Mylonites, Cataclasites, and Associated Rocks along the Alpine Fault, South Island, New Zealand

Abstract

The cataclastic rocks in the Alpine Fault Zone fall into three distinct groups: (a) fault pug, fault breccia, and shattered rock, (b) cataclasite, mortared and brecciated rock, and (c) mylonite, augen mylonite, ultramylonite, and blastomylonite. The first group is related to Quaternary movements at the junction of schist and granite; the zone of shattered schist is several chains or more in width, and the fault pug and fault breccia a few feet to more than 100 ft wide. The second group is conspicuous in the granitic rocks, the structureless cataclasite being 100–200 ft thick, and the zone of brecciated and mortared granite a mile or more in width, The mylonite group is characterised by fluxion banding and forms a belt 1,000–2,500 ft wide, largely within the granite. The age of formation of the mylonite and cataclasite is uncertain but a key tectonic horizon is the blastomylonite, which shows features of both dislocation and regional metamorphism. The evidence of the blastomylonite, and the comparable mineral assemblages of the schist and mylonite, point to the mylonitisation being penecontemporaneous with the regional metamorphism of the Rangitata Orogeny (late Jurassic and early Cretaceous). Formation of the cataclasite group is attributed to movements during the late Tertiary phase of the Kaikoura Orogeny, a phase to which retrograde metamorphism of the schist, granite, and mylonite is also ascribed. The Alpine Fault to which most attention has been paid in recent years forms the eastern boundary of a fault zone, a mile or more in width, recognised by Morgan (1908), who considered the western boundary to be the “main overthrust”.