1. Impetus for this contribution

The Deccan volcanic province (DVP) is one of the world's largest LIPs and perhaps the best studied continental flood basalt (CFB). However, its genesis and evolution are still poorly understood.

Recently, Sheth (1999) convincingly refuted the plume model as a basis for the genesis of the DVP (see The Deccan beyond the plume hypothesis). However, his suggestion that the DVP and Laccadive-Chagos ridge formed as a consequence of southward crack propagation along the Vishnu fracture zone is not consistent with data concerning the geomagmatic and tectonic history of the Indian peninsular plate. The current status of knowledge thus represents a shortfall of understanding. The need to fill this gap in our knowledge, and to establish a genetic model for the DVP based on the vast volumes of existing scientific data, was the impetus for the present contribution.

2. Introduction

The DVP is one of the Earth's giant continental flood basalts and has a total exposed area of about half a million square kilometers, between latitudes 16° - 24° N and longitudes 70° - 77° E. In the northwestern, central and southern Indian peninsula, the approximate volume of the DVP is about 2 x 10⁶ km³ and its estimated age is 64-65 Ma. It is generally believed that the DVP originated during Gondwanaland breakup as part of the Seychelles-India separation event. Another important belief concerning this CFB is that it is the “head” of a plume which is currently active as Reunion volcanism, with the “tail” consisting of the rather irregular chain of volcanic islands extending from Reunion to and along the Laccadive-Maldive-Chagos ridges. The latter model has been refuted convincingly, as mentioned above. However, a viable alternative model has not yet been proposed. In this contribution we attempt to provide an alternative hypothesis based on existing geological information about the DVP.

3. The Deccan volcanic province: existing scientific data

The DVP erupted on the Archean-Proterozoic shield areas of south, north-west and central India and the adjoining offshore area off the west coast (Figure 1) (Devey & Stephens, 1991). The volcanics cover two cratonic areas – the Dharwar craton of the south Indian shield and the central Indian craton. Apart from this, the DVP is associated with four major rift zones of peninsular India (Figure 1). It is juxtaposed with the east-west-running Narmada-Satpura-Tapi rift which is a horst-and-graben-type rift zone that trends ENE-WSW for > 1600 km along central India (Mishra, 1977). In northwest India, the DVP is in contact with the Cambay, Kutch and West Coast rifts.

Figure 1: Geological map showing the location of the Deccan volcanic province and its relationship with the geo-tectonic features of the region. Modified after Sheth (2005).

To evaluate the causes of DVP genesis, it is essential to understand the geological context of its hosts. Therefore, to interpret the data from the Deccan volcanics, we first review the nature of the hosts.

Along the West Coast rift, the south Indian shield witnessed several prior phases of magmatism before it hosted the Deccan volcanics. The first recorded event occurred at 678 Ma when gabbro, granophyre and anorthosite magmatism occurred (Nair & Vidyadharan, 1982). This was followed by granitic plutonism, almost 128 My later, i.e., at 550 Ma (Soman et al., 1983). Subsequently, the region hosted pegmatitic intrusions at 460 Ma (Soman et al., 1982). These pegmatites mark the end of the major phases of the magmatic episode because there was then a complete hiatus of magmatic events in the region until 93 Ma (date averaged from six differing K-Ar dates), i.e., for more than 350 My! At 93 Ma, the West Coast region again experienced magmatism, this time involving rhyolitic and dacitic volcanics (Valsangkar et al., 1980). This was followed by vast mafic volcanism and plutonism which resulted in the DVP and associated dyke swarms during the period 64-65 Ma, and covered the huge area mentioned above.

Along the Narmada-Tapi rift zone, prior to the Deccan episode, no major magmatic event is reported. Instead, the horst (Satpura) and grabens (Narmada and Tapi) paved the way for enormous sedimentation, which gave rise to the Mahakoshal group of rocks (Chanda & Bhattacharya, 1966). These metasediments recorded several phases of shearing implying that the Narmada – Tapi rift is associated with intense shear deformation along an east-west trend. These metasediments are followed by Jurassic-early Cretaceous sediments on top of which the Deccan volcanics lie (Chanda & Bhattacharya, op.cit).

