizontal dikecomplexes located along the entire crestof the Sierra Nevada,which emanate
dzsplaeement might be largely accommodated in meltedor partiallymeltedrock,in whichno offsetmayform;and/or (2)
earlyfabricsmightbe severelyoverprinted by subsequent, solid-statedeformation; and/or (3) later granitieintrusions
wouldpreferentially takeplacealongthesemegashear zones, erasingthe early formedfabrics. Strike-slipoffsets, in particular, maynotbepreserved because theytendtocolnettie with the axzsof magmatism(e.g., the Great Sumarran fault) [Bellletand Sebrier,1994].
fromtheseLateCretaceous plutons.Thesedikesareextremely The existenceof elongateplutons,ratherthanoffsetof w•despread but are usuallyrecognized only whenintruding hthological contacts between adjacent granites, is observed dloritic,metavolcanlc, andmetasedimentary hostsbecause of withintheRFSZ andmaybe a commoncharacteristic of shear the difficulty of seeinggramticdikesin granitichostrock. zonesin magmaticarc settings. Giventhe high heatflow, The dikes emanatingfrom the easternside of Mono Creek
particularly next to an emplacingmagmaticbody,older plutonsmay respond to deformation by ductileratherthan brittleprocesses, In particular, theshapeof theLakeEdison granodiorite, twobulbous-shaped endsanda thinmiddle which W andextendfor up to 2 km from theedgeof the granite corresponds to the locationof the RFSZ, alsosupports this [Bateman,1965a]. Thesedikesareclearlytemporally related idea. Only when the oldestplutons,far from intruding to emplacementof the granitoids,as dikes from the Mono magmas, havehadenough timeto coolcompletely is discrete Creekintrudethe RoundValleyPeakgranodiorite (87 Ma) offsetof plutoncontacts possible,suchas seenalongthe granite are particularlyImpressive,locallyconstituting 50% of the exposurenear the contact[Sizerlockand Hamilton, 1958]. Theseaplitieandgraniticdikesdipbetween 0ø and30ø
[Stem et al., 1981].
cataclasticsectionof the RFSZ betweenthe Lamarck andTurret
Thesedikescut across theregional steeply dipping, NW orientedstructuraltrendwithinthe roof pendants andare
Peak plutons.
locatedon both the eastand westsidesof the Mono Pass, Tuolumne,andWhitneyIntrusive Suitegranitoids andthusare not a resultof preexisting weaknesses in the rock. Dikesare
The lack of plutoncontactoffsetwzththe RFSZ is also
consistent withtheideathatemplacement of theplutons may be tectonically controlled [Tikoffand Teyssier, 1992].If tectonic spacewascreated for a plutonthrough a structural generally assumed to betheresultof magmatic wedging and process, suchasa pull-apart void[Tikoffand Teysster, 1992] can therelorebe utilizedas an indicatorof infimtesimalstrain or large-scale tensional crack[e.g, Castro,1987],pervaszve (stress) direction [e.g, Hutton, 1988]. The horizontal deformation of theadjacent material is notrequired because orientationof thesedikesimpliesa maximuminfinitesimal creationof spacefor the magmais accommodated by strain (least compresszve stress)in the verticaldirection, translation of surrounding wallrocks/older granites.Further•
TIKOFF AND SAINT BLANQUAT: SIERRANEVADA MAGMATIC ARC
453
settingfor this phaseof the Late Cretaceous thereis clearevidence fromwithintheRFSZthatshearing transpressional arc. facilitated plutonism.Oneexample, citedin theappendix, is SierraNevadamagmatic theintrusion of theelongate porphyritic Laraarck granodiorite In particular, the existence of theRESZsupports the ideaa
(Figure13) [Tikoffand (Figure 7) [Lockwood andL•vdon, 1975],which is onlyfound strike-slippartitionedtranspression adjacent to andwithintheRFSZ. Therefore, in theeastern Teyssier,1994b;Tevssieret al., 1995]. Duringstrike-slip SierraNevadabatholith,it appearsthat the emplaccmcnt of
partitioning, a percentage of thestrike-slip component
by platemotionis takenup alonglargelhults/shear theplutons andtectonics of themagmatic arcareintimately imposed zoneswithin the magmaticarc. The remainderof the strikeslip •notion and all of the contractional motion is
interrelated.
