Parallelism between the maximum exhumation belt and the Moho ramp along the eastern Tibetan Plateau margin: Coincidence or consequence?

Knowledge of how strain is partitioned and accommodated in crustal shortening provides fundamental constraints on evaluating tectonic models of orogenic plateaus. A number of tectonic models have been proposed for the central Longmen Shan along the eastern Tibetan Plateau margin, including mid-crustal channel flow, upper crustal shortening, and whole crust shearing. However, it remains controversial whether these models work in the Min Shan - northern Longmen Shan area, and if so, how the models function in three dimensions. One straightforward test is to evaluate the timing and rate of uplift on the principle crustal faults. Here we present 36 fission track dates from this region. Our data show that the southeastern Min Shan has experienced the most rapid exhumation in this area since the late Miocene, due to slip along the Huya thrust and coeval northwestward tilting of the Min Shan fault block. Further, we identify a maximum exhumation belt (MEB) along Min Shan and Longmen Shan that is characterized by the greatest amount of exhumation in the late Cenozoic and the formation of 5000+ m peaks along the eastern Tibetan Plateau margin. Interestingly, the MEB overlies the Moho ramp between the Tibetan Plateau and the Sichuan Basin. The parallelism between the MEB and the Moho ramp leads us to a crustal-scale tectonic model, in which the Tibetan upper crust undergoes shortening and thrusting and the lower crust undergoes pure-shear thickening. Moreover, a similar to 15 degrees angle exists between the trend of the MEB and that of the Longmen Shan, implying that the Longmenshan thrust belt is a reactivated structure, which explains the along-strike variations of fault kinematics and activity. We conclude that the eastern Tibetan crust is partially decoupled, and future studies need to quantify strain partitioning in both the upper and the lower crust during continental orogenesis. Published by Elsevier B.V.