The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism

Lithos 79 (2005) 385 – 405 www.elsevier.com/locate/lithos

The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism Dougal A. Jerrama,*, Mike Widdowsonb a

Dept. of Earth Sciences, The University of Durham, South Rd., Durham DH1 3LE, UK b Dept. of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Received 1 March 2004; accepted 9 September 2004 Available online 26 November 2004

Abstract The internal architecture of the immense volumes of eruptive products in Continental Flood Basalt Provinces (CFBPs) provides vital clues, through the constraint of a chrono-stratigraphic framework, to the origins of major intraplate melting events. This work presents close examination of the internal facies architecture and structure, duration of volcanism, epeirogenetic uplift associated with CFBPs, and the potential environmental impacts of three intensely studied CFBPs (the Parana-Etendeka, Deccan Traps and North Atlantic Igneous Province). Such a combination of key volcanological, stratigraphic and chronologic observations can reveal how a CFBP is constructed spatially and temporally to provide crucial geological constraints regarding their development. Using this approach, a typical model can be generated, on the basis of the three selected CFBPs, that describes three main phases of flood basalt volcanism. These phases are recognized in Phanerozoic CFBPs globally. At the inception of CFBP volcanism, relatively low-volume transitional-alkaline eruptions are forcibly erupted into exposed cratonic basement lithologies, sediments, and in some cases, water. Distribution of initial volcanism is strongly controlled by the arrangement of pre-existing topography, the presence of water bodies and local sedimentary systems, but is primarily controlled by existing lithospheric and crustal weaknesses and concurrent regional stress patterns. The main phase of volcanism is typically characterised by a culmination of repeated episodes of large volume tholeiitic flows that predominantly generate large tabular flows and flow fields from a number of spatially restricted eruption sites and fissures. These tabular flows build a thick lava flow stratigraphy in a relatively short period of time (c. 1–5 Ma). With the overall duration of flood volcanism lasting 5–10 Ma (the main phase accounting for less than half the overall eruptive time in each specific case). This main phase or dacmeT of volcanism accounts for much of the CFBP eruptive volume, indicating that eruption rates are extremely variable over the whole duration of the CFBP. During the waning phase of flood volcanism, the volume of eruptions rapidly decrease and more widely distributed localised centres of eruption begin to develop. These late-stage eruptions are commonly associated with increasing silica content and highly explosive eruptive products. Posteruptive modification is characterised by continued

* Corresponding author. Tel.: +44 191 334 2281; fax: +44 191 334 2301. E-mail addresses: [email protected] (D.A. Jerram)8 [email protected] (M. Widdowson). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.09.009

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episodes of regional uplift, associated erosion, and often the persistence of a lower-volume mantle melting anomaly in the offshore parts of those CFBPs at volcanic rifted margins. D 2004 Elsevier B.V. All rights reserved. Keywords: Flood basalt; Mantle melting; CFBP; Etendeka; Deccan; North Atlantic igneous province

1. Introduction The products of continental flood basalt (CFB) volcanism represent the largest outpourings of magma in Earth’s history. They are important in not only offering an insight into how the planet operates (e.g., Mahoney and Coffin, 1997; and references therein), and with respect to their likely environmental impact (Wignall, 2001; and references therein), but also because they provide analogues for eruptive materials, and styles of eruption observed on the surfaces of the other terrestrial planets and planet-like bodies (Keszthelyi et al., 2000). Continental flood basalt provinces (CFBPs) are commonly associated with spatially constrained melting anomalies located within the upper mantle (e.g., Ernst and Buchan, 2003). These anomalies are, geologically speaking, long-lived, and during their

early stages of activity are capable of extraordinarily high rates of melt production, in the formation of the CFBP at the initiation stages of the anomaly. Once initiated, the melting anomaly is thought to be spatially fixed over time, and is largely uninfluenced by tectonic processes or by movements operating within and affecting the lithosphere lying above. Whether this anomaly is defined as a dmantle plumeT sensu stricto, or some other mantle anomaly, remains a polemic issue and is beyond the scope of this paper. Accordingly, debate has largely become polarised between dplumeT and dnonplumeT hypotheses, with much of the argument conducted using elaborate geophysical and geochemical models. For any model to be demonstrably robust, it must be capable of explaining and predicting readily observed geological phenomena. In other words, it must explain the nature of the eruptive products and their spatial and temporal

Fig. 1. Location of the three Continental Flood Basalt Provinces used in this study. (a) Parana˜–Etendeka; (b) Deccan Traps; (c) North Atlantic Igneous Province.

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distribution. Therefore, in order to further constrain and test available models, it becomes crucial to establish the basic observations regarding the nature of CFBPs. These observations include: what are the geological features that characterise the onset and early stages of flood basalt volcanism?; for how long do CFBs continue to erupt, and what is the nature of output variation during their eruptive duration?; how is a CFBP edifice constructed over time, and what characterises their internal structure at the meso- (c. 1– 103 m) and macro- (103–105 m) scales? In other words, what is the anatomy of a CFBP? This contribution offers a new overview of the key geological characteristics of CFBPs, presenting examples outlining the construction of the volcanic stratigraphy and architecture, and ultimately to describe the anatomy of CFBPs. The observations presented are based upon detailed volcanological, stratigraphical and geochemical work conducted primarily upon the Parana˜–Etendeka, Deccan and North Atlantic Igneous Provinces (NAIP) and, to a lesser extent, other CFBPs including the Columbia River Basalt Province (CRBP) and the Ethiopia– Yemen Province (Fig. 1). In closing, discussion is aimed at the key features of flood volcanism which should help to further resolve and refine geophysical and petrogenetic models for the origin of CFBPs.

2. What are the key geological features of CFBPs? What is a CFBP? A CFBP can be defined as a series of volcanic outpourings erupted onto areas of continental crust and which are comprised predominantly of great thicknesses of basaltic lava flows. They are a major, but separate, subset of large igneous provinces (LIPs), which may include eruptive and intrusive bodies displaying a wide range of chemistries (i.e., from mafic to silicic) and which may affect either continental (e.g., Parana–Etendeka, Deccan, etc.) or oceanic crust (e.g., Ontong Java; Fig. 1). Whilst this definition is widely accepted, there nevertheless remain a number of questions regarding the nature of CFPBs, their eruptive products, and the manner in which they were erupted. What is the nature of the internal stratigraphy of CFBPs—are they simple dlayer cakeT structures?

