Late Oligocene–Early Miocene initiation of shortening in the Southwestern Chinese Tian Shan: Implications for Neogene shortening rate variations

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Abstract

The Cenozoic Tian Shan is one of the preeminent examples of an intracontinental orogen. However, there remains a significant controversy over when deformation related to the India-Asia collision commenced and therefore how shortening within the mountains has been partitioned over time. One approach has been to look at the modern shortening rate as measured by geodetic studies, combined with estimates of the total shortening across the range and extrapolate backwards. This approach suggests that the onset of range construction was ca. 10 Ma [K.Y. Abdrakhmatov, S.A. Aldazhanov, B.H. Hager, M.W. Hamburger, T.A. Herring, K.B. Kalabaev, K.B. Kalabayev, V.I. Makarov, P. Molnar, S.V. Panasyuk, M.T. Prilepin, R.E. Reilinger, I.S. Sadybakasov, B.J. Souter, Y.A. Trapeznikov, V.Y. Tsurkov, A.V. Zubovich, Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates, Nature 384 (6608) (1996) 450–453]. An alternate method is to determine the age of the onset of exhumation using thermochronology. We present 19 new apatite fission-track (AFT) results from the southwestern Chinese portion of the belt; this region represents the first area exhumed during the late Tertiary along a transect at ca. 76°E. Exhumation commenced at the Oligocene–Miocene boundary (∼ 24 Ma) along the Maidan and Muziduke thrusts, which bound the southern side of the Kokshaal range. Subsequently, deformation propagated ca. 20 km south to the Kashi basin-bounding thrust (KBT), which was exhumed by no earlier than 18.9 ± 3.3 Ma. Three detrital AFT samples from Plio-Pleistocene strata deposited ca. 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20.9 + 7.0/− 5.3 Ma to 15.9 + 5.4/− 4.0 Ma with a fairly constant lag time of 16 to 18 Ma. These ages, combined with structural data, suggest that both the hanging wall and the footwall of the KBT experienced a renewed episode of exhumation during the latest Cenozoic. The discrepancy between the Late Oligocene–Miocene initiation of significant exhumation shown herein and the 10 Ma initiation estimate from geodesy suggests that the Tian Shan has undergone a complex Late Cenozoic shortening history. Assuming that the present shortening rate could account for the total amount of Cenozoic shortening in 10 Ma and realizing that shortening initiated at least 15 Myr earlier, we conclude that the shortening rate must have varied over time, possibly in pulsed-southward migrating events, and that the present rate may not reflect the average rate since initiation of range uplift.

Introduction

The 2500 km long Tian Shan comprise the most significant Central Asian topography north of the Tibetan Plateau, with several peaks exceeding 7000 m. Situated between the enormous Tarim basin to the south and the Chu and Junggar basins to the north (Fig. 1), this region forms one of the world's best expressed intracontinental mountain belts. Deformation is driven by the active continental collision between India and Asia, located up to 1500 km to the south. We focus on the portion of the range between the Talas Ferghana fault and the region with the highest peaks: the Kyrgyz Central Tian Shan and the Chinese Western Tian Shan. There, the present-day structure of the reverse- and thrust-fault bounded Tian Shan consists of roughly E–W-trending ranges separated by roughly parallel thick late Cenozoic sedimentary basins (e.g., [1]). At present, dominantly contractile seismicity and deformation are distributed across the width of the orogen (e.g., [2], [3], [4]) and geodetic studies document a continuous gradient of shortening between Kashgar and Bishkek [5], [6].

Although present-day deformation is evenly distributed, the early stages of range growth are poorly constrained. Key questions are when and where this shortening commenced. One approach has been to look at the modern shortening rate as measured by geodetic studies [5], [6], [7], estimates of the total shortening across the range [8], and extrapolate backwards. Modern shortening rates across the entire range are estimated to be 19 ± 3 mm/yr at 76°E (Fig. 1). This value [5], [6] comprises a significant proportion of the present geodetically-determined India-Eurasia convergence rate of 35 mm/yr (REVEL, [9]). Avouac et. al. [8] estimated 203 ± 50 km of shortening across 76°E longitude based on area-balancing of the assumed crustal thickening under the mountain belt and adding sediment lost by erosion, taking into account the isostatic response. Using 200 km of shortening and 20 mm/yr deformation rate, this approach suggests that the onset of range construction was ca. 10 Ma [5]. While much of the GPS data have been collected in the Kyrgyz Tian Shan, the majority of the geological evidence has been obtained from regions ca. 500 km to the east. This raises the possibility that the onset of shortening could vary along strike within the Tian Shan.

