A near-surface fine-velocity structure in the western Hoh Xil Basin revealed by first-arrival travel-time tomography from a deep seismic reflection profile

Abstract

The Hoh Xil Basin (HXB) is a key site for continental sedimentation from the Late Cretaceous to the Neogene on the northern-central Tibetan Plateau and provides a record of regional tectonic activity before and after the onset of the India-Asia collision in the early Paleogene. On the basis of a deep seismic reflection profile spanning the western HXB, we used the first-arrival travel-time tomography method to study the shallow upper crustal structure beneath the area in detail. Our images revealed a high-resolution 2D P-wave velocity structure at an approximate depth of 4 km. The P-wave velocity structure constrains the shallow upper crustal tectonics of the area and allows some speculation regarding its geodynamics. P-wave velocities exceeding 4.0 km/s were observed in the widely distributed Triassic flysch deposits within the western HXB. Lateral velocity perturbations are based on P-wave velocity structure calculations and can increase the lateral resolution of the profile and accurately delineate stratigraphic boundaries across different geological ages in the shallow upper crust. During the Late Cretaceous to Early Eocene (81–50 Ma), the amount of crustal shortening within the Tanggula thrust was estimated to have been greater than 50 % throughout the HXB, reflected by the complex P-wave velocity structure of the Kangtuo Formation. During the middle Eocene to Oligocene (45–28 Ma), the Hoh Xil thrust fault and Fenghuoshan thrust fault controlled the deposition of the Suonahu Formation in the west and the Yaxicuo Formation in the east, respectively, as well as the subsequent tectonic deformation. Our results indicate that the tectonic deformation and shortening of the upper crust during this period led to crustal thickening and elevation uplift. The Neogene Wudaoliang Formation in the eastern HXB and the Quaternary sedimentary layers in the western HXB show no signs of tectonic deformation. These results indicate that the uplift of the HXB after the Neogene was unlikely to be related to upper crustal tectonic deformation.