geologic offsets across the northern karakorum fault ......slivers of carbonate 25–35 km long,...

Earth and Planetary Science Letters 279 (2009) 123–130

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Earth and Planetary Science Letters

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Geologic offsets across the northern Karakorum fault: Implications for its role andterrane correlations in the western Himalayan-Tibetan orogen

Alexander C. Robinson ⁎Department of Earth and Atmospheric Sciences, 312 Science & Research 1, University of Houston, Houston, TX, 77204-5007, USA

terrane

⁎ Tel.: +1 713 743 3571; fax: +1 713 748 7906.E-mail address: [email protected].

0012-821X/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.epsl.2008.12.039

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While the N1000 km long

Received 4 September 2008Received in revised form 23 December 2008Accepted 24 December 2008Available online 3 February 2009

Editor: T.M. Harrison

Keywords:KarakorumoffsetTibetstrike-slip

Karakorum right-slip fault is one of the most prominent structures in theHimalayan-Tibetan orogen, considerable disagreement exists as to the total magnitude of offset across thefault and the role it has played in accommodating convergence between India and Asia. Using satelliteimages, I have correlated a well defined carbonate unit, the Late Triassic-Early Jurassic Aghil formation, acrossthe northern portion of the Karakorum fault from the southwest Pamir to the Tianshuihai terrane of westernTibet. The northern and southern exposure limits of the Aghil formation have 149–167 km of separation,which I interpret to represent the magnitude of displacement on the northern Karakorum fault. These valuesoverlap with estimates correlating the Bangong-Nujiang and Shyok sutures across the central Karakorumfault and maximum offsets of the Miocene Baltoro Granite and rule out correlations of the Bangong-Nujiangsuture and the Rushan Pshart suture. The displacement yields a geologic slip rate of 10.8±1.3 mm/yr using aninitiation age of 14.7±1 Ma, or 6.89±0.8 mm/yr with an initiation age of 23±1 Ma. This result supportsprevious work showing limited offset across the Karakorum fault and indicates that the fault that has notaccommodated either large magnitudes (i.e. 100's of km) of eastward lateral extrusion of Tibet or hundreds ofkilometers of offset between terranes of the western and central portions of the orogenic belt.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Although the N1000 km long right-slip Karakorum fault is one ofthe most morphologically prominent structures along the westernportion of the Himalayan-Tibetan orogen (e.g. Molnar and Tapponnier,1978) (Fig. 1), there is considerable debate regarding its roll in theorogenic belt. In one set of models, the Karakorum fault plays a majorrole in accommodating north–south convergence between India andAsia by facilitating eastward lateral extrusion of the Tibetan Plateauand/or accommodating significant northward displacement of thePamir-Karakorum region relative to the Tibetan Plateau (Tapponnieret al., 1982; Peltzer and Tapponnier, 1988; Armijo et al., 1989; Lacassinet al., 2004; Schwab et al., 2004; Valli et al., 2007; Valli et al., 2008). Keyto these models is the prediction of hundreds of kilometers of slipalong the Karakorum fault. In another set of models, the role of theKarakorum fault is more limited, either acting as a transfer structurelinking trust belts in the Pamir andwestern Tibet and/or accommodat-ing outward radial growth of the Himalayan arc (Burtman andMolnar,1993; Searle,1996; Searle et al.,1998; Seeber and Pecher,1998;Murphyet al., 2000). These models require far less displacement along theKarakorum fault (∼66–150 km) and predict the fault has accommo-

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dated minor offset of terranes between the Pamir-Karakorum regionandwestern Tibet. Determining themagnitude of displacement acrossthe Karakorum fault is thus critical for understanding: (1) the role ofthe Karakorum fault, and regional strike-slip faults in general, in theHimalayan-Tibetan orogen, (2) whether the fault has accommodatedsignificant eastward lateral extrusion of the Tibetan Plateau, (3) thecorrelation of tectonic terranes between the Pamir-Karakorummountains to the west and the Tibetan plateau to the east, and (4)long-term slip rates along the Karakorum fault (e.g. Molnar andTapponnier, 1975; Tapponnier et al., 1982; Armijo et al., 1989).

