Sedimentary Records Of Cenozoic Tectonic Events In The Pamir And South Tian Shan

The western part of Kashi Depression sits in the conjection area of the Tarim Basin, the Pamir Syntax and the south West Tian Shan and recived thick sediments from the Pamir Syntax and the south West Tian Shan, which could be used to document the Paleogene activities of the Pamir, late Cenozoic tectonic process of South Tian Shan and interaction beteew Pamir and South Tian Shan, and to reveal the regional climatic and environmental events.Based on the Cenizoic magnetostratigraphy, sedimentology and particle size analysis of the Tierekesazi succession in the foreland region of west South Tian Shan, and regional sedimentary facies analysis and comparison of Cenozic stratigraghic columns in the study area, we attempted revealing tectonic and paleoclimatic event with these sedimentary records. Through this, the following conclusions have been achieved.1. Via sedimentary characteristics comparison of Cenozic stratigraghic columns in the Northeast Pamir and Kashi Depression, this article propose that Pamir might already be active in the Paleogene with high altitude and initial curvature shaped in the early Paleogene or even earlier. Three stages of Paleogene tectonic activeites could be recognized in the Pamir. Stage I:the Aertashi Formation (earliest Paleocene-ca.56.5 Ma), alluvial sediments of conglomerate and coarse sandstone are deposited at foreland of North Pamir. Activities are only found at the North Pamir, being responded to the far-field effect of northward Andean subduction of Neotethy. Marine facies of mainly gypsum layers deposits at other area. Stage II:the Qimugen Formation (ca.56.5-45.5 Ma), alluvial facies of conglomerate, coarse sandstone and pebbly sandstone are deposited at foreland area from North Pamir to West Kunlun Mountain. Coarse deposit expand eastwardly indicating strengthen of the Pamir activities triggered by India-Asia collision. The other areas are dominated by interlayers of gypsum-mudstone and mudstone which could be divided into lower green part and upper red part. Stage III:the Kalataer, Wulagen and Bashibulake Formation (ca.45.5 Ma-roughly O-M boundary), coarse sediments deposited at north edge of Pamir and West Kunlun Mountain and deliver terrigenous clasts to the Kashi Depression, indicating continuous enhancement of tectonic activities of the Pamir. This event is consistent with large scale crustal shortenting, thickening and exhumation in the Pamier, triggered by far-field effect of continuous convergence of India and Asia.Based on our model, this article argues for the initial curvature of Pamir already exsits in the early Paleocene which resembles present Pamir. The tectonic activities of Paleogene Pamir at its north boundary are mainly represented by early Main Pamir Thrust (MPT). The ca.300 km northward intrusion of Pamir inferred by Burtman and Molnar (1993) and ca.280 slip of the KYTS inferred by Cowgill (2010) in the late Cenozoic should be reconsidered.2. Sedimentary analysis of Tierekesazi profile in the foreland of Southwest Tian Shan reveals a section of generally upwardly increasing particle sizes. Three striking events have been identified, represented by the first appearances of (1) sandstone of meandering river facies at the bottom of the Keziluoyi Formation, (2) pebble gravel sheet of fan delta plain facies in the middle of the Pakabulake Formation, and (3) massive cobble conglomerate at the bottom of the Atushi Formation. These events imply three episodes of increasing particle size and water power, divide the Cenozoic succession into four lithofacies:(ⅰ) Paleogene marine lithofacies from the Aertashi to Bashibulake Formations, (ⅱ) lacustrine to fluvial (plain) lithofacies from the Keziluoyi to middle Pakabulake Formations, (ⅲ) alluvial sand-gravel sheet lithofacies in the upper Pakabulake Formation, and (ⅳ) conglomerate lithofacies from the Atushi to Xiyu Formations.3. Our magnetostratigraphic results, combined with biostratigraphic correlations, provide the chronologic constraints for each lithofacies and also the sediment accumulation rates (SAR). These results indicate lithofacies (ⅰ) aged ca.65-34 Ma with SAR of ca.2.4/3.3-3.5 cm/ka (compacted/decompacted), lithofacies (ⅱ) aged ca. 22.1-12 Ma with SAR of ca.12.3/16-17 cm/ka (compacted/decompacted), lithofacies (ⅲ) aged ca.12-5.2 Ma with SAR of ca.16.3/19.5-20.6 cm/ka (compacted/decompacted), and lithofacies (ⅳ) aged ca.5.2 Ma-present (?) with SAR of>22.8 cm/ka. Regional correlation of SAR from foreland of South Tian Shan has suggested a linkage of the sedimentary event to the tectonic activity along South Tian Shan. We propose that the earliest Miocene event may represent the initial response of the far-field effect of Indian-Eurasian convergence, but more directly and likely marks the initial underthrusting of the Tarim block beneath the South Tian Shan. The mid-Miocene and Mio-Pliocene boundary events have different structural expression in the opposite regions east and west to the Talas-Fergana fault (TFF). These events are likely resulted from significant basinward propagation of foreland fold-and-thrust belts along the South Tian Shan in the regions east to the TFF. In the studied Tierekesazi region west to the TFF, the events are tentatively related to accelerated mountain building in the south West Tian Shan. The difference of the shortening amounts in opposite foreland regions east and west to the TFF is proposed to cause the dextral slipping of the TFF, with its dextral slipping amount at least 60-70 km. More fundamentally, such structural contrast and the activity of the TFF are likely driven by the northward indentation of the Pamir at this time.4. Paricle size analysis and mass susceptibility (Ms) results of Cenozoic samples in the Tierekezazi profile could correlate with sedimentary facies evolution. Mean diameter, deviation and Ms of the samples arise at ca.34-30 Ma. At ca.16-12 Ma, the mean diameter, deviation decreased with at peak at ca.14 Ma; skewness and kurtosis of the samples arise at this time; Ms also show a significant peak at ca.14 Ma. At ca. 8-7 Ma and 5 Ma, the mean diameter decrease and the deviation and Ms increase. The whole samples show a trend of upwardly coarsening with coarse component increasing upwardly.Similar viariation trend could be found through comparision of combination of magnetostratigraphy, parcile distribution parameters and Ms curve and global change of CO2, oxygen (δ18O) and carbon (δ13C) isotope records, espesically at ca.34-30 Ma, 14-12 Ma,8-7 Ma and 5 Ma. Ms show a sharp rise and decrease at ca.34-30 Ma, particle size decrease at ca.33-31 Ma, reflecting a relative warm period after Oi-1 Glaciation of ca.34 Ma. Since Miocene, the Ms shows a decreasing trend and skewness shows an increasing trend, indicating strengthen of aridification caused by intensification of Asia monsoon. The mean diameter and Ms increase significantly at ca.14-12 Ma,8-7 Ma and 5 Ma, might indicate three intensification events of Asia winter monsoon. These results account for strengthen of aridification caused by intensification of Asia monsoon in the study area. Meanwhile, uplift acceleration events of the South Tian Shan might also facilitate the regional aridification.