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What is the duration of a CFPB? Are effusion rates consistent through the duration of the CFBP? What are the likely environmental implications of a CFBP emplacement? etc. In the following section, we will review some of the key geological features of CFBPs in terms of: internal facies architecture and structure, duration of flood volcanism, uplift associated with CFBPs, and the potential environmental impacts of flood volcanism. In order to do this, we will examine three wellconstrained examples: the Parana˜–Etendeka, the Deccan Traps, and the North Atlantic Igneous Province (Fig. 1). In each case, a figure depicting the pre-, onset, and main phases during the evolution of the provinces is presented, thus summarising important observations on their development. The aim is to use provinces that the authors have worked on extensively, in order to document as completely as possible the anatomy of these CFBPs. These dcase studiesT are then used as a guide to highlight the important geological constraints that must be incorporated when considering models of CFBP formation. 2.1. Internal facies architecture and structure—the anatomy of a CFBP During the past two decades, CFBP studies have largely fallen into two main categories. The first has concentrated upon petrogenetic arguments which aim to explain the origin and evolution of large volumes of mafic magma through their chemical and isotopic characteristics (e.g., Cox, 1980; Cox and Hawkesworth, 1985; Hawkesworth et al., 2000). The second, largely promulgated by rapid CFBP eruption models (i.e., 0.5–1 Ma) as revealed through radiometric and palaeomagnetic dating techniques, focuses upon the environmental impact that these types of eruption may have had in terms of their effect upon climatic deterioration (Erba et al., 2004) and, most newsworthy, their effect upon faunal and floral collapse or extinction (e.g., Courtillot, 1999; Hallam and Wignall, 1997; Wignall, 2001). A third, rapidly developing field aims to explore the stratigraphical and structural architecture of CFBP edifices at a variety of scales. This latter area of research offers great potential since it may assist in evaluating the magnitude of environmental effect on an eruption-by-

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eruption basis. Until relatively recently, few studies had investigated either the internal volcanic architecture, or the nature of intercalated weathering and sedimentary horizons which are common within CFBP successions (e.g., Widdowson et al., 1997; Jerram, 2002; Jolley and Bell, 2002; Single and Jerram, 2004). The latter are proving especially crucial in helping to further understand the palaeoenvironments contemporaneous with these massive eruptions (e.g., Jolley, 1997), and ultimately the effect that CFBP volcanism exerts upon the hydrosphere and atmosphere. 2.1.1. Examples of CFBP anatomy 1—Parana˜ – Etendeka The Etendeka forms the most eastern preserved extent of the Parana˜–Etendeka CFBP. Due to the erosion level and wealth of exposure, the Etendeka province on the African margin provides an excellent example in which to explore the onset of flood volcanism associated with the breakup of Africa with South America. Fig. 2 is a summary evolutionary panel diagram for the Etendeka side of the Parana˜– Etendeka CFBP, and highlights the palaeoenvironment directly prior to eruption, during the onset of flood volcanism, and during the main phase of flood volcanic effusion. Before eruption, Parana˜–Etendeka was an extensive aeolian desert (Jerram et al., 2000a; Scherer, 2002). Several episodes of rifting and extension are recorded in the sedimentary sequences which underlie the flood basalts, and these extend back for several tens to hundreds of Ma, prior to continental breakup (Stollhofen, 1999; Jerram et al., 1999a). This is an important observation since these earlier episodes of extension did not lead to flood basalt effusion, suggesting the role of some later mantle anomaly was required in order to have triggered and ultimately driven the flood volcanism. Flood volcanism in Parana˜–Etendeka comprises olivine-rich to picritic basaltic lavas, basaltic andesites, and rhyolite/quartz latite volcanics (Jerram et al., 1999a,b). The earliest manifestations of flood volcanism in the Etendeka are relatively low-volume lavas which are highly magnesian, known locally as the Tafelkop geochemical type. These early lavas erupted onto, and interacted with, an active aeolian sand sea (Jerram et al., 2000a; Jerram and Stollhofen, 2002),

forming low-level shield features around volcanic centres (e.g., the Doros volcano; Jerram et al., 1999a; Jerram and Robbe, 2000; Marsh et al., 2001). Locally, this resulted in dynamic sediment lava interaction and the preservation of many palaeo-desert features, such as 100-m-high sand dunes (Mountney et al., 1999; Jerram et al., 2000a; Jerram and Stollhofen, 2002; Scherer, 2002). The sediment interlayers have been key in locally preserving pahoehoe textures associated with these early lavas, indicating their mode of eruption (Jerram et al., 2000a,b; Jerram and Stollhofen, 2002). The topography of the dune environment trapped many early flows, leading to localised thick ponded units of ~100 m maximum thickness at the base of the Parana˜–Etendeka. To the north and south of the Huab Basin, the lavas were erupted onto Karoo or basement rocks, suggesting a prevolcanic landscape comprising rifts filled with sediments flanked by horst and basement highs (Fig. 2a). These early shield features were later buried by more voluminous basaltic andesite flows, known as Tafelberg-type in the Etendeka, during the onset of the main phase of the volcanic episode (Fig. 2b). These flows formed more typical or dclassicT tabular flow facies (Jerram, 2002), with rubbly flow tops. These latter have massive flow cores reaching thicknesses of 50 m, and are more akin to aa lavas (Jerram et al., 1999b). As the flood volcanic pile built up, an increasing frequency of high-volume silicic volcanism marked the development of large shallow-level chambers (e.g., the Messum igneous complex), which erupted silicic volcanic materials with volumes of individual events of up to 6340 km3(Milner et al., 1995; Fig. 2c). It is important to note that these units are larger than the largest individual recorded mafic flows (e.g., Roza member, 1300 km3; Self et al., 1997). In addition, these later, more evolved volcanics have been instrumental in establishing a trans-South Atlantic correlation of the lava stratigraphy between the separated Etendeka and Parana˜ provinces (Milner et al., 1995). The main phase of volcanism in the Parana˜–Etendeka has also been linked with climatic changes as recorded by carbon isotope excursions (Erba et al., 2004), and it is possible that these large individual eruptions would play a significant role in potential climatic effects of the flood volcanism. The volcanic stratigraphy of the Parana˜–Etendeka, as with most of the world’s CFBPs, has been mapped

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Fig. 2. Evolutionary panel of (a) pre-, (b) onset, and (c) main phase development of Etendeka CFB (adapted from Jerram et al., 2000a,b). (See text for details.)

