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1 57 2 58 3 59 Journal of Structural Geology xx (0000) xxx–xxx 4 60 www.elsevier.com/locate/jsg 5 61 6 62 7 63 8 Detachment folding in the Southwestern Tian Shan–Tarim foreland, 64 9 65 10 China: shortening estimates and rates 66 11 67 12 a,* b c a d c c 68 13 K.M. Scharer , D.W. Burbank , J. Chen , R.J. Weldon , C. Rubin , R. Zhao , J. Shen 69 14 aDepartment of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403, USA 70 15 bDepartment of Geological Sciences, University of California, Santa Barbara, CA 93106, USA 71 16 cState Key Laboratory of Earthquake Dynamics, Institute of Geology, China Seismological Bureau, P.O. Box 9803, Beijing 100029, P.R. China 72 d 17 Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA 73 18 Received 17 February 2003; received in revised form 15 January 2004; accepted 26 February 2004 74 19 75 20 76 21 77 Abstract 22 78 23 Geologic observations of the Chinese Tian Shan foreland reveal active, contractional deformation along the entire margin. To quantify the 79 24 amount of shortening and understand the structural style, we mapped the stratigraphy and structure of four folds expressed at the western end 80 25 of the foreland, north of Kashi. In this region, upper Tertiary through Quaternary sediments are conformable, but an abrupt transition from 81 26 parallel to unconformable bedding in the uppermost strata suggests a transition from pre-fold strata to strata deposited on a growing fold. The 82 folds have very steep (60–908) limbs and are box-like to isoclinal, suggestive of detachment folding. Total north–south shortening across the 27 83 center of the region is .9 km, of which 5–7 km occurred in the Kashi–Atushi fold system. Shortening estimates determined with excess 28 84 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 29 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 85 30 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 86 31 with foreland deformation east of the study area, suggesting that the regional stresses or response of the foreland stratigraphy are unique to 87 32 the Kashi–Atushi fold system kinematics. 88 33 q 2004 Published by Elsevier Ltd. 89 34 90 Keywords: Kashi Depression; Xiyu conglomerate; Lateral propagation; Growth strata; Anticline; Kepingtage 35 91 36 92 37 93 38 1. Introduction yr, of the total modern convergence between the Indian and 94 39 Eurasian plates is accommodated by shortening in the Tian 95 40 The Tian Shan (Fig. 1) has been the focus of recent Shan (Abdrakhmatov et al., 1996; Holt et al., 2000; Wang 96 41 geologic investigations concerning the timing, sequence, et al., 2000). The geologic and geodetic observations agree 97 42 and geometry of active, intracontinental mountain building well in the Kyrgyz Tian Shan, which suggests that the 98 43 (Sobel and Dumitru, 1997; Yin et al., 1998; Allen et al., remaining, geodetically calculated shortening of ,7– 99 44 1999; Burchfiel et al., 1999; Sobel et al., 2000; Abdrakh- 10 mm/yr should occur south of the Aksay Basin in the 100 45 matov et al., 2002; Thompson et al., 2002). These studies southern Tian Shan and northern Tarim Basin (Fig. 1). 101 46 suggest that the onset of deformation in the Tian Shan began Studies in the central and eastern Tian Shan foreland 102 47 103 about 20 Ma, or ,35 Myr after the initial Indo-Asian have estimated the Cenozoic shortening, but provided loose 48 104 collision, and that north–south directed shortening con- control on the timing of the deformation in the western 49 105 tinues to be active across the entire range. Thompson et al. foreland (Yin et al., 1998; Allen et al., 1999; Burchfiel et al., 50 106 (2002) demonstrate that at late Pleistocene time scales, 1999). In the southwestern Chinese Tian Shan foreland, up 51 107 ,13 mm/yr of shortening is accommodated on 5–6 discrete to 12 km of Cenozoic strata (Bally et al., 1986) provide a 52 108 structures distributedUNCORRECTED across the Kyrgyz Tian Shan. pristine succession PROOF in which the style, age, and rates of 53 Geodetic studies indicate that up to 50%, or 20–24 mm/ 109 54 shortening can be investigated. This paper presents 110 structural, stratigraphic and geomorphic data to define the 55 * Corresponding author. Tel.: þ1-514-346-4573; fax: þ1-514-346-4692. 111 56 E-mail address: [email protected] (K.M. Scharer). style of folding in this region. In combination with a 112

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113 169 114 170 115 171 116 172 117 173 118 174 119 175 120 176 121 177 122 178 123 179 124 180 125 181 126 182 127 183 128 184 129 185 130 186 131 187 132 188 133 189 134 190 135 191 Fig. 1. Elevation map of the Tarim Basin, Tian Shan, Pamir Mountains, and Northern Tibetan Plateau. Major rivers designated with dashed lines, lake (Issyk- 136 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 192 137 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, 193 138 Kuche, and Boston Tokar/Kalasu River sections of Allen et al. (1999), Yin et al. (1998), and Burchfiel et al. (1999), respectively. 194 139 195 140 196 141 magnetostratigraphic study presented by Chen et al. (2002), subsequently felt the effects of at least three Mesozoic 197 142 we estimate the timing of initiation of folding and quantify collisions south of the Tarim craton (Hendrix et al., 198 143 regional shortening rates. We show that structures in the 1992). The period between the last Mesozoic collision, 199 144 northwest Tarim Basin are detachment folds that began when the Kohistan–Dras arc–forearc complex was 200 145 deforming during the Pleistocene and have accommodated accreted to southern Asia (,70 Ma), and the initiation 201 146 much of the recent shortening predicted geodetically to of Cenozoic deformation in the Tian Shan (,25 Ma) was 202 147 occur south of southern Kyrgyzstan. a time of tectonic quiescence. During this ,45 Myr 203 148 period, the ancestral Tian Shan was beveled by erosion. 204 149 1.1. Geographic and geologic setting Stratigraphically, this period is represented by a wide- 205 150 spread, planar denudation surface in the Tian Shan 206 151 The Tian Shan extends over 2000 km eastward from (Abdrakhmatov et al., 2002) and by deposition of lower 207 152 Tajikistan, covers most of Kyrgyzstan, and tapers to the east Tertiary strata along its margins (Bally et al., 1986; Hu, 208 153 in the Xinjiang province of western China At the study 1992). Within the Tian Shan proper, the Cenozoic 209 154 location, the range is ,500 km wide. The Tarim Basin is an deformation has been quantified by studying the uplift 210 155 internally draining, trapezoid shaped basin 1500 km long, and faulting of the early Tertiary peneplain (Burbank 211 156 pinched between the Tian Shan on the north, the Pamir et al., 1999; Bullen et al., 2001; Abdrakhmatov et al., 212 157 Mountains to the west, and the Tibetan Plateau to the south. 2002; Thompson et al., 2002). At the southern margin of 213 158 The field area is located in the northwest corner of the Tarim the Tian Shan, the peneplain is absent, but deformation of 214 159 Basin, approximately 250 km from the Alai Valley where foreland fold-and-thrust sequences has been used to 215 160 the Tian Shan and the Pamir Mountains meet (Fig. 1). The understand the timing and style of the outward growth 216 161 arid climate and the paucity of vegetation provide excellent and uplift of the Tian Shan (Sobel and Dumitru, 1997; 217 162 exposure for structural and stratigraphic mapping. Yin et al., 1998; Allen et al., 1999; Burchfiel et al., 1999; 218 163 The Tian Shan and the Tarim Basin blocks have been Chen et al., 2001, 2002). 219 164 proximate sinceUNCORRECTED the Devonian, when an intervening ocean Geophysical PROOF evidence, numerical modeling, and both 220 165 basin separating the two began subducting under the geologic and geomorphic evidence suggest that the Tarim 221 166 ancestral Tian Shan (Watson et al., 1987; Carroll et al., Basin is being coherently thrust under the Tian Shan 222 167 1995). The two blocks were sutured by the late Permian (Molnar and Tapponnier, 1975; Burov et al., 1990; Avouac 223 168 (Carroll et al., 1995; Yin and Nie, 1996) and have et al., 1993; Burtman and Molnar, 1993; Neil and House- 224

