Geological Society of AmericaSpecial Paper 388

2005

From Deccan to Réunion: No trace of a mantle plume

Hetu C. ShethDepartment of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai, Mumbai 400 076 India

ABSTRACT

The widely accepted mantle plume model postulates that (1) the currently vol-canically active Réunion Island in the Indian Ocean is fed by the narrow “tail” of amantle plume that rises from the core-mantle boundary, (2) the Deccan continentalflood basalt province of India originated from the “head” of the same plume duringits early eruptive phase near the end of the Cretaceous, and (3) the Lakshadweep-Chagos Ridge, an important linear volcanic ridge in the Indian Ocean, is a product ofthe plume. It is not generally appreciated, however, that this “classic” case of a plumecontradicts the plume model in many ways. For example, there is little petrologicalevidence as yet that the Deccan source was “abnormally hot,” and the short (~1.0–0.5 m.y.) duration claimed by some for the eruption of the Deccan is in conflict withrecent Ar-Ar age data that suggest that the total duration was at least ~8 m.y. The Dec-can continental flood basalts (CFB) were associated with the break-off of the Sey-chelles microcontinent from India. Geological and geophysical data from the Deccanprovide no support for the plume model and arguably undermine it altogether. Theinterplay of several intersecting continental rift zones in India is apparently responsi-ble for the roughly circular outcrop of the Deccan. The Lakshadweep-Chagos Ridgeand the islands of Mauritius and Réunion are located along fracture zones, and theapparent systematic age progression along the ridge may be a result of southwardcrack propagation through the oceanic lithosphere. This idea avoids the problem of a10o paleolatitude discrepancy which the plume model can solve only with the ad hocinclusion of mantle roll. Published Ar-Ar age data for the Lakshadweep-Chagos Ridgebasalts have been seriously questioned, and geochemical data suggest that they likelyrepresent postshield volcanism and so are unsuitable for hotspot-based plate recon-structions. “Enriched” isotopic ratios, such as values of 87Sr/86Sr higher than those fornormal mid-ocean ridge basalts, which have been observed in basalts of the ridge andthe Mascarene Islands, may mark the involvement of delaminated enriched continen-tal mantle instead of a plume. High values of 3He/4He also do not represent a deepmantle component or plume. The three Mascarene islands (Mauritius, Réunion, andRodrigues) are not related to the Deccan but reflect the recent (post-10 Ma) tectonic-magmatic development of the Africa Plate. I relate CFB volcanism to continental rift-ing, which often (but not always) evolves into full-fledged seafloor spreading. I ascribethe rifting itself not to mantle plume heads but to large-scale plate dynamics them-selves, possibly aided by long-term thermal insulation beneath a supercontinent thatmay have surface effects similar to those predicted for “plume incubation” models.

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*E-mail: jcsheth@iitb/ac/in.

Sheth, H.C., 2005, From Deccan to Réunion: No trace of a mantle plume, in Foulger, G.R., Natland, J.H., Presnall, D.C., and Anderson, D.L., eds., Plates, plumes,and paradigms: Geological Society of America Special Paper 388, p. 477–501. For permission to copy, contact editing@geosociety.org. © 2005 Geological Societyof America.

Nonplume plate tectonic models are capable of explaining the Deccan in all its great-ness, and there is no trace of a mantle plume in this vast region.

Keywords: PLEASE SUPPLY FIVE OR SIX KEYWORDS.

478 H.C. Sheth

INTRODUCTION

Many continental flood basalt (CFB) provinces of the worldformed during the rifting and breakup of continents (see e.g.,Storey et al., 1992 and references therein). An excellent exam-ple is the ca. 65–60 Ma Deccan province of India (Fig. 1). Itis one of the larger and better-preserved CFB provinces of theworld, with a present-day areal extent of ~5 × 105 km2 and anestimated original area of at least 1.5 × 106 km2 (e.g., Wadia,1975). The Deccan CFB formation was associated with conti-nental rifting and the break-off of the Seychelles microcontinentfrom India (e.g., Norton and Sclater, 1979; Mahoney, 1988;Devey and Stephens, 1991; Fig. 1). Greater India (India plus the

Seychelles) was involved in two other continental breakup eventsduring the Mesozoic prior to the Deccan episode, both of whichwere also associated with major flood basalt volcanism. Thus,Greater India broke off from Madagascar at ca. 88–85 Ma, andthis was associated with the formation of the Indo-Madagascarflood basalt province. This province, though considerably erodedtoday, is represented by extensive lavas and dike swarms inMadagascar, the submarine Madagascar Plateau south of Mada-gascar, and some volcanics and dike swarms in southern India(e.g., Storey et al., 1995, 1997; Anil Kumar et al., 2001; Pandeet al., 2001; Fig. 1). Prior to this event, at ca. 120–116 Ma,the Rajmahal-Sylhet flood basalt province formed in eastern-northeastern India (Baksi, 1995) as part of the early Cretaceous

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Figure 1. Map showing the approximateboundaries of the Precambrian cratonsmaking up the Indian basement crust(e.g., Naqvi and Rogers, 1987; Pandeyand Agrawal, 1999), the granulite ter-rain (stippled), the Precambrian structuraltrends (heavy broken lines), the riftzones crossing peninsular India (e.g.,Biswas, 1982), and the present outcropareas of the Deccan and Rajmahal floodbasalts (shaded). The Saint Mary’s is-lands felsic volcanics and some of themany Karnataka-Kerala dike swarms arerelated to the India-Madagascar breakup(e.g., Anil Kumar et al., 2001; Pandeet al., 2001); there are many Precambriandike swarms throughout southern Indiaas well. The inset shows the breakup ofthe Seychelles microcontinent (black)from India at ca. 65 Ma (after Norton andSclater, 1979; Mahoney, 1988).

Rajmahal-Kerguelen Province, and its formation succeeded bya few million years the break-off of India from Australia (e.g.,Kent et al., 1997).

During the last decade, the mantle plume model for floodbasalt volcanism (Richards et al., 1989; Campbell and Griffiths,1990) has been widely accepted (e.g., Hooper, 1990; Hill, 1991;Kent et al., 1992). However, observations anomalous with re-spect to the predictions of the model are increasingly coming tolight from the flood basalt provinces of the world. For example,regional prevolcanic lithospheric uplift of 1–4 km is a prerequi-site in all thermal models, such as the plume model (e.g., Camp-bell and Griffiths, 1990; Farnetani and Richards, 1994), but suchuplift was absent in the largest CFB, the Siberian Traps (Cza-manske et al., 1998), and absent or far less than expected in theworld’s largest oceanic plateau, the Ontong-Java Plateau (Tejadaet al., 2002, 2004). The very existence of mantle plumes hasbeen questioned (e.g., Anderson et al., 1992; Smith, 1993; An-derson, 1994, 1999; Sheth, 1999a,b; Smith and Lewis, 1999).Numerous nonplume mechanisms and explanations have beenforthcoming, including volatile-rich mantle sources (e.g., Bon-atti, 1990; Smith, 1993; Janney et al., 2000), fertile (eclogite-rich)mantle sources (e.g., Foulger et al., 2005 and this volume), crack-related volcanism (e.g., Smith, 2003 and this volume; Natland andWinterer, this volume), top-down lithospheric control and edge-driven convection (EDC; e.g., Mutter et al., 1988; King andAnderson, 1995, 1998; Anderson, 1998a), and asteroid impacts(e.g., Jones et al., 2002). Besides, with new data, especiallygeophysical data, many regions traditionally considered classicplume areas are now attributed by several workers to nonplumeplate tectonic processes. Two prominent examples of such areIceland (e.g., Foulger et al., 2000 and this volume; Lundin andDoré, this volume) and Yellowstone (e.g., Humphreys et al.,2000; Christiansen et al., 2002). Iceland is the world’s classicridge-centered “hotspot,” and Yellowstone is generally consid-ered the type example of a continental “hotspot.”

The Deccan Traps of India constitute one of the world’sbest-developed and best-known CFB provinces, and manyworkers have invoked or supported a plume origin for the Dec-can. As first put forth by Morgan (1972, 1981), this model pos-tulates that (1) the Deccan originated from the ancestral Réunionhotspot, which upwelled beneath India in the late Cretaceous,and (2) the hotspot, now located on the Africa Plate, is fed by adeep mantle plume. Fluid dynamic modeling (e.g., Campbelland Griffiths, 1990) postulated that plumes rise buoyantly fromthe core-mantle boundary (CMB) and, by entrainment of sur-rounding mantle, develop large bulbous “heads” that remainconnected to the source region by narrow “tails.” In the plumemodel, the break-off of the Seychelles from India and the as-sociated Deccan volcanism were both consequences of the im-pingement of the mantle plume on the Indian lithosphere (e.g.,Hooper, 1990; Courtillot et al., 1999). In the present contribu-tion, I extend my previous arguments (Sheth, 1999a,b, 2000,2005) that a wealth of geological and geophysical evidence fromthe Deccan CFB province, and from the subsequent hypothe-

sized products of the Réunion hotspot, is at odds with the pre-dictions of the plume model. I begin by briefly stating what Imean by a plume.

WHAT IS A PLUME?

