Collaborative Research: Testing Mechanical Models of Himalayan Orogenesis in NW India

The Himalaya and Tibet stand as the type example of a continent-continent collision, thus the Indo-Asian orogen serves as a paradigm for collisional orogenesis and a testing ground for tectonic models. One overriding tectonic issue is what controls the structural evolution of an orogen. Classic models of thrust development (the critical taper model) do not preclude weak portions of the crust but suggest that these do not work in concert to induce flow; in contrast recent thermal-mechanical models (channel flow models) treat the middle and lower crust as profoundly weak and in concert with erosion dominate metamorphic and structural development. This project, carried out by researchers from the University of Alabama and Boise State University in collaboration with scientists at the Wadia Institute of Himalayan Geology, aims to discriminate between these two opposing views by exploiting the radically different pressure-temperature-time kinematic histories predicted by these models. Specifically, the team will carry out integrated structural, petrologic, stratigraphic, and geochronologic investigations in the Himachal-Uttaranchal in northwestern India to address three basic questions regarding Himalayan thrust transport and tectonic evolution: (1) How do stratigraphic and structural packages correlate along strike from Nepal? Field mapping and structural analysis will permit stratigraphic correlations from Nepal to India and define thrust geometries and shortening amounts via balanced cross sections. Thin section, Nd isotope, and detrital zircon analysis will help define major lithotectonic units, which is critical to accurate placement of different thrust sheets. (2) What was the pressure-temperature-time evolution of the major Himalayan thrusts? Petrologic and chronologic characterization of monazite grains via electron and ion microprobes will be related to the temperature-time evolution of each rock, yielding timing of thrust movement and rates of heating and cooling. New more accurate and precise trace element thermometers are use to determine peak pressure-temperature conditions and distributions and paths. (3) How does the kinematic history in Himachal-Uttaranchal compare with Nepal?

The main purpose of this research is to address a fundamental and controversial question in modern tectonics: Is the overall behavior of the crust during mountain building better explained by the critical taper model or the more recent channel flow model. The critical taper theory argues that a wedge-shaped mass of deformed rocks develops in front of a rigid backstop resulting in forward propagating thrust faults. The channel flow model argues that focused erosion at the mountain range front forces deformation in a weak channel in the mid- to lower crust and partially molten rocks tunnel toward the mountain front. This project also support the research education of graduate and undergraduate students, fosters improved scientific collaborations among institutions both within the United States and in India, and supports an underrepresented group among the geosciences professoriate.