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225 man, 1997; Burchfiel et al., 1999; Abdrakhmatov et al., 2001, 2002). Fission track dating by Sobel and Dumitru 281 226 2002). Sediments express the convergence in a series of (1997) suggests that deformation propagated southward 282 227 folds and thrust faults along the northern margin of the from the Tian Shan, exhuming this part of the Kepingtage 283 228 Tarim Basin (Rubin et al., 2000; Scharer et al., 2000; Zhao system since ,14 Ma. Total shortening in this part of the 284 229 et al., 2000; Chen et al., 2001). The topographic front of the Kepingtage has not been calculated, but folded sediments in 285 230 Tian Shan proper is delineated by the South Tian Shan fault, piggyback basins provide evidence for a component of late 286 231 a reverse fault with unknown displacement magnitude (Yin Tertiary deformation. 287 232 et al., 1998). Basinward of the South Tian Shan fault, two The southern area, the Kashi–Atushi fold system, is 288 233 areas with contrasting deformational styles are clear in south of the Kepingtage system. It includes the towns of 289 234 Landsat Multispectral Scanner imagery (Fig. 2). The Kashi (Kashgar) and Atushi (Artush), and is characterized 290 235 northern area, called the Kepintage–Yishilakekalawuer by two rows of sinuous folds within the Kashi Depression, a 291 236 thrust fault (Kepingtage), represents the westernmost deep Tertiary sub-basin. Major rivers draining the Tian 292 237 expression of the Kashi–Akesu system as identified by Shan cross the Kashi–Atushi fold system. Large earth- 293 238 Yin et al. (1998). It is characterized by south verging thrust quakes have occurred at the perimeter of this area, but none 294 239 faults that place Paleozoic strata on top of Mesozoic and are recorded within the fold system (Fig. 3). We 295 240 Cenozoic sediments (XBGM, 1985; Sobel and Dumitru, differentiate this area from the Kashi–Akesu system of 296 241 1997; Rubin et al., 2000; Zhao et al., 2000; Chen et al., Yin et al. (1998) due to the striking difference between the 297 242 298 243 299 244 300 245 301 246 302 247 303 248 304 249 305 250 306 251 307 252 308 253 309 254 310 255 311 256 312 257 313 258 314 259 315 260 316 261 317 262 318 263 319 264 320 265 321 266 322 267 323 268 324 269 325 270 326 271 327 272 328 273 329 274 330 275 331 276 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 332 Kepingtage–YishilakekalawuerUNCORRECTED Thrust Fault system (Kepingtage). The timing and displacements of thesePROOF faults are poorly constrained. TFF is the Talas– 277 333 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 278 image. Upper Tertiary strata (light color) comprise most of the folds, which are rimmed by resistant Quaternary conglomerates (dark color distinct on the 334 279 southern limbs). The Talas–Ferghana fault projects into the middle of the fold system, which appears unaffected, suggesting right-lateral strain on the Talas– 335 280 Ferghana fault is partitioned into the South Tian Shan fault or the Muziduke fault rather than the Tarim craton. 336

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337 393 338 394 339 395 340 396 341 397 342 398 343 399 344 400 345 401 346 402 347 403 348 404 349 405 350 406 351 407 352 408 353 409 354 410 355 411 356 412 357 413 358 414 359 415 360 416 361 417 362 418 363 419 364 420 365 421 366 422 367 423 368 424 369 425 370 426 371 427 372 428 373 429 374 430 375 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 431 376 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 432 377 Atushi–Talanghe anticline. Focal mechanisms (Harvard CMT, 2003) of major earthquakes show north-directed, low angle reverse faulting; the 1902 ,M 7.7 433 378 Artux earthquake is located just east of the map area (Molnar and Ghose, 2000). 434 379 435 380 imbricate thrust stacks of Mesozoic and Paleozoic strata that 2. Stratigraphy 436 381 form the northern band of deformation in the Tarim Basin 437 382 and the simply folded upper Cenozoic strata in the study Although not exposed in the study area, we review the 438 383 area. The Kashi–Atushi fold system includes the Atushi– lithology and interpreted depositional setting of the pre- 439 384 Talanghe, Mutule, Mingyaole and Kashi anticlines. We Tertiary sediments because they are involved in, and to 440 385 present fieldwork from the summers of 1999, 2000, and some extent may control, the modern deformation and 441 386 2003, which includes six structural transects across the folds structural behavior of the system. As a depocenter, the field 442 387 (Fig. 2). We incorporate the magnetostratigraphic dating on area shares many attributes with the Southwest Depression 443 388 the Atushi–TalangheUNCORRECTED anticline presented by Chen et al. that borders thePROOF Kunlun Shan (Fig. 1; Hu, 1982; Sobel and 444 389 (2002) to constrain the timing and rates of this shortening. Dumitru, 1997). The Paleozoic rocks across the western 445 390 The discussion evaluates the mechanisms of detachment Tarim Basin comprise ,4000 m of neritic carbonates and 446 391 folding and the regional setting that produced this unique shallow marine carbonates interbedded with mudstone and 447 392 deformation. shale, recording a series of marine transgressions and 448

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449 regressions. No Triassic sediments are reported in the Kashi small pebble conglomerate layers. In the field area, 505 450 region, whereas Jurassic coal bearing sediments range in paleocurrent directions in the upper Tertiary clastics are 506 451 thickness from 1400 to 3800 m. Lower Cretaceous mud- oriented predominantly southeast. 507 452 stone and sandstone approximately 1000 m thick was The top of the Atushi Formation is marked by an abrupt 508 453 deposited before an eastward transgression of the Tethys coarsening and darkening of the lithology that is easily 509 454 Sea occurred in the late Cretaceous (Hu, 1982; Sobel, 1999). mapped in the study area and indicates transition into the 510 455 Isopachs depict Tertiary through modern clastic sedi- Xiyu Formation (Fig. 5). At the base of the Xiyu Formation, 511 456 ments exceeding 10,000 m in the Kashi Depression (Bally 0.3–7-m-thick pebble conglomerate beds alternate with 512 457 et al., 1986); in a different study a lower value of ,8000 m poorly sorted sand lenses and discontinuous silt and sand 513 458 is cited (Hu, 1992). These strata are subdivided into the layers with scarce pebbles. Higher in the section, the 514 459 Paleogene Kashi Group, the Neogene Wuqia Group and the conglomerate beds are thicker and comprised of well 515 460 Atushi and Xiyu Formations (Fig. 4; Hu, 1982; Mao and rounded, clast supported pebbles and cobbles; less common 516 461 Norris, 1988). The Paleogene Kashi Group records a series are beds with thicknesses of 5–15 m, which are matrix 517 462 of alternating shallow marine, hypersaline lagoon, and supported. The Xiyu Formation varies from 0 to 2500 m 518 463 fluvial deposits. The thickest members of the Kashi Group thick. Clast composition of the Xiyu Formation is 519 464 include the 200-m-thick massive gypsum and limestone of dominated by limestone and sandstone clasts derived from 520 465 the Aertashi Formation, the red, purple and gray–green Paleozoic to Mesozoic strata in the Kepingtage system and 521 466 mudstone and siltstone of the Bashibulake Formation the southern Tian Shan, and is similar to the modern, 522 467 (.268 m) and the Oligocene Kezilouyi Formation, a southeast flowing rivers in the study area. 523 468 brown mudstone and gray–green sandstone with numerous The Xiyu and Atushi Formations appear conformable, 524 469 gypsum interbeds (.280 m; Mao and Norris, 1988; Yang, but the stratigraphic level of the contact varies along strike. 525 470 1996). The Kashi Group is exposed in the southern edge of The character of the contact can be seen in the Landsat 526 471 the Kepingtage system (Fig. 3). imagery (Fig. 2) where, for example, west of transect line 527 0 0 472 Neogene through modern strata are exposed by the F–F the contact creeps down section or east of the B–B 528 473 Kashi–Atushi fold system (Fig. 3; Zhou and Chen, 1990; traverse on the Atushi–Talanghe anticline, the dark Xiyu 529 474 Chen et al., 2001, 2002). Red mudstone exposed in the core Formation conglomerate tapers out within the lighter Atushi 530 475 of the Atushi fold is inferred to correlate with the sediments. These along-strike variations suggest interdigi- 531 476 multicolored Wuqia Group, dated elsewhere by faunal tation of different depositional systems during systems 532 477 assemblage as late Oligocene through Miocene (Hu, 1982). during the late Cenozoic. 533 478 Conformably above the Wuqia Group, the Atushi Formation 534 479 is dominated by yellow–gray to tan mudstone, siltstone, and 2.1. Progradation 535 480 fine- to medium-grained sandstone. Thin beds, 5–50 mm 536 481 thick, of gypsum and gypsiferous mudstone are preserved The stratigraphic height of the Atushi–Xiyu contact also 537 482 within the lower section; higher the section contains rare appears to vary in a north–south direction, but consistently 538 483 539 484 540 485 541 486 542 487 543 488 544 489 545 490 546 491 547 492 548 493 549 494 550 495 551 496 552 497 553 498 554 499 555 500 UNCORRECTED PROOF 556 501 557 Fig. 4. Cenozoic stratigraphy exposed by folding includes over 5 km of Neogene through mid-Pleistocene fine-grained sediments capped by lower to middle 502 Pleistocene conglomerates. The Kashi Group is exposed along parts of the southern edge of the Kepingtage. The combined thickness of the Tertiary sediments 558 503 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, 559 504 1965a,b). Pleistocene ages presented in Chen et al. (2002). 560

