Distribution of crustal extension and regional basin architecture of the Albertine rift system, East Africa

By applying a kinematic and flexural model for the extensional deformation of the lithosphere, and using a recently available EROS Data Center topography DEM of Africa in conjunction with new and previous gravity data from Lakes Albert, Edward and George, we have determined the distribution, amplitude, and style of deformation responsible for the formation of the Albertine rift system, East Africa. Further, we have been able to approximate the three-dimensional architecture of the Albertine rift basin by analyzing a series of profiles across and along the rift system for which we also estimate the flexural strength of the rifted continental lithosphere and its along-strike variation. Previous modeling studies of the Lake Albert basin either overestimated the flexural strength of the extended lithosphere and/or underestimated the crustal extension. The single most important factor that compromised the success of these modeling efforts was the assumption that crustal extension was limited to the present-day distribution of the rift lakes. The style of deformation appears to have changed with time, beginning with a regionally distributed brittle deformation across the region that lead progressively to the preferential growth and development of the major border faults and antithetic/synthetic faults within the collapsed hangingwall block. Minor fault reactivation within the footwall block appears to be related to the release of bending stresses associated by the flexural uplift of the rift flank topography. By simultaneously matching the observed and modeled topography and free-air gravity across the Albertine rift system, we have determined a cumulative extension ranging from 6 to 16 km with the maximum extension occurring in the central and northern segments of the basin. Crustal extension is not constrained to the lake proper, but extends significantly to the east within the hangingwall block. Effective elastic thickness, T_e, varies between 24 and 30 km and is unrelated to either the amount of extension or the maximum sediment thickness. The variation of T_e relates possibly to small changes in crustal thickness, heterogeneities in crustal composition, and/or variations in radiogenic crustal heat production. Maximum sediment thickness is predicted to be 4.6 km and occurs within the central region of Lake Albert. Low bulk sediment densities, correlating with the location of major lake deltas, may be indicative of present-day sediment overpressures. Our results show that basin geometry is strongly dependent on the cumulative (and distribution) of lithospheric extension and the flexural rigidity of the lithosphere. Thus, in order to determine the total amount of extension responsible for the formation of a basin system, it is necessary to independently constrain the flexural strength of the lithosphere both during and after extension. Conversely, in order to determine the rigidity of extended lithosphere using the stratigraphy and/or geometry of rift basins and passive margins, it is necessary to independently constrain the cumulative extension of the lithosphere.