Thermochronologic constraints on the breakup of the Pacific Gondwana margin: The Paparoa metamorphic core complex, South Island, New Zealand

Abstract

Continental extension preceding the breakup of Gondwana in the Cretaceous produced a metamorphic core complex preserved in the Paparoa Range on South Island, New Zealand. Most features of classic Cordilleran core complexes are present including high metamorphic grade lower plate rocks separated from low‐grade upper plate rocks by detachment faults, syntectonic granitic intrusions and volcanism, and thick sequences of subaerial breccias and conglomerates. Dating of lower plate rocks by the40Ar/39Ar method indicates rapid cooling rates up to 110 °C Myr−1from temperatures of ∼500°–170 °C during the Cretaceous interval from ∼110 to 90 Ma, followed by lower cooling rates (∼5 °C Myr−1) beginning at ∼90 Ma. In contrast, granites intruding the upper plate underwent slow cooling (<5 °C Myr−1) during extended cooling histories (>200 Ma) beginning in the Devonian and ending in the Cretaceous. Combined with published U/Pb and fission track dates, the K/Ar‐ and40Ar/39Ar data define complete thermal histories (∼700°–100 °C), indicating rapid unroofing of lower plate rocks during a brief interval around 100 Ma. Rapid cooling rates recorded in lower plate rocks contrast with the extended slow cooling histories of upper plate rocks. Cooling ages for core rocks relative to distance from the southern (Pike) detachment fault indicate extension rates of ∼4 mm yr−1and suggest that the Pike detachment was responsible for most of the unroofing. The presence of syntectonic granitic plutons supports models in which magmatism is intimately associated with core complex formation. Slower cooling rates beginning at ∼90 Ma may record cessation of continental extension and the inception of seafloor spreading in the Tasman Sea (oldest basaltic crust, ∼84 Ma). These data establish a temporal and spatial link between continental extensional tectonics of Gondwana at ∼110–90 Ma and inception of seafloor spreading in the Tasman Sea (∼90–80 Ma) leading to separation of New Zealand from Australia and Antarctica.