A late Cenozoic regional mesofracture system in southern England and northern France

Abstract

The Upper Cretaceous and Palaeogene rocks of S England and N France are cut by a NW-trending system of fractures, characterized by swarms of vertical extension joints accompanied by conjugate sets of hybrid and shear joints, and normal mesofaults. Many major photolineaments exhibit the same NW trend as the fractures, and are particularly conspicuous in N France where they are coincident with rectilinear drainage channels, suggesting a relationship between fracture orientation and physiography. In S England, this relationship is absent; possibly because of the influence of additional fracture systems related to major flexures trending obliquely to the NW-SE direction. These major E-W trending flexures and associated fracture systems were formed during the Oligocene to Early Miocene 'Helvetic' phase of deformation, and are cross-cut obliquely by fractures in the NW-trending system. The NW-trending fractures and lineaments are parallel to neotectonic normal faults in the Lower Rhine embayment, and become less well developed to the west with increasing distance from these major structures. The Lower Rhine faults were reactivated and propagated into the Quaternary cover as a consequence of NE-SW regional tension generated during the late Neogene to Recent 'Jura' phase of NW-SE Alpine