J. geol. Soc. London, Vol. 142, 1985, pp. 1047-1057, 12 figs. Printed in Northern ___ Some buried Tertiary dykes of Britain and surrounding waters deduced by magnetic modelling and seismic reflection methods

S. R. Kirton & J. A. Donato* British Geological Survey, 19 Grange Terrace, Edinburgh EH9 2LF, UK; *Goal Petroleum plc, New Bond Street House, 1 New Bond Street, London W1Y OSD, UK SUMMARY: Dykes of Tertiary age have been identified by modelling offshore marine and aeromagnetic data and onshore aeromagnetic data. Dykes have been found in the North Sea off NE , and in the Irish Sea. Published aeromagnetic maps have also been used to locate non-outcropping Tertiary dykes in the Scottish Borders. Many of the offshore anomalies are caused by multiple dykes that cannot be separately resolved. All dykes retain the dominant NW-SE trend of the known Tertiary dykes although this trend approaches E-W near the east coast of England.Under the North Seathe trend swings back and becomes increasingly southerly trendlng. The dykes may be the result of NW-SE compressional or NE-SW tensional stress with deep crustal weaknesses causing changes in trend.

Inrecent years the British Geological Survey (former- Lake District and Anglesey have been investigalted ly Institute of Geological Sciences) has run extensive (Fig. 1). These have been modelled and comparec 1 to geophysicalsurveys over the UK mainlandand an anomaly of aknown Tertiary dyke in the Scottish surroundingcontinental shelf. For this papermarine Borders. and aeromagnetic surveys have been used in coniunc- tion with commercial multichannel seismic surveys to Selection of interpreted anomalies define the extent of some DroDosed non-outcrominp II Y Tertiary dykes in Britain and surrounding waters. In Data coverage is excellent for the entire area shown particular,some examples with pronounced linear in Fig. 1. Marine traverses generally form a grid with a magnetic anomalies off the coasts of NE England, the line spacing rarely greater than 6 km in any direction.

FIG. 1. Location of Tertiary dykes. Includes dykes marked on published geological maps and dykes inferred from magnetic anomalies. The distribution is generalized in west of . The dashed line indicates central line of the zone of linear anticlinal and synclinal features of mid-Tertiary to Zechstein age referred to in the text. The dot-dash line indicates area of Fig. 2.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1047/4888956/gsjgs.142.6.1047.pdf by guest on 30 September 2021 1048 S. R. Kirton 6; J. A. Donato The aeromagnetic data, in addition to being acquired Borders, and by inferencesimilar magnetic anomaly over the mainland, werealso collected overthe inshore profiles may be used to locate non-outcropping dykes, areasshown in Fig. 1. The E-W aeromagnetic lines as they will have the same shape. (However, increas- rarely have a spacing of more than 3 km with N-S tie ing depth of burial will increasethe wavelength but lines at various intervals, depending on the particular decrease the amplitude.) This area is thus essential in survey. The aeromagneticdata were collectedat testingobserved and modelled anomalies to known approximately 300 m above topography. dykes. In the Scottish Borders the dykes are part of Most of the above data have been contoured and the southeasterly extension of the swarm centred on publishedat 1: 625,000 scale (Stubblefield 1965; Mull (Richey 1961) and are recognized by their strong Dunham 1972) or asa series of mapsat 1:250,000 WNW-ESE trend (Fig. 1) which iswell defined by scale (e.g.Brown 1980, 1981). The anomaliesmod- aeromagneticanomalies (Brown 1980) (Fig. 2).Such elled here were all selected because of their marked anomalies(e.g. near Moffat) are characterized by a linear appearance on such maps. significant magnetic minimum to the south and a small maximum to the north,typical of intrusions of Tertiary Magnetic anomalies studied age (Robson 1964) (Fig. 3). Scottish Borders Offshore NE England Linearmagnetic anomalies are directly correlated This area is the most extensively studied, as there is with outcroppingTertiary dykes inthe Scottish excellentmarine andaeromagnetic coverage. The

