Post-collisional exhumation of the Indus-Yarlung suture zone and Northern Tethyan Himalaya, Saga, SW Tibet

Abstract

The Cretaceous-Eocene sedimentary sequences of the Sangdanlin section at Saga, SW Tibet, within the India-Asia collisional zone, have been studied in detail for constraining the timing of initial collision. However, the post-depositional history has not been investigated. Here, we report new low temperature thermochronology data along the classic Sangdanlin section, to reveal the cooling/exhumation history of the Indus-Yarlung suture zone (IYSZ) and northern Tethyan Himalaya. Samples from the IYSZ, all yield latest Oligocene-Miocene ages, with most zircon (U-Th)/He ages ranging between ~18 and 27 Ma and apatite fission track (AFT) ages of ~12–9 Ma. Thermal history modelling indicates a consistent Middle-Late Miocene cooling episode related to post-Great Counter Thrust (GCT) incision of the Yarlung River. To the south, samples from the northern Tethyan Himalaya Mesozoic sequences yield older ZHe ages of ~32–36 Ma, signifying activity of the Tethyan Himalayan fold-thrust zone, while two granites linked to formation of the North Himalayan gneiss domes yield Late Miocene AFT ages. The regional low temperature thermochronology data reveal different cooling patterns for tributaries on the northern and southern sides of the Yarlung River. This points to the strong influence of Early Miocene tectonism along the GCT, as a major controlling factor on river drainage reorganization at this time.