TECTONICS, VOL. 21, NO. 6, 1059, doi:10.1029/2001TC001300, 2002

Cenozoic deformation of the Tarim plate and the implications for mountain building in the Tibetan Plateau and the Tian Shan

Youqing Yang and Mian Liu Department of Geological Sciences, University of Missouri, Columbia, Missouri, USA Received 11 May 2001; revised 14 March 2002; accepted 8 June 2002; published 17 December 2002.

[1] The Tarim basin in NW China developed as a the geology and tectonics of the Himalayan-Tibetan orogen complex foreland basin in the Cenozoic in association [Dewey and Burke, 1973; Molnar and Tapponnier, 1975; with mountain building in the Tibetan Plateau to the Allegre et al., 1984; Armijo et al., 1986; Burchfiel et al., south and the Tian Shan orogen to the north. We 1992; Harrison et al., 1992; Avouac and Tapponnier, 1993; reconstructed the Cenozoic deformation history of the Nelson et al., 1996; England and Molnar, 1997; Larson et Tarim basement by backstripping the sedimentary al., 1999; Yin and Harrison, 2000]. However, some funda- mental questions, such as how stress has propagated across rocks. Two-dimensional and three-dimensional finite the collisional zone and how strain was partitioned between element models are then used to simulate the flexural crustal thickening and lateral extruding of the lithosphere, deformation of the Tarim basement in response to the remain controversial. One end-member model, approximat- sedimentary loads and additional tectonic loads ing the Eurasia continent as a viscous thin sheet indented by associated with overthrusting of the surrounding the rigid Indian plate [e.g., England and McKenzie, 1982; mountain belts. The results suggest that uplift of England and Houseman, 1986, 1988], predicts progressively both the western Kunlun Shan and the Tian Shan northward propagation of crustal thickening and plateau orogens started during or before Oligocene. Since the uplift, with limited lateral lithosphere extrusion. Another Miocene, mountain building accelerated in the end-member model, based on indentation of a plastic Asian western Kunlun Shan but showed segmented continent, suggests that a major part of the convergence was development in the Tian Shan. The results also taken up by eastward extrusion of the lithosphere along major strike-slip faults activated by the collision [Tappon- indicate reduced tectonic load along the Altyn Tagh nier et al., 1982]. Constraints on the timing and spatial fault since late Miocene, consistent with geological distribution of crustal deformation in the Tibetan plateau and and geophysical evidence of the fault cutting the surrounding regions, which are essential for illumination of entire lithosphere and migrating southward during its the dynamics of collisional crustal deformation, remain cause of the Cenozoic evolution. INDEX TERMS: 8105 sparse and often controversial [Coleman and Hodges, Tectonophysics: Continental margins and sedimentary basins; 1995; Harrison et al., 1995; Chung et al., 1998; Yin and 8102 Tectonophysics: Continental contractional orogenic belts; Harrison, 2000]. 8120 Tectonophysics: Dynamics of lithosphere and mantle— [3] The Cenozoic deformation history of the Tarim basin general; 9320 Information Related to Geographic Region: Asia; in northwestern China (Figure 1) may provide useful 9604 Information Related to Geologic Time: Cenozoic; insights into the problems of stress propagation and strain KEYWORDS: Tibet, Tarim, Tian Shan, continental deformation, partitioning following the Indo-Asian collision. Located Indo-Asian collision. Citation: Yang, Y., and M. Liu, Cenozoic between the Tibetan plateau and the Tian Shan orogen, deformation of the Tarim plate and the implications for mountain the relatively rigid Tarim block behaved as a secondary building in the Tibetan Plateau and the Tian Shan, Tectonics, ‘‘indenter,’’ transmitting collisional stresses to the Tian Shan 21(6), 1059, doi:10.1029/2001TC001300, 2002. [Molnar and Tapponnier, 1975; Neil and Houseman, 1997]. The Cenozoic deformation of the Tarim basin was closely coupled with mountain building in the northern Tibetan 1. Introduction plateau and the Tian Shan, which caused major subsidence of the Tarim basin near its northern and southwestern [2] The continued plate convergence between India and margins. The nearly 10-km-thick Cenozoic sedimentary Asia since 50–70 million years ago resulted in the uplift of rocks derived from the surrounding mountain belts [Li et the Himalayan-Tibetan plateau, reactivated the Tian Shan al., 1996; Jia, 1997] provide useful constraints on the orogen, and affected regions perhaps as far north as the deformation history of the Tarim basin and mountain Baikal [Molnar and Tapponnier, 1975; Yin and Harrison, building in the surrounding orogens. 2000]. Following the pioneering work of Argand [1924] and [4] In this study we investigate the Cenozoic deforma- Gansser [1964], significant progresses have been made on tion history of the Tarim basin and its implications for mountain building in the northern Tibetan plateau and the Tian Shan orogen. The basement deformation during three Copyright 2002 by the American Geophysical Union. periods (Paleogene, early-middle Miocene, late Miocene- 0278-7407/02/2001TC001300$12.00 Pliocene) is reconstructed by backstripping the sedimen-

