Crustal-upper mantle velocity structure from the North Qilian to Beishan block and the tectonic significance of the crustal deformation

Abstract. The Qilian Shan constitutes a Cenozoic fold-thrust belt characterized by multi-stage tectonic deformation since the Paleozoic era. The Hexi corridor basins and the Beishan block, located north of the Qilian Shan, are the southern segment of the Central Asian Orogenic Belt. The crustal-mantle structure of the study area serves as a transition zone, crucial for comprehending the deep processes of accretion and crustal deformation. This study introduces a novel 460-km deep seismic sounding profile spanning the North Qilian Shan to the Beishan block. The P-wave velocity structure of the crust and upper mantle indicates a crustal thickness between 47.5 km and 60 km, segmented into five strata. The central Jiuquan basin displays the most substantial crust, measuring 59.5–60 km in thickness. The average crustal velocity varies between 6.24 and 6.43 km/s, while the Pn velocity ranges from 7.7 km/s to 8.1 km/s. The considerable variance in crustal velocity indicates heterogeneity in the crustal composition of the Qilian Shan and Beishan block. The southern edge fault of the Beishan block delineates the area, where northward and southward thrusting induces upper crustal deformation on opposing sides. The crust of the southern Beishan block is weaker than that of the bordering regions. Additionally, we propose a north-direction subduction polarity of the Qilian Ocean based on the northward-tilted velocity contour in Paleozoic.