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Fig. 3. Scales of vertical and horizontal heterogeneity in the Parana˜–Etendeka CFBP. (a) Lava flow front of large tabular flow overlying compound thin pahoehoe flows, Etendeka (after Jerram et al., 1999a,b). (b) Volcanic disconformity between tabular flows above and compound pahoehoe flows interbedded with aeolian sediments, Etendeka (after Jerram et al., 1999a,b). (c) Volcanic stratigraphy in the Huab Basin NW Namibia (After Jerram et al., 1999a,b), highlighting volcanic disconformities mapped out. (d) Large-scale chemostratigraphic variation on Etendeka (adapted from Marsh et al., 2001) and Parana˜ sides (from Peate, 1997).

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on a gross scale using chemostratigraphic relationships (e.g., Peate, 1997 and references therein; Marsh et al., 2001). Increasingly detailed field-based volcanological correlations of the lava sequences have also identified disconformities within lava sequences of the same chemostratigraphic type (e.g., Jerram et al., 1999a). Fig. 3 highlights heterogeneity elements within the flood volcanic sequences in the Parana˜– Etendeka CFBP, at a variety of different scales from lava flow (Fig. 3a) through lava field km–10s km (Fig. 3b and c), to the basin wide chemostratigraphic packages identified on the Etendeka and Parana˜ sides (Fig. 3d). Syn-volcanic rifting also occurred during the emplacement of the upper parts of the Etendeka, with high-silica quartz latite flows ponding in north– south-orientated rift structures in the coastal zone of the Etendeka (Milner et al., 1995). This ponding characteristic has been further confirmed by AMS studies through lava sequences which similarly show an N–S trend (Glen et al., 1997). 2.1.2. Examples of CFBP anatomy 2—Deccan Traps Much of the northwest peninsula of India is covered by the Late Cretaceous Deccan Traps CFBP. The Deccan Traps represent perhaps the most widely known and discussed CFBP due to its c. 4 Ma eruption period across the Cretaceous–Tertiary boundary (Courtillot et al., 1988; Mitchell and Widdowson, 1991; Widdowson et al., 2000).Whilst the Deccan CFBP displays many of the characteristics typical of CFBPs, the basalt lava succession of the main Deccan province (MDP) also exhibits many unique features, due largely to the unusual asymmetric evolution of the volcanic edifice. The pre-, onset, and main phase evolution of the Deccan CFBP is summarised in Fig. 4. Following the breakup of Gondwana during the Early Cretaceous, the western part of the Indian craton was subject to two major rifting events, beginning with the splitting of Madagascar (c. 88 Ma; Storey et al., 1995) and, 20–25 Ma later, the separation of the Seychelles–Mascarene microcontinent (c. 65–64 Ma; Hooper, 1990). The later, Seychelles, rifting event is widely considered to be associated with the arrival of the Reunion dplumeT beneath the northwestern margin of the Indian continent. Consequently, the plume–rift interaction is thought to have given rise to the rapid eruption of prodigious volumes of basalt (c. 106 km3;

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Courtillot et al., 1986; White et al., 1987; White and McKenzie, 1989). Much of the terrain onto which the Deccan Traps lavas were erupted consisted of the rifted flank (c. 88 Ma) of the ancient (i.e., Archaean–Proterozoic) Dharwar Craton. In many instances, the lavas were erupted onto a landscape that had lain exposed to continental weathering effects for a considerable period of geological time. As a result, these dGondwananT surfaces were characterised by relatively low-relief topography, thin and laterally impersistent sedimentation, and localised weathering profiles. The earliest tholeiite lavas were erupted at ~ 67 Ma, reaching an eruptive acme at 66–65 Ma, and thus rapidly drowned this landscape, filling in any remaining valleys and depressions, and overwhelming low hills. In some instances, especially in localities at the eastern and southernmost fringes of the MDP, the initial lava sheets preserve these Late Cretaceous sediments and palaeoweathering profiles. In the south and southeast of the MDP, the lavas erupted mainly on to basement rocks spanning 3 Ga, with occasional local examples of lacustrine sediments beneath and intercalated with the early lavas. By contrast, in the Kutch region in the north, and along the valleys of the major westerly flowing Narmada and Tapti rivers, the pre-Deccan basement predominantly consists of Cretaceous sediments, commonly marine sandstones, shales, and limestones. These form the uppermost successions of a long-lived Mesozoic depocentre which reaches a thickness of c. 5 km, and located within an east– west orientated rift. Local tilting, folding, and erosion of these sediments produced an angular unconformity between the uppermost Cretaceous units and overlying Deccan basalts. This clearly demonstrates that regional deformation and uplift occurred during the Late Cretaceous prior to the onset of flood volcanism. It is important to note that the formation of lava fields was often diachronous in the different regions of the Deccan, since the lavas comprising the SW periphery did not erupt until c. 66–65 Ma, some 1–2 Ma after the initial flood eruptions in the northwest. The basalt succession of the MDP is broadly lensoid, with the greatest thicknesses (z3 km) inland of the coast along the Western Ghats escarpment (Fig. 5). The succession thins both

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Fig. 4. Evolutionary panel of pre-, onset, and main phase development of the Deccan CFB. (See text for details.)

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Fig. 5. Lava flows stack at Mahabaleshwar, high in the Western Ghats of India. The image shows a c. 300 m succession of the Mahabaleshwar Fm which 40Ar/39Ar and palaeomagnetic data demonstrates was erupted across the Cretaceous–Tertiary boundary (KTB). Individual flows together comprise flow fields which are often separated by thin weathering or ash horizons corresponding to periodic hiatuses in eruption. It should be noted that this is not a layer-cake stratigraphy. Instead, inspection reveals complex thinning and thickening of individual units, together with on-lap and off-lap stacking patterns.

eastward and southward, since relatively few lava fields ever extended into these peripheral regions. Detailed stratigraphical investigation reveals that development of the province is one of southwards overstep by progressively younger units (Fig. 4d), a structure which is considered consistent with the northward movement of India over the young dReunion plumeT. This idea has been elaborated both qualitatively (Devey and Lightfoot, 1986; Widdowson and Mitchell, 1992) and quantitatively (Watts and Cox, 1989). After c. 64 Ma, the region of active volcanism, now apparently much reduced in activity, emerged from beneath the attenuated and rifted western Indian margin (c. 158N), manifesting itself as a chain of progressively younger minor volcanic edifices forming the Maldive–Laccadive ridge, and culminating in the currently active volcano of Piton de la Fournaise (Reunion Island), located south of the Indian Ocean spreading ridge (Duncan, 1990).

