Tectono-stratigraphic history of the Chatham Islands, SW Pacific—The emergence, flooding and reappearance of eastern ‘Zealandia’

The Chatham Islands, SW Pacific, provide a significant window into the Paleozoic–Cenozoic geology and palaeontology of this remote part of eastern Zealandia, and today, they are the only emergent tracts of an otherwise submerged Chatham Rise east of New Zealand. As part of early 19th Century intensive surveys by the New Zealand Company, German scientist Johann Karl Ernst Dieffenbach (1811–1855) spent the winter months of May–July 1840 studying the remote eastern outpost of the Chatham Islands archipelago at 43.7° S and 176.7° W, recording the first geological observations in this part of the Southwest Pacific as part of the land surveys to determine the suitability of natural resources and settlement. Dieffenbachʹs descriptions of the Chathams sparked some interest in England, but it would take some 130 years before the first geologic map of the Chathams was published in 1970 as a result of research by the Institute of Geological and Nuclear Sciences (New Zealand). The Chathams, today the only emergent parts of the Chatham Rise extending eastward from the New Zealand mainland for 1000 km, host the only outcrops of the Riseʹs geology, with deep roots in Gondwana. The geology comprises basement rocks of Permo-Triassic metasediments and schists which correlate to the Torlesse Supergroup of New Zealand, having been connected to Marie Byrd Land, West Antarctica. Through phases of complex uplift and accretion along the Gondwanan landmass, a convergent margin ensued with associated back-arc volcanism within Antarctica for at least the Permian to mid-Jurassic time, which subsequently evolved into a passive margin, giving the Chatham Rise its distinctive finger-like shape. Divergent plate motions, resulting from the final fragmentation of Gondwana, pulled the Zealandia block away from the West Antarctic margin ca. 85–80 Ma concurrently with rifting phases that created large horsts and grabens forming a basin and range style landscape. Tectonism also formed intraplate volcanic provinces, including the establishment of a massive alkaline basalt stratovolcano in the Late Cretaceous with associated alkali olivine basalts and tuffs. From ca. 83 Ma the Chathams were positioned at 70° S to about 54° S at the K–Pg boundary, at a rate of approximately 0.9°/m.y. Crustal thinning and subsidence resulted in the deposition of a thick sequence of associated terrestrial to paralic rift-fill sediments in the mid-Cretaceous, overlain by Upper Cretaceous to Paleogene shallow marine volcaniclastics, greensands and minor interstitial limestones. However, the presence of terrestrial fauna and flora of Paleocene age provides evidence of at least some emergent tracts of land (Chatham Peninsula) along the Rise until at least the mid- to late Paleocene. The lower Cenozoic successions are characterised by thin and discontinuous marine sandstones, tuff and limestones, reflecting regional subsidence of the Chatham Rise, concomitant with pulsed volcanic activity, which persisted intermittently throughout the late Cenozoic. Limestone deposition and volcanic sediments waned before the end of the Pliocene, and only thin accumulations of littoral sands, peats, dune sand, and marine terrace gravels, were lain down after the islands’ uplift in the latest Pliocene. Current knowledge infers that the Chatham Islands have been emergent only for the last three to four million years. Varying terrigenous and carbonate sedimentation, reflecting intense palaeoclimatic cycles from global cooling during the Pliocene to Holocene, continue to the present day.