FIG.2. Part of ‘Borders’ aeromagnetic anomaly map (Brown 1980) (N-S line spacing is approximately 10 km; E-W line spacing approximately 2 km) showing dykes as marked on geological maps and inferred from magnetic data. Location given in Fig. 1. Modelled anomaly is aeromagnetic line T-1541255 (Fig. 3). The contour interval is 10nT.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1047/4888956/gsjgs.142.6.1047.pdf by guest on 30 September 2021 Tertiary dykes Tertiary of Britain 1049 SO UTH NORTH SOUTH NORTH 10 km.This grid, and the contoured aeromagnetic W survey (Fig. 4), show two distinct, but discontinuous, V linear anomalies extending up to 150 km offshore. The most northerly is approximately along strike from the Acklington Dyke, and hence is termed the Acklington Dyke Group, while the most southerly is along strike nT -10 T-154/255 froma number of dykes atBlyth. The Acklington -20 - Observed Dyke Group anomaly is the greater of the two, with a maximummeasured amplitude of 900 nT (at sea -3 0 Calculated level), whereas the greatest recorded amplitude at sea -40 4 l I I I I level for the Blyth Dyke Group anomaly is only just 0.0 0.5 1.0 1.5 2.0 over 200nT. The greatest amplitude for both anoma- Horizontal scale (km) lies occurs close to the coast and decreases eastwards as the dykes presumably become more deeply buried FIG. 3. Observed (at 300 m above topography) and (Figs 5a and b). Nevertheless, an anomalywith a calculated aeromagneticanomaly over known relatively short wavelength and a high amplitude can Tertiarydyke near Moffat. Arrow indicates posi- betraced for up to 150 km offshore,where the tion of dyke as marked on geological map. Position causative dykes are estimated, by multichannel seismic of line shown on Fig. 2. See Fig. 10 formodel data, to be covered by 30CL500 m of sediment (Fig. 4). magnetization vector. Some of the individualmarine traverses show a minor peak near the baseof the minimum. Thiscan be aeromagnetic lines have an E-W line spacing of modelled as the result of two or more closely spaced approximately 3 km with N-S tie lines approximately dykes and is detailed in the section on the modelling of 37 km apart.The marine surveyswhich have been the observed anomalies (Figs 5a and b). used in the modelling form a N-S/E-W grid with a line To theeast of, and along strike from, the Blyth spacing of approximately 6 km but never greater than DykeGroup is long-wavelength,a low-amplitude

55ON-

SCARBOROUGH b

FIG.4. Aeromagnetic anomaly map (unpublished Aeroservices Corporation data) showing Acklington and Blyth Dyke Groups. The E-W flight line spacing is approximately 3 km; N-S tie line spacing approximately 37 km. Dashed line indicatesthe central line of thezone of linear anticlinal and synclinal features of mid-Tertiaryto Zechstein age referred to in the text. The letters A, B,C, D, E, F and G refer to marine magnetic lines 76/04-23153, 76/04-5/200,77/02-51/29,76/04-23/62,76/04-5/211,76/04-33/16,81/03-20/03, respectively (Figs 5a and b). The letters H and J refer to seismic line CUK 12 (Fig. 11). The contour interval is 25 nT.

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SOUTH NORTH SOUTH NORTH Position of dyke (depth in km) D A 200-

150-

100 - 76104-23162 nT 0- 50 -

nT 0- 76104-23153 E -50 -

-100-

-150 - 76104-51211

-200 7 B 4001 0.2 1

200{

O\ O\ FF-76/04-33/16-~ 76/04-5/200

-400

-600 l I I I 1 0.0 0.5 1.0 1.5 2.0 nT-2001 v Horizontal scale (km) anomalies D,E,F

77/02-51/29 :::l I -5 0 0.4 l - Observed -100 - Calculated G

0-0 0.5 1.0 1.5 2.0 8 l /03-20/03 Horizontalscale (km)

