Volcanoes of Zealandia and the Southwest Pacific

ABSTRACT

New Zealand and the southwest Pacific region have a well-preserved geological record of Late Cretaceous to Holocene volcanism. Volcanoes and volcanic fields formed in six main tectonic settings: (1) oceanic crust; (2) large igneous provinces; (3) subduction-related volcanic chains; (4) intraplate related to Late Cretaceous rifting of Zealandia from Gondwana; (5) intraplate in Cenozoic age progressive chains that define so-called hotspot tracks; and (6) intraplate with a scattered, Cenozoic, non-age progressive, non-rift-related distribution. At least 500 volcanic fields, stratovolcanoes and seamounts, active within the last 105 million years, can be identified within the c. 5 million km² area of Zealandia continental crust. Volcanic rocks from onland New Zealand are the best-studied and dated but represent material from less than a fifth of Zealandia’s volcanoes and volcanic fields.

KEYWORDS:

• Zealandia
• Southwest Pacific Ocean
• volcanoes
• tectonics
• subduction
• intraplate
• hotspot

Introduction

In the first account of New Zealand geology, Hochstetter (1864) drew special attention to the ‘Taupo Zone’ of the North Island with its ‘formations of rhyolitic and trachytic lava, obsidian and pumice, developed on a tremendous scale’. Hochstetter also mentioned Quaternary volcanic rocks in Taranaki, Auckland and Northland, and older Cenozoic volcanic formations near Christchurch, Dunedin and elsewhere (Figure 1, inset). By the early 1900s, the submarine New Zealand Plateau (later Zealandia) had been recognised, as had the Tonga–Kermadec ridge and trench (Marshall 1910). Using petrography and major element chemistry, Marshall (1910) had also identified the presence of alkaline volcanic suites in Auckland, Dunedin and the Subantarctic Islands (Marshall 1910). The rock name ignimbrite, along with an account of the fragmentation and welding processes that generate it, was first introduced from the Taupō area by Marshall (1935). The ‘Taupo Volcanic Zone’ was named by Steiner (1958), and the acronym TVZ has been in common use ever since.

Figure 1. Generalised igneous map of the Zealandia region showing nature of crust, active and extinct volcanoes and volcanic fields, and plutonic belts. Inset shows onland North and South Island volcanoes. Compiled from numerous sources (see text and Mortimer 2020) principally Cole (1986), Weaver and Smith (1989), Hoernle et al. (2006), Timm et al. (2010), Global Volcanism Program (2013), Mortimer et al. (2010, 2017, 2018), Tulloch et al. (2009, 2019) and the Petlab database (Strong et al. 2016). Young end of hotspots chains and tracks shown by yellow letters: Cg, Cosgrove; Cb, Comboyne; B, Balleny; T, Tasmantid; LH, Lord Howe; Lv, Louisville; S, Samoan. Cret, Cretaceous; Bas, Basin; TVZ, Taupo Volcanic Zone; CVZ, Coromandel Volcanic Zone. * Offshore ages are mostly inferred. ** For this subset of intraplate rocks, the circle symbols are confidently identified and the irregular polygons are > 300 nT magnetic anomalies inferred to be rift-controlled mafic igneous rocks (Tulloch et al. 2019). *** Median Batholith comprises contiguous plutons, with only minor volcanic rocks; Australian equivalents are not shown. Mercator projection.

The extent, stratigraphy and broad petrological characteristics of almost all of onland New Zealand’s volcanic rock formations had been mapped by the 1960s (Challis and Watters 1978). Papers in the Smith (1986) and Johnson et al. (1989) volumes reported X-ray fluorescence trace element data, K–Ar ages and Sr and Nd isotopic data, and interpreted them using plate tectonic concepts.

In the past 40 years, there have been only a few new discoveries of onland volcanic formations (eg Kopi Boninite, Wood 1980; Eocene Grasseed Volcanics, Reay 1993; Te Hukui Basalt, Kósik et al. 2017). By contrast, there has been vastly increased surveying and sampling of active and extinct offshore volcanoes (Wright et al. 2006; Graham et al. 2008; Mortimer et al. 2010; Timm et al. 2010; de Ronde et al. 2012). The past few decades have also seen a substantial increase in the application of high-precision trace element, Ar–Ar geochronological and additional tracer isotope data to investigate New Zealand’s volcanic rocks. Volcanic formations, groups and supersuites have recently been formally integrated into a high-level sedimentary stratigraphy of New Zealand (Mortimer et al. 2014).

The present-day tectonic framework for volcanism is shown in Figure 1. A c. 5 million km² region of continental crust, Zealandia, is located in the southwest Pacific around New Zealand and New Caledonia (Mortimer et al. 2017). A comparison of Figure 1 with Marshall (1910 third figure) and Johnson and Wellman (1989, fig. 1.2.1) shows how both the numbers of known volcanoes and their offshore regional tectonic context have changed. Plate tectonic control on volcanism is exerted by the relative and absolute motions of the Pacific and Australian plates, whose boundary cuts through New Zealand.

The purpose of this paper is to contribute a broad overview of the age and distribution of volcanism within and near the Zealandia continent (Figure 1). The emphasis on long spatial and temporal baselines provides a useful background for detailed studies.

Settings of volcanism

The volcanoes and volcanic centres shown in Figure 1 are symbolised according to the age and interpreted regional tectonic setting of each magmatic system. The scale of compilation, however, has demanded much merging and simplification of mapped volcanic rocks, especially on land. For example, some South Island Cenozoic volcanic fields comprise dozens of dikes and outliers that outcrop across many tens of kilometres (Coombs et al. 2008, fig. 2), and the Auckland Volcanic Field comprises > 50 separate small volcanoes (Lindsay et al. 2011). For the purposes of this continental-scale compilation, each point symbol on Figure 1 represents: (1) an individual seamount, guyot, stratovolcano or caldera (eg Mt Ruapehu); (2) a single volcanic field comprising a number of monogenetic cones or mapped formations (eg Auckland Volcanic Field, Mount Somers Volcanics Group); or (3) more rarely, a dike swarm (eg Alpine Dike Swarm).

Oceanic crust

Basins, c. 3–5 km deep and floored by oceanic crust, surround the Zealandia continent. These basins range in age from Early Cretaceous to Holocene and include some back-arc basins. The oldest oceanic crust near Zealandia was formed at the Osbourn Trough spreading centre between c. 120 and c. 100 Ma (Figure 1). The Tasman and Southwest Pacific basins opened during the separation of Zealandia from Australia and Antarctica from c. 85 Ma. Seafloor spreading ceased at c. 55 Ma in the Tasman Basin but continues to this day in the southwest Pacific. The other, smaller oceanic basins in Figure 1 all formed during the Cenozoic, many of them as back-arc basins. The North Fiji and Lau basins are still actively opening today.

Not shown in Figure 1 are allochthonous thrust slices of Cretaceous to Oligocene back-arc basin basalts that are locally present in onland North Island and New Caledonia (Te Raupua Supersuite of Mortimer et al. 2014). These are parts of back-arc basin ophiolite suites which were obducted in the Eocene and Oligocene (Whattam 2009).

Large igneous provinces

Two Early Cretaceous (c. 125–110 Ma) large igneous provinces are present north and east of Zealandia, the Hikurangi and Manihiki plateaus (Hoernle et al. 2010). Probably, these were formerly contiguous with the Ontong Java Plateau (Taylor 2006). The Hikurangi Plateau played a major geodynamic role in Zealandia tectonics by jamming the Cretaceous trench and halting long-lived subduction along this portion of Gondwana, now southern Zealandia (Reyners 2013). This influence continues today as it is the Hikurangi Plateau, not normal oceanic crust, that is being subducted under the TVZ.