While early estimates suggested total magnitude of slip along theKarakorum fault of up to 1000 km (Peltzer and Tapponnier, 1988),recent studies addressing offset features along the fault have proposeda range of slip magnitudes from 65 km (Murphy et al., 2000) to≥400 km (Lacassin et al., 2004; Schwab et al., 2004; Valli et al., 2008).Larger estimates for the magnitude of displacement are based oncorrelating the Bangong-Nujiang suture from central Tibet with theRushan-Pshart zone of the Central Pamir which yields ≥400 km of slip(Fig. 1). This correlation is based in part on similarities between thetwo suture zones as well as interpreted correlations of magmatic beltsbetween the South Pamir and Lhasa terranes (Schwab et al., 2004).Another proposed offset feature supporting larger offsets across theKarakorum fault is the correlation of antiformal domes in the CentralPamir with the Qiangtang anticlinorium which yields ∼250 km ofslip (Schwab et al., 2004). However, these domes have also been

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Fig. 1. Simplified tectonic map of the Indo-Asian collision zone showingmajor active structures and suture zones (modified after Burtman andMolnar, 1993; Yin and Harrison, 2000).MPT–Main Pamir thrust; IYS–Indus-Yalu suture; BNS–Bangong-Nujiang suture; JS–Jinsha suture; KS–Kunlun suture; SS–Shyok Suture; RPZ–Rushan Pshart Zone; TS–Tanymas Suture .Terranes of the western Indo-Asian collision zone are: 1–Northern Pamir; 2–Central Pamir; 3–South Pamir–Karakorum; 4–Kohistan arc.

124 A.C. Robinson / Earth and Planetary Science Letters 279 (2009) 123–130

interpreted to continue east of the inferred Karakorum fault into thefootwall of the active Kongur Shan extensional systemwith no lateraloffset (Robinson et al., 2007). Lower estimates of displacement acrossthe fault have instead correlated the Bangong-Nujiang suture with theShyok suture (Fig. 1) which yields ∼120 km of offset (Searle, 1996).This correlation is consistent with other interpreted offsets along thecentral portion of the Karakorum fault such as the Miocene Baltorogranite which is offset between 40 and 150 km (Searle, 1996; Searleet al., 1998; Phillips et al., 2004) and the 120 km offset of the course ofthe Indus river (Searle, 1996). However, there is disagreement as towhether these features post-date the initiation of the fault and wouldtherefore represent minimum displacements only (Lacassin et al.,2004; Valli et al., 2007; Valli et al., 2008). The lowest slip estimates arefrom the southern portion of the Karakorum fault where a north-directed thrust fault (the South Kailas thrust) has been correlatedacross the Karakorum fault yielding 66±5.5 km of right-lateraldisplacement (Murphy et al., 2000). However, this correlation hasalso been challenged, with other studies interpreting the faultsmapped by Murphy et al. (2000) to be part of a flower structurerelated to the Karakorum fault (Lacassin et al., 2004).

Most studies involving direct field observations along theKarakorum fault have focused on the southern half due in part todifficult terrane and sensitive political borders along its northern half.In this paper I present observations from satellite images that identifya well defined lithologic unit, the carbonate Aghil formation, whichcan be reliably correlated across the northern half of the Karakorumfault from the southeastern Pamir to the Tianshuihai terrane ofwestern Tibet (the westward continuation of the Qiangtang terrane).

2. Geologic setting

2.1. The Karakorum fault

The right-slip Karakorum fault runs for N1000 km across thewestern margin portion of the Himalayan-Tibetan orogenic belt from

the southwestern Tibetan Plateau to the Pamir, separating the Pamir-Karakorum mountains from the Tibetan Plateau (Fig. 1). At itssoutheastern end, the Karakorum fault links with the Gurla Mandhatadetachments system and continues into the Himalayas (Murphy et al.,2002; Murphy and Copeland, 2005). A portion of the strain on theKarakorum fault is also interpreted to continue along the Indus-Yalusuture zone to the east (Lacassin et al., 2004) (Fig. 1). At its northernend, the Karakorum fault is interpreted link with north directed thrustfaults of the Rushan Pshart zone in the central Pamir (Burtman andMolnar, 1993; Strecker et al., 1995) (Figs. 1 and 2).

