Friday, November 8, 2013

volcano (geology) :: Hot springs and geysers -- Encyclopedia ...
Hot springs and geysers also are manifestations of volcanic activity. ... Yellowstone National Park in the United States is one of the most famous areas of hot springs and geysers in ... Hot springs and geysers · Volcanism and tectonic activity.

Youth drowns in hot water springs lake in Manor

MUMBAI: A youngster drowned in a natural hot water springs lake in Sativali of Manor after he suffered an epilepsy attack on Monday evening.
The charred body was found on Tuesday.
Deepak Gaikar (20) a resident of Lokmanya Nagar in Thane entered the hot springs lake in Varai village in Sativali, around 95km from Mumbai. In the water he suffered an epilepsy attack and drowned.


Deccan Traps - Wikipedia, the free encyclopedia
The bulk of the volcanic eruption occurred at the Western Ghats (near Mumbai) some 65 million years ago. This series of eruptions may have lasted less than ...

Geysers are hot springs that intermittently spout a column of hot water and steam into the air. This action is caused by the water in deep conduits beneath a geyser approaching or reaching the boiling point. At 300 metres (about 1,000 feet) below the surface, the boiling point of water increases to approximately 230 °C (450 °F) because of the increased pressure of the overlying water. As bubbles of steam or dissolved gas begin to form, rise, and expand, hot water spills from the geyser’s vent, lowering the pressure on the water column below. Water at depth then momentarily exceeds its boiling point and flashes into steam, forcing additional water from the vent. This chain reaction continues until the geyser exhausts its supply of boiling water.
After a geyser stops spouting, the conduits at depth refill with groundwater, and reheating begins again. In geysers such as Yellowstone’s Old Faithful, the spouting and recharge period is quite regular. This famous geyser has gushed to heights of 30 to 55 metres (100 to 180 feet) about every 90 minutes for more than 100 years. If Old Faithful’s eruption lasts only a minute or two, the next interval will be shorter than average, while a four-minute eruption will be followed by a longer interval. Other geysers have much more erratic recharge times.

It was no volcanic activity on Sunday in Baramati, say experts

Thursday, May 15, 2008, 3:06 IST
The mystery behind the geological eruption at the Murti village in Baramati taluka on Sunday remained unsolved, even as a three-member geologists' team from Pune visited the place.
Witnesses say a lava-like substance was oozing out from the six-inch radius hole
BARAMATI: The mystery behind the geological eruption at the Murti village in Baramati taluka on Sunday remained unsolved, even as a three-member geologists' team from Pune visited the place on Wednesday and ruled out any volcanic activity.
The team from Deccan Volcanological Society (DVS) was led by its president Anant Phadke.

After inspecting the spot thoroughly, Phadke said, "The eruption is neither a volcano nor an earthquake. Villagers need not worry."
"Onlookers have stepped on the spot and have destroyed vital clues. We should have visited the spot much early. We have taken a few samples of the solidified matter," Phadke said.
When asked why the area was not cordoning off to keep it safe from public interference, police sub-inspector Rajendra Kale said he had informed the higher authorities about the incidence. "Tehsildar Sameer Shingte visited the place on Monday," he said.
Confirming that it was not a natural phenomenon, DVS secretary Sampada Joshi said, "The melting of rock-like substances had taken place at the electricity pole. It later solidified in the form of hard matter."
Explaining a possibility, she said since the eruption occurred at the bottom of an iron electricity pole, perhaps it has to do with electricity. The molten lava-like fluid may have taken place due to residual current under the earth surface.
According to witnesses, a lava-like substance was oozing out from the six-inch radius hole.
Akbar Pathan, who was among the first to visit the spot after the eruption, said initially there was hot, brown fluid and vapour was continuously
coming out.
Sunil Nalawade, in whose field the incident took place, said that he saw the eruption of a red-hot fluid.

 "The government authorities will decide their next course of action after studying the geologists' report," said Sub-divisional officer Nandkumar Katkar.
Volcanic Eruption Articles By Date
October 20, 2013 | PTI
IMPHAL: A suspected volcano-like eruption has been reported in a remote village of Manipur near the India-Myanmar border which forced locals to evacuate the area, official sources said on Sunday. According to locals in Tusom village in Ukhrul district of Manipur, a deafening sound was followed by the rolling down of a huge boulder from a nearby hilltop which then released a lava-like liquid that charred trees and plants on the hill slopes. Although the incident...

Volcanoes in India, not meteorite, killed dinosaurs: Study - Economic ... › Collections
Dec 9, 2012 - NEW YORK: Volcanic activity in the Deccan Traps near modern-day Mumbai, and not an asteroid, may have killed the dinosaurs about ...

Volcanoes in India, not meteorite, killed dinosaurs: Study - Economic ... › Collections
Dec 9, 2012 - NEW YORK: Volcanic activity in the Deccan Traps near modern-day Mumbai, and not an asteroid, may have killed the dinosaurs about ...
Coastal ecosystem responses to late stage Deccan Trap volcanism:
the post K–T boundary (Danian) palynofacies of Mumbai
(Bombay), west India
J.A. Cripps
, M. Widdowson
, R.A. Spicer
, D.W. Jolley
School of Earth Sciences and Geography, Kingston University, Kingston-upon-Thames, KT1 2EE, United Kingdom
Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
Centre for Palynology, University of Sheffield, Sheffield, S3 7HF, United Kingdom
Received 24 March 2004; received in revised form 23 August 2004; accepted 12 November 2004
The Deccan Trap continental flood basalt eruptions of India occurred c. 67–63 Ma, thus spanning the Cretaceous–Tertiary
boundary (65 Ma). Deccan eruptions were coeval with an interval of profound global environmental and climatic changes and
widespread extinctions, and this timing has sparked controversy regarding the relative influence of Deccan volcanism upon end-
Cretaceous catastrophic events. If Deccan Trap activity was capable of affecting global ecosystems, evidence should be present
in proximal Indian sedimentary facies and their palaeontological contents. The impact of late stage Deccan volcanism upon
biota inhabiting Mumbai (Bombay) Island’s post K–T boundary lagoonal systems is documented here. Sediments (or
) which accumulated within these lagoons are preserved between Trap lavas that characterise the closing stages
of this flood basalt episode.
Mumbai Island Formation intertrappean faunal and floral communities are conspicuously distinct from those common to
many pre K–T boundary, late Maastrichtian intertrappeans across the Deccan province. The latter sedimentary intercalations
mostly developed in cognate semiarid, palustrine ecosystems; by contrast, those around Mumbai evolved in sheltered,
peripheral marine settings, within subsiding continental margin basins unique to this late Deccan stage, and under an
increasingly humid Danian climate. Geochemical analyses reveal that Mumbai sedimentation and diagenesis were intimately
related to local explosive volcanic and regional intrusive activity at c. 65–63 Ma. Although tectonic and igneous events
imprinted their signatures throughout these sedimentary formations, organisms usually sensitive to environmental perturbations,
including frogs and turtles, thrived. Critically, palynofacies data demonstrate that, whilst plant material deposition was
responsive to environmental shifts, there were no palpable declines in floral productivity following Mumbai pyroclastic
discharges. Therefore, it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally.
2004 Elsevier B.V. All rights reserved

Volume 172, Issues 1–2, 10 May 2008, Pages 3–19
Physical Volcanology of Large Igneous Provinces
Research paper

Correlation of the Deccan and Rajahmundry Trap lavas: Are these the longest and largest lava flows on Earth?

  • a Volcano Dynamics Group, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK
  • b Astrogeology Research Program, US Geological Survey, 2255 N. Gemini Dr. Flagstaff, AZ 86001, United States


We propose that the Rajahmundry Trap lavas, found near the east coast of peninsular India, are remnants of the longest lava flows yet recognized on Earth (∼ 1000 km long). These outlying Deccan-like lavas are shown to belong to the main Deccan Traps. Several previous studies have already suggested this correlation, but have not demonstrated it categorically. The exposed Rajahmundry lavas are interpreted to be the distal parts of two very-large-volume pāhoehoe flow fields, one each from the Ambenali and Mahabaleshwar Formations of the Wai Sub-group in the Deccan Basalt Group. Eruptive conditions required to emplace such long flows are met by plausible values for cooling and eruption rates, and this is shown by applying a model for the formation of inflated pāhoehoe sheet flow lobes. The model predicts flow lobe thicknesses similar to those observed in the Rajahmundry lavas. For the last 400 km of flow, the lava flows were confined to the pre-existing Krishna valley drainage system that existed in the basement beyond the edge of the gradually expanding Deccan lava field, allowing the flows to extend across the subcontinent to the eastern margin where they were emplaced into a littoral and/or shallow marine environment. These lavas and other individual flow fields in the Wai Sub-group may exceed eruptive volumes of 5000 km3, which would place them amongst the largest magnitude effusive eruptive units yet known.
We suggest that the length of flood basalt lava flows on Earth is restricted mainly by the size of land masses and topography. In the case of the Rajahmundry lavas, the flows reached estuaries and the sea, where their advance was perhaps effectively terminated by cooling and/or disruption. However, it is only during large igneous province basaltic volcanism that such huge volumes of lava are erupted in single events, and when the magma supply rate is sufficiently high and maintained to allow the formation of very long lava flows. The Rajahmundry lava fields were emplaced around 65 Ma during the later times of Deccan volcanism, probably just after the K/T environmental crisis. However, many lava-forming eruptions of similar magnitude and style straddled the K/T boundary.