Morgan (1972) proposed plumes that were fixed in theEarth so that they could be used as reference frames to recordplate motions, and because of the perceived fixity of hotspots,the deep plume explanation was considered superior to otherexplanations for island-seamount chains, such as crack-relatedvolcanism (see Anderson and Natland, this volume). Now it isthought, however, that plumes sway in the mantle wind andneed not be fixed (Steinberger and O’Connell, 1998). Manythink of “hotspots”—locations of intraplate volcanism—as ther-mal plumes, but most hotspots are thought to be no hotter thanaverage (Stein and Stein, 2003; DeLaughter et al., this volume;Green and Falloon, this volume). Several classic “plumes” showno evidence for an initial head phase (e.g., Hawaii) or a tailphase (e.g., Siberia, Ontong-Java), and several plume propo-nents now suggest that the classic model is too restrictive andthat not all plumes need have heads or tails, and not all plumesneed be deep-sourced (e.g., Cserepes and Yuen, 2000). Taillessupper-mantle “plumes” of the type conceived by Cserepes andYuen (2000) still cannot explain observations such as the absentor insignificant prevolcanic lithospheric uplift in Siberia orOntong-Java. Campbell and Griffiths (1990), originators of themodern model of plume heads and tails based on fluid dynamicmodeling, postulated that the inflated heads of new plumes con-tain plume source material into which surrounding cooler mid-ocean ridge basalt (MORB) mantle becomes entrained duringupwelling and that plume tails contain hot plume source mate-rial. Van Keken (1997), relying on numerical modeling, cameto the opposite conclusion, however, saying that the heads ofplumes should contain primitive plume source material, and thetail should entrain surrounding mantle.

A glossary of plume definitions and many technical articleson the subject can be found at www.mantleplumes.org. Clearly,plume means different things to different workers, but thisshould not divert us from the main issue—whether there is ev-idence for (1) “abnormally hot” mantle that is (2) actively up-welling, and (3) has its origin in the deep mantle or at the CMB.These three qualities are argued and believed to be the primecharacteristics of plumes that distinguish them from shallowermechanisms. Nevertheless, Courtillot et al. (2003) have definedthree different types of plumes: their “primary” or “Morganian”plumes come from the deepest mantle or the CMB, and a sec-ond category of plumes originates at the base of the mantle Tran-sition Zone, whereas a third “Andersonian” category includesplumes that are shallow-sourced. Courtillot et al. (2003) findthat only seven out of the forty-nine hotspots of the world sat-isfy their criteria for deep origin, and Réunion is one of them.However, as Anderson (this volume) shows, these criteria aresubjective.

From Deccan to Réunion: No trace of a mantle plume 479

The hy-phen inOntong-Java wasused ineveryother in-stance inthe vol-ume, soI’ve left ithere.

In this chapter I take plume to mean an active upwelling thatis narrow (relative to the plates), anomalously hot, and deep-sourced (originating in the lowermost mantle or at the CMB).As the modern plume model is based essentially on fluid dy-namical experiments, I also expect that a plume has a bulboushead connected to the source region by a narrow, pipe-like tail.Having defined a plume, I now examine field data from the Dec-can and India.

GEOLOGY OF THE DECCAN AND ITS BASEMENT

Continents consist of cratons amalgamated by networks oforogenic belts that contain oceanic, island-arc, and continentalmargin rocks besides local fragments of older cratons, and a truecontinent must contain long-stable cratons (Rogers, 1996). Ac-cording to Rogers (1996), a block of continental crust is a cra-ton only if it has been sufficiently stable to provide a basementfor the deposition of shallow-water or subaerial volcano sedi-mentary suites on platforms, in broad unrifted basins, or in riftvalleys. The Indian subcontinent has a rich rock record from theEarly Archean up to Recent time. At least six Archean to earlyProterozoic cratonic nuclei are recognized. These are the Ar-avalli, Bundelkhand, Singbhum, Bastar, and Dharwar cratonsand the high-grade granulite terrane in the far south (e.g., Naqviand Rogers, 1987; Rogers, 1996; Pandey and Agrawal, 1999;Fig. 1). These nuclei may have been sutured together in Archeanand Proterozoic time by proto–plate tectonic processes (e.g.,Naqvi et al., 1974; Valdiya, 1984; Radhakrishna, 1989).

Several major rift zones traverse the subcontinent (Fig. 1).The Godavari and Mahanadi Rifts lie in the east, the CambayRift in the north-northwest, and the Kachchh Rift in the north-west. The Narmada-Tapi Rift Zone, a major extensional zonewithin the Indian peninsula, runs in an ENE-WSW directionfor >1600 km along the central part of India (e.g., Mishra, 1977).This comprises the Narmada and Tapi grabens, which are sepa-rated by an upraised horst block, the Satpura range. The Indianrifts are known to run along major Precambrian tectonic trends(e.g., Katz, 1978). The Narmada zone is a prominent, ancientline of weakness and is considered a Proterozoic protocontinen-tal suture between a northern (Aravalli) and a southern (Dharwar)protocontinent (Naqvi et al., 1974). The western Indian coastand the Cambay Rift also developed by faulting parallel to theNNW-SSE Dharwar orogenic trend of Precambrian age (Raju,1968; Biswas, 1987). Another major Precambrian orogenic trend,the NE-SW Aravalli trend, splays into two trends at its south-ern end: the east-west Delhi trend (along which the MesozoicKachchh Rift has developed) and the main NE-SW Aravallitrend, which continues right across the Cambay Rift into theSaurashtra peninsula (Biswas, 1982). Two more rifts, the NW-SE-trending Koyna and Kurduvadi Rifts were postulated underthe southern Deccan region (Fig. 1) on the basis of linear grav-ity lows (Krishna Brahmam and Negi, 1973), but these gravitylows may instead represent upwarps of the sub-Deccan base-

ment. The extensions of these two hypothesized rifts to the northand the south are not clear.

Figure 2 shows the main rock formations that constituteIndian geology: a large portion of the Indian Shield is made upof Precambrian rocks, and there are many younger sedimentarybasins. The Deccan lava pile, which obscures the basement fromobservation over 0.5 million km2, is thickest (~2000 m) alongthe Western Ghats region (Sahyadri range) adjacent to the west-ern coast and thins progressively eastward and southeastward,such that along the eastern fringes of the province the lava pileis only ~200 m thick. Whereas the Deccan lava pile in the West-ern Ghats region and in the interior areas of the province is madeup almost completely of tholeiitic basalts (e.g., Beane et al.,1986), felsic and alkaline magma types are also prominent alongthe rift zones and along the west coast. Considerable volumesof acid and basic tuffs and rhyolite and trachyte lavas exist alongthe coast, as at Bombay (Lightfoot et al., 1987; Sheth et al.,2001a; Sheth and Ray, 2002). The west coast and the rift zonesare also where significant tectonic-structural disturbances suchas listric faulting and monoclinal flexing have affected the lavapile (e.g., Guha, 1995; Sheth, 1998). The coastal dikes formregional, dominantly north-south-oriented swarms and are mostlyof basalts, dolerites, and alkaline rocks such as lamprophyres(e.g., Dessai, 1987, 1994; Murthy, 1987). Significant volumes offelsic rocks and many alkaline complexes (several of which in-clude carbonatites) are found along the Narmada Rift and in thenorthwest of the Deccan proper (the Tavidar felsic volcanics andthe Mer Mundwara and Sarnu-Dandali alkaline complexes; e.g.,Basu et al., 1993, and Roy, 2003). The Narmada-Satpura-Tapizone also contains major linear dike swarms (e.g., Deshmukh andSehgal, 1988; Sant and Karanth, 1990; Keshav et al., 1998).

Drilling in the Latur area (Fig. 2), the epicenter of a disas-trous earthquake (M = 6.3) in 1993 and situated on the proposedKurduvadi Rift, directly encountered Precambrian basement (thePeninsular Gneiss of the southern Indian Shield) at 338 m depth(Gupta et al., 1998). Many believe that the pronounced linearityof the west coast and the continental margin suggests structuralcontrol (e.g., Biswas, 1987). The newly formed continental mar-gin and the rift zones may have constituted major vent areas forthe Deccan lavas, as inferred from abundant mafic dike swarmsand intrusions, high heatflow, and aligned thermal springs (e.g.,Sheth, 2000). Seismic studies and drilling for oil have shownthat the Deccan basalts continue beyond the west coast and ontothe continental shelf (Biswas, 1982; Chandrasekharam, 1985).Eruptive centers undoubtedly existed along the present-day sub-merged shelf. The Cambay Rift and the region offshore of thewest coast are regions of productive oil and gas fields. Much ofthe Cambay region is today covered by Tertiary and Quaternarysediments, and the underlying igneous rocks are not exposed.Boreholes drilled by the Oil and Natural Gas Commission havepenetrated thick (5 km) Tertiary sediments, and at places theunderlying basalts are known, based on seismic data, to be over4 km thick (Kailasam and Qureshy, 1964; Mahadevan, 1994).

480 H.C. Sheth

THE PREVOLCANIC UPLIFT ISSUE

It has been argued (e.g., White and McKenzie, 1989) that thehuge volumes erupted in CFB provinces such as the Deccan re-quire a large mass of “abnormally hot” mantle. Such argumentsare based on the assumption that the mantle is normally cold andsubsolidus and has no lateral temperature variations. However,lateral temperature variations of ~200 oC are evident in the man-tle from seismic tomography (e.g., Anderson et al., 1992) and area natural consequence of “normal” plate tectonic processes (An-derson, 2000b). Also, the plume model, which is primarily a ther-mal model, requires substantial (1–4 km) broad-scale prevolcanicuplift of the lithosphere ~5 m.y. before the onset of flood vol-canism (Campbell and Griffiths, 1990; Farnetani and Richards,1994). If such regional domal uplift was absent, a thermal mech-anism would be violated. Such uplift did not predate magmatism

in many flood basalt provinces (e.g., Menzies, 2000). Regionaldomal uplift was absent during the enormous CFB event thatformed the Siberian Traps (ca. 250 Ma) and absent or far from ad-equate during the formation of the Ontong-Java oceanic plateau,the world’s largest, which remained completely submergedduring its construction (e.g., Czamanske et al., 1998; Tejada et al.,2004). In fact, the Siberian CFB are underlain almost every-where by terrigeneous sediments of the Tungusskaya Series(320–250 Ma), which include the Tunguska coaliferous basin, theworld’s largest (Czamanske et al., 1998). Regional subsidence,not regional uplift, is thus seen (Elkins-Tanton and Hager, 2000).

