Structure and distribution of fault rocks in the Alpine Fault Zone, New Zealand

Abstract

The Alpine Fault Zone, which is the dominant strand in a dextral system of transform faults along the boundary between the Indo-Australian and Pacific plates, forms a pronounced lineament about 1 km in width. Within the fault zone, the cross-strike passage from cataclasite through augen mylonite (both derived from largely granitoid basement to the NW) to mylonites derived from the high-grade Alpine Schists on the upthrown south-eastern side, is thought to reflect the original distribution of fault rocks with depth below near-surface gouge zones. Structural data suggest that the fault rocks had their fabrics impressed during the late Cenozoic phase of oblique compression when the Southern Alps were ‘ploughed-up’ along the fault. Mylonitic foliation indicates a dip approaching 50°SE for the fault zone at depth, while penetrative stretching lineations plunge in a direction sub-parallel to the present-day interplate slip vector, consistent with dextral-reverse-oblique shear across the fault zone. Pseudotachylyte friction-melt is fairly widely distributed and can be found cutting all other fault rock types apart from gouge. Models for the evolution of the fault zone are considered.