  • basalt lava flows;
  • pāhoehoe;
  • Rajahmundry Traps;
  • flood basalts;
  • Deccan;
  • long lava flows;
  • large igneous provinces

Figures and tables from this article:
Full-size image (77 K)
Fig. 1. 
Sketch map of main Deccan province on NW peninsula India and outlying related lava-covered areas of the Deccan large igneous province (shaded), with the location of the Rajahmundry Traps shown. Dots indicate towns. Godavari and Krishna drainage systems and Western and Eastern Ghats also shown; on-shore part of Godavari Basin is approximately area containing buried lavas. Boxed area is that of Fig. 7.
Full-size image (65 K)
Fig. 2. 
Sketch map of central to southern part of main Deccan province showing inferred original extent of Deccan province and Wai Sub-group lava flows, including mapped and inferred boundaries of Ambenali and Mahabaleshwar Formation lavas (see text for discussion and source of information). Exposure area of Rajahmundry lavas (RT) and outlines of crustal structures along which developed drainage systems that lavas may have followed are also shown.
Full-size image (61 K)
Fig. 3. 
View of quarry at Gouripatnam (N 17° 2′ 3.8″; E 81° 36′ 14.2″) exposing hackly jointed core of a sheet lobe of the Rajahmundry Trap lavas. Uppermost quarry level shows deep weathering of the lava; second lower level is formed of back-filled material. Trucks and people lower right give scale. Photo courtesy of Matthias Raab (University of Melbourne).
Full-size image (58 K)
Fig. 4. 
Five logs of sections through lava flows around the Mahabaleshwar Plateau, Western Ghats (see location of Mahabaleshwar on Fig. 1), in Deccan volcanic province, that straddle the Chron 29R/29N paleomagnetic boundary (after Jay, 2005). Names are road-cut traverses up the ghats along which the lava flows were logged; elevations are metres above sea level. Column to right of each log is palaeomagnetic polarity; white Chron 29R, black is Chron 29N (grey shows parts of sections where no samples were obtained). Note that Chron29R/29N transition falls in lower Mahabaleshwar Formation. Logs show varying numbers of lava units between paleomagnetic reversal horizon, along which logs are aligned, and top of Ambenali chemotype lava flows.
Full-size image (60 K)
Fig. 5. 
Logged section of lavas in upper part of the Ambenali Ghat traverse, Mahabaleshwar Plateau (as for Fig. 4, Ambenali log). Elevations (left of column) are metres above sea level. Note that there are fewer good quality exposures of lava units towards top due to deep weathering and topographic bench formation. Note also occurrence of upper Mahabaleshwar chemotype lava flows, with similar compositional character to the upper RT lava. Right column continues from left.
Full-size image (67 K)
Fig. 6. 
Concentrations of Nb, Zr, and Ba in main series Deccan lava samples from (left column) Ambenali Ghat near Mahabaleshwar (see Fig. 5) after Jay (2005), and (three other columns) from Rajahmundry Trap lavas, labelled C&J Unpub (data from J. Cripps and A.E. Jay, unpublished data), Knight 2003 (data from Knight et al., 2003), and Baksi 2001 (data from Baksi, 2001). Sample labels are: A is Ambenali Formation, AE is SE Deccan Ambenali Formation; M is Mahabeleshwar Formation, Mu is upper Mahabeleshwar chemotype, ME and MuE are SE Deccan Mahabaleshwar and upper Mahabeleshwar chemotypes, respectively. Grey bar gives range of 16 samples from bottom to top of one Mahabeleshwar Formation sheet lobe (Jay, 2005). U and L designate Upper and Lower Rajahmundry Trap lavas; KU is Kolhapur unit of Mahabeleshwar Formation discussed by Baksi (2001). M'war and Amb averages are average values of these elements in Ambenali and Mahabeleshwar Formation lavas from sample data set compiled by Widdowson et al. (2000b); M'war/Amb. discriminant value is that adopted for this study (see also Table 3).
Full-size image (71 K)
Fig. 7. 
Simplified geological map of southeastern main Deccan province with outcrop areas of Deccan formations shown, together with the Kagnar and Bhima rivers which connect to the Krishna River. Only exposure of Bushe Fm is in NE of area shown; laterite mapped is upper-level Tertiary laterite of Widdowson (1997). Sample collection sites for Ambenali and Mahabaleshwar chemotype lavas (see text) shown as squares. Towns are shown as dots; dark area on Manjra River is a lake.
Full-size image (33 K)
Fig. 8. 
Plot of total lava lobe thickness vs down-flow change in temperature from thermal flow model with conditions appropriate for Deccan-Rajahmundry lavas, see text. Maximum allowable temperature decrease of 0.05 °C/km, permitting < 50 °C cooling over 1000 km is indicated.
Table 1. Stratigraphic sequence of main sub-groups, assigned volumes, paleomagnetic chrons, and typical thicknesses for the Deccan Basalt Group
View Within Article
Table 2. Major and trace element analyses by XRF (except ) for main Deccan series lavas and Rajahmundry Trap lavas
RT lava: lower RT or upper RT flow as designated by author. Chemotype designation as discussed in text; Amb = Ambenali type; Mwar = Mahabaleshwar type; upper Mwar = upper Mahabaleshwar type. (J&W, 2007) = Jay and Widdowson, 2007; (Cripps and Jay, unpublished) = AE Jay and J Cripps, unpublished data; (Baksi) = Baksi (2001); (Knight) = Knight et al. (2003), see text; Knight: trace elements by IC-PMS, all others by XRF; ⁎⁎ total Fe as FeO. ⁎⁎⁎ Fe calculated as Fe2O3 for the normalised recalculation; LOI = loss on ignition; nr = not reported.
View Within Article
Table 3. Thicknesses and criteria used to distinguish the geochemical formations (chemotypes) in Deccan Basalt Group in the Western Ghats, India, after 71, 72 and 8, and this study (Ambenali and Mahabaleshwar Formations only). No entry (dash) indicates an element or ratio not useful for discrimination for that formation
View Within Article
Corresponding author contact information
Corresponding author. Tel.: +44 1908 659773.

Deccan Volcano - John Seach

The Deccan Traps is located in central west India and dates from 66 million years ago. The lava flows are some of the largest on earth covering 900 km and meet the coast at the Arabian Sea. Deccan volcanism coincided with the decline of the dinosaurs raising the possibility that the Indian volcanoes were involved with their decline.