Clues in the Sub-Deccan Rocks

In the case of the Deccan, evidence for regional domal up-lift is absent as well. Over most of the Deccan province today,

From Deccan to Réunion: No trace of a mantle plume 481

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the lava-basement contact is at considerable depths in the sub-surface. As noted, there is a huge thickness (~1700 m exposed)of Deccan basalts in the Western Ghats (Sahayadri) region andan additional ~500 m in the subsurface (as identified from seis-mic studies, e.g., Kaila et al., 1981b,c). The Deccan lavas over-lie two linear Mesozoic sedimentary basins in the Narmada-Tapiregion (Kaila, 1988; Sridhar and Tewari, 2001). The northern,the Narmada basin, is 1000 m thick, and the southern, the Tapibasin, is 1800 m thick. The thick Mesozoic sedimentary pileunder the Deccan (Upper Gondwana Supergroup, Triassic) inthis region is exposed in the Pachmarhi region by tectonic upliftof the Satpura range (Fig. 2).

Toward the western part of the Narmada Rift, the Deccanlavas overlie sandstones and limestones of the Bagh Formationdeposited during a marine transgression in the Late Cretaceous(Sheth, 1999b). This is where the center of the plume head andmaximum uplift should have been. The fact is that there wereboth local uplifts and subsidences just before volcanism in thisarea (Tandon, 2002). Interestingly, Maastrichtian sediments ofthe Lameta Formation (which, like the Bagh Formation, under-lies the Deccan lavas in this area) include clays derived in partfrom the Deccan basalts themselves; clearly, the Deccan lavaswould have erupted and then undergone some uplift to form thesource areas for the Lameta clays (Salil and Shrivastava, 1996,Salil et al., 1997; Tandon, 2002). Different conditions are foundin the Lametas of the Dongargaraon Basin of the Nagpur area(Fig. 2). Tandon (2002) has recorded a clear “shallowing up”trend from shallow lake deposits to a paleosol before the terrainwas buried by the first lava flow. He has related this to pre-volcanic surface uplift of the area on the order of meters only,and it is possibly also related to mock aridity (Harris and VanCouvering, 1995; Khadkikar et al., 1999). Again, local uplifts andsubsidences (e.g., Jerram and Widdowson, 2005) cannot be usedto support or refute the plume model, because they are easilyrelated to the filling and emptying of magma chambers, to em-placement of intrusions, to faulting, and to related processes.

Campbell and Griffiths (1990) cited Pachmarhi as the cen-ter of a broad, uplifted dome caused by the plume, mentioningthat the lava-basement contact at Pachmarhi is over 1 km abovesea level. They did not consider it important, or were unaware,that the contact is hundreds of meters in the subsurface overmost of the province. Pachmarhi is an isolated case. Indeed,even over large parts of the Satpura range (e.g., at Toranmal;Fig. 2) the lava-basement contact is in the subsurface. Further-more, the uplift of the Pachmarhi block appears to be a result notof prevolcanic doming but of postvolcanic, recent uplift becauseof the very youthful landscape (kilometer-high escarpments inthe Gondwana sandstones, V-shaped ravines and gorges, and tor-rential rivers). Choubey (1971) recognized successive planationsurfaces here, and the highest is at ~1300 m above sea level. AsOllier and Pain (2000) argued, such surfaces must form near thebase level of erosion of a river, which is at sea level in mostcases, and because there is no geomorphic process capable of

creating a planation surface at a high elevation, a planation sur-face at high elevations above sea level must indicate recent,rapid uplift. Dixey (1970, reprinted in Subbarao, 1999), basedon field work in this region, gave evidence that the Deccan lavaswere erupted over an old, regional, flat-lying erosion surfacedeveloped over older rocks, and noted that this surface could beput to good use in deciphering the subsequent tectonic evolutionof the region.

Casshyap and Khan (2000) provided evidence for “pre-Deccan doming” of the Indian subcontinent, again based on fieldstudies in the Pachmarhi region. They identified three separateuplift events, the latest of which resulted in Late Jurassic–earliest Cretaceous sediments with a source in northwesternIndia. Clearly, uplift centered on northwestern India and preced-ing Deccan CFB volcanism by a long time (~70 m.y.) cannot beconsidered evidence for Deccan plume-related prevolcanic uplift.Broad-scale domal, prevolcanic uplift of the Indian lithospherejust prior to Deccan volcanism has yet to be demonstrated. How-ever, the old and extensive erosion surface beneath the Deccanlavas in central India (e.g., Dixey, 1970), suggests that such up-lift did not occur.

Clues in the Indian Rivers

Many major Indian rivers originate in the Western Ghats,not far from the west coast, and yet flow for hundreds of kilo-meters eastward to eventually meet the Bay of Bengal, which isa remarkable fact. Cox (1989) speculated that the pronouncedeasterly drainage of the Indian peninsula (Fig. 3) was a conse-quence of plume-caused lithospheric doming. The Narmada andthe Tapi, two of India’s major rivers, flow westward, however,and Cox ascribed this to their exploiting a rift system in thedome. He did not address why such a rift system should producea westerly drainage (toward the topographically high center ofthe uplifted dome). Summerfield (1990) discussed problemswith Cox’s views, with examples from Africa.

Ollier and Powar (1985) observed that the drainage patternof the Indian peninsula is dendritic over both the region of theDeccan lavas and the older basement. This led them to suggestthat the drainage developed subsequent to the eruption of theDeccan lavas. The uplift of the Western Ghats must have beenstill later, noting their very youthful topography and the evi-dence that formation of several of the major east-flowing Indianrivers was antecedent. The Western Ghats (Sahyadri range)constitute a roughly north-south-trending, 1500 km–long, nearlycontinuous “Great Escarpment” that reaches heights of >2.5 kmin the Nilgiri and Palni-Kodaikanal massifs of southern India(Ollier, 1990; Gunnell and Radhakrishna, 2001; Fig. 3). Noticethat the Cauvery River originates on the western flank of the West-ern Ghats and flows east through the highest part of the plateau.This is impossible unless the formation of the river was an-tecedent to the uplift of the plateau (Ollier and Powar, 1985).Widdowson and Cox (1996) provided similar observations and

482 H.C. Sheth

arguments for the area near Mahabaleshwar. The uplift of theWestern Ghats and the associated tilting of the Indian peninsulafrom west to east are young, ongoing, and postvolcanic.

In summary, there is no evidence as yet from the Deccan forregional prevolcanic uplift that can be related to an upwellingthermal plume. Existing data offer two possibilities: (1) pre-volcanic lithospheric uplift occurred, and the presently buriedlava-basement contact over most of the province is due to rapiddecay of the thermal uplift and loading of the crust by the lavapile (Campbell and Griffiths,1990), or (2) prevolcanic litho-spheric uplift never occurred, and a thermal mechanism is in-valid. The second scenario is similar to the situation at Siberiaand Ontong-Java and is the explanation I prefer. Campbell andGriffiths (1990) argued that prevolcanic regional domal upliftdue to a plume head may not be significant due to lateral mi-gration of magma in the crust. Jerram and Widdowson (2004)argue that while an area may truly be undergoing uplift on thescale of hundreds of kilometers, areas within it may be under-going subsidence on the scale of tens of kilometers, and thusevidence for subsidence cannot be used to disprove uplift. Itseems to me that, given the observations, the plume might aswell not have been there. Fortunately, it is possible to conclude

that broad-scale prevolcanic uplift did not occur, because (1) thereis no evidence for such uplift, and (2) there is actual evidenceagainst it (such as the old erosion surface beneath the Deccanlavas in central India). Apatite fission track data can also help.Some have recently become available, but remain inconclusiveregarding the timing of Western Ghats uplift (Gunnell et al.,2003).

THE DECCAN: SOME MYTHS AND FACTS

Having considered the issue of prevolcanic uplift, I find itis instructive to consider some of the main arguments repeatedlyoffered in favor of a plume origin of the Deccan. These argu-ments have little support in the data themselves.

An Anomalously Hot Mantle Source?

There is no evidence for an “abnormally hot” mantle sourcefor the Deccan lavas. Some alkaline picritic liquids (identifiedbased on appropriate whole-rock Mg #s and olivine composi-tions) are encountered in boreholes in the northwestern Deccan(around Botad; see Fig. 2) and in parts of the Narmada Rift

From Deccan to Réunion: No trace of a mantle plume 483

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(Melluso et al., 1995; Krishnamurthy et al., 2000). Campbelland Griffiths (1990) thought the borehole lavas were high-temperature, high–melt fraction liquids from the plume axis.Peng and Mahoney (1995), however, found that they are some-what alkaline and are consistent with high-pressure, low–meltfraction melting. The thick sequence of the Deccan basalts in theWestern Ghats (Figs. 1 and 2) includes picritic basalts, but theseare enriched in cumulus olivine and clinopyroxene and do notrepresent liquid compositions. The parental melts of these pi-crites are estimated to have contained only ~9%–10% MgO(Beane and Hooper, 1988; Sheth, 2005). It is possible that thesub-Indian mantle was warmer than average during Deccanvolcanism; it could have been warm over a very broad region(thousands of kilometers) as a consequence of long-term ther-mal insulation under Gondwana.

A Very Short-Lived Catastrophic Eruption?

Very rapid, catastrophic emplacement of the Deccan Trapshas been one of the key arguments for a plume origin, but theduration of the volcanism remains one of the most debated is-sues. Recent 40Ar-39Ar data for trachyte and basalt flows fromBombay (Sheth et al., 2001a,b) suggest that the total durationwas no less than ~8–9 m.y. There may have been a major, rapid,short-duration eruptive phase in the Western Ghats, estimatedby some to have lasted only 1.0–0.5 m.y. (e.g., Courtillot et al.,1988; Duncan and Pyle, 1988; Hofmann et al., 2000) and byothers to have been more protracted, 4–5 m.y. (Venkatesanet al., 1993; Pande, 2002). Allègre et al. (1999) reported an Re-Os isochron age of 65.6 ± 0.3 Ma (2s) for basalt lava flows sam-pled over a large area of the province, arguing for intense, veryshort-lived (~0.5 m.y. or less) volcanism right at the Cretaceous-Tertiary boundary. That random, noncomagmatic samples col-lected over an area 1000 km across and occupying varioustopographic-stratigraphic levels should define an isochron isremarkable, but the goodness-of-fit value (F) for the claimedisochron, which was not reported, is 22 (Baksi, 2001a). The lineis clearly an “errorchron” (Faure, 1986), and the age uncertaintyis much greater than the 0.3 m.y. claimed by Allègre et al.(1999).

