Palaeomagnetic studies of Cenozoic sedimentary rocks of the northwest Nelson area, New Zealand, and implications for the tectonic history of the region

Abstract

Palaeomagnetic data from Cenozoic sedimentary rocks have been collected at thirteen localities in the Northwest Nelson region in order to obtain a vertical axis rotation chronology and to develop constraints on the tectonic evolution of the area. The lithologies sampled were often very weakly magnetised and showed a scattered behaviour of remanence directions during demagnetisation experiments. Diagenetic iron sulphide formation at the expense of detrital iron oxide minerals is the main process that has altered the detrital magnetic remanence in fine grained Cenozoic mudstones. Mean palaeomagnetic vectors of three sampling localities were interpreted as to represent a detrital characteristic magnetisation. This new palaeomagnetic data in conjunction with re-examined data from previous studies indicates that Northwest Nelson has not been significantly affected by vertical axis rotations of crustal blocks within the last 36 Ma. The eastern boundary of this palaeomagnetic domain is most likely the Alpine-Wairau Fault, which acted as a shear zone, accommodating most of the relative plate motion through Cenozoic time.