Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet, yet coarse gravel in these rivers vanishes within approximately 10-40 kilometres on entering the ...Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes reduced channel capacity and increased flood inundation. Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.
Changes in sediment flux to continental margins are commonly interpreted in terms of tectonic growth of topography or climatic change. Here, we show that variations in sediment yield from orogenic ...systems, previously considered as resulting from climate change, drainage reorganisation or mantle processes can be explained by intrinsic mechanisms of mountain belt/foreland basin systems naturally evolving during post‐orogenic decay. Numerical modelling indicates an increase of sediment flux leaving the orogenic system synchronous with the cessation of deposition in the foreland basin and the transition from late syn‐ to post‐orogenesis. Experiments highlight the importance of lithospheric flexure that causes the post‐orogenic isostatic rebound of the foreland basin. Erosion of the rebounding foreland basin combined with continued sediment flux from the thrust wedge drives an acceleration in sediment outflux towards continental margins. Sediment budget records in natural settings such as the Northern Pyrenees or Western European Alps also indicate accelerated post‐orogenic sediment delivery to the Bay of Biscay and Rhône Delta respectively. These intrinsic processes that determine sediment yield to continental margins must be accounted for prior to consideration of additional external tectonic or climatic controls.
The initial period following post‐orogenesis of an orogenic system is characterized by an increase of sediment flux toward continental margins. The mechanisms that drive higher sediment flux are a reduction of accomodation space in the foreland basin and the combination of sediment yield from the range and sediment eroded from the uplifted basin. This evolution is observed for the Western Alp and Rhône Delta system and the Northern Pyrenees and Bay of Biscay system.
The potential link between erosion rates at the Earth's surface and changes in global climate has intrigued geoscientists for decades
because such a coupling has implications for the influence of ...silicate weathering
and organic-carbon burial
on climate and for the role of Quaternary glaciations in landscape evolution
. A global increase in late-Cenozoic erosion rates in response to a cooling, more variable climate has been proposed on the basis of worldwide sedimentation rates
. Other studies have indicated, however, that global erosion rates may have remained steady, suggesting that the reported increases in sediment-accumulation rates are due to preservation biases, depositional hiatuses and varying measurement intervals
. More recently, a global compilation of thermochronology data has been used to infer a nearly twofold increase in the erosion rate in mountainous landscapes over late-Cenozoic times
. It has been contended that this result is free of the biases that affect sedimentary records
, although others have argued that it contains biases related to how thermochronological data are averaged
and to erosion hiatuses in glaciated landscapes
. Here we investigate the 30 locations with reported accelerated erosion during the late Cenozoic
. Our analysis shows that in 23 of these locations, the reported increases are a result of a spatial correlation bias-that is, combining data with disparate exhumation histories, thereby converting spatial erosion-rate variations into temporal increases. In four locations, the increases can be explained by changes in tectonic boundary conditions. In three cases, climatically induced accelerations are recorded, driven by localized glacial valley incision. Our findings suggest that thermochronology data currently have insufficient resolution to assess whether late-Cenozoic climate change affected erosion rates on a global scale. We suggest that a synthesis of local findings that include location-specific information may help to further investigate drivers of global erosion rates.
The timing of formation of the low‐gradient, internally drained landscape of the Tibetan Plateau is fundamental to understanding the evolution of the plateau as a whole. Well‐dated sedimentary ...records of internal drainage of rivers into lakes are used to reveal the timing of this evolution. Here we redate the youngest continental sedimentary successions of central Tibet in the Lunpola Basin and propose a new age range of ca. 35 to 9 Ma, significantly younger than previously thought. We demonstrate long‐standing internal drainage in central Tibet since the late Eocene and stable sedimentary environments, source regions, and low topographic relief since at least the early Miocene. We suggest that sediment aggradation of internal drainage and reduction of hillslope gradients by erosion dominate the formation of low‐relief landscapes and that the late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere.
Plain Language Summary
Internal drainage of rivers into lakes is a characteristic of the high plateaus of the world and, most notably, the Tibetan Plateau. Internal drainage generates local perched base levels for Tibetan rivers, enabling geomorphic isolation from the rapidly incising rivers of the Himalaya and surrounding regions. However, the question of when the low‐relief plateau topography was initiated has been largely ignored, and its formation mechanism is controversial. Here we report a detailed investigation in the Lunpola Basin of central Tibet and propose a new depositional age range of ca. 35–9 Ma. We demonstrate that the internal drainage kept eroding the mountain ranges and filling the surrounding lowlands since at least the late Eocene. By no later than the early Miocene, a gentle landscape formed in central Tibet. The late Cenozoic basins in central Tibet developed in response to deep crustal or mantle flow and associated upper crustal deformation.
