EXTRACTING THE UNROOFING HISTORY FROM FORELAND BASIN SEDIMENTS

What data can be extracted and exploited from the foreland basin sediments to determine the tectonic history of a mountain belt? The Himalaya provide a natural laboratory to address this question because of the rich archive of the tectonic history stored in the foreland basin. As a mountain belt erodes, sediment is transported and deposited in the foreland basin. Thus, the sedimentary archive reveals what was at the surface in the mountain belt at any given time. However, note that when data are collected along a transect in the sedimentary archive, one profile is extracted in a huge river system. That one profile may be a unique snapshot of that river, not repeatable a short distance offset from the profile. The collected samples are not useful until put into context with the age of the rocks. Coupled measured sections and magnetostratigraphy bracket the age of the rock units. Once the age is known, geochemical, geochronologic or mineralogic tracers can be used to determine what kind of rock was at the surface provided that the tracer being used is distinct in the rock units of the mountain belt (the provenance of the foreland basin sediments). Several tracers are distinct in the rock units that build the Himalaya to provide an unroofing history – petrography and dense minerals, mineral geochemistry, isotopic dating of detrital minerals (Ar-Ar dating, U-Th-Pb dating, fission track dating) and using distinct isotopic values (Sm-Nd, Rb-Sr, Pb). In the Himalaya, two isotopic tracers are particularly useful – Nd isotopic signatures and U-Pb detrital zircon ages. Shifts in the values and ages in the Siwalik Group, the foreland basin, indicate that different parts of the mountain belt were exposed at different time periods. These values and ages derived from the foreland basin, which are tied to different rock types, indicate emplacement of thrust sheets, growth of a duplex, motion over buried ramps and first motion on particular faults. Using these data in conjunction with cross-section reconstructions as well as exhumation data allows a first-order visualization of how the mountain belt eroded through time. Reconstructions using data from the foreland basin sediments and reconstructions in western Nepal reveal the evolution of the Himalaya thrust belt.