The effects of differential uplift and sediment supply on major Himalayan river systems at the mountain front
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Abstract:
It is well documented that in tectonically active regions, fluvial morphology responds to changes in base level. Vertical incision rates are adjusted through changes in channel morphology to balance imposed rates of rock uplift. It is common for responses in channel width, slope, grain size distribution and stream power to reflect spatial and temporal changes in rock uplift rates. Channels within tectonically active orogenic belts, such as the Himalayas, may be influenced by changes in discharge or sediment supply in addition to tectonic controls. Identifying the causes of morphological response in such areas is therefore of first order importance to enhance our understanding of landscape evolution. The Nepalese Himalayan foreland presents the ideal location to undertake this investigation, with its distinct tectonic frameworks heavily influenced by the style of foreland basin development. Thin-skinned thrust faulting over the past 1.6 Ma, has facilitated the recycling of foreland basin fill into hanging wall deposits of the frontal thrust (HFT), producing topographic entities now recognised as the Siwalik Hills. Above weak basal decollements, Dun valleys separate the frontal Siwalik Hills, and have rapidly filled with erosional detritus from the rising Himalaya. Poorly consolidated lithologies within the Siwalik Hills and Dun valleys are now being remobilised by modern incision of Himalayan River systems. It is unknown whether patterns of sediment storage and release within the foreland affect river morphology.
To understand the controls behind Himalayan river morphology, longitudinal profiles and channel slope were extracted from 90 m digital elevation models along the Gandak and Kosi Rivers about the Himalayan mountain front. Remotely sensed channel width measurements have also been made, and further supplemented with grain size data derived in the field and analysed using photo sieving techniques. Short-lived increases in channel slope are noted at the Main Boundary Thrust and Main Dun Thrust (MDT) of both rivers, in addition to a decrease in slope upstream of the HFT and Kosi Main Central Thrust (MCT). Increases in channel width upstream of the HFT and MCT (Kosi) are also consistent with morphological response to tectonic uplift. Where characteristic responses in morphology are absent at identified tectonic structures, it is likely that changes in lithology or anthropogenic modification of flow have overwhelmed tectonic influences. No increase in grain size upstream of recognized fault locations was noted on the Gandak, and it is proposed that an alternative mechanism dictates grain size patterns at the mountain front. On passing downstream of the MDT, an absence of direct hill slope inputs and a greater proportion of seasonal tributary inputs is reflected by a narrowing of grain size distributions, loss of Greater Himalayan lithologies, and decrease in D84 (by 50 mm).
These differences between geometry and grain size of the Gandak and Kosi Rivers are interpreted in terms of the style of foreland basin evolution. A lack of foreland accommodation above the strong basal decollement of the Kosi River facilitates continuous exportation of erosional detritus out of the mountain front. Widely spread D84 grain size distributions along the Kosi are dominated by regular inputs of coarse hill slope material from unstable relief produced by exceptional rates of uplift, above closely spaced frontal tectonic structures. It is interpreted that the weak basal decollement characterising the Gandak region has produced more stable hillslopes and accommodation for sediment within the Chitwan Dun. The fine grained and well sorted grain-size distributions of the Gandak noted between the MDT and HFT reflect an absence of direct hill slope inputs and a presence of seasonal tributary derived material. This study concludes that active tectonic structures strongly influence channel geometry at the mountain front. Grain size patterns are believed independent to differential uplift, and are considered a function of lateral sediment inputs, the nature of which reflects the structural evolution and tectonic history of the foreland basin.Keywords:
Main Central Thrust
Stream power
Tectonic uplift
Lithology
River morphology
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Alluvial fan
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Main Central Thrust
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Anticline
Orogeny
Isostasy
Ravine
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Landform
Anticline
Sinuosity
Main Central Thrust
Alluvial fan
Thrust fault
River terraces
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This paper aims to unveil neotectonic imprints in topography, drainage and sediments in the 46.25 km long course of the River Chel from its source down to its alluvial fan at the base of the Himalayan Mountain Front in the Darjeeling–Jalpaiguri districts of India. A semi-circular ridge delimits its primary catchment. Within confinement of this watershed basin the drainage pattern is composite being convergent along the periphery and divergent on a butte inside. All these geomorphic neotectonic imprints are accompanied by ramp and flat structures and spectacular mylonitization of rocks. High hypsometric index and convex shape of the hypsometric curve derived from the central near-straight course of the river between the primary catchment and the Main Frontal Thrust (MFT) also reflects tectonic youthfulness of the river course. It is well manifested also in widely variable stream index and stream gradient index ratios (SL/K) often exceeding 2. In response to neotectonism, this river course as a whole shifted westward between 1962 and 2007. Maximum reduction of the stream gradient on top of the MFT is eloquent enough about recent uplift of the thrust ridge. The high average slope gradient of canyon wall about 45.68° is well consistent with this uplift. Very low channel-width/valley-height ratio along the river further corroborates the uplift. The alluvial fan system of the River Chel is comprised of five morphogenetic fans stacked one above another with a tendency to shrink and shift progressively upslope. They differ from each other in terms of tilt, axial orientation, primary depositional surface gradient and convexity in transverse section and thus present a writ of ongoing tectonism. Progressive upward increase in the share of distal crystalline rocks in clast composition within alluvial fan package is a clear proxy for southerly advancement of the MFT. Concomitant increase in maximum clast size is in good agreement with sediment source uplift. All the five fans are, however, dormant now. Present-day River Chel deeply incises through all of them and suggests further basement uplift in the context of frequent evidences of neotectonism all around, although the role of climate remains uncertain in absence of adequate data.
