A SHORTENED INTRAPLATE RIFT SYSTEM IN THE PROTEROZOIC MOUNT ISA TERRANE, NW QUEENSLAND, AUSTRALIA

The Leichhardt River Fault Trough of the Mount Isa terrane developed a complex extensional architecture between approximately 1800 and 1600 Ma, forming the underlying template upon which compressional structure s were superimposed during the 1590 to 1500 Ma Isan Orogeny Basin-fill material accumulated during at least five multiphase periods of rifti ng and associated postrift subsidence forming a stacked succession of unconformity-bounded sequences. Initial E-W extension was associated w ith a massive magmatic event. Half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites. Thereafter a period of N-S extension resulted in southward tapering north tilted half graben in which synrift basalt ic and siliciclastic strata accumulated. N-S extension was followed by regional postrift subsidence and the deposition of a laterally contin uous quartzite-carbonate package. A multiphase period of E-W to NW-SE extension ensued during which time two unconformity-bounded sequences accumulated. The stratal architectures of these sequences are strongly asymmetric in cross section, exhibiting a pronounced rotational thick ening toward the east, consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width. Fina lly, a period of N-S extension resulted in the development of E-W tren ding F1 drag synclines in the highest level cover rocks. The associati on of angular unconformities and block-bounding faults, E-W trending s ynclines and E-W striking faults, and the unique internal fold geometr ies of fault blocks suggest that many fault-bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening.