Testing the effects of basic numerical implementations of water migration on models of subduction dynamics
Subduction of oceanic lithosphere brings water into the Earth's upper mantle. Previous numerical studies have shown how slab dehydration and mantle hydration can impact the dynamics of a subduction system by allowing a more vigorous mantle flow and promoting localisation of...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-06-01
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| Series: | Solid Earth |
| Online Access: | http://www.solid-earth.net/5/537/2014/se-5-537-2014.pdf |
| Summary: | Subduction of oceanic lithosphere brings water into the Earth's upper
mantle. Previous numerical studies have shown how slab dehydration and
mantle hydration can impact the dynamics of a subduction system by
allowing a more vigorous mantle flow and promoting localisation of
deformation in the lithosphere and mantle. The depths at which dehydration
reactions occur in the hydrated portions of the slab are well
constrained in these models by thermodynamic calculations. However,
computational models use different numerical schemes to simulate
the migration of free water. We aim to show the influence of
the numerical scheme of free water migration on the dynamics of the
upper mantle and more specifically the mantle wedge. We investigate
the following three simple
migration schemes with a finite-element model: (1)
element-wise vertical migration of free water, occurring independent
of the flow of the solid phase;
(2) an imposed vertical free water velocity; and
(3) a Darcy velocity, where the free water velocity is
a function of the pressure gradient caused by the difference in density
between water and the surrounding
rocks. In addition, the flow of the solid material
field also moves the free water
in the imposed vertical velocity and Darcy schemes. We first test the
influence of the water migration scheme using a simple
model that simulates the sinking of a cold, hydrated cylinder into
a dry, warm
mantle. We find that the free water migration scheme has
only a limited impact on the water distribution after 1 Myr in these
models. We next investigate slab dehydration and mantle hydration with
a thermomechanical subduction model that includes brittle behaviour
and viscous water-dependent creep flow laws.
Our models demonstrate that the
bound water distribution is not greatly influenced by the water
migration scheme whereas the free water distribution is. We find that a
bound water-dependent creep flow law results in a broader area of
hydration in the mantle wedge, which feeds back to the dynamics of the
system by the associated weakening. This finding underlines
the importance of using dynamic time
evolution models to investigate the effects of (de)hydration. We also
show that hydrated material can be transported down to the base of the
upper mantle at 670 km. Although (de)hydration processes
influence subduction dynamics, we find that the exact numerical
implementation of free water migration is not important
in the basic schemes we investigated.
A simple implementation of water migration
could be sufficient for a first-order impression of the effects of water
for studies that focus on large-scale features of subduction
dynamics. |
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| ISSN: | 1869-9510 1869-9529 |