EXPERIMENTAL MODELING OF THE GEOMETRY AND EVOLUTION OF STRUCTURES ABOVE DUCTILE AND FRICTIONAL DETACHMENTS

Fold-thrust belts formed above ductile units like salt are typically characterized by detachment folds, whereas those formed above frictional detachments contain fault-related folds, such as fault-bend folds and duplexes. Scaled analog models, using silica sand to represent sediments and silicone gel to represent salt, were conducted to study the fold geometry, fold-fault relations, and sequential development of structures formed in each setting and at the boundaries between the two settings. Variations in the geometry of the boundary between the two settings and different thickness ratios between the ductile substrate and the overburden layers were investigated. Different structure styles were observed for the two settings: detachment folds with both forethrusts and backthrusts for ductile detachments, and duplex structures with only forethrusts for frictional detachments. Larger thickness ratios between the ductile substrate and overburden layers resulted in more asymmetric folds than smaller thickness ratios. A relatively steeper wedge developed above a frictional detachment, so that the deformation front above a ductile detachment propagated farther forward than the deformation front above a frictional detachment. Experiments with different orientations of the boundary between the frictional and ductile detachments confirm that the geometry of the boundary strongly controls the geometry of overburden structures. Thrust faults connect across the two boundaries with significant changes in position and resulting changes in orientation. Backthrusts above ductile detachments typically terminate at the boundary. The experimental models were compared with surface and subsurface examples of salt basin boundaries from both continental and deep water settings. The results are directly applicable to the mapping of fold-thrust structures in areas of poor subsurface data quality.