taylor 2003 tectonics

8/11/2019 Taylor 2003 Tectonics

1/25

Conjugate strike-slip faulting along the Bangong-Nujiang suture

zone accommodates coeval east-west extension and north-south

shortening in the interior of the Tibetan Plateau

Michael Taylor,1 An Yin,1 Frederick J. Ryerson,2 Paul Kapp,1,3 and Lin Ding4

Received25January 2002; revised 13January 2003; accepted 5 March2003; published 28August 2003.

[1] Geologic investigations of how the Tibetanplateau is currently deforming have focusedprimarily on its boundary faults. Consequently, howthe interior of the plateau deforms remains poorlyunderstood. To fill this gap in knowledge, weconducted field mapping, analysis of remote sensingand digital topographic data, and reinterpretation ofexisting geologic maps in central Tibet. This studyreveals a200300 kmwideand 15001800 kmlong

easttrendingzoneof conjugatestrike-slip faultsacrosscentral Tibet. Thecentral Tibet conjugatefault zoneiscomprisedof northeaststrikingleft-slip faultsnorthofthe Bangong-Nujiang suture and northwest strikingright-slip faultssouthof thesuturezone. Thesestrike-slip faultsarekinematically linkedwithnorth trending

Tibetan rifts located north and south of the conjugatefault systems. Without exception, all conjugate faultsintersect or merge toward one another along theBangong-Nujiangsuturezone. Motion onthesefaultsaccommodates coeval east-west extension and north-south contraction. To determine the fault kinematicsand the magnitudeof fault slip, we investigated threeconjugate fault sets in the central Tibet fault zone.

These include from east to west, the Dong Co, BueCo, and Aishi Co conjugatefault systems, which areadjacent to the Bangong-Nujiang suture zone andseparated by a distance of 400 and 70 km,respectively. The average magnitude of fault motionon individual strike-slip faults is 12 km asdetermined by offsets of Tertiary thrusts andPaleozoic-Mesozoic lithologic units. The conjugatefault configuration requires 12 km of north-southcontractionacrossthe200300kmfault zonesinceitsinitiation. Because the conjugatestrike-slip faults are

kinematically linked with the north trending Tibetanrifts which initiated between 14 and 8 Ma, ourestimated magnitude of north-south contractionimplies a contraction rate of 12 mm/yr acrosscentral Tibet. Therelatively closely spaced (


2/25

[3] In light of recentresultsof geologic andgeophysicalinvestigations in Tibet, these end-member views mayrequire a level of refinement. M ey er e t a l.[1998], Owensa n d Z a n d t [1997], and C a t t in a n d A vo u a c[2000] point outthat discrete continental subduction may have been thedominant mode of deformation in accommodating theIndo-Asiancollisionalongthenorthernandsouthernedgesof theTibetan plateau. This implies that extrusiontectonicsmay haveplayedalesser rolein accommodatingtheoverallconvergencebetweenIndiaandAsia. Thisis consistentwiththeresults of recentgeologic andgeodetic studiesonmajorstrike-slip faults in the Indo-Asian collision zone. Forexample, studies along the southern and central segmentof theactiveright-slipKarakoramfaultsuggestthat its totaldisplacement is between 60 and 150 km, respectively[Searle, 1996; S ea rl e e t a l., 1998, 1999; Mu rp hy e t a l.,2000]. Theseestimatesaremuchlessthanthe1000kmofright-slip motion predicted by theextrusion model [Peltzerand Tapponnier , 1988]. Using Quaternary slip rates alongmajor strike-slip faults to test the extrusion hypothesis hasyielded ambiguous results. At a timescale of a few years,GPS studies suggest that the Altyn Tagh fault moves at arateof10mm/yr orless[Bendick et al., 2000; S h e n e t a l .,2001]. However, long-termslip rates alongtheAltyn Taghfault could be as high as30mm/yr [e.g., M er i au x e t a l .,1999]. In addition, arecentstudy of thelong-termslip ratealongtheKarakoramfault suggeststhat it may beas low as3 mm/yr, based on dating offset geomorphic landformsusingcosmogenic 10Be[Brown et al., 2002]. Why slipratesalong theAltynTagh and Karakoramfaults vary with timeremainspoorly understood.

[4] Smaller magnitudes of displacement and slower sliprates along major strike-slip faults in Asia imply that east-wardmotionof Tibetmaybelessimportantthannorth-south

shortening [England and M olnar, 1998]. However, thisinterpretation may be oversimplified, because eastwardmovement of Tibet could be accommodated by a greaternumber of strike-slip faultsdistributed over a largeareaof

Tibet [Rothery and Drury, 1984; M o ln a r a n d L y on - C a en,1989; Yin, 2000]. If correct, then an alternative to theend-member models of discretelateral extrusion anddistributedcrustal thickening arises. That is, eastward motion of Tibetcould be accommodated by a large number of strike-slipfaultsdistributedacrosscentral Tibet [Kong and Bird, 1996;Yin, 2000]. This interpretationimpliesthattheleft-slipAltyn

Tagh and right-slip Karakorum faults may be only one ofmany conjugate fault systems that assist in the eastwardspreading of the Tibetan crust (Foldout 1 and Figure 1).

A key to testing this possibility is to establish thegeometry and kinematics of active faults in the interiorof the Tibetan plateau related to the Indo-Asian collision.

Armijo et al.[1986, 1989] viewed the Qiangtang terraneas a rigid block moving eastward relative to the Lhasaterrane via a series of en-echelon right-slip faults (theKarakorum-J iali fault zone) directly south of the Ban-gong- Nujiang suture zon. This inter-pretation has been questioned by both geophysical andgeologic observations. First, active deformation is occur-ring in the Qiangtang terrane as expressed by numerous

fault-bounded blocks with each having complex kine-matics [M olnar and Lyon-Caen, 1989]. Second, fieldstudies by Yi n e t a l. [1999a] and B li sn iu k e t a l. [2001]suggest that the Qiangtang terrane has experienced mea-surable east-west extension. In addition, a sparse GPSnetwork that spans Tibet near 95E longitude suggeststhat a measurable amount of coeval north-south contrac-tion and east-west extension is presently occurring [Wange t a l., 2001].

[5] To address the question of how Tibet is deforminginternally, we conducted geologic investigations of active-to-recently active faults using field mapping, analysis ofdigital topography (GTOPO30), andinterpretationof Land-sat-7andCORONA satelliteimages. Our resultsallowustodevelop a coherent kinematic model that emphasizes theroleof conjugatestrike-slip faults in accommodatingcoevalnorth-south contraction and east-west extension in central

Tibet.

2. Regional Geology

[6] TheLhasaandQiangtangterraneswereaccretedontothe southern margin of Eurasia beginning in the north,during the LateTriassic-Early Jurassic and the LateJuras-sic-Early Cretaceous[Dewey et al., 1988; Y i n a n d H a r r i s o n,2000]. Fromnorthtosouth, theboundariesof theseterranesare marked by the Jinsha, Bangong-Nujiang, and Indus-

Yalusuturezonesandhavebeenreactivated by bothstrike-slip and contractional deformation during the CenozoicIndo-Asian collision [Armijo et al., 1986, 1989; M e ye r e t al., 1998].

[7] A salient topographic featureof theLhasa terraneisthenorthtrending rift systems, which dissect theotherwiselow-relief surface. Whether the rifts are an expression of

upper crustal deformation or involvethemantlelithospherehasbeendebated[M asek et al., 1994; Yin, 2000]. It hasalsobeen considered that Tibetan rifts arethe result of gravita-tional collapse[M olnar and Tapponnier, 1978; Tapponnier e t a l., 1981] or caused by aconvectiveevent in themantle[England and Houseman, 1989]. Except the Yadong-Gulurift that extends into the Himalaya, all rifts in the Lhasaterrane terminate before they reach the Indus-Yalu andBangong-Nujiang suture zones [Armijo et al., 1986]. Thisobservation implies that the sutures on both sides of theLhasaterranemay actastransfer structuresconvertingeast-westextensional strain totheQiangtangterranein thenorthand southward into the Himalayan range [Yin, 2000]. Theinitiation of the Yadong-Gulu rift is constrained to be 8

1 Ma along the Nyainquentanghla Shan [Harrison et al.,1995], with an upper age bound of 12 Ma along itssouthernmost segment [Edwards and Harrison, 1997; W ue t a l., 1998]. Minor east-weststretchingof theLhasacrustat an earlier stage is inferred from a north trending dikeswarmnear Xigazeand was dated at 1813Ma[Yi n e t a l .,1994; Wi l li a ms e t a l ., 2001]. TheThakkola graben in thecentral Himalaya south of the Lhasa terrane initiatedbetween 1114 Ma based on ages of the oldest synriftsediments[G a r iz o n e e t a l ., 2000] andnorthtrending exten-sional fractures [C o l em a n a n d H o d ge s, 1995].

