Controls on early-stage, magma-poor rifting from top-to-bottom seismic imaging of the Malawi (Nyasa) Rift

Few constraints are available on variations in extension with depth and along-strike in early stage continental rift systems, leaving many questions on the mechanisms of extension and the controlling factors. The Malawi (Nyasa) Rift in the southern East Africa Rift System exemplifies an active, magma-poor, weakly extended continental rift. Between 2014-2016, we collected a multi-faceted, amphibious, active- and passive-source seismic dataset across the northern Malawi Rift as a part of the SEGMeNT (Studies of Extension and maGmatism in Malawi aNd Tanzania) interdisciplinary experiment. Together, analysis and integration of these seismic imaging datasets provide a comprehensive portrait of the style and amount of stretching throughout the lithosphere and along strike.  Broadband scattered-wave imaging and wide-angle seismic reflection/refraction data reveal substantial variations in extension with depth, with much more thinning of the lithospheric mantle than the crust (stretching factors of 3.8 and 1.7, respectively). The modest observed reduction in velocity below the rift from both broadband surface- and body-wave imaging can be explained with small thermal perturbations and without melt. Lower velocities and complex patterns of anisotropy underlie the Rungwe Volcanic Province to the north of the Malawi Rift, suggesting focused lithospheric modification, melting and complex mantle flow below this localized volcanic province.  Active-source seismic refraction and multi-channel seismic (MCS) reflection data quantify cumulative extension accommodated by the border faults and intrarift faults. Border faults have throws up to ~8 km and bound half graben basins. Intrarift faults are also relatively large (throws up to 2.5 km) and active, and they are estimated to account for ~20-25% of cumulative upper crustal extension. Along-strike variations are observed in faulting and in crustal and lithospheric stretching. In this presentation, we will synthesize these seismic imaging results and compare them with complementary constraints, including from other parts of the SEGMeNT project .