Crustal-upper mantle velocity structure from the North Qilian Shan to Beishan Orogenic Collage: tectonic significance of crustal deformation

Abstract

The Qilian Shan represents a Cenozoic fold-thrust belt characterized by multi-stage tectonic deformation since the Paleozoic. North of it lies the Hexi corridor basins and the Beishan Orogenic Collage (BOC), which constitute the southern segment of the Central Asian Orogenic Belt. The crustal-mantle structure of the study area, a transition zone, is crucial to understanding the deep processes of accretion and crustal deformation. This study presents a newly acquired 460 km wide-angle reflection and refraction profile traversing from the North Qilian Shan to the BOC. P-wave velocity structure reveals a 47.5-60 km thick crust divided into five layers. The deepest Moho (60 km) lies beneath the central Jiuquan. Average crustal velocities (6.24-6.43 km s-1) and Pn values (7.7-8.1 km s-1) reveal strong lateral heterogeneity. North-dipping velocity contours from 20 km to the uppermost mantle beneath the Qilian Shan, coupled with a lower-crust-upper-mantle low-velocity corridor beneath the Hexi Basin, support early Paleozoic north-dipping subduction of the Qilian Ocean. A positive upper-mantle anomaly (8.0-8.3 km s-1, 45-70 km depth) aligns with the Hongliuhe-Xichangjing ophiolite mélange likely represents a fossil slab broken off after north-dipping subduction of the Beishan Ocean. Crustal velocity contrasts across the southern margin fault of the Beishan affirms its role as a regional strike-slip structure. Integrating geological and geophysical evidence, we suggest that the Altyn Tagh Fault does not terminate at the Qilian Shan front but rather extends east-northeast along the southern margin of the BOC into the Beishan and Alxa regions.