Tectonic Accretion Controls Erosional Cyclicity in the Himalaya

Abstract

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.

Tectonic Controls on Himalayan Denudation?

“…The envelope of accumulation rates that are determinable from Mandal et al's magnetostratigraphic record does expand at time scales less than 1.3 million years, 10.1029/2021AV000539 2 of 3 as would be expected for a strong periodicity among hiatuses in the stratigraphic section itself (Sadler & Strauss, 1990). Mandal et al (2021) identify fluctuations in erosion rate that approach long timescales at which several interrelated environmental forcings might be dominant: Milankovitch grand cycles of climate change, source area uplift, and the tectonic accommodation of resulting sediments in a non-marine setting. In the proximal Himalayan foreland, the authors apply cosmogenic 10 Be analysis and magnetostratigraphy for independent dating of locally unsteady, upstream erosion and downstream accommodation rate, respectively.…”

“…More recently, Mandal et al. (2021) analyzed cosmogenic 10 Be concentrations in foreland sediments from the north‐western Indian Himalaya dating back 6 Ma. Their detailed erosion record suggests an approximately one‐million‐year periodicity in erosion rates with a gradual increase towards the present.…”

“…To minimize environmental signal buffering associated with complex sediment routing systems, Mandal et al. (2021) target a proximal foreland basin that is, intimately linked to the eroding hinterland. The high erosion rates characterizing tectonically active mountain ranges (Codilean et al., 2018) mean that cosmogenic 10 Be response times to changes in erosion rates are short in these settings.…”

“…It is impossible to unravel the complex histories of sediment grains as they travel from the erosional source areas to the depositional sinks from the abundance of a single cosmogenic nuclide (e.g., 10 Be). To minimize environmental signal buffering associated with complex sediment routing systems, Mandal et al (2021) target a proximal foreland basin that is, intimately linked to the eroding hinterland. The high erosion rates characterizing tectonically active mountain ranges (Codilean et al, 2018) mean that cosmogenic 10 Be response times to changes in erosion rates are short in these settings.…”

A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif

“…The physical and thermal evolution of mountain belts is governed by the interplay between tectonic deformation, the production and transfer of heat, crustal metamorphism, and surface denudation. The extent to which surficial processes (e.g., climate and erosion) can modulate deep crustal metamorphic and tectonic processes is poorly known (1)(2)(3). The places on Earth where surface denudation is most likely to exert control on deeper crustal processes are the syntaxial massifs of the Himalaya-the Namche Barwa massif (NBM) in the east and the Nanga Parbat massif (NPM) in the west (Fig.…”