Kristoffer T. Walker

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

Received 28 October 2003; revised 2 April 2004; accepted 21 May 2004; published 6 August 2004. [1] East Africa is a tectonically complex region owing to the presence of a rigid craton, paleothrust belts and shear zones, active magmatism and rifting, and possibly even a mantle plume. We present new splitting results of teleseismic shear phases recorded by 21 broadband seismic stations in Tanzania, seven broadband stations in Kenya, and three permanent broadband Global Seismic Network stations in Kenya and Uganda. Inconsistent apparent splitting is observed beneath the craton and along its southern and southeastern flank in Tanzania. Splitting at stations elsewhere in the rifts and orogenic belts is more consistent. We test between different models of anisotropy for stations with inconsistent splitting (single layer with horizontal fast axis, single layer with dipping fast axis, and two layers with horizontal fast axes). However, we show that these more complicated models do not reasonably explain the data. The data are explained better by a laterally (and/or in some places vertically) varying single-layer model with a horizontal fast axis. We arrive at a conceptual model of anisotropy in Tanzania and Kenya that is controlled by (1) active shear along the base of the plate associated with asthenospheric flow beneath and around the moving craton keel, (2) asthenospheric flow from a plume north of central Kenya, (3) fossilized anisotropy in the lithosphere due to past orogenic events, and possibly (4) aligned magma-filled lenses beneath the rifts. Our most robust conclusion is that we can rule out an extension-induced lattice preferred orientation of olivine as a dominant factor, which is surprising given the long history of extension in the region. This indicates that mantle-lithospheric extension in East Africa occurs via dikeintrusion and/or ductile thinning within narrow rift zones and is possibly facilitated by a mechanical lithospheric anisotropy imparted by fossilized north/south structural or mineralogical fabrics. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; KEYWORDS: anisotropy, shear wave splitting, extension

Shear‐wave splitting in Ethiopia: Precambrian mantle anisotropy locally modified by Neogene rifting

[1]xa0Twenty-six broadband seismic stations in an areal array spanning 500 × 500 km across Ethiopia were used for shear-wave splitting studies. Our results show small-to-moderate delay times (0.5–1.7s) with fast-polarization azimuths sub-parallel to the orientation of the East African Rift (NNE-SSW) and also to the Proterozoic tectonic fabric across the entire studied area. Our results imply Ethiopian upper-mantle anisotropy is controlled largely by the Proterozoic accretion of the Mozambique belt, with possible minor effects within the rift due to aligned cracks or melt pockets parallel to the rift axis. Our observations are not consistent with anisotropy created by asthenospheric flow parallel either to the Cenozoic extension direction (NW-SE) or to the modern absolute plate motion direction (NNW-SSE), or to asthenospheric radial flow from the “Afar” plume.

Shear-wave splitting to test mantle deformation models around Hawaii

Seismic anisotropy allows us to study mantle deformation, and it can thus help to constrain mantle flow in the vicinity of hotspots. Hypotheses for the cause of seismic anisotropy in this environment include the “parabolic asthenospheric flow” (PAF) model: radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion. In map view, this gives a parabolic pattern of flow, opening in the direction of plate motion. We present new shear-wave splitting observations from land and ocean stations around the Hawaiian Islands that can be explained by the parabolic flow model. The observations suggest asthenospheric anisotropy under the Hawaiian islands, which may be explained if dislocation-creep persists to deeper depths there than in other regions, perhaps due to the higher temperatures near hotspots.

Shear wave splitting around hotspots: Evidence for upwelling-related mantle flow?

