A Permian-Triassic Retroforeland Thrust System—The New England Orogen and Adjacent Sedimentary Basins, Eastern Australia

Abstract

From the Late Devonian to the Triassic, eastern Australia was part of eastern Gondwana, where an active, convergent plate margin was influenced by a west-dipping subduction system. This system terminated as the result of global plate reorganization in the Middle to Late Triassic. The southern New England orogen changed from a prowedge (P) mode (terminology of Beaumont et al., 1999) in the Devonian-Carboniferous to an uplifted plug (P-U) mode in the Permian to Triassic. In contrast, the northern New England orogen was dominated by the retrowedge (P-U-R) mode in both time periods. This led to the development, in the Permian–Triassic, of a major west-directed retroforeland thrust belt in northern New England, with the formation of a thick foreland-basin phase in the adjacent Bowen Basin to the west. Thick-skinned and thin-skinned processes operated simultaneously. In the orogen, the thrusts are dominantly thick-skinned and planar, cutting deep into the crust. The eastern part of the Bowen Basin, however, is dominated by thin-skinned thrusting that, in places, propagated a considerable distance into the basin, with the formation of an imbricate thrust fan. The thick-skinned and thin-skinned thrusts are hard-linked into a single thrust system by a major middle-crust detachment surface. Contractional events at the plate margin associated with the formation of the thrust system were also responsible for the propagation of far-field stresses and the reactivation of older extensional faults as thrusts. These reactivated faults are well inboard of, not physically attached to, and soft-linked to, the retroforeland thrust system. Contraction in the Denison Trough in the western Bowen Basin produced a variety of geometries, including reactivation of Early Permian extensional faults as thrusts and the growth of fault-propagation folds. These inversion structures often house commercial quantities of hydrocarbons.