Gravity-driven deep-water fold and thrust belts: sediment-structure interaction

Fold and thrust belts are found in deep-water on many passive margins associated with large Tertiary deltas and a long way from any plate boundary. They are analogous in size to the thin-skinned parts of fold and thrust belts (e.g. Pyrennees, Apennines) or accretionary prisms (Makran, Barbados) found in zones of plate convergence. As opposed to being driven by plate tectonic forces these passive margin ‘mountain belts’ form due to gravitational forces, where the entire sedimentary section, in places up to 10 km thick, has undergone gravity-driven deformation above weak salt or shale detachments. The resultant structures can conceptually be considered somewhat analogous to enormous landslides extending 200-300 km from onshore to the ocean floor at water depths around 4 km. Large, kilometre-scale, listric, extensional growth faults at the landward side of the margin are balanced by kilometre -scale fold and thrust belts and allochthonous salt sheets at the oceanward end of the system. Our knowledge of these systems has improved significantly in the last 10-15 years due to hydrocarbon exploration in the 'deep-water' plays of the West African, Brazilian and Gulf of Mexico passive margins largely driven by the improvements in seismic imaging, and the collection of large 3D seismic reflection datasets by the oil and gas industry. I will show some examples of imaging improvements and then I will discuss the interaction between deep-water sedimentary systems and structural growth. Work on three study areas, in the Gulf of Mexico, offshore Angola and on the Niger Delta respectively, show slope channels that modify their downstream course as they interact with growing structures. Ponded deposits are common in each area and local slumping associated with structural relief is observed in all three areas. We have quantified the shortening rate of thrust-related folds, salt-cored anticlines and faulted salt-detachment folds with the aim of investigating whether there are any predictive relationships between structural parameters such as structural relief, growth rate versus sediment accumulation rate, and depositional patterns. Our results for the Niger Delta show channels have been able to keep pace with the time integrated uplift since 1.7 Ma and have likely reached a local topographic steady-state. Entrenchment of the submarine channels upstream of growing folds helps to drive this process and we estimate that bed-shear stresses > 120 Pa are sufficient to keep pace with structural strain rates of –0.005 Ma-1.