Alluvial fan
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Tectonic uplift
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The pattern of fluvial incision across the Himalayas of central Nepal is estimated from the distribution of Holocene and Pleistocene terraces and from the geometry of modern channels along major rivers draining across the range. The terraces provide good constraints on incision rates across the Himalayan frontal folds (Sub‐Himalaya or Siwaliks Hills) where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces. Farther north and upstream, in the Lesser Himalaya, prominent fill terraces were deposited, probably during the late Pleistocene, and were subsequently incised. The amount of bedrock incision beneath the fill deposits is generally small, suggesting a slow rate of fluvial incision in the Lesser Himalaya. The terrace record is lost in the high range where the rivers are cutting steep gorges. To complement the terrace study, fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate. This approach allows quantification of the effect of variations in channel slope, width, and discharge on the incision rate of a river; the determination of incision rates requires an additional lithological calibration. The two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared. In the Sub‐Himalaya, river incision is rapid, with values up to 10–15 mm/yr. It does not exceed a few millimeters per year in the Lesser Himalaya, and rises abruptly at the front of the high range to reach values of ∼4–8 mm/yr within a 50‐km‐wide zone that coincides with the position of the highest Himalayan peaks. Sediment yield derived from the measurement of suspended load in Himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range. The observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat‐ramp‐flat geometry of the Main Himalayan Thrust fault. The morphology of the range reflects a dynamic equilibrium between present‐day tectonics and surface processes. The sharp relief together with the high uplift rates in the Higher Himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the Tibetan Plateau driven by late Cenozoic climate change, or late Miocene reactivation of the Main Central Thrust.
River terraces
Bedrock
Terrace (agriculture)
Anticline
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Lithology
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The location and magnitude of Himalayan tectonic activity has been debated for decades, and several aspects remain unknown. For instance, the spatial distribution of crustal shortening that ultimately sustains Himalayan topography and the activity of major fault zones remain unknown at Ma timescales. In this study, we address the spatial deformation pattern in the data-scarce western Himalaya. We calculated catchment averaged, normalized river-steepness indices of non-glaciated drainage basins with tributary catchment areas between 5 and 200 km2 (n = 2138). We analyzed the spatial distribution of the relative change of river steepness both along and across strike to gain information about the regional distribution of differential uplift pattern and relate this to the activity of distinctive fault segments. For our study area, we observe a positive correlation of averaged ksn values with long-term exhumation rates derived from previously published thermochronologic datasets combined with thermal modeling as well as with millennial timescale denudation rates based on cosmogenic nuclide dating. Our results indicate three tectono-geomorphic segments with distinctive landscape morphology, structural architecture, and fault geometry along the western Himalaya: Garhwal-Sutlej, Chamba, and Kashmir Himalaya (from east to west). Moreover, our data recognize distinctive fault segments showing varying thrust activity along strike of the Main Frontal Thrust, the Main Boundary Thrust, and in the vicinity of the steep topographic transition between the Lesser and Greater Himalaya. In this region, we relate out-of-sequence deformation along major basement thrust ramps, such as the Munsiari Thrust with deformation along a mid-crustal ramp along the basal décollement. We suggest that during the Quaternary, all major fault zones in the Western Himalaya experienced out-of-sequence faulting and have accommodated some portion of crustal shortening.
Denudation
Main Central Thrust
Thrust fault
Basement
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ABSTRACT This article investigates landscape characteristics and sediment composition in the western Greater Caucasus by using multiple methods at different timescales. Our ultimate goal is to compare short‐term versus long‐term trends in erosional processes and to reconstruct spatio‐temporal changes in sediment fluxes as controlled by partitioning of crustal shortening and rock uplift in the orogenic belt. Areas of active recent uplift are assessed by quantitative geomorphological techniques [digital elevation model (DEM) analysis of stream profiles and their deviation from equilibrium] and compared with regions of rapid exhumation over longer time intervals as previously determined by fission‐track and cosmogenic‐nuclide analyses. Complementary information from petrographic and heavy‐mineral analyses of modern sands and ancient sandstones is used to evaluate erosion integrated throughout the history of the orogen. River catchments displaying the highest relief, as shown by channel‐steepness indices, correspond with the areas of most rapid exhumation as outlined by thermochronological data. The region of high stream gradients is spatially associated with the highest topography around Mount Elbrus, where sedimentary cover strata have long been completely eroded and river sediments display the highest metamorphic indices and generally high heavy‐mineral concentrations. This study reinforces the suggestion that the bedrock–channel network can reveal much of the evolution of tectonically active landscapes, and implies that the controls on channel gradient ultimately dictate the topography and the relief along the Greater Caucasus. Our integrated datasets, obtained during a decade of continuing research, display a general agreement and regularity of erosion patterns through time, and consistently indicate westward decreasing rates of erosional unroofing from the central part of the range to the Black Sea. Copyright © 2014 John Wiley & Sons, Ltd.
Denudation
Heavy mineral
Cosmogenic nuclide
Bedrock
Fission track dating
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