Journal of the Geological Sociery, London, Vol. 151, 1994, pp. 931-944, 11 figs. Printed in Northern Ireland
Basin structure and Tertiary magmatism on Skye, NW Scotland
R. W. H.BUTLER’ & D. H. W. HUTTON* ‘Department of Earth Sciences, The University, Leeds LS2 9JT, UK 2Department of Geological Sciences, The University, Science Site, South Road, Durham DH1 3LE, UK
Abstract: The emplacement of igneousmaterial into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic,Mesozoic structures aretraced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland.This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW-SE-trending faults that transfered Mesozoic displace- ments between the principal fault strands. A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed intothe sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that althoughthe higher parts of the basinfaults werereactivated to facilitate thedoming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation.
The extension of continental lithosphere which controls the may drive basin-wide advective systems in formation waters, development of sedimentary basins is associated commonly influencingburial diagenesis and consequentlymodifying withsyntectonic igneous activity. In recent years, mantle- carrierbed, reservoir and seal characteristics. These meltingmodels have emphasized the importance of processes andphenomema willbeinfluenced by the asthenosphericplumes which elevatetemperatures at the duration and sites of magmatism within the lithosphere. Of base of the lithosphere (McKenzie & Bickle 1988; White & these it is the site of magmatism and its structural controls McKenzie1989). Plumes may promotesurface uplift that we wish to address, with special reference to the upper (Griffiths et al. 1989), increase the geothermal gradient and crust. generate substantial volumes of basaltic magma (McKenzie Despite the importance of igneousrocks forthe & Bickle1988). If plumes coincidewith regions of geological evolution of some sedimentary basins, there has lithospheric extension this magma may be emplaced into the been little attention directed at the structureof such igneous crust. This may have a profound effect on basin geometry, complexes. Since the pioneering work of Anderson (1936), by increasingcrustal volume through intrusion. Normal studies of the structural geology of igneousrocks, faults generally createsedimentary basins.Syn-tectonic particularlylinking intrusion mechanisms and tectonics, magma may fill space otherwise available to accommodate have largely been directed at examples exhumed from the sedimentsthrough eruption of lava piles. It mayalso fill middle and lower crust wheredeformation hasbeen intra-crustal ‘pull-apart’ sites along faults thus reducing the dominantlyductile (e.g. Hutton 1982,1988; Hollister & depth of overlyingbasins. The advective heattransfer Crawford 1986; Brun et al. 1990; McCaffrey 1992). In these associated with emplaced basalticmagma may promote deep settings,the interplay between tectonic structures, melting of the continental crust and will certainly modify the tectonicand magmabuoyancy forces andthe interplay rheology of the lithosphere, its strength and the manner in between magma and country-rockrheologies provide the which it responds to tectonicboundary conditions and key controls on the styles and sites of intrusion. However, gravitational loads. In addition to these geometric aspects, within most sedimentary basins, the close proximity of the magmatismwill strongly affect thethermal structure of Earth’ssurface, as a free surface of stressrelease and sedimentary basins, thereby influencing hydrocarbon matu- deformation, is critical. Deformation of this surface caused ration(e.g. England et al. 1993). Thethermal anomalies bymagma spacecreation in thenear subsurfacethrough 93 1
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processes such as cauldron subsidence and doming is widely Scotland, with the greatest amalgam of centres occurring on documentedfrom modem volcanic centres.thisIn Skye (Fig. 1).Recent reviews of theTertiary geology of contribution we investigate the extent to which pre-existing Skye are provided by Bell & Hams (1986) andEmeleus basinstructures control the siting and geometry of large (1991). The igneousrocks arerepresented by asuite of magma chambersthat represent sub-volcanic complexes. plateau lavas into which are intruded a,layered complex of Our examples come fromthe British Tertiary Volcanic basicand ultrabasic composition (collectively formingthe Province of NW Scotland, especially the Isle of Skye. Cuillin complex) and a later groupof granitic intrusions (the Red Hills complex). Most of these units both cut and are themselves intruded by the major NW-SE-trending swarm Geological background of, dominantly basaltic, dykes. Thus there is a well-known The igneous centres of the Isle of Skye are classic ground, igneous ‘stratigraphy’: our conclusions will not modify it. not only for British geology but also for the development of TheHebridean province collectively experiencedpeak igneous studies in general. The classification of major and igneous activity from c. 63 to 59 Ma (Mussett et al. 1988). minorintrusions and their importance in building up On Skye, theplateau lavas were erupteddominantly at volcanic edifices owes much to the seminal studies of Harker about 59 Ma with the youngest Red Hills granite dated at (1904), Richey (1932), Bailey (1947) and others. These and 53.5 Ma (Dickin & Exley 1981). These rocks were emplaced earlierstudies (e.g. Geikie 1897) identifieda number of into the Sea of Hebrides basin which had experienced a long largeigneous complexes along the western coast of history of relatively minor subsidencethroughout late
Fig. 1. Simplified map of the geology of the Sea of Hebrides area, onshore and offshore NW Scotland (modified after BGS 1986u, b).Mesozoic sediments, wide stipple; Oligocene sediments, fine stipple; Palaeocene lavas, horizontal ruling; principal igneous centres, dashes. Pre-Mesozoic rocks are unornamented. Mesozoic faults (‘lolly-pop’ symbolson downthrown side): CF, Camasunary Fault; RF, Raasay Fault; AF, Apple- cross Fault; KF, Kishorn Fault; MF, Minch Fault. Boxed area shows Fig. 4. Section line of Fig. 2b is indicated (b-b’). Location of critical outcrops of Raasay fault at Balachuirn is indicated (f). Inset: British Isles, location map
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Palaeozoicand Mesozoic times with activefaulting thatthe dyke swarm hasno geometric relationship to continuing intothe Cenozoic(Peach et al. 1910; Hallam previousstructures but they do suggest thatthe major 1983; Morton 1987). centres are spatially connected with pre-existing faults; this Since the early studies, much of the igneous research has spatial association is also recognized by Emeleus (1991). been of a geochemical nature. This has suggested that at Ourpurpose here is toexamine the relationships least the later portions of the magmatism were derived from between upper crustal structures and magmatic activity as thelowermost lithosphere andupper asthenosphere indicatedprimarily by surfacegeology, not to assess the (Morrison et al. 1980,' Thompson & Morrison 1988). plumemodel orthe processes on alithospheric scale. Thompson & Gibson (1991) stressthe importance of the Specifically we will addressthe links between Mesozoic coincidence of theproto-Icelandic mantle plume and the structuresand the geometry of thevarious Palaeocene pre-existing 'thinspot' of attenuated continental lithosphere igneous intrusions on Skye. Therefore wefirst discuss the in the Sea of