Anne Replumaz

The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1,100 km) of a composite “proto‐Tibet,” itself the product of a long Paleozoic and Mesozoic history of accretion of Gondwana‐derived continental fragments and volcanic arcs against the Asian continental margin. The difficult access and the scarcity of outcrops have long limited the possibilities of studying these Mesozoic suture zones in the heart of the Plateau. In this work, we present new U‐Pb and 40 Ar/ 39 Ar ages from the highly deformed units of the Yushu mélange, along the Jinsha Suture in the northeastern Qiangtang terrane. Early Triassic (c. 253 Ma) to Middle Jurassic ages (c. 165 Ma) complement the existing data set and help to refine the chronology of the Paleo‐Tethyan oceanic subductions which have structured the northeastern part of the Qiangtang terrane. The Yushu mélange records at least three successive tectono‐magmatic events. The opening of a back‐arc basin during the northward Paleo‐Tethyan subduction along the Longmu Co‐Shuanghu Suture during Early to Middle Triassic; then its closure during the southward subduction of the Songpan‐Ganze Ocean along the Jinsha Suture in Late Triassic. Finally, a shortening phase related to the continental collision of the Songpan‐Ganze and Qiangtang blocks from Late Triassic to Early‐Middle Jurassic. No evidence for any high‐ or mid‐temperature Cenozoic reactivation of the Jinsha suture in our study area is recorded.

To test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed.