Paleomagnetism of the Carboniferous Gresford Block, Tamworth Belt, southern New England Orogen: minor counter-clockwise rotation of a primary arc segment

Abstract

Four oroclinal structures have been identified from structural, magnetic and gravity trends across a Carboniferous continental arc, forearc basin [Tamworth Belt (TB)] and conjugate accretionary complex in the southern New England Orogen (SNEO) of eastern Australia. None of the structures has yet been confirmed conclusively by paleomagnetism as oroclinal. Ignimbrites are common within the forearc basin and have been demonstrated to retain primary magnetisations despite prevalent overprinting. They are well exposed across six major tectono-stratigraphic blocks with partly interlinked stratigraphies, making the forearc basin highly prospective to oroclinal testing by comparing pole path segments for individual blocks across curved structures. Paleomagnetic studies have shown no noticeable rotation across the western/southwestern TB (Rocky Creek, Werrie and Rouchel blocks), but documented herein is a minor counter-clockwise rotation of the Gresford Block of the southern TB. This study details paleomagnetic, rock magnetic and magnetic fabric results for 87 sites (969 samples) across the southern Gresford Block. Predominantly thermal, also alternating field and liquid nitrogen, demagnetisations show a widely present low-temperature overprint, attributed to regional late Oligocene weathering, and high-temperature primary and overprint components residing in both mainly magnetite and mainly hematite carriers. Subtle, but systematic, directional differences between magnetite and hematite subcomponents show the latter as the better cleaned, better-defined, preferred results, detailing nine primary poles of middle and late Carboniferous ages and Permian and Permo-Triassic overprints as observed elsewhere in the western/southwestern TB. The primary poles update a poorly defined mid-Carboniferous section of the SNEO pole path and demonstrate counter-clockwise rotation, quantified at about 15° ± 13° from comparison of mid-Carboniferous Martins Creek Ignimbrite Member poles for the Rouchel and Gresford blocks, that may not necessarily have been completed prior to the Hunter–Bowen phase of the Gondwanide Orogeny. This minor counter-clockwise rotation of the Gresford Block accentuates a primary curvature of the southwestern/southern TB and heralds further, more complex, rotations of the Myall Block of the southeastern TB.