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Passive Margin Fold and Thrust Belts

Lebit, Hermann D.1; Jensen, Luke 2
1 Int. New Ventures, Marathon Oil Company, Houston, TX.
2 Shell International E&P, Houston, TX.

Marine folds and thrusts form extensive belts along ocean slopes and are associated with prolific hydrocarbon basins. For the Atlantic Basin, including Gulf of Mexico, these sedimentary systems have common boundary conditions: They are located along passive continental margins with or without post/syn-rift evaporite sequences followed by Cretaceous to present siliciclastic accumulations, provided by large drainage systems.

Post-rift tectonic stresses play a minor role at passive margins and the sediment influx (load) and rate of deposition are the major driving forces that shape the internal architecture of the shelf to deep basin sections.

The prograding sedimentary systems drive progressive gravitational collapse, manifested by growth faulting dominating the shelf section, which again provide new sediment accommodation space. Rapid burial and disequilibrium compaction in the growing stratigraphic sequence may cause conditions that trigger salt withdrawal and fluid migration, forming significant reservoirs at structural traps.

Normal faults commonly bottom out at basal décollements that translates to contractional regimes farther outboard - where thin-skinned folds and imbricates dominate. Regional seismic sections capturing the complete passive margin sequence were used for sequential kinematic restoration that demonstrate balance between the extension in the shelf region and corresponding shortening near the slope toe.

Horizontal net displacement rates are one order of magnitude less than in comparable tectonic belts. Though bulk flow is relatively slow it fluctuates ubiquitously in these basins. This may be due to varying sediment supply related to climatic conditions or plate tectonic activities.

The thin-skinned architecture of the passive margin sequences is comparable with gravitational flow in large-scale mass wasting processes or the movement in glaciers. The latter reveals similar boundary conditions and loading processes and represents a suitable analog for modeling displacement fields in sedimentary wedges.

Mechanical instabilities, however, may cause local heterogeneous flows with dramatic rates such as rapid lateral salt extrusion along thrust planes and the formation of allochthonous salt canopies. In this respect the various styles of salt displacement (withdrawal or extrusion) are consider to be a consequence rather than a control of the deformation and the resulting architectural style of these basins.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009