Geophysical domains, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
D M Reay
The shaded relief images of the aeromagnetic and gravity anomalies can be divided into major geophysical anomaly domains and interpreted qualitatively. The magnetic anomaly data yield information about relatively magnetic basement and igneous rocks, whereas the gravity anomalies reflect the contrast between less dense sedimentary and silicic rocks and more dense metamorphic rocks and basic igneous rocks.
Anomalies on the aeromagnetic image (P947883) range from -700 to +700nT, with most of the variation caused by the highly magnetic basalt lavas of the Antrim Plateau. The colour scale on the image has been stretched to show the lower amplitude anomalies that characterise less magnetic rocks in Northern Ireland (P948102).
The negative anomaly MD1, associated with Pre-Dalradian Basement rocks of the Lough Derg Inlier, extends north of the Lough Derg Slide thus confirming their extension below this shallow structure (see Central Highlands (Grampian) Terrane - metamorphic basement article). MD2 is a less well-defined belt of moderately positive anomalies extending from Ballybofey (Co. Donegal) into the Sperrin Mountains. They probably reflect a higher proportion of metabasic rocks in the Dalradian compared to MD3 where quartz-rich Dalradian and Carboniferous rocks around and under Lough Foyle give slightly negative anomalies. The strong positive anomalies of MD4 offshore from Malin Head in Co. Donegal may indicate high-density Lewisian basement rocks beneath the Dalradian in this area.
Positive anomalies extend from the Dalradian inlier in northeast Co. Antrim (MD5a) southwestwards under the basalt lavas of the Antrim Plateau to the Sperrin Mountains, the Lack Inlier and Fintona Block (MD5b - c). Strong positive anomalies coincide with mafic rocks in the Tyrone Igneous Complex (see Midland Valley Terrane article) (MD5d). The extension to the northeast of similar anomalies may indicate the presence of these rocks at shallow depths. A lozenge-shaped negative anomaly coincides in part with tonalite intrusions in the Tyrone Igneous Complex and may reflect the extension of these bodies at depth, beneath the Omagh Thrust Fault (MD5e).
In the southwest there is a prominent WNW-ESE belt of three positive anomalies (MD6a-c). Magnetic basement, possibly early Palaeozoic volcanic rocks, below the Carboniferous sedimentary basin is the probable source of these anomalies. MD7 is an area of negative magnetic anomaly over the Devonian and Carboniferous rocks between the Tempo-Sixmilecross and Clogher Valley faults (see Variscan (Hercynian) Orogenic Cycle article).
The southern margin of the Midland Valley Terrane in Scotland is marked by a strong positive anomaly over the Ballantrae (Ophiolite) Complex and late Ordovician magnetite-bearing greywacke. This anomaly can be traced southwestwards from Scotland across the North Channel (MD8a) into Northern Ireland between Larne and the north shore of Belfast Lough. However, its character is masked by the strong magnetic response of the basalt lavas of the Antrim Plateau, although a similar positive anomaly (MD8b) emerges along strike southwest of Lough Neagh. The anomaly amplitude decreases to the southwest until this belt intersects the WNW-ESE Fermanagh Highlands zone to produce the Slieve Rushen magnetic high (MD6c). It can be inferred that ultramafic rocks, similar to those of the Ballantrae Complex, lying at relatively shallow depths, are the source of this magnetic anomaly (MD8a-b). South of magnetic high MD8, the early Palaeozoic sedimentary rocks of the Southern Uplands-Down-Longford Terrane are characterised by negative anomalies (MD9).
In the southeast of Northern Ireland the image is dominated by a high amplitude positive anomaly over the Slieve Gullion Central Complex (see Palaeogene intrusive igneous rocks article) (MD10). Subsidiary positive anomalies also occur over the Carlingford Central Complex and over both the western and eastern centres of the Mourne Mountains Central Complex (MD11a-c). Zone MD12 in Co. Louth (Republic of Ireland) represents the northeastern end of an anomaly that resembles positive anomalies across the Central Belt of the Southern Uplands-Down-Longford Terrane in Scotland. Granodiorite associated with the Newry Igneous Complex show a lower amplitude positive anomaly (MD13a), with a small high amplitude anomaly associated with the Ultramafic-Intermediate Complex at Slieve Croob (see Late Palaeozoic intrusives article) (MD13b).
