Cenozoic Wedge Tectonics in eastern Tibet: Evidence from Surface Mapping and Reflection Seismology

Abstract

Previous studies of eastern Tibet have focused on its range-bounding structures, but the structures controlling the uplift of the plateau interior are still poorly known. We address this issue by integrating surface mapping, crustal-scale seismic-reflection profile analysis, and balanced cross-section construction of the tectonically active Min Shan region in the interior of eastern Tibet. The Min Shan is a N-S-trending mountain range about 50 km wide with elevations of over 4.5 km in the west that decrease gradually to less than 1.5 km in the east. It is bounded by the Minjiang fault along its western margin and the Huya-Jiuzhaigou fault along its eastern margin, yet these structures do not correspond to the surface topography. Previous studies show that the west-dipping Minjiang fault thrusts Triassic rocks over Quaternary deposits, but the Min Shan is in its footwall. The west-dipping Huya-Jiuzhaigou fault has the Min Shan in its hanging wall, but this fault is not associated with substantial surface deformation nor high topography and cannot explain the uplift of the Min Shan. Our surface mapping shows that the NNE-striking Minjiang fault consists of a series of steeply west-dipping, oblique right-slip brittle thrust faults that cross-cut earlier folds within mostly Triassic strata. Our mapping of the roughly NNW-striking Huya-Juzhaigou fault shows that it is a steeply west-dipping, oblique left-slip thrust fault with minor surface expression to the north and little influence on surface structures over 5-10 km west into the Min Shan region. A crustal-scale seismic reflection profile across the western Min Shan region shows west-dipping reflectors beneath the Minjiang fault zone, as well as an anticlinal structure beneath the western Min Shan. Combining our mapped surface geology with the seismic reflection data highlights a west-directed thrust wedge beneath the Min Shan with the active Minjiang thrust fault as its roof structure. Balanced cross-section construction and restoration based on these data results in a minimum of 60% shortening accommodated by Cenozoic brittle thrusting. The newly discovered thrust wedge could explain the uplift and asymmetric topography of the Min Shan, while this amount of shortening is sufficient to explain the current crustal thicknesses and elevations of this region. These findings contribute to a better understanding of the Cenozoic tectonics and uplift of eastern Tibet.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)