Structural geometry of a thick-skinned fold-thrust belt termination: the Olary Block in the Adelaide Fold Belt, South Australia

The Olary Block comprises a set of Palaeoproterozoic to Mesoproterozoic bas ement inliers that were deformed together with the Neoproterozoic sedimenta ry cover of the Adelaide Geosyncline during the ca 500 Ma Cambro-Ordovician Delamerian Orogeny. Balanced and restored structural sections across this region show shortening of less than 20%. These basement inliers represent t he interface between a region of thick-skinned deformation bordering the Cu rnamona Craton to the north and a region of thin-skinned deformation to the south and west in the Nackara Are. The basement inliers represent upthrust segments of the subsided basin margin with the sedimentary package thicken ing to the south and to the west. Earlier formed extensional faults provide d the major strain guides during Delamerian shortening. An early phase of e ast-west shortening is interpreted to be synchronous with dextral strike-sl ip deformation along basement-relay structures (e.g. Darling River lineamen t). During progressive shortening the tectonic transport direction rotated into a northwest to north direction. coeval with the onset of the main phas e of thin-skinned fold deformation in the adjacent Nackara Are.