18 - 2 TAYLOR ET AL.: CONJUGATE STRIKE-SLIP FAULTING WITHIN TIBET


3/25

Foldout 1. Regional tectonic map of theTibetan plateau modifiedafter Armijo et al.[1986, 1989], M eyArrowsmith and Strecker[1999], Y i n e t a l . [1999b],andourownobservations. Inactivestructures(e.g., thein central Tibet havebeenomitted for clarity. Aneast-west trendingbelt of conjugatestrike-slip faults is pBangong-Nujiangsuturezonein central Tibet.Seetextfordescriptionof thefault geometryanddistributionstars indicatethe locations of the8 November 1997, M7.6 left-slip Manyi earthquake(left), andthe14NM7.9 left-slip Kokoxilli earthquake (right). Labels A, B, and C indicatethe intersections of the Dong CAishi Co conjugate strike-slip fault systems. Seeenlarged version of this figurein theHTML.


4/25

Figure1

.

ColorshadedreliefmapwithacompressedpaletteofcentralTibets

howingabroadeast-westtrending

depressio

n(indarkblue)alongtheBangong-Nujiangsuturezone.

Notethatthed

epressionismarkedbyaseriesof

discontin

uousbasins.

Strike-slipfaultsarerepresentedbynarrowandlineartopograp

hicstructurestrendingnortheastand

northwes

t,northandsouthofthecentraldepression.Theselinearfeaturesarethelef

t-slipfaultsintheQiangtangterrane

andright-slipfaultsintheLhasaterrane.Notethatright-slipfaultsintheLhasaterraneboundthenorthernmarginof

severaln

orthtrendingrifts,whereasleft-s

lip

faultsintheQiangtangterraneareconnectedwithtopographicallyless

pronounc

ednorthtrendingrifts.Thepoortop

ographicdefinitionofriftsinQiangtangmayresultfromtheinternally

drainedb

asinfilling[Yin

etal.,

1999b],

orbe

causeofhorizontalmotionassociated

withstrike-s

lipfaulting.

Seecolor

versiono

fthisfigureatbackofthisissue.



5/25

[8] Activefaultingwithin theQiangtangterranehasbeenrecognized previously through fault plane solutions andanalysis of satellite imagery [M olna r and Lyon-Ca en,1989; M o l na r a n d Ta p p o n ni e r , 1978; R o th e ry a n d D ru r y,1984]. Thesestudies revealed that active-to-recently activefaults in theQiangtang vary in strike and fault kinematics.

The latter ranges from strike-slip to normal faulting. Aminimum initiation age of normal faulting was obtainedby Blisniuk et al.[2001],whousedthe40Ar/39Ar methodtodatesyn-kinematicallygrownmuscovitefromtheShuanghunormal fault zone. Their results suggest that the onset ofeast-west extension may have occurred at or prior to13.5 Ma [Blisniuk et al., 2001]. Similar to rifts in theLhasaterrane, rifts andtheir associated faultsin theQiang-tang terrane do not appear to extend across theBangong-Nujiang suturezone, suggesting that it has been actingas atransferzonelinkingextensional structures in theLhasaandQiangtangterranes[Yin, 2000]. It is thequestionof howtheextensional structures are geometrically and kinematicallyrelated across the Bangong-Nujiang suture zone that hasmotivated this study as presented in detail below.

3. Conjugate Faults Along the Bangong-

Nujiang Suture Zone

[9] From field observations, topography, and satelliteimages, faults that appear to beactive and located directlynorth of the Bangong-Nujiang suture zone are exclusivelynortheaststrikingwhilethosedirectly southof thesuturearenorthweststriking[Rothery and Drury, 1984; Armijo et al.,1986]. These faults merge toward one another along thesuture(Foldout1andFigure1).Becausenorthtrendingriftsin Lhasa and Qiangtang accommodate nearly east-westextension [M ercier et al., 1987; Yi n e t a l., 1999a], an

inference is that northeast striking faults are left-slip,whereas the northwest striking faults are right-slip. Thiswould imply that these structures are conjugate fault setsthat accommodate coeval east-west extension and north-southcontraction. In thefollowing, weshow that conjugatestrike-slip fault systems are common structures in central

Tibet, which all together accommodate a portion of Indo-Asian convergence.

3.1. Dong Co Conjugate System

[10] The east facing Dong Co conjugate fault system iscomprised of the northeast striking Yibug Caka fault zonetothenorth and thenorthwest striking Gyaring Co fault to

the south (Foldout 1 and Figure 1). Wehave mapped thebedrockgeologyof theconjugatefault systemon1:100,000scale topographic maps. In thevicinityof active-to-recentlyactive faults, we mapped the Quaternary stratigraphy onCORONA satellite photographs. Quaternary units weredelineated based on their relative surface heights androughness, degree of surface vegetation and incision, andsurfacealbedoobserved in satellite images andin thefield.3.1.1. Left-Slip Yibug Caka Fault Zone

[11] The340kmlongYibugCakafaultzoneisS-shapedin map view, with the northern and southern segments

striking N70E and the central segment striking N30-40E (Foldout 1 and Figures 1 and 2). The northern andsouthern fault segments are dominantly left-slip, whereasthe central fault segment is dominantly normal-slip (Fig-ures 2b2d). Associated with along-strike variation offault strike and kinematics is a change in topographicrelief. Local relief is 500 m along the strike-slip seg-ments of the fault zone, but increases to >1000 m alongthenormal-fault segment (Figure 2a).

[12] The Riganpei Co fault defines the southern Yibug-Caka fault zone and strikes N70E. Locally, it exhibitssub-horizontal fault striations (Figure 2). The fault offsetsleft-laterally a north dipping thrust at its northeast end for7 km (Figure 2a). The thrust juxtaposes Permian lime-stones over folded Tertiary strata. The Riganpei Co faultlocally offsets alluvial fans and paleo-shorelines, asobserved near Riganpei Co (Figure 6). The Riganpei Cofault becomes buried, or tips out at its southern end at theZhaxi Co and Dong Co basins (Figure 2a). Althoughnortheast striking fault scarps are locally observed nearZhaxi Co, thesefaults disappear to thesouth. Thenorthernend of the Riganpei Co fault links with an extensionalsystem in thecentral Yibug-Caka fault zone.

[13] Locally, pressure ridges, sag ponds, and offsetalluvial fans are observed along the Riganpei Co fault(Figure 3). Field observations allow us to identify fourchronostratigraphic units (Qa1Qa4 fromoldest to young-est, Figure3). The right stepping, en echelon alignmentofthe fault traces and ridges is consistent with left-slipmotion. South flowing streams arealso offset left laterally(Figure 4). Field measurements of the offset gullies andbeheaded channels range from 30100 m. The variationsin horizontal displacement presumably reflects therelativeages of the offset geomorphic features with the least

incised and smoothest (i.e., youngest) and relatively loweralluvial surfaces displaying the smallest offsets. Locally,shutter ridges are present along the active trace of theRiganpei Co fault withan averagetrendof N70E. Shutterridges are typically several hundred meters in length andtens of meters in height. They are asymmetric in cross-section, where the north sides of the shutter ridges aresteep and coincident with the sub-vertical fault surface(Figure 4).

[14] In thevicinity of Riganpei Co, regressiveshorelinesareobserved with overlyingyounger alluvial sediments. Inturn, thealluvial sedimentsarecutbyseveral anastomosingstrands of the Riganpei Co fault (Figure 5). Left-lateraloffsetof thealluvial fansisobservedontheir easternmargin

where a north facing fault scarp indicates left-lateral anddip-slip displacements of 25 5 m and 2 0.5 m,respectively, asdetermined in thefield withatapemeasureandBrunton clinometer (Figure5).A left-lateral offset of achannel is also 255 mat this site(Figure5a). Most lakesand closed basins in Tibet are surrounded by regressiveshorelines. An example fromwest central Tibet is LongmuCo, where the shorelines were leveled from 75250 mabove the present lake level [Avouac et al., 1996]. AmodifiedPenman formulawasusedtocalculateasyntheticcurveof theshorelineregression at Longmu Co [Avouac et

TAYLOR ET AL.: CONJUGATE STRIKE-SLIP FAULTING WITHIN TIBET 18-5


6/25

Figure2

.

