Evolution of the Isan Orogeny at the southeastern margin of the Mt. Isa Inlier

We present the results of field mapping, structural and metamorphic analysis in an area at the southeastern margin of the Mt Isa Inlier. The results provide insight into the evolution of the Isan Orogeny and a comparison with structural studies conducted along the line of the Mt Isa Deep Seismic Transect. Our observations are consistent with a two-stage orogenic scheme in which there was a transition from early thin-skinned to later thick-skinned deformation as the orogen evolved. Metasedimentary rocks of the Maronan Supergroup (ca 1700-1650 Ma) were deposited in a basin marginal to the presently exposed Mt Isa Inlier. They were then thrust toward the north and west during the early stages of shortening, ca 1600-1580 Ma. This produced an arcuate fold and thrust belt with east-west-trending folds in the east of the study area and north-south-trending folds in the west. Amphibolite to upper amphibolite facies, high-temperature-low-pressure metamorphic conditions, evidenced by garnet, andalusite and sillimanite porphyroblasts, were reached during the early phase of deformation. Subsequent deformation, ca 1550-1500 Ma, resulted in upright to steeply inclined folding of the earlier fabrics and steeply east-dipping reverse faults. This orogenic phase was characterised by the growth of staurolite in aluminous schists, and its subsequent replacement by biotite, consistent with a distinct cycle of prograde metamorphism at higher pressures than the first. Based on the differing orientation and style of structures and the association with separate metamorphic events, we argue that the two phases of deformation, ca 1600-1580 Ma and ca 1550-1500 Ma, represent discrete tectonic events that may have had different driving forces and boundary conditions.