Reactivation of relay ramps and structural evolution of the Mere Fault and Wardour Monocline, northern Wessex Basin

Abstract

New geological field mapping along a 24-km-long portion of the Mere Fault, in the northern part of the Wessex Basin, together with seismic reflection and other subsurface data, allow an analysis of displacement, both along the length and down the dip of the reactivated fault. The principal segments of the Mere Fault dip south at about 70° and display components of both syndepositional normal displacement and later reversal of movement during basin contraction. Minimum estimates of the largest down-to-the-south displacements range from less than 100 m at the surface to 350 m at the top of the pre-Permian basement and these values decrease to zero toward the fault segment tips. Estimates that allow for reverse movement along the fault suggest that there must have been at least 500 m of normal displacement along the central portion of the segment. Stratigraphical separation at the surface indicates that the largest down-to-the-north displacements, associated with later fault reversal, are at least 200 m and occur in the east, where reversal of movement has taken place on an early, high-angle fault segment. In the west, the principal fault strands are eroded to deeper stratigraphical levels where largely normal slip is preserved and segments are linked by normal and oblique transfer faults. The Wardour Monocline was developed during basin contraction, in part by movement along a concealed fault segment, overstepping from the Mere Fault at the surface, and in part over a relay ramp between the two fault segments.