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561 617 562 618 563 619 564 620 565 621 566 622 567 623 568 624 569 625 570 626 571 627 572 628 Fig. 5. Time-transgressive nature of major lithostratigraphic boundaries 573 629 illustrated by interfingering Atushi and Xiyu Formations at the Boguzihe 574 water gap. View is to the SSW, across a terrace of the Boguzihe on the 630 575 southern limb of the Talanghe anticline. Hill at left is ,250 m high. 631 576 632 577 across the region This variation is shown in the distance 633 578 between the axial surface and the Xiyu Formation on the 634 579 north versus the south flank of each fold (Figs. 2 and 4). 635 580 Despite the essentially identical geometry of the fold limbs Fig. 6. Models to account for map trace of Xiyu Formation. (A) Shear 636 581 and an absence of major thrust faults, the Atushi–Xiyu during folding thins units in the forelimb. In this model, the base of the gray 637 582 contact on the north side of each fold is consistently 1.5–2 unit in the forelimb is ,2.5 times closer to the fold axis than the base of the 638 583 times closer to the core than the contact is on the south side. gray unit on the backlimb. (B) When folded, a progradational contact will 639 be farthest from the axial surface on the distal side (assuming no limb 584 Along the Atushi–Talanghe anticline, the contact on the 640 thinning). In this cartoon, the backlimb contact is twice as far from the core 585 northern limb is ,1.5 km lower stratigraphically than on as the forelimb contact. 641 586 the southern limb. The difference is reduced to ,0.5 km 642 587 across the more southerly Mingyaole and Kashi anticlines. stratigraphic thickness across the fold (Fig. 6b). Given 643 588 This spatial variation in the thickness of the Xiyu Formation both the observed variations in the thickness of the contact 644 589 demands explanation because the conglomerate unit offers and absence of penetrative shear preferential to either limb, 645 590 the only potential stratigraphic control with which to we interpret that the Xiyu Formation is wedge shaped. 646 591 produce cross-sections across the folds (the gradual color Furthermore, the contact appears to be conformable. A 647 592 change between the Wuqia Group and Atushi Formation conformable, wedge shaped unit in this setting can be 648 593 makes this older contact ambiguous). Two mechanisms, created by progradation into a subsiding foreland basin. If 649 594 structural or stratigraphic, could account for the spatial the unit is a progradational wedge, the contact should be 650 595 variation. older at the northern limb and younger at the southern limb. 651 596 A structural model suggests that beds on the northern Given these stratigraphic and structural considerations, we 652 597 limbs have been thinned during folding (Fig. 6a). The interpret the contact to be progradational and relatively 653 598 structural model predicts that the Xiyu Formation would be younger on the southern limb than on the northern limb of 654 599 thinned on the northern side and that the axial traces would each fold. A similar appearing conglomerate, commonly 655 600 not bisect the neighboring kink panels. Poor preservation called the Xiyu Formation, is widespread across the Tian 656 601 and/or exposure of the conglomerate on the northern side of Shan foreland and, without quantitative constraints, has 657 602 the folds make these observations difficult, so we consider been interpreted to correlate with a ,2.5 Ma climate 658 603 variation in mesoscopic deformation across the fold. The change (Avouac et al., 1993; Burchfiel et al., 1999; Zhang 659 604 shear needed to thin the northern limb by ,1.5 km should et al., 2001). Our interpretation at the Kashi–Atushi fold 660 605 require significant penetrative shear across the bedding. system indicates, however, that the base of the conglomerate 661 606 Shear strains of ,15% can be observed in mesoscopic is temporally variable, ranging from 2.8 to 1.9 Ma across the 662 607 studies around fault zones (Jamison, 1989), suggesting that Talanghe anticline (Chen et al., 2002). 663 608 thinning the forelimb by 50% should create obvious shear 664 609 and deformation within the attenuated limb. During 2.2. Growth strata 665 610 structural mapping, we observed no significant difference 666 611 in mesoscopic deformation between the limbs. On the Bedding is conformable within the folds but not at the 667 612 contrary, beddingUNCORRECTED planes were well maintained in each limb southernmost exposedPROOF flanks at each of the transects across 668 613 even where wide panels of the limb were steeply dipping or the Atushi–Talanghe and Kashi anticlines At these 669 614 overturned. transects, field observations show that in the last hundreds 670 615 The stratigraphic model relies on a wedge shaped upper of meters of exposed strata, the bedding inclinations 671 616 unit (in cross-section) to cause the difference in the decrease abruptly up section, and in most places form 672

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673 angular unconformities suggestive of off lapping. The bedding orientation at each traverse were derived from the 729 674 Ganhangou paleomagnetic section was located to capture DEM and used as bases for our cross-section interpretations. 730 675 this transition (Fig. 7). In contrast, mapping at the The cross-sections are constrained by structural measure- 731 676 Mingyaole anticline revealed that a few closely spaced ments, stratigraphic thickness based on field measurements, 732 677 unconformities are located just below the base of the Xiyu progradation of the conglomerate unit, and unpublished 733 678 Formation. A transition from conformable beds to off seismic data. Our interpretations suggest most of the folds 734 679 lapping or angular unconformities is frequently interpreted would reach 3–4 km above the land surface, yet modern 735 680 to result from the transition from static conditions to active topography exhibits less than 1 km of relief (Figs. 8 and 9). 736 681 folding (e.g. Poblet et al., 1997). We argue that the gently Due to the vigorous erosion and lack of distinctive 737 682 dipping outer beds are syntectonic strata, deposited after the stratigraphic marker beds, two techniques were used to 738 683 initiation of folding in the region. quantify the amount of shortening across the region. We 739 684 At the Seven Mills and Atushi transects, the southern review the general method for creating the cross-sections, 740 685 flanks show continuous reduction in dips of off lapping summarize general observations from the seismic reflection 741 686 strata; however, the northern flanks revealed dramatic data, discuss the common structural style of the folds, and 742 687 changes in dip. At the Atushi transect, for example, a then report details of each fold individually. 743 688 wide, 758 north dipping panel is overlain by a 158 dipping The basic fold geometry was determined by fitting a form 744 689 panel. We infer that, as observed today, rivers eroded the line to the structural data. Where canyon walls reveal cross- 745 690 north side of the folds as they grew, and that growth strata sectional views, the Kashi–Atushi fold system exhibits 746 691 were not preserved until later in the development of the fold, sharp kink axes in the limbs, while sheared zones were 747 692 when these sections had risen above the grade of the river. It common in the core of tight folds. These features are well- 748 693 appears that the southern sides of the folds were sheltered modeled using kink style fold axes to separate dip panels 749 694 from this effect, and growth strata were deposited (Marshak and Mitra, 1988). Dip panel width and inclination 750 695 continuously. were determined by converting dip measurements into 751 696 apparent dip with respect to the cross-section trend, 752 697 grouping the dip measurements into internally consistent 753 698 3. Structure sets (variability #158) and calculating the average angle 754 699 (Figs. 8 and 9). Where we were able to measure bedding 755 700 We conducted detailed field mapping on six traverses thicknesses across a kink axis, the thicknesses did not 756 701 across the Atushi–Talanghe, Mutule, Mingyaole and Kashi change, indicating that the kink axis bisects the kink panels. 757 702 anticlines to investigate the variation in shortening and In each cross-section, the stratigraphic height of the growth 758 703 deformational style across the Kashi–Atushi fold system strata within each fold limb is accurate, but the geometry of 759 704 (Fig. 2). Transects were located along water gaps or roads the growth strata package is idealized. For simplicity (and 760 705 cutting orthogonal to the structures to maximize access to lacking evidence to the contrary), we assumed that there is 761 706 structures and bedding exposures. Structural and sedimen- no change in structural height from north to south across the 762 707 tological data were mapped at ,1:25,000 onto CORONA structures. 763 708 satellite photographs, aerial photos, and topographic maps. 764 709 Additional structural data were interpreted from Chinese 3.1. Seismic reflection data 765 710 geologic maps (XBGM, 1965a,b). Field maps were later 766 711 transferred onto a geographic information system database We examined three unpublished, confidential seismic 767 712 with the Landsat image and CORONA photographs tied to a reflection images from transects across the Atushi– 768 713 90 m digital elevation model (DEM) base. Topographic Talanghe and Kashi anticlines The seismic profiles are 769 714 profiles oriented perpendicular to the average strike of the basically co-located with three traverses (Fig. 3): (1) over 770 0 715 the Atushi–Talanghe anticline along the road at A–A and 771 716 continuing through the Kashi anticline along the Baishiker- 772 0 717 emu River, (2) over the Atushi–Talanghe anticline at B–B 773 718 and crossing the eastern edge of the topographic expression 774 719 of the Kashi anticline, and (3) over the Atushi–Talanghe 775 0 720 anticline at C–C and continuing ,20 km into the evaporite 776 721 deposits ,15 km east of the end of the topographic 777 722 expression of the Kashi anticline. These profiles fortuitously 778 723 provide this study with a second perspective for interpreting 779 724 Fig. 7. Interpreted photograph of growth strata on the southern limb of the the structures at depth. 780 Talanghe anticlineUNCORRECTED at the Ganhangou paleomagnetic transect. View is to the PROOF 725 The most valuable information in the seismic lines was 781 east. South of a ,3-km-wide panel dipping 508, off lapping dips of siltstone 726 and small-pebble conglomerates (foreground) are interpreted to record the the presence of long, gently dipping (,128) reflectors at the 782 727 initiation of folding. Pale hill under skyline is the eastward continuation of outermost flanks of the folds, where young alluvial deposits 783 728 Talanghe anticline. on the surface prohibited access to the structure. The gently 784

