cenozoic right-slip faulting along the eastern margin …...right-slip faulting, eastern pamir...

ABSTRACT

The Pamir salient defi nes the western end of the Himalayan-Tibetan orogen and has overthrust the Tarim-Tajik basin to the north by ~300 km along a late Cenozoic, south-dipping intracontinental subduction zone. Field mapping, structural measure-ments, and analysis of mesoscale structures along a 32-km-long reach of the Yarkand River document the tectonic evolution of the east fl ank of this salient, between the North Pamir to the west and the Western Kunlun Shan to the east. The study area is cut by a set of four, north-northwest–striking, steeply dipping brittle faults. Microstructures and asymmetric outcrop- to map-scale folds in-dicate right slip along these faults. Between these structures, fault-bounded panels of Phanerozoic strata are deformed by en eche-lon folds with axes that trend more westerly than the adjacent faults, consistent with dextral transpression. The fault system de-scribed here extends for ~350 km along the eastern fl ank of the Pamir salient. Transpres-sional right slip along this set of faults, here called the Kashgar-Yecheng transfer system, appears to have accommodated late Ceno-zoic separation of the North Pamir from the Western Kunlun Shan during south-directed intracontinental subduction beneath the leading edge of the Pamir salient. Correlation of major faults suggests total slip along the Kashgar-Yecheng transfer system is likely on the order of ~280 km. This offset estimate im-plies long-term slip rates of 7–15 mm/a along the Kashgar-Yecheng transfer system when combined with previous sedimentologic, stratigraphic, and thermochronologic data that indicate deformation along the east fl ank of the Pamir started between the late Eocene and early Miocene. These results imply that the fi rst-order structures on the western and eastern fl anks of the Pamir are asymmetric: previous work has shown that deformation

in the west was accommodated by anticlock-wise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin. This rotation is generally interpreted to refl ect northwest-directed radial thrusting, in con-trast to the transpressional right-slip transfer faulting on the east side reported here.

INTRODUCTION

Determining the spatial distribution and temporal evolution of intracontinental strain re-sulting from the Cenozoic Indo-Asian collision is critical for developing a better understanding of how the continental crust and lithosphere deforms (e.g., England and Houseman, 1985; Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1976; Vilotte et al., 1982). Central to this problem is determining the geometry and kinematics of the fi rst-order structural systems within the collision zone. The Pamir-Kunlun fault system is one such system and lies be-tween two major, left-slip faults, the Chaman fault in the west, and the Altyn Tagh fault to the east (Fig. 1A).

The Pamir-Kunlun system separates the Pamir mountains and Tibetan Plateau to the south from the Tarim Basin to the north (Fig. 1A), and can be separated into three segments on the basis of structural trend. The central third of the system (75°–77°E) lies along the eastern fl ank of the Pamir salient and is defi ned by the northwest-southeast–trending Kashgar-Yecheng segment of the Western Kunlun Shan (Figs. 1 and 2). The western third of the system (71°–75°E) is defi ned by the east-west–striking, north-directed Trans-Alai–Pamir thrust belt (Arrowsmith and Strecker, 1999; Brunel et al., 1994; Burtman and Molnar, 1993; Coutand et al., 2002; Pan, 1996; Pavlis et al., 1997; Strecker et al., 1995). Like-wise, the eastern third of the system (77°–80°E) is defi ned by the Hotan fold-thrust belt, a region of east-west–striking, north-directed thrusts that lies along the northern margin of the West-ern Kunlun Shan (Avouac and Peltzer, 1993; Cowgill , 2001; Gilder et al., 1996; Matte et al., 1996; Pan, 1996; Sengör and Okurogullari, 1991;

Yin et al., 2002). Although the Kashgar-Yecheng segment of the Pamir salient links north-directed thrusts within the Pamir and Hotan thrust belts, its kinematics remain unclear.

The systematic along-strike changes in ori-entation of the Pamir-Kunlun system make it a type of structure variably called a bent orogen (Marshak, 1988), a map-view curve (Marshak, 2004), or a curved orogen (Weil and Sussman, 2004). Although in detail numerous kinematic models have been proposed to explain the devel-opment of such curved orogens, in general these models defi ne two broad classes, depending on whether the curvature results from vertical-axis rotation or not (Carey, 1955; Marshak, 1988, 2004; Ries and Shackleton, 1976; Weil and Sussman, 2004). For example, reviews by Marshak (1988, 2004) and Macedo and Mar-shak (1999) distinguish nonrotational models, in which the thrust belt initiates with a curved trajectory, from rotational (orocline) models, in which curvature develops by bending of a linear belt. Likewise, the review by Weil and Sussman (2004) differentiates primary arcs, which initiate with curved trajectories, from progressive arcs, which acquire their curvature during thrusting and oroclines, in which an originally linear belt is bent during a separate phase of deformation (e.g., Carey, 1955). For clarity, it should be noted that orocline is used variably in the above two systems: in the former it refers to all orogenic curves formed by vertical-axis rotation, but in the latter it is restricted to those bent during the subsequent event. Here I follow the former.

As Figure 3 indicates, these general models lead to two basic options for the kinematics of the Kashgar-Yecheng segment. One possibility is that the segment forms part of a rotational curve (Fig. 3A). In detail, such rotation could be due to radial thrusting (Strecker et al., 1995), such as that deduced for the western fl ank of the Pamir on the basis of paleomagnetic data from the Tajik basin (Fig. 1) (Bazhenov and Burtman, 1982; Bazhenov et al., 1994; Bourgeois et al., 1997; Burtman, 2000; Burtman and Molnar, 1993; Thomas et al., 1994; Thomas et al., 1996). Rotation also could be due to oroclinal buckling,

For permission to copy, contact [email protected]© 2009 Geological Society of America

145

GSA Bulletin; January/February 2010; v. 122; no. 1/2; p. 145–161; doi: 10.1130/B26520.1; 9 fi gures.

†E-mail: [email protected]

Cenozoic right-slip faulting along the eastern margin of the Pamir salient, northwestern China

Eric Cowgill†

Department of Geology, University of California, Davis, California 95616, USA

Cowgill

146 Geological Society of America Bulletin, January/February 2010

as used to explain deformation of inferred ribbon continents in Alaska (Johnston, 2001) and along the New Caledonia–d’Entrecasteaux ridge in the southwest Pacifi c (Johnston, 2004). In either case, this fi rst scenario predicts clockwise rota-tion of the east fl ank of the Pamir, possibly asso-ciated with east-northeast–directed thrusting of the Kashgar-Yecheng segment over the Tarim basin to the east (Fig. 3A). The second possi-bility is that the change in strike of the moun-tain front could refl ect ~200 km of separation of the northern Pamir from the Western Kunlun Shan along a north-northwest–striking dextral strike-slip system (Fig. 3B) (Brunel et al., 1994; Sobel, 1999; Sobel and Dumitru, 1997; Tappon-nier et al., 1981). This second scenario requires right-slip faulting along the Kashgar-Yecheng segment. Although such dextral strike-slip fault-ing has been inferred for some time (Burtman and Molnar, 1993; Ding et al., 2004; Sobel, 1999; Sobel and Dumitru, 1997) and a regional map by Brunel et al. (1994) shows right-slip

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Figure 1. (A) Simplifi ed map of major faults within the Indo-Asian collision zone, show-ing the location of the Pamir salient at the west end of the orogen (after Robinson et al., 2004). R and S refer to recess and salient , respectively, as defi ned by Mar-shak (2004) and Weil and Sussman (2004). (B) Simplifi ed map of major tectonic do-mains within the Pamir and western Tibet. Abbreviations: AKS—Anyimaqen-Kunlun; ATS— Akbaytal-Tanymas; BNS—Bangong-Nujiang; JS—Jinsha; MCT—Main Central Thrust; RPS—Rushan-Pshart; STDS—South Tibetan Detachment System (after Robinson et al., 2004). (C) Diagram illus-trating competing correlations of terranes (boxes) and their bounding sutures (heavy black lines) between the Pamir and the Tibetan Plateau to the east. Although both models agree that markers in the Pamir have been displaced northward relative to Tibet, correlation 1 (Burtman and Molnar, 1993; Searle, 1996; Yin and Harrison, 2000) implies smaller magnitudes of relative dis-placement than correlation 2 (Lacassin et al., 2004; Schwab et al., 2004). (D) Cartoon illus-trating how subduction of Tarim beneath the Western Kunlun Shan does not explain the observed ~475 km offset of basement units in the Western Kunlun Shan from those in the Eastern Kunlun Shan, because such a fault geometry places both markers within the same plate and therefore does not permit large relative motions between them (modifi ed from Cowgill et al., 2003).

Right-slip faulting, Eastern Pamir

Geological Society of America Bulletin, January/February 2010 147

faulting along the northwestward continuation of what is here called the Kusilaf fault (Fig. 2), documented fi eld evidence for such structures has generally been lacking.

To address this problem, I conducted 1:100,000-scale structural mapping along a 32-km-long section of the Yarkand River Valley (38.0°N, 76.5°E), in the middle of the Kashgar-

Yecheng segment of the Western Kunlun Shan (Figs. 1 and 2). This drainage trends subperpen-dicular to the Kashgar-Yecheng system and pro-vides excellent exposures of structures along and to the west of the range front. The following sec-tions fi rst present the regional geologic context of the Kashgar-Yecheng system and then report the results of the structural mapping. As the data

presented below indicate, the Kashgar-Yarkand system comprises dextral transfer faults that link thrusts in the northern Pamir to the northwest with those in the western Kunlun Shan to the southeast. I conclude by discussing this result in the context of previous work and then explore implications of this result on models for the for-mation of the Pamir salient. This study, together

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Figure 2. Simplifi ed geologic map of the Kashgar-Yecheng transfer system, based on Pan et al. (2004), and compiled from my own work and previous results (Liu, 1988; Robinson et al., 2007; Sobel and Dumitru, 1997; Xinjiang Bureau of Geology and Mineral Resources, 1993).