Hebrides basin. Probably, on a large scale, the basin structure. site of magmatism is strongly influenced by pre-existing variations in the thicknessand thus structure of the lithosphere. Basin structure and early geological evolution Vann (1979) and latterly England (1988,1992) showed The principal structures controlling Mesozoic basins are the thatthe regional dyke swarms of theBritish Tertiary Minch and Camasunary faults in the south, with the Minch, VolcanicProvince can be related to regional NE-SW Raasay, Applecross and Kishorn faults in the north (Fig. 1). extension.Speight et al. (1982) demonstratedthat the Thesefaults trend NNE-SSWwith down-to-the ESE swarms occur as groups of closely spaced dykes recording throws. The basinsappear to be simple half graben between 4% and 20% extensionover a few kilometres. developed on the downthrown hanging walls (Fig. 2). This is These high strain extensional zones separate less intruded, confirmed from deep seismic reflection profiles acquired by little extended regions. Mussett et al. (1988) proposed that the BIRPS group (Stein & Blundell 1990; Flack & Warner dyke intrusion throughout the basin occurred over a c. 5 Ma 1990). Stein and co-workers (Stein 1988; Lailey et al. 1989; period, with timea averaged strain rate of10-15 to Stein & Blundell 1990; Blundell et al. 1989) proposed that 10-l6s-'. Bell (1976) andVann (1979) proposedthat the the Mesozoic faultswere reactivated Precambrian struc- main igneouscentres on Skye are localized by the tures.The Mesozoic Sea of Hebrides basin is floored by intersection of the more dilatant parts of the dyke swarm Torridoniansediments, preserved within thehalf-graben. with theCamasunary fault, an important structure that The Minch Fault may have been active from late Palaeozoic operated during the Mesozoic. Gass & Thorpe (1976) noted times although there is little evidence for thick sequences of
W E Leac Camasunary Loch Slapin Loch Camasunary An Leac
",'/
Torridohian
? DurnessLimestone 2 km 0 - WN W ESE lsles Sea .Outer lsles Cuillins of Hebrides
Fig. 2. (a) Simplified cross-sectionthrough southern Skye, showing the geometry of Mesozoic normal faults, the basinfill and the sub-lava unconformity. Tr, Triassic;J, Jurassic. a-a' on Fig. 4. (b) Schematic and simplified crustal cross-section across the Seaof Hebrides basin to Skye, illustrating the site of the Cuillin complex (subsurface structure schematicbut consistent with the gravity data of Bott& Tuson 1973) in the footwall to the Camasunary fault (CF); MF, Minch Fault; M, Mesozoic sediments. b-b' on Fig. 1.
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Devonian or Carboniferous sediments as has beensuggested basinsis the compressionalreactivation of the basin- elsewhere(Stein 1988). The principalfaulting episodeon boundingfaults that initially operatedduring crustal the basin-defining faultsappears to be of Triassic age, extension. This ‘inversion’ model has been applied widely to controlling the wedge-shapedaccumulation of continental earlyTertiary geological histories aroundthe British Isles deposits of this age. Assuming that the subsequent, Jurassic (e.g. Zeigler 1987). However, using the criteria of Williams deposition occurred during a phase of thermal subsidence, et al. (1989), there is no evidence for compressional Morton (1987) calculateda near-invisible stretchingfactor reactivation of the basin faults on Skye, such as anticlines (p) of 1.025, so that the Triassic ‘syn-rift’ subsidencewas developed in the half graben fillsimmediately adjacent to just 500 m.There is someevidence however, that fault the faults or contractionalstratigraphic separations across activity continued during the Jurassic. the faults. Indeed, the continent-wide uplift is more likely to Morton (1987, 1989) recognizes that tectonic subsidence relateto a truly regionalprocess ratherthan local fault controlled thedevelopment of depositionalsequences displacements. duringJurassic sedimentation. Peach et al. (1910) Regional uplift associatedwith volcanism may be recognized that Jurassicunits thicken towardsthe explained by theoretical models formantle plumes (e.g. Camasunaryfault. Neither features are consistentwith Vann 1979; White & McKenzie 1989; Griffiths et al. 1989). simplemodels of thermalsubsidence. Evidence for These predict thatthe elevation of the immediately post-Triassic fault activity comes from the Raasay fault at sub-lithosphericgeotherms willbe recorded by the Balachuirn(Fig. where1) theBearreraig Sandstone generation of up to 2 km of surface topography, although Formationin the hangingwall abuts against Torridonian this assumes thatthe earth’s surfacelay at or above sea sandstone in the footwall. The Bearreraig shows fault- level. This explanation was proposed by Vann (1979) for the vergentslump folds and is draggedinto the fault zone. Hebrideanarea. He notedthat the uplift, as recorded by Fault-adjacentdeformation occurs throughindependent erosion of pre-existing rocks, was centred on theFaeroe particulate flowwhich disruptsand transposes primary region. The plume and the uplift created by elevated upper bedding in a 10m wide zone.These deformation fabrics, mantle temperatures are likely to pre-date immediately the suggestingdominantly down-dip movements, wrap early production and eruption of basaltic magmas generated from diagenetic doggers. Clearly this deformation occurred while partial melting of lower lithosphere and asthenosphere. The substantialpore water volumeswere present in the uplift is transient, only maintained by the elevated mantle Bearreraig sandstone, prior to any substantial compaction. temperatures.When the plumesubsides SO too will the Subsequent Tertiary movements on the fault are indicated lithosphere. by a deformed basaltic dyke found within the fault zone. Upliftmay result from plumes by a mechanism other than merely elevatingupper mantle temperature. The addition to the lithosphere of magma of a lower density than Origin of the sub-lava unconformity the average for the lithosphere will generate surface uplift. Asnoted earlier, the emplacement of the mainigneous This magmacould be eruptedto increasesurface centres on Skye was preceded by the eruption of a suite of topography by building volcanic edifices. But it could also plateaubasalts. These lavas overlieasubaerial, erosive be added as underplate intrusions prior tovolcanic eruptions unconformity which oversteps all units, from Cretaceous at the surface. The geochemistry of Tertiary igneous rocks down tothe pre-Mesozoicbasement (e.g. Bailey1954). of NW Scotland indicate that magmas evolved at or near the Walker (1975) proposedthat this unconformity records base of the crust prior to eruption (Thompson & Morrison denudation and uplift of central Skye caused by the diapiric 1988, Dicken et al. 1984). It is likely that this occurred in ascent of graniticplutons through the crust early in the large magma bodies implying that crustal thickness has been evolution of the main centres. However, Bell (1982) pointed increased during the early Tertiary by magmatic processes. outthat the exposedTertiary granites throughout the The isostatic uplift generated by this underplating will be provincewere generated within theupper few km of the permanent, in contrast tothat generated by thethermal crust (Brown 1963). Thus they did not experience a diapiric anonaly of the plume, provided theunderplated material ascent. Perhaps Walker’s timing of granite emplacement was retains its igneous mineralogy and density. inspired by the discovery of graniticboulders within the The thermal plume and underplate models require uplift