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785 841 786 842 787 843 788 844 789 845 790 846 791 847 792 848 793 849 794 850 795 851 796 852 797 853 798 854 799 855 800 856 801 857 802 858 803 859 804 860 805 861 806 862 807 863 808 864 809 865 810 866 811 867 812 868 813 869 814 870 815 871 816 872 817 873 818 874 819 Fig. 8. Combined cross-section of Atushi and central Kashi transects. The depth to the detachment surface is determined from excess area calculations; the 875 820 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 876 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 821 877 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 822 represent growth strata. Where observed in the field the fanning dips are drawn on the image, otherwise the Quaternary sediments are unmarked. ‘S’ indicates 878 823 approximate location of kink axis determined from seismic reflection profile. Dip of kink panels with no structural data is determined from seismic data (if 879 824 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 ) 880 825 identifies the bed used to calculate line length shortening. 881 826 882 827 dipping outer limbs bracketed regions of discontinuous and Formation and other gypsiferous units within the Kashi 883 828 unresolved reflectors positioned directly below the topo- Group (Yang, 1996). The variation in deformation with 884 829 graphic expression of the folds, and correlate with measured depth indicates several horizons may have accommodated 885 830 bedding dips greater than ,458. In the cross-section different amounts of strain in this convergent setting. 886 831 interpretations (Figs. 8 and 9), the gently dipping flanks 887 832 seen in the seismic surveys are included, resulting in 3.2. Field observations and structural model 888 833 anticlines that are wider than evidenced in the topography. 889 834 Another common feature in each of these profiles is an At the surface, most of the folds exhibit steep limbs 890 835 essentially flat lying, strong reflector at 9–10 km below land dominated by dip panels inclined between 60 and 858 891 836 surface, withUNCORRECTED the exception of Seven Mills, which was (overturned) ThePROOF similarity between the largest, steepest, 892 837 12 km below land surface. From 5 km below land surface to dip panel on each side of a fold obscures vergence, but in 893 838 the strong reflector at ,9 km, the reflectors are gently general, a subtle north vergence of the folds is exhibited in 894 839 warped or discontinuous. Stratigraphically, 9–10 km is the the cross-sections. The fold cores are tight, resulting in box- 895 840 approximate depth of a massive gypsum unit of the Aertashi like to isoclinal knobs that rise off the low angle flanks (Figs. 896

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897 953 898 954 899 955 900 956 901 957 902 958 903 959 904 960 905 961 906 962 907 963 908 964 909 965 910 966 911 967 912 968 913 969 914 970 915 971 916 972 917 973 918 974 919 975 920 976 921 977 922 978 923 979 924 980 925 981 926 982 927 983 928 984 929 985 930 986 931 987 932 988 933 989 934 990 935 991 936 992 937 993 938 994 939 995 940 996 941 997 942 998 943 999 944 1000 945 1001 946 1002 947 1003 948 UNCORRECTED PROOF 1004 949 1005 950 1006 951 1007 952 1008

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1009 8 and 9). To accommodate the steep bedding dips of the structural mapping along the southern limb east transect 1065 0 1010 inner panels, the folds can be modeled as fault propagation A–A (Fig. 8) showed that bedding on this limb continues in 1066 1011 folds with complex, imbricate blind faulting histories (Al a ,808 SE dipping panel and nowhere forms a syncline 1067 1012 Saffar, 1993) or as detachment folds. Basic observations required for separate folds. The parallel curvature of the 1068 1013 suggest the anticlines are not fault propagation folds. (1) bedding strike suggests the Atushi and Talanghe anticlines 1069 1014 There is no evidence for long thrust ramps above 6 km form over a common, linear structure and are not offset, 1070 1015 below land surface in the seismic images. (2) Along some linked anticlines (e.g. Sattarzadeh et al., 2000). We 1071 1016 transects, fault scarps were observed near the core of the designate the entire anticline as the Atushi–Talanghe 1072 1017 folds, but never at the base of the forelimb where a thrust anticline, but refer to the Atushi and Talanghe anticlines 1073 1018 fault is predicted from traditional fault propagation fold to identify the western and eastern halves, respectively. 1074 1019 geometry (e.g. Suppe and Medwedeff, 1990). Instead, the At transect A–A0, the Atushi anticline is isoclinal, 1075 1020 geometry of the anticlines is consistent with models of dominated by an 808 dip panel on the southern limb and a 1076 1021 disharmonic detachment folds (Mitra and Namson, 1989)or 758 dip panel on the north (Fig. 8). The characteristic, dark 1077 1022 faulted detachment folds with moderate strain (Figure 3b in Xiyu Formation conglomerate is clearly visible on the 1078 1023 Mitra, 2002). Common to both of these models is a contrast northern side but is absent on the southeast side (Fig. 2). 1079 1024 between relatively competent layers deforming primarily by Without the Atushi–Xiyu contact, the southern contact for 1080 1025 flexural slip overlying incompetent units deforming by the gravel was located by projecting beds in from ,4kmto 1081 1026 distributed or inhomogeneous deformation. The strati- the east. In the cross-section, the width of the fold was 1082 1027 graphic units at the Kashi–Atushi fold system exhibit a determined by the southernmost structural measurements 1083 1028 similar pattern, best observed at the Boguzihe profile (B–B0, and the requirement of no net structural relief between the 1084 1029 Fig. 3). At the hinge zone of the fold, the lowest river terrace synclines bounding the fold. The top of the anticline was 1085 1030 risers reveal that Wuqia Group bedding is obliterated, modified to have a horizontal dip panel. This alteration 1086 1031 showing discontinuous blocks of bedded strata floating and contributes to a conservative shortening estimate, and is 1087 1032 chaotically rotated within a sheared mudstone matrix. consistent with dip panels observed at the Boguzihe water 1088 1033 Above this, the older river terrace risers reveal the Atushi gap and Mingyaole sections, and the gentle character of the 1089 1034 Formation bending unbroken across the axial trace. anticlines at their tips. 1090 1035 The cross-sections (Figs. 8 and 9) were constructed as At transect B–B0, the Boguzihe water gap (Fig. 9), the 1091 1036 detachment folds, constrained by structural measurements northernmost dip measurements for the 808 panel were 1092 1037 and the low dip panels observed in the seismic sections. measured ,1.7 km west of the profile on bedrock islands in 1093 1038 Specific information on each profile is presented in the an ephemeral channel. The southern limb contains an 1094 1039 sections below. Each cross-section interpretation was drawn overturned panel that dips 878 to the north, which is 1095 1040 reducibly, meaning that axial planes were located to connected to a steeply dipping panel (748) by a long, gently 1096 1041 minimize the length of steeply dipping panels and maximize dipping kink hinge visible in the lowest terrace riser. Where 1097 1042 gently dipping panels. Short sections of moderate to gently the Boguzihe enters New Atushi (Fig. 2), a remnant knob of 1098 1043 dipping beds on the crest of the fold were included, resulting sediments records the initiation of shortening in an abrupt 1099 1044 in rounded folds with minimized shortening. set of reducing dips interpreted as syntectonic deposition 1100 1045 (Chen et al., 2002). Paleomagnetic samples along this 1101 1046 3.2.1. Atushi–Talanghe anticline profile constrain the age of the strata (Figs. 2 and 4). Two 1102 1047 The topographic expression of the Atushi–Talanghe fault scarps are located ,100 and ,400 m north of the core 1103 1048 anticline extends eastward over 100 km from the margin of of the fold, offsetting several fluvial terraces of the 1104 1049 the Pamir Mountains Constrained by our structural transects Boguzihe. The geomorphic expression of the scarps in 1105 1050 and a strike line trace, the axial trace curves up to the CORONA images indicates that the south dipping faults 1106 1051 northeast in several gentle arcs, lost finally to a series of have been recently active. The location of these faults 1107 1052 1108 deflected channels that drain southward from the Mutule within the hinge zone of the anticline suggests that these 1053 1109 anticline (Fig. 2). The fold is oriented approximately N72E. faults accommodate deformation in the core, but are not 1054 1110 Previous mapping (Scharer et al., 2000) separated the causal thrusts of a fault propagation fold. 1055 1111 western, Atushi fold from the eastern Talanghe fold at an The southern limb of the Ganhangou traverse, located 1056 1112 eastward plunging core visible to the east of the Atushi ,10 km west of the Boguzihe, has a uniform strike of N70E 1057 1113 transect line (A–A0)inFig. 2.However,subsequent and a ,508 dip for 3 km that ends in off-lapping growth 1058 1114 1059 1115 1060 UNCORRECTED PROOF 1116 1061 1117 Fig. 9. Cross-section interpretations for the Kashi–Atushi fold system. Bedding orientations and lithologic data were obtained by field mapping; (D) and (E) 1062 were improved with data on Chinese geologic maps (XBGM, 1965b) and (F) was constructed solely with the information from Chinese geologic maps 1118 1063 (XBGM, 1965a). Conventions as in Fig. 8. The asterisk ( p ) identifies the beds used to calculate line length shortening. Part (B) includes double detachment 1119 1064 planes and low-angle, deep thrust faults interpreted from seismic survey. 1120

