Crustal structure beneath the Malawi and Luangwa Rift Zones and adjacent areas from ambient noise tomography

Abstract

Crustal shear wave velocity structure beneath the Malawi and Luangwa Rift Zones (MRZ and LRZ, respectively) and adjacent regions in southern Africa is imaged using fundamental mode Rayleigh waves recorded by 31 SAFARI (Seismic Arrays for African Rift Initiation) stations. Dispersion measurements estimated from empirical Green's functions are used to construct 2-D phase velocity maps for periods between 5 and 28 s. The resulting Rayleigh wave phase velocities demonstrate significant lateral variations and are in general agreement with known geological features and tectonic units within the study area. Subsequently, we invert Rayleigh wave phase velocity dispersion curves to construct a 3-D shear wave velocity model. Beneath the MRZ and LRZ, low velocity anomalies are found in the upper-most crust, probably reflecting the sedimentary cover. The mid-crust of the MRZ is characterized by an ~3.7% low velocity anomaly, which cannot be adequately explained by higher than normal temperatures alone. Instead, other factors such as magmatic intrusion, partial melting, and fluid-filled deep crustal faults might also play a role. Thinning of the crust of a few kilometers beneath the rifts is revealed by the inversion. A compilation of crustal thicknesses and velocities beneath the world's major continental rifts suggests that both the MRZ and LRZ are in the category of rifts beneath which the crust has not been sufficiently thinned to produce widespread syn-rifting volcanisms.