Nick Mortimer

New Zealand's Geological Foundations

Abstract New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths. The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (>100 km 2 ) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatism from ca. 375–110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Melanges bound the Pahau Terrane. In the South Island, small areas ( 2 total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.

Jurassic tectonic history of the Otago Schist, New Zealand

A regional reanalysis of the Otago Schist shows that ductile mesoscopic structures can be interpreted as elements of a single, progressive, heterogeneous, noncoaxial, Jurassic deformation related to juxtaposition of the Caples and Torlesse terranes. Stretching lineations are parallel, oblique, and perpendicular to the orogen in various parts of Otago, but changes in trend are generally unrelated to the terrane boundary. Structural geometry and rare shear criteria indicate that the Caples terrane overthrust the Torlesse terrane from the south and west, but precise transport directions are not known, being model-dependent. Macroscopic recumbent folds may be present in the schist, but mainly in the form of partially developed nappe structures truncated by zones of retransposed foliation and/or high strain.

Textural zones in the Haast Schist—a reappraisal

Abstract Field subdivision of schist using textural appearance is valuable as a means to readily identify post‐metamorphic faults and to subdivide monotonous schist, at 1:50 000 scale or smaller. Recent regional mapping in the Haast Schist of South Island, New Zealand, has revealed ambiguities and shortcomings in the existing field‐based systems of textural subdivision. We propose a revised textural zonation scheme that is broadly compatible with the previous Hutton‐Turner and Bishop systems, yet overcomes their deficiencies. The main criteria for identifying textural zones (TZ) are white mica grain size and foliation development. For field use, the main features are: (1) restriction to first generation penetrative textures and fabrics; (2) clarification of the definition and usage of segregation to better distinguish TZIIB from III schist; and (3) grouping of TZIIIB and IV rocks because of problems of protolith identification and quartz veining. The revised system is applicable to both sandstone and mudstone protoliths.

An episodic Cretaceous cooling model for the Otago‐Marlborough Schist, New Zealand, based on 40Ar/39Ar white mica ages

Abstract New 40Ar/39Ar ages of 12 white mica samples from deep levels of the Otago and Marlborough Schists, together with previously published whole‐rock K‐Ar ages yield a nonlinear age‐depth profile suggesting a fossil partial retention zone for argon. In contrast to earlier studies, we interpret (1) the peak of Otago Schist metamorphism to have occurred in the Middle Jurassic (170–180 Ma) rather than Early Jurassic; and (2) subsequent cooling to have been episodic, not slow and continuous. These data cannot be modelled uniquely but support a model where the schist was held at mid to lower crustal depths until c. 135 ± 5 Ma, after which it was rapidly unroofed at 0.6–1.0 mm/yr during regional crustal thickening along the eastern margin of New Zealand. We infer that there were also one or more younger periods of argon loss affecting deep levels of the Otago/ Marlborough Schists in a spatially heterogeneous way after c. 120 Ma. Late Cretaceous argon loss at <75–84 Ma coincided with seafloor spreading offsh...

Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

Abstract We describe two shear zones in the Otago Schist of the Torlesse accretionary wedge, South Island, New Zealand: the north‐dipping Rise‐and‐Shine Shear Zone (RSSZ) and the south‐dipping Cromwell Gorge Shear Zone (CGSZ). Kinematic indicators (shear bands and asymmetric folds) indicate top‐north movement for the RSSZ and top‐south transport for the CGSZ. Back rotation of the shear zones into their Late Cretaceous orientation and consideration of the relationship of the shear zones to arching of the Otago Schist show that both shear zones are extensional. Offset of textural zones suggests up to 15 km of dip‐slip displacement on the RSSZ and probably a similar amount of slip for the CGSZ. We speculate that the gold‐mineralised RSSZ may be the western continuation of the late Mesozoic gold‐bearing Hyde‐Macraes Shear Zone in eastern Otago, forming a c. 100 km long extensional shear zone on the northern flank of the Otago Schist. Age constraints suggest that shear zone formation took place between 135 and 105 Ma. The shear zones aided the final exhumation of the deeper parts of the Otago Schist. We discuss whether normal shearing is related to syn‐orogenic supercritical tapering of the Torlesse wedge, or due to post‐orogenic New Zealand‐wide Albian rifting.

