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The main structural and metamorphic features of the Lewisian rocks of the Outer Hebrides are outlined and compared with those of the Lewisian on the Scottish mainland. In each area three structural zones are recognized, characterized by folding of different styles and periods. The three zones of the Outer Hebrides are closely analogous to the Scourian and Laxfordian zones of the mainland.
Evidence of three periods of folding and metamorphism in varying degrees of intensity may be seen in the Outer Hebrides. The first and second periods were separated in time by the intrusion of a non-orogenic suite of basic sills and dykes of tholeiitic affinities, possibly associated with the plutonic centre in South Harris. This basic suite is regarded as equivalent to the Scourie dykes of the mainland.
The first period of folding and (granulite-facies) metamorphism is recognized in the gneisses above the Outer Isles thrust-plane and in a narrow belt along the Sound of Harris, and is correlated with the Scourian of the mainland. After the intrusion of the basic suite, the rocks were affected by a second period of folding and granulite-facies metamorphism which converted the basic intrusions to garnet-pyroxene-amphibolites. The Scourie dykes of the mainland retain evidence of this metamorphism in the form of corona garnets, garnetiferous border zones, and clouded feldspars. The second folding and metamorphism is regarded as Early Laxfordian because the folding has the same trend as that accompanying the third and last period of metamorphism, which is a retrograde type associated with migmatization and intrusion of granites and pegmatites and is known from absolute age-determinations to be equivalent to the Lax-fordian of the mainland.
The positions of the Lewisian metamorphic and structural zones on the opposite shores of the Minch do not coincide, and the offset can be adequately explained only by later wrench-faulting. Evidence is presented to support the existence of the Minch fault, a sinistral wrench-fault with an approximate displacement of 77 miles, comparable in direction and magnitude with the Great Glen fault.
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