With respect to eruption style and architecture of flows and flow fields, the tholeiites of the MDP typically comprise inflated pahoehoe sheets up to 20– 30 m thick. Thicker flow units are very uncommon, whilst thinner units of 1–10 m can dominate in some regions, and are often observed as thick stacks of welded flows indicative of rapid repetition of separate flow-field-forming eruptions. Individual flows may be traced for kilometres, and exceptionally for tens of kilometres, but commonly tend to pinch out, or otherwise change in character and thickness. Packages of flows, comprising wider flow fields, and probably representing individual eruptive events, are more readily traced over much larger areas (z102–103 km2) and may represent eruptive volumes of z103 km3. The dominance of pahoehoe observed in the Deccan is common to many CFBPs, as is the scarcity of aa flows. Interestingly, it is high eruption rates (N100s m3 s 1) that typically result in rapidly advancing channel-fed aa flows; by contrast, low

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eruption rates (b100s m3 s 1) promote the development of pahoehoe and tumuli fields, and result in tube-fed emplacement and inflation which are most characteristic of the Deccan flows. Therefore, even during the eruptive peak, the rates of magma supply are unlikely to have been exceptionally high, but, in order to generate the vast flow fields, must have occurred over prolonged periods of time, and probably from multiple vent and fissure sources along the southerly advancing volcanic front. Thus, the Deccan, as with other CFBPs, also contains significant lateral volcanological heterogeneity at a variety of scales (e.g., Widdowson and Mitchell, 1992; Duraiswami et al., 2001; Bondre et al., 2004). 2.1.3. Examples of CFBP anatomy 3—North Atlantic Igneous Province (NAIP) The Palaeogene flood volcanism and associated intrusions in the North Atlantic region define one of the most widely studied and discussed of all CFBPs, the North Atlantic Igneous Province (NAIP). The NAIP rifted apart during and after the main eruptive phase and, consequently, extensive remnants may be found from West Greenland through to East Greenland, the Faeroe Islands, the British Tertiary Igneous Province (BTIP; e.g., Skye, Mull, Ardnamurchan, etc.), and associated subcrops in the offshore rifted margins on either side of the North Atlantic Rift. The pre-, onset, and main phase of flood volcanism for the NAIP is summarised in an evolution panel (Fig. 6). The onset of the NAIP occurred c. 62–61 Ma, with coeval eruptions occurring throughout the NAIP from Greenland to the British Tertiary igneous centres. Directly prior to the onset of flood volcanism, there is conflicting evidence for both uplift and subsidence, since elevated, emergent areas and localised basins and major seaway environments have been identified (Pedersen et al., 1998; Dam, 2002; Peate et al., 2003). For instance, sedimentary sequences prior to the eruption of the lavas in West Greenland show evidence for sea-level fall and incision, yet the lava eruptions occurring at the onset of flood volcanism in this same area were erupted as hyaloclastites into marine conditions typical of hundreds of metres of water depth (Dam, 2002). Similar examples of eruptions into large water bodies are found in the West Greenland (Pedersen et al., 1998, 2002) and in

the Faeroe–Shetland basin (Ellis et al., 2002). By contrast, other parts of the NAIP were clearly erupted subaerially, and in some cases preserved coeval continental sediments (Jolley, 1997). Thus, the regional basement directly prior to the onset of CFBP volcanism must have been characterised by a complex mosaic of exposed basement and sediment highs, with localised continental and lacustrine sedimentary environments, but dissected by major seaways in basement lows and in incipient rift-related grabens. These major seaways had water depths up to ~700 m in places (Pedersen et al., 2002) and, interestingly, were often close to the locus of the early eruptions (Peate et al., 2003). This suggests that basin bounding faults in horst-graben type settings may have influenced the distribution and occurrence of the early volcanism (Fig. 6a). The NAIP shows a large variety of volcanic styles and architectures. There are the 20- to 30-m-thick classic dtrap-likeT flows on Greenland; transitions between classic sheet flows and compound-braided pahoehoe systems occur in the Faeroes; and more intricate relationships between lava fields and igneous centres occur in the BTIP. Early eruptive materials often show interactions with water bodies, as discussed above, and volcanism of intermediate and silicic types also occurs in small volumes throughout the duration of the NAIP (Bryan et al., 2002). Detailed investigation of the lava flows in the BTIP reveals the presence of small-volume lava flows, typical of observed eruptions on Hawaii, which would have produced shield features around each of the erupting volcanic centres (Single and Jerram, 2004). Consequently, significant volcanic disconformities do exist within the volcanic succession in a fashion similar to those observed in the Etendeka (Jerram et al., 1999a). This variety of eruptive style and architecture suggests that there was a large spatial and temporal variation in volcanic style and eruptive volume/rates in the NAIP during its formation. Whilst much of the volume of the NAIP consists of flood basalt eruptions of various types and facies architecture, the final phases of the volcanic episode were characterised by highly explosive eruptions (Peate et al., 2003). Numerous ashes from this explosive phase (c. 54.5–54.0 Ma) are preserved within the Balder Fm sediments of the Faeroe–

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Fig. 6. Evolutionary panel of (a) pre-, (b) onset, and (c) main phase development of the NAIP. (See text for details.)

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Shetland and Rockall basins, as well as equivalent successions in the North Sea basin. The change in volcanic style, from flood basalt lava fields, hyaloclastites, and associated seaward-dipping reflector sequences, to one of phreatomagmatic explosivity, is thought to be a consequence of inundation and associated water–magma interaction along the opening North Atlantic rift. Importantly, this change in volcanic style may also have initiated a short-lived, Northern Hemisphere cooling episode recorded in the floral diversity at key localities (Jolley and Widdowson, 2005). Since foundering and inundation are a common consequence of postrift evolution at many rifted CFBPs, such fundamental changes in volcanic style, and associated climate change, may not necessarily be unique to the NAIP. As stated earlier, some of the earliest eruptives in the NAIP erupted into a mixture of subaerial and subaqueous environments (Fig. 6b), and the early volcanics in East Greenland were initially restricted to localised basins and tended to form shallow shield volcanic features of 30–40 km diameter. These volcanoes were later ddrownedT by the more voluminous flood basalt sequences that characterise the plateau basalts (Peate et al., 2003). Similar internal variations are displayed in West Greenland and the Faeroe–Shetland basin (Fig. 6c). The location of a number of volcanic sites identified throughout the NAIP (e.g., Skye-Mull, British Tertiary; Faeroes; East Greenland) clearly indicates multiple centres of eruption and, as such, requires a number of volcanic disconformities to be present through the lava fields in a similar fashion to those in the Parana˜–Etendeka CFBP. In Fig. 6c, an examination of the juxtapositions of the different lava and igneous centres in a schematic cross section of the British Tertiary Mull– Rum–Skye areas is presented. Also indicated are the known age ranges. Attempts to reconstruct the eroded stratigraphy across the BTIP must, therefore, take into account these relative ages, the presence of volcanic disconformities, and the juxtaposition of the Rum igneous centre. Detailed work is already revealing the existence of important volcanic disconformities within the eroded lava field (as interpreted in Fig. 6; Single and Jerram, 2004). Indeed, such volcanic disconformities, often between lava batches of the same geochemical type, are clearly an inevitable structure that will develop in CFBPs

