71-1 Milin Thrust: A Segment of Gangdese Thrust in Southeastern Tibet

Session: The Geodynamic Evolution of the Himalaya: From Mountain Building to Modern Seismicity and Climate Change (Posters)

Presenting Author:

Authors:

Abstract:

Crustal deformation related to the Asia-India collision has been partially accommodated by multiple orogen-parallel shear zones across the Himalaya and southern Lhasa terrane. The Gangdese Thrust (GT) is an important Oligocene to Early Miocene contractional structure along the southernmost edge of the Gangdese Batholith that has been proposed to have played a major role in the uplift of Tibet in 30-20 Ma. Originally mapped by Yin et al. (1994), the GT is well exposed in the eastern portion of the Gangdese batholith (Xigaze and Zedong regions), where it juxtaposes Gangdese plutons in the north over forearc sediments or suture zone mélange to the south. Yet, the exact location of the GT in the easternmost batholith near the eastern Himalayan syntaxis has remained unclear. Our preliminary fieldwork and analytical results from the Milin region identify a ~35 km long, suture-parallel, steeply NW-dipping shear zone, here termed the Milin Thrust (MT), that likely represents a segment of the GT at a deep crustal level.

The upper plate of the MT comprises mid- to lower-crustal Gangdese granitoids, granitic to dioritic orthogneiss of amphibolite to granulite facies, migmatitic gneiss, and Paleozoic Lhasa terrane basement. Shear indicators such as subsolidus S-C fabrics and asymmetrical folds and clasts show top-to-the-SE or NW-side-up kinematics. The lower plate consists of clastic metasedimentary rocks, mostly sandstone and argillite, metamorphosed to phyllite, schist, and paragneiss, locally migmatitic. Detrital zircon ages from a lower-plate garnet–rutile gneiss suggest a potential Cretaceous mélange origin, while zircons from a meta-sedimentary unit indicate a forearc sedimentary source. Zr-in-rutile thermometry from the gneiss just beneath the MT yields peak temperatures of ~660 °C. The lower plate is characterized by intense bedding-parallel shortening, isoclinal folding, and occasional indicators for normal, thrust-sense, and sinistral shears. A published Ar-Ar biotite cooling age of 24 Ma from a 50 Ma pluton in the upper plate near the MT suggests Oligocene exhumation. Based on similarities in lithology, kinematics, P–T conditions, and cooling ages, we propose that the Milin Thrust, together with the Lohit Thrust farther southeast, represent a mid- to lower-crustal segment of the Gangdese Thrust.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8492

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.