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Structural Segmentation, Inversion, and Salt Tectonics on a Passive Margin: Tectonic Evolution of the Inner Kwanza Basin, Angola

Abstract
The Kwanza Basin, Angola, is divided into the Inner and Outer Kwanza salt basins by the Central Platform, a synrift basement high on which Aptian salt is thin or absent. Despite their location on a passive margin, basement faults in the Inner Kwanza Basin have repeatedly been reactivated since Neocomian rifting. Structures formed during reactivation include three fold-and-thrust belts, all located near basement uplifts. The thrust belts are bounded by northeast-trending transfer zones that segmented the basin during rifting and were apparently reactivated during subsequent contraction. At least three episodes of postrift basement-involved contraction are documented in the Inner Kwanza Basin: (1) Albian-Cenomanian, (2) Senonian, and (3) Oligocene-Recent. The Senonian event may coincide with a plate reorganization, but the other two appear unrelated to orogeny. We relate the Albian-Cenomanian event to ridge push and Oligocene-Recent shortening to uplift of the African Superswell.

Structural segmentation of the Inner Kwanza Basin played a central role in 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. In other areas, broad salt "megawalls" 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 megawalls began to subside. Withdrawal of salt from the megawalls 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 in a single trough.

For more information, please contact Michael Hudec. Telephone 512-471-1428. E-mail michael.hudec@beg.utexas.edu.
March 2003
 
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