Modelling slab age and crustal thickness: numerical approaches to drivers of compressive stresses in the overriding plate in Andean style subduction zone systems.

Abstract

The Andes Mountains are formed at a destructive plate margin, where dense oceanic crust descends beneath relatively buoyant continental crust. There are regions of the Andes where the range is narrow, but there is also the high plateau of the Central Andes. The reasons for this variation in structure of the overriding plate are not clear. Previous numerical modelling shows that slabs typically roll back, so that continents are stretched, causing tension and potentially back-arc extension. This model does not fit the Andes. Instead, the formation of a thick crust and sustained elevation of the Andes has been hypothesized to be due to a range of different processes, including anchoring of the slab in the lower mantle, subduction of buoyant features in the Nazca plate, or compression driven by large-scale convection cells underneath South America. The enigmatic formation of the Andes is the central theme of this thesis.

Previous research suggests a clear correlation between slab age and overriding plate crustal thickness, globally, but in particular for South America. In this project, we hypothesize that this age variation plays a significant role in the formation of the Andes. As subducting slabs descend into the mantle, their properties differ in conjunction with their age affecting their buoyancy and strength, thereby generating different dynamics, surface tectonics, and slab morphologies. Using numerical modelling code ASPECT, we examined the role of slab properties and related dynamics on the state of stress in the overriding plate.

We quantify how much compression occurs in the overriding plate, and use this as a proxy for topographic growth. Typically, older slabs cause more rollback and therefore extension. The models in this study however, predict that the stronger pull force from older slabs causes more vigorous subduction, in which mantle convection contributes to corner flow in the mantle wedge, thereby increasing compression.

An increase in overriding plate thickness from 50 to 100km increases its overall density and therefore the amount of compression in the overriding plate by 10 MPa, while an increase in slab age from 40 to 80 Myr generates a similar increase in compression. Finally, slab morphology affects the geometry and vigour of convection cells beneath the overriding plate, which, in turn, affects the compressional state of the plate. This is affected by variation of parameters such as mantle viscosity and changes to frictional coupling, which is in qualitative agreement with previous work.