While the trace of the southern half of the Karakorum fault is welldefined by active fault scarps, active deformation along the northernhalf of the fault has not been documented. However, geologicmappingand observations from satellite images along the northern half of theKarakorum fault in the Shaksgam Valley and southern TashkorganValley regions have documented the trace of the fault. In theShaksgam Vally region, the Karakorum fault zone consists of severalsplays which surround the main trace of the fault (Searle and Phillips,2007). The most prominent of these is the Shaksgam fault whichstrikes ∼N20°W and extends for N50 km to the east into theTianshuihai terrane (Searle and Phillips, 2007) (Fig. 2). Althoughdextral offset of glaciers along the Shaksgam fault have been reported(Searle and Phillips, 2007), exposures of the Aghil formation (the focusof this paper) show left-lateral separation leaving the kinematics ofthe Shaksgam fault unclear. Other than the Shaksgam fault, all otherminor splays related to the Karakorum fault in the Shaksgam valleyregion do not appear on published geologic maps of the region andlikely have minimal displacement.

To the north along the southern end of the Tashkorgan Valley inthe southeast Pamir, the Karakorum fault splits into two identifiablestrands (Figs. 2 and 3); an eastern strandwhich is themain trace of theKarakorum fault (previously referred to as the Kalagilu fault, Robinsonet al., 2007), and a western strand referred to as the Achiehkopai fault.These strands bound a block ∼100 km long and up to 15 km wide(Fig. 2). Further north the trace of the faults are difficult to discern but

Fig. 2. Simplified tectonic map of the western end of the Himalayan-Tibetan orogen on top of a mosaic of ASTER SWIR images (RGB: bands 4, 6, and 8), Landsat images, and SRTMshaded relief 90 m DEM projected in UTM zone 43N. The Late Triassic-Early Jurassic Aghil formation is seen as the prominent yellow unit on the ASTER SWIR images.

Fig. 3. (A) Subset of Fig. 2 focusing on the Karakorum fault and displaced Aghil formation. (B) Interpretation of the satellite images showing distribution of the Aghil limestone, trace ofthe Karakorum and Achiehkopai fault and associated faults, and offset markers.

125A.C. Robinson / Earth and Planetary Science Letters 279 (2009) 123–130

Fig. 4. ASTER SWIR image from the northern end of the Achiehkopai Fault. Southwardbending of the Aghil formation into the Achiehkopai fault indicates a component ofdistributed simple shear next to the fault (i.e. drag folding) with ∼7 km of displacementwithin the zone.


are interpreted to reconnect, becoming the East Pamir fault beforelinking with thrust faults along the Rushan Pshart Zone (Burtman andMolnar, 1993; Strecker et al., 1995). Within the southeast Pamiranother fault zone, the Aksu-Murgab fault zone, splays off theAchiehkopai fault (Fig. 2). While the fault has been cited as havingup to 60 km of displacement (Burtman and Molnar, 1993), myobservations of satellite images don't show any significant displace-ment across the fault zone. Several other small faults spay off theKarakorum fault in the southeast Pamir north of the Achiehkopai fault(i.e. the Karasu fault, Strecker et al., 1995). The magnitude of right-slipon these faults is not well determined, but may be up to 20 km(Burtman and Molnar, 1993).

2.2. Distribution of the Aghil formation

The Late Triassic-Early Jurassic Aghil formation is part of theregional Shaksgam sedimentary belt and consists of thick bedded tomassive fossiliferous carbonates (Gaetani et al., 1990a,b; Burtman andMolnar, 1993; Gaetani, 1997). A critical feature of carbonates is thatthey have a distinct signature in the SWIR (short wavelength infrared)bands recorded by the ASTER (Advanced Spaceborn Thermal Emissionand Reflection Radiometer) instrument on the Earth ObservationSystem Terra satellite. This signature yields a prominent yellow onfalse color images using an RGB combination of SWIR bands 4, 6, and 8(Figs. 2 and 3). Due to lack of suitable ASTER images in some regions(i.e. the Tianshuihai terrane) interpretations are augmented byLandsat images and regional geologic maps to extrapolate theobservations further from the Karakorum fault (Figs. 2 and 3).Attempts to match other geologic features across the fault based onspectral signature are ambiguous at best.

