On crustal and lithospheric structures of rift basins formed within the Turkana Depression in the east Africa and the Trans-Southern African Orogen in the southwest Africa

This doctoral dissertation examines the crustal and lithospheric structures of rift basins formed within the Turkana Depression (TD) in east Africa and the Trans-Southern African Orogen (TSAO) in southwest Africa. Both regions have undergone complex structural and petrological evolution since the Archean eon and exhibit belts of deformations that resulted from the assembly and fragmentation of both Rodina and Pangaea. Since the Cenozoic, the crust and the sub-continental lithospheric mantle beneath the TD and the TSAO have been significantly weakened and stretched by plume related dynamics triggering the southward and the southwestward propagation of the East African Rift System. The Precambrian suture zones and the Mesozoic rift basins have also influenced these zones of extensional deformations. This doctoral dissertation is designed to explore the spatial extent of these structurally complex tectonic entities and investigate the role of these structures in the nucleation and development of various zones of extensional deformations, particularly rift basins related to the East African Rift System. To this end, geophysical, remote sensing, and numerical methods have been implemented to model the upper crustal-, crustal-, and lithospheric-scale structures beneath the TD and TSAO. For the first time, this doctoral work also introduces improved methods to estimate depth to the Moho and the Precambrian crystalline basement using potential field data. The details are presented in the following chapters summarized as three independent projects

Evolution of the Broadly Rifted Zone in Southern Ethiopia Through Gravitational Collapse of Dynamic Topograpy

The Broadly Rifted Zone (BRZ) is a 300 km wide diffused zone of extension in southern Ethiopia that connects the narrow (50-65 km wide) South Main Ethiopian Rift and the Eastern Branch of the East African Rift System (EARS) represented by the Kenya and Turkana rifts. The topographic features of the BRZ resemble those of the Basin-and-Range in the western United States in that they are characterized by the presence of NE-trending ridges and valleys superimposed on regionally uplifted (~2 km average elevation) terrain. This rift morphology, which is unique to the BRZ within the EARS, resulted from the presence of tilted blocks of Eocene-Pliocene volcanic rocks bounded by steep normal faults that exhibit half-grabens geometry. The tilted blocks rest unconformably on Precambrian crystalline basement and the half-grabens are filled with Miocene-Pliocene sedimentary rocks. The geodynamic mechanism through which the BRZ was developed is not well understood. Previous studies proposed that the BRZ hosts both tectonically active and inactive rift basins resulting from northward propagation of the Kenyan Rift and southward propagation of the Southern Main Ethiopian Rift. In order to understand the relationship between crustal and upper mantle structures beneath the BRZ and its unique morphological expression and extensional style, we first estimated the Moho depths using two-dimensional (2D) radially-averaged power spectral analysis of the World Gravity Map (WGM 2012) satellite gravity data. Subsequently, to verify results from the 2D radially averaged power spectral analysis, we developed lithospheric-scale 2D forward models along E-W profiles across the BRZ and extensional structures to north and south of it. We found that the Moho topography depicts a dome shape beneath the BRZ and it shallows to a minimum depth of 27 km in the central part of this dome. We suggest that the Moho doming, topographic uplift and arching of the crust underlying the BRZ is the result of mantle upwelling beneath the BRZ from a deeper asthenospheric source that changed to northeast lateral flow at shallower depth. This is supported by seismic tomography results which show the presence of low velocity anomaly of shear wave at lithospheric depth of 0-100 km and 100-175 km stretching in a NE-direction from beneath the BRZ to the Afar Depression. At depths between 175-250 km and 250-325 km the low shear wave velocity anomaly became only a broad elliptical feature centered beneath the BRZ. This created gravitationally unstable dynamic topography that triggered extensional collapse leading to the formation of the BRZ as a wide rift within the narrow rift system of the EARS.Geolog

Evolution of the Broadly Rifted Zone in Southern Ethiopia through Gravitational Collapse and Extension of Dynamic Topography

The Broadly Rifted Zone (BRZ) is a ~ 315 km wide zone of extension in southern Ethiopia. It is located between the Southern Main Ethiopian Rift and the Eastern Branch of the East African Rift System (EARS) represented by the Kenya-Turkana Rift. The BRZ is characterized by NE-trending ridges and valleys superimposed on regionally uplifted (~ 2 km average elevation) terrain. Previous studies proposed that the BRZ is an overlap zone resulted from northward propagation of the Kenya-Turkana Rift and southward propagation of the Southern Main Ethiopian Rift. To understand the relationship between the BRZ\u27s extensional style and its crustal and upper mantle structures, this work first estimated the Moho depth using the two-dimensional (2D) radially-averaged power spectral analysis of the World Gravity Map. Verification of these results was accomplished through lithospheric-scale 2D forward gravity models along E-W profiles. This work found that the Moho topography beneath the BRZ depicts a dome-like shape with a minimum depth of ~ 27 km in the center of the dome. This work proposes that the Moho doming, crustal arching underlying the BRZ and associated topographic uplift are the result of asthenospheric mantle upwelling beneath the BRZ. This upwelling changed to a NE-directed lateral mantle flow at shallower depth. This is supported by seismic tomography imaging which shows slow S-wave velocity anomaly at lithospheric depth of 75 km to 150 km stretching in a NE-SW direction from beneath the BRZ to the Afar Depression. This work proposes that the asthenospheric upwelling created gravitationally unstable dynamic topography that triggered extensional gravitational collapse leading to the formation of the BRZ as a wide rift within the narrow rift segments of the EARS

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

info:eu-repo/semantics/publishe

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift.

info:eu-repo/semantics/publishe