Earth and Planetary Science Letters 271 (2008) 202–208

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Earth and Planetary Science Letters

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A new mode of inner volcano growth: The “flower intrusive structure”

Alessandro Tibaldi a,⁎, Federico A. Pasquarè b

a Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano Bicocca, Milan, Italyb Dipartimento di Scienze Chimiche e Ambientali, Università degli Studi dell'Insubria, Como, Italy

Iceland

⁎ Corresponding author. Tel.: +39 2 64482052; fax: +E-mail address: alessandro.tibaldi@unimib.it (A. Tiba

0012-821X/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.epsl.2008.04.009

A B S T R A C T

A R T I C L E I N F O

Article history:

A multiple-sill laccolith nest Received 29 October 2007Received in revised form 28 March 2008Accepted 8 April 2008Available online 22 April 2008

Editor: C.P. Jaupart

Keywords:laccolithsillsheetstructural modelEsja

ed within a centrally-dipping sheet swarm is a recently discovered sub-volcanicstructure, resembling a “flower” in section-view, which we have found in some eroded volcanoes in Iceland,suggesting that this structure can have some general relevance in deforming rock successions in volcanicareas. After a brief summary of the main characteristics of these examples, with details on Stardalur volcano,we present an evolutionary model to explain the whole structure. Both sills and sheets are fed by multiplepulses of magma following radial paths from a shallow chamber; the earlier vertical set of the radial sheetsabove the middle of the chamber are abruptly deflected into sills by the overlying lava succession. A proto-laccolith starts developing, composed of different, vertically stacked sills, originating from the central verticalfeeder zone. Each sill contributes to the deformation of the overburden; room for intrusions is provided bythe higher strain that can be attained by the widespread hyaloclastites and breccias which have a “softer”behaviour (with a lower Young's modulus) than the stiffer (higher Young's modulus) lava flows, combinedwith the doming of the overlying lavas. Later dikes get deflected outwards along the contact with the proto-laccolith and acquire a convex-upward shape, in cross-section.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Investigating the conditions and mode of emplacement of sub-volcanic intrusions is crucial for the interpretation of the internalstructure of volcanoes and to understandmagmamigration processes.Different emplacement styles result in the emplacement of differentsub-volcanic bodies, ranging from horizontal sills to inclined sheetsand to vertical dikes, that produce different deformation of the hostrocks. The correct interpretation of such deformations, in terms ofsurface signals at active volcanoes, can contribute to volcanic hazardassessment. Understanding the mode of formation and geometry ofshallow intrusive bodies also provides information on the factorscontrolling whether or not intrusions develop into shallow magmachambers. The internal growth of volcanoes may represent animportant contribution to their build-up (Annen et al., 2001).

Recently, Pasquarè and Tibaldi (2007) have demonstrated, by fielddata, a new type of sub-volcanic structure: The association of amultiple-sill laccolith nested within a centrally-dipping sheet swarm below theeroded Stardalur volcano, in SW Iceland. The structural condition for sillemplacement is related to the effect of neutral buoyancy forces (Corry,1988), stress barriers (Gretener,1969), discontinuities (Weertman,1980;Gaffney et al., 2007), or to horizontal compressive stresses higher thanthe vertical stress (Gudmundsson, 1995). The latter enables magma topropagate horizontally by fracturing and opening up the host rock in thevertical direction where a lower force is required. At divergent plate

39 2 64482073.ldi).

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boundaries, where the greatest principal stress (σ1) is vertical, theemplacement of sills should not take place. However, several sills arefound at extinct volcanoes in the rift zone of Iceland, such as in thePliocene terrains located immediately west of the southwestern rift arm(Fig. 1) and in the Pleistocene terrains along the rift (Forslund andGudmundsson,1991); outside Iceland, for example, sills were emplacedduring the Basin and Range extensional phase (Valentine and Krogh,2006). Probably, sills in volcanoes aremorewidespread than commonlythought because, in the absence of detailed field observations of eachsingle layer, they can be confused with lavas.

Centrally-dipping sheets have long been recognized elsewhere, sincethe pioneerwork byAnderson (1936); however, the presence ofmultiple,vertically stacked sills forming a laccolith has been only recentlyrecognized in the field (Horsman et al., 2005; Pasquarè and Tibaldi,2007) and conceptually corroborated (Menand, 2008). Herewe highlightthe novelty of the association of a multiple-sill laccolith nested withincentrally-dipping sheets, an occurrence almost undocumented in theliterature. In the present paper we first briefly summarise the main dataon Stardalur volcano, according to Pasquarè and Tibaldi (2007), we thenprovide some new field observation on the Stardalur area and anothertwo similarfield examples, Thverfell andKjalarnes, located in SWIceland.Finally, we propose a new, general evolutionary model for interpretingthe emplacement of this peculiar intrusive architecture.

2. Geological and structural setting

The geology of Iceland is dominated by Tertiary flood basalts andcentral volcanoes surrounding the active rift system. The studied area

Fig.1. A. Simplified sketchof theoriginal 1:10,000 scale geologicalmapof the Stardalur volcano area after PasquarèandTibaldi (2007), andB.N–S geological cross-section (A–B). Thearrowsindicate themodal dip direction of 114 sheetsmeasured at 31 sites. All the sheets dip below the laccolith, according to a centrally-dipping swarmgeometry. The proposed “flower intrusivestructure” is documented in the cross section. C. Schmidt's stereogramprojection, lowerhemisphere, showing the poles to inclined sheets (diamonds) and dikes (triangles). Note that polesto inclined sheets clearly depict a conical geometry. Inset shows location of the Stardalur volcano and the Kjalarnes–Thverfell area; they are both located in the Esja Peninsula.

203A. Tibaldi, F.A. Pasquarè / Earth and Planetary Science Letters 271 (2008) 202–208

(known as Esja Peninsula, Fig. 1A,), west of the Quaternary rift zone, iscomposed of a succession of basaltic lava flows and hyaloclastites,known as the Plio-Pleistocene Formation (3.3-0.8 Ma BP, Fridleifsson,1977; Johannesson and Saemundsson, 1998). The lavas composing thebulk of this peninsula (cumulative thickness=1.3 km) were erupted bythe Kjalarnes, Hvalfjordur and Stardalur volcanoes, active 2.8, 2 and1.7 Ma ago, respectively. Stardalur volcano was affected by a calderacollapse before the emplacement of the documented intrusions(Fridleifsson and Kristjansson, 1972).

Detailed lithostratigraphic and structural mapping (at the 1:10,000 scale) of the Stardalur area has been carried out by Pasquarèand Tibaldi (2007), as briefly summarized below. A succession ofbasaltic lavas, hyaloclastites and volcano/sedimentary breccias andfluvial deposits, belonging to the older terms of the Plio-PleistoceneFormation, is cut by several intrusions (Fig. 1A and B) and covered bylavas b0.8 Ma old. The intrusions include dikes, inclined sheets and amajor intrusive body, regarded as a laccolith by Fridleifsson (1977).Dikes and inclined sheets are all made of grey to brown dolerite. Noother petrographic or chemical data are available on these intrusions.Dikes and faults mainly strike E–W and NNE–SSW. The E–W-strikingstructures are minor dikes and faults cutting pre-Quaternary rocks, inagreementwith the structures documented by Villemin et al. (1994) inthe whole Esja peninsula and the nearby Snaefells peninsula. TheNNE–SSW structures are long dikes and major normal faults linked tothe main Icelandic rift and widespread in most of Iceland.

