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Earth and Planetary Science Letters 279 (2009) 123–130
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j ourna l homepage: www.e lsev ie r.com/ locate /eps l
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
a b s t r a c t
a r t i c l e i n f o
Article history:
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-
l rights reserved.
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
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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
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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.
126 A.C. Robinson / Earth and Planetary Science Letters 279
(2009) 123–130
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
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127A.C. Robinson / Earth and Planetary Science Letters 279
(2009) 123–130
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.
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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.
128 A.C. Robinson / Earth and Planetary Science Letters 279
(2009) 123–130
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
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(2009) 123–130
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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2004
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