The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland

Abstract

Structures deforming Llandovery turbidites of the Gala Group in the Southern Uplands terrane are spectacularly exposed in the Berwickshire coastal section, southeastern Scotland. The upward-facing, upright to NW-vergent folds and associated structures appear to record a single regional phase of subhorizontal NW–SE contractional deformation, with a steeply dipping direction of bulk finite extension. These structures are markedly different from those developed in rocks correlated with the Upper Llandovery Hawick Group exposed some 5 km to the south in the Eyemouth–Burnmouth coastal section. Here a highly domainal system of sinistral transpressional strain occurs, with zones of steeply plunging curvilinear folds, clockwise cleavage transection and bedding-parallel sinistral detachment faults. The markedly different bulk strain patterns in the Berwickshire coastal sections are thought to reflect the regionally diachronous nature of transpressional deformation in the Southern Uplands terrane. There are striking similarities in the structures recognized in the Berwickshire coastal sections and those developed in stratigraphically equivalent units along strike in southwestern Scotland and Northern Ireland. This confirms the lateral structural continuity and correlation of tracts and tract boundaries along the entire length of the Southern Uplands terrane. The regional structure suggests that a phase of top-to-the-NW backthrusting and backfolding associated with the southern margin of the Gala Group outcrop marks the transition from orthogonal contraction to sinistral transpression in the Southern Upland thrust wedge during late Llandovery times.

Late Carboniferous dextral transpressional reactivation of the crustal-scale Walls Boundary Fault, Shetland: the role of pre-existing structures and lithological heterogeneities

“…Many studies (e.g. Tavarnelli and Holdsworth 1999;Holdsworth et al 2002;Tavarnelli et al 2004;McCaffrey et al 2005) have demonstrated that transpression can be partitioned into end-member wrench and shortening components across a wide range of scales. Many workers (e.g.…”

“…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 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 also consideration of the effects of strain partitioning into domains of monoclinic wrench-and contraction-dominated deformation (Tavarnelli 1998;Teyssier and Tikoff 1998).…”

Geometry and impact of transpressional faulting in polyphasic metamorphic orogenic belts: the Viù Deformation Zone (inner Western Alps)

“…Transpressional deformation is also commonly associated with the partitioning of the bulk strain into distinct domains characterized by orogen parallel strike-slip and reverse faults (Oldow et al 1990;Molnar 1992;Tikoff and Teyssier 1994;Jones and Tanner 1995;Teyssier et al 1995;Dewey et al 1998;Jiang et al 2001;Holdsworth et al 2002b;Jones et al 2004).…”