morphology and emplacement of flows from the deccan volcanic province, india

7/29/2019 Morphology and emplacement of flows from the Deccan Volcanic Province, India

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Bull Volcanol (2004) 66:2945DOI 10.1007/s00445-003-0294-x

R E S E A R C H A R T I C L E

Ninad R. Bondre Raymond A. Duraiswami Gauri Dole

Morphology and emplacement of flowsfrom the Deccan Volcanic Province, India

Received: 14 December 2001 / Accepted: 20 April 2003 / Published online: 5 July 2003 Springer-Verlag 2003

Abstract The present study is probably the first of itskind in the Deccan Volcanic Province (DVP) that deals indetail with the morphology and emplacement of the

Deccan Trap flows, and employs modern terminology andconcepts of flow emplacement. We describe in detail thetwo major types of flows that occur in this province.Compound pahoehoe flows, similar to those in Hawaiiand the Columbia River Basalts (CRB) constitute theolder stratigraphic Formations. These are thick flows,displaying the entire range of pahoehoe morphologyincluding inflated sheets, hummocky flows, and tumuli.In general, they show the same three-part structureassociated with pahoehoe flows from other provinces.However, in contrast to the CRB, pahoehoe lobes in theDVP are smaller, and hummocky flows are quite com-mon. Simple flows occur in the younger Formations and

form extensive sheets capped by highly vesicular, weath-ered crusts, or flow-top breccias. These flows have fewanalogues in other provinces. Although considered to beaa flows by previous workers, the present study clearlyreveals that the simple flows differ considerably fromtypical aa flows, especially those of the proximal variety.This is very significant in the context of models of floodbasalt emplacement. At the same time, they do not displaydirect evidence of endogenous growth. Understanding theemplacement of these flows will go a long way indetermining whether all extensive flows are indeedinflated flows, as has recently been postulated.

Most of the studies relating to the emplacement ofContinental Flood Basalt (CFB) lavas have relied onobservations of flows from the CRB. Much of the current

controversy surrounding the emplacement of CFB flowscenters around the comparison of Hawaiian lava flows tothose from the CRB. We demonstrate that the DVPdisplays a variety of lava features that are similar to thosefrom the CRB as well as those from Hawaii. This suggeststhat there may have been more than one mechanism orstyle for the emplacement of CFB flows. These need to betaken into account before arriving at any general modelfor flood basalt emplacement.

Keywords DVP Flows Pahoehoe Compound Simple Inflation Emplacement

Introduction

Some recent models for the emplacement of continentalflood basalts (CFB) hypothesize that many of these flowsare inflated pahoehoe flows, and that they may have beenemplaced in a gentle fashion analogous to inflatedpahoehoe flows from Hawaii and Iceland (e.g., Thordar-son and Self 1998). This has, however, been contended byother studies that propose that CFB flows have beenemplaced rapidly (e.g., Anderson et al. 1999). Differentmodels have been proposed to explain the exact nature ofendogenous growth in basaltic lava flows and lobes, andaspects such as the time scales and pulses of inflationhave been debated (Anderson et al. 2000; Self et al.2000). These studies have focused only on the ColumbiaRiver Basalts (CRB) and active volcanoes such as inHawaii.

In this paper, we seek to integrate the observations andinferences of our ongoing studies in the Deccan VolcanicProvince (DVP; Fig. 1). While we have made observa-tions on the northern and eastern parts of the province, wefocused on detailed documentation of features from thewestern part (Fig. 2) where exposures are excellent andmost of the previous stratigraphic work has been done.

Editorial responsibility: T. Druitt

N. R. Bondre ())Department of Geology,Miami University,114 Shideler Hall, Oxford, Ohio, 45056, USAe-mail: [email protected].: 513-529-3227Fax: 513-529-1542

R. A. Duraiswami G. DoleDepartment of Geology,University of Pune,411007 Pune, India


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Fig. 1 Map showing the extentof the Deccan Volcanic Pro-vince in India (modified afterDeshmukh 1988). The distribu-tion of compound pahoehoe andsimple flows is also shown

Fig. 2 Map depicting locationswhere observations were made

during the past 3 years. Boxesshow regions for which detailedmaps are shown in Fig. 3. Sev-eral of these have been referredto in the text. The principalrivers and the position of theWestern Ghats Escarpment arealso shown

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We describe lava flow morphology from the DVP byconsidering examples of each from specific locationswithin this region. We then discuss the emplacement offlows and formation of associated features, and theimplications that they may have for the evolution of the

DVP in particular, and CFB provinces in general. Fourareas have been the focus of most of our studies. Detailsof tumuli and small pahoehoe lobes around Dhule andDaund (Fig. 2) have been described elsewhere (Du-raiswami et al. 2001, 2002). In this paper, we focus onstudies of lava flow morphology from the Pune andSangamner areas (Fig. 3) in addition to observations fromnumerous other localities. This work is largely qualitativeas a consequence of the lack of even the most basic dataon various aspects of the Deccan flows that would haveaided a more quantitative approach. In spite of this, wefeel that it is a good basis for more comprehensive studiesin the near future.

The Deccan Volcanic Province

The DVP is one of the largest CFB provinces in theworld, covering an area of more than 500,000 km2 in thewestern and central parts of India (Fig. 1). It is postulatedthat an equivalent area of the Deccan Traps has beendown-faulted along the western coastal region and thevolume of erupted material could have been well over amillion km3. At places along the western part of theprovince, a continuous vertical succession of basaltic

flows with a thickness of more than 1,200 m can beobserved, and geophysical studies have indicated that thethickness attained by the lava pile is over 2,000 m in thewestern part of the province (Kaila 1988). On the fringesof this province around Nagpur (Fig. 1), the lava pile

decreases considerably in thickness to as little as 10 m. Ithas been postulated that the DVP formed around 65 Ma inresponse to the passage of the Indian plate over theReunion hotspot (e.g., Beane et al. 1986; Courtillot et al.1986; Cox and Hawkesworth 1985; Morgan 1972). Theeruption of the Deccan lavas is intimately related to theformation of the passive margin along the western coastof India, as the Seychelles micro-plate separated from theIndian plate. The DVP is constituted dominantly oftholeiitic basaltic lava flows, nearly horizontal, stackedone above the other. The sequence exhibits a low dip ofabout 1 to the southeast. The basaltic flows are intrudedat a number of locations by essentially doleritic dykes,some of which are speculated to represent feeders.Occasional alkaline intrusions have also been noted fromthis province (see Auden 1949; Powar 1987).

Considerable work has been done on the Deccanbasalts in the past two decades, especially with respect totheir stratigraphy, geochemistry, geochronology, andstructural geology (e.g., Subbarao 1988, 1999 and refer-ences therein). One of the most important achievementsof this work has been the establishment of a chemostratig-raphy for a large portion of the DVP (e.g., Beane et al.1986; Cox and Hawkeshworth 1985; Devey and Lightfoot1986; Mitchell and Widdowson 1991; Subbarao et al.