At Stardalur, a centrally-dipping sheet swarm geometry has beenconstrained by mapping and describing 114 sheets at 31 sites(Pasquarè and Tibaldi, 2007). All the sheets are basaltic in composi-

tion, with an intergranular (doleritic) texture. The original data aredepicted in a stereogram (Fig. 1C) where poles to inclined sheetsclearly indicate a conical geometry; poles to planes for the feederdikes are also shown. The data collected at the 31 sites are displayed(Fig. 1A) in terms of the modal dip direction of the sheets at each site,which is tangential to the depth trajectory of the sheets and might becalled “sheet trajectory” (Klausen, 2004). The arrows in Fig. 1A, whichrepresent the calculated sheet trajectories, point towards the centralportion of the study area, where the laccolith crops out (Fig. 1A):Hence, they define a centrally-dipping sheet geometry. In the centralportion of the swarm, sheets are sub-vertical to vertical and theiraverage dip decreases outwards (Fig. 1B). Above this central zone thelaccolith is composed of different, vertically stacked sills.

3. Sheets, sills and central laccoliths

Further field investigations enabled us to understand that theStardalur sheet system is even larger than previously thought. Moresheets belonging to this system have been found farther west,encompassing a total distance of 12 km from the eastern end of thesystem to the western one. Most of the sheets that reach up to higherelevations in the host rock are gently-dipping (b20°) or regarded as sills,being parallel to the general dip of the host rock and sub-horizontal.Some compose the laccolith and some are external to it, beingconcentrated in the upper peripheral sectors of the centrally-dippingsheet system. These outer and uppermost sheets intruded up to aboutthe same crustal level as the central laccolith: No sheets have beenobserved cropping out at a higher elevation than the main laccolith.

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The laccolith (Fig. 1A) at the centre of the mapped sheet swarm hasan exposed area of 3 km2 plus a surface hidden by glacial and fluvialdeposits (not reported for the sake of clarity in Fig.1A) that,most likely,is around 1.5 km2. Since the laccolith average minimum thickness is200m, its volume is at least 0.9 km3. All its rock samples correspond toa microgabbro with a subophitic texture revealing shallow emplace-ment. The overburden is mainly composed of lavas which arepreserved only at the margins of the major intrusion. Here, thegeometry of the lava layers is clearly unrelated to the regional tectonicdip that is on average 8° to the SE: The lavas along the laccolithmarginsshow different dips (up to 25–35°) and dip directions which might bepartly related to the caldera downsagging and partly to the doming ofthe overburden due to the growth of the major intrusion.

The intrusive rocks are pervaded by vertical and horizontaldiscontinuities (e.g. Fig. 2A and B). Vertical discontinuities are mostlyrepresented by well-developed columnar cooling joints that fre-quently and abruptly stop at the horizontal planar surfaces, as shownin Fig. 2B. The magmatic foliation, defined by the iso-orientation ofphenocrysts and elongated vesicles, is parallel to these horizontalsurfaces, as commonly documented at themargins of sheet intrusions.Some chilled margins are locally observable along the horizontal

Fig. 2. A. Example of chilled margins separating single sills that represent different emplaceC. Example of amountain cliff entirely composed of stacked sills, composing a laccolith. A dikesome of the main planar surfaces between single intrusive units can be observed. The laccdirections from a vertical feeder zone.

surfaces (e.g. Fig. 2A) which, therefore, separate single emplacementunits whose thickness mostly ranges from 10 to 30 m (e.g. Fig. 2C andD), although some thinner sheets locally occur. Approximately in thecentral portion of the laccolith, along an E–W-trending sector thatstretches from site B to site C in Fig. 3A, columnar joints are sub-horizontal and the planar surfaces corresponding to fine-grained/chilled margins are sub-vertical, suggesting the presence of a vertical,E–W-trending feeding zone. On either side of the E–W feeder, at ahigher elevation, columnar joints gradually rotate and acquire avertical plunge; correspondingly, the planar surfaces, chilled marginsand magmatic foliation gradually become horizontal, indicating a sill-like geometry of the intrusive units (e.g. Fig. 3B and C). Most of the sillsare directly into contact with each other without interlayered hostrocks. In several cases, the absence of chilledmargins and the presenceof foliation suggest the emplacement of magma into units that werenot completely cooled yet. The local presence of chilled marginssuggest that in some cases the sills were emplaced when the previousones had already cooled, in such a way as to hinder amalgamation.

