THE ROLE OF PRE-EXISTING STRUCTURES IN THE INITIATION OF THE NORTHERN WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM

The northern Western Branch of the East African Rift (EAR) consists of two segments: the magma-poor Albertine-Rhino graben and the magma-rich Lakes George-Edward graben. Recent studies of the northern Western Branch suggest that rifting is unlikely to be initiated by sublithospheric melt in the Albertine and Rhino grabens, implying that pre-existing structures may play a dominant role in facilitating the extension of this graben. In this study, we investigate the role of pre-existing structures in the initiation of the northern Western Branch using the 3D finite element code ASPECT. We use fault trace data available in the study region together with the average fault dip angles obtained from structural geologic mapping to create a 3D fault geometry using the Geodynamic World Builder (GWB) software package. The fault geometry created with GWB is used as an initial condition in a 3D regional ASPECT model, where deformation is driven by extensional velocity boundary conditions: a free surface at the top, tangential at the bottom, and a prescribed velocity boundary on the remaining four other sides based on the average extension rates provide by the Saria et al. (2014 kinematic model). Fault scarp heights are then calculated using the modeled velocities in the z-direction and the age of the rift. A comparison between the modeled fault scarp heights and the real-world (observed) fault scarp heights is used to infer whether or not pre-existing structures are the weakening mechanism facilitating the rifting. Our preliminary results suggest that pre-existing structures are the dominant weakening mechanism facilitating rifting in the Albertine graben.