scharer jsg tien shan 04.pdf

UNCORRECTED PROOF

Detachment folding in the Southwestern Tian Shan–Tarim foreland,

China: shortening estimates and rates

K.M. Scharera,*, D.W. Burbankb, J. Chenc, R.J. Weldona, C. Rubind, R. Zhaoc, J. Shenc

aDepartment of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USAbDepartment of Geological Sciences, University of California, Santa Barbara, CA 93106, USA

cState Key Laboratory of Earthquake Dynamics, Institute of Geology, China Seismological Bureau, P.O. Box 9803, Beijing 100029, P.R. ChinadDepartment of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA

Received 17 February 2003; received in revised form 15 January 2004; accepted 26 February 2004

Abstract

Geologic observations of the Chinese Tian Shan foreland reveal active, contractional deformation along the entire margin. To quantify the

amount of shortening and understand the structural style, we mapped the stratigraphy and structure of four folds expressed at the western end

of the foreland, north of Kashi. In this region, upper Tertiary through Quaternary sediments are conformable, but an abrupt transition from

parallel to unconformable bedding in the uppermost strata suggests a transition from pre-fold strata to strata deposited on a growing fold. The

folds have very steep (60–908) limbs and are box-like to isoclinal, suggestive of detachment folding. Total north–south shortening across the

center of the region is .9 km, of which 5–7 km occurred in the Kashi–Atushi fold system. Shortening estimates determined with excess

area methods for individual folds decrease from a maximum of 6.8 km in the northwest to a minimum of 0.7 km in the southeast. Timing

derived from a paleomagnetic study shows that the transition to syn-folding strata occurred ,1.2 Ma in the middle of the study area, resulting

in an average shortening rate for the Kashi–Atushi fold system of ,5 mm/yr if folding was coeval. The shortening rate is high compared

with foreland deformation east of the study area, suggesting that the regional stresses or response of the foreland stratigraphy are unique to

the Kashi–Atushi fold system kinematics.

q 2004 Published by Elsevier Ltd.

Keywords: Kashi Depression; Xiyu conglomerate; Lateral propagation; Growth strata; Anticline; Kepingtage

1. Introduction

The Tian Shan (Fig. 1) has been the focus of recent

geologic investigations concerning the timing, sequence,

and geometry of active, intracontinental mountain building

(Sobel and Dumitru, 1997; Yin et al., 1998; Allen et al.,

1999; Burchfiel et al., 1999; Sobel et al., 2000; Abdrakh-

matov et al., 2002; Thompson et al., 2002). These studies

suggest that the onset of deformation in the Tian Shan began

about 20 Ma, or ,35 Myr after the initial Indo-Asian

collision, and that north–south directed shortening con-

tinues to be active across the entire range. Thompson et al.

(2002) demonstrate that at late Pleistocene time scales,

,13 mm/yr of shortening is accommodated on 5–6 discrete

structures distributed across the Kyrgyz Tian Shan.

Geodetic studies indicate that up to 50%, or 20–24 mm/

yr, of the total modern convergence between the Indian and

Eurasian plates is accommodated by shortening in the Tian

Shan (Abdrakhmatov et al., 1996; Holt et al., 2000; Wang

et al., 2000). The geologic and geodetic observations agree

well in the Kyrgyz Tian Shan, which suggests that the

remaining, geodetically calculated shortening of ,7–

10 mm/yr should occur south of the Aksay Basin in the

southern Tian Shan and northern Tarim Basin (Fig. 1).

Studies in the central and eastern Tian Shan foreland

have estimated the Cenozoic shortening, but provided loose

control on the timing of the deformation in the western

foreland (Yin et al., 1998; Allen et al., 1999; Burchfiel et al.,

1999). In the southwestern Chinese Tian Shan foreland, up

to 12 km of Cenozoic strata (Bally et al., 1986) provide a

pristine succession in which the style, age, and rates of

shortening can be investigated. This paper presents

structural, stratigraphic and geomorphic data to define the

style of folding in this region. In combination with a

0191-8141/$ - see front matter q 2004 Published by Elsevier Ltd.

doi:10.1016/j.jsg.2004.02.016

Journal of Structural Geology xx (0000) xxx–xxx

www.elsevier.com/locate/jsg

* Corresponding author. Tel.: þ1-514-346-4573; fax: þ1-514-346-4692.

E-mail address: [email protected] (K.M. Scharer).

SG 1558—7/7/2004—17:10—MNICHOLLS—110002— MODEL 5 – pp. 1–19

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http://www.elsevier.com/locate/jsg

UNCORRECTED PROOF

magnetostratigraphic study presented by Chen et al. (2002),

we estimate the timing of initiation of folding and quantify

regional shortening rates. We show that structures in the

northwest Tarim Basin are detachment folds that began

deforming during the Pleistocene and have accommodated

much of the recent shortening predicted geodetically to

occur south of southern Kyrgyzstan.

1.1. Geographic and geologic setting

The Tian Shan extends over 2000 km eastward from

Tajikistan, covers most of Kyrgyzstan, and tapers to the east

in the Xinjiang province of western China At the study

location, the range is ,500 km wide. The Tarim Basin is an

internally draining, trapezoid shaped basin 1500 km long,

pinched between the Tian Shan on the north, the Pamir

Mountains to the west, and the Tibetan Plateau to the south.

The field area is located in the northwest corner of the Tarim

Basin, approximately 250 km from the Alai Valley where

the Tian Shan and the Pamir Mountains meet (Fig. 1). The

arid climate and the paucity of vegetation provide excellent

exposure for structural and stratigraphic mapping.

The Tian Shan and the Tarim Basin blocks have been

proximate since the Devonian, when an intervening ocean

basin separating the two began subducting under the

ancestral Tian Shan (Watson et al., 1987; Carroll et al.,

1995). The two blocks were sutured by the late Permian

(Carroll et al., 1995; Yin and Nie, 1996) and have

subsequently felt the effects of at least three Mesozoic

collisions south of the Tarim craton (Hendrix et al.,

1992). The period between the last Mesozoic collision,

when the Kohistan–Dras arc – forearc complex was

accreted to southern Asia (,70 Ma), and the initiation

of Cenozoic deformation in the Tian Shan (,25 Ma) was

a time of tectonic quiescence. During this ,45 Myr

period, the ancestral Tian Shan was beveled by erosion.

Stratigraphically, this period is represented by a wide-

spread, planar denudation surface in the Tian Shan

(Abdrakhmatov et al., 2002) and by deposition of lower

Tertiary strata along its margins (Bally et al., 1986; Hu,

1992). Within the Tian Shan proper, the Cenozoic

deformation has been quantified by studying the uplift

and faulting of the early Tertiary peneplain (Burbank

et al., 1999; Bullen et al., 2001; Abdrakhmatov et al.,

2002; Thompson et al., 2002). At the southern margin of

the Tian Shan, the peneplain is absent, but deformation of

foreland fold-and-thrust sequences has been used to

understand the timing and style of the outward growth

and uplift of the Tian Shan (Sobel and Dumitru, 1997;

Yin et al., 1998; Allen et al., 1999; Burchfiel et al., 1999;

Chen et al., 2001, 2002).

Geophysical evidence, numerical modeling, and both

geologic and geomorphic evidence suggest that the Tarim

Basin is being coherently thrust under the Tian Shan

(Molnar and Tapponnier, 1975; Burov et al., 1990; Avouac

et al., 1993; Burtman and Molnar, 1993; Neil and House-

Fig. 1. Elevation map of the Tarim Basin, Tian Shan, Pamir Mountains, and Northern Tibetan Plateau. Major rivers designated with dashed lines, lake (Issyk-

kul) in stripes. The study area is located in the western corner of the Tarim Basin, surrounded to the north, west, and south by .4000 m peaks. The box north of

Kashi locates Fig. 2. Number 1 locates the Aksay Valley described by Thompson et al. (2002) and 2 is the Alai Valley. Numbers 3–5 locate the Kepingtage,

Kuche, and Boston Tokar/Kalasu River sections of Allen et al. (1999), Yin et al. (1998), and Burchfiel et al. (1999), respectively.