- Observed - Calculated

FIG.Sa. Serial observed and calculated marine 1 I I I I 0.0 1.0 2.0 3.0 4.0 magneticanomalies over the inferred Acklington Dyke Group. The most westerly anomaly is at top Horizontal scale (km) anomaly G of the figure. Line of traversesshown on Fig. 4. Magnetization vectors shown in Fig. 10. Note how FIG. Sb.Serial observed and calculatedmarine 76/04-23/53 can be calculated as two closely spaced magneticanomalies over inferred Blyth Dyke dykes and 76/04-Y200 has been calculated as one Group.The most westerly anomaly is attop of dyke but may be the result of two or more very diagram. Line of traverses shown on Fig. 4. closely spaced dykes. Magnetization vectors shown in Fig. 10.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1047/4888956/gsjgs.142.6.1047.pdf by guest on 30 September 2021 Tertiary dykes of Britain 1051 anomaly extending for a further 170 km (Fig. 4). This west of Fleetwood (e.g. Woodland 1977a, 1978) (Fig. anomaly may be caused by numerous dykes or a large 6). N-S aeromagnetic lines with a 2 km spacing show dyke at depth, but modelling has not proved satisfac- the anomaly to have a maximum measured amplitude tory in determining the depth or number of dykes. of almost 300 nT 300 mabove sea level. West of Multichannelseismic datado not suggest the approximately 3’32’W (Figs 6 and 7) the anomaly is a presence of dykesbut reveal thatthe magnetic broad feature up to 7 km in width, whereas east of anomaly is associated with a series of linear anticlinal 3”32’W it is much narrower (approximately 3 km). features of mid-Tertiary to Zechstein age. These folds are possibly caused by movements of underlying salt, themovement of which may becaused by deeper, Offshore Anglesey unresolvedstructural features,e.g. faults. Thus, the West and north-west of Anglesey are a number of magneticanomaly may be caused by the folded linear anomalies with the same NW-SE trend as the sediments(unlikely), the deep structural feature, Tertiarydykes at outcrop on Anglesey (Woodland Tertiary dykes, or a combination of all three. 19778). Theabundance of anomalieson individual traversesmakes correlation with other traverses Offshore LakeDistrict (Fleetwood Dyke uncertain.Thus, a number of dykepatterns can be Group) inferred but the interpretation by Wright et al. (1971), This consists of a pronounced WNW-ESE trending where sparker records have been used to follow dykes anomaly 50 km long, and commencing 25 km north- at subcrop, will he followed (Figs 8 and 9).

FIG.6. Parts of ‘Lake District’ and ‘ Bay’ aeromagnetic anomaly maps (Woodland 1977a, 1978) (N-S line spacing is approximately 10 km, but 2 km over the Fleetwood Dyke anomaly; E-W line spacing approximately 2 km) showing Fleetwood Dyke Group inferred from the magnetic anomaly map. Modelled anomalies A and B refer to TL-6/217 and TL-7/152, respectively, and seismic lines A and B refer to MB-148 and MB-48, respectively (Fig. 11). The contour interval is 10 nT.