(a)Simplifiedtectonicmapofthe

YibugCakafaultzone,

Qiangtang.(b

),(c),and(d)arelowerhemisphere

stereographicprojectionsoffaultkinematicda

ta.

GTF=Gangtangfault,WYF=W

estYibugCakafault,EYC=East

YibugCakafault,JF=Jiamoufault,BZF=B

uZangAifault.Seecolorversionof

thisfigureatbackofthisissue.



7/25

al., 1996]. It was determined that a rapid shorelineregres-siondroppedthelakelevel byabout160m, whichoccurredfor a duration of 120 years after the lakes reached theirhighest stand at 7.6 ka [Avouac et al., 1996]. Althoughthe exact causefor theshorelineregression is unknown, itmay bearesult of a regional climatic effect[Avouac et al.,1996].

[15] The Riganpei Co fault terminates into the exten-sional Riganpei Co basin (Figures 2a and 6). Faulting isobserved along the western margin of the basin and isexpressed by several southeast and northeast facing scarpscutting alluvial fans (Figure6). Three topographic profileswere surveyed across the southeast facing scarps using atape measure and Brunton clinometer, which indicate a7.5 mvertical offset (Figure6). The profiles consist of 67(profile 1), 76 (profile 2), and 97 (profile 3) measured

points, with an uncertainty derived fromreading the slopeangle, which is 1.

[16] The central Yibug-Caka fault zone trends N30-40E and is characterized by several fault-bounded tiltedblocks and interlying NNE trending basins in between(Figure 2a). The range-bounding faults include the Gang-tang Co, West Yibug Caka, East Yibug Caka, and Jiaomufaults, whichstrikeNNE and cut both bedrock and alluvialsediments (Figure 7). The West Yibug Caka fault is eastdippingandis geometrically linkedtotheRiganpei Co fault(Figure 2a). To the north, the West Yibug Caka faultbecomes antithetic to the East Yibug Cakafault and trans-fers WNW-ESE extension to two west dipping normalfaults: theEastYibugCakaandJiaomufaults. Tothenorth,the north striking faults merge with Bu Zhang Ai left-slipfault in thenorthernsegmentof theYibug-Cakafault zone.

Figure 3. Relationships between tectonic landforms, Quaternary units, and the Riganpei Co fault.

(a) CORONA satellite imageof thefault traceassociatedwith transpressional structures.Theenechelonalignmentof theridgesandtheright-steppingfaulttracesindicatesalocal strain field consistentwithleft-slip motion. (b) Quaternary geologic map based onfield observations made in thesouthwestern part ofthemap areaandinterpretationof CORONA imagery in (a). Themap shows locations of fault strands,offset drainages, asag pond, andobliquely oriented pressureridges withrespect to thefault trace. Notethat apressureridgeis observed wheretheleft-slip fault steps right. Seetext for details oncriteriausedfor separatingmapped units. SeeFigure2afor location.



8/25

[17] The NNE striking faults in the central fault zoneare normal-slip as indicated by dip-slip fault striations(Figure 2c), toolmarks, mullions, and Riedel shears(Figure 8a). The striation data were collected only fromthe range-bounding faults, as minor faults located awayfrom the main fault zone may be unrelated to the YibugCaka fault. The best fit mean slip direction for westdipping faults is 326, 56(Figure 2c).

[18] Thecentral block in thenorthern half of thecentralYibugCakafault zoneexposesblueschist-bearing melange.Its exhumation is associated with motion along the early

Jurassic Rongma detachment fault, which is related to aregional early Mesozoic extensional event that occurredthroughout thecentral Qiangtang[Kapp et al., 2003; Kapp,2001]. TheRongmadetachmentfault is regionallyextensiveandformsakey marker horizonfor restoringthemagnitudeof extension across theactive normal faults in the central

YibugCakafaultzone. Althoughtheuncertaintiesarelarge,aminimumof 2kmof east-westextension is accommodatedby the NNE striking faults that are distributed across thecentral Yibug Cakafault zone[see Kapp et al., 2003].

[19] The Bu Zang Ai fault segment of the Yibug Cakafault zone strikes N73E and is dominantly left-slip.

Locally, sub horizontal striations and tool marks areobservedonfault surfacesandindicateameanslip directionoriented 073, 09(Figure2d). Weestimatethemagnitudeof left slip along this fault segment tobe14kmbased onthe left-separation of a north dipping Tertiary thrust. Thethrust juxtaposing Paleozoic limestones over Tertiary redbeds is truncated by two strands of the strike-slip systemthat strike N55E (Figures 2a and 9). The thrust isoroclinally folded as it approaches the fault zone from amoderatedip of 47in theeast, andbecomes subvertical inthewest(Figure9).Wesuggestthatsynkinematic transformmotion related to thrusting in theQuaternary is unlikely, asno thrust fault scarps areobserved, and locally Quaternarysediments unconformably overlie the thrust contact.

[20] Although weonly visited thesouthernmost part ofthe northern rift segment, the morphologically defined

Yibug Caka rift extends for at least another 50 km to thenorth (Figure 2a). From topography and existing activetectonic maps of the region [Liao, 1990], it appears thatthe Yibug Caka rift transfers displacement to a series ofnorthtrendingriftsin thenorth.Theserifts in turn, appear toterminate into a series of ENE striking left-slip faults thatweconsider asthewesternmostextension of theseismically

Figure 4. (a) CORONA satellite imagery of neotectonic features alongapotentially active fault zone.Landforms include offsets of drainages, shutter ridges, and sag ponds that areconsistent with left-slipmotion. (b) L ine drawing of the major geologic features observed in the field and shown in (a). SeeFigure2a for location.



9/25

active left-slip Kunlun fault system [Kidd and M olnar,1988; Yin, 2000; Va n d e r Wo e rd e t a l ., 2000, 2002]. Thisinferenceis consistentwiththeleft-slipfocal mechanismsofthe M 7.6 and M 7.9 Manyi and Kokoxilli earthquakes,whichoccurred alongtheinferred left-slip faults(Foldout1and Figure1) [Peltzer et al., 1998; L a ss e r re e t a l ., 2002].3.1.2. Right-Slip Gyaring Co Fault

[21] From regional mapping, topographic maps, andLandsat images, the left-slip Riganpei Co fault in thesouthern Yibug Caka fault zone merges to the south withtheright-slipGyaringCo fault in thenorthernLhasaterrane(Foldout1andFigure1).This relationshipsuggests that thetwo faults may form a conjugate strike-slip set, whichwould require a comparable magnitude of displacementacross the two faults. Right-slip faults directly south ofthe Bangong-Nujiang suture, namely the Karakorum-J ialifault zone [Armijo et al., 1986, 1989], are kinematicallyrelatedtonorthtrendingrifts further tothesouth[M ercier etal., 1987; Burchfiel et al., 1991; Ratschbacher et al., 1994;

Harrison et al., 1995; C og an e t a l., 1998]. Four of thesestrike-slipfaults, theKarakorumfault, theGyaringCo fault,

theJiali fault, andtheBengCofault werestudiedby Armijoet a l. [1989]. However, that study emphasized the neo-tectonic activity of these structures and provides no con-straintsonthemagnitudeof faultslip.Thus,our study, withanemphasisondeterminingpiercingpointsacrosstheright-slip faults in the northern Lhasa terrane compliments thework of A rm i jo e t a l .[1989]. Theresults of our investiga-tion ontheGyaring Co fault aredescribed below.

