A test of deep seismic reflection profiling across the central uplift or metamorphic belt of the Qiangtang （羌塘） terrane, Tibet plateau, provides a first image of the crustal structure. Complex reflection patterns in the upper crust are interpreted as a series of folds and thrusts, and bivergent reflections in the lower crust may represent a convergence between the Indian and the Eurasian plates.
The Qinghai （青海）-Tibet plateau is the newest and biggest orogenic belt in the world and a natural laboratory for researching continental geodynamics, such as continent-continent collision, convergence, subduction, and plateau uplift. From the 1950s to the present, there have been many active-source （deep seismic sounding and deep seismic reflection profiling） and passive-source seismic probing （broadband seismic observations） implemented to reveal the crust-mantle structure. In this article, the authors mainly summarize the three seismic probings to discuss the Moho depth of the Qinghai-Tibet plateau based on the previous summaries. The result shows that the Moho of the Qinghai-Tibet plateau is very complex and its depth is very different; the whole outline of it is that the Moho depth is deeper beneath the south than the north and deeper in the west than in the east. In the Qiangtang （羌塘） terrane, the hinterland of the Qinghai-Tibet plateau, the Moho is shallower than both the southern and the northern sides. The deepest Moho is 40 km deeper than the shallowest Moho. This trend records the crustal thickening and thinning caused by the mutual response between the India plate and the Eurasia plate, and the eastward mass flow in the Qinghai-Tibet plateau.
The Dabashan nappe structural belt links the Hannan block to the west with the Huangling block to the east between Yangxian and Xiangfan. The Dabashan arc-shaped fold belt formed during late Jurassic and was superposed on earlier Triassic folds. To achieve an improved understanding of the deep tectonics of the Dabashan nappe structural belt, we processed and interpreted the gravity and magnetic data for this area using new deep reflection seismic and other geophysical data as constraints. The results show that the Sichuan basin and Daba Mountains lie between the Longmenshan and Wulingshan gravity gradient belts. The positive magnetic anomalies around Nanchong-Tongjiang-Wanyuan-Langao and around Shizhu result from the crystalline basement. Modeling of the gravity and magnetic anomalies in the Daba Mountains and the Sichuan basin shows that the crystalline basement around Nanchong-Tongjiang-Wanyuan-Langao extends to the northeast underneath the Wafangdian fault near Ziyang. The magnetic field boundary in the Zhenba-Wanyuan-Chengkou-Zhenping area is the major boundary of the Dabashan nappe thrusting above the Sichuan Basin. This boundary might be the demarcation between the south Dabashan and the north Dabashan structural elements. The low gravity anomaly between Tongjiang and Chengkou might be partly caused by thickened lower crust. The local low gravity anomaly to the south of Chengkou-Wanyuan might result from Mesozoic strata of low density in the Dabashan foreland depression area.
