Zealandia: Earth’s Hidden Continent
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
ABSTRACT A 4.9 Mkm2 region of the southwest in which to investigate processes of conti- continental fragments that were stranded Pacific Ocean is made up of continental nental rifting, thinning, and breakup. in the ocean basins during supercontinent crust. The region has elevated bathymetry breakups (e.g., Buck, 1991; Lister et al., relative to surrounding oceanic crust, INTRODUCTION 1991; Gaina et al., 2003; Franke, 2013; diverse and silica-rich rocks, and rela- Earth’s surface is divided into two types Eagles et al., 2015). But what about the tively thick and low-velocity crustal struc- of crust, continental and oceanic, and into major continents (Fig. 1)? Continents are ture. Its isolation from Australia and large 14 major tectonic plates (Fig. 1; Holmes, Earth’s largest surficial solid objects, and it area support its definition as a conti- 1965; Bird, 2003). In combination, these seems unlikely that a new one could ever nent—Zealandia. Zealandia was formerly divisions provide a powerful descriptive be proposed. part of Gondwana. Today it is 94% sub- framework in which to understand and The Glossary of Geology defines a con- merged, mainly as a result of widespread investigate Earth’s history and processes. tinent as “one of the Earth’s major land Late Cretaceous crustal thinning preced- In the past 50 years there has been great masses, including both dry land and conti- ing supercontinent breakup and conse- emphasis and progress in measuring and nental shelves” (Neuendorf et al., 2005). quent isostatic balance. The identification modeling aspects of plate tectonics at It is generally agreed that continents have of Zealandia as a geological continent, various scales (e.g., Kearey et al., 2009). all the following attributes: (1) high eleva- rather than a collection of continental Simultaneously, there have been advances tion relative to regions floored by oceanic islands, fragments, and slices, more cor- in our understanding of continental rifting, crust; (2) a broad range of siliceous igne- rectly represents the geology of this part continent-ocean boundaries (COBs), and ous, metamorphic, and sedimentary rocks; of Earth. Zealandia provides a fresh context the discovery of a number of micro- (3) thicker crust and lower seismic velocity
Figure 1. Simplified map of Earth’s tectonic plates and continents, including Zealandia. Continental shelf areas shown in pale colors. Large igneous province (LIP) submarine plateaus shown by blue dashed lines: AP—Agulhas Plateau; KP—Kerguelen Plateau; OJP—Ontong Java Plateau; MP—Manihiki Plateau; HP—Hikurangi Plateau. Selected microcontinents and continental fragments shown by black dotted lines: Md—Madagascar; Mt—Mauritia; D—Gulden Draak; T—East Tasman; G—Gilbert; B—Bollons; O—South Orkney. Hammer equal area projection.
GSA Today, v. 27, doi: 10.1130/GSATG321A.1 structure than oceanic crustal regions; and (4) well-defined limits around a large enough area to be considered a continent rather than a microcontinent or continental fragment. The first three points are defin- ing elements of continental crust and are explained in many geoscience textbooks and reviews (e.g., Holmes, 1965; Christensen and Mooney, 1995; Levander et al., 2005; Kearey et al., 2009; Condie, 2015). To our knowledge, the last point—how “major” a piece of continental crust has to be to be called a continent—is almost never dis- cussed, Cogley (1984) being an exception. Perhaps this is because it is assumed that the names of the six geological continents— Eurasia, Africa, North America, South America, Antarctica, and Australia— suffice to describe all major regions of continental crust. The progressive accumulation of bathy- metric, geological, and geophysical data since the nineteenth century has led many authors to apply the adjective continental to New Zealand and some of its nearby submarine plateaus and rises (e.g., Hector, 1895; Hayes, 1935; Thomson and Evison, 1962; Shor et al., 1971; Suggate et al., 1978). “New Zealand” was listed as a continent by Cogley (1984), but he noted that its continental limits were very sparsely mapped. The name Zealandia was first proposed by Luyendyk (1995) as a collec- tive name for New Zealand, the Chatham Rise, Campbell Plateau, and Lord Howe Rise (Fig. 2). Implicit in Luyendyk’s paper was that this was a