Effective elastic plate thickness beneath the East African and Afar plateaus and dynamic compensation of the uplifts

Abstract

The broad topographic domes, or plateaus, of East Africa and Afar are characterized by long‐wavelength negative Bouguer gravity anomalies and volcanically active rift valleys. Gravity and topography data from the East African and Afar plateaus and data from the stable cratonic regions to the west were subdivided into 17 smaller regions to study the variation of elastic plate thickness within part of the African continent, its relation to rifting processes within these intracontinental plateau regions, and compensation mechanisms for the broad uplifts. Assuming that loads at the surface, within, and beneath the base of a thin elastic plate contribute to the observed Bouguer gravity anomalies, the wavelength dependence of the coherence between gravity and topography was used to determine the effective elastic plate thickness in each of the subregions. Estimates of elastic plate thickness were found to be 64–90+ km in the stable cratonic areas, provided that surface and subsurface loads are uncorrelated. Lower estimates (43–49 km) were obtained in the largely unfaulted regions encompassing the broad uplifted plateaus and the narrower Darfur dome to the west of the Afar plateau. Estimates of 21–36 km correspond to regions that include the severely faulted and commonly volcanically active Kenya, Western, and Ethiopian rift valleys as well as unfaulted regions adjacent to the rift valleys. We attribute the smallest estimates of elastic plate thickness (21–36 km) to averaging unfaulted topography with mechanically weakened topography within the severely faulted Kenya, Western, and Ethiopian rifts. The linear transfer function between gravity and topography within the uplifted East African plateau region at wavelengths longer than 1000 km can be explained by a dynamic uplift mechanism and associated heating of the thermal lithosphere above a convecting region within the asthenosphere. These isostatic and dynamical compensation mechanisms are consistent with existing geological and geophysical data and with constraints on the timing of volcanism and uplift within the East African plateau region.