It has been possible to document in the field the transition zonebetween the feeder dikes and the horizontal sills composing thelaccolith, exposed in deep gullies. The dikes cropping out at a deeper

ment phases. B. Example of a sill's brittle margin where columnar joints abruptly stop.cuts through thewhole sequence. For location see Fig. 3. D.Whole view of the laccolith;olith grew by vertical stacking of several, 10 to 30 m-thick sills. Arrows indicate flow

Fig. 3. A. Location of the laccoliths nested within centrally-inclined sheet systems found in the Esja peninsula, suggesting that the flower intrusive structure can be a commonintrusive feature. Note the black arrows indicating the average strike and dip of the sheets. The Kjalarnes and Thverfell case studies are very similar to the described Stardalur example(letters indicate the sites shown in the photos 3B to 3G and photo 2C). B. and C. main vertical feeder dikes gradually rotate into horizontal sills; this geometry has been commonlyobserved in sheets which are not offset by any other intrusions and hence are younger. In D. a dike is abruptly deflected into a horizontal sill; this geometry has been observed morerarely. In E. inclined sheets gradually bend into sills. F. Photo of the area where the west-dipping sheets of Kjalarnes and the east-dipping sheets of Thverfell crosscut each other.G. Photo of the area where the west-dipping sheets of Thverfell and the east-dipping sheets of Stardalur crosscut each other.

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erosion level, upwards acquire a shallower dip. Twomain types of dike-sheet-sill transitionhavebeenobserved:Thefirst one is representedbyarotation (e.g. in Fig. 3B, C and E) of sub-vertical sheets gradually turninginto horizontal sills, hence creating a convex-upward geometry. Thesheet thickness does not change with the decrease in sheet dip.Gradually-rotating sheets represent themost common typeof transitionobserved and are generally not offset by other intrusions, hencesuggesting a later phase of emplacement. Their host rock is mostlyrepresented by previous intrusions, or by previous intrusions above andhyaloclastites or breccia deposits below. Offset markers observed atsome sites indicate that the sheets filled Mode-I fractures, as no shearoccurred prior to sheet emplacement. The second type of transition is

represented by the abrupt deflection froma lower, vertical feederdike toan upper, horizontal sill (e.g. Fig. 3D). Also in this case the sheetsoriginated as Mode-I fractures. The sharp dike-sill transition was lessfrequently observed than the gradual rotation, and occurs at the contactwith an upper layered succession composed of lava beds. Most of theabrupt dike-sill associations were cut by successive intrusions, hencethey were emplaced at an earlier stage. It is also important to highlightthat, conversely, most of the NE-striking long dikes belonging to theyounger Quaternary rift system, cut across the entire lava overburden.

Farther west (Fig. 3A) we have found another two examples of thistype of intrusive architecture, including both the sill-composed laccolithand the centrally-dipping sheet systems. The first case, Thverfell, is

206 A. Tibaldi, F.A. Pasquarè / Earth and Planetary Science Letters 271 (2008) 202–208

represented bya 9-km-in-diameter sheet swarmcomposed of hundredsof sheets dipping inward toward a common area. At the center of thesheet system occurs a sill-composed laccolith, (e.g. Fig. 3C, detailed datain Tibaldi et al., 2008), 4 km in diameter and 150m thick. At the easternand western end of this sheet system, two sets of sheets with differentopposite dips crosscut each other (Fig. 3F andG). To the east they belongto the Stardalur and Thverfell sheet systems. To the west they belong tothe Thverfell system and to another one, located farther west, calledKjalarnes. Also at Kjalarnes a system of sheets dip towards a commonzone; however a complete picture of the structure is hindered by thepresence of the sea. The available data indicate that the Kjalarnescentrally-dipping sheet system is at least 4.5 km in diameter. At aboutthe center of this system, another main intrusive body crops out. In allthese three cases, field data on the intrusion cross-cutting relationshipsindicate that the centrally-inclined sheets and nested laccoliths havebeen emplaced in a short time span.