Transpressional
Setting
and
Simultaneous
accommodated by localizedmovement alongtheplatecontact (the
Plutonism
Bentoff zone) and/or distributed contractional
deformationthroughoutthe overridingcontinentalplate. Since RFSZ records strike-slip tectonism in the Sierra
Within the Late CretaceousSierraNevadaarc, the SierraCrest
Nevada magmatic arc, it is criticalto observethatthe shearing shearzone systemindicates arc-paralleldextralshearing. is localized along the trend of the active magmatism.
Dextral shearingwithin the arc is coincidentwith continued
in otherpartsof theorogen,includingthe Sevier Magmatic arcsmayoccupya uniqueposition withinorogenic contraction belts,becausethey are continuouszones of high heat flow
probably acts as a zone of lithospheric weakness [e.g.,
thrust belt [e.g., Miller et al., 1992]. These observations generally supportthe modelof Sunda-styletectonicsof Beck [1986], at least during post-90 Ma plutonsirewithin the
Jarrard, 1986]. Since the roots of magmatic arcs extend
Cretaceous Sierra Nevada batholith.
into which fluids (or melt) are continually injected, which
vertically through the crust and the orientation of the
magmaticarcs are orogen parallel, the locus of active magmatism shouldaccommodate the strike-slip component of obliqueplate convergence [Beck, 1986]. As suggested by manyauthors,one may expecta positivefeedbackloop between strike-slipshearzonesoccurringin the magmaticarc andplutonsbeingeraplaced intoa magmatic arcbecause of the strike-slipshearzones (e.g., Glazt,er [1991]; Tikoff and Teyssier[1992] for the Sierranarc). Deformationwithinthe arc also has a componentof contractional,in addition to the transcurrent,movement.
The simultaneous combination of
contractionaland strike-slip shearing suggestsan overall
A raodernanalog for strike-slippartitirafingwithin the Late Cretaceous Sierran batholith is the modern Sumatra
magmaticarc [Fitch, 1972]. In this setting,approximately 1/3 of the strike-slipcomponent of platemotionoccursalong the Great Sumatranfault whosepositionroughlyparallelsthe trend of magmatism [Tikoff and Teyssier, 1994b]. Contractionaldeformationstructures(e.g., folds and faults), whichare parallelonbothsidesof theGreatSumatran faultbut obliqueto the trendof the fault [Mount and Suppe, 1992], record a combinationof contractionand strike-slip motion [Tikoff and Teyssier, 1994b]. Elliptical to rectangular calderas(e.g., Toba and Ranau)form in seriesof en &helon,
PARTITIONIN
Figure 13. Cartoonof strike-slippartitioningin an obliquelyconvergent magmatic arc, particularly for the Late Cretaceous (92-83 Ma) SierraNevada. A component o; thestrike-slipdisplacemen! is partitioned
intothemagmatic arc,leadingto a transpressional arcsetting.A combination of strike-slip andcontraction is also found in the forearc and back arc areas.
