Lithospheric Dripping in Central Tibet: Underappreciated Factor in Orogenic Plateau Development?

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The lower part of a continental plate is more dense than underlying asthenosphere. If it gets thickened as a result of plate convergence, such as within the India-Asia collision zone, it may become gravitationally unstable and sink into the deeper mantle as a large sheet or smaller-sized drips. This project will test the hypothesis that 45-20 million year old elliptical-shaped basins in central Tibet formed above sinking lithospheric drips, while mantle-derived volcanic rocks simultaneously erupted along the basin-bounding topographic bulges. If validated, the lithospheric drip theory would provide an appealing mechanism to explain the multitude of enigmatic elliptical basins found in all of Earth’s major orogenic plateaus. In collaboration with the University of Arizona’s Assistant Dean of Military and Veteran Engagement, this project will engage three undergraduate-student veterans in geological field and laboratory research. Military veterans are returning from active duty to college in increasing numbers and are underrepresented in the Science-Technology-Engineering-Math workforce. The effectiveness of the student-veteran engagement program will be assessed, along with which factors influence veteran career pathways, by delivering entrance and exit surveys. The project will also train two graduate students and facilitate continued academic, scientific, and cultural exchange among people and institutions in the United States, China, and Tibet.In addition to the lithospheric drip theory, the project will test the alternative hypotheses that Cenozoic shortening, volcanism, and basin formation in central Tibet are manifestations of (i) southward propagation of regional (600 km long) thrust systems and lithospheric thickening followed by lithospheric root removal or (ii) long-lived (active from Eocene through early Miocene time) ~200-250-km-long thrust belts and associated flexural loading across central Tibet, with unrelated mantle processes generating localized volcanic highlands. The hypotheses make distinct predictions about the temporal-spatial distribution of shortening, age of shortened basin fill relative to the age of adjacent volcanic rocks, basin geometries and the nature of their margins, and sedimentary provenance and dispersal patterns. A combination of geological mapping, basin analysis, structural analysis and restorations, and U-Pb igneous and detrital zircon geochronology will be employed to test these predictions. Field research will include the principal investigators, graduate students, undergraduate student-veterans, and collaborators from the Institute of Tibetan Plateau Research in Beijing, and will focus on Cenozoic geology within and between three basins in central Tibet. The spatial coverage will permit assessment of whether (i) shortening structures are localized to the basins or extend regionally along strike, (ii) the basins were previously integrated as long and narrow E-W or more regional basins, or developed throughout their history as isolated depocenters, and (iii) maximum basin subsidence and shortening were coeval with eruption of adjacent volcanic rocks. Ultimately, insofar as Tibet is the world’s archetypical collisional plateau, project results will have relevance for other orogenic plateaus worldwide.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.