From the east, the Aghil formation can be traced within theTianshuihai terrane of western Tibet for ∼250 km trending ∼N55°Wbefore it is abruptly truncated along the trace of the ∼N35°W strikingKarakorum fault, immediately northeast of K2 (Figs. 2 and 3). Severalslivers of carbonate 25–35 km long, 2–5 km wide, and striking sub-parallel to the Karakorum fault are exposed along the fault trace for∼100 km to the north (Fig. 3). These slivers are interpreted to beportions of the Aghil formation caught up within a ∼5 kmwide shearzone along this portion of the Karakorum fault. To the west of theKarakorum fault the Aghil formation is exposed at the southern end ofthe Tashkorgan valley within the central portion of block bounded bythe Karakorum and Achiehkopai faults. The Aghil formation thencontinues west of the Achiehkopai fault into the southeast Pamir for∼150 km, initially trending ∼N55°W but changing to ∼N70°W 25 kmwest of the Karakorum fault zone (Fig. 2).

As the documented exposures the Aghil formation are the onlyregionally extensive carbonate unit observed in the satellite images, itis highly unlikely that the correlation of exposures identified on eitherside of the Karakorum fault is incorrect. In further support of theproposed correlation, the aerial distribution of the Aghil formation issimilar on both sides of the Karakorum fault. Within ∼40 km of theKarakorum fault to both the east and west, exposures of the Aghilformation are ∼25 km in width perpendicular to the trend of the belt(Fig. 3). Exposures of the Aghil formation also getwider in north–southextent further from the Karakorum fault in both directions to ∼35–45 km (Fig. 2). A final aspect to note is that in both the Tianshuihaiterrane and the southeastern Pamir exposures of the Aghil formationare bounded to both the north and south by Carboniferous to Triassicsedimentary units (Liu, 1988; Yin and Bian, 1992; Upadhyay, 2002)demonstrating that the basal contact of the formation is exposed on allmargins of the belt.

3. Displacement calculations

In addressing the magnitude of displacement across the northernKarakorum fault zone, I focus on displacements across the Karakorum

and Achiehkopai faults. While the Shaksgam fault may have accom-modated significant displacement (i.e. kilometers to tens of kilometers)the unknown sense of slip on the faultmakes evaluating its contributionto strike-slip displacement on the Karakorum fault impossible.Additionally, while some slip may have been partitioned onto theAksu-Murgab fault, the amount of displacement is likely low andwouldnot affectmy results outside theuncertainties present. All othermappedfault splays of the Karakorum fault zone in the southeastern Pamirbranch off north of the exposures of the Aghil formation and do notaffect the calculated displacements of the formation.

I use three different measurements to evaluate the magnitude ofdisplacement of the Aghil formation across the northern portion of theKarakorum fault: (1) separation along the fault of the northernmarginof the Aghil formation exposures; (2) separation along the fault of thesouthern margin of the Aghil formation exposures; and (3) northwarddisplacement of the along-strike projection of themargins of the Aghilformation across the fault.

1) Separation of the northern margin of the Aghil formation can beaccurately measured as its intersection with the Karakorum fault andAchiehkopai faults are well defined in the satellite images. Across theKarakorum fault the northern margin of the Aghil formation is offset129 km (A–A′, Fig. 3). An additional 14 km of offset along theAchiehkopai fault striking N10°W yields 13 km of separation in theregional direction of strike of the Karakorum fault (N35°W) (B–B′,Fig. 3). Another important feature is that the Aghil formation appearsto have undergone a component of distributed simple shear (i.e. dragfolding) east of the Achiehkopai fault (Fig. 4). This distributed shearhas resulted in ∼7 km of additional northward displacement of thenorthern margin of the Aghil formation. These offsets yield a total of149 km of right-lateral separation of the Aghil formation across thenorthern Karakorum fault zone.

2) Separation of the southern margin of the Aghil limestone ismore difficult to determine as its intersection with the Karakorum


fault is heavily glaciated in the Shaksgam Valley to the south, coveredby surficial deposits in the southern Tashkorgan Valley to the north,and its intersection and interaction with the Achiehkopai fault ispoorly resolved. Additionally, a small spur of limestone south of theintersection of the Shaksgam and Karakorum fault complicates theinterpretation of where the southern boundary of the Aghil formationlies. The southern margin of the Aghil limestone is separated by 130–147 km along the main trace of the Karakorum fault (C–C′ and C–C″respectively, Fig. 2B) yielding a total of 150–167 km of separation(adding the separation from the Achiehkopai fault and distributedshear determined above).