9 - 1 9 - 2 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE tary strata. We then simulate the basement deformation Zheng, 1994]. The time of initiation of the Altyn Tagh fault using two-dimensional (2-D) and three-dimensional (3-D) is suggested to be Oligocene [Wang, 1990], middle Miocene finite element methods. The results show uplift of the west [Peltzer and Tapponnier, 1988; Yin and Nie, 1996] or Kunlun Shan and the Tian Shan orogens starting in the Pliocene [Bally et al., 1986; Burchfiel et al., 1991] based Paleogene and accelerating since Miocene, with significant on various geological arguments. internal variations, and reduced tectonic load along the Altyn Tagh fault since late Miocene. 2.3. Tian Shan and Southern Tian Shan Thrust Belt [9] Bordering the Tarim basin to the north is the Tian 2. Geological Background Shan mountain belt, which extends broadly east-west for over 2500 km across central Asia, with peaks exceeding [5] The rhomb-shaped Tarim basin is surrounded by the 7000 m and basins lower than 100 m below sea level Tian Shan (‘‘Shan’’ means mountain ranges in Chinese) to (Figure 1). A middle Paleozoic Tian Shan orogen may have the north, the western Kunlun Shan and the Altyn Tagh developed from one or more arc-continental collisions Shan of the northern Tibetan plateau to the south and [Coleman et al., 1992; Allen et al., 1993; Carroll et al., southeast, and the Pamir to the west (Figure 1a). These 1995], followed by reactivation of the older faults caused by mountain ranges originated in the late Paleozoic [Allen et successive collision of island-arc systems onto the southern al., 1993] or even earlier [Nakajima et al., 1990] and margin of Asia in the late Paleozoic and the Mesozoic rejuvenated following the India-Eurasia collision 50–70 [Hendrix et al., 1992; Sobel, 1995]. million years ago [Molnar and Tapponnier, 1978; Windley [10] During the Cenozoic the Tian Shan was rejuvenated et al., 1990; Yin and Harrison, 2000]. Cenozoic deforma- by the Indo-Asian collision [Tapponnier and Molnar, 1979; tion of the Tarim basin was coupled with mountain building Burchfiel et al., 1991; Lu et al., 1994]. The present Tian in the surrounding orogenic belts through the bounding fault Shan is flanked by east-west trending, active thrust systems systems: the western Kunlun thrust belt and the Altyn Tagh on its north and south sides [Tapponnier and Molnar, 1979; fault to the south, and the southern Tian Shan thrust belts to Burchfiel et al., 1991; Avouac and Tapponnier, 1993; the north (Figure 1b). Molnar et al., 1994; Abdrakhmatov et al., 1996]. Between the Tian Shan and the Tarim basin is the southern Tian Shan 2.1. Western Kunlun Shan and Western Kunlun thrust belt, which may be divided into four segments based Thrust Belt on their deformation styles and structural trend [Yin et al., [6] The western Kunlun Shan is composed mostly of 1998]. From west to east (Figure 1b), they are (1) the Kashi- Precambrian schist and gneiss overlain by Paleozoic and Aksu thrust system, (2) the Baicheng-Kuche thrust system, Mesozoic carbonate and clastic sequences [Yin and Harri- (3) the Korla transfer system, and (4) the Lop Nor thrust son, 2000]. The present western Kunlun Shan is defined as a system [Yin et al., 1998; Allen et al., 1999]. The western- Cenozoic fold and thrust belt in the north and the Karakaxi most Kashi-Aksu system is characterized by the occurrence fault in the south (Figure 1b). Southward subduction of the of evenly spaced (12–15 km) imbricate thrusts. The Bai- Tarim plate beneath the northern Tibetan Plateau is sug- cheng-Kuche and Korla systems are expressed by a major gested as the cause of the western Kunlun thrust belt [Lyon- north dipping thrust (the Kuche thrust) that changes its Caen and Molnar, 1984]. Geologic mapping in the eastern strike eastward to become a NW striking oblique thrust part of the western Kunlun thrust belt [Cowgill et al., 2000] ramp (the Korla transfer zone). The Lop Nor system suggest that the magnitude of north-south shortening across consists of widely spaced thrusts, all involved with base- the eastern part of the thrust belt is between 50 and 100 km. ment rocks. The age of initial thrusting is estimated to be This magnitude appears to increase westward based on 20–25 Ma [Hendrix et al., 1994; Yin et al., 1998]. paleomagnetic studies [Yin and Harrison, 2000]. 2.4. Tarim Basin 2.2. Altyn Tagh Fault [11] The Tarim basin has a complex and protracted [7] The Altyn Tagh fault connects the western Kunlun history of deformation since the late Precambrian [Hendrix Shan in the west and the Nan Shan in the east. It extends et al., 1992; Wang et al., 1992; Carroll et al., 1995; Li et al., over 1200 km and bounds the northern boundary of the 1996]. The basement consists of Archean and Proterozoic Tibetan Plateau (Figure 1b). metamorphic rocks [Tian et al., 1989; Chai et al., 1992]. [8] The Altyn Tagh fault may have played a key role in The sedimentary cover, including Proterozoic and Phaner- eastward extrusion of Tibet during the Indo-Asian collision ozoic sequences, ranges from 17 km within major depres- [Tapponnier and Molnar, 1977]; however, many aspects of sion centers to 5 km in the central uplift [Li et al., 1996]. the fault remain controversial. The displacement history of The Paleozoic sequence is dominated by platform carbo- the Altyn Tagh fault has been derived mainly from off- nates and fine-grained clastics [Carroll et al., 1995]. Mes- setting of Quaternary features. The estimated Quaternary ozoic strata of north central Tarim are interpreted to record left-slip rates range from 4 to 30 mm/yr in various segments deposition as the result of multiple episodes of contraction of the fault [Molnar et al., 1987; Peltzer et al., 1989; Meryer and uplift of the margins of the basin [Hendrix et al., 1992]. et al., 1996]. Estimates of the total magnitude of left slip on These periods of conglomerate deposition may be related to the Altyn Tagh fault varies from 1200 km to 200 km the accretion of the Qiangtang block in the Permian-Juras- [Peltzer and Tapponnier, 1988; Burchfieletal., 1991; sic, the Lhasa block in the latest Jurassic, and the Kohistan- YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 3

Figure 1. (a) Topographic relief map of the Tarim basin, NW China, and surrounding regions. The dots mark the approximate locations of some of the industrial wells drilled before 1990 [Zhao et al., 1997]. (b) Simplified geological map of the Tarim basin and surrounding regions [after Yin et al., 1998].

Dras arc complex in the Late Cretaceous [Hendrix et al., the Indo-Asian collision. Basinward thrusting of the Tian 1992]. Since then it has been largely associated with fore- Shan and western Kunlun Shan caused further subsidence land depressions caused by basinward thrusting and over- of the foreland depressions where Cenozoic sedimentary loading of the Tian Shan in the north and the western rocks are up to 10 km thick (Figure 2). During the Eocene, Kunlun Shan in the south [Jia, 1997; Metivier and Gau- sedimentation was mainly centered in the Hotan (or the dermer, 1997]. southwestern) depression adjacent to the western Kunlun [12] During the Cenozoic the Tarim basin developed into thrust belt. By the end of Pliocene, both the Hotan and the a complex system of foreland basins largely in response to Kuche depressions adjacent to the Tian Shan orogen had 9 - 4 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE received >5000 m sediments (Figure 2). Cenozoic sedi- A before layer B was deposited (Figure 3). In the calcu- mentsinthebasinweremainlyterrestrialfacies.The lations we used the values of f0 = 0.429 and c = 0.675 sedimentary strata in the Tarim basin have been well kmÀ1 determined by Liu et al. [1996] based on drill hole documented from dense oil wells drilled over the past data from the Tarim basin. decades [Jia, 1997]. [15] Figure 4 shows the reconstructed history of basement deformation for the Paleogene and the early-middle Mio- cene. During Paleogene the Hotan depression was sepa- 3. Basement Deformation History rated from the broad southwest depression centered near [13] To understand the causes of Cenozoic deformation of Yecheng, where the basement depression was up to 2200 m the Tarim basin we need to reconstruct the basement (Figure 4a). Minor depression centers also developed along deformation history. The basic data are the isopach of the the southeastern margin of the Tarim basin, near Minfeng Cenozoic sedimentary rocks. Petroleum and gas exploration and Qiemo. By early Miocene both the southwestern and in the Tarim basin for the past three decades has produced the Kuche depression centers deepened and propagated detailed information about the sedimentary cover of the basinward (Figure 4b). Note that the result in Figure 4b is basin. Based on industrial drilling, shallow seismic reflec- the incremental subsidence during the period of early- tion, and geological studies, the Chinese Petroleum and Gas middle Miocene. We consider the Paleogene sedimentary Corporation produced the isopach maps [Jia, 1997] (Figure rocks as part of the basement by this time. The Hotan 2) that are used in this study. Because of industrial secrecy, depression became part of the southwestern depression no details of the location of the oil wells or seismic lines where the maximum subsidence reached 3500 m near used to construct the isopach maps were given by Jia Yecheng. The incremental basement subsidence since late [1997]. We obtained the locations of 55 industrial oil wells Miocene can be estimated directly from the isopach of this drilled before 1990 from Zhao et al. [1997] and show them period (see Figure 2c). It is clear that basement subsidence in Figure 1a. This information is incomplete, and the accelerated since Miocene. Near Yecheng the incremental isopach maps (Figure 2) were constructed in 1995, and basement depression was up to 5500 m during late Mio- presumably were based on data from more drill wells than cene-Pliocene. Similar basement depression occurred in the those shown in Figure 1a. Both Jia [1997] and Figure 1a Kuche depression, which branched southwestward to form indicate that there are no drill well data in the Kepingtage another depression center near Aksu (Awati) where the area. The isopachs in the Kepingtage area are generated by subsidence reached 5000 m. Not much subsidence occurred interpolation and therefore are not used in our calculations along the southeastern margin of the Tarim basin during late of basement deformation or sedimentary loads. The general Miocene-Pliocene. isopach patterns in Figure 2, including the separated depo- centers at Yecheng and Hotan in the Paleogene, are con- 4. Causes of Basement Deformation: sistent with other published data [Metivier and Gaudermer, 1997; Allen et al., 1999; Metivier et al., 1999]. Whereas the Sedimentary and Tectonic Loads lack of detailed information of the source data makes it [16] It is clear from Figures 2 and 4 that basement difficult to assess the errors in these isopachs, the isopach deformation of the Tarim basin through the Cenozoic was maps in Figure 2 are probably sufficient for a first-order dominated by the development of two major foreland basins modeling. adjacent to the western Kunlun Shan and the Tian Shan [14] The depth of the basement (relative to present basin mountain ranges. The theory of foreland basin formation surface) during the youngest period (late Miocene-Pliocene) resulting from flexural deflection of the lithosphere has been may be estimated from the thickness of sedimentary rocks well developed [Turcotte, 1979; Beaumont, 1981]. Flexure formed during this time (Figure 2c). Basement deflection of a thin elastic plate can be described by [Turcotte, 1979] during the earlier periods can be reconstructed using back- 4 2 stripping [Lerche, 1990; Makhouse et al., 1997]. As illus- Dr w þ pr w ¼ q; ð3Þ trated in Figure 3, the original depth of layer A can be where w is deflection, r2 and r4 are the Laplace and the restored by stripping off the overlying layer B and correct- biharmonic operators, respectively, p is horizontal load and ing for compaction. Assuming the porosity f decreases with q is vertical load per unit area on the lithosphere. For the depth following the empirical relation [Falvey and Middle- flexure of continental lithosphere the vertical load may be ton, 1981]: written as q = qt + rsgw0 À rmgw, where qt is the tectonic f ¼ f0 expðÞÀcz ; ð1Þ load, and (rsgw0 À rmgw) is the net force of sediment loads and the buoyancy force arising from the displaced mantle. where z is depth and c is a constant, mass conservation of Here rs is the density of the overlying sedimentary material, the solid material in layer A can be written as g is gravitational acceleration, w0 is the thickness of the f f sediment layer, and rm is the density of the underlying z À z þ 0 ½Š¼exp ÀðÞÀcz expðÞÀcz d þ 0 ½ŠexpðÞÀÀcd 1 ; mantle. Thus equation (3) could be written as 2 1 c 2 1 c 4 2 ð2Þ Dr w þ pr w þ rmgw ¼ qt þ rsgw0: ð4Þ where z1 and z2 are the depth to the bottom of layer B and For a perfectly compensative basin of elastic flexure, i.e., all layer A, respectively, and d is the original thickness of layer deflection is filled with sediments, w0 and w would be YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 5

Figure 2. Isopach of sedimentary rocks for (a) Paleogene; (b) early-middle Miocene; (c) late Miocene to Pliocene (data are from Jia [1997]). identical and equation (4) can be written in the more w0 is derived directly from the sedimentary records, familiar form [Turcotte, 1979]: whereas w is the theoretical basement deflection. The parameter D is the flexural rigidity: 4 2 Dr w þ pr w þ ðÞrm À rs gw ¼ qt: Eh3 However, equation (4) is more useful for illustrating our D ¼ ; ð5Þ 12 1 n2 approach: we used rsgw0 as the sedimentary load because ðÞÀ 9 - 6 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE

where E is the Young’s modulus, n is the Poisson’s ratio, and h is the thickness of the elastic plate. For lithospheric flexure an effective elastic thickness, Te, may be used to replace h [Watts, 1976; McNutt, 1984]. Equation (3) can be modified for other rheologies [Turcotte, 1979; McMullen et al., 1981; Burov and Diament, 1992]. Various rheologies have been applied to foreland basins [Beaumont, 1981; Jordan, 1981; Royden and Hodges, 1984; Patton and O’Connor, 1988] and other tectonic settings [Walcott, 1970; Melosh, 1978; Turcotte, 1979; Karner and Watts, 1983; Watts and ten Brink, 1989], with varying degrees of success. In many cases an elastic rheology has been shown Figure 3. Sketch showing reconstruction of basement to be a good first-order approximation [Turcotte, 1979; subsidence using backstripping. The original depth of layer Burov and Diament, 1992], and the effects of various factors A is restored through peeling off layer B and correcting for on the lithospheric rheology, such as thermal structures and compaction. mechanical weakening, may be lumped into Te [McNutt, 1984; Burov and Diament, 1995]. Note that equations (3) and (4) are for thin elastic plate only, and we used these

Figure 4. Basement deformation in (a) Paleogene and (b) early-middle Miocene derived from backstripping. Basement deformation for the youngest period (late Miocene-Pliocene) can be estimated directly from the isopach in Figure 2c. The two straight lines in (a) show the location of the 2-D models in Figures 6 and 7. YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 7

