Shortening and Erosion Across the Indus Valley, NW Himalaya
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Squeezing river catchments Squeezing river catchments through tectonics: Shortening and erosion across the Indus Valley, NW Himalaya H.D. Sinclair1,†, S.M. Mudd1, E. Dingle1, D.E.J. Hobley2, R. Robinson3, and R. Walcott1,4 1School of GeoSciences, The University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP, UK 2Department of Geological Sciences/Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, Colorado 80309-0399, USA 3Department of Earth Sciences & Environmental Sciences, University of St. Andrews, Irvine Building, St. Andrews, KY16 9AL, UK 4Department of Natural Sciences, National Museums Scotland, Chambers Street, Edinburgh, EH1 1JF, UK ABSTRACT lake development. Conglomerates beneath and Molnar, 2001). Similarly, the river catch- some of the modern alluvial fans indicate ments of the Southern Alps of New Zealand are Tectonic displacement of drainage divides a northeastward shift of the Indus River understood to have been deformed to their pres- and the consequent deformation of river channel since ca. 45 ka to its present course ent shape during oblique convergence (Koons, networks during crustal shortening have against the oppo site side of the valley from 1995; Castelltort et al., 2012). Tectonically been proposed for a number of mountain the Stok thrust. The integration of structural, induced changes in catchment shape may be fur- ranges, but never tested. In order to pre- topographic, erosional, and sedimentological ther modified by river capture and progressive serve crustal strain in surface topography, data provides the first demonstration of the migration of drainage divides in response to fac- surface displacements across thrust faults tectonic convergence of drainage divides in tors such as variability in rock strength (Bishop, must be retained without being recovered by a mountain range and yields a model of the 1995), changing river base levels (Mudd and consequent erosion. Quantification of these surface processes involved. Furbish, 2005), and ridge-top glaciation (Dortch competing processes and the implications for et al., 2011a). The competition between tec- catchment topography have not previously INTRODUCTION tonic deformation of river catchments and the been demonstrated. Here, we use structural response of the rivers is highlighted across the mapping combined with dating of terrace The topography of active mountain ranges Himalaya, where all of the big rivers are charac- sediments to measure Quaternary shortening records surface uplift in response to crustal terized by steepened reaches and more localized across the Indus River valley in Ladakh, NW thickening countered by erosion (e.g., Dahlen, knick zones as they respond to variable rock Himalaya. We demonstrate ~0.21 m k.y.–1 of 1990). The horizontal velocities that drive uplift fields (Seeber and Gornitz, 1983; Wobus horizontal displacement since ca. 45 ka on crustal thickening are commonly an order of et al., 2006a). The smaller river catchments the Stok thrust in Ladakh, which defines magnitude higher than the vertical velocities, near the foothills of the Himalaya exhibit vari- the southwestern margin of the Indus Val- and so it is expected that this should be recorded able catchment geometries in response to lateral ley catchment and is the major back thrust by the topography (Pazzaglia and Brandon, advection over thrust ramps (Champel et al., to the Tethyan Himalaya in this region. We 2001; Willett et al., 2001; Miller and Slinger- 2002; Miller et al., 2007). Large-scale catch- use normalized river channel gradients of the land, 2006). Model experiments have indicated ment deformation has broad implications for the tributaries that drain into the Indus River to that the broad asymmetry of many small moun- topographic form of active mountain ranges and show that the lateral continuation of the Stok tain ranges, such as the Southern Alps of New the distribution of erosion and transported sedi- thrust was active for at least 70 km along Zealand, the Pyrenees, and ranges in Taiwan, ment to surrounding sedimentary basins. Any strike. Shortening rates combined with fault may be explained by the horizontal translation modification of catchment shape also has impli- geometries yield vertical displacement rates of deforming rock from the side of the range cations for the scaling of upstream catchment that are compared to time-equivalent ero- dominated by accretion toward the opposing area with channel length and hence the long pro- sion rates in the hanging wall derived from side (Willett et al., 2001; Sinclair et al., 2005; file of rivers (Whipple and Tucker, 1999; Willett published detrital 10Be analyses. The results Herman and Braun, 2006). et al., 2014). demonstrate that vertical displacement rates It is reasonable to suggest that such large- Fluvial erosion into bedrock can be approxi- across the Stok thrust were approximately scale forcing of topography must also play a mated by a power-law relationship between twice that of the time-equivalent erosion role in determining the geometry of river catch- channel slope and river discharge (Howard rates, implying a net horizontal displace- ments and their channel courses. At the largest et al., 1994; Whipple and Tucker, 1999). In this ment of the surface topography, and hence scale, it is proposed that the extraordinarily stream power model, the fault offset generates narrowing of the Indus Valley at ~0.1 m elongate forms of the rivers draining eastern an oversteepened channel reach (knickpoint or k.y.