Experimental Modeling of Geometry and Evolution of Structures above Ductile and Frictional Detachments

Abstract

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. Analog models, using silica sand to represent sediments and silicone gel to represent salt, are 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 orientations of the boundary between the frictional and ductile detachments, different thickness ratios between the ductile layer and overburden layers and the width of deformable backstop are investigated in this study. The results of experiments show different structure styles for the two settings: detachment folds with both forethrusts and backthrusts for ductile detachments, and duplex structures with only forethrusts for frictional detachments. A relatively steeper wedge was 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. Thrust faults connect across the two settings with significant changes in position and resulting changes in orientation. Backthrusts above ductile detachments typically terminate at the boundary. The experimental models are compared with surface and subsurface examples of salt basin boundaries. The results are applicable to the mapping of fold-thrust structures in areas of poor subsurface data quality.