Deepwater Angolan Toe-Thrust Kinematics and Structural Styles

Abstract

Toe-thrusts are common structural features at the tail end of gravitationally driven systems found in several passive margins, including some of the world's most prolific oil basins. These structures accommodate the updip extensional collapse of the sediment load and often record significant magnitudes of shortening in the absence of true tectonic stresses. In the deepwater offshore Angola, this process has occurred through the Cretaceous to the modern, where a thick stratigraphic section has been deposited upon ductily deforming post-rift salt. Near shore, the shelf has collapsed through time and salt has been extruded out into a deepwater fold and thrust belt. We document the three-dimensional development of this fold and thrust belt throughout its deformational history. Our analysis demonstrates that the fold and thrust belt grew through a combination of simultaneous shortening, allochthonous salt emplacement, and salt evacuation. Following an initial phase of detachment fold buckling directly above the salt layer, formation of the thrust faults allows for the emplacement of allochthonous salt bodies, continuous with fault slip. Salt body aggradation at or near the paleo-seafloor is driven by additional lithostatic loading from hanging wall emplacement and intra-structural sedimentation. Subsequent salt withdrawal causes the faults to “collapse” over time, eventually sealing the salt conduit and terminating local allochthonus salt growth. Inter-basin growth kinematics record this process, indicating downward displacements of up to 900m in fault hanging walls during salt withdrawal. Restoring synkinematic growth strata deformed by this process suggests that the regional pre-deformation salt detachment may have formed a ramp that dipped ∼2° towards the near-shore shelf. Variations in deep, pre-salt architecture and the subsequent pre-deformational salt thickness likely influence the sequencing of thrusting and localization of intra-basinal sedimentation.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014