The tectonics of the northwest Indian peninsula and the offshore region are inter-related. The summary of the tectonic history of this region given here is based on the studies of several workers (Glennie, 1932; Qureshy, 1971; Owen, 1976; Kaila et al., 1979; 1981; Biswas, 1982; 1987; Harbison & Bassinger, 1973; Gupta et al., 1998). As shown in Figure 1, four major rift zones are in contact with the Deccan volcanics, the Narmada-Tapi rift, the West Coast rift, the Cambay rift and the Kutch rift. Kutch rifting occurred in the late Triassic to early Jurrasic followed by early Cretaceous Cambay rifting. The Narmada-Tapi and West Coast rifts were reactivated in the late Cretaceous. The Narmada-Tapi rift zone is believed to be extending along its trend into the offshore area of the Indian west coast.

Concerning geophysics (Chandrasekharam, 1985), the Bouguer gravity anomaly pattern and seismic profiles along the West Coast rift indicate:

4. The new proposed hypothesis: an effort to bridge the gap in knowledge

4.A: The first phase of magmatism, 678 to 460 Ma: Of the four rifts, only the West Coast rift exclusively hosts magmatic rocks older than the Deccan volcanic event at 64-65 Ma. These magmatic rocks, which formed at 678-460 Ma, show a clear trend of fractional melting (gabbro-rhyolite to pegmatites). This implies the presence of a magma chamber beneath the Indian lithosphere under the west coast. In this magma chamber, magma could have remained stored and preserved its primary chemistry. The primary magma could have risen from its depth of segregation which was beneath or within the west coast lithosphere, but certainly not deeper than 200 km because the first magmatic event was gabbroic. The hiatus in igneous activity from 460 Ma to 93 Ma along the West Coast rift indicates that during this period the magma pressure in the chamber was less than lithospheric, and/or the temperature was too low for magma to cross the liquidus of its compositions. By 93 Ma, that is 350 My later, this problem was removed.

4.B The beginning of the second phase of igneous activity, at 93 Ma: This phase, along the West Coast rift, is marked by a magmatic event which was not plutonic, like previous ones, but rather volcanic. Unlike the previous episode, it began with magma of felsic composition – the rhyolites. It is relevant to note that Madagascar-Indian plate breakup took place at around 93 Ma. This breakup also occurred along the western continental margin of India. Obviously, such a megascale rifting event will leave its signature in the form of volcanic, not plutonic activity, which can explain the simultaneous rhyolitic volcanism. The composition, felsic instead of mafic, probably indicates that the temperature was too low to melt mafic components in the magma chamber beneath the West Coast region, but elevated enough to produce the rhyolitic melts, i.e., it was around 1000°C.

4.C The Deccan volcanism, at 64-65 Ma: The second event of this phase is the Deccan volcanism which occurred after a hiatus of 30 Ma. Also, after exactly the same hiatus, and at the same time of 64-65 Ma, the Indian plate experienced breakup from yet another partner at Gondwanaland time – the Seychelles. This probably indicates that the second stage of magmatism, which began at 93 Ma, was controlled by the breakup events between Gondwanaland microplates and the Indian plate. Also, it shows that the breakup process was a gradual and progressive phenomenon, starting with Madagascar-India separation and, after another 30 My, Seychelles-India separation. Since this process controlled West Coast magmatism, which tectonism is expected to do, we deduce that the magmatism was also progressive. Thus, we conclude that (i) the rhyolitic volcanism at 93 Ma resulted from Madagascar-India breakup, and (ii) that this breakup event was a continuous process which led to Seychelles-India breakup after another 30 My and to Deccan volcanism at the same time. The progressive chemical trend of the volcanics, i.e., from rhyolite to basalt, indicates gradual progressive increase in temperature and/or gradual progressive lowering of the liquidus in the magma chamber as a result of gradual progressive rifting/breakup of the Indian plate with Madagascar and the Seychelles respectively.

The shift from plutonism during first phase to volcanism during the second phase perhaps indicates the presence of direct, uninterrupted conduits from the magma chamber to the surface of the continental crust during the second phase. This is what is expected during extensive rifting events such as Madagascar-India and Seychelles-India separation which did not occur during the first phase.