4. Discussion

The data here presented, together with those of Pasquarè andTibaldi (2007), document the presence, at Stardalur volcano, of a sill-composed laccolith surrounded by a centrally-dipping sheet swarm.Since no inclined sheet cuts the laccolith, it can be hypothesized thatthe major intrusion was emplaced immediately after or at the sametime as the sheets; this fact together with the central location of thelaccolith with respect to the sheets indicate that they belong to thesame intrusive system. It is likely that the occurrence of a calderacollapse at Stardalur had no influence whatsoever on the sheet/laccolith association, as it took place before the intrusions. Moreover,the very same association of a sill-composed laccolith within acentrally-dipping sheet swarm occurs at the Thverfell and Kjalarnesintrusive systems, located west of the study area; none of themexperienced caldera collapse.

To the best of our knowledge, this type of association has neverbeen reported in the literature: Centrally-dipping sheets, in fact, havelong been recognized elsewhere, since the pioneering work byAnderson (1936), but never associated with a centrally-located, coevallaccolith. Moreover, laccoliths have been considered classically to formas a major expanding intrusion (for a review see Corry, 1988 andreferences therein), whereas the association of multiple, verticallystacked sills forming a laccolith has been only recently recognized inthe field (Horsman et al., 2005; Pasquarè and Tibaldi, 2007). Hence,this association represents a new intrusive structure worth beingdistinguished from other ones. We hereby propose to call it “flowerintrusive structure” a definition that adequately describes the flower-shaped appearance of the sheet system, in cross-sectional view,within which the laccolith is nested.

We propose a mechanical explanation for the whole flowerstructure, after summarising the main studies conducted in the paston its individual intrusive units.

4.1. Previous theories

The stress field controlling the geometrical arrangement of conicalsheets is generally given by a local stress field generated by themagmachamber dominating over the regional tectonic stress field. Anderson(1936) and Phillips (1974) suggested that conical sheets are related tocollapse of a magma chamber along conical fractures providing spacefor sheets. In the present case this possibility must be discardedbecause: 1) a caldera occurs only at Stardalur volcano; 2) even here,caldera formation took place long before sheet emplacement, 3) noevidence of shear has been found along the sheet walls, and 4) sheetsoccur also at the center of the system.

Following Gudmundsson (1998), the excess pressure of themagmachamber creates a local compressive stress field around the reservoirwith trajectories of the greatest principal stress (σ1) departing radially,

suitable for the intrusion of concentric cone sheets. The radial sheetsabove the middle of the chamber are vertical, suggesting a localvertical σ1. The condition for sill emplacement is that horizontalcompressive stresses are larger than the vertical stress (Gudmunds-son, 1990).

Concerning the dike-sill transition, a classical explanation calls forthe effect of neutral buoyancy forces (Corry, 1988). Magma movesvertically upward due to the density contrast with the surroundinghost rock, until it reaches a neutral buoyancy level along which itspreads laterally. The studied intrusions at Stardalur, Thverfell andKjalarnes were injected through a host rock succession dominantlycomposed of fragmented deposits (i.e. hyaloclastites and breccias) thedensity of which is no higher than the density of magma. In Iceland,thousands of high-density basaltic dikes propagated upwards withoutany difficulty through low-density sediments, hyaloclastites andrhyolites, so neutral buoyancy cannot be the main explanation fordike bending and sill formation. The upward propagation of dikes andsheets can be stopped due to the generation of stress barriers, i.e.layers with local stresses unfavourable for the intrusion propagation(Gretener,1969; Gudmundsson,1986,1990; Parsons et al., 1992); sheetintrusions can also become arrested at discontinuities (Gudmundsson,2002; Gudmundsson and Brenner, 2005). In other cases, such as theones here documented, dikes and sheets do not actually becomearrested, but are bended into sills: This can occur at the intersectionwith an already existing horizontal, freely slipping joint (Weertman,1980) or, similarly, at the intersection between a weak bedding planeand a steep normal fault at shallow depths (Gaffney et al., 2007). Thesudden deviation from a dike to a sill has also been reproducedexperimentally by Kavanagh et al. (2006) at the interface betweenupper, rigid layers overlaying lower, weaker layers. Finally, Valentineand Krogh (2006) related dike bending and sill formation to localstress rotation along 3-D variations on a normal fault plane.