The magnetic anomaly field of the Antrim Plateau (MD14) is characterised by large amplitude negative anomalies which reflect the dominance of the NRM acquired by the basalt lava during a period of reversal of the Earth’s magnetic field. In many areas the plateau margin is defined by positive anomalies which may, in part, be an edge effect. These are often most pronounced where zones of positive magnetic anomalies disappear beneath the plateau but may be poorly developed when adjacent areas, such as around Lough Foyle, have low or negative anomalies.
Areas of the Antrim Plateau with positive anomalies occur over the concealed Dalradian rocks of the Highland Border Ridge, an arcuate anomaly around the Tardree Rhyolite Complex (see Mantle plumes, ocean spreading and the North Atlantic Igneous Province, Palaeogene extrusive igneous rocks article), and as several small areas covered by the subsurface extension of the Ballantrae magnetic anomaly (between MD6a and MD6b).
At least three main dyke swarms are recognised in Northern Ireland. Late Palaeozoic lamprophyre dykes in the Southern Uplands-Down-Longford Terrane do not have a significant magnetic expression (see Late Palaeozoic intrusives article). Two major Late Cretaceous to Palaeogene dyke swarms occur in Northern Ireland (P947872); the northwest to southeast Donegal-Kingscourt dyke swarm and the NNW-SSE trending dykes crossing Counties Antrim and Down. The larger dykes of the former swarm commonly have characteristic reverse anomalies that can be traced through Counties Fermanagh, Tyrone and Armagh although not all dykes have strong anomalies and their magnetisation patterns can be complex. A broader linear positive anomaly (MD15) running northwest-southeast across Co. Down is associated with the St. John’s Point – Lisburn dyke swarm but may also represent an older major crustal fracture zone (the Antrim – Down Lineament).
Bouguer gravity anomaly values in Northern Ireland range from -10 over Lough Neagh to +57 mGal at Slieve Gullion, against a regional or background level of 20–23 mGal. The main structural elements of Northern Ireland are clearly evident on the gravity images, with blue and purple negative anomalies representing sedimentary basins and granitic intrusions and red positive anomalies representing high-density basement and mafic intrusions (P947884).
A negative anomaly (GD1) over the Lough Derg Inlier reflects the relatively low density of those predominantly quartzose rocks. GD2, just to the north, coincides with the Barnesmore granite which has been modelled as a batholith at least 5km deep . The area from eastern Co. Donegal to the northern part of the Sperrin Mountains (GD3) is characterised by Bouguer anomaly values of +20–23mGal reflecting high density Dalradian rocks. The overlapping gravity high (GD4) and positive magnetic anomaly MD4 in the north of Inishowen, Co. Donegal also indicates higher density basement rocks.
The Rathlin (GD5a) and Foyle (GD5b) sedimentary basins are characterised by strong negative anomalies, separated by a ridge of shallow Dalradian basement rocks extending northeastwards from the Sperrin Mountains. The gravity anomaly minima, adjacent to the Tow Valley and Foyle faults, indicate the areas of thickest preserved sedimentary rocks in these basins and demonstrates their half-graben profile.
The Dalradian inlier in northeast Co. Antrim is characterised by a high positive anomaly (GD6a) although higher values do occur to the southwest beneath the basalt lavas of the Antrim Plateau (GD6b), possibly reflecting metabasic Dalradian rocks at shallow depths. To the southwest, beneath the Antrim Plateau, the presence of small Carboniferous and Permo-Triassic basins on the Highland Border Ridge, with north-south orientated bounding faults, can be inferred from the anomaly pattern (GD6c).
The south Sperrin Mountains show small elongated anomalies >+25mGal (GD7a-c) superimposed on a background field of +20–23mGal which correlate with outcrops of basic rocks of the Tyrone Igneous Complex but also occur beneath the Pomeroy Inlier (see Midland Valley Terrane article) and Devonian rocks to the south (see Devonian article).
