Integrated tectonic and quantitative thermochronometric investigation of the Xainza rift, Tibet

The Himalayan Tibetan orogeny is a superlative in many respects and has drawn a lot of attention because of its unrivaled landscape and geologic attractiveness. Many processes including ocean-continent and continent-continent collision, mountain building, plateau uplift, and E-W extension can be studied. This study utilizes a variety of different techniques to improve understanding of the history of the Lhasa terrane from its collision with the Qiangtang terrane to the north, subsequent amalgamation of the Indian subcontinent to the south, and late-stage extensional tectonics.;The Xainza rift in the central Lhasa terrane, an about 200 km long N-S trending structure, provides access to deeper crustal rocks enabling the study of magmatic evolution as well as timing and magnitude of footwall uplift during E-W extension. Zircon U/Pb dating reveals three distinct stages of magmatism at ~140-110 Ma, ~65-50 Ma, and ~15 Ma. The Cretaceous magmatism is triggered by southward subduction of the Bangong ocean slab whereas early Tertiary rocks are emplaced as a result of northward subduction of the Neo-Tethyan slab. The Miocene magmatic rocks result from additional heat influx following delamination of an over-thickened Lhasa lithosphere and show signs of significant assimilation of surrounding early Tertiary plutons. Whole rock geochemistry reveals that the Lhasa terrane has ancient and thicker crust in its interior and more juvenile crust going outward, which has a first order effect on the observed isotopic ratios. Metamorphosed basaltic melts, under-plated during the early Tertiary, play a major role in the observed elemental patterns (high La/Yb and Sr/Y ratios) in post-collisional rocks.;Low-temperature thermochronology results from vertical transects as well as single samples, reveal that E-W extension initiated in the middle Miocene. Rift morphology combined with decreasing apatite (U-Th)/He ages from north to south support the proposed model of zipper-like opening of the rift triggered by right-lateral slip on the Gyaring Co fault. Major phase of rift shoulder uplift is constrained at ~12-10 Ma in the north and ~8 Ma in the southernmost segment. These dates coincide with the waning stages of south-directed thrusting along major faults in the Himalayas, suggesting a causal relationship between N-S shortening and E-W extension.;Thermal history modeling of (U-Th)/He data is a critical component of this study and the results are based on improved analysis of these data utilizing a newly developed software package. Modeling samples from vertical transects together provides superior control over the time-temperature evolution of the sampled crustal sections allowing for better constraint estimates of initiation and magnitude of rifting.