Cowgill


with previous work, suggests that regional defor-mation within the Pamir salient is asymmetric, with radial thrusting on the west and strike-slip transfer faulting on the east.

GEOLOGIC SETTING

The following sections summarize the re-gional geology of the Pamir-Kunlun system from west to east.

Trans-Alai–Pamir

The northernmost edge of the Pamir is de-fi ned by the south-dipping Main Pamir Thrust, which accommodates north-directed thrusting of the Trans-Alai range over the Alai Valley to the north (Arrowsmith and Strecker, 1999; Burtman and Molnar, 1993; Coutand et al., 2002; Hamburger et al., 1992; Pavlis et al., 1997; Strecker et al., 1995) (Figs. 1B and 2). Global positioning system (GPS) results indicate

13 ± 4 mm/a of present-day convergence be-tween the northern Pamir and the southern Tian Shan (Reigber et al., 2001) that is most likely accommodated by the Main Pamir Thrust. The Holocene dip-slip rate along the fault is at least ~6 mm/a (Arrow smith and Strecker, 1999). Coutand et al. (2002) determined horizon-tal shortening rates across the Alai Valley of 0.7–0.8 mm/a since 25 Ma ago using balanced cross sections, although these sections yield minimum estimates of total shortening (and thus minimum rates) because they do not ac-count for slip along basement-involved thrusts at the ends of the sections.

Paleomagnetic data from the eastern Tajik basin indicate systematic anticlockwise vertical-axis rotations that progressively de-crease in magnitude with increasing distance from the western margin of the Pamir salient (Bazhenov et al., 1994; Burtman, 2000; Burt-man and Molnar, 1993; Thomas et al., 1994). These rotations are early Miocene or younger,

based on the similarity of the Cretaceous and pre-Miocene paleomagnetic vectors within the Tajik basin (Thomas et al., 1994). The active, northeast-striking Darvaz fault separates the Pamir to the east from the Tertiary thrust belts in the Tajik basin to the west (Fig. 1A), ap-proximately defi ning the western margin of the Pamir salient (Burtman and Molnar, 1993). The kinematics, slip rate, and total offset along the Darvaz fault are poorly constrained. Displaced landforms of probable Holocene age indicate left slip at an inferred rate of 10–15 mm/a (Burt-man and Molnar, 1993, and references therein), although the ages of these features do not appear to be well constrained. In contrast, Thomas et al. (1994) show the Darvaz fault as a thrust.

Several observations imply that Eurasian lithosphere has been subducted for ~300 km beneath the Pamir along the Main Pamir Thrust and associated faults during the Cenozoic, in-cluding a south-dipping zone of seismicity that projects ~250 km downdip from the Main Pamir Thrust and Alai Valley, sinistral separation of Cretaceous and Paleogene facies boundaries be-tween the Tajik basin and northern Pamir, and northward defl ections of sutures in the Pamir relative to positions in Afghanistan and Tibet to the west and east, respectively (e.g., Burtman and Molnar, 1993; Fan et al., 1994; Hamburger et al., 1992; Schwab et al., 2004). The subduc-tion polarity implied by the zone of seismicity is disputed, and several workers have argued that the south-dipping zone of seismicity represents a segment of northward-subducting Indian litho-sphere that has been locally overturned (Pavlis and Das, 2000; Pegler and Das, 1998). Accord-ing to Burtman and Molnar (1993) the southern Pamir has been displaced a total of ~600 km northward relative to the Alai Valley, owing to ~300 km of south-directed subduction and ~300 km of internal shortening within the Pamir.

Although there is agreement that Mesozoic and Paleozoic terranes in the Pamir have been displaced north relative to their along-strike con-tinuations to the west and east (Fig. 1B), the de-tails of the suture correlations are disputed. Two main scenarios have been advocated (Fig. 1C), with one implying larger magnitudes of north-ward translation than the other. In the fi rst, the Qiangtang terrane of central Tibet is correlated with the South Pamir–Karakoram–Hindu Kush block to the west, such that the Jinsha suture corresponds to the Rushan-Pshart zone, and the Bangong-Nujiang suture is equivalent to the Shyok suture (Burtman and Molnar, 1993; Searle, 1996; Yin and Harrison, 2000). In con-trast, the second model implies larger magnitudes of translation because the Qiangtang terrane is correlated with the Central Pamir block, in which case the Jinsha suture corresponds to

final

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Figure 3. Cartoons illustrating end-member kinematic models of Pamir salient formation. Models involve (A) vertical-axis rotation of the fl anks of the salient via radial thrusting or oroclinal buck-ling, or (B) deformation via conjugate strike-slip transfer fault systems.



the Akbaytal-Tanymas fault and the Bangong-Nujiang suture is equivalent to the Rushan-Pshart zone (Lacassin et al., 2004; Schwab et al., 2004). The fi rst scenario is preferred here because previous work in the Western Kunlun Shan, North Pamir, and Paropamisus regions (Fig. 1B) (Boulin , 1988; Burtman and Molnar, 1993; Girar-deau et al., 1989; Sengör, 1984; Stöcklin, 1989) suggests that the Akbaytal fault (Cowgill et al., 2003; Girardeau et al., 1989; Stöcklin, 1989) sepa rat ing north and central Pamir correlates to the west with the Herat fault in Afghanistan and to the east with the Anyimaqen suture, which is now represented by the active, left-slip Karakax fault separating the western Kunlun Shan to the north from the Tianshuihai-Songpan-Ganzi fl ysch belt to the south (Cowgill et al., 2003). If these correlations are accurate, this marker shows ~270 km of dextral separation between the Pamir and the western Kunlun Shan (Cowgill et al., 2003). The north Pamir is also characterized by active, approximately north-south–striking ex-tensional systems (Brunel et al., 1994; Robinson et al., 2004, 2007; Strecker et al., 1995).

Kashgar-Yecheng System

Previous work along the Kashgar-Yecheng system has emphasized paleomagnetic and sedi-mentologic aspects of the Cretaceous and Ter-tiary deposits, although Brunel et al. (1994) show right-slip faulting along the northwestward continuation of what is here called the Kusilaf fault. Recent work has focused particularly on the depositional and structural evolution of the late Tertiary Kashi foreland basin along the southern margin of the Tian Shan in the vicinity of Kashgar (Fig. 1B) (Chen et al., 2002; Chen et al., 2007; Heermance et al., 2008; Heermance et al., 2007; Scharer et al., 2006; Scharer et al., 2004). Chen et al. (1992) determined paleomag-netic poles for Cretaceous deposits at Yingjisha and Oytag (also spelled “Uytak” or “Wuyitake,” Fig. 2) and found a large dispersion in decli-nations from Oytag, which they attributed to faulting. Rumelhart et al. (1999) determined a paleomagentic pole for Oligocene strata in the Aertashi area (see Fig. 4 for location)1. When corrected for an error in the original publica-tion, these data indicate that the Aertashi beds record 7.4° ± 5.8° of clockwise vertical-axis rotation (Yin et al., 2000). Magnetostratigraphi-cally constrained ages and basin analysis of the Aertashi section suggest that subsidence accel-erated between 37 and 33 Ma ago, interpreted to refl ect initiation of thrust loading along the frontal part of the western Kunlun Shan at this

time (Yin et al., 2002), consistent with a cluster of single-grain detrital apatite fi ssion-track ages at 30.0 ± 4.8 Ma in a Miocene(?) sandstone sug-gesting that exhumation of rocks in the source area was under way by this time (Sobel and Dumitru , 1997). Using apatite fi ssion-track data and subsidence analysis, Sobel and Dumitru (1997) investigated the timing of Cenozoic defor ma tion along the western margin of Tarim and concluded that the Main Pamir Thrust and the Kumtag fault together accommodated >200 km of northward displacement of vol-canic rocks at Oytag relative to those south of Kudi (stars in Fig. 2), with northward indenta-tion of the Pamir likely under way by at least 20 Ma ago. Likewise, Sobel (1999) carried out basin analysis of Jurassic through Cretaceous sections in western Tarim and concluded that the Kashgar-Yecheng segment underwent dex-tral transtensional deformation in the Middle Jurassic, followed by Late Jurassic to Early Cretaceous contractional(?) deformation. Both Sobel and Dumitru (1997) and Sobel (1999) ar-gued for ~200 km of right-lateral offset along the Kashgar-Yecheng system. However, as dis-cussed below, this reconstruction is problematic because the correlated markers show sinistral, rather than dextral, separation along the fault cited to explain their displacement. Correlations between ophiolitic materials in the Oytag and Kudi areas have also been made (Pan, 1996; Sobel and Arnaud, 1999), although more recent work indicates that the Kudi ophiolite was em-placed in the Middle Ordovician to the Middle Devonian, whereas the Oytag ophiolite formed during the Late Devonian to Carboniferous (Xiao et al., 2002).

