Mylonites in the fault zone comprise kyanite-orthoamphibole hornblendite, garnet amphibolite, chlorite -hornblende schist, kyanite-plagioclase schist, and talc-tremolite rocks, are characterised by ultrabasic to intermediate chemistry, show extreme enrichment in Cr and Ni, and collectively include kyanite, margarite, fuchsite, scapolite, and the first reported occurrence of gedrite and anthophyllite from the Eastern Province.

These kyanite-bearing, metasomatised mylonites are juxtaposed with quartzofeldspathic and calcsilicate mylonites, and with amphibolites derived from ocean-floor basalt, within the Alpine Fault Zone. The Makawhio River kyanitebearing mylonites and associated rocks probably represent a tectonised sliver of the Pounamu Ultramafic Belt ophiolite, which is exposed to the north and south as part of the imbricated basement to the Torlesse Terrane along the western margin of the Southern Alps.