Sen (2001) calculated an eruptive duration for the thickWestern Ghats lava pile that was ten times shorter than even thatpostulated by most “rapid volcanism” proponents. Based on theestimated formation times of large plagioclase crystals in someof the Deccan flows (by comparisons to Hawaiian plagioclases)and on the derivation of a “one-dimensional eruption rate,” heproposed that the eruption of the entire Western Ghats sequencetook only ~55 k.y. Sheth (2002) analyzed this approach, arguingthat one must use a volumetric eruption rate (presently unknown)and that the one-dimensional eruption rate was a meaninglessquantity and therefore the calculated duration of 55 k.y. couldbe grossly in error.

Physical volcanology can also offer clues. Some authorshave invoked hot plume heads to explain the extremely high

lava eruption rates of CFB, though such eruption rates have yetto be demonstrated. A large proportion of the Deccan basalts ismade up of pahoehoe compound lava flows (e.g., Deshmukh,1988). If new models of protracted, gradual emplacement ofsimilar lava flows from the Columbia River Province (Self et al.,1997) are correct, most of the Deccan could have formed at loweruption rates. In fact, Bondre et al. (2004) find that the scaleof the individual lava lobes in the Deccan pahoehoe flows is thesame as that of modern Hawaiian flows, though the Hawaiianflows themselves are much smaller than the Columbia River andthe Deccan lava flows. The large volumes of individual Deccanflows require explanation, however, and may reflect an extra-fertile mantle source (relative to peridotite) (Sheth, 2005), ahydrous or CO2-rich mantle (e.g., Presnall et al., 2002; neverevaluated), a lithospheric regime dominated by extension (Sheth,2000), and great lengths of the fissure systems (Self et al., 1997).Dikes 50–60 km in length are common in the Narmada-Satpura-Tapi region (e.g., Deshmukh and Sehgal, 1988; Keshav et al.,1998).

A Systematic Southward Stratigraphic Younging?

Southward stratigraphic younging of the various formationswithin the Western Ghats region, apparent in the earlier years ofDeccan geochemical stratigraphy, has also been widely used insupport of the plume model. Stratigraphically younger lavashave been said to have erupted in progressively more southerlylocations consistent with the passage of the plate over a plume(Cox, 1983; Mitchell and Widdowson, 1991). The latest workson geochemical stratigraphy have provided new data from otherparts of the province that contradict this view: thick lava pilesclosely resembling (in both elemental and isotopic composition)some of the youngest formations of the Western Ghats stratig-raphy are now known to outcrop in far northern areas of the Dec-can (e.g., Mahoney et al., 2000; Sheth et al., 2004), and there isno evidence to suggest that the source of the lavas or the erup-tive centers moved systematically southward with time.

A Cambay Triple Junction?

Originally included by Burke and Dewey (1973) in theirlist of plume-generated triple junctions worldwide, the Cambaytriple junction is not real, because the Narmada Rift Zone con-tinues into the Saurashtra peninsula and it, the west coast rift,and the Cambay Rift form a cross (Sheth, 1999a; Fig. 1). TheKachchh Rift is another structure that the Cambay triple junc-tion idea does not address. Nevertheless, papers subsequent tothat of Burke and Dewey (1973) have popularized this triplejunction and the Réunion plume model for the Deccan. Anotherunfortunate development is the proliferation of model-dependentinterpretations by which every geological and geophysical ob-servation from the Deccan is interpreted as an effect of theRéunion plume. For example, low–seismic velocity mantleunderlying the Cambay Rift of the Deccan is interpreted as a

484 H.C. Sheth

remnant of the plume (Kennett and Widiyantoro, 1999) insteadof as simply passive upper mantle upwelling.

Geophysical data, like geological data, can offer valuableclues and insights, and to these we now turn.

GEOPHYSICS OF THE DECCAN AND ITS BASEMENT

Heatflow

The thermal structure of the large stable region of thesouthern Deccan is characterized by a heatflow in the range of40–70 mWm–2, which is the normal low cratonic heatflowfound over the southern Indian basement shield (Gupta andGaur, 1984). Heatflow values are 75–93 mWm–2 in the Broach-Ankleshwar area, between Cambay and Surat (Fig. 4). Heatflowis high (average 83 mWm–2) in the northern part of the Cambaygraben, where temperature gradients are >70 oC/km in somezones (Pandey and Negi, 1995). At 3 km depth, in situ temper-atures were estimated, using drill hole data, to be as high as 175± 25 oC. In the Bombay offshore region, the average heatflowis ~83 mWm–2 and temperature gradients are 36–78 oC/km. Theexpected temperature at a depth of 3 km is ~175 ± 50 oC. In theKonkan plain between the west coast and the Western Ghats,some sixty thermal springs are distributed over a linear N-S dis-

tance of 300 km, with temperatures ranging from 34 to 71 oC(Pandey and Negi, 1995) (Fig. 4). However, along the Narmada-Tapi Rift, the Saurashtra peninsula coast, and the west coast, theheatflow structure is also determined by convective heat trans-fer, and the measurements may not accurately reflect the crustalheat production (Roy and Rao, 2000). Biswas (1987) has sug-gested that the current high thermal regime of the Cambay andBombay offshore regions marks a renewed rifting phase. Royand Rao (2000) reported high heatflow values from the CambayRift; however, they stated that there was no evidence for ther-mal transients associated with Deccan volcanism in the Deccanregion proper (south of the Tapi Rift).

A hot plume under the lithosphere is expected to cause ther-mal erosion of the lithosphere and thereby produce a thinnedlithosphere and high heatflow. Negi et al. (1986) proposed adrastically thinned and anomalously hot Indian lithosphere, withestimates of present-day lithospheric thickness as low as 60 km(and 40 km under Cambay). They suggested that the Indian litho-sphere was both greatly thinned and abnormally hot, and thiswas the reason for its supermobility (at Cretaceous–Tertiary timeIndia was moving northward at superfast rates of 15–20 cm/yr;e.g., Patriat and Achache, 1984). Gupta (1993) questioned thisview, stating that the arguments of Negi et al. (1986) for pro-nounced thermal erosion of the Indian lithosphere were basedon the scarce data available then. With much more geothermaldata compiled for Precambrian shield areas of India, Africa,Australia, and Brazil (all of which formed parts of Gondwana-land), Gupta (1993) found no support for the notion that theIndian Shield was hotter than other shields. He concluded thatthe Indian landmass is no hotter than the other Gondwana land-masses and that its supermobility was not a consequence of be-ing hot, and apparently not related to its thermal characteristics.

Gravity Studies and Deep Seismic Sounding:Normal or Upwarped Moho?

The most striking gravity feature, not only of the westernIndian margin but of the entire Deccan province, is perhaps the60 km–wide high Bouguer anomaly close to the Bombay, Surat,and Saurashtra coasts (Fig. 4; Mahadevan, 1994). The Bougueranomaly values reach extreme lows of –100 mgal some 150 kmeast of Bombay and highs of as much as +50 mgal at the Bom-bay coast. The high gravity anomaly near Bombay was attributedby Glennie (1951) to a 26 km–wide mafic dike off Bombay andby Takin (1966) to a differentiated magma chamber (of olivinegabbro bulk composition and a density contrast of 0.4 g/cm3

with its surroundings). Kaila (1986) suggested that these grav-ity highs resulted from Moho upwarps and that the crust underthis region was abnormally thin. This suggestion gained furthersupport from deep seismic sounding (DSS) studies (Kaila et al.,1981a; Kaila and Krishna, 1992).

There is a broad relative gravity high over the SatpuraMountains, and the strong gravity high beneath Navsari, locatedon the west coast, is particularly noticeable (Fig. 5). The Satpura

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gravity high has been interpreted variably in the past. Qureshy(1971) felt that this indicated a horst-type structure for the Sat-puras or was related to the migration of upper-mantle materialinto the crust (Moho upwarp?). Verma and Banerjee (1992)postulated high-density mafic intrusive material at midcrustallevels. Singh and Meissner (1995) carried out 2D density mod-eling along the four DSS profiles across the Narmada-Satpura-Tapi zone (Fig. 5), proposing an upwarped Moho and igneousaccretion at the base of a thinned crust. Later Singh (1998) car-ried out 3D gravity modeling and considered the Satpura gravityhigh anomalous because a topographically high region ought tohave a low Bouguer anomaly due to expected isostatic compen-sation. Whereas the DSS results indicated a Moho shallowerthan 25 km, Singh (1998) found a normal crustal thickness(Moho at 38 km) along the west coast of India. This is directlycounter to the earlier beliefs that the crust along the west coastis thin. Singh also proposed an accreted igneous layer (of 15–20 km thickness and 3.02 g/cm3 density) at the base of the crustunder the Satpura-Tapi region (Fig. 6). This layer is aligned east-west, and its thickness varies from 8 km beneath the eastern partto ~16 km beneath the central part of the region. The thicknessof the layer under Navsari is 24 km, and this greater thickness ex-plains well the gravity high over Navsari. Singh (1998) suggestedthat what had been imaged by the DSS studies as the Moho mayactually represent the high-density or velocity discontinuitywhere normal continental crust transforms into transitional-typecrust, and concluded that the crust in this region is not thinnedbut of normal thickness.

Seismic Tomography: Thin or Thick Crust and Lithosphere?

Seismic tomography does not support the notion of plume-related thermal damage to the Indian lithosphere. For the south-ern Indian peninsular shield, the lower-crustal shear wave(S-wave) velocities are higher than those for the Baltic, African,and Canadian shields, but the upper-mantle velocities are lower.Also, compared to the southern Indian shield, the Deccan region

exhibits marginally higher S-wave velocity both in the lowercrust and in the upper mantle (Mohan et al., 1997). Compres-sional wave (P-wave) analyses have given similar results (Iyeret al., 1989). The seismic signature of the hypothesized causativeplume head is absent under the Deccan proper, and P-wave datasuggest a lithospheric thickness of at least 300 km under theDeccan region as well as the southern Indian shield (Rameshet al., 1993). Crustal thickness in the Indian shield is normal aswell. Ravi Kumar et al. (2001) determined that the dominantlyArchean crust forming the southern Indian shield has a very sim-ple structure without any prominent intra-crustal discontinuities,has an average Poisson’s ratio close to 0.25, and is 33–39 kmthick. They found that the predominantly Proterozoic crust form-ing the northern Indian shield is complex, with several seismicdiscontinuities, and has a crustal thickness of >40 km.

Mutter et al. (1988) and Anderson (1994) have argued thatlarge-volume basaltic provinces and volcanic rifted margins formwhere the transition from thick to thin lithosphere is abrupt, be-cause such an abrupt transition sets up high lateral temperaturegradients. These gradients, in turn, induce small-scale convec-tion and rapid movement of mantle material. An abrupt lateralchange in the lithospheric thickness of a plate focuses both strainand magma ascent. Anderson et al. (1992) and King and Ander-son (1995, 1998) have pointed out that every CFB province issituated on the margin of a Precambrian craton. In contrast toIndia, which is made up of several Precambrian cratons, theNorth American continent has only one Precambrian craton (theWyoming craton), and the only post-Precambrian CFB provincein North America, the Columbia River flood basalt province, islocated along the edge of the Wyoming craton. Clearly there arefundamental lithospheric controls on the location of CFB, andthese must be discussed by any realistic geodynamic model forCFB volcanism. Because of the geological and geophysicalcharacteristics of the west coast region of India (volcanic erup-tive centers, dike swarm concentrations, high heatflow, alignedthermal springs, etc.), many authors have considered it a regionof crustal thinning (e.g., Chandrasekharam, 1985; Sheth, 1999a,b).

486 H.C. Sheth

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Understandably, thinned lithosphere can permit the underlyingmantle to upwell to shallow depths and decompress, resultingin extensive melting and a thick lava pile like the one exposedin the Western Ghats. Therefore, the recent findings of Mohanand Ravi Kumar (2004), that the crust along the west coast isthick, not thin, are interesting.

Mohan and Ravi Kumar (2004) performed a receiver func-tion analysis of teleseismic data recorded from 1998 to 2003 bya ten-station network deployed near Bombay (Fig. 7). The net-work comprised one broadband station and nine short-periodstations spread over an area 50 km × 100 km across. The receiverfunction analysis reveals a continental crustal thickness varyingfrom 36 to 41 km, which is quite “normal” and shows that thiscrust cannot be called thin crust. The observed thick crust isapparently not due to underplating by basaltic magma, becausesuch mafic underplated material would be identifiable from itscharacteristic Poisson’s ratio (the ratio of lateral strain to lon-gitudinal strain in an elastic body due to uniaxial longitudinalstress). Mohan and Ravi Kumar (2004) obtained a value of 0.26(± 0.01) for the Poisson’s ratio for the sub-Deccan crust in thisregion and suggested a felsic to intermediate composition simi-lar to that of the Precambrian southern Indian Shield. (The valueof this ratio is at least 0.28 for mafic rocks and 0.30 and higherfor ultramafic rocks.) It is likely, however, that if there wereDeccan-related underplating of a thinned crust by Deccan felsic

magmas, the geophysical methods used would not be able todistinguish such an underplated crust from true basement crust,given their closely similar densities and Poisson’s ratios. Asnoted, Deccan rhyolite and trachyte flows and dikes are indeedfound along the west coast, particularly at Bombay (Sheth et al.,2001a; Sheth and Ray, 2002). However, these felsic magmasprobably developed in magma chambers established at upper-crustal levels (shallower than ~10 km) (Sheth and Ray, 2002),and they cannot have underplated a thinned (say 15–20 km thick)crust all the way to a depth of 40 km, as is required by the data.Mohan and Ravi Kumar (2004) note that the crustal thickness,Poisson’s ratio, and average crustal S-wave velocity (3.7 km/s)are all similar to the values for the Precambrian Indian Shield,concluding that the crust under the Deccan basalts in this regionhas not been affected by the Deccan volcanism. It is likely, there-fore, that the concentrations of eruptive centers and thermalsprings, dikes, and intrusions and the high heatflow along thewest coast are related not to crustal thinning but to fracture con-trol in an otherwise thick crust and lithosphere.

The Cambay Seismic Anomaly

Kennett and Widiyantoro (1999) reported low-P-wave-velocity mantle under the northern part of the Cambay Rift andinterpreted it as a remnant of the Deccan plume. This mantle is

From Deccan to Réunion: No trace of a mantle plume 487

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2% slower than the seismically fast Indian Shield, defines aregion of roughly circular cross-section that is ~250 km across,of depths between 80 and 250 km (Fig. 8), and appears to con-nect with a larger (600 km across) slow region that extends from250 km down to 500 km depth. Kennett and Widiyantoro (1999)did not rule out seismic anisotropy as a cause of the low seismicvelocity, but felt that the velocity anomaly could also reflect hightemperatures (supporting evidence for which comes from thehigh heatflow along the Cambay Rift). They suggested that theanomalous mantle region represents a conduit of the Deccan-Réunion plume, because some of the earliest Deccan rocks out-crop in alkaline complexes in this region, and these have beenargued to be plume-derived (Basu et al., 1993) because of theirhigh 3He/4He ratios (up to 14 times the atmospheric 3He/4Heratios in pyroxenes).

Anderson (2000a) has presented a lucid discussion of cur-rent 3He/4He ratio fallacies and a priori data-filtering practicesdetrimental to rigorous statistical averaging. 4He is producedby radioactive decay of U and Th, and Anderson argues thathigh 3He/4He ratios do not reflect high 3He abundance (as in theplume model) but rather low 4He (i.e., revealing a source poorin U + Th). This conclusion is supported by Natland (2003) andMeibom et al. (2003), who argue that during crystallization of amagma, olivine crystals trap 3He along with CO2 in fluid inclu-sions, and because olivine is a mineral very poor in U + Th, thereis negligible growth of 4He over time. Olivine crystals thereforeact as “He time capsules.” Thus, mantle-derived mafic or ultra-mafic rocks with large amounts of olivine are likely to have high3He/4He ratios. 3He/4He ratios in mantle-derived rocks cannot

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Italicsaren’tneeded forforeignterms thathave madetheir wayinto Eng-lish dic-tionaries.

be used to infer mantle source ratios, and definitely do not con-stitute evidence for plumes.

The age of the Cambay seismic anomaly is unknown. As-suming it dates from the time of Deccan volcanism, it is locatedwhere expected for passive models involving rift convection(e.g., Sheth, 1999b). Kennett and Widiyantoro (1999) infer thata 65 Ma anomaly would have to be at least 50 km across orig-inally, with a temperature excess of 300 oC or more (giventhermal diffusion over time). Such hot sources arguably wouldproduce high–melt fraction subalkaline picritic liquids (as theplume model postulates). However, there is no petrological ev-idence from the rocks exposed in this region (or indeed any-where in the Deccan) for such high mantle source temperatures.Although picrites are encountered in boreholes near Cambay,these are somewhat alkaline and are low-degree melts (Peng andMahoney, 1995). Even the mafic-ultramafic rocks analyzed byBasu et al. (1993) are low-degree, high-pressure alkaline melts(as inferred, for example, from rare earth element patterns sug-gestive of residual garnet).

There are two suggestions as to possible origins of the Cam-bay seismic anomaly outside of the plume model: (1) the anom-aly is of Deccan age and related to passive rifting and relatedupper-mantle convection, or, more likely, (2) the anomaly is ofpost-Deccan age and much more recent. Kennett and Widiyan-toro’s study ran out of resolution for the area along the westcoast south of 20oS, but they considered a southerly extensionof the anomaly likely. If so, it would support the view that theage of the anomaly is post-Deccan and that the spectacular post-Deccan uplift of the Western Ghats and the development of thisseismic anomaly may be related in some way. The Cambay Rifthas received several km of Tertiary sediments and continues tobe a low-lying area today. This may be because crustal extensionremoves the need for uplift. Biswas (1982) has considered theCambay Rift a true active rift in India today. If it is one, the Cam-bay low-velocity anomaly may represent warm, expanded, rel-atively less dense mantle that has risen to depths of 80–100 kmbecause of extension in the overlying lithosphere.

We shall now leave the Deccan and India behind as weenter the Arabian Sea and the Indian Ocean.

THE LAKSHADWEEP-CHAGOS RIDGE

The Lakshadweep-Chagos Ridge is a linear north-southridge that runs for 2200 km in the western Indian Ocean (Fig. 9)and has been postulated to have been produced by the “tail” ofthe Réunion plume after the head of the plume was consumedin the production of the Deccan Traps. The Lakshadweep islandsat the northern end of the ridge are capped by coral reefs. Theridge takes off where the crudely circular outcrop of the Deccanlavas ends, and the pair has been considered a classic exampleof a plume head and tail. However, new Ar-Ar data have shownthat the systematic southward age progression required in theplume model does not apply. Sheth et al. (2001a,b) dated three

Deccan rocks from Bombay at 19oN by the 40Ar-39Ar tech-nique. Two of them are trachyte lava flows and yielded ages of60.4 ± 0.6 Ma (2σ) and 61.8 ± 0.6 Ma (2σ), respectively. Theirthird sample came from the well-known, thick (>25 m), andcolumnar-jointed Gilbert Hill basalt and was dated at 60.5 ± 1.2Ma (2σ). The very well-developed plateau spectra, isochronswith good mean square of weighted deviates (MSWD) values,and atmospheric 40Ar/36Ar intercepts indicate that these datesare reliable crystallization ages. But these ages are troublesomefor the plume model because, according to the model, the plumehead was consumed at 65–66 Ma and only the narrow plume tailremained. How can a plume tail 100–200 km wide have pro-duced volcanism simultaneously in Bombay and at OceanDrilling Program (ODP) site 715, two locations that are 1000km apart (Fig. 9)?

