The concealed Tamworth Belt (New England Orogen)

Abstract

The subsurface geometry and tectonic development of the Devonian-Carboniferous Tamworth Belt, a fore-arc basin in the New England Orogen, Eastern Australia, has been examined using seismic reflection, drill core data, aeromagnetic and gravity data. In the study area, in southern Queensland, the belt is covered discordantly by up to nearly 2000 metres of Jurassic-Cretaceous sediments. The Tamworth Belt is affected in the west by the Moonie Fault, a short-cut thrust fault, which exhibits a fault-bend-fold geometry. An elongated positive magnetic anomaly is associated with the Moonie Fault, although the fault strikes beneath the thick Mesozoic cover. The western extent of the fore-arc basin lies far to the west of the study area, but probably is bounded by the Leichhardt Fault, a thrust fault that seems to root onto the same thrust flat at depth.<br /> A major westward-dipping structure forms the main eastern boundary of the belt, with a series of eastward-dipping backthrusts located farther to the west. The eastern margin also coincides with a gravity and magnetic ridge, similar to the gravity and magnetic pattern of the Lower Devonian serpentinites and iron-enriched rocks that are exposed along the Peel Fault to the south.<br /> In the study area, the Tamworth Belt is over 75 km wide and has been shortened by at least 35 km across strike. The sedimentary succession is moderately folded, and, due to thrust sheets sliding on top of each other, at least 12 km thick. Within the succession, six seismic sequences were identified, each of which is separated by a major sequence boundary.<br /> Three major deformational events have been recognised - a Mid Permian period of compressional deformation, the Mid-Late Triassic Goondiwindi Event and the Late Cretaceous Moonie Event.<br /> To the west of the Moonie Fault, Bowen-Gunnedah Basin rocks of Permian-Triassic age occur, comprising sediments that originate to the east of the Tamworth Belt succession and conformably overly at least the uppermost sequences of the Tamworth Belt succession.<br /> Within the Tamworth Belt succession, a hanging-wall syncline immediately to the east of the Moonie short-cut thrust fault may be correlated with the Rocky Creek Syncline in the northernmost exposed part of the fore-arc basin, approximately 170 km farther south.<br /> The eastern boundary separates the forearc basin sediments from rocks of the Tablelands Complex, which is a subduction-related accretionary wedge assemblage. The overall geometry of the subsurface Tamworth Belt in the southern study area is similar to that seen on the deep seismic survey and on geological cross sections farther south, but is a wider belt to the north.