Inferences of Complex Anisotropic Layering and Mantle Flow Beneath the Malawi Rift Zone from Shear-Wave Splitting

Measuring the magnitude and orientation of seismic anisotropy beneath actively extending rift zones provides invaluable estimates of the influence of numerous geodynamic parameters upon their evolution. In order to infer the character and origin of extensional forces acting upon the Malawi Rift Zone (MRZ) and Luangwa Rift Zone (LRZ) of southern Africa, we installed 33 Seismic Arrays For African Rift Initiation (SAFARI) three-component broadband seismic stations in Malawi, Mozambique, and Zambia between 2012-2014. Shear-wave splitting parameters, including the fast-component polarization orientation and the splitting time, are extracted from 142 events recorded during that time period for a total of 642 well-defined PKS, SKKS, and SKS phase measurements. Polarizations trend NE-SW along the western flank of the LRZ, whereupon they demonstrate an abrupt shift to N-S within the rift valley and the eastern flank. SWS orientations shift increasingly counterclockwise toward the east until, at 33°E, they shift from WNW-ESE to ENE-WSW, suggesting a systematic change in dominant mantle fabric orientation. The resulting fast orientations demonstrate remarkable variability within the MRZ, with E-W measurements in the north rotating counterclockwise toward the south to N-S within the southernmost MRZ. Measurements revert to E-W and NE-SW orientations toward the east in Mozambique, suggesting the presence of complex two-layer anisotropy. Azimuthal variations of SWS parameters recorded by stations within the central MRZ exhibit excellent 90° periodicity, further suggesting complex anisotropic layering. Lateral variation of measurements between the northern and southern MRZ imply the modulation of the mantle flow system beneath the active rift zone.