TIME-SCALES OF MAGMATISM, METAMORPHISM AND DEFORMATION DURING THE INITIATION OF INTRAPLATE RIFTING IN THE LOWER CRUST OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND

Structural data and zircon ²⁰⁶U/²³⁸Pb ion probe (Stanford-USGS SHRIMP-RG) ages from a lower crustal section in Fiordland allowed us to investigate the space-time patterns of deep crustal processes as arc magmatism and subduction gave way to continental rifting.

The transition coincided with emplacement of the Malaspina Orthogneiss (MO) at ~40 km depth. Emplacement began by 120.2 ± 2.2 Ma on the NW side of the MO where a 6 km wide zone of vertical flow foliations record the movement of gabbroic-dioritic magma through the lower crust. The central sector records slightly younger ages (115.9 ± 1.2 Ma) and gently to moderately dipping layered sills up to 2 km thick. The initiation of extension was marked by (1) the formation of gently to moderately dipping shear zones within the MO that record crustal thinning and NE-SW stretching that match extension directions outside Fiordland, (2) a change from deformation styles involving vertical and highly variable displacements to ones involving mostly horizontal flow, and (3) decompression with cooling. Four stages of extension are discernable. During stage 1 shear zones localized within a >200 km² area in the core of the MO and record deformation synchronous with partial melting and high-T (>850°C) granulite-facies metamorphism at 113.5 ± 1.7 Ma (met. zircon rim ages). The Doubtful Sound shear zone (DSSZ) formed during stage 2, also in the central MO. This structure records decompression, retrogression to upper amphibolite facies, and cooling through 650°C mostly during 106-102 Ma. The DSSZ was abandoned by 97.8 ± 1.3 Ma. Stage 3 formed a new extensional shear zone system SW of the DSSZ that includes the Resolution Island shear zone and records upper amphibolite facies deformation at 98-88 Ma. The emplacement of gabbroic dikes at this time signal waning arc magmatism and/or intracrustal melts resulting from relaxing geotherms. Stage 4 saw the rupture of continental lithosphere, the initiation of seafloor spreading, and the formation of new ocean crust in the Tasman Sea by ~85 Ma.

These four stages correlate well with episodes of silicic volcanism and normal faulting outside Fiordland. The en echelon stepping of extensional shear zones in Fiordland suggests that rifting was controlled by regional tectonic forces rather than local buoyancy-driven flow linked to hot, partially molten crust.