Controls on the flow regime and thermal structure of the subduction zone mantle wedge: a systematic 2-D and 3-D investigation

Abstract

Arc volcanism at subduction zones is likely regulated by the mantle wedge’s flow regime and thermal structure and, hence, numerous studies have attempted to quantify the princi- pal controls on mantle wedge conditions. In this thesis, we build on these previous studies by undertaking the first systematic 2-D and 3-D numerical investigations, across a wide parameter-space, into how hydration, thermal buoyancy and toroidal flow around the slab edge influence the wedge’s flow regime and associated thermal structure. We find that small- scale convection (SSC), resulting from Rayleigh-Taylor instabilities, or drips, off the base of the overriding lithosphere, is a typical occurrence, if: (i) viscosities are < 5 × 1018 Pa s; and (ii) hydrous weakening of wedge rheology extends at least 100-150 km from the forearc cor- ner. In 2-D models, instabilities generally take the form of ‘drips’. In 3-D, two separate, but interacting, longitudinal Richter roll systems form (with their axes aligned perpendicular to the trench), the first below the arc region and the second below the back-arc region. These instabilities result in transient and spatial temperature fluctuations of 100-150K, which are sufficient to influence melting, the stability of hydrous minerals and the dehydration of crustal material. Furthermore, they are efficient at eroding the overriding lithosphere, par- ticularly in 3-D and, thus, provide a means to explain observations of high heat flow and thin back-arc lithosphere at many subduction zones. A preliminary study into the effects of a finite-width slab on the wedge’s flow regime, which allows for toroidal flow around the slab edge, highlights that the toroidal cell can locally increase or decrease temperatures suf- ficiently to either enhance or shut down wet melting, while a hydrated wedge corner may channel trench-parallel flow. The dynamic complexities of wedge flow revealed by our mod- els may help explain the diversity in geophysical and geochemical subduction signatures.