Structural and Isotopic studies of Moine Rocks: Moine Thrust to the Sgurr Beag Slide

Abstract

During Caledonian amphibolite grade metamorphism in the Fannich/Ullapool area of N.W. Scotland, high grade migmatitic gneisses were overthrust northwestward along the Sgurr Beag Slide, onto lower grade rocks. A minimum displacement of about 50km is indicated. Features associated with the slide include: the presence of slivers of Lewisian basement along the slide at Achnasheen; the intensification of strain in a 2km zone adjacent to the slide; differences in the early structural and metamorphic histories of the crustal blocks lying above and below the slide; and local inversion of isograds.

The slide zone, and all peak metamorphic fabrics, were reworked by a set of major N-S trending, westward verging asymmetrical folds. Associated deformation fabrics intensify towards the Moine Thrust zone, culminating in the development of greenschist facies mylonites. Within 4km of the thrust, folds and fabrics of this generation become sub-parallel to banding and fold axes are rotated towards the movement direction of thrusting (i.e. the W.N.W.). Shear bands are developed in pelitic horizons whilst in psammitic lithologies, strong quartz c-axis girdle fabrics characterise mylonitic rocks.

K-Ar dates on muscovite and biotite pairs from pelites (200--250um grain-size), along an 18km E-W traverse through the area indicate that the whole of the area cooled virtually simultaneously between 424Ma and 421Ma. However uplift possibly as early as 430Ma is indicated close to the thrust. During the same period an excess argon atmosphere was developed in a zone within 4km of the Moine Thrust. The K-Ar study reveals a relationship between age, reproducibility and grain-size. Fine grain-sizes (125--200um) give reproducible ages with a mean of 423.5Ma (muscovite) and 421.5Ma (biotite). Whereas coarser grain-sizes (250--500um)give unreproducible ages at around 433Ma, significantly older than the finer. Rb-Sr mica pairs of all grain-sizes give ages compatible with the K-Ar ages from the finest grain-sizes. It is suggested that the micas suffered a partial resetting event late in their cooling history.

The major conclusion of the study is that the development of the Moine Thrust zone significantly post-dates peak metamorphism and movement upon the Sgurr Beag Slide. The slide zone is thought to have moved during peak metamorphic conditions around 460Ma whereas the Moine Thrust developed and moved between 430Ma and 420Ma.