I wonder if there is zircon in asteroids. Specifically the large volcano being coincidental on the same spot. That makes me wonder if the presence of basaltic zircon could also be explained by the impact of a large body that punched its way down through the crust to the mantle. Essentially creating a tube which allowed magma to flow to the surface creating the volcano. The resulting dislocation of material causing it to subside below sea level.
That could account for the additional gravitational mass (especially if it was a very dense meteorite) but that then starts me thinking about ways to falsify that hypothesis. What would have to be true if the origin was an impactor vs what could rule out an impactor. The dating sort of lines up with the Permian-Triassic extinction (231MMyr)
Isotopic analysis and dating from samples taken from cores from within and without the area in question could establish extra-terrestrial origin, especially in conjunction with correlation of possible ejecta worldwide. Geological evidence of stressed material with consistent dates? Maybe ocean floor topography? I think those are what they used to pinpoint the more recent K-T event impactor. I don't know if you could use that to confirm or deny your specific scenario though.
The Times has gone steeply downhill the last few years. It's hard to balance cost, profits, and quality - the coverage of local issues in particular is weirdly off. Sometimes it can feel like someone in Oregon describing my neighborhood: it's not wrong or mean, it's just not "local"
Zealandia [1] is the largest continental fragment [2]. It broke off from Australia and then largely (or wholly) sank. Later uplifts from the subcontinent include the north and south islands of New Zealand and New Caledonia. 93% of the microcontinent remains submerged.
That could account for the additional gravitational mass (especially if it was a very dense meteorite) but that then starts me thinking about ways to falsify that hypothesis. What would have to be true if the origin was an impactor vs what could rule out an impactor. The dating sort of lines up with the Permian-Triassic extinction (231MMyr)