The large volume of the Deccan volcanics and the high rate of volcanism during the Deccan episode indicate:

The geochemical variation within the Deccan volcanics, as mentioned above, perhaps indicates differences in the chemistry of the host rocks. For example, along the Narmada-Tapi rift zone the magmatic melt must have interacted with the host sediments, which are Mahakoshal Jurassics along with Archean metamorphics. Similarly along the West Coast, Cambay and Kutch rifts, the melt would have interacted with Archean-Precambrian metamorphics. Consequential changes in the chemistry would be reflected in the geochemistry of the DVP volcanics.

4.D Post Deccan

4.D.1: 61 Ma: The rifting which had started at 93 Ma resulted in the opening of the Carlsberg ridge at about 61 Ma. Deccan volcanism also continued, as suggested by the age of DVP rocks from the Bombay area (Sheth & Ray, 2002). This is the time when the Laccadive ridge also experienced Deccan volcanism.

4.D.2: 55-50 Ma: Sea-floor spreading along the Carlsberg ridge resulted in the emplacement of ocean floor between the ridge and the continental margin of the Indian West Coast. Deccan volcanism along the West Coast rift produced the Maldive ridge at around 55 Ma and the Chagos ridge at around 50 Ma. Clearly, the Maldive and Chagos ridges formed by interaction of the West Coast rift magma with Carlsberg ridge mantle magma because the Laccadive ridge, at 61 Ma, is characterised by melt with the same chemistry as melt from the magma chamber beneath the western continental crust of India.

The locations of these ridges mark the position of India at a given point in time. At this time, the Indian plate was moving northwards at a velocity of 18-19 cm/year. Thus, the position of Indian plate was controlled by (a) its own velocity in a northerly direction, and (b) the speed of Carlsberg ridge propagation.

4.D.3: 45-35 Ma: By this time, the northward movement of India had slowed considerably because the Indian plate had collided with the Eurasian plate. As a result, intraplate tectonics were solely responsible for deformational events. Along the West Coast rift, two types of forces were important (Figure 2), the north-south-trending West Coast and Cambay rift forces and the east-west-trending Narmada-Tapi rift force. The net vector force of these two combined resulted in shearing and stretching along the West Coast rift zone (Figure 2). Simultaneously, isostatic balancing forces resulting from the emplacement of a huge volume of volcanics resulted in vertical movements along the west coast. These conclusions are based on the fact that the West Coast rift is bounded by intersecting sets of faults and fractures which extend up to the Laccadive ridge (Figure 2). The fracture system north of 16°N formed during the late Cretaceous whereas the systems to the south of 16°N formed during the middle to late Tertiary. This indicates that deformation along the coast had started in the late Cretaceous and gradually progressed southwards during the late Tertiary. Probably, the combined outcome of these forces resulted in Laccadive ridge separation (due to stretching), southward displacement (due to shearing) and subsidence (due to isostatic balancing) during the late Tertiary.

Figure 2: Diagrammatic representation of the proposed hypothesis described herein for the geological history of the Indian plate from 65 Ma onwards. The major stress directions along western continental margin of India 45-35 Ma are indicated by red arrows.

Summary

We propose that a magma chamber underlies the West Coast rift of the Indian peninsular. In support of this idea, we point to evidence for continuous magmatism along the west coast at 678 – 460 Ma, which shows a continuous trend of fractional melting. This was followed by a lull in magmatism for almost 350 My. Magmatism started again at around 93 Ma when Madagascar broke away from the Indian plate. Since that rifting took place along the west continental margin of India (i.e. today’s western coast), we conclude that the associated magmatism along the western continental margin of the Indian plate was related to Madagascar-India break up. Rifting led to decompression which in turn led to partial melting in the magma chamber beneath the western continental margin of India. This is supported by the fact that magmatism during this phase started with felsic volcanism and not mafic.

As rifting proceeded along the western margin of the Indian plate, the rate of partial melting in the magma chamber increased proportionately leading to the mafic Deccan volcanism. The high rate of Deccan volcanism was due to the fact that along with the West Coast rift, three other deep crustal rifts were activated simultaneously. This could have been due to continuous rifting along the west coast from 93 Ma onwards. We propose that the high volume of Deccan volcanism was because: