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.