9419086 Lundberg Recent work on the Himalaya suggest that significant crustal shortening, metamorphism, uplift and erosion began well before the mid-Miocene and younger record that has been well documented in the Siwaliks and distal Bengal fan. This project proposes to investigate the poorly-constrained initiation and ensuing history of orogenesis and tectonic sedimentation in the eastern Himalaya, by analyzing detrital geochronology and mineralogy of late Eocene through Recent strata exposed and drilled in Assam and the Bengal basin. These strata represent a missing link in an active clastic wedge that has been used to exemplify orogenic sedimentation and to inspire collisional models, yet they remain virtually unexploited for stratigraphic information on mountain-building and erosional processes. The objectives of this research include (1) determining the onset of rapid unroofing and evaluating temporal/and or regional patterns in rapid unroofing in the eastern Himalayas, (2) distinguishing sediment input from 3 major river systems that join in the Bengal basin, and (3) evaluating changes in sediment source terranes in the major drainages through time. Cooling ages of detrital grains will be constrained by fission-track analysis of zircon, complemented by 40AR/39Ar dating of potassium feldspar from selected horizons in pre-18 Ma strata. Detrital composition will be analyzed by determining sand (stone) modes, especially focusing on lithic fragments, and heavy-mineral populations, complemented by microprobe analyses of tourmalines, garnets, and amphiboles. Sediment input from the major river systems will be documented using an actualistic approach, beginning with modern sands from the Ganges, Brahmaputra, and Meghna (and related) rivers, and working back through Cenozoic sequences to assess changes in source rocks since inception of the Himalaya. The significance of this project lies in the largely unexploited stratigraphic record accessible onland in the eastern Himalayan foredee p, which provides the opportunity to reconstruct orogenesis and surface processes that pre-data strata studied from the Siwaliks and the distal Bengal fan, as well as to evaluate along-strike and downstream changes in sediment composition in the Neogene part of this immense clastic wedge. The ultimate objective of this project is to improve understanding of tectonic and surface processes in a well-constrained modern continental collision. The proposed geochronologic and compositional analyses, complemented by improved stratigraphic control and related ongoing studies, will extend existing data sets through the entire development of the Himalaya, providing ground truth necessary for evaluating models of collisional processes.