[22] TheGyaring Co fault is expressed as a linear valley250 km long that strikes N70W. Its northern endmerges withthesouthernendof theYibug Cakafault zonealongtheBangong-Nujiangsuture, whereasitssouthernendis kinematically linked with the north trending Pum-Qu-Xainza rift (Foldout 1 and Figure 1). Our fieldwork con-centrated on the northern fault segment where the faultoffsets Mesozoic and Early Tertiary east trending contrac-tional structures and strata (Figure 10a). The fault is sub-vertical and well exposed along the northwest shore ofGoman Co. Striations in the fault zone are dominantlyhorizontal andthefault zonegougecanreachtensof metersin thickness. Locally, the trace of the fault cuts alluvial

Figure 5. (a) Field photo looking WSW along a fault scarp that cutsalluvial fans along the northernsegment of theRiganpei Co fault. Thefault scarp is northfacing and offsetsadrainagefor30 mleftlaterally. (b) CORONA imagery of the fault trace. (c) Linedrawingof geologic relationships based onfield mapping andinterpretation of (b). Notethat younger alluvial deposits, which arecut by the fault

strands,depositionally overlie theregressiveshorelinesasshownbythegreen arrows. SeeFigure2aforlocation.



10/25

sedimentsandisassociatedwith sagponds, pressureridges,offset alluvial fans and terrace risers, all consistent withright-slip motion.

[23] The magnitude of slip on the Gyaring Co fault isconstrained by restoring a piercing point defined by theintersection linebetween a Tertiary unconformity (definedby Tertiary stratadeposited on top of Mesozoic strata) andthe Goman-Caibu Co thrust system, both of whicharecutby the strike-slip fault (Figures 10 and 11). The Goman-Caibu Co thrust system juxtaposes Early Cretaceous lime-stonesoveryoungerCretaceousvolcanicrocks, andTertiarysandstonesandconglomerates,all offset bytheGyaringCofault. Theageassignmentof stratigraphic units observed inthe study area is adopted from Liu[1988]. Because theGoman-Caibu Co thrust system cuts Tertiary strata, theoffset of this structure by the Gyaring Co fault can beattributed to its LateCenozoic motion. A best fit planetothe Goman-Caibu Co thrust is oriented N75E, 24NW(Figure 10d). Measurements of the footwall stratigraphicbeddingandtheunderlyingunconformity indicateabestfitplaneoriented N81E, 14NW (Figure10c). Thetrendandplunge of the intersection line between the two best fitplanes is 257, 01(Figure10b). Theoffset calculation ofthis intersection line uses a best fit orientation of theGyaringCo fault.Faultzonefabricsarewell exposedalong

the north shore of Goman Co. Measurements of faultsurfaces indicate a best fit fault plane oriented N70W,89NE (Figure 10d). We project the intersection linebetween the Goman-Caibu Co thrust systems and the

Tertiary unconformity into the plane of the Gyaring Cofault. The intersection line pierces the Gyaring Co fault inthe west and east sides at the 4710-m and the 4610-mstructure contours, respectively. A calculated net slip is12.54 kmoriented 110, 5. A minor normal componentof 100 m is calculated from the total slip vector. Theuncertainty in the slip calculation is introduced due touncertainties in the location of the intersection line, whichresult fromerrors associated with the fault orientation andtheattitude of theTertiary unconformity.

[24] A similar magnitude of right separation along theGyaringCo faultmay beinferredtothenorthwest, basedonthe 12 km of right separation of a north dipping thrust

juxtaposing Mesozoic carbonates over Mesozoic volcanicrocks(Figure10). Theinitiationageof theGyaringCo faultis unknown.

3.2. Bue Co Conjugate System

[25] The Bue Co conjugate fault system is located400 km west of the Dong Co conjugate system andconsists of two fault segments that conjoin at Bue Co

Figure 6. (a) CORONA imageryof theRiganpei Co basinat thenorthernendof theRiganpei Co fault.(b) Line drawing of key geologic relationships as observed in the field and interpreted from(a). Bothnormal and strike-slip faults cut Quaternary alluvial deposits in the transtensional structure. (c) Faultscarpprofiles surveyed withtapemeasureandBrunton clinometer. See(b) for locationof scarpprofilesand Figure2a for location.



11/25

Figure 7. (a) CORONA imagery showingwesternmargin of arangefront bounded byawest dippingnormal fault. Northwesttrending, whitearrowindicates location of normal fault in Figure7b. Southeast

trending, whitearrow at therangefront indicates locationof Figure8a; symbol onalluvial fan indicatesviewing location and viewing direction for Figure 8b. (b) A west dipping normal fault offsetting thecontactbetween Quaternary fan deposits aboveandthePaleozoic stratabelow, view is tothenorth. See(a) for location.

Figure 8. (a) Downdipfault striations, toolmarksandreidel shearsonsurfaceof awestdippingnormalfault, view to the east. Location of this outcrop is indicated in Figure 7aby southeast trending whitearrow. (b) West dipping normal fault, view to the ESE. SeeFigure7a for location.



12/25

(Figure11). TheBueCo fault strikes N70E and forms thenorthern branch of thestrike-slip conjugatesystemandtheLamu Co fault strikes N55W and forms the southernbranch. The Bue Co fault forms a broad valley up to10kmwideand80kmlong. Alongstriketothenortheast,the fault merges with two northeast dipping faults thatbound the elongate basins (Figure 12). The main Bue Cofault displays an apparent left-oblique normal-slip compo-nent (Figure 14). This interpretation is based on the pres-

enceof a25 kmlong and 7 kmwide crescent-shapedbasin that is bounded on its east sideby the southern BueCo fault. We interpret this basin geometry to result fromleft-slip motion along a releasing bend (Figure 12). Theactive nature of the fault is suggested by its trace cuttingseveral alluvial fans (Figure 13a). The magnitude of left-lateral motion along the Bue Co fault is 12 km, asestimated fromleft separation of a Permian limestoneunit[C he ng a nd X u, 1987] observed in Landsat-7 imagery(Figure13).

[26] TheLamu Co fault strikesN55W and consists oftwo segments (Figure13). Theoverlapping areaof thetworight stepping fault segments resembles a pull-apart basin.If correct, this implies that theLamu Co fault is right-slip.

The northern fault segment strikes N50W and is45 kmlong. Fault scarps and triangular facets are visible inLandsat images, suggestingthat thisstructuremay beactive(Figure13a). Jurassic volcanicstrata[C h e n g a n d X u, 1987]at location A-A0 are truncated by the fault and display11 kmof right separation (Figure14). Thesouthern faultsegment of theLamu Co fault is also visibleonLandsat-7images(Figure13a). It iscurvilinear inmapview, is>95kmlong, andtruncates east-west striking Jurassic strata.

[27] Distinct Jurassic volcanic rocks appear as adark redlithologic unit in the center of Figure 13a. This unit

coincideswithasouth-directedthrustfault thatwasmappedon the western side of the Lamu Co fault [C he ng a nd X u,1987]. This contact is truncated by the Lamu Co fault atlocationB in Figure13b. A red lithologic unit with similarcharacteristicsandgeometry is observedontheeasternsideof the Lamu Co fault as recognized from the Landsat-7image (B0 on Figure 13b). Because the two units appearsimilar, wesuggest that they wereoncethe same unit thathasbeen cut by theLamu Co fault. This implies 175km

of right separation along theLamu Co fault.[28] Thetriangular block bounded by the conjugateBue

Co and Lamu Co faultsappears tobeundergoingeast-westextension. This inference is suggested by the presence ofthree north trending range fronts with triangular facetsbounding their western margins (Figure 13a). Althoughthedataaresparse, weinterpret this rangefrontmorphologyto represent active-to-recently active faulting (Figure 13a).Basedonthelocationsof theadjacentbasinswithrespecttothe north-south strike of the range-bounding features, weinfer theseto represent westdipping extensional structures.A basin is located at the tip of the wedge-shaped blockbounded by the Bue Co and Lamu Co faults. This basinappearsbetranstensional [Harland, 1971] in originbecauseof theeastward extrusion of thesmall wedge-shaped blockthat leaves a spaceat its trailing end. A prediction of thismodel is thatthekinematicsof range-boundingfaultswouldhaveadominant componentof obliquestrike-slip motion.

3.3. Aishui Co Conjugate System

[29] The Aishui Co conjugate fault system is located70 kmwest of theBue Co conjugate fault system. Thisfault systemconsists of theleft-slip Xianqie Co fault in thenorth and the right-slip Awong Co fault in the south.Investigation of this conjugate fault system is based on

Figure 9. Geologic map of the Bu Zhang Ai fault zone. Strike-slip faults cut a north dipping thrustalong thenorthern rift segment of theYibug Cakarift. See Figure2a for location.



13/25

Figure10.

