Abstract
The Walls Boundary Fault in Shetland, Scotland, formed during the Ordovician–Devonian Caledonian orogeny and underwent dextral reactivation in the Late Carboniferous. In a well-exposed section at Ollaberry, westerly verging, gently plunging regional folds in the Neoproterozoic Queyfirth Group on the western side of the Walls Boundary Fault are overprinted by faults and steeply plunging Z-shaped brittle–ductile folds that indicate contemporaneous right-lateral and top-to-the-west reverse displacement. East of the Walls Boundary Fault, the Early Silurian Graven granodiorite complex exhibits fault-parallel fractures with Riedel, P and conjugate shears indicating north–south-striking dextral deformation and an additional contemporaneous component of east–west shortening. In the Queyfirth Group, the structures are arranged in geometrically and kinematically distinct fault-bounded domains that are interpreted to result from two superimposed tectonic events, the youngest of which displays evidence for bulk dextral transpressional strain partitioning into end-member wrench and contractional strain domains. During dextral transpressional deformation, strain was focused into pelite horizons and favourably aligned pre-existing structures, leaving relicts of older deformation in more competent lithologies. This study highlights the importance of pre-existing structures and lithological heterogeneity during reactivation and suggests the development of a regional transpressional tectonic environment during the Late Carboniferous on the Shetland Platform.
Highlights
Oblique tectonic interactions are an inevitable consequence of the motion of lithospheric plates on the surface of a sphere (Dewey et al 1998)
Numerous additional modifications to this basic model have been proposed (e.g. Tikoff and Fossen 1993; Tikoff and Teyssier 1994; Teyssier et al 1995; Fossen and Tikoff 1997, 1998; Jones et al 1997; Jones and Holdsworth 1998; Holdsworth et al 2002). These include the incorporation of an inclined deformation zone (Jones et al 2004), which leads to transpressional strains with triclinic symmetry (Jones et al 2005), and consideration of the effects of strain partitioning into domains of monoclinic wrench- and contractiondominated deformation (Tavarnelli 1998; Teyssier and Tikoff 1998)
We propose that the Queyfirth Group initially developed a ‘Proto’ cleavage under garnet grade metamorphic conditions, overprinted by mesoscale tight-isoclinal ‘Early’ folds characterized by westerly verging, shallowly north- or south-plunging fold hinges and east-dipping axial planes formed in an east–west contractional regime
Summary
Oblique tectonic interactions are an inevitable consequence of the motion of lithospheric plates on the surface of a sphere (Dewey et al 1998). The WBF is widely viewed as being the northwards continuation of the Great Glen Fault (GGF) of mainland Scotland (McGeary 1989; Flinn 1992), a major lithospheric strike-slip fault active during the sinistrally transpressional collision of Laurentia and Baltica during the Caledonian orogeny and subsequent transtensional collapse during the Devonian (Dewey and Strachan 2003; Watts et al 2007). The Wester Keolka Shear Zone, which separates the Uyea Gneiss Complex and the Sand Voe Group, has been correlated with the Moine Thrust, the westernmost limit of Caledonian deformation in mainland Scotland (Andrews 1985; Flinn 1992; Flinn et al 1979; McBride and England 1994; see Walker et al 2016). The Queyfirth Group has been correlated with the Neoproterozoic–Cambrian Dalradian Supergroup, which underlies much of the Grampian Terrane in mainland Scotland (Flinn 1988)
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