Palaeomagnetic data from New Zealand

Abstract

Palaeomagnetic studies in the New Zealand region have lagged behind those in eastern Australia, due mainly to the more complex problems posed by tectonic rotations (about both horizontal and vertical axes) associated with the late Cainozoic Kaikoura orogenic movements. These tectonic movements affect principally a 50 km-wide belt either side of the present Indian-Pacific plate boundary (Alpine Fault in part). Within this tectonically disturbed belt, Cretaceous-Cainozoic volcanic rocks are generally unsuitable for palaeomagnetic work because of varying degrees of magnetic instability and uncertainty in estimation of tilt corrections.By concentrating on undisturbed or gently tilted volcanic sequences away from the Alpine Fault some progress has been made over the past five years. At the Chatham Islands on the eastern end of the Chatham Rise, Late Cretaceous (70-80 m.y. BP), Eocene-Oligocene (35-40 m.y.) and latest Miocene-Pliocene (5 m.y.) alkaline basaltic vo/canics lie flat and have provided data for an apparent Polar Wander Path for the eastern New Zealand region (part of Pacific Plate) over the past 75 m.y. (Grindley et. al., 1977). Closer to the Alpine Fault at Oamaru in North Otago, Late Eocene-Early Oligocene basalts have provided good N.R.M. data (Coombs and Hatherton, 1959) from which a pole position has been calculated which compares closely with the age-equivalent pole position from the Chatham Islands. At Mt Somers in Mid-Canterbury only 75 km from the Alpine Fault, an excellent 95 m.y. pole position has recently been obtained from early Late Cretaceous caic-alka/ine andesitic to rhyolitic lavas, tuffs and ignimbrites (Oliver, 1977; Oliver et al., in press., and compared with the apparent Polar the New Zealand Polar Wander Path provided no rotation is allowed between Mt Somers and the Chatham Islands.West of the Alpine Fault, few reliable results have been obtained due to strong tectonism and related magnetic instability. In the Buller gorge area of North Westland, early Late Cretaceous (80-90 m.y.) basalts, iamprophyres and acid tuffs have recently yielded promising results comparable with those at Mt Somers which are still being investigated.Younger rocks in the New Zealand region have yielded pole positions indistinguishable from those of the present day. The Upper Miocene Dunedin Volcanics have provided N.R.M. data from which a pole position close to the present axial dipole can be inferred (Coombs and Hatherton, 1959). The Upper Miocene Akaroa Volcanics and the Stoddart Formation of the Lyttleton Volcano have yielded results statistically different from the present axial dipole (Evans, 1970) but in the case of the Stoddart Formation at least, this can only be regarded as a virtual geomagnetic pole, secular variation not being eliminated. Ignimbrites and andesites of the Taupo Volcanic Zone, Auckland and Taranaki all less than 1 m.y. old have yielded a pole within 8° of the present pole with a circle of confidence of T (Cox, 1969).All the reliable New Zealandpalaeomagnetic data are shown in Table 1 using the conventions of McElhinny (1973). The significant palaeopoJe positions are plo tted on Figure 1 (after Oliver et al., in prep.).and compared with the apparent Polar Wander Path for Eastern Australia (McElhinny, 1973). The New Zealand Polar Wander Path lies some 1000-1500 km east of the Australian Polar Wander Path, the differences corresponding to the finite rotational displacements across the Indian-Pacific and other plate boundaries (such as the Tasman spreading) over the past 90 m.y. Finite poles for these rotations lie on great circles bisecting the lines joining age-equivalent positions on the separate Polar wander paths (Figure 1).