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SOUTH NORTH vector, until agreement between observed and calcu- lated profiles was obtained. In all cases it was assumed A Position of dyke 0.34 that the total magnetizationvector lay within avertical planealong the N-S magnetic axis. Formodels in which thedirection of theobserved profile was not approximatelyat right angles to the strike of the postulated dyke, calculated values were resolved along thedirections of theobservations. It was found necessary to remove small, linear, regional gradients I I 1 50 from the observed profiles in an attempt to eliminate 0.0 1.0 2.0 3.0 4.0 any long wavelength variations present in the vicinity Horizontolscale (km) of the modelled dyke. Generally, single, well-defined anomalieswere modelled; however, this wasnot always possible, and it was sometimes found necessary B toinclude two ormore closelyspaced dykes in the model in anattempt to match themore complex observed anomalies. The anomalyover aknown Tertiary dyke near Moffat wasmodelled (Figs 2 and 3) initially. Our model shows a dyke as having a depth to the top of 300m below the flight level (i.e.approximately at outcrop) and a width of 36 m. Since the susceptibility andremanent magnetization of thedyke are not - Observed reliablyknown, this model,and all others discussed - Calculated here, mustbe regarded as representing only very generalizedsolutions. Equally good fits canbe r l I I I 0.0 2.0 4.0 6.0 8.0 achieved by using slightly modifiedcombinations of Horizontol scale (km) parameters.Also, the width of adyke cannot be reliably resolved if the depthiwidth ratio is significantly FIG. 7. Observed (at 300m abovesea level) and greater than 1. calculated aeromagneticanomalies over inferred However,it is notpossible to achieveacceptable Fleetwood dyke group. Line of traverses shown on agreement using a total magnetization vector direction Fig. 6. Magnetization vectors shown in Fig. 10. that is significantly differentfrom that used (i.e. southwarddeclination and inclined slightly upward; Fig. 10). Such an orientation produces the form of the Magnetic modelling of observed anomalies with a main negative trough and the observed anomalies an associated smaller peak to the north. This is true for all the anomalies modelled here. To testthe suggestion thatthe magnetic anomalies Magnetic anomalies over the Acklington, Blyth and observedoffshore may beassociated with buried Fleetwood Dyke Groups and dykes to the north and Tertiarydykes, aseries of magneticmodels were northwest of Anglesey are shown in Figs 5a,5b, 7 produced.Inaddition, modela over a known and 9, respectively. It was found necessary to model outcroppingTertiary dyke was calculated to allowa someanomalies (e.g. profiles A and G over the comparison to be made. Acklington and Blyth Dyke Groups, respectively (Figs Owing to the linear nature of the anomalies, it is 5a and 6)) as double dykes. In other examples (profile adequateto apply a simpletwo-dimensional model. B over the Acklington Dyke; Fig. sa) it was necessary The model used considers the dyke as vertically sided to model thedyke as one wide dyke, although the and calculates the magnetic anomaly along a profile at anomaly may be the result of two or more narrower right angles to the postulated dyke. In all models the dykes magnetically interfering. In addition, the west- observed profiles were taken directly from the marine ern part (west of 3'32'W) of the Fleetwood anomaly oraeromagnetic analogue records, which were not may be the result of an inclined dyke because of its always normal to the trend of the proposed dyke. In broadmagnetic anomaly. In fact, a previous inter- suchcases this results in theanomaly havingan pretation of theFleetwood anomaly showed, using apparently longer wavelength than in reality, but this marine magnetic data, thatit could either be causedby was taken into account in the modelling. a sill or a dyke (Young 1965). In the latter case, the Profileswere calculated using different sets of twomodels Young produced gave depths belowsea parameters of dyke dimensions, depth of burial, and level of 65 and 101 m with widths of 150 and 282 m, the strength and inclination of the total magnetization respectively. Our results of 50 and 250 m below sea

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FIG.8. Part of 'Anglesey' aeromagnetic anomaly map (Woodland 1977b) (N-S line spacing is approximately 2 km; E-W line spacing is approximately 10 km) showing dykes deduced by Wright er al. 1971 from marine magnetic and sparkerrecords. Modelled anomalies A and B referto lines 70101-22/37and 70101-21125, respectively (Fig. 9). Contour interval is 10 nT.

level with widths of 100 and 600m arethus in Thereare two mainphases of post-Precambrian moderateagreement, bearing in mind the extremely basic dyke intrusion in the British Isles, these being of simplified nature of the models involved. Carboniferousand Tertiary ages. Powell (1963) has Nevertheless,the magnetization vectors of the examinedmagnetic profilesacross Permo- Fleetwood and other anomalies modelledby us are all Carboniferousdykes and describes the anomaly as inclined upwards and point southwards in agreement characteristically consisting ofa positive peak with a with themodelled profileover the knownTertiary small negativetrough, usually tothe south. This dyke near Moffat (Fig. 10). contrasts markedly with the anomalies described here Wilson et al. (1974) and Dagley (1969) have from the known Tertiary dyke and from the offshore measuredthe remanent magnetization of various anomalies. Tertiarydykes within Britain.These measurements In summary, despite thevery simplified and general- generally show a remanentvector pointing approx- ized nature of the magnetic models, they do indicate imately southward and inclined typically between 40" that it is possible to account for the observed offshore and 70" upward. Such directions are clearly compatible anomaliesin terms of dyke-like bodies. Inaddition, with our modelled values, allowing for the addition of thedirections of thetotal magnetization vectors thecontribution from the magnetization induced by required by the models are consistent with the the earth's present-day field. postulated dykes having a Tertiary age.

