Migration Imaging of Mid-Upper Mantle Discontinuities

Brown University, Providence RI

Investigators

Abstract

Fischer EAR-0208284 The investigators propose to better image seismic velocity discontinuities in the mid-upper mantle using broadband Ps and other scattered teleseismic phases. They will focus on structure in the 150-400 km depth range, where a rich variety of discontinuity structures have been observed. Testable hypotheses for the origin of these features include: the transition from dislocation creep to diffusion creep, layers of rheological weakness due to volatile enrichment, layers of partial melt, mechanical boundaries within the continental lithosphere, and mineralogical phase transformations. The various mechanisms imply differences in discontinuity depth, polarity, strength and anisotropic properties, and some of these predictions vary between tectonic environments due to changes in mantle temperature. These predictions will be compared to observed discontinuity structure globally (using IRIS/Global Seismic Network stations), and with focused regional studies in continental cratons, actively deforming continental regions, subduction zones, and beneath a mid-ocean ridge (using data from PASSCAL experiments and other arrays). Two types of migration will be employed: determination of receiver functions by simultaneous deconvolution of data from different geographic or back-azimuth bins, and formal 2D migration. Receiver function binning as a function of back-azimuth is particularly effective at resolving anisotropy, and the proposed 2D migration has great potential to constrain both P-wave and S-wave velocity contrasts at discontinuities. This work will help to characterize mid-upper mantle rheology, deformation mechanism, anisotropic fabric, and volatile and melt distributions.