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1121 strata (Fig. 7). One of the magnetostratigraphic sections 3.2.3. Kashi anticline 1177 1122 used in this paper was collected along this transect (Chen The axial trace of the Kashi anticline has a similar 1178 1123 et al., 2002). A cross-section of this transect was not orientation as the Mingyaole anticline (N85E) but their tips 1179 1124 produced due to the erosion of the northern limb. are offset by ,5 km The Kashi anticline is over 60 km long 1180 0 1125 Transect C–C , along the Seven Mills water gap, and is bisected by the Baishikeremuhe. As at the Talanghe 1181 1126 provides the most complete record of both limbs of the anticline, deflected streams at the eastern end suggest the 1182 1127 Talanghe anticline (Fig. 9). Similar to the Boguzi River fold continues farther into the basin than indicated by its 1183 1128 section, the fold is tight, dominated by ,758 dipping limbs. topographic expression or the Landsat image (Fig. 2). This 1184 1129 The seismic reflection profile indicates that the total fold interpretation is supported by a gentle fold exhibited in the 1185 1130 width is ,13 km, almost twice the size of its topographic seismic reflection profile that crossed ,15 km east of the 1186 1131 expression (,7 km). Along the canyon walls, the hinge topographic end of the fold. The width of the fold is constant 1187 0 0 1132 zone exhibits ,500 m of folded and disrupted bedding, across the fold, ,13 km at transects F–F and G–G , but the 1188 0 1133 bound on the southern side by a south dipping fault that is fold amplitude is ,1.5 km lower at transect G–G . Fanning 1189 1134 parallel to bedding in the hanging wall. Without strati- strata are observed in the seismic profile on the north and 1190 1135 graphic marker beds, the offset on the fault is not south sides of the fold. 1191 1136 quantifiable beyond the ,200 m offset provided by the 1192 1137 exposure. Unique to this section, the seismic reflection 3.3. Calculated shortening 1193 1138 profile contained two reverse faults and fault bends between 1194 1139 depths of 9 and 12 km (Fig. 9). Beds translated across the Slickenlines on bedding planes were observed through- 1195 1140 268, south dipping fault ramp are offset by approximately out the study area, but were not considered pervasive 1196 1141 2 km. Similarly, small-scale accommodation features were scarce. 1197 1142 In the northeast corner of the study area, the opposing We did not observe evidence of consistent deformation 1198 1143 tips of the Mutule and Talanghe anticlines fold the Xiyu domains that suggest the folds formed by tangential 1199 1144 Formation into an open syncline along transect D–D0 (Figs. longitudinal strain (e.g. neither tensile cracks in outer arcs 1200 1145 3 and 9). The blunt nose of the open Mutule anticline nor reverse faults in deeper arcs). Due to the poorly 1201 1146 plunges more steeply than the tight Talanghe anticline. The indurated stratigraphy and vigorous erosion of the uplifted 1202 1147 Xiyu Formation is folded in the syncline to the east of the structure, we lack observational control on the internal 1203 1148 profile, and has been represented in the cross-section by character of the anticline, and thus on the mechanism(s) of 1204 1149 locating the bottom of the contact just above the remaining shortening [e.g. homogeneous strain, second order faulting 1205 1150 topography along the transect. Growth strata were observed or second order folding (Epard and Groshong, 1995)]. The 1206 1151 on the southern limb of the Talanghe anticline, but the bulk of each limb, however, maintained broad, planar kink 1207 1152 northern limb of the Mutule anticline has been eroded. panels, and the limbs appear to control the largest scale 1208 1153 morphology of the anticlines. We therefore present short- 1209 1154 ening estimates from two methods: excess area calculations 1210 3.2.2. Mingyaole anticline 1155 and line length balancing (Table 1). Both methods assume 1211 1156 The Mingyaole anticline is the smallest in the study area, that flexural slip is the dominant deformation mechanism. 1212 1157 ,40 km long and 10 km wide at the widest section The fold 1213 1158 has an arcuate shape and is impinged upon its southwest 3.3.1. Excess area 1214 1159 edge by the Kazikeaete thrust system and north verging Flexural slip is accommodated by layer parallel shear, 1215 1160 Mushi anticline (Fig. 3). A fault associated with the which can be displayed graphically in an apparent short- 1216 1161 Kazikeaete thrust ruptured in 1985 during the M 7.2 ening plot (Fig. 10a; Mitra and Namson, 1989). All of the 1217 1162 Wuqia earthquake, creating a 1.3 m dextral and ,1m anticlines show a decrease in apparent shortening profile 1218 1163 vertical offset (Chen et al., 1997; Molnar and Ghose, 2000). with depth, which suggests interbed shear increases in 1219 1164 In cross-section, the Mingyaole anticline exhibits north stratigraphically higher layers. In the case of disharmonic 1220 1165 vergence. Unlike the Atushi–Talanghe anticline, which detachment folds, Mitra and Namson (1989) suggest the 1221 1166 expresses very tight folding at the surface, the Mingyaole concentricity of the anticline is lost in the lower layers 1222 1167 anticline is broad. Also unique to the Mingyaole anticline, a because space accommodation problems result in penetrat- 1223 1168 series of unconformities we have interpreted to indicate the ive shortening, which changes the deformed bed length. In 1224 1169 initiation of folding in this area were found below the base the Kashi–Atushi fold system, erosion limits our ability to 1225 1170 of the Xiyu Formation. Moving up section at the southern evaluate macroscopic deformation in stratigraphically lower 1226 1171 end of the Mingyaole transect, a .2 km thick, 588 dip panel layers, so we assume that the differential penetrative 1227 1172 ends as bedding dips abruptly decrease to 278, and then are shortening in lower layers balances the increase in interbed 1228 unconformablyUNCORRECTED replaced by 138 dipping beds over a 50 m PROOF 1173 shear in higher layers. 1229 1174 interval (Fig. 9). To produce this cross-section, the growth Excess area, or the area encompassed by a fold under a 1230 1175 strata were modeled to deform progressively by limb particular stratigraphic level divided by the total shortening 1231 1176 rotation. at that level, was first used by Chamberlin (1910) to 1232

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1233 1289 1234 1290 1235 1291 1236 1292 1237 1293 1238 1294 1239 1295 1240 0.8 1296 , Shortening rate (mm/yr) assuming lateral propagation 1241 1297 1242 1298 1243 1299

1244 (Ma) 1300 b 1245 1301 1246 1.9 1302 .

1247 Inferred initiation of folding 1303

1248 re robust than the line length shortening 1304 1249 1305 1250 1306 1251 1307 1252 1308 1253 1309 1254 1310

1255 (mm/yr) using excess 1311 a 1256 1312 Slowest/fastest shortening rate area shortening 1257 1313 1258 1314 1259 1315 1260 1316 ). 1261 1317 1262 1318 Fig. 12 1263 1319 1264 1320 1265 1321

1266 Shortening rate (mm/yr) using excess area shortening, assuming uniform initiation at 1.2 Ma 1322 1267 1323 1268 1324 1269 1325 1270 1326 1271 1327 1272 1328 Line length shortening (km) 1273 1329 1274 1330 1275 1331 1276 1332 1277 1333 1278 1334 1279 1335 1280 1336 (km) (preferred) 9.3 9.2 7.8 5.3/11.9 – – 1281 1337 1282 Fig. 10. Results of (a) apparent shortening profile and (b) excess area 1338 ) 4.1 3.2 3.4 2.3/5.3 1.1 3.7 0

1283 ) 0.7method for 0.2 each transect. From 0.6 a maximum at the uppermost 0.4/0.9 layer, the 1339 0.7 1.0 ) 1.5 1.4 1.3 0.8/1.9 0 0 ) 4.3 4.7 3.6 2.4/5.5 1.4 3.1 0

c apparent shortening profiles reduce with depth, indicating a gradient in ) 2.4 1.8 2.0 1.4/3.1 0.8 3.0 )

1284 ) 2.5 4.1 2.1 1.4/3.21340 0.9 2.8 ) 6.8 5.5 5.7 3.8/8.7 2.0 3.4 0 0 0 UNCORRECTED0 interbed shear. The PROOF true shortening profiles are unknown, because interbed 1285 1341 shear can be balanced by penetrative deformation. (b) Excess area methods 1286 (H–H of Epard and Groshong (1993) calculate the shortening (the slope) and the 1342

1287 Slowest rate calculated with foldInferred initiation from at lateral 1.77 propagation Ma, rateIncludes fastest (50 line rate km/Myr) length calculated defined shortening with between from fold the the initiation Boguzihe southern at and limb 0.78 Ganhangou of Ma. magnetostratigraphic the See sections Keketamu text (see anticline for Section (4.3 explanation. 4). km, depth to the detachment (the x intercept). Dashed lines show regression for 1343 a c b Boguzihe (B–B Kashi Central (F–F Transect Excess area shortening Table 1 Summary of shortening and shortening rate estimates for structural transects. The shortening estimates determined by excess area are considered mo 7 Mills (C–C Kashi East (G–G Mutule (D–D Mingyaole (E–E Atushi (A–A 1288 Fig. 12 the linear, upper part of the series, solid lines use all data from the transect. 1344