(a)Simplifiedgeologicmapofthe

northwestsegmentoftheGyaringCofaultlocatedintheLhasaterrane.See

Foldout1

forlocation.

(b),(c),(d):Lowerhemi

sphere,

equal-areastereonetsshowing

orientationdataoftheGoman-Caibu

Cothrust

faults,T

ertiaryunconformity,andtheGyaringCofaultusedintheslipestimateoftheGyaringCofault.GMCT=

GomanC

othrust,

CCT=CaibuCothrust.

Seecolorversionofthisfigureatbackofthisissue.



14/25

digital topography, satellite images, and reinterpretation ofexisting geologic maps[Liu, 1988; C he n g a n d X u, 1987].3.3.1. Left-Slip Xianqie Co Fault System

[30] TheXianqie Co fault is comprised of two principalfault segments that strike N65E and are defined by thepresence of three lakes (Figure12). The southern segmentof the Xianqie Co fault system begins at the QuaternaryAishui Co basin in the south and links with the northernfault segment at Xianqie Co. The northern fault segmentcontinues from Xianqie Co in the south and appears toterminate at Lumajangdong Co in the north (Figure 12).

The Aishui Co basin may be associated with an easttrending contractional structure whereas the Xianqie Cobasin may be related to the N25W striking J ieze Cakafault. The latter displays normal slip as suggested bytriangular facets observed in CORONA satellite imagery(Figure14a). The JiezeCaka fault continues further to thewest and changes strike to N70W (Figure 12). Thenorthern segment of the Xianqie Co fault zone continuesto the northeast and links with an apparent west dippingtranstensional fault. This range-bounding fault strikesN10E and appears to control thelocation of the Luma-

jangdong Co basin (Figure 12).[31] Left-slip kinematics along the Xianqie Co fault is

indicated by CORONA satellite photographs that show aprominent south facing curvilinear fault scarp and associ-atedleft-slipdisplacementof aterraceriser (Figures14cand14d).Threedifferentagesof terracesurfacescanbeinferredbased on the surface texture and albedo observed in theCORONA images(Figure14d). They rangefromT3for theephemeral to active drainage, to T1 for the oldest terracesurface. TheT1-T2 terraceriser displaysaleft-lateral offsetof100 montheeastern sideof thesouth-flowing stream(Figure14c). Thewesternterraceriser displaysnoapparent

offset, which we attribute to erosion by the active streamchannel andwhich is consistentwith left-slip motion.3.3.2. Awong Co Fault System

[32] Located south of the Bangong-Nujiang suture zoneis theAwong Co fault, whichweinterpret to be thestrike-slip conjugate to theXianqie Co left-slip fault (Figure12).

Therange-bounding Awong Co fault strikes northwest andis curvilinear in map view. The northern segment of theAwong Co fault strikes N65W and is sub parallel tothe Bangong-Nujiang suturezone. However, to the south,the Awong Co fault becomes more oblique to the suturezone and strikes N45W. Spatially corresponding to thechangein faultstrikearereleasingbendsasindicatedbythelocation of Awong Co and Nier Co. This fault geometrysuggests right-slip motion along the Awong Co fault. Anorth dipping thrust is shown in themap of C h e n g a n d X u[1987] juxtaposing Jurassic flysch over Cretaceous strata,which is truncated by the Awong Co fault (Figures 13aand 13b). We infer horizontal separation along the appa-rently active structure, however this inference does notpreclude vertical motion. The magnitude of right lateralmotion alongthesouthern segmentof the AwongCo faultis unknown.

4. Discussion

4.1. Kinematics of Active Deformation in Central Tibet

[33] The geologic observations presented above suggestthat conjugate strike-slip faults are significant structuresalong the Bangong-Nujiang suture zone in central Tibet.Observations of strike-slip and normal faults near theBangong-Nujiang suture zone are consistent with recentlyobtained fault-planesolutions of earthquakes in the region[Langin et al., 2001]. Thepresenceof conjugatestrike-slipfaultsalongan elongatebelt across central Tibet raises thequestion of their role, both in accommodating convergencebetween India and Asia in general, and specifically, theirrelationshiptonorthtrendingriftslocatednorthandsouthofthestrike-slipsystems.IntheQiangtangterrane, theleft-slipfaults are linked with NNE striking normal faults, asdemonstrated along the Yibug Caka, Bue Co, and AishiCo conjugatefaultsystems. IntheLhasaterrane, it hasbeenlong recognized that right-slip faults near the Bangong-Nujiang suture zone are kinematically linked with northtrending rifts [Armijo et al., 1986, 1989] (Foldout 1 andFigure1). Thusrifts in theQiangtangandLhasaterranesarekinematically linked by aseries of conjugatesetsof strike-slip faultsinbetween. Thismodeof transferringextensionisdifferentfromthat alongmid-oceanridgeswheretransformfaults oriented perpendicular to normal faults is the rule.

The difference may be explained by contrasting strainfields. Along themid-ocean ridges, ridge-parallel compres-sion is probably negligible. However, in Tibet rift-parallel(i.e., north-south) compression is significant as induced bythe Indo-Asian collision.

[34] Thepresenceof conjugatestrike-slip faultsin centralTibet isarefinementof thesimpleeastwardextrusionmodelof arigid Qiangtang[Armijo et al., 1989]. Ourobservationsincorporatethe left-slip faults of the Qiangtang, where the

Figure 11. A simple block diagramillustrating the offsetrelationshipsof theGoman-CaibuCo thrustsystemsalongasimplified traceof theGyaring Co fault.



15/25

conjugatestrike-slip fault geometry together with theTibe-tan rifts facilitates coeval north-south contraction andeast-west extension. Because east-west extensional strain isdistributed over central Tibet and is accommodated by theeastward extrusion of aseriesof small wedges bounded byconjugate strike-slip faults, the overall mode of east-westextensionmay beexpressedaseastwardspreading of the

Tibetan plateau (Figure 15). Because the magnitude ofstrike-slip displacement is similar to a first-order for theactive northeast and northwest striking strike-slip faults incentral Tibet, we interpret this kinematic pattern to besuggestiveof aconstrictional strain, asoriginally postulatedfor Tibet by M ercier et al.[1987].

[35] Figure 15a is a schematic kinematic model for theactive tectonics of central Tibet. The left-slip Kunlunfault in the north and the right-slip Karakorum-J iali fault

zone in the south assist eastward spreading of Tibetancrust. To the east, crustal material turns toward thesoutheast and is bounded by the Ganzi-Xianshuihe-Xiao-

jiang fault system in the north and the J iali-Red Riverfault system in the south. This kinematic pattern isconsistent with the model of R a t sc h ba c he r e t a l .[1996]and recent GPS results from eastern Tibet [Royden et al.,1997; Ch en e t a l., 2000].

[36] If fault-bounded blocks are rigid, conjugate strike-slip faultingalongtheBangong-Nujiangsuturezonewouldresult in a gap at the trailing edge of the eastward movingwedge(Figure16). Thespacemay beclosed bymovementalongeast strikingthrust faults(i.e., atrailingzipper), orbyvertical-axis rotation of the surrounding rock massandthestrike-slip faults thatboundthewedge-shapedblock.Thrustearthquakes and folding in the late Cenozoic basins along

Figure 12. A simplifiedfaultmapshowingthegeometryof theBueCoandAishui Coconjugatestrike-slip fault systems. SeeFoldout 1 for location.



16/25

the Bangong-Nujiang suture zone would beexpected withnorth-south contraction of thecentral Tibetan crust. How-ever, this prediction is inconsistent with the absence ofthrust earthquakemechanisms in central Tibet [M olnar and

Lyon-Caen, 1989; L an g in e t a l ., 2001] andthe lack of easttrending folds in the late Cenozoic strata along the Ban-gong-Nujiangsuturezonebased onour own field observa-tions. Thus, we favor the geometry and kinematic patternshown in Figure 16. This would explain the presence ofnumerous Quaternary basins such as theSiling Co, Lum-pola, and Dong Co basins along the Bangong-Nujiang

suturezone (Foldout 1 and Figures 1 and 15). Perhaps thelow elevation of the Bangong-Nujiang suture zone isinduced by the extrusion of small crustal wedges boundedby conjugate strike-slip faults. Oblique-slip kinematics ispredicted for thebasin-bounding faults in this model.

