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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
plate margins. There is also abundant sup-      entirely overlooked in comparative studies       Zealandia illustrates that the large and
porting sedimentary basin evidence that         of continental rifting and of COBs (e.g.,      the obvious in natural science can be over-
Zealandia experienced widespread Late           Buck, 1991; Menzies et al., 2002; Franke,      looked. Based on various lines of geologi-
Cretaceous (ca. 105–85 Ma) extension            2013). By including Zealandia in investiga-    cal and geophysical evidence, particularly
prior to Gondwana supercontinent breakup        tions, we can discover more about the rhe-     those accumulated in the last two decades,
(e.g., Luyendyk, 1995; Klingelhoefer et al.,    ology, cohesion, and extensional deforma-      we argue that Zealandia is not a collection
2007; Bache et al., 2014; Mortimer et al.,      tion of continental crust and lithosphere.     of partly submerged continental fragments
2014; Higgins et al., 2015). The situation of                                                  but is a coherent 4.9 Mkm2 continent
Zealandia’s Phanerozoic orogen overlying          Gondwana breakup along the paleo-            (Fig. 1). Currently used conventions and
Precambrian mantle (Liu et al., 2015) pos-      Pacific margin resulted in continents with     definitions of continental crust, continents,
sibly suggests major tectonic detachments       wide, thinned shelves, such as Zealandia       and microcontinents require no modifica-
along the Moho.                                 and West Antarctica (Figs. 1 and 3). In        tion to accommodate Zealandia.
                                                contrast, breakup of Gondwana’s core
  Thermal relaxation and isostatic balance      resulted in continents with narrow shelves,      Satellite gravity data sets, New Zealand’s
of the thinned continental crust of Zealandia   such as Africa and its neighbors (Fig. 1).     UNCLOS program, and marine geological
and West Antarctica ultimately led to their     Various lithospheric versus mantle controls    expeditions have been major influences in
submergence. Despite the pervasive thin-        on styles of continental rifting and breakup   promoting the big picture view necessary
ning, the only part of Zealandia that might     are still debated (Ebinger and van Wijk,       to define and recognize Zealandia (Fig. 2).
qualify as a hyper-extended zone (i.e.,         2014; Whittaker et al., 2016). The broad       Zealandia is approximately the area of
stretched by a factor of 3–4 with crustal       spatial association of stretched continental   greater India and, like India, Australia,
thinning to 8 km or less; Doré and Lundin,      crust with a pre-softened, Mesozoic, paleo-    Antarctica, Africa, and South America,
2015) is the New Caledonia Trough.              Pacific convergent margin from the             was a former part of the Gondwana super-
Zealandia and West Antarctica seemingly         Falkland Plateau, through West Antarctica      continent (Figs. 3 and 5). As well as being
record a mode of continental crust defor-       and Zealandia to the Marion Plateau            the seventh largest geological continent
mation in which extension, although sub-        (Fig. 3), is possibly no coincidence (cf. Rey  (Fig. 1), Zealandia is the youngest, thinnest,
stantial, is more distributed and less focused  and Müller, 2010). Other proposed controls     and most submerged (Fig. 4). The scientific
than in most examples of continental            on the localization of Zealandia-Gondwana      value of classifying Zealandia as a conti-
breakup. Zealandia has a widespread syn-        breakup include a mantle plume (Weaver         nent is much more than just an extra name
rift Late Cretaceous volcanic record (Tulloch   et al., 1994), plate capture (Luyendyk,        on a list. That a continent can be so sub-
et al., 2009; Mortimer et al., 2014); thus,     1995), and/or impingement of an oceanic        merged yet unfragmented makes it a useful
processes that operate at volcanic rifted       spreading ridge (Mortimer et al., 2006).       and thought-provoking geodynamic end
margins (Menzies et al., 2002) may be                                                          member in exploring the cohesion and
applicable to the broad area of Zealandia.        Gaina et al. (2003) proposed that micro-     breakup of continental crust.
                                                continents are created by plume-controlled
Significance                                    ridge jumps during the early stages of         ACKNOWLEDGMENTS
                                                supercontinent breakup. The general cohe-
  Zealandia once made up ~5% of the area        sion of continental crust in extension is         We thank Belinda Smith Lyttle for GIS work
of Gondwana. It contains the principal          attested to by the contrast in size between    and Patti Durance, Ron Hackney, and Brendan
geological record of the Mesozoic conver-       Zealandia and its neighboring continental      Murphy for comments. Formal reviews by Peter
gent margin of southeast Gondwana               fragments of East Tasman, Gilbert, and         Cawood, Jerry Dickens, and an anonymous ref-
(Mortimer et al., 2014) and, until the Late     Bollons seamounts (Figs. 2 and 4). Condie      eree greatly improved the focus and content. This
Cretaceous, lay Pacificward of half of          (2015) postulated that ancient and modern      paper is based on work supported by New Zealand
West Antarctica and all of eastern              continent-continent collisions were a lead-    Government core funding grants to GNS Science.
Australia (Figs. 3 and 5). Thus, depictions     ing cause of continental elevation. The
of the Paleozoic-Mesozoic geology of            geological history of Zealandia would sup-     REFERENCES CITED
Gondwana, eastern Australia, and West           port this hypothesis: The Paleozoic and
Antarctica are both incomplete and mis-         Mesozoic orogens of Zealandia are non-         Adams, C.J., and Griffin, W.L., 2012, Rodinian
leading if they omit Zealandia.                 collisional (Mortimer et al., 2014), and          detrital zircons in Late Cretaceous sandstones
                                                                                                  indicate a possible Precambrian basement under
                                                                                                  southern Zealandia: Precambrian Research,
                                                                                                  v. 212–213, p. 13–20, doi: 10.1016/j.precamres
                                                                                                  .2012.04.003.

                                                www.geosociety.org/gsatoday                                                                       33
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