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

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.

在澳大利亚东部新英格兰造山带南部（southern New England Orogen, SNEO），研究者通过构造、磁学与重力趋势分析，在石炭纪大陆弧、弧前盆地[塔姆沃思带（Tamworth Belt, TB）]及共轭增生杂岩中识别出4处弧形构造（oroclinal structure）。目前尚无一处构造可通过古地磁学（paleomagnetism）确凿证实为弧形构造。熔结凝灰岩（ignimbrites）在该弧前盆地中广泛分布，尽管普遍存在磁化叠加现象，但其仍被证实可保留原生磁化强度。该类凝灰岩在6个主要构造-地层地块中出露良好，各地块地层存在部分连通性，这使得通过对比弧形构造两侧不同地块的极移路径段开展弧形构造验证的弧前盆地具备极高研究潜力。古地磁研究表明，塔姆沃思带西部/西南部地块（洛基克里克、沃里与鲁谢尔地块）未出现显著旋转，但本文记录了塔姆沃思带南部格雷斯福德地块存在小幅逆时针旋转。本研究详述了格雷斯福德地块南部87个采样点（共计969件样品）的古地磁、岩石磁学与磁组构测试结果。实验主要采用热退磁，辅以交变场退磁与液氮退磁，结果显示样品普遍存在低温叠加磁化信号，该信号归因于渐新世晚期的区域风化作用；同时还存在以磁铁矿为主、以赤铁矿为主的两类高温原生磁化与叠加磁化组分。磁铁矿与赤铁矿亚组分之间存在细微但系统性的方向差异，其中赤铁矿亚组分的清洗效果更佳、信号定义更清晰，为优选结果；研究识别出9个中、晚石炭世原生极位，以及二叠纪与二叠-三叠纪叠加磁化极位，该结果与塔姆沃思带西部/西南部地块的已有观测一致。该原生极位更新了新英格兰造山带南部极移路径中原本界定模糊的中石炭世段，并通过对比鲁谢尔与格雷斯福德地块的中石炭世马丁斯溪熔结凝灰岩成员极位，证实存在约15°±13°的逆时针旋转；该旋转未必发生于冈瓦纳造山运动亨特-鲍文期之前。格雷斯福德地块的此次小幅逆时针旋转强化了塔姆沃思带西南部/南部的原生弧形构造，同时预示塔姆沃思带东南部迈尔地块存在更为复杂的后续旋转。