LITHOSPHERIC THICKNESS INDUCED MODULATION OF MANTLE ANISOTROPY BENEATH THE MALAWI AND LUANGWA RIFT ZONES FROM SHEAR-WAVE SPLITTING OBSERVATIONS

Continental rifting is often characterized by a dichotomous environment insomuch that rifted lithosphere often deforms either in response to active thinning or far-field extensional stresses. With the ambition of assessing whether progressive mantle deformation is coupled to lithospheric tectonism in eastern Africa, 50 PASSCAL broadband seismic stations were installed during the summer of 2012 across the Malawi, Luangwa, and Okavango rift zones and were demobilized in the summer of 2014. We examine a data set consisting of 33 SAFARI (Seismic Arrays For African Rift Initiation) stations throughout Malawi, Mozambique, and Zambia combined with adjacent stations from local networks and AfricaArray to conduct a preliminary shear-wave splitting (SWS) analysis to investigate the behavior of anisotropy beneath incipient southern East African Rift segments. Over 800 high-quality pairs of SWS measurements were recorded from nearly 300 events by 73 stations in a 12° by 12° quadrangle centered on the Malawi rift. Splitting times throughout the region are predominantly between 0.8 and 1.2 s, with higher delay times to the N and NW of the Luangwa rift and to the east of the Malawi rift zone. Fast directions to the west of the Luangwa roughly parallel the rift axis with inclusive oblique measurements. Measurements from east of the Luangwa rift are predominantly NNW-SSE which shift abruptly to NE-SW over a 60 km lateral distance on the western Malawi flank. Orientations within the southern Malawi rift zone and Mozambique are prevailingly oblique and are more strongly E-W in northern Malawi. Measurements along the Rukwa basin strongly parallel the strike of the rift axis. Fast directions observed beneath the triple junction at the northern tip of the Malawi rift, whereupon the Victoria, Rovuma, and Nubian plates intersect, are highly variable and may indicate multiple flow regimes. No complex anisotropy is demonstrated in the SWS measurements obtained for stations in the vicinity of variable fast orientations, indicating the presence of a lateral shift in anisotropic character as a consequence of modulated asthenospheric deformational flow into channelized plate-boundary lithosphere. These preliminary results suggest that the Malawi, Luangwa, and Rukwa rift zones are subject to passive, far-field extensional processes.