Salt detachment deformation: the influence of salt thickness and proximity on structural geometry

Abstract

Structural style above detachment zones has been related to thickness and proximity to the basal detachment layer. Fieldwork, structural measurements and seismic interpretation of evaporite horizons in the Amadeus Basin, Central Australia, have given insight into the impact of salt properties on structural geometry of surrounding rocks during compressional deformation. The lower Gillen Member of the Bitter Springs Formation has acted as a detachment horizon since the Late Proterozoic. Paleo-stress orientations, calculated from conjugate fracture sets in outcrop, express the mechanical detachment of younger packages via a 90˚ rotation in maximum horizontal stress. Synformal deflection beneath the Ross River Syncline has evacuated salt to the north, providing a greater amount of compensation for compressive stresses in the detachment horizon. Vertical dip-slip displacement along thrust faults has been sufficient enough to exhume basal sedimentary units where grounding between competent layers has occurred as a result of lateral salt exhaustion. Aspect ratios of folds indicate that a decrease in both amplitude, wavelength and arc length can be attributed to the proximity of the layer to a detachment horizon. Fold geometry consists of small-scale isoclinal and large-scale tight folding, thereby increasing geometrical complexity toward the detachment horizon. Detachment of overriding layers has also appeared to eliminate fold vergence.