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The continent of Zealandia (Mortimer, 2006) was once located on the outermost margin of Gondwana until its extensional fragmentation and separation in the Cretaceous. The present landmass of New Zealand represents the subaerial portion of this continent. The North Island is underlain by a series of basement rocks that represent Mesozoic accretionary terranes consisting chiefly of sandstone (“greywacke”), argillite and schist (Kaimanawa Schist; Fig. 2.2a). Here we focus on the low-temperature thermochronology of Triassic to Early Cretaceous rocks of the Torlesse Supergroup (in the axial ranges) and Waipapa Supergroup (west of Taupo Volcanic Zone, TVZ) of central North Island (Figs. 2.1a and 2.2a).

Based on different lines of evidence, many authors have proposed the timing of initiation of the break-up of Zealandia from eastern Gondwana and the cessation of the Mesozoic subduction. This was well summarised by Laird and Bradshaw (2004).

Estimates of the timing of the cessation of Mesozoic subduction range from 105 Ma (Bradshaw, 1989) to about 85 Ma (Kamp, 1999; Mazengarb and Harris, 1994). Laird and Bradshaw (2004) concluded that the subduction ceased and extension started at about 100 Ma, based on tectonic, geochronological and stratigraphic evidence along the east coast of North Island and north-western South Island. Based on U-Pb and

40Ar/³⁹Ar ages of volcanic rocks from across Zealandia, Tulloch et al. (2009) suggested that the crustal extension within Gondwana may have started at about 112 Ma and that thinning by normal faulting continued until 82 Ma. The range in age estimates may well be due to the fact that such a major event was probably diachronous both in time and space.

Inception of Cenozoic subduction of the Pacific Plate along the Hikurangi Trough beneath the eastern margin of the Australian Plate caused thickening and uplift of the hanging wall of that juvenile subduction zone, leading to subaerial exposure of what are now the islands of New Zealand (King, 2000; Sutherland et al., 2009). The fore-arc of this margin includes, on its eastern side, Neogene–Recent accretionary wedges that young eastward into the still actively deforming belt of the offshore region (Fig.

2.1b); a discontinuously preserved cover of Neogene sedimentary basins (Figs. 2.1 and 2.2a); and, farther west, the axial ranges, which consist of the uplifted Mesozoic basement. Today, the axial ranges are cut by the active North Island Fault System

(NIFS; Figs. 2.1 and 2.2a)(Beanland, 1995; Nicol et al., 2007)(New Zealand Active Faults Database, GNS). Farther west is the actively extending TVZ (Wilson et al., 2010), with uplands and basins along the western margin of that rift (Figs. 2.1 and 2.2a).

Fig. 2.2 Study area in central North Island, New Zealand. (a) Simplified geological map, modified from the Geologicla map of New Zealand 1:10⁶ project, produced by GNS Science, New Zealand. Major faults in the North Island Fault System (NIFS) are named. Numbers depict AFT (bold), AHe (italic) and ZFT (magenta) ages (Ma). (b) Shaded relief and sample locations. Topography is from the Shuttle Radar Topography Mission (SRTM), 3 arc-second resolution. Locations of whole-rock K-Ar ages (Adams et al., 2009) cited in this study are

The axial ranges trend northeast-southwest subparallel to the Hikurangi subduction trough. They include the Kaimanawa, Kaweka and Ahimanawa Ranges in central North Island (Fig. 2.2b). These narrow (about 60 km at the widest segment), elongate ranges reaching heights over 1.6 km (Fig. 2.2b) are chiefly composed of low-grade (prehnite-pumpellyite facies) metamorphosed greywackes of the Torlesse Supergroup (here mostly of Jurassic stratigraphic age, (Adams et al., 1998; Adams et al., 2009)).

Higher grade, pumpellyite-actinolite facies occurs on the western flank of the Kaimanawa Mountains (Fig. 2.2a) (Adams et al., 2009; Beetham and Watters, 1985), immediately to the east of the TVZ. Remnants of a pre-Pliocene erosional surface (Lee et al., 2011) are inferred to control the elevation of aligned summit tops in the Kaimanawa Mountains. From these uplifted deposits, a mean surface uplift rate has been calculated for the axial ranges of ~1.0–1.3 mm/yrover the last ~1–5 Myr (Beu et al., 1981; Litchfield and Berryman, 2006).

Previous geochronological studies on the basement terranes provided some constraints on the timing of their low-grade metamorphism during their accretion to Gondwana in the Mesozoic (Adams and Maas, 2004; Adams et al., 2009; Mortimer, 1994; Mortimer, 2004). Whole-rock K-Ar slate ages (Figs. 2.3a and 2.3b), which vary inversely with metamorphic grade, have been interpreted as estimates for the timing of cooling of the terranes from their maximum metamorphism (Adams, 2003; Adams et al., 2009). The Kaimanawa Schist yielded K-Ar ages between 150 and 125 Ma (Fig.

2.3b), distinctly younger than such ages measured in other parts of the Torlesse (and Waipapa) Terranes with lower grade metamorphic grade, which are typically 175–155 Ma (Figs. 2.3a and 2.3b) (Adams et al., 2009). Torlesse rocks in the Ahimanawa Range in the northeast of the study area yielded K-Ar ages of 220–180Ma (Fig. 2.3a).

These ages overlap with those of detrital zircons as measured by U-Pb methods and are older than the age of deposition of the strata in which they occur (Adams et al., 2009). This observation implies that rocks in the Ahimanawa Ranges were never heated to K-Ar closure temperature subsequent to their deposition.

West of TVZ, the basement rocks of the Waipapa Supergroup (Late Triassic–Late Jurassic age, (Adams et al., 2009)) crop out in the rolling and relatively low-relief uplands including the Hauhungaroa Range (Figs. 2.2a and 2.2b). Here, Cenozoic sedimentary sequences unconformably overlying the Mesozoic basement in the adjacent King Country basin have been extensively preserved. Sedimentation in the

basin was continuous between the Late Oligocene and Early Pliocene (Fig. 2.2a) (Kamp et al., 2004).

Fig. 2.3 Thermochronological ages along profiles across central North Island (see Fig. 2.2b for locations). Terrane depositional age and K-Ar whole-rock slate ages are taken from Adams et al. (2009).