Domainal deformation patterns and strain partitioning during transpression: an example from the Southern Uplands terrane, Scotland

Abstract

The partitioning of deformation into wrench- and contraction-dominated deformation domains is a widely reported but poorly described phenomenon in ancient transpression zones. This paper documents spectacularly exposed examples of such partitioning from the Southern Uplands terrane in SE Scotland (Berwickshire), which was deformed during late Llandovery to Wenlock time. A well-exposed coastal section from Eyemouth to Burnmouth preserves a broadly homoclinal sequence in which a highly heterogeneous array of contemporaneous structures formed during regional triclinic transpression. The deformation involved components of NW–SE contraction with subvertical extension, top-to-the-SE thrusting and top-to-the-SW sinistral shear. In the northern third of the section studied these components are partitioned into a series of fault-bounded, metre- to kilometre-scale structural domains that contain geometrically and kinematically distinct assemblages of variably curvilinear folds, strike-slip detachments and locally transecting cleavages. The structures are all broadly contemporaneous and, in individual domains, record either non-coaxial contractional- or sinistral wrench-dominated strains. Similar highly heterogeneous, domainal structural patterns are likely to be found in other regions of oblique convergence in both ancient and modern settings.

Transitional geometries between gently plunging and steeply plunging folds: an example from the Lower Palaeozoic Brabant Massif, Anglo-Brabant deformation belt, Belgium

“…Probably, the dextral displacement at depth is accommodated in a more gradual fashion at the present-day outcrop level by means of the type B folds and the fold transition zone. This is compatible with observations by Holdsworth et al (2002b) in the sinistral transpression zone in the Southern Uplands of steeply plunging S-shaped folds rooting downwards into sinistral strike-slip detachments.…”

“…Similar zones containing asymmetric, steeply plunging folds adjacent to gently plunging folds with strong variations in facing directions and curvilinear fold hinge-lines have been attributed to strain partitioning in transpression zones (Holdsworth et al 2002b). On the basis of cleavage-fold relationships, Holdsworth et al (2002b) distinguished four domains in a 500 m wide sinistral transpression zone in the Southern Uplands Terrane: domain 1, with steeply SW-plunging folds with an S-fold geometry; domain 2, with subhorizontal, NE-SW-trending, upward facing folds; domain 3, with moderately to steeply plunging, strongly curvilinear folds with a sinistral vergence within a NE-SW-striking, subvertical homocline; domain 4, with subhorizontal to moderately plunging folds, locally strongly curvilinear, affected by sinistral detachments (Holdsworth et al 2002b).…”

“…On the basis of cleavage-fold relationships, Holdsworth et al (2002b) distinguished four domains in a 500 m wide sinistral transpression zone in the Southern Uplands Terrane: domain 1, with steeply SW-plunging folds with an S-fold geometry; domain 2, with subhorizontal, NE-SW-trending, upward facing folds; domain 3, with moderately to steeply plunging, strongly curvilinear folds with a sinistral vergence within a NE-SW-striking, subvertical homocline; domain 4, with subhorizontal to moderately plunging folds, locally strongly curvilinear, affected by sinistral detachments (Holdsworth et al 2002b). The type B folds of the Brabant Massif resemble the folds of domain 1, the type A folds closely correspond to the folds of domain 2, and the fold transition zone shows similarities to both domains 3 and 4.…”

“…Furthermore, the relative position of the fold transition zone is comparable with that of domains 3 and 4, situated between domains 1 and 2 and between two domains 2, respectively. However, whereas Holdsworth et al (2002b) depicted the various domains with abrupt boundaries, and suggested that the deformation in domain 1 is partitioned into domains 2 and 3, in the Brabant Massif gradual transitions are observed and the zone resembling domains 3 and 4 is considered as a transition zone between the type A and type B folds.…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…This combination of structural features indicates that, from the mid-Llandovery onwards, a significant component of sinistral shear was intermittently present during the accretionary D 1 deformation at the thrust front, and at the same time was probably responsible for the imposition of the steeply plunging D 3 folds within the thrust hinterland. The domainal nature of this transpressive deformation has been stressed, most recently by Holdsworth et al (2002b) who describe, from the Berwickshire coastal sections, its partitioning into strike-slip and contraction-dominated zones. The approximate temporal coincidence of this increase in transpression affecting the Hawick Group, with back-thrusting and out-of-sequence thrusting affecting the southernmost (youngest) Gala Group tracts, led to the suggestion by Rushton et al (1996) that during the Late Llandovery the accretionary thrust belt had to accommodate to an obstacle to its forward propagation.…”