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0.24 I

70/01-22/37

I W -1504

s55ow N55'E

01 09 B.

O2a nT -20 70/01-21/25 SOUTH ,o NORTH - Observed -40 v -0 I - Calculated - l i 0.0 0.5 1.0 1.5 2.0 MAGNETIZATION Horizontal scale (km) ~XIO-* amp/m l - FIG. 9. Observed and calculated marine magnetic I anomalies overinferred dykesnorthwest of DOWN Anglesey. Line of traversesshown on Fig. 8. FIG. 10. Plot of strengthand direction of total Magnetization vectors shown in Fig. 10. magnetizationvectors as calculated by the mod- elled profiles. Numbers refer to models as follows: Independent evidence for the 1, Moffat, T-1541255; 2a andb, Acklington, age of the non-outcropping 76104-23153; 3, Acklington, 76104-51200; 4, Ackling- ton, 77102-51129; 5, Blyth, 76104-23162; 6, Blyth, dykes 76104-51211; 7, Blyth, 76104-33116; 8a and b, Blyth, 81103-20103; 9,Fleetwood, TL-61217; 10,Fleet- Independent evidence is also available to establish the wood, TL-71152; 11,Anglesey, 70101-21125; 12, age of the dykes. First, the strikeof the Carboniferous Anglesey, 70101-22137. dykes is broadly E-W, whereasthe strike of the Tertiary swarm is broadly NW-SE. Thus, the parallel- Mull. The former dyke group can be correlated with ism and extrapolation of the observed anomalies to the reasonable certainty to the onshore Acklington Dyke. known Tertiarydykes stronglyimplies thatthe Conversely, the latter dyke group may be correlated anomalies are caused by dykes of this age. withseveral onshore dykes (Fig. 1). The useof Secondly, and more conclusively, multichannel seis- realistic values for the totalmagnetization intensity for micsections show that the Acklington,Blyth and both the Acklington and Blyth Dyke Groups results in Fleetwood Dyke Groups cut post-Carboniferous sedi- individual dykes having to be modelled at up to 75 m ments; the dykes must therefore postdate the Carbo- wide. This is significantly greater than the outcropping niferous(Fig. 11). Unfortunately, multichannel seis- coastal dykes that have average widths of 6m and a mic sections are not available northwest of Anglesey. known maximum of 21 m (Land 1974). However, the The Fleetwood Dyke is especially clear as a vertical thin coastal dykes do not cause magnetic anomalies on featureon multichannel seismicsections east of 1:250,000 aeromagnetic maps (e.g. Brown 1981) and 3"32'W and is at outcrop on the seabed(Fig. 11). West it is thought that readily observable offshore magnetic of 3"32'W it is not visible on the seismic sections even anomalies reflect an increase in the width or number though the magnetic anomaly is broader. This may be of individual dykes offshore. in keeping with oursuggestion that the western partof The numerous dykes off the northwest of Anglesey theFleetwood anomaly is relatedto a more deeply areintermediate between the swarm at Carlingford buried and possibly inclined dyke. and the outcropping dykes of Anglesey (Fig. 12). The relationship of theFleetwood Dyke Group is less Significance of the offshore certain; it maybe related tothe swarmscentred in dykes Northern Ireland, Mull or Arran or to a centre in the North Channel postulated to explain the presence of The Acklingtonand Blyth Dyke Groups form a the N-S trending North Channel dykes (Figs 1 and 12) southeasterly extension to the dyke swarm centred on (Caston 1975).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1047/4888956/gsjgs.142.6.1047.pdf by guest on 30 September 2021 Tertiary dykes of Britain 1055 theybecome increasingly easterly, cutting across S Scotland and N England, where they have a WNW- ESE trend. Further east,this study has shown that the I ...... - dykes quite unexpectedly become increasingly south- erly trending. The total length of the Scottish-centred dykes may be up to 800 km (Fig. 12). The trend of the dyke swarm is continued by a zone of devolatalized Westphalian coals (outgassing caused by Tertiarydyke intrusion) and may be taken as indicating that the dykes extend across the southern North Sea to bealong strike with the Ruhr Graben ' .. (Fig. 12) (Dewey 1982; Ziegler 1981).If this is correct, the Scottish-centreddyke swarm may besome 1500 km in length. The second group of dykes is to the south and west of the first group and comprises the dykes of Ireland, I... . . Anglesey and isolated dykes in England. These dykes I .. all have a NW-SE trend. . -. M.

