Tectonic Accretion Controls Erosional Cyclicity in the Himalaya

Indian Institute of Science Education and Research Kolkata, Mohanpur, India(3);The evolution of Earth’s climate over geological timescales is linked to surface erosion by 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. A key question is 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 cosmogenic 10Be-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 towards 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 model, 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. In addition, we suggest that the long-term increase in erosion rates was likely driven by successive basal accretion and the commensurate topographic growth in the interior of the Himalayan thrust wedge. Because tectonic accretion processes are inherent to collisional orogenesis, they likely confound climatic interpretations of erosion rate histories.