454
TIKOFFANDSAINTBLANQUAT:SIERRANEVADAMAGMATIC ARC
pull-apart structures related to releasing stepovers of theGreat Sumarran fault[BellierandSebrier, 1994].Sincethese pull-
remain relatively high, aswithmost plate motions during the
LateCretaceous [Engebtetson etal, 1985; Kelley, 1993] apartstructures wouldrepresent spacefor tectonicintrusion of Theerrormargins for thistypeof platemotion granitoids at depth,tectonic space forplutons in theSumarran reconstruction are approximately + 5-10 my. [DC magmatic arcis created in anoverall transpressional setting Engebretson, personal communication, 1996]Regardless of Theideaof strike-slip partitioning of magmatic arcsis in theuncertainties, wepostulate thatthetransition from directcontradiction to someearliermodels. Smith[1981],in contractional shearing todextral shearing within theSierra particular, proposed thatonlynormalcompressire forcesare Nevada batholith at-90Mamaybearesult ofFarallon / North transmitted acrossthe arc to the backarc region,with the American interactions. The100-90 Macontractional shear strike-slipcomponent beingentirelyaccommodated within zones [e.g.,Tobisch et al., 1995]mayresult fromalmost the are Whileit is clearthatmagmatic arcsare zonesof head-on convergence between the Farallon andNorth mechanical weakness andpreferred strike-slip motion,both American plates.Dextral shearing withintheRFSZ •s contractional andstrike-slip movement occuron bothsidesof consistent witha resolved component of dextral displacement activearcs(e.g.,AndesandSumatra)Rather,strike-slip causedby plate interactions. partitioningof are systemsis more consistentwith the
Theangle ofrelative convergence oftheFarallon and North
orogenic floatmodel, where theentireorogen isunderlain bya American plates during activation of theRFSZwas75o-90 ø lithospheric basaldetachment anddisplacement is partitioned [Kelley,1993;D C Engebretson, personal communication throughout the orogen[Oldowet al., 1989,1990], In this 1995]Thiscalculation •sbased onthepresent orientation o• model,normaldisplacement (contractional/extensional), as the centralSierra Nevadabatholith(N25øW)and well as strike-shp movement, is distributed throughout the paleomagnetm evidence fora lackofsubsequent rotatton [Fret orogen, although a component of strike-slip movement may et al., 1984]. Thisresultsuggests that evena small
be concentrated in thearc(Figure13),
component oftranscurrent motion imposed byplate boundary
canpotentially resultin strike-shp partitioning withinanarc setting.For instance, strike-shp partitioning withinthe Dextral Shearing in the Sierran Arc and Plate Sumarran areoccurs withanangleof convergence of- 50o Motions Another interesting resultinolves approximate calculation of the transcurrent motion imposed by plate interaction, which TheLateCretaceous Sierra Nevada magmatic archasbeen interpretedto exhibitdifferentstylesof deformation before
results in a -22 km/Madextral component [Kelley,1993,
personal commumcatlon, 1995] If dextral and after-90 Ma [Tobtsch et al., 1995]. As summarized Engebretson, shearing occurred for 10 m.y. at this rate, upto 220kmof above, after90 Ma theactive trendof plutonism isdominated dextral offset is permissible. However, if we assume that only by dextraltranspressional deformationContrarlly, 1/3of thetranscurrent platemotionis translated intothearc NWoriented, contractional shearzones(Bench Canyon, [Tikoff andTeyssier, 1994b], approximately Courtright-Wishon, KaiserPeak,andQuartzMountain) (e.g.,Sumatra) be expected alongtheaxisof Sierra [Toblsch et al., 1995;McNulty,1995]areactivebefore90 73 kmoœoffsetwould withother Ma. Therefore a change inSierra Nevada aredynamics occurs Nevadamagmatism.This estimatets consistent geologically-based offsetwithinthe SierraCrestshearzone system of 20 km (minimumestimate)[Greene and Therearedifferent possibilities forchanging theplate-
at approximately 90 Ma