3) The northern and southern exposure margins of the Aghilformation strike ∼N55°W within the Tianshuihai terrane of westernTibet, as well as within 25 km to thewest of the Karakorum fault in thePamir (Fig. 3). If the margins of the Aghil formation are projectedwestward from the Tianshuihai terrane along a strike of N55°Wacrossthe Karakorum fault, they lie 65–67 km to the south of their equivalentmargins in the southeastern Pamir which is equivalent to 160–156 kmof displacement of the Karakorum fault (Fig. 3). It should be notedhowever, that the strike of both the Karakorum fault and trend ofthe Aghil limestone are not perfectly linear. Taking these uncer-tainties into account and using a 35±2° strike for the Karakorum faultand a N55±3°W strike for the margins of the Aghil formation yields159+27/−35 km of displacement. Though less precise than directlymeasuring displaced features along the fault (given the possibility ofblock rotations during slip along the Karakorum fault and otherregional deformation), this measurement has the advantage that ittakes into account possible distributed shear around the fault notaccounted for in the calculations above. As the results are consistentwith the separations measured directly along the Karakorum andAchiehkopai faults, it suggests that there is no significant componentof distributed deformation along the northern Karakorum fault zonethat is unaccounted for in my calculations.

Finally, there are several sources of uncertainty regarding thegeologic history of the region that could result in my calculatedseparations not accurately representing the amount of strike-slipdisplacement on the northern Karakorum fault. Nonetheless, I arguethat these uncertainties are not likely to be significant, and thatthe separations measured represent an accurate determination ofthe amount of slip on the northern Karakorum fault. 1) The Aghillimestone has been subjected to internal shortening since deposition(Gaetani et al., 1990b; Burtman and Molnar, 1993; Gaetani et al.,1993) as has the rest of the southeast Pamir and Tianshuihai terrane(e.g. Matte et al., 1996). If shortening occurred during slip along theKarakorum fault, different amounts of deforamtion on either side ofthe fault could result in separations that do not represent theamount of slip. However, much of the deformation in the Pamirand Tianshuihai terrane is interpreted to be pre-Cenozoic in age,(Burtman and Molnar, 1993; Matte et al., 1996), indicating this is nota significant source of error as internal shortening likely occurredprior to initiation of slip on the Karakorum fault. 2) As the Aghillimestone is not a vertical feature and the base of the formation isexposed on both the northern and southern margins of the belt ofexposures, differential erosion of its margins during motion on theKarakorum fault could also result in apparent offsets that do notrepresent the amount of slip. An argument against this significantlyaffecting the calculated displacements is that the north–south widthof the exposures perpendicular the trend of the Aghil formation isroughly the same (∼25 km) on either side of the Karakorum faultzone for ∼40 km (Figs. 2 and 3). If there had been differential erosionof the margins during motion on the Karakorum fault, the north–south width of the exposures would likely be different on either sideof the fault. Based on these arguments, I feel that the 149–167 km ofseparation of the Aghil formation accurately represents the amountof strike-slip displacement on the northern half of the Karakorumfault.

4. Discussion

4.1. Correlation of suture zones and tectonic terranes

The identified offset of the Aghil formation along the northernKarakorum fault of 149–167 km resolvesmuch of the debate regardingpreviously matched offset features across the fault. One of the morepersistent of these is whether the Bangong suture zone correlates withthe Shyok suture zone of the Karakorum, or the Rushan Pshart Zone ofthe Pamir. A critical point to make is that regardless of the accuracy ofmy calculated magnitude of displacement along the northernKarakorum fault, the relative position of the exposures of the Aghillimestone to these suture zones directly addresses this issue. As thedocumented offset Aghil formation lies south of the Rushan PshartZone west of the Karakorum fault and north of the Bangong-Nujiangsuture east of the fault (Fig. 5) the correlation between the two suturezones is no longer viable. Instead, my result supports correlations ofthe Jinsha suture to the Rushan Pshart Zone, and Shyok to Bangong-Nujiang suture zones (Searle, 1996; Searle et al., 1998; Yin andHarrison, 2000; Phillips et al., 2004), the latter of which is offset by asimilar amount as the Aghil formation (85–120 km, Searle, 1996;Phillips et al., 2004). Additionally, exposures of the Aghil formation lieroughly the same distance south of the Rushan Pshart Zone as theJinsha suture zone of western Tibet (Fig. 3) supporting the conclusionthat they are equivalent sutures.

