Selective reverse-reactivation of normal faults, and deformation around reverse-reactivated faults in the Mesozoic of the Somerset coast

Abstract

Normal faults exposed in the Triassic–Jurassic limestones and shales of the Somerset coast were formed during the Mesozoic development of the Bristol Channel Basin. Reverse-reactivation of some of these normal faults occurred during Late Cretaceous to Early Tertiary north–south contraction. The contraction is also evident from thrusts and conjugate strike-slip faults. Preferential reactivation of the normal faults is attributed to: (1) decreased fault-plane friction, (2) domino block rotation, (3) displacement magnitude, and (4) fault connectivity. The geometries of overlapping and underlapping zones in reactivated fault zones are dependent on the existing structural geometry. Two distinctive styles of displacement accommodation occur between reverse-reactivated normal faults: (1) formation of a network of strike-slip faults, conjugate about NNE–SSW, and (2) oblique steeply-dipping reverse faults. Interaction between strike-slip and an existing fault is dependent on whether the normal fault was reactivated. The range of structures related to the north–south contraction has been incorporated into a single deformation model, controlled by the northwards movement of the hanging wall of the Quantock's Head Fault. Pure dip-slip movement occurred in the centre of its curved fault trace, with a sinistral component at the western tip, and a dextral component of displacement and strike-slip block rotations occurred at the eastern tip. Shortening of these blocks was achieved through development of a strike-slip fault network and NW-striking thrusts. In an underlap zone, loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique, steeply-dipping reverse faults.