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1345 determine the depth to the detachment horizon. For this probably results from the tightness of the fold. Poor 1401 1346 study, we use the excess area method as adapted by Epard correlation at the Kashi East transect may reflect the 1402 1347 and Groshong (1993). This method compares the excess kinematics of the early stages of fold development (Poblet 1403 1348 area at several stratigraphic levels with the depth from each and McClay, 1996). 1404 1349 level to a constant reference horizon (Fig. 10b). A plot of the 1405 1350 relationship between the excess area and depth to the 1406 1351 reference horizon gives the estimated shortening (the slope) 4. Discussion 1407 1352 and the depth to the detachment (the X-intercept). For this 1408 1353 study, 6–8 excess areas were determined for each anticline The style and form of these folds, as drawn, result from a 1409 1354 and the land surface at each cross-section was plotted as the set of interpretations concerning their deposition and 1410 1355 reference horizon. Shortening calculated by excess area geometry. Described in previous sections, the interpretation 1411 1356 varied from 6.8 km at Atushi to 0.7 km at the East Kashi of the nature of the Xiyu–Atushi contact, conventions 1412 1357 transect (Table 1). followed in reconstructing the anticlines, and the nature of 1413 1358 Excess area graphs of the Atushi, Boguzihe, Seven Mills, the growth strata all affect the shortening estimates. 1414 1359 and Kashi cross-sections show a common feature, in that the 1415 1360 slope (i.e. the shortening) is linear to a depth of 5.5–6.0 km 1416 4.1. Structure 1361 below land surface at each profile, respectively (Fig. 10b). 1417 1362 Above this depth, the slopes have a high correlation 1418 1363 coefficient, suggesting that deformation mechanisms are A basic assumption that influences the shortening 1419 1364 balanced and bed lengths remain constant with deformation. estimates is the lack of structural height that developed 1420 1365 Below this depth, the slope shallows, suggesting reduction across the folds Although the cross-sections were developed 1421 1366 in strain. This pattern is produced if the detachment horizon to minimize the height of the folds (principally by restricting 1422 1367 is raised over time, or if the detachment occurs over a zone the length of steeply dipping panels), not modeling the 1423 1368 with differential strain rather than a single detachment imbricate thrusts commonly required that the width of each 1424 1369 plane. Alternatively, the deformation gradient may change fold was broadened from its surface expression by 1425 1370 at lower depths, resulting in an imbalance between interbed increasing the distance between the outer kink axes. The 1426 0 1371 shear and differential penetrative shortening. For this Kashi East transect (G–G ) is the only profile unaltered by 1427 1372 reason, two detachment horizons are considered. The this convention. The seismic lines, however, provide 1428 1373 upper detachment horizon is ,6 km below land surface, independent support for this modification, as they show 1429 1374 at the transition from constant bed lengths to imbalanced that in the subsurface the folds are broader than their 1430 1375 deformation mechanisms. Using all data, the maximum topographic expression, and lack clear evidence of thrust 1431 1376 estimated depth to detachment is 8.6 km below land surface faults in the upper 6 km. Where seismic lines are available, 1432 2 1377 [due to very poor correlation (R ¼ 0.6) and an anom- the location of the outer kink axes in the cross-section 1433 1378 alously low depth to detachment estimate (13.4 km), the interpretations does not contradict the seismic reflection 1434 1379 Kashi East transect is not included in the average]. We data. 1435 1380 suggest that between 6 and 9 km, there is a reduction in Interpretation of the Kashi–Atushi fold system as a set of 1436 1381 north directed shortening and an increase in northward detachment folds is supported by the strong competency 1437 1382 translation of material under the Tian Shan. contrasts in the deformed beds (Dahlstrom, 1990; Mitra, 1438 1383 2002). The Kashi Group contains a significant proportion of 1439 1384 3.3.2. Line length balancing gypsiferous units, and shows severe small-scale folding and 1440 1385 For comparison with other regional estimates of short- pervasive faulting where exposed in the southern Keping- 1441 1386 ening (Yin et al., 1998; Allen et al., 1999; Burchfiel et al., tage system. The Wuqia Group is rich in siltstones, and 1442 1387 1999), we also calculated line length changes for each shows a ductile behavior in areas of high strain, such as the 1443 1388 profile (Table 1). This method assumes that the folding core of the Boguzihe transect. The Atushi Formation 1444 1389 process does not alter the bedding from its original length. exhibits second order faulting at the core of the Mingyaole, 1445 1390 To capture the shortening of the youngest pregrowth Atushi, and Boguzihe transects and second order folding at 1446 1391 deposits, the uppermost reconstructed bedding horizon Seven Mills. In the limbs of each anticline, however, the 1447 1392 was used (Figs. 8 and 9). With the exception of the Seven Atushi Formation forms parallel-bedded kink panels with 1448 1393 Mills and Kashi East transects, the correlation between the scarce to no faults. Finally, the Xiyu Formation appears to 1449 1394 excess area and line length balancing methods is very good be the most competent, commonly forming the highest 1450 1395 (^20%). Poor correlation at the Seven Mills transect portions of the modern topography. Following the schema 1451 1396 UNCORRECTEDdeveloped by DahlstromPROOF (1990), we interpret that the 1452 1397 gypsiferous Kashi Group acts as a lower detachment plane, 1453 The shortening is linear for all transects to a depth of ,6 km below land 1398 surface, indicating interbed shear and penetrative deformation are balanced. the upper Kashi and Wuqia Groups behave as a ductile 1454 1399 Below ,6 km, the shortening is reduced, suggesting a gradient in folding detachment zone, and the Atushi and Xiyu Formations 1455 1400 down to the detachment plane, ,9 km below land surface. constitute a brittle upper layer that deforms by flexural slip 1456

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1457 in the limbs and second order faulting and folding in the initiation are determined from the polarity sequence 1513 1458 cores. correlation to the GPTS (Fig. 11). Assuming both sites are 1514 1459 This mechanical stratigraphy permits limb rotation as a correctly tied at the base of the Olduvai chron, the long 1515 1460 mechanism for fold growth (Dahlstrom, 1990), but the reversed period containing the syntectonic unconformity at 1516 1461 geometric similarity between these folds and symmetric both sites cannot be older than the end of the Olduvai chron 1517 1462 faulted detachment folds suggests that limb lengthening also (1.77 Ma) and no younger than the beginning of the Bruhnes 1518 1463 occurs (Mitra, 2002). Additional insight on the method of normal chron (0.78 Ma). Therefore, we choose upper and 1519 1464 fold growth is provided by the anticlinal morphology. lower limits of 1.77 and 0.78 Ma, respectively, to bracket 1520 1465 Detachment folding by limb rotation should be the initiation of folding. Records of small magnitude 1521 1466 accompanied by a reduction in width and increased limb earthquakes (USGS Earthquake Hazards Program, 2003), 1522 1467 dips with advanced shortening. In contrast, shortening by geomorphic expression of fault scarps, (Zhao et al., 2000), 1523 1468 limb lengthening should be accommodated by an increase in warping of fluvial terrace surfaces, and geodetic studies 1524 1469 the height of the fold and a minor increase in fold width (Wang et al., 2000) indicate that folding continues to the 1525 1470 (Poblet and McClay, 1996). For each anticline, the fold present. 1526 1471 widths remain fairly constant along strike, but the fold 1527 1472 heights and limb dips are correlated with increased short- 4.3. Shortening rates 1528 1473 ening (Figs. 8 and 9). This suggests that over the life of the 1529 1474 folds, both mechanisms play a role in fold development. A simplistic approach to quantifying the geologic 1530 1475 Unfortunately, the growth strata are insufficiently exposed shortening rate is to identify a representative location and 1531 1476 to more precisely evaluate the kinematics of fold develop- apply a regional initiation of folding at 1.2 Ma Expanding 1532 0 1477 ment (e.g. Salvini and Storti, 2002). In a future study, we the region to a central profile (H–H ) including shortening 1533 1478 hope to use deformed fluvial terrace surfaces to illuminate accommodated in the southern limb of the Keketamu 1534 1479 this question. anticline (4.3 km) and the Boguzihe and Kashi East 1535 1480 transects, the total shortening across the Kashi–Atushi 1536 1481 4.2. Paleomagnetic constraints fold system is 9.3 km (Fig. 12). If shortening across the 1537 1482 three structures in this transect initiated contemporaneously, 1538 1483 Magnetostratigraphic sampling across the Boguzihe and a mean shortening rate of 7.8 mm/yr has continued for the 1539 1484 Ganhangou sections provides temporal constraints on the past 1.2 Myr (Fig. 13). Extreme minimum (5.3 mm/yr) and 1540 1485 initiation of shortening (Figs 2 and 4; Chen et al., 2002) and maximum (11.9 mm/yr) rates result from upper and lower 1541 1486 allows an estimation of the shortening rate. The sections are age limits, respectively (Table 1). Shortening rates for each 1542 1487 linked together by tracing bedding along intervening strata transect reduce to the east if folding was coeval (Table 1; 1543 1488 and then matched to the geomagnetic polarity timescale Fig. 13). 1544 1489 (GPTS) of Cande and Kent (1995) constrained by the local While synchronous folding provides a reasonable 1545 1490 Plio-Pleistocene fossil assemblage (Fig. 11). Sampling estimate of the shortening rate, it is an unlikely condition 1546 1491 strategies and statistics, demagnetization processes, and because the stratigraphic height of the growth strata changes 1547 1492 correlation to the GPTS are described in detail in Chen et al. across the study area. At the Mingyaole profile, the growth 1548 1493 (2002). We apply their results to evaluate the initiation of strata are located just below the base of the Xiyu Formation 1549 1494 shortening across the Kashi–Atushi fold system, and whereas at the Boguzihe transect and the Ganhangou 1550 1495 discuss implications for shortening rates. section, the growth strata are younger than the Xiyu 1551 1496 The Boguzihe section is a good location to constrain the Formation. To integrate these observations, we estimate 1552 1497 age of the base of the Xiyu Formation (,1.9 Ma), but the the geologic shortening rate at each transect using the 1553 1498 cobble conglomerate prevents paleomagnetic sampling difference in growth strata ages at Boguzihe and Ganhangou 1554 1499 through the syntectonic unconformity. Consequently, the (1.4 and 1.2 Ma, respectively). Dividing the distance 1555 1500 age of the syntectonic unconformity at Boguzihe (,1.4 Ma) between the sites (,10 km) by the difference in initiation 1556 1501 is calculated both from a linear extrapolation of the of folding at Boguzihe and Ganhangou (0.2 Myr) results in 1557 1502 sedimentation rate of the underlying, dated strata and by an eastward lateral propagation rate of ,50 km/Ma. The 1558 1503 strike line correlation to dated strata in the Ganhangou eastern end of the Talanghe anticline shows geomorphic 1559 1504 section (Fig. 11). At the Ganhangou section, finer grained evidence of eastward lateral propagation (Fig. 14). The 1560 1505 sedimentsallowustosampleacrossthesyntectonic geomorphic expression of the anticline is 25 km shorter than 1561 1506 unconformity and capture the age of the growth sediments. expected if the lateral propagation rate has been constant for 1562 1507 The age of the unconformity at Ganhangou (,1.2 Ma) is the last 1.2 Ma, suggesting that the extrapolated rate 1563 1508 younger thanUNCORRECTED at the Boguzihe section. Ages of the overestimates PROOF the true lateral propagation rate. If the 1564 1509 syntectonic unconformity are maxima; if some pregrowth propagation rate decreased to the east because stress was 1565 1510 strata are missing, the true age of the unconformity could be accommodated on the Mutule anticline, the lateral propa- 1566 1511 younger. gation rate can be used to estimate the initiation of folding 1567 1512 Upper and lower limits on the 1.2 Ma estimate for fold across the Kashi–Atushi fold system, recognizing that 1568