1995] and70km[Tikoff andTeyssier, 1992]. scaledynamics of a magmatic aresystem, thesimplest of Schwetckert, which isa change inthedirection and/or rateofrelative plate motion withsubruction of thesame oceanic plate.Recent Conclusions
relative plate motionsbetweenthe Farallonand North
American platesforMesozoic California mayexplain this transition between contractional andstrike-shp structures at
Dextralshearing in theRosyFinchshearzonewasactive
from88-80Ma, simultaneously withemplacement of the
-90 Ma. Kelley[1993] andKelley andEngebretson [1994] youngest plutonsof LateCretaceous SierraNevadabathohth wereableto betterresolve platemotions during the118-83 Dextral shearing in theStarran arcappears to behmtted to Ma Cretaceous LongNormalusinga combination of satellite youngest (eastern) section of theCretaceous arc.Thestyle of altimetry, revised marine magnetic stratigraphy, andmore dextral shearing in theRosyFinchshear varies along strike complete platemotion andhotspot analyses. Thebiggestfromwidezones of ductile deformation in theyoungest
change between thenewplatemotions [Kelley,1993]and plutons to narrowzonesof cataclastic deformation in theolder previously published studies [e.g,Engebretson etal.,1985] plutons. TheRosyFinchshear zoneis partof the310kin Involves theorientation ofplatemotion during theCretaceouslongSierra Crest shear zonesystem, which records strike-shp LongNormal.In particular, a slmstral component of shearing along the axis of magmatism. A strong transcurrent motioncharacterizes the Farallon/North America contractional component of deformation accompanied this
boundary until -95Ma,rather than until 125Maaspreviously dextralshearing, suggesting a regional transpressional cited After~95Ma,anincreasing dextral component of setting
transcurrem motioncharacterized thisplateInteraction. More recentreevaluauon of thedatasuggests thattheswitchfrom
TheRosy Finch shear zoneisinterpreted tobetheresult of strike-slip partitioning, allowingpreferential strike-slip sinistml todextral occurs aslateas90Ma[D,C.Engebretson, movement to occurwithinthe activetrendof magmatlsm, personal communication, 1995]. The ratesof relative whichprobably actsasa lithospheric weakness Theoverall convergence between theFarallon andNorth American plates setting oftheLate Cretaceous Slerran arc sugge'sts that strike-
TIKOFFAND SAINTBLANQUAT'SIERRANEVADAMAGMATICARC
indicatebulkdextralshear(Figure4), slippartitioning: themagmatic arcmayaccommodate a cleavages
455
Shearbands,
certain percentage of thestrike-slip motion imposed by plate consistingprimardyof quartzand mica, vary slightly m astheywraparoundlargerfeldspar crystals(Figure motton, buttheforearcandbackarcregions alsoexperienced orientation 4b). Although the deformationis d•smbutedrelatively transpression during thistime. overthe widthof the shearzone(up to 4 kin), Recent platemotionsstudies[Kelley,1993;Kelleyand homogeneously some localizalion of shearoccurs, generallytowardthemtddle Engebretson, 1994]record a change fromoblique simstral convergence to obliquedextralconvergence between North of the zoneor adjacentto a wall rockcontact. In thesezones America/Farallon interaction at ~95 Ma. We correlatethis change in platemotionto a change between activation of contractional shearzonesand strike-slipshearzones,which occursat ~90 Ma within the CretaceousSierra Nevada batholith. The timingof the RFSZ,the transpressional nature ofthedeformation, andtheamountof dextraldeformation are allconsistent withtheseplatemotionstudies. Acknowledgments.
We would like to thank D.
of greaterdeformation, K-feldspar porphyroclasts are broken, andquartzaggregates aremoreelongate,S-C anglesarelower, and the microstincturebecomesgenerallymore mylonttic. Quartzgrainsthroughoutthe shearzone definethe fohation and are elongate,consistent with the elongation of rare marie enclaves, and show uniform stretching direction and magnitudeon the outcropscale. Ductile-brittleslnistralshear zones,generallylessthat 0.5 m thick, are developedwithin the RFSZ.