large region of conti- nental crust, although this was only men- Figure 2. Spatial limits of Zealandia. Base map from Stagpoole (2002) based on data from Smith and Sandwell (1997). Continental basement samples from Suggate et al. (1978), Beggs et al. (1990), Tull- tioned in passing and he did not character- och et al. (1991, 2009), Gamble et al. (1993), McDougall et al. (1994), and Mortimer et al. (1997, 1998, ize and define Zealandia as we do here. 2006, 2008a, 2008b, 2015). NC—New Caledonia; WTP—West Torres Plateau; CT—Cato Trough; Cf— In this paper we summarize and reassess Chesterfield Islands; L—Lord Howe Island; N—Norfolk Island; K—Kermadec Islands; Ch—Chatham Islands; B—Bounty Islands; An—Antipodes Islands; Au—Auckland Islands; Ca—Campbell Island. a variety of geoscience data sets and show Mercator projection. that a substantial part of the southwest Pacific Ocean consists of a continuous expanse of continental crust. Furthermore, the 4.9 Mkm2 area of continental crust is and Campbell, 2014; Graham, 2015). regarded as part of the Australian continent, large and separate enough to be considered However, it is still not well known to the although the geographic term Australasia not just as a continental fragment or a broad international science community. A often is used for the collective land and microcontinent, but as an actual conti- correct accounting of Earth’s continents is islands of the southwest Pacific region. In nent—Zealandia. This is not a sudden important for multiple fields of natural the following sections, we summarize the discovery but a gradual realization; as science; the purpose of this paper is to for- four key attributes of continents and assess recently as 10 years ago we would not have mally put forth the scientific case for the how Zealandia meets these criteria. had the accumulated data or confidence in continent of Zealandia (Figs. 1 and 2) and interpretation to write this paper. Since it explain why its identification is important. Elevation was first proposed by Luyendyk (1995), Continents and their continental shelves the use of the name Zealandia for a south- ZEALANDIA AS A CONTINENT vary in height but are always elevated rela- west Pacific continent has had moderate New Zealand and New Caledonia are tive to oceanic crust (Cogley, 1984). The uptake (e.g., Mortimer et al., 2006; Grobys large, isolated islands in the southwest elevation is a function of many features, et al., 2008; Segev et al., 2012; Mortimer Pacific Ocean. They have never been fundamentally lithosphere density and thickness, as well as plate tectonics (e.g., Kearey et al., 2009). The existence of positive bathymetric features north and south of New Zealand has been known for more than a century (Farquhar, 1906). The accuracy and precision of seafloor map- ping have improved greatly over the past decades (Brodie, 1964; Smith and Sandwell, 1997; Stagpoole, 2002) and a deliberately chosen color ramp on a satel- lite gravity-derived bathymetry map pro- vides an excellent visualization of the extent of continental crust (Fig. 2). The approximate edge of Zealandia can be placed where the oceanic abyssal plains meet the base of the continental slope, at water depths between 2500 and 4000 m below sea level. The precise position of the foot of the continental slope around Zealandia was established during numer- ous surveys in support of New Zealand’s Law of the Sea submission (Wood et al., 2003; UNCLOS, 2008). Zealandia is everywhere substantially elevated above the surrounding oceanic crust. The main difference with other con- tinents is that it has much wider and deeper continental shelves than is usually the case (Fig. 1). Zealandia has a modal elevation of Figure 3. Present day map of CRUST1.0 crustal thickness (Laske et al., 2013) showing the dispersed - Gondwana continents of Australia, Zealandia, East and West Antarctica, and South America. Note ~ 1100 m (Cogley, 1984) and is ~94% sub- thin continental crust in vicinity of Mesozoic arc. M—Marion Plateau; R—Ross Sea; W—Weddell Sea; merged below current sea level. The high- F—Falkland-Malvinas Plateau. LIP abbreviations: KP—Kerguelen Plateau; OJP—Ontong Java Pla- est point of Zealandia is Aoraki–Mount teau; MP—Manihiki Plateau; HP—Hikurangi Plateau. Thick coastlines in Antarctica are isostatically corrected ice-free coastlines (Jamieson et al., 2014). Orthographic projection. Cook at 3724 m.
