HR: 11:50h
AN: S51F-06    [PDF]
TI: Anisotropic complexity in Tanzania and Kenya from teleseismic shear-wave splitting
AU: * Walker, K T
EM: ktwalker@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Mitchell Building, Stanford, CA 94305 United States
AU: Nyblade, A A
EM: andy@geosc.psu.edu
AF: Department of Geosciences, Penn State University, 503 Deike Building, University Park, PA 16802 United States
AU: Bokelmann, G H
EM: goetz@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
AU: Owens, T J
EM: owens@sc.edu
AF: Department of Geological Sciences, University of South Carolina, 701 Sumter Street, Columbia, SC 29201 United States
AB: Previous studies have found that shear-wave splitting fast directions in western Kenya are roughly parallel to the Kenya Rift, part of the East Africa rift system. This anisotropy can be explained by three different mechanisms: fossilized anisotropy in the lithosphere from previous orogenic events, vertically oriented magma-filled lenses in the lithosphere and asthenosphere, and asthenospheric flow along the base of the plate that is guided by a background asthenospheric flow current and/or channeled by upwarped basal lithospheric topography beneath the rift. Preliminary results from Ethiopia appear to be similar to those in Kenya (M. Kendall, pers. commun., 2003). Resolving between these mechanisms has major implications on the evolution of rifting, the geodynamics associated with rift/hotspot interactions, and the origin of anisotropy detected by shear-wave splitting. Tanzania and Kenya comprise a unique, tectonically complex region due to the presence of a rigid craton, paleo-thrust belts and shear zones, active magmatism and rifting, and possibly even a nearby mantle plume. We present new results of teleseismic splitting analysis of SKS, SKKS, PKS, and S recorded by 20 broadband seismic stations in Tanzania, 7 broadband stations in Kenya, and 3 permanent broadband GSN stations in Kenya and Uganda. Fast directions (phi) from most of the craton stations are oriented WNW and are subparallel to both absolute plate motion and the strike of the well-defined surficial fold axes. The delay times (dt) are $\sim$0.5 s, and when combined with previously published surface-wave results, suggest anisotropy in the lower lithosphere and asthenosphere beneath the craton, which would be expected due to relative motion between the WNW moving plate and a fixed underlying mantle. However, the fast direction/structural fabric correlation at the surface, and the lack of WNW plate-motion parallel fast directions in the Kaapvaal craton in southern Africa suggests that vertically coherent deformation best explains craton splitting. In the mobile belts and rifts, phi is roughly parallel to the strike of the rifts, the paleo-shortening directions, and the edge of the craton, with significant variations between stations. For most of these stations, dt ranges from $\sim$0.8-1.5 s, and suggests that at least a significant part of the anisotropy is in the asthenosphere. We interpret the dominant off-craton mantle anisotropy in Kenya and Tanzania to be due to shear along the base of the plate associated with plume flow around the craton keel. Significant variations between nearby stations are assumed to be due to the secondary effects of fossilized anisotropy or anisotropy due to magma-filled lenses or partially molten dikes in the lithosphere/asthenosphere. Our data strongly suggest that extension has not led to the development of a resolvable LPO in the lithospheric mantle, which is surprising given the long history of extension in the region. This suggests that mantle-lithosphere extension in East Africa occurs within narrow rift zones via ductile thinning and/or dike intrusion, possibly facilitated by mechanical lithospheric anisotropy from fossilized N-S fabrics that originated from prior orogenic events.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8162 Rheology--mantle
SC: Seismology [S]
MN: 2003 Fall Meeting