Quantitativedeformationanalysisof the RFSZ within the Mono Creekgraniteis difficultdue to the lackof widespread enclaves or other markers. In an attempt to quantify deformauon•n porphyriticgranites,Ttkoff and Teysster [1994a] createda model of porphyroclast (megacrysticKandfabricdevelopment. The modeluses BradMurray,LoganColby,andthePineCreekpackerstation feldspar)lmbfication different types of clast rotations to simulate sohd-state, staffaregratefullyacknowledged for theirassistance withthe intermediate, and magmaticconditions and differenttypesof fieldwork.This work wassupported by NSF EAR-9305262. two-dimensionalstrain histories,from pure shearto simple shear. Tracingsfrom the Mono Creek graniteon planes Appendix parallel to fincation and perpendicularto foliation were compared to modelresults.Theanalysis indicated thata solidRFSZ in the Mono Creek Granite state, simple sheardeformationhistory of up to ¾=4 was superimposedon an earlier strain hislory, either a TheRFSZ m the Mono Creekgraniteis easilyrecognized in submagmaticor htgher-temperature, solid-statedeformation thefieldas an orthogneiss in which foliationandhneationare This idea is also supportedby the observation of tmbricated developed. The shear zone parallels the Mono Creek feldspar porphyroclasts recordingdextral shear, observed granite/Mount Mornson pendantcontacton the northeastern immediatelyoutsidethe RFSZ Together,the modelingand sideof the granite (N20øW) and takes a N-S orientation field observations suggestthat the lower-temperature RFSZ through the middleof the pluton. At the southend of the concentratedalong a zone of previoushigher-temperature pluton,lhe shearzonebranches into a main shearzonethat deformation. Averaging their analysisof porphyroclast continues into the Lake Edisongranodioriteand a minor zone imbncation,quartzelongation,and S-C fabric development whichparallelsthe SW contactof the Mono Creek granite overthe widthof the shearzone,Tikoffand Teyssier[1994a] (N20øW)(Figure2). Althoughthe shearandfoliationplanes esttmate 8 km of solid-state offset superimposedon an withinthe shearzone are consistently subvertical,the plunge unknown amount of offset during higher-temperature of the hneation varies along strike (Figure 3) Lineation deformation.
Engebretson for providinginformation on Cretaceous plate motionreconstructions.K. Basset,S. Cruden,D. Greene,J. Maxson, and C. Teyssierare thankedfor helpfulcommentson earlierversionsof the manuscript,and D. Cowan, H. Maher, andO. Tobischare acknowledgedfor constructivereviews.
plungesmoderatelysouthwardin the northernsection, plungessubhorizontally in the middle, and plungesshallowly northward m southern section.
The foliation in the Mono
Creek granite outside of the shear zone is consistently subvertical and cutsacrossplutoncontacts.
The style of deformationwithin the RFSZ in the Mono Creek granite also varies along strike, but m general, fohatlon is definedby the elongauon of quartzaggregates and by mina foha wrapping aroundwell-preservedfeldspar porphyroclasts (Figure4a). The hneationis typicallydefined by theelongation of quartzgrainsandby a streakylinearion on mica-richfolia, resemblinga slickenside. Within these foha,biottteand/orhornblende are generallyretrogressed to chloriteand/orepidote,whichalsooccurin thediscreteshear
RFSZ
in the Lake
Edison
Granodiorite
The RFSZ passes continuously from theMonoCreekinto the Lake Edison pluton as a zone of ductile deformation characterizedas an orthogneiss. Although the RFSZ is