Figure 5. Sketch showing Cenozoic deformation of the Tarim basin resulting from the distributed sedimentary loads and tectonic loads near the margins of the basin associated with overthrusting of the surrounding mountain belts. Given the sedimentary loads for each of the three periods (Figure 2), the optimal tectonic loads may be estimated by fitting the observed basement deformation. equations here to illustrate the roles of sedimentary and parison with the analytic solutions of numerous simple 2-D tectonic loads in basement deformation. Although the thin problems. elastic plate may be a viable approximation of the Tarim plate, we have developed a fully three-dimensional model 4.1. Two-Dimensional Models basedontheelastictheory[Timoshenko and Goodier, [19] Most previous studies of foreland basin formation 1970]. The models solve for the full sets of 15 linear were based on 2-D models [Beaumont, 1981; Burov and equations (six stress and six strain components plus three Diament, 1992], because they are easy to compute and displacement components) without requiring the thin-plate useful for illustrating the effects of major model parameters. approximation. We also used a viscoelastic model and will We conducted a series of 2-D finite element modeling to compare the results below. explore the model parameters and the relative roles of [17] The major loads causing the flexural foreland basin sedimentary and tectonic loads in the basement deformation development in the Tarim basin include sedimentary loads of the Tarim basin. Figure 6 compares the predicted and distributed across the basin and tectonic loads near the observed Paleogene basement subsidence along a profile margins of the basin associated with overthrusting of the from Yecheng to Kuche. Using the sedimentary loads along surrounding mountain belts (Figure 5). Horizontal load is the profile (see Figures 4a and 2a), we could determine the usually neglected in elastic flexure models because of the required tectonic loads by fitting the observed basement unreasonably high values of the load (6.4 GPa for a 50 km deformation. The problem here is Te, the effective elastic thick elastic plate) required to cause buckling [Turcotte and thickness of the Tarim plate, that is also unknown. There are Schubert, 1982]. However, Karner [1986] pointed out that horizontal load could have significant effects for plates with preexisting deformation, such as sedimentary basins. Compressional horizontal load tends to amplify the plate deformation; the amount of amplification is a function of the elastic thickness and the wavelength. We do not include horizontal load in the model because its value is difficult to constrain and doing so would increase the number of free parameters, thus introduce large uncertainties to the tec- tonic loads, the major target of this study. Applying Karner’s [1986] results to the Tarim basin, we estimated that <5% of error may be introduced by neglecting the horizontal load. Although bending moments may also contribute to flexure, they are difficult to constrain from geological data and therefore are not included in our calculations. [18] Because the spatial distribution of the sedimentary rock (sedimentary loads) for each of the three periods are known (see Figure 2), the optimal tectonic loads for each period may be derived from matching the observed base- ment deformation for a given rheological structure. For this study we developed both two-dimensional (2-D) and three- dimensional (3-D) finite element models using FEPG (available at http://www.fegensoft.com/english/index.htm), Figure 6. Comparison of the observed and predicted an automatic finite element codes generating system [Yang, Paleogene basement flexure along the Yecheng-Kuche 1998]. Both the 2-D and 3-D models solve for the full set of profile (location in Figure 4a) by the 2-D model. The top linear elastic equations without requiring the thin-plate plot shows the location and magnitude of the optimal approximation. We verified the numerical results by com- tectonic loads. The optimal Te is 60 km for the Tarim plate. 9 - 8 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE

[22] The good fits between the predicted and observed basement deformation in Figures 6 and 7 also illustrate the limitations of 2-D models for the Tarim basin. In 2-D models we are free to choose the tectonic loads and Te, consequently we can always find a combination of these parameters that produce close fit to the observed basement deformation. However, different results may be obtained for different profiles. For example, the optimal tectonic load at Kuche is 3 MPa over an 80-km width for the Yecheng-Kuche profile (Figure 6), but 45 MPa over a 20-km zone for the Qiemo-Kuche profile (Figure 7). Furthermore, the optimal Te is 60 km for the Yecheng-Kuche profile but 15 30 km for the Qiemo-Kuche profile, and there is no geological evidence supporting such a sharp lateral change of the lithospheric rheology over the Tarim basin. Part of the problem may be the location of the 2-D profiles. The Yecheng-Kuche profile is perpendicular to the Southwestern depression but not to the Kuche depression, thus is not an ideal representation for deformation in the Kuche region. The particular geometry of the Tarim basin, with limited Figure 7. Results of a 2-D flexural model for the Qiemo- spatial extension in all directions, and the complex internal Kuche profile (location in Figure 4a) showing the effects of deformation patterns, are difficult for 2-D representations. In the following we shall focus on three-dimensional models. Te. The top plot shows the location and magnitude of the optimal tectonic loads. 4.2. Three-Dimensional Models

[23] Figure 8 shows the numerical mesh of the 3-D model. certain trade-offs between the Te value and the tectonic loads (see below), and the method of trial and error was The rheology of the Tarim plate is assumed to be either used to find the best combination of these two variables. elastic or viscoelastic. We focus on elastic flexure here and The density of the sedimentary material and the mantle are discuss the results of viscoelastic models later. The elastic 3 3 model of the Tarim plate has two layers (Figure 8). The assumed to be 2600 kg/m and 3300 kg/m , respectively, 10 and the other elastic constants are represented implicitly by Young’s modulus is taken to be 3 10 Pa for the top layer and 5 1010 Pa for the lower layer. The somewhat lower the Te value. The Winkler spring mattress is used at the bottom to simulate restoring body forces induced by dis- placement of density boundaries [Williams and Richardson, 1991]. For this profile the best fit is found when Te =60km (Figure 6), and nearly 70% of the observed basement subsidence can be explained by the sedimentary loads. The largest misfits are near the margins of the basin where additional loads may result from overthrusting of the adjacent fold and thrust belts, i.e., tectonic load. Applying the optimal tectonic loads significantly improved the fit (Figure 6). [20] We should note that the boundary between the Tarim basin and the surrounding mountain ranges are not clear-cut. Thus, it is difficult to completely separate the sedimentary load from tectonic load. Some of the sedimentary rocks near the margins of the basin, while being counted as sedimen- tary load in our model, are part of the fold and thrust belts associated with mountain building and hence perhaps should be regarded as tectonic load. [21] For elastic flexures one controlling parameter is the Figure 8. Numerical mesh of the 3-D finite element effective elastic thickness. The physical effects of various Te model, which includes two layers with different elastic can be seen from Figure 7, which shows the predicted and properties. Tectonic loads within each of the labeled observed Paleogene basement deformation along a profile boundary domains are assumed to be uniform for each from Qiemo to Kuche (see location in Figure 4a). A thicker period. KL1–KL4: western Kunlun Shan; AT1–AT3: Altyn plate (larger Te) allows the effects of tectonic load to Tagh fault; and TS1–TS7, Tian Shan. Five weak zones propagate further into the basin, resulting in smoother and shown in white were included to simulate the effects of broader flexure. A thinner plate, on the other hand, tends to faults for the late Miocene-Pliocene period. See text for localize subsidence to where the loads are applied. detailed discussion. YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 9 value for the top layer is to reflect a relatively weaker upper Table 1. Optimal Tectonic Loads crust because of fractures and the sedimentary cover. We used multiple layers in the model to explore the effects of Early-Middle Late Miocene- Paleogene Miocene Pliocene rheologic variations with depth, although for elastic models Boundary the results are dependent mainly on the total effective elastic Domain MPa kma MPa kma Mpa kma thickness. The Poisson’s ratio is taken as 0.25 for all elements. These elastic constants are within the typical KL1 26 0.96 46 1.7 À57 À2.1 values for crustal rocks [Turcotte and Schubert, 1982]. The KL2 43 1.6 101 3.7 286 10.6 KL3 21 0.78 À27 À1.0 62 2.3 model includes 3102 nodal points and 1932 isoparameteric KL4 31 1.1 59 2.2 46 1.7 solid elements, providing a horizontal element size of 25–30 AT1 4 0.15 5 0.18 86 3.2 km. We divide the margin zone of the Tarim basin into 14 AT2 10 0.37 2 0.07 À40 À1.5 domains. Domains KL1 – KL4 are along the western AT3 18 0.67 36 1.3 66 2.4 TS1 23 0.85 28 1.0 197 7.3 Kunlun Shan thrust belt, AT1–AT3 are along the Altyn TS2 À15 À0.55 À101 À3.7 À152 À5.6 Tagh Shan and fault system, and TS1–TS7 line from west to TS3 8 0.3 37 1.3 61 2.3 east along the Tian Shan thrust belt. Within each domain the TS4 32 1.2 40 1.5 À54 À2.0 tectonic load is assumed to be uniform for a given period, TS5 5 0.18 18 0.67 94 3.5 TS6 À93 À3.4 À38 À1.4 À180 À6.7 and the optimal values are obtained by regression stated TS7 8 0.3 5 0.18 49 1.8 below. The domain division roughly reflects the general geological settings, but the number of boundary domains Early-Middle Late Miocene- and their boundaries are somewhat arbitrary. Paleogene Miocene Pliocene [24] We constrain the horizontal displacement along the R 0.80 0.78 0.76 eastern side, which is allowed to move only vertically. This F test 1.86 1.72 1.39 boundary condition prevents artificial rigid-body translation Confidence 97% 94% 85% b and rotation. Other sides of the model plate are free, and the Te 60 km 84 km 108 km 20 21 21 Winkler spring mattress is used at the bottom to simulate D 7.6 10 Nm 2.1 10 Nm 4.4 10 Nm restoring body forces induced by displacement of density aTectonic loads converted to the weight of a rock column with a density boundaries [Williams and Richardson, 1991]. of 2700 kg/m3. 25 b [ ] Similar to the approach used in 2-D models, we Te is 24 km near Aksu and Minfeng. determine the optimal tectonic loads by fitting the predicted flexure to the observed basement deformation. For a given rheology structure, the tectonic load on each of the boun- the linear correlation between W*(x, y) and the tectonic dary domains during each geological period may be deter- loads on the 14 boundary domains. A high confidence level mined. A major problem we face here, as in 2-D models, is from the F-test will indicate the assumption of linearity is that we have to determine the trade-off between the tectonic acceptable. loads and the effective elastic thickness. In 2-D models we [27] We started with a uniform Te for the entire Tarim used the method of trial and error to look for the combina- plate. For a given Te value, the optimal tectonic load on each of the boundary domain can be derived by linear regression, tion of tectonic loads and Te that produces the best fit to the observed basement deformation. This caused the problems and the F-test is conducted to score the linear correlation between observed basement deflection and those caused by of having large variations of Te along different profiles, as discussed above. The situation is greatly improved here by the tectonic and sedimentary loads. The process is repeated having an additional constraint, the requirement of linearity. systematically with different Te values. The best Te value is Because elastic flexure is a linear process, the basement the one with which the highest confidence level of the F-test deformation should be the sum of deformation caused by is reached. the sedimentary loads over the whole basin and the tectonic [28] The results for the three Cenozoic periods are shown load on each of the boundary domains. Thus we can choose in Table 1. For the Paleogene the best fit is reached when Te = 60 km. The corresponding confidence level of the F-test (a) the Te value that gives the highest linearity, determine by the F-test (see below), and obtaining the optimal tectonic load is above 97%, and the correlation coefficient (R) is 0.80, so on each of the margin domains by linear regression. the linearity assumption is acceptable. In other words, the Paleocene basement deformation can be satisfactorily [26] Assuming Wi(x, y) is the response to tectonic load of explained by the summation of flexural response to the unit magnitude on the ith boundary domain, W0(x, y)isthe response to sedimentary load, and W*(x, y) is the observed sedimentary loads and tectonic loads on the margins of a basement deflection (see Figure 4), the linear assumption uniform elastic plate. The optimal tectonic load on each of can be expressed as the boundary domain for this period is shown in Table 1 and discussed later. Similar quality of the linear regression X14 is obtained for the early-middle Miocene period. However, ; ; ; ; W*ðÞ¼x y W0ðÞþx y aiWiðÞx y ð6Þ the assumption of linearity is unacceptable for the Late i¼1 Miocene-Pliocene period: the best confidence level from where ai is the magnitude of tectonic load on the ith the F-test is only 35% with the optimal Te = 156 km, and boundary domain. We use the method of multivariate linear unreasonably high tectonic loads (up to 1110 MPa or a regression to determine the values of ai, and the F-test to test 41 km high mountain on some boundary domains) are 9 - 10 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE

