By Rob Strachan, Bob Holdsworth, Clark Friend, Ian Burns and Ian Alsop
Fig. 13.1 Simplified geological map of north Sutherland together with the localities for the excursion.Fig. 13.4 Locality 13.2. A. The Ben Hope Thrust and basement-cover relations in the Ben Hope to Kinloch area of the Moine Nappe; B. detailed map of the Kinloch area; C.Interpretative sketch of D3 structures in the Kinloch area (from Moorhouse et al., 1988).
Excursion 13 North Sutherland
Purpose:
A general traverse across the Caledonian thrust nappes that outcrop between the Moine Thrust and the sedimentary cover of the Devonian Orcadian Basin.
Aspects covered:
Various metasedimentary lithologies, metamorphic minerals and migmatites, Lewisianoid basement gneisses, Precambrian and Lower Palaeozoic (meta)igneous intrusions, Caledonian ductile structures, late to post-Caledonian brittle faults, Devonian and probable Permo-Triassic sedimentary rocks.
Useful information:
Hotel and B&B accommodation and camping are available at Talmine, Tongue and Bettyhill.
The traverse along the well-exposed north coast of Sutherland (Fig. 13.1) provides the best opportunity to examine the complex regional structure of the Moine and Naver nappes, as well as a number of the Archaean basement inliers that crop out in the area. Particular points of interest include the nature of the relationships of these inliers with the Moine, and the evidence for their pervasive reworking during the Caledonian orogeny. These basement inliers have often been correlated with the Lewisian gneisses of the foreland, although unambiguous correlation remains to be demonstrated. Accordingly, use of the less specific term ‘Lewisianoid’ is employed here to denote basement that is of similar age and lithology to that of the foreland. Both the basement inliers and the Moine have undergone a complex polyphase deformation history. Complications in attempting to erect a consistent chronology across the traverse arise from the recognition that in the Moine Nappe the earliest structures and metamorphic fabrics recognizable in the field are Neoproterozoic in age, whereas in the Naver Nappe they are Ordovician (Grampian). It is assumed that Neoproterozoic fabrics were originally present within the Naver Nappe, but were extensively reworked during Ordovician high-grade metamorphism and migmatization (Table S.1). The Grampian event has so far not been recognized within the Moine Nappe. In the deformation chronology presented in Table S.1, we therefore recognize D1 (Moine Nappe) and D1N (Naver Nappe) events that are of different ages. Both thrust nappes record a similar polyphase Silurian (Scandian) history: sets of structures assigned to D2, D3 and D4 episodes are thought to have resulted from a continuous, progressive deformation.