Subduction-related volcanism

The Holocene and Pleistocene (< 2.6 Ma) volcanoes of the Taupo–Kermadec–Tonga arc, the Vanuatu–Solomons arc and Solander Island (red triangles, Figure 1) lie above present-day Benioff zones and have an unambiguous subduction-related origin (Cole 1986; Wilson et al. 1995). The older volcanic chains of Taranaki Basin, northern North Island, and the Three Kings, Loyalty and Colville-Lau ridges are interpreted as being subduction-related on the combination of their spatial distribution and geochemical (ie low Nb and Ta) characteristics (Mortimer et al. 2010; Booden et al. 2012). Confidence in a subduction-related origin is provided by overall eastward younging of these northern volcanic chains, which is interpreted to have arisen from eastward arc migration and Pacific trench rollback (Herzer 1995; Mortimer et al. 2010). On land this eastward migration is represented by the transition from Miocene–Pliocene volcanoes of the Coromandel Volcanic Zone to the Pleistocene TVZ (Figure 1 inset; Cole 1986; Kear 2004). The Holocene potassic volcano of Mt Taranaki lies west of the main active TVZ arc volcano axis above a deeper part of the subducting slab. It has many geochemical features of subduction-related volcanoes, and is shown as such on Figure 1, but other explanations for Taranaki volcanism involving different mantle sources and/or lithospheric delamination are possible (see Price et al. 2016 for discussion).

The Pacific Plate is inferred to have started to tectonically subduct under northern Zealandia in the Eocene (Sutherland et al. 2017), but the oldest subduction-related volcanic rocks at New Zealand latitudes are Early Miocene. The development of the Cenozoic arcs and back-arc basins north of New Zealand may have involved some subduction polarity flips (Whattam 2009). Only one subduction-related volcano, on Solander Island, is known from south of New Zealand (Mortimer et al. 2013).

Subduction-related volcanoes have erupted basalt, andesite, dacite and rhyolite, and show a wide range of terrestrial and submarine stratovolcano and caldera forms (Gamble et al. 1996; Leonard et al. 2010). Hochstetter’s (1864) comment about the notable abundance of rhyolitic lava, obsidian and pumice in the Taupo Zone (as opposed to elsewhere in the area of Figure 1) still holds true today; nowhere else in the area of Figure 1 are silicic volcanic rocks so common. In TVZ they are accompanied by active geothermal systems. Stratigraphically, subduction-related lavas in New Zealand are called Whakaari Supersuite (Mortimer et al. 2014). About 220 individual ancient and modern subduction-related volcanoes and volcanic fields lie within the inferred continental crust of Zealandia, and 44 of these are exposed above sea level.

In addition to the red symbols in Figure 1, Zealandia also has a well-documented record of 500–105 Ma subduction-related igneous rocks (Mortimer et al. 2014). Because of exhumation levels, these are mostly plutonic rocks that commonly occur in a linear Cordilleran-style batholith, the Median Batholith (dark pink band in Figure 1).

Intraplate volcanism

Late Cretaceous to Holocene volcanoes and volcanic fields unrelated to plate boundaries are shown by green and yellow symbols in Figure 1. In New Zealand these are termed Horomaka Supersuite (Mortimer et al. 2014). Most volcanoes in the offshore region remain unsampled, and are identified by seamount or guyot shape, gravity anomaly or reflection seismic interpretation. There are many ways to categorise intraplate volcanoes to explore the causes volcanism, eg eruptive style, volume, geochemical and isotopic composition, crustal and lithospheric thickness. For the purposes of this paper, we use age and long-baseline spatial criteria to identify three subgroups of intraplate igneous rocks: (1) non-age progressive, Gondwana-rift related, Late Cretaceous volcanoes; (2) age progressive, Cenozoic volcanic chains; and (3) non-age progressive, non-rift, Cenozoic volcanoes. A total of at least 280 individual intraplate-related volcanic centres have erupted onto the continental crust of Zealandia. Of these, c. 60 are currently exposed above sea level. Most of the Zealandia continent’s intraplate volcanoes are non-rift and non-age progressive.

Late Cretaceous Gondwana rift-related volcanism

Where dating information is available, Late Cretaceous 105–66 Ma volcanoes can be identified as a distinct intraplate subgroup (light green circles in Figure 1). They occur both close to and distant from continent–ocean boundaries. The Late Cretaceous was a time of crustal stretching, rifting and metamorphic core–complex formation along the south Gondwana margin, and followed the cessation of long-lived subduction. This extensional deformation event culminated in the opening of the Southwest Pacific and Tasman ocean basins and the splitting of Zealandia from Gondwana (Mortimer et al. 2017). Compositions of sampled syn-rift igneous rocks include basalt, gabbro, lamprophyre, syenite and rhyolite. Alkaline and subalkaline suites are represented, and some show HIMU (high radiogenic Pb) isotopic characteristics (Panter et al. 2006; Tulloch et al. 2009).

