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 perio 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 contrusts 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.
ASJC Scopus subject areas
- Geochemistry and Petrology