Regional volcanism of northern Zealandia: post-Gondwana break-up magmatism on an extended, submerged continent

Abstract

Volcanism of Late Cretaceous–Miocene age is more widespread across the Zealandia continent than previously recognized. New age and geochemical information from widely spaced northern Zealandia seafloor samples can be related to three volcanotectonic regimes: (1) age-progressive, hotspot-style, low-K, alkali-basalt-dominated volcanism in the Lord Howe Seamount Chain. The northern end of the chain (c. 28 Ma) is spatially and temporally linked to the 40–28 Ma South Rennell Trough spreading centre. (2) Subalkaline, intermediate to silicic, medium-K to shoshonitic lavas of >78–42 Ma age within and near to the New Caledonia Basin. These lavas indicate that the basin and the adjacent Fairway Ridge are underlain by continental rather than oceanic crust, and are a record of Late Cretaceous–Eocene intracontinental rifting or, in some cases, speculatively subduction. (3) Spatially scattered, non-hotspot, alkali basalts of 30–18 Ma age from Loyalty Ridge, Lord Howe Rise, Aotea Basin and Reinga Basin. These lavas are part of a more extensive suite of Zealandia-wide, 97–0 Ma intraplate volcanics. Ages of northern Zealandia alkali basalts confirm that a late Cenozoic pulse of intraplate volcanism erupted across both northern and southern Zealandia. Collectively, the three groups of volcanic rocks emphasize the important role of magmatism in the geology of northern Zealandia, both during and after Gondwana break-up. There is no compelling evidence in our dataset for Late Cretaceous–Paleocene subduction beneath northern Zealandia.

Crustal Structure Across the Lord Howe Rise, Northern Zealandia, and Rifting of the Eastern Gondwana Margin

“…A problem with invoking plume-driven processes as a driver for eastern Gondwana fragmentation is the lack of evidence for high-velocity underplating or igneous intrusion in the lower crust of our crustal seismic velocity model (Figure 4), which suggests that northwestern Zealandia is a magma-poor margin. This is consistent with the volcanotectonic regime of this region discussed by Mortimer et al (2018), who show that other than the time-progressive Oligocene-Recent Lord Howe and Tasmantid seamount chains, low volume and widely scattered Late Cretaceous to Holocene volcanism in this region is not related to plumes but could instead be derived from diffuse asthenospheric or lithospheric sources.…”

“…Other wide margins such as the South China Sea exhibit less uniform thinning at similar distances from the continent‐ocean transitions (Pichot et al, ). Previous work suggests that higher than normal heat flow is needed to create a wide margin (Buck et al, ), which in Zealandia could be related to the formation of Whitsunday Volcanic Province that was active during the Early Cretaceous prior to the onset of extension across northern Zealandia (Bryan et al, ), or to the presence of a diffuse alkaline magmatic province in the southwest Pacific that is unrelated to a plume or specifically to rifting (Finn et al, ; Mortimer et al, ).…”

Reconnaissance Basement Geology and Tectonics of South Zealandia

“…Magnetic susceptibility data from ~9,000 New Zealand igneous rocks (Table S7 and Figure S6) shows that most of these basaltic‐gabbroic rocks have susceptibilities 1–2 orders of magnitude greater than that of most rhyolitic‐granitic igneous rocks, supporting our suggestion that the magnetic source is more likely to be mafic, rather than felsic. Within Zealandia, there are several scattered occurrences of Late Cretaceous igneous rocks (Tulloch, Ramezani, Mortimer, et al, ; Timm et al, , Mortimer, Gans, et al, , and references therein). The only onshore rocks of South Zealandia that might be related to the CMAS are circa 85 Ma alkaline basalt‐trachybasaltic Southern Volcanics of Pitt Island, The 97 Ma weakly A‐type granites of the Auckland Islands and Takahe dredge site probably are also representative of this widespread magmatism within South Zealandia.…”

Subduction Duration and Slab Dip

“…The much more refined geochronological constraints allow us to assign an age of 50 Ma to Tonga, substantially older than the 24 Ma age of Jarrard (). Although Tonga‐Kermadec is thought to have arisen at the position of an older arc (Sutherland et al, ), there is no compelling evidence for Late Cretaceous to Paleocene subduction regionally (Mortimer et al, ); thus, there is an approximately 60 million interval between the old subducting boundary and the new one.…”