Speculatively included in this Gondwana-rift group on Figure 1 are areas of high positive magnetic anomaly on the Zealandia continental shelf (light green patches in Figure 1). The sub-orthogonal orientation of the anomalies with respect to nearby continent–ocean margins leads to the interpretation that they are caused by mafic Gondwana rift-related igneous rocks (Tulloch et al. 2019). For the most part these features are unsampled but if the correlation with syn-continental breakup mafic magmatism is correct then this age and style of magmatism is far more voluminous and widespread than previously recognised.

Cenozoic age progressive volcanism

Seven chains of subaerial and submarine volcanoes in Figure 1 show demonstrable change in age over hundreds of kilometres along their length. The Cenozoic Tasmantid, Lord Howe, Cosgrove, Comboyne and (more speculatively) the Balleny volcanic chains (McDougall and Duncan 1988; Crawford et al. 1997; Cohen et al. 2013; Davies et al. 2015; Seton et al. 2019) are on the Australian Plate and their volcanoes get younger to the south. The latest Cretaceous–Cenozoic Louisville and Cenozoic Samoan chain volcanoes are on the Pacific Plate and their ages generally get younger to the east and southeast (Jackson et al. 2010; Koppers et al. 2011). For the most part, all erupted lavas are mafic and alkaline in composition. Controversy about deep mantle plumes notwithstanding (Anderson and Natland 2005), the seven volcanic chains approximately record the absolute motion of Pacific and Australian lithospheric plates above the deep mantle (Courtillot et al. 2003). Some western North Island Plio-Pleistocene volcanoes in Auckland, Waikato and Taranaki show age-progressive trends over tens of km that do not match absolute motions of the Pacific Plate. These tracks are too short to show in Figure 1.

Cenozoic non-rift, non-age progressive volcanism

The remainder of the intraplate volcanoes and volcanic fields in Figure 1 are of Cenozoic age and erupted while Zealandia was drifting north away from Gondwana. The ones within Zealandia have been referred to as a diffuse alkaline magmatic province (DAMP, Finn et al. 2005), and include the volcanoes of the Holocene Auckland Volcanic Field and Pleistocene Antipodes Islands (Figure 1). However, intra-oceanic seamounts are even more widespread and more poorly sampled. Volcanism is mostly mafic and alkaline, with ultra-alkaline and sub-alkaline compositions known from some places (Coombs et al. 2008; Timm et al. 2010). Some of these lavas contain mantle xenoliths and provide a window into the composition and heterogeneity of the deep Zealandia lithosphere (Scott et al. 2016, 2019). The Precambrian Re–Os ages of many Zealandia mantle xenoliths pose a conundrum as they are significantly older than the overlying Phanerozoic crust (McCoy-West et al. 2013; Liu et al. 2015).

As indicated by their name, the intraplate non-rift, non-age progressive volcanoes have defied attempts to establish any relationship between their age, composition, tectonic setting and/or spatial distribution. Geochemistry-based interpretive models appeal to variants of edge-driven disturbances in the mantle to explain why the mantle melted (Hoernle et al. 2006; Timm et al. 2010), but the ultimate cause and timing of these instabilities, or whatever else led to sporadic mantle melting, remains enigmatic.

Conclusions

A wide range of Late Cretaceous to Holocene volcanoes and volcanic rocks is present on the Zealandia continent and the surrounding oceanic lithosphere. There are more than 500 individual subduction-related and intraplate volcanic fields, stratovolcanoes, caldera volcanoes and seamounts present within the Zealandia continental limits (Figure 1). Of these, c. 100 are exposed on land in New Zealand, New Caledonia and smaller islands. With the passage of time, progressively more sample data are being acquired and more patterns in Zealandia and southwest Pacific volcanism are being revealed.

Acknowledgements

Pre-submission reviews by Kevin Faure and Geoff Kilgour, and journal reviews by George Zellmer and Gabor Kereszturi led to improvements in the paper. Mortimer acknowledges core funding to GNS Science from the Ministry of Business, Innovation and Employment.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Ministry of Business, Innovation and Employment.