Shear wave splitting measurements in northeastern Uganda and southeastern Tanzania: corroborating evidence for sublithospheric mantle flow beneath East Africa

Abstract

SUMMARY Several interpretations of shear wave splitting measurements in East Africa have been proposed, but a comprehensive understanding of seismic anisotropy is lacking vis-à-vis geodynamic processes affecting the African Plate, in part because of geographically limited data coverage. Here, we report new shear wave splitting observations using PKS, SKS and SKKS phases recorded on 17 temporary AfricaArray seismic stations in northeastern Uganda and southeastern Tanzania that help fill key gaps in data coverage in non-volcanic regions of East Africa. The stations in northeastern Uganda straddle the Aswa shear zone, which separates the Neoproterozoic Saharan Metacraton (to the east) and the Palaeoproterozoic Northern Uganda Terrane (to the west). The stations in southeastern Tanzania are located mostly in Karoo basins formed within the Neoproterozoic Mozambique Belt. Our results show NNE fast polarization directions in northeastern Uganda, with splitting times between 1.0 and 1.5 s, and NE directions in southeastern Tanzania, with splitting times between 0.6 and 1.3 s. The NNE fast polarization directions in Uganda cannot be explained with oriented melt pocket (OMP) anisotropy in the lithosphere, as the stations are far from any volcanic fields, or fossil anisotropy resulting from the Neoproterozoic East African orogeny. They are, however, parallel to the flow direction of the African superplume and corroborate previous measurements from northern Uganda that were used to argue that the overall northerly orientation of fast polarization directions across East Africa can be attributed to sublithospheric mantle flow associated with the African superplume. The NE fast polarization directions in Tanzania cannot be explained with SE-oriented fossil anisotropy expected from the Mesozoic rifting of Madagascar from Africa or OMP anisotropy in the lithosphere. However, they are consistent with the overall northerly orientation of fast polarization directions in East Africa and can be attributed to either fossil anisotropy in the Mozambique Belt lithosphere resulting from the East African Orogeny or sublithospheric mantle flow associated with the African superplume.