which are fed by more than one centre or fissure system (Jerram, 2002). 2.1.4. Anatomy of other CFB provinces Many of the architectural elements described in the present study are also found within other flood volcanic sequences. The Karoo, for example, contains early eruptions into active sedimentary environments with extensive flood lahars in places (Skilling, 2001); higher in the stratigraphy, silicic eruptions begin to appear, but the top of the Karoo, and hence its late stage volcanic characteristics, are not preserved. The Yemen–Ethiopia province also contains significant silicic eruptions, and potentially provides additional volcanological information because a large part of the end phase of the CFBP remains preserved. In the case of the Yemen–Ethiopia province, the transition from the main phase of volcanism to the end stages is marked by a decrease of magma flux and the establishment of shield volcanoes on top of the main flood sequence (Kieffer et al., 2004). The 174,000 km3 Columbia River flood basalt province is the youngest flood basalt province, with dates for the entire succession ranging from 17.5 to 6.0 Ma. Greater than or equal to ninety percent of its volume erupted in a pulse of 1 Ma or less at c. 16 Ma (Tolan et al., 1989; Baksi, 1989). Although smaller than the larger CFBPs (N1–2106 km3), it contains some of the largest documented single mafic eruptions (i.e., z1000 km3 dsuper-eruptionT lava units, such as the Roza Member, ~1300 km3). Using detailed observations from the ~14.7 Ma Roza flow field, Self et al. (1997) and Thordarson and Self (1998) proposed that continental flood basalt (CFB) lavas were predominantly emplaced as inflated compound pahoehoe flow fields via prolonged, episodic eruptions. Many of these features can be seen in lavas from other CFBPs (e.g., Deccan) and it has been suggested that this emplacement style is typical of most CFBPs (Self et al., 1998). Detailed examination of the architectural elements of the large CFBPs in the present study have shown a variety of flow types from pahoehoe style inflated units, aa units, rhyolites– rheoignimbrites, volcaniclastic and hyaloclastic/pillow lavas. Therefore, although the inflation model of Self et al. (1998) is applicable to many flows in CFBPs, a variety of styles of eruption and emplacement may be identified.

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3. Duration of flood volcanism Despite the immense size (i.e., 105–106 km3) of the larger CFBPs, the repeated, high-volume eruptive episodes that generate them occur over a relatively short period of geological time. The typical dlifetimeT of a CFBP is b10 Ma but, importantly, the rate of eruption is not uniform throughout this lifetime. In many instances, a peak tholeiitic basalt output is achieved during a b0.5–5 Ma acme in which N70% of the products may be erupted. However, where CFBPs are associated with continental breakup, the meltingvolcanic anomaly often persists to the present day, albeit in a much reduced form, and over time its products result in the development of submarine volcanic ridges or volcanic chains (e.g., Tristan de Cunha, Reunion, Iceland). In the following sections, the three examples of CFBP will be considered in terms of their known duration, and a discussion about the general pattern of CFBP duration is presented. 3.1. Example 1; age and duration of the Parana˜– Etendeka The Parana˜ –Etendeka province has been the subject of a number of studies aimed at constraining eruption age and duration (e.g., Renne et al., 1992, 1996; Turner et al., 1994; Stewart et al., 1996; Kirstein et al., 2001). Eruption began at ~138 Ma and, based on the age range preserved in the Parana˜ (Peate, 1997), continued over the following ~10 Ma period. The most voluminous phase of eruption occurred between 134 and 129 Ma, and coincided with the main phase of magmatism preserved in the Etendeka where the flood volcanism occurred between 133 and 131 Ma (Milner et al., 1995; Renne et al., 1996; Jerram et al., 1999a). Younger igneous activity is preserved within the Mesozoic igneous complexes in Namibia (Milner et al., 1995). Mean effusion rates have been estimated at ~ 0.1 km3 year 1 (based on the 10 Ma duration of Turner et al., 1994) and as high as 1.5 km3 year 1 (based on the limited age range preferred by Renne et al., 1992). A more reasonable estimate probably lies somewhere between these values, being in the order of 0.2–0.25 km3 year 1 during the acme of volcanism which occurred over a 4–5 Ma duration. The location of magmatism also shifted northward during the evolution of the

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province, and is clearly evident from the Parana˜ chemostratigraphy (Fig. 3d; after Peate, 1997). This change in eruption and magma-generation locus, as well as changes in the gross geochemical signature of the flood volcanism, have been attributed to variation in the stress regime and relative contributions of lithospheric and asthenospheric melting associated with lithospheric thinning (Hawkesworth et al., 2000; Kirstein et al., 2001). 3.2. Example 2; age and duration of the Deccan The precise age and duration of Deccan volcanism still remains a source of considerable discussion, due primarily to its close temporal association with Cretaceous–Tertiary (K–T) boundary events. The earliest eruptions began in the northwest region, and probably exploited inherited structural weaknesses associated with a putative triple junction (Burke and Dewey, 1973) comprising the intersection between the Gulf of Cambay and Narmada and Son river valleys. The earliest volcanism began with the emplacement of isolated alkaline complexes (e.g., Amba Dongar and Phenai Mata, dated at c. 68 Ma; Basu et al., 1993) and associated regional dyke injection, thought to represent mantle metasomatism above an arriving mantle plume and small degrees of associated lithospheric mantle melting (Simonetti et al., 1995). Later, components of this alkaline activity were coeval with the eruption of the earliest picritic basalt lavas, including a series of Krich picrites documented by West (1958) from a deep borehole at Dhanduka, and those exposed lying directly upon Cretaceous sandstones in the western region of the Narmada valley (Krishnamurthy and Cox, 1977; Krishnamurthy et al., 2000; Gibson et al., 2000). The composition of these lavas is considered typical of the earliest stages of CFB eruption, when particularly hot tholeiitic material is erupted via nascent dyke conduits developing through relatively undepleted lithosphere (Cox, 1980). These early lavas yield 40Ar/39Ar ages of c. 67 Ma, and represent the first eruptions of the MDP tholeiitic lava succession. Progressively younger ages are found southwards within the MDP, culminating with the youngest ages of c. 64.0 Ma near Belgaum at the southernmost periphery (Widdowson and Kelley, in prep.).

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Much of the MDP tholeiitic succession was erupted during chron 29R (Courtillot et al., 1986, 1988), suggesting that the duration of the main pulse of volcanic activity was probably less than 1 Ma. Compilation of reliable 40Ar/39Ar studies (Baksi, 1994) provides a weighted mean age of 65.5F0.5 Ma, which agrees very well with KTB age estimates, and lends further support to the postulated rapidity of the eruptions during chron 29R. This timing, coupled with a short duration for this major volcanic event, has clear implications for environmental stress and, inevitably, their association with KTB boundary floral and faunal extinctions (e.g., Wignall, 2001, and references therein). Elsewhere, at Anjar in the NW Deccan, a proposed KTB horizon within the lava succession consists of a thin sequence of lacustrine limestones and shales with elevated Ir concentrations. These are considered consistent with the Ir anomalies which elsewhere are believed to be indicative of the Chicxulub impact ejecta fallout (e.g., Bhandari et al., 1995). Whilst dating indicates these lavas are demonstrably coeval with eruption of the Wai subgroup (c. 65 Ma; Courtillot et al., 2000), the unique geochemistry of the basalt succession of the Kutch region precludes direct stratigraphical comparison with the MDP. The final Palaeocene stages of MDP lava eruption occur in the southwest of the province (Widdowson et al., 2000) and are coeval with renewed mafic and felsic volcanic activity in the Bombay region. At Bombay, extrusive volcanism (c. 64 Ma) consists of intercalated subaerial and subaqueous basalt and rhyolite flows and explosive tuffs (Widdowson and Kelley, unpublished data), and the later intrusion (60– 62 Ma) of trachytic and doleritic bodies (Sheth et al., 2001). These later phases of activity are thought to be associated with the foundering of the Deccan volcanic margin during the final separation of the Seychelles– Mascarene plateau. 3.3. Example 3; age and duration of the NAIP There has been considerable recent debate over the age and duration of the NAIP (Pearson et al., 1996; Hamilton et al., 1998; Chambers and Pringle, 2001; Jolley and Bell, 2002; Jolley et al., 2002). The age range for the entire NAIP is between 62 and 54 Ma,

but with a main phase of volcanism at ~61–58 Ma, and a later stage of volcanism associated with formation of seaward-dipping reflector sequences terminating at 54 Ma (Jolley et al., 2002). The earliest volcanism appears to have started almost simultaneously across both Greenland and the BTIP, and the pattern of active sites at this time suggests that there was a large lithospheric control in the sites of magmatism (Pearson et al., 1996). Age studies using radiometric and palynological dating techniques have proven invaluable in the NAIP in helping to refine the Palaeogene time scale and calibrating the main phase of volcanism with the late Palaeocene thermal maximum (Jolley et al., 2002). 3.4. Is there a general pattern of duration for CFBPs? The examples given serve to demonstrate that the construction of the tholeiitic succession of CFBP characteristically begins with limited basaltic lava eruptions (e.g., the Tafelkop of the Etendeka, Naramada–Dhanduka picrites of the Deccan). These may have been preceded by small volume, exotic magmatism (e.g., lamprophyres and carbonatites) associated with sublithospheric melting above inferred mantle anomalies. Early lava successions, and other coeval exotic magmas, are then rapidly succeeded by repeated, high-volume tholeiitic eruptions which then construct much of the succession before diminishing in both frequency and volume during the closing stages of the CFBP episode. Late-stage eruptions are, in some instances, associated with more evolved, explosive magmatism. However, at the original locus of CFBP eruption, small volume basaltic eruptions are commonly sustained for considerable periods of geological time. These manifest as so-called dhotspotsT, forming the active part of chains of progressively older volcanic edifices. Another advance in understanding the eruptive duration of CFBPs is derived from eruption rate and cooling measurements and calculations, typically conducted on Hawaiian or Icelandic analogues. Since the fundamental unit of CFBP stratigraphies are the individual flows which, during a single eruptive episode, build together to form a cogenetic flow field, it becomes crucial to understand the time scales required to generate such units. Using an empirical expression for the rate of crust growth of Hawaiian

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inflated sheet flows, Self et al. (1997) estimated that individual Roza flows of the Columbia River Basalts were emplaced over 5 to 50 months, and that the Roza flow field was constructed over a period of 6 to 14 years. Importantly, this estimate is considerably longer than the few days or weeks that had previously been postulated and largely accepted (e.g., Swanson et al., 1975). However, even with this longer eruption duration, the average lava effusion rate of ~4000 m3 s 1 to construct such flow fields is similar to the highest recorded effusion rate during the 1783–1784 Laki eruption in Iceland. Calculations such as these could potentially be combined with volcanological and petrographic data to better constrain the durations of individual volcanic episodes, and hence the relative proportion of repose, during the lifetime of the CFBP. There has been considerable debate regarding the geologically dshortT duration for the construction of CFBPs, and this had had fundamental implications upon models of melt generation and extraction (e.g., White et al., 1987; Richards et al., 1989; White and McKenzie, 1989). Invariably, systematic dating combined with detailed stratigraphical and volcanological studies have revealed a far more complex history than at first thought, and estimates have since had to be rationalised with respect to these new geochronological and field data. There is now a requirement for more elegant explanatory models that can address the different phases and rates of volcanic activity, and that will ultimately aid in understanding the coeval development of different geological environments and complex ecosystems. A useful manner in which to begin this process is first to consider the evolution of a CFBP in terms of its temporal hierarchy. For instance— ! !

!