Another previously proposed offset feature across the northernKarakorum fault is the correlation of the metamorphic-rock coredcentral Pamir anticlines with the Qiangtang anticlinorium of northernTibet (Schwab et al., 2004) which yields ∼250 km of displacement. Aswith the suture zones, the exposures of the Aghil formation lie to thesouth of the Central Pamir antiforms west of the fault and north of theinferred western continuation of the Qiangtang anticlinorium east ofthe fault (Fig. 5). This shows that the antiformal structures of thePamir and Qiangtang are not equivalent and provides further supportto the interpretation that the Central Pamir antiforms are not offset bythe Karakorum fault, but rather continue east into the footwall of theKongur Shan normal fault (Robinson et al., 2007).

The documentation of the offset Aghil formation also partiallyresolves the correlation of tectonic terranes across the Karakorumfault. This study clearly documents the correlation of the South Pamir-Karakorum terrane to the Tianshuihai-Qiangtang terrane of theTibetan Plateau (e.g. Gaetani et al., 1990b; Burtman and Molnar,1993; Gaetani, 1997; Yin and Harrison, 2000; Upadhyay, 2002; Searleand Phillips, 2007) (Fig. 5), and invalidates correlations between theSouth Pamir and Lhasa terranes (e.g. Schwab et al., 2004). Thedocumented offset Aghil formation also show that the southeasternPamir are not the westward continuation of the Songpan-Ganziterrane (Yin and Harrison, 2000; Robinson et al., 2004) as the Aghilformation lies south of the Jinsha suture in the Tianshuihai terrane(Fig. 5). Further, my results support the interpretation that ultramaficrocks exposed near Shiquanhe in western Tibet are an allochthonouspart of Bangong suture zone (Kapp et al., 2003) rather than a separatesuture zone (Matte et al., 1996). The latter interpretation had beenargued for by proponents of the correlation of the Rushan-Pshart andBangong suture zones (Lacassin et al., 2004; Valli et al., 2007, 2008) astheir reconstruction left no other correlatable feature for the Shyoksuture zone. This interpretation left the difficulty of having hugegradients in the magnitude of displacement across the Karakorumfault with the Shyok-Shiquanhe suture zones offset only ∼200 kmwhile the Rushan Pshart-Bangong suture zones were offset ≥400 kmdespite the Bangong and Shiquanhe sutures being separated by only∼80 km (e.g. Valli et al., 2008). While this led Valli et al. (2008) tosuggest the presence of a proto Karakorum fault zone along thenorthern half of the fault, the documented 149–167 km of displace-ment on the northern Karakorum fault of the late Triassic-earlyJurassic Aghil formation firmly rules out this possibility.

Fig. 5. Simplified tectonic map of the western end of the Himalayan-Tibetan orogen with offset features along the Karakorum fault from previous work consistent with this study.Offset features: 1) Murphy and Copeland, 2005; 2) Murphy et al., 2002; 3) Murphy et al., 2000; 4) Searle, 1996; 5) Searle et al., 1998 and Phillips et al., 2004; 6) Searle et al., 1998,Phillips et al., 2004; 7) This study; 8) Robinson et al., 2007.


Several important features should be noted however whichsuggest a one to one correlation of terranes and pre-Cenozoic tectonichistory between the Pamir-Karakorum region and Tibetan plateau isoverly simplistic. Two of the most important of these are; 1) thepresence of ophiolitic fragments immediately south of the Aghilformation in the southern Pamir (Schwab et al., 2004), which havebeen interpreted to represent a major terrane boundary (e.g. Yin andHarrison, 2000), and 2) a possible suture zone identified in thewestern Karakorum region, consisting of ultramafic rocks exposedalong the Tirich Mir Fault Zone (Zanchi et al., 2000). While the latterwere interpreted to represent a fragmented crust-mantle boundaryfrom a thin portion of continental crust rather than a true suture zone(Zanchi et al., 2000), exposures of ophiolitic material in the southernPamir point to a complicated regional geologic history that may differfrom that along strike to the east in the Tibetan Plateau. Furthercomplicating the tectonic architecture of the Pamir is the terraneaffiliation of the Central Pamir, which is bound to the south by theRushan Pshart Zone and to the north by the Tanymas suture (Figs. 1and 5) (Burtman and Molnar, 1993). This region has been variouslycorrelated with the Songpan-Ganzi terrane or Qiangtang terrane ofthe Tibetan Plateau (Yin and Harrison, 2000; Schwab et al., 2004), orhas been interpreted to be a separate terrane not found east of the

