Preferential localized thinning of lithospheric mantle in the melt-poor Malawi Rift

The forces required to initiate rifting in cratonic plates far exceed the available tectonic forces. High temperatures and resultant melts can weaken the lithosphere, but these factors do not readily explain the extension of old and strong lithosphere in magma-poor rifts, such as the Malawi Rift. Here, new seismic converted-wave imaging shows that even in this magma-poor rift, upper-crustal rift basins are associated with localized preferential thinning of the lithospheric mantle. We calculated the beta factor, the ratio between current and prerift thickness, beneath the rift axis and found crustal beta of 1.7 +/- 0.3 and lithospheric-mantle beta of 3.8 +/- 1.7. Purely mechanical stretching cannot explain the preferential lithospheric mantle thinning-instead, thinning of the rheological lithosphere was probably augmented by thermochemical rejuvenation and erosion. Although local surface-wave-derived shear-wave velocities preclude a substantially elevated temperature and partial melt today, fusible materials preserved in the lower lithosphere that underlie the Ubendian Belt and its bounding subduction-related sutures in which the Malawi Rift nucleated may have provided an early supply of melt that enabled localized lithospheric alteration and/or removal. A plume-related or other asthenospheric perturbation would preferentially melt the more fusible lithospheric materials and the rising melts would heat and weaken progressively shallower parts of the lithosphere, which spatially localizes weakening (hence the lithospheric-mantle thinning) and enables the onset of rifting. The mantle lithosphere has thinned more than the crust beneath the Malawi Rift despite being melt-poor, according to seismic wave imaging; this suggests early melting of fusible mantle material.