Gneiss domes, vertical and horizontal mass transfer, and the initiation of extension in the hot lower-crustal root of a continental arc, Fiordland, New Zealand

Abstract

Structural analyses and sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) zircon 206Pb/238U dates reveal the tectonic evolution of the deep (40–65 km) root of a Cretaceous continental arc as subduction beneath Gondwana ended and rifting began. By ca. 123 Ma, a dense root, composed partly of garnet pyroxenite and omphacite granulite, had formed. At 118–115 Ma, during regional contraction, a magma flare-up thermally and mechanically rejuvenated the base of the arc, resulting in widespread crustal melting, granulite-facies metamorphism, and the circulation of hot partially molten lower crust. By ca. 114 Ma, the flow formed two different styles of migmatitic gneiss domes. At the deepest (∼65 km) levels, the Breaksea domes record the upward flow of material in diapirs balanced by a sinking garnet pyroxenite root. At shallower (∼40 km) levels, the Malaspina domes record lateral flow in 1–2-km-thick channels beneath a roof of Paleozoic gneiss. At ca. 106 Ma, regional extension began with the formation of the Doubtful Sound shear zone (106–97 Ma), followed by the Resolution Island shear zone (95–89 Ma). These structures overprint migmatitic fabrics in the gneiss domes and record lower-crustal thinning, decompression with cooling (<730 °C), and horizontal flow oblique to the arc. They also show a migration of deformation toward the Gondwana interior that was driven by differences in the thermal structure and viscosity of the lower crust. In our model, gneiss doming and root detachment were precursors to rifting and triggered by magmatism. The evolution of extension occurred in three stages that reflect both the rheological structure of the lower crust and the influence of propagating spreading ridges.