The geology and structural evolution of a Caledonian fold and ductile thrust zone, Kyle of Tongue region, Sutherland, northern Scotland

Abstract

The Middle to Upper Proterozoic metasedimentary rocks of the northern Highland Moine in Sutherland are disposed in a series of major Caledonian ( c . 470–430 Ma) metamorphic thrust nappes produced by NW/WNW-directed shortening and reworking of parts of a pre-existing Precambrian (c. 1000 Ma) orogen. Lewisian inliers occur throughout Sutherland and represent tectonically interleaved units of the original basement gneisses upon which the Moine cover sequence was deposited. In the western Moine Nappe of the Kyle of Tongue region, the rocks are deformed by two main episodes, D1 and D2, which are thought to represent the effects of Precambrian and Caledonian orogenesis respectively. Little basement-cover interleaving or large-scale folding occurred during the D1 event in the Moine Nappe, but the associated metamorphism (to ut leust garnet grade) produced peak metamorphic mineral assemblages. Subsequent Caledonian reworking (D2) produced a foreland-propagating system of WNW-directed ductile thrusts and related major F2 sheath folds, both of which resulted in much of the observed Moine-Lewisian interleaving. Metamorphism associated with D2 decreases in grade westwards, occurring at similar, or slightly lower grades, than the earlier D1 event; this retrogressive overprint is most obvious at the W margin of the Moine Nappe. The D2 folds and ductile thrusts are geometrically and kinematically similar to the Caledonian structures recognized within the Moine Thrust Zone to the west, the very low (≫5°) ramp angles suggesting that the thrusts must have very shallow-dipping trajectories through the crust. F2 fold formation is attributed to an early phase of WNW-directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued WNW-directed thrusting. The occurrence of internal extension in the orogenic wedge during active thrust-related foreshortening implies that the shear strength of the material close to the basal detatchment(s) was continuously reduced, i.e. wedge motion became supercritical . In this context, strain softening processes within the ductile thrust mylonites were probably of major significance.