Cenozoic orogenesis of the Qilian Mountain and the lithosphere mantle tectonic framework beneath it

Abstract : Based on the DSS（Deep Seismic Sounding） and BB（Broad Band Earthquake seismograms）research along the INDEPTH-IV line accomplished in 2013, the authors investigated the crustal thickening and orogenic mechanism of the Qilian Mountain, and revealed 4 faults in the block which was reactivated by the distant effect induced by the Indo-Asia collision since Cenozoie, especially since 23 Ma. The upper crust was shortened by about 30%, and hence Qilian crust 40km in thickness was thickened to 57 km. The lower crust was thickened by the intrusion of the Qaidam lower crust, which caused the variation of the crust thickness of Qilian Block from 57 km to the present 60 - 74 km. According to the P-wave receiver function (PRF) image, the authors obtained the Lithosphere Mantle（LM） Tectonic Framework beneath the Qilian Block, which comprises four parts: the southern part was from the southern Kunlun Block of 60km thickness with a two-layer structure and subducted northward, whereas the northern one was from Alxa Block with a thickness of 50 km and subducted southward. Both of them collided beneath the Qilian Block at the depth of 120 km . The other two parts are the Qaidam and Qilian blocks, with the relic texture of two old sutures. All of the four lithosphere mantle sections constructed a reverse delta structure. On the two old sutures（north fringe of the Qaidam Block and north fringe of the Middle Qilian Sub-Block）, the authors found “two double phase” phenomenon, and also revealed two ecologite belts at the ground surface along the suture zones.