Figure 9. Misfit between the predicted flexural deformation and the observed basement deflection for the Paleogene. (a) The predicted flexure is caused by sedimentary loads alone. (b) The predicted flexure is caused by sedimentary loads and the optimal tectonic loads. predicted. These results indicate that the basement defor- reduced. Inclusion of these factors for earlier periods, how- mation during this period cannot be satisfactorily explained ever, resulted in poorer fit, suggesting that these features by flexure of a uniform elastic plate. probably developed after late Miocene. [29] Numerous factors not included in these calculations, [30] Figure 9 shows the misfit between the predicted such as internal faulting and the basinward growth of the flexural deformation and the observed basement deflection thrusting belts, may contribute to the poor fit in this case. We for the Paleogene. Note that 70% of the basement defor- modified the model to (1) include five major faults in the mation can be explained by sedimentary loading, and the areas where the fit is particularly poor (see Figure 8), (2) largest misfits are near the margins of the basin (Figure 9a). widen the area of tectonic load in the Kepingtage region The fits are systematically improved by applying the opti- (boundary domains TS1–TS3) where geological evidence mal tectonic loads on the margin domains (Figure 9b). The indicating a significant basinward growth of the thrust belt relative roles of sedimentary and tectonic loads are further [Lu et al., 1998], and (3) lower the Te value (to 24 km) near illustrated in Figure 10. Figures 10a–10c shows the the Aksu (Awati) and Minfeng depression where the Tarim response to sedimentary loads alone for the three Cenozoic plate may have been weakened by intensive late Cenozoic periods. Although sedimentary loads can explain much of faulting [Jia, 1997]. With all these factors included in the the observed basement deformation, there are systematic model, the confidence level of linearity for late Miocene- misfits for deep depressions that are concentrated near the Pliocene improved to 85%, and the misfit is significantly margins of the basin (Figures 2 and 4). The predicted YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 11

loads broadened significantly, consistent with geological data [Lu et al., 1998]. Tectonic loads along the western Kunlun Shan thrust belt continued to accelerate, while moderate unloading (negative tectonic loads) is predicted for the central section of the Altyn Tagh fault system. These results may also be compared with the general topography of these mountain ranges. For the period of late Miocene- Pliocene, the average tectonic load over western Tian Shan thrust belt (boundary domains TS1–TS4) is larger than that over the eastern segments (TS5–TS7), consistent to higher elevations to the western part of south Tian Shan. The predicted large tectonic load over the western Kunlun Shan and relative small tectonic loads over the Altyn Tagh Shan are also comparable to the relative topography of these mountain ranges.However,cautionisneededwhenFigure11is compared with topography because (1) the results in Figure 11 are the incremental tectonic loads for each period, (2) the division of the boundary domains is somewhat arbitrary, and (3) other factors may influence the tectonic loads. Some of these factors are discussed below.

5. Discussion

[32] We have constructed the temporal and spatial evo- lution of the optimal tectonic loads on the margins of the Tarim basin through the Cenozoic that may be associated with the collision-driven mountain building of the northern Tibetan Plateau and the Tian Shan. Here we briefly discuss the effects of model parameters and the implications for the collisional tectonics in this region.

