CALEDONIAN TRANSPRESSION FROM THE PERI-GONDWANAN MARGIN OF WALES, UK

Britain, south of the Iapetus suture, comprises a collage of Neoproteozoic and Lower Palaeozoic units transected by strike parallel fault zones. These are interpreted as long lived transcurrent structures that reflect a protracted history of oblique convergence and partitioned transpression along the Gondwanan margin.

The Neoproterozoic basement comprises a mosaic of terranes formed in a supra-subduction zone setting between 700 and 530 Ma. Cambrian to earliest Ordovician times saw the formation of contrasting sedimentary successions juxtaposed along the Menai Strait, Bala and Welsh Borderland fault systems, associated with accretionary thrust tectonics and uplift (ca. 475 Ma Monian Orogeny).

There is no conclusive evidence for strike slip displacement, although new U-Pb dating from the Stanner Hanter Complex (710.8 ± 1.5 Ma), within error of an Rb-Sr isochron, shows that it remained unaffected by local 650 Ma tectonism and may be an exotic microplate entrained within the Welsh Borderland Fault System (WBFS). Similarly, new provenance studies from the Arfon Basin and the Harlech Dome demonstrate the presence of contrasting and exotic Cambrian successions juxtaposed along significant strike faults.

Ordovician to Silurian times saw the development an ensialic back-arc, Welsh Basin. Strike-slip reactivation along the continental margin of this subduction complex (WBFS) has been inferred from development of short lived wrench basins and local deformation (ca. 440 Ma Shelvian Orogeny) as well as stacking patterns of laterally sourced turbidite fans.

Caledonian deformation in Wales (ca. 400 Ma Acadian Orogeny) has traditionally been interpreted as reflecting Iapetan terminal collision. However, recent studies suggest that an advancing Rheic Ocean subduction zone may have led to inversion of the eastern Avalonian upper plate. Deformation was achieved by a combination of approximately co-axial shortening and transcurrent movement along major faults to produce a strongly partitioned transpressional strain. However, new field observations from southwest Wales reveal superimposed deformations which indicate that thrust tectonics also operated within the WBFS.