Along‐axis segmentation and isostasy in the Western rift, East Africa

Abstract

Structural variations along the southern sectors of the Western rift, East Africa, have previously been described, but subsurface structures in the northern sector (Uganda, Zaire) are virtually unknown. Our aims are to investigate the along‐axis segmentation of the northern sector, thereby adding to the structural picture of the Western rift, and to study the isostatic compensation of the varying rift morphology along the sector's length. This study describes the first gravity survey to be carried out on the shallow Lake Albert, forward models of these and existing gravity data, and the results from inverse modeling of existing aeromagnetic data designed to delimit border and transfer fault systems. Our tectonic model shows that the northern rift sector is segmented along‐axis into five 25 to 65‐km‐wide, 80 to 100‐km‐long rift segments, characterized by closed‐contour Bouguer anomaly lows, and bounded by steep gravity, aeromagnetic, and topographic/bathymetric gradients. Werner and Euler deconvolution results and gravity anomaly data reveal that some faulted basins are separated by structural highs and cross‐rift ramps or faults and suggest sedimentary basin depths of 4–6 km. Forward modeling of structural and free‐air gravity profiles across individual basins and flanks using a model that assumes flexural compensation also suggests sediment thicknesses of up to 5.5 km, similar to the estimates from magnetic data. The basin and flank morphology can be explained by 6–9 km of extension of a lithosphere with an effective elastic thickness (Te) of 25 km (equivalent to a flexural rigidity of 1.4 × 1023 N m), similar to results in other Western rift basins. Potential field data and lithospheric strength estimates in the Western rift system show small along‐axis variations in lithospheric structure, regardless of the presence or absence of Cenozoic magmatism.