basalinterlava sediments (documented by Emeleus 1991). to be the harbinger of volcanismand thatthe two However, these boulders are only found in SW Skye and are phenomenashould beclosely linked intime. Forthe most likely derived from the neighbouring Rum centre (Fig. Hebrideanprovince, the sub-lavaunconformity should l). There is no other indication of acid magmatism on Skye immediately pre-date the overlying basalts. Critical coastal that pre-dates the lavas. outcrops along SW Skyesuggest that this isnot the case. Marine seismic reflection profiles acquired since Stratigraphicrelationships for this area are presented on Walker’s (1975) proposalindicate that the sub-lava chronostratigraphic columns (Fig. 3), using the time-scale of unconformity is widespread across the Sea of Hebrides basin Harland et al. (1990). The unconformity is decorated by thin (e.g. Stein 1988) and is not restricted to Skye. Basal Tertiary sequences of shallow-marine, belemnite-bearing, glauconitic unconformities are recognized throughout NW Europe and sandstones and chalks. These rocks are thought to be Late Greenland(Vann 1979).Available evidence therefore Cretaceous in age (e.g. Bell & Harris 1986), and at An Leac suggests that uplift and denudation were of regional extent. (Fig. 4) theCretaceous units overstep Liassicshales and Regional uplift requiresa regional explanation. so all limestones, Triassicbreccias andconglomerates to rest available data militate against Walker’s (1975) model. The directly onTorridonian basement. To thesouth of the unconformity is nota localized phenomenonrelated to Camasunaryfault, Cretaceous sediments rest unconfor- granite emplacement focused at an igneous centre. mably on Oxfordian shales. Note that the Late Cretaceous An alternative mechanism of uplifting sedimentary was aperiod of high eustaticsea levels (e.g.Haq et al.
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smallmelt fraction.This pre-Tertiary meltingmay have frozen to underplate NW Scotland. However, Morrison et al. (1980)suggest thatthis igneous activity occurred in NW SE Permo-Carboniferous times and has a surface manifestation An hac Camasunary Strarhaird Fad1 in strongly alkalic dykes of this age in NW Scotland. Mesozoic igneous activity appears unlikely for this area. Regardless of the origin of the uplift, we can explain the geological relationshipson Skyeby aperiod of pre-Late Cretaceous denudation across the Camasunary fault blocks. At this time, the high-standing footwall area was eroded to expose Torridonian Sandstones while the hanging-wall area preserves much of the Jurassic basin fill. We can use these stratigraphic relationships to reconstruct fault profiles across Skye.
Tracing the Mesozoic faults on Skye The principalMesozoic faults(Raasay, Camasunary and Minch) trend NE-SW, dipping SE. Thesefaults have half-graben on their hanging walls. However, the linkages between two of the faults (Raasay and Camsunary, Fig. 1) havebeen unclear. The key to understanding the basin structurethrough Skye andthe adjacent islands lies in mapping these faults, yet much of their original geometry hasbeen obscured by the large Tertiary intrusions. However, insights can be gained by examining the residual screens, roof pendants and more dismembered volumes of country rocks betweenthe intrusions. In these preserved remnants, the relationships between the basaltic lavas and the substrata on which they lie unconformably can be used to reconstruct the basin architecture. We have seen earlier in SW Skye (Fig. 3) thatthe unconformity steps across different material from one side of the Camasunary fault to a the other. On the NW (footwall) side the lavas rest directly uponexhumed Proterozoic basement while onthe SE (hanging-wall) side they lie onUpper Jurassic sediments. Fig. 3. Vertical chronostratigraphic sections for selected sites in SW Similarly in central Skye, lavas rest directly on Torridonian Skye. Locations shown on Fig. 4. (Bailey 1954) and Lewisian basement (Bell 1982). To illustratethe structural relationships between the lavas and their substrate adjacent to the main intrusions we presenta series of cross-sections (Fig. 5), linked tothe simplifiedgeological map (Fig. 4). Onthe CreaganDubh 1987). So although wecan explain the transgression that transect lavas rest directly on Lewisian basement yet several allowed marineconditions to be establishedwe cannot hundred metres of Jurassic sediments outcrop immediately invokeeustatica argument to explain the preceding to the northwest. This strongly suggests a pre-lava normal unconformity and significant period of subaerialerosion. fault,downthrowing towards NW. Thisstructure canbe Thus, prior to the deposition of the late Cretaceous rocks, traced to the Strollamus district where on the NW side of there must have been a period of uplift relative to sea level. the fault Jurassic and Cretaceous sediments are preserved, Using(1990) chronology of Harland et al, the uplift while tothe SE lavas rest directly onTorridonian (King pre-dates the lavas by between 45 and 10 Ma, depending on 1954). FurtherNE the fault can betraced tothe isle of the precise age of the sediments. Scalpay (Fig. 4), where Jurassic and Cretaceous sediments So the origin of the sub-lavaunconformity remains together with the lava carapace are preserved in a graben, unclear.Certainly, Walker's(1975) recognition of en- bounded on eitherside by Torridonian. Here it is difficult to hanced, pre-lava denudation in central Skye requires resolve the timing of fault activity, although the SE fault of modification. Buta plume model (e.g. Vann 1979)would thegraben cuts lavas. It is likely that in the Strollamus- requirethe initial thermalpertubation within thesub- Creagan Dubh area there was a NW-dipping normal fault Hebridean mantle to pre-date the emplacement of magma terminatinga wedge of Jurassic sedimentsbeneath the into the upper crust by a considerable period. Alternatively, sub-lava unconformity. there may have been an entirely separate uplift mechanism, It is more difficult to trace the Camasunary fault directly perhapsan earlier plume, that acted inearly Cretaceous through the region. On the eastern flank of Blaven (Fig. 4) times. There is an indication of this possibility in the igneous the footwallblock is entirely consumed by themajor geochemistry of Tertiary lavas in the Hebridean province. gabbroicintrusions. However, some information maybe Morrison et al. (1980) suggested that the Tertiary magmas gleanedfrom examining the contents of the agglomerate werederived from mantle that had previouslyyielded a body at Belig (Fig. 4). Bell &L Harris (1985) note that in the
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Palaeocene lavas
Intrusive centres r\Mesozoic sediment I 1 we-Mesozoic basement
Fig. 4. Geological sketchof the Skye district. Locationshown in Fig. 1. Location of stratigraphic sections of Fig. 3 are indicated (A-H) as are the profiles of Fig. 5 (X,y, z), the base of the Raasay granophyre(s) and thelocation of Fig. 9 (boxed). Thecross-section of Fig. 2a runs from An Leac to Loch Slapin (a-a'). The intrusive centres are ornamented: dashes, granites; crosses,basic rocks. NW SE
1x Scalpay
,:X- l --,.. -
Strollamus Fig. 5. Structuralsections through the margins of the Red Hills granites, showing the pre-intrusive relationships between Tertiarylavas and their sub- strata. Locations shown on Fig. 4. b, basement (mostly Torridonian, except on the Creagan Dubh transect where the basement is Lewisian gneiss); J, Jurassic; K, Cretaceous. Lavas are indicatedby the double-brick ornament and granites 3z Creagan Dubh by random flecks.