K.M. Scharer et al. / Journal of Structural Geology xx (0000) xxx–xxx2

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UNCORRECTED PROOF

man, 1997; Burchfiel et al., 1999; Abdrakhmatov et al.,

2002). Sediments express the convergence in a series of

folds and thrust faults along the northern margin of the

Tarim Basin (Rubin et al., 2000; Scharer et al., 2000; Zhao

et al., 2000; Chen et al., 2001). The topographic front of the

Tian Shan proper is delineated by the South Tian Shan fault,

a reverse fault with unknown displacement magnitude (Yin

et al., 1998). Basinward of the South Tian Shan fault, two

areas with contrasting deformational styles are clear in

Landsat Multispectral Scanner imagery (Fig. 2). The

northern area, called the Kepintage–Yishilakekalawuer

thrust fault (Kepingtage), represents the westernmost

expression of the Kashi–Akesu system as identified by

Yin et al. (1998). It is characterized by south verging thrust

faults that place Paleozoic strata on top of Mesozoic and

Cenozoic sediments (XBGM, 1985; Sobel and Dumitru,

1997; Rubin et al., 2000; Zhao et al., 2000; Chen et al.,

2001, 2002). Fission track dating by Sobel and Dumitru

(1997) suggests that deformation propagated southward

from the Tian Shan, exhuming this part of the Kepingtage

system since ,14 Ma. Total shortening in this part of the

Kepingtage has not been calculated, but folded sediments in

piggyback basins provide evidence for a component of late

Tertiary deformation.

The southern area, the Kashi–Atushi fold system, is

south of the Kepingtage system. It includes the towns of

Kashi (Kashgar) and Atushi (Artush), and is characterized

by two rows of sinuous folds within the Kashi Depression, a

deep Tertiary sub-basin. Major rivers draining the Tian

Shan cross the Kashi–Atushi fold system. Large earth-

quakes have occurred at the perimeter of this area, but none

are recorded within the fold system (Fig. 3). We

differentiate this area from the Kashi–Akesu system of

Yin et al. (1998) due to the striking difference between the

Fig. 2. Landsat image of the southwestern Tian Shan. The study area is located south of the Southern Tian Shan fault (STS), the Muziduke fault (MF), and the

Kepingtage–Yishilakekalawuer Thrust Fault system (Kepingtage). The timing and displacements of these faults are poorly constrained. TFF is the Talas–

Ferghana fault, a right lateral fault with ,10 mm/yr Cenozoic slip rate (Burtman et al., 1996). The Kashi–Atushi fold system covers the lower half of the

image. Upper Tertiary strata (light color) comprise most of the folds, which are rimmed by resistant Quaternary conglomerates (dark color distinct on the

southern limbs). The Talas–Ferghana fault projects into the middle of the fold system, which appears unaffected, suggesting right-lateral strain on the Talas–

Ferghana fault is partitioned into the South Tian Shan fault or the Muziduke fault rather than the Tarim craton.


K.M. Scharer et al. / Journal of Structural Geology xx (0000) xxx–xxx 3

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UNCORRECTED PROOF

imbricate thrust stacks of Mesozoic and Paleozoic strata that

form the northern band of deformation in the Tarim Basin

and the simply folded upper Cenozoic strata in the study

area. The Kashi–Atushi fold system includes the Atushi–

Talanghe, Mutule, Mingyaole and Kashi anticlines. We

present fieldwork from the summers of 1999, 2000, and

2003, which includes six structural transects across the folds

(Fig. 2). We incorporate the magnetostratigraphic dating on

the Atushi–Talanghe anticline presented by Chen et al.

(2002) to constrain the timing and rates of this shortening.

The discussion evaluates the mechanisms of detachment

folding and the regional setting that produced this unique

deformation.

2. Stratigraphy

Although not exposed in the study area, we review the

lithology and interpreted depositional setting of the pre-

Tertiary sediments because they are involved in, and to

some extent may control, the modern deformation and

structural behavior of the system. As a depocenter, the field

area shares many attributes with the Southwest Depression

that borders the Kunlun Shan (Fig. 1; Hu, 1982; Sobel and

Dumitru, 1997). The Paleozoic rocks across the western

Tarim Basin comprise ,4000 m of neritic carbonates and

shallow marine carbonates interbedded with mudstone and

shale, recording a series of marine transgressions and

Fig. 3. Geologic map of Kashi–Atushi fold system, modified from Chen et al. (2002). Note that the Xiyu Formation pinches out along strike and is located

closer to the axial trace on the north side of each anticline. The Boguzihe (‘he’ means river in Chinese) has eroded the northern limb of the eastern end of the

Atushi–Talanghe anticline. Focal mechanisms (Harvard CMT, 2003) of major earthquakes show north-directed, low angle reverse faulting; the 1902 ,M 7.7

Artux earthquake is located just east of the map area (Molnar and Ghose, 2000).



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UNCORRECTED PROOF

regressions. No Triassic sediments are reported in the Kashi

region, whereas Jurassic coal bearing sediments range in

thickness from 1400 to 3800 m. Lower Cretaceous mud-

stone and sandstone approximately 1000 m thick was

deposited before an eastward transgression of the Tethys

Sea occurred in the late Cretaceous (Hu, 1982; Sobel, 1999).

Isopachs depict Tertiary through modern clastic sedi-

ments exceeding 10,000 m in the Kashi Depression (Bally

et al., 1986); in a different study a lower value of ,8000 m

is cited (Hu, 1992). These strata are subdivided into the

Paleogene Kashi Group, the Neogene Wuqia Group and the

Atushi and Xiyu Formations (Fig. 4; Hu, 1982; Mao and

Norris, 1988). The Paleogene Kashi Group records a series

of alternating shallow marine, hypersaline lagoon, and

fluvial deposits. The thickest members of the Kashi Group

include the 200-m-thick massive gypsum and limestone of

the Aertashi Formation, the red, purple and gray–green

mudstone and siltstone of the Bashibulake Formation

(.268 m) and the Oligocene Kezilouyi Formation, a

brown mudstone and gray–green sandstone with numerous

gypsum interbeds (.280 m; Mao and Norris, 1988; Yang,

1996). The Kashi Group is exposed in the southern edge of

the Kepingtage system (Fig. 3).

Neogene through modern strata are exposed by the

Kashi–Atushi fold system (Fig. 3; Zhou and Chen, 1990;

Chen et al., 2001, 2002). Red mudstone exposed in the core

of the Atushi fold is inferred to correlate with the

multicolored Wuqia Group, dated elsewhere by faunal

assemblage as late Oligocene through Miocene (Hu, 1982).

Conformably above the Wuqia Group, the Atushi Formation

is dominated by yellow–gray to tan mudstone, siltstone, and

fine- to medium-grained sandstone. Thin beds, 5–50 mm

thick, of gypsum and gypsiferous mudstone are preserved

within the lower section; higher the section contains rare

small pebble conglomerate layers. In the field area,

paleocurrent directions in the upper Tertiary clastics are

oriented predominantly southeast.

The top of the Atushi Formation is marked by an abrupt

coarsening and darkening of the lithology that is easily

mapped in the study area and indicates transition into the

Xiyu Formation (Fig. 5). At the base of the Xiyu Formation,

0.3–7-m-thick pebble conglomerate beds alternate with

poorly sorted sand lenses and discontinuous silt and sand

layers with scarce pebbles. Higher in the section, the

conglomerate beds are thicker and comprised of well

rounded, clast supported pebbles and cobbles; less common

are beds with thicknesses of 5–15 m, which are matrix

supported. The Xiyu Formation varies from 0 to 2500 m

thick. Clast composition of the Xiyu Formation is

dominated by limestone and sandstone clasts derived from

Paleozoic to Mesozoic strata in the Kepingtage system and

the southern Tian Shan, and is similar to the modern,

southeast flowing rivers in the study area.

