Geology

doi: 10.1130/G22758A.1 2006;34;929-932Geology

 Joseph Cartwright and Dorthe Møller Hansen Magma transport through the crust via interconnected sill complexes  

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� 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; November 2006; v. 34; no. 11; p. 929–932; doi: 10.1130/G22758A.1; 4 figures; Data Repository item 2006208. 929

Magma transport through the crust via interconnectedsill complexes

Joseph CartwrightDorthe Møller Hansen*

3D Lab, School of Earth, Ocean and Planetary Sciences, Park Place, Cardiff University, Cardiff CF10 3YE, UK

ABSTRACTThree-dimensional seismic data from the volcanic margin offshore Norway have been

used to map the complex three-dimensional geometry of two major sill complexes. Thesedata show that the complexes consist of bowl-shaped or inclined, sheetlike discordant sillsthat intersect with one another over a vertical distance of 8–12 km and transect basementand sedimentary rocks of the middle to upper crust. Using a simple graphical technique,we demonstrate the interconnectedness of the sill complex, and hence the maximum timefor sill emplacement. We show that sills are interconnected by junctions occurring system-atically at the lowest points of the overlying sills, and, hence, that the sill complex couldhave acted as a single throughgoing magmatic plumbing system, transporting mafic meltsfrom mid-lower crustal depths to near surface. Based on this imaging of the magmaticplumbing system, we conclude that sills can form viable and efficient conduits for crustalmagma transport in volcanic margins, and may play a role in feeding large extrusivecomplexes.

Keywords: magma, transport, sills, 3D, seismic.

INTRODUCTIONTransport of mafic magma through the up-

per brittle crust is widely viewed as beingdominated by dikes (Lister and Kerr, 1991;Rubin, 1995), which are often invoked as themain feeders for flood basalt sequences (Cof-fin and Eldholm, 1994; Ernst et al., 1995).Transport via dikes is strongly supported bythermal-mechanical models of dike emplace-ment (Lister and Kerr, 1991), by geochemicalfingerprinting of feeders and eruptive products(Hanghoj et al., 2004; Klausen and Larsen,2002), and by outcrop-based petrological-structural studies in numerous igneous prov-inces (e.g., Anderson, 1951).

In contrast to dikes, sills are widely regard-ed as having only a limited role in verticalmagma transport because the limited outcropdimensions of sills show them to be almostexclusively formed of bedding-parallel sheetswith only limited vertical extent (Francis,1982), thus limiting their apparent role invertical magma transport. Recently, three-dimensional multichannel seismic (MCS) dataacquired for hydrocarbon exploration alongthe NE Atlantic volcanic margin have chal-lenged this fundamental tenet because the fullextent of the sills is imaged by the data, whichshow that the sills are often bowl-shaped withmargins inclined at 10–35�, crossing vertical

*Current address: School of Earth, Atmosphericand Environmental Sciences, University of Man-chester, Williamson Building, Oxford Road, Man-chester M13 9PL, UK.

sections of more than a kilometer (Smallwoodand Maresh, 2002; Wilson and Wheeler, 2002;Hansen et al., 2004).

Here we present evidence from high-resolution three-dimensional (3D) MCS dataof individual sills that have ascended vertical-ly over much greater distances than previouslyrecognized, up to 6 km through upper-crustalrocks. Critically, we also demonstrate that in-terconnected sill complexes can define contin-uous magma networks of �10 km in verticalascent. We propose that large-scale sill com-plexes can therefore represent a hitherto un-recognized mode of vertical long-range mag-ma transport through the upper crust,supporting earlier suggestions by Larsen andMarcussen (1992) and Wilson and Wheeler(2002).

REGIONAL SETTINGThe study area is located in the Norwegian

sector of the rifted NE Atlantic margin (Fig.1). The margin underwent several phases ofrifting in the Mesozoic leading to breakup�55 million years ago (Brekke, 2000). Themargin was a locus for intense magmatismimmediately before and during breakup, withextensive development of seaward-dipping re-flectors outboard of a marginal lava escarp-ment (Berndt et al., 2001). Mafic sills wereintruded into Cretaceous and older rocks overan area of 80,000 km2 as part of this regionalmagmatic event, and their distribution extendsover the outer parts of the Møre and VøringBasins (Berndt et al., 2000; Planke et al.,

2005). Following breakup, the margin subsid-ed passively, and thick depocenters ofEocene–Holocene clastic and biogenic sedi-ments accumulated in the two main basins(Brekke, 2000).