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non-plutonic clasts, the NW outcrops of theagglomerate Raasay. Given this map pattern, it appears likely that the contain only lavas and Torridonian sandstone. However,on Camasunary fault links kinematically onto the Raasay fault theSE quarter clastpopulations include metamorphosed via abrupt, NW-SE-trending, transfer zones. It is within this sandstones and carbonates plausibly interpreted as having a region of lateral transfer that NE-SW-trending faults, such Jurassicorigin. Assuming these blocks are emplaced as occur betweenCreagan Dubh andScalpay, have vertically, the agglomerate ‘ghost stratigraphy’ suggests that developed antithetically to the main fault strands, creating Jurassic sediments are only preserved beneath the lavas on small ‘key-stone’ basins. We will see later that the NW-SE the SE side of the Belig subcrop and that the fault trace transfer fault trend is followed by later movements that are continued this far to the NE. contempory orlater than the Tertiary lava pile. The Now we consider the connection for the Raasay fault to reconstructed Mesozoic basin geometry, preserved beneath Skye. The footwall to this fault is characterized by a tilted the Tertiary lavas, is illustrated on Fig. 6. Mesozoic sedimentary sequence overstepped by lava. The lava rests on progressively older rocks from north to south and these relationships can be traced to the south of Loch Emplacement of major intrusions Sligachan (Fig. 4). Here Mesozoic sediments rest on older In a companion study, England (1992, England et d. 1993) basement with lavas overstepping both units progressively has investigated the structure of the regional dyke swarm towards the south. On the northern flanks of Glamaig the and associated minor intrusions together with the lava fields lavas rest directly on the Triassic and Torridonian. So the on Skye in relationship to Tertiary tectonics. Here we are footwall tothe Raasayfault continues at least as far as concerned with the major intrusive centreson the island and Glamaig and the fault trace cannot intersect mainland Skye their relationship to the Mesozoic basin structure outlined directly.Rather it must bedeflected or, more likely, above. transfer along a NW-SE segment through the Narrows of Major basic intrusions Gass & Thorpe (1976) suggestedthat both the Rum and main Cuillin centres were linked to the Camasunary fault. It has long beenrealized that the main centres lie on an upliftedridge of pre-Mesozoicrocks (e.g. Richey 1937). This ridge runs fromSkye via the islands of Soay, Rum, Col1 andTiree, along the NW flank of thefault (Fig. 1). However,todemonstrate any structural link requires consideration of morethan just a map view. Thedeep structure of bothigneous complexes is understoodfrom gravity data to be crudely cylindrical, penetrating to at least 16 kmin the subsurface(e.g. Bott & Tuson 1973). The acquisition of deep seismic reflectiondata renders the picture somewhat clearer by imaging the deep structure of the majorthe basin faults.Regionally, these faults, Camasunaryincluded, dip away fromthe ridges whichin turnrepresent the uplifted footwall blocks. Consequently the Camasunary fault cannot intersect the either the Cuillin or Rumcentres in the subsurfaceand thus cannot have controlled geometrically their intrusion.
Ellis & King’s (1991) model Simulations of continentalextensional faulting by Ellis & King(1991) may providean explanation. By modelling large-scalefaulting within a flexing elasticbeam, a dilatationalsite is predictedto occur at the base of the elasticlayer, in thefootwall tothe fault. Although compressive sites are predicted in their model in the upper part of the plate, Ellis & King suggest that melt-enhanced crack propagation can allow a conduit to link to surface, leading to volcanism. The separationbetween volcanism and the surface trace of the fault in the Ellis & King model is controlled by theelastic strength of the flexing plate, expressed as the effective elastic thickness (Q. The values of T, across continents has been examined by Bechtel et al. (1990). sUsing a value of 15 km, a low value for Bechtel et Fig. 6. The map pattern of Mesozoic basins (stippled) preserved al., Ellis & King predict the site of maximum dilatation to beneath the sub-lava unconformityon Skye and in adjoining areas. occur c. 25 km away from the fault trace at outcrop. CF, Cam’asunary Fault; RF, Raasay Fault; AF,Applecross Fault; The Ellis & King (1991) model may be applied to Skye. KF, Kishorn Fault. Offshore geology after BGS (1986a, b) The centre of the Cuillin complex is located c. 5 km from
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the surface trace of the Camasunary fault. Thus the T. of the Hebrideanlithosphere must be substantially smallerthan the 15 kmused for Ellis & King’scalculations. There is currently much controversyconcerning the T, of actively rifting lithosphere because different methods of estimating it yieldradically differentresults. Hendrie et al. (in press) report various T, values for rift basinsin Kenya, with the lowestvalues being estimating frombasin modelling (T,= 3.5 km) while isostatic modelling of gravity anomalies yield values an order of magnitude higher. At present there are no independent tests available for values of Te in the Hebridean area for the early Tertiary. As well as predicting lithosphere rheology, an applica- tion of the Ellis & King (1991) model also makes geological predictions. If their model is appropriate, the Camasunary fault must have been active during at least the initial stages of intrusion of the Cuillincomplex. Cross-sections across Camasunary Bay (e.g. Fig. 2a) do not suggest large offsets Fig. 7. Photograph of the base of the principal granophyre on after the sub-basalt unconformity which predates the Cuillin Raasay. The underlying rocks are Liassic location[GR: complex. However, the ideal elastic modelonly requires NG552 3511, see Fig. 4. small displacements to create large dilatational stresses. The availablesurface outcrops permit, but do not themselves demonstrate, displacements on the Camasunary fault at its and roofs can be seen in the Western Red Hills complex type locality of less than c. 50m after the intrusion of the (e.g. Thompson 1969), where detailed mapping has shown lava pile. the domes to have a concentric sheeted structure and that To summarize, although there may not be a geometric the sheets were emplacedin a sequence from the outside in. control by the Camasunary fault to create the space required Thegeneral sub-horizontal, laccolithic nature of the for the emplacement of the Cuillin complex, it is possible Skye granites isconfirmed by seismic reflection workby that a minor renewal of extensionaldisplacements on the Goulty et al. (1992) in the Western Red Hills. An Camasunaryfault mayhave perturbedthe distribution of experiment at the head of Loch Ainort (Fig. 3) imaged a stress in the lithosphere. The dilatationpredicted by the