Lithospheric Structure of the Malawi Rift: Implications for Magma-Poor Rifting Processes

Our understanding of how magma-poor rifts accommodate strain remains limited largely due to sparse geophysical observations from these rift systems. To better understand the magma-poor rifting processes, we investigate the lithospheric structure of the Malawi Rift, a segment of the magma-poor western branch of the East African Rift System. We analyze Bouguer gravity anomalies from the World Gravity Model 2012 using the two-dimensional (2-D) radially averaged power-density spectrum technique and 2-D forward modeling to estimate the crustal and lithospheric thickness beneath the rift. We find: (1) relatively thin crust (38-40 km) beneath the northern Malawi Rift segment and relatively thick crust (41-45 km) beneath the central and southern segments; (2) thinner lithosphere beneath the surface expression of the entire rift with the thinnest lithosphere (115-125 km) occurring beneath its northern segment; and (3) an approximately E-W trending belt of thicker lithosphere (180-210 km) beneath the rift's central segment. We then use the lithospheric structure to constrain three-dimensional numerical models of lithosphere-asthenosphere interactions, which indicate similar to 3-cm/year asthenospheric upwelling beneath the thinner lithosphere. We interpret that magma-poor rifting is characterized by coupling of crust-lithospheric mantle extension beneath the rift's isolated magmatic zones and decoupling in the rift's magma-poor segments. We propose that coupled extension beneath rift's isolated magmatic zones is assisted by lithospheric weakening due to melts from asthenospheric upwelling whereas decoupled extension beneath rift's magma-poor segments is assisted by concentration of fluids possibly fed from deeper asthenospheric melt that is yet to breach the surface.