Taking into account our field observations that document thegradual rotation from dikes to sills, and considering that hyaloclastitesand breccias are not affected by any preferential discontinuities orsystematic anisotropy, we believe that only some of the abovemechanisms can help to explain part of the structure and that thewhole flower intrusive structure can be explained as follows.

4.2. A model for the emplacement of flower intrusive structures

The centrally-dipping sheets, the vertical dikes underneath thelaccolith and the laccolith itself (fed by the dikes) belong to the sameradial sheet system (i.e. radial pattern of σ1) departing from a shallowmagma chamber. The set of inclined sheets represent the trajectoriesalong the upper flanks of the magma chamber (Fig. 4A). The E–Wvertical dikes feeding the laccolith propagated from the central, uppersector of the magma chamber. The presence of the rigid lava coverinduced a sharp deflection of the earlier dikes into sills whichoriginated a proto-laccolith (Fig. 4B). The vertical dikes were not ableto cut across the overlying succession of lavas for two main reasons: i.the higher stiffness of the lavas, consistent with the experiments ofKavanagh et al. (2006), and ii. because the tectonic stress state in thisregion was not characterised by extension at that time (Tibaldi et al.,2008), also documented by the occurrence of strike-slip faults in thestudy area (Villemin et al., 1994) and adjacent areas (Bergerat et al.,1990; Gudmundsson et al., 1992; Passerini et al., 1997). This is alsoconsistent with the stress barrier model (Gretener, 1969; Gudmunds-son, 1986). For dense magmas such as those of basaltic composition inthe study area, having small buoyancy, Pinel and Jaupart (2004)suggest that the upwards propagation of dikes can also be inhibitedbecause they cannot force their way against the compressive stressesgenerated by the presence of a volcano edifice above them.

Deflected by the lava pile, the sheets propagated into thehyaloclastites and breccias; this occurred both at Stardalur and at theother two intrusive systems of Thverfell and Kjalarnes. We highlight

Fig. 4. Model explaining the development of the whole flower intrusive structure. A. Thecentrally-inclined sheets intrude with trajectories departing radially from a shallowmagma chamber. B. Sheets propagate upwards and the ones outpoured vertically from themiddle of the magma chamber are deflected into horizontal sill, giving rise to a proto-laccolith. Room for emplacement is mainly provided by deformation in the hyaloclastite/breccia deposits and arching of the overburden. C. Further diking favours the developmentof the proto-laccolith into a thick laccolith made of vertically stacked sills, through theemplacement of upward-convex sheets and more intense arching of the overburden.

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that magma injection is favoured in fragmented deposits that can bedeformed more easily than rigid lava flows. In fact, volcanic brecciasand hyaloclastites have different physical properties than lava flows:They are “softer” (with a lower Young's modulus) than the stiffer(higher Young's modulus) lava flows, they have a larger voidpercentage and hence are less dense. Moreover, they have a very lowtensile strength, as demonstrated by in situ and laboratory geotechni-cal tests (Apuani et al., 2005a,b). A higher strain in fragmented depositscan be attained by compaction of voids, rotation of clasts and rigidblock motions. These conditions might favour a rotation of the stresstrajectories at the contacts between the “softer” and the stiffer rocks asalso suggested by Gudmundsson (2006).

