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

Continental extension preceding the breakup of Gondwana in the Cretaceous p roduced a metamorphic core complex preserved in the Paparoa Range on South Island, New Zealand. Most features of classic Cordilleran core complexes ar e present including high metamorphic grade lower plate rocks separated from low-grade upper plate rocks by detachment faults, syntectonic granitic int rusions and volcanism, and thick sequences of subaerial breccias and conglo merates. Dating of lower plate rocks by the Ar-40/Ar-39 method indicates ra pid cooling rates up to 110 degrees C Myr(-1) from temperatures of similar to 500 degrees-170 degrees C during the Cretaceous interval from similar to 110 to 90 Ma, followed by lower cooling rates (similar to 5 degrees C Myr( -1)) beginning at similar to 90 Ma. In contrast, granites intruding the upp er plate underwent slow cooling (<5 degrees C Myr(-1)) during extended cool ing histories (>200 Ma) beginning in the Devonian and ending in the Cretace ous. Combined with published U/Pb and fission track dates, the K/Ar and Ar- 40/Ar-39 data define complete thermal histories (similar to 700 degrees-100 OC), indicating rapid unroofing of lower plate rocks during a brief interv al around 100 Ma. Rapid cooling rates recorded in lower plate rocks contras t with the extended slow cooling histories of upper plate rocks. Cooling ag es for core rocks relative to distance from the southern (Pike) detachment fault indicate extension rates of similar to 4 mm yr(-1) and suggest that t he Pike detachment was responsible for most of the unroofing. The presence of syntectonic granitic plutons supports models in which magmatism is intim ately associated with core complex formation. Slower cooling rates beginnin g at similar to 90 Ma may record cessation of continental extension and the inception of seafloor spreading in the Tasman Sea (oldest basaltic crust, similar to 84 Ma). These data establish a temporal and spatial link between continental extensional tectonics of Gondwana at similar to 110-90 Ma and inception of seafloor spreading in the Tasman Sea (similar to 90-80 Ma) lea ding to separation of New Zealand from Australia and Antarctica.