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1569 1625 1570 1626 1571 1627 1572 1628 1573 1629 1574 1630 1575 1631 1576 1632 1577 1633 1578 1634 1579 1635 1580 1636 1581 1637 1582 1638 1583 1639 1584 1640 1585 1641 1586 1642 1587 1643 1588 1644 1589 1645 1590 1646 1591 1647 1592 1648 1593 1649 1594 1650 1595 1651 1596 1652 1597 1653 1598 1654 1599 1655 1600 1656 1601 1657 1602 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 1658 1603 paleomagnetic sections. The sections were linked by tracing distinct beds ,10 km from the Boguzihe to the Ganhangou section (wavy line), and then 1659 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 1604 1660 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 1605 absent at the Ganhangou transect. Modified from Chen et al. (2002). 1661 1606 1662 1607 1663 1608 interpreting the sequence of fold development across the formed successively and deformation propagated eastward. 1664 1609 folds is speculative given vigorous erosion of the anticlines From this construct, folding initiated at ,0.7 Ma at the East 1665 1610 and uncertainty in the kinematics of fold growth. Kashi transect, ,1.1 Ma at the Central Kashi transect, and 1666 1611 To arrive at local estimates of the shortening rate, we at least ,1.9 Ma at the Mingyaole transect (Table 1). 1667 1612 simply extend the 50 km/Myr lateral propagation rate from Because the growth strata are older than the Xiyu Formation 1668 1613 the Ganhangou location back in time to the west and at Mingyaole, this scenario suggests that the Xiyu 1669 1614 forward to the east (Table 1; Fig. 13). Under this Formation at Mingyaole is younger than ,1.9 Ma, the 1670 1615 assumption, the initiation of folding would occur ,2.0 Ma same age as the base of the Xiyu at the Boguzihe transect 1671 1616 at the Atushi transect, ,0.9 Ma at the Seven Mills and (1.9 Ma). A western source for this conglomerate suggests 1672 1617 ,0.8 Ma at the Mutule transect. Although considerably the Pamir salient had reached this area by ,1.9 Ma, 1673 1618 more speculative, we estimate the ages along the Mingyaole consistent with estimates for development of the Trans Alai 1674 1619 and Kashi anticlines using a similar technique and the same Range around ,4Ma(Arrowsmith and Strecker, 1999). 1675 1620 propagation rate.UNCORRECTED A seismic reflection profile ,20 km east While local shortening PROOF rates decrease to the east along the 1676 1621 of the topographic expression of the Kashi anticline shows Atushi–Talanghe anticline, they are variable across the 1677 1622 low amplitude folding. If folding began at this end recently, Mingyaole and Kashi anticlines. Restricted to the Kashi– 1678 1623 the initiation of folding can be extrapolated westward from Atushi fold system, we consider representative a regional 1679 1624 this location, assuming the Mingyaole and Kashi anticlines average rate of ,5 mm/yr (Fig. 13). 1680

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1681 1737 1682 1738 1683 1739 1684 1740 1685 1741 1686 1742 1687 1743 1688 1744 1689 1745 1690 1746 1691 1747 1692 1748 1693 1749 1694 1750 1695 1751 1696 1752 1697 1753 1698 1754 1699 1755 1700 1756 1701 1757 1702 Fig. 12. Generalized cross-section across southern Tian Shan (H–H0 on Fig. 2). Total Pleistocene through modern shortening between the Keketamu and Kashi 1758 1703 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 1759 p 1704 calculated by line length balancing. The bed used to measure the shortening is noted with an asterisk ( ). Shortening calculated for the Boguzihe and Kashi 1760 East anticlines was determined by excess area methods. Arrows above detachment surface show decreasing slip to the north while the Tarim craton is 1705 1761 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 1706 Tian Shan are not known and drawn schematically. 1762 1707 1763 1708 1764 1709 1765 1710 1766 1711 1767 1712 1768 1713 1769 1714 1770 1715 1771 1716 1772 1717 1773 1718 1774 1719 1775 1720 1776 1721 1777 1722 1778 1723 1779 1724 1780 1725 1781 1726 1782 1727 1783 1728 1784 1729 1785 1730 1786 1731 1787 1732 1788 Fig. 13. SimplifiedUNCORRECTED maps of shortening rates determined along the Kashi–Atushi fold system assuming PROOF (A) uniform initiation (average rates for structure- 1733 1789 perpendicular transects across the entire Kashi–Atushi fold system are presented under the double line) and (B) lateral propagation. We have not mapped the 1734 Mushi anticline, but its size and appearance suggest it has accommodated at least as much deformation as the Mingyaole anticline. These estimates are shown 1790 1735 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 1791 1736 laterally, the Atushi anticline formed first, followed by the Mushi and Mingyaole anticlines, then the Talanghe and Kashi anticlines. 1792