A traditional method for determining the onset of shortening is to examine the sediments stored in the adjacent foreland basin. The Tarim and Junggar basins lie on the south and north sides of the Tian Shan, respectively. Unfortunately, non-marine Oligo-Miocene stratigraphy from these basins has poor biostratigraphic resolution and a complete absence of volcanic ashes, making accurate stratigraphic age determinations difficult. Yin et al. [10] proposed an Early Miocene age of shortening based on rough magnetostratigraphic dating of the onset of conglomerate deposition in the Kuqa region; however, a new, higher resolution magnetostratigraphic data for this section (the Qiulitagh anticline) yields a Middle to Late Miocene age for this conglomerate [11]. Indeed, magnetostratigraphic studies in the Kashi Basin show that prominent range-bounding conglomerates are time-transgressive [12], [13], [14]; therefore, the onset of more rapid exhumation and/or more proximal thrusting could have started even earlier than any single age reported from a basin. Based on an evaluation of mass accumulation in these basins, Métivier and Gaudemer [15] suggested that the onset of shortening was ca. 16 Ma, although the underlying stratigraphic resolution can be debated. Reflection seismic data from the subsurface south of Kuqa shows an angular unconformity above the late Oligocene Suweiyi Formation [16], indicating that the onset of deformation at this locality (slightly) post-dates this time.

Alternatively, one can examine thermochronologic data to constrain the onset of cooling. We assume that this cooling is linked with exhumation driven by crustal shortening, although cooling may lag behind the onset of deformation (e.g., [17] and references therein). Apatite fission-track cooling ages from a vertical profile along the north flank of the Tian Shan adjacent to the Junggar Basin, near Manas, and from detrital grains deposited in Miocene sandstones from the NW Tarim basin suggest that shortening commenced around the Oligocene–Miocene boundary [18], [19].

Using apatite fission-track thermochronology, we have studied the cooling history of every major topographic range along a transect between Kashgar and Bishkek ([19], [20], [21], Sobel, unpublished); in this study, we report on the area hosting the oldest Cenozoic results. These data constrain the onset of exhumation along the southern flank of the Tian Shan at 76°E. In turn, we address whether the shortening rate has varied since the onset of deformation.

Section snippets

Geologic setting

The Tian Shan records a complex Paleozoic history of island arc accretion (e.g., [22], [23]). The final ocean closure occurred in the Late Carboniferous–Early Permian, represented by ophiolites and eclogite in the Atbashi range in southern Kyrgyzstan, ca. 100 km north of our study area [24], [25]. Suturing was followed by Permian orogen-parallel strike-slip deformation (e.g., [26], [27]). Although episodes of intracontinental deformation driven by distal plate margin tectonism occurred during

Fission-track methodology

The study area is characterized by thrusts and folds, suggesting that exhumation driven by contractile deformation and erosion is the primary mechanism for late Cenozoic cooling. In this setting, apatite fission-track cooling ages from rocks that have been exhumed from above the total annealing temperature can provide a close approximation of the deformation age (e.g., [36]). For moderately rapidly cooled (10 °C/Ma) apatites with between 0.03 and 0.10 wt.% Cl, typical of this study, the total

Results

Two sampling transects were collected across the southern margin of the range (Figs. 2 and DR1 in Appendix A; Table 1). Together, these samples are from both the South Tian Shan (Maidan) and the Muziduke faults. Three apatite-bearing samples were collected from the hanging wall of the Maidan fault along the western transect. Samples 99WT-11 and 03-FT4 are about 2.4–2.7 km north of the fault and have ages of 25.8 ± 2.4 and 22.7 ± 2.0 Ma, respectively; sample 03-FT3 is located 1.1 km from the fault

Discussion

We interpret the AFT ages from the hanging wall of the Maidan and Muziduke faults as reflecting thrust-driven exhumation beginning in the latest Oligocene–Early Miocene and continuing for at least several million years. The quality of the data do not conclusively show that the southern fault is the younger of the two, although this would be the predicted deformation pattern if we assume southward propagating deformation [46]. The pattern of faulting depicted in Fig. 2B suggests that

Acknowledgments

We thank Yin Jinhui, Nan Lin, Tian Qinjian, and Kate Scharer for their help with sample collection and Doug Burbank for thoughtful discussions. Funding was provided by the National Science Foundation of China Grants 403720081 and 49834005 and US National Science Foundation Grant EAR-0230403. Dieter Rhede and Oona Appelt helped collect the electron microprobe data. Jean-Philippe Avouac, Marc Jolivet, Stuart Gilder and an anonymous reviewer provided helpful critiques.

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