Two-stage development of the Paparoa Metamorphic Core Complex, West Coast, South Island, New Zealand: Hot continental extension precedes sea-floor spreading by ∼25 m.y.

The Paparoa Metamorphic Core Complex (PCC) developed in the mid-Cretaceous due to continental extension, which conditioned the crust for the eventual breakup of the Gondwana Pacific margin and formation of the Tasman Sea. The PCC has two detachment systems with opposite senses of shear: the top-to-the-NE Ohika Detachment in the north and the top-to-the-SW Pike Detachment in the south. Rb-Sr dating on mylonite shows that the Pike Detachment was active before 116.2 ± 5.9 Ma. It was the dominant detachment exhuming Cretaceous synextensional migmatites and was synchronous with the intrusion of the Buckland Granite, from which U-Pb zircon crystallization ages between 110.41 and 109.73 Ma were obtained. The ductile shear zone beneath the Pike Detachment records upper-amphibolite to lower-greenschist facies metamorphism and cataclastic deformation. Pronounced hydrothermal alteration at 108.91 ± 0.04 Ma is interpreted to be related to initial movement on the Ohika Detachment. The structural hinge separating top-to-the-SW from top-to-the-NE shearing has been located in the northern part of the PCC, also indicating that the Pike Detachment is the master detachment of the PCC. Fission-track data indicate a period of enhanced heat flow resulting in reset and partially reset apatite and zircon fission-track ages at ca. 75 Ma concurrent with the onset of sea-floor spreading in the Tasman Sea. Our data show that initial extension in the mid-Cretaceous proceeded under high-temperature conditions and preceded continental breakup by ∼ 25 m.y.