Hotan Thrust Belt

The Hotan thrust belt is characterized by east-west–striking, north-directed thrusts that emplaced the western Kunlun Shan to the south over the Tarim Basin to the north (Avouac and Peltzer, 1993; Cowgill, 2001; Gilder et al., 1996; Matte et al., 1996; Pan, 1996; Sengör and Okuro-gullari, 1991; Yin et al., 2002). The exposed por-tion of the foreland fold-thrust belt is narrow, with a cross-strike width of only ~30 km. Within the belt, ~12-km-thick thrust sheets constituting the sedimentary cover of Tarim and its underly-ing metasedimentary basement are tightly folded and juxtaposed along steeply to moderately south-dipping faults (Cowgill, 2001). A balanced cross section based on both 1:100,000-scale structural mapping and thermochronological constraints on the magnitude of thrust sheet unroofi ng indicates that total Cenozoic shorten-ing in the Hotan thrust belt is at least 100 km (Cowgill, 2001). The Southwest Depression lies

to the north of the Hotan thrust belt, forming an active foreland basin to the range (Jia, 1997; Li et al., 1996). Well-log and seismic data indicate that the depression contains a total of ~12 km of supracrustal deposits, ~7–9 km of which are Ceno zoic in age (Hu, 1992; Jia, 1997; Lee, 1985; Li et al., 1996; Nishidai and Berry, 1990).

A number of workers have argued that, like the Pamir, the western Kunlun Shan also repre-sents an area of south-directed intracontinental subduction, on the basis of gravity (Jiang et al., 2004; Lyon-Caen and Molnar, 1984), tomo-graphic (Wittlinger et al., 2004), and geologic studies (Arnaud et al., 1992; Avouac and Peltzer , 1993; Matte et al., 1996; Tapponnier et al., 2001). Although these data clearly indicate south-directed underthrusting of Tarim, they do not require subduction, and no south-dipping zone of seismicity analogous to that seen in the Pamir is evident beneath the Western Kunlun Shan. In addition, although subduction of Tarim is commonly cited to absorb left-slip along the Altyn Tagh fault, as Figure 1D illustrates, it can-not explain 475 ± 70 km of post–Early Juras-sic sinistral separation between the southern edges of the Western and Eastern Kunlun Shan along the Altyn Tagh fault (Cowgill et al., 2003; Ritts and Biffi , 2000; Yin and Harrison, 2000). In particular, the problem is that calling upon a fault geometry in which Tarim subducts beneath Tibet predicts only minor separation between the Western and Eastern Kunlun Shan, because such a geometry places both markers within the same block (Fig. 1D). However, this predicted minor offset is incompatible with the observed large-magnitude (475 km) displacement ob-served between these markers along the Altyn Tagh fault. Thus, an alternative fault geometry is required. To account for the 475 km displace-ment, Cowgill et al. (2003) proposed that left slip along the Altyn Tagh fault fed into a south-directed thrust belt inferred to lie to the south of the displaced marker in western Tibet, in which case north-directed thrusting along the northern margin of the Western Kunlun Shan adds to the 475 ± 70 km of separation along the Altyn Tagh fault as recorded by the displaced marker.

GEOLOGY ALONG THE YARKAND RIVER

The main structure along the range front in the Aertashi-Kusilaf region has been interpreted as an east-directed thrust (Fig. 5 of Sobel and Dumitru, 1997), a fault with unspecifi ed kine-matics (Xinjiang Bureau of Geology and Min-eral Resources, 1993), or a depositional contact (Liu, 1988). New geologic mapping along the Yarkand River (Fig. 4) indicates that this area is dominated by northwest-striking and steeply

1Figure 4 is on a separate sheet accompanying this issue.

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2164

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2133

2297

2787

2672

2605

*

*

*

*

* *

*

*

2837

*

2117

23101928

2293

2570

*

*

*

*

*

*

3117

3252

2755

3247

3050

2582

20062121

2202

2116

2516

2340

2032

2044

1918

2030

1906

1932 2004

2052

2058

2234

1896

1978

1850

1934

2082

1762

1628

1605

2338

2530

2598

2934

3233

2316

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

**

*

*

*

*

1800

1600 1600

1800

2000 2200

2000

2140

75

53

Y913-1Y913-3

78

80

50

65Y913-2

C913-1

C913-441

31

47

71

40

38 30

30

3540

35

30

40

25

721525

35

40

25

58

60 55

50 20

C915-1 45

69

80

85 75

66

50

81 88

76

C914-1

80 80

77

30

2550

70

66 5861

C914-2

55

3569 65 87

70

84

70

833080

Y914-1

55

65

60

41

5368

76

52 59 Y914-2

61

75

75

31

29

6271

55 62

74

7668

64

75

79

50

53

81

24 35

76

66

39

76

70

80

84

Measured Section of Yin et al., 2002

20

3065

67

7050

46

70

8584

76 80

83 80

58

25

66

35

48

50

1 km

0 km

2 km

3 km

-1 km

1 km

0 km

2 km

3 km

A (west) A′′ (east)

N85°E

O?J

CT2D?

gr

phyK-T1

K-T1

Aertashi faultYarkand faultKumtagfault

Kusilaffault

V = H

1 km

1 km

5 km

76° 35′76° 30′76° 25′76° 20′

76° 20′ 76° 25′ 76° 30′ 76° 35′

37° 55′38° 00′38

° 00

′37

° 55

′

Units: Symbols:

Key to Mapping:

85

65

ATTITUDES: Broken where projected along strike

Bedding:

Cleavage:

CONTACTS: Solid where well-located, dashed where approximate, dotted where concealed. Arrows show fault dip direction and magnitude (decorated where projected).

Depositional:

STATION NUMBER: Format is NMDD-# where N is geologist (Y = An Yin, C = Eric Cowgill) M is month, DD is day, and # is station number

Y913-2

Fault:

FOLD AXES: Solid where well-located, dashed where approximate, dotted where concealed. Arrows point in direction of limb dip.

MARKER BED: Distinctive limestone marker bed underlain by gypsum within the lower part of the Cretaceous-Paleogene section. Dotted where concealed.

Syncline (upright, overturned):

Anticline (upright, overturned):

4145

70Overturned bedding:

60

GEOGRAPHY: CROSS-SECTION SYMBOLS

Town:

Yarkand River:

Road:

Peak: 3824 m*

N

Monocline (short arrow on steeper limb):

Phyllite: Massive phyllite with bands of quartzite, marble, and metaigneous rocks. Intruded by dikes at station Y913-3.

Devonian(?) sandstone, shale, and minor limestone: Well-bedded sandstone intervals are 0.5–5 m thick, greenish gray to brown, internally laminated, and have current-rippled tops. Burgundy shale/mudstone intervals are 2–10 m thick, well-bedded, and phyllitic. Sands are more abundant down section. Near contact with Carboniferous strata there are 1–2 m thick gray limestone intervals which grade into recessive, well-bedded brown sandstones and shales. In the same area some of the mudstone intervals show mudcracks.

Ordovician(?) carbonate and conglomerate: Carbonate and pebble to cobble conglomerate. Conglomerate clasts are predominantly angular and consist of quartzite, marble, phyllite, and locally schist.

Carboniferous shallow marine carbonates: At least 1200 m thick. Upper and lower contacts are both fault bounded. Lower ~800 m is well-bedded, dark gray to tan limestone with layers up to 5 m thick and interlayerd with well-bedded, tan, quartz sandstone beds up to 1 m thick. Upper ~400 m is dominated by massive, mottled gray carbonates with lumpy bedding planes.

Quaternary alluvium: Alluvial and fluvial deposits containing abundant plutonic and gneissic clasts.

Jurassic terrestrial siliciclastic deposits: Interlayered, well-bedded shale, mudstone, and arkosic sandstone with minor coal-bearing zones. Locally contains poorly sorted, clast-supported, massive granule to boulder conglomerate zones. Clasts within the conglomerates are well-rounded and are composed of white and tan quartzite/vein quartz as well as red and green siltstone and sandstone similar to the Devonian(?) strata farther east. Many of the clasts are heavily fractured and show mm-scale offsets of clast margins. Both clasts and matrix are cross-cut by mm to cm scale veins.

Tertiary marine to alluvial redbed deposits: Interbedded sandstone and boulder conglomerate of alluvial fan facies. Overall, units K-T1 and T2 show an upwards coarsening and thickening sequence.

Cretaceous-Tertiary marine to alluvial deposits: Lower part of unit contains interlayered shale, siltstone, sandstone, gypsum, and fossiliferous limestone of marine to marginal marine facies. Prominent limestone marker bed near base of unit is underlain by distinctive gypsum interval that is up to several hundred meters thick in fold hinges. Upper part of the unit contains interlayered red siltstone, sandstone, and conglomerate showing a facies progression from meandering fluvial through braided fluvial facies (Sobel, 1995; Yin et al., 2002). granite: Two-mica peraluminous granite intrudes Ordovician(?) strata along an

irregular contact. Ordovician(?) carbonates are metamorphosed to marble along the contact. Sample Y96-9-13-2, collected at station Y913-2, has yielded an ion microprobe 206Pb/238U zircon age of 456 ± 4 Ma (Cowgill et al., 2003).

A

A′

An Yin

Eric Cowgill

Mapped 9/13-17, 1996 by An Yin and Eric Cowgill

T2

K-T1

C

phy

O?

gr

D?

J

Qal

Yarkand River

Kusilaf

Aertashi

phy

phy

O?

O?

Qal

Qal

gr

Kumtag fault

Yarkand fault

Kusilaf fault

T2

K-T1

K-T1

C

C

D?

D?

D?

J

J

D?

D?

