Paleomagnetism and geochronology of volcanics in the northern North Island, New Zealand

Abstract

This thesis constitutes an exploratory paleomagnetic study of 11 major Permian to Recent volcanic formations in the northern part of the North Island, New Zealand. The study was devised to clarify the tectonic evolution of this geologically active region. The data base comprises approximately 1500 oriented core samples taken from 152 sites in an area 225 km by 270 km. To supplement available geochronology, 13 sites in the Miocene volcanics were dated using the K-Ar method. A paleomagnetic coring drill rig and a demagnetising oven were developed and used in conjunction with an existing coil-type spinning magnetometer. Archeomagnetic ages deduced for lava flows imply that the prominent Recent Rangitoto Island volcano was most active at about 1400 AD. From the paleomagnetic results an apparent polar wander path (APWP) can be plotted which relates closely to the Australian APWP of Embleton (1981). It is therefore inferred that no significant relative displacement of the northern North Island and Australia has occurred except for seafloor spreading which is supposed to have occurred in the Tasman Sea about 80 million years ago. Thus there is no evidence to substantiate current theories calling for post Miocene rotation greater than 10° or displacement greater than 1100 km of the northern North Island relative to the Australian continent. However, post-Miocene anticlinal "arching" about the axis of the Hauraki Depression is confirmed by a fold test. Various modern theories regard the Tangihua Volcanics as far-travelled allochthonous bodies. However, none of three adjacent massifs within the Tangihua Volcanics appears have undergone significant rotation since their final magnetisation was acquired, probably by overprinting in the late Cretaceous. The Permian Waipapa Group spilites also appear to have been magnetically overprinted in the late Cretaceous. Paleosecular variation has an angular standard deviation of approximately 20° for all the formations,' which is 20% greater than expected from geomagnetic field models summarised by Brock (1971). The natural remanent magnetisation (NRM) for basalts was found to decrease with their age,which is consistent with the Earth's magnetic field having increased with time. Conversely it could be due to decay of magnetisation if the paleomagnetic field were approximately constant. The latter hypothesis has been developed and it is suggested that such a decay may be simply described by representing the Koenigsberger Q ratio as a function of magnetite grainsize distribution, and time.