The Xiyu and Atushi Formations appear conformable,

but the stratigraphic level of the contact varies along strike.

The character of the contact can be seen in the Landsat

imagery (Fig. 2) where, for example, west of transect line

F–F0 the contact creeps down section or east of the B–B0

traverse on the Atushi–Talanghe anticline, the dark Xiyu

Formation conglomerate tapers out within the lighter Atushi

sediments. These along-strike variations suggest interdigi-

tation of different depositional systems during systems

during the late Cenozoic.

2.1. Progradation

The stratigraphic height of the Atushi–Xiyu contact also

appears to vary in a north–south direction, but consistently

Fig. 4. Cenozoic stratigraphy exposed by folding includes over 5 km of Neogene through mid-Pleistocene fine-grained sediments capped by lower to middle

Pleistocene conglomerates. The Kashi Group is exposed along parts of the southern edge of the Kepingtage. The combined thickness of the Tertiary sediments

is ,10 km in this region (Bally et al., 1986). According to Sobel (1999) and Chinese geological maps, the oldest exposed sediments are Miocene (XBGM,

1965a,b). Pleistocene ages presented in Chen et al. (2002).



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UNCORRECTED PROOF

across the region This variation is shown in the distance

between the axial surface and the Xiyu Formation on the

north versus the south flank of each fold (Figs. 2 and 4).

Despite the essentially identical geometry of the fold limbs

and an absence of major thrust faults, the Atushi–Xiyu

contact on the north side of each fold is consistently 1.5–2

times closer to the core than the contact is on the south side.

Along the Atushi–Talanghe anticline, the contact on the

northern limb is ,1.5 km lower stratigraphically than on

the southern limb. The difference is reduced to ,0.5 km

across the more southerly Mingyaole and Kashi anticlines.

This spatial variation in the thickness of the Xiyu Formation

demands explanation because the conglomerate unit offers

the only potential stratigraphic control with which to

produce cross-sections across the folds (the gradual color

change between the Wuqia Group and Atushi Formation

makes this older contact ambiguous). Two mechanisms,

structural or stratigraphic, could account for the spatial

variation.

A structural model suggests that beds on the northern

limbs have been thinned during folding (Fig. 6a). The

structural model predicts that the Xiyu Formation would be

thinned on the northern side and that the axial traces would

not bisect the neighboring kink panels. Poor preservation

and/or exposure of the conglomerate on the northern side of

the folds make these observations difficult, so we consider

variation in mesoscopic deformation across the fold. The

shear needed to thin the northern limb by ,1.5 km should

require significant penetrative shear across the bedding.

Shear strains of ,15% can be observed in mesoscopic

studies around fault zones (Jamison, 1989), suggesting that

thinning the forelimb by 50% should create obvious shear

and deformation within the attenuated limb. During

structural mapping, we observed no significant difference

in mesoscopic deformation between the limbs. On the

contrary, bedding planes were well maintained in each limb

even where wide panels of the limb were steeply dipping or

overturned.

The stratigraphic model relies on a wedge shaped upper

unit (in cross-section) to cause the difference in the

stratigraphic thickness across the fold (Fig. 6b). Given

both the observed variations in the thickness of the contact

and absence of penetrative shear preferential to either limb,

we interpret that the Xiyu Formation is wedge shaped.

Furthermore, the contact appears to be conformable. A

conformable, wedge shaped unit in this setting can be

created by progradation into a subsiding foreland basin. If

the unit is a progradational wedge, the contact should be

older at the northern limb and younger at the southern limb.

Given these stratigraphic and structural considerations, we

interpret the contact to be progradational and relatively

younger on the southern limb than on the northern limb of

each fold. A similar appearing conglomerate, commonly

called the Xiyu Formation, is widespread across the Tian

Shan foreland and, without quantitative constraints, has

been interpreted to correlate with a ,2.5 Ma climate

change (Avouac et al., 1993; Burchfiel et al., 1999; Zhang

et al., 2001). Our interpretation at the Kashi–Atushi fold

system indicates, however, that the base of the conglomerate

is temporally variable, ranging from 2.8 to 1.9 Ma across the

Talanghe anticline (Chen et al., 2002).

2.2. Growth strata

Bedding is conformable within the folds but not at the

southernmost exposed flanks at each of the transects across

the Atushi –Talanghe and Kashi anticlines At these

transects, field observations show that in the last hundreds

of meters of exposed strata, the bedding inclinations

decrease abruptly up section, and in most places form

Fig. 5. Time-transgressive nature of major lithostratigraphic boundaries

illustrated by interfingering Atushi and Xiyu Formations at the Boguzihe

water gap. View is to the SSW, across a terrace of the Boguzihe on the

southern limb of the Talanghe anticline. Hill at left is ,250 m high.

Fig. 6. Models to account for map trace of Xiyu Formation. (A) Shear

during folding thins units in the forelimb. In this model, the base of the gray

unit in the forelimb is ,2.5 times closer to the fold axis than the base of the

gray unit on the backlimb. (B) When folded, a progradational contact will

be farthest from the axial surface on the distal side (assuming no limb

thinning). In this cartoon, the backlimb contact is twice as far from the core

as the forelimb contact.



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UNCORRECTED PROOF

angular unconformities suggestive of off lapping. The

Ganhangou paleomagnetic section was located to capture

this transition (Fig. 7). In contrast, mapping at the

Mingyaole anticline revealed that a few closely spaced

unconformities are located just below the base of the Xiyu

Formation. A transition from conformable beds to off

lapping or angular unconformities is frequently interpreted

to result from the transition from static conditions to active

folding (e.g. Poblet et al., 1997). We argue that the gently

dipping outer beds are syntectonic strata, deposited after the

initiation of folding in the region.

At the Seven Mills and Atushi transects, the southern

flanks show continuous reduction in dips of off lapping

strata; however, the northern flanks revealed dramatic

changes in dip. At the Atushi transect, for example, a

wide, 758 north dipping panel is overlain by a 158 dipping

panel. We infer that, as observed today, rivers eroded the

north side of the folds as they grew, and that growth strata

were not preserved until later in the development of the fold,

when these sections had risen above the grade of the river. It

appears that the southern sides of the folds were sheltered

from this effect, and growth strata were deposited

continuously.

3. Structure

We conducted detailed field mapping on six traverses

across the Atushi–Talanghe, Mutule, Mingyaole and Kashi

anticlines to investigate the variation in shortening and

deformational style across the Kashi–Atushi fold system

(Fig. 2). Transects were located along water gaps or roads

cutting orthogonal to the structures to maximize access to

structures and bedding exposures. Structural and sedimen-

tological data were mapped at ,1:25,000 onto CORONA

satellite photographs, aerial photos, and topographic maps.

Additional structural data were interpreted from Chinese

geologic maps (XBGM, 1965a,b). Field maps were later

transferred onto a geographic information system database

with the Landsat image and CORONA photographs tied to a

90 m digital elevation model (DEM) base. Topographic

profiles oriented perpendicular to the average strike of the

bedding orientation at each traverse were derived from the

DEM and used as bases for our cross-section interpretations.

The cross-sections are constrained by structural measure-

ments, stratigraphic thickness based on field measurements,

progradation of the conglomerate unit, and unpublished

seismic data. Our interpretations suggest most of the folds

would reach 3–4 km above the land surface, yet modern

topography exhibits less than 1 km of relief (Figs. 8 and 9).

Due to the vigorous erosion and lack of distinctive

stratigraphic marker beds, two techniques were used to

quantify the amount of shortening across the region. We

review the general method for creating the cross-sections,

summarize general observations from the seismic reflection

data, discuss the common structural style of the folds, and

then report details of each fold individually.

The basic fold geometry was determined by fitting a form

line to the structural data. Where canyon walls reveal cross-

sectional views, the Kashi–Atushi fold system exhibits

sharp kink axes in the limbs, while sheared zones were

common in the core of tight folds. These features are well-

modeled using kink style fold axes to separate dip panels

(Marshak and Mitra, 1988). Dip panel width and inclination

were determined by converting dip measurements into

apparent dip with respect to the cross-section trend,

grouping the dip measurements into internally consistent

sets (variability #158) and calculating the average angle

(Figs. 8 and 9). Where we were able to measure bedding

thicknesses across a kink axis, the thicknesses did not

change, indicating that the kink axis bisects the kink panels.

In each cross-section, the stratigraphic height of the growth

strata within each fold limb is accurate, but the geometry of

the growth strata package is idealized. For simplicity (and

lacking evidence to the contrary), we assumed that there is

no change in structural height from north to south across the

structures.

3.1. Seismic reflection data

We examined three unpublished, confidential seismic

reflection images from transects across the Atushi–

Talanghe and Kashi anticlines The seismic profiles are

basically co-located with three traverses (Fig. 3): (1) over

the Atushi–Talanghe anticline along the road at A–A0 and

continuing through the Kashi anticline along the Baishiker-

emu River, (2) over the Atushi–Talanghe anticline at B–B0

and crossing the eastern edge of the topographic expression

of the Kashi anticline, and (3) over the Atushi–Talanghe

anticline at C–C0 and continuing ,20 km into the evaporite

deposits ,15 km east of the end of the topographic

expression of the Kashi anticline. These profiles fortuitously

provide this study with a second perspective for interpreting

the structures at depth.

The most valuable information in the seismic lines was

the presence of long, gently dipping (,128) reflectors at the

outermost flanks of the folds, where young alluvial deposits

on the surface prohibited access to the structure. The gently

Fig. 7. Interpreted photograph of growth strata on the southern limb of the

Talanghe anticline at the Ganhangou paleomagnetic transect. View is to the

east. South of a ,3-km-wide panel dipping 508, off lapping dips of siltstone

and small-pebble conglomerates (foreground) are interpreted to record the

initiation of folding. Pale hill under skyline is the eastward continuation of

Talanghe anticline.



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UNCORRECTED PROOF

dipping outer limbs bracketed regions of discontinuous and

unresolved reflectors positioned directly below the topo-

graphic expression of the folds, and correlate with measured

bedding dips greater than ,458. In the cross-section

interpretations (Figs. 8 and 9), the gently dipping flanks

seen in the seismic surveys are included, resulting in

anticlines that are wider than evidenced in the topography.

Another common feature in each of these profiles is an

essentially flat lying, strong reflector at 9–10 km below land

surface, with the exception of Seven Mills, which was

12 km below land surface. From 5 km below land surface to

the strong reflector at ,9 km, the reflectors are gently

warped or discontinuous. Stratigraphically, 9–10 km is the

approximate depth of a massive gypsum unit of the Aertashi

Formation and other gypsiferous units within the Kashi

Group (Yang, 1996). The variation in deformation with

depth indicates several horizons may have accommodated

different amounts of strain in this convergent setting.

3.2. Field observations and structural model

At the surface, most of the folds exhibit steep limbs

dominated by dip panels inclined between 60 and 858

(overturned) The similarity between the largest, steepest,

dip panel on each side of a fold obscures vergence, but in

general, a subtle north vergence of the folds is exhibited in

the cross-sections. The fold cores are tight, resulting in box-

like to isoclinal knobs that rise off the low angle flanks (Figs.

Fig. 8. Combined cross-section of Atushi and central Kashi transects. The depth to the detachment surface is determined from excess area calculations; the

widths of the structures are constrained by surface exposures and the outer limit of folding expressed in the seismic images. Deeper horizons are dashed because

they are used in area calculations; we do not prescribe the actual deformation style at these levels. Detachment folding occurs either due to an abrupt decrease in

the magnitude of slip across the structure or a horizontal fault tip where displacement stops. Fanning dips at the outer edges of the folds are interpreted to

represent growth strata. Where observed in the field the fanning dips are drawn on the image, otherwise the Quaternary sediments are unmarked. ‘S’ indicates

approximate location of kink axis determined from seismic reflection profile. Dip of kink panels with no structural data is determined from seismic data (if

bordered by kink axis labeled with an S) or results from the constraint that there is zero structural height developed across a fold (dashed). The asterisk ( p )

identifies the bed used to calculate line length shortening.



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UNCORRECTED PROOF

8 and 9). To accommodate the steep bedding dips of the

inner panels, the folds can be modeled as fault propagation

folds with complex, imbricate blind faulting histories (Al

Saffar, 1993) or as detachment folds. Basic observations

suggest the anticlines are not fault propagation folds. (1)

There is no evidence for long thrust ramps above 6 km

below land surface in the seismic images. (2) Along some

transects, fault scarps were observed near the core of the

folds, but never at the base of the forelimb where a thrust

fault is predicted from traditional fault propagation fold

geometry (e.g. Suppe and Medwedeff, 1990). Instead, the

geometry of the anticlines is consistent with models of

disharmonic detachment folds (Mitra and Namson, 1989) or

faulted detachment folds with moderate strain (Figure 3b in

Mitra, 2002). Common to both of these models is a contrast

between relatively competent layers deforming primarily by

flexural slip overlying incompetent units deforming by

distributed or inhomogeneous deformation. The strati-

graphic units at the Kashi–Atushi fold system exhibit a

similar pattern, best observed at the Boguzihe profile (B–B0,

Fig. 3). At the hinge zone of the fold, the lowest river terrace

risers reveal that Wuqia Group bedding is obliterated,

showing discontinuous blocks of bedded strata floating and

chaotically rotated within a sheared mudstone matrix.

Above this, the older river terrace risers reveal the Atushi

Formation bending unbroken across the axial trace.

The cross-sections (Figs. 8 and 9) were constructed as

detachment folds, constrained by structural measurements

and the low dip panels observed in the seismic sections.

Specific information on each profile is presented in the

sections below. Each cross-section interpretation was drawn

reducibly, meaning that axial planes were located to

minimize the length of steeply dipping panels and maximize

gently dipping panels. Short sections of moderate to gently

dipping beds on the crest of the fold were included, resulting

in rounded folds with minimized shortening.

3.2.1. Atushi–Talanghe anticline

The topographic expression of the Atushi–Talanghe

anticline extends eastward over 100 km from the margin of

the Pamir Mountains Constrained by our structural transects

and a strike line trace, the axial trace curves up to the

northeast in several gentle arcs, lost finally to a series of

deflected channels that drain southward from the Mutule

anticline (Fig. 2). The fold is oriented approximately N72E.

Previous mapping (Scharer et al., 2000) separated the

western, Atushi fold from the eastern Talanghe fold at an

eastward plunging core visible to the east of the Atushi

transect line (A–A0) in Fig. 2. However, subsequent

structural mapping along the southern limb east transect

A–A0 (Fig. 8) showed that bedding on this limb continues in

a ,808 SE dipping panel and nowhere forms a syncline

required for separate folds. The parallel curvature of the

bedding strike suggests the Atushi and Talanghe anticlines

form over a common, linear structure and are not offset,

linked anticlines (e.g. Sattarzadeh et al., 2000). We

designate the entire anticline as the Atushi–Talanghe

anticline, but refer to the Atushi and Talanghe anticlines

to identify the western and eastern halves, respectively.

At transect A–A0, the Atushi anticline is isoclinal,

dominated by an 808 dip panel on the southern limb and a

758 dip panel on the north (Fig. 8). The characteristic, dark

Xiyu Formation conglomerate is clearly visible on the

northern side but is absent on the southeast side (Fig. 2).

Without the Atushi–Xiyu contact, the southern contact for

the gravel was located by projecting beds in from ,4 km to

the east. In the cross-section, the width of the fold was

determined by the southernmost structural measurements

and the requirement of no net structural relief between the

synclines bounding the fold. The top of the anticline was

modified to have a horizontal dip panel. This alteration

contributes to a conservative shortening estimate, and is

consistent with dip panels observed at the Boguzihe water

gap and Mingyaole sections, and the gentle character of the

anticlines at their tips.

