Deformation in the deep Canadian shield and plate-mantle coupling
Bokelmann(1), G.H.R., Silver (2), P.G.
(1) Stanford University;
(2) DTM, Carnegie Institution of Washington
The Canadian shield is an ideal place for studying mechanical
interaction between lithospheric plates and deeper mantle.
It has very thick lithosphere which may act as an indentation
into the deeper mantle if lithosphere and deeper mantle
move relative to each other. Also the Canadian shield has a
high absolute plate velocity of about 2cm/yr. The central
Canadian shield gives rise to very strong shear wave
splitting which indicates strong deformation. Indeed, no other
shield produced stronger shear wave splitting so far.
We also find an anomalous behaviour of P and S travel time delays.
While S wave delays through the Canadian shield lithosphere vary
considerably from station to station P wave delays hardly vary.
Solving for anisotropic structure under the Canadian shield
we find that both the anomalous relative variation of P and S wave
delays and the strong shear wave splitting can be explained if the
Canadian shield lithosphere comprises two anisotropic layers.
The model constrains the deeper layer to have nearly horizontal foliation
plane orientation. Such an anisotropy/deformation would be expected
from plate-mantle interaction.
To understand why the anisotropy appears to be stronger under the shield
than elsewhere we construct a rheological model for the Canadian shield
with typical geothermal gradients for the shield and adjacent areas.
Based on the geological record we assume coherent motion between
these two areas. Then the vertically integrated strain rate is the same.
On the other hand, viscosity and hence the level of stress is larger
under the shield. The rheological model suggests that dislocation creep is
more dominant under the shield than in adjacent areas. This leads
to strong anisotropy under the shield.
The observations also suggest that shear stresses under the shield can be
constrained using seismic anisotropy. The level of shear stress at the base
of the plate is very important since it constrains the relative size of the
different forces which drive the plates.
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