We review evidence for plumelike upwellings beneath the Eifel, Great Basin, Hawaii, Afar, and Iceland hotspots by using teleseismic shear wave splitting to resolve the anisotropy associated with mantle flow. An approximately parabolic pattern of fast polarization azimuths (f) is consistent with splitting observations around the Eifel, Great Basin, and Hawaii hotspots, and this pattern may be explained by a model of upwelling material that is being horizontally deflected or sheared in the direction of absolute plate motion (parabolic asthenospheric flow, PAF). The source of splitting beneath Iceland and the Afar is not clear, but the data are not inconsistent with a plumelike upwelling. The success of the upwelling model in explaining the splitting data for the Eifel and the Great Basin, regions far from plate boundaries, suggests that a mantle anisotropy pattern exists for at least some hotspots driven by plumelike upwellings and that splitting can be a useful diagnostic to differentiate between plumelike and alternative sources (e.g., propagating cracks, leaky transform faults) for mantle hotspots. Furthermore, the PAF pattern provides two useful tectonic and geodynamic parameters: the direction of relative motion between the lithosphere and asthenosphere and the stagnation distance, which is proportional to the strength of the upwelling and the speed of the moving plate. When this pattern is used with other types of geophysical data such as seismic velocity tomographic images, one can estimate the plate speed, upwelling volumetric flow rate, buoyancy flux, asthenospheric thickness, excess temperature, and viscosity.

Reply to “Shear-wave splitting to test mantle deformation models around Hawaii” by Vinnik et al.

INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8120 Tectonophysics: Dynamics of lithosphere and mantle— general; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8130 Tectonophysics: Heat generation and transport; 8162 Tectonophysics: Rheology—mantle. Citation: Walker, K. T., G. H. R. Bokelmann, and S. L. Klemperer, Reply to ‘‘Shear-wave splitting to test mantle deformation models around Hawaii’’ by Vinnik et al., Geophys. Res. Lett., 30(13), 1676, doi:10.1029/2002GL016712, 2003.

Seismic Anisotropy in the Asthenosphere Beneath the Eifel Region, Western Germany

We provide evidence for a plume-like upwelling beneath the Eifel hotspot, Western Germany, by using teleseismic shear-wave splitting to resolve the anisotropy associated with upwelling flow that is spreading laterally into the asthenosphere. The variation in fast-polarization azimuth we find across the Eifel hotspot is explained by a model of slowly upwelling material that is horizontally being deflected or sheared in a parabolic asthenospheric flow (PAF) pattern toward west-southwest, a direction that correlates with Eurasian absolute plate motion. We suggest that the lack of an age progression for Eifel volcanism, which is expected for a fixed-upwelling model, is a result of (1) sporadic volcanism due to a low excess plume temperature and/or varying crustal stresses that periodically relax and facilitate eruption, and (2) complex upwelling flow pathways and/or Late Tertiary changes in the slow Eurasian plate motion. The success of the PAF model in fitting the data is remarkable given the small number of parameters (four) and the consistency with the plate motion direction determined from geology and/or geodesy. This suggests that a predictable mantle-anisotropy pattern may exist also for other hotspots driven by plume-like upwellings, and that splitting can be a useful diagnostic to differentiate between plume-like and alternative sources for mantle hotspots.

Tectonic Evolution of the Bristol Bay basin, southeast Bering Sea: Constraints from seismic reflection and potential field data: GEOPHYSICS OF THE BRISTOL BAY BASIN

[1]xa0We interpret the tectonic evolution of the Bristol Bay basin, also known as the North Aleutian basin, on the basis of a deep seismic reflection profile, lithologic data from a well, unreversed seismic refraction profiles, a bathymetry profile, a magnetics profile, forward modeling of a gravity profile, and flexural modeling of a basin-wide paleosurface. We present evidence that (1) an early or middle Eocene through late Miocene phase of extension led to fault-controlled subsidence; (2) a late Eocene through early Miocene phase of volcanic-arc loading led to flexural subsidence, which was amplified by additional factors possibly including lithospheric cooling, tectonic compression, reverse or thrust faulting, or small-scale intrusions of dense magma; and (3) a late Miocene through Holocene resurgence of arc volcanism and a northward prograding delta continued (or possibly increased) flexural subsidence in the back arc region. Our interpretations imply that the fault-controlled subsidence in the Bristol Bay basin is genetically linked to that found in the other outer Bering Shelf basins, but the subsequent flexural subsidence of these basins is not related. More fundamentally, our observations suggest that the basins evolution is unusual and has components typically found in both back arc (extension) and retroarc foreland (flexure) basins.