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1793 along the Kepingtage and Kuche thrust systems. In 1849 1794 particular, the Boston Tokar and Kalasu transects of 1850 1795 Burchfiel et al. (1999) contain tight, upright folds and 1851 1796 abrupt lithologic changes across the folds. Cross-sections on 1852 1797 these transects yield shortening estimates of ,10–20 km, 1853 1798 1–4 times larger than estimated for the Kashi–Atushi fold 1854 1799 system. To quantify the shortening rates, Burchfiel et al. 1855 1800 (1999) used a range of initiation between 2.5 and 1.0 Ma 1856 1801 (based on a climate inferred Quaternary age of a conglom- 1857 1802 erate), resulting in estimated shortening rates between 3 and 1858 1803 21 mm/yr. 1859 1804 Given uncertainty in the age of fold initiation at the 1860 1805 Boston Tokar and Kalasu sections, the mean shortening rate 1861 1806 (5.1 mm/yr) estimated for the Kashi–Atushi fold system is 1862 Fig. 14. CORONA satellite photo of confluence of Mutule (M) and high compared with the more proximal Kepingtage and 1807 Talanghe (T) anticlines. Light-colored rocks are Atushi Formation and 1863 1808 darkest gray rocks are Xiyu Formation. Most of the folded Xiyu Formation Kuche regions, and folding probably initiated more 1864 1809 in the syncline pre-dates the initiation of folding. The series of deflected recently. This suggests that the setting of the Kashi–Atushi 1865 1810 drainages across the nose of the Talanghe anticline suggest its eastward fold system may be unique. Indeed, it is located above the 1866 propagation. 1811 deepest depocenter in the Tarim Basin and shows little 1867 1812 seismicity in comparison with the other systems. 1868 1813 4.4. Regional kinematics We envision the Kashi–Atushi fold system as part of an 1869 1814 accretionary wedge, formed as the Tarim craton is forced 1870 1815 It is informative to compare timing and rate estimates north and underthrust below the Tian Shan. Approximately 1871 1816 presented here with those from other areas at the boundary 9 km of the Tarim craton has been underthrust below the 1872 1817 between the Tarim Basin and the Tian Shan, beginning near Tian Shan in the last 1–3 Myr, which is resolved by the 1873 1818 Wenguri, just north of the study area, and proceeding folding of the top 6 km of Cenozoic strata (Fig. 12). 1874 1819 clockwise around the Southern Tian Shan Sobel and Structural evidence of parallel folding and geodetic 1875 1820 Dumitru (1997) sampled locations within the Kepingtage evidence for northward directed shortening (Holt et al., 1876 1821 system for apatite fission track provenance and exhumation 2000), combined with geophysical models of Bouger 1877 1822 dates. They infer that thrusting north of Wenguri (Fig. 3) gravity anomalies (Burov et al., 1990) support this model. 1878 1823 was active before 14 Ma and has propagated southward The two interpretations for initiation of shortening indicate 1879 1824 through time. This is consistent with our results and that either the eastern end has been shortening more slowly 1880 1825 indicates that there was a ,12 Ma gap between initiation or for less time (Fig. 13). In either case, the patterns of 1881 1826 of activity on the Kepingtage system and the Kashi–Atushi shortening rates may be facilitated by the Pamir Salient, 1882 1827 fold system. which imposes a load on the westernmost Tarim Basin 1883 1828 Allen et al. (1999) report on shortening to the east of the (Burov et al., 1990). In response to the load, the Tarim 1884 1829 Kashi–Atushi fold system, along the Kepingtage thrust craton would be flexed more in the west, which potentially 1885 1830 zone (Fig. 1). At that location, the shortening is expressed as facilitates underthrusting of the western Tarim craton below 1886 1831 thin-skinned, imbricate thrusts that have accommodated the Tian Shan. In the eastern portion of the study area, the 1887 1832 ,35 km of shortening within a ,100 km long zone. From reduced effects of the load may inhibit underthrusting of the 1888 1833 the magnetostratigraphic results of Yin et al. (1998), they Tarim craton, and northward directed convergence would be 1889 1834 estimate that thrusting initiated around 20 Ma, resulting in taken up within the Tian Shan proper. The higher 1890 1835 an average shortening rate of ,1.8 mm/yr, if shortening concentration of folds in the west may also be controlled 1891 1836 spans the entire interval to the present. Farther east in the by the geometry of the Kashi Depression, which was 1892 1837 Kuche region (Fig. 1), magnetostratigraphic results by Yin thickest in the west and thinned to the east. 1893 1838 et al. (1998) are used to date a lacustrine to fluvial transition 1894 1839 inferred to represent regional thrust initiation, between 24 1895 1840 and 21 Ma. When applied to the 20–40 km of total 5. Conclusions 1896 1841 shortening, the rate (1–1.9 mm/yr) is similar to the results 1897 1842 in Kepingtage, but much lower than the rates at the Kashi– The Kashi–Atushi fold system occupies a unique 1898 1843 Atushi fold system. location in the Indian–Eurasia collision. The folds are 1899 1844 Burchfiel etUNCORRECTED al. (1999) present cross-sections from both located in an , 80PROOF8 corner between the north verging frontal 1900 1845 the northern and southern perimeters of the eastern Tian thrust of the Pamir (the Kazikeaete thrust system) and the 1901 1846 Shan (Fig. 1). These cross-sections show a structural style Southern Tian Shan fault. We model the Atushi–Talanghe, 1902 1847 that is similar to our interpretation for the Kashi–Atushi Mutule, Mingyaole, and Kashi anticlines as detachment 1903 1848 fold system, and contrasts with imbricate thrusting observed folds due to very steep limbs (60–908), an absence of 1904

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1905 emergent, fold bounding fault scarps or faults in proprietary References 1961 1906 seismic surveys. The results do not prescribe a kinematic 1962 1907 history for the deformation, but limit the shortening Abdrakhmatov, K.Y., Aldazhanov, S.A., Hager, B.H., Hamburger, M.W., 1963 1908 estimates to the most constrained data, namely the dips of Herring, T.A., Kalabaev, K.B., Makarov, V.I., Molnar, P., Panasyuk, 1964 S.V., Prilepin, M.T., Reilinger, R.E., Sadybakasov, I.S., Souter, B.J., 1909 the beds and the stratigraphic depth of the syngrowth 1965 Trapeznikov, Y.A., Tsurkov, V.Y., Zubovich, A.V., 1996. Relatively 1910 sediments. recent construction of the Tien Shan inferred from GPS measurements 1966 1911 Shortening estimates calculated from the cross-sections of present-day crustal deformation rates. Nature 384(6608), 450–453. 1967 1912 show that total shortening decreases towards the east and Abdrakhmatov, K.Y., Weldon, R., Thompson, S., Burbank, D., Rubin, C., 1968 1913 south; the Atushi anticline accommodated the most short- Miller, M., Molnar, P., 2002. Origin, direction, and rate of modern 1969 compression of the Central Tien Shan. Geology and Geophysics 1914 1970 ening (6.8 km), while the Talanghe anticline transects (Russian) 42(10), 1585–1609. 1915 1971 exhibit 4.3 and 2.5 km at the Boguzihe and Seven Mills Al Saffar, M., 1993. Geometry of fault-propagation folds; method and 1916 water gaps, respectively. The transect crossing the eastern application. Tectonophysics 223(3–4), 363–380. 1972 1917 tip of the Talanghe anticline and the western end of the Allen, M.B., Vincent, S.J., Wheeler, P.J., 1999. Late Cenozoic tectonics of 1973 1918 Mutule anticline yields a total shortening of 2.4 km. The the Kepingtage thrust zone: interactions of the Tien Shan and Tarim 1974 1919 Basin, northwest China. Tectonics 18, 639–654. 1975 southern set of folds generally accommodates less than the Arrowsmith, J.R., Strecker, M.R., 1999. Seismotectonic range-front 1920 1976 northern sets, with the Mingyaole anticline shortened by segmentation and mountain-belt growth in the Pamir–Alai region. 1921 1.5 km and the Kashi anticline by 4.1 km in the middle and Kyrgyzstan (India–Eurasia collision zone). Geological Society of 1977 1922 0.7 km at the east end. Shortening across the Talanghe America Bulletin 111(11), 1665–1683. 1978 1923 anticline reduces to the east, suggesting that either (1) the Avouac, J.-P., Tapponnier, P., Bai, M., You, H., Wang, G., 1993. Active 1979 1924 thrusting and folding along the northern Tien Shan and late Cenozoic 1980 folds are propagating eastward over time, or (2) shortening rotation of the Tarim relative to Dzungaria and Kazakhstan. Journal of 1925 1981 across the anticlines began everywhere at the same time, but Geophysical Research 98(4), 6755–6804. 1926 the strain rate reduces to the east. In total, at least 6 km of Bally, A.W., et al., 1986. Notes on sedimentary basins in China; report of 1982 1927 the Tarim Craton has underplated the southern Tian Shan in the American Sedimentary Basins Delegation to the People’s Republic 1983 1928 the last 1–2 Myr. of China. U.S. Geological Survey Open-File Report 86-327, pp. 93–95. 1984 1929 Bullen, M.E., Burbank, D.W., Abdrakhmatov, K.Y., Garver, J., 2001. 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If deformation at the Keketamu anticline is 1998 included, the shortening of rate (7.8 mm/yr) across the Society of America Special Paper 281, 76. 1943 Burtman, V.S., Skobelev, S.F., Molnar, P., 1996. Late Cenozoic slip on the 1999 1944 region plus the previously computed shortening rates in the Talas–Ferghana fault, the Tien Shan, central Asia. Geological Society 2000 1945 Tian Shan (,13 mm/yr; Abdrakhmatov et al., 2002; of America Bulletin 108(8), 1004–1021. 2001 Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic 1946 Thompson et al., 2002) agrees with the geodetic rates of 2002 ,20–24 mm/yr (Abdrakhmatov et al., 1996; Holt et al., polarity timescale for the Late Cretaceous and Cenozoic. Journal of 1947 Geophysical Research 100, 6093–6095. 2003 1948 2000) across the western Tian Shan. Carroll, A.R., Graham, S.A., Hendrix, M.S., Ying, D., Zhou, D., 1995. 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Three-dimensional architecture of growth strata 439pp. 2122 2067 2123 2068 UNCORRECTED PROOF 2124 2069 2125 2070 2126 2071 2127 2072 2128

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