Structural Styles in Deepwater Gravitational Fold-and-Thrust Belts — Mobile X Non-Mobile Detachment Zones

Pedro V. Zalán
E&P, Petrobras, Rio de Janeiro, Brazil

Continental margins are characterized by basal thinned crust and tilted/rotated rift blocks overlain by thermal subsidence sedimentary packages deformed by linked extensional-compressional systems dominated by gravitational stresses. As flexural bending evolves the depositing sedimentary sequences of the platform/upper slope regions start to slide down under the influence of gravity, via normal listric faults, and move towards the lower slope/abyssal realms. Detachment zones nucleated in plastic lithologies provide the gliding media to the moving allochthonous rocks. In the deep/ultra-deep waters shortening and compression develop due to diverse buttressing effects. These gravitational fold-and-thrust belts (GFTBs) resemble in every aspects, but the dimensions, the orogenic foldbelts of collision zones. In the last decade they have been the primary targets of several petroleum exploration campaigns of success all over the world. The structural styles developed in GFTBs vary a great deal and are primarily a function of the mobile (intruding) or non-mobile (non-intruding) nature of the detachment zone. As a consequence, the type/geometry/size of the associated anticlines can be predicted in advance of the more detailed seismic acquisition/interpretation steps. In GFTBs dominated by mobile salt (halite) the detachment media bends, folds and intrudes the overlying allochthons, greatly increasing the complexity of the overall compressional deformation. Salt bubbles/sills/canopies and bodies of diverse geometries cut and override the sedimentary strata creating overturned and vertical beds and enhancing folds and thrust faults. GFTBs developed on non-mobile shales present a more organized deformation constituted by a predictive succession of fault-related folds. Examples from the margins of Brazil and Africa will be presented and their petroleum potential discussed.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery