Earth system interactions across mountain belts : a case study of the Himalaya

Abstract

This thesis is comprised of three studies that together enhance our understanding of the influence of mantle flow on subduction dynamics, landscape evolution, and biogeographical patterns in the largest mountain range in the world, a region that is one of the world's most significant biodiversity hotspots.

The first study explores the role of mantle drag at convergent boundaries, where oceanic subduction is followed by continental collision. Through seven three- dimensional analogue models, the study investigates the effects of horizontal mantle flow velocity and the length of a subducting oceanic tectonic plate preceding continental collision on the deformation of the overriding continental plate and subduction dynamics. Results indicate that mantle drag, induced by unidirectional horizontal mantle flow perpendicular to the trench, profoundly affects the depth of Indian oceanic slab subduction and deformation patterns of the overriding plate. Furthermore, the initial geometric configuration of the convergent margin, particularly the relative lengths of the subducting plates, is found to be crucial in dictating subduction efficiency and the degree of deformation experienced by the overriding continental plate.

The second part of this thesis addresses the influence of dynamic topography, driven by mantle flow, on the surface processes and landscape evolution of tectonically active regions, with a particular focus on the Himalaya. Through numerical landscape evolution modeling, this research illuminates the mechanisms behind the observed course reversal of the Yarlung-Indus river network, a phenomenon attributed to dynamic uplift in response to mantle flow-driven Indian tectonic slab break off between ~ 26 and ~ 12 million years ago. The study identifies dynamic topography, with a wavelength of ~ 600 – 950 km and an uplift rate of ~ 3 – 5 mm/year, as a plausible explanation for the drainage divide relocation observed in the Himalaya. This finding significantly contributes to our understanding of how mantle processes influence surface topography and fluvial systems in tectonically active landscapes which has not been considered before.

The final study investigates the biotic assembly of the Himalaya. Performing a cross-taxonomic biogeographic analysis of species from ten major groups, this research integrates the present day geographical distributions of ~ 3,333 species with large-scale phylogenies to infer the timing and origin of Himalayan biota. Whilst the study demonstrates that the Himalayan mountains have acted as a species attractor of immigration in all groups of organisms included in this study, the geological hypothesis of progressive Himalayan uplift cannot exclusively provide a mechanism for Himalayan biotic assembly of most groups of organisms in this study, apart from snakes. Collectively, the studies in this thesis provide a multidisciplinary perspective on the geodynamic, geological, geomorphological, hydrological, and ecological dynamics of the Himalaya, offering insights into the complex interactions between geophysical processes and biodiversity in tectonically active regions.