Pjj. Kamp et Ij. Liddell, Thermochronology of northern Murihiku Terrane, New Zealand, derived from apatite FT analysis, J GEOL SOC, 157, 2000, pp. 345-354
Murihiku Terrane is the least deformed of a series of tectonostratigraphic
terranes that have been amalgamated to form the basement underlying New Zea
land. Apatite fission-track (FT) thermochronology has been applied to the y
oungest 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 stratigr
aphic ages, and remarkably, decrease in age upsection. Interpretation and m
odelling of the maximum palaeotemperatures experienced by the sample horizo
ns and the timing of cooling requires a wedge of Cretaceous sedimentary roc
ks to have accumulated within the axis of the major fold (Kawhia Syncline).
Deposition of Murihiku Terrane sequences continued therefore into the Cret
aceous and probably until c. 100 Ma. Deposition may have ended as late as 8
5 +/- 10 Ma, when cooling via denudation started. The upper parts of the fo
rmer succession probably accumulated during folding. In addition, interpret
ation of the FT data require a marked increase in palaeogeothermal gradient
from a subnormal value of 15 degrees C km(-1) during the Late Jurassic, up
to c. 30 degrees C km(-1) at 85 +/- 10 Ma. This is attributed to the termi
nation of subduction along the Pacific margin of Gondwanaland, and a transi
tion to a rift-transform tectonic setting in western New Zealand during Tas
man Sea spreading. The latest Cretaceous-Palaeocene eastward tilting of the
Murihiku basement at Port Waikato may have been accommodated on the Tarana
ki Fault or another coast-parallel fault; the Waikato Fault probably acted
as a transfer fault at that time accumulating a significant amount of throw
.