Surface topography of the overriding plates in bi-vergent subduction systems: A mechanical model

Abstract

Using a two-layer mechanical model we ran real time computational fluid dynamic (CFD) simulations to explore the underlying dynamics of the overriding plate (OP) topography in a bi-vergent subduction system. Our model topography develops typically a continental scale plateau, flanked by two distinct fore-arc highs and topographic depressions. We show that large scale flow vortices produced by the two subducting plates (SP) can sustain such first-order elevated plateau topography, whereas compressive stress localization in the OP lithosphere decides the higher order fore-arc highs. Both symmetric and asymmetric subduction models were used to perform a quantitative analysis of the surface topography of OP as a function of the inter-trench distance (λd), and convergence plate velocity (Vc) on a time scale (t) 1 to 10Ma. For t=10Ma, the relative plateau elevation (ΔHOP) is 1250m, which multiplies to nearly 1800m on an increase of Vc from 1 to 5cm/yr. In contrast, ΔHOP varies inversely with λd. Bi-vergent subduction with the two trenches differing in Vc, slab dip (θs) and mechanical coupling (slip versus non-slip condition) of SP gives rise to asymmetric OP topography. Among these parameters, the mechanical coupling is found to have the strongest control in switching the symmetric to asymmetric transition. We finally take two natural examples, the Philippines and the Caribbean subduction systems, to discuss the applicability of our bi-vergent model to natural settings.