Silurian subduction-related assembly of fault-defined tracts at the Laurieston Fault, Southern Uplands accretionary terrane, Scotland, U.K.

Abstract

Subduction-related accretion of fault-defined tracts built up the Southern Uplands terrane during the final stages of closure of the Iapetus Ocean (Llandovery to Wenlock). Contrasts in depositional environment and pronounced differences in geochemical composition, provenance studies and metamorphic grade across the Laurieston Fault between the Gala and Hawick groups, suggests that it has a greater regional significance than most other tract-bounding structures. Initiated by underthrusting, and acting as a locus for subsequent sinistral strike-slip, the fault overlies a regional gravity anomaly gradient that is interpreted to be due, in part, to a concealed NW-ward dipping shallow basement surface. This is modelled as an open ramp in the NE that steepens to a near-vertical step along-strike to the SW. A change in structural geometry noted at the Laurieston Fault, with excision of accretionary tracts, is related to a period of oblique closure of the Iapetus Ocean. The youngest Gala Group tracts were accreted during a period of intense transpression to form a regional strike-slip duplex over the shallow basement ramp with termination of the tracts at the Laurieston Fault, its surface expression. The ramp acted as an obstacle to forward-breaking thrust progress, forcing the out-of-sequence thrusting and repetitive thrust imbrication noted in the eastern Southern Uplands. Upper Palaeozoic reactivation of this basement structure may have transferred strain between extensional Permian basins.

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

“…Our findings, taken together with those of Holdsworth et al (2002), entirely support recent suggestions that the late Llandovery was a key period in the evolution of the Southern Uplands terrane (e.g. Akhurst et al 2001) and further highlight the regional importance of the Laurieston Fault (Fig. 6).…”

“…This structure presumably corresponds to the along-strike projection of the Laurieston Fault which is recognized in southwestern Scotland ( Fig. 1a; Akhurst et al 2001). The normal fault that currently separates the Silurian basement south of Eyemouth from the Devonian and Carboniferous rocks to the north (Fig.…”

Geotectonic evolution of Midland Scotland from Cambrian to Silurian: a review

“…However, some faults, originally with considerable strikeslip displacements, may well have been over-run by later thrusting, the images of which may be seen as flat surfaces, undetected or possibly wrongly attributed to other structures by conventional geophysics. Discontinuities deeper in the crust, as recorded by Akhurst et al (2000), when matched with existing outcrop patterns, point to anomalies that strongly suggest strike-slip faulting (in this case along the Laurieston Fault, although the Orlock Bridge and Moniaive shear zones are also important). Hall et al (1983) also pointed out that the depth to basement in the Southern Uplands varies considerably and might be the result of strike-slip faulting.…”