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AAPG Bulletin, V.
Evolution of the Cretaceous Astrid thrust belt in the ultradeep-water Lower Congo Basin, Gabon
1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713; [email protected]
2Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713; [email protected]
3Apache Corporation, 2000 Post Oak Boulevard, Houston, Texas 77056; [email protected]
4Shell Exploration and Production Company, 900 Louisiana Street, Houston, Texas 70002; [email protected]
The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africa's Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeirogenic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleoslope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward.
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