CRUSTAL AND MANTLE STRUCTURE AND ANISOTROPY BENEATH THE YOUNG AND INCIPIENT SEGMENTS OF THE EAST AFRICAN RIFT SYSTEM (Invited Presentation)

The SAFARI (Seismic Arrays for African Rift Initiation) experiment, which was conducted over the period of 2012-2014, consists of 50 broadband seismic stations deployed across the Malawi and Okavango rifts with a total profile length of about 2500 km. Shear wave splitting and H-k receiver function stacking analyses indicate the absence of mantle upwelling beneath either of the rift zones. Instead, we have found that the fast polarization directions of the split shear wave, which reflect the direction of simple shear originating from mantle flow patterns in the asthenosphere, are consistent with the absolute plate motion direction computed under the non-net rotation reference frame and with local deflections along the edges of thick lithospheric blocks. The crust beneath either rift is thinned by about 5 km relative to immediately adjacent unstretched areas, resulting in a beta factor of about 1.1. No significant increase in the crustal Vp/Vs ratio is observed at stations in or near the rift valleys, although possible exceptions for the northernmost part of the Malawi Rift close to the Rungwe Volcanic Province are detected. In addition, no systematic along-strike variations in the crustal thickness or Vp/Vs are found along the Malawi Rift. The abovementioned seismological and gravity modeling results and other results obtained using the SAFARI data set, including from seismic tomographic imaging and determinations of the mantle transition zone topography, support a passive rifting model. This model advocates rift development along pre-existing zones of lithospheric weakness wherein abrupt deviations of the lithospheric thickness are observed. Lateral variations in the dragging stress applied to the lithospheric base are the most likely cause for the initiation and development of the young and incipient segments of the EARS.