Deccan lava meets the Arabian Sea at Goa. The lava flows cover 900 km throughout central and western India. The Reunion mantle plume was responsible for the lava flows which covered 500 000 sq km.
lava at goa india
Lava meets the sea at Goa, India
lonar meteorite crater india
Lonar meteorite crater, Deccan volcano basalt, India

Deccan Volcano Eruptions

66 million years ago

Roadmap | Home
   Deccan traps
The Deccan beyond the plume hypothesis
Hetu C. Sheth
Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai, Bombay (Mumbai) 400 076 India. or

Click here to download a PDF version of this webpage

The widely accepted mantle plume model (e.g., Morgan, 1981; Richards et al., 1989; Campbell & Griffiths, 1990) postulates that (i) the currently active Réunion Island, in the Indian Ocean, is fed by the narrow “tail” of a mantle plume that rises from the core-mantle boundary, (ii) the Deccan continental flood basalt (CFB) province of India originated from the “head” of the same plume during its early eruptive phase near the end of the Cretaceous, and (iii) the Lakshadweep-Chagos Ridge, an important linear volcanic ridge in the Indian Ocean, is a product of this plume. It is not generally appreciated, however, that this so-called “classic” case of a plume contradicts the plume model in many ways. For example, there is little petrological evidence as yet that the Deccan source was abnormally hot, and the short (~ 1.0 – 0.5 Myr) duration claimed by some for the eruption of the Deccan is in conflict with recent Ar-Ar age data that suggest that the total duration was at least ~ 8 Myr (Sheth et al., 2001a,b). The Deccan CFB was associated with the breakup of the Seychelles microcontinent from India (e.g., Mahoney, 1988). Geological and geophysical data from the Deccan provide no support for the plume model and arguably undermine it altogether (Sheth, 2005a,b). The interplay of several intersecting continental rift zones in India is apparently responsible for the roughly circular outcrop of the Deccan. The Lakshadweep-Chagos Ridge, and the islands of Mauritius and Réunion, are located along fracture zones, and the systematic southerly age progression along the Ridge (though questioned) may be a result of southward crack propagation through the oceanic lithosphere. This idea avoids the problem of a 10° palaeolatitude discrepancy which the plume model can only solve with the ad hoc inclusion of mantle roll. Published Ar-Ar age data for the Lakshadweep-Chagos Ridge basalts have been seriously questioned (Baksi, 1999, 2005), and geochemical data suggest that they likely represent post-shield volcanism (Sheth et al., 2003) and so are unsuitable for hotspot-based plate reconstructions. “Enriched” isotopic ratios such as higher-than-N-MORB values of 87Sr/86Sr, observed in basalts of the Ridge and the Mascarene Islands may mark the involvement of delaminated enriched continental mantle instead of a plume (Smith, 1993). High values of the 3He/4He ratio also do not represent a deep mantle component or plume (Anderson, 1998a; 1998b). The three Mascarene Islands (Mauritius, Réunion, and Rodrigues) are not related to the Deccan but reflect the recent (post-10 Ma) tectonic-magmatic development of the African Plate.
I relate CFB volcanism to continental rifting, which often (but not always) evolves into full-fledged sea-floor spreading (Sheth, 1999a, 2005a). I ascribe the rifting itself not to mantle plume heads but to large-scale plate dynamics, possibly aided by long-term thermal insulation beneath a supercontinent which may have surface effects similar to those predicted for “plume incubation” models. Non-plume, plate tectonic models are capable of explaining the Deccan in all its greatness.
Figure 1. The 1,200-m-thick exposed section through the Deccan basalt pile at Mahabaleshwar, Sahyadri (Western Ghats) region. Grand! Photo by Hetu Sheth.
Since the rapid rise to dominance of the plume-head/plume-tail model for flood basalts (Richards et al., 1989; Campbell & Griffiths, 1990), hundreds of papers have invoked, or supported, a plume head origin for the Deccan Traps of India. These papers are in unanimous agreement on two issues: (i) the Deccan originated from the ancestral Réunion hotspot which upwelled beneath India in the late Cretaceous, and (ii) the hotspot, now located on the African plate, is fed by a deep mantle plume. The overall appearance of the Deccan, with its roughly circular outcrop, and the linear Laccadives-Chagos (more correctly, Lakshadweep-Chagos) Ridge to the south of India, looks very much like what is expected for a spherical plume head and a narrow plume tail (Figures 2 & 3). Nevertheless, the following observations and deductions suggest that the plume model is not valid for the Deccan (Sheth, 1999a,b, 2005a).
Figure 2. Map showing the approximate boundaries of the Precambrian cratons making up the Indian shield (e.g., Pandey & Agrawal, 1999; Naqvi & Rogers, 1987), the granulite terrain, the Precambrian structural trends (heavy broken lines), rift zones crossing peninsular India (e.g., Biswas, 1987), and the present outcrop areas of the Deccan and Rajmahal flood basalts. Inset shows the breakup of the Seychelles microcontinent, situated along the northern tip of the Mascarene Plateau (black), from India, soon after the Deccan flood basalt episode (after Norton and Sclater, 1979; Mahoney, 1988). The Koyna and Kuruduvadi “rifts” have been proposed based on gravity surveys and may represent humps of the granitic basement rather than rifts.
Figure 3. Prominent structural-tectonic features of southern Asia and the Indian Ocean basin (based on Mahoney et al., 2002). Abbreviations for localities are: Q, Quetta; Z, Zhob; B, Barmer, M, Mundwara; D, Dhandhuka; B, Bombay; R, Rajahmundry. WG is the Western Ghats region (ages from Venkatesan et al., 1993 and others). ~ 64 Ma age for Rajahmundry basalts is from Baksi (2001a). G, ~ 61 Ma Goa dykes (Widdowson et al., 2000). KK, ~ 90-69 Ma Karnataka-Kerala dykes (e.g., Radhakrishna et al., 1994; Anil Kumar et al., 2001). SMI are the St. Mary's Islands volcanics (85.5 Ma, Pande et al., 2001), part of the Indo-Madagascar CFB which in India is otherwise represented by the KK dykes. The associated flood basalt lavas are not represented or known in India; there are many Precambrian dyke swarms throughout southern India as well. 72-73 Ma ages for Quetta and Zhob rocks and 65 Ma age for Dhandhuka-Botad lavas are from Mahoney et al. (2002), as also the modelled hotspot track showing expected ages in Ma. Note the rift zones underlying the Deccan, and the absence of any triple junction. OFZ, Owen Fracture Zone; MFZ, Mauritius Fracture Zone; VFZ, Vishnu Fracture Zone. Click here for enlargment.
 Abnormally hot mantle? There is no evidence for “abnormally hot” mantle sources for the common and voluminous Deccan basalts (Figure 4). Some picritic liquids are encountered in boreholes in the northwestern Deccan and in the Narmada region (Krishnamurthy et al., 2000). The borehole lavas were conjectured by Campbell & Griffiths (1990) to be high-temperature, high-melt-fraction liquids from the plume axis. Peng & Mahoney (1995), however, found that they are somewhat alkalic and could be high-pressure, low-degree melts. The Deccan flood basalt sequence is best developed in the Western Ghats region with ~3 km of stratigraphic thickness (Figures 1-3), and picritic basalts are found there, but these are enriched in cumulus olivine and clinopyroxene and do not represent liquid compositions. The parental melts of these picrites are estimated to have contained only ~ 9-10% MgO (Beane & Hooper, 1988; Sheth, 2005b).
Figure 4. (a) A plot of 624 samples of Deccan basalts of the Western Ghats (data of Beane, 1988; courtesy J. J. Mahoney) on the well-known TAS diagram. Note the complete absence of compositions other than basalt and basaltic andesite, and the nearly exclusive subalkalic (tholeiitic) nature. Dividing lines between alkalic and subalkalic fields proposed by Macdonald & Katsura (1964) and Irvine & Baragar (1971) are also shown. (b) Plot of the same samples on the familiar AFM diagram, showing the Fe enrichment trend typical of tholeiitic basalts. Typical tholeiite trend (Thingmuli, Iceland) and calc-alkaline trend (Cascades) are also shown, along with boundaries between the two fields proposed by Kuno (1968) and Irvine & Baragar (1971). See Sheth (2005b) for an extended petrological discussion.
Very short (1 – 0.5 Myr) eruptive duration? Very rapid emplacement of the Deccan Traps is one of the key arguments for a plume origin, though also not incompatible with plate-related and stress-caused mechanisms. The duration of volcanism has also been one of the most debated issues. Recent 40Ar-39Ar data for trachyte and basalt flows from Bombay (Sheth et al., 2001a,b) suggest the total duration to have been no less than ~ 8 – 9 Myr. There may have been a major, rapid, short-duration eruptive phase in the Western Ghats, estimated by some to have lasted only 1.0 – 0.5 Myr (e.g., Duncan & Pyle, 1988; Courtillot et al., 1988; Hofmann et al., 2000), and by others 4 – 5 Myr (Venkatesan et al., 1993; Pande, 2002). Also, the data do not always justify the arguments advanced. Allègre et al. (1999) report an Re-Os isochron age of 65.6 ± 0.3 Ma (2σ) for several lava flow samples, arguing for a very short duration for the volcanism. That random, non-comagmatic samples collected across an area 1000 km wide and and at various topographic-stratigraphic levels should define an isochron is remarkable, but the goodness-of-fit (F) value for the claimed isochron, which was not reported, is 22 (Baksi, 2001b); the line is clearly an “errorchron” (Faure, 1986).
Catastrophic eruption rates? Some authors have explained “the extremely high lava eruption rates” in CFBs by hot plume heads, though there is no direct and simple relationship between melt production and melt eruption (Th. Thordarson, pers. comm., 2005). A large proportion of the Deccan basalts comprise pahoehoe compound lava flows (e.g., Walker, 1970; Bondre et al., 2004a,b). My own fieldwork at scores of places in the Deccan, and on the Kilauea volcano, Hawaii (Sheth, 2003), shows that the size and scale of individual flow units of many large Deccan compound flows are the same as those of modern Kilauea lava flows (Figure 5).The large volumes of the individual Deccan lava flows compared to the Hawaiian flows may reflect in part the great amount of decompression during India-Seychelles continental breakup (Figure 2 inset), considerable lengths (40 – 50 km) of the fissure systems (Figure 6; see also Self et al., 1997), excess source fertility (Sheth, 2005b), mantle volatiles such as CO2 (Presnall & Gudfinnsson, 2005), and similar features.