At the southwest end of the Fintona Block is the ‘Dromore High’ (GD8). This prominent 35 x 25 km positive anomaly has a maximum of +37mGal in the vicinity of Dromore, Co. Tyrone. The subsidiary high on its northwest flank coincides with Dalradian rocks of the Lack Inlier. The rocks at surface across most of the Dromore High are low-density Devonian and Carboniferous sedimentary rocks that cannot be the source of the anomaly. In order to account for the anomaly two main models have been proposed . The preferred model envisages the anomaly as reflecting an extension of ophiolitic rocks similar to those in the Tyrone Igneous Complex. The other model is based on the occurrence of a Palaeogene gabbroic intrusion similar to that beneath the Slieve Gullion Central Complex. However, in contrast to Slieve Gullion, the magnetic anomaly associated with the Dromore High is low frequency and low amplitude. Seismic reflection data across the northwest flank of the anomaly show high amplitude reflectors between 0.9–1.5secs TWT, equivalent to a depth of 2.3–4 km, using the seismic stacking velocities. The southwesterly-dipping reflectors are almost planar and fit a model of an obducted ophiolite sheet better than a pluton of basic igneous rocks.
The thick succession of Carboniferous sedimentary rocks southwest of Lower Lough Erne is marked by a Bouguer anomaly low (GD9). The major gravity lows GD10 and GD11 reflect low-density sedimentary rocks in the Larne and Lough Neagh basins respectively. The Lough Neagh Basin extends northeastwards offshore (into the North Channel and Firth of Clyde basins) and the gravity anomaly indicates that the thickest onshore sequence occurs near Larne. The lowest Bouguer anomaly values in the Lough Neagh Basin occur beneath the northeast and southwest corners of the present day Lough Neagh. These gravity minima probably reflect the greatest thickness of both the Oligocene Lough Neagh Group and Carboniferous to Early Jurassic sedimentary rocks.
The Southern Uplands-Down-Longford Terrane (GD12a) is characterised by Bouguer anomaly values of +20–22mGal in north Co. Down and +15-18mGal in south Co. Armagh (see Southern Uplands-Down-Longford Terrane article). The Caledonoid Newry Igneous Complex is marked by the low anomaly values (GD13) expected from the granodiorite rocks. Northwest of the Complex, lower than usual values of +12–15mGal over the early Palaeozoic envelope rocks may reflect a subsurface extension of the pluton (GD12c).
The major feature in the southeast of Northern Ireland is the gravity high (GD14) associated with the Slieve Gullion and Mourne Mountains central complexes. The coincidence of the gravity anomaly (maximum of +55mGal) and aeromagnetic anomaly with Slieve Gullion indicates that a massive mafic body is concealed beneath the exposed granitic and gabbroic rocks. Using a density contrast of +0.28Mg/m3 with the early Palaeozoic country rocks the anomaly can be modelled as a basic body c. 10 km thick with its top at a depth of about 2–3 km. Large positive gravity anomalies characterise a number of the basic intrusive centres of the North Atlantic Igneous Province (see Mantle plumes, ocean spreading and the North Atlantic Igneous Province, Palaeogene extrusive igneous rocks article). Bouguer anomaly values rise southeastwards towards the Co. Down coast and an offshore high with values up to +30mGal (GD15). In the southeast corner of the image Bouguer anomaly values decrease towards the Peel sedimentary basin between Co. Down and the Isle of Man.
- Cook, A H, and Murphy, T. 1952. Measurements of gravity in Ireland. Gravity Survey north of the line Sligo-Dundalk. Geophysical Memoirs No. 2, Part 4, Dublin Institute of Advanced Studies.
- Carruthers, R M, Beamish, D, Heaven, R E, Legg, I C, Mitchell, W I, Reay, D M, and Walker, A S D. 1999. Regional interpretation of gravity and aeromagnetic data from Northern Ireland. GSNI Technical Report 99/1.