sub-horizontalzone of alternating velocities, 250 m thick Ellis & King model may haveacted with the NNE-SSW starting at 2 km depth.They interpret this zone to be extension associated with the regional dyke swarm (Speight sheeted granites and basic intrusives. These data support the et al. 1982; England 1992) to locate preferentially the Cuillin notion that the Red Hills constitute a c. 3 km thick bulbous centre in the footwall to the Camasunary fault. The Rum body of granite. Note that a similar conclusion was reached intrusivecomplex (Fig. 1) mayhave beencontrolled in a for the structure of granitic intrusions associated with the similar manner.But equally, there is no conclusive Mull centre(Bott & Tantrigoda 1987),based on gravity geological evidence to support the model. data. Despite the fact that these granite sheets, emplaced at different levels in thesedimentary pile, inmany cases Geometry of the Red Hills and associated granites forcefully updomed their roofs, we can find no substantial Althoughthe tectonic setting of the gabbroiccentres on evidence of emplacement-related deformation fabrics (either Skyeis ambiguous, there is amore appealing correlation magmatic state/pre-full crystallization fabrics or high between the site of granitic intrusions and basin structure. temperature solid state/crystal plastic strain fabrics: see The granites of the Western and Eastern Red Hills granite Hutton 1988 for criteria and definitions). A similar situation form a NW-SE-trending belt that broadly coincides with the pertains for the Arran granitic diapir (England1988) and we NW-SE-trending transfer zone linking the Camasunary and conclude similarly thatemplacement wasachieved with Raasay-Applecrossfault systems (Fig. 6). It is likely that substantial meltremaining to be crystallized. Weenvisage thegranites represent the siliceous topto a largebasic emplacement of thesesheets wascontrolled in siting magmaticplug, identified on gravity data (Bott & Tuson primarily by a major, or series of major, NW-SE-trending 1973). Geochemically the granites show signs of substantial steep transferfaults, possiblythemselves controlled by crustal contamination through partial melting of Proterozoic majorfaults along thistrend within theProterozoic material (see Dickin er al. 1984) and thus were derived in basement(see Watson 1985).Magma migrated up these part from fractionation of the parental, basic magma and in primary steep fracturesand entered the higherlevel part from partial melting of country-rock (Brown 1963). sediments propagating out along bedding planes and other The granitesform a collection of domes, pillows and weaknesses as vertically inflating laccolithic sills. The lenses. A variety of exposure levels through these intrusions intrusions chiefly occur at veryhigh levels in thecrust, allows a composite picture to be built up. Some bodies lie as primarily created space by doming, tiulting and faulting the bulbous upswellings from a thin magma sheet intruded into Earth’sfree surface. The heterogeneities in upper crustal the Mesozoic basin fill. The base of such a sheet can be seen structure that provide weak zones for magma intrusion are on Raasay(Fig. 4), sub-concordant to bedding in the nowdiscussed forthe case of the Coire Uaigneich underlying Liassic sediments (Fig.7). The dome-like sides granophyre.
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The structure of the Coire Uaigneich granophyre mapped intruding into and chilling against the same basic The Coire Uaigneich granophyre is a ribbon-like body that material.This indicates at least twoages of granophyre lies along the SE margin of the Cuillin gabbro complex (Fig. intrusions developed at different stages in the cooling history 8), above and along strike from outcrops of the Camasunary of the Cuillin gabbro. Clearly the outcrop exposures of the fault (Bell1976). Petrogeneticstudies byDickin & Exley Cuillin (Blaven) gabbros at the Coire Uaigneich site must (1981) suggest the granophyre is in part an acid differentiate predate the granophyre and have cooled by at least c. 50% froma basic parentmagma and in parta partial melt of of theiremplacement temperature (from c. 1100°Cto Torridoniansandstones. Fieldevidence for thelatter substantiallybelow 600°C). There is no such requirement (Brown 1963) can be found around Camasunary Bay where for the granites at the Camasunary Bay site. Nevertheless, (a) the granophyre contains partially ingested Torridonian both sites show that cone sheets, spatially associatedwith sediments and (b) a lattice-work of pre-granite basic dykes, the Cuillin centre, cross-cut thegranophyre. Substantially interpreted as refractory material remaining after melting of more work is required to differentiatethe varying the surrounding Torridonian country-rocks, can be mapped petrogenesis of thesegranophyres using a combination of throughthe granophyre. It is likely that magmahas geochemistry and field mapping. migratedfrom these sites along fracture systemswhich The granophyres atthe two outcrop localities show back-vein laterally,without chilled margins, intothe differentemplacement mechanisms. At Camasunarythe adjacent gabboic bodies for at least 5 km. Fracturing is also country rocks have been deformed by fracturing. However, recognized in the adjacent Torridonian sandstone. structure contours on thebase of the lava pile link across the Field relations at Camasunary Bay are consistentwith fault in a simple fashion indicating that the syn-intrusional the hypothesis of partial melting of Torridonian sandstone overburden was not significantly domed.The Mesozoic due to the emplacement of a basic magma (Cuil'lin gabbro) Camasunary fault is demonstrably not reactivated on a large to generate the Coire Uaigneich granophyre. However, in scale, being stitched by pre-and post-Cuillinbasic dykes. itstype area along strike (Fig. 8), the granophyre can be However,in Coire Uaigneich the continuation of the Camasunary fault, and tear faults, accommodate differential uplift of the base lava pile (Fig. 9) implying that the granite lies in thetrace of the faultand that this structure was reactivated during the intrusion. This conclusion differs from that of Bell(1976) who suggests that pre-existing features are not offset across the granite. Structure contours on the basal lava unconformity,and preserved beddingin the Mesozoic sediments, define folds cored by granophyre (Fig. 9). The unconformity is offset across Coire Uaigneich itself and by faults elsewhere. On close inspection many of these faultscontain thin seams of granite, suggestingagain a syn-magmatic origin. Additionallylarger branches of the granophyreintrude as dykes and inclined sheetsinto the gabbro and lava pile, generating rotations of layering. We conclude that the Coire Uaigneich (sensu stricto) body was intruded into the Camasunary fault at a later stage than the smaller body at Camasunary Bay (Coire Uaigneich sensu luto) and has deformed the surrounding country rocks.