Tectonic significance of the dykes

EarlyTertiary reconstructions show the northern Atlantic as a narrow strip of oceanic crust bounded by Greenland to the northwest and the British Isles and Norway to the southeast and east, respectively (Fig. 12; Bell1976). Both continental margins had been intruded by complex dyke swarms related to Atlantic opening, although each is a result of different stress patterns.The first swarm is theGreenland coast parallel swarm dated at 5650 Ma and is interpreted as a rift parallel continental spreading centre at the time of anomaly 24 (Larsen1978). The secondstress pattern resulted in the emplacement of dykes associ- ated with the Scottish and Irish igneous centres, i.e. dykes intruded obliquely tothe Atlantic margin. FLEETWOOD These have been dated at 55.8 f 0.3 Ma on samples FIG. 11. Seismic sections across Acklington, Blyth from the Cleveland Dyke Echelon (Fig. 1; Fitch et al. and FleetwoodDykes. Location of seismic lines 1978). given in Figs 4 and 7. Vertical scale is two-way Thelatter swarmclearly cannot be the result of time in seconds. TZ represents top Zechstein, BZ simpleAtlantic rift paralleldilation but is possibly represents base Zechstein, TC represents top either (1) relatedto very complex stress patterns Carboniferous. Note how dykes disturb sediments related to Atlantic opening such as skewing caused by younger than Carboniferous and must therefore differential movement of plates(Bell 1976) or (2) postdate the Carboniferous. caused byNW-SE oriented compressive stress (or NE-SW tensionalstress). Thelatter idea has been Distribution of Tertiary dyke developed by Vann (1978) who modelled the dykes by swarms of Britain consideringhomogeneous, isotropic plates subjected toa NW-SE oriented compressivestress and equal The Tertiary dykes of Britain may be crudely divided magma pressure at each of the centres. Such a stress is into two groups (Fig. 12). The first of these is the most surprising in view of the NW-SE tensionthat must intensely studied,and comprises the swarmsassoci- have been related to Atlantic opening. Nevertheless, ated with the western Scottish igneous centres. These this model yields a very good fit to the observed dykes dykes are recognizedfrom up to 100 km north of aroundthe centres, but discrepanciesoccur as the Scotland, where they are N-S trending, to as far south dykesbecome increasingly distant. These discrepan- as the igneous centres into which they converge (Fig. cies are best accounted for by assuming that the plates 12) (Dunham 1972, Vann 1978). South of the igneous are heterogeneous and that stress and dyke patterns centres they diverge again, but within afew kilometres have been influenced by megascopic crustal features.

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3

FIG. 12. Tertiaryreconstruction of North Atlanticareas showing generalizedTertiary dyke pattern. Based on diagram in Bell (1976).