Some workers have suggested that the 60–61 Ma volcanicactivity in Bombay was of minor volume and hence not prob-lematic (e.g., Mahoney et al., 2002; Courtillot and Renne, 2003).Courtillot and Renne (2003) noted the young, 60–61 Ma, agesof Sheth et al. (2001a) but stated that Sheth et al. (2001a) haddated trachyte dikes and that such late-stage dike activity wasexpected and should not render the plume model questionable.Jerram and Widdowson (2005) also imply that the dated tra-chytes were intrusions. Sheth et al. (2001a) clearly stated thatthey were dating trachyte flows, and the Ar-Ar ages on these dip-ping lava flows provided age constraints on the formation of thePanvel flexure (Sheth, 1998), of which they are a part. In anycase, we do not know that this volcanic activity was minor involume: large volumes of Deccan lava exist in the subsurfacealong the west coast, and there is a scarcity of geochronologicaldata; the three Ar-Ar dates of Sheth et al., (2001a,b) are the onlyexisting ones for this region of the Deccan. In comparison, theWestern Ghats section has been heavily sampled and dated. Fi-nally, whatever its magnitude, the late-persisting volcanismmust be still explained without ad hoc auxiliary hypotheses. Butit is not. Suggestions such as northward dragging of the plumetail by the plate (made by several colleagues in personal com-mun.) are ad hoc, and such drag and tilting would make any sys-tematic age progression impossible in the first place.

Interestingly, there was Deccan-age magmatism in the ca.116 Ma Rajmahal Traps of eastern India (Figs. 1 and 9). Kent etal. (2002) dated a ferro-tholeiite dike in the Rajmahal Traps at65.4 ± 0.3 Ma (2s). If Deccan volcanism was caused by a plumeunder western India, how did it generate magmatism 1500 kmaway? Long-distance mantle flow or magma flow may be in-voked, but this also significantly reduces the value of a fixedplume model. Besides, there are mafic dikes in Kerala, south-western India (Fig. 9), dated at ca. 69 Ma (Radhakrishna et al.,1994), and if they are a part of the Deccan event, which seemslikely, they do not support a systematic southerly age progres-sion from the Deccan. They also complicate the spherical plumehead–narrow plume tail picture by virtue of their location (notethat the Lakshadweep-Chagos Ridge departs from the western

From Deccan to Réunion: No trace of a mantle plume 489

Indian coast far north of the Kerala dike occurrences). Thesedikes support “passive,” rifting-related volcanism.

A model for the origin of the Lakshadweep-Chagos Ridgemust address the following:

1. Visually, the Lakshadweep-Chagos Ridge, Mauritius, andRéunion together do not form a picture similar to that of theHawaiian chain.

2. The Ridge runs along the Vishnu fracture zone (Fig. 9).

3. Baksi (1999 and this volume) has critically evaluated theAr-Ar ages of the basalts forming the Ridge (and those ofother “hotspot tracks” in the Atlantic and Indian Oceans)and has stated that out of about thirty-four published (androutinely used) ages, only three satisfy the statistical cri-teria for acceptable ages. If he is correct, we do not knowif the perceived age progression along the Lakshadweep-Chagos Ridge is even real.

4. In the fixed plume model, the 33 Ma date for ODP site 706

490 H.C. Sheth

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Figure 9. Prominent structural-tectonicfeatures of southern Asia and the IndianOcean Basin (based on Mahoney et al.,2002). Abbreviations for localities areas follows: Q—Quetta; Z—Zhob; B—Barmer; M—Mundwara; D—Dhand-huka; B—Bombay; R—Rajahmundry.WG indicates the Western Ghats region(ages from Venkatesan et al., 1993 andothers). Ages (in Ma) are in boldfacenumbers. The ca. 64 Ma age for theRajahmundry basalts is from Baksi(2001b). G indicates the ca. 61 Ma Goadikes (Widdowson et al., 2000). KK in-dicates the ca. 90–69 Ma Karnataka-Kerala dikes (e.g., Radhakrishna et al.,1994; Anil Kumar et al., 2001). SMI in-dicates the 85.5 Ma Saint Mary’s Islandsvolcanics (Pande et al., 2001), part ofthe Indo-Madagascar continental floodbasalts, which in India are otherwise rep-resented by the KK dikes. The 72–73 Maages for the Quetta and Zhob rocks andthe 65 Ma age for the Dhandhuka-Botadlavas are from Mahoney et al. (2002), asis the modeled hotspot track showingexpected ages in Ma (italics). Note therift zones underlying the Deccan and theabsence of any triple junction. OFZ—Owen fracture zone; MFZ—Mauritiusfracture zone; VFZ—Vishnu fracturezone.

on the “track” on the Africa Plate, across the SouthwestIndian Ridge, has been considered evidence that a northerlyjump of the Central Indian Ridge transferred the Réunionhotspot under the Africa Plate. The alternative explanationis that the Central Indian Ridge jumped over a southward-propagating crack (track) at 30 Ma and split the track.

5. The 450 km–long Rodrigues Ridge that lies east of Mauri-tius is oriented at right angles to the predicted hotspot track.The Rodrigues Ridge was hypothesized to have been pro-duced by lateral flow of Réunion plume material as theCentral Indian Ridge moved away from it, and it has noage progression itself (the whole ridge is dated at 8–10 Ma;Duncan, 1990).

6. The geochemistry of the basalts along the track indicatesmixing of “plume” and “MORB” mantle (Fisk et al., 1988;White et al., 1990), or, alternatively, these basalts are notshield-stage rocks but postshield basalts (Sheth et al., 2003,and later discussion). In ocean islands, the postshield andposterosional (rejunvenated) stages of volcanism occur 1 m.y.to a few m.y. after the main shield-building stage (e.g.,Clague, 1987). If these basalts are not shield-stage, theycannot be used to locate the plume and their ages cannot beused for plate reconstructions based on hotspots.

7. The paleolatitudes of the basalts along the chain are vari-able and do not themselves support the plume model with-out the introduction of further ad hoc parameters (see laterdiscussion).

THE PALEOLATITUDE VARIATION:MANTLE ROLL OR CRACK PROPAGATION?

The Kerala mafic dikes (Radhakrishna et al., 1994) com-plicate the simple picture of a plume head (Deccan Province)and a plume tail (Lakshadweep-Chagos Ridge). The roughlycircular outcrop of the Deccan does not require or indicate aspherical plume head beneath, but may be a consequence of theintersecting rift zones (the western Indian margin, the CambayRift, and the Narmada-Tapi Rift). Sheth (1999a) argued that theage progression along the Lakshadweep-Chagos Ridge and upto Réunion Island may be explained by southward crack propa-gation through the oceanic lithosphere and that the narrow“hotspot track” may represent localized melting and magmafocusing from a wider area (the “transform-fault effect”; Lang-muir and Bender, 1984). In support of this is the fact that theLakshadweep-Chagos Ridge lies along the Vishnu fracture zone(Fig. 9). It is not necessary to explain how a crack would havepropagated across two separate plates, because it is quite pos-sible that the current volcanism at Réunion Island is unrelatedto the Deccan and to the Lakshadweep-Chagos Ridge geo-dynamically, though it taps delaminated Indian continental man-tle brought beneath the Africa Plate by a ridge jump at ca. 30 Ma(see also Burke, 1996, for a similar interpretation).

In my view, relating the Deccan and Réunion to each other,and both to a mantle plume, has created more problems than it

has solved. The Deccan lavas erupted at a latitude of ~30°S(Clegg et al., 1956), but Réunion Island is at 21° S today (Fig. 9and 10A). This large discrepancy has promoted further ad hocspeculation. Vandamme and Courtillot (1990) proposed truepolar wander (TPW) of the Earth’s mantle. In their view, sub-sequent to the Deccan eruptions the Réunion plume remainedfixed in the mantle while the mantle itself “rolled” as a ball in-side the lithospheric shell, in a northerly direction. Accordingto these workers, this has brought the Réunion plume northwardfrom 30oS to 21oS (a distance of ~1000 km) over 65 m.y. Burke(1996) questioned this postulated TPW and suggested that theDeccan plume died out at 30 Ma and that the Réunion plume isnew and unrelated to the Deccan plume.

A simpler alternative is illustrated by the schematic cartoonin Figure 10B. This alternative is that the systematically chang-ing paleolatitudes between the Deccan and ODP site 706 (33 Ma)

From Deccan to Réunion: No trace of a mantle plume 491

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Figure 10. (A) Palaeolatitude variation from the Deccan to Réunion Is-land through the Ocean Drilling Program leg 115 sites (Vandamme andCourtillot, 1990). (B) Schematic cartoon showing the development ofvolcanism resulting from a crack’s propagating more slowly southwardthan the plate moves northward in the southern hemisphere.

indicate southward crack propagation in a northward-movingplate with the condition that the northward plate motion wasfaster than the southward crack propagation. The situation shownin Figure 10B is for a plate in the southern hemisphere. At timeT1, there was an active volcano 1 at the crack tip at latitude60° S. Between times T1 and T2, the crack tip moved southwardby 10°, but because the plate itself had moved north by 20°, thenew volcano 2 at the crack tip had the latitude of 50° S. A sim-ilar progression occurred between times T2 and T3. Therefore,although the crack tip is propagating southward, the paleo-latitudes systematically become more northerly. Thus, thereis not necessarily a contradiction between southward crackpropagation and progressively northerly paleolatitudes, and theLakshadweep-Chagos ridge is better interpreted as a crack thanas a track. The ridge may mark the location of a major Gond-wanic transform (Reeves and de Wit, 2000; Reeves et al., 2004).