3D SEISMIC INTERPRETATIONWe focus on detailed mapping of sills in

two subareas with extensive 3D seismic cov-erage and well control within the Møre andVøring Basins, respectively (A and B in Fig.1A). In both survey areas A and B, sills arewell imaged by 3D seismic data across a depthrange of 8–11 km, i.e., spanning approximate-ly half the present-day crustal thickness. Thesills are easily identified by their characteristichigh-amplitude seismic response and bowl-shaped or discordant reflection geometry (Fig.2; GSA Data Repository1). For sill interpre-tation and definition, we followed the meth-odology outlined by Smallwood and Maresh(2002), Thomson and Hutton (2004), andHansen et al. (2004). No well calibrations forthe sills were available in either survey area,but similar high-amplitude reflections in near-by areas are calibrated as mafic sill intrusionswith a doleritic composition (Berndt et al.,

1GSA Data Repository item 2006208, uninter-preted versions of seismic lines presented in Figs. 2and 3, and three-dimensional visualization of sillcomplex from Area A, is available online at www.geosociety.org/pubs/ft2006.htm, or on request fromediting@geosociety.org or Documents Secretary,GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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930 GEOLOGY, November 2006

Figure 1. Location of study area on Norwegian margin and representative crustal structure. A: Regional map showing location of surveyareas A and B and distribution of intrusive and extrusive magmatism on mid-Norwegian continental margin based on Berndt et al. (2001).‘‘Inner Flow’’ region refers to region of subaqueous extrusives emplaced landward of escarpments (Svensen et al., 2004). B: Depth-converted cross section of Vøring Margin transecting survey area B located over North Gjallar Ridge close to seaward-dipping reflectors(SDR) of Vøring marginal high (from Gernigon et al., 2004). T reflection marks top of lower-crustal High Velocity Body (HVB)(>7 km/s)interpreted as being magmatic in origin (Mjelde et al., 2001; Corfield et al., 2004). DK—Denmark; N—Norway; S—Sweden.

Figure 2. Interconnectivity of sill complex in cross section and in planform. A: Represen-tative seismic section from survey area A, illustrating fully interconnected geometry of sillcomplex. Vertical resolution varies from ~30 m at 3 km depth to ~100 m at 8 km depth.Lateral resolution is related to bin size, which is 25 m. Individual sills are discordant tostratal reflections and exhibit bowl-shaped cross-sectional geometry with vertical relief ofup to 6 km. Sills are linked through sill-sill junctions (numbered) and a continuous flowpath can be traced through network from deepest to shallowest sill. The paloseafloor attime of intrusion was constrained from interpretation of hydrothermal vent horizons (Plankeet al., 2005). B: Plan view display of outline of sills shown in (A) and numbered sill-silljunctions. Sill-sill junctions account for ~10% of length of sill margins. TWT—two-waytraveltime.

2000; Planke et al., 2005). Some thin sills re-corded in wells have no high-amplitude seis-mic expression (Planke et al., 2005). Strati-graphic markers were calibrated by wellsdrilled in each of the survey areas.

We mapped a total of 33 sills in area A and90 in area B. Individual sills range in areafrom 1 to 500 km2. Some sills are sufficiently

thick (�100 m) to have a discrete reflectionfrom upper and lower contacts, but most sillshave a tuned seismic response, and sill thick-ness is estimated as being less than the verticalresolution (Smallwood and Maresh, 2002) andranges from �60 to 100 m. Most (�90%) ofthe sills mapped are highly discordant to thehost stratal reflection configurations, dipping

10�–35� to the horizontal. The intruded unitshave dips that are generally �10� (Figs. 2 and3). Sills intruded into crystalline basementrocks exhibit similar geometry to those in-truded into the sedimentary cover (Fig. 3).The vertical extent (H) of individual sills rang-es from 100 m to 6.4 km. Sills intruded atdeeper crustal levels are larger and ascend ver-tically further than those intruded at shallowerlevels, suggesting a link between the emplace-ment depth and H.

