Towards a model of the Cenozoic tectonic development of New Zealand

Abstract

New Zealand has a regionally complex and diverse Cenozoic geological record. However, few attempts have been made so far to formulate a model of the Cenozoic tectonic development of New Zealand, which reconciles this regional complexity and diversity. The objective of this thesis is to work towards such a model. This has involved the identification and resolution of five critical and interrelated problems: (1) The age of inception of the New Zealand sector of the Australia-Pacific plate boundary. (2) The tectonic setting during the Paleogene. (3) The nature and location, north of the Alpine Fault proper, of the relative plate motion which is evident as dextral fault displacement on the Alpine Fault. (4) The total amount of Cenozoic horizontal displacement through New Zealand. (5) The Neogene and Quaternary extent and geometry of the subducted Pacific Plate beneath northern New Zealand. The proposed solutions to these problems are as follows: (1) The Australia-Pacific plate boundary formed during the early Miocene, about 23 My B.P. (2) A continental rift system developed through western New Zealand during the Paleogene. (3) Northeast of the Alpine Fault the relative plate motion was expressed as a combination of brittle and ductile shearing that formed a recurved arc. (4) There has been a total of 500 km of horizontal displacement through New Zealand. (5) The subducting slab of Pacific Ocean lithosphere progressively increased its extent to the southwest beneath northern New Zealand and concomitantly increased its dip. Based on these solutions, it is proposed that the Cenozoic geological development of New Zealand may be modelled as a tectonic succession, involving the oblique dislocation and tectonic overprinting of a Paleogene north-south trending continental rift system through western New Zealand, by a Neogene-Quaternary transform to obliquely convergent plate boundary. In this context most of the regional complexity and diversity arises for the following reasons: (1) The rift system developed as two independent segments, a North Island segment and a South Island segment, and each in different ways. (2) Rifting continued along some parts of the rift system after the transform plate boundary had started to dislocate the South Island rift segment in the early Miocene. Adjacent to the Alpine Fault in central Westland, rifting ceased immediately following inception of the plate boundary. However, the effects of the change in tectonic regime were recorded later and to a lesser degree in the rift system at localities further away from the Alpine Fault. (3) While the relative plate motion was accommodated on a continent-continent transform fault in the South Island, ocean-continent convergence progressively emplaced a slab of Pacific Ocean lithosphere beneath the North Island. The shallow dip of this slab beneath eastern North Island is responsible for a portion of the compression which dominates the Neogene and Quaternary record there. In northern and western North Island, the emplacement of the slab and changes in its geometry are responsible for the tectonic overprinting of the western rift system; the southwestward direction of slab emplacement accounts for the north to south overprinting of the North Island segment of the former rift system. (4) A major contributor to the regional complexity of the Cenozoic geological record is the nature of the basement. Differences in the competence of the Tuhua Orogen (late Precambrian-early Paleozoic) versus the Rangitata Orogen (late Paleozoic-Mesozoic) caused the relative plate motion to be expressed as Alpine Fault movement in the South Island but as the formation of a mega brittle-ductile shear zone in Marlborough and eastern North Island.