Figure 10. Comparison of the predicted and the observed 5.1. Effects of Model Parameters basement subsidence. Wobs is the observed basement [33] One of the most important parameters for elastic subsidence, W1 is the predicted subsidence by sedimentary flexure is the effective elastic thickness. Many factors that loads alone, W2 is the predicted subsidence by sedimentary and tectonic loads. The straight lines represent perfect affect lithospheric rheology (e.g., geothermal condition, fitting. Notice the systematic misfit for deep depressions in lithospheric composition, and mechanical weakening due (a)–(c). (d)–(f) show the significantly reduced misfits when to densely developed basement faulting), can be implicitly the optimal tectonic loads are applied. represented by Te [Burov and Diament, 1995]. In our models the optimal Te values were determined by minimiz- ing the misfit between the observed and predicted basement subsidence by sedimentary loads is always lower than the deformation in 2-D models and by producing the highest observed values for deep depressions, indicating the need linearity in 3-D models. Our 2-D models predicted the for additional tectonic loads. The misfit is greatly reduced optimal Te varying from 60 to 15 km for different profiles, when the optimal tectonic loads (Table 1) are applied similar to the 2-D flexural results of Burov and Diament (Figures 10d–10f ). [1992]. However, the Te values obtained from the 3-D [31] The temporal and spatial evolution of the optimal models are generally greater than 2-D values: they range tectonic loads is plotted in Figure 11. During the Paleogene from60kminthePaleogeneto108kmintheLate the tectonic loads on the Tarim basin were relatively minor. Miocene-Pliocene for most parts of the Tarim basin. The The maximum load is 43 MPa, roughly the weight of a 1.6- large discrepancies of the Te values among different profiles km mountain range, in western Kunlun Shan and 32 MPa in indicate the limitations of 2-D models for the Tarim basin. the central segment of the Tian Shan thrust belt. On the The high Te values from the 3-D models are similar to that boundary domains TS2 and TS6 the predicted tectonic loads of the India plate (90 km) [Jin et al., 1996] and consistent are negative (upward force or unloading). Tectonic loads with the notion of the Tarim plate behaving as a secondary accelerated since the early Miocene, especially in the west- indenter during the Indo-Asian collision [Molnar and Tap- ern segment of western Kunlun Shan thrust belt and the ponnier, 1975]. A stiff Tarim plate is also consistent with central segment of the Tian Shan thrust belt (note the the large contrast of deformation styles between the Tarim different vertical scales in Figure 11). During the late basin and the surrounding mountain belts, and is supported Miocene, tectonic loads along the Tian Shan thrust belt split by seismic data that indicate abnormally high velocity from the central Tian Shan. In the Kepingtage region tectonic structures extending >220 km depth beneath the Tarim plate 9 - 12 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE

Figure 11. The predicted incremental tectonic loads (see Table 1) for the periods of (a) Paleogene, (b) early-middle Miocene, and (c) late Miocene-Pliocene. Note the different scales for tectonic loads in these plots. YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 13

[Liu and Jin, 1993]. On the other hand, the optimal Te 1993], and suggest that elastic flexure is a reasonably good values for regions around the Kuche and the Minfeng first-order approximation of the Cenozoic deformation of depression (see Figure 1) are abnormally low (24 km) the Tarim basin. for the late Miocene-Pliocene period. This may reflect intensive faulting along the central Tian Shan thrust belt and the Altyn Tagh fault zone in the late Cenozoic. Such 5.2. Implications for Mountain Building in the Tibetan localized mechanical weakening near the margins of the Plateau and the Tian Shan Tarim plate may also explain the low Te (<45 km) derived [36] We have shown that the major part of the Cenozoic from fitting gravity profiles transecting the Tibetan Plateau deformation of the Tarim basin can be explained by flexure and the southeastern margin of the Tarim basin [Jin et al., under the sedimentary loads. Because these sediments were 1996]. For the most part of the Tarim plate, our results mainly derived from the surrounding orogenic belts [Hen- indicate significant increase of the Te values through the drix et al., 1992; Metivier and Gaudermer, 1997], the basin Cenozoic (see Table 1). The cause of the increasing stiffness deformation can be directly related to the collisional moun- of the Tarim plate is not clear, but the results are consistent tain building in the surrounding regions. The additional with the paleogeothermal studies that suggest accelerated tectonic loads may provide further insights into the geo- decrease of paleogeothermal gradients, from 3.1–2.5C/ dynamic coupling between the Tarim basin and the sur- 100 m in the early Tertiary to 2.7–2.2C/100 m during the rounding Tibetan Plateau and the Tian Shan. late Tertiary to 2.1C/100 m at present [Jia, 1997]. 5.2.1. Uplift of the Western Kunlun Shan [34] For the earlier periods (Paleogene-mid-Miocene) a and the Pamir Plateau uniform Te for the entire Tarim plate produced satisfactory [37] The timing of tectonic uplift of the western Kunlun fit to the observed basement deformation. This may suggest Shan is important for understanding the fundamental that internal faulting of the basement had not caused mechanics of the growth of the Tibetan Plateau [England significant lateral heterogeneity of the mechanical properties and Houseman, 1988; Yin and Harrison, 2000]. 40Ar/39Ar of the Tarim plate. However, since late Miocene the base- and fission track dating [Arnaud et al., 1993; Matte et al., ment deformation showed strong local variations that reflect 1996; Sobel and Dumitru, 1997] suggest multiple events of control of individual fault systems and cannot be satisfac- cooling of the western Kunlun Shan in the Cenozoic, but the torily fit by an intact elastic plate. One example is the Bachu initial age of the western Kunlun Shan uplift in response to Uplift in the northwestern part of the basin (Figure 1b), the Indo-Asian collision remains unclear. The 1800-m where the fit is particular poor with an intact elastic plate. Paleogene sedimentary rocks in the southwestern depres- The Bachu Uplift is bounded by a system of faults. Since sion (Figure 2a) and the average 30 MPa tectonic load along Miocene this area experienced multiple phases of block the southwestern margin of the Tarim basin (Table 1) uplift as evidenced by the angular unconformity between suggest that mountain building in western Kunlun Shan different Tertiary strata [Jia, 1997]. When weak zones started at least in the Oligocene, consistent with recent simulating the Bachu fault systems are included in the stratigraphic studies in the southwestern depression that model (Figure 8), the fit over this region is significantly place the initiation age of the western Kunlun thrust belt improved. The poor fit to the late Miocene-Pliocene base- in 30 Ma [Yin and Harrison, 2000]. Since the Miocene ment deformation using an intact elastic plate indicates mountain building in the western Kunlun Shan may have stronger fault activity since late Miocene. accelerated. The thickness of the sedimentary rocks in the [35] We have also tested the effects of different model southwestern depression reached 2800 m during the early- rheology. Over the long history of Cenozoic deformation of middle Miocene and more than 5000 m since the late the Tarim basin viscous flow and stress relaxation may be Miocene, and the incremental tectonic loads over the entire expected, which cannot be simulated in elastic models. We western Kunlun belt, averaged over margin domains KL1– thus designed a linear (Maxwell) viscoelastic flexure model KL4, are 44 MPa and 84 MPa respectively for these two with four layers of different material simulating the mechan- periods. Through the Cenozoic the maximum tectonic load ical properties of the upper and lower crust, the strong layer has been on the boundary domain KL2 (Figure 8), in front in the uppermost mantle and the underlying mantle. The of the depression center near Yecheng. The accelerating boundary conditions around the Tarim plate are free for tectonic loads are consistent with the change in depositional vertical displacement but restricted for horizontal displace- facies from distal alluvial plains to proximal alluvial fans ment. Lacking detailed age control on the sedimentary near Yecheng in the western Kunlun Mountains during the strata, we assumed the sedimentary and tectonic loads were past 4.5 Myr [Zheng et al., 2000]. applied linearly during each of the three Cenozoic periods, [38] The accelerated tectonic load in this region has and a full range of rheological parameters are explored to important implications for tectonic evolution and climate seek for the best fit between the predicted and observed change. The increasing sedimentation rate and the associ- basement deformation. In general the results are worse than ated change of facies of sediments in this region may those of the elastic models. The results are generally better indicate accelerated tectonic uplift or climate change, and when the effective viscosity of the crust and the uppermost a number of recent studies have interpreted these evidence mantle is higher. These results are consistent with the notion as indicators of climate change in the past few million years of the Tarim plate being a relatively cold and stiff block [Molnar and England, 1990; Zhang et al., 2001]. In this through the Cenozoic [Lyon-Caen, 1986; Liu and Jin, study the sedimentary loads have been considered explicitly. 9 - 14 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE

The increasing tectonic load derived here is independent of load from the central Tian Shan thrust belt (domain TS4 in sedimentation rates and thus provides an unambiguous Table 1) indicate that rejuvenation of the Tian Shan started evidence for accelerated tectonic uplift of the western before the end of Oligocene. Since the Miocene the tectonic Kunlun Shan since late Miocene. loads along the Tian Shan-Tarim boundary accelerated and [39] Whether the Tarim plate subducted beneath the migrated westward, probably related to the indentation of western Kunlun Shan has been an issue of debate. The the Pamir plateau. The major phase of uplift in the Tian south dipping western Kunlun Shan thrust belt is believed to Shan probably occurred after late Miocene, indicated by the have developed in response to the Indo-Asian collision >5000-m sedimentary rocks in the Kuche and Aksu (Awati) [Lyon-Caen and Molnar, 1984]. Recent seismic survey depressions and the tectonic loads in this period (Figure 11c shows no clear evidence of a subducting Tarim plate near and Table 1). There are also significant variations within the 80.5E[Kao et al., 2001]. This seems consistent with the Tian Shan belt, such as tectonic unloading in domain TS2 predicted relatively small tectonic load (46 MPa) since late since late Miocene that may be correlated to the Bachu Miocene on boundary domain KL4, where the seismic block uplift [Jia, 1997]. The predicted basinward broad- profile was located. However, the predicted tectonic load ening of the Kashi-Aksu thrust belt (Figure 11c) is consis- increases westward, jumps to 286 MPa on domain KL2 tent with geological studies [Lu et al., 1994, 1998]. These (near Yecheng). This would be equivalent to have a 10-km- internal variations of the optimal tectonic loads may reflect high rock column on the domain. Because the division of the tectonic segmentation along the Tian Shan and the the boundary domains in the model is arbitrary, this result Tarim boundary [Yin et al., 1998]. The accelerated mountain may be interpreted as requiring a lower tectonic load spread building in the Tian Shan since late Miocene is also over a broader region. The high tectonic load may have consistent with the deformation rates extrapolated from resulted partly from overestimation of the basement sub- geodetic measurements [Abdrakhmatov et al., 1996]. sidence near Yecheng, where the present land surface is [42] Whereas positive tectonic loads may be related to about 500 m higher than the average elevation of the basin. overthrusting of mountain belt onto the margins of the By assuming a completely compensative basin and deter- basin, the meaning and origins of negative tectonic loads mining the basement depth from the isopachs, we may have are not always clear. For the boundary domains TS2 and overestimated the basement deformation near Yecheng by TS6, negative tectonic loads are predicted for the entire 10%. Nonetheless, the high tectonic load over the western Cenozoic. Although some of these results may be model- segments of the boundary (KL2 and KL3) since late dependent artifacts, an inevitable consequence of arbitrarily Miocene seems real and would be difficult to explain by dividing the boundary domains and making a first-order overthrusting of the western Kunlun thrust belt alone. It approximation, we cannot dismiss all the negative tectonic seems necessary to have at least part of the Tarim plate loads as artifacts, because many of the predictions are extended southward to beneath the western Kunlun Shan, consistent with geological features in these regions. As especially near the western end of the Tarim basin where the mentioned above, the negative tectonic load on TS2 is predicted tectonic load is the high through the Cenozoic. correlated with the Bachu Uplift, an uplifted-region since [40] The negative tectonic load on the boundary domain late Paleozoic. However, the cause of the uplift is unclear. KL1 since late Miocene (Table 1) seems inconsistent with The Upper Cambrian to Permian strata was exposed in TS2, the indentation and uplift of the Pamir plateau since early the Kepingtage area [Jia, 1997], suggesting strong erosion Oligocene, which has caused up to 180 km crustal short- that could explain the negative tectonic loading. The neg- ening in central western Kunlun Shan [Rumelhart et al., ative load on domain TS2 may also be related to an 1999]. One possible explanation is that the Karakaxi fault abnormally hot mantle beneath the central Tian Shan where (Figure 1b) may have deepened in the past 10 Myr or so, mantle upwelling is inferred from seismic evidence [Make- weakening mechanical coupling between the Tarim basin yeva et al., 1992]. The boundary domain TS6 is geograph- and the Pamir plateau in this region. This would cause shift ically correlated with the Yanji Basin (Figure 1). However, of tectonic loads to the two sides and hence help to explain the origin of the Yanji basin is not clear. the large tectonic loads on the adjacent boundary domains 5.2.3. Tectonics of the Altyn Tagh Fault System KL2 and TS1. The origin of the negative tectonic load on [43] The Altyn Tagh fault has been the focus of intensive the boundary domain KL3 during early-middle Miocene is studies because of its hypothesized role in facilitating large- not clear. scale eastward extrusion of Asian continent during the Indo- 5.2.2. Uplift of the Tian Shan Mountains Asian collision [Tapponnier and Molnar, 1977]. Closely [41] The timing of the rejuvenation of the Tian Shan related to the current controversy of the displacement orogen by the Indo-Asian collision is critical for under- history of the Altyn Tagh fault is the nature of the fault standing the propagation of collisional stresses [Tapponnier activity and the timing of its initiation, which ranges from and Molnar, 1979; Burchfiel et al., 1991; Lu et al., 1994]. the Oligocene to Pliocene in various studies [Bally et al., Estimates based on fission tracks and stratigraphic analysis 1986; Peltzer and Tapponnier, 1988; Wang, 1990; Burchfiel place the minimum age around 25 Ma [Hendrix et al., et al., 1991]. Some workers regard the Altyn Tagh fault as a 1994; Yin et al., 1998]. Our results do not show significant fundamental lithologic boundary [Tapponnier et al., 1982; delay of mountain building in the Tian Shan relative to the Deng, 1989; Arnaud et al., 1993; Wittlinger et al., 1998]; western Kunlun Shan. The >900-m Paleogene sediments in others consider it to be a crustal structure [Burchfiel et al., the Kuche depocenter (Figure 2a) and 30 MPa tectonic 1989]. It remains controversial as whether or not the Tarim YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE 9 - 15

between the Tarim basin and northern Tibetan Plateau in this region. A young deep-cutting Altyn Tagh fault is consistent with a low-velocity anomaly extending to 140- km depth based on teleseismic imaging across the central Altyn Tagh fault [Wittlinger et al., 1998], and Quaternary basaltic eruptions along the trace of western Altyn Tagh fault [Deng, 1989]. [45] The evolution of tectonic loads in the western Kunlun Shan, the central part of the Tian Shan (TS3 TS5) and the central segment of the Altyn Tagh fault are compared in Figure 12. Uplift in both western Kunlun Shan and Tian Shan may have started in Paleogene and accel- erated since Miocene, where the Altyn Tagh fault is char- acterized by tectonic unloading since late Miocene.