Fig. 3 Map of geology and lo-calities studied in a the Puneregion and b the Sangamnerregion

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1988). Flows have been grouped into Formations andSubgroups based on their field relations and geochemis-trymajor elements, trace elements, and trace elementratios. This stratigraphy (Table 1) is best constrainedalong the western part of the province, while mapping andgeochemical studies are ongoing in the northern andeastern regions (e.g., Malwa Traps and Mandla Lobe;Fig. 1). These studies have indicated that the mappedFormations extend over distances of hundreds of kilome-ters and occupy areas of the order of tens of thousands of

square kilometers. A lithostratigraphic classification ofthese lavas also exists, as formulated by the GeologicalSurvey of India (e.g., Godbole et al. 1996), which is basedentirely on the field characters of the lava flows. Crucialto both these classification schemes are regionallyextensive flows containing giant phenocrysts of plagio-clasetermed Giant Phenocryst Basalts in the Dec-can which have been used to separate the Formations.

It was widely believed that the Deccan Traps representa fissure type of eruption and that sheets of lava eruptedthrough numerous fissures/cracks in the crust and spreadlaterally over considerable distances, sometimes acrosshundreds of kilometers (West 1959). Based on the plume

model, it has been suggested that the main phase oferuption originated mainly from one major eruptioncenter, which was located in the northwestern part ofthe province (Beane et al. 1986). The observed southwardmigration and overstepping of the chemostratigraphicFormations is believed to be a consequence of thenorthward movement of the Indian Plate over the Reunionhotspot (Watts and Cox 1989; Mitchell and Widdowson1991). However, several other views regarding the sourceof the Deccan lavas exist, and multiple eruption centers

have also been proposed (Bhattacharjee et al. 1996; Kaleet al. 1992).

Some notable work on the Deccan lava flows and theirmorphology has been done in the past few decades (e.g.,Karmarkar 1978; Marathe et al. 1981; Walker 1969;Rajarao et al. 1978; Deshmukh 1988). Walker (1969)made some very important observations on the Deccanlavas and his analysis of various volcanogenic features is

one of the best in the province so far. He was the first tointroduce the terms Simple and Compound to flowsfrom the Deccan, and these terms are still prevalent,although they have not always been used in their originalsense. Karmarkar (1978) and Marathe et al. (1981)discussed the characters and emplacement of flows fromthe western DVP, describing the flows have beendescribed as Compact, Amygdaloidal, etc, which areterms without any genetic significance. Rajarao et al(1978) provided an informative description of the Deccanlavas and described them as pahoehoe and aa. They alsomentioned the occurrence of flows with mixed charactersand the fact that aa flows from the Deccan lack the basal

breccia horizon. Deshmukh (1988) discussed the generalcharacters of compound pahoehoe flows from the DVPand the mechanisms responsible for petrographic varia-tions in pahoehoe flow lobes. In the past few years,sinuous lava tubes and channels have been reported fromsome parts of the province (e.g., Thorat et al. 1996; Misra2002). Dole et al. (2002) have, however, commented onthe validity of these observations.

In general, though, studies of the physical characters ofthe Deccan lava flows and their emplacement havedeveloped in isolation from similar studies in other partsof the world. This has resulted in the development of localterminology and has resulted in considerable confusion.

Although several workers made detailed observations oflava flow morphology, a plethora of terms have been usedand this renders any comparison and meaningful inter-pretation difficult. Virtually no work has been done onsuch aspects of the Deccan lava flows and theiremplacement as vesicle distribution, segregation struc-tures, viscosity, and effusion rates. Hence, while volumi-nous information is published regularly on the physicalvolcanology of lava flows world-wide, this is not the casefor the Deccan lavas. Recently, some studies (Bondre etal. 2000; Duraiswami et al. 2001, 2002, 2003) have triedto document the character and emplacement style of flowsfrom the DVP and compare them with those from otherprovinces. These studies suggest that the pahoehoe flowsand certain associated features from the DVP are verysimilar in nature and scale to those from Hawaii.

Flows from the Deccan Volcanic Province

Although detailed maps of individual flows are notavailable for most Deccan Trap flows, it is well knownthat they extend over great distances. For example,Rajarao et al. (1978) mention certain flows that have beenseen to extend over areas of 3,000 km2 without pinching

Table 1 Chemostratigraphic classification of the Deccan Trapbasalts (Subbarao and Hooper 1988). This classification evolvedfrom concerted efforts on the part of several groups of researchersthat mapped the DVP during the early 1980s and 1990s. Eachformation refers to a mappable package of lava flows that displaysimilar physical, textural, and geochemical characters, and whichmay be separated by giant phenocryst basalts. Distinctive flowswithin each formation have been classified as members (not shownin this table). All formations have been grouped into threesubgroups based on some common characters, while these three

subgroups together define the Deccan Trap Group. See text formore details

Group Subgroup Formation

D Wai DesurE PanhalaC MahabaleshwarC AmbenaliA Poladpur

N Lonavala BusheB Khandala

A Kalsubai BhimashankarS ThakurwadiA NeralL Igatpuri

T Jawhar

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out. As mentioned earlier, the chemostratigraphic forma-tions extend over distances of hundreds of kilometers(Fig. 4). It is possible to treat many of these Formations asindividual flow fields, as defined by Keszthelyi et al.(1999). Flows from the DVP can be grouped into twomain categories, Compound pahoehoe and Simpleflows. The former type is almost exclusively confinedto older Formations (Kalsubai Subgroup; Table 1), while

the latter dominates in the younger Formations (WaiSubgroup; Table 1). In addition to this vertical disposi-tion, the two types occupy distinct geographical areas(Fig. 1). The area depicted as dominantly compoundexposes the older Formations, while that labeled dom-inantly simple exposes the younger Formations. Walker(1969) suggests that flows are compound close to thesource and become simple further away from it. This maybe why compound pahoehoe flows in the DVP occuraround the proposed source region in the northwest, whilethe simple flows occupy peripheral positions. However, ithas yet to be demonstrated that the simple flows didindeed originate in the northwestern part of the provinceand flowed to the south and east. Compound pahoehoeflows also occur in the Saurashtra region (Fig. 1), andsuch flows have been reported from the Mandla Lobe(Fig. 1; Solanki et al. 1996).

Compound pahoehoe flows

As Prof. G.P.L. Walker observed during his visit to theDeccan (Walker 1969), compound pahoehoe flows haveprobably been developed on a unique scale in the DVPand this makes it an excellent place to study such flows.

