CRUSTAL STRUCTURE OF THE EAST-AFRICAN PLATEAU FROM RECEIVER FUNCTIONSAND RAYLEIGH-WAVE PHASE VELOCITIES

The origin of the East African Plateau and rift valleys is poorly unde rstood largely because little is known about the crust and upper mantl e beneath East Africa. The center of the plateau is composed of the Ar chean Tanzania Craton and is flanked by the Proterozoic Kibaran, Ubend ian and Mozambique Belts to west, southwest, and east, respectively. C enozoic faults of the East African rift system lie primarily within th e mobile belts. New estimates of Moho depths, mean shear velocity ((V) over bar s), and Poisson's ratio for the crust of the East African Pl ateau are presented and used to address questions related to the tecto nic development of the plateau and rift valleys. The new constraints o n crustal structure are obtained by separately modeling receiver funct ions and Rayleigh wave phase velocities from teleseismic earthquakes r ecorded by a deployment of 20 broadband seismic stations spread across Tanzania in 1994 and 1995 and then by combining the results of the se parate analyses to obtain estimates of mean crustal structure that sat isfy both sets of observations. For the Tanzania Craton, (V) over bar s is 3.79 km/s, Moho depths lie between 37 and 42 km, and estimates of Poisson's ratio are between 0.24 and 0.26. For the Mozambique Belt, ( V) over bar s is 3.74 km/s, Moho depths range between 36 and 39 km, an d estimates of Poisson's ratio are between 0.24 and 0.27. Parameter un certainties are +/-0.10 km/s for (V) over bar s, +/-4 km for Moho dept h, and +/-0.02 for Poisson's ratio. Results from stations in the Ubend ian Belt indicate a (V) over bar s of similar to 3.74 km/s and Moho de pths between 40 and 45 km. Based on a comparison of these results to g lobal averages for Precambrian crust, it can be concluded that there a re no regional scale anomalies in the crustal structure that can easil y explain the isostatic uplift of the East African Plateau and that Ar chean and Proterozoic crust in East Africa may be slightly more felsic than Precambrian crust elsewhere. In addition, patterns of crustal th inning beneath rifted areas in East Africa appear to be consistent wit h amounts of extension deduced from surface structures.