Tectonic Evolution of the Yarlung Suture Zone, Lopu Range Region, Southern Tibet

Andrew Laskowski, University of Arizona
Paul Kapp, University of Arizona

The Lopu Range, located ~600 km W of Lhasa in southern Tibet, exposes a continental high-pressure (HP) metamorphic complex structurally beneath India-Asia (Yarlung) suture zone assemblages. New geologic mapping, 14 detrital U-Pb zircon (n=1895 ages), 11 igneous U-Pb zircon, and nine zircon (U-Th)/He samples reveal the structure, age, provenance, and time-temperature histories of Lopu Range rocks. From N to S, the region exposes Eocene Gangdese arc rocks, the Oligocene—Miocene Kailas Formation, U. Cretaceous—Paleogene forearc strata, ophiolitic and sedimentary-matrix mélange, and metasedimentary rocks of Indian passive margin (Tethyan) affinity. In the range core, Cambrian high-grade and Permian low-grade meta-Tethyan rocks are in the footwall of a domal, top-N shear zone below mélange—similar to other continental HP metamorphic complexes in the NW Himalaya.
A depositional contact between basal forearc strata (MDA 97 ± 1 Ma) and ophiolitic mélange indicates ophiolite formation in a forearc setting prior to L. Cretaceous time. A hbl-plag-ep paragneiss block in ophiolitic mélange along the top-N, S-dipping Great Counter thrust (GCT) was deformed during L. Jurassic—E. Cretaceous time and cooled below ~180 C by ~110 Ma based on U-Pb and zircon (U-Th)/He data. We interpret this sample to record Lhasa terrane—Neo Tethyan subduction initiation followed by forearc extension. Five Gangdese arc granitoids yielded U-Pb ages between 49-37 Ma. They intrude subduction-accretion mélange, and may record southward trench migration and/or steep subduction following collision initiation. The GCT cuts older suture zone structures, placing forearc on Kailas Fm. and mélange on forearc during the latest stage of Kailas Fm. deposition. The ~N-S oriented Lopukangri and Rujiao faults bound the range, together comprising a horst that cuts the GCT. The faults initiated by ~16 Ma based on a U-Pb zircon age of a dike that cuts a footwall splay of the Lopukangri fault and are likely related to Miocene—recent orogen-parallel extension. Five other leucogranite U-Pb ages between 17-15 Ma record crustal melting during extension onset. Seven zircon (U-Th)/He ages from the horst block record 12-6 Ma cooling below ~180 C. The tectonic evolution of the Yarlung suture in the Lopu Range region can be explained by cycles of slab rollback, breakoff and underthrusting during Cretaceous-Paleogene time, suggesting that slab dynamics played an important role in the Himalayan—Tibetan orogen.

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