Permian crater in Antarctica...maybe

I was way too busy last week finishing off marking and vivas to comment on this story about the possible discovery of a 300-mile wide impact crater which seems to be about the right sort of age to have possibly contributed to the Permian extinction, an event 250 million or so years ago which wiped out about 90 % of all species on Earth (see here and here for some discussion and links).

The method is indirect, using the GRACE gravity satellites (which I've mentioned before) to find a positive mass anomaly beneath Antarctica which seems to indicate a massive upwelling of material from deep in the Earth. This has been linked to a ring like structure beneath the ice (pictures here).

Dean Armstrong points to a Nature article which emphasises that there is as yet little direct evidence of this being an impact structure. In particular there is little evidence in Antarctic Permian rocks (at least, those which aren't under lots of ice) for any sort of impact - no ash flows, impact breccias, or megatsunami deposits which can be found around the KT impact site. Which leads to another thought: the researchers tried to make a link between their putatative crater and the mantle upwelling, and the fact that this is close to the rifting event that began the break-up of Gondwana. If there is no crater, this may not be support for the (controversial) idea that large impacts can cause rifting, but instead could be telling us something about the internal processes that triggered that rifting.