RÉUNION, MAURITIUS, AND RODRIGUES

Burke (1996) has questioned the Deccan-Réunion link. Ac-cording to him, Réunion, Mauritius, and Rodrigues are membersof the youthful (<30 Ma) population of hotspots on the Africaplate. He gave evidence that that the Africa plate came to restover the underlying mantle circulation at 30 Ma and has movedlittle since. If so, the manifestation of the Deccan hotspot shouldhave remained in the same place over the last 30 m.y. However,there has been no volcanism in this area (the Saya de Malha Bank,Nazareth Bank or Cargados Carajos Bank) since 30 Ma. Burketherefore argued that the Deccan plume died out at 30 Ma, thatit was a different plume from the Réunion plume, and thereforethat the position of Réunion today at 21oS did not provide evi-dence of true polar wander.

This suggestion is thought-provoking. The first volcanicactivity in the area after ca. 30 Ma clearly began at 8–10 Ma(Mauritius Shield and Rodrigues stage). Thus, in the Réunionplume model, the plume must have been inactive for 20 m.y.Burke (1996) suggested that each of the three Mascarene islands(Réunion, Mauritius, and Rodrigues) is underlain by a youngplume. However, the distances between these islands, if they areunderlain by deep plumes, are too small for each of them to havea CMB origin (Anderson, 1998b). Mauritius and Réunion arelocated along the Mauritius fracture zone (Fig. 9), and a smallseamount west of Réunion (Fig. 9) (Duncan, 1990) complicatesthe picture. Hirn (2002) has discussed structural studies ofRéunion Island using seismic methods and noted significant de-partures from the Hawaiian case, to which it is traditionallycompared. There is no moat around Réunion of the type that sur-rounds a Hawaiian Shield volcano, but crustal doming is seeninstead. Two radial reflection lines to the SSW, close to eachother, detect a difference in depth of the oceanic basement, andthis may be related to the presence of a fracture zone, suggestedfrom the magnetic anomaly pattern seen from reconstruction ofthe seafloor spreading history. The magnetic anomaly patternpreviously has been interpreted to indicate that the western part

of Réunion developed atop a Paleogene fossil accretionary cen-ter. The same was also suggested for Mauritius Island, i.e., Mau-ritius and Réunion are both fossil accretionary centers that wereactive on different sides of a triple junction and were carriedaway from each other along the later fracture zone in between.The seismic data available support the idea that the location ofRéunion is related to the structural heterogeneity of the under-lying lithosphere (Charvis et al., 1999; de Voogd and Pontoise,1999).

The Deccan and Réunion Island are probably unrelated. Itis true that some Réunion-type chemical and isotopic signaturesare found in some lavas in the Deccan (e.g., Peng and Mahoney,1995) and in ophiolitic rocks predating the Deccan (Mahoneyet al., 2002). However, these signatures may be continental man-tle signatures.

GEOCHEMICAL-ISOTOPIC DATA

In many ocean island volcanoes worldwide (e.g., Hawaii,Mauritius), shield-stage lavas that make up most of the vol-cano’s volume (99%) have systematically greater 87Sr/86Sr ra-tios and lower 143Nd/144Nd ratios than post-shield-stage andpost-erosional-stage (rejuvenated) lavas (e.g., Clague, 1987). Itis the shield-stage lavas that, in the framework of the plumemodel, are derived from plumes, and by dating the shield stageone can infer the location of the plume at that time. Although theages of the ODP leg 115 (Indian Ocean) basalts have been usedby plate motion modelers such as Müller et al. (1993), almost allof these ages were questioned by Baksi (1999). A further prob-lem is that the close Nd-Sr isotopic similarity of some of theODP leg 115 basalts to post-shield lavas on Mauritius (the Inter-mediate and Younger Series) (Fig. 11) suggests that the leg 115basalts could similarly be post-shield lavas (Sheth et al., 2003).If so, their ages cannot be used for modeling plate motions basedon the plume model, because the age of the shield stage wouldremain unknown (it would be variably older.) We cannot com-pare these ODP leg 115 basalts to the modern Réunion basalts,because Réunion Island is currently only in the active shield-building stage. White et al. (1990) argued for progressively more“enriched” geochemistry southward along the Réunion plumetrack, based on Nd and Sr isotopic compositions. Their datapoints define a gentle, broad array in each isotopic plot (see theirFig. 5). But if the analytical error bars are considered, and alsothe fact that post-shield volcanism on Réunion has yet to takeplace, the array in each plot may be statistically no differentfrom a flat array, and the time-progressive systematic variationin isotopic characteristics that White et al. (1990) argued for isnot apparent.

Are mantle plumes the cause of the “enriched” geo-chemistry? Smith (1993) proposed that ocean island volcanismis derived from enriched continental mantle, often a part of an-cient sutures, delaminated from beneath a breaking continentand dispersed in the new oceanic mantle. “Enriched” isotopicratios, such as higher-than-normal-MORB values of 87Sr/86Sr,

492 H.C. Sheth

are usually taken as plume signatures. However, such composi-tions may instead mark the involvement of shallow-level enrichedcontinental mantle. High values of 3He/4He may be explainedby shallow models (e.g., Anderson, 2000a).

The (enriched) plume model is not required to explain con-tinental intraplate volcanism, given the abundance of enrichedmantle domains within the continental lithosphere itself. Theplume model was extrapolated to continental magmatism fromthe ocean basins based on the worldview that the oceanicmantle was entirely “depleted,” MORB-like, convecting, andhomogeneous. As a consequence, anything enriched or anom-alous was assumed to come from plumes (Smith and Lewis,1999). However, if continental mantle can be introduced intothe oceanic mantle, e.g., by delamination during continentalbreakup (e.g., Smith, 1993), enriched plumes are not requiredto explain either continental or oceanic intraplate volcanism,and enriched “plums” of continental mantle dispersed within theoceanic mantle would constitute a better scenario. The wholeplume argument is unnecessary, then, from a geochemistry pointof view. Rather than accepting that the Deccan formed from adeep mantle plume now located under Réunion Island, we cannow see that Réunion volcanism may be in part sourced fromdelaminated Indian continental mantle. Réunion Island is cur-rently in the active shield-building stage. Modern Réunion basaltsare closely similar isotopically to the Older Series (shield-stage)basalts of Mauritius (Mahoney et al., 1989; White et al., 1990;Sheth et al., 2003). Both the plume model and the “plum” or“blob” model (e.g., Sleep, 1984) can explain this.

Mahoney et al. (2002) reported Réunion-like elemental andisotopic compositions for mafic ophiolitic rocks in Pakistan datedat 72–73 Ma (Fig. 9). They proposed that some of these rep-resent pre-Deccan oceanic seamounts. The associated intrusions

were emplaced in continental shelf-and-slope-type marine sed-iments along the northern margin of India. Mahoney et al. (2002)considered the continental mantle delamination model, but feltthat it did not explain Réunion-type volcanism occurring onthe updrift side of India at 72–73 Ma. They concluded that theplume model was the most viable option.

Nevertheless, the intrusions in question are located withinthe boundary of the Indian continental mantle, and the purelyoceanic seamounts may not have been far from the northernmargin of India. Delaminated continental mantle could havemigrated northward ahead of India in a radial, outward extru-sion and fed the seamounts built on oceanic lithosphere. Thecontinent followed behind, and when it converged upon Asia ittrapped them along the suture. This is a better explanation forthe observations than the plume-head-impact (not incubation)model favored by Mahoney et al. (2002), despite the ~8 m.y.time gap between the Pakistani rocks and the 66–65 Ma volu-minous volcanism in the Deccan. Mechanisms such as lateral floware required even by the plume model, e.g., for the RodriguesRidge (Fig. 9).

DISCUSSION AND CONCLUSIONS

The traditional mantle plume model for CFB like the Dec-can suffers from many contradictions and shortcomings. Manygeological and geophysical features of the Deccan have gener-ally been considered to require a mantle plume origin. Singh(1999), for example, discussed geophysical data for the LaxmiRidge in the northeastern Arabian Sea (Fig. 9). The Laxmi Ridgeis 700 km long and 100 km across, parallels the western Indiancontinental margin, and consists of isolated submarine structuralhighs. Singh related the Laxmi Ridge to Deccan volcanism and

From Deccan to Réunion: No trace of a mantle plume 493

0.7026 0.7028 0.7030 0.7032 0.7034 0.7036 0.7038 0.7040 0.7042 0.7044

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noted that both active and passive models were available thatcould explain the geophysical observations. He opted for theplume model, however, “there being ample independent evi-dence for a plume origin.” The aim of the present paper is toshow that no evidence requires a plume. Rather, a “passive,”nonplume model, such as rifting and related mantle convection,explains the observations equally well, if not better, and is freeof the contradictions and problems of the plume model. A mas-sive amount of decompression melting would have taken placewhen the Central Indian Ridge jumped onto the northerly drift-ing Greater India, splitting the Seychelles (Fig. 1). This canalso explain why the Deccan basalts closely resemble mid-oceanridge basalts in many elements, a long-noted fact (e.g., Chandra-sekharam and Parthasarathy, 1978; Shrivastava and Pattanayak,1995). The magma type from the Deccan that is least contami-nated by continental material is the Ambenali, and the Ambenalibasalts are evolved ferrobasalts (e.g., Mahoney, 1988). The tran-sitional MORB characteristics (e.g., a small Nb peak in a nor-malized multielement pattern) of the Ambenali do not requirean “enriched” mantle plume mixing with entrained or ambientMORB source mantle. They have been proposed to reflect aslight amount of contamination by phlogopite-bearing peridotite(Sheth, 1999a), or might represent shallow-residing ancienteclogite (Sheth, 2005). When a mature, steady-state spreadingridge suddenly jumps to a new location, strong lateral pressure-temperature gradients must be created in the mantle relativelyinstantaneously. A quick burst of voluminous volcanism wouldbe a natural outcome, because a focused flow of mantle and par-tial melt would be distributed over a wide region toward the newridge. A large plume “head,” invoked in current plume theories,is not indicated or required.

