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 Recent published and unpublished information relating to the lineament is synthesized and an evolutionary model is proposed.Geophysical data suggest it was initiated as a strike-slip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 1984 Lleyn earthquake and other historical seismicity denote neotectonic activity on the same structures.

Tectonic controls of Ordovician arc and marginal basin volcanism in Wales

“…Although the Rhobell Fracture dominated development of the AVG, the locations of the volcanic centres suggeSt that magma may have been channelled at intersections with faults or deeper discontinuities trending NE-SW. There is evidence in the Aran Mountains, close to the Cader Idris-Arans centre, of NE-SW flexure and faulting during the Ordovician (see above), and Fitches & Campbell (1987) have demonstrated the importance of the Bala Fault in post-Ordovician times, arguing that it reflects a pre-existing crustal discontinuity. Furthermore, to the west, major faults of the RFZ (e.g.…”

“…It might have been terminated to the south by some NE-SW fault lying southeast of the present AVG outcrop (see above ;Dunkley 1978Dunkley , 1979, or transferred on such a structure, but this remains uncertain. The term 'Aran Graben' has been used by Fitches & Campbell (1987) for a graben variously identified (not named) by Ridgway (1976), Kokelaar (1977Kokelaar ( , 1979 and Dunkley (1978), and lying in a position partly coincident with the Rhobell Graben. The previous term is not adopted here because the Rhobell Graben is in many ways differently defined, is more extensive, and is in parts…”

“…3), which is a splay of the (post-AVG) Bala Fault, was active in AVG times (Cox & Wells 1921, 1927Dunkley 1978; cf. Fitches & Campbell 1987, fig. 4).…”

Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques

“…The coincidence of the magnetic Euler anomaly with the surface disturbance, however, implies that more than one structure is involved. The continuity and narrowness of the strong gravity and aeromagnetic Euler anomalies, and their distribution across Wales, the Borderlands and off-shore, strongly suggest that they define a network of deep faults supporting the views of other researchers (for example : Jones, 1938;Shackleton, 1954;James & James, 1969;Woodcock, 1984;Kokelaar et al 1984;Smith, 1987;Fitches & Campbell, 1987;Wilkinson & Smith, 1988;Kokelaar, 1988;Woodcock & Gibbons, 1988;Howells, Reedman & Campbell, 1991). The majority of the Euler solution depths are less than 4 km which indicates that the anomalies generated in the cover sequences are overprinting any deeper solutions from the basement.…”

“…It has been suggested for many years that deep structures in the Precambrian basement have strongly influenced the framework, sedimentation, volcanism and tectonics of the Lower Palaeozoic Basin of Wales (for example ;Jones, 1938;Shackleton, 1954;James & James, 1969;Woodcock, 1984;Kokelaar et al 1984;Smith, 1987;Fitches & Campbell, 1987;Wilkinson & Smith, 1988;Kokelaar, 1988;Woodcock & Gibbons, 1988;Howells, Reedman & Campbell, 1991). The existence of these structures has been inferred mainly from the geological modelling of the exposed rocks except in the few cases where seismic data or major changes in gravity and magnetic parameters have allowed more positive identification.…”