Crustal Discontinuity Structure Beneath the Sichuan Basin and Adjacent Areas Based on Receiver Function Migration

The crustal structure of the Sichuan Basin, the core of the Yangtze Craton, was investigated by P-wave receiver function (P-RF) imaging. Teleseismic data used come from 151 broadband seismic stations that form two linear arrays nearly perpendicular with each other crossing the Sichuan Basin. The SW-NE-trending linear array ends at the Qinling Orogen in the NE and Yungui Plateau in the SW. The NW-SE-trending one ends at the Songpan-Ganzi Block in the NW and western Hunan-Hubei Fold Belt in the SE. Crustal discontinuity structure images along both arrays were constructed by applying the wave-equation based migration technique to the corresponding P-RFs. Along the SW-NE profile, the Moho is the shallowest ( 45km) in the southwestern Sichuan Basin and deepening to both sides with the deepest under the Yungui Plateau ( 60km). A sharp Moho step ( 20km) is present in between the northern and southern Qinling Orogen, whereas a gradual SW-ward uplift of the Moho is observed across the boundary between the Yangtze Craton and the Daba Mountain Orogen. Along the NW-SE profile, the Moho depth increases rapidly from the Songpan-Ganzi Block ( 55km) to the northwestern margin of the Sichuan Basin ( 45km) , spatially coincident with the Longmen Shan Thrust Belt. The Moho is almost flat( 43km) beneath the Sichuan Basin except an apparent deepening under its northwestern part. An uplift of the Moho appears beneath the western Hunan-Hubei Fold Belt with the shallowest depth up to 32km. Besides the Moho, two nearly parallel intra-crustal discontinuities are imaged within the Sichuan Basin along both profiles. While the shallow one probably corresponds to the bottom of the sedimentary cover, the deeper one may result from a long-term crustal differentiation of the craton. Inclination of these intra-crustal discontinuities might have been associated with the paleo-uplift of the basin basement since the Permian, and our image suggests that the basement uplift was probably controlled by the structure and processes in the deep crust. Large-scale structure of the crust beneath the Sichuan Basin reflects its stability relative to the surrounding blocks.