ABSTRACT: The Northern Kwanza Basin--An Unusual Example of Raft Tectonics

LUNDIN, ERIK R., Conoco Norway, Stavanger, Norway

The Northern Kwanza Basin, Angola, displays a rare style of thin-skinned raft tectonics. Large rigid blocks, termed rafts, of the late Aptian to latest Cretaceous post-salt section have been extended approximately 55 km on the Aptian salt detachment. Separating the rafts are grabens containing syndeformational Tertiary to Holocene sedimentary fill. In cross section, the grabens display a peculiar geometry, which includes growth sequences that alternate in expansion direction, demonstrating a "rocking chair-like" motion during growth. Regionally, the grabens become younger toward the west, revealing basinward progression of the deformation. Extension on the shelf and upper slope is coupled with compression near the slope rise.

Geometric and temporal relationships provide the basis for a new structural model. Cross-section restoration and forward modeling reveal the kinematic history and the key role salt has played during rafting. This rare expression of raft tectonics is controlled by a unique combination of (1) a stable basement configuration permitting quiescent accumulation of the raft section, (2) the presence of a thin salt layer acting as a glide surface, and (3) rapid Tertiary shelf progradation driving the deformation by gravity spreading. During deformation, salt has been propelled basinward, thereby leaving the front of the extensional region free to move while the rear successively has locked. Geometries within the grabens are related to the local availability of salt.

Field analogs from other parts of the world are not known to the author, but published scaled deformation experiments show several similarities with the model.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)