Integrated geophysical study of the crustal structure of the Southern Kenya rift

A significant amount of geophysical and geological data have been recently gathered around the Kenya rift, primarily associated as a result of the Kenya Rift International Seismic Project (KRISP). These data indicate the presence of a major mantle seismic velocity anomaly and crustal thickening beneath the Kenya dome, an uplift centered on the southern part of the Kenya rift. Detailed crustal studies north of the dome show thinning of the crust northward with an increase in lithospheric extension. This study employed refraction and teleseismic data from KRISP, surface geology, gravity data from the UTEP data base and 5,600 new gravity measurements, drill hole data from geothermal exploration wells, and mantle xenolith and petrochemical results to constrain the construction of integrated cross-sectional models of lithosphere by gravity modeling. The approach taken in this study was to first interpret the deep regional tectonics affecting the whole of the East African Plateau on which the Kenya rift is superimposed. It was important to understand rifting in the broader context of the East African Plateau and the interplay between mantle and crustal structures and their contribution to the full spectrum of gravity anomalies. The long wavelength anomalies associated with mantle structures were modeled in a regional sense and the remaining short wavelength anomalies of crustal origin were then modeled in detail. An updated data base of more than 156,000 gravity reading was used to construct a Bouguer anomaly map, a suite of filtered gravity maps and regional profiles across the East African Plateau which were modeled. These models together with gravity maps demonstrate lithospheric thinning beneath the rift valleys and suggest the existence of a deep mantle anomaly centered beneath the East African Plateau associated with the Tanzania craton. The mantle anomaly is widely believed to be a plume and our models indicate that the diameter of its head at 130km depth is about 600 km. In our models, two arms from the plume head with a diameter less than 250 km penetrate the lithosphere to shallow levels under the rifts. In the western rift and the southern part of the Kenya rift (Tanzania), the mantle anomaly is deeper than under Kenya. This study provides additional evidence that the long wavelength Bouguer gravity anomaly over the East African Plateau can be explained by lithospheric heating and thinning by thermal erosion of the cratonic root. A regional model along the axis of the Kenya rift suggests that the rift is propagating southwards into central Tanzania. The result of detailed integrated interpretation of the crustal structure of the southern Kenya rift show the following: (1) there is no north-south crustal thickness symmetry with respect to the apex of the Kenya dome along the rift axis; (2) the rift graben master fault is on the western flank and there is no evidence for half-graben polarity (master fault side) reversals along the entire southern part of the Kenya rift; (3) the pre-existing lithospheric contrast between the Archean and Precambrian tectonic grains plays a significant role in the rift\u27s location and structural geometry; (4) along axis crustal thickness is less related to crustal thickening by underplating than pre-rift crustal type and thickness