HR: 10:20h
AN: V21F-01    [PDF]
TI: Shear-wave splitting as a diagnostic tool for resolving plume-related mantle flow around hotspots
AU: Bokelmann, G H
EM: goetz@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Mitchell Building, Stanford, CA 94305 United States
AU: * Walker, K T
EM: ktwalker@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Mitchell Building, Stanford, CA 94305 United States
AU: Klemperer, S L
EM: sklemp@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Mitchell Building, Stanford, CA 94305 United States
AB: The plate tectonics hypothesis successfully explains most of Earth's geological and geophysical features. However, mantle hotspots, regions often associated with large magmatic provinces, linear age progressions of volcanism, and/or large topographic swells, do not fit into a simple plate-tectonic model. Most hotspots are explained by a simple plume model, i.e. a conduit of hot buoyant upwelling material that originates from a deeper thermal boundary layer, the origin of which is often assumed to be in the lower mantle. Past global seismic tomography has had little success in resolving plume-like structures due to resolution limitations. Recent regional teleseismic imaging above hotspots has had limited success in imaging plume-like conduits down to depths of $\sim$400 km. Receiver function and SS precursor studies have also had limited success in detecting the thinning of the transition zone beneath hotspots, which is expected on physical grounds due to the penetration of hot plume material from below. It has been proposed recently that some of these features associated with hotspots can be explained by more complicated plate tectonic models, leading to significant debate. We analyze shear-wave splitting of upward propagating shear waves through the seismically anisotropic upper mantle around the Hawaii and Eifel hotspots, and southwest of Yellowstone along the hotspot axis. If plumes exist beneath these hotspots, these data may resolve the geometry and magnitude of upper-mantle flow and anisotropy associated with the interaction between the moving plate and upwelling plume material. Each of the hotspots we investigate is in a unique geophysical setting. For each study region, we observe an approximately parabolic pattern in map view of the splitting fast directions that is predicted by a simple kinematic plume model. The common pattern we observe, along with inferences about the location of anisotropy, suggests that plume conduits exist in at least the upper mantle beneath Hawaii, Eifel, and eastern Nevada. The best plume model for Hawaii fits the few observations well, but more data are needed to critically test it. The optimum Eifel plume model predicts the splitting fast directions fairly well, with complexity observed between nearby stations that suggests contributions from lithospheric and/or shallow asthenospheric sources. Fast directions from six stations in Idaho, in addition to splitting data collected across eastern Idaho, western Utah, and Nevada are predicted well by a plume model centered in eastern Nevada, but not beneath Yellowstone. We show that shear-wave splitting may be resolving plume-related mantle flow around some hotspots, and therefore if enough splitting data are collected, they could be used as a diagnostic tool to help resolve between plume and non-plume sources for other hotspots. A helpful complement to such future splitting investigations are regional surface-wave anisotropy investigations to better determine the depth extent of azimuthal anisotropy, and the development of better tools that more accurately predict fast directions and delay times from numerical mantle flow models.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
DE: 8162 Rheology--mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting