CENOZOIC DEVELOPMENT OF EASTERN TIBET BY PURE-SHEAR SHORTENING

A key issue in the studies of the eastern Tibetan plateau is that its high relief (>4 km) and thick crust (>50 km) cannot be adequately explained by the slow geodetic slip rates (<3 mm/yr), late Cenozoic initiation (~30 Ma), and 10s km Cenozoic shortening along its easternmost edge. The channel-flow model may explain this discrepancy, but it discounts the role of deformation in the upper mantle and the upper crust in the interior of eastern Tibet. Here we address this problem by conducting field mapping, interpreting crustal-scale seismic-reflection profiles, and constructing balanced cross sections in the Longmen Shan and Min Shan of western Sichuan. Our mapping reveals pervasive development of NE-striking oblique right-slip thrusts characterized by (1) preferred development in Triassic-folded Silurian and Devonian strata, and (2) discrete shear planes with unknown offsets. The parallelism, similar kinematics, and similar brittle mode of deformation between the Cenozoic plateau-bounding structures and the mapped faults suggest a Cenozoic age. Our seismic interpretations consistent with regional field relationships suggest: (1) two-phase deformation first by Triassic thin-skinned folding followed by later Cenozoic thick-skinned faulting, (2) the presence of a west-directed thrust wedge in the Min Shan with the east-directed Minjiang thrust as its roof structure, and (3) pure-shear ductile shortening in the middle and lower crust in contrast to brittle faulting in the upper crust and the uppermost mantle. A preliminary balanced cross section across the Min Shan reveals >67% shortening accommodated by brittle thrusting. Integrating field observations and seismic data leads to a tectonic model that involves pure-shear thick-skinned crustal thickening for the Cenozoic development of eastern Tibet. If the 67% shortening from the Min Shan represents the average Cenozoic strain in eastern Tibet, this would be sufficient to explain the current crustal thickness and high relief.