Figure 5. (a) The newborn toe of a compound pahoehoe basalt flow that has emerged from under the solidified lava crust, as a “breakout”. The front is about 1 m from the camera and 0.5 m wide. Ropes are forming in the frontal part and satellite breakouts emerge at right and left (bright yellow portions). Kilauea, Hawaii, May 2002. Photo by Hetu Sheth. This is how I believe the compound pahoehoe flows of the Deccan were emplaced. (b) Broader view of the actively inflating pahoehoe compound flow containing the lobe shown in (a). Note how numerous lobes are juxtaposed laterally and vertically. I am seen opening with the hammer the solidified roof of a lobe which yellow-hot magma (at ~ 1200°C) is filling. Kilauea, May 2002. Photo by Jyotiranjan Ray. This is how many compound pahoehoe flows of the Deccan look. Compare with (c). (c) Section across part of a compound pahoehoe lava flow of the Deccan, showing the distribution of vesicles and pipe vesicles. Some 17 flow units are seen. Modified from Walker (1970). Compare with (a) and (b).
Figure 6. Map of the Dediapada dyke swarm in the Narmada-Satpura region of the Deccan (after Krishnamacharlu, 1970). This is one of the large, spectacular oriented dyke swarms of the Deccan Traps. Geochemical studies of these dykes are currently underway. Elevations are in metres.
Internal age progression? None exists within the Deccan (Figure 3). Courtillot & Renne (2003) suggested that the 60–61 Ma volcanic activity well within the Deccan (e.g., at Bombay, Sheth et al., 2001a,b), was “minor”, and that the duration of Deccan volcanism was indeed very short. However, (a) this activity is not minor; large volumes of lava are emplaced in the subsurface along the west coast, and there is a scarcity of geochronological data. In comparison, the Western Ghats section has been heavily sampled and dated. (b) Whatever its magnitude, the late-persisting volcanism must be still explained without ad hoc auxiliary hypotheses. It is not. For example, according to prevalent views the plume head was all consumed in a quick phase around 66 – 65 Ma, and the predicted 60 Ma volcanic basement to the south of India, on top of the Maldives Ridge, should have formed from the narrow (100 – 200 km wide) plume tail. It is not clear how this plume tail could produce basalt in Bombay, 1,000 km to the north, at 60.5 Ma (Sheth et al., 2001b). Suggestions such as northward dragging of the plume tail by the plate are ad hoc, and such drag and tilting would make impossible any systematic age progression in the first place. Furthermore, if the ~ 69 Ma mafic dykes reported from Kerala, southernmost India (Radhakrishna et al., 1994) do represent early Deccan-related magmatism, as Sheth (1999b) considered likely, an entirely different, non-plume, passive, continental-breakup-related model for Deccan volcanism is even more attractive.
Enriched mantle: plume or continent? Smith (1993) proposed that ocean-island volcanism is derived from enriched continental mantle delaminated from a continent rifted along an ancient suture (see also Lithospheric delamination page). “Enriched” isotopic ratios such as higher-than-N-MORB values of 87Sr/86Sr, for example, are usually taken as plume signatures. However such compositions may instead mark involvement of shallow-level, enriched continental mantle. High values of 3He/4He may also be explained by shallow models (e.g., Anderson, 1998a, 1998b; see also Helium fundamentals page). The ~ 68.5 Ma alkalic complexes (Mundwara, Barmer) in the northern part of the Deccan province, related by Basu et al. (1993) to the Réunion plume based on Sr and He isotopic ratios, could thus be derived from the continental mantle.
The “enriched” plume model was never required to explain continental intraplate volcanism, given the abundance of “enriched” mantle domains within the continental lithosphere itself. The plume model was extrapolated to continental magmatism from the ocean basins based on the world view that the oceanic mantle was entirely ”depleted“, MORB-like, convecting and homogeneous. The reasoning was that anything “enriched” or anomalous had thus to come from plumes (Anderson, 1996; Smith & Lewis, 1999). However, if continental mantle is introduced into the oceanic mantle, e.g., by delamination during continental breakup (e.g., Smith, 1993), enriched plumes are not required to explain either continental or oceanic intraplate volcanism, and the whole argument can be turned around. Rather than the Deccan having formed from a deep mantle plume now located under Réunion island, Réunion volcanism may be in part sourced from delaminated Indian continental mantle.
Mahoney et al. (2002) recently reported Réunion-like elemental and isotopic compositions for mafic ophiolitic rocks, dated by them at 72 – 73 Ma, and outcropping in Pakistan. They opined that some of these may represent pre-Deccan oceanic seamounts. The associated intrusions were emplaced in continental shelf-and-slope-type marine sediments along the northern margin of India. Mahoney et al. (2002) considered the continental mantle delamination model, but argued that it does not explain Réunion-type volcanism occurring on the updrift side of India at 72 – 73 Ma, and concluded that the plume model is the most viable option. Notably, they supported the plume-head-impact model rather than the plume-head-incubation model, despite the ~ 8 Myr age gap between the Pakistani rocks and the 66 – 65 Ma voluminous basalt volcanism of the Deccan.
Nevertheless, the analyzed intrusions are located within the boundary of the Indian continental mantle, and the true oceanic seamounts may not have been far from the northern margin of India. Continental mantle delaminated during the early stages of India-Seychelles breakup could have migrated northward ahead of India and fed the seamounts built on oceanic lithosphere. The continent followed behind, and when it converged upon Asia it simply overrode these seamounts. This is a better explanation for the observations than the plume model. If the lateral flow of continental mantle proposed here seems ad hoc, note that the mechanism of long-distance lateral flow is required even by the plume model, as for the Rodrigues Ridge (Morgan, 1981; Figure 3) which is not located along the conjectured Deccan-Réunion hotspot track and trends roughly E-W. The rocks analyzed by Mahoney et al. (2002) and Basu et al. (1993) are undersaturated and alkalic, and have ocean-island-basalt-type characteristics (e.g., Sr isotopic ratios), but rather than being melts from a hot plume, they may be melts of carbonated lherzolite (see Keshav & Gudfinnsson, 2004).
The “hotspot track”: plume under plate, or crack propagation? The claimed southerly younging age progression along the Chagos-Laccadive Ridge and up to Réunion Island (though duly questioned by Baksi, 1999, 2005) does not require a lithospheric plate moving over a fixed plume. It may be explained by southward crack propagation through the oceanic lithosphere (see below). The narrow “hotspot track” may represent localized melting and magma focusing from a wider area (the “transform-fault effect”, Langmuir & Bender, 1984). In support of this, I note that the Chagos-Laccadive Ridge lies along the Vishnu Fracture Zone. The Ridge may mark the location of a major Gondwanic transform (Reeves & de Wit, 2000; Reeves et al., 2004).
It is possible that the current volcanism at Réunion Island may be unrelated to the Deccan geodynamically, though it taps delaminated Indian continental mantle brought beneath the African plate by the ridge jump at ~ 30 Ma (Sheth, 2005a; see also Burke, 1996). Burke (1996), a plume proponent, argued that the Deccan plume died out at 30 Ma and the Réunion plume is a different plume.
The Cambay triple junction and other fiction. Originally included by Burke & Dewey (1973) in their world-wide list of plume-generated triple junctions, the Cambay triple junction has been popularized by several subsequent papers supporting the Réunion plume model for the Deccan. However, the triple junction is not real (Sheth, 1999b, 2005a; Figures 2 & 3). Another unfortunate development is proliferation of model-dependent interpretations by which every geological and geophysical observation from the Deccan is interpreted an effect of the Réunion plume. For example, low-seismic velocity mantle underlying the Cambay rift of the Deccan is interpreted as a remnant of the plume (Kennett & Widiyantoro, 1999) instead of warm, low-density upper mantle welling up due to rift-related convection. This geophysical feature may even be a recent (post-Deccan) development (Sheth, 2005a).
Pre-volcanic lithospheric uplift, or lack thereof? Pre-volcanic lithospheric uplift of up to a few kilometres is an essential prerequisite for all thermal models such as the plume model. This is yet another issue on which specialists of different flood basalt provinces have come to diametrically opposed conclusions (e.g., Czamanske et al., 1998; He et al., 2003; Tejada et al., 2004; Saunders et al., 2005; see also Dhanjori page). Campbell & Griffiths (1990) cite the Deccan as a good example of a flood basalt with pre-volcanic uplift, but the Pachmarhi area mentioned by them as evidence for this appears instead to show the very opposite (recent uplift). Pachmarhi is on the Satpura horst between the Tapi and Narmada rifts. The very youthful landscape (e.g., kilometre-high escarpments in the basement Gondwana sandstones) and several planation surfaces (as high as 1,300 m above MSL) indicate very recent uplift (Ollier & Pain, 2001).
The same is true of the Deccan plateau region, where the Deccan-basement contact is in the subsurface over vast areas. Major rivers draining the Deccan plateau are of the antecedent type, i.e., they were in existence before the Western Ghats (Sahyadri Range) rose in their way, and the popular dome-flank drainage picture of the Indian drainage painted by Cox (1989) is highly speculative. The uplift of the Western Ghats is post-volcanic and recent (possibly Miocene and younger), and not pre-volcanic uplift produced by a plume (Sheth, 2005a). There are two possible interpretations: (1) Pre-volcanic lithospheric uplift occurred and then completely decayed and was overprinted by post-volcanic uplift. This is what plume proponents advocate. (2) Pre-volcanic uplift never did take place and the plume explanation is invalid. Option (2) is more plausible, and there is in fact actual support for it in the form of an uplifted, extensive planation surface below the Deccan lavas in central India (Dixey, 1970; see Sheth, 2005a). Note that the Western Ghats rise much higher in southern India (the region little or not affected by Deccan volcanism) than they do in the Deccan plateau region (Figure 7).

