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Mechanics and Kinematics of Backthrusting in Deepwater Fold and Thrust Belts: Observations from the Niger Delta

Ana Krueger and Ed Gilbert
Devon Energy, Houston, TX

The decollement surface for deep water fold-thrust belts (FTBs) can be either autochthonous salt (central Gulf of Mexico, SE Brazil) or massive shales (Niger Delta, Mexican Ridges of the Gulf of Mexico). Back thrusting – the formation of thrust structures vergent toward the hinterland – was described very early from continental foreland FTBs, where they are occur both as both discrete structures and as components of hydrocarbon-producing “triangle zones”. The formation of backthrusts is mechanically complex, and several models have been proposed. We conclude that the backthrusts of the Niger Delta periphery can be modeled as a critical taper wedge (Davis, 1983) in a near-isotropic medium. Backthrusts are therefore a predictable result of a dynamic interaction between (1) the slope of mechanical basement (controlled by the flexural rigidity of oceanic crust under sediment loading) and its associated detachment, (2) surface slope (the dominant parameter, controlled by syntectonic deltaic sedimentation), and (3) the relative changes of these properties through time as structures develop. High quality 3D seismic from the Niger Delta FTB illustrates a structural development sequence in which the sedimentary cover initially shortens by simple shear, with bi-directional thrusts. With the construction of a forward surface slope by both structural elevation and delta slope progradation, pure shear becomes dominant, and foreland-vergent thrusts become dominant. Where no significant surface slope is developed, backthrusts may develop preferentially, and even locally dominate the structural style. The development of backthrusts has significant impact upon hydrocarbon maturation and migration pathways.