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Figure 5. Zealandia as part of the former Gondwana supercontinent. Upper panel shows Mesozoic Zealandia would, much earlier, have been
orogen convergent margin that was active until ca. 105 Ma. Lower panel shows pre-breakup intra­ investigated and identified as one of
continental extension of Zealandia and West Antarctica from 105 to 85 Ma; seafloor spreading sub- Earth’s continents. Even relatively recently,
sequently split Gondwana into its present-day constituent continents (Fig. 3). Orthographic projec- some papers refer to the offshore ridges
tions with East Antarctica fixed. From Mortimer and Campbell (2014). and plateaus of Zealandia as an amalgam
of continental fragments and slivers (e.g.,
paper would be describing the scientific 4 makes a case for a natural twofold group- Gaina et al., 2003; Blewett, 2012; Higgins
advance that the Australian continent was ing of continents and microcontinents. et al., 2015) with the explicit or implicit
4.9 Mkm2 larger than previously thought. notion that oceanic crust intervenes
DISCUSSION AND IMPLICATIONS between the continental fragments. The
Being >1 Mkm2 in area, and bounded by way in which Zealandia has been divided
well-defined geologic and geographic lim- Recognition into blocks to make it amenable to rigid
its, Zealandia is, by our definition, large plate reconstructions and the way in which
enough to be termed a continent. At 4.9 Satellite gravity-derived bathymetry coastlines and outlines have been drafted
Mkm2, Zealandia is substantially bigger maps (e.g., Fig. 2) have been of immense as “floating” in the Pacific Ocean (e.g.,
than any features termed microcontinents use in visualizing Zealandia, clarifying Gaina et al., 1998, 2003; Lisker and Läufer,
and continental fragments, ~12× the area its limits, focusing attention on intra- 2013; Higgins et al., 2015) has probably
of Mauritia and ~6× the area of Madagascar Zealandia structures, and planning sustained this false impression of remote
(Fig. 4). It is also substantially larger than research voyages. If the elevation of and discombobulated tectonic allochthony
the area of the largest intraoceanic large Earth’s solid surface had first been mapped and poorly defined COBs. In contrast, we
igneous province, the Ontong Java Plateau in the same way as those of Mars and view Zealandia as a coherent, albeit
(1.9 Mkm2). Zealandia is about the same Venus (which lack the arbitrary datums of thinned and stretched, continent with
area as greater India (Figs. 1 and 4). Figure opaque liquid oceans), we contend that 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
known Cenozoic volcanic arcs (Graham,
2015), and the Mellish Rise and Louisiade
and West Torres plateaus. However, no
continental basement rocks have yet been
sampled from any of these features, so
their continental nature remains unproven.

Development and Submergence

As shown in Figure 4, ~94% of the area
of Zealandia currently is submerged. It is
not unique in this regard: an ice-free, iso-
statically corrected West Antarctica would
also largely be submerged (Figs. 3 and 4;
Jamieson et al., 2014). Zealandia and West
Antarctica were formerly adjacent to each
other along the southeast Gondwana mar-
gin and, prior to thinning and breakup, the
orogenic belts, Cordilleran batholiths, and
normal continental crustal thickness of
eastern Australia would have projected
along strike into these areas (Figs. 3 and 5).

32 GSA Today | March–April 2017