The use of detrital mineral cooling ages to evaluate steady state assumptions in active orogens: An example from the central Nepalese Himalaya

Abstract

The distribution of detrital mineral cooling ages in modern sediments has been proposed as a proxy for long‐term, catchment‐averaged erosion rates in developing orogens. However, the applicability of this potentially valuable tool hinges on restrictive assumptions regarding a catchment's steady state thermal and topographic evolution. In this paper, we outline a method by which these assumptions can be tested through statistical comparisons of cooling age distributions for detrital minerals and the hypsometric curves for their source regions using cumulative synoptic probability density functions. Our approach is illustrated with new detrital muscovite 40Ar/39Ar dates from the Marsyandi River valley, in the central Nepalese Himalaya. One of three studied catchments (Nyadi Khola) showed the strong correlation of hypsometry and cooling ages expected for steady state conditions over the 11 to 2.5 Ma time frame. The pattern of mismatch between hypsometry and cooling age distribution in the other catchments suggests that spatially nonuniform and transient erosional processes may be responsible for departure from steady state. Cooling age distribution comparisons for samples collected from nearby localities, samples collected in different years, and different grain size fractions from the same sample were used to evaluate sampling fidelity over a range of spatial scales (200 to 2590 km2). We found that approximately 50 analyses from a single sediment sample adequately characterize the cooling age signal for tributary catchments with simple erosional histories. However, because of temporally and spatially transient erosion, a specific detrital sample is unlikely to adequately characterize the complex signal in trunk stream sediments that integrate information from several large tributaries.

Evidence for Pleistocene Low-Angle Normal Faulting in the Annapurna-Dhaulagiri Region, Nepal

“…Previous efforts to estimate Late MiocenePleistocene exhumation rates in the Annapurna Range from thermochronometric data have involved direct interpretations of apparent age-elevation profiles Blythe et al 2007), modeling of detrital mineral cooling ages from samples of modern river deposits (Brewer et al 2003(Brewer et al , 2006Ruhl and Hodges 2005;, and one-, two-, and three-dimensional thermokinematic modeling of distributed data, particularly apatite and ZrnFT dates Nadin and Martin 2012). All of these estimates are based on minerals in bedrock or detrital samples from Greater Himalayan Sequence lithologies collected from structural levels most comparable to the collection sites of the Annapurna-Dhumpu footwall samples we studied.…”

Natural experiments in landscape evolution

“…In any event, cosmogenic nuclide analysis and low-temperature thermochronology (e.g. Ruhl and Hodges, 2005) provide tools to confirm or refute the existence of quasi-steady topography.…”

Climate and topography control the size and flux of sediment produced on steep mountain slopes

“…Thus, sediment collected from the creek should have apatite helium ages that reveal the relative contributions of different elevations to the sediment flux at the sampling point (24). In the reference case of uniform sediment production and erosion, each point on the landscape is equally prone to producing a sediment particle and delivering it to the creek (29,30). In that case, the measured age distribution Significance Rivers carve through landscapes using sediment produced on hillslopes by biological, chemical, and physical weathering of underlying bedrock.…”