Geology Batholith and Haast Schist are parts, can be orogenic belts on which a broad exten- By itself, relatively high elevation is not tracked through onland New Zealand and sional province and several narrow rift enough to establish that a piece of crust is across Zealandia (Fig. 2). Thus, there is a zones have been superimposed (Mortimer continental. Oceanic large igneous prov- predictable regional coherency and conti- and Campbell, 2014). inces such as the Ontong Java Plateau nuity to the offshore basement geology. Atop its geological basement rocks, (Fig. 1; Coffin and Eldholm, 1994) are Traditionally, continents have been sub- Zealandia has a drape of at least two dozen elevated but not continental. Rocks of the divided into cratons, platforms, Phanerozoic spatially separate Late Cretaceous to modern oceanic crust typically comprise orogenic belts, narrow rifts, and broad Holocene sedimentary basins. These typi- basalt and gabbro of Jurassic to Holocene extensional provinces (Levander et al., cally contain 2–10-km-thick sequences of age. In contrast, continents have diverse 2005). Eurasia, Africa, North America, terrigenous and calcareous strata (Zealandia assemblages of Archean to Holocene igne- South America, Antarctica, and Australia Megasequence of Mortimer et al., 2014) and ous, metamorphic, and sedimentary rocks, all contain Precambrian cratons. The old- include a widespread continental breakup such as granite, rhyolite, limestone, quartz- est known rocks in Zealandia are Middle unconformity of ca. 84 Ma age (Bache et ite, greywacke, schist, and gneiss, arranged Cambrian limestones of the Takaka al., 2014). The Zealandia Megasequence in orogenic belts and sedimentary basins. Terrane and 490–505 Ma granites of the provides a Zealandia-wide stratigraphic Essential geological ground truth for Jacquiery Suite (Mortimer et al., 2014). record of continental rifting, and marine Zealandia is provided by the many island Precambrian cratonic rocks have not yet transgression events, similar to that seen in outcrop, drill core, xenolith, and seabed been discovered within Zealandia, but formerly conjugate east Australian basins dredge samples of Paleozoic and Mesozoic their existence has been postulated on the (Blewett, 2012). greywacke, schist, granite, and other sili- basis of Rodinian to Gondwanan age ceous continental rocks that have been detrital zircon ratios (Adams and Griffin, Crustal Structure found within its limits (Fig. 2). Many of 2012). Furthermore, some Zealandia man- Continental crust varies considerably these have been obtained from expeditions tle xenoliths give Re-Os ages as old as in thickness and physical properties. in the past 20 years (see Fig. 2, caption). 2.7 Ga (Liu et al., 2015). Geologically, Christensen and Mooney (1995) give an Orogenic belts, of which the Median Zealandia comprises multiple Phanerozoic average P wave velocity of 6.5 km-1 and mean density of 2830 kgm-3 with an aver- age thickness of 46 km for orogens and 30 km for extended crust. In contrast, oceanic crust is typically 7 km thick, and, in its lower part typically has a P wave velocity of 7.5 km-1 (White et al., 1992). From geophysical work, we know that Zealandia has a continental crust velocity structure, Vp, generally <7.0 km-1, and a thickness typically ranging from 10 to 30 km throughout its entire extent to >40 km under parts of South Island (Shor et al., 1971; Klingelhoefer et al., 2007; Grobys et al., 2008; Eberhart-Phillips et al., 2010; Segev et al., 2012). Whereas most of Zealandia’s crust is thinner than the 30–46 km that is typical of most conti- Figure 4. Areas and submergence of all of Earth’s geological con- tinents (red symbols) along with microcontinents (brown symbols) nents, the above studies show that it is and intraoceanic large igneous provinces (LIPs, blue symbols) everywhere thicker than the ~7-km-thick shown in Figures 1 and 2. Note x-axis is log scale. Data mainly after Cogley (1984) except Zealandia data from Mortimer and crust of the ocean basins. This result is Campbell (2014); microcontinents after Gaina et al. (2003) and visible in the global CRUST1.0 model of Torsvik et al. (2013). Emergent land area for Antarctica is the iso- statically-corrected ice-free bedrock surface from Jamieson et al. Laske et al. (2013) shown in Figure 3. (2014). New Guinea and Greenland are arbitrarily given the same Collectively, the crustal structure results submergence value as their parent continents. AP—Agulhas Plateau; KP—Kerguelen Plateau; OJP—Ontong Java Plateau; show that the rock samples of Figure 2 are MP—Manihiki Plateau; HP—Hikurangi Plateau; N Am—North not from separate continental fragments or America; S Am—South America. blocks now separated by oceanic crust, but are from a single continental