The Plumbing Beneath The Deccan Volcanic Province

All that lava had to have come from somewhere.

A faculty friend invited me for field work leading up to an international conference on dike swarms of the Deccan Volcanic Province. There are thousands of dikes intruding the Deccan Basalts and many of them feeder to the lavas but they can be grouped into three broad clusters.

A roughly east-west trending cluster parallel to the Narmada-Tapi-Satpura rift which is a very ancient rift zone active intermittently since the Proterozoic. A roughly N-S trending cluster parallel to the West Coast rift which likely originated through the rifting of India with Madagascar around 90 mya., and a not so well oriented cluster in the region northeast of Mumbai.

The map below shows these dike clusters. NTDS is the Narmada Tapi Dike Swarm...WCDS is the West Coast Dike Swarm......WGE is the Western Ghat Escarpment.

 Source: Building a Continental Flood Basalt Province

Chemical affinities of these various dike swarms with the Deccan lava formations suggest that the east-west trending dike complexes likely were feeders to the older and middle part of the lava pile, while the ones along the west coast and northeast of Mumbai likely fed the younger lava formations. Lots of dikes though show no affinities and may represent feeders to now eroded lava flows or may not be feeders but intrusives that terminated within the lava pile.

Matching dike swarms to lava formations is important since it can tell us about which regional stress fields were active and influential in controlling volcanism during a particular eruptive phase and secondly how far did the lavas flow from the source vents. These kind of studies suggest that the feeders dike systems were not always situated close to the formations they have an affinity with, indicating that lavas flowed for tens to a hundred odd km away from the source vents.

I missed the first part of the field work which took place around the cluster in the northern part i.e. north of Dhule in the E-W dike zone. But you can get an idea of the immensity of these dike swarms in the image below which is around Nandurbar north of Dhule.

...I  jumped on the bus for the second part which was along the west coast south of Mumbai in the area between Alibag and Murud.  Here the dikes fell broadly into two compositional categories. An older tholeiitic composition stage and likely feeders to the younger Deccan formations and a later alkaline stage younger than the youngest of the flood basalts represented by lamprophyres, nephelinites and phonolites. These represent post Deccan intrusives.

Overall these dikes were smaller in extent that the east-west trending cluster but more varied in composition. The locations were spectacular, exposed along wave cut benches, accessible only at low tides. Here are some examples:

Exposed along the beach at Korlai village south of Alibag

Same beach..notice the reddened margins which define the contact of the dike with the host rock

..like a serpent..

.. exposed at Murud village further south...

..you can see this Lamprophyre dike continuing into the overlying flow..

 
The trip was great fun. I have not been involved really with the Deccan Basalts since my M.S. days, so I got to learn quite a bit about the advances in the field and features which I had always seen on the wayside but not given much thought to made more sense to me. 

The most important part of the trip was off course the group lunches.. Yummy coastal food under a coconut plantation.

I will be posting more pictures and stuff about this trip in the weeks to come.