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 fa ult blocks which exhibit different local deformational histories and s tructural orientations. Many structural patterns in the stratigraphica lly highest cover sequences do not occur in adjacent fault blocks that expose older and mechanically stronger rocks. Within the Mount Isa Gr oup sediments of the Crystal Creek block for example, tight E-trending upright folds appear to be at odds with the surrounding north-south r egional trends. In this paper it is proposed that an inherited extensi onal fault geometry played a significant role in controlling the struc tural patterns developed during later shortening. Through the integrat ion of detailed surface mapping and forward magnetic modelling, an inv erted half-graben was identified beneath the Crystal Creek block, expo sing the underlying origin of local structural-complexities. Structura l inversion and buttressing against pre-existing faults simultaneously removed extensional displacement across pre-rift rocks and induced fo lds and faults in overlying sag-phase sediments. This led to the devel opment of two structural levels characterized by different deformation al styles and apparently distinct deformational histories.