The lifetime of a CFBP is between 5 and 10 Ma. Throughout its lifetime, a CFBP is composed of a series of discrete volcanic stages which can be broadly grouped into three identifiable phases, each lasting c. 105–106 Ma. These are (i) an initiation phase, (ii) a main phase, dpulseT or acme, and (iii) a diminishing phase. Stratigraphical detail reveals periods of quiescence during these phases, as evidenced by the presence of weathering horizons, sedimentation, and erosion surfaces occurring at different times at different

399

places within the CFBP. These hiatuses typically occur over periods of 10–104 years. ! Individual eruption episodes produce lava flow fields. These episodes are considered to be continual eruptions emanating from vent or fissure systems over periods lasting months, years, or, in exceptional cases, decades. ! It is possible that more than one lava field may be active during any eruptive phase. Evolution of multiple lava fields is most likely during the eruptive acme. To date, this temporal hierarchy has commonly been overlooked, partly through lack of detailed investigation or availability of comprehensive data, and partly because periods of =105 years have been considered as geologically insignificant with respect to the long-term evolution of CFBPs. However, such an approach is clearly timely, since it is now known that volcanically induced climate and many associated forcing factors that result in profound oceanographic and environmental change (e.g., the destruction and reestablishment of flora and fauna) occur over 10–102 year periods.

4. Pre-, syn- and post-eruption uplift One of the fundamental tenets of the plume model for CFBP eruption is evidence of dpre-eruption upliftT. This is the assumption that immediately prior to the onset of eruptions, the lithosphere above the plume will be heated and domed by the dynamic buoyancy of hotter, less dense material within the rising and decompressing plume head (e.g., White and McKenzie, 1989; Campbell and Griffiths, 1990; Ernst and Buchan, 2003). A number of studies have sought to demonstrate that such doming and associated preeruptive surface uplift does occur through the exploration of river drainage patterns (e.g., Cox, 1989; Kent, 1991), and it can be shown that rifting can occur for millions of years prior to the onset of flood volcanism (Kent, 1991; Stollhofen, 1999). Other studies have used the sedimentary record as a proxy for recording uplift patterns (White and Lovell, 1997; Nadin et al., 1997). In many instances, however, local evidence of preeruption uplift directly prior to flood volcanism is circumstantial because the lava eruptions have inevi-

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tably buried the surface and hence the geological evidence for these effects, or in others contradictory as the earliest eruptive units are subaqueous. It is clear that when incorporating the term dupliftT in models, that a clear indication of the scale, wavelength, and timing of the uplift is critical. For example, a region may be undergoing uplift on the wavelength of several hundreds of kilometres but, due to the reactivation of complicated preexisting major structures, large parts of the region are represented by subsidence on the scale of several tens of kilometres. A common feature of CFBPs is the fact that, eventually, they form regions of elevated topography. There does not appear to be a unanimous agreement as to why this should be the case (e.g., Crough, 1979) but it may be a consequence of a net gain of hotter material in the region of the sublithosphere affected by the CFBP volcanism (e.g., McKenzie, 1984). Alternatively, thickening of the crust by lavas erupted onto the surface, and by injection of sill-like bodies within and at the base of the lithosphere (Cox, 1980) will result in net gain in regional elevation. For such thickening to occur, a significant amount of additional material is required other than that available from directly below the CFBP. This requirement implies addition of melt from a wider mantle source and/or the dynamic movement of mantle to feed the CFBP. Whilst initial regional uplift is often considered to be a response of the overlying lithosphere to the thermal and dynamic effects resulting from the arrival of a dplumeT, in many instances, components of uplift considerably postdate the volcanic episode. One mechanism by which uplift may be perpetuated is through isostatic uplift as a consequence of onshore erosion, and concomitant offshore sedimentary loading (e.g., Gilchrist and Summerfield, 1990). Moreover, because denudational unloading is independent of plume effects, it provides a long-term mechanism which permits the generation of permanent and continuing uplift over geological time. In the case of the Deccan, the lava pile has undergone considerable erosion during the c. 65 Ma since their eruption. Thicknesses of 1–1.5 km have been removed from the western edge of the rifted Deccan CFB province, thus creating a lowland coastal plain fringing much of northwest peninsular India. This erosion is largely the consequence of eastward retreat of the Western Ghats escarpment,

and the observed uplift is therefore interpreted as a consequence of lithospheric flexuring of the entire margin (Widdowson, 1997; Widdowson and Gunnell, 1999). This combination of macromorphological components (i.e., coastal plain–escarpment–plateau) and its associated erosion history is not unique to the Deccan CFBP, and the similarities can be demonstrated in the broad structure and pattern of largescale denudation at other CFBP rifted margins of different ages (e.g., Karoo and Parana´ ). These similarities clearly indicate a common pattern of postrift evolution (Cox, 1988). Moreover, this common pattern represents a fundamental long-term control upon continental-scale drainage patterns and the rates of offshore sedimentation.

5. Establishing the environmental impact of CFBP eruptions Flood basalt lava flows and their associated volcanic effects have been implicated by many studies for their role in mass extinctions and other serious environmental impacts (see reviews by Hallam and Wignall, 1997; Rampino and Self, 1999; Wignall, 2001). For example, initial volatile release estimates from single eruptions, such as the Roza member of the Columbia River Basalt, indicate that prodigious amounts of S, Cl, and F were injected into the upper troposphere and lowermost stratosphere (Thordarson and Self, 1996). Therefore, even single eruptive episodes (i.e., those generating individual flow fields) could have had a significant effect on the global atmosphere. If other CFBPs produced similar amounts of volatiles from just a few of the episodes that comprise their stratigraphies, then volatile release may thus provide a link between flood basalt eruptions and major climate perturbations. Indeed, some workers believe that there is a one-to-one correlation between mass extinctions and periods of flood basalt volcanism (e.g., Courtillot et al., 1996), thus implying a causal effect. The most likely manner in which such flood basalt eruptions can influence the environment on a global scale is by degassing of sulphur (as opposed to the release of CO2, see Caldeira and Rampino, 1990), and it is known that basaltic flood eruptions released huge quantities of sulphur (Thordarson et al., 1996). It has also been suggested that

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CO2 from flood volcanism may cause atmospheric perturbations which contribute to global climatic signals (e.g., Erba et al., 2004). However, it is not necessarily the total volume of gas delivery, but rather the rate, duration, and efficiency of its delivery into the higher atmosphere that constrain the degree and distribution of environmental damage. Currently, there remains considerable debate regarding the rate at which flood basalt provinces were erupted (see Self et al., 1998), yet such arguments are pivotal to the idea of whether these eruptions fundamentally altered climate. Thus, there are several aspects concerning CFBP volcanism that future work should seek to address. These include: !

