Metamorphic belts and orogenesis in southern New Zealand

Abstract

Rocks of a Lower Paleozoic geosyncline, named here the Buller geosyncline, occur as remnants on the west side of the South Island. They underwent mid-Paleozoic orogenesis. Sediments accumulated in the New Zealand geosyncline to the east of these rocks in the Late Paleozoic and Mesozoic, local low-grade metamorphism, uplift, and erosion proceeding more or less simultaneously with sedimentation. Major orogenesis affected the rocks of both geosynclines in Early to mid-Cretaceous times. The eastern and western provinces constitute paired metamorphic belts. On the west, the Tasman Metamorphic Belt was characterized by relatively high-temperature-low-pressure metamorphism. The Wakatipu Metamorphic Belt on the east was characterized by relatively low-temperature-high-pressure metamorphism. The lowest thermal gradient, about 14°/km, developed during the Cretaceous in a strip of Permian rocks at least 300 km long and not more than 10 km wide in which pervasive law-sonite-albite-chlorite assemblages were produced. The two belts are in contact along a fault system referred to as the median tectonic line. It marks a zone of rapid change in thermal gradient during metamorphism and a change probable continental- to oceanic-type basement during prior sedimentation. Cenozoic crustal unrest culminated in the Kaikoura orogeny of Late Miocene to Recent age. The two metamorphic belts and the median tectonic line are cut and displaced 480 km by the still-active Alpine fault.