J. geol. Soc. London, Vol. 142, 1985, pp. 1047-1057, 12 figs. Printed in Northern Ireland ___ 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 England, 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 Scotland. 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 inference similar 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 increase the 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- aeromagnetic anomalies (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 Tertiary dyke 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 indicates the central line of the zone of linear anticlinal and synclinal features of mid-Tertiary to 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/6/1047/4888956/gsjgs.142.6.1047.pdf by guest on 30 September 2021 1050 S. R. Kirton & J. A. Donato 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 traverses shown 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.