Passive Margin Fold and Thrust Belts

Marine folds and thrusts form extensive belts along ocean basin slope toes and are associated with prolific hydrocarbon provinces. For the Atlantic Basin, including the Gulf of Mexico, these sedimentary systems have common boundary conditions: they are located along passive continental margins, sometimes comprising pre/syn-rift evaporite sequences, followed by Cretaceous to present siliciclastic accumulations provided by ample drainage systems.

Post-rift tectonic stresses are considered insignificant at passive margin transitions; therefore sediment influx and rate of deposition are the major driving forces that shape the shelf-to-basin architecture.

These prograding systems drive progressive gravitational collapse in the sedimentary sequence, in shelf sections dominated by growth faulting, which provides additional sediment accommodation space. Rapid burial in the growing stratigraphic sequence may trigger salt withdrawal and fluid migration, forming significant reservoirs and structural traps.

Shelf normal faults commonly bottom out at basal décollements that translate into contractional regimes farther outboard – where thin-skin folds and imbricates dominate. Regional seismic sections capturing the complete passive margin sequence were used for sequential kinematic restoration that illustrate equilibrium between the extension in the shelf region and corresponding shortening near the toe of slope. Horizontal net displacement rates are ~one order of magnitude less than those in tectonic belts. Though bulk flow is relatively slow it fluctuates ubiquitously in these basins due to varying sediment supply.

The mechanics of these passive margin sequences is analogous to gravitational flow during mass wasting processes or in ice sheets, which represent suitable analogs for modeling displacement fields in sedimentary wedges.

However, localized dramatic flow rates are evidenced by rapid lateral salt extrusion along thrusts and/or may generate allochthonous salt canopies. In this respect the various styles of salt emplacement are consider a consequence, rather than a control of the deformation and resulting architectural style of these basins.