Basement structure of southeastern boundary region ofSichuanYunnan active block：Analysisresult of Yanyuan-Xichang-Zhaojue-Mahu deep seismic sounding profile

Yanyuan-Xichang-Zhaojue-Mahu wide-angle reflection/refraction deep seismic profile has been completed in the southeastern boundary region of Sichuan-Yunnan active block. The images of P-wave velocity structure and the basement interface along the profile are obtained by interpreting the arrival times of refracted Pg wave and using finite difference inversion and time-term inversion. The result shows that the basement structure is significantly inhomogeneous and different tectonic zones reveal different deformation features. The Yanyuan basin is characterized by caprock nappe thrust deformation, while its basement is the rigid base of Kangdian axis. In the range of Jinhe basin to Xichang Mesozoic basin, the caprock deformation is characterized by fold, thrust and uplift, while the basement reveals renewed dislocation and crush deformation of preexisting fault. In Daliang Mountain region deformation is mainly represented by renewed dislocation and crushing of basement fault. Yanyuan nappe structure is composed of caprock low-velocity body, gently west-dipping basement interface and deep high velocity basement. It thrusts toward northeast with the west-dipping Jinhe-Qinghe fault zone as the nappe front. The detachment surface of the nappe structure is a gently west-dipping basement interface, which is 2mdash;3 km deep and outcrops in the west margin of Jinhe basin. The basement depth varies generally and changes more significantly in the range from Jinhe basin to Xichang basin than in its western and eastern adjacent areas, with maximum depth being about 6 km there. The faults in the upper crust behave as the low velocity zones or velocity gradient zones, while the travel-time curves of Pg wave through the faults occur variation in negative slope and there is significant difference in the depth of basement on both sides of the faults. Faulting feature can be clearly seen on Anninghe fault, Zemuhe fault and Daliangshan fault. Anninghe fault and Zemuhe fault behave as low velocity belts, where strong concave velocity isolines appear and sharp velocity differences as compared with their two sides can be distinguished. The two faults both dip northeastward. Daliangshan fault is composed of two branches, with the west branch dipping southwestward and the east one northeastward, being characterized by rapid velocity variation structure in a narrow belt. Mopan shan fault dips toward southwest. Heishuihe fault is divided into two branches, with west one dipping toward southwest and east one dipping toward northeast. The east edge fault of Xichang Mesozoic basin dips toward southwest.