Downstream development of a detrital cooling-age signal: Insights from ⁴⁰Ar/³⁹Ar muscovite thermochronology in the Nepalese Himalaya

Abstract

The character and distribution of cooling ages in modern river sediment provide useful constraints on rates and patterns of uplift and erosion within actively deforming mountain ranges. Such sediment effectively samples all locations within the catchment area, irrespective of remoteness. We evaluate how successfully detrital cooling ages may be used to constrain hinterland erosion rates by examining the modern catchment of the Marsyandi River in central Nepal. Over the 100-200-km-length scale of the catchment, laser fusion 40 Ar/ 39 Ar data for detrital muscovite collected from 12 separate sites illustrate the downstream development of a detrital cooling-age signal that is both systematic and representative of the contributing area. Comparisons of paired samples indicate that, at short spatial (tens of meters) and temporal (hundreds of years) scales, the detrital cooling-age signal is consistent. The distribution of bedrock cooling ages in a subcatchment and the resulting detrital signal at the catchment mouth can be modeled as a function of the erosion rate, relief, hypsometry, catchment area, and muscovite distribution. Given that independent constraints are available for most of these variables, the detrital age signal should be a robust indication of the spatially averaged erosion rate. In the Marsyandi, our model predicts erosion rate differences of approximately twofold, with higher rates (>2 mm/yr) along the southern topographic front of the Himalaya.

Laser 40Ar/39Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

“…Monazites from graphic schists in the Lesser Himalayas revealed a range of early to mid-Miocene age (15.8-11 Ma) [Catlos et al, 2001]. Cooling ages of detrital muscovites from two catchment areas in Nepal also provided age ranges between 11 to about 20 Ma, having the upper ages that drained from the top of the Higher Himalayas and the Manaslu Granite [Brewer et al, 2006]. Fission track and U/Pb studies provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0 ± 1.4 Ma, that is likely related to a combination of tectonic and erosional activity, including movement on MCT and Southern Tibetan Detachment System .…”

“…Both studies have suggested a Himalayan contribution of the detritus. Based on thermochronometry, Brewer et al [2006] show that modern sediment from the Higher Himalayan sequences yield muscovite ages of circa 4-10 Ma, indicating recent erosion rates of ∼2 mm/yr or higher. The present study indicates there was a major influx of sediment to the Bengal basin at circa 16-18 Ma, consistent with the rapid unroofing event deduced by Harris et al [2004] for the Sikkim Himalaya.…”

“…For many Miocene-Pliocene foreland-basin samples of the Siwalik molasse and the Bengal fan, this interval is statistically indistinguishable from zero, indicating that sediment residence times in fluvial systems on the southern flank of the Himalayas are remarkably brief [Copeland and Harrison, 1990]. To the extent that rapid transport is related to high relief in the source region, this phenomenon may indicate a well-developed Himalayan orogenic "front" existed and uplift and unroofing ages may come close to be the depositional ages of the clastic wedges [Hodges et al, 2005;Brewer et al, 2006].…”

Thermochronology in Orogenic Systems

“…Thermochronology of detrital minerals is a valuable tool for the reconstruction of regional patterns of erosion (Carrapa et al, 2009;Garver et al, 1999;Lonergan and Johnson, 1998;Najman et al, 2008;Rahl et al, 2007;Renne et al, 1990;Stuart, 2002;Vermeesch et al, 2006). A relatively recent development has been the use of thermochronologic data for modern stream sediment samples to approximate bedrock cooling age distributions (Huntington andHodges, 2006), paleorelief (McPhillips andBrandon, 2010;Stock and Montgomery, 1996), and erosion rates (Brewer et al, 2003(Brewer et al, , 2006Bullen et al, 2001). Ruhl and Hodges (2005) demonstrated how comparisons of detrital mineral cooling age distributions from modern sediments could be compared with catchment hyposometry to evaluate the evidence for topographic steady state and, if steady-state conditions were indicated, to extract robust estimates of Steady-state temperature structure beneath a periodic topography as calculated using the algorithms of Mancktelow and Grasemann (1997).…”

Focused erosion in the Alps constrained by fission-track ages on detrital apatites

“…Provenance studies based on detrital geochronological techniques are a powerful tool to investigate the present-day pattern of erosion in mountain ranges (e.g. Hurford & Carter 1991;Garver et al 1999;Bernet & Spiegel 2004;Brewer et al 2006). In the Alpine belt, systematic analysis of bulk composition, ranking of metamorphic lithic grains, heavy-mineral assemblages and detrital zircon fission-track (FT) ages suggest that modern river sediments are characterized by specific end-member detrital modes, and they derive from small portions of the drainage where erosion rates are one order of magnitude higher than in surrounding areas (Bernet et al 2001(Bernet et al , 2004aVezzoli 2004;Vezzoli et al 2004;Malusà & Vezzoli 2006).…”