In spite of the ancient nature of this province, typicalpahoehoe features are extremely well-preserved. Exhu-mation by drainage has acted in such a fashion that atseveral places it is possible to study the surface

morphology of flow lobes. Delicate ropy structures andfestoons observed in the DVP are identical to similarfeatures in very young volcanoes.

Each Formation of the Kalsubai Subgroup, and theBushe Formation of the Lonavla Subgroup, consist of asequence of thick (>150 m at places) compound pahoehoeflows. Compound pahoehoe flows are constituted ofmultiple flow lobes (flow units; Nichols 1936) of varyingdimensions. In the DVP, although individual flows areoften >100 m thick (Walker 1969; Deshmukh 1988), theyare strongly compounded on a local scale, similar to theirHawaiian counterparts (Bondre et al. 2000; Duraiswamiet al. 2001, 2002). Walker (1969) measured 31 flow lobes

in 120 m thickness of a compound pahoehoe flow in theIgatpuri region, giving an average thickness of 3.7 m perlobe (ranging from 0.3 to13 m). Observations made inwells from the Igatpuri region corroborate these observa-tions (Fig. 5). Observations of these flows in the westernDVP can be made both in vertical sections and on surfaceexposures (exhumed). Studies of vertical sections of flowlobes are facilitated by stream and road cuts, whileexcellent exposures of exhumed surfaces are observedalong the channels and banks of streams. Some of theconstraints on observations include discontinuous out-

Fig. 5 Logs constructed from observations in wells from theIgatpuri region (Fig. 2). Note the thickness of flow lobes (as definedby a core and crust pair) and the relative thickness of cores andupper crusts of lobes

Fig. 4 Map showing the extent of two of the chemostratigraphi-cally defined formations in the Deccan Volcanic Province. Otherformations have been omitted for clarity

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crops due to highly dissected terrain, difficulty inobserving and accessing flows exposed along highvertical cliffs, and thick soil mantle and lack of exposurein some regions. We have documented characters of theseflows from the Dhule, Daund, Sangamner, and Pune areasof the DVP. As mentioned before, we focus in this paperon the latter two areas.

In regions of relatively higher reliefe.g., around

Sangamnerthe thicker flows form vertical to subverticalcliff faces. Natural caves often form by weathering alongthe boundaries between flow lobes (Bondre et al. 2000).In areas of relatively low relief, such as around Pune city,these flows can be studied in a number of isolated hills.Each flow in vertical section characteristically consists ofa few thick (! 10 m), laterally extensive (up to 100 mobserved in field exposures) sheet lobes, and numerousmuch smaller lobes of restricted lateral extent. The largerlobes (and many smaller ones too) display a characteristicthree-tiered structure (Aubele et al. 1988; Thordarson and

Self 1998) with a thick, vesicular upper crust, a densecore, and a thin lower crust (Fig. 6a). The tops of many ofthe smaller lobes display oxidized glassy rinds and ropes(Fig. 7a), while pipe amygdales (pipe vesicles filled withsecondary silica) are almost always present at the base(Fig. 7b). Lobes can be classified as either S-type or as P-type lobes of Wilmoth and Walker (1993). S-type lobeslack pipe vesicles and are vesicular throughout their

vertical extent. P-type lobes are characterized by pipevesicles and display a typical internal structure with avesicular base and top, and a relatively vesicle poor core(Fig. 6a, b, c). In the DVP, flow lobes of S-type are lesscommon than the P-type lobes. Table 2 depicts represen-tative measurements of sheet lobes from the westernDVP, while Table 3 shows the dimensions of smallerlobes measured in a road cut in Karhe Ghat (Fig. 3).Typical hummocky flows with tumuli are also observed inseveral parts of the DVP (Fig. 7c), especially along thechannels and banks of rivers where they have been

Fig. 6. a Field sketch of a vertical section of a compound pahoehoeflow observed at Warje (Fig. 3a). Note the typical subdivision offlow lobes into upper crust, core, and lower crust. The figure also

shows part of a simple flow that rests on the underlying compoundpahoehoe flow. b Field sketch of a vertical section through P-typeflow lobes observed east of Ganore (Fig. 3b). c Sketch of flow lobesexposed along the walls of the gorge of the Mahalungi River at

Nimgaon Bhojapur (Fig. 3b). Note the termination oflobe 2 againstlobe 4. Only the upper crust of lobe 4 is exposed, while the basalcrust, crudely jointed and spheroidally weathered core, and part of

upper crust of lobe 1 are exposed. d Sketch of small, bun-shapedpahoehoe lobes exposed along the walls of a gorge at Dapur(Fig. 3b)

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Fig. 7. a Ropy structure observed at Kondhwa Hill (Fig. 3a). b Pipe vesicles exposed east of Ganore (Fig. 3b). c Hummocky pahoehoeflow exposed at Daund (Fig. 2). d Squeeze-up occupying the inflation cleft of a lobe at Kondhwa Hill (Fig. 3a)

Table 2 Data on representative sheet lobes from different parts of the western Deccan Volcanic Province

Location Total thickness(m)

Crust(m)

Core(m)

Remarks

Pune University hill 19 6 13 Base not exposed. Median horizontal joint plane well exposed.Core with segregation veins, rare vesicle cylinders. Extensivein length (>200 m).

Warje 5.5 3.1 2.4 Fairly extensive sheet lobes with segregation features (>50 m inlength exposed in the section)

7.0 2.0 5.04 1.70 2.302.05 1.20 0.85

Kondhwa hill 12 6 6 Thick, extensive (exposed length >50 m) sheet lobe. Crust withnumerous squeeze-ups and some tumuli. Squeeze-ups intrudedhorizontally into the crust and have twisted forms. Pipe-like

vesicle cylinders. Base is smooth and horizontal, contact withlower flow marked by red horizonKatraj Ghat 6.9 5 1.9

South of Sangamner 2.12 0.92 1.0 Some brecciated patches are observed in the otherwise smoothcrust of this sheet lobe. Segregation veins display branchingpatterns.

Northwest of Sangamner 7.5 4.50 3.0

South of Sangamner 7 5 2 Gradational between sheet lobe and tumulus. Fairly steep dipsof crustal slabs observed

Karhe Ghat Partially exposed 6.5 3.1 Segregation veins are found at four different levels and showbranching. Numerous vesicle cylinders are present; domedvesicles are found at 2 different levels

South of Sangamner Partially exposed 2.5 5.5 Lower surface is hummocky in nature

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exposed due to exhumation. The tumuli in the DVP arevery similar to the Hawaiian tumuli in morphology anddimensions (Duraiswami et al. 2001, 2002). Tumuli invertical sections can be recognized by the virtue ofsteeply dipping crustal slabs and inflation clefts occupiedby squeeze-ups (Fig. 7d) or crack fillings.

