Extensional strain in salt-influenced basins

Abstract

The structural style of salt-rich extensional basins may significantly differ from those in salt-free settings. One key observation is growth folds, developed above the upper tips of propagating normal faults, may persist throughout extension in salt-rich settings, whereas they are largely transient features in salt-free settings, which are often breached during early extension. A second difference is salt-rich basins containing salt diapirs may structurally overprint regional extension and impart further strain on the surrounding country rock, typically as drape folding or radial faulting. Growth folds, drape folds and radial faults are ubiquitous in these salt-rich settings, however, uncertainties remain with respect to: (i) how extensional strain is accommodated above and below salt; (ii) how growth folds evolve and the underlying controls on geometry; (iii) how strain is accommodated around rising salt diapirs; and (iv) the implications for these structures for hydrocarbon exploration. To address these issues, three-dimensional seismic reflection data from the Halten Terrace (offshore Norway) and the Santos Basin (offshore Brazil), a compiled database of growth fold geometry and occurrence in models and in nature, and a series of kinematic trishear models are used. The results demonstrate that: (i) growth folds may accommodate significant amounts of extensional strain in salt-rich settings, affecting our ability to determine kinematic coherence and deformation related to thick-skinned, whole-plate stretching and independent, gravity-driven deformation; (ii) growth folds establish their near-final width early during folding, whereas fold amplitude increases with fault throw, and thus, the shape and size dramatically changes throughout folding; (iii) the causal mechanism for radial faulting will likely change as roof thickness varies during diapirism reflecting the interplay of the sedimentation rate and salt volumetric flux; and (iv) growth folds and radial faults adjacent to diapirs provide a rare opportunity to target vertically-stacked hydrocarbon reservoirs.