Interactions between granites and faults
SE NW Mapping around Coire Uaigneich suggests that the carapace Bloven of Mesozoic sediments and Tertiary lavas has been domed, uplifted and faulted by the emplacement of the underlying granite. Large panels of country rock are incorporated into the main intrusion suggesting subordinatea sheeting *" S'or Uorgneach mechanism (in the sense of Hutton 1992). This processis not passive, asindicated by therotational deformations experienced by the panels and by the overlying carapace of Mesozoic sedimentsand lavas. We believe that folds revealed by the structure contour patterns on the sub-lava unconformity are related to this doming process. It is now possible to explore the applications of this style 500 m of emplacement to the late RedHills and associated granites of Skye. The clearest indications of doming comes from the v =h Beinnan Dubhaich intrusion. This granite lies within the Fig. 8. (a) View looking WSW into Coire Uaigneich from the head core of a major (Ben Suardal) antiform as defined by its of Loch Slapin (0 m). The large, snow-speckled hill is Blaven mantle of Palaeozoic carbonates (the Durness Limestone). (928 m). The col to itsleft (SE) is the head of Foinne-choire This structure is also shown by the Triassic sediments which (c. 600 m) with the lava cliffs of An Stac (528 m) to the left again. overlie unconformably the Palaeozoicrocks, with critical (b) Sketch profile from An Stac to Blaven. exposureson the eastern shore of LochSlapin. The
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Fig. 9. Simplified map of the Coire Uaigneich area (location shownon Fig. 4), depicting the relationships between the outcrop distribution of granites and structures in the country rocks. The associated foldsand faults are deter- 500 m I mined from structure contours on the sub-lava unconformity. The map is Structure contours on base of lava (m). simplified from 1 :10 OOO field slips.
composite nature of the intrusion is indicated by apparently state (Fig. 10). In essence this method is a modification of later portions of the granite truncating deformation fabrics the stratigraphic separation technique used to analyse fault within the country rocks that may relate to early parts of systems (Elliott & Johnson 1980). So, the ‘hanging wall’ of domedevelopment. The granite itself intersects themap thegranite must restore back onto its ‘footwall’. The trace of a pre-Tertiary basin fault (Fig. 4). This fault traces restoration defines an initial emplacement surface. northeastwards probably to link with the Kishorn fault on Wehave constructed a restorationthrough the Skye the Scottish mainland (Fig. 1). granites along a NW-SE transect, from Glamaig to Suardal (Fig. 10). The originalbasin geometries are preserved, slightly deformed but not dismembered by intrusion, in the Activation of NW-SE-trending faultsduring granite northwest sector. At Glamaig and the Western Red Hills, emplacement the granites have a roof in the basalts. Consequently, the pre-lava substrata remain buried at depth. In the Eastern The Coire Uaigneich map (Fig. 9) shows that major offsets Red Hills the roof and walls of the granites contain in the level of the sub-lavaunconformity occuracross Mesozoic andolder rocks. Atthe northern flank of the NW-SE-trending features. These can be mapped into the Beinn na Caillich granite (8 on Fig. 10) the country rocks Cuillin (Blaven)granite as a suite of granitedykes (Bell are in basalts. However, the Beinn an Dubhaich granite is 1976). On Raasay the principalgranite has an abrupt intruded into Palaeozoic strata. These various relationships northeast margin (Fig. 4). NW-SE-trending faults offset the imply that the granites were emplaced at several different Raasay granite contacts and underlying markers.A similarly levels in the basin structure. The profile shows the initial trending fault terminates the small graben on Scalpay and is emplacementsurface stepping upand across basement marked by minor volumes of granite along its trace. blocks inherited from the rift geometry (Fig.10). The pre-existing faultshave not onlyinfluenced the initial site of granite magma delivery, but also the way in Reconstructing the granite country rocks which the intrusions have been able to deform the overlying By assuming vertical (or hinged), coherent movement of the rocksduring doming and uplift. At Glamaig, the faults country rock duringmagma emplacement, the roof and definepanels of country rock that have experienced walls of intrusionscan be restored back totheir original differential rotations. At Beinn an Dubhaich, the basin fault
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NW SE V U GlamaigOubh Creagan Loch Ainort Suardal
Beinn an Western Red Hills Eastern Red Hills -- Dubhaich - -- early basic complex - - - - Fig. 10. Restored geometryof the country rocks of the Red Hills granites, Skye. The upper figure a is cross-section (U-V on Fig.4). the granitic intrusions (random flecks) are numbered in intrusion sequence (after& Bell Harris 1986). The lower figure shows the restored basin architecture, assuminga sub-horizontal base lava unconformity. The inferred emplacement level for the intrusionsis shown by the pecked line. Doming from this surface creates the accommodation spacefor the emplacement of the granites.
may havepermitted the doming of the granite by earlier zonesthemselves possibly located along steep,pre- having preserved the more deformable Durness Limestone Torridonianbasement faults. The importance of granite in its hanging wall. However,it is the NW-SE-trending dyking was noted by Hutton (1992) and the phenomenon is transfer faults that may have exerted a stronger influence. well-displayed on Skye where wings of the Coire Uaigneich granophyrehave intruded into the Blavengabbros. Continued delivery was accommodated in two ways: doming What controbthe emplacement level? of the rocks above the sill; lateral propagation and lateral Thereare twobroad controls on emplacement level of injection dominated by structural and lithological heteroge- graniticmelts in the sediment pile. The first of these is neities. The presence of pre-existing faults canplay two buoyancy. The magma has the potential to rise to a state of principal roles: by offsetting suitable stratigraphic horizons lithostatic equilibrium whereby the integrated mass of unit and by providing weaknesses to enhance the jacking up of columns of country rock and magma are balanced. roof segments. However,the rheology of the sediment pile is also Aspects of ourinterpretations are reminiscent of a important in permittinglateral propagation of sills and recently proposed model of granite emplacement (Brun et sheets, and accommodating the deformation during doming al. 1990). Pillow-like bulbousintrusions canform where (seeBrun et al. 1990). The lack of deformationfabrics granitic magmas emplace into a multi-layer of well lithified within the granites clearly indicates that intrusion occurred and poorly lithified sediments. Thewet, poorly lithified prior to any significant crystallization of the magma as is sedimentspreferentially localize granitic sills. Delivering typical of high-levelforceful intrusion. At suchshallow more magma to a particular horizon can be accommodated crustal levels, crystallization of granitic magmas is predicted by deforming the free surface, creating pillows.A similar tobe extremelyrapid. Therefore the strain rates in the effect has been created by the Beinn an Dubhaich granite. countryrocks during doming must have been veryhigh. Its country rocks are carbonates of the Durness Formation The principal emplacementaccommodation structures are which behave incompetently at high temperatures, thereby expected tobe high strainrate phenomena, specifically permitting lateral magma injection, followed by doming. faulting aided by high volatile pressures and associated rigid Clearly rheological contrastsin the sedimentary pile body rotations. So confining pressure and the presence of stronglyinfluenced the initial emplacement of granites on pre-existing weaknesses within the sediment-lava pile will Skye.However, the simple lithological pattern in the be importantbe controls on theamount of doming. sedimentshas been disrupted bypre-Tertiary basin Additionally the fluid content and effectiveness of fluid flow structure.Although the principalNE-SW-trending basin to drive advective cooling of the magma are also important. faultstrands were not reactivated atdepth by the NNE-SSW Tertiary extension, the overall sites of granitic intrusion coincidewith the proposedtransfer fault zone. Thesefaults disrupted likely emplacement levels for the Granite emplacement model granites at high levels in the crust. We proposethat the Our model of graniteemplacement is schematically effect of these faults was to inhibit the lateral emplacement illustrated on Fig. 11. Granitic magmais delivered to an of magma. Consequently the spacecreated for granite initial emplacement level, essentially alarge sill, probably emplacement was by doming, tilting and faulting - vertical by vertical ascent in dykes linking fromthe tops of movements that could be accommodated by deformation of differentiated,dominantly basic, magma bodies and melt the Earth’s free surface.
