A team of scientists from the University of Arizona and the University of Houston are investigating the nature of the transition from contraction to extension in the Zada basin area of the Himalayan uplift. The Zada basin is the largest late Tertiary basin in the Himalaya and is broadly constrained to be Late Miocene to Pleistocene in age. The basin fill and its underlying basement are spectacularly exposed owing to incision by the Sutlej River, a major southward-flowing trans-Himalayan drainage. The basin currently lies at approximately 3500 m elevation in the hinterland of the Himalayan thrust belt and developed on top of a regionally extensive Early to Middle Miocene thrust system. The thrust system flanks a second major basin, the Kailas basin, which is considered to have developed during mid-Tertiary time, but remains poorly dated. Together, the Zada and Kailas basins archive the collapse of this region from high to relatively low elevation and the transition from erosional denudation to thick sediment accumulation in only a few million years. The multidisciplinary study is using structural analysis, thermobarometry, argon-argon thermochronology, basin analysis, and oxygen isotope paleoaltimetry to place constraints on: (1) geometry, kinematics, and magnitude of slip of the Qusum detachment, South Tibetan Detachment, and Great Counter thrusts; (2) provenance and subsidence history of the Kailas thrust basin and Zada basin; (3) paleoelevation history of the Zada basin; (4) pressure-temperature-time paths recorded in the rocks exhumed by the major faults; and (5) the depth to which the major structures operated. Results are being used to determine the: (1) timing of onset of high-elevation extension in this part of the Himalaya; (2) mode of kinematic accommodation of extension in and around the Zada basin; (3) rates of arc-perpendicular and arc-parallel extension; (4) elevation at which extension began, and the evolution of paleoelevation and paleoenviornments of deposition in the Zada basin in response to ongoing extension; and (5) date at which the paleodrainage pattern in Zada basin become integrated into the headwaters of the modern Sutlej River. By answering these questions, three theories to explain the transition from arc-normal contraction and to arc-parallel extension in major orogenic belts are being tested: (1) extension occurs in regions of overthickened crust; (2) extension is due to outward expansion of the arc-shaped thrust front; or (3) extension is caused by extrusion of the middle crust. The project involves undergraduate and graduate students in the research, collaboration with Chinese scientists, and education and outreach efforts in both the United States and China.
