Quantifying the basal accretion-induced erosion rate variability of collisional orogens

Das, Sayan (2022) Quantifying the basal accretion-induced erosion rate variability of collisional orogens. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Sayan Das (17MS147)) 17MS147_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

The Himalaya is a collisional orogen that was formed as a result of the Indian plate started underthrusting the Eurasian plate around 55 million years ago. The subsequent thickening and shortening of the crust lead to the development of a network of discreet thrust faults that makes the processes of frontal and basal accretion dominant in mountain building. However, the Late Cenozoic growth of the Himalaya is mainly attributed to the formation of duplex structure in the Himalayan subsurface due to basal accretion over a mid-crustal ramp. The response of the Himalayan topography to the tectonic control of basal accretion and the resulting variation in erosion rates over geological time remains ambiguous. In this thesis, we build on the hypothesis presented in Mandal et al., 2021, that the cyclic nature of basal accretion along the MHT perturbs the landscape out of equilibrium and results in periodic temporal variations in erosion rate patterns. Furthermore, on studying elevation profiles across the Himalayan range, we can make first order observations regarding the variations in the topography. This indicates that the surface expression of basal accretion is non-uniform in nature. We hypothesize that the dip of the ramp and the subsequent thickness of the accreted sediment layer over it influences the development of surface topography. We run simulations to study the changes in topography and subsequent changes in precipitation pattern by linking the orographic precipitation model proposed by Hergarten and Robl, 2022, to the Duplex Model used in Mandal et al., 2021. A migrating zone of high uplift (HUZ) is introduced in the model generated landscape with uplift rates ~5 times greater than the background uplift rate. The orographic precipitation model distributes the incoming water volume throughout the cells in the model based on the changes in topography due to overall surface uplift patterns. We consider the uplift rate and difference in topography between the current modified grid and the previous grid to calculate the spatially-averaged erosion rates, integrated over the time step length. Various experiments were run involving migration velocity of the ramp, its width and accretionary period taken together with precipitation parameters obtained from studying the Himalayan orographic precipitation. We determined the effect of changes in precipitation pattern over erosion rate to be transient in nature and feedbacks among the basal accretion-driven rock uplift, river steepening, and erosion rate proved to be more prominent.

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