Thursday, July 21, 2016

Conversations: Tempo Of Deccan Traps Eruptions

My friend V.V Robin who is an evolutionary biologist and researches the genetics of bird species along the Western Ghats also takes a keen interest in geology. He pointed me to this new work on the tempo of Deccan Volcanism;

Tempo of the Deccan Traps eruptions in relation to events at the Cretaceous-Paleogene boundary - Renne, Paul et al 2016; Presented at the meeting of the EGU General Assembly 2016, held 17-22 April, 2016 in Vienna Austria

Robin wrote- I found this quite cool - that all the volcanism was possibly within one million time frame! Must have also had a huge impact on the organisms living then.

But it was not clear from this where the epicentre was - "Poladpur or uppermost Bushe Fm., near the base of the laterally extensive Wai Subgroup"

Right and this is very important... this refers to when the style of magmatism changed in relation to the K-Pg boundary event. Within the enormous pile of lava, in the upper parts of the section subdivided based of geochemical signatures into the Bushe (older), Poladpur and Ambenali (youngest) formations lies the K-Pg boundary. See figure to the left (Source: Earth Magazine; credit K. Cantner AGI) which shows the chemical stratigraphy of the Deccan Basalts as defined primarily by sampling the Western Ghat sections. The change in magmatism style is somewhere near the top of the Bushe Formation. Lava of the entire Wai Subgroup may have erupted in just a few hundred thousand years.  It is key to know at what stratigraphic level the K-Pg boundary is. If the boundary lies say near the base of Bushe, then this would strengthen the hypothesis that the Chicxulub impact triggered a more effusive phase of Deccan volcanism. In this scenario, since it would have taken hundreds to thousands of years before volcanic rates increased, the more effusive phase of volcanism may not have caused the mass extinction, but played a role in stifling recovery of ecosystems for hundreds of thousands of years after. On the other hand if the boundary lies higher up, say somewhere in upper part of Poladpur or Ambenali, then the implication would be that the more effusive phase of volcansim started before the impact and that would mean a more direct causal link between volcanism and mass extinction.

Robin though had read the abstract a little differently and he answered back -

So, they dont really say much about WHERE the epicentre was, as suggested in the initial part of the abstract. Maybe that's ongoing research. Very exciting stuff! This would certainly be interesting to folks studying older taxa - centipedes, frogs etc

He wanted to know where the volcanic epicentre was geographically. He was referring to these sentences from the abstract -

 40Ar/39Ar geochronologic study is providing the first indications of variable time-averaged eruption rates in the important Western Ghats region, in addition to providing the first precise location of the KPB within the Deccan pile

" radioisotope geochronology has failed to clarify the tempo of the eruptions or to delineate where the KPB age-equivalent horizon occurs within the eruptive sequence"

 Well, in the language of stratigraphy the authors are not referring to a geographic place but where in the stratigraphic sequence.  They are referring to its temporal position .

Geographically, the K-PG age equivalent horizon, meaning a lava flow which erupted 66.04 million years ago (coincident with the Chixculub impact) could be preserved anywhere. Near Lonvala, near Mahabaleshwar, in southern Konkan or Goa or all of these places. We haven't found it yet, but it could be a matter of time, with better radiometric resolution now allowing finer and finer sampling.

Off course it is a perfectly reasonable question to ask if there was an geographic epicenter of Deccan volcanism. Here is a cross section of the Western Ghat with formation boundaries and structural features.

Source: M Widdowson  and K.G Cox 1996

Note that there is a large vertical exaggeration (40X) to the profile which is about 650 km in length. The long wavelength fold and dips are not apparent at outcrop scale and have been revealed through accurate measurements of topographic levels of formation boundaries. What you do see is that the older formations (Kalsubai Subgroup)  are to the north and the younger to the south (Wai Subgroup). Is this due to a southerly migration of volcanism or is this due to deeper erosion of the lava pile to the north which has stripped away younger lavas and exposed the older section of the lava pile?
Likely there was not one epicentre of volcanism in the Deccan volcanic province. Rather, there would have been many many eruptive centers spread out along the N-S oriented western rifted margin and the E-W oriented Narmada Tapi rift zone and more in the central parts of the plateau too. Within the Western Ghat section there have been some suggestions that the lava thickness and laterite geochemistry might be consistent with the idea of a south moving focus of volcanism during the eruption of the Wai Subgroup. But on a larger scale, taking into account the entirety of the province it is hard to make a case of any one epicentre.

Take a look at this map with plotted ages of Deccan basalts. Younger and older basalt ages are scattered all over.

Source: Hethu Sheth - The Deccan Beyond The Plume Hypothesis

The Western Ghats have been the most intensely studied and sampled region of the province and hence its age distribution is now better constrained. But the volcanic province is spread over large areas in Madhya Pradesh, Gujarat, eastern Maharashtra and importantly a large area under the Arabian sea covered by hundreds of feet of Cenozoic sediment.

Here is a map which shows the distribution of selected Deccan lava sections beyond the Western Ghats. Many sections to the north, northeast and the east contain the Bushe, Poladpur and Ambenali Formation lavas which are thought to span the K-Pg boundary. Geochemical work has identified dykes feeding these lavas, and they are distributed over wide regions. A few have been identified in the Narmada rift region which may have erupted upper formation lavas present in the north and northeast of the province. There is also a cluster between Nasik and Pune and between the coast to Sangamner on the plateau to the east of the Western Ghats also. Some of these feeder dykes likely gave rise to the bulk of the Western Ghat upper formations. So, eruptive centers of lavas spanning the mass extinction was occurring in regions well away from the Western Ghat region too, suggesting there wasn't any one epicenter of volcanism during this time.

 Source: The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry be continued as more questions come in!

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