REACTIVATION OF RELAY RAMPS AND STRUCTURAL EVOLUTION OF THE MERE FAULT AND WARDOUR MONOCLINE, NORTHERN WESSEX BASIN

New ogical field mapping along a 24-km-long portion of the Mere Fault, in the northern part of the Wessex Basin, together with seismic refle ction and other subsurface data, allow an analysis of displacement, bo th along the length and down the dip of the reactivated fault. The pri ncipal segments of the Mere Fault dip south at about 70 degrees and di splay components of both syndepositional normal displacement and later reversal of movement during basin contraction. Minimum estimates of t he 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 the se values decrease to zero toward the fault segment tips. Estimates th at allow for reverse movement along the fault suggest that there must have been at least 500 m of normal displacement along the central port ion of the segment. Stratigraphical separation at the surface indicate s that the largest down-to-the-north displacements, associated with la ter fault reversal, are at least 200 m and occur in the east, where re versal of movement has taken place on an early, high-angle fault segme nt. In the west, the principal fault strands are eroded to deeper stra tigraphical levels where largely normal slip is preserved and segments are linked by normal and oblique transfer faults. The Wardour Monocli ne was developed during basin contraction, in part by movement along a concealed fault segment, overstepping from the Mere Fault at the surf ace, and in part over a relay ramp between the two fault segments.