Quantifying sand provenance and erosion (Marsyandi River, Nepal Himalaya)

We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of theMCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100±3400 t/ km and 5.1±1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6±0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of theGreater Himalaya, where rainfall decreases rapidly to b2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4± 0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to b0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion ⁎ Corresponding author. Tel.: +39 2 64482088; fax: +139 2 64484273. E-mail addresses: [email protected] (E. Garzanti), [email protected] (G. Vezzoli), [email protected] (S. Andò), [email protected] (J. Lavé), [email protected] (M. Attal), [email protected] (C. France-Lanord), [email protected] (P. DeCelles). 1 Tel.: +39 2 64482088; fax: +139 2 64484273. 2 Now at Institute of Earth Science, School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland. Tel.: +33 0476635919; fax: +33 0476514058. 3 Tel.: +33 383 594220: fax: +33 383 511798. 4 Tel.: +1 520 6214910: fax: +1 520 6212672. 0012-821X/$ see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.04.010 501 E. Garzanti et al. / Earth and Planetary Science Letters 258 (2007) 500–515 and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the central Nepal eight-thousanders may be viewed as topographic anomalies in cold desert climate at the southern edge of the Tibetan rain shadow. © 2007 Elsevier B.V. All rights reserved.