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

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 subducti on along the Pacific margin of eastern Gondwanaland. The cover includes all ochthonous Late Cretaceous-Oligocene passive margin slope to basin facies e mplaced 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 baseme nt and cover sediments. The detrital zircon FT ages constrain the maximum d epositional ages of the low grade (zeolite and pumpellyite facies) basement . Pahau Terrane is no older than 152 +/- 13 Ma, this zircon FT mode and old er ones (198 +/- 8 Ma, 232 +/- 58 Ma) recording cooling/denudation in the s ource areas, probably including Rakaia Terrane. Waioeka Terrane, located oc eanward of Pahau Terrane, is no older than 125 +/- 2 Ma, being the age of v olcanic-derived zircons, and supports Early Cretaceous dinoflagellate ages reported previously. Omaio petrofacies comprises the youngest part of the W aioeka Terrane, and has a zircon FT age of 108 +/- 6.3 Ma. This indicates t hat basement sequences accumulated in the northeast contemporaneously with cover sequences (Koranga Fm) in the southwestern part of the peninsula. The se 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-Cretaceou s until about 85 Ma, as proposed earlier, when Mesozoic subduction terminat ed. Apatite FT data indicate for all basement samples a phase of Neogene he ating and cooling associated with basin formation, sediment accumulation an d subsequent uplift and erosion, resulting in almost complete removal of th e Neogene stratigraphic record over western parts of the peninsula. Forward modelling of the data imply rapid earliest Miocene downwarping of the crus t in Bay of Plenty, coinciding with initiation of subduction of Pacific pla te, followed by emplacement of a 1.5-2.0 km thick wedge of East Coast Alloc hthon from the northeast. Early Miocene sediments accumulated in a foredeep along the front (SW margin) of the allochthon, probably sourced from a blo ck of Pahau basement west of Whakatane Fault, which was exhumed and denuded by 4 km contemporaneously. During the late-Early to Middle Miocene the bas in depocentre migrated further to the southwest, while areas previously dow nwarped (northeast) were uplifted and eroded. The depocentre migration and subsequent inversion is a pattern that affected most of North Island, and i s related to mechanical or dynamical interaction of the two plates as the s lab of Pacific oceanic lithosphere was emplaced beneath North Island. This may be a general feature of young subduction zones. (C) 1999 Elsevier Scien ce B.V. All rights reserved.