Using ambient noise to create a regional shear wave velocity model for the Northern East African Rift

The East African Rift system is one of the only places on Earth where we see the transition from continental rifting to insipient seafloor spreading. The main aim is to understand where melt migration and storage occur in the upper mantle and crust during continental rifting and breakup. Moreover, we aim to provide insights into how the lithosphere is modified by plate thinning, intrusion and heating. Here we use ambient noise tomography to generate a regional absolute shear wave velocity model for the crust and uppermost mantle of the Main Ethiopian Rift (MER) and Afar Depression. We use a combination of 157 temporary and permanent seismic stations active from 1999-2011 to generate vertical component noise correlation functions. We measure Rayleigh wave phase velocity dispersion from 7-45s period, and invert for phase velocity maps at each period. The phase velocity maps are inverted for a 3D shear wave velocity model of the crust and uppermost mantle. In our shear velocity models at 15km depth, the average velocity beneath the plateau is 3.45±0.07kms-1, consistent with continental crust. The MER is slower at 3.25±0.05kms-1. These velocities are too slow to be accounted for by temperature or composition, given the widely acknowledged presence of gabbro intrusions at this depth in the rift, which should result in velocities >3.81kms-1. Additionally, a temperature anomaly of >800°C would be required. We therefore infer the presence of partial melt or fluids to explain lower velocities in the rift. Afar has the fastest shear velocities in the crust, with values of 3.7±0.05kms-1 consistent with shear velocities for mafic intrusions. At lower crustal/upper mantle depths, we still image anomalously slow Vs beneath the MER. Our S-wave images provide strong evidence for the presence of partial melt beneath the rift, especially at mid-lower crustal depths in the MER. The deep crust may therefore be an important melt storage region during early magma-rich continental breakup.