Structural and Geochronological Investigations into Mid-Crustal Shear Zones, Shetland, Scottish Caledonides

Strain in the lithosphere localises into weakened regions known as shear zones, which are often sites of repeated reactivation, reworking and poly-metamorphism. Mature shear zones are composite areas that include spatially and temporally heterogeneous combinations of coaxial and non-coaxial deformation, such as transpression and transtension. Due to their heterogeneous nature, determining the structural evolution and regional significance of shear zones is challenging. Such areas of ductile deformation are associated with the SE-dipping North Roe Nappe and the N-S striking Walls Boundary Fault (WBF) on NW Mainland Shetland. Here, the metamorphic basement and bounding strike-slip WBF provide pre-Permian geological links between the Scottish, Norwegian and Greenland sectors of the Caledonian orogen. Field mapping, microstructural characterisation and Rb-Sr geochronology in combination with other published work in the area show that NW Mainland Shetland was subject to Neoarchaean (c. 2800 – 2700 Ma), Neoproterozoic (827 Ma – 697 Ma) and Caledonian (487 – 404 Ma) deformation. Weakly-defined Neoarchaean deformation is preserved in the structurally lowest sections of the Uyea Gneiss Complex below the North Roe Nappe. Within the North Roe Nappe, Neoproterozoic deformation is mainly found in coaxially-deformed decimetre-scale horizons. The main ductile fabrics found on NW Mainland Shetland formed during top-to-the NW thrusting and top-to-the N/sinistral shearing, dated to be broadly Caledonian in age. Bounding the coaxially-deformed Neoproterozoic horizons are decimetre-scale horizons of non-coaxial top-to-the W thrusting and NNE-SSW sinistral shearing interleaved with antithetic top-to-the E extensional and NNE-SSW dextral shears. Due to a lack of overprinting evidence and overlapping geochronological ages, non-coaxial deformation in the North Roe Nappe is interpreted to have developed broadly coevally during the Caledonian (c. 440-404 Ma). The results from this study suggest that Shetland was subject to inclined transpression during the Caledonian (Grampian-Scandian) orogeny. During transpression, Neoproterozoic material was locally vertically and laterally extruded along foliation-parallel channels in unconfined transpression. As Caledonian deformation is observed throughout NW Mainland Shetland, the westernmost limit of Caledonian deformation and the northern continuation of the Moine Thrust is either obscured by post-Caledonian brittle faults or lies offshore to the west of Shetland. Post-Caledonian dextral reactivation of the WBF resulted in localised strain being focused into bedding-parallel and fold limb-parallel shears. This study highlights the importance of combining field, microstructural and geochronological data in determining the structural evolution of reactivated, transpressional shear zones.