Figure 7. The main elements of the physiography of the Indian peninsula. The Western Ghats escarpment is shown by the heavy broken line. Note the pronounced easterly drainage. After Ollier & Powar (1985) and Sheth (2005a).
Palaeolatitudes: true polar wander or crack propagation? The Deccan lavas erupted at ~ 30°S latitude. Réunion Island is at 21°S today (Figure 8a). To explain this significant discrepancy in the framework of the plume model, some workers have proposed true polar wander (TPW) of the Earth's mantle (e.g., Vandamme & Courtillot, 1990). In this view, subsequent to the Deccan eruptions, the Réunion plume remained fixed in the mantle, while the mantle itself rolled like a ball, inside the lithospheric shell, in a northerly direction. Such speculation indicates well the extent of special pleading permitted within the plume model. Burke (1996) has questioned this postulated TPW.
I propose a much simpler alternative to the TPW, illustrated by the schematic diagram shown in Figure 8b. This is that the systematically changing palaeolatitudes between the Deccan and ODP Site 706 (33 Ma) indicate southward crack propagation in a northward-moving plate with the condition that the northward plate motion was faster than the southward crack propagation. The situation shown in Figure 8b is for a plate in the southern hemisphere. At time T1, there is an active volcano 1 at the crack tip at latitude 60°S. Between times T1 and T2, the crack tip has moved southward by 10°, but because the plate itself has moved north by 20°, the new volcano 2 at the crack tip has the latitude of 50°S. A similar progression occurs between times T2 and T3. Thus, although the crack tip propagates southward, the palaeolatitudes systematically become more northerly.
I conclude, based on diverse evidence, that the Réunion plume model for the Deccan is wrong.
Figure 8: (a) Palaeolatitude variation from the Deccan to Réunion Island through the ODP Leg 115 sites (Vandamme & Courtillot, 1990). (b) Schematic cartoon showing the development of volcanism resulting from a crack propagating more slowly southward than the plate moves northward, in the Southern Hemisphere.
Additional thoughts
Deccan volcanism was associated with the separation of the Seychelles microcontinent from India (Figure 2, inset), and this breakup itself is often ascribed to the Réunion plume head impact. I propose instead that the breakup occurred because of prolonged continental extension and an eventual ridge jump.
One interesting question is whether eclogite, a mantle rock more fusible than peridotite, could have been a source in part for the Deccan lavas. The rifted western continental margin of India follows the NNW-SSE Dharwar structural trend of the Precambrian southern Indian shield (e.g., Biswas, 1987). Also, the Narmada zone that crosses India has been proposed as an ancient suture between the southern (Dharwar) and northern (Aravalli) protocontinents (e.g., Naqvi et al., 1974; Naqvi & Rogers, 1987; Radhakrishna, 1989). Such an ancient suture may have contained trapped, eclogitized oceanic crust. Foulger et al. (2005) recently proposed such a model for Icelandic volcanism, and Sheth (2005b) explored it in considerable detail for the Deccan. If eclogite constituted a major source for the Deccan, mantle fertility and not high mantle temperatures are implicated (see also Yaxley, 2000).
I have already explained why, despite all problems and anomalies, the whole Deccan province resembles so much what a plume-head/plume-plume tail is expected to create. The interplay of the Deccan rift zones is responsible for this. Several older sedimentary rift basins underlie the Deccan (Figures 2 & 3). These are the Cambay, Kachchh, Narmada-Tapi and Godavari rifts. They are not arranged radially, are much older than the Deccan, being of at least Jurassic age, and certainly were not produced by whatever produced the Deccan. The presence of several such major rifts means that lithospheric control (Anderson, 1998c), and lithospheric extension (Sheth, 2000) were important in Deccan volcanism. The nearly circular outcrop of the Deccan proper does not reflect a spherical plume head beneath, but simply results from the confluence of numerous rift zones and the continental margin in west-central India. This is a likely explanation because elsewhere, in the peripheral/outlier parts of the Deccan along individual isolated rifts, the lava outcrop is linear or localized (e.g., the Deccan outliers of the Kachchh and Rajahmundry areas, the latter being on the Godavari rift near the east coast of India). The “hotspot track” on the oceanic crust, as argued above, may be related to melting and magma focusing along a southward-propagating fracture. The available seismic data for Réunion Island support the idea that its location is related to structural heterogeneity of the underlying lithosphere (Charvis et al., 1999; de Voogd & Pontoise, 1999; Hirn, 2002).
In conclusion, a non-plume, plate-tectonic model involving continental breakup and related mantle convection and decompression melting, is suitable for the Deccan. If radial, focused flow of the upper mantle occurs (instead of vertical flow as in the plume model), a potentially unlimited volume of the mantle is available for processing.
The plume model was proposed for the Deccan more than 30 years ago, when little was known about the Deccan and about the plume mode of convection. Today we know a lot more about the Deccan, and the plume model becomes less tenable as our knowledge grows. The originators and champions of the plume model had little or no personal knowledge of the Deccan, and their broad generalizations should have been put to critical tests by regional experts on the Deccan. The tendency has been to assume a plume origin and infer the plume properties and characteristics based on whatever is required by the observations, but if volatiles, mantle fertility, continental geology, and dynamic, evolving plates are considered, one no longer needs mantle plumes (Sheth, 2005b). The new voices asking for new explanations (e.g., are a good sign. The plume model for the Deccan has been around for over thirty years, and has failed. A new, scientifically tenable non-plume model for the Deccan could be rapidly developed if only a few of the many Deccan/flood basalt enthusiasts share the task. We live in interesting times.

Why the Deccan Traps are Important!

Map of India with Deccan Traps LabeledThe timing of when the bulk of the eruptions occurred, 65 million years ago, is interesting because this is at the same time as what is known as the K-T boundary. Found in the rock at the K-T boundary is the presence of enriched iridium, an element rare in Earth's crust but abundant in meteorites. An impact event now identified at what is named Chicxulub Crater is widely believed to have caused the extinction of the dinosaurs. There is strong evidence though that this could not have been the only factor though. So scientists are now looking at the Deccan Traps as a contributing factor perhaps in the extinction of the dinosaurs.
The impact of the meteorite at Chicxulub Crater would have caused massive damage around the Earth and triggered an impact winter. The Deccan Traps would have contributed a further global 2°C drop in temperature and the realease of massive amounts of sulphuric gasses into the atmosphere. There is data supporting a killing off of foraminifera with the main eruptions at the Deccan Traps.

