Cenozoic tectonic sequence and tectonic events at?the eastern margin of the Qinghai? Juan Tibet plateau

Abstract: The eastern margin of the Qinghai-Tibet plateau can be divided into four tectonic units; from northwest to southeast, they are the Songpan-Garzê orogenic belt, Longmenshan-Jinpingshan thrust belt, foreland basin and Longquanshan forebulge, which are cut by the Xianshuihe fault. Cenozoic intermontane basins are developed in the inner part of the Longmenshan? Juan Jingpingshan Thrust Belt, and a Foreland basin occurs along the frontal part of the thrust belt. The Cenozoic sedimentary record along the margin is a key to constraining the plateau uplift and evolution during the collision and continued convergence of India and Eurasia during the last 65Ma. Cenozoic sediments in the basin of the eastern margin can be classified into five tectonic sequences. Sequence 1 is composed of Paleocene to early Eocene red beds (65 ~ 55Ma) and represents a fining? Juan the Upward retrogressive cyclothem in the Foreland basin, with alluvial fan sediments in the lower part and playa sediments in the upper part. Sequence 2 is composed of Middle-Late Eocene red beds (50 ~ 40Ma) and represents a retrogressive and progressive cyclothem in the Yanyuan intermontane basin, with alluvial fan sediments in the lower and upper parts and fluvial sediments in the middle. Sequence 3 is composed of Miocene coal? Juan of bearing beds (23 ~~ 16mA) and a represents a coarse-Upward progressive cyclothem in the Foreland basin, with alluvial fan and lacustrine sediments. Sequence 4 is composed of the Pliocene? Juan Early the Pleistocene Yanyuan Formation (4.29 to 2.58Ma) or Dayi conglomerate (3.6 to 1.7mA) and represents a fining-Upward retrogressive cyclothem, in the Foreland basin with alluvial fan sediments in the lower part and lacustrine sediments in the upper part. Sequence 5 is composed of the Middle Pleistocene Ya'an conglomerate (0.65 ~ 0.20Ma) and represents a fining-upward retrogressive cyclothem in the foreland basin. Based on the filling succession and discordant contacts between Cenozoic strata in the basin, six tectonic events along the eastern margin of the Qinghai-Tibet plateau are recognized. During each tectonic event, thrusting was followed by strike-slip deformation, which controlled basin formation, sedimentation and tectonic sequence. During the second and third tectonic events related to the Late Eocene collision between the Indian and Eurasian plates, deposition occurred within small intermontane basins. A hot, dry climate gave rise to sediments dominated by red beds interbedded with gypsum and eolian sandstone. The fourth tectonic event in Early Miocene time resulted in deposition within a foreland basin. A warm, humid climate gave rise to the deposition of lignite-bearing microclastic rocks, which was subsequently uplifted to an elevation of 2000m. The fifth began in Pliocene time, during which a large volume of fan conglomerate was deposited in the foreland basin. The sixth tectonic event in Middle Pleistocene time resulted in deposition of conglomerate in the Chengdu foreland basin.