A characteristic feature is that most sillsconnect with sills above and below via silljunctions (Figs. 2 and 3). This important ob-servation means that individual sills were em-placed as part of larger sill complexes. Im-portantly, we observe that all sills formingjunctions connect with shallower sills neartheir deepest point, which strongly impliesthat the shallower sills were intruded by mag-ma fed via the deeper sills (Hansen et al.,2004).

Sill Complex Connectivity and VerticalExtent

The vertical extent and degree of intercon-nectivity of the sill complexes in the two sur-vey areas is graphically portrayed in Figure 4.Only a subset of the total number of sillsmapped in each area is shown for clarity. Inarea A, the sill complex forms a fully inter-connecting network through sill-sill junctions(Fig. 4A), extending almost 8 km vertically,from 150 m below the paleosurface to the baseof the 3D seismic profiles. Deeper-level sillsare observed to extend some 2–3 km deeper,to a paleodepth of �10–11 km on nearby re-gional two-dimensional (2D) seismic profilesthat tie with the 3D seismic data. Individual

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GEOLOGY, November 2006 931

Figure 3. Representativeseismic section from sur-vey area B, showing high-amplitude reflections in-terpreted as sills intrudedboth in basement andsedimentary host rocks.This shows link betweenHigh Velocity Body (HVB)and overlying sill com-plex. Vertical magmatransport from level ofHVB was partially facili-tated along shallowly in-clined fault plane fromwhich sills intruded atshallower levels and part-ly through sill-sill junc-tions. BTU—Base TertiaryUnconformity.

Figure 4. Graphical representation of connectivity and vertical extent of individual sills com-prising sill complexes in three-dimensional (3D) seismic survey areas. Vertical axis is depthbelow paleoseafloor. Horizontal axis is not scaled and is used to show relative positions ofspecified mapped sills with interconnecting geometry. Datum for vertical scale is themapped paleoseafloor at time of sill intrusion at ca. 55 Ma (Svensen et al., 2004). Depthconversion calculations used linear velocity functions derived from interval velocities fromexploration well 6704/12-1 and velocities derived from wide-angle seismic data (Skogseidand Eldholm, 1987). Error in depth conversion is estimated as 10%. Data have not beencorrected for compaction. A: Sill complex in survey area A. B: Sill complex in survey areaB. Deepest level of sills is equivalent in depth to level of top of High Velocity Body (HVB)shown in Figure 1B. Sill complex can be traced across vertical distance of >10 km fromHVB to within a few hundred meters of paleoseafloor.

sills range from hundreds of meters to 3 kmin vertical ascent.

In area B, 80% of the total vertical rangeof 11 km of the sill complex is interconnectedby junctions (Fig. 4B). The gaps in verticalconnectivity coincide with mapped intersec-tions between faults and sills, and it is likelythat these provided the means by which sillslinked to adjacent bodies in a minority of cas-es (cf. Wilson and Wheeler, 2002; Hansen,2004; Trude et al., 2003). The sill network canbe traced vertically from within 200 m of thepaleoseafloor down to the level of an inferredlower-crustal magmatic body (Mjelde et al.,2001; Torne et al., 2003) (Fig. 4 and HVB ofFig. 1B), where the host rocks have been in-terpreted as continental crystalline basementby Gernigon et al. (2004) and Corfield et al.(2004). This body seems to be the likeliest

source of the magma feeding the sills (Cor-field et al., 2004), but it is not universally in-terpreted as a body of underplated magma(Gernigon et al., 2004). Our mapping thusprovides a direct observational link between amajor intrusive complex (sill network) and itsputative source (the High Velocity Body[HVB]), and suggests that the mapped sill net-work spans virtually the entire vertical extentof the thinned continental crust from the verynear surface down to the lower-crustal magmasource.