Although no systematic studies documenting texturaland petrographic variations across pahoehoe lobes havebeen undertaken, the petrography of a few lobes from thepresent study reveal interesting insights into texturalvariations and cooling history of the lava. The outermost1 to 3 cm quenched selvage of the toes and the lobes showa predominantly hypohaline texture. A near total absenceof microphenocrysts and the absence of opaques (il-menite/titanomagnetite) characterize this zone (also seeThordarson and Self 1998). The upper crust and the basalcrust exhibit a typical hypocrystalline texture with fineFeTi oxide minerals. The core tends to be morecrystalline but small intra-mineral voids lend a dikty-taxitic texture to the core. Intergranular, ophitic andsubophitic textures are often seen in thick sheet-lobes. Inthe core, FeTi oxides are seen as subhedral skeletalcrystals. Interesting petrographic variations in tumulihave been discussed by Duraiswami et al. (2001).

We measured vesicle sizes and documented theirdistribution in more than 20 lobes in the western DVP.The cores of Deccan lobes are very sparsely vesicular andhence only vesicles from the upper and lower crustsprovide any meaningful data. In most cases the vesicleparameters were measured in vertical sections or in handsamples, while in some cases, exhumed surfaces wereused. Measurements were made at numerous levels withinindividual flow lobes. Wherever distinct vesicle bandingwas observed, vesicles in those bands were measured. Ateach level, the long and short axes (L1 and L2) of around

50 vesicles were measured using a vernier calliper.Vesicle frequency (F) was determined by counting allvesicles within areas marked on the outcrops or on handspecimens, and calculating the means of these. It is likelythat in some cases, the true three-dimensional nature ofthe vesicles was not documented. In the future, we hopeto improve upon this by measuring vesicles by usingsections oriented at different angles within the same lobe,and other more advanced techniques. Implications ofthese patterns are discussed in the next section, while amore detailed account of vesicle distribution studies fromthe western DVP is currently under preparation. Thecharacters of each of the three divisions, namely the uppercrust, core and the lower crust of lobes, sheets, and tumuliare described in detail below.

Upper crust

The thickness of the upper crust ranges from about a thirdto half the thickness of the lobe, and it commonly displaysvesicle layering. The largest sheet lobes that we havemeasured have crustal thickness of around 10 m. Fig. 8shows the observed vesicle distribution from the crusts of

two lobes exposed in the Fergusson College Hill, Pune(Fig. 2a). Lobe 6 displays a downward increase in theaverage vesicle diameter [(L1+L2)/2], and a dramaticdecrease in the frequency (F; number of vesicles/cm2),similar to observations by Cashman and Kauahikaua(1997). Lobe 4 shows a similar trend, although the patternfor F versus depth below the top suggests alternatingvesicle-rich and vesicle-poor bands. This is corroboratedby field observations of the crust of the flow lobe. Theelongation ratio (E=L1/ L2; ratio of long axis of vesicle tothe short axis) appears to vary according to the thickness

Table 3 Measurements of di-mensions of smaller lobes ob-served in a road cut in KarheGhat. The section is about 10 mthick and consists of a couple ofsheet lobes and numerousoverlapping smaller lobes.P andS refer to P-type and S-typelobes, respectively. The pre-ponderance of relatively small

P-type lobes is evident.

Length of lobe (m) Thicknessof lobe (m)

Description Type

4.9 0.8 No distinct core, vesicles present throughout S0.95 0.28 Pipes are present at the distal end of the toe P-type toe0.2 0.12 No pipes present, unit is entirely glassy S-type toe3.9 0.54 P2.2 0.44 P

Partially exposed 2.68 P8.6 1.02 P

22.5 1.73 P0.54 0.3 Pipes are present at the distal end of the toe P-type toe4.45 1 Highly undulating lower surface S6.78 0.8 S1.9 0.3 S4.8 0.95 P1.7 0.95 P1.65 0.6 P3.75 1.1 P0.2 0.12 P-type toe3.2 2.15 P0.3 0.2 P-type toe2.6 0.5 P6.2 1 Associated 10 p-type break-outs P3 0.5 P1.5 1.5 Associated 1 s-type and 9 p-type break-outs P

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of the lobe. In thicker lobes such as Lobe 4 and Lobe 6, Eincreases slightly with depth below the top, and mayincrease dramatically near the transition with the core.The transition from the crust to the core in such cases ischaracterized by some large (>2 cm) flattened, elongatedor domed vesicles. In thinner lobes (~ 3 m), E decreasesmarkedly with distance below the top. In another paper

(Duraiswami et al. 2003) we have described the vesicledistribution and deformation in a slabby pahoehoe flow,for which vesicle deformation due to shear was muchmore significant.

Crusts of pahoehoe sheet lobes from the CRB typicallydisplay considerable vertical, hackly jointing (e.g., Thor-darson and Self 1998). In contrast to this, well-developedvertical jointing is rarely observed in crusts of the Deccanlobes. If present, vertical jointing is poorly developed. Asheeted appearance due to differential weathering ofvesicle-rich and vesicle-poor bands is more common andin such cases, this zone is extensively calcretized.Anderson et al. (1999) describe the jointing of Hawaiian

tumuli and inflated sheet lobes as consisting of threedistinct zones: an upper zone of crude to well-definedcolumnar joints, a middle zone of planar fracture surfaceswith no distinct fracture surface features, and a lowerzone of fracture surfaces that show evidences of brittleand ductile deformation. Some tumuli in the Deccan (e.g.,Daund) were seen to exhibit a similar structure, but mostlobe crusts are poorly jointed.

Thick squeeze-ups often break through the crust andsometimes form small pahoehoe toes. They may occupyaxial clefts of tumuli on the crusts of the lobes.

Sometimes, offshoots of squeeze-ups are seen to haveintruded horizontally into the crust (e.g., Fig. 5, Du-raiswami et al. 2001). Wherever exhumed surfaces areseen, as along river channels, they are usually gentlyhummocky in nature and riddled with squeeze-ups thatmay be as much as a meter wide. We are currentlyinvolved in generating high-scale surface maps as well as

documenting the morphology of these features in verticalsections. The crust has a much finer texture as comparedto that of the core, with glass forming a significantcomponent of the groundmass. The crusts of smaller lobesand toes are essentially thin glassy rinds, often displayinga red to orange color due to oxidation and alteration. Thecrusts of some lobes are characterized by the presence ofpatches of breccia, or slabs. Such lobes representtransitional pahoehoe, but are relatively minor con-stituents of the Deccan flows. In another paper (Du-raiswami et al. 2003), we describe a slabby pahoehoe flowtransitional to aa and discuss its emplacement andimplications.

