LAUSR

The western Junggar is situated in the southwestern Central Asian Orogenic Belt, and its origin is believed to be related to either intracontinental arc or intraoceanic arc-related subduction system. However, the mode of subduction is still contentious. In this study, we build a high-resolution 3-D model of the crust of the western Junggar to constrain the subduction mode. By deploying a seismic array consisting of 31 portable broadband seismic stations from September 2013 to December 2013, we determine the Moho depth beneath every station using a H-κmethod and the Moho variations along three NW-SE profiles using a common conversion point stacking method. We also construct a 3-D crustal S wave velocity model using ambient noise tomography. Our results reveal significant variations of Moho depths across the study region and a pronounced Moho offset between the Zaire mountains and the western Junggar basin. Ambient noise tomography shows that a high shear velocity layer (>3.9 km/s) is observed in the middle/lower crust of the western Junggar Basin, which is overlaid by a low velocity upper crust. The high-velocity body gradually dips northwestward with depth beneath the Zaire mountains. The speed of this high-velocity body is consistent with the calculated S wave speed of various metamorphic facies of mid-ocean ridge basalt under the P/T conditions of middle/lower crust in the western Junggar. Our seismic observations support a northwest paleosubduction model for the evolution of the western Junggar, in which a remnant oceanic slab is still trapped in the western Junggar.