Tracking crustal processes by FT thermochronology in a forearc high (Hikurangi margin, New Zealand) involving Cretaceous subduction termination and mid-Cenozoic subduction initiation

Abstract

Raukumara Peninsula is a forearc high on the Pacific margin of New Zealand. Its basement comprises two sedimentary terranes that formed in subduction prisms during the Late Jurassic–mid-Cretaceous, marking the end of subduction along the Pacific margin of eastern Gondwanaland. The cover includes allochthonous Late Cretaceous–Oligocene passive margin slope to basin facies emplaced into the forearc during the Early Miocene initiation of the current subduction regime, as well as autochthonous Neogene sediments. Apatite and zircon fission track (FT) data are reported and interpreted for the basement and cover sediments. The detrital zircon FT ages constrain the maximum depositional ages of the low grade (zeolite and pumpellyite facies) basement. Pahau Terrane is no older than 152±13 Ma, this zircon FT mode and older ones (198±8 Ma, 232±58 Ma) recording cooling/denudation in the source areas, probably including Rakaia Terrane. Waioeka Terrane, located oceanward of Pahau Terrane, is no older than 125±2 Ma, being the age of volcanic-derived zircons, and supports Early Cretaceous dinoflagellate ages reported previously. Omaio petrofacies comprises the youngest part of the Waioeka Terrane, and has a zircon FT age of 108±6.3 Ma. This indicates that basement sequences accumulated in the northeast contemporaneously with cover sequences (Koranga Fm) in the southwestern part of the peninsula. These zircon FT ages together with the results of thermal history modelling of certain Waioeka Terrane samples are consistent with migration of a thrust system in a subduction prism environment during the Late Jurassic–Cretaceous until about 85 Ma, as proposed earlier, when Mesozoic subduction terminated. Apatite FT data indicate for all basement samples a phase of Neogene heating and cooling associated with basin formation, sediment accumulation and subsequent uplift and erosion, resulting in almost complete removal of the Neogene stratigraphic record over western parts of the peninsula. Forward modelling of the data imply rapid earliest Miocene downwarping of the crust in Bay of Plenty, coinciding with initiation of subduction of Pacific plate, followed by emplacement of a 1.5–2.0 km thick wedge of East Coast Allochthon from the northeast. Early Miocene sediments accumulated in a foredeep along the front (SW margin) of the allochthon, probably sourced from a block of Pahau basement west of Whakatane Fault, which was exhumed and denuded by 4 km contemporaneously. During the late-Early to Middle Miocene the basin depocentre migrated further to the southwest, while areas previously downwarped (northeast) were uplifted and eroded. The depocentre migration and subsequent inversion is a pattern that affected most of North Island, and is related to mechanical or dynamical interaction of the two plates as the slab of Pacific oceanic lithosphere was emplaced beneath North Island. This may be a general feature of young subduction zones.