Evolution of central eastern Australia during the late Palaeozoic and early Mesozoic

Abstract

Palaeogeographic reconstructions and structural analysis of the Late Carboniferous to Triassic of central eastern Australia indicate that sedimentation and deformation were responses to the prolonged application of a dextral rotational force couple to the craton margin and to eustatic sea‐level changes. The force couple distorted the craton margins and adjacent Yarrol‐New England geosyncline and orogen into an incipient coupled orocline. The influence of the couple commenced in the Late Devonian and continued with varying effect until the Late Triassic, when it reversed to a sinistral system, part of a completely different stress regime that controlled sedimentation and structure during the Early Jurassic. Within the craton, deformation mainly took the form of a series of en echelon depressions, such as the Drummond Basin, Koburra, Denison and Taroom Troughs. A lineament between Longreach and Roma marks the southern boundary of this type of strain, although crust beyond its limit was not so rigid as to be unaffected by the force couple. The Yarrol‐New England region during the Devonian and the Early Carboniferous was the site of geosynclinal deposition where a thick and typically volcanogenic wedge lay along the eastern border of the craton. During the Late Carboniferous and Early Permian comparable wedges were formed farther to the east, in effect building outwards into the geosyncline. The same tensional regime that created the geosyncline is seen as the means for thinning crust below the sediment wedge and thus provided thermal instability, and for the igneous diapirism expressed as both intrusion and extrusion that characterizes the orogen from the Late Carboniferous onwards. The dextral force couple was responsible for most of the deformation and for controlling final emplacement of plutons. Sea‐level rises were marked in the late Early Permian and again in the early Late Permian.