!

!

What are the volumes produced during individual eruptive episodes that comprise flows and flow fields? If so-called dsuper-eruptionsT produce in the order of 300–1000 km3 of erupted material (Rampino, 2002), then do mega-eruptions, single flood basalt episodes yielding 5–10,000 km3 of lava, exist within CFBP stratigraphies? Currently, the largest known eruption is that of the SringbokPAVunit B, in the Parana˜–Etendeka province N6340 km3 (Milner et al., 1995). How long might be the durations of such eruptions? To date, dgreat flowsT z1000 km3 have been identified the Columbia River (e.g., Tolan et al., 1989) and the Parana˜–Etendeka. There are suggestions that some Deccan lava flow dunitsT (i.e., flow fields; Thordarson and Self, 1998) are of the order of 10,000 km3 (Widdowson and Self, unpublished data), but no current dating method can resolve such short time intervals that occurred 65 Ma. Consequently, rates and volumes of environmentally damaging volatile release remain speculative. Determination of the time interval between groups of eruptions and individual major flood lava eruptions. The use of evidence from the fossil vegetation and soils preserved between successive eruptive units will help establish degree, type, and extent of floral recolonisation between lava supereruptions. The environmental impact of CFBP eruptions can thus be further resolved from analysis of interlava flow sediments and soils and determination of their provenance and development in relation to the lava flows. Such work is

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fundamental if we are to establish whether and how such eruptions may influence biota and environment. Most of the volume of the Deccan and Columbia River Basalt lava piles was erupted in a time interval that, until recently, lay within the errors of dating techniques (Tolan et al., 1989). However, work by Widdowson and Kelley (in preparation) demonstrates that it is practicable to resolve duration and timing of the different phases of CFBP eruption that comprise the Deccan. If these techniques are applied to the younger, better preserved Columbia River Basalts, it may become possible to resolve difference in age between individual eruptive units. Little information exists regarding the time elapsed between the eruption of major flows, flow fields, or even between major groups of flows. This information is vital to know if we are to improve assessments of the environmental impact of the eruptions. In the Parana˜–Etendeka CFBP, the silicic eruptions are of particular interest, due to their volume, possible mode of emplacement, and generation within the context of CFBPs. There has been debate as to whether large volume silicic units are erupted as lava flows or as rheoignibrites (Milner et al., 1992; Manly, 1995; Kirstein et al., 2001). Clearly, such large volume eruptions, if formed by pyroclastic mechanisms (Milner et al., 1992), would have had a much more profound effect on the environment than effusions of basaltic lava flows, due the massive cloud that would have been ejected high into the atmosphere (Jerram, 2002). Clearly, establishing methodological approach to describing CFBP architecture, improving dating techniques for the lavas and intrusives, and more thorough examination of the interlava flow sediments, soils, and flora will improve our knowledge of the manner in which a CFBP edifice is constructed and the time scales required for the development of its different components. It is clear that without such knowledge, the understanding of flood basalt volcanism cannot advance much more than at present. Such work is of high priority since the results will find application in aiding our understanding of the nature of Earth’s response to past environmental catastrophe, and also in the possible mitigation of future climate change.

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6. Closing remarks Whether or not CFBPs are the products of plumes or other mechanisms of generating mantle melting anomalies, it is clear that if we are to further understand the processes by which these large volumes of melt are generated in the mantle, we first need to fully determine the temporal, spatial, and volcanological characteristics. In effect, any complex geochemical or geophysical models for the mantle must first address the field observations, volcanology, and, importantly, recognize the salient similarities of CFBPs that help identify the underlying geological processes. The present study has sought to examine the temporal, spatial, and volcanological characteristics of three classic CFBPs (the Parana˜–Etendeka, Deccan Traps and North Atlantic Igneous Province) with additional information from other CFBPs. The key conclusions are summarised as follows: !

!

!

!

Detailed stratigraphical correlations combined with a full understanding of the temporal and spatial variations reveal a prolonged duration of magmatism (i.e., 5–10 Ma) comprising onset, acme, and closing or waning phases during the evolution of CFBPs. Earliest lava flows often erupt onto a mixture of exposed cratonic basement, continental sediments, long-lived sedimentary basins and into open seaways (as with the NAIP). During the pre-eruption, and early syn-eruption stages, the geological relationships of the lava-basement contact in these terrains is often very different. The scale, wavelength, and timing of preonset uplift is complicated by existing major structures, as reactivation of such structures during regional uplift (wavelength of several hundreds of kilometres) can result in localised subsidence on the scale of several tens of kilometres. This results in initial volcanism erupting onto a variety of environments from exposed basement to seaways and sedimentary basins. CFBPs initially begin with low-volume eruptions. These are often highly modified both geochemically and architecturally by the crust through which they pass, and the surfaces upon, and environments into which, they erupt. The main

!

!

phase of eruption, or acme, has a relatively short duration (b1–5 Ma) and is typically characterised by large volume tabular flows and extensive flow fields. The latter part of a CFBP eruption is characterised by a significant decrease in erupted volume, longer hiatuses between successive eruptive episodes, and, in many cases, an increase in the silicic components of the eruptive units or products. Where CFBPs are on volcanic rifted margins, a continuing melting anomaly remains. Hierarchical scales of heterogeneity exist within all CFBP stratigraphies. These are often preconditioned by: the nature of the crust through which the magma is passing since more eruptive centres will preferentially develop in areas of thinned crust or inherent structural weakness; the nature of the surface topography (i.e., the basement high and lows in the NAIP); and temporal variations of eruptive output during the lifetime of the CFBP. Regional uplift occurs after the emplacement of the CFBP due to increased crustal thickness, and may be perpetuated over periods of N107 years by processes associated with denudational isostasy and concomitant offshore loading of the attenuated rifted margin.

Acknowledgements This work was supported by funding provided to DAJ by Elf GRC and the EU 5th Framework Project SIMBA (CONTRACT N8: ENK6-CT-2000-00075), and to MW by NERC (Grant No. GR3/11474), and the Open University Research Development Fund (RDF). James Day, Jo Garland, Graham Thompson, Richard Single, Henry Emeleus, and Stephen Self are thanked for their help during manuscript preparation. Keith Cox is remembered for his encouragement with helping MW develop Deccan research themes. Helpful review comments were provided by Ray Kent, Ray McDonald, and Andrew Kerr.

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