Core

The interface between the crust and the core in thick lobesis usually sharp, marked by a change in the style ofjointing and the distribution of vesicles. The core has adense appearance and is sparsely vesicular. It rarelydisplays well-developed columnar jointing, while crude,four-sided columns or irregular joints are more common.Spheroidal weathering is commonly observed along these

Fig. 8 Vesicle distribution pat-terns from two lobes exposed atthe Fergusson College Hill.Average vesicle size [(L1+L2)/2], average frequency (F) andelongation ratio (E) have beenplotted against D, the distancein centimeters below the ex-posed upper surface of the flowlobe. L1 and L2 refer to the long

and short axes of a vesicle,average frequency is the num-ber of vesicles per unit area,while the elongation ratioequals L1/L2

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joints. Different types of segregation structures areobserved in the core. Flow lobes around Pune and inthe area around Sangamner afford detailed observations tobe made on these features. The segregations are usuallydarker than the host and slightly coarser in texture.Vesicle cylinders (Goff, 1996) are seen in the lower andmiddle parts of the core, while segregation veins (vesiclesheets; Thordarson and Self 1998) occur near the interface

between the crust and the core. Figure 9a is a sketch ofpart of a flow lobe containing segregation veins from theKarhe Ghat near Sangamner. Most veins are up to 5 cm inthickness, but thicker (>25 cm) veins are not uncommon.The segregation veins display an irregular form andbranching patterns, which indicates that they occupyjoints formed at the early stages of cooling. It is alsopossible that they propagated by hydraulic fracturing(Walker 1993). Their sinuous nature may also indicatethat the viscous lava during later stages of cooling wasstill mobile, which led to the deformation of the veins.The vesicle cylinders are almost always rootless, and arenot connected with the pipe vesicles in the basal crust,

unlike in the CRB (Thordarson and Self 1998). A typicalcylinder in the Deccan is broader in the middle and taperstowards the top and the bottom. However, mushroom-shaped and pipe-like forms have also been observed(around Kurundwad and at Kondhwa, respectively).Cylinders in thick lobes can be up to 1 m tall. They aresometimes observed to connect to segregation veins. Inthe cores of some flow lobes, these structures areessentially trails of vesicles and are not occupied bysegregated material. Cylinders are also often bent or at anangle to the flow-top, probably in the direction of flow.The core usually displays ophitic or subophitic textures.

Basal crust

The basal crust generally does not vary greatly inthickness irrespective of the thickness of the lobe (exceptin very small lobes). A measurement of the thickness ofthe basal crust from 46 lobes indicates that the meanthickness is around 0.3 m. This may, in part, have been aresult of a sampling bias in favor of smaller lobes thathave thinner basal crusts or have only a glassy rind. In thethicker lobes (>3 m), the thickness of this zone is usuallyaround 0.5 m. The lowermost portion of this zone ischilled and glassy. Pipe vesicles (almost always showingan inverted Y geometry) filled by cryptocrystallinesilica are very common (Fig. 7b). Pipe vesicles/amygdalesare observed even in the smallest lobes. Although manyworkers consider pipe vesicles as belonging to the samecategory as vesicle cylinders, those in the Deccan nevercontain segregated material. Small (around 2 mm), moreor less spherical vesicles also occur in this zone.Measurements on two lobes (Lobe 1 and Lobe 2) fromthe Fergusson College Hill (Fig. 3a) yield values of 2.28and 3.24 mm for average size, 1.4 and 1.37 for E, and 2/cm2 and 3/cm2 for F, respectively. Pipe vesicles are notalways restricted to the base but sometimes extend well

into the lower core. Pipe vesicles occurring at twodifferent levels are not uncommon and excellent exam-ples are seen in the Karhe Ghat near Sangamner (see alsoFig. 7b). These features are indicative of multipleinjections of lava and suggest endogenous growth(Wilmoth and Walker 1993). Recently, we have observedspiracles at the base of a flow lobe northwest of Pune thathave incorporated clay, and this is being documented indetail. The geometry of the contact with underlying lobesdepends primarily on the microtopography on which thelobe/lobes were emplaced, as described by Duraiswami et

Fig. 9. a Sketch of part of a flow lobe at Karhe Ghat (Fig. 3b). A,B, C, and D refer to the crust, zone of elongated vesicles, jointedcore, and segregation veins respectively. b Sketch of a thick sheetlobe observed at Kondhwa Hill (Fig. 3a). Note the relatively sharpand flat contact of the flow lobe with underlying lobes. c Sketch ofa lobe exposed to the southwest of Sangamner (Fig. 3b). Theundulating basal contact is clearly visible

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al. (2002) for flow lobe tumuli. This is underlined by thefact that in two lobes that are vertically superposed, thepipe vesicles may bend in completely different directions.Larger sheet lobes are usually characterized by planarbases (Fig. 9b); however, the contact can be quiteundulating if they are underlain by hummocky flows(Fig. 9c).

Simple flows

The term simple flow was first coined by Prof. G.P.L.Walker during his visit to the Deccan in 1969 (Walker1969, 1971), and has since been used to describe thesecond major category of flows in the DVP. As definedby Walker (1971), the term simple refers to flows thatare themselves not constituted of smaller flow lobes.These flows occur in the northern, southern and easternparts of the DVP where compound pahoehoe flows arerarely observed (Fig. 1). This includes the youngerstratigraphic Formations (Khandala Fm of the Lonavla

Subgroup and all Formations of the Wai Subgroup).Simple flows vary in thickness generally from a fewmeters to several tens of meters. Flows greater than ahundred meters thick have been reported from thenorthern parts of the province (Rajarao et al. 1978).Thick exposed sections of simple flows are observed inregions such as Mahabaleshwar and Toranmal. Mahoneyet al. (2000) describe a 870-m-thick section from theToranmal area, consisting of simple flows ranging inthickness from 10 to 120 m. Most such studies have,however, focused on the chemostratigraphy of the flows,and detailed descriptions of simple flows are sparse. Eachsimple flow tends to maintain a relatively constant

thickness over considerable distances (Fig. 10a, b). It isdifficult to quantify these distances in view of thedissection in the province as well as poor exposure inmost of the eastern parts. An example of the distancesinvolved is a simple flow that can be observed for most ofthe way from Pune to Daund (approximately 80 km).Each simple flow appears to be a single cooling unit andthere is presently no evidence to indicate the presence ofmultiple flow lobes. Keszthelyi et al. (1999) state thatmost simple flows may actually be large sheet lobeswhich, when traced over a considerable distance, willterminate against other lobes. Although this observationhas been verified for some of the CRB flows, we have yetto observe something of this nature in the DVP. In theabsence of a suitable alternative term to describe theseflows (given that they are sufficiently different frompahoehoe flows from the Deccan), they have beenreferred to as simple flows in this paper. We madedetailed observations of several outcrops in the PuneareaKatraj Ghat and Dive Ghat; flows belonging to thePoladpur and Ambenali Formationsand around Sataraand Kurundwadflows belonging to the Poladpur, Am-benali, and Mahabaleshwar Formations. The thickness offlows ranges from 10 to 50 m, and flows can be followedalong cliffs in the vicinity for kilometers. Figure 11

displays the internal structure of a typical simple flowfrom the DVP, based on our observations from thewestern DVP and observations of previous workers fromthis and other regions. A typical simple flow displaysthree zones crustal, central, and basal as describedbelow.