C

C

K-T1

QalQal

Aertashifault

Qal

E. Cowgill & A. YinGeologic Map and Cross Section of the Yarkand Fault System, Aertashi-Kusilaf Area, Western Kunlun Shan, Northwestern China

cleavage dip-tick

bedding dip-tick

right-slip fault

Figure 4. Geologic map and cross section of the region adjacent to the Yarkand River between Kusilaf and Aertashi, based on mapping by Eric Cowgill and An Yin, as well as previous work (Liu, 1988; Sobel and Dumitru, 1997; Xinjiang Bureau of Geology and Mineral Resources, 1993).

Cenozoic right-slip faulting along the eastern margin of the Pamir salient, northwestern China

CowgillFigure 4

Supplement to: Geological Society of America Bulletin, v. 122, no. 1/2, doi: 10.1130/B26520.S1© Copyright 2009 Geological Society of America

Cowgill


dipping brittle, dextral strike-slip faults that jux-tapose narrow panels of basement and Phanero-zoic strata. These panels are deformed by en echelon folds with axes that trend more westerly than the adjacent faults, consistent with right-lateral deformation (e.g., Sylvester, 1988, and references therein). There are four main faults

in the map area. From east to west, these are the (1) Aertashi, (2) Yarkand, (3) Kumtag, and (4) Kusilaf faults. The Kusilaf fault was called the Main Pamir Thrust by Sobel and Dumitru (1997). The following sections describe the geol ogy of these faults and the units they bound, proceeding from east to west.

Aertashi Fault and Cretaceous-Tertiary Units

At the east end of the map area the Aertashi fault separates Cretaceous-Tertiary units to the east from folded Carboniferous strata to the west (Figs. 4 and 5).

StratigraphyTo the east of the Aertashi fault the lowermost

part of the Cretaceous-Tertiary section com-prises >550 m of poorly exposed Cretaceous de-posits, including >350 m of Lower Cretaceous thin-bedded, very fi ne sandstone and siltstone with calcisol horizons tentatively interpreted as alluvial-plain deposits (Sobel, 1999), overlain by ~200 m of Upper Cretaceous shale, siltstone, gypsum, and limestone (Sobel, 1995). I did not differentiate the Cretaceous strata from the overlying Paleocene through Miocene deposits. The latter record a progression from Paleocene-Eocene marginal marine deposition to meander-ing, and then braided fl uvial deposition in the Oligocene, followed by alluvial fan deposition in the late Oligocene and Miocene (Rumelhart, 1998; Sobel, 1995; Sobel and Dumitru, 1997; Yin et al., 2002). In general, the Cretaceous-Tertiary section to the east of the Aertashi fault defi nes an east-dipping homocline (Fig. 4A and B). However, at distances ≤3 km east of the fault this unit is deformed by a set of chevron folds with steep axial planes that strike subparallel to the fault (Fig. 4). A distinctive limestone marker bed delineates the structure in this area (Fig. 4) the top of which appears to correspond to the boundary between the Paleocene Aertashi and the Paleocene-Eocene Qimugen Formations described by Sobel (1995). Gypsum that under-lies this marker bed is tectonically thickened within the hinges of the folds, suggesting that the ~300 m stratigraphic thickness of this unit as reported by Sobel (1995) may be too high.

StructureBoth the macroscopic geometry of the

Aertashi fault and structural measurements indicate this structure to be a dextral strike-slip fault (Figs. 4A, 5, and 6A). All structural measurements are reported using azimuth no-tation. The Aertashi fault strikes 155° and dips 75°W near station Y914–2, north of the Yarkand River, but reverses dip direction along strike, such that on the south side of the river it strikes 012° and dips 75°E (Fig. 4). As such, the fault shows reverse separation (Carboniferous over Cretaceous-Tertiary ) in the north and normal separation (Cretaceous-Tertiary over Carbon-iferous) in the south. Along-strike reversals in separation sense along a steeply dipping fault are typical of strike-slip faults (Sylvester, 1988).

View to westCarboniferous

A

B View to west

horizontalstriae

rotatedblock

Cretaceous & Tertiary

View to west

horizontalstriae

rotatedblock

Cretaceous & Tertiary

Figure 5. Photographs of the Aertashi fault. (A) Overview of the fault, looking northwest, showing Carboniferous carbonates in the background juxtaposed against Cretaceous-Tertiary strata in the foreground. Height of ridge is ~150 m. (B) Subhorizontal striae on an approximately vertical fault surface within Carboniferous strata directly adjacent to the Aertashi fault. Both photographs were taken ~500 m east of station Y914–2 (Fig. 4A).



Structural measurements made ~500 m east of station Y914–2 (Fig. 4A) characterize the style of deformation along the Aertashi fault. Sub-horizontal striae on steeply dipping, secondary fault surfaces that parallel the main trace of the Aertashi fault at this locality indicate strike-slip displacement (Fig. 5B). At this site the main fault strikes almost due north (350°) and dips 85° east (Fig. 6A). A set of north-northeast–striking and steeply northwest-dipping faults with horizontal striae and centimeter-scale offsets record dextral shear sense, as indicated by brittle shear-sense indicators such as torn calcite fi bers (Fig. 6A). Based on their slip sense and orientation with respect to the main fault, I interpret these small faults as Riedel shears formed by dextral slip on the main structure. Dilational veins at this site strike 240° and dip 80° north, such that their poles are subhorizontal and trend more westerly than the strike of the Aertashi fault. These pole orientations are also indicative of dextral motion along the fault, presuming that the poles approx-imate the maximum extension direction and that the intermediate extension direction lay in the plane of the Aertashi fault (i.e., approximately vertical) at the time the veins formed.

Macroscopic structures also attest to right-slip motion along the Aertashi fault. For ex-ample, dextral drag has refolded a fold that is defi ned by the limestone marker bed within the Cretaceous-Tertiary section 1–2 km east of the Aertashi fault (Fig. 4A). North of the river this overturned, west-vergent fold has a north-striking axial surface (Fig. 4A and B). However, south of the river the axial trace curves by 90°, such that it is west striking where it is truncated by the Aertashi fault (Fig. 4A).

Yarkand Fault and Flanking Units

In the center of the map area the Yarkand fault juxtaposes Devonian(?) clastic deposits to the west against folded Carboniferous carbonates to the east (Fig. 4).

StratigraphyThe rocks to the west of the Yarkand fault com-

prise a thick sequence of well-bedded mudstones and fi ne-grained sandstone of apparently Devo-nian(?) age (Sobel and Dumitru, 1997), although they have also been mapped as Proterozoic (Liu, 1988; Xinjiang Bureau of Geology and Mineral Resources, 1993). Within this section, sand-stone zones are most abundant in the lower part of the sequence, whereas in the upper part, near the Yarkand fault, the unit contains gray lime-stone interbeds that grade into sandstone and shale zones, some of which preserve mudcracks.

East of the Yarkand fault the Carboniferous deposits are shallow marine carbonates. The

A

B

C

Equal Area

Y914-2Aertashi fault

Dextral faults

N

Fault(350, 85)

Dilational veins (240, 80)

Equal Area

C914-2Yarkand fault

Fault(225, 80)

N

Equal Area

Y913-1Kusilaf fault

Fault (326, 84)

N

Figure 6. Lower hemisphere, equal area stereoplots showing fault-slip data from the Aertashi-Kusilaf area. Bold great circles indicate lo-cal attitude of primary fault, and thinner lines correspond to subsid-iary faults showing centimeter- to meter-scale offsets. Striae plotted as triangles, with arrows indicating motion of hanging wall block where known. Shear sense was generally determined from torn calcite fi ber lineations. (A) Structural measure-ments from the Aertashi fault. See text for discussion. (B) Slip direc-tions of secondary faults adjacent to the Yarkand fault are more varied than those from the Aertashi fault, but generally show dextral or dextral-reverse slip. Note that the secondary faults along the Yarkand and Aertashi faults show the same strike. Secondary faults showing dip-slip motion may have formed during folding of the Carboniferous strata. Data col-lected at station C914–2 (Fig. 4A). (C) Most secondary faults adja-cent to the Kusilaf fault show strike-slip motion, as indicated by the preponderance of steeply dipping planes with subhorizontal striae. Slip sense is not deter-mined, however. Data collected at station Y913–1 (Fig. 4A).

Cowgill


lower part of this section, adjacent to the De-vonian(?) strata, contains ~1-m-thick interbeds of tan quartz sandstone and minor, mudcracked shale zones. The lithologic similarities between the upper part of the Devonian(?) unit and the lower part of the Carboniferous section sug-gest that the contact between the two units may have originally been a gradational depositional boundary prior to formation of the Yarkand fault. I have assigned a Devonian(?) age to the rocks west of the Yarkand fault because of this appar-ent gradational contact and their gross similarity to units present along the road between Yecheng and Mazar (Fig. 2), which were mapped as Devo nian (Matte et al., 1996) (Fig. 2). To the south of the Yarkand River, the Carboniferous strata are reportedly overlain by Early Permian deposits (Liu, 1988; Xinjiang Bureau of Geol-ogy and Mineral Resources, 1993), suggesting that an originally continuous section of Devo-nian through Permian strata lay west of the Aertashi fault (Fig. 2).

StructureThe Yarkand fault is a steeply dipping, poorly

defi ned fault zone that locally strikes 225° and dips 80°N at station C914–2 (Figs. 4 and 6B). The mapped fault trace trends more northerly than this local measurement (Fig. 4A), suggest-ing that the fault is not planar. Folding is more intense up to 300 m on either side of the fault than it is at greater distances, attesting to localized defor ma tion along the contact between the Devo-nian(?) and Carboniferous units. Striated surfaces within the fault zone strike more northerly than the local fault measurement and show both right slip and reverse slip (Fig. 6B). The latter may have resulted either from local partitioning of transpressional deformation within the fault zone or fl exural-slip folding. The poor defi nition of the fault zone, the heterogeneous fault-slip measure-ments, and the lithologic similarities between the upper part of the Devonian(?) section and the lower part of the Carboniferous sequence all sug-gest that the magnitude of displacement along the Yarkand fault is likely to be small, probably on the order of a few kilometers or less.