slightlywider(~2 km wide)thanmapped by Lockwoodand Lydon [1975], both their mapping and our fieldwork consistentlyshow steeplydipping,NNW orientedfoliation and shallowlyplunginghneation(Figure 3). The RFSZ is found primarily m the thinnedmiddle of the Lake Edison
pluton,although deformation alsocontinues alongtheNE and SW contactsof the pluton. Foliattonwithinthe Lake Edison granodiorite is recordedby elongationof quartzaggregates, zones of locahzed strain. alignedmarieenclaves, andalignment of themaficminerals. Meso-scale and microscale structures, such as S-C W•thin the shearzone, bratitcand hornblendeare partially to chlorite,andfeldsparts locallyrecrystalltzed relationships, folded aplitm dikes, megacrystlmbrieation retrogressed [Ttkoff and Teyssier,1994a], mica fish, sigmaand delta (althoughnot extcnsively).In thin section,thin shearbands dextralshear(Figure5). porphyroelasts, and "bookshelP'displacementon feldspar (0.1 - 0.5 into)indicatedominantly
456
TIKOFF AND SAINTBLANQUAT: SIERRA NEVADA MAGMATIC ARC
The quartz aggregates are commonly dynamically recrystallizedin polycrystalline ribbonsand displaya strong crystallographicpreferredorientation Toward the Mono
Creekcontact,the quartzaggregates displayirregular,lobate
intoa series of sheet-like bodies, thelargest of which isthe porphyritic Lamarck granodionte (Klp of theMr. Abbot quadrangle) [Lockwood andLydon, 1975].Allthese units are pervasively sheared in a dextralfashion (Figure4) The
shapes, perhaps indicating high temperatures during
elongatenatureof thesebodiesparallelto the Lake Edison/Lamarck contact, theiroccurrence onlywithinor immediately adjacent to theRFSZ,andtheiroverallductile behavior (compared to the brittlebehaviorof themain Lamarck pluton, seebelow)suggests late-stage intrusion of 6). Between 5 and 12 marc enclaves were measuredat each these bodies intoa weak zonecontrolled bytheshearing. site and a harmonicmean[e.g.,Lisle, 1977]wastakenasthe AstheRFSZcontinues southward, it cutsdiagonally across averagestrain. The resultsare only semlquantitative, because themainLamarck granodiorite (Figure7) Shearing within
deformation causedby proximityto theyoungerpluton. Widespreadmafic enclaves,commonthroughout the Lake Edison granodiorite,are used for estimatingstrain [e.g, Hutton, 1988], as shownfor the Bear Creektraverse(Figure
of the low numberof enclavesmeasuredat any location. Within fl•e BearCreektraverse(Figure6) andthroughout the
thiszone generally occurs asboth distributed dextral sheanng
as well as discretelnylonltlcand cataclastic shearzones Withinthedistributed shearing, shearbands areoriented N20o. fohatlon with the maximumelongationparallel to the 30øE,fohation is subvertlcal, andlineations aresubhorizontal hneationdirection.The shortaxisof thestrainellipsoidalso to shallowly northplunging (Figures 3 and8a) Thediscrete lles in the horizontalplane, makinga horizontalsurface mylomtic shearzonesare approximately 2-3 m wide,strike Rosy Finch shear zone, mafic enclavesare flattened in the
approximately parallelto the XZ plane Sincemaficenclaves approximately N-S to N25øE,andhavesteeply dipping m the L•e Edisonplutonfar away fromthe shearzoneare foliations and shallowlyplunging lineations. These
quali!atively roundto weaklyelliptical,elongation withinthe shearzonegivesan approximation of thecombined effectof magmaticandsolid-state strain. Sincethemaficenclaves are elongate towardthewestern edgeof theRFSZ(Figure6), where sohd-state deformation is relativelyweak,someelongation of the mafic enclavesis probablydue to high-temperature or magmaticdeformation,again pointingto a close relation betweenshearingandplutonemplacement.
