The basin-mountain system in the Sichuan Basin(SCB) reflects the main tectonic activity and the orogenic denudation in this region. The seismic probing work reveals the deep structure of the basin-mountain system. The seismic work was re-sampled to the Moho depth and the sedimentary thickness as well as the P-wave velocity-depth function to analyze the deep structure of the SCB and adjacent orogens. The results show two deposit centers in the SCB: the Deyang area in the west and the Nanchuan area in the east and depression uplift exists in the southwestern part of the SCB; the Moho shallowers gradually from the west to east(ca. 62-36 km deep),the South-North seismic belt(SNSB) is very distinctive: the Moho depth is much shallower(< 50 km)to the east of the SNSB, whereas it is much deeper(>50 km)to the west of the SNSB, suggesting that the SNSB rather than the Longmen Shan tectonic belt is a main Moho transition belt; the topography and the top interface of the basement have the same undulation trend when the sedimentary thickness and the Moho depth have a mirror relationship; the low velocity zone developed in the Kangdian thrust and fold belt and Songpan-Garzê belt implied a soft, weak and thick crust there showing tectonic activity in these areas.
The Yinchuan basin, located on the western margin of the Ordos block, has the characteristics of an active continental rift. A NW-striking deep seismic reflection profile across the center of Yinchuan basin precisely revealed the fine structure of the crust. The images showed that the crust in the Yinchuan basin was characterized by vertical stratifications along a detachment located at a two-way travel time(TWT) of 8.0 s.The most outstanding feature of this seismic profile was the almost flat Mohorovicˇic′ discontinuity(Moho) and a high-reflection zone in the lower crust. This sub-horizontal Moho conflicts with the general assumption of an uplifted Moho under sedimentary basins and continental rifts, and may indicate the action of different processes at depth during the evolution of sedimentary basins or rifts.We present a possible interpretation of these deep processes and the sub-horizontal Moho. The high-reflection zone, which consists of sheets of high-density, mantlederived materials, may have compensated for crustal thinning in the Yinchuan basin, leading to the formation of a sub-horizontal Moho. These high-density materials may have been emplaced by underplating with mantlesourced magma.