The panel of Carboniferous carbonate is deformed by a set of angular, upright, and sub hori zontally plunging folds that trend northeast-southwest, oblique to the bound-ing Aertashi and Yarkand faults (Fig. 4A). The obliquity and orientation of these folds relative to the panel-bounding faults are consistent with their formation in a zone of dextral shear (e.g., Moody and Hill, 1956; Sylvester, 1988). The fold spacing is tighter at distances ≤3 km from the Yarkand and Aertashi faults than in the interior of the panel of Carboniferous strata, suggesting that the folds are genetically related to slip along these faults.

The Devonian(?) strata are deformed by a broad anticline with secondary folds on the west limb and at the crest (Fig. 4). This unit has a well-developed cleavage that lies subperpendicular to bedding and strikes parallel to the axial plane of the anticline (Fig. 4). As with the panel of Car-boniferous strata to the east, fold hinges within the Devonian(?) section trend obliquely to the bound-ing faults, consistent with dextral wrenching.

Kumtag and Kusilaf Faults and Flanking Units

At the west end of the map area, in the vicin-ity of the village of Kusilaf, the Kusilaf and Kumtag faults separate phyllite to the west from the Devonian(?) section to the east, with slivers of Jurassic and Ordovician(?) sediments lying between the two faults (Fig. 4). The Kusilaf fault and its along-strike continuation were pre-viously called the Main Pamir Thrust in this area (Sobel, 1999; Sobel and Dumitru, 1997). I have renamed this fault because it appears to be kine-matically distinct from the Main Pamir Thrust.

StratigraphyLower Middle Jurassic strata to the west of

the Kumtag fault at Kusilaf were described by Sobel (1999), who reported >300 m of green, laminated, organic-rich shale interbedded with coals and thin, tabular sandstones interpreted as swamp, overbank, and lacustrine deposits. In addition to these units, I also observed well-bedded coarse-grained sandstone, pebbly sand-stone, granule conglomerate, and poorly sorted, massive, clast-supported boulder to cobble conglomerate. Clasts within this conglomerate were rounded and comprised white, tan, and red quartzites along with abundant clasts of aqua-green sandstone and siltstone similar to litholo-gies within the Devonian(?) strata to the east of the Kumtag fault. Two samples from the Jurassic section yielded a pooled apatite fi ssion-track age of 18.0 ± 0.8 Ma (Sobel and Dumitru, 1997).

The Jurassic deposits are juxtaposed against Ordovician(?) (Liu, 1988) carbonate and pebble to boulder conglomerate to the west along a steeply dipping, northwest-striking contact (Fig. 4). Although this contact was not di-rectly observed in the fi eld, I infer that it is a fault, based on the stratigraphic juxtaposition across it, and its proximity to the Kumtag and Kusilaf faults. Clasts within the Ordovician(?) conglomerate are angular and comprise quartz-ite, marble, phyllite, and locally schist. These sediments are intruded by a peraluminous, two-mica granite the intrusive margin of which is highly irregular (Fig. 4). Within the adjacent metamorphic aureole, limestone beds have been metamorphosed to marble. Ion micro-

probe zircon analyses yielded a 206Pb/238U age of 475.1 ± 9.7 Ma (2σ) for this granite (Cowgill et al., 2003), indicating that the sediments it in-trudes must be Early Ordovician or older.

The Ordovician(?) deposits are juxtaposed against phyllite to the west across the steeply dipping Kusilaf fault. The phyllite is massive and contains bands of quartzite, marble, and metaigneous rocks, and is intruded by dikes at station Y913–3 in Figure 4. The unit is mapped as Proterozoic on the basis of previous work (Liu, 1988; Xinjiang Bureau of Geology and Mineral Resources, 1993).

StructureThe Kumtag fault is a steeply dipping, brittle

fault, against which the Jurassic strata to the west are tightly folded (Fig. 4). The Jurassic strata are deformed by a set of asymmetric, meso scale folds directly adjacent to, and up to several hundred meters west of, the Kumtag fault (Fig. 7). These folds have steeply plunging, nearly vertical hinge lines, and their sense of asymmetry indicates clockwise rotation about these axes. As such, they indicate dextral strike slip along the Kumtag fault.

The Kusilaf fault is also a steeply dipping brittle fault. This fault reverses both dip direc-tion and separation sense along strike, dipping ~80°E and placing Ordovician(?) over phyl-lite on the north side of the Yarkand River but ~70°W (with phyllite over Jurassic) to the south. Both the reversal in separation sense along strike (e.g., Sylvester, 1988) and structural measure-ments suggest the structure to be a strike-slip fault. In particular, near station Y913–1, a set of small faults with orientations similar to the main Kusilaf fault have shallowly plunging lineations (Figs. 4 and 6C). Although shear sense on the Kusilaf fault was not determined, it seems likely that it is a dextral fault, since it is oriented simi-larly to the Aertashi and Kumtag dextral faults.

DISCUSSION

Understanding the tectonic evolution of the Kashgar-Yecheng segment of the Western Kun-lun Shan is important for understanding the development of the Pamir salient and the evolu-tion of intracontinental subduction in the Pamir. In the following sections I (1) suggest that the right-slip faults described above are part of a 350-km-long, transpressional right-slip transfer system along the eastern margin of the Pamir salient ; (2) discuss constraints on the magnitude of slip along the fault system; (3) review previ-ous data that constrain the timing of deforma-tion within this fault system; and (4) explore implications of these results on the kinematic evolution of the Pamir salient.



Kashgar-Yecheng Transfer System

Dextral strike-slip along the east fl ank of the Pamir salient has been inferred for some time (Brunel et al., 1994; Burtman and Molnar, 1993; Sobel, 1999; Sobel and Dumitru, 1997) and was advocated by Sobel and Dumitru (1997). The structural observations presented here pro-

vide documented fi eld evidence of such struc-tures. In conjunction with geologic (Burtman and Molnar, 1993; Cowgill et al., 2003; Pan, 1996; Schwab et al., 2004; Sengör, 1984; Sobel and Dumitru, 1997), paleomagnetic (Chen et al., 1992; Rumelhart et al., 1999; Yin et al., 2002), thermochronologic (Sobel and Dumitru, 1997), and sedimentologic data (Sobel, 1999;

Yin et al., 2002), the right-slip faults mapped in the Aertashi-Kusilaf area suggest that Cenozoic separation between the north Pamir and West-ern Kunlun Shan occurred by right slip along a set of dextral strike-slip faults herein called the Kashgar-Yecheng transfer system (Fig. 8). This strike-slip system links north-directed thrusts at the leading edge of the northern Pamir to the west (Arrowsmith and Strecker, 1999; Brunel et al., 1994; Burtman and Molnar, 1993; Pavlis et al., 1997; Strecker et al., 1995) with north-directed faults in the Western Kunlun Shan to the east (Figs. 1, 2, and 8). In terms of its overall geometry and kinematics, the Kashgar-Yecheng transfer system is somewhat analogous to the Altyn Tagh fault system (Fig. 1A), which links northeast-directed thrusts within the Nan Shan and Qilian Shan at its northeastern end with north-directed thrusts in the Western Kunlun Shan at its southwestern terminus (Burchfi el et al., 1989; Cowgill et al., 2003; Yin et al., 2002).

The Kashgar-Yecheng transfer system is ~350 km long and comprises a complex net-work of anastamosing faults with a cross-strike width up to 50 km (Fig. 8). The system separates Paleozoic basement of the North Pamir–Kunlun terrane to the west from Paleozoic through Ceno zoic sedimentary cover of the Tarim basin to the east (Figs. 2 and 8). Several observations suggest that the system is transpressional. First, the new structural mapping reported above in-dicates that the fault-bounded panels within the Kashgar-Yecheng system are deformed by folds indicating ENE-WSW maximum shorten-ing (Fig. 4), consistent with structural measure-ments that indicate both reverse- and right-slip faulting in the Aertashi-Kusilaf area (Fig. 6B). Second, structural patterns on previously pub-lished regional geologic maps (Liu, 1988; Pan et al., 2004; Xinjiang Bureau of Geology and Mineral Resources, 1993) indicate that north of the Yarkand River the fault-bounded panels within and to the east of the Kashgar-Yecheng transfer system are deformed by regional folds with axes that also indicate ENE-WSW shorten-ing and that trend oblique to the bounding faults (Figs. 2 and 8). If either these folds, or those in the study area, were due to basinward thrusting orthogonal to the range front, then their hinges should trend subparallel to the bounding faults, as they do at the northwestern end of the sys-tem where it connects with east-west–striking thrusts along the leading edge of the Pamir (Figs. 2 and 8). Third, stratigraphic depth gen-erally increases from east to west across the Kashgar-Yecheng system (Fig. 2), consistent with a component of west-side-up deformation along the transfer system.

While the right-slip component of the transpres-sional deformation along the Kashgar-Yecheng

View to S55oW

Yarkand River

Kumtag fault

Kusilaf

~50 m

axial traces

J

J

J

J

Plan view

Figure 7. Photograph (top panel) and tracing (bottom panel), showing Jurassic strata de-formed by asymmetric, steeply plunging mesoscale folds adjacent to the Kumtag fault. Fold-asymmetry and steeply plunging hinge lines together indicate dextral slip. Inset to bottom panel shows plan view of fold asymmetry expected in dextral shear zone that strikes parallel to the long axis of the framing box. Photograph was taken at station C913–4 (Fig. 4A), look-ing toward the southwest (235°). Kusilaf village and the Yarkand River lie in the distance.

