An oblique subduction and transform faulting model for the evolution of the Broken River Province, northern Tasman Orogenic Zone

Abstract

Formation of the Broken River Province, an important element of the northern Tasman Orogenic Zone, can be rationalized in terms of oblique subduction and strike‐slip faulting. Its two broadscale terranes, the smaller southwestern Graveyard Creek Subprovince and larger Camel Creek Subprovince, are regarded as Silurian‐Devonian forearc basinal, and subduction complex, assemblages, respectively. They are divided by the Gray Creek Fault, interpreted as a dextral strike‐slip structure of Siluro‐Devonian age. Part of the multiphase structural history of the Camel Creek Subprovince is ascribed to the partitioning of shortening and shear stress regimes known to characterize continental margins experiencing oblique‐slip subduction. Late Devonian (Famennian) crustal shortening is ascribed to a change in the direction of plate motion and an increase in its rate. A sheet of basement is thought to have been thrust eastwards to cover most of the forearc basinal assemblage with the southwestern part of the Burdekin River Fault developing as a tear structure at its southern margin. Development of the Clarke River Fault Zone, regarded as a transform offsetting the continental margin in a sinistral sense, is also inferred for Famennian time. Northeasterly trending folds characteristic of the Graveyard Creek Subprovince and recognized as a discrete fold phase in the southern Camel Creek Subprovince are considered to have been induced by shear adjacent to the Clarke River Fault Zone and to wedging of the continental margin sedimentary assemblage between basement blocks. Oblique subduction appears to have resumed in Early Carboniferous (Tournaisian) time. Two previously separate structures, the southwestern Burdekin River Fault and part of the older Gray Creek Fault, united to form a single dextral strike‐slip structure. Movement on this composite fault, the Burdekin River Fault of current nomenclature, sponsored the development of a deep Early Carboniferous pull‐apart molasse basin which retained stratigraphic and structural continuity with the preceding forearc basin. Extensive shallow Early Carboniferous molasse basins developed in response to vertical movements on the Gray Creek and Clarke River Fault Zones. With subsequent vertical movement coupled with erosion, the Burdekin River Fault has come to mark the eastern limit of overthrust basement.