Crustal zone

Our observations suggest that the nature of the crustalzone of simple flows varies considerably. Although someflows do display preserved crusts, many are characterized

by reddened (oxidized), rubbly tops (Fig. 12a) or flow-topbreccia, as observed around Kurundwad and Satara. Thehighly vesicular/amygdaloidal nature (almost scoriaceousat places) and fine to glassy texture are immediatelyapparent. The frequency of vesicles appears to be highthroughout the crust, unlike the crusts of inflated pahoe-hoe lobes where it decreases with depth below the top.The vesicles are also much smaller (~12 mm). Quanti-tative studies on vesicle distribution patterns of theseflows are ongoing. Lava blisters, filled with cryptocrys-talline silica are sometimes observed. Small lobes and

Fig. 10. a Photo showing a laterally continuous simple flow ( darkband towards the top marked by arrow) observed close to Satara(Fig. 2). b Simple flows seen along cliffs in the Satara region(Fig. 2). The dark bands are thickly vegetated flows

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squeeze-ups that are ubiquitous in the pahoehoe flows, arealmost never observed in the crustal zone of the simpleflows. The flow-top breccia is usually constituted ofirregular, highly vesicular/amygdaloidal fragments in amatrix of fine, often glassy material (Fig. 12a) and rangesfrom 0.85 to 5.00 m thick. This zone is invariablyreddened and often takes on a crumbly appearance. The

flow-top breccia grades into the underlying flow and israrely entrained within the coherent part of the flow. Thegeneral appearance is of a well-developed, vesicular crustthat has been disrupted at some stage in the emplacementof the flow. It is very difficult to trace a single flow and toascertain if the flow-top breccia is a continuous horizon,or if it grades into something else. Along several cliffsections (e.g., Mahabaleshwar), however, the contactbetween two flows is clearly visible for a considerabledistance due to the differential weathering of flow tops.This topographic expression probably owes its origin tothe softer flow-top breccia and suggests that suchhorizons are fairly continuous.

Central zone

This is usually the thickest part of the flow (Fig. 12b); it isoften well jointed and displays columnar jointing (some-times multi-tiered). In the localities that we have studied(mentioned earlier) the thickness can be up to 20 m. In thenorthern parts of the province (Malwa Traps, and east ofDhule), where much greater thickness of simple flows(>50 m) have been observed, the central zone isproportionately greater in thickness. Our study has notrevealed the presence of segregation structures in the

central zones of simple flows. The frequency of vesiclesis much lower than that in the crustal zone. In severalflows, elongated or flattened silica-filled vesicles havebeen observed near the interface of the crustal zone andthe central zone (Fig. 11). Simple flows in the DVPgenerally can exhibit well-developed jointing patterns(Fig. 13). These patterns are generally persistent laterallyand are referred to as colonnade and entablature (Tom-kieff 1940; Long and Wood 1986). Entablature refers to0.30- to 0.50-m- thick, irregular columns that occur in theupper portions of flows. They commonly deviate from a

perpendicular orientation and may form radiating pat-terns. In certain flows, two or more levels of entablatureare seen, usually separated by platy joints (Fig. 13b, c).Well-developed colonnade structures are usually seen in

Fig. 12. a Photo of flow-top breccia exposed close to Kurundwad(Fig. 2). White amygdales are visible, while individual fragmentsand horizontal joints filled with calcium carbonate have beenmarked for clarity. b Central zone (~25 m) of a thick flow exposed

in a quarry close to Kurundwad (Fig. 2). Note the relatively well-developed columnar jointing and the considerable extent of theflow in this exposure. c Basal zone of a flow exposed in theDhadgaon area (Fig. 2). Note the smooth nature of the base. Xrefers to a zone of closely spaced joints, while Y indicates darkglassy bands

Fig. 11 Morphology of a typical simple flow. The upper parts ofthe upper flow, as well as the base of the lower flow have not beendepicted, since this is how such flows are exposed in most low-relief regions

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the lower portions of most simple flows. Columns arecommonly perpendicular to the base of the flow and theirwidth ranges from 0.50 to 2.0 m. An upper colonnadezone is also present in some flows. The entablature isoften glassy and sometimes shows horizontal vesicularzones. In other flows, no clear distinction between thecolonnade and entablature can be made, and the entireflow shows highly irregular jointing and a fine texture.

Good examples of entablature and colonnade structuresare seen at Nanand-Bhavjai Ghat, Toranmal, etc. (see alsoDe 1996) and the southern parts of the Western Ghats(Katraj Ghat, Dive Ghat, Mahabaleshwar, etc.). Thecolumns often fan out in a radial manner, encirclingbreccia. The proportion of flows displaying well-devel-oped columnar jointing is, however, much less than thatin the CRB.

Basal zone

Most flows display a sharp, slightly undulating, glassy

contact with the top of the underlying flow. This zone hasa variable thickness and often shows close spaced jointingand a bluish-black sheen as seen in (Fig. 12c). It is rarelyred as in the pahoehoe flows. This is particularly wellobserved in Dive Ghat and around Kurundwad. Plagio-clase phenocrysts can often be observed in this zone.Although this zone can be highly vesicular, pipe vesicleshave not been observed. The simple flows rest on groundslopes less than 4 and satisfy the criteria for thedevelopment of pipe vesicles (Walker 1987). Hence, theabsence of pipe vesicles must be related to the rheology of

the lavas and their emplacement style. Thin patches ofbreccia are sometimes present at the base, but these areusually not extensive. Sometimes it is very difficult toascertain whether the breccia at the base of these flows isthe flow-bottom breccia of the upper flow, or the flow-topbreccia of the lower flow. It is worth noting that noevidence of scouring of the base has been observed in anyof the flows (e.g., Fig. 12c).

