The Leannan fault

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Abstract
A powerful north-east-trending fault cuts right across the north-west corner of Ireland and forms the most important member of a system of steep wrench-faults that dissect the Dalradian rocks of Donegal into parallel strips. The contrast between the rocks flanking this Leannan fault is very marked. On the north-western side the outcrops of the same succession of metasediments run parallel to the fault-line for over 60 miles; over this distance they show little change in lithology, structure, or metamorphism. On the other side the geology varies considerably along the fault, and, except in the north-east, the outcrops run at a high angle to the fault-line; as a result, several different rock-groups are brought against the fault. The metamorphism increases progressively to the south-west so that rocks of kyanite grade are finally brought against others of chlorite grade. The main movements on the fault are shown to have occurred between the deposition of red beds, presumably of Lower Old Red Sandstone age, and the deposition of Viséan marine sediments. The fault-zone generally consists of one main break accompanied by two others of lesser importance: together these fractures isolate narrow strips of metasediments in which inverted sequences are the rule. It is argued that the inversion of these strata predates the faulting, and that they were derived from the now eroded limb of a great recumbent fold. Along the main fault a total of 60 yards of intense deformation is usual, and black schist, platy siliceous mylonites, and quartzitic breccias are the result; the steep platiness is always set at an acute angle to the fault. Two separate movements are shown to have occurred; a downthrow to the south-east amounting to several thousand feet is combined with a left-handed displacement, the amount of which, though difficult to estimate, may be as much as 25 miles. The Leannan fault is directly in line with the Great Glen fault and the possibility that it is a continuation of this structure is discussed.

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