JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B3, PAGES 2883-2901, MARCH 10, 1989 Effective Elastic Plate Thickness Beneath the East African and Afar Plateaus and DynamicCompensation of the Uplifts C. J.EBINGER1,2, T. D. BECHTEL 3, D. W. FORSYTH 4, AND C. O. BOW• 5 The broad topographic domes,or plateaus, of East Africa and Afar are characterized by long-wavelength negative Bouguer gravity mtomaliesand volcanically active rift valleys. Gravity and topography data from the East African and Afar plateaus and data from the stable cratonicregions 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. Assumingthat loads at the surface, within, and beneath the base of a thin elastic plate contribute to the observedBouguer 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 obtainedin the largely unfaultedregionsencompassing the broad uplifted plateausand the narrowerDarfur dome to the west of the Afar plateau. Estimates of 21-36 km correspond to regionsthat include the severelyfaulted and commonlyvolcanically 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 topographywithin the severely faulted Kenya, Western, and Ethiopian rifts. The linear transfer function between gravity and topography within the uplifted EastAfricanplateau regionat wavelengths longer than 1000 km can be explainedby 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 volcanismand uplift within the East African plateauregion. INTRODUCTION Broad uplifted plateaus and narrow rift valleys characterize the topographicrelief of East Africa (Figure 1). A representative cross section through EastAfrica (Figure 1) illustrates the approximately 1300-km-wide,1200-m-high EastAfrican dome,or plateau, andthe muchnarrower rift valley and volcanic topography of the Kenya andWestern rift systems that is superimposed on the plateau. Similarly, the narrow Ethiopian rift system and Afar depression are superimposed on theAfar plateau, whichlies northeast of the EastAfricanplateau, and to the southwest 1Now atGeodynamics Branch, Goddard Space Flight Center, Greenbelt, Maryland. 2Massachusetts Institute of Technology/Woods Hole Joint Program in Oceanography, Department of Earth,Atmospheric, and PlanetarySciences, Cambridge. 3Kurz Associates, Inc., Providence, Rhode Island. 4Department of Geological Sciences, Brown University, Providence, Rhode Island. 5Department ofGeology and Geophysics, Woods Hole Oceanographic Institution, Massachusetts. Copyfight 1989 by the American Geophysical Union. Paper number 88IB03946. 0148-0227/89/88 JB- 03946505.00 andsoutheast of theRed Sea andGulf of Adenspreading centers (Figure 1). In contrast to the two broad plateaus, the 500-km-wide Darfurdome in thenorth central partof the study region hasno associated rift valleytopography. Broad, negative Bouguer gravity anomalies coinciding with the topographic domes havebeenused to indicate that the upliftsare isostatically compensated [e.g.,Bullard, 1936; Girdler and Sowerbutts, 1970;Khan and Mansfield,1971; Makris et al., 1972;Fairhead, 1976;Bermingham et al., 1983], but thereis no systematic correlation between the shorter wavelength features of thetopography and Bouguer gravityanomalies (Figure 1, inset). The lackof correlation between gravity and topography at short wavelengths suggests that a regional or flexural compensation model maybe appropriate, with the short wavelength surface and internal loads supported by elastic stresses within the lithosphere. The broad domal uplifts and Tertiary to Recent volcanism of EastAfrica have been attributed to elevated geotherms within thelithosphere beneath these regions [e.g., Thiessen et al., 1979; Wendlandt and Morgan, 1982; Crane and O'Connell, 1983]. In earlier interpretations of gravity data from East Africa and Afar, the plateau topography is supported by buoyancy forces dueto low-density material beneath theuplifted plateaus, andthislow-density material produces the associated negative Bouguer gravity anomaly [Girdler and Sowerbutts,1970; Searle and Gouin, 1971; Fairhead, 1976; Bermingham et al., 1983]. In support of 2883