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Shear-Enhanced Compaction Folds: A New Class of Folds Interpreted From Observations in the Colombian Basin, Caribbean Sea

Abstract

A new 3D seismic survey images a series of subtle folds in the eastern Colombian Basin, Caribbean Sea, that lie at the western margin of the South Caribbean deformed belt (SCDB). The folds exist within an undrilled sequence of interpreted Tertiary deep-water sediments deposited on basement, and trend roughly parallel to the strike of the SCDB (~ENE-WSW). The folds are 10-15 km along strike. Typical limb dips are less than five degrees, with steeper dipping forelimbs that dip to the NNW, and shallower backlimbs that dip SSE. A NNW vergence direction interpreted from the steeper forelimb dips is consistent with the shortening direction within the SCDB. Maximum fold heights are ~400 m. The folds are associated with a sequence of strike-parallel faults that form keystone collapse graben in the middle to lower portion of the folded sediments. Parts of the graben bounding faults are inverted within folds in close proximity to the SCDB. The folds lie directly above basement highs that represent horst blocks, suggesting they are clearly influenced by the basement geometry. However, no clear detachment surface is apparent between the basement and overlying sediments, and the folds are not fault-related (i.e., fault-bend fold or fault-propagation folds). All of the strata within the folded sections are lifted above the regional elevations. In contrast to models of detachment and fault-related folds that exhibit a systematic decrease in excess area (i.e., area lifted above the regional elevation) with depth, these folds exhibit an increase. Furthermore, we observe fold amplitudes to decrease with distance above the basement. A series of two-dimensional sequential restorations and forward models suggest a large portion of the folding can be explained by compactional drape folding above the basement highs, consistent with observed increase in excess area and fold amplitude with depth. However, the asymmetric fold geometry, with consistently steeper NNW dipping forelimbs, cannot be explained by the basement geometry alone and require a component of shear that is modeled using bed-parallel flexural slip. Furthermore, folds within close proximity to the SCDB experience enhanced shortening that result in greater fold amplitudes than cannot be explained solely by compactional drape folding. We refer to this new class of folds as Shear Enhanced Compaction Folds because they require a combination of kinematic models to accommodate both compaction drape folding and shear.