Download Mantle Flow at a Subduction

Mantle Flow at a Subduction-Transfrom Plate Boundary
Margarete Jadamec and Magali Billen, Dept. of Geology, Univ. of California, Davis
Using high-resolution, instantaneous 3D viscous deformation models run with CitcomCU,
we constrain how Neogene deformation in southern Alaska is linked to the subducted Pacific
slab in the subsurface. The slab thermal structure incorporates the plate age at the time of
subduction and the thermal structure of the overriding North American plate is constrained
by heat flow and seismic observations of crust and lithosphere thickness (Fig. 1a). The model
rheology is a composite dislocation-diffusion creep viscosity with a depth-dependent plastic
yield stress. Plate boundaries are modeled as low viscosity shear zones.
We compare the mantle flow and surface deformation for two end-member slab shapes to
observations of shear-wave splitting in the mantle, the surface distribution of volcanism and
dynamic topography, and plate motions. We find that a slab shape missing the northwest
extension of the slab from 100–300 km depth, as constrained by seismicity and seismic tomography, is more consistent with the entire suite of observations, than a model that includes this
portion of the slab, as would be predicted based only on past plate motions. We also find that
models with only a Newtonian mantle viscosity (diffusion creep) have a significantly different
flow field and surface deformation than models using the non-Newtonian composite viscosity.
Figure 1. Slab Struca)
ture and Flow Field. (a)
3D view of slab shape
indicating location of two
viscosity slices along slabperpendicular arcs, BB’
(beneath the Wrangell
b)
Volcanics) and DD’ (beMap view:
neath Mt. Denali). (b)
flow at ~130 km
depth
Map view of flow at approximately 100 km depth.
(c) View, from the northeast, of flow around the
slab edge.
Effective viscosity along two slab profiles.
c)
View from the
northeast showing flow
toward the slab nose.
For both slab shapes,
the resulting flow exhibits the typical 2D corner-flow pattern away from the slab edge, with
flow being drawn into the wedge corner and upwelling beneath the fore-arc. A 3D map view
of flow field at a depth of 130 km, for the full-slab shape, shows a strong component of
along-slab flow towards the nose of the slab (Fig. 1b, c). This flow is driven by the pressuredifference between the regions beneath and above the slab which draws flow around the slab
edge, and the increased mass in the slab nose, which further draws this flow along the slab.
References. Jadamec, M. A., 3D Numerical Models of Lithosphere and Mantle Deformation in
southern Alaska, Ph.D. thesis, University of California, Davis, 2008.
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