Cowgill


?

?

?

?

? ?

?

Kudi

Oytag

Oytag

Tam Karaul

Tam Karaul

Kudi-Tam Karaul correlationimplies minimal separation:

Oytag-Tam Karaul correlationimplies dextral separation:

Oytag-Kudi correlationimplies sinistral separation:

Kudi

50 km

Thrust/reverse fault

Strike-slip fault

Normal fault

Detachment fault

Syncline

Anticline39°N

75°E 76°E 77°E

38°N

37°N

39°N38°N

37°N

Map Symbols:

75°E 76°E 77°E

Site discussed in text

Kudi-Oytag

Anyimaqen-Kunlun

StudyArea

Tiklik F.

Aertashi F.

KongurShan F.

Kum

tag F.

Kusilaf F.

Aksu-Murgab F.

East Pamir F.

Karakoram F.

Main Pamir Thrust

N Tam Karaul F.

Karakax F.

Oytag(Wuyitake)

Kudi

Kashgar-Yechengtransfer system

Sutures:

Figure 8. Simplifi ed map of the transpressional, right-slip Kashgar-Yecheng transfer sys-tem and other major structures along the eastern fl ank of the Pamir salient. Dotted gray lines show sutures, and stars indi-cate occurrences of distinctive volcanic rocks discussed in the text. Schematic diagrams at the base illustrate implications re-garding the sense of fault sepa-ration of previously suggested geologic correlations. See text for discussion.



transfer system is apparently accommodated at the northwestern end of the fault by south-directed subduction of the Tarim-Tajik basin beneath the south-dipping Main Pamir Thrust and associated faults to the south (e.g., Burt-man and Molnar, 1993), it is not presently clear how deformation is absorbed at the southeastern end of the fault, in the Western Kunlun Shan. Although the fault system appears to link with the north-directed Hotan fold-thrust belt and the south-dipping Tiklik and Tam Karaul faults (Cowgill, 2001; Matte et al., 1996; Sobel and Dumitru, 1997), as shown in Figures 2 and 8, this structural geometry alone does not produce dextral separation of the North Pamir relative to the Western Kunlun Shan, because both belts lie within the upper plate of the fault system. A similar problem was documented for the southwestern termination of the Altyn Tagh fault (e.g., Fig. 1D) at the eastern end of the western Kunlun Shan (Fig. 1A) (Cowgill et al., 2003). The defl ection of major terranes and su-tures within the Pamir (Fig. 1B) (Burtman and Molnar , 1993; Schwab et al., 2004) may suggest that oroclinal bending of units at the southeast-ern termination of the Kashgar-Yecheng transfer system might resolve this problem. Before ex-ploring this question, a brief explanation of the orocline concept and associated terminology is needed for clarity relative to prior work.

By defi nition, oroclines form by vertical-axis rotation of the fl anking limbs (Carey, 1955); however, the detailed kinematics by which such rotations occur was not specifi ed when the con-cept was fi rst introduced. Thus, whereas some authors treat all orogenic curves formed by vertical-axis rotations as oroclines (Macedo and Marshak, 1999; Marshak, 1988, 2004), others differentiate those that evolve into a curved form during progressive deformation (progressive arcs) from those bent during a later event (oro-clines) (Weil and Sussman, 2004). Because here I follow the fi rst use, oroclines include orogenic curves formed by the wrapping of an orogen around an obstacle, radial thrusting, and buck-ling owing to shortening along strike (Johnston, 2004; Marshak, 1988). Kinematic similarities between oroclines and folds have long been noted (e.g., Carey, 1955; Johnston, 2001; Mar-shak, 1988; Ries and Shackleton, 1976), al-though such comparisons do not mean to imply a similarity in terms of mechanics. A key dif-ference in these kinematic models is whether rotation is accommodated by tangential exten-sion (i.e., increase in arc length between the endpoints shown in Fig. 9A), such as during ra-dial thrusting, or by strike-parallel shortening of the belt (i.e., convergence of endpoints with no change in arc length), such as during oroclinal buckling (Fig. 3A). To explore the implications

of comparing fold kinematics with those of an orocline, Figure 9A shows two simple kinematic models for the way in which oroclinal bending might be accommodated. In the fi rst, deforma-tion is analogous to that within a parallel fold, in which case the map-view width of markers (gray bands in Fig. 9A) does not change. In the second, deformation is somewhat analogous to that within a similar fold, such that distributed shearing of the limbs of the orocline results in attenuation of the map-view width of markers (Fig. 9A). The potential importance of such distributed shearing and/or strike-slip faulting in accommodating deformation along the limbs of map-view curves has been previously rec-ognized by a number of workers (Macedo and Marshak, 1999; Marshak, 1988, 2004; Ries and Shackleton, 1976). It seems most reasonable to apply the simple shear model to the southeast-ern termination of the Kashgar-Yecheng transfer system, because the parallel-fold model predicts left-slip faulting along the eastern fl ank of the Pamir that is incompatible with the right-slip faulting observed in the Aertashi-Kusilaf area. In addition, the parallel-fold model predicts endpoint convergence along the reference line, for which there is no evidence. I return to this latter point below.

Estimated Displacement

Three different markers have previously been used to infer the magnitude of deformation along the eastern margin of the Pamir salient, although two of these are problematic. The fi rst, and most reasonable, estimate is based on corre-lation of the North Pamir with the Western Kun-lun Shan, a link that has been made by a number of authors because of similarities in the Paleo-zoic geology of the two regions (Burtman and Molnar, 1993; Cowgill et al., 2003; Pan, 1996; Sengör, 1984; Sobel and Dumitru, 1997; Tap-ponnier et al., 1981; Yin and Harrison, 2000). U-Pb zircon ages from plutonic rocks in the Western Kunlun Shan and eastern Pamir sup-port this correlation (Cowgill et al., 2003, and references therein; Schwab et al., 2004). Geo-logical and geophysical data indicate that sepa-ration between the North Pamir and Western Kunlun Shan was accommodated by ~300 km of Cenozoic intracontinental subduction of the Tajik-Tarim basin beneath the North Pamir and an additional ~300 km of convergence between North and South Pamir owing to shortening within the upper plate (e.g., Burtman and Mol-nar, 1993; Hamburger et al., 1992). As such, the Main Pamir Thrust and Tiklik fault appear to be equivalent structures that form the lead-ing edge of the Pamir-Kunlun system, with the Main Pamir Thrust having moved north relative

to the Hotan thrust belt. Correlation of the Main Pamir Thrust with the Tiklik fault (Figs. 2 and 8) suggests ~280 km of total right slip along the Kashgar-Yecheng transfer system.

The second offset estimate is based on correla-tion of ophiolitic rocks at Oytag, Kudi, and along the Tam Karaul fault in the Western Kunlun Shan (Deng, 1996; Pan and Bian, 1996; Sobel and Arnaud , 1999) (Fig. 8). The ophiolitic materials at Kudi are intensely deformed, and no complete section is preserved. They have been interpreted as lower Paleozoic or Proterozoic oceanic crust intruded by an Ordovician magmatic arc (Deng, 1996; Matte et al., 1996) and have been inter-preted as a lower Paleozoic arc ophiolite (Xiao et al., 2002). Kudi lies along strike from ultra-mafi c bodies mapped to the east (Xinjiang Bu-reau of Geology and Mineral Resources, 1993) that generally coincide with the Tam Karaul fault (Matte et al., 1996; Norin, 1946; Xinjiang Bu-reau of Geology and Mineral Resources, 1993). However, there are problems with correlating the Oytag, Kudi, and Tam Karaul regions. Most importantly, they appear to be of different ages, with Middle Ordovician to Middle Devonian emplacement of the Kudi ophiolite and Carbon-iferous or younger emplacement of the Oytag arc (Xiao et al., 2002), although the age of the Oytag arc is not well constrained. The structural separations are also problematic (Fig. 8): The Kudi–Tam Karaul correlation across the south-eastern extension of the Kashgar-Yecheng trans-fer system implies no signifi cant displacement, whereas the Oytag–Tam Karaul correlation im-plies major dextral separation along the Kusilaf fault, and the Oytag-Kudi correlation implies an equivalent sinistral separation along the same fault (Fig. 8). In summary, more detailed work in the area is needed before these localities can be correlated confi dently.

Finally, based on a correlation of porphyritic rhyolites near Oytag and south of Kudi (Figs. 2 and 8), Sobel and Dumitru (1997) and Sobel (1999) argued for ~200 km of dextral offset on what they call the Main Pamir Thrust–Kumtag fault system. In this interpretation, the Main Pamir Thrust (here called the Kusilaf fault) is thought to terminate into the Tam Karaul fault near Kudi (Figs. 2 and 8) (Sobel and Dumitru, 1997). However, as Figures 2 and 8 indicate, both rhyolite localities lie west of the Kumtag fault, and their separation on the Main Pamir (here Kusilaf) fault is sinistral, rather than dex-tral. Thus, it is not clear how these samples sup-port right-lateral separation on this fault system as originally argued by Sobel (1999) and Sobel and Dumitru (1997).

