DEPTH AND TIME DEPENDENCE OF STRAIN PARTITIONING IN THE DEVELOPMENT OF MESOZOIC AND CENOZOIC EXTENSIONAL STRUCTURES WITHIN THE TURKANA DEPRESSION, EAST AFRICA

The Turkana depression is a NW-trending narrow topographic corridor between the Ethiopia – Yemen plateau in the northeast and the East Africa plateau to the southwest and it appears to suppress the surface expression of the East African Rift System (EARS). It extends in northern Kenya, southern Ethiopia and eastern South Sudan between the southern Main Ethiopian Rift in the northeast and the Kenya rift to the southwest. The depression is dominated by outcrops of Precambrian crystalline basement rocks, Mesozoic and Cenozoic rift-related sediment, and Cenozoic volcanic rocks of the EARS. The Mesozoic Anza rift extends within the Turkana depression and it representes a NW-trending failed arm of the Lamu paleo triple junction. The other two rifts of the triple junction resulted in the separation of Madagascar from the African continent. Geophysical studies suggest that the Anza rift is connected to the Mesozoic Sudan – South Sudan rifts, especially the Muglad – Jonglei – Mongala rift. The Anza rift is intersected at high angle by the N- and NE-trending rift segments of the EARS represented by the Turkana rift and the Kino - Sogo rift. In this work, we imaged the crustal thickness beneath the Turkana depression using two-dimensional (2D) inversion of the Bouguer gravity anomalies of the European Improved Gravity Model of the Earth by New Techniques (EIGEN – 6C4) as well as 2D forward gravity modeling of these Bouguer gravity anomalies. In addition, we modeled in three-dimension (3D) the upper crustal density distribution of the Kino - Sogo rift through the inversion of the residual gravity data obtained from the EIGEN – 6C4. Our results show thinner crust ranging between 24 and 28 km beneath the Turkana depression along the Anza rift and the Sudan – South Sudan rifts as well as localized crustal thinning beneath the Turkana rift. This observation is consistent with results from the 3D upper crustal density model which shows broad NW-trending density anomalies between 5 and 10 km depth beneath the Anza rift and narrow NE-trending density anomalies associated with the Kino Sogo rift between 0 m and 5 km depth. The findings suggest that the dominant crustal structure of the Turkana depression is associated with the Mesozoic Anza rift and the Sudan - South Sudan rifts whereas the Cenozoic EARS is expressed only at shallower upper crustal level.