Estimating fault slip rates in the Himalaya and Tibet over 10 - 10^6 year timescales

Abstract

The Himalayan-Tibetan orogen is the highest on the modern Earth and an archetypal region for studying continental collisions. As such, its characteristics have been the basis for many different models for orogenesis, in spite of a lack of data on deformation styles and rates in many parts of the orogen. More data on deformation rates and histories are needed to create, modify or reject hypotheses seeking to explain aspects of this orogeny. This work comprises four studies of deformation over vastly different temporal scales, with a spatial emphasis on the western Himalaya and Tibet. The first study combines global positioning system (GPS) geodesy and structural field observations to study arc-parallel extension and translation of the Himalaya. Arc-parallel extension is estimated at ~3 cm yr-1 over the length of the Himalaya, with the highest rates in eastern Nepal. Arc-parallel translation is expressed as slip on the Karakoram fault and decreases in rate and magnitude from northwest to southeast. Results from this study indicate that a model of variably-oblique convergence between the Indian plate and the Himalaya is likely responsible for the observed deformation, while other models considered fail to match observations. The second study combines field mapping, zircon (U-Th)/He thermochronology, zircon U-Pb geochronology and thermokinematic modeling to determine the deformation history of the previously unstudied South Lunggar Rift in southwestern Tibet. Results indicate that extension started in the middle Miocene and accelerated in the late Miocene to modern horizontal extension rates of 1-3 mm yr-1. Cumulative extension in the rift varies from 3 to 21 km along strike. The third study extends the thermokinematic modeling performed in the South Lunggar Rift to the North Lunggar Rift; key results include a northward propagation in rapid extension that may result from the underthrusting of the Indian plate beneath Tibet. The fourth study is a neotectonic slip rate study on the southeastern Karakoram fault. Mapping results suggest that late Quaternary offsets of geomorphic features may be considerably lower than previously estimated. Slip rate estimates await laboratory results but are likely much lower than earlier estimates, consistent with the oblique convergence hypothesis for Himalayan deformation. These combined results provide much-needed data on deformation rate and style in the orogen and highlight the role of the Indian plate in driving orogenesis.