No abstract

Provenance analysis of the Sub-Himalayan Late Miocene-Pleistocene foreland basin deposits (Siwaliks) from the Dehradun reentrant area provides a 10-Myr long record of the denudation history and tectonic evolution of the northwestern Indian Himalaya. We studied Siwalik sediments exposed along the Mohand-Rao and Haripur-Khol sections, using detrital zircon U-Pb geochronology, major and trace elements, and Sr-Nd isotope geochemistry. Results suggest that the erosion pattern has been relatively stable since the Late Miocene with sediments derived from the Tethyan Himalayan (THS), Greater Himalayan (GHS), and outer-(oLHS) and inner-Lesser Himalayan (iLHS) sequences. Provenance data indicate that erosional unroofing of the Lesser Himalayan Crystalline sequences (LHCS) initiated around 6 Ma, possibly related to out-of-sequence movement of the Ramgarh-Munsiari Thrust. Our data also suggest erosional recycling of older foreland basin deposits into younger Siwaliks since~5.5 Ma, which may indicate the time of thrust propagation from the Lesser Himalaya into the foreland basin. While the iLHS has been exposed to erosion since at least~10 Ma, the Siwaliks were dominated by materials derived from the GHS and THS sources. We interpret these results as an indication that tectonic uplift and erosion of the orogenic wedge occurred in response to duplexing of the iLHS and concomitant high topography and rock uplift rate in the Greater and Tethyan Himalaya. Comparing the provenance of the Siwalik sediments with that of the modern Ganga and Yamuna river sediments further indicates that deposition during the Late Cenozoic was most likely accomplished by southward flowing transverse Himalayan rivers, analogous to the modern ones.

The evolution of Earth's climate over geological timescales is linked to surface erosion via weathering of silicate minerals and burial of organic carbon. However, methodological difficulties in reconstructing erosion rates through time and feedbacks among tectonics, climate, and erosion spurred an ongoing debate on mountain erosion sensitivity to tectonic and climate forcing. At the heart of this debate is the question of whether late Cenozoic climate cooling has increased global erosion rates or not. The Himalaya plays a prominent role in this debate as its erosion produces a large fraction of global sediments delivered to ocean basins. We report a 6‐Myr‐long record of normalBnormale10‐derived erosion rates from the north‐western Himalaya, which indicates that erosion rates in this region varied quasi‐cyclically with a period of ∼1 Myr and increased gradually toward the present. We hypothesize that the observed pattern of erosion rates occurred in response to the tectonic growth of the Himalaya by punctuated basal and frontal accretion of rocks from the underthrusting Indian plate and concomitant changes in topography. In this scenario, basal accretion episodically changes rock‐uplift patterns, which brings landscapes out of equilibrium and results in quasi‐cyclic variations in erosion rates. We used numerical landscape evolution simulations to demonstrate that this hypothesis is physically plausible. We attribute the long‐term increase in erosion rates to the erosional response of topography due to frequent basal accretion relative to frontal accretion. Because tectonic accretion processes are inherent to collisional orogenesis, they likely confound climatic interpretations of erosion rate histories.

The NNW Iranian Plateau and west Alborz within the Arabia‐Eurasia collision zone are characterized by three main tectono‐stratigraphic zones, crosscut by the Qezel‐Owzan River (QOR) Basin. The interplay between present‐day deformation and climate, which control the landscape evolution of the region, is still poorly constrained. We addressed this gap by measuring millennial‐scale erosion rates from 10Be‐concentration in the QOR sands along with topographic/climatic metrics analyses. Results reveal low erosion rates in the Plateau and relatively high in the west Alborz. The regional consistency of topographic parameters with geomorphology suggests that they control sediment fluxes in the Plateau, while the surface uplift, active thrust‐faulting, and shallow crustal seismicity in the west Alborz are the main controlling factors. Climate has a secondary role on erosion rates. Furthermore, we calculated exhumation rates from published thermochronometric AFT/AHe ages to determine their relationship with 10Be short‐term data. Results imply that the exhumation rates increased slightly in the Plateau and west Alborz from ∼26 to ∼10 Ma, simultaneous with hard collision processes between the Arabia‐Eurasia. This trend accelerated from ∼10 to ∼2.8 Ma due to the isolation of the Caspian Sea and extreme base‐level fall. From ∼2.8 to ∼2 Ma, base‐level rise occurred under climate influence, and erosion rates decreased. Millennial‐scale data show the erosion rate decreased from ∼2 Ma to the Present‐day, which is attributed to the change in deformation style and fault kinematics from fold/thrusting to mainly strike‐slip faulting. The significantly lower erosion rates in the Plateau compared to west Alborz suggest a relatively stable plateau surface.

