It has to be said that it’s never been a particular ambition of mine to mix with the luminaries of the American Association of Petroleum Geologists. Still, lots of interesting research does get done in the name of finding and extracting oil: if you want to know where it is, and how much there is, and the best way to get it out of the ground, you need to understand the depositional systems that generate source and reservoir rocks, and how the tectonic development of particular regions has generated hydrocarbons and the structural nooks and crannies that allow them to accumulate. And sometimes, getting to grips with these questions forces you to look at some quite fundamental aspects of Earth Sciences.
This is why on my first morning at the Cape Town Conference I heard a serious of talks discussing how continental crust finally breaks apart to from a new ocean basin. The early stages of rifting, where there is just stretching of continental crust, can be described fairly well by a simple shear model, where the crust and lithosphere are equally stretched and thinned, with extension in the brittle upper crust being accommodated by high angle normal faulting. However, at very high levels of stretching, this model seems to break down. What the session I attended indicated is that a consistent picture is starting to emerge of the general features of the ttransition between thinned continental crust and new oceanic crust at rifted margins, both modern (e.g., the margins of the present day Atlantic) and ancient (a particularly cool talk described how you can find a complete section through both margins of the now-closed Tethyan Ocean exposed in the Alps). As stretching gets more extreme, you start to get extreme thinning of continental crust without any faulting to accommodate it, and exhumation of mantle rocks along large low angle detachment faults that penetrate far below the Moho. The break is often also asymmetric; a pair of margins which have been separated by sea-floor spreading often look very different from each other.
I found this interesting because when I was being lectured about this almost a decade ago, some of these features were only just starting to be recognised, and there was no coherent picture of what was going on. Now, it seems, there is a much better idea of what has happened, even if the why is still being debated; there were three distinct models presented to explain how the observed margin geometries were generated. Still, there is a real sense of progress in this area
The afternoon session I attended was a lot less integrated: there was a common theme, which seemed to be extreme global changes, but each talk was pretty much a stand-alone. Topics included a possible “Snowball Earth” event at around 2.2-2.3 billion years ago, and its possible linkage to the evolution of photosynthesis (as I understand it, because it took a while for aerobic respiration to evolve, all of the organic carbon being produced would get buried, leading to a major CO2 drawdown from the atmosphere); why the late Neoproterozoic (600-700 million years ago, and the time of the more well-known “Snowball Earth” glaciations) may have been a time where the whole Earth’s crust and mantle were shifting around relative to the rotation axis (this is the idea of ‘True Polar Wander’, which I really need to get around to blogging about at some point), and a detailed study of the Carboniferous-Permian glaciation of Gondwana, which made the very interesting point that the geologically quite rapid waxing of waning of the ice sheets we can see in the last couple of million years mean that you shouldn’t really assume that previous glaciations just involved one big ice sheet sticking around for 10 million years before melting. Oh, and Brian’s colleague seemed to upset everyone by suggesting the boundary between the Permian and Triassic in the Karoo basin didn’t in fact happen at the stratigraphic boundary that it was normally taken as, but wandered diachronously a long way below it (earlier) in the west, and above it (later) in the east. All in all, there was lots of interesting stuff to think about, but there was no overarching take-home message from the afternoon, which means I’m still mulling over a lot of it.
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