Karakorum fault (Burtman and Molnar, 1993). These results correlat-ing the Southern Pamir with the Qiangtang terrane, and evidenceshowing the northern Pamir are most likely equivalent to theSongpan-Ganzi terrane (Schwab et al., 2004), support the latterinterpretation. Lastly, the northward deflection of contemporaneousCretaceous magmatic belts from the western portion of the orogenicbelt in the South Pamir-Karakorum region relative to the centralportion of the orogenic belt in the Lhasa terrane documented bySchwab et al. (2004) illustrates significant along-strike differences inthe pre-Cenozoic tectonic evolution of the Himalayan-Tibetan orogen.

4.2. Geologic slip rates

Using offset geologic features across a fault to determine long-termslip rates requires; 1) the feature to predate initiation of slip, and 2)knowing the initiation age of the fault. While the Late Triassic-EarlyJurassic Aghil formation clearly predates initiation of the CenozoicKarakorum fault, recent studies along the fault have yielded verydifferent initiation ages. Dating different generations of igneousintrusions with variable amounts of deformation along the Bangongtranspressional zone, as well as petrologic evidence against submag-matic deformation of the oldest Miocene granites, yielded an


initiation age for the Karakorum fault of 15.7–13.7 Ma (Phillips et al.,2004; Phillips and Searle, 2007). However, similar studies on igneousrocks with variable amounts of ductile deformation in the Ayi Shanalong the southern Karakorum fault yielded an older initiation ageof ≥22–25 Ma (Lacassin et al., 2004; Valli et al., 2007, 2008). Usingthese two different initiation ages (14.7±1 Ma and 23±1 Ma) the 149–167 km of displacement of the Aghil limestone along the northernKarakorum fault yields long-time slip rates of 10.8±1.3 mm/yr and 6.8±0.8 mm/yr respectively. These two possible rates are consistent withactive slip rates determined from GPS, InSAR, and dating of offsetquaternary landformswhichyield slip rates of∼10mm/yr (11±4mm/yr,Banerjee and Burgmann, 2002; and 10.7±0.7 mm/yr, Chevalier et al.,2005) and ∼4 mm/yr (4±1 mm/yr, Brown et al., 2002; 3.5±5.0 mm/yr,Jade et al., 2004; and 1±3 mm/yr, Wright et al., 2004). These results arealso broadly consistent with the long-term slip rates of 3–10 mm/yrproposed by Phillips et al. (2004). Finally, while the results of thisstudy cannot resolve the debate on the initiation age of the Karakorumfault, they rule out the possibility of high slip rates on the order of 21–27 mm/yr (e.g. Valli et al., 2008).

4.3. Evolution of the Karakorum fault

A compilation of offset features across the Karakorum fault which donot conflict with the results from this study show a relatively welldefined decrease in displacement from north to south (Fig. 5).Displacement across the northern portion of the Karakorum faultdefined by the separation of the Aghil limestone is 149–167 km.Displacement decreases slightly along the central portion of theKarakorum fault, with offsets of ∼120 km for the Bangong-Shyok suturezones and Indus River (and the poorly defined displacement of 40–150 km of the Baltoro granite) (Searle, 1996; Phillips et al., 2004). Thedecrease gets more pronounced at the southern end of the Karakorumfault with only 65 km of offset of the Kailas Thrust (Murphy et al., 2000),35–66 km of displacement on the Gurla Mandhata detachment system(Murphyet al., 2002) and ≥21 kmof strike-slip displacementof theMainCentral Thrust where the Karakorum fault is interpreted to propagateinto the Himalayas (Murphy and Copeland, 2005). However, with thefinal two displacements it should be noted that some portion of thestrain on the southern Karakorum fault is partitioned into strike-slipdeformation along the Indus suture zone north of the Gurla Mandhatadetachment system (Lacassin et al., 2004).