At transect B–B0, the Boguzihe water gap (Fig. 9), the

northernmost dip measurements for the 808 panel were

measured ,1.7 km west of the profile on bedrock islands in

an ephemeral channel. The southern limb contains an

overturned panel that dips 878 to the north, which is

connected to a steeply dipping panel (748) by a long, gently

dipping kink hinge visible in the lowest terrace riser. Where

the Boguzihe enters New Atushi (Fig. 2), a remnant knob of

sediments records the initiation of shortening in an abrupt

set of reducing dips interpreted as syntectonic deposition

(Chen et al., 2002). Paleomagnetic samples along this

profile constrain the age of the strata (Figs. 2 and 4). Two

fault scarps are located ,100 and ,400 m north of the core

of the fold, offsetting several fluvial terraces of the

Boguzihe. The geomorphic expression of the scarps in

CORONA images indicates that the south dipping faults

have been recently active. The location of these faults

within the hinge zone of the anticline suggests that these

faults accommodate deformation in the core, but are not

causal thrusts of a fault propagation fold.

The southern limb of the Ganhangou traverse, located

,10 km west of the Boguzihe, has a uniform strike of N70E

and a ,508 dip for 3 km that ends in off-lapping growth

Fig. 9. Cross-section interpretations for the Kashi–Atushi fold system. Bedding orientations and lithologic data were obtained by field mapping; (D) and (E)

were improved with data on Chinese geologic maps (XBGM, 1965b) and (F) was constructed solely with the information from Chinese geologic maps

(XBGM, 1965a). Conventions as in Fig. 8. The asterisk ( p ) identifies the beds used to calculate line length shortening. Part (B) includes double detachment

planes and low-angle, deep thrust faults interpreted from seismic survey.



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UNCORRECTED PROOF

strata (Fig. 7). One of the magnetostratigraphic sections

used in this paper was collected along this transect (Chen

et al., 2002). A cross-section of this transect was not

produced due to the erosion of the northern limb.

Transect C–C0, along the Seven Mills water gap,

provides the most complete record of both limbs of the

Talanghe anticline (Fig. 9). Similar to the Boguzi River

section, the fold is tight, dominated by ,758 dipping limbs.

The seismic reflection profile indicates that the total fold

width is ,13 km, almost twice the size of its topographic

expression (,7 km). Along the canyon walls, the hinge

zone exhibits ,500 m of folded and disrupted bedding,

bound on the southern side by a south dipping fault that is

parallel to bedding in the hanging wall. Without strati-

graphic marker beds, the offset on the fault is not

quantifiable beyond the ,200 m offset provided by the

exposure. Unique to this section, the seismic reflection

profile contained two reverse faults and fault bends between

depths of 9 and 12 km (Fig. 9). Beds translated across the

268, south dipping fault ramp are offset by approximately

2 km.

In the northeast corner of the study area, the opposing

tips of the Mutule and Talanghe anticlines fold the Xiyu

Formation into an open syncline along transect D–D0 (Figs.

3 and 9). The blunt nose of the open Mutule anticline

plunges more steeply than the tight Talanghe anticline. The

Xiyu Formation is folded in the syncline to the east of the

profile, and has been represented in the cross-section by

locating the bottom of the contact just above the remaining

topography along the transect. Growth strata were observed

on the southern limb of the Talanghe anticline, but the

northern limb of the Mutule anticline has been eroded.

3.2.2. Mingyaole anticline

The Mingyaole anticline is the smallest in the study area,

,40 km long and 10 km wide at the widest section The fold

has an arcuate shape and is impinged upon its southwest

edge by the Kazikeaete thrust system and north verging

Mushi anticline (Fig. 3). A fault associated with the

Kazikeaete thrust ruptured in 1985 during the M 7.2

Wuqia earthquake, creating a 1.3 m dextral and ,1 m

vertical offset (Chen et al., 1997; Molnar and Ghose, 2000).

In cross-section, the Mingyaole anticline exhibits north

vergence. Unlike the Atushi–Talanghe anticline, which

expresses very tight folding at the surface, the Mingyaole

anticline is broad. Also unique to the Mingyaole anticline, a

series of unconformities we have interpreted to indicate the

initiation of folding in this area were found below the base

of the Xiyu Formation. Moving up section at the southern

end of the Mingyaole transect, a .2 km thick, 588 dip panel

ends as bedding dips abruptly decrease to 278, and then are

unconformably replaced by 138 dipping beds over a 50 m

interval (Fig. 9). To produce this cross-section, the growth

strata were modeled to deform progressively by limb

rotation.

3.2.3. Kashi anticline

The axial trace of the Kashi anticline has a similar

orientation as the Mingyaole anticline (N85E) but their tips

are offset by ,5 km The Kashi anticline is over 60 km long

and is bisected by the Baishikeremuhe. As at the Talanghe

anticline, deflected streams at the eastern end suggest the

fold continues farther into the basin than indicated by its

topographic expression or the Landsat image (Fig. 2). This

interpretation is supported by a gentle fold exhibited in the

seismic reflection profile that crossed ,15 km east of the

topographic end of the fold. The width of the fold is constant

across the fold, ,13 km at transects F–F0 and G–G0, but the

fold amplitude is ,1.5 km lower at transect G–G0. Fanning

strata are observed in the seismic profile on the north and

south sides of the fold.

3.3. Calculated shortening

Slickenlines on bedding planes were observed through-

out the study area, but were not considered pervasive

Similarly, small-scale accommodation features were scarce.

We did not observe evidence of consistent deformation

domains that suggest the folds formed by tangential

longitudinal strain (e.g. neither tensile cracks in outer arcs

nor reverse faults in deeper arcs). Due to the poorly

indurated stratigraphy and vigorous erosion of the uplifted

structure, we lack observational control on the internal

character of the anticline, and thus on the mechanism(s) of

shortening [e.g. homogeneous strain, second order faulting

or second order folding (Epard and Groshong, 1995)]. The

bulk of each limb, however, maintained broad, planar kink

panels, and the limbs appear to control the largest scale

morphology of the anticlines. We therefore present short-

ening estimates from two methods: excess area calculations

and line length balancing (Table 1). Both methods assume

that flexural slip is the dominant deformation mechanism.

3.3.1. Excess area

Flexural slip is accommodated by layer parallel shear,

which can be displayed graphically in an apparent short-

ening plot (Fig. 10a; Mitra and Namson, 1989). All of the

anticlines show a decrease in apparent shortening profile

with depth, which suggests interbed shear increases in

stratigraphically higher layers. In the case of disharmonic

detachment folds, Mitra and Namson (1989) suggest the

concentricity of the anticline is lost in the lower layers

because space accommodation problems result in penetrat-

ive shortening, which changes the deformed bed length. In

the Kashi–Atushi fold system, erosion limits our ability to

evaluate macroscopic deformation in stratigraphically lower

layers, so we assume that the differential penetrative

shortening in lower layers balances the increase in interbed

shear in higher layers.

Excess area, or the area encompassed by a fold under a

particular stratigraphic level divided by the total shortening

at that level, was first used by Chamberlin (1910) to



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UNCORRECTED PROOF

Tab

le1

Su

mm

ary

of

sho

rten

ing

and

sho

rten

ing

rate

esti

mat

esfo

rst

ruct

ura

ltr

anse

cts.

