Tectonic evolution of the Bala Lineament in the Welsh Basin

Abstract

The Bala Lineament is one of several NE–SW major linear fault zones traversing the Lower Palaeozoic Welsh Basin. Recent published and unpublished information relating to the lineament is synthesized and an evolutionary model is proposed. Geophysical data suggest it was initiated as a strike-slip structure, probably in late Precambrian times. Unlike the other lineaments, it had no separate Lower Palaeozoic identity; during that period, sedimentation was strongly influenced by contemporary faulting but there is no reason to suspect that deformation was confined to a long, narrow, NE–SW belt or that neighbouring regions were not similarly affected. Moreover, the lineament is not distinguishable from adjoining regions by anomalous end-Caledonian strains. In the Phanerozoic, the lineament became a recognizably discrete tectonic zone during or after the end-Caledonian regional deformation. Then, cleavage and folds along part of the lineament were rotated, and overprinted by a local cleavage, in a broad NE–SW dextral shear zone. This ductile event perhaps denotes the onset of simple shear deformation which affected Carboniferous sedimentation in northeast Wales and culminated in major strike-slip faulting. These faults, which dominate the geological maps of this part of Wales and include the Bala Fault itself, are essentially Variscan structures; their sense and amount of displacement remains uncertain. Previously underemphasized N–S Lower Palaeozoic faults are shown to be as important as the more familiar NE–SW structures in influencing sedimentation patterns along the lineament. It is suggested that the Lower Palaeozoic combination of N–S and NE–SW structures is widespread in the Welsh Basin and is due to distributed regional sinistral shear. Both sets of structures were reactivated repeatedly. In Cardigan Bay, Mesozoic–Tertiary sedimentary basins subsided on these structures, whilst the 1934 Lleyn earthquake and other historical seismicity denote neotectonic activity on the same structures.