Basement and crustal structure from magnetic and gravity anomalies in the Songpan-Garzê and adjacent areas, China

The Songpan-Garzê orogenic belt is located in the eastern part of the Tibetan Plateau and west of the Sichuan basin. It is bounded by the South China, North China, Kunlun-Qaidam and Qiangtang (North Tibet) continental blocks. To the east, the Longmen Shan thrust-nappe belt separates the Songpan-Garzê fold belt from the Sichuan basin. The Songpan-Garzê basin and adjacent area are filled with a thick sequence of Triassic flyschoid sedimentary rocks. Within the Songpan-Garzê area, the Neoproterozoic basement only crops out in the southern part, in the Danba antiformal structure and to the east, along the Longmen-Shan belt, in the Xuelongbao metamorphic complex. To the north, below the Triassic sedimentary rocks, the nature of the basement (oceanic or continental) remains unknown. In order to research the range and the affinity of the basement of Songpan-Garzê orogenic belt, we ascertain the distribution range of the basement of Songpan-Garzê orogenic belt with aeromagnetic and seismic data, and discuss the affinity of the basement of the Songpan-Garzê orogenic belt with the geochemical data. Before identifying the boundaries of the basement we reduced the magnetic anomalies to pole and continued them upward. The results show that the basement in Songpan-Ganzi areas belongs to Yangzi block. The boundaries are Erdaogou-Yushu-Litang in the west-south, Qingchuan-Dujiangyan in the east, and the line of southern Kunlun to Xinghai-Xiahe-Xihe. Under the restrictions of deep seismic sounding and rock density characteristics, we calculate the density structure of the crust across the Songpan-Garzê orogenic belt and adjacent areas along the gravity profile A-A', which trends 400NE. The density model of the crust in this area is divided into two parts, upper and lower crust, vertically. The depth of the Moho is about 62 km in the southwest, and 54 km in the northeast. This model shows that gravity isostasy obeys the Airy theory approximately within the Songpan-Garzê orogenic belt although a buried load in the bottom of lower crust. While a buried load remain beneath the Moho to the north of Songpan-Garzê orogenic belt. During constructing these buried loads we computed local decompensative gravity anomalies on the condition that the depth of the lithospheric bottom is 120km. This work was supported by Crust Probe Project of China (SINOPROBE-02, SINOPROBE-08-02), the Natural Science Foundation of China (Nos. 40830316, 40774026, 40874045 ), China Geological Survey (Nos. 1212010611809, 1212010711813, 1212010811033), and scientific research project for public welfare from the Ministry of Land and Resources of China (No. 200811021,201011042).