Reconnaissance Basement Geology and Tectonics of North Zealandia

Abstract

New rock dredge samples supply key information to establish the tectonic and geological framework of the northern two‐thirds of the 95% submerged Zealandia continent. The R/V Investigator voyage IN2016T01 to the Fairway Ridge, Coral Sea, obtained poorly sorted poly‐lithologic pebbly to cobbly sandstones, well sorted fine grained sandstones, mudstones, bioclastic limestones, and basaltic lavas. Post‐cruise analytical work comprised petrography, whole rock geochemical and Sr and Nd isotopic analyses, and U‐Pb zircon, Rb‐Sr, and Ar‐Ar geochronology. A Fairway Ridge cobbly sandstone has a ∼95 Ma (early Late Cretaceous) depositional age; two biotite granite cobbles are 111 ± 1 and 128 ± 1 Ma in age, and some volcanic pebbles are also likely Early Cretaceous. Fairway Ridge basalts have intraplate alkaline chemistry and are of Late Eocene age (∼40–36 Ma). By analogy with South Zealandia, we interpret strong positive continental magnetic anomalies of North Zealandia to mainly result from Late Cretaceous to Cenozoic intraplate basalts, many of them rift‐related lavas. A new basement geological map of North Zealandia shows the position of the Mesozoic Gondwana magmatic arc axis (Median Batholith) and other major geological units. This study completes onland and offshore reconnaissance geological mapping of the entire 5 Mkm2 Zealandia continent.

The Thermal Structure of Central Te Riu a Māui/Zealandia Continent as Determined From the Depth to Base of Magnetic Sources

“…(2020b), and Mortimer, Williams, et al. (2023), many of the positive magnetic anomalies in the continental offshore region are likely to mark the location of <100 Ma gabbroic and basaltic intraplate igneous rocks. Anomalies with rectilinear shapes are plausibly related to Late Cretaceous Gondwana‐Zealandia rifting.…”

“…Magnetically low and quiet regions over the Chatham Rise and Challenger and Campbell Plateaus are the along-strike offshore continuations of onland greywacke and schist terranes. Following Tulloch et al (2019), Bischoff et al (2020), Mortimer et al (2020b), andMortimer, Williams, et al (2023), many of the positive magnetic anomalies in the continental offshore region are likely to mark the location of <100 Ma gabbroic and basaltic intraplate igneous rocks. Anomalies with rectilinear shapes are plausibly related to Late Cretaceous Gondwana-Zealandia rifting.…”

Why the pounamu? Low- to medium grade metabasites and metaultrabasites of New Zealand from a geoheritage perspective

“…The formation of various greenstones including the highly valued pounamu is associated with the unique geotectonical evolution of New Zealand within the Zealandia geotectonic context [41][42][43]. New Zealand's high-order stratigraphy framework shows well the geotectonic context of the Late Paleozoic-Mesozoic tectonic evolution of the basement rocks forming distinct lithospheric terrains that amalgamated over 100 million of year time scale [44][45][46]. In this process, plate margin processes and closure of ocean basins lead to the formation of narrow bands of wrapped oceanic crustal components into mafic to ultramafic lithological zones following more or less the plate boundary along convergence then collision took place [33,36,43].…”