DISCUSSIONThe most remarkable observation from both

survey areas is that the mapped sills form al-most continuous networks linked by junctionsspanning �50% of the vertical extent of thethinned continental crust. Vertically stacked

sills have been recognized before, but only intwo previous studies (Larsen and Marcussen,1992; Wilson and Wheeler, 2002) have theybeen documented as occurring over a similardepth range as those identified here. Becausethey only had access to 2D seismic data, Lar-sen and Marcussen (1992) were not able toshow three-dimensional evidence that theirvertically distributed sills were physicallyconnected. However, they were the first topropose that the sills might be instrumental infeeding the volumetrically impressive floodbasalts located in the same region of theGreenland margin. This latter proposal hassubsequently been challenged by Hald andTegner (2000), but it remains an interestingand potentially far-reaching hypothesis. Wil-son and Wheeler (2002, p. F1407) previouslyinferred a connection between the HVB andthe overlying sill complex in area B, notingthat magma ‘‘appears to move from sill to sillvia a complex system of narrow, dyke-likefeeders.’’

Our data allow us to demonstrate the con-nectivity between sills at depth and in the nearsurface because we can identify and map thejunctions, and thus at least open the way toreexamine the hypothesis advanced by Larsenand Marcussen (1992). As such, the sillsmapped in survey areas A and B represent themost extensively developed and best-imagedsill network described to date. The intercon-nectedness of the sill networks and the obser-vation of a potential link between magmasource and sill complex in survey B allow usto address the important question raised byLarsen and Marcussen (1992) as to what ex-tent the sills could have acted as feeders forsurface extrusion of magma. For this to hap-pen, we suggest that two conditions must bemet: (1) the sills must be conduits for magmaflow along a continuously linked network pri-or to their solidification (Spera, 1980), and (2)the shallowest sills in this network must in-tersect the paleosurface, or come close to suchan intersection. Our mapping of the seismicdata clearly verifies the second condition, withshallow-level sills emplaced within 150–400m of the surface in the two study areas. Forcondition 1, we note that all deeper-level sillsforming junctions with shallower sills do sonear their deepest points. This argues stronglyfor the sequential feeding of shallower sills bydeeper sills via their junctions. It also arguesfor the maintenance of the sills as open con-duits throughout the entire period of emplace-ment of the full vertical extent of all the sillsthat are interconnected via this type of junc-tion (Hansen et al., 2004). In the case of sur-vey A, this condition is certainly fulfilled, butfor survey B, it requires an additional role offaults in providing connectivity for magmathroughput, as previously noted by Wilson andWheeler (2002).

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How long could these sills have remainedopen under the pressure-temperature condi-tions extant at their emplacement depths?From sill cooling theory (Jaeger, 1959) ap-plied to published data on sill thicknesses inthe NE Atlantic igneous province (Smallwoodand Maresh, 2002; Svensen et al., 2004), weestimate maximum cooling times of 103–104

yr. This is probably an upper limit for the lon-gevity of individual sill complexes as trans-crustal magma transport networks, althoughwe note the possibility that conduits wereopen longer than suggested by the simplecooling time calculation because it is based onan instantaneous single intrusion of magma,and does not take into account the possibilityof convective cooling. It should be noted thatthere are many sill complexes along the Nor-wegian margin that could have acted as mag-ma supply routes, although these lack com-prehensive 3D seismic coverage at present(Svensen et al., 2004).

Sills have not previously been widely con-sidered as an efficient transport mechanism forvertical magma ascent in crustal settings, al-though recent studies of the Ferrar province inAntarctica have linked shallow-crustal silltransport routes to flood basaltic magmatism(Marsh, 2004). We suggest that the combina-tion of their large apertures (some fraction oftheir final thickness of 20–100 m) and theirlarge surface area is sufficient to offset anyloss in magma pressure head due to their 10–35� angle of climb (Spera, 1980). In conclu-sion, this study raises the viability of trans-gressive sills as conduits for long-range ver-tical magma transport through the crust.Critical to this view is our observation of sucha large vertical depth range of continuous sillintrusion. Given the limited seismic coverageof volcanic margins in general, vertically ex-tensive sill complexes are possibly much morecommon than indicated by the few descrip-tions to date. The close spatial association ofmajor sill complexes with high-flux extrusivesystems in regions such as the NE Atlanticmargin, the Karoo igneous province, and theSiberian Traps justifies the future testing of theintriguing proposal advanced by Larsen andMarcussen (1992) that sills play a much moresignificant role in feeding large-scale extrusivemagmatism than currently envisaged.