mylonitlcshearzonesgradefrom S-C myloniteson the outside to completely recrystallized or eventhin(1 cm)zones of cataclasis in thecenter.Thesesmallmylonltic shear zones all showdextralstrike-slipmotionwith subverticalfoliation
anda shallowly plunging hneatlon.Quartzaggregates form
elongate ribbons, which areoftenobserved tobebroken by
fractures. Feldsparsare bent, broken,and often serlcitized
Biotite,andparticularly hornblende, grainsare brokenand retrogressed to chlorite. Mylonitm to ultramylomtlc ENE-NE with subhorizontal hneations, exposedin the Lake fragments are alsofoundas lithic fragments insidethese Edisongranodioritehavebeenextensivelystudied[e.g, cataclasticzones, suggestingthat significant ductile LockqoodandMoore,1979;Martelet al., 1988,Segallet deformationprecededcataclasis(Figure 8b). These 1990. Burgmannand Pollard, 1994] and providesome microstructural observations suggesta periodof ductile constraints on movement in the RFSZ The 40Ar/39Ar deformation (e.g., quartzrecrystallization andfeldspar analysis on secondary mica,up to severalhundred micrometers folding)followedby brittledeformation (e.g., pervasive long, withinthesesmistmlfaultsprovideagesof 79-75Ma. fracturing),as previouslyarguedfor on the basisof field Thesefaultsare interpreted as a conjugate setto late-stage relatlons. movement on the RFSZ, offset the Mono Creek/Lake Edison Thesecataclastlc/mylonltic zonestendto converge toward the West PinnaclesCreek valley, where there is a Ncontact(Figure6) [Tikoff,1994],andgenerally increase in ductilitytowardthe MonoCreekgranite[Bargmannand S oriented, 10 m wide zone of cataclastic deformation is Pollard, 1994]. In theBearCreekarea(Figure6), a single [Lockwoodand Lydon,1975]. The zoneof ½ataclasis slmstralductile fault from within the RFSZ in the Mono Creek composed entirely of chlorlte, mica,quartz,andfeldspars and granitecan be followedcontinuously into the Lake Edison severe grainsizereduction hasoccurred; fragments of quartz andfeldspar grainsarenolargerthan1.0mm. Kinematics are granodiorite within the RFSZ where it becomesa brittleBrittle and brittle-ductile sinistral fault zones, oriented
ductilefractureand,finally,a brittlefaultoutsideof theRFSZ in the LakeEdisongranodiorite.However,evenwhereductile withintheMonoCreekgranite,thesinistmlfaultsshowlower
difficult to determinewithin the N-S oriented,subvertical
theendof ductile,dextralshearing in theRFSZat 79-75Ma.
slightlylarger. The NW orientedcontactbetweentheTarret
cataclastic zone.Although slickensides arefoundm a variety of orientations withinthezone,meter-scale grooves corrugate faultsurfaces andplunge12øN,suggesting temperaturedeformationthan the surrounding dextral the subvertical strike-shp movement. Towardtheedgeof this shearing, for example,increased cataclasis andpervasive predominantly is moremylonitic,quartzis fracturing of K-feldspars. Therefore thecross-cutting nature of zonewherethe deformation dynamically recrystallized, andthelithicfragments are thesesimstral faultsin theBearCreekareaeffectively dates locally
PeakandLamarck plutons is dextrally offset-1.2 kmalong RFSZ in the Lamarck Granodiorite
the singlemylontic/cataclastic zone in West PinnaclesCreek
valley,theonlyplacealongtheRFSZwherea discrete dextral
TheRFSZwithinandadjacent to theLamarck granodlonteoffset ofa phiton contact isobserved (Figure 7). is quitecomplex (Figures 3 and7), because of compositional TheLamarckgranodionte is alsodeformed in theRFSZas variationwithinthe plutonandtheexistence of bothbrittle farsouth asthePiuteCreek traverse (Figure 10). In thiszone, andductile features. Thefabrics in theLamarck granodmnteonlythewestern most200m of theLamarck granodiorite is willbediscussed fromnorth tosouth.In thenorthern section, affectedby dextral deformation. Foliation is subvertical, theRFSZcontinues south fromtheLakeEdison granodioritedefinedby bothmaficmineralandmaficenclaves, andthe
TIKOFFAND SAINTBLANQUAT. SIERRANEVADAMAGMATICARC
lineations plungeshallowly northward.Thedeformation ss
457
enclavesappearingas elongatepencfi-hkestructureson a
horizontalplane(XZ planeof deformatton).Movingfarther pervasive but generallyexhibitslow-temperature
microstructures suchasdescribed above.
eastward, the deformation continues to decrease, with the exceptionof the cataclasticzones These cateelasticzonesare
RFSZin the Turret Peak Granite
up to I m wide,andshowno visiblestructure, excepta drag consistent with dextral shear and occasional cataelastlc S-C
TheTurretPeakgraniterecordsmorepervastve deformation structures in thsnsectson (Figures 9b and9c). Themineralogy mtheRFSZthantheLamarck granodiorite, TheTurretPeak consistsof ohiorite, mica, occasionalductfiely deformed granite hasa mapped extentof approximately 7.5 km m quartz,and feldsparactingas lithie fragments.The intensity decreases eastward, althoughtheentirewidthof length (NW-SE)anda wtdthof 1.5kmin theMountAbbot of deformatson
quadrangle [Lockwood and Lydon,1975]butwasnot
the TurretPeakis sheared alongthePtuteCreektraverse.