Key Points
Robust age constraints of the youngest continental stratigraphic unit from the Lunpola Basin in central Tibet are reported
Aggradation and erosion of internal drainage dominated the formation of low‐relief topography in central Tibet by the early Miocene times
Late Cenozoic drainage basins in central Tibet developed in response to flow in the lower crust and/or mantle lithosphere
Abstract
The Gangetic Plains comprise steep gravelly river channels that transition to low gradient sandy channels 10-40 km downstream of the mountain front. This “gravel-sand transition" is ...characterized by an abrupt greater-than-one-order-of-magnitude drop in both gradient and sediment grain size, suggesting a degree of long-term stability. However, the stratigraphic record of the gravel-sand transition in the Miocene Siwalik Group demonstrates intermittent transport of coarse gravels tens of kilometres downstream of the transition; such events in contemporary channels would drive channel avulsion(s) and increase flood risk, devastating communities across the plains. We combine sedimentological analysis of Siwalik deposits with entrainment calculations which demonstrate that hyperconcentration is required to transport coarse bedload over low-gradient plains. Transport conditions are attainable when intense monsoon precipitation (a 200- to 1000-year event) is combined with increased suspended sediment concentrations in channels. Predicted climate change and ongoing seismicity increase the likelihood of such extreme events within this century.
The transition to a post‐orogenic state in mountain ranges has been identified by a change from active subsidence to isostatic rebound of the foreland basin. However, the nature of the interplay ...between isostatic rebound and sediment supply, and their impact on the topographic evolution of a range and foreland basin during this transition, has not been fully investigated. Here, we use a box model to explore the syn‐ to post‐orogenic evolution of foreland basin/thrust wedge systems. Using a set of parameter values that approximate the northern Pyrenees and the neighbouring Aquitaine foreland basin, we evaluate the controls on sediment drape over the frontal parts of the retro‐wedge following cessation of crustal thickening. Conglomerates preserved at approximately 600‐m elevation, which is ~ 300 m above the present mountain front in the northern Pyrenees are ca. 12 Ma, approximately 10 Myrs younger than the last evidence of crustal thickening in the wedge. Using the model, this post‐orogenic sediment drape is explained by the combination of a sustained, high sediment influx from the range into the basin relative to the efflux out of the basin, combined with cessation of the generation of accommodation space through basin subsidence. Post‐orogenic sediment drape is considered a generic process that is likely to be responsible for elevated low‐gradient surfaces and preserved remnants of continental sedimentation draping the outer margins of the northern Pyrenean thrust wedge.
Schematic representation of a mountain range and foreland basin system for four time frames and highlight continental sediment accumulation that can drape over the frontal portions of the thrust wedge.
This article presents combined stratigraphic, sedimentological, subsidence and provenance data for the Cretaceous–Palaeogene succession from the Zhepure Mountain of southern Tibet. This region ...records the northernmost sedimentation of the Tethyan passive margin of India, and this time interval represents the transition into continental collision with Asia. The uppermost Cretaceous Zhepure Shanpo and Jidula formations record the transition from pelagic into upper slope to delta‐plain environments. The Palaeocene–lower Eocene Zongpu Formation records a carbonate ramp that is overlain by the deep‐water Enba Formation (lower Eocene). The upper part of the Enba Formation records shallowing into a storm‐influenced, outer shelf environment. Detrital zircon U–Pb and Hf isotopic data indicate that the terrigenous strata of the Enba Formation were sourced from the Lhasa terrane. Unconformably overlying the Enba Formation is the Zhaguo Formation comprising fluvial deposits with evidence of recycling from the underlying successions. Backstripped subsidence analysis indicates shallowing during latest Cretaceous‐earliest Palaeocene time (Zhepure Shanpo and Jidula formations) driven by basement uplift, followed by stability (Zongpu Formation) until early Eocene time (Enba Formation) when accelerated subsidence occurred. The provenance, subsidence and stratigraphy suggest that the Enba and Zhaguo formations record foredeep and wedge‐top sedimentation respectively within the early Himalayan foreland basin. The underlying Zongpu Formation is interpreted to record the accumulation of a carbonate ramp at the margin of a submarine forebulge. The precursor tectonic uplift during latest Cretaceous time could either record surface uplift over a mantle plume related to the Réunion hotspot, or an early signal of lithospheric flexure related to oceanic subduction, continental collision or ophiolite obduction. The results indicate that the collision of India with Asia occurred before late Danian (ca. 62 Ma) time.
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