ACKNOWLEDGMENTSWe thank David Rickard, Julian Pearce, Andrew

Kerr, and Huw Davies for discussions; Steve Cor-field, Arild Lie, Else Grandal, and Walter Wheelerof Norsk Hydro for access to the three-dimensionalseismic data and well data; and Statoil (Lars Jensen)for access to regional two-dimensional seismic data.Marjorie Wilson, John Smallwood, Hans ChristianLarsen, and Sverre Planke are thanked for thought-ful and constructive reviews.

REFERENCES CITEDAnderson, E.M., 1951, The dynamics of faulting

and dyke formation with applications to Brit-ain: Edinburgh, Oliver & Boyd, 206 p.

Berndt, C., Skogly, O.P., Planke, S., Eldholm, O.,and Mjelde, R., 2000, High-velocity breakup-related sills in the Vøring Basin, off Norway:Journal of Geophysical Research, v. 105, no.B12, p. 28,443–28,454.

Berndt, C., Planke, S., Tsikalas, F., and Rasmussen,T., 2001, Seismic volcanostratigraphy of theNorwegian margin: Constraints on tectono-magmatic breakup processes: Journal of theGeological Society of London, v. 158,p. 413–426.

Brekke, H., 2000, The tectonic evolution of Nor-wegian Sea continental margin with emphasison the Vøring and Møre Basins on the Nor-wegian Sea continental margin, in Fleet, A.J.and Boldy, S.A.R., eds., Petroleum geology ofnorthwest Europe: Proceedings of the 5thConference: London, Geological Society[London], p. 261–274.

Coffin, M.F., and Eldholm, O., 1994, Large igneousprovinces: Crustal structure, dimensions andexternal consequences: Reviews of Geophys-ics, v. 32, p. 1–36, doi: 10.1029/93RG02508.

Corfield, S.M., Wheeler, W., Karpuz, R., Wilson,M., and Helland, R., 2004, Exploration 3Dseismic over the Gjallar Ridge: Visualizationof structures on the Norwegian volcanic mar-gin from Moho to seafloor, in Davies, R.J.,Cartwright, J.A., Stewart, S.A., Lappin, M.,and Underhill, J.R., eds., 3D seismic technol-ogy: Application to the exploration of sedi-mentary basins: Geological Society [London]Memoir 29, p. 177–185.

Ernst, R.E., Head, J.W., Parfitt, E., Grosfils, E., andWilson, L., 1995, Radiating dyke swarms onEarth and Venus: Earth-Science Reviews,v. 39, p. 1–58, doi: 10.1016/0012-8252(95)00017-5.

Francis, E.H., 1982, Emplacement mechanism oflate Carboniferous tholeiite sills in northernBritain: Journal of the Geological Society ofLondon, v. 139, p. 1–20.

Gernigon, L., Ringenbach, J.C., Planke, S., and LeGall, B., 2004, Deep structures and breakupalong volcanic rifted margins: Insights fromintegrated studies along the outer Vøring Ba-sin: Marine and Petroleum Geology, v. 21,p. 363–372, doi: 10.1016/j.marpetgeo.2004.01.005.

Hald, N., and Tegner, C., 2000, Composition andage of tertiary sills and dykes, Jameson LandBasin, East Greenland: Relation to regionalflood volcanism: Lithos, v. 54, p. 207–233,doi: 10.1016/S0024-4937(00)00032-3.

Hanghoj, K., Storey, M., and Stecher, O., 2004, Anisotope and trace element study of the EastGreenland dyke swarm: Journal of Petrology,v. 44, p. 2081–2112.

Hansen, D.M., 2004, 3D seismic characterization ofigneous sill complexes in sedimentary basins:North-east Atlantic margin [Ph.D. Thesis]:Cardiff, Wales, Cardiff University, 448 p.

Hansen, D.M., Cartwright, J.A., and Thomas, D.,2004, 3D seismic analysis of the geometry ofigneous sills and sill junction relationships, inDavies, R.J., Cartwright, J.A., Stewart, S.A.,Lappin, M., and Underhill, J.R., eds., 3D seis-mic technology: Application to the explorationof sedimentary basins: Geological Society[London] Memoir 29, p. 199–208.

Jaeger, J.C., 1959, Temperatures outside a coolingintrusive sheet: Science, v. 257, p. 44–54.