[37] Ithasbeenpostulatedthatmajor Cenozoicstrike-slipsystems in central Tibet reactivate older suture zones[Allegre et al., 1984]. However, from field mapping, wefoundnoevidencethat theconjugatestrike-slip faults in thecentral Tibet fault zone have been reactivated from olderstructures such as Mesozoic sutures and Tertiary thrusts.

Figure 13a. Landsat-7 image was processed with an RGB color combination using channels 7-4-1.Note the prominent northeast and northwest striking faults. See Figure 12 for location and text for adiscussion. Seecolor version of this figureat back of this issue.



17/25

Instead they consistently truncate and offset these olderstructures. If the strike-slip faults formed by theCoulombfracturemechanism, their intersection angleacross theeastfacing fault-bounded wedge should be about 120, assum-ing a typical internal friction angle of 30. However, theobserved intersection angle between conjugate strike-slipfaults is only 7090. This implies that left-slip faultsandfault-boundedblocksintheQiangtangterranemayhaverotated 1525clockwise, whereas right-slip faults andfault-bounded blocksin theLhasaterranemay haverotated1525counterclockwise. However, the magnitude offault slip along thestrike-slip faultsis only12km, which

would be inconsistent with a predicted fault rotation of1525. Thus,wepreferamechanismthat includesafault-bendfold mechanismonmapview that would not requirealarge magnitude of fault rotation (Figure 16) [Peltzer andTapponnier, 1988]. This prediction may betested by futurepaleomagnetic studies.

4.2. North-South Contraction and East-WestStretching of Central Tibet

[38] If weuse12kmas theaveragemagnitudeof strike-slipdisplacementalongindividual faults, atotal of 12kmof

Figure 13b. Simple linedrawingof geologic relationshipsshownin Figure13a. AA0 andB B0 showaright separation of adistinct lithologic units. Seetext for adiscussion.



18/25

Figure1

4.

(a)CORONAimageoftheXianq

ieCofault.Notetheconjoiningfault

segmentscomprisedofthenortheast

strikingXianqieColeft-s

lipfaultandanorthweststrikingnormalfaultthatdefinestherangefront.SeeFigure12for

location.

(b)Linedrawingofkeygeologicrelationshipsinterpretedfromtheimageshownin(a).(c)CORONAimagery

showing

left-s

lipdisplacementofaterraceris

eralongtheXianqieCofault.(d)Interpretivelinedrawingshowingkey

geologic

relationshipsin(c).See(a)forlocation.



19/25

north-south contraction should have been absorbed bymotion on the conjugate left- and right-slip faults acrossthe Bangong-Nujiang suture zone between 31240N and33300N along the longitude of 86E. Using the onset ageof Tibetan rifts as a proxy for the initiation age of thekinematically linkedconjugatestrike-slipsystems in central

Tibet, which isbetween 14and8Ma[Blisniuk et al., 2001;Harrison et al., 1995], the corresponding rates of north-

south contraction across the strike-slip fault zone are12 mm/yr. This rateof contraction accounts for 4% of theIndo-Asian convergence rate if one assumes a total con-vergence rate of 3747 mm/yr [Demets et al., 1990;G ord on e t a l., 1999; S he n e t a l., 2000]. However, eventheupper bound of our long-termrateis significantly lowerthanthatdeterminedbyaGPS surveyacrossthesamerangeof latitudewhereour estimatewascarriedout.Althoughthe

Figure 15. Schematic kinematic model of theactive tectonics in central Tibet. (a) Conjugatestrike-slipfaults accommodate coeval east-west extension and north-south contraction in the interior of Tibet.Closely spaced (


20/25

uncerta inties are large, the north-south contraction rate is5 3 mm/yr as determined by the GPS survey of Wa ng e t al. [2001]. This implies that the interior of Tibet may haveaccomm odated an insignificant amount of north-sou th con-traction on a timescale of millions of years than thatdetermined by GPS studies on a timescale of several years.

[39] In comparison to the Karakoram fault , individualconjug ate strike-slip fault systems in central Tibet displayless finite strain by at least a factor of 5 (i.e., 12 km versus65 km) [M urphy et al., 2000]. The Altyn Tagh fault exhibitsthe largest displaceme nt with500 km of left-slip motion[Peltzer et al., 1988; Yi n a nd H ar ri so n, 2000; Yin et al.,2002] . However, the summation of eastward motion alongthe strike slip faults in central Tibet may be signific ant(Figure 15a). Specifically, inte grating the east-west compo-nent of individual fault s in this study accounts for 33 kmof east-west stretching, which is more significant t hannorth-s outh co ntracti on by app rox imately a f acto r of 3.We su gg est t hat thi s is li kel y a mi ni m um est im at e , a snorthea st and northwest striking strike-slip faults are presentto the east and west of the study area presented in this paper.

Thus, the magnitude of east-west stretching may be as highas60 km if one includes slip along the Beng Co and Jialistrike -slip faults in east central Tibet, and the Longmu Cofault to the west (Foldo ut 1 and Figure 1). If our correlationis correct, it implies that the cumulative magnitude of east-west stretchi ng across central Tibet could be of sim ilarmagnitude to the Karakoram fault. In turn, the Qiangtangterrane could not behave as a rigid block because, if thiswere the case, the magnitude of horizontal motion alongindividual faults (i.e., the Karakoram , Gyaring Co, BengCo, and Jiali faults) [Armijo et. al., 1989] would each be65 km (Figure 15b). When summed together, the totalamoun t of eastward motion along the Karakora m fault and

the central Tibet conjugate fault zone is at least

125 km.[40] At a timescale of a few years, geodetic studies suggestthat the slip-rate along the Altyn Tagh fault is10 mm/yr orless [Bendick et al., 2000; Shen et al., 2001], and the long-term behavior of this structure suggests that it may slip at9 mm/yr, which would be consiste nt with steady sta tebehavior [Yi n et a l., 2002]. A recent study of the interm edi-ate-t erm behavior of the Karakoram fault suggests a rela-tively low slip rate along this structure at3mm/yr[Brown etal., 2002]. In central Tibet a recent InSAR investigationsuggeststhesouthernsegmentof theYibugCakafault zone(i.e., theRiganpei Co fault) may bemovingat ashort-termratethat may beashighas5mm/yr [Ta y l o r e t a l ., 2002].

5. Conclusions

[41] Thisstudy reveals a200300kmwideand1500kmlongeasttrendingzoneof conjugatestrike-slip faultsacrosscentralTibet.ThecentralTibetconjugatefaultzoneconsistsofnortheast striking left-slip faults north of the Bangong-Nujiangsutureandnorthwest strikingright-slip faultssouth

ofthesuture.TheconjugatefaultsmergetowardtheBangong-Nujiang suture zone and accommodate coeval east-westextension andnorth-southcontraction.ThreeconjugatefaultsetsincentralTibetwereinvestigated.TheDongCoconjugatesystemadjacenttothecentral Bangong-Nujiangsuturezoneconsists of thenortheast striking left-slip Yibug Cakafaultnorth of the suture and the northwest striking right-slipGyaring Co fault south of the suture. TheBue Co and theAishui Co conjugatesystems arelocated along thewesternBangong-Nujiang suture400 km west of the Dong Coconjugatesystem. TheBueCo fault systemconsists of thenortheast striking left-slip Bue Co fault in the north andthe northwest striking right-slip Lamu Co fault in the south.

TheAishui Cofaultsystemconsistsof thenortheaststriking

left-slip Xianqie Co fault in the north and the northweststriking right-slip Awong Co fault in the south. Weuse anaveragemagnitudeof horizontal fault motion of12km, asdeterminedbyoffsetsandseparationsof TertiarythrustsandPaleozoic-Mesozoic lithologic units. This conjugate faultconfiguration requires a minimum of12 km of north-southcontraction and >31 km of eastward extrusion to have beenaccommodated across the 200300 kmwide central Tibetconjugatefault zone. Becausetheconjugatestrike-slipfaultsarekinematically linkedwithnorthtrendingTibetanriftsthatinitiated between 14 and8Ma, our estimated magnitudeofcontraction implies a north-south contraction rate of 12 mm/yr and >2 4 mm/yr of east-west extension acrosscentral Tibet. The presence of a series of conjugate fault

systemsintheinteriorofTibetsuggeststhatthecentralTibetancrust has been spreading eastward via distributed eastwardextrusionof small (


21/25

Burchfiel, B. C., Z. Chen, L. H. Royden, Y. Liu, andC. Deng, Extensional development of the Gabovalley, southern Tibet, T e c t o n o p h y s i c s, 194 ,187193, 1991.

