Wrench-faulting in Cornwall and south Devon

Abstract

A system of wrench-faults has been recognised in Cornwall and south Devon on which there has been a cumulative dextral displacement of at least twenty-one miles. These north-north-westerly faults are thought to be largely Armorican tensional structures rejuvenated during the Alpine orogeny. By removing the fault shifts the geological map of south-west England has been redrawn. The most remarkable change is the shift of the southern margin of the Dartmoor granite into the same latitude as the southern margin of the adjacent Bodmin Moor granite. This now gives a uniformity, hitherto lacking, to the presumed form of the south-west England batholith, removes the cranked shape of the granite outcrops from ENE.-WSW. in Cornwall to E.-W. between Bodmin Moor and Dartmoor, and makes unnecessary the former emphasis given to the presence of mainly E.-W. mineral veins in the latter region. A straightened Lizard-Start fault, avoiding the Dodman, now fits into the regional setting much more comfortably than the former great arc. It is a matter for speculation whether this fault, so close to the required trend for mid-Tertiary sinistral wrench-faults, could have accommodated a part of the deformation associated with the Alpine orogeny and in so doing moved the metamorphic regions of the Start and the Lizard into their present positions.