ALONG-AXIS SEGMENTATION AND ISOSTASY IN THE WESTERN RIFT, EAST-AFRICA

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 aim s are to investigate the along-axis segmentation of the northern secto r, thereby adding to the structural picture of the Western rift, and t o study the isostatic compensation of the varying rift morphology alon g the sector's length, This study describes the first gravity survey t o be carried out on the shallow Lake Albert, forward models of these a nd existing gravity data, and the results from inverse modeling of exi sting aeromagnetic data designed to delimit border and transfer fault systems, Our tectonic model shows that the northern rift sector is seg mented along-axis into five 25 to 65-km-wide, 80 to 100-km-long rift s egments, characterized by closed-contour Bouguer anomaly lows, and bou nded by steep gravity, aeromagnetic, and topographic/bathymetric gradi ents, 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 acro ss individual basins and flanks using a model that assumes flexural co mpensation also suggests sediment thicknesses of up to 5.5 km, similar to the estimates from magnetic data, The basin and flank morphology c an be explained by 6-9 km of extension of a lithosphere with an effect ive elastic thickness (T-e) of 25 km (equivalent to a flexural rigidit y of 1.4 X 10(23) N m), Similar to results in other Western rift basin s. Potential field data and Lithospheric strength estimates in the Wes tern rift system show small along-axis variations in lithospheric stru cture, regardless of the presence or absence of Cenozoic magmatism.