Cattin, R., andJ. Avouac, Modelingmountain buildingand the seismic cycle in the Himalaya of Nepal,

J . Ge ophy s. Re s., 105, 13,389 13,407, 2000.Chen, Z., B. C. Burchfiel, Y. Liu, R. King, L. H.

Royden,W. Tang, E. Wang, J. Zhao,andX. Zhang,

Global positioning system measurements fromeastern Tibet and their implications for India/Eur-asiaintercontinental deformation, J . Ge ophy s. Re s.,105, 16,215 16,227, 2000.

Cheng, J., andG. Xu, Geologicmapof theRituregionwith report (in Chinese), 598 pp., Tibetan Bur. ofGeol. and Miner Resour., Chengdu, Peoples Re-public of China, 1987.

Cogan, M. J., et al., Shallow structures of theYadong-Gulu Rift, southern Tibet, fromrefraction analysisof Project INDEPTH common midpoint data, Tec-tonic s, 17 , 46 61, 1998.

Coleman,M. E., andK . V. Hodges,EvidenceforTibe-tan plateau uplift before 14 m.y. ago froma newminimumagefor east-westextension, Nature, 374,4952, 1995.

Demets, C., R. G. Gordon, D. F. Argus, andS. Stein,Current platemotions, G e o p h ys . J . I n t ., 101, 425478, 1990.

Dewey, J. F., andK. Burke, Tibetan,Variscan andPre-cambrianbasement reactivation: Products of conti-nental collision, J . G e o l og y, 81, 683 692, 1973.

Dewey,J .F.,R. M. Shackleton,C. Chengfa,andS.Yiyin,ThetectonicevolutionoftheTibetanPlateau, Philos.

T r a n s . R . S o c . L o n d o n , S e r . A, 327 , 379 413,1988.Edwards,M. A., andT. M. Harrison,Whendidtheroof

collapse? Late Miocene north-south extension inthe High Himalaya revealed by Th-Pb monazitedating of the Khula Kangri granite, Ge ology, 25,543546, 1997.

England, P., andG. Houseman, Extension duringcon-tinental convergence,withapplicationtotheTibetanplateau, J . Ge ophy s. Re s., 94,17,561 17,579,1989.

England,P., andP. Molnar,Thefieldof crustal velocityin Asia calculated fromQuaternary rates of slip onfaults, G e o ph y s , J . I n t ., 130, 551 582, 1998.

Garizone, C. N., D. L. Dettman, J. Quade, P. DeCelles,andR. F.Butler, HightimesontheTibetanplateau:Paleoelevation of the Thakkola graben, Nepal,Ge ology, 28, 339 342, 2000.

Gordon, R. G., D. F.Argus, andM. B. Heflin, Revisedestimateof theangular velocity of Indiarelative toEurasia, Eos T rans. AGU , 80(46), F273, Fall Meet.Suppl., 1999.

Harland, W. B., Tectonic Transpression in CaledonianSpitzbergen, G e o l . M a g ., 108, 27 42, 1971.

Harrison, T.M., P. Copeland, W. S. F.Kidd, andO. M.Lovera, Activation of the Nyainqen-tanghla shearzone:Implicationsforuplift of thesouthernTibetanPlateau, Tectonics, 14, 658 676, 1995.

Kapp, P. A., Tectonic evolution of theQiangtang ter-raneand theBangong-Nujiang suturezone, central

Tibet, 398 pp., Ph.D thesis, Univ. of Calif., LosAngeles, 2001.

Kapp, P.,A. Yin,C. E. Manning, T.M. Harrison, M. H.Taylor, and Lin Ding, Tectonic evolution of theearly Mesozoic blueschist-bearingQiangtangmeta-morphic belt, central Tibet, T e c t o n i c s, 22 ,doi:10.1029/2002TC001383, in press, 2003.

Kidd, W. S. F., and P. Molnar, Quaternary and activefaulting observed on the 1985 Academia Sinica-Royal Society Geotraverse of Tibet, Philos. T rans.

R. Soc . L ondon, Se r. A, 327 , 337 363, 1988.Kong,X., andP.Bird,NeotectonicsofAsia:Thin-shell,

finite-elementmodelswithfaults,in T h e T e c t o ni c s o f Asia, editedby A. Yin andT.M. Harrison, pp. 1835, CambridgeUniv.Press, New York,1996.

Langin,W. R.,L. Brown, andE. Sandvol,SeismicityofCentral Tibet from Project INDEPTH II I seismicrecordings, E o s T r a n s. A G U , 82(47), Fall Meet.Suppl., Abstract S31D-06, 2001.

Lasserre, C., G. Peltzer, J . Van der Woerd, andY. Kl inger, Coseismic deformation from the

M w=7.8 Kokoxilli, Tibet earthquake, FromERS

InSAR data, E o s T ra n s. A G U , 83(47), Fall Meet.Suppl., Abstract G61B-0987, 2002.

Liao,K., (Ed.),Atlasof theQinghai-XizangPlateau,237pp., Inst. ofGeogr.,Sci.Publ. House,Beijing,1990.

Liu, Z., Geologic map of theQinghai-Xizang Plateau,Geol. Publ. House, Beijing, 1988.

Masek, J. G., B. L. Isacks, andE. J. Fielding, Rift flankuplift in Tibet: Evidence for aviscous lower crust,

Tectonics, 13, 659667, 1994.Mercier, J. L., R. Armijo, P. Tapponnier, E. Carey-Gailhardis, and T. L. Han, Change from Tertiarycompression to Quaternary extension in southern

Tibet during the India-Asia collision, Tectonics, 6 ,275304, 1987.

Meriaux, A.-S., P. Tapponnier, F. J. Ryerson, J . vander Woerd, C. Lasserre, X. Xu, R. Finkel, andM. Caffee, Large-scale strain patterns, great earth-quake breaks, and late Pleistocene slip-rate alongtheAltyn TaghFault (China), J . Conf. Abstr. EUG,4(1), 55, 1999.

Meyer, B. P., P. Tapponnier, L. Bourjot, F. Metivier,Y. Gaudemer, and G. Peltzer, Crustal thickening intheGansu-Qinghai, lithospheric mantle-subductionand oblique, strike-slip controlled growth of the

Tibetan plateau, G e o p h ys . I n t ., 135, 1 47, 1998.Molnar,P., andH. Lyon-Caen,Fault planesolutionsof

earthquakesandactivetectonics of theTibetanPla-teauanditsmargins, G e o p h y s . J . I n t ., 99, 123 153,1989.

Molnar, P.,andP.Tapponnier,Activetectonicsof Tibet, J . Ge ophy s. Re s., 83, 5361 5373, 1978.

Murphy, M., A. Yin, T. M. Harrison, Ding Lin, andGuo J inghui, Southward propagation of theKara-koramfault system, southwest Tibet: Timing andmagnitudeof slip, Ge ology, 28, 451 454, 2000.

Owens, T. J., and G. Zandt, Implications and crustalproperty variations for models of Tibetan plateauevolution, Nature, 387 , 37 43, 1997.

Peltzer, G., andP.Tapponnier,Formationandevolutionof strike-slip faults, rifts, and basins during theIndia-Asia collision: An experimental approach,

J . Ge ophy s. Re s., 93, 15,085 15,117, 1988.Peltzer, G., F. Crampe, and G. King, Evidence of

nonlinear elasticity of the crust from the M w7.6

Manyi (Tibet) Earthquake, S c i e n c e, 286 , 272276, 1998.

Ratschbacher, L., W. Frisch, G. Liu, and C. C. Chen,Distributed deformation in southern and western

Tibet during and after the India-Asian collision, J . Ge ophy s. Re s., 99, 19,917 19,945, 1994.

Ratschbacher, L., W. Frisch, Changsheng Chen, andGuitang Pan, Cenozoic deformation, rotation, andstresspatternsin easternTibetandwesternSichuan,China, in T h e T e c t on i c s o f A s i a, edited by A. YinandT. M. Harrison, pp. 227 249,CambridgeUniv.Press, NewYork, 1996.

