Burbank 9627865 Traditional methods of defining hinterland deformation and associated denudation typically have involved site-by-site calibration of bedrock cooling histories and/or P-T-t paths. Results from a few sites are then sometimes extrapolated regionally in order to speculate on broader deformational/denudational histories. The ability to date individual detrital minerals has provided a new technique to calibrate deformation and erosion, whereby ages are generated for populations of detrital minerals which were collected from strata of known age in a basin adjacent to a range. The measured temporal difference between the time of cooling below an annealing or closure temperature and the time of deposition can be interpreted as representing the time required to uplift a mineral through its annealing/closure isotherm to the erosional surface, transport it across the cooling and erosional history from an entire drainage basin. Recent Himalayan studies have shown that some grains in nearly every sample have experienced very rapid cooling, erosion, transport, and deposition (often <1-2 My from cooling to deposition). At present, the interpretation of detrital mineral studies is strongly limited by numerous uncertainties. The detrital record integrates age information from all tributary basins. Often the drainage catchment contributing to a given site in the past is unknown, changing through time, or poorly defined. The relative importance of point sources (perhaps rapidly uplifting and denuding) within a catchment is unknown. The impact of topographic and climatic variability on the total detrital contribution for any given catchment is unknown. These uncertainties permit detrital age data to be reliably analyzed only in broad, generalized terms. If the relationships among spatial contributions, and downstream mixing were known, much more useful information could potentially be extracted from detrital age populations. We propose a baseline calibration of age distributions of detrital minerals and related topography, geomorphology, and climate in order to understand the interactions among factors which control the detrital age populations as observable today. We will study a Himalayan drainage that extends from the modern foreland to the Tibetan drainage divide and that includes areas with strong climatic and topographic contrasts, areas for which marked differences in bedrock uplift rates have been inferred or observed, and areas in which the bedrock geology is quite well known and dated. The catchment is largely covered by a 90-m digital elevation data set that provides a basis for analysis of topographic effects. Through documentation of the changing detrital age population along the river course and the contributions from tributary catchments, we will assess the dependence of the detrital signal on bedrock ages, inferred or defined bedrock uplift or erosion rates, topographic characteristics, and climatic influences. This calibration will allow us to assess the key controls exerted by these interacting factors in a tectonically dynamic landscape and will form a conceptual basis for improved interpretation of subsequent detrital mineral age studies. Moreover, this integration should permit us to evaluate the potential for utilizing detrital mineral ages to extract relative bedrock erosion rates: a key problem in geomorphic studies. Given the global climatic effects attributed to Himalayan and Tibetan uplift, as well as tectonic significance of the Himalayan collision, a firmer basis for interpreting integrated uplift and erosion histories is clearly needed.
