Reconnaissance Basement Geology and Tectonics of South Zealandia

We report new U‐Pb zircon ages, geochemical and isotopic data for Mesozoic igneous rocks, and new seismic interpretations of mostly submerged South Zealandia (1.5 Mkm2). We use these data, along with existing geological and geophysical data sets, to refine the extent and nature of geological units. Our new 1:25 M geological map of South Zealandia provides a regional framework to investigate the rifting and breakup that formed Zealandia, Earth's most submerged continent. Samples of prerift (pre‐100 Ma) plutonic rocks can be matched with on‐land New Zealand igneous suites and indicate an east‐west strike for the subduction‐related 260 to 105‐Ma Median Batholith across the Campbell Plateau. The plutonic chronology of formerly contiguous plutonic rocks in West Antarctica reveals similar pulses and lulls to the Median Batholith. Contrary to previous interpretations, the Median Batholith does not coincide with the 1,600‐km‐long Campbell Magnetic Anomaly System. Instead we interpret the continental magnetic anomalies to represent a mainly mafic igneous unit, whose shape and extent is controlled by synrift structures related to Gondwana breakup. Correlatives of some of these unsampled igneous rocks may be exposed as circa 85 Ma alkalic volcanic rocks on the Chatham Islands. Extension directions varied by up to 65° from 100 to 80 Ma, and we suggest this allowed this large area to thin considerably before final rupture to form new oceanic crust. Synrift (90–80 Ma) structures cut the oroclinal bend in southern South Island and support a pre‐early Late Cretaceous age of orocline formation. Key Points The first geological map of 1.5‐Mkm2 submerged South Zealandia reveals an E‐W striking batholith with similar age pattern to Marie Byrd Land The 1,600‐km‐long Campbell Magnetic Anomaly System is interpreted as due to failed rift mafic magmatism that preceded 80 Ma Zealandia breakup Variation of extension direction over 20 Ma allowed this large area to thin considerably before breakup and formation of new ocean crust