mass. The thinnest crust within Zealandia is in continental geology is continuous across Mauritia, and Gulden Draak microconti- the 2200-km-long and 200–300-km-wide Nares Strait between northernmost nents (Gaina et al., 2003; Torsvik et al., New Caledonia Trough, where the water Greenland and Ellesmere Island 2013; Whittaker et al., 2016). Discriminating depth varies from 1500 to 3500 m (Fig. 2). (Pulvertaft and Dawes, 2011). Tectonic between what is a continent and what is a This raises the question as to whether the plate boundaries, with or without interven- microcontinent may be considered an arbi- trough is floored by oceanic crust or is a ing oceanic crust, provide the basis for trary exercise. Nonetheless, maps like failed continental rift. Two wide-angle continent-continent boundaries between Figure 1 need labels. Therefore, following seismic profiles across the trough near Africa and Eurasia, and North and South Cogley (1984) and the vagaries of general New Caledonia (Klingelhoefer et al., 2007) America (Fig. 1). Large area is an inherent conventional usage, we propose that the both show ~2–5 km of sedimentary cover part of the definition of a continent sensu name continent be applied to regions of over 8.5 km of crustal basement that has a stricto (Neuendorf et al., 2005). Cogley continental crust that are >1 Mkm2 in area velocity of ~7 km-1 throughout much of its (1984) defined Central America (1.3 Mkm2), and are bounded by well-defined geologic thickness. Klingelhoefer et al. (2007) noted Arabia (4.6 Mkm2), and greater India limits. By this definition India, prior to its these profiles as atypical of normal oce- (4.6 Mkm 2) as modern-day continents. collision with Eurasia, would be termed a anic crust. Sutherland et al. (2010) and This schema has not been generally continent. Hackney et al. (2012) interpreted the New adopted, probably because Central The edges of Australia and Zealandia Caledonia Trough as continental crust that America (the Chortis block) is a piece of continental crust approach to within 25 km was thinned in the Late Cretaceous and displaced North America, and Arabia and across the Cato Trough (Fig. 2). The Cato re-deepened in the Eocene due to litho- India are transferring to, and are now Trough is 3600 m deep and floored by oce- sphere delamination. contiguous with, Eurasia and have clearly anic crust (Gaina et al., 1998; Exon et al., defined COBs in the Red Sea and Indian 2006). The Australian and Zealandian Limits and Area Ocean (Fig. 1). The six commonly recog- COBs here coincide with, and have been Where oceanic crust abuts continental nized geological continents (Africa, created by, the Cato Fracture Zone along crust, various kinds of continent-ocean Eurasia, North America, South America, which there has been ~150 km of dextral boundaries (COBs) define natural edges to Antarctica, and Australia) are thus not only strike slip movement, linking Paleogene continents (Fig. 1; Eagles et al., 2015). large but they are also spatially isolated by spreading centers in the Tasman and Coral Despite its large area, Greenland is uncon- geologic and/or bathymetric features. seas (Fig. 2; Gaina et al., 1998). This spatial troversially and correctly regarded as part At the other end of the size spectrum, a and tectonic separation, along with inter- of North America (Figs. 1 and 4). This is number of continental crust fragments in vening oceanic crust, means that the because, despite oceanic crust intervening the world’s oceans are referred to as Zealandia continental crust is physically between southern Greenland and Labrador microcontinents. Examples include the separate from that of Australia. If the Cato and Baffin Island, North American Madagascar, East Tasman, Jan Mayen, Trough did not exist, then the content of this Zealandia would, much earlier, have been investigated and identified as one of Earth’s continents. Even relatively recently, some papers refer to the offshore ridges and plateaus of Zealandia as an amalgam of continental fragments and slivers (e.g., Gaina et al., 2003; Blewett, 2012; Higgins et al., 2015) with the explicit or implicit notion that oceanic crust intervenes between the continental fragments. The way in which Zealandia has been divided into blocks to make it amenable to rigid plate reconstructions and the way in which coastlines and outlines have been drafted as “floating” in the Pacific Ocean (e.g., Gaina et al., 1998, 2003; Lisker and Läufer, 2013; Higgins et al., 2015) has probably sustained this false impression of remote and discombobulated tectonic allochthony and poorly defined COBs. In contrast, we view Zealandia as a coherent, albeit thinned and stretched, continent with interconnected and throughgoing geologi- cal provinces (Figs. 2 and 5; Mortimer