Geometric Analysis of Faulted Rollovers: Contrasting Styles of Extensional Growth Folding in the Gulf of Mexico and Offshore Trinidad
Peter A. Bentham
Rapid sedimentation rates combined with the presence of seaward-dipping detachment levels in both the U.S. Gulf of Mexico (GOM) and the Columbus Basin (Offshore Trinidad) have facilitated the development of large extensional growth faults and their associated hanging-wall rollover folds. While detachment in the GOM is often associated with salt and/or overpressured shale, shale is believed to be responsible for detachment and translation of large stmctural blocks within the Columbus Basin. Although, gravity-driven extension seems to be the primary mechanism driving structural development in both regions, the detailed structural geometries and deformational mechanisms present seem to be quite different.
In the Columbus Basin, extensional rollovers are generally dominated by the presence of pervasive synthetic faults. Restoration of these structures suggests that these synthetic faults are accommodating the bulk hanging-wall deformation as the hanging-wall slides basinwards above an underiying listric fault surface. This synthetic simple shear deformation is not typically observed in Gulf of Mexico rollovers. In contrast, although synthetic faults are present, often spatially associated with the master growth fault, antithetic or mixed antithetic/synthetic faulting dominates the crest of the stwctures. An antithetic simple shear deformation is borne out by simple geometric modeling and restoration. The synthetic faulting seem in Trinidadian rollovers to be influenced by the presence o important vertical pressure seals (major unconformities) that may act as internal detachment levels within the larger folds. Synthetic faults are often observed to sole or die downwards into these intervals. Generally, lateral closure of rollover anticlines in the GOM is controlled by the underlying scoop-like shape of the major listric growth faults. This is not the case in Trinidad where Iistric faults usually have very linear trends in map view. Instead, lateral closure at the field-scale seems to be controlled by the pre- and/or syn-extensional development of linear WSW-ENE trending highs. These structural ridges trend nearly perpendicular to the strike of the major extensional faults. It is probable that the location of these ridges is related to the structural influence of underly ng transpressive strike-slip faults.
The on-going interplay of strike-slip faulting and gravity-driven extension may account for the both differences in extensional styles observed between the GOM and Trinidad study areas.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California