Tectonic Evolution of the Yarlung Suture Zone, Lopu Range Region, Southern Tibet
The Lopu Range, located ~600 km west of Lhasa, exposes a continental high-pressure metamorphic complex beneath India-Asia (Yarlung) suture zone assemblages. 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. A hornblende-plagioclase-epidote paragneiss block in ophiolitic mélange, deposited during Middle Jurassic time, records Late Jurassic or Early Cretaceous subduction initiation followed by Early Cretaceous forearc extension. A depositional contact between forearc strata (MDA 97 ± 1 Ma) and ophiolitic mélange indicates that the ophiolites were in a suprasubduction-zone position prior to Late Cretaceous time. Five Gangdese arc granitoids that intrude subduction-accretion mélange yield U-Pb ages between 49-37 Ma, recording Eocene southward trench migration after collision initiation. The south-dipping Great Counter thrust system cuts older suture zone structures, placing forearc strata on the Kailas Formation, and sedimentary-matrix mélange on forearc strata during early Miocene time. The north-south, range-bounding Lopukangri and Rujiao faults comprise a horst that cuts the Great Counter thrust system, recording the early Miocene (~16 Ma) transition from north-south contraction to orogen-parallel (E-W) extension. Five early Miocene (17-15 Ma) U-Pb ages from leucogranite dikes and plutons record crustal melting during extension onset. Seven zircon (U-Th)/He ages from the horst block record 12-6 Ma tectonic exhumation. Jurassic—Eocene Yarlung suture zone tectonics, characterized by alternating episodes of contraction and extension, can be explained by cycles of slab rollback, breakoff and shallow underthrusting—suggesting that subduction dynamics controlled deformation.