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Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola

¹ Bureau of Economic Geology, University of Texas, Box X, University Station, Austin, Texas 78713-8924, USA

The Kwanza Basin, Angola, is divided into the Inner and Outer Kwanza salt basins, separated by a chain of synrift basement highs on which Aptian (112–122 Ma) salt is thin or absent. North- to northwest-trending basement structures in the Inner Kwanza Basin have repeatedly been reactivated since Neocomian (144–127 Ma) rifting. Reactivation formed three northwest-striking fold-and-thrust belts near basement uplifts. The thrust belts are bounded by northeast-striking rift-related transfer-fault zones that were apparently reactivated during subsequent shortening. Three episodes of postrift, basement-involved shortening are documented in the Inner Kwanza Basin: (1) Albian–early Cenomanian (112–96 Ma), (2) Senonian (89–65 Ma), and (3) Oligocene–Holocene (34–0 Ma). We relate the Albian–early Cenomanian event to ridge push, the Senonian event to global-plate reorganization, and Oligocene–Holocene shortening to uplift of the African superswell.

Structural segmentation of the Inner Kwanza Basin controlled the evolution of salt structures. Adjacent to basement uplifts, diapirs were initiated as buckle folds. Some anticlines were unroofed by erosion and evolved into passive salt walls. Elsewhere, broad salt walls were triggered by either detached extension or basement-block uplift. These walls grew until they exhausted their supply of salt. Thereafter, dissolution rates exceeded rates of salt inflow, so the walls began to subside. Withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous. Trough fill ranges in age from Cenomanian to Pliocene, and this age varies greatly from trough to trough and along strike within troughs.

Key Words: Angola • basement tectonics • Kwanza Basin • passive margins • salt tectonics • segmentation

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