Structural evolution and deformation near a tertiary salt weld, Willouran Ranges, south Australia

Abstract

Abstract We combine insights from subsurface studies and published numerical models to propose a three-phase conceptual model that considers the potential deformation patterns that form during the emplacement, evacuation and welding, and post-welding phases of tertiary weld formation. The proposed model aims to: 1) help establish the timing of deformation adjacent to a tertiary weld based on the geometry of deformation patterns, and 2) serve as a guide for future work that examines deformation adjacent to tertiary welds and beneath allochthonous salt. We apply the model, along with new, 1:10,000 scale geological mapping and mesostructural analysis of deformation patterns along the Kingston tertiary salt weld and contiguous Witchelina-Breaden Hill salt canopy in the Willouran Ranges of South Australia to constrain the evolution of the allochthonous salt system and the style, extent, and timing of adjacent deformation. Deformation near the weld is dominated by opening mode fractures that exhibit spatial heterogeneity in their orientation both along the strike of allochthonous salt and in suprasalt versus subsalt strata. A halokinetic drape fold, decimeter to decameter scale folding, and localized debris flow deposits are present in subsalt strata, whereas no evidence of thrust imbricates, or laterally continuous shear and/or rubble zones was observed anywhere below the canopy-weld complex. We hypothesize that remnant evaporite distribution and the juxtaposition of units exert the primary, largest-scale control on the hydrological behavior of tertiary welds. The influence of deformation occurs at a much smaller scale and is more heterogeneous and difficult to predict. Higher resolution models and additional field studies are required to identify and rank the many variables that control this deformation.