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

Abstract

[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.

Magma-assisted rifting in Ethiopia

“…10). The generally rift-parallel alignment is in agreement with other SKS [12][13][14][15] and surface-wave 16 studies of anisotropy along the East African Rift and in Afar. A detailed study of SKS splitting at the permanent Ethiopian IRIS station FURI reveals little dependence on incoming back-azimut (direction from station to earthquake), thus suggesting a uniform layer of anisotropy with a horizontal symmetry axis 13 .…”

“…In contrast, our data show rift-parallel fast directions (f) that are perpendicular to tectonic extension. The rift-parallel anisotropy could be caused by channelled horizontal mantle flow along the rift 23 , or the preferred alignment of melt-filled cracks, or dykes emanating from the upper mantle and aligned parallel to rifting [12][13][14]25 . We consider both models in the light of independent data.…”

The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly

“…The presence of Cenozoic volcanism in some places clearly suggests that partial melts may locally influence mantle velocities, and compositional differences between Archean and younger lithosphere could also contribute to mantle wave speed variations. The pattern of anisotropy is complex, and it is less obvious how this might affect mantle velocities [Bagley and Nyblade, 2013;Walker et al, 2004].…”

Historical Volcanism and the State of Stress in the East African Rift System

“…Crustal anisotropy is more variable, and reflects major structural features, for example in Afar, north of the Tendaho-Gobad Discontinuity (TGD), there is a high degree of anisotropy and the fast direction is oriented NNW, but to the south, the anisotropy is more moderate and the fast direction is orientated NNE (Keir et al, 2011a). In Kenya and Tanzania there are regional teleseismic event surveys (e.g., Walker et al, 2004), but only a few shallow crustal seismic anisotropy studies south of Ethiopia, and in the Western Rift, for example in the Ruwenzori segment (Batte et al, 2014). Preliminary teleseismic results from the VVP/Kivu, indicate a deep northeasterly oriented fabric (Zal et al, 2014).…”