High-level stratigraphic scheme for New Zealand rocks

We formally introduce 14 new high-level stratigraphic names to augment existing names and to hierarchically organise all of New Zealands onland and offshore Cambrian–Holocene rocks and unconsolidated deposits. The two highest-level units are Austral Superprovince (new) and Zealandia Megasequence (new). These encompass all stratigraphic units of the countrys Cambrian–Early Cretaceous basement rocks and Late Cretaceous–Holocene cover rocks and sediments, respectively. Most high-level constituents of the Austral Superprovince are in current and common usage: Eastern and Western Provinces consist of 12 tectonostratigraphic terranes, 10 igneous suites, 5 batholiths and Haast Schist. Ferrar, Tarpaulin and Jaquiery suites (new) have been added to existing plutonic suites to describe all known compositional variation in the Tuhua Intrusives. Zealandia Megasequence consists of five predominantly sedimentary, partly unconformity-bounded units and one igneous unit. Momotu and Haerenga supergroups (new) comprise lowermost rift to passive margin (terrestrial to marine transgressive) rock units. Waka Supergroup (new) includes rocks related to maximum marine flooding linked to passive margin culmination in the east and onset of new tectonic subsidence in the west. Māui and Pākihi supergroups (new) comprise marine to terrestrial regressive rock and sediment units deposited during Neogene plate convergence. Rūaumoko Volcanic Region (new) is introduced to include all igneous rocks of the Zealandia Megasequence and contains the geochemically differentiated Whakaari, Horomaka and Te Raupua supersuites (new). Our new scheme, Litho2014, provides a complete, high-level stratigraphic classification for the continental crust of the New Zealand region.

Ferrar magmatic province rocks discovered in New Zealand: Implications for Mesozoic Gondwana geology

On the basis of similar ages, major and trace element concentrations, and Sr, Nd, and Pb isotopic ratios, we correlate the Kirwans Dolerite of western New Zealand with Jurassic low-Ti tholeiites of the well-known Ferrar magmatic province of Gondwana. Field relations and preliminary paleomagnetic data also support this correlation. Although the 1 km 2 Kirwans Dolerite is the first, and so far only, Ferrar correlative to be reported from New Zealand, it considerably increases the known shape and areal extent of the Ferrar province to close to the paleo-Gondwana margin. The presence of Ferrar rocks in New Zealand can be used to test several models of Ferrar magma genesis and Gondwana regional tectonics.

Metamorphic zones, terranes, and Cenozoic faults in the Marlborough Schist, New Zealand

Abstract The northeast‐striking Picton Fault Zone is a major structural and metamorphic break that divides the Marlborough Schist into two separate blocks. The northwestern (Kaituna) block shows a regular increase in textural and metamorphic grade to the southeast from prehnite‐pumpellyite to greenschist facies and textural zone I to IV. The southeastern (Arapawa) block shows an irregular increase in grade to the southeast from prehnite‐pumpellyite to pumpellyite‐actinolite facies and textural zone I to IIB. The Kaituna block is correlated with the Caples and Torlesse Terranes and the Otago Schist of the southern South Island. The Arapawa block is correlated with the Waipapa Terrane (possibly also the Torlesse Terrane) and Kaimanawa Schist of the North Island. Isotects and the Picton Fault Zone are dextrally offset by 10–15 km across the ENE‐striking Queen Charlotte Fault Zone. The latter should be included with the late Cenozoic Marlborough faults.

Zealandia: Earth’s Hidden Continent

A 4.9 Mkm2 region of the southwest Pacific Ocean is made up of continental crust. The region has elevated bathymetry relative to surrounding oceanic crust, diverse and silica-rich rocks, and relatively thick and low-velocity crustal structure. Its isolation from Australia and large area support its definition as a continent—Zealandia. Zealandia was formerly part of Gondwana. Today it is 94% submerged, mainly as a result of widespread Late Cretaceous crustal thinning preceding supercontinent breakup and consequent isostatic balance. The identification of Zealandia as a geological continent, rather than a collection of continental islands, fragments, and slices, more correctly represents the geology of this part of Earth. Zealandia provides a fresh context Nick Mortimer, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Hamish J. Campbell, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Andy J. Tulloch, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Peter R. King, Vaughan M. Stagpoole, Ray A. Wood, Mark S. Rattenbury, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Rupert Sutherland, SGEES, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand; Chris J. Adams, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Julien Collot, Service Géologique de Nouvelle Calédonie, B.P. 465, Nouméa 98845, New Caledonia; and Maria Seton, School of Geosciences, University of Sydney, NSW 2006, Australia in which to investigate processes of continental rifting, thinning, and breakup.

Relationships between the brook street Terrane and Median Tectonic Zone (Median Batholith): Evidence from Jurassic conglomerates

Abstract U‐Pb zircon ages of 237–180 Ma and c. 280 Ma of seven granitoid clasts from the Rainy River Conglomerate which lies within the eastern Median Tectonic Zone (Median Batholith) in Nelson, and the Barretts Formation of the Brook Street Terrane in Southland, constrain the depositional ages of both units to be no older than c. 180–200 Ma (Early Jurassic). The minimum age of the Rainy River Conglomerate is constrained by the 147 +2 ‐1 Ma (latest Jurassic) emplacement age of the One Mile Gabbronorite (new name: previously western Buller Diorite). The ages and chemistry of five of the granitoid clasts are broadly compatible with derivation from rocks that are now represented by Triassic plutons of the Median Tectonic Zone (Median Batholith), although ages as young as 180 Ma are slightly outside the range of the latter as currently exposed in New Zealand. The age (273–290 Ma, 237 ± 3 Ma) and chemistry of the other two clasts (one each from Rainy River Conglomerate and Barretts Formation) suggest derivatio...