Upper Mantle Heterogeneity Beneath Continental Africa and Its Implications for Lithospheric and Asthenospheric Processes

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes
Type: Oral
Date: 4/29/2020
Time: 08:30 AM
Room: 120 + 130
Description: 

Our understanding of mantle processes in Africa is hindered by unevenly distributed seismic instrumentation throughout the continent. In order to better image remote regions and obtain a fuller sense of the connections between the cartoons, rifts and hotspots, we used long-period ambient noise full waveform tomography on data collected by seismic stations throughout Africa and surrounding regions. We extracted empirical Green’s functions from ambient seismic noise using a frequency-time normalization method and retrieved coherent signal at periods of 7-340 seconds. We simulated wave propagation through a heterogeneous Earth using a spherical finite-difference approach to obtain synthetic waveforms, measured the misfit as phase delay between the data and synthetics, calculated numerical sensitivity kernels using the scattering integral approach and iteratively inverted for structure. Our results provide isotropic, absolute shear-wave velocities in the upper mantle beneath continental Africa, revealing more heterogeneity than typically imaged in continental-scale models and also showing close agreement with regional models from temporary seismic deployments. We image clear separation between large, cratonic blocks and find evidence that the highest wavespeeds closely underlie Archean outcrops. We also find evidence for distinct upwellings in the upper mantle along the East African Rift System and in North Africa; these proposed upwellings often underlie areas of volcanism and, in some locations, appear to divert around lithospheric structures or connect to low-wavespeed anomalies near mantle transition zone depths. The absolute velocities associated with several low-wavespeed anomalies are lower than expected for purely thermal features. This suggests that melt and possibly also compositional variations contribute to the anomalies, which we interpret as an indication for thermochemical plumes beneath Africa.

Presenting Author: Erica L. Emry

Authors