The areas of crustal weakness primarily utilized by Graben (Fig. 12). In S Scotland and N England, dykes the dykes are the igneous centres but, given a NW-SE initiatedfractures uniting the pre-existing zones of oriented compressivestress as postulated by Vann weakness,i.e. those of theNorth Seaand of the (1978), it is unlikely thatdykes could be intruded igneouscentres of Scotland,and did not utilize along thedominant NE-SW orientedCaledonian existing crustal weaknesses. structures. It is far more likely that the dykes utilized pre-existing ordeveloping major crustal weaknesses Conclusions approximatelyparallel tothe compressivestress. Structuralfeatures of this trendare present in the Modelling of magneticanomalies suggests the exist- North Sea as indicated by the anticlinal and synclinal ence of Tertiary dykes in the western partof the North axes shown on Figs 1 and 4; these features may have Sea and the Irish-Sea. Most of the latter are probably provided suitable zones for dyke intrusionin the North related to the centres of Ireland, whilethose in the Sea. If Tertiary dykes are present at depth as indicated North Sea are part of the swarm centred on Mull. The by the zone of devolatalized coals, the dykes may have total length of the swarms radiating from the Scottish beeninfluenced by featuresas remote as theRuhr centres can be demonstrated to be at least 800 km and

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a zone of devolatilized coals in the southern North crustal weaknesses have determined local variations in Sea may be associated with Tertiary dykes extending, the strike of dykes. at depth, to the European mainland. Althoughall the dykes retain a generalNW-SE trend, the Scottish-centred dykes describe a sinusoidal ACKNOWLEDGEMENTS.We wish tothank Western Geo- physical for permitting us to use seismic sections shown in curve.The most northerly are N-S trendingbut Fig. 11,and D. I. Jackson andA. E. Andersonfor their becomeincreasingly easterly trending to the south. interpretation of thesesections. Fig. 4 is based on an Near theNorth Sea coast they once more start to aeromagneticsurvey flownby Aeroservices Corporation in return to a more southerly trend. 1963. This paper is published with the approval of the The swarms are thought to be the result of NW-SE Director, British Geological Survey (NaturalEnvironment compressive (NE-SW tensional) stress, but underlying Research Council), and the Department of Energy. References BELL,J. D. 1976. The Tertiary intrusive complex on the Isle magnetometer survey. Proc. geol. Soc. 34, of Skye. Proc. Geol. Assoc. 81, 247-71. 293-308. BROWN,G. M. 1980. Bordersaeromagnetic anomaly map, STUBBLEFIELD,J. 1965. Aeromagneticmap of Great . 1: 250,000 series. Institute of Geological Sciences, Sheet 2,1:625,000; 1st edn,The Geological Survey. Natural Environment Research Council. Southampton, Ordnance Survey.

~ 1981. Farne magneticanomaly map, 1:250,000series. VANN,I. R. 1978. The siting of Tertiary vulcanicity. In: Institute of Geological Sciences, NaturalEnvironment BOWES, D.R. & LEAKE,B. E. (Editors) Crustal Research Council. Evolution in North-Western Britain and adjacent regions. CASTON,G. F, 1975. Igneous dykes and associated scour Spec. Issue geol J. 10. hollows of the North Channel, Irish Sea. Mar. Geol. 18, WILSON, R.L., ADE-HALL, J. M,,SKELHORN, R. R., M77-85. SPEIGHT,J. M. &L DAGLEY, P.1974. The British Tertiary DAGLEY,P. 1969. Palaeomagnetic results from some British igneous province:Palaeomagnetism of theVaternish Tertiary dykes. Earth planet. Sci. Lett. 6, 349-54. dyke swarm onNorth Skye,Scotland. Geophys.J.R. DEWEY, J. F. 1982. Platetectonics and evolution of the astron. Soc. 37, 23-30. British Isles J. geol. Soc. London, l39, 371-412. WOODLAND,A. W. 1977a. Lake District aeromagnetic DUNHAM,K. C. 1972. Aeromagnetic map of , anomaly map, 1: 250,000 series. Institute of Geological Sheet1, 1:625,000, 1st edn, Institute of Geological Sciences, Natural Environment Research Council.

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Received 17 September 1984; revised typescript accepted 12 June 1985.

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