Thermochronology of the Torlesse accretionary complex, Wellington region, New Zealand

The Torlesse Complex comprises several Mesozoic accretionary prism complexe s together forming continental basement over large parts of New Zealand. Th is study focuses on the thermal history of relatively low grade graywacke r ocks exposed in a transect in southern North Island that crosses the struct ural grain of the Torlesse Complex, including its older and younger parts. Zircon fission track (FT) ages for the Late Triassic Rakaia Terrane, which is the most inboard of the accretionary complexes, are partially annealed, some possibly reset, and may indicate early Cretaceous (134 +/- 10 Ma) cool ing from maximum temperatures (T-max), probably related to imbrication of y ounger complexes of the Pahau Terrane. Numerical modeling of the zircon FT ages and published Ar-40/Ar-39 muscovite and biotite ages for the Rakaia Te rrane suggest T-max values of 265-310 degrees C and exhumation from depths of 10-12 km. The rocks underlying the Aorangi Range and involving the young est accretionary complex have experienced much lower T-max values of less t han or equal to 210 degrees and greater than or equal to 110 degrees C, bra cketed by reset apatite FT ages and detrital zircon FT ages. The occurrence of a circa 100 Ma component of zircon FT ages in both the weakly and highl y indurated rocks beneath the Aorangi Range, as well as in remnants of an o verlying Albian accretionary slope basin (Whatarangi Formation), imply mult istorey accretion and incorporation of sediment into the youngest prism. Th is circa 100 Ma zircon FT age component also places a maximum age on the te rmination of Mesozoic subduction beneath the New Zealand region. The occurr ence of reset apatite FT ages across the whole of the Wellington transect i ndicates that at least 4 kin of exhumation occurred during the late Miocene .