Quantifying bedrock cooling history is crucial for understanding the long‐term landform evolution across passive margins and its control onto the sediment routing system. To constrain the low‐temperature cooling history and its relationships to the Phanerozoic tectonic events of southern Peninsular India, we present new apatite (U‐Th‐Sm)/He (AHe) analyses of 39 Precambrian basement samples. The new AHe ages range from 38.1 ± 6.8 to 364.2 ± 44.6 Ma: they are younger than 50 Ma in the Palghat Gap region and older than 200 Ma in the interior of the Deccan Plateau. Thermal modeling based on AHe data indicates enhanced cooling and exhumation in the interior of the Deccan Plateau by Permian‐Triassic times followed by gradual cooling up to the Present. This discrete episode of Permian‐Triassic cooling is associated with continental extension that preceded the Early Jurassic breakup of Gondwana. Bedrock cooling and exhumation on the southeastern and southern limits of the Deccan Plateau was likely accomplished by Late Cretaceous drainage reorganization. The distribution of old (>200 Ma) AHe ages over the >2600 m high Nilgiri Plateau reflects very low erosion/exhumation rates and adds to examples of long‐lived postorogenic topography. The relatively younger AHe ages from the ∼30 km wide low mountain pass (Palghat Gap) within the Western Ghat Mountains attest for intense Cenozoic erosion likely facilitated by the erodible lithological backbone of the Neoproterozoic shear zone. AHe ages across the western coastal plain challenge the widely hold notion of ∼3 km of post‐breakup isostatic rebound in response to erosion of the margin. Instead, the new AHe data are more compatible with less than 1–1.5 km of crustal denudation along the coastal strip.

The morphology of 19 adjacent westward‐flowing and four eastward‐flowing major catchments draining across the Western Ghat escarpment in southern Peninsular India was studied to examine how the channel patterns and their longitudinal profiles reflect the landscape evolution. Field surveys were complemented with the quantitative morphometric analysis of fluvial channel profiles using digital topographic data. Results show distinctive differences between eastward‐ and westward‐flowing drainage systems. The channel profiles of eight westward‐flowing drainage basins show an apparent morphological equilibrium characterized by concave upward shape, whereas most channels from the other westward‐flowing basins display knickpoint(s). All studied eastward‐flowing drainage basins display the morphological signature of disequilibrium in the form of knickpoints. The eastern and western margins of southern Peninsular India have experienced major tectonic events ~120 and ~65 Ma ago, respectively, and are since then tectonically quiescent. All studied westward‐flowing basins share the same Arabian Sea base level, flow over comparable lithologies and developed under similar climatic conditions. The studied eastward‐flowing basins have the Bay of Bengal as base level and similar climatic conditions. Therefore, the spatial variations in catchment morphometry is interpreted as a dynamic response to complex interactions and feedbacks between (i) pre‐existing topography along and across the escarpment margins, and (ii) vigorous drainage piracy in more recent times. We hypothesize that the studied drainage basins have experienced different forcing magnitudes that can be quantified to a first order using the present day topography. Copyright © 2016 John Wiley & Sons, Ltd.

<p>Rivers are one of the most dynamic features on the Earth's surface. Over time, river channels migrate across its floodplain either gradually or rapidly in response to erosion, accretion, sediment transport, or high-water flow events, respectively. These lateral migrations of the river channel could be detrimental to the human settlements, infrastructure, and ecological elements in the floodplain region. Indo-Gangetic plain is the world's largest alluvial tract, drained by rivers such as Ganga, Brahmaputra, Indus, and their tributaries. Most of these rivers are known to be very dynamic and have the potential of affecting a large population. Previous studies focused on individual rivers to understand the spatiotemporal patterns of channel migration. However, regional-scale analysis becomes necessary to understand the large-scale controls on river dynamics and determine their response to future climate change and anthropogenic activities. This study intends to map and measure migration rates of all the major river channels in the Himalayan foreland basin using Landsat imagery from 1990 to 2020. We generated annual active channel binary masks from Landsat imagery using Google Earth Engine. We delineated the centerline of channels and calculated channel migration rates between consecutive years using the RivMAP toolbox in MATLAB. Here we show that the elevation difference between the river channel and its floodplain acts as a spatial constraint and controls the relationship between channel patterns and migration rates. Channel segments with higher elevation differences correspond to less channel movement and vice versa. Additionally, we explore the effects of anthropogenic activities on river dynamics in the study area.</p>

<p>The Late Cenozoic growth of the Himalaya is mainly thought to be a result of basal accretion due to duplexing at the subsurface. However, over geological time, the complex nature of the response of Himalayan topography and erosion rates to the basal accretion along the Main Himalayan Thrust (MHT) fault remains ambiguous. Mandal et al. 2021 hypothesized that the punctuated basal accretion along the MHT brings the landscape out of equilibrium and results in periodic temporal variations in erosion rates. We seek to build on this idea by exploring the growth of the topography and resulting erosion rates due to long-term basal accretion processes along the MHT.  To simulate the changes in topography and consequent variation in precipitation pattern, we are linking an orographic precipitation model (Hergarten & Robl, 2021) to the landscape evolution model used in Mandal et al. 2021. We introduce a migrating zone of high uplift (HUZ) in the model landscape, where the uplift rates are ~5 times greater than the background uplift rate. The orographic precipitation model works by controlling the influx of water in the cells of the model space and subsequently distributing the water volume based on the changes in topography due to overall surface uplift patterns. We calculate the spatially-averaged erosion rates, integrated over the time step length, by considering the uplift rate and the elevation difference between the previous time step and the current, updated elevation grid. In Mandal et al. 2021, feedbacks among the basal accretion-driven rock uplift, river steepening, and erosion rate were observed with the upstream migration of knickpoints and the migration of the ramp over time. With the introduction of the orographic precipitation model, we aim to understand the coupling between duplex-induced growth of the topography and rainfall variation and consequent temporal variability in erosion rates. </p>