To sink a continent: Exploring the implications of Zealandia’s fate

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Campbell, Hamish۱; Begg, John۱; Mildenhall, Dallas۱; Landis, Charles۲; Paterson, Adrian۳; Trewick, Steve۴
Holding Date 11 October 2008

In crustal terms, New Zealand and New Caledonia are continental islands. They are the biggest remnants of a much larger tract of continental crust that is below sea level. They are emergent parts of a largely submerged eighth continent, Zealandia.
In terms of surface area, the 2,500metre isobath is a proxy for defining the limits of Zealandia. On this basis, the continental crust of Zealandia is almost half the size of Australia, or about the size of India. New Zealand and New Caledonia represent less than 7% of this area, thus almost 93% of Zealandia is under the sea.
What happened? The implication is that Zealandia was a continent that has subsequently sunk, and indeed the geological record strongly supports this idea. Why did it sink and when? And why are New Zealand and New Caledonia emergent? Why are they too not submerged?
These questions were brought into sharp focus during research exploring the antiquity of the land surface in the Chatham Islands (176° W, 44° S) located c.850 kilometres due east of Christchurch (South Island of New Zealand), on the Pacific Plate, well in-board of the active Australia-Pacific plate boundary that runs through mainland New Zealand.
This research shows that the Chatham Islands became emergent less than 3 Ma. The mechanism for uplift is as yet uncertain but active mantle inflation of regional extent is suspected rather than a localised volcanic effect or far-field plate boundary tectonic collision effect. For all that, the Chatham Islands have been the locus of dominantly terrestrial intra-plate basaltic volcanism during Late Cretaceous time, and much smaller scale largely submarine intra-plate basaltic volcanism during Paleocene to Eocene and Miocene to Pleistocene time.
More importantly, our investigations in the Chatham Islands have led to the realisation that they offer a unique subaerial glimpse of undeformed Zealandia. By contrast, New Zealand and New Caledonia are highly deformed.
Subsequent research in mainland New Zealand has shown that the geological evidence for continual presence of land since Zealandia rifted away from Gondwanaland c.85 Ma is inconclusive. Furthermore, a reasonably compelling geological argument can be made for maximum submergence c.23 Ma and acknowledges the possibility of total submergence. Though unproven one way or the other, we encourage the scientific community to explore logically the idea that the New Zealand region of Zealandia may have been totally submerged c.23 Ma. Seminal to this idea is the recognition and significance of a regional marine-cut geomorphic feature within the New Zealand landscape, the Waipounamu Erosion Surface.
This idea, the drowning of Zealandia, sets a new paradigm for understanding the antiquity and origins of native biotas of New Zealand and New Caledonia. The potential implications are profound and the sinking of a continent during Late Cretaceous to Miocene time represents a geological phenomenon of global significance.