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esh

ort

enin

ges

tim

ates

det

erm

ined

by

exce

ssar

eaar

eco

nsi

der

edm

ore

rob

ust

than

the

lin

ele

ngth

sho

rten

ing

Tra

nse

ctE

xce

ssar

eash

ort

enin

g

(km

)(p

refe

rred

)

Lin

ele

ngth

sho

rten

ing

(km

)

Sh

ort

enin

gra

te(m

m/y

r)

usi

ng

exce

ssar

eash

ort

enin

g,

assu

min

gu

nif

orm

init

iati

on

at1

.2M

a

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t/fa

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ort

enin

g

rate

a(m

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sin

gex

cess

area

sho

rten

ing

Infe

rred

init

iati

on

of

fold

ing

b(M

a)

Sh

ort

enin

gra

te(m

m/y

r)

assu

min

gla

tera

lp

rop

agat

ion

Fig

.1

2(H

–H

0 )c9

.39

.27

.85

.3/1

1.9

––

Atu

shi

(A–

A0 )

6.8

5.5

5.7

3.8

/8.7

2.0

3.4

Bog

uzi

he

(B–

B0 )

4.3

4.7

3.6

2.4

/5.5

1.4

3.1

7M

ills

(C–

C0 )

2.5

4.1

2.1

1.4

/3.2

0.9

2.8

Mu

tule

(D–

D0 )

2.4

1.8

2.0

1.4

/3.1

0.8

3.0

Min

gy

aole

(E–

E0 )

1.5

1.4

1.3

0.8

/1.9

.1

.9,

0.8

Kas

hi

Cen

tral

(F–

F0 )

4.1

3.2

3.4

2.3

/5.3

1.1

3.7

Kas

hi

Eas

t(G

–G

0 )0

.70

.20

.60

.4/0

.90

.71

.0

aS

low

est

rate

calc

ula

ted

wit

hfo

ldin

itia

tio

nat

1.7

7M

a,fa

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on

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eso

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Fig

.1

2).

Fig. 10. Results of (a) apparent shortening profile and (b) excess area

method for each transect. From a maximum at the uppermost layer, the

apparent shortening profiles reduce with depth, indicating a gradient in

interbed shear. The true shortening profiles are unknown, because interbed

shear can be balanced by penetrative deformation. (b) Excess area methods

of Epard and Groshong (1993) calculate the shortening (the slope) and the

depth to the detachment (the x intercept). Dashed lines show regression for

the linear, upper part of the series, solid lines use all data from the transect.



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UNCORRECTED PROOF

determine the depth to the detachment horizon. For this

study, we use the excess area method as adapted by Epard

and Groshong (1993). This method compares the excess

area at several stratigraphic levels with the depth from each

level to a constant reference horizon (Fig. 10b). A plot of the

relationship between the excess area and depth to the

reference horizon gives the estimated shortening (the slope)

and the depth to the detachment (the X-intercept). For this

study, 6–8 excess areas were determined for each anticline

and the land surface at each cross-section was plotted as the

reference horizon. Shortening calculated by excess area

varied from 6.8 km at Atushi to 0.7 km at the East Kashi

transect (Table 1).

Excess area graphs of the Atushi, Boguzihe, Seven Mills,

and Kashi cross-sections show a common feature, in that the

slope (i.e. the shortening) is linear to a depth of 5.5–6.0 km

below land surface at each profile, respectively (Fig. 10b).

Above this depth, the slopes have a high correlation

coefficient, suggesting that deformation mechanisms are

balanced and bed lengths remain constant with deformation.

Below this depth, the slope shallows, suggesting reduction

in strain. This pattern is produced if the detachment horizon

is raised over time, or if the detachment occurs over a zone

with differential strain rather than a single detachment

plane. Alternatively, the deformation gradient may change

at lower depths, resulting in an imbalance between interbed

shear and differential penetrative shortening. For this

reason, two detachment horizons are considered. The

upper detachment horizon is ,6 km below land surface,

at the transition from constant bed lengths to imbalanced

deformation mechanisms. Using all data, the maximum

estimated depth to detachment is 8.6 km below land surface

[due to very poor correlation (R 2 ¼ 0.6) and an anom-

alously low depth to detachment estimate (13.4 km), the

Kashi East transect is not included in the average]. We

suggest that between 6 and 9 km, there is a reduction in

north directed shortening and an increase in northward

translation of material under the Tian Shan.

3.3.2. Line length balancing

For comparison with other regional estimates of short-

ening (Yin et al., 1998; Allen et al., 1999; Burchfiel et al.,

1999), we also calculated line length changes for each

profile (Table 1). This method assumes that the folding

process does not alter the bedding from its original length.

To capture the shortening of the youngest pregrowth

deposits, the uppermost reconstructed bedding horizon

was used (Figs. 8 and 9). With the exception of the Seven

Mills and Kashi East transects, the correlation between the

excess area and line length balancing methods is very good

(^20%). Poor correlation at the Seven Mills transect

probably results from the tightness of the fold. Poor

correlation at the Kashi East transect may reflect the

kinematics of the early stages of fold development (Poblet

and McClay, 1996).

4. Discussion

The style and form of these folds, as drawn, result from a

set of interpretations concerning their deposition and

geometry. Described in previous sections, the interpretation

of the nature of the Xiyu–Atushi contact, conventions

followed in reconstructing the anticlines, and the nature of

the growth strata all affect the shortening estimates.

4.1. Structure

A basic assumption that influences the shortening

estimates is the lack of structural height that developed

across the folds Although the cross-sections were developed

to minimize the height of the folds (principally by restricting

the length of steeply dipping panels), not modeling the

imbricate thrusts commonly required that the width of each

fold was broadened from its surface expression by

increasing the distance between the outer kink axes. The

Kashi East transect (G–G0) is the only profile unaltered by

this convention. The seismic lines, however, provide

independent support for this modification, as they show

that in the subsurface the folds are broader than their

topographic expression, and lack clear evidence of thrust

faults in the upper 6 km. Where seismic lines are available,

the location of the outer kink axes in the cross-section

interpretations does not contradict the seismic reflection

data.

Interpretation of the Kashi–Atushi fold system as a set of

detachment folds is supported by the strong competency

contrasts in the deformed beds (Dahlstrom, 1990; Mitra,

2002). The Kashi Group contains a significant proportion of

gypsiferous units, and shows severe small-scale folding and

pervasive faulting where exposed in the southern Keping-

tage system. The Wuqia Group is rich in siltstones, and

shows a ductile behavior in areas of high strain, such as the

core of the Boguzihe transect. The Atushi Formation

exhibits second order faulting at the core of the Mingyaole,

Atushi, and Boguzihe transects and second order folding at

Seven Mills. In the limbs of each anticline, however, the

Atushi Formation forms parallel-bedded kink panels with

scarce to no faults. Finally, the Xiyu Formation appears to

be the most competent, commonly forming the highest

portions of the modern topography. Following the schema

developed by Dahlstrom (1990), we interpret that the

gypsiferous Kashi Group acts as a lower detachment plane,

the upper Kashi and Wuqia Groups behave as a ductile

detachment zone, and the Atushi and Xiyu Formations

constitute a brittle upper layer that deforms by flexural slip

The shortening is linear for all transects to a depth of ,6 km below land

surface, indicating interbed shear and penetrative deformation are balanced.

Below ,6 km, the shortening is reduced, suggesting a gradient in folding

down to the detachment plane, ,9 km below land surface.



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UNCORRECTED PROOF

in the limbs and second order faulting and folding in the

cores.

This mechanical stratigraphy permits limb rotation as a

mechanism for fold growth (Dahlstrom, 1990), but the

geometric similarity between these folds and symmetric

faulted detachment folds suggests that limb lengthening also

occurs (Mitra, 2002). Additional insight on the method of

fold growth is provided by the anticlinal morphology.

Detachment folding by limb rotation should be

accompanied by a reduction in width and increased limb

dips with advanced shortening. In contrast, shortening by

limb lengthening should be accommodated by an increase in

the height of the fold and a minor increase in fold width

(Poblet and McClay, 1996). For each anticline, the fold

widths remain fairly constant along strike, but the fold

heights and limb dips are correlated with increased short-

ening (Figs. 8 and 9). This suggests that over the life of the

folds, both mechanisms play a role in fold development.

Unfortunately, the growth strata are insufficiently exposed

to more precisely evaluate the kinematics of fold develop-

ment (e.g. Salvini and Storti, 2002). In a future study, we

hope to use deformed fluvial terrace surfaces to illuminate

this question.

