Crust and upper mantle structure beneath Southwest China and its implications for Mesozoic multistage gold deposits

Abstract

The Youjiang Basin (YB) on the southwestern margin of the Yangtze Craton (YC) contains >200 Triassic or Cretaceous gold deposits, but deep dynamic processes contributing to large-scale gold accumulation remain unknown. Based on a south–north trending linear broadband seismic array, we obtained the crust and mantle structure beneath the YB and southwestern YC by P-wave and S-wave receiver functions and ambient noise tomography. Our results show a remarkable lithospheric boundary beneath the northern margin of the YB, which separates the YB with felsic thin crust and thin mantle lithosphere from the YC with both a thick crust and thick mantle lithosphere. A new structural image shows that the southern end of the YC Moho dips southward and overlaps with the YB Moho. A 5-km-thick high-velocity layer is also found at a depth of 10 km beneath the northern YB. Combining this information with previous tectonic and geological results, we reveal the process of Mesozoic lithospheric evolution in the southwestern South China Block. We infer that both the pre-Mesozoic weak crust and mantle lithosphere and the intense intracontinental orogenesis in the Early-Middle Triassic contributed to the formation of first-stage Au deposits in the YB. Lithospheric delamination in the Early Cretaceous thinned the lithosphere of YB rather than YC, forming a steep lithosphere–asthenosphere boundary beneath the transition zone between YB and YC, which favored upwelling of Au-rich asthenosphere below the thick cratonic root to the bottom of the crust in the northern YB and contributed to the second-stage Au deposits in the YB.