Since the Turret Peak graniteis older thanthe Lamarck recognized m the earliermappedBlackcapMountain quadrangle [Bateman, 1965b].Thustheextent of thispluton granodiortite,basedon xenolithsinclusions[Tikoff, 1994],
onemightexpectmoredistributed deformatson m theyounger
is unknown.
TheRFSZ w•denswithin theTurret Peak pluton;thisunit is
pluton. However,we ascribethispatternof deformation to the
locally ductiley deformed across stsentirewidth,suchasiv the
differingcomposition of theplutons.The TurretPeakpluton
P•uteCreek traverse(Figure 10). However, the cateelastic zonethatformsthe centerof West Pinnaclescreekvalley also continues southward into theTurretPeakpluton. This zoneof intense shearingbecomesan ultracataclastic zone within the
containsmorequartzthan theLamarckplutonand as suchis expectedto deformmore easfiy]n uppercrustalconditions Thts same type of relation, an older, quartz-richpluton
TurretPeak plutonand continuessouthinto Piute Canyon
seen elsewherem the SterraNevada [e.g., Patersonet al.,
deformingmoreeasilythana younger,quartz-poor pluton,ts
iF•gure 10). FarthersouthwithintheTurretPeakgranod•ofite, 1989]. the pervasiveductile deformationand the cateelastic deformationswings into NNW orientation adjacentto the RFSZ in the Evolution Basin Alaskite Evolution Basrapluton. The cateelasticzonein this southern section •s locallyintrudedby basalticdikes. Distributedsheanng,m the form of well-developed solidA representative traverseof deformation sntheTurretPeak statefoliation,is foundalongthe NE contactof the Evolution pluton ts shownin Figure10. The RFSZwithintheTurret Basinpluton,adjacentto shearingin the TurretPeak. Sohdis characterized primarilyby elongatequartz Peak plutoncontains bothbrittleandductilefeatures.As with statedeformation grains, which display weak undulose extinction and theLamarckgranodsorite, numerous smaller-scale (up to 1 m wsde), N-S orientedmyloniticzonesare foundadjacentto the elongation. Within the shearzone, quartz,feldspars,and centraleataelasticzone. These snylonitic zones all show biotite all showpervasivefracturingbut very little alteration •hallowlyplunginglineationsand dextralstake-slipmotion. (such as sericite replacingfeldsparor chlorite replacsng Thepervasiveductiledeformation within the Turret Peak biotite). We interprettheseareasof solid-statefoliation as gramte is easilyquantifiedby the presence of marieenclaves, continuationof the RFSZ into the Evolution Basra pluton treated in a similarfashionto thosein the Lake Edisonpluton (Figure 2), as the style ts ssmfiarto the deformationof the d•scussed above Againstthe contactof the MountGoddard sectionof theRFSZ in partsof the MonoCreekgranite. S-C metevolcanic roof pendant,theTurretPeakplutonis locallya fabric ts not seenwithinthe alask•te,probablybecauseof the gneiss,with asymmetricfolds developedindicatingdextral lack of megacrysts,which apparently act to localize senseof shear(Figure 9a). To the east, st is a sheared deformationand causeshearbands.The extentof shearingin porphyriticgranite, with a strongS-C fabric and marie this plutonis not known References Ague,R, and G H Bumhall, Magmaticarc a•ymmetryand distnbutronof anomalous plutohiebelt• in the batholtthsof Cahfomia' Effects of assimilation. crustal thickness,
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