Interaction of strong lower and weak middle crust during lithospheric extension in western New Zealand

Abstract

Exhumed sections of the middle and lower crust in western New Zealand reveal how deformation was partitioned within a thermally and rheologically evolving crustal column during Cretaceous continental extension. Structural data, P‐T determinations, and U‐Pb geochronology from central Fiordland and the Paparoa Range in Westland show that extension initiated in the lower crust by ∼114 Ma as a period of arc‐related magmatism waned. Initially, deformation was localized into areas that were weakened by heat and magma. However, these hot, weak zones were ephemeral. During the period 114–111 Ma, lower crustal fabrics record a rapid progression from magmatic flow to high‐temperature deformation at the garnet‐granulite facies (T > 700°C, P = 12 kbar) to cooler deformation at the upper amphibolite facies (T = 550–650°C, P = 7–9 kbar). Lower crustal cooling and compositional contrasts between mafic granulites and hydrous metasedimentary material resulted in a middle crust that was weak relative to the lower crust. Between circa 111 and circa 90 Ma, focused subhorizontal flow and vertical thinning in a weak middle crust led to the collapse of the upper crust and the unroofing of midcrustal material. During this period, arrays of conjugate‐style shear zones transferred displacements vertically and horizontally through the crust, resulting in a structural style that resembles crustal‐scale boudinage. The New Zealand example of continental extension shows that a weak middle crust and a relatively cool, highly viscous lower crust can result in a localized style of extension, including the formation of metamorphic core complexes that exhume the middle crust but not the lower crust.