Detrital-mineral thermochronology: Investigations of orogenic denudation in the Himalaya of central Nepal.

摘要

This investigation examines the fundamental processes that determine the distribution of cooling ages observed in detrital minerals eroded from orogenic belts. A detrital cooling-age sample collected from a riverbed represents an integration of information from the upstream area. Within orogenic belts that contain glacial cover and high relief, detrital minerals provide an easy method to sample the range of cooling ages found within a basin. In addition, detrital-mineral thermochronology can be used to extract information from the foreland stratigraphic record, which extends the temporal applicability of the technique beyond traditional bedrock thermochronology. For example, individual mineral grains can be extracted from a stratigraphic horizon and dated. Following correction for the stratigraphic age of the horizon, the detrital mineral ages provide a proxy for the erosion rates contained within the catchment area at the time the rock was deposited. However, before reliable interpretations of the stratigraphic record are made, a modern calibration of the technique was needed.; We investigated the spatial development of a modern cooling-age signal in the Marsyandi valley of central Nepal with muscovite grains dated using 40Ar/39Ar thermochronology. Over 500 individual grains were dated from both the trunk stream and tributaries over a ∼100-km transect along the Marsyandi. These provide a database that displays striking contrasts along the length of the Marsyandi River. The first stage of the investigation focused on the interaction of geological parameters that control the distribution of detrital cooling ages from an individual basin. The range of bedrock cooling ages contained within a catchment is determined by the erosion rate and the depth of the closure isotherm (∼350°C for muscovite). With a 2-D thermal model, we investigated the effects of the vertical erosion rate and topography on the depth of the closure isotherm. Increasing the erosion rate and/or topographic relief decreased the depth of the closure isotherms below valley floors, and re-equilibration following sustained changes in the erosion rate took ∼10 My. Once the range in cooling ages had been determined for a basin, the distribution of detrital cooling ages in sediment at the basin mouth was calculated as a function of catchment hypsometry. This approach was applied to two sub-catchments of the Marsyandi River. The predicted probability distribution of cooling-ages matched the observed data better in the more slowly eroding basin, than in the more rapidly eroding basin.; A more integrated approach was used to predict the spatial distribution of bedrock cooling ages within the 3-D landscape, and the distribution of detrital cooling ages resulting from the erosion of that landscape. A 2-D kinematic-and-thermal model, using the assumption of a single orogen-scale decollement, was developed to predict the depth of the closure isotherm as a function of the ramp geometry and the relative partitioning of convergence between the Indian Plate underthrusting and southern Tibet over-thrusting. The thermal result was extrapolated laterally and combined with a digital elevation model to predict the distribution of bedrock cooling ages. (Abstract shortened by UMI.)