Building A Continental Flood Basalt Province: Key Significance Of The

Building a continental flood basalt province: Key significance of the Deccan Trap dyke swarms HETU SHETH Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai, Mumbai 400 076 (email: [email protected])

Continental flood basalts (CFBs) represent fissure eruptions on a grand scale. The plumbing systems of these vast lava fields can be both extensive and complex, and dyke and sill networks are an important part of these. CFBs that are eroded exhibit dense swarms of mafic dykes that arguably represent the magma-filled fissures through which the lavas poured out. Mafic dyke swarms are of great current international interest, as they provide a lot of valuable information on the mantle regions of the basalts, the interaction of the magmas with the wall rocks en route to the surface, and the prevalent stress conditions in the lithosphere, and can also often be linked to large-scale geodynamic processes like continental rifting and break-up. The ~65-million-year-old Deccan CFB of India (with a present-day areal extent of 500,000 km2; Fig. 1) has been extensively studied over the last two decades in terms of geochemistry, palaeomagnetism, and stratigraphy. The basalt pile is best developed in the Western Ghats (Sahyadri) region in the southwestern part of the province, where it reaches a stratigraphic thickness of 3 km. A question that has often been asked is where the eruptive centres of these huge lava flows lay. Arguably, the large Deccan lava flows were fed by regional dykes. But which dykes? This question can be answered with detailed field, petrographic, geochemical (major and trace element and isotopic) and palaeomagnetic studies of individual dyke swarms in the province, followed by statistical comparisons of the geochemical data to well-characterized lava packages. Once the feeder dykes of a particular lava sequence are thus identified, bounds can be placed on the distances of magma transport, whether in the crust, or after eruption on the surface. The Deccan province has three major zones where mafic (dolerite-basalt) dyke swarms attain profuse development (Fig. 1; Auden 1949; Deshmukh and Sehgal 1988; Bhattacharji et al. 1996; Sheth 2000). One of them is the Narmada-Satpura-Tapi region, in the north of the province, containing thousands of dykes that form sub-regional swarms with a general ENE-WSW strike. The second major zone of dykes is the Konkan coastal plain, between the Arabian Sea to the west and the Western Ghats escarpment to the east. Here, dykes are abundant and have a general NNW-SSE strike, parallel to the western Indian rifted margin. The third important zone of dykes is the region in the Western Ghats northeast of Mumbai. Here, dykes do not show as strong a preferred orientation as in the other two areas (Beane et al. 1986), and yet, individual swarms in this large region do show preferred orientations (Bondre et al. 2006). I and my colleagues and students are currently engaged in active research on the Deccan dyke swarms, specifically Rangpur (near Phenai Mata), Dediapada, Dhadgaon, Nandurbar-Dhule, Sangamner, and Dahanu-Umargam (Gujarat and Maharashtra). In this talk, I will be summarizing our recent detailed geochemical-isotopic study of the Sangamner dyke swarm in the Western Ghats (Bondre et al. 2006), and will also present field and

structural aspects of the dykes from all the above-mentioned areas. Many dykes we have encountered are several kilometres long, a few dykes are several tens of kilometres long, and the largest dyke we have encountered so far is 79 km long, in the Dhule region. Theoretical considerations suggest that a substantial Deccan lava flow (1000 km3 or larger in volume) could have formed at low eruption rates of 1 m3 per metre length of fissure, from a 50-kmlong dyke with consecutively active 5-km-long segments, in about 7 years. Clearly, focussed field, petrographic and geochemical (element and isotopic) studies of individual, sub-regional Deccan swarms have the potential to answer many long-standing questions about the volcanological, structural-tectonic and geochemical evolution of the Deccan lavas and CFBs worldwide.

References AUDEN, J.B. (1949) Dykes in western India – A discussion of their relationships with the Deccan Traps. Transactions of the National Institute of Sciences of India, v. 3, pp. 123-157. BHATTACHARJI, S., CHATTERJEE, N., WAMPLER, J.M., NAYAK, P.N. and DESHMUKH, S.S. (1996) Indian intraplate and continental margin rifting, lithospheric extension, and mantle upwelling in Deccan flood basalt volcanism near the K/T boundary: Evidence from mafic dike swarms. Journal of Geology, v. 104, pp. 379-398. BEANE, J.E., TURNER, C.A., HOOPER, P.R., SUBBARAO, K.V. and WALSH, J.N. (1986) Stratigraphy, composition and form of the Deccan Basalts, Western Ghats, India. Bulletin of Volcanology, v. 48, pp. 61-83. BONDRE, N.R., HART, W.K. and SHETH, H.C. (2006) Geology and geochemistry of the Sangamner mafic dyke swarm, western Deccan volcanic province, India: Implications for regional stratigraphy. Journal of Geology, v. 114, pp. 155-170. DESHMUKH, S.S. and SEHGAL, M.N. (1988) Mafic dyke swarms in the Deccan volcanic province of Madhya Pradesh and Maharashtra. In: Subbarao KV (Ed.), Deccan Flood Basalts. Geological Society of India Memoir 10, pp. 323-340. SHETH, H.C. (2000) The timing of crustal extension, dyking, and the eruption of the Deccan flood basalts. International Geology Review, v. 42, pp. 1007-1016.