A shortened intraplate rift system in the Proterozoic Mount Isa terrane, NW Queensland, Australia

Abstract

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 structures were superimposed during the 1590 to 1500 Ma Isan Orogeny. Basin‐fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity‐bounded sequences. Initial E‐W extension was associated with 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 basaltic and siliciclastic strata accumulated. N‐S extension was followed by regional postrift subsidence and the deposition of a laterally continuous 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 thickening toward the east, consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width. Finally, a period of N‐S extension resulted in the development of E‐W trending F1 drag synclines in the highest level cover rocks. The association of angular unconformities and block‐bounding faults, E‐W trending synclines and E‐W striking faults, and the unique internal fold geometries 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.