Upper mantle structure of the Congo Craton and the East African Rift from full wave ambient noise tomography

Abstract:

The relationship between lithospheric structure, mantle flow, and continental rifting along the East African Rift is the subject of ongoing discussion. The upper mantle beneath the Main Ethiopian Rift and the East African Rift farther south has been seismically imaged following the deployment of several temporary regional arrays. However, due to uneven distribution of seismic arrays, key questions regarding a connection between these upper mantle anomalies at the Turkana Depression and the effect that the thick Congo Craton has on diverting upwelling material towards the East African Rift are poorly resolved. We use overlapping records from several temporary and permanent broadband seismic arrays (1980-2014) located throughout the African continent and surrounding regions in order to image the upper mantle beneath the East African Rift and the Congo Craton where regional seismic arrays have not been deployed. We do this by seismic ambient noise tomography using the recently developed frequency-time normalization (FTN) method to extract empirical Green’s functions (EGFs) at periods of 7-250 seconds. We cross correlate the normalized continuous records and stack them to obtain EGFs for each temporally coincident station-station pair. We simulate wave propagation through a spherical Earth using a finite-difference method, measure phase delay times between synthetics and EGFs, and invert them for velocity perturbations with 3D Rayleigh wave sensitivity kernels. We will present results from full-wave ambient noise inversions that illuminate upper mantle structure throughout the continent, with particular focus on the Congo Craton and northern sections of the East African Rift System.