–1. A fill terrace records debris-flow em- Tibet (Salween, Mekong, and Yangtze) repre- knick zone) that migrates upstream as a kine- placement linked to thrust activity, result- sent highly strained forms of previously more matic wave. Additionally, the model predicts ing in damming of the valley and extensive regularly shaped catchments in response to dis- that sustained differential rock uplift across a tributed crustal shortening and rotation around fault will generate increased channel steepness †hugh .sinclair@ ed .ac .uk the eastern corner of the Indian indentor (Hallet (for a given upstream area) on the upthrown GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–15; doi: 10.1130/B31435.1; 11 figures; Data Repository item 2016164.; published online XX Month 2016. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Sinclair et al. block. Analysis of channel steepness has been sedimentological data provides the first quanti- The Indus molasse records sedimentation in a used to assess fault activity in mountain ranges fication of the tectonic convergence of drainage forearc basin that evolved into an intramontane (e.g., Hodges et al., 2004; Kirby and Whipple, divides in a mountain range and yields a model basin following continental collision (Garzanti 2012), with relative rock uplift in the hanging of the surface processes involved. and Van Haver, 1988; Searle et al., 1990; Sin- wall of a thrust fault leading to increased stream clair and Jaffey, 2001). The Ladakh batholith power generated by channel steepening. REGIONAL BACKGROUND forms part of the Gangdese batholith complex Little is known of the interaction between at the boundary between the northern mountains thrust shortening and the consequent deforma- The Indus River of Ladakh flows northwest- of the Himalaya and the Tibetan Plateau. It rep- tion of catchment shape, as opposed to the off- ward (Fig. 2) between the highly deformed resents the magmatic arc prior to continental set of individual channels by faults. As yet, there Cretaceous to Miocene sediments of the Indus collision and consists of a succession of grano- has been no demonstration of the horizontal molasse, which are thrust northeastward against dioritic rocks overlain by a volcanic succession convergence of drainage divides in response to the relatively undeformed Cretaceous and Paleo- that forms the southern wall of the Shyok Valley shortening on a thrust fault that bisects a catch- gene Ladakh batholith complex (Figs. 2 and 3). to the north (Weinberg and Dunlap, 2000). ment. The challenge that this sets is the require- ment to quantify both the shortening and the time-equivalent erosional response. A The objective of this study is to test whether SW NE rates of horizontal displacement across a thrust drainage drainage fault are capable of driving the horizontal con- divide vergence of opposing drainage divides when divide moderated by the erosional response to fault Mean topographic displacement. We examine the Indus River val- slope (α) ley in Ladakh, NW India, which is one of the x x x largest longitudinal river catchments of the x x x Himalaya, with an average width of around x Indus x x x 35 km and a length of ~200 km parallel to the x x x River x x x mountain range in this region. The aim is first to River long x x x x x x x test for the presence of active shortening across x x x profile x the Indus Valley, which has never been demon- x x x x x strated. This is regionally significant because the valley follows the line of the main back thrust in the region, carrying Tethyan Himalaya units northeastward toward the Gangdese batholith (van Haver, 1984; Searle et al., 1990). Large B portions of the Indus and Tsangpo Rivers fur- ther east in the Himalaya also follow this struc- Displacement of thrusted tural feature. Thrust displacement rates were α hanging-wall measured using mapped and dated alluvial and lacustrine terraces, and by documenting dis- V v placement of these terraces across faults. Sec- ond, having presented evidence for Quaternary deformation, we compare the vertical compo- V nent of rock displacement in the hanging wall V of the main back thrust relative to the magnitude β h of erosion at similar time scales (Fig.