Imaging Reflectors in the Deep Mantle; Post-Perovskite or Not?

Details Imaging Reflectors in the Deep Mantle; Post-Perovskite or Not? Imaging Reflectors in the Deep Mantle; Post-Perovskite or Not?

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.u32a..03l&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #U32A-03

Mathematics

Logic

3924 High-Pressure Behavior, 7207 Core And Mantle, 8121 Dynamics, Convection Currents And Mantle Plumes, 8147 Planetary Interiors (5430, 5724)

Scientific paper

The discovery of the post-perovskite phase transition presents a new context for interpreting seismological observations of deep mantle structure. As is the case for seismic velocity discontinuities detected in the upper mantle transition zone, interpretation as an expression of a phase change provides an opportunity to infer thermal and dynamical effects that are otherwise not directly sensed by seismology. Extensive observations of P and S wave velocity increases several hundred kilometers above the core-mantle boundary in different regions of the deep mantle appear to be generally compatible with the presence of a post-perovskite transition. The tendency for S wave reflections to be stronger and for the ratio of Vs/Vp to be higher in the D" layer beneath the velocity increases can be accounted for by the phase transition. If the post-perovskite transition is indeed the root cause of the seismic discontinuity, small scale variations in the discontinuity may provide a probe of thermal and chemical variations. One of the most intensively studied regions of the deep mantle is beneath the Cocos plate, where P and S waves from deep earthquakes under South America bottom in the deep mantle before being observed at the dense BDSN and TRINet networks of broadband stations in California. We apply simplified seismic migration procedures to image the P and S velocity structure under the Cocos plate. Use of 3D tomographic models to account for volumetric heterogeneity allows us to constrain topography on the seismic reflectors and their relative strengths. S reflections are stronger and more readily migrated than P reflections in this region, and an extensive lateral reflector is found in the S images. While this appears to be rather uniform in depth over a 700 km extent, an abrupt change in depth occurs near 5N, with perhaps 50 km change in depth of the reflector. Explaining this abrupt change in the framework of post-perovskite requires some chemical contribution beyond a thermal effect. We also find that there is no evidence for a radial decrease in velocity below the D" discontinuity. There is however, evidence for scattering from a localized low velocity structure offset from the reflector. Implications for post-perovskite interpretations will be considered.

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