Controls on the flow regime and thermal structure of the subduction zone mantle wedge: a systematic 2-D and 3-D investigation
File(s)
Author(s)
Le Voci, Giuseppe
Type
Thesis
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.
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.
Version
Open Access
Date Issued
2013-10
Date Awarded
2014-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Davies, Rhodri
Goes, Saskia
Piggott, Matthew
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)