DEVELOPMENT OF BROADLY RIFTED ZONE THROUGH GRAVITATIONAL COLLAPSE OF DYNAMIC TOPOGRAPHY AND THREE-DIMENSIONAL (3D) STRAIN PARTITIONING WITHIN CRUSTAL DENSITY STRATIFICATION AND OVERLAPPING RIFTS

No two rift segments of the East African Rift System (EARS) are alike since they do not occupy the same space simultaneously nor rest upon the same lithospheric structure. Consequently, each segment of the EARS evolves uniquely within its own distinct lithospheric settings. This study focuses on the evolution of two specific rift segments within the EARS: 1) the Broadly Rifted Zone (BRZ) of southern Ethiopia, known for its unusual breadth spanning 315 km, and 2) the Turkana Rift, situated within the Turkana Depression (TD) characterized by an anomalously low topographic corridor. Using the data from the World Gravity Model – 2012 model within the BRZ, the depth to the Moho using two-dimensional (2D) radially-averaged power spectral analysis and 2D forward gravity models was calculated. The results showed a dome-like shallowing of the Moho directly beneath the BRZ which correlated with S-wave velocity anomalies within the lithosphere and the uppermost part of the asthenosphere through weighted depth-profile slices of 0-100 km, 100-175 km, 175-250 km, and 250-325 km. The combined gravity and seismic models showed that the elliptical topography profile and the crust's dome-like arching beneath the BRZ are consequences of an ascending mantle body from approximately 325 km in depth, deflecting northeastward at shallow depths to form a mantle flow. This observation attributes the formation of the BRZ to the collapse of gravitationally unstable dynamic topography. In contrast, 2D and 3D gravity inversions, along with 2D forward gravity models, show that the Turkana Rift develops within the TD, where the lithosphere is significantly weakened and stretched by the Kenya-Sudan Rift (KSR). The oblique superimposition of the Turkana Rift on the KSR could potentially involve the reactivation of their E-W trending structures through dike intrusions beneath areas such as the Kino Sogo fault system. Both the BRZ and the Kenya Rift exemplify that, despite being part of the same transcontinental rift, rift segments evolve uniquely owing to the heterogeneity of the lithosphere on which they rest.