Anisotropy, Deformation, and Driving Forces of Plate Tectonics
Bokelmann, G.H.R., Stanford University
The mechanism of plate tectonics is one of the most important
yet open questions in geosciences. While there is a consensus
that the mechanism is internal to the Earth, it is not clear
whether the tectonic plates are pulled/pushed from the side
or whether mantle convection helps to drive the plates from below.
Large-scale tectonic motions of the surface plates don't help to
discriminate between the two mechanisms, but seismology can,
through inferring the sense of shear deformation in the
lithosphere/asthenosphere system.
The sense of shear is recorded in the directions in which seismic
fast axes dip. We discuss recent seismological measurements of the
dips of fast axes. Interestingly, seismological stations on the
stable North American craton show P-wave fast axes that dip into
the third quadrant (direction of North American plate motion),
while stations off the craton have fast axes dips that are
distributed onto the other three quadrants, while the horizontal
projections of the fast P-axes are generally consistent with
shear-wave splitting. If the plate were driven from the side we'd
expect fast axis dips in the opposite direction (first quadrant).
The fast axes dips are consistent with a simple-shear deformation of
the lithosphere under the North American craton, and they suggest
that the driving component from below, by mantle convection, is
the dominant one. This mechanism can explain several seemingly
unrelated phenomena in North American tectonics, including why
the westward motion of North America has slowed down dramatically
throughout the past 100 million years.
(Invited Talk, Incorporated Research Institutions in Seismology (IRIS) meeting 2001, Jackson Lake, Wyoming)
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