Structural evolution of the semischist–schist transition in an accretionary complex: the Otago Schist section at Lake Hāwea, New Zealand
Permo-Triassic, Rakaia Terrane, pelitic and psammitic schists at Lake Hāwea, Otago, show progressive metamorphism from TZII, pumpellyite–actinolite facies to TZIV, greenschist facies. The sequence shows evidence of five phases of deformation. D1 and D2 are progressive synmetamorphic phases, forming macroscopic nappe-like folds, interpreted to have formed by internal deformation within the Rakaia–Caples accretionary wedge during terrane collision. Thick S1 quartz veins are the form surface for definition of D2 folding, intensification of which progressively destroys evidence of D1 at higher textural and metamorphic grades. D3–5 are post metamorphic, less pervasive, kink folds (commonly conjugate) and brittle fault zones that predate the intrusion of lamprophyre magma and carbonatitic fluids associated with the Alpine Dike Swarm (c. 25 Ma). They relate to tectonic relaxation and exhumation of the schist in Cretaceous–Paleogene times. Fluid flow has been ubiquitous throughout the deformation, resulting in extensive quartz veining with diffusion of silica from the immediate country rock.