The intra-cratonic evolution of the central eastern Mount Isa Inlier, northwest Queensland, Australia

Abstract

The central eastern part of the Mount Isa Inlier can be subdivided into three major tectono-stratigraphic domains, from west to east: (i) the Mitakoodi Anticlinorium, a tight, northeast-plunging, composite anticline, which is developed within subaerial to shallow-marine, continental-rift facies lithologies; (ii) the Marimo-Staveley Block, in which tight to isoclinal folds are developed within a marine shelf; (iii) the central part of the Soldiers Cap Group belt, which is dominated by a series of metaturbidites, and which hosts a fold nappe refolded about N-S to NE-SW trending axes. The transition from subaerial sediments in the west through marine shelf sediments to turbidites in the east has been one of the main arguments used by advocates for a Proterozoic continental margin at this locality. Here, however, it is argued that folding and differential uplift along late fault and shear zones led to the juxtaposition of different stratigraphic levels, and to the apparent deepening of palaeobasins from subaerial in the west to deep water in the east. The age of the Soldiers Cap Group is poorly constrained, but it is likely to be older than that of rocks cropping out in the Marimo-Staveley Block. The Marimo-Staveley Block lithologies overlie those of the Mitakoodi Anticlinorium, so here time-transgression rather than eastward deepening may be deduced. Thus, a palinspastic reconstruction based on an eastwards progressive deepening, is not justified. This adds to other evidence that refutes the continental margin model previously proposed for the eastern margin of the Mount Isa Inlier. Instead, the lithostratigraphic sequences reflect several stages of intra-continental rifting, all without formation of oceanic crust. The pervasive post-depositional deformational event is associated with regional low-P-facies metamorphism, which indicates a further ensialic history.