et al., 2006; Grobys et al., 2008; Tulloch et al., 2009; Adams and Griffin, 2012; Bache et al., 2014; Graham, 2015). Like parts of North America and Eurasia, Zealandia has undergone active deformation in a zone between two essentially rigid plates—in Zealandia’s case, the Pacific and Australian (Fig. 2). Several elevated bathymetric features north of Zealandia are possible candidates for Zealandia prolongations or separate microcontinents (Fig. 2). These include the Three Kings, Lau-Colville, and Tonga- Kermadec ridges and Fiji, which are Figure 5. Zealandia as part of the former Gondwana supercontinent. Upper panel shows Mesozoic known Cenozoic volcanic arcs (Graham, orogen convergent margin that was active until ca. 105 Ma. Lower panel shows pre-breakup intra 2015), and the Mellish Rise and Louisiade continental extension of Zealandia and West Antarctica from 105 to 85 Ma; seafloor spreading sub- sequently split Gondwana into its present-day constituent continents (Fig. 3). Orthographic projec- and West Torres plateaus. However, no tions with East Antarctica fixed. From Mortimer and Campbell (2014). continental basement rocks have yet been sampled from any of these features, so paper would be describing the scientific 4 makes a case for a natural twofold group- their continental nature remains unproven. advance that the Australian continent was ing of continents and microcontinents. 4.9 Mkm2 larger than previously thought. Development and Submergence Being >1 Mkm2 in area, and bounded by DISCUSSION AND IMPLICATIONS As shown in Figure 4, ~94% of the area well-defined geologic and geographic lim- of Zealandia currently is submerged. It is Recognition its, Zealandia is, by our definition, large not unique in this regard: an ice-free, iso- enough to be termed a continent. At 4.9 Satellite gravity-derived bathymetry statically corrected West Antarctica would Mkm2, Zealandia is substantially bigger maps (e.g., Fig. 2) have been of immense also largely be submerged (Figs. 3 and 4; than any features termed microcontinents use in visualizing Zealandia, clarifying Jamieson et al., 2014). Zealandia and West and continental fragments, ~12× the area its limits, focusing attention on intra- Antarctica were formerly adjacent to each of Mauritia and ~6× the area of Madagascar Zealandia structures, and planning other along the southeast Gondwana mar- (Fig. 4). It is also substantially larger than research voyages. If the elevation of gin and, prior to thinning and breakup, the the area of the largest intraoceanic large Earth’s solid surface had first been mapped orogenic belts, Cordilleran batholiths, and igneous province, the Ontong Java Plateau in the same way as those of Mars and normal continental crustal thickness of (1.9 Mkm 2). Zealandia is about the same Venus (which lack the arbitrary datums of eastern Australia would have projected area as greater India (Figs. 1 and 4). Figure opaque liquid oceans), we contend that along strike into these areas (Figs. 3 and 5). Several continental metamorphic core The importance of Zealandia is not so there is only incipient collision between complexes (Lister and Davis, 1989) of Late much that there is now a case for a for- northern and southern Zealandia across the Cretaceous age have been identified in merly little-known continent, but that, by present-day Pacific-Australian plate Zealandia and West Antarctica, but not in virtue of its being thinned and submerged, boundary. Ironically, for a continent so Australia or East Antarctica (Figs. 3 and 5; but not shredded into microcontinents, it is thoroughly shaped by extensional pro- Kula et al., 2007). These have been a new and useful continental end member. cesses and subsidence, it is the more explained by Lister et al. (1991) and Kula Zealandia started to separate from widely recognized and better-studied con- et al. (2007) in terms of an asymmetric Gondwana in the Late Cretaceous as an vergence across the Cenozoic Pacific- continent-scale detachment fault model in ~4000-km-long ribbon continent (Fig. 5) Australian plate boundary that has resulted which Zealandia and West Antarctica are but has since undergone substantial intra in any of Zealandia being above the sea. highly extended, lower-plate passive conti- continental deformation, to end up in its nental margins, and Australia and East present shape and position (Figs. 1–3). To CONCLUSIONS Antarctica are relatively unstretched upper date, Zealandia is little-mentioned and/or Zealandia illustrates that the large and plate margins. There is also abundant sup- entirely overlooked in comparative studies the obvious in natural science can be over- porting sedimentary basin evidence that of continental rifting and of COBs (e.g., looked. Based on various lines of geologi- Zealandia experienced widespread Late Buck, 1991; Menzies et al., 2002; Franke, cal and geophysical evidence, particularly Cretaceous (ca. 105–85 Ma) extension 2013). By including Zealandia in investiga- those accumulated in the last two decades, prior to Gondwana supercontinent breakup tions, we can discover more about the rhe- we argue that Zealandia is not a collection (e.g., Luyendyk, 1995; Klingelhoefer et al., ology, cohesion, and extensional deforma- of partly submerged continental fragments 2007; Bache et al., 2014; Mortimer et al., tion of continental crust and lithosphere. but is a coherent 4.9 Mkm2 continent 2014; Higgins et al., 2015). The situation of Gondwana breakup along the paleo- (Fig. 1). Currently used conventions and Zealandia’s Phanerozoic orogen overlying Pacific margin resulted in continents with definitions of continental crust, continents, Precambrian mantle (Liu et al., 2015) pos- wide, thinned shelves, such as Zealandia and microcontinents require no modifica- sibly suggests major tectonic detachments and West Antarctica (Figs. 1 and 3). In tion to accommodate Zealandia. along the Moho. contrast, breakup of Gondwana’s core Satellite gravity data sets, New Zealand’s Thermal relaxation and isostatic balance resulted in continents with narrow shelves, UNCLOS program, and marine geological of the thinned continental crust of Zealandia such as Africa and its neighbors (Fig. 1). expeditions have been major influences in and West Antarctica ultimately led to their Various lithospheric versus mantle controls promoting the big picture view necessary submergence. Despite the pervasive thin- on styles of continental rifting and breakup to define and recognize Zealandia (Fig. 2). ning, the only part of Zealandia that might are still debated (Ebinger and van Wijk, Zealandia is approximately the area of qualify as a hyper-extended zone (i.e., 2014; Whittaker et al., 2016). The broad greater India and, like India, Australia, stretched by a factor of 3–4 with crustal spatial association of stretched continental Antarctica, Africa, and South America, thinning to 8 km or less; Doré and Lundin, crust with a pre-softened, Mesozoic, paleo- was a former part of the Gondwana super- 2015) is the New Caledonia Trough. Pacific convergent margin from the continent (Figs. 3 and 5). As well as being Zealandia and West Antarctica seemingly Falkland Plateau, through West Antarctica the seventh largest geological continent record a mode of continental crust defor- and Zealandia to the Marion Plateau (Fig. 1), Zealandia is the youngest, thinnest, mation in which extension, although sub- (Fig. 3), is possibly no coincidence (cf. Rey and most submerged (Fig. 4). The scientific stantial, is more distributed and less focused and Müller, 2010). Other proposed controls value of classifying Zealandia as a conti- than in most examples of continental on the localization of Zealandia-Gondwana nent is much more than just an extra name breakup. Zealandia has a widespread syn- breakup include a mantle plume (Weaver on a list. That a continent can be so sub- rift Late Cretaceous volcanic record (Tulloch et al., 1994), plate capture (Luyendyk, merged yet unfragmented makes it a useful et al., 2009; Mortimer et al., 2014); thus, 1995), and/or impingement of an oceanic and thought-provoking geodynamic end processes that operate at volcanic rifted spreading ridge (Mortimer et al., 2006). member in exploring the cohesion and margins (Menzies et al., 2002) may be Gaina et al. (2003) proposed that micro- breakup of continental crust. applicable to the broad area of Zealandia. continents are created by plume-controlled ridge jumps during the early stages of ACKNOWLEDGMENTS Significance supercontinent breakup. The general cohe- We thank Belinda Smith Lyttle for GIS work Zealandia once made up ~5% of the area sion of continental crust in extension is and Patti Durance, Ron Hackney, and Brendan of Gondwana. It contains the principal attested to by the contrast in size between Murphy for comments. Formal reviews by Peter Cawood, Jerry Dickens, and an anonymous ref- geological record of the Mesozoic conver- Zealandia and its neighboring continental eree greatly improved the focus and content. This gent margin of southeast Gondwana fragments of East Tasman, Gilbert, and paper is based on work supported by New Zealand (Mortimer et al., 2014) and, until the Late Bollons seamounts (Figs. 2 and 4). Condie Government core funding grants to GNS Science. Cretaceous, lay Pacificward of half of (2015) postulated that ancient and modern West Antarctica and all of eastern continent-continent collisions were a lead- REFERENCES CITED Australia (Figs. 3 and 5). Thus, depictions ing cause of continental elevation. 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