Possible Jurassic age for part of Rakaia Terrane: implications for tectonic development of the Torlesse accretionary prism

Greywacke sandstone and argillite beds comprising Rakaia Terrane (Torlesse Complex) in mid Canterbury, South Island, New Zealand, are widely regarded as Late Triassic (Norian) in age based on the occurrence of Torlessia trace fossils, Monotis, and other taxa. This paleontological age assignment is t ested using published Ar-40/Ar-39 mica and U-Pb zircon ages for these rocks and published and new zircon fission track (FT) ages. The youngest U-Pb zi rcon ages in the Rakaia Terrane rocks in mid Canterbury are Norian, whereas 10-20% of the Ar-40/Ar-39 muscovite ages are younger than Norian. Numerica l modelling of these mica ages shows that they cannot have originated from partial thermal overprinting in the Torlesse prism if the thermal maximum w as short-lived and early in the prism history (210-190 Ma), as commonly inf erred for these rocks. The young component of mica ages could, however, be explained by extended residence (200-100 Ma) at 265-290 degreesC in the pri sm. Early Jurassic (c. 189 Ma) zircon FT ages for sandstone beds from Arthu r's Pass, the Rakaia valley, and the Hermitage (Mt Cook) are interpreted no t to have experienced maximum temperatures above 210 degreesC, and therefor e cannot have been reduced as a result of partial annealing in the Torlesse prism. This is based on identification of a fossil Cretaceous, zircon FT, partial annealing zone in low-grade schists to the west, and the characteri stics of the age data. The Early Jurassic zircon FT ages and the young comp onent of Ar-40/Ar-39 mica ages are regarded therefore as detrital ages refl ecting cooling in the source area, and constrain the maximum depositional a ge of parts of the Rakaia Terrane in mid Canterbury. The zircon FT data als o show the initiation (c. 100 Ma) of marked and widespread Late Cretaceous cooling of Rakaia Terrane throughout Canterbury, which is attributed to upl ift and erosion of inboard parts of the Torlesse prism due to continuing su bduction accretion at its toe. The critical wedge concept is proposed as a new framework for investigating the development of the Torlesse Complex. The Rakaia Terrane may have forme d the core of an accretionary wedge imbricated against the New Zealand marg in during the Middle or Late Jurassic. Late Jurassic nonmarine sediments (e .g., Clent Hills Formation) accumulated upon the inner parts of the prism a s it enlarged, emerged, and continued to be imbricated. Exhumation of Otago Schist from c. 135 Ma may mark the development of a balance (steady state) between sediments entering the prism at the toe and material exiting at th e inboard margin. The enlargement of the area of exhumation to all of Cante rbury from c. 100 Ma may reflect a dynamic response to widening of the pris m through the accretion of Cretaceous sediments. The model of a dynamic cri tical wedge may help to explain the various expressions of the Rangitata Or ogeny.