Rothery, D. A., and S. A. Drury, The neotectonics oftheTibetan plateau, Tectonics, 3, 19 26, 1984.

Royden, L. H., B. Burchfiel, R. King, E. Wang,Z. Chen, F. Shen, and Y. Liu, Surfacedeformationand lower crustal flow in eastern Tibet, Sc ie nc e,276 (5313), 788790, 1997.

Searle, M. T., Geological evidenceagainst large-scalepre-Holocene offsets along the Karakoram Fault:Implications for the limited extrusion of the Tibe-tan, Tectonics, 15, 171 186, 1996.

Searle, M. P., R. F.Weinberg, andW. J. Dunlap,Trans-pressional tectonics along the Karakoram faultzone, northern Ladakh: Constraints on Tibetan ex-trusion, in Contine ntal T ranspre ssional and T rans-te nsional T e ctonic s, edited by R. E. Holdsworth,R. Strachan, and J . F. Dewey, G e ol . S o c. S p ec .

Publ., 135, 307 326, 1998.Searle, M. P., M. Asif Khan, J. E. Fraser, S. J. Gough,

and M. Qasim Jan, The tectonic evolution of theKohistan-Karakoramcollisionbelt alongtheKara-koramHighwaytransect,NorthPakistan, Tectonics,18, 929 949, 1999.

Shen, Z., C. Zhao, A. Yin, Y. Li, D. J ackson, P. Fang,and D. Dong, Contemporarycrustal deformation ineastAsia constrainedbyGlobal PositioningSystem

measurements, J . G e o p h y s . R e s ., 105, 57215734,2000.

Shen, Zheng-Khang, M. Wang, Y. Li, D. Jackson,A. Yin, D. Dong, andP. Fang, Crustal deformationalong theAltynTagh fault system, western China,fromGPS, J . G e o p h y s . R e s ., 106 , 30,607 30,621,2001.

Tapponnier, P., J. L. Mercier, R. Armijo, T. Han, andT.J . Zhao, Fieldevidenceforactivenormal faulting

in Tibet, Nature, 294, 410 414,1981.Taylor, M.,G.Peltzer, A. Yin, F. J. Ryerson, R. Finkel,and D. Lin, Active deformation in Central Tibet:Quantitative results from InSAR and geologic ob-servations, E o s T ra n s. A G U , 83(47), Fall Meet.Suppl., Abstract T51B-1153, 2002.

Van der Woerd, J., F. J. Ryerson, P. Tapponnier, A.-S.Meriaux, Y. Gaudemer, B. Meyer, R. Finkel,M. Caffee, Zhao Gouguang, and Xu Zhiqin, Uni-formslip-ratealongthe Kunlun fault: Implicationsfor seismic behavior and large-scale tectonics,G e o p hy s . R e s . L e t t ., 27 , 2353 2356, 2000.

Van der Woerd, J., P. Tapponnier, F. J. Ryerson, A.-S.Meriaux, B. Meyer, Y. Gaudemer, R. C. Finkel,M. W. Caffee, andX. Zhiqin, Uniformpost-glacialslip-rate along the central 600 km of the Kunlunfault (Tibet), from 26Al, 10Be, and 14C dating ofriser offsets, and climatic origin of the regionalmorphology, G e o p h y s . J . I n t ., 148, 356 388,2002.

Wang, Q., et al., Present-day crustal deformation inChina constrained by global positioning systemmeasurements, Sc ie nc e, 294, 574 577, 2001.

Williams, H., S. Turner, S. Kelley, and N. Harris, Ageand composition of dikes in southern Tibet: Newconstraintsonthetimingof east-westextensionandits relationship topost-collisional volcanism, Ge ol-ogy, 29, 339 342, 2001.

Wu, C.,K. D. Nelson,G.Wortman, S.Samson,Y. Yue,J. Li, W. S. F. Kidd, and M. A. Edwards, YadongcrossstructureandSouthTibetandetachment in theeast central Himalaya (8990E), Tectonics, 17 ,28 45, 1998.

Yin, A.,Modeof Cenozoiceast-westextensioninTibetsuggestsacommonoriginof riftsinAsiaduringtheIndo-Asian colli sion, J . G e o ph y s . R e s ., 10 5,21,745 21,759, 2000.

Yin, A., andT.M. Harrison, Cenozoicevolutionof theHimalayan-Tibetan orogen, A n n u. R e v. E a r t h , P l a -n e t S c i ., 28, 211 280, 2000.

Yin,A.,T.M. Harrison,F.J .Ryerson,W.J. Chen,W.S.F.Kidd,andP.Copeland,Tertiary structural evolutionof theGangdesethrustsysteminsoutheasternTibet,

J . Ge ophy s. Re s., 99, 18,175 18,201,1994.Yin, A., P. A. Kapp, M. A. Murphy, T. M. Harrison,

M. Grove, L. Ding, X. Deng,andC. Wu,SignificantlateNeogeneeast-westextension in northernTibet,Ge ology, 27 , 787 790,1999a.

Yin, A., T. M. Harrison, M. A. Murphy, M. Grove,S. Nie, F. J. Ryerson, X. F. Wang, and Z. L. Chen,

Tertiary deformation history of southeastern andsouthwestern Tibet during the Indo-Asian colli-sion, G S A B u l l ., 111(11), 1644 1664, 1999b.

Yin, A., et al., A Tectonic history of the Altyn Taghfault systemin northern Tibet inferred fromCeno-zoic sedimentation, G S A B u ll ., 114(10), 12571295, 2002.

L. Ding, Institute of Geology and Geophysics,

Chinese Academy of Sciences, Beijing 100029, Peo-ples Republic of China.

P. Kapp, Department of Geosciences, University ofArizona, Gould-SimpsonBuilding, Tucson, AZ 85721,USA.

F. J. Ryerson, Instituteof GeophysicsandPlanetaryPhysics, Lawrence L ivermore National Laboratory,Livermore, CA 94550, USA.

M. Taylor and A. Yin, Department of Earth andSpaceSciences,University of California, 3651GeologyBuilding, Los Angeles, CA 90095-156702, USA.([email protected])



22/25

Figure

1.

ColorshadedreliefmapwithacompressedpaletteofcentralTibets

howingabroadeast-westtrending

depressio

n(indarkblue)alongtheBangong-Nujiangsuturezone.

Notethatthed

epressionismarkedbyaseriesof

discontin

uousbasins.

Strike-slipfaultsarerepresentedbynarrowandlineartopograp

hicstructurestrendingnortheastand

northwes

t,northandsouthofthecentraldepre

ssion.Theselinearfeaturesarethelef

t-slipfaultsintheQiangtangterrane

andright-slipfaultsintheLhasaterrane.Notethatright-slipfaultsintheLhasaterraneboundthenorthernmarginof

severaln

orthtrendingrifts,whereasleft-s

lip

faultsintheQiangtangterraneareconnectedwithtopographicallyless

pronouncednorthtrendingrifts.Thepoortop

ographicdefinitionofriftsinQiangtangmayresultfromtheinternally

drainedb

asinfilling[Yinetal.,

1999b],

orbec

auseofhorizontalmotionassociatedwithstrike-s

lipfaulting.

TAYLOR ET AL.: CONJUGATE STRIKE-SLIP FAULTING WITHIN TIBET

18- 4


23/25

Figure 2. (a) Simplified tectonic map of theYibug Caka fault zone, Qiangtang. (b), (c), and (d) are lostereographic projectionsof faultkinematicdata. GTF =Gangtangfault, WYF =WestYibugCakafault, EYCakafault, JF =Jiamou fault, BZF =Bu Zang Ai fault.

18

-

6


24/25

Figure 10. (a) Simplifiedgeologic mapof thenorthwestsegmentof theGyaringCo fault locatedin theLhFoldout1for location. (b), (c), (d): Lower hemisphere, equal-areastereonetsshowingorientationdataof thCothrustfaults, Tertiaryunconformity, andtheGyaringCofault usedintheslipestimateof theGyaringCo

Goman Co thrust, CCT =CaibuCo thrust.

18

-

13


25/25

Figure 13a. Landsat-7 image was processed with an RGB color combination using channels 7-4-1.Note the prominent northeast and northwest striking faults. See Figure 12 for location and text for adiscussion.

TAYLOR ET AL.: CONJUGATE STRIKE-SLIP FAULTING WITHIN TIBET

taylor 2003 tectonics

Documents