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gronife sill separation between the main centres and the trace of the \ d.. Camasunary fault is only a few km. For the application to be valid, the Hebridean lithosphere must have had a very low effective elastic thickness (T,
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References HALLAM, A.1983. Jurassic, Cretaceous and Tertiary sediments. In: CRAIG, G.Y. (ed.) Geology of Scotland. ScottishAcademic Press, Edinburgh, ANDERSON,E.M. 1936. Dynamics of cone-sheets, ring-dykes andcauldron 343-356. subsidences. Proceedings of the Royal Society of Edinburgh, 56,128-157. HAO. B.U., HARDENBOL,& J. VAIL, P.R.1987. Chronology of fluctuating sea BAILEY,E.B. 1947. Chilledand 'baked' edges are criteria of relative age. levels since the Triassic. Science, 235, 1156-1167. Geological Magazine, 89, 369-375. HARKER,A. 1904. The Tertiary igneous rocks of Skye. Memoirs of the - 1954. Contact of Tertiary lavas with Torridonian near Broadford. Skye. Geological Survey of Great Britain. Geological Magazine, 91, 105-115. HARLAND, W.B., ARMSTRONG, R.L., Cox, A.V., CRAIG, L.E., SMITH, A.G. BECHTEL,T.D.. FORSYTH, D.W., SHARFTON,V.L. & GRIEVE, R.A.F.1990. & SMITH,D.G. 1990. A geological time scale 1989. Cambridge University Variations in effective elasticthickness of theNorth American Press. lithosphere. Nature, 343, 636-638. HENDRIE,D.B., KUSZNIR, N.J.. MORLEY, C.K. & EBINGER,C.J. in press. BELL,B.R. 1982. The evolution of the Eastern Red Hills Tertiary igneous Cenozoic extension in northern Kenya: a quantitative model of rift basin centre, Skye, Scotland. PhD thesis. University of London. development in the Turkana region. Tectonophysics. - & HARRIS. J.W.1986. An excursion guide to the geology of the Isle of HOLLISTER,L.S. & CRAWFORD,M.L. 1986. Melt-enhanceddeformation: a Skye. Geological Society of Glasgow. major tectonic process. Geology. 14, 558-561. BELL,J.D. 1976. TheTertiary intrusive complex onthe Isle of Skye. HUTTON,D.H.W. 1982. A tectonic model for the emplacement of the Main Proceedings of the Geologists' Association. 87, 247-271. Donegalgranite, NW Ireland. Journal of the Geological Society of BINNS, P.E., MCQUILLAN, &R. KENOLTY.N. 1974. The geology of the Sea of London, 139, 615-63 1. Hebrides. Institute of Geological Sciences report. 73/14. - 1988. Graniteemplacement mechanisms andtectonic controls: BLUNDELL,D.J, RESTON, T.J. & STEIN, A.M. 1989. Deepcrustal structural inferences from deformation studies. Transactions of the Royal Society of controls on sedimentary basin evolution. In: PRICE, R.A. (ed)Origin and Edinburgh, Earth Sciences, 79, 245-255. evolution of sedimentary basins and their energy and mineral resources. ~ 1992. Granitesheeted complexes: evidencefor the dyking ascent American Geophysical Union Geophysical Monographs, 48, 57-64. mechanism. Tramactions of the Royal Society of Edinburgh, Earth Bon, M.H.P. & TANTRIGODA, D.A.1987. Interpretation of the gravity and Sciences, 83,377-382. magneticanomalies over the Mull Tertiaryintrusive complex, NW KING,B.C. 1954. The Strollamusarea of Skye. GeologicalMagazine, 91, Scotland. Journal of the Geological Society of London. 144, 17-28. 255-256 -& TUSON,J. 1973. Deep structure beneath the Tertiary volcanic regions LAILEY,M,, STEIN. A.M. & RESTON,T.J. 1989. The Outer Hebrides fault, a of Skye, Mull andArdnamurchan, north-west Scotland. Nature, 242, majorProterozoic structure in NW Britain. Journal of the Geological 114-116. Society of London, 146, 253-259. BROWN, G.M.1963. Melting relations of Tertiary granitic rocks in Skye and MCCAFFREY,J.K.W. 1992. Igneousemplacement in a transpressive shear Rhum. Mineralogical Magazine, 33, 533-562. zone,the Ox Mountains igneous complex. Journal of the Geological BRUN, J.P., GAPAIS, D.,COGNE, J.P., LEDRU,P. & VIGNERESSE, J.L.1990. Society, London, 144. TheRamanville granite (NW France): an unequivocal example of a MCKENZIE, D.& BICKLE, M.J. 1988. The volume and composition of melt syntectonically expanding pluton. Geological Journal. 25, 271-286. generated by extension of thelithosphere. Journal of Petrology, 29, BGS 198Q. Little Minch sheet 57"N-O8"W, Solid Geology. 1 :25OooO series. 625-679. British Geological Survey. MORRISON,M.A., THOMPSON, R.N., GIBSON. I.L. & MARINER,G.F. 1980. -19866. Tiree sheet, 56"N-O8"W, Solid Geology. 1 :25OooO series. British Lateral chemical heterogeneity in the Palaeocene upper mantle beneath Geological Survey. the Scottish Hebrides. Philosophical transactions of the Royal Society of DICKEN,A.P. & EXLEY, R.A. 1981. Isotopicand geochemical evidencefor London. A297, 229-244. magma mixing in the petrogenesis of the Coire Uaigneich Granophyre, MORTON,N. 1987. Jurassic subsidence history in the Hebrides, NW Scotland. Isle of Skye. Contributions to Mineralogy and Petrology, 76, 98-108. Marine and Petroleum Geology, 4, 226242. -. BROWN, J.L., THOMPSON, R.N., HALLIDAY, A.N.