Tectonic Evolution of the Yarlung Suture Zone, Lopu Range and Lazi Regions, Central Southern Tibet

Abstract

The Yarlung (India-Asia) suture zone in southern Tibet records Middle Jurassic—Late Cretaceous development of the Lhasa terrane (Eurasian) convergent margin and subsequent India-Asia collision beginning in Paleocene time. This dissertation reports data from field-based geologic investigation of the Yarlung suture zone in the Lopu Range and Lazi Regions, ~600 and ~300 km west of the city of Lhasa, respectively. Field data were combined with new geochronology (detrital and igneous zircon U-Pb, garnet Lu-Hf), thermochronology (white mica Ar-Ar and zircon U-Th/He), and metamorphic petrology data to develop a tectonic model involving multiple episodes of shallow underthrusting, rollback, and breakoff of both oceanic and continental lithosphere. Switches between extensional and contractional deformation along the Yarlung suture zone appear to be controlled by changes in subduction dynamics. If this tectonic model is representative, then the tectonic process of inter-continental collision is responsible for much larger magnitudes of crustal recycling that previously thought. A hornblende-plagioclase-epidote paragneiss block in ophiolitic mélange, deposited during Middle Jurassic time, records Late Jurassic or Early Cretaceous subduction initiation along the Eurasian margin followed by Early Cretaceous forearc extension. Detrital zircons from Xigaze forearc basin strata deposited unconformably atop ophiolitic mélange produced a maximum depositional age of 97 ± 1 Ma, providing a minimum age for establishment of an arc-forearc-trench convergent margin along the southern Lhasa terrane. Metasedimentary rocks that were originally deposited along the Indian passive margin were subducted beneath the Lhasa terrane to upper-mantle depths, reaching high-pressure (HP), low-temperature conditions (≥1.4 GPa at T≤600 °C). Garnet Lu-Hf geochronology indicates that prograde metamorphism of the Indian metasedimentary rocks was ongoing at 40.4 ± 1.4 Ma while white mica Ar-Ar thermochronology indicates exhumation to mid-crustal depths between 39-34 Ma. Gangdese arc magmatism persisted after the onset of India-Asia collision, producing plutons that intruded sedimentary-matrix mélange of the southern Lhasa terrane subduction-accretion complex between 49-37 Ma. These data suggest steep subduction or southward trench retreat immediately prior to shutdown of arc magmatism along the Yarlung suture zone (37 Ma), shortly after the onset of high-pressure rock exhumation. We interpret that these data record a Paleocene—Eocene episode of southward rollback, breakoff, and underthrusting. During Oligocene—Miocene time, nonmarine strata were deposited along the Yarlung suture zone immediately prior to shortening across a system of out-of-sequence, top-north reverse faults. Based on our data and previous work, we interpret that sedimentation was driven by a second episode of rollback and breakoff of Indian continental lithosphere, whereas subsequent contractional deformation was driven by renewed shallow subduction. Compilation of regional thermochronological data and interpretation of seismic reflection data from previous investigations suggests that the top-north reverse faults comprise a foreland-dipping passive roof duplex above the leading edge of a structurally deeper, hinterland-dipping duplex beneath the southern Lhasa terrane. The Yarlung suture zone switched from north-south contraction to east-west extension by ~16 Ma based on a crosscutting relationship between a leucogranitic dike and a normal fault related to a larger horst structure in the Lopu Range region. Tectonic exhumation in the footwall block of the horst drove cooling through zircon (U-Th)/He closure temperature (~180 °C) between 12-6 Ma.