A systematic 2‐D investigation into the mantle wedge's transient flow regime and thermal structure: Complexities arising from a hydrated rheology and thermal buoyancy

G Le Voci,C R Wilson,S Goes,S C Kramer,D R Davies

doi:10.1002/2013gc005022

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 principal controls on mantle wedge conditions. In this paper, we build on these previous studies by undertaking a systematic 2‐D numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge's flow regime and associated thermal structure. We quantify the role of a range of plausible: (i) water contents (0–5000 H/106Si); (ii) subduction velocities (2–10 cm/yr); and (iii) upper‐plate ages (50–120 Myr), finding that small‐scale convection (SSC), resulting from Rayleigh‐Taylor instabilities, or drips, off the base of the overriding lithosphere, is a typical occurrence. The morphology of SSC varies with viscosity and subduction parameters, with drips at their most prominent when subduction velocities and wedge viscosities are low. Our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner‐flow regime, and strong upper‐plate erosion below the arc region. By contrast, we find that back‐arc upper‐plate erosion by SSC is largely controlled by wedge viscosity, occurring when: (i) viscosities are < 5·1018 Pa s; and (ii) the length of the upper plate, available for destabilization, exceeds the characteristic wavelength of instabilities. Thus, if hydrous weakening of wedge rheology extends at least 100–150 km from the trench, our 2‐D models predict an unstable flow regime, resulting in temperature fluctuations of 50–100 K, which are sufficient to influence melting and the stability of hydrous minerals.

Highlights

Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA [1] Arc volcanism at subduction zones is likely regulated by the mantle wedge’s flow regime and thermal structure and, numerous studies have attempted to quantify the principal controls on mantle wedge conditions
[17] To unravel the dominant controls on the mantle wedge’s flow regime and thermal structure, we examine a suite of simulations, where we systematically vary a range of subduction parameters
We find that higher subduction velocities, higher wedge viscosities, and older upper plates all result in decreased slab-surface temperatures (SSTs), which is consistent with recent studies by Lee and King [2009] and Syracuse et al [2010]

Summary

Le Voci

Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA [1] Arc volcanism at subduction zones is likely regulated by the mantle wedge’s flow regime and thermal structure and, numerous studies have attempted to quantify the principal controls on mantle wedge conditions. We build on these previous studies by undertaking a systematic 2-D numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge’s flow regime and associated thermal structure. Our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner-flow regime, and strong upper-plate erosion below the arc region. R. Wilson (2014), A systematic 2-D investigation into the mantle wedge’s transient flow regime and thermal structure: Complexities arising from a hydrated rheology and thermal buoyancy, Geochem.

Introduction

Additional Controls

Findings

Conclusions and Future