& MORRISON, M.A. - 1989. Jurassicsequence stratigraphy in theHebrides basin, NW 1984. Crustalcontamination and the granite problem in the British Scotland. Marine and Petroleum Geology, 6, 243-260. TertiaryVolcanic Province. Philosophical Transactions of the Royal MUSSETT,A.E., DAGLEY, P.& SKELHORN, R.R.1988. Time and duration of Society of London, A310, 755-780. activity in the British TertiaryIgneous Province. In: MORTON,A.C. ELLIOTT,D. & JOHNSON, M.R.W.1980. Structural evolution in the northern & PARSON,L.M. (eds) Early Tertiary Volcanism and Opening of part of the Moine thrust zone, NW Scotland. Transactions of the Royal the NE Atlantic. Geological Society, London, Special Publications, 39, Society of Edindurgh, Earth Sciences. 71, 69-%. 337-348. ELLIS, M. & KING, G. 1991. Structuralcontrol of flank volcanism in PEACH, B.N., HORNE, J., WOODWARD, H.B., CLOUGH, C.T.. HARKER, A. continental rifts. Science, 839-842. WEDD, C.B. 1910. Geology of Glenelg, Lochalsh and south-east Skye. EMELEUS,C.H. 1991. Tertiaryigneous activity. In: CRAIG,G.Y. (ed.) Memoirs of the Geological Survey of Great Britain. Geology of Scotland. 3rd. Edition. The Geological Society of London, RICHEY,J.E. 1932. Tertiary ring structures in Britain. Transactions of the 455-502. Geological Society of Glasgow, 19, 42-140. ENGLAND, R.W.1988. The ascent and emplacement of granitic magma: the SPEIGHT, J.M., SKELHORN, R.R., SLOAN,& T. KNAAP, R.J.1982. The dyke northern Arran granite. PhD thesis, University of Durham. swarms of Scotland. In: SUTHERLAND, D.S. (ed.)Igneous rocks of the -1992. The role of Palaeocene magmatism in the tectonic evolution of the British Isles. Wiley, Chichester, 449-459. Sea of HebridesBasin: Implications for basin evolutionon the NW STEIN,A.M. 1988. Basementcontrols upon basin development in the seaboard. In: PARNELL,J. (ed.) Basins on the AtlanticSeaboard: Caledonian foreland, NW Scotland. Basin Research. 1, 107-119. petroleumgeology, sedimentology and basin evolution. Geological -& BLUNDELL, D.J.1990. Geological inheritance and crustal dynamics Of Society, London, Special Publications, 62, 163-174. the northwest Scottish Continental Shelf. Tectonophysics, 173, 65-67. -, BUTLER,R.W.H. & HUTTON, D.H. 1993. The role of Palaeocene THOMPSON, R.N.1969. Tertiary granites and associated rocks of the Masco magmatism in the Tertiary evolution of basins on the NW seaboard. In: area, Isle of Skye. Quarterly Journal of the Geological Society Of PARKER,J.R. (ed.) Petroleum Geology of Northwest Europe: London, W, 349-385. Proceedings of the 4th Conference. The Geological Society,London, - & GIBSON,S.A. 1991. Subcontinentalmantle plumes, hotspotsand 97-105. preexisting thinspots. Journal of the GeologicalSociety. London, 148, FLACK, C.A.& WARNER, M.R.1990. Three-dimensional mapping of seismic 973-977. reflectionsfrom the crust andupper mantle, northwest of Scotland. -& MORRISON. M.A.1988. Asthenosphere and lower lithospheric mantle Tectonophysics, 173, 469-481. contributions to continentalextensional magmatism: anexample from GAS. I.G. & THORPE,R.S. 1976. Igneous case study: The Tertiary igneous the British Tertiary Province. Chemical Geology, 68, 1-15. rocks of Skye,NW Scotland. In: APRAHAMIAN,F. (ed.) Earth Science VANN,I.R. 1979. Thesiting of Tertiary vulcanicity. In: BOWES.D.R. & topics and methodr.Open University Science course S336. Open LEAKE, B.E. (eds)Crustal evolution in northwestern Britain and adjacent University Press, Milton Keynes. regions. Special publications of the Geological Society of London, 10, GEIKIE, A.1897. The ancient volcanoes of Great Britain. MacMillan, London. 393-414. GOULTY. N.R.. LEGGETT, M., DOUGLAS, T.& EMELEUS. C.H.1992. Seismic WALKER, G.P.L.1975. A new concept of the evolution of the British Tertiary reflection test on thegranites of theSkye Tertiary Centre. Geological intrusivecentres. Journal of the Geological Society of London. 131, Magazine. 129, 633-636. 121-141. GRIFFITHS.R.W., GURNIS, M. & EITELBERG,G. 1989. Holographic WA~ON,J.V. 1985. Northern Scotland asan Atlantic-North Sea Divide. measurements of surfacetopography in laboratorymodels of mantle Journal of the Geological Society of London, 142, 221-243. hotspots. Geophysical Journal, %, 477-495. WHITE,R.S. & MCKENZIE, D.1989. Magmatism at rift zones: The generation
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/6/931/4890044/gsjgs.151.6.0931.pdf by guest on 26 September 2021 944 R.BUTLER W. H. & D. H. W. HUTTON
of volcanic continental margins and flood basalts. Journal of Geophysical of London, 44, 3-15. Research. WB, 7685-7729. ZEIGLER, P.A.1987. Late Cretaceous and Cenozoic intra-plate compressional WILLIAMS,G.D., POWELL,C.M. & COOPER,M.A. 1989.Geometry anddeformations in the Alpine foreland - a geodynamic model. kinematics of inversion tectonics. In: COOPER,M.A. & WILLIAMS,G.D. Tectonophysics, U7,389-420. (eds) Inversion Tectonics. Special publications of the Geological Society
Received 14 June 1993; revised typescript accepted 28 February 1994.
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