THIN-SKINNED EXTENSIONAL TECTONICS ON A SALT DETACHMENT, NORTHERN KWANZA BASIN, ANGOLA

Thin-skinned extension on a salt detachment, of a style known as raft tectonics, characterizes deformation of the late Aptian to Recent section in the northern Kwanza Basin, Angola. Large rafts and intervening grabens are interpreted to have developed as a result of gravity gliding (increased dip) and gravity spreading (asymmetrical loading of a sedimentary wedge). Major raft movement occurred primarily from the Middle Tertiary, but some deformation appears to have started as early as late Cenomanian or early Turonian times. Individual grabens measure up to 70 km in length, 15 km in width and nearly 3 km in depth, and show switching polarities in the sense of thickening of the Tertiary sedimentary infill. Graben geometries were controlled by syndepositional extension and the local availability of salt. From the data examined, which includes both seismic and well data, the grabens appear to become younger from east to west, reflecting basinward progression of the deformation. As the age relationships could be related to insufficient sampling of strata deposited in complex sedimentation patterns, they may have no relevance to the timing of the deformation; the diachronous deformation is therefore speculative. Extension in the northern Kwanza Basin is estimated at 60%; the outermost raft is translated almost-equal-to 55 km westwards. In addition to the extensional structures, compressional folds, thrust faults and salt structures mark the toe of the gravity gliding and spreading system. All of the rafts lie within the African portion of the Aptian Salt Basin, on rifted continental crust, and they are generally unrelated to structures below the salt. Biostratigraphy and lithostratigraphy indicate that the grabens formed on the shelf and upper slope from the Middle Eocene. Associated compressional features developed near the slope rise. The unusual expression of raft tectonics is attributed to a combination of: (1) the presence of a salt layer; (2) a stable basement configuration that allowed quiescent accumulation of the raft sequence; and (3) an increase in basin dip and sediment supply.