Emplacement of the flows

Compound pahoehoe flows

The compound pahoehoe flows in the DVP displayexcellent, unambiguous evidence of endogenous growth.In this sense, they are very similar to inflated pahoehoeflows in Hawaii (Hon et al. 1994), the Columbia RiverBasalt Province (Thordarson and Self 1998), and Aus-tralia (Whitehead and Stephenson 1998). The featuresindicating endogenous growth are (1) compound nature of

the flows with sheet lobes displaying thick crusts andvesicle layering. The rapid downward decrease in vesic-ularity accompanied by an increase in vesicle sizeobserved in the Deccan is a characteristic feature ofinflated lavas (Cashman and Kauahikaua 1997);( 2) anabundance of local inflation features such as tumuli. Thegreat thickness and abundance of compound pahoehoeflows in the western DVP strongly suggests a slow, ratherthan rapid emplacement during sustained eruptive epi-sodes. The preliminary vesicle distribution studies indi-cate that two neighboring lobes may display different

Fig. 13. a Field sketch of asimple flow exposed in DiveGhat (Fig. 3a). A refers to theentablature, a zone of randomlyoriented jointing that imparts achaotic appearance to the zone.B refers to the colonnade, char-acterized by more regularcolumnar joints. b Sketch ofpart of a simple flow exposed in

Katraj Ghat (Fig. 3a) displayingthree tiers of joints, each sepa-rated by platy joints. c Log of asimple flow from Katraj Ghat(Fig. 3a) showing multi-tieredcolumnar jointing. CN iscolonnade, EN is entablature,PJ is platy joints, while BZrefers to the basal zone

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patterns. While more studies are needed to comment onthis, it suggests that subtle variations in volatile content,degree of endogenous growth or pulses of inflation, rateof cooling, formation of break-outs, etc., may all controlvesicle distribution in pahoehoe lobes.

The Deccan Trap flows extend for tens to hundreds ofkilometers and the area occupied by these compoundpahoehoe flows is almost half of the total area of the

province (Fig. 1). However, it is not clear if the presenceof inflation features is alone sufficient to explain the greatareal extent of flows from the DVP, particularly in theabsence of well-developed tube systems. Flows from theCRB are constituted of some thick and extensive (kilo-meter scale) sheet lobes (Keszthelyi et al. 1999), while thelongest flow lobes in the DVP are generally observed toextend over a few hundred meters. There is a predom-inance of smaller lobes over larger ones (Table 3). It isdoubtful if such small-scale inflation features would besufficient to enable the long distance transfer of lava. Thelarge areal extent of flows in such a scenario cantheoretically be explained by:

1. The efficient delivery of lava through well-developedtube systems. The efficacy of lava tubes in transferringlava over long distances and in minimizing cooling iswell known and has been discussed in detail byKeszthelyi (1995); Keszthelyi and Self (1998); Kaua-hikaua et al. (1998), etc. Recently, some sinuous lavatubes have been reported by Misra (2002) in parts ofthe western DVP. Evidence for a small tube system hasalso been documented by Duraiswami et al. (2001),based on the disposition of tumuli. If these features areindeed tubes, they could shed considerable light on theemplacement of the flows. However, in a large part of

the DVP, tubes (sensu stricto) have not been discov-ered. Tubes in areally extensive systems could also bemore like sheets in geometry (R.A.F Cas, personalcommunication). Jim Kauahikaua (personal communi-cation) has pointed out that the term tube need not beused in a restrictive sense and that other features suchas elongate tumuli could also play the same role (e.g.,Kauahikaua et al. 1998). The question in the DVP,however, is whether elongate tumuli could transferlava over the distances that the flows are presentlyseen to have flowed. This is further elaborated upon inthe next point.

2. The long distance transfer of lava via sheet flows:Keszthelyi and Self (1998) discuss the efficacy ofsheet flows in transporting lava over long distances.Their modeling suggests that a sheet with a 5-m-thickcrust emplaced on a slope of 1% with an effusion rateof 72 m3/s will, in theory, lead to conditions of zerocooling. However, in this case, a single sheet greaterthan 100 km long may be expected. So far, there isvery sparse evidence of the existence of such featuresin CFB provinces including the CRB. On the contrary,the longest lobes in the CRB are of the order of a fewkilometers, while in the DVP, they are even smaller.Could the transfer have been achieved by a system of

interconnected and overlapping lobes, now anasto-mosed, rather than well-defined tubes? In such ascenario, the flow field might advance by the constantbudding of new pahoehoe sheets at the fronts or tops ofpreviously inflated ones, and by ephemeral ventformation. Some evidence in favor of this mechanismmay be obtained from the ubiquity of large, dyke-likesqueeze-ups that were described earlier (see also

Karmarkar 1978). The important question, though, iswhether these conduits could be thermally efficient tocontinue to feed lava from one lobe to the other overlong periods of time and over large distances.

3. The assumption that several emplacement units werefed simultaneously from different fissure segments oflong vent systems (Keszthelyi et al. 1999): this impliesthat the eruptions were truly polycentric and that lavaextruded out of widely scattered vents throughout theprovince as proposed by Marathe et al. (1981) andKale et al. (1992). However, concrete evidence for thisis presently lacking.

Simple flows

The thick, laterally extensive simple flows in the southernand eastern parts of the Deccan have been considered tobe aa flows by previous workers (e.g., Godbole et al.1996). A study of these flows in the western DVPindicates that most of these flows do not satisfy thecriteria (e.g., Macdonald 1972; Peterson and Tilling 1980)for classifying them as aa flows sensu stricto, especiallyproximal-type aa (Rowland and Walker 1987). They donot show the presence of flow-bottom breccia (althoughin rare cases it has been observed) or irregular, twisted

vesicles that characterize true aa flows. Similarly,entrains of the flow-top clinker are often found withinthe massive cores of aa flows, a feature which is notobserved in the DVP. A strong piece of evidence againsttheir being aa flows is their broad (at least a few km)sheet-like morphology and the absence of levees andother channelized flow-related features. This is quiteunlike aa flows, which tend to have a narrow, elongatemorphology owing to rapid emplacement in open chan-nels (see Keszthelyi et al. 1999). The flow tops of thesimple flows in the DVP contain a high frequency ofsmall, almost spherical vesicles and are hence more akinto disrupted pahoehoe crusts. The appearance of the FTBis very different from the jagged and spinose clinker thatcharacterizes aa flows.

There is a possibility that the simple flows thickenedinitially by inflation and developed the flow-top brecciaslater (Duraiswami et al. 2003). The brecciation may havebeen initiated as a response to an increase in flow rate orviscosity during the later stages of emplacement. In otherwords, these flows are somewhat similar to slabbypahoehoe flows/pahoehoe flows with rubbly tops. How-ever, features indicative of inflation are rarely observed.Each simple flow appears to have been emplaced as abroad sheet over a low but consistent gradient during a

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single eruptive pulse. The volume erupted during eachpulse as well as the volumetric flow rate was sufficient toproduce a flow of considerable lateral extent. Thisprobably prevented the break down of the flow/flowfront into discrete flow lobes. The presence of flow-topbreccias and elongated vesicles in the cores of these flowssuggests the influence of substantial shearing duringemplacement, as well as relatively higher flow rates.