Klausen, M.B., and Larsen, H.C., 2002, EastGreenland coast-parallel dike swarm and itsrole in continental breakup, in Menzies,M.A., et al., eds., Volcanic Rifted Margins:Geological Society of America Special Pa-per 362, p. 133–158.

Larsen, H.C., and Marcussen, C., 1992, Sill intru-sion, flood basalt emplacement and deep crust-al structure of the Scoresby Sund region, East

Greenland, in Storey, B.C., Alabaster, T.,and Pankhurst, R.J., eds., Magmatism andthe causes of continental break-up: GeologicalSociety [London] Special Publication 68,p. 365–386.

Lister, J.R., and Kerr, R.C., 1991, Fluid-mechanicalmodels of crack propagation and theirapplication to magma transport in dykes: Jour-nal of Geophysical Research, v. 96,p. 10,049–10,077.

Marsh, B., 2004, A magmatic mush column RosettaStone: The McMurdo dry valleys of Antarc-tica: Eos (Transactions, American GeophysicalUnion), v. 47, p. 497–508.

Mjelde, R., Digranes, P., van Schank, M., and Shin-amura, H., 2001, Crustal structure of the outerVøring Plateau, offshore Norway margin fromocean bottom seismic and gravity data: Jour-nal of Geophysical Research, v. 106,p. 6769–6791.

Planke, S., Rasmussen, T., Rey, S.S., and Mykle-bust, R., 2005, Seismic characteristics and dis-tribution of volcanic intrusions and hydrother-mal vent complexes in the Vøring and MøreBasins, in Dore, A.G., and Vining, B., eds.,Petroleum geology: N.W. Europe and globalperspectives: London, Geological Society[London], p. 833–844.

Rubin, A.M., 1995, Propagation of magma filledcracks: Annual Review of Earth and PlanetarySciences, v. 23, p. 287–336, doi: 10.1146/annurev.ea.23.050195.001443.

Skogseid, J., and Eldholm, O., 1987, Early Ceno-zoic crust at the Norwegian continental mar-gin and the conjugate Jan Mayen Ridge: Jour-nal of Geophysical Research, v. 92,p. 11,471–11,491.

Smallwood, J.R., and Maresh, J., 2002, The prop-erties, morphology and distribution of igneoussills, in Jolley, D.W. and Bell, B.R., eds., TheNorth Atlantic igneous province: Stratigraphy,tectonic, volcanic and magmatic processes:Geological Society [London] Special Publi-cation 197, p. 271–306.

Spera, F.J., 1980, Aspects of magma transport, inHargraves, R.B., ed., Physics of magmaticprocesses: Princeton, New Jersey, PrincetonUniversity Press, p. 265–323.

Svensen, H., Planke, S., Malthe-Sørensen, A., Jamt-veit, B., Myklebust, R., Eidem, T.R., and Rey,S.S., 2004, Release of methane from a volca-nic basin as a mechanism for initial Eoceneglobal warming: Nature, v. 429, p. 542–545,doi: 10.1038/nature02566.

Thomson, K., and Hutton, D., 2004, Geometry andgrowth of sill complexes: Insights using 3Dseismic from the North Rockall Trough: Bul-letin of Volcanology, v. 66, p. 364–375, doi:10.1007/s00445-003-0320-z.

Torne, M., Fernandez, M., Wheeler, W., and Karpuz,R., 2003, 3D crustal structure of the VøringMargin (NE Atlantic): A combined seismicand gravity image: Journal of Geophysical Re-search, v. 108 (B2), ETG 16-1-11, doi: 10.1029/2002JB001838.

Trude, K.J., Cartwright, J.A., Davies, R.J., andSmallwood, J.R., 2003, A new technique fordating igneous sills: Geology, v. 31,p. 507–532.

Wilson, M., and Wheeler, W., 2002, Magma migra-tion through the continental crust: 3-D seismicand thermo-mechanical constraints on sites ofcrustal contamination: Eos (Transactions,American Geophysical Union), v. 47, Supple-ment, p. F1407.

Manuscript received 24 February 2006Revised manuscript received 30 May 2006Manuscript accepted 31 May 2006

Printed in USA

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