The north fiordland transcurrent convergence

Abstract

The crystalline basement of New Zealand shows a chi‐shaped combination of the straight Alpine Fault and a curved late Paleozoic continental margin, so that the plutonic basement of northern Fiordland converges toward the plane of the Alpine Fault. On the ground along Milford Sound the petrography suggests (1) a gradual change from amphibolite and granulite fades in the west to igneous fabrics decreasingly overprinted by epidote amphibolite facies towards east, and (2) a continuum of deformational styles from early solid flow and constructive metamorphism at depth to late outright rupture near the surface. Of structures close to the surface, the postulated Pembroke Fault, 3 km east of the Alpine Fault, marks the main increase of relief and also marks the eastern limit of a zone of intense lamination. The Anita Ultramafites continue between the Alpine and Pembroke Faults to at least 5 km north of Milford Sound. Further east, a several kilometres wide section of the Milford profile converges northward to the Kaipo Fault, which sharply juxtaposes western amphibolite facies and eastern gabbroic rocks, so that on this suture most of the metamorphic transition is missing. The Milford Gneiss fabric, where undisturbed, is >70 Ma old, based on hornblende K‐Ar dates. Since that fabric merges well with the northward cut‐off by the Alpine and Pembroke Faults, an Alpine Fault megashear may have been active already in the Late Cretaceous. Two genetically well‐defined lithologic units are the Milford Gneiss with its relicts of isochemical Pembroke Granulite, and the Darran Leucogabbro with its western hornblende diorite facies. The two magmas are chemically similar and share their oxygen and strontium isotope signatures, and possibly Paleozoic protoliths. A Cretaceous final emplacement of these magmas was followed by continental extension and rifting, with formation of garnet zone joints in granulite at depth, and initial deep‐seated displacement in the Alpine Fault Zone that continues with its associated surface expressions (faults) until the present. © 1991 Taylor & Francis Group, LLC.