The upper mantle seismic velocity structure beneath the Arabian Shield and East Africa.

摘要

Broadband seismic data from Ethiopia and the Arabian Shield have been used to image the seismic structure of the upper mantle beneath ∼70% of the Afar hotspot. In the northernmost area of the hotspot, the Arabian Shield, the results from a P wave tomography and a receiver function analysis of the mantle transition zone discontinuities reveal a low velocity anomaly in the upper mantle. The anomaly appears to extend to ∼300 kin depth but does not reach the transition zone. In contrast, the P and S wave velocity structure of the upper mantle and transition zone discontinuities beneath Ethiopia, also imaged using body wave tomography and receiver function analysis, reveal a broad, low wave speed anomaly in the upper mantle that extends to at least 660 km depth. Temperatures within the anomalous mantle, inferred from the tomographic models and relief on the 410 and 660 km discontinuities, are ∼300 K higher than normal. These results suggest that the Afar hotspot is caused by a broad, deep seated thermal upwelling in the mantle, possibly related to the African Superplume, as opposed to a shallow plume head.; The southern extent of uplift associated with the Afar hotspot has also been examined. Rayleigh wave group velocities have been used in a grid search algorithm to model crustal and upper most mantle structure beneath a region of low elevation in northern Kenya and southeastern Sudan between the Ethiopian and East African Plateaus. Model results reveal a thinned crust and slow upper most mantle velocities, suggesting that the low elevations are an isostatic response to thinned (i.e., rifted) crust, instead of a boundary in lithospheric structure marking the southern end of the Afar hotspot.