Lithospheric structure beneath the eastern Junggar Basin (NW China), inferred from velocity, gravity and geomagnetism

Abstract

A comprehensive geophysical profile stretching northward from near Tianchi (on the southern margin of the Junggar Basin) to Zalat (near the northern margin of the Junggar Basin) was carried out in an attempt to probe the basement structure and properties of the eastern Junggar Basin, NW China. Along the profile the seismically-converted waves were used to determine the P- and S-wave velocity structures of the crust and uppermost mantle. A joint inversion of gravitational and magnetic data was also performed along the profile to acquire the density and magnetic intensity. The results revealed that the crust of the eastern Junggar Basin is composed of the upper, middle and lower layers. In particular, deformation mainly occurs within the middle crust, a process which has affected the crustal thickness. The middle crust, with a relatively low velocity and low density, changes laterally. It thins at its center but thickens at both ends of the profile. The crystalline crust along the profile is structurally convex, with some thrust faults, indicating that it has experienced a strong north-south compression. Combining geological and geochemical data, we found that the eastern Junggar Basin comprises the Manas Terrane in the south and the Ulungu Terrane in the north, separated by the Dishuiquan-Sangequan Suture Zone (DSSZ). The Ulungu Terrane has a double-layered basement: the upper layer is a Hercynian folded basement; the lower is a crystalline basement of Mid-Late Proterozoic age. The Manas Terrane has a single-layered crystalline basement. The north-south compression causes the eastern Junggar Basin’s convex structure and the consequent subduction southward beneath the Northern Tianshan Orogenic Belt, giving rise to a rapid uplift of the Bogda Mountains and the frequent occurrences of major earthquakes. Based on the observations of the 2D velocity structure beneath the eastern Junggar Basin, the 2D density structure and 2D geomagnetic intensity structure are also observed. Combining these observations with geological and geochemical data, we assume that the eastern Junggar Basin has an oceanic basement, but with some small micro-continental blocks. In addition, basic to ultra-basic rocks from the uppermost mantle may have squeezed into the crust through the deep faults and mixed with the crustal material already there, leading to the relatively thin crust, as well as its high seismic velocity, high density and high magnetic intensity.