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The Deccan Volcanic Province:
Thoughts about its genesis
S. Rajan, Anju Tiwary & Dhananjai Pandey
National Centre for Antarctic & Ocean Research,
Headland Sada, Vasco-da-Gama, Goa-403 804, India,
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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:
  • thinning of the continental crust along the western coast. This implies delamination of the lithosphere beneath the west coast,
  • rifting of the coast in a horst-and-graben pattern, and
  • shear displacement of the West Coast fault.
Deep Seismic Sounding (DSS) investigations have been carried out in the Indian peninsula (Reddy et al., 1999). This study indicates that the crust seems to become thinner (24 km) towards the northern parts of the west coast, that is north of 15°N. The west coast was also characterized by upwarp of the Moho during the late Cretaceous period.
Vertical crustal movements are recognized along the West Coast rift, and shear displacement along the Narmada-Tapi rift zone and its extension into the offshore areas (Biswas, 1982).
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:
  1. higher rate of adiabatic decompression due to continental scale rifting,
  2. consequently, higher rate of melting of magma in the chamber,
  3. further, continental delamination of the western continental crust due to elevation in temperature and decrease in viscosity caused by the presence of a heat source in the form of a magma chamber at its base, and
  4. a direct plumbing system between the melt and the surface, during eruption.
Considering that the West Coast and Narmada-Tapi rift zones were reactivated at the time of Deccan volcanism and the Cambay and Kutch rifts were also available as direct conduits for the upward movement of melt, we infer that the presence of these four rifts and geophysical evidence of lithosphere thinning beneath the westen coast explains the size, volume and eruption rate of the Deccan volcanics.
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.
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:
  1. The magma chamber accumulated magma for almost 350 My, between 460 Ma and 93 Ma, during which time magma was added to the chamber and crystallized, and
  2. Delamination of the Indian continental crust occurred above the magma chamber.
We envisage the magma chamber process to be as follows. A huge magma chamber progressively accumulates melt, first from the underlying mantle and eventually from both mantle and continental crust (by delamination). The molten material solidifies with time and remains in the chamber during the period 460-93 Ma. When continental breakup of greater India starts (including Madagascar rifting away) the solified magma begins to melt. Since the melting point of felsic components is lowest, these are melted first and rhyolites typically comprise the first phase of volcanics. Alkaline magma is not initially formed because it has a higher melting point.
Following Deccan volcanism, the Carlsberg ridge formed and the Indian plate continued to move north. The systematic time progression of volcanism between the Carlsberg ridge and the Indian plate is due to sea floor spreading and not to plate movement above hotspot.
When the Indian plate collided with the Eurasian plate, its velocity decreased considerably and intraplate forces started to play a lead role. The main forces were then an EW force along the Narmada-Tapi rift and a NS force along the West Coast rift. The combined affect of these forces led to rifting and shearing along the West Coast rift and resulted in subsidence, pull-apart and en echelon deformation along the Laccadive ridge.
To reiterate, the proposition that ~ 106 km3 of Deccan basalts was erupted from a magma chamber in which it was stored is radical. However, there are several strong points that support this suggestion:
  1. The long history of magmatism along the same zone which culminated in the eruption of the Deccan Traps,
  2. A lull in the magmatism for 350 My and its subsequent reactivation,
  3. DSS results which show that the Moho upwarped beneath the west coast during the late Cretaceous. This could be associated with formation of the magma chamber,
  4. Continental scale rifting (Seychelles-India) and Deccan volcanism are contemporaneous. The timing of the major phase of Deccan volcanism is considered to be ~ 66-62 Ma (Courtillot et al., 1986; Venkatesan et al., 1993) and the timing of Seychelles-India breakup is also thought to be late Cretaceous (Biswas, 1982). Note that the breakup of Gondwanaland was a continuous process which started (in the case of greater India) with the separation of Madagascar (at ~ 93 Ma) and by 63 Ma, Seychelles also separated from mainland India (Biswas, 1982; Gombos et al., 1995). The breakup of Seychelles-India is thought to be the main rifting phase associated with Deccan flood basalt eruption and the available data suggest that they were contemporaneous. This age data provides strong evidence in favor of stored magma which underwent progressive melting, in proportion to advancement in rifting (and consequent lowering of liquidus of the magma in the chamber), and resulted in volcanism whose rate and volume corresponded to the rate and scale of rifting.
  5. The time of reactivation of the deep crustal continental rifts coincided with breakup events elsewhere in greater India,
  6. The volcanism in the second phase started out felsic and gradually became mafic over a 30-My period, which points toward progressive lowering of the liquidus, and not a sudden increase in temperature, and
  7. Geophysical evidence points towards thinning of the continental crust north of 15°N, that is exactly beneath the Deccan volcanic province. This provides additional support for the existence of delaminated continental crust there.
We offer these ideas for further work and discussion.
The authors appreciate the continuous encouragement, valuable discussions and critical comments of Dr. Harsh K. Gupta, Secretary, Department of Ocean Development, Government of India and Dr. P.C. Pandey, Director, NCAOR. The authors are thankful to Prof. John J. Mahoney who very kindly provided his critical comments which helped us to improve the manuscript. We were inspired to work on the Deccan by the thought-provoking arguments and iconoclastic views of Dr. H. C. Sheth. We thank our colleagues at NCAOR for their remarkable contribution all through the brainstorming sessions and discussions of the Deccan.

Dinosaur Deaths Outsourced to India?

Boulder, CO, USA - A series of monumental volcanic eruptions in India may have killed the dinosaurs 65 million years ago, not a meteor impact in the Gulf of Mexico. The eruptions, which created the gigantic Deccan Traps lava beds of India, are now the prime suspect in the most famous and persistent paleontological murder mystery, say scientists who have conducted a slew of new investigations honing down eruption timing.
"It's the first time we can directly link the main phase of the Deccan Traps to the mass extinction," said Princeton University paleontologist Gerta Keller. The main phase of the Deccan eruptions spewed 80 percent of the lava which spread out for hundreds of miles. It is calculated to have released ten times more climate altering gases into the atmosphere than the nearly concurrent Chicxulub meteor impact, according to volcanologist Vincent Courtillot from the Physique du Globe de Paris.
Keller's crucial link between the eruption and the mass extinction comes in the form of microscopic marine fossils that are known to have evolved immediately after the mysterious mass extinction event. The same telltale fossilized planktonic foraminifera were found at Rajahmundry near the Bay of Bengal, about 1000 kilometers from the center of the Deccan Traps near Mumbai. At Rajahmundry there are two lava "traps" containing four layers of lava each. Between the traps are about nine meters of marine sediments. Those sediments just above the lower trap, which was the mammoth main phase, contain the incriminating microfossils.
Keller and her collaborator Thierry Adatte from the University of Neuchatel, Switzerland, are scheduled to present the new findings on Tuesday, 30 October, at the annual meeting of the Geological Society of America in Denver. They will also display a poster on the matter at the meeting on Wednesday, 31 October.
Previous work had first narrowed the Deccan eruption timing to within 800,000 years of the extinction event using paleomagnetic signatures of Earth's changing magnetic field frozen in minerals that crystallized from the cooling lava. Then radiometric dating of argon and potassium isotopes in minerals narrowed the age to within 300,000 years of the 65-million-year-old Cretaceous-Tertiary (a.k.a. Cretaceous-Paleogene) boundary, sometimes called the K-T boundary.
The microfossils are far more specific, however, because they demonstrate directly that the biggest phase of the eruption ended right when the aftermath of the mass extinction event began. That sort of clear-cut timing has been a lot tougher to pin down with Chicxulub-related sediments, which predate the mass extinction.
"Our results are consistent and mutually supportive with a number of new studies, including Chenet, Courtillot and others (in press) and Jay and Widdowson (in press), that reveal a very short time for the main Deccan eruptions at or near the K-T boundary and the massive carbon dioxide and sulfur dioxide output of each major eruption that dwarfs the output of Chicxulub," explained Keller. "Our K-T age control combined with these results strongly points to Deccan volcanism as the likely leading contender in the K-T mass extinction." Keller's study was funded by the National Science Foundation.
The Deccan Traps also provide an answer to a question on which Chicxulub was silent: Why did it take about 300,000 years for marine species to recover from the extinction event? The solution is in the upper, later Deccan Traps eruptions.
"It's been an enigma," Keller said. "The very last one was Early Danian, 280,000 years after the mass extinction, which coincides with the delayed recovery."
Keller and her colleagues are planning to explore the onset of the main phase of Deccan volcanism, that is, the rocks directly beneath the main phase lavas at Rajahmundry. That will require drilling into the Rajahmundry Traps, a project now slated for December-January 2007/2008.


