Dierdre Mallyon

1^st Class Honours

According to the critical taper model, there are three processes that determine the morphology of foreland fold-and-thrust belts: 1) the removal of material by erosion, 2) the accretion of material, determined by sedimentation and plate convergence rates, and 3) steady-state critical growth, determined by the mechanical properties of the materials that comprise the wedge. The Himalayan orogen provides a unique opportunity to study collisional orogens and active fold-and-thrust belts (FTB) in an environment where processes 1 and 2 (above) vary systematically along strike. The Siwalik Group consists of synorogenic sediments of Miocene to Pleistocene age, and constitutes the presently active foreland FTB of the Himalayan orogen. Recently constructed balanced cross-sections have revealed two important observations concerning foreland FTB morphology across the Himalayan arc: 1) west to east increase in strain and strain rate correlates with plate convergence rates, and 2) annual rainfall amounts are inversely correlated with FTB morphology. From these observations, it was proposed that surface processes exert greater control on wedge morphology than tectonic processes in the Himalayan FTB. In this study, the critical taper model is used to test the relative contribution of surface processes to the morphology of the present-day foreland FTB of the Himalaya. Internal parameters such as; friction, both along the wedge (μ) and décollement fault plane (μb); the Hubbert-Rubey pore fluid ratio, both within the wedge (λ) and along the basal décollement (λb); and the critical taper (α+β) were held constant. Key parameters including: thickness of accreted foreland material (T), depth at which the décollement flattens (D), rock erodibility (K) and material flux into the wedge (vT) were varied within the range of observed values along-strike of the Himalayan FTB. Comparisons between experiments allow estimation of model sensitivity to each parameter individually. The calculated critical taper morphology and growth rate are consistent with those observed in the Himalayan FTB, indicating that the FTB developed in accordance with critical Coulomb wedge theory. Comparative analysis indicates that within a narrow range of parameters and high values of the erodibility factor K (1×10-5≤ K ≤ 1×10-4), climatically induced erosion is the principal control on Himalayan foreland FTB morphology. Conversely, when the erodibility factor (K) is low, tectonic accretion of the material (vT) is the dominant parameter in controlling FTB morphology.

Keywords: critical taper, steady-state, FTB, surface processes, tectonic processes, critical Coulomb wedge
Pages: 99
Supervisor: Djordje Grujic