The critical components of this compilation are; 1) the pronouncedchange in magnitude of displacement across the Karakorum faultalong its southern portion south of the Bangong-Nujiang Suture zone,2) the broadly similar displacement magnitudes along the northernand central segments of the fault, and 3) the interpreted unin-terrupted continuation of the Central Pamir antiforms into thefootwall of the Kongur Shan normal fault (Robinson et al., 2007)which suggests all displacement along the northern end of the faulthas been fed into thrusting along the Rushan Pshart Zone. Theseobservations support the evolutionary model for the Karakorum faultproposed by Murphy et al. (2000), in which the Karakorum faultinitiates as a transfer structure linking thrust belts in the Pamir andwestern Tibet (i.e. the Shiquanhe thrust belt) and subsequentlypropagats southward into southwestern Tibet in the Late Miocene(Murphy et al., 2000), possibly to accommodate radial expansion ofthe Himalayan arc (Ratschbacher et al., 1994; Seeber and Pecher, 1998;Murphy and Copeland, 2005).

In regard to the regional role of the Karakorum fault, the observed149–167 kmoffset of theAghil formation along the northern Karakorumyields 122–137 km of north–south relative displacement, and only 85–96 km of east–west relative displacement of Tibet and the Pamir-Karakorum region. These results show that the Karakorum fault cannothave accommodated large magnitudes (i.e. hundreds of kilometers) ofeastward lateral extrusion of the Tibetan plateau (Tapponnier et al.,1982; Peltzer and Tapponnier, 1988; Armijo et al., 1989), or hundreds of

kilometers of northward displacement of tectonic terranes between thewestern and central portions of the Himalayan-Tibetan orogen (e.g.Lacassin et al., 2004; Schwab et al., 2004; Valli et al., 2008). Rather, theseresults are consistent with models in which the right-slip Karakorumfault has accommodated a relatively small portion of the strain related tothe north–south between India and Asia (e.g. Searle, 1996). Finally, thelimited displacement on the Karakorum fault shows the ∼450 km ofnorthward displacement of tectonic terranes in the Pamir-Karakorumregion relative to the central Tibetan Plateau is the result of distributeddeformation throughout the western portion of the Himalayan-Tibetanorogen rather than localized deformation along an individual structure.

5. Conclusions

Interpretations of satellite images along the northern right-slipKarakorum fault have identified a distinct carbonate unite, the LateTriassic-Early Jurassic Aghil formation, which shows 149–167 km ofright lateral separation across the fault. I interpret this observedseparation to be a reliable measurement of the amount of displace-ment across the northern Karakorum fault. This result shows that theSouth Pamir-Karakorum terrane is equivalent to the Qiangtang terraneof the Tibetan Plateau, rather than the Lhasa terrane. This is consistentwith previous studies which yield limited displacement across theKarakorum fault (i.e. b200 km) and demonstrates the Karakorum faultcannot have accommodated either large-scale displacement oftectonic terranes between the western and central portions of theHimalayan-Tibetan orogen or eastward lateral extrusion of the TibetanPlateau. Thus, while an important component of the structuralevolution of the western portion of the Himalayan-Tibetan orogenicbelt, the Karakorum fault has not accommodated a large portion of thestrain related to the Cenozoic convergence between India and Asia.

Acknowledgments

I thankMichaelMurphy for discussionswhich greatly improved thepresentation of the ideas in this manuscript. I thank Maurizio Gaetaniand an anonymous reviewer for helpful comments on an earlier draftof this manuscript, and comments from two anonymous reviewerswhich substantially improved themanuscript. I thank ShuhabKhan forassistance in obtaining the ASTER images used in this study.

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Geologic offsets across the northern Karakorum fault: Implications for its role and terrane cor.....IntroductionGeologic settingThe Karakorum faultDistribution of the Aghil formation

Displacement calculationsDiscussionCorrelation of suture zones and tectonic terranesGeologic slip ratesEvolution of the Karakorum fault

ConclusionsAcknowledgmentsReferences

geologic offsets across the northern karakorum fault ......slivers of carbonate 25–35 km long,...

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