Shifts in the locus of crustal thickening during mesoproterozoic orogenesis in the Mt. Isa Terrane

A fundamental change from thin-skinned to thick-skinned tectonics in the Mt Isa Terrane is interpreted to reflect a westward shift in the locus of crustal thickening during Mesoproterozoic orogenesis in northeastern Australia. Geochronological constraints indicate that west-directed, thin-skinned deformation in the eastern part of the terrane occurred before ca 1540 Ma. This upper crustal system is interpreted to link with deeper crustal thickening in the Georgetown Inlier to the east, where synorogenic granites intruded at ca 1550 Ma. This early system was dissected by basement-cutting reverse faults across the entire Mt Isa Terrane after 1540 Ma. The nature of deeper crustal tectonics within the Mt Isa Terrane is determined by evaluating possible origins for the crustal structure revealed in a seismic-refraction profile. In particular, alignment of shallowly west-dipping, mid-crustal, high-velocity layers is interpreted to reflect tectonic duplication of underplated crust. This crustal stacking fault is inferred to be the controlling structure for the thick-skinned deformation associated with the Isan Orogeny.