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 topogra phy DEM of Africa in conjunction with new and previous gravity data from La kes Albert, Edward and George, we have determined the distribution, amplitu de, and style of deformation responsible for the formation of the Albertine rift system, East Africa. Further, we have been able to approximate the th ree-dimensional architecture of the Albertine rift basin by analyzing a ser ies 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 und erestimated the crustal extension. The single most important factor that co mpromised the success of these modeling efforts was the assumption that cru stal extension was limited to the present-day distribution of the rift lake s. The style of deformation appears to have changed with time, beginning wi th a regionally distributed brittle deformation across the region that lead progressively to the preferential growth and development of the major bord er faults and antithetic/synthetic faults within the collapsed hangingwall block. Minor fault reactivation within the footwall block appears to be rel ated to the release of bending stresses associated by the flexural uplift o f the rift flank topography. By simultaneously matching the observed and mo deled topography and free-air gravity across the Albertine rift system, we have determined a cumulative extension ranging from 6 to 16 km with the max imum extension occurring in the central and northern segments of the basin. Crustal extension is not constrained to the lake proper, but extends signi ficantly to the east within the hangingwall block. Effective elastic thickn ess, 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 relat es possibly to small changes in crustal thickness, heterogeneities in crust al composition, and/or variations in radiogenic crustal heat production. Ma ximum sediment thickness is predicted to be 4.6 km and occurs within the ce ntral region of Lake Albert. Low bulk sediment densities, correlating with the location of major lake deltas, may be indicative of present-day sedimen t overpressures. Our results show that basin geometry is strongly dependent on the cumulative (and distribution) of lithospheric extension and the fle xural rigidity of the lithosphere. Thus, in order to determine the total am ount of extension responsible for the formation of a basin system, it is ne cessary to independently constrain the flexural strength of the lithosphere both during and after extension. Conversely, in order to determine the rig idity of extended lithosphere using the stratigraphy and/or geometry of rif t basins and passive margins, it is necessary to independently constrain th e cumulative extension of the lithosphere. (C) 2001 Elsevier Science Ltd. A ll rights reserved.