However, neither channelized flow nor scouring of thesubstrate seems to have occurred. It is improbable that thesimple flows were emplaced in a manner analogous totypical aa flows, or as turbulent sheets at extremely highdischarge rates as proposed by Shaw and Swanson (1970).Duraiswami et al. (2003) explore the possibility of theseflows being akin to either inflated pahoehoe sheet flows,or distal type aa flows. They discuss the possible impactof plume-generated uplift and steepening gradients toexplain the large areal extent of simple flows. Whilecertain simple flows may have been emplaced as distaltype aa, many of them are sufficiently distinct inmorphology from true aa and true pahoehoe flows, and

warrant a more systematic and detailed study.It is likely that these flows originated as relatively fluidlava with low or no yield strength. The highly vesicularnature of the crust, whether preserved or disrupted,suggests that these lavas were rich in dissolved volatiles.The effect of volatiles on the viscosity of lava is not verywell constrained (Keszthelyi and Self 1998). In adissolved state, volatiles may decrease the viscosity ofthe lava (Sparks and Pinkerton 1978). Exsolution ofvolatiles and the presence of bubbles will generally leadto an increase in viscosity since the bubbles act as rigidspheres (Keszthelyi and Self 1998). However, at highstrain rates the bubbles may be sheared and this will

actually lower the viscosity (op cit). The high initialfluidity of the simple flows may have been a result of thepresence of a high proportion of dissolved gases, and evenafter exsolution, the strain rates were probably highenough to maintain the pre-exsolution viscosity. Evidencefor high strain rate comes from the ubiquitous FTB. Thefact that fragments in the FTB are often highly vesicularsuggests that substantial degassing preceded the breccia-tion. However, stretched vesicles indicative of such strainrates are not observed in the FTB, but are observed in thecentral zones of such flows. It is now imperative toascertain whether the present thickness of these flows isthe same as that when they were erupted, or is a result ofsome process of endogenous growth. Similarly, it isimportant to determine the duration of emplacement for asingle simple flow. Unfortunately, since no quantificationof morphology of these flows exists, and since they havepoorly preserved crusts, the equation of Hon et al. (1994)cannot be directly used to calculate the duration. Themorphology of the simple flows in the DVP needs to bestudied in greater detail since they may prove to be crucialin answering questions regarding flow rates, and mech-anisms of emplacement of CFB lavas.

Macdonald (1972) has described similar flows fromthe Yakima basalts of the CRB. He mentions that many

Yakima basalt flows do not fit the descriptions of typicalaa flows, neither are they identical to pahoehoe flows.These flows possess flow-top breccias similar to those ofthe simple flows, which constitute 2035% of the flowthickness. Other characters of the Yakima basalt flows asdescribed in Macdonald (1972) are also similar to those ofthe simple flows in the DVP. It is likely that the two are aproduct of a similar emplacement process.

Some of the columnar flows from the DVP may havebeen emplaced in pre-existing topographic lows/rivervalleys. The localized ponding probably led to thedevelopment of multi-tiered columnar jointing withglassy entablatures and fanning columns. According toDe (1996), the presence of entablature is probably a resultof the near vertical nature of isotherms during the laterstages of cooling of lava. Such flows are not as commonin the DVP as compared to the CRB, where they seem tobe quite common (Long and Wood 1986). In the DVP,they occur in the northern fringes of the province, close toan ancient zone of weakness. It is quite possible that thepaleotopography was favorable for the ponding of flows.

However, this may not necessarily be the only factorresponsible. Thick flows with multi-tiered columnarjointing commonly occur higher up in the stratigraphicsequence, which means that they cannot have beenponded in the pre-eruptive topographic depressions.Although the duration of hiatuses between the emplace-ment of two successive flows are not known, the absenceof intervening sediments and/erosion suggests that thesemay not have been substantial enough to allow significanttopography to develop. Hence, the origin of the multi-tiered jointing may need to be explained by factors otherthan ponding and damming of drainage.

Implications for the emplacementof flood basalt lavas

The observations of flows from the DVP emphasize theneed for physical volcanological studies in CFB provincesother than the CRB. It is probable that such studies willreveal the similarities as well as differences in the stylesof eruption of flows from different provinces. In a recentstudy, Anderson et al. (1999) observe that CFB flowstypically display spectacular columnar jointing, as op-posed to Hawaiian inflated flows that have crudelycolumnar tops and massive interiors. They use thisobservation to suggest that the emplacement of floodbasalts is fundamentally different from that of Hawaiianpahoehoe flows. Self et al. (2000), in their discussion ofthe pulsed inflation model of Anderson et al. (1999), drawa distinction between hummocky pahoehoe flows asfound in Hawaii and large pahoehoe sheet flows/lobesseen in the CRB. They argue that CFB provinces aredominantly constituted of sheet flows, and hence conclu-sions derived from hummocky flows cannot be directlyapplied to the emplacement of CFB lavas.

The present study clearly highlights the fact that theCRB is not representative of all CFB provinces, as has

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also been mentioned by Long and Wood (1986) in theirdiscussion of multi-tiered jointing. Previous studies(Bondre et al. 2000; Duraiswami et al. 2001, 2002,2003), as well as the present study suggest that flows verysimilar to those in Hawaii are abundant in the DVP. Smallflow lobes and tumuli are as common as, if not moreabundant than sheet flows. Models of pahoehoe lavaemplacement in the CRB (e.g., Thordarson and Self 1998)

deal with extensive, thick inflated sheets (>25 m thick). Itremains to be investigated whether such models may bedirectly applicable to the strongly compound pahoehoeflows from the DVP. At the same time, simple flows fromthe DVP are similar to certain flows from the CRB. It thusseems likely that there were considerable variations in thestyles of emplacement of flows from different CFBprovinces, and even within a single province as isexemplified by the DVP. It is therefore critical to insurethat any general model of flood basalt emplacement takesall CFB provinces into account; the possibility remains,however, that there may not really be any such generalmodel (J. Kauahikaua, personal communication). At the

same time, it is very important to document thesevariations and model their emplacement. These variationsmay ultimately reflect upon the differences in thetectonics of formation of the CFB provinces, as has beendiscussed by Duraiswami et al. (2003).

As compared to other provinces, studies of flowmorphology and quantitative studies of lava flow em-placement are lagging far behind in the DVP. The presentstudy is only a small attempt to review the existing stateof knowledge about flows from this province, and needsto be followed up with rigorous quantification.

Acknowledgements We have benefited greatly from our corre-spondence with Profs. G.P.L. Walker, Stephen Self, Laszlo

Keszthelyi and Jon Stephenson. We thank Dr. J. Kauhikaua forhis encouraging comments, and Dr. R.A.F Cas and Dr. Tim Druittfor their critical and careful reviews. Discussions with Dr. VivekKale and Shreyas Mangave were highly illuminating. Ninad Bondreand Gauri Dole are grateful to Prof. K.V. Subbarao for giving theman opportunity to participate in the Penrose Deccan 2000 fieldconference. Ninad Bondre would also like to thank his parents andZu Watanabe for their encouragement.

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morphology and emplacement of flows from the deccan volcanic province, india

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