Thermochronology of northern Murihiku Terrane, New Zealand, derived from apatite FT analysis

Abstract

Murihiku Terrane is the least deformed of a series of tectonostratigraphic terranes that have been amalgamated to form the basement underlying New Zealand. Apatite fission‐track (FT) thermochronology has been applied to the youngest beds (Late Jurassic, Hettangian–Tithonian Stages) of this terrane, located at Port Waikato in western North Island, which are broadly folded. The apatite FT ages are partially annealed, are younger than their stratigraphic ages, and remarkably, decrease in age upsection. Interpretation and modelling of the maximum palaeotemperatures experienced by the sample horizons and the timing of cooling requires a wedge of Cretaceous sedimentary rocks to have accumulated within the axis of the major fold (Kawhia Syncline). Deposition of Murihiku Terrane sequences continued therefore into the Cretaceous and probably until c. 100 Ma. Deposition may have ended as late as 85 ± 10 Ma, when cooling via denudation started. The upper parts of the former succession probably accumulated during folding. In addition, interpretation of the FT data require a marked increase in palaeogeothermal gradient from a subnormal value of 15°C km −1 during the Late Jurassic, up to c. 30°C km −1 at 85 ± 10 Ma. This is attributed to the termination of subduction along the Pacific margin of Gondwanaland, and a transition to a rift‐transform tectonic setting in western New Zealand during Tasman Sea spreading. The latest Cretaceous–Palaeocene eastward tilting of the Murihiku basement at Port Waikato may have been accommodated on the Taranaki Fault or another coast‐parallel fault; the Waikato Fault probably acted as a transfer fault at that time accumulating a significant amount of throw.