Crustal Extension and Rift Convection

Lithospheric extension was operative before, during, and af-ter Deccan flood volcanism (Sheth, 2000). Geological and geo-physical data suggest that intense eruptive and intrusive activityoccurred along the rift zones and the new rifted continental mar-gin. The Deccan lavas were produced under the imminent con-tinental margin, along the boundary between thinned-extendedand thick crust, and flowed on thick cratonic lithosphere imme-diately adjacent to the margin. More supporting evidence forthis is the relative scarcity of dikes in the Deccan in the more in-terior parts (eastern and southeastern) of the province, whereasdikes and intrusions are far too numerous along the west coastand along the Narmada-Satpura-Tapi zone and the KachchhRift (they are buried beneath Tertiary sediments in the CambayRift). Also, in the eastern and southeastern parts of the Deccanprovince the lava pile is very thin (~400 m maximum, includ-ing exposed and subsurface portions), compared to the massive1700 m exposed along the Western Ghats, with an additional500 m or so in the subsurface (e.g., Tiwari et al., 2001, and ref-erences therein). A huge thickness of the lavas along the Ghats

is exactly what is expected in the rift convection model. (Notethat the present position of the Western Ghats escarpment,~50 km east of the coast, is a result of erosion-induced parallelscarp retreat; e.g., Ollier and Powar, 1985; Widdowson andMitchell, 1999). The role of rift convection in causing Deccanvolcanism was evidently not confined to the west coast alone butalso significant along the Deccan rifts.

Besides, we have noted that, contrary to earlier views (basedon DSS) that the continental crust along the western Indian coastis greatly thinned, recent gravity modeling and seismic tomo-graphic studies indicate a continental crust of normal thickness(36–40 km) along the west coast (Mohan and Ravi Kumar,2004). If, as in the edge-driven convection model of King andAnderson (1995, 1998), rifted-margin flood basalts like thoseof the Deccan largely form and erupt where cratonic lithospheresuddenly changes thickness, it is important to explain how thislithospheric asymmetry forms in the first place. The juxtaposi-tion of thin and thick crust along the present west coast of Indiais not difficult to explain, because, as already noted, prior to theDeccan episode, Greater India (India plus the Seychelles) brokeoff from Madagascar at ca. 85 Ma along the coast (e.g., Storeyet al., 1995; Anil Kumar et al., 2001; Pande et al., 2001; Fig. 1).This does not mean, of course, that the crust was thinned. Itis possible that strike-slip motions were involved in India-Madagascar separation (e.g., Chand and Subrahmanyam, 2003;Raval and Veeraswamy, 2003), and it is not known whethercrustal thinning was involved besides crustal extension and frac-turing along the then–western continental margin of India. Kingand Anderson (1995) argued that CFB events may be the prod-ucts of lithospheric splitting along preexisting discontinuitiesrather than of plume-caused uniform lithospheric thinning ordistributed stretching. The splitting of the lithosphere permitsadiabatic ascent from great depth (>150 km) and extensive melt-ing and yields a high melting column.

The present Indian continental shelf, where many horst-graben complexes with basement ridges and Deccan lava out-liers occur, is a region of thinned, extended crust that subsidedbelow sea level subsequent to Deccan volcanism and has receivedseveral kilometers of Tertiary sediments, and the present westcoast of India is apparently the eastern limit of this stretchedand thinned crust. Thus, lithospheric extension during the India-Madagascar breakup event, and also immediately preceding theDeccan event, created the pronounced lithospheric asymmetry.The strongly structure-controlled volcanism itself occurred atthe time of the ridge jump that India experienced (Fig. 1).

An Asteroid Impact?

Why should the ridge have jumped? Plume proponentswould propose that the impingement of a plume head at the baseof the lithosphere weakened the lithosphere and provided thestress necessary for rifting it, which would also explain, for them,Deccan flood volcanism itself. An alternative active mechanism

494 H.C. Sheth

to a plume is asteroid impact. Hartnady (1986) proposed a LateCretaceous asteroid impact in the Indian Ocean Basin, and Chat-terjee (1992) proposed that this impact resulted in a large impactcrater (which he coined as the “Shiva Crater”) off western Indiaand the jump of the Central Indian Ridge to the new location,which caused the Seychelles to split off from India. The ShivaCrater, according to Chatterjee, was split in two by the ridge, andthe two halves were carried away from each other by subsequentseafloor spreading. Large asteroid impacts have been proposedto have caused the formation of flood basalt provinces, andquantitative modeling of the process has been performed (Joneset al., 2002). An impact model has also recently been proposedfor the Ontong-Java Plateau (Ingle and Coffin, 2004), becauseseveral first-order features of the plateau are at variance withwhat is expected from a mantle plume-head origin (Tejada et al.,2004). However, ridge jumps need not be caused by “active” up-wellings from below, but may be related to the dynamics of theplates themselves, including evolving plate boundaries, conti-nental collisions, and crustal thickening.(e.g., Hamilton, 2002).If so, it is of interest to note that the Indian shield is supposed tohave already contacted Asia by 70 Ma or so (Jaeger et al., 1989),and the early stages of the collision with Asia may have changedthe long-distance stress field across the plate.

Early Alkaline Magmatism Due to Incipient Ridge Jumps?:The Deccan and Mexico

Important alkaline mafic magmatism, though comparativelyof smaller volume, took place before, during, and after the mainflood basalt phase (e.g., Basu et al., 1993; Sheth, 1999b; Ray et al.,

2003) and was apparently restricted to the rift zones, which werein existence well before volcanism and contained thick Meso-zoic sediments. On the other side of the globe, the Mexicanvolcanic belt provides an interesting comparison (Fig. 12). Thisis a linear belt ~1000 km long, and Miocene to Recent in age,with many monogenetic cone fields and several active andesiticstratovolcanoes (Verma, 2001, 2002). Ocean island basalt (OIB)–like alkaline magmatism is relatively low in volume comparedto the widespread andesitic volcanism, but is found throughoutthe belt. There is a prominent triple junction in the western partof the belt (the Guadalajara triple junction), and both the triplejunction and the OIB-type magmatism in the belt have beenascribed to a mantle plume (e.g., Márquez et al., 1999). Someothers (e.g., Sheth et al., 2000) propose the origin of the OIB-type magmatism in an enriched sub-Mexican mantle lithosphere.Earlier Luhr et al. (1985) proposed that the recent and ongoingmagmatism in western Mexico is related to abortive attempts ofthe East Pacific Rise to relocate onto the continent. They did notconceive an “active” cause. The Deccan-Indian scenario priorto the flood basalt episode is strikingly similar—several rifts thathosted thick sediments and in which low-volume, OIB-typemagmas were emplaced (Sheth, 1999b). Can these rifting eventsbefore the flood volcanism be related , then, to incipient jumpsof the Central Indian Ridge onto the Indian subcontinent, as hasbeen argued for western Mexico? If so, it was only the last suc-cessful event that eventually split the Seychelles from India.

In summary, the Deccan volcanic episode was significantlycontrolled by lithospheric structure and was the end product oflong-duration continental rifting and alkaline magmatism, fol-lowed by full continental breakup and decompression melting

From Deccan to Réunion: No trace of a mantle plume 495

Ce

MIDDLE AMERICA TRENCH

Sierra Madre del Sur

Gulf ofMexico

Guadalajara

VeracruzMexico

City

0 200 400

km

25o N

20o N

110o W 100o W

SLP

Sierra M

adre Occidental

Sierra M

adre Oriental

RIVERA PLATE

PACIFIC PLATE

COCOSPLATE

Rivera FZ

Orozc

o FZE

ast P

acifi

c R

ise

MG

Tamayo FZ

Altiplano

TZR

CoR

ChR

Figure 12. The tectonic setting of theMexican volcanic belt (shaded). FZ—fault zone; MG—Mesozoic granitoids;TZR—Tepic-Zacoalco Rift; CoR—Colima Rift; ChR—Chapala Rift. Theboundaries of proposed terranes formingsouthern Mexico are also shown as dashedlines. Based on Sheth et al. (2000).

(Fig. 13). A “passive” model of rifting-induced convection fitsthe observations best.

ACKNOWLEDGMENTS

I dedicate this paper to Don Anderson for his monumental con-tributions to geodynamics, his efforts for a plume-free planet,and the inspiration he has provided. Participating in the 2003Geological Society of America Penrose Conference, Plumes IV:Beyond the Plume Hypothesis, in Iceland, was invaluable. I amgrateful to the conference conveners, Gillian Foulger, JamesNatland, and Don Anderson, for inviting me to it, and to the GSAand the International Association of Volcanology and Chemistryof the Earth’s Interior (IAVCEI) for the financial support thatenabled my participation. Interaction with the conference dele-gates was an enriching experience and a pleasure. I also thankG. Mohan for discussions and Mike Widdowson, Godfrey Fit-ton, Gill Foulger, and an anonymous referee for helpful reviewcomments (on a different manuscript submitted earlier to this

volume). I also thank an anonymous reviewer and Gill Foulgerfor helpful comments on the present manuscript. This work wassupported in part by Research Grant 03IR014 from the Indus-trial Research and Consultancy Centre (IRCC), IIT Bombay.

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496 H.C. Sheth

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Enriched Mantle (EM)

Depleted Mantle (DM)

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FLOOD BASALT PILE

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A

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