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1 Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138; corredor{at}fas.harvard.edu
2 Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138
3 ChevronTexaco, 4800 Fournace Place, Bellaire, Texas 77041; present address: Unocal E&E, 14141 Southwest Freeway, Sugar Land, Texas 77478
Freddy Corredor is a Colombian national and a Ph.D. candidate in structural geology and tectonics at Harvard University. He received an M.S. degree in structural geology from the University of Colorado at Boulder in 1999 and a B.S. degree in geosciences from the National University of Colombia in 1994. He worked as exploration geologist at Occidental Oil and Gas from 1993 to 1996. Freddy's research is focused on three-dimensional structural analysis and restoration of contractional and extensional fault-related fold systems and the interaction between tectonics and growth sedimentation.John H. Shaw is professor of structural and economic geology at Harvard University. He received a Ph.D. from Princeton University in structural geology and applied geophysics and was employed as a senior research geoscientist at Texaco's Exploration and Production Technology Department. Shaw's research interests include complex trap and reservoir characterization in fold and thrust belts and deep-water passive margins. He heads the Structural Geology and Earth Resources Program at Harvard, an industry-academic consortium that supports student research in petroleum systems.
Frank Bilotti received a Ph.D. in structural geology from Princeton University in 1997 and a B.S. degree in geology and applied mathematics from the University of Miami. He worked as a structural consultant in Texaco E&P Technology from 1996 to 2001. He is presently at Unocal E&E Technology where he is a structural geology specialist. Frank's current technical interests are in Gulf of Mexico salt tectonics and three-dimensional restoration technology.
The deep-water Niger Delta includes two large fold and thrust belts, products of contraction caused by gravity-driven extension on the shelf that exhibit complex styles of thrusting. These contractional structures formed above multiple detachment levels in the overpressured shales of the Akata Formation. Using the patterns of growth sedimentation, fold shapes, fault-plane seismic reflections, and combined conventional and shear fault-bend folding theories, we describe and model the structural styles and kinematics of the fault-related folds and imbricate thrust systems that compose these belts. Individual fault-related folds, involving both forethrusts and backthrusts, are characterized by long planar backlimbs that dip less than the associated fault ramps, with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation. Forelimbs are short compared to backlimbs, but growth strata show more consistent dips that suggest a component of folding by kink-band migration. Thus, we employ a combination of classic and shear fault-bend fold theories to describe these structures, including the influence of a weak basal detachment zone in the overpressured shales. We expand upon these theories to model the kinematics of imbricate thrust systems, which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta. Regional patterns of folded growth strata are used to define break-forward, break-backward, and coeval thrusting involving single and multiple detachment levels. We define two main types of imbricate thrust systems: type I system with a single basal detachment level and type II imbricate system with multiple basal detachment levels, which cause massive structural thickening of the Akata Formation and refolding of shallow thrust sheets. Through the sequential restoration of two regional cross sections across these systems, we resolve the structural styles, the timing and sequences of thrusting, as well as the regional amounts of shortening, all of which have important implications for hydrocarbon maturation and charge in the deep-water Niger Delta.
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