Main Deccan Volcanism Phase Ends at K-T Mass Extinction: Evidence from the Krishna-Godavari Basin, SE India
Colorado Convention Center Room 506
Tuesday, October 30, 11:00 a.m. - 11:15 a.m.
[ view abstract ]
Paleoenvironment After Main Deccan Volcanism Ended at K-T Mass Extinction: Evidence From The Krishna-Godavari Basin, SE India.
Colorado Convention Center Room 407
Tuesday, October 30, 11:15 a.m. - 11:30 a.m.
[ view abstract ]
Age and Paleoenvironment of Deccan Volcanism and the K-T Mass Extinction
Colorado Convention Center Exhibit Hall E/F
Wednesday, October 31, 8:00 a.m. - 12:00 p.m.
[ view abstract ]


Gerta Keller
Professor, Dept. of Geosciences
Princeton University, Guyot Hall, Princeton, NJ 08544, USA
Telephone: 609-258-4117
Thierry Adatte
Professor, Geological Institute
University of Neuchatel, Neuchatel, CH-2007, Switzerland.
Telephone: 41 32 726-2617
Cell phone: 41 79 371-2715
Sunil Bajpai
Professor, Department of Earth Sciences
Indian Institute of Technology, Roorkee 247 667, Uttarakhand, India
For information and assistance during the GSA Annual Meeting, 27-31 October, contact Ann Cairns in the onsite newsroom, Colorado Convention Center Room 604, +1-303-228-8486,


1) Regarding the dating of 80 percent of the Deccan Traps to within 300,000 years of the K-T boundary and the greenhouse gas releases
Anne-Lise Chenet
Cambridge University, UK
Vincent Courtillot
Director, Institut de Physique du Globe de Paris
Université Paris 7, et Institut Universitaire de France, Paris.
Telephone: 0033(0)14427-3908
Frederic Fluteau
Professor, Institut de Physique du Globe de Paris
Université Paris 7, et Institut Universitaire de France, Paris.

History Carved Out of the Deccan Traps

Author Rasoul Sorkhabi, Ph.D.
Ancient cave temples carved out of the Deccan basalts are some of the best places to view both the world-renowned Deccan Traps and the Indian mythology narrated on these rocks.

This statue of Shiva depicts four faces representing Mahadeva (the calm “great lord,” central figure), Aghora (the frightful or destructive aspect of Shiva, on the left), Uma (the beautiful feminine aspect, on the right), and Nandin (the sacred bull as the mouth or doorkeeper of Shiva, not visible). This sculpture is in Cave No. 1 on Elephanta Island. Photo: Rasoul Sorkhabi

The Deccan Traps, one of the Earth’s largest igneous provinces, cover over 500,000 km2 of west-central India. Erupted about 66 million years during the extinction of the dinosaurs, these flood basalts, in cooperation with the sea, rains and rivers, have shaped the landscape of west-central India. Ancient cave temples have been carved out of the Deccan basalts in many places and the Elephanta Caves located on a small island offshore Mumbai (Bombay) is one such place.

Flood Basalts in Central India

The triangular peninsula of India is largely a Precambrian shield, with a central flat area, the so-called Deccan Plateau, surrounded by the mountain ranges of the Eastern and Western Ghats. The name Deccan is derived from the Sanskrit word ‘dâkshin’, meaning “south.” The west-central parts of the Indian peninsula are dominated by flood basalts which form a prominent terraced landscape; this form of flood basalt is called ‘trap’, after the Dutch-Swedish word ‘trappa’, meaning ‘stairs’.

A large number of geochronological data have been reported from the Deccan Traps over the past four decades, and the data cluster between 69 and 63 Ma (corresponding to the magnetic polarity epochs of 31 Reverse and 28 Normal) suggests that the main phase of eruption was at 66.9 ±0.2 Ma, shortly before the Cretaceous-Tertiary (K-T) boundary at 65.5 ±0.2 Ma. This age range is also consistent with paleontological data from the interbedded sediments. Aside from terraces, the Deccan basalts also form numerous dikes, some of which represent the youngest phase of the volcanic activity. While some scientists support a several million year duration, others have argued that the eruption occurred within a million years at the K-T boundary.

The original extent of the Deccan Traps has been estimated as 1.5 million km3, but the latter is highly imprecise as erosion on land and undersea subsidence on the western Indian margin have altered the rock volume accessible to us. The Deccan Traps are thickest on the Western Ghat Range (over 2,000 km thick) or in fault-bounded grabens in west-central India, but become thinner (less than 100 m) close to the margin of the trap province. Over 95% of these lavas are tholeiitic basalts (tholeiite, named after Tholey, Germany is a type of basalt rich in silica). Mantle xenoliths in the Deccan Traps have been reported from a few places.

Most scientists believe that the Deccan Traps poured out as the Indian plate, on its northward journey after the Gondwana breakup, passed over the Reunion hotspot, a still active volcanic island located in the south-west Indian Ocean. Coeval with (or probably as a result of) this event, there was also a continental rift-drift between India and the Seychelles Islands. Indeed, flood basalts of similar age also occur on the Seychelles. (For Seychelles see the article “An Oil Prone Frontier Basin,” GEO ExPro, Vol. 4, No. 3). The occurrence of petroleum reservoirs below the Deccan Traps remains unexplored.

Distribution of the Deccan Traps in India and their linkage in space and through time to the Reunion Hotspot. Inset: A simple paleotectonic sketch map showing the outpouring of the Deccan basalts at 66 Ma (K-T boundary) related to the impingement of Reunion plume beneath the Indian continental plate, and subsequent rifting between Seychelles and India. Image: Rasoul Sorkhabi

A view of two of the caves on Elephanta Island. The Deccan basalts are prominently seen in the photo. Photo: Rasoul Sorkhabi

Cave Temples in Deccan Traps

One can see exposures of the Deccan Traps in the Indian states of Gujarat, Madhya Pradesh, and Maharashtra in India, but vegetation, soil cover, and land development often mask these rocks. Cliffs of lavas on the Western Ghats and hill caves in Maharashtra perhaps provide the best outcrops to examine these formations. The hill caves are particularly important as many of these are also ancient Hindu or Buddhist temples, centuries old and portraying the Indian myths on rocks.

Some of the best known Deccan Trap caves are close to Mumbai (Bombay), including Ajanta (perhaps the oldest one dating back to 200 B.C.), Mandapesvara Caves, Kanheri Caves, Jogeshwari Caves, Mahakali Caves, and of course, the Elephanta Caves, which are our subject here.

Cave No. 1 or the Great Cave is the largest and most celebrated of all the Elephanta caves. This cave temple (restored in the 1970s) contains many statutes and sculptures of Lord Shiva and his life stories in Hindu mythology. Photo: Rasoul Sorkhabi

Elephanta Island

The Elephanta Caves are located on Elephanta Island, offshore Mumbai, precisely 11 km north-west of Apollo Bunder near the Gateway of India, where numerous ferries take visitors to the island daily. The entire island, about 2.5 km long and 7 km in circumference, is made up of the Deccan basalts, covered with trees and bushes. Three villages on the island house a few thousand people engaged in farming, fishing, and tourism.

Through centuries, the island has come under the rule of various Indian dynasties. In 1534, the Portuguese occupied it. In 1661, when Charles II of England married Catherine of Braganza, daughter of King John IV of Portugal, Elephanta Island was given to the British royal court as a marriage dowry, thus beginning British control of the island until 1947, when India gained independence.

The native name for the island is “Gharapuri” – the “town of Ghari priests (those priests belonging to the Shudra or laborer and artisan class, and devoted to Lord Shiva). But the Portuguese called it Fontis (Elephanta) after a huge elephant statute that once stood on the island.

There are seven temple caves. The first five, on the western part of the island, are Hindu temples dedicated to Shiva, a deity which along with Brahma (‘creator’) and Vishnu (‘preserver’) forms the supreme Hindu pantheon. Shiva - literally the ‘Auspicious One’- is often translated as the ‘lord of destruction’ but as one observes his various sculptures in Elephanta Caves he plays a far more varied role in Hindu mythology. The rock architecture of these Hindu caves has been dated between the 5th and 8th centuries.

The other two caves are Buddhist temples dating back to the 3rd century or even older and are not open to visitors. The Buddhist Stupa on the eastern part of the island is the highest point of the island; it is called the Stupa Hill and is about 173m in elevation.

