Paleomagnetism of the Upper Cretaceous Mount Somers Volcanics, Canterbury, New Zealand

Abstract

The Mount Somers Volcanics of mid Canterbury in eastern South Island are a high-K calcalkaline andesite–dacite–rhyolite suite erupted subaerially approximately 95 Ma ago during the long Late Cretaceous episode of normal geomagnetic field. Thermoremanent magnetic directions at 46 sites, corrected for post–eruption tectonic tilt, are stable and group well (a95= 3·8°, K = 31·7) with a very steep mean inclination (I = –85°, D = 354°). The resulting paleomagnetic South Pole position is at 52° S 174° E, indicating that Canterbury was within 10° of the South Pole 95 Ma ago, and that little finite rotation of Canterbury relative to the South Pole has occurred since. The Mount Somers Volcanics were erupted prior to the separation of eastern New Zealand from West Antarctica along the Pacific–Antarctic ridge. They provide a mid–Cretaceous pole position that can be compared with pole positions of similar age from Australia and with the polar–wander path previously reported for the Chatham Islands over the past 75 Ma. The Mount Somers pole fits neatly onto the New Zealand polar-wander path, which is consistent with the Australian polarwander path when convergent displacements across the Pacific-Indian plate boundary, and spreading of the Tasman Sea, are allowed for. The paleomagnetic data also support the proposition that regional folding and oroclinal bending of the pre-Cretaceous Torlesse basement rocks mainly predates the extrusion of the Mount Somers Volcanics and took place primarily during the Early Cretaceous Rangitata Orogeny and not during the Cenozoic as implied in recent plate-tectonic reconstructions.