6. Conclusions

[46] The deformation history of the Tarim basin during the Cenozoic provides useful insights into mountain build- ing in the Tibetan Plateau and the Tian Shan mountains in response to the Indo-Asian collision. Major conclusions we may draw from this study include the following: 1. The Cenozoic history of basement deformation in the Figure 12. Summary of the evolution of tectonic loads in Tarim basin can be largely (up to 70%) accounted for by the western Kunlun Shan, the central segments of the Tian foreland flexure under the sedimentary loads. Near major Shan, and the central segment of the Altyn Tagh fault. depression centers additional tectonic loads are needed, Mountain building in both the western Kunlun Shan and the suggesting that these depressions were associated with Tian Shan started in the Paleogene and accelerated since tectonic uplift and erosion of the surrounding mountain Miocene, whereas the Altyn Tagh fault is characterized by ranges. The fit between the observed and the predicted tectonic unloading since mid-Miocene. basement deformation using a uniform elastic plate was unsatisfactory for late Miocene-Pliocene and required involvement of additional factors, such as internal faulting. The rhomb-shape geometry of the Tarim basin and large plate subducted beneath the Altyn Tagh fault [Lyon-Caen internal heterogeneity make 3-D models particularly useful. and Molnar, 1984; Cowgill et al., 2000]. 2. The Tarim plate is indeed cold and rigid throughout [44] Because horizontal loading and shearing are not the the Cenozoic. The optimal thickness of the effective elastic dominant causes of the flexure of the lithosphere and are not plate is 60 km in the Paleogene, 84 km in the early-middle included in the model, the results cannot provide much Miocene, and >100 km in most part of the Tarim plate since constraint on the timing and magnitude of shearing along late Miocene. Abnormally low Te (24 km) near the Aksu the Altyn Tagh fault. Nonetheless, some useful insights of (Awati) and Minfeng depression since late Miocene is tectonic activity along the fault can be derived from the probably caused by intensive and localized faulting. The Cenozoic sedimentation and tectonic loads along the south- rigid Tarim plate is consistent with seismic tomography eastern margin of the Tarim basin. The model predicts indicating a high velocity anomaly extending to >220 km increasing tectonic loads in the boundary domains AT1 near depth under the Tarim basin. The increasing Te throughout Minfeng and AT3 near Ruoqiang (Table 1 and Figures 8 and the Cenozoic is consistent with the cooling history of the 1) throughout the Cenozoic, probably related to continuous Tarim basin. mountain building in the eastern end of the western Kunlun 3. Tectonic uplift of the western Kunlun Shan mountain Shan and Altyn Tagh Shan. On the contrary, tectonic load ranges occurred during Oligocene or earlier. Rapid uplift along the central segment of the Altyn Tagh fault (domain started around early-middle Miocene and has been accel- AT2, Table 1) decreased since Miocene and become neg- erating ever since. The large tectonic loads required for this ative (unloading) during late Miocene. This unloading in the region cannot be entirely explained by the loads of the central segment of the Altyn Tagh fault is reflected in the western Kunlun Shan thrust belt, and suggest that part of the change of the dipping direction of the basement along this Tarim plate has subducted underneath the western Kunlun part of the basin boundary, which was southeastward from Shan mountains. The strongest underthrusting of the Tarim the Paleogene to middle Miocene, but was flipped to plate may have occurred under the western end of the northwestward after late Miocene, as one may infer from western Kunlun thrust belt. the isopach maps (Figure 2). The tectonic unloading may be 4. Tectonic rejuvenation of the Tian Shan mountains in explained by deep cutting of the Altyn Tagh fault since response to the Indo-Asian collision is not significantly later Miocene, which may have caused dynamic decoupling than mountain building in the western Kunlun Shan. More 9 - 16 YANG AND LIU: CENOZOIC DEFORMATION OF THE TARIM PLATE tectonic loads were predicted in the western part of the Tarim basin and the surrounding regions. However, prom- southern Tian Shan thrust belts, probably related to the ising progress has been made in recent years by combining indentation of the Pamir plateau. The large along-strike the lithostratigraphy, biostratigraphy, magnetostratigraphy, variation of tectonic loads since early Miocene is consistent and 39Ar/40Ar thermochronology [Yin et al., 1998; Yin and with geological evidence of fault-controlled segmentation of Harrison, 2000]. Future studies, with improved age con- the southern Tian Shan thrust belt. The basinward broad- straints and 3-D models that explicitly include the fault ening of the Kepingtage thrust belt initiated in Late systems bounding the Tarim basin, should be able to Miocene. significantly refine the picture of temporal-spatial develop- 5. Throughout the Cenozoic tectonic loads near Minfeng ment of crustal deformation in central Asia in response to and Ruoqiang increased moderately, indicating continued the Indo-Asian collision and help to address many remain- uplift in the eastern end of the western Kunlun Shan and the ing questions, such as the control of strain partition between Altyn Tagh Shan. However, along the central segment of the crustal thickening and tectonic extrusion following the Altyn Tagh fault tectonic loads started to decrease since collision. Miocene and become negative (unloading) since late Miocene. These results may be explained by dynamic decoupling between the Tarim basin and the northern Tibetan Plateau, probably due to the Altyn Tagh fault [48] Acknowledgments. We thank Wang Liangshu for turning our cutting the entire lithosphere and the southward migration interests to the Tarim basin, and Lu Huafu, Jia Dong, Gao Rui, Luo Zhaohua, and Michael Hamburger for helpful discussion on the regional of the active fault zone. The same results do not favor geology and geophysics. An Yin kindly provided preprints, commented on subduction of the Tarim plate under the Altyn Tagh fault. the earlier version of the manuscript, and helped us to understand the age [47] Most studies of collisional tectonics in central Asia problems of the sedimentary strata in the Tarim and surrounding regions. Ding Zhongyi helped us during the initial stage of the model development. have been focused on the Tibetan Plateau and other moun- We thank Kelin Whipple (AE) and the Tectonics reviewers, Dennis Harry tain belts in this region. Our results suggest that much about and Roger Buck, for their very detailed and helpful comments. This study is the collisional orogenesis can be learned from the Tarim supported by NSF grant EAR-9805127 and the Research Board of the basin and probably the numerous other intermountain basins University of Missouri. Visit to China was partly supported by Chinese NSF grant 49832040 to Lu Huafu as part of the collaborative effort for the in central Asia. At present one major hurdle is the lack of Chinese NSF’s key project ‘‘dynamic coupling between the Tarim basin and age constraints on the Cenozoic sedimentary strata in the Tian Shan.’’

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