ARTICLE IN PRESS 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 0191-8141/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.jsg.2004.02.016 SG 1558—7/7/2004—17:10—MNICHOLLS—110002— MODEL 5 – pp. 1–19 ARTICLE IN PRESS 2 K.M. Scharer et al. / Journal of Structural Geology xx (0000) xxx–xxx 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 SG 1558—7/7/2004—17:10—MNICHOLLS—110002— MODEL 5 – pp. 1–19 ARTICLE IN PRESS K.M. Scharer et al. / Journal of Structural Geology xx (0000) xxx–xxx 3 225 man, 1997; Burchfiel et al., 1999; Abdrakhmatov et al., 2001, 2002).