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

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 cont raction is also evident from thrusts and conjugate strike-slip faults. Pref erential 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 underlappin g zones in reactivated fault zones are dependent on the existing structural geometry. Two distinctive styles of displacement accommodation occur betwe en reverse-reactivated normal faults: (I) formation of a network of strike- slip faults, conjugate about NNE-SSW, and (2) oblique steeply-dipping rever se faults. Interaction between strike-slip and an existing fault is depende nt on whether the normal fault was reactivated. The range of structures rel ated to the north-south contraction has been incorporated into a single def ormation model, controlled by the northwards movement of the hanging wall o f 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, a nd a dextral component of displacement and strike-slip block rotations occu rred at the eastern tip. Shortening of these blocks was achieved through de velopment of a strike-slip fault network and NW-striking thrusts. In an und erlap zone, loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique, steeply-dipping reverse faults. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.