Once the earlier centrally-dipping sheets and sills are emplaced,deformation can no longer take place through readjustements withinthe host rock and more room is required for further intrusions. At thispoint a major role is played by the doming of the overburden inducedby magma driving pressure, resisted by internal, shear and frictionalforces (Pollard and Johnson, 1973). However, our observation of several,vertically stacked sills forming a major laccolith suggests a meaningfuldifference from the classical models. In other cases (e.g. Jackson andPollard, 1988, 1990) it has been suggested that: 1) multiple sill

emplacement can precede laccolith development, 2) the sills areinterlayered at different levels in the host rocks, 3) the development ofthe laccolith occurs via the gradual thickening of one major intrusion,4) the laccolith has a circular shape in plan view. In our case, instead:1) the sills are no older than the laccolith but they actually compose thelaccolith, 2) the sills are vertically stacked3) the laccolith has anellipticalshape inplanview, 4) a centrally-dipping sheet swarm is associatedwiththe laccolith.

In our case it seems that magma applied its pressure on theoverburden rock along specific zones: At the beginning of theintrusion process the contact horizon between fragmented depositsand lavas played the role of a weak discontinuity. This is testified bythe older intrusions which developed along this contact. The magmaspreading laterally in the form of each sill could not eventually gainenough leverage on the overlying strata to form a single laccolith.Rather, each sill injection resulted in slight deformation of theoverburden. During this stage, most of the feeder dikes and sillswere emplaced in the central zone creating a sort of bulge, i.e. a proto-laccolith (Fig. 4B) that was thicker in its central, lower portion (see alsocross-section in Fig. 1B). We also suggest that this bulging started toinduce the upward-directed offset of the overburden, which occurredthrough brittle fracturing and rigid rotation of blocks, as evidenced bythe dips of lava layers measured around the flower intrusive structurethat are steeper than in the surrounding area and locally dip outwardsfrom the caldera. However, a complete picture of the overburdendeformation cannot be provided because the host rock geometry wasmodified also by the caldera collapse.

When the later dikes approached the gradually-thickened proto-laccolith, they were deflected along the contact surface (Fig. 4C)between the previous intrusions and the hyaloclastites/breccias. Sinceall these sheets end at the same crustal level, this should correspondto the boundary of influence of the magma chamber stresses. Thisstep-by-step emplacement style generated sufficient strain in the hostrock, and hence room, for the final development of a thick sheetedintrusion made of stacked sills (Fig. 4C). Sheeted emplacement pro-cesses have been observed or inferred also in large plutons (Hutton,1992; Barton et al., 1995; Coleman et al., 2004; Annen et al., 2006;Lipman, 2007) and at laccolith types different from the present one(Rocchi et al., 2002).

Finally, when the later regional rifting phase affected also this area,propagation of long NE-striking dikes across the entire sequence oflavas and sills took place, being favoured by the horizontal tectonic σ3

and the vertical σ1.

5. Conclusion

Field data in SW Iceland document the association of multiple-silllaccoliths nested in the middle of centrally-dipping sheets that built upflower intrusive structures in the lowermost portion of presently-eroded volcanoes. The geometry of the radial portion of the systemwascontrolled by a local radial σ1 linked to a shallow magma chamberoverpressure. Underneath the laccolith, magma was injected upwardsthrough vertical dikes rotating into sills. The abrupt deflection of theearlier dikes into sills was guided by structural barriers induced byoverlying lava beds. The gradual rotation of dikes into inclined sheetsand finally into sills has been explained in terms of barriers posed byproto-laccoliths. The barriers were due to the higher strain resistanceposed by the overlying lava flows or earlier-formed sills with respect tothe fragmented deposits (hyaloclastites and breccia) and to the possibleinfluence of a horizontal tectonic σ1. The final architecture of thelaccolith was built through piecemeal and cumulative upward-directeddisplacements of the overburden section. We believe that this model offlower intrusive structures can be of broad application in similar areasthat are characterised by the combination of large rheology contrasts inthe rock succession and the occurrence of a transcurrent or compres-sional tectonic regime, possible also close to rift systems.

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Acknowledgments

We wish to thank A. Gudmundsson and three anonymousreviewers, whose comments have significantly improved our manu-script. This workwas carried out in the framework of the InternationalLithosphere Programme - Task Force II project “New tectonic causes ofvolcano failure and possible premonitory signals”.

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