THE ROLE OF DUCTILITY CONTRAST AND BASEMENT ARCHITECTURE IN THE STRUCTURAL EVOLUTION OF THE CRYSTAL CREEK BLOCK, MOUNT ISA INLIER, NW QUEENSLAND, AUSTRALIA

The Leichhardt River Fault Trough of the Mount Isa Inlier comprises fault blocks which exhibit different local deformational histories and structural orientations. Many structural patterns in the stratigraphically highest cover sequences do not occur in adjacent fault blocks that expose older and mechanically stronger rocks. Within the Mount Isa Group sediments of the Crystal Creek block for example, tight E-trending upright folds appear to be at odds with the surrounding north-south regional trends. In this paper it is proposed that an inherited extensional fault geometry played a significant role in controlling the structural patterns developed during later shortening. Through the integration of detailed surface mapping and forward magnetic modelling, an inverted half-graben was identified beneath the Crystal Creek block, exposing the underlying origin of local structural-complexities. Structural inversion and buttressing against pre-existing faults simultaneously removed extensional displacement across pre-rift rocks and induced folds and faults in overlying sag-phase sediments. This led to the development of two structural levels characterized by different deformational styles and apparently distinct deformational histories.