Abstract

Synopsis

The Gairloch shear zone (GSZ) has long been regarded as a dextral strike-slip shear zone with a northwesterly plunging shear direction. However, the original strain analysis applies only to the northeastern third of the zone and the structure of the remaining two-thirds is considerably more complex. The 6.5 km wide GSZ was formed during late Laxfordian (D3) deformation at c. 1.7 Ga. It was a product of the interaction of D3 folding and dextral strike-slip shear under retrogressive greenschist-facies metamorphism on rocks previously intensely deformed by early Laxfordian (D2) deformation in amphibolite-facies.

The GSZ is divided into four units across strike: the Northeastern shear zone with NW-plunging D3 linear fabric; the Flowerdale belt with SE-plunging D2 linear fabric and low D3 strain; the Ard belt with steeply plunging F3 folds; and the Southwestern shear zone, with SE-plunging D3 lineation. Thus the GSZ consists of two high-strain D3 shear zones on either side of a central low-strain belt. Although consistently dextral strike-slip throughout, the D3 shear regime varies from NE-up in the Northeastern shear zone through sub-horizontal strike-slip in the Ard belt to SW-up in the Southwestern shear zone. This variation may have been caused by local uplift or depression induced by variations in geometry of the walls of the strike-slip system. A revised very approximate minimum estimate of the horizontal component of dextral shear is c. 9.5 km with a net NE-up vertical component of c. 5.5 km. Similarities in structure between the GSZ and the South Harris belt strengthen the case for these two zones to have been part of the same structure.

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