Other major Caledonoid lineaments and faults, Northern Ireland

Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

D M Reay

Other major Caledonoid lineaments and faults

Horizontal gradient of Bouguer gravity anomaly after upward continuation by 500 m. Shaded-relief illumination from the north. (P947886)

Geological map of the Newry Igneous Complex. (P947810)

Many aeromagnetic and gravity lineaments of Caledonoid trend can be correlated with known faults and may be used to delineate these faults where they are not exposed. The interpretation of aeromagnetic and gravity horizontal gradient images is particularly helpful in tracing the Tow Valley Fault on the Antrim Plateau where it juxtaposes identical basalt lavas of the Upper Basalt and Lower Basalt formations.

In addition to mapped faults and their extensions other faults and structures may be inferred from lines with high gravity gradients. For example, there is a suite of small faults beneath Lough Neagh and a number of lineaments in the Southern Uplands-Down-Longford Terrane which may be correlated with tract boundaries.

NNW-SSE to NNE-SSW trending faults and lineaments

In addition to the dominant Caledonoid trending features there are a significant number of NNW, north-south and NNE-SSW trending geophysical lineaments, some of which may be correlated with mapped faults and their concealed extensions. The alignment of four Irish base metal deposits along a north-south trend led to the hypothesis that major north-south geofractures may have controlled the siting of some mineral deposits . Four such geofractures, linking mineral deposits and abutting pre-Carboniferous inliers, were identified. This theory has been modified and extended into the north of Ireland where the alignment of geological features and geophysical lineaments has been used to define major deep crustal fractures . The geophysical characteristics of these and other lineaments are discussed below.

Omagh Lineament

This lineament, which trends NNE from Tempo in Co. Fermanagh to Drumahoe, south of Lough Foyle, is a rather weak feature on both gravity and magnetic anomaly images but is recognised by its dislocation of other Caledonoid anomalies. It is coincident with two mapped faults and is associated with an increase in the number of mantle-derived tholeiitic sills and mineralised veins and causes an abrupt change in the orientation of bedding and the dominant foliation in Dalradian rocks at the eastern end of the Lack Inlier . It is interpreted as one of a series of ancient basement structures, maybe as old as 1800Ma , which have been reactivated on several occasions and may be associated with mineralisation. Recent high resolution data have delineated a persistent, narrow, low amplitude aeromagnetic anomaly coincident with the northern part of the originally defined lineament.

Draperstown Lineament

The Draperstown Lineament trends from Portrush on the north coast of Co. Antrim south past Draperstown towards the mineral deposits in south Co. Armagh and Co. Monaghan . It was recognised from an alignment of faults and intrusions, and the orientation of minor structures. In this case a zone of sub-parallel geological and geophysical features (the Draperstown Lineament corridor) is interpreted as lying above a major deep crustal discontinuity and are genetically related to episodic reactivation of this structure. It also forms the northern section of a proposed geofracture that passes south through the Kingscourt Fault and abuts the Kildare Inlier in the Republic of Ireland. Although originally identified as a linear gravity feature it is poorly defined on the gravity horizontal gradient image where it forms a zone of discontinuous linear gradient segments. Unlike the Omagh Lineament it does not have an obvious magnetic signature, although this may be because it extends mostly through cover, rather than basement, rocks.

Antrim-Down Lineament

A major linear anomaly extends northwest from St John’s Point on the southeast coast of Co. Down to Lisburn in Co. Antrim and can be traced continuously to the Sixmilewater Fault. It is unusual in that it is a positive anomaly and has been attributed to the presence of the St John’s Point-Lisburn Palaeogene dyke swarm that is most evident in coastal outcrop. An alternative interpretation is that the magnetic anomalies caused by the dykes are superimposed on a longer wavelength positive anomaly representing an older, major fault or fracture zone . Although the anomaly is discontinuous north of the Sixmilewater Fault the lineament may continue along a NNW or northerly trend and terminate against the Tow Valley Fault near Ballymoney.

Garron Point - Conlig Lineament

The most easterly of the defined geofractures was taken to run N6oW through the Conlig - Whitespots Mines (see Mineral resources article) and follow a 40 km long Palaeogene dyke just off the east coast of Co. Antrim. On the aeromagnetic and gravity horizontal derivative images, however, the most prominent lineament trends NNW, is marked by dykes on the Ards Peninsula, follows the east coast of Co. Antrim past Garron Point and offsets the gravity gradient associated with the southeast flank of the ‘Highland Border Ridge’ at Cushendun.

However, a caveat must be added to this lineament analysis in that most of the linear features are quite short and the selection of continuous lineaments is a subjective process, influenced by factors such as the scales and processing parameters used to generate the images, and personal bias in the interpreter. It is equally possible to pick features that cross-cut or intersect the above lineaments — for example, the north-south lineament from Dundrum, Co. Down, to Armoy, Co. Antrim that cuts the Antrim-Down Lineament (P947886).

Other trends

There are few examples of east-west trending structures in the geology of Northern Ireland. In north Co. Antrim, near the Giant’s Causeway, the WSW-ENE trending Portbradden Fault is marked by a magnetic anomaly and is accompanied by several other short, parallel anomalies offshore.

Intrusive bodies

The margins of major intrusive centres such as the Newry Igneous Complex and the Palaeogene Mourne Mountains, Slieve Gullion and Carlingford complexes are picked out by high gravity gradients. The gravity and magnetic gradients developed over the latter two centres are consistent with the model of a horizontal cylindrical basic intrusion with a WNW-ESE oval cupola beneath Slieve Gullion and a subsidiary cupola beneath Carlingford . In the Newry Igneous Complex interpretation of the aeromagnetic image identifies the three main granodiorite intrusions (P947810) but also indicates that the main batholith may be composed of two separate intrusions. Minor igneous intrusions such as dykes produce a significant response on the magnetic image but not on the gravity image except for the St John’s Point swarm on the Antrim-Down Lineament.