The Elephanta Caves were originally colour-painted but today only traces remain on the bare rock. Much damage has been done to the caves through centuries of weathering but also by the Portuguese soldiers who fired shots into the caves (to test the echo of their big guns), thus breaking some sculptures and pillars. In 1909, the Elephanta Caves came under the authority of the Archaeological Survey of India, and in 1987 UNESCO included it in the World Heritage list.

A trip to Mumbai is not complete without a visit to the amazing Elephanta Islands, where a portion of India’s ancient history and mythology are preserved and displayed by the Deccan basalts – a fine sight, especially for geologists.

The stepped nature of the layered basalts of the Deccan Traps is clearly seen inland at Matheran, 90 km from Mumbai Photo: Nichalp, via Wikimedia Commons

Deccan plume, lithosphere rifting, and volcanism in Kutch, India

  • a Florida International University, Miami, FL 33199, USA
  • b Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA
  • c Presidency College, Kolkata, India


Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.


  • Deccan Traps;
  • mantle xenoliths;
  • plume;
  • volcanism;
  • rifting;
  • lithosphere;
  • Kutch

Figures and tables from this article:
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Fig. 1. 
Simplified geological map of Kutch (bottom; Biswas 2005). Our area of study is around Bhuj. There are several major roughly E–W faults (KHF: Kutch Highland fault, KMF — Kutch Mainland fault, NPF — Nagar Parker fault, IPF — Island Belt fault) that slice up the geology. The xenolith-bearing alkalic bodies occur as plugs inside shallow marine Mesozoic sediments at Bhuj and along a WNW trending belt south of the KMF. The inset shows the location of Bhuj in India.
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Fig. 2. 
(a) Composition of olivines in Kutch wehrlite and lherzolite xenoliths are compared. Filled circles — this study; and unfilled circles — Krishnamurthy et al. (1989). Also shown is a calculated equilibrium melting residue trend from a hypothetical source (circle with cross). The field for olivine phenocrysts in Deccan picrites from the northwest (Source: Krishnamurthy et al., 2000) is shown for comparison purpose. This plot suggests that primitive Deccan picrites, considered by many to be the parental magma to the tholeiites, may have been derived by about 10% partial melting of lherzolite. (b) Clinopyroxenes in the wehrlite and lherzolite xenoliths from Kutch are compared (our data). Increased partial melting should result in strong depletion in Na2O with increasing Mg# (i.e., Mg/Mg + Fe) in the residual clinopyroxenes. The lack of such correlation in Kutch cinopyroxenes suggests that these are not simple products of partial fusion, and metasomatic enrichment in Na2O is offered as an explanation. See text for further discussion.
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Fig. 3. 
Primitive mantle (McDonough and Sun, 1995) normalized trace element patterns in Kutch volcanics (our data). The alkalic rocks show prominent negative Pb anomaly whereas tholeiites clearly show a positive Pb spike that is similar to the basalts from Saurashtra area, which is further south of our study area. The Saurashtra data are from Melluso et al. (1995).
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Fig. 4. 
(a). Initial Nd–Sr isotope compositions of Kutch basalts are compared with Deccan basalts from northwestern India and some select formations (Ambenali, Mahabaleshwar, Thakurvadi) from the Western Ghats (11, 30, 49 and 51). Reunion hot spot generated lavas and Central Indian Ocean Ridge basalts data are also shown for comparison (source of data: GEOROC). Initial ɛSr is calculated using present day bulk earth 87Sr/86Sr = 0.7047 and 87Rb/86Sr = 0.08168, and ɛNd using chondritic earth with 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967. (b). Pb vs Sr isotope plot for the Kutch volcanics compared with other basalts. (c). Initial ɛHfɛNd diagram comparing the Kutch volcanics with Indian MORB, Reunion and Mauritius lavas, and Group 1 kimberlites. Kutch tholeiites plot over a wide area that covers much of the Indian MORB field and Deccan basalts from other areas (not shown). Two alkalic basalts plot inside the kimberlite field and perhaps represent very small degrees of melts from an enriched sublithospheric source. Other alkalic basalts plot between Indian MORB and Reunion lavas, suggesting that they were derived from a mixed source of depleted asthenosphere and the Reunion plume. Data sources: Reunion: Bosch et al. (2008); Indian MORB: Meyzen et al. (2007) and the GEOROC database; Mauritius: Paul et al. (2005); G1 kimberlites: Nowell et al. (2004).
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Fig. 5. 
Schematic block diagram showing inferred geological relationships between the main structural elements of Kutch rift zone and Deccan volcanism (the structural elements are mostly based on 4 and 5). The subsurface Deccan Trap ridge at the center of the diagram is based on gravity data interpretations (Chandrasekhar and Mishra, 2002). We consider the lithosphere to be about 90–100 km thick on either side of this paleo-rift zone, and under such conditions it should have a garnet peridotite lower layer. Such a layer is missing in beneath the rift zone as evident from the absence of garnet peridotite xenoliths in the alkalic basalts. Therefore, we suggest that the rift was already extended and thinned during prior Late Triassic–Jurassic rifting. Isotope data suggest that the alkalic basalts were produced from a mixed asthenospheric (Indian MORB-like) and plume (Reunion-like) source (this is shown as black “blobs”).
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Fig. 6. 
A geodynamic model of magma generation in Kutch is presented based on peridotite-CO2 melting relations (Presnall and Gudfinnsson, in press). In all three figures the bottom diagram shows a schematic geological cross-section (north is approximately to the right and south is to the left), and the top shows magma production in pressure–temperature phase diagrams. In all three figures volatile free (gray) and CO2-saturated (black) lherzolite solidi are shown as solid lines and geothermal gradient is shown as a dashed curve. The position of the geotherm changes in response to rifting and later on due to arrival of deeper, hotter, Reunion-like bodies. Magma generation is shown in three stages (I, II, and III) as initial rifting (stage I), arrival and melting of CO2-rich peridotitic blobs (stage II, alkalic melt production), and generation of tholeiitic picrites from the main plume head (stage III). Lithospheric thinning due to rifting causes CO2-bearing asthenosphere to rise and cross the volatile bearing peridotite solidus, generating carbonatitic melt. As these melts rise through the lithosphere they freeze, releasing CO2-rich vapor. These melts and associated vapor metasomatizes the lithosphere, converting spinel lherzolite wall rock to spinel wehrlite along veins used by such fluids. Stage II shows the arrival of Reunion-like bodies that break off the leading edge of main Deccan plume head and begin to melt once their solidus is crossed around ∼ 75–90 km. In this stage the geotherm rises higher and produces alkalic magmas, which ascend along pathways created by deep lithospheric rift faults and erupt to form small bodies distributed along the strike of major east–west fault systems. In the lowermost diagram the plume head is shown as thermally zoned with a hot core that produces tholeiitic magmas and a cooler rim, which feeds the alkalic magmas. The thick arrow shows direction of plate movement. Stage III represents production of picritic tholeiite magmas from the hotter core of the plume head as the volatile-free solidus is crossed by the hot plume geotherm. It is speculated that the tholeiite magmas were not generated at Kutch but the lavas arrived from elsewhere further south.
Table 1. Trace elements in Kutch volcanics
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Table 2. Isotopic composition of Kutch volcanic rocks
Strontium, Nd and Pb isotope compositions were determined on a Finnigan MAT 262 TIMS at FSU. Sr isotope ratios were corrected for fractionation using 86Sr/88Sr = 0.1194 and are reported against the measured value of the E&A standard: 87Sr/86Sr = 0.708000 ± 14 (2SD, n = 11). Nd isotope ratios are corrected for fractionation using 146Nd/144Nd = 0.7219, and are reported against the measured value of the La Jolla standard: 143Nd/144Nd = 0.511846 ± 11 (2SD, n = 8). The NBS-981 Pb standard was measured at 206Pb/204Pb = 16.90 ± 0.02, 207Pb/204Pb = 15.45 ± 0.02, 208Pb/204Pb = 36.60 ± 0.04 (n = 18) and the reported Pb isotope ratios are corrected for fractionation relative to the NBS-981 values reported by Todt et al. (1996). The JMC 475 Hf standard was measured at 176Hf/177Hf = 0.282185 ± 19 (2SD, n = 11) and the Hf isotope compositions are reported relative to the widely accepted JMC value of 176Hf/177Hf = 0.282160. Initial (in.) isotope ratios, and ɛNd and ɛHf values are calculated at 65 Ma, using the Rb/Sr, Sm/Nd and Lu/Hf ratios from the trace element data, and present day values for CHUR: 143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967, 176Hf/177Hf = 0.282772, 176Lu/177Hf = 0.0332.
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