Field relations and movement history of the Coniston Fault, Cumbria

Abstract

The Coniston Fault is one of the largest and most fundamental faults in the Lake District. It is most easily appreciated near to Coniston where the Coniston Limestone is displaced horizontally (north to south) by 1.5 km, but, although the fault complex is easily traced for more than 40 km from the Vale of St John to well south of Coniston Water, the displacements in other areas are not always easy to determine. It is unlikely that the fault is entirely strike-slip, but if this were the case a dextral displacement at Coniston would be theoretically wrong for a north-south fault formed in the NW-SE Caledonian stress field, although such anomalies are common in geology. A possible explanation would be for a shallow wedge between the Coniston Fault and Brathay Fault to have been pushed forward during the Acadian movements, giving rise to the Coniston Thrust where it was overridden by the Coniston Grits. In any case the field evidence shows that the Coniston Fault had a long history of movement which started during, or before, the Borrowdale Volcanic episode, with any strike-slip (wrench) component most probably belonging to the late Caledonian (Acadian) phase. To the east of Coniston the fault, together with the adjacent Brathay Fault, maps out as a composite structure, bending into an ENE-bedding thrust, and then bending back again into the Beacon Tarn wrench fault. To the north the Coniston Fault extends through the Borrowdale Volcanics to Grasmere where it splits, one branch following the line across Dunmail Raise to Thirlmere and the Vale of St John, and the others crossing the fells to Ullswater. One concern of this paper is to consider the relationship of the Coniston Fault and other structures to the Borrowdale Volcanic sequences, parts of which change abruptly at the fault. Much of this I believe to have been a result of fault movements during the volcanic episode, possibly related to the caldera formation further west. Regional folding was also influenced by the Coniston Fault since the well-defined NE-trending Wrynose Anticline and Fault, also likely to have been initiated during the volcanic episode, terminate at the fault. Renewed post-volcanic-pre-Coniston Limestone movement of the Coniston Fault, followed by erosion, resulted in the absence of the Tarn Hows Formation west of the fault, a clear demonstration of pre-Coniston Limestone movement. The Tarn Hows Formation is the highest part of the volcanic sequence in this area. The latest demonstrable movements were post-Bannisdale Slates and of presumed Acadian age, although by analogy with adjacent areas there may have been posthumous activity in post-Carboniferous and post-Triassic times.