The upper mantle transition zone beneath southern California
Simmons, Nathan Alan
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The upper mantle transition zone contains key information about Earth-scale convection processes. Seismic discontinuities in the transition zone, such as the 410-and 660-km discontinuities, are expected to react to thermal anomalies based on the thermodynamic properties of the hypothesized mineralogy. Therefore, temperature anomalies should be detectable by determining the topography of major seismic discontinuities within the transition zone. To sufficiently image these discontinuities, tremendous amounts of data are required. Southern California currently has 21 permanent broadband stations that have been deployed for multiple years. The spatial distribution of these stations and large volume of available data allow us to adequately image the transition zone in three dimensions. Since P-to-S conversions occur at seismic discontinuities, we generated PdSy, PPds, PcPds and PKPdfds receiver functions for all significant earthquakes recorded in southern California from 1981 to 1998. The transition zone was then imaged using 3D receiver function stacking algorithms. One of our major discoveries is that the base of the transition zone (near 660 km) is affected by phase transformations in the gamet mineralogy. The implication is that the seismic signature at these depths may not be solely dependent on olivine transformations and that convection may not necessarily be inhibited in this region. Another major finding is that the discontinuities respond to low-wavelength temperature variations in a linear fashion. This has provided significant statistical evidence that the major discontinuities in the transition zone are temperature dependent. This implies that these boundaries are generated by pressure-induced phase transformations in a combined olivine-gamet mineralogical system and can therefore support (but not require) whole mantle convection.