It may be that some Pamir-Kunlun separa-tion predates the Cenozoic. Basin analysis of Mesozoic deposits along the eastern fl ank of the

Cowgill


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Pamir led Sobel (1999) to conclude that lower Middle Jurassic strata were deposited in a nar-row, northwest-trending elongate basin that he interpreted to have formed within a trans-tensional step between the right-slip Talas-Ferghana fault and an ancestral Main Pamir Thrust (my Kusilaf fault in Fig. 4). The mag-nitude of proposed Jurassic right slip along the ancestral fault was not specifi ed, and structural evidence for this phase of deformation remains to be reported. One important problem with this tectonic interpretation of the Jurassic strata is the likelihood that the present distribution of Jurassic deposits does not represent the original geometry of the Jurassic basin: Thrusting of the North Pamir over the Tajik-Tarim basin would have removed any Jurassic deposits to the west of those described by Sobel (1999), giving the impression that the original basin was elongate along the younger strike-slip faults that trun-cated it. For these reasons I have not attempted to remove Mesozoic right slip from my esti-mate of 280 km of total displacement along the Kashgar-Yecheng transfer system.

Timing and Implied Long-Term Slip Rates

Regional stratigraphic data and apatite fi ssion-track analyses suggest that Cenozoic dextral slip along the Kashgar-Yecheng system initiated be-tween the late Eocene and early Miocene. Burt-man and Molnar (1993) argued that separation of the North Pamir from the Western Kunlun Shan initiated in the late Eocene , based on re-gional stratigraphic data documenting both the retreat of a Late Cretaceous to Paleogene marine incursion in the composite Tajik-Tarim basin and isolation of the Tajik basin from southwest-ern Tarim. Likewise, basin analysis of Ceno zoic deposits at Aertashi indicates that rapid sub-sidence initiated at ca. 37 Ma ago at this site (Yin et al., 2002). Detrital apatite fi ssion-track data from the Aertashi section are consistent with this result, with a cluster of single-grain ages at 30.0 ± 4.8 Ma in a probable Miocene sandstone sample (Sobel and Dumitru , 1997), indicating that cooling in the source terrain was under way by the early Oligocene. Considering the tectonic setting of the Aertashi section, such denudation and basin subsidence are most rea-sonably attributed to crustal thickening within the Pamir-Kunlun system and asso ciated load-ing of the Tarim. Presuming that the Kashgar-Yecheng system was active by ca. 37 Ma ago, and has accommodated ~280 km of right slip, then the average dextral slip rate along the east-ern margin of the Pamir has been on the order of 7–8 mm/a.

Additional data indicate that deformation within the Kashgar-Yecheng system was under

way in the vicinity of Aertashi by 25–18 Ma ago. Sedimentologic and magnetostratigraphic analy-sis of Oligocene-Miocene deposits at Aertashi indicates that the basin underwent a transition to proximal, coarse-clastic deposition at ca. 25 Ma ago (Yin et al., 2002), indicating that the Pamir-Kunlun system was topographically expressed by this time. Samples of the folded Jurassic de-posits between the Kusilaf and Kumtag faults at Kusilaf (Fig. 4) yielded tightly clustered popu-lations of single-grain apa tite fi ssion-track ages with a pooled age of 18.0 ± 0.8 Ma (Sobel and Dumitru, 1997). Thermal modeling of these data indicates that the samples cooled below ~120–100 °C between 21 and 19 Ma ago (Sobel and Dumitru, 1997). Likewise, apatite fi ssion-track ages of folded Cretaceous-Paleogene samples east of the Aertashi fault (Fig. 4B) yield a pooled, minimum age of 20.0 ± 3.1 Ma for the time of cooling below ~120–100 °C (Sobel and Dumitru , 1997). At a regional scale, paleo-magnetic data indicate that vertical-axis rotation of thrust sheets within the Tajik basin did not start until after the early Miocene, suggesting that northward migration of the Pamir relative to SW Ghissar (Fig. 1A) did not start until this time (Thomas et al., 1994).

If the Kashgar-Yecheng transfer system did not initiate until 25–18 Ma ago, its average dex-tral slip rate has been 11–15 mm/a. This rate is broadly similar to GPS results that indicate 13 ± 4 mm/a of convergence between the north-ern Pamir and the southern Tian Shan (Reigber et al., 2001), which is presumably accommodated by the Main Pamir Thrust. Horizontal shorten-ing rates since the Holocene (Arrowsmith and Strecker, 1999) and 25 Ma ago (Coutand et al., 2002) are lower, but are also minimum bounds.

Kinematic Model of the Pamir

In contrast to the right-slip transfer model I advocate for the eastern margin of the Pamir salient, an alternative scenario is that the northwest-southeast trend of the margin results either from radial thrusting and associated clock-wise vertical-axis rotation of the Pamir-Kunlun belt over the Tarim basin to the east, or orocli-nal buckling (e.g., Fig. 3A). Paleomagnetic data from the eastern Tajik basin indicate systematic anticlockwise vertical-axis rotations that pro-gressively decrease in magnitude with increas-ing distance from the Pamir in a pattern that is generally interpreted to refl ect radial thrusting along the western margin of the salient (Bazhe-nov and Burtman, 1982; Bazhenov et al., 1994; Burtman, 2000; Burtman and Molnar, 1993; Thomas et al., 1994) but which are also com-patible with oroclinal buckling. Importantly, the similarity of Cretaceous and Tertiary paleomag-

netic vectors within the Tajik basin indicates that rotation of the western fl ank of the Pamir is early Miocene or younger (Thomas et al., 1994). Several studies have developed palin spastic re-constructions of the Tajik basin and the west fl ank of the Pamir to explain both these paleo-magnetic data and structural geometries within the basin (Bourgeois et al., 1997; Burtman, 2000; Thomas et al., 1996). Bourgeous et al. (1997) developed a particularly elegant least-squares numerical restoration of fault-bounded blocks in map view. These authors were able to determine fi elds of fi nite horizontal transla-tion and vertical-axis rotation within the Tajik basin by restoring in three dimensions deforma-tion from faulting and folding as recorded by a single stratigraphic layer. In this reconstruction, at least 240 km of northwestward motion of the Pamir relative to the SW Gissar (Fig. 1A) was accommodated by anticlockwise vertical-axis rotations and rigid-body translations of indi-vidual thrust sheets, with more westerly sheets subject to larger rotations than those to the east, which matches the paleomagnetic data. Thus, both reconstruction of structures and paleomag-netic data independently indicate radial thrust-ing within the Tajik basin.

However, analogous radial thrusting along the eastern margin of the Pamir is not supported by the right-slip fault systems along the Yarkand River described above (Figs. 4–7). In addition, paleomagnetic analyses of Cretaceous and Ter-tiary deposits at Wuqia, Yingjisha, and Aertashi (Fig. 2) preclude vertical axis rotation by orocli-nal bending along the eastern fl ank of the salient (Chen et al., 1992; Rumelhart et al., 1999; Yin et al., 2000). Oytag is the only measured site along the eastern margin that shows signifi cant vertical-axis rotation (Chen et al., 1992). How-ever, this locality lies between the Kusilaf and Kumtag faults (Fig. 2), and its clockwise rota-tion is more reasonably explained by right slip along these structures and rotation of the panel they bound, rather than large-magnitude radial thrusting equivalent to that along the western margin of the Pamir.

Radial thrusting around the entire Pamir sa-lient is also inconsistent with observed magni-tudes of tangential extension. In radial-thrusting models for orocline formation in which the endpoints remain fi xed (Fig. 9A), material lines parallel to the thrust front undergo tangential extension as the amplitude of the orocline in-creases (Marshak, 1988). With reference to the two endpoint locations shown in Figure 1, the edge length (i.e., the distance between two endpoints as measured around the salient arc) is ~1310 km, whereas the chord length (i.e., the straight-line distance between the endpoints) is ~930 km. Together, these values predict on

Cowgill


the order of a ~380 km extension if the Pamir salient formed by pure radial thrusting. How-ever, this predicted extension is an order of mag-nitude larger than that reported from the two main extensional systems within the Pamir, the Karakul graben (a minimum of ~1.2 km throw; Strecker et al., 1995) and the Kongur Shan ex-tensional system (a minimum of ~34 km E-W extension; Robinson et al., 2004, 2007).

An alternative possibility is that the Pamir salient has formed by oroclinal buckling. In pure oroclinal buckling, tangential extension is zero because the increase in orocline amplitude is balanced by convergence of the endpoints along the reference line (Johnston, 2001, 2004), meaning the difference between the edge and chord lengths indicates the magnitude of end-point convergence (Figs. 3A and 9A). Thus, when applied to the Pamir, such a model im-plies ~380 km of roughly east-west shorten-ing of the Paropamisus–Northern Pamir–South Kunlun belt. However, there is no independent evidence of such east-west shortening, and con-sidering that Indo-Asian convergence is directed generally north-south, such an along-strike convergence seems unlikely. More important, however, are the implications for deformation within the crustal blocks to the north and south of the Paropamisus–Northern Pamir–South Kunlun belt: If the belt is to have shortened by ~380 km along strike, the continental crust and lithosphere surrounding it should have as well, but structures accommodating such east-west shortening are not evident. Other discus-sions of oroclinal buckling have recognized the same problem of accommodating shortening in the surrounding lithosphere during the orogen-parallel shortening associated with oroclinal buckling (Johnston, 2004; Schellart and Lister, 2004). Although decoupling of the orogen from surrounding oceanic lithosphere during oro-clinal buckling of a ribbon continent may work in the intra-oceanic settings to which the model has previously been applied (Johnston, 2001, 2004), it is diffi cult to envision the kinematics of such deformation within the intracontinental setting of the Pamir.

