The structure of the Myponga River – Carrickalinga Creek area, southern Adelaide Fold Belt, Fleurieu Peninsula, South Australia

Abstract

The area between Myponga Beach and Carrickalinga Creek on the southern Fleurieu Peninsula was the subject of detailed geological and structural mapping. Early-middle Proterozoic Basement Complex and an overlying sedimentary prism of Late Proterozoic to Cambrian age were redistributed during the Delamerian compressional event. Southeast over northwest movement induced thrusting and folding which produced a southeast hindward dipping thrust system. The deformational processes and structural geometries are similar to other classic thin-skinned fold and thrust belts such as the Moine Thrust Zone in NW Scotland. Major thrusts are generally unseen but associated with intense folding and layer parallel shear in hanging wall zones and less intense deformation in footwall zones. Structural geometry commonly changes along strike from frontal to oblique and lateral orientations and minor folds have complex axial trace trend variations. These features are related to the morphology of the basal detachment surface which may form oblique and lateral ramps. Individual thrusts are noted to have differential displacements along their traces (up to 4.8 km) and the spatial variation in displacement is used to reconstruct thrust sheet interactions during emplacement. Fourteen closely spaced (50 m) imbricate thrusts define a duplex system in the more external zone. Each thrust has minor displacement and horses contain northwest vergent anticline­syncline pairs. Major and minor folds occur throughout the study area. Minor folds mirror the larger scale structures and are generally northwest-vergent steep to overturned closed folds. A homoclinal, phyllitic, discrete, spaced cleavage was predominant throughout the area. Its intensity increased (spacing became smaller) in proximity to major thrusts. Local crenulation cleavages and small S2 folds indicate the occurrence of a second phase of deformation. A 45% shortening has produced high strain ratios of up to 11: 1 and oblate strain ellipsoids which are recorded by elongate ooids within the Brighton Limestone.