4.2. Paleomagnetic constraints

Magnetostratigraphic sampling across the Boguzihe and

Ganhangou sections provides temporal constraints on the

initiation of shortening (Figs 2 and 4; Chen et al., 2002) and

allows an estimation of the shortening rate. The sections are

linked together by tracing bedding along intervening strata

and then matched to the geomagnetic polarity timescale

(GPTS) of Cande and Kent (1995) constrained by the local

Plio-Pleistocene fossil assemblage (Fig. 11). Sampling

strategies and statistics, demagnetization processes, and

correlation to the GPTS are described in detail in Chen et al.

(2002). We apply their results to evaluate the initiation of

shortening across the Kashi–Atushi fold system, and

discuss implications for shortening rates.

The Boguzihe section is a good location to constrain the

age of the base of the Xiyu Formation (,1.9 Ma), but the

cobble conglomerate prevents paleomagnetic sampling

through the syntectonic unconformity. Consequently, the

age of the syntectonic unconformity at Boguzihe (,1.4 Ma)

is calculated both from a linear extrapolation of the

sedimentation rate of the underlying, dated strata and by

strike line correlation to dated strata in the Ganhangou

section (Fig. 11). At the Ganhangou section, finer grained

sediments allow us to sample across the syntectonic

unconformity and capture the age of the growth sediments.

The age of the unconformity at Ganhangou (,1.2 Ma) is

younger than at the Boguzihe section. Ages of the

syntectonic unconformity are maxima; if some pregrowth

strata are missing, the true age of the unconformity could be

younger.

Upper and lower limits on the 1.2 Ma estimate for fold

initiation are determined from the polarity sequence

correlation to the GPTS (Fig. 11). Assuming both sites are

correctly tied at the base of the Olduvai chron, the long

reversed period containing the syntectonic unconformity at

both sites cannot be older than the end of the Olduvai chron

(1.77 Ma) and no younger than the beginning of the Bruhnes

normal chron (0.78 Ma). Therefore, we choose upper and

lower limits of 1.77 and 0.78 Ma, respectively, to bracket

the initiation of folding. Records of small magnitude

earthquakes (USGS Earthquake Hazards Program, 2003),

geomorphic expression of fault scarps, (Zhao et al., 2000),

warping of fluvial terrace surfaces, and geodetic studies

(Wang et al., 2000) indicate that folding continues to the

present.

4.3. Shortening rates

A simplistic approach to quantifying the geologic

shortening rate is to identify a representative location and

apply a regional initiation of folding at 1.2 Ma Expanding

the region to a central profile (H–H0) including shortening

accommodated in the southern limb of the Keketamu

anticline (4.3 km) and the Boguzihe and Kashi East

transects, the total shortening across the Kashi–Atushi

fold system is 9.3 km (Fig. 12). If shortening across the

three structures in this transect initiated contemporaneously,

a mean shortening rate of 7.8 mm/yr has continued for the

past 1.2 Myr (Fig. 13). Extreme minimum (5.3 mm/yr) and

maximum (11.9 mm/yr) rates result from upper and lower

age limits, respectively (Table 1). Shortening rates for each

transect reduce to the east if folding was coeval (Table 1;

Fig. 13).

While synchronous folding provides a reasonable

estimate of the shortening rate, it is an unlikely condition

because the stratigraphic height of the growth strata changes

across the study area. At the Mingyaole profile, the growth

strata are located just below the base of the Xiyu Formation

whereas at the Boguzihe transect and the Ganhangou

section, the growth strata are younger than the Xiyu

Formation. To integrate these observations, we estimate

the geologic shortening rate at each transect using the

difference in growth strata ages at Boguzihe and Ganhangou

(1.4 and 1.2 Ma, respectively). Dividing the distance

between the sites (,10 km) by the difference in initiation

of folding at Boguzihe and Ganhangou (0.2 Myr) results in

an eastward lateral propagation rate of ,50 km/Ma. The

eastern end of the Talanghe anticline shows geomorphic

evidence of eastward lateral propagation (Fig. 14). The

geomorphic expression of the anticline is 25 km shorter than

expected if the lateral propagation rate has been constant for

the last 1.2 Ma, suggesting that the extrapolated rate

overestimates the true lateral propagation rate. If the

propagation rate decreased to the east because stress was

accommodated on the Mutule anticline, the lateral propa-

gation rate can be used to estimate the initiation of folding

across the Kashi–Atushi fold system, recognizing that



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UNCORRECTED PROOF

interpreting the sequence of fold development across the

folds is speculative given vigorous erosion of the anticlines

and uncertainty in the kinematics of fold growth.

To arrive at local estimates of the shortening rate, we

simply extend the 50 km/Myr lateral propagation rate from

the Ganhangou location back in time to the west and

forward to the east (Table 1; Fig. 13). Under this

assumption, the initiation of folding would occur ,2.0 Ma

at the Atushi transect, ,0.9 Ma at the Seven Mills and

,0.8 Ma at the Mutule transect. Although considerably

more speculative, we estimate the ages along the Mingyaole

and Kashi anticlines using a similar technique and the same

propagation rate. A seismic reflection profile ,20 km east

of the topographic expression of the Kashi anticline shows

low amplitude folding. If folding began at this end recently,

the initiation of folding can be extrapolated westward from

this location, assuming the Mingyaole and Kashi anticlines

formed successively and deformation propagated eastward.

From this construct, folding initiated at ,0.7 Ma at the East

Kashi transect, ,1.1 Ma at the Central Kashi transect, and

at least ,1.9 Ma at the Mingyaole transect (Table 1).

Because the growth strata are older than the Xiyu Formation

at Mingyaole, this scenario suggests that the Xiyu

Formation at Mingyaole is younger than ,1.9 Ma, the

same age as the base of the Xiyu at the Boguzihe transect

(1.9 Ma). A western source for this conglomerate suggests

the Pamir salient had reached this area by ,1.9 Ma,

consistent with estimates for development of the Trans Alai

Range around ,4 Ma (Arrowsmith and Strecker, 1999).

While local shortening rates decrease to the east along the

Atushi–Talanghe anticline, they are variable across the

Mingyaole and Kashi anticlines. Restricted to the Kashi–

Atushi fold system, we consider representative a regional

average rate of ,5 mm/yr (Fig. 13).

Fig. 11. Paleomagnetic results for the Boguzihe and Ganhangou sections compared with the GPTS (Cande and Kent, 1995). See Fig. 4 for location of the

paleomagnetic sections. The sections were linked by tracing distinct beds ,10 km from the Boguzihe to the Ganhangou section (wavy line), and then

correlated to the GPTS. The basal age of growth strata sampled at Ganhangou is inferred to be 1.2 Ma, whereas the age at Boguzihe (1.4 Ma) was calculated

extrapolating a constant sedimentation rate. At the Boguzihe section, the contact between the Xiyu and Atushi Formations is ,1.95 Ma. The Xiyu Formation is

absent at the Ganhangou transect. Modified from Chen et al. (2002).



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Fig. 12. Generalized cross-section across southern Tian Shan (H–H0 on Fig. 2). Total Pleistocene through modern shortening between the Keketamu and Kashi

anticlines is 9.2 km. Due to uncertainties in the geometry of the northern margin of the Keketamu anticline, shortening accommodated by this structure was

calculated by line length balancing. The bed used to measure the shortening is noted with an asterisk ( p ). Shortening calculated for the Boguzihe and Kashi

East anticlines was determined by excess area methods. Arrows above detachment surface show decreasing slip to the north while the Tarim craton is

underthrust below the Tian Shan. The fault geometry of the Kepingtage, South Tian Shan fault, and Muziduke fault, and the angle of the Tarim craton under the

Tian Shan are not known and drawn schematically.

