DIFFERENTIAL UNROOFING ACROSS THE SOUTHEASTERN LHASA BLOCK AND NAMCHE BARWA ANTIFORM

During collision between the rigid Indian plate corner and relatively warm and deformable rocks in the southeasterrn Lhasa Block, vertical strain became focused within the region surrounding the Namche Barwa antiform. When this focusing of vertical strain occurred is important to constrain, as this has been identified as a primary mechanism driving rapid localized exhumation in the Namche Barwa massif. Here we combine new zircon (U-Th)/He, and K-feldspar ⁴⁰Ar/³⁹Ar, and biotite ⁴⁰Ar/³⁹Ar thermochronometric data from the southeastern Lhasa block with forward thermokinematic modeling and thermal history modeling to investigate patterns in exhumation in order to inform models of plateau evolution in the vicinity of this indenter corner. Rocks in the northernmost part of the southeastern Lhasa block record near isothermal conditions in this part of the Tibetan plateau between the early Cretaceous and Eocene, suggesting widespread erosional quiescence over this time period. Modeling results incorporating multiple thermochronometer systems indicate that a period of enhanced exhumation initiating in the Miocene was variable in magnitude and timing across the southeastern Lhasa Block. A period of high exhumation rates (>1 km/Ma) is observed west of the Nari Yun Chu Rift between 18 and 14 Ma, while comparably high exhumation rates are observed within and to the east of the Nari Yun Chi Rift after 14 Ma. During this entire period, rocks from west of the Nari Yun Chu Rift experienced less total exhumation than those to the east, proximal to the Namche Barwa antiform. Our zircon (U-Th)/He and biotite ⁴⁰Ar/³⁹Ar data and other thermochronometer data from the region are well fit to a forward thermokinematic model in which there is a gradient in rock uplift rates increasing from west to east across the Lhasa Block in the middle to late Miocene, driving enhanced exhumation in the east over this period. We use the present-day depth of the Moho as a marker for magnitude of rock uplift and suggest from these results that a broad warping of the crust focused within the region surrounding the Namche Barwa antiform drove enhanced exhumation in the region and initial development of the antiform beginning in the middle Miocene.