Abstract
Using a topography-dependent tomographic scheme, the seismic velocity structure of the Eastern Tibetan Plateau, including the uplifted Longmenshan (LMS) orogenic belt, is accurately imaged in spite of the extreme topographic relief in the LMS region and thick sedimentary covers in the neighbouring Sichuan Basin. The obtained image shows a high-resolution upper crustal structure on a 500 km-long profile that is perpendicular to the LMS. The image clearly shows that the crystalline basement was uplifted within the LMS orogenic belt, and that the neighbouring Songpan-Ganzi Terrane was covered by a thick flysch belt, with evidence of near-surface thrust faults caused by convergence between Eastern Tibet and the Sichuan Basin. The indication that the lower crust beneath the LMS was folded and pushed upwards and the upper crust was removed by exhumation, supports the concept of a lower crustal channel flow beneath Eastern Tibet. The image also reveals that the destructive Wenchuan earthquake of year 2008 occurred in the upper crust, directly at the structural discontinuity between Eastern Tibet Plateau and the Sichuan Basin.
Highlights
The collision between the Indian subcontinent and Asia in the late Eocene resulted in crustal thickening and the high elevation of the Tibetan Plateau[1,2,3,4,5,6,7]
This orogenic belt is characterised by a steep topographic gradient, as the elevation rises from ~500 m on the eastern part to over 4 km in height on the western part within a short horizontal distance of less than 50 km[11, 12]
It has been speculated that the build-up of this orogenic belt experienced a two-phase growth during the Cenozoic[5, 12,13,14,15,16] and was a result of the eastward flow of the deep crustal viscous channel within Eastern Tibet[5, 14,15,16,17,18] or lithospheric thrust faulting with large amounts of slip[19,20,21]
Summary
The elevation of sources and receivers along the 500-km long DSS profile is variable from 4.2 km to 0.3 km. Because of the irregular topography, only pseudo-orthogonal grids were defined in the tomographic inversion[29]. These pseudo-orthogonal grids in the Cartesian coordinate were transformed to a curvilinear coordinate. Topography-dependent tomography produced a high-resolution upper crustal velocity model (Fig. 3a), which is well-constrained according to the illumination of ray paths penetrating through the media (Fig. 3b). Beneath the LMS orogenic belt, the penetration depth is much shallower (about 6 km thick, depth from +2 km to −4 km), demonstrating the high velocity of the upper crust. The penetration depth of the reflection ray paths is about 17 km This updated velocity model confirms that the high-velocity anomaly is reliable at a depth greater than 5 km beneath the Longmenshan faults. The sub-vertically continuous high-velocity column and the uplifted interface beneath Longmenshan orogenic belt may be caused by the lower crustal extrusion
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
