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Fault Controlled Extension Regime — A Case of the Albertine Graben, East African Rift System, Uganda


The Albertine Graben forms the northernmost termination of the western arm of the East African Rift System. Rifting was initiated during the Miocene and thick sediments have accumulated in asymmetric basins along strike of the rift system. The rift is highly segmented and bordered by en–echelon border faults typically ten to forty kilometers long. Border faults have been linked to form deep seated and long fault segments. The Albertine Graben can be divided into three structural domains based on structural geometry and trend; the southern, central and northern domains. The structural elements in the southern domain trend in a NNE–SSW direction, structural elements in the central domain change to a NE–SW direction while the structural elements in the northern domain return to a NNE–SSW trend. These structural domains are separated by accommodation zones. The southern structural domain is volcanic and structures that have been created in this part of the Albertine Graben have been influenced by volcanism in part. On the contrary, the central and northern structural domains are non-magmatic. The southern structural domain consists of the Lakes Edward-George basins and is controlled by two synthetic parallel deep seated extensional faults that dip to the west. The central domain is controlled by two antithetic, deep seated faults that create some elements of transpressional forces between them. The northern domain is asymmetrical and controlled by one basin bounding fault. Unlike the southern structural domain, the basins in the central and northern structural domains are not volcanic. The structures formed in these basins have been mainly by extension processes without volcanic influence. The internal structures within the sedimentary succession are characterized by NW–SE and NE–SW Miocene fault trends that penetrate basement through to recent strata. The NW-SE trend is an inherited structure of the Pre-Cambrian lineaments. The Late Pliocene NE–SW fault trends crosscut this trend and are up to 5 km long. The extension episode was followed by a mild Pleistocene inversion where the older NE–SW faults were reactivated producing anticlinal structures in the hanging walls of extensional faults. This article will use geological and geophysical data acquired for petroleum exploration to discuss fault control on structural evolution of the Albertine Graben.