To reconcile the right-slip Kashgar-Yecheng system I describe here along the eastern margin of the Pamir, with the radial thrusting along the western margin suggested by the paleomagnetic and structural data from the Tajik basin, I sug-gest that regional deformation within the Pamir salient is asymmetric (Fig. 9B and C). In this model, subduction of the Tajik-Tarim basin be-neath the Pamir was absorbed along the western margin of the salient by radial thrusting, and along the eastern margin by dextral-slip along the Kashgar-Yecheng system. Figure 9C sche-matically shows the proposed sequence.

In the Paleocene-Eocene confi guration, prior to the formation of the Pamir salient, the North Pamir–Kunlun terrane formed a single, sub-linear belt that lay along the southern margin of the continuous Tajik-Tarim basin (Burtman and Molnar, 1993; Cowgill et al., 2003) (fi rst panel in Fig. 9C). The southern margin of the North Pamir–Kunlun terrane is now represented by the Herat, Akbaytal, and Karakax faults (Cowgill et al., 2003). In addition, restoration of ~300 km of upper-plate shortening within the Pamir sug-gests that the north-south width of the central Pamir in the early Cenozoic was several hun-dred kilometers larger than at present.

In the late Oligocene to early Miocene con-fi guration, the North Pamir has moved ~150 km north relative to the Paropamisus and Kunlun belts to the west and east, respectively (second panel in Fig. 9C). In the center of the salient this motion was absorbed by south-directed subduction of the Tarim-Tajik basement along the Main Pamir Thrust or equivalent, older structures (e.g., Burtman and Molnar, 1993; Hamburger et al., 1992). To the west, northwest-directed radial thrusting resulted in anticlock-wise, vertical-axis rotation of the west fl ank of the Pamir over the Tajik basin. In contrast, the transpressional dextral Kashgar-Yecheng trans-fer system formed along the eastern fl ank of the Pamir. Sedimentologic and thermochronologic data from the Aertashi area (Sobel and Dumitru , 1997; Yin et al., 2002) suggest that this site had been juxtaposed against the Pamir by the Kashgar-Yecheng system by 25–18 Ma ago. In this intermediate stage the Western Kunlun Shan have moved north ~25–50 km relative to the Paropamisus, owing to north-directed thrusting within the Hotan thrust belt and along the Tiklik fault, to the east of the Kashgar-Yecheng trans-fer system (Cowgill, 2001; Matte et al., 1996). I also speculate that shortening of the central Pamir was facilitated by the northward propa-gation of the left- and right-slip Chaman and Karakoram faults along the western and eastern margins of the Pamir salient, respectively.

Currently the western fl ank of the Pamir has undergone >50° of anticlockwise vertical-axis rotation from radial thrusting, and the Kashgar-Yecheng system has accommodated ~280 km of right-lateral northward motion of the lead-ing edge of the Pamir relative to the Tiklik fault to the east (third panel in Fig. 9C). Distributed defor ma tion within the upper plate of the Pamir subduction system has been accommodated by oroclinal bending of the Pamir-Kunlun terrane, shortening and possible intracontinental sub-duction within the interior of the Pamir (e.g., Ducea et al., 2003; Robinson et al., 2007), and strike-slip faulting along the Chaman, Kara-koram, and Karakax faults. Oroclinal bending

and associated along-strike elongation of the Pamir-Kunlun belt probably reduced the cross-strike width of this marker, as indicated sche-matically in Figure 9A by the “similar-fold” model of oroclinal bending.

It is interesting to note that the distribution of extensional faults also shows marked east-west asymmetry. In contrast to the western margin of the Pamir, where no major extensional systems have been reported, the northeastern margin of the Pamir is cut by major north-south–striking extensional systems, including the Kongur Shan detachment (Robinson et al., 2004, 2007) and the Karakul extensional system (Strecker et al., 1995) (Figs. 1 and 2). This spatial correlation suggests that there may be a genetic relation-ship between these extensional fault systems and the Kashgar-Yecheng fault system. While direct structural linkages are not evident, the exten-sional systems may help accommodate tangential extension asso ciated with a component of oro-clinal bending south and west of the leading edge of the Pamir-Kunlun belt (i.e., south and west of the Main Pamir Thrust, Kashgar-Yecheng trans-fer, and Hotan fold-thrust-belt system).

The need for either large tangential exten-sion or along-strike shortening within the Pamir would be reduced if the total offset along the left-slip Darvaz fault is large (200–300 km). In this case, the Pamir would be a nonrotational arc bounded by the left-slip Darvaz and right-slip Kashgar-Yecheng transfer systems on its west-ern and eastern fl anks, respectively (Fig. 3B). In this scenario, radial thrusting in the Tajik basin to the west of the Darvaz fault resulted from lo-cal rotations of thrust sheets during advance of the Pamir.

CONCLUSIONS

The following conclusions can be drawn from the present structural investigation of the Kashgar-Yecheng segment of the Pamir-Kunlun range.

(1) Structural mapping of a 32-km-long reach of the Yarkand River indicates that the area is deformed by a series of northwest-striking , brittle, dextral strike-slip faults that juxtapose narrow panels of basement and Phanerozoic strata at, and to the west of, the Pamir-Kunlun range front. From west to east, the four main faults in the area are the Aertashi, Yarkand, Kumtag, and Kusilaf faults. The fault-bounded panels between these structures are deformed by en echelon folds with axes that trend more westerly than the adjacent faults, consistent with dextral transpression. Structural measurements and asymmetric folds with near-vertical hinge



lines document right-slip shear sense along the Aertashi and Kumtag faults, and suggest that the Yarkand and Kusilaf faults are also right-slip structures.

(2) The right-slip faults in the Aertashi-Kusilaf area indicate that Cenozoic separation be-tween the North Pamir and Western Kun-lun Shan was accommodated by the dextral Kashgar-Yecheng transfer system, a ma-jor transpressional fault system that links north-directed thrusts at the leading edge of the Pamir with similarly oriented faults in the Western Kunlun Shan to the east. This transfer system is ~350 km long and com-prises a complex network of anastamosing faults ~50 km wide across strike.

(3) Integrating these results with previous work suggests that regional deformation within the Pamir salient is asymmetric. Deformation along the western fl ank of the Pamir appears to have been accommodated by northwest-directed radial thrusting and associated anti-clockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin (Bazhenov et al., 1994; Burtman, 2000; Burt-man and Molnar, 1993; Thomas et al., 1994), along with a component of left-slip faulting along the Darvaz fault (Burtman and Molnar, 1993), although it remains unclear how the tangential extension predicted by such radial thrusting has been absorbed. In contrast, the present study and previous paleomagnetic work (Chen et al., 1992; Rumelhart et al., 1999; Yin et al., 2000) together indicate that subduction of the Tajik-Tarim basin beneath the Pamir was absorbed along the eastern margin of the salient by dextral-slip along the Kashgar-Yecheng transfer system.

(4) On the basis of the defl ection of major ter-ranes, sutures, and structures within the southeastern Pamir and western Tibet, I sug-gest that Pamir-Tarim relative motion may have been absorbed at the southeastern end of the dextral Kashgar-Yecheng transfer sys-tem by oroclinal bending. I speculate that distributed shearing of the limbs of the oro-cline may have resulted in reduction of the cross-strike width of regional markers, in a pattern analogous to that seen on the limbs of similar folds.

(5) Total right slip along the Kashgar-Yecheng transfer system appears to be ~280 km, based on correlation of the Main Pamir Thrust and the Tiklik fault at the northwestern and southeastern ends of the system, respectively. Previous correlations of porphyritic rhyolites at Oytag and south of Kudi (Sobel, 1999; Sobel and Dumitru, 1997) are problematic, because both rhyolite localities lie west of the Kumtag fault, and their separation on the

Kusilaf–Main Pamir Thrust is sinistral rather than dextral. As such, it is unclear how these samples can support right-lateral separa-tion along these faults as previously argued (Sobel , 1999; Sobel and Dumitru, 1997).

(6) Dextral slip along the Kashgar-Yecheng sys-tem may have initiated as early as the late Eocene on the basis of regional stratigraphic data (Burtman and Molnar, 1993; Yin et al., 2002) and detrital apatite fi ssion-track analy-ses (Sobel and Dumitru, 1997), suggesting a long-term slip rate of 7–8 mm/a. Alterna-tively, data indicating that deformation was under way by 25–18 Ma ago near Aertashi leads to an average slip rate of 11–15 mm/a, if the Kashgar-Yecheng transfer system had initiated at this time.

ACKNOWLEDGMENTS

This project was supported by U.S. National Sci-ence Foundation grants EAR9725599 to An Yin and EAR0310415 to Cowgill. Detailed and constructive reviews by Stephen Johnston, Alexander Robinson, and an anonymous reviewer helped improve the paper signifi cantly. Nickolas Raterman, Ryan Gold, Eric Kirby, Lindsay Schoenbohm, and an anonymous re-viewer also kindly provided helpful comments on ear-lier drafts. My understanding of the region has been greatly improved by discussions with Peter Rumel-hart, Paul Kapp, Mike Murphy, and Ed Sobel. I grate-fully acknowledge An Yin for the use of unpublished fi eld mapping and structural data and thank Zhang Qing, Laura Veirs, and Kari Cooper for their assis-tance in the fi eld. Stereoplots were generated with Stereonet v. 4.9.6, developed by Rick Allmendinger. The fi eld research reported here formed part of my Ph.D. work and was partially supported by grants-in-aid of research from The Geological Society of America, the American Association of Petroleum Geologists, and the U.S. National Academy of Sciences through Sigma Xi, The Scientifi c Research Society.

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