Fig. 13. Simplified maps of shortening rates determined along the Kashi–Atushi fold system assuming (A) uniform initiation (average rates for structure-

perpendicular transects across the entire Kashi–Atushi fold system are presented under the double line) and (B) lateral propagation. We have not mapped the

Mushi anticline, but its size and appearance suggest it has accommodated at least as much deformation as the Mingyaole anticline. These estimates are shown

in parentheses, and added to the cross structure averages. (A) If folding was coeval, shortening rates reduce to the east by a factor of four. (B) If the folds grew

laterally, the Atushi anticline formed first, followed by the Mushi and Mingyaole anticlines, then the Talanghe and Kashi anticlines.



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UNCORRECTED PROOF4.4. Regional kinematics

It is informative to compare timing and rate estimates

presented here with those from other areas at the boundary

between the Tarim Basin and the Tian Shan, beginning near

Wenguri, just north of the study area, and proceeding

clockwise around the Southern Tian Shan Sobel and

Dumitru (1997) sampled locations within the Kepingtage

system for apatite fission track provenance and exhumation

dates. They infer that thrusting north of Wenguri (Fig. 3)

was active before 14 Ma and has propagated southward

through time. This is consistent with our results and

indicates that there was a ,12 Ma gap between initiation

of activity on the Kepingtage system and the Kashi–Atushi

fold system.

Allen et al. (1999) report on shortening to the east of the

Kashi–Atushi fold system, along the Kepingtage thrust

zone (Fig. 1). At that location, the shortening is expressed as

thin-skinned, imbricate thrusts that have accommodated

,35 km of shortening within a ,100 km long zone. From

the magnetostratigraphic results of Yin et al. (1998), they

estimate that thrusting initiated around 20 Ma, resulting in

an average shortening rate of ,1.8 mm/yr, if shortening

spans the entire interval to the present. Farther east in the

Kuche region (Fig. 1), magnetostratigraphic results by Yin

et al. (1998) are used to date a lacustrine to fluvial transition

inferred to represent regional thrust initiation, between 24

and 21 Ma. When applied to the 20–40 km of total

shortening, the rate (1–1.9 mm/yr) is similar to the results

in Kepingtage, but much lower than the rates at the Kashi–

Atushi fold system.

Burchfiel et al. (1999) present cross-sections from both

the northern and southern perimeters of the eastern Tian

Shan (Fig. 1). These cross-sections show a structural style

that is similar to our interpretation for the Kashi–Atushi

fold system, and contrasts with imbricate thrusting observed

along the Kepingtage and Kuche thrust systems. In

particular, the Boston Tokar and Kalasu transects of

Burchfiel et al. (1999) contain tight, upright folds and

abrupt lithologic changes across the folds. Cross-sections on

these transects yield shortening estimates of ,10–20 km,

1–4 times larger than estimated for the Kashi–Atushi fold

system. To quantify the shortening rates, Burchfiel et al.

(1999) used a range of initiation between 2.5 and 1.0 Ma

(based on a climate inferred Quaternary age of a conglom-

erate), resulting in estimated shortening rates between 3 and

21 mm/yr.

Given uncertainty in the age of fold initiation at the

Boston Tokar and Kalasu sections, the mean shortening rate

(5.1 mm/yr) estimated for the Kashi–Atushi fold system is

high compared with the more proximal Kepingtage and

Kuche regions, and folding probably initiated more

recently. This suggests that the setting of the Kashi–Atushi

fold system may be unique. Indeed, it is located above the

deepest depocenter in the Tarim Basin and shows little

seismicity in comparison with the other systems.

We envision the Kashi–Atushi fold system as part of an

accretionary wedge, formed as the Tarim craton is forced

north and underthrust below the Tian Shan. Approximately

9 km of the Tarim craton has been underthrust below the

Tian Shan in the last 1–3 Myr, which is resolved by the

folding of the top 6 km of Cenozoic strata (Fig. 12).

Structural evidence of parallel folding and geodetic

evidence for northward directed shortening (Holt et al.,

2000), combined with geophysical models of Bouger

gravity anomalies (Burov et al., 1990) support this model.

The two interpretations for initiation of shortening indicate

that either the eastern end has been shortening more slowly

or for less time (Fig. 13). In either case, the patterns of

shortening rates may be facilitated by the Pamir Salient,

which imposes a load on the westernmost Tarim Basin

(Burov et al., 1990). In response to the load, the Tarim

craton would be flexed more in the west, which potentially

facilitates underthrusting of the western Tarim craton below

the Tian Shan. In the eastern portion of the study area, the

reduced effects of the load may inhibit underthrusting of the

Tarim craton, and northward directed convergence would be

taken up within the Tian Shan proper. The higher

concentration of folds in the west may also be controlled

by the geometry of the Kashi Depression, which was

thickest in the west and thinned to the east.

5. Conclusions

The Kashi–Atushi fold system occupies a unique

location in the Indian–Eurasia collision. The folds are

located in an ,808 corner between the north verging frontal

thrust of the Pamir (the Kazikeaete thrust system) and the

Southern Tian Shan fault. We model the Atushi–Talanghe,

Mutule, Mingyaole, and Kashi anticlines as detachment

folds due to very steep limbs (60–908), an absence of

Fig. 14. CORONA satellite photo of confluence of Mutule (M) and

Talanghe (T) anticlines. Light-colored rocks are Atushi Formation and

darkest gray rocks are Xiyu Formation. Most of the folded Xiyu Formation

in the syncline pre-dates the initiation of folding. The series of deflected

drainages across the nose of the Talanghe anticline suggest its eastward

propagation.



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emergent, fold bounding fault scarps or faults in proprietary

seismic surveys. The results do not prescribe a kinematic

history for the deformation, but limit the shortening

estimates to the most constrained data, namely the dips of

the beds and the stratigraphic depth of the syngrowth

sediments.

Shortening estimates calculated from the cross-sections

show that total shortening decreases towards the east and

south; the Atushi anticline accommodated the most short-

ening (6.8 km), while the Talanghe anticline transects

exhibit 4.3 and 2.5 km at the Boguzihe and Seven Mills

water gaps, respectively. The transect crossing the eastern

tip of the Talanghe anticline and the western end of the

Mutule anticline yields a total shortening of 2.4 km. The

southern set of folds generally accommodates less than the

northern sets, with the Mingyaole anticline shortened by

1.5 km and the Kashi anticline by 4.1 km in the middle and

0.7 km at the east end. Shortening across the Talanghe

anticline reduces to the east, suggesting that either (1) the

folds are propagating eastward over time, or (2) shortening

across the anticlines began everywhere at the same time, but

the strain rate reduces to the east. In total, at least 6 km of

the Tarim Craton has underplated the southern Tian Shan in

the last 1–2 Myr.

The geometry of the folds, organized like a pleat in a

folded fabric, proximity to the Pamir Mountains, and the

north vergence of the system suggest that the fold system

results from the expected convergence between the Tarim

Basin and the Tian Shan, but is affected by northward

movement of the Pamir Salient as well. The young and

relatively fast deformation of the Kashi–Atushi fold system

appears to be related to the abnormally thick Tertiary

sediments in the Kashi foredeep and regional forces

imparted by the Pamir Salient. Assuming coeval initiation,

the shortening rates across the Kashi Atushi fold system

reduce to the west, from ,9 to 2 mm/yr, with an average of

,5 mm/yr. If deformation at the Keketamu anticline is

included, the shortening of rate (7.8 mm/yr) across the

region plus the previously computed shortening rates in the

Tian Shan (,13 mm/yr; Abdrakhmatov et al., 2002;

Thompson et al., 2002) agrees with the geodetic rates of

,20–24 mm/yr (Abdrakhmatov et al., 1996; Holt et al.,

2000) across the western Tian Shan.

Acknowledgements

This study was greatly improved by the detailed reviews

of Mark Allen and Josep Poblet. Thanks also to Dennis

Fletcher for GIS assistance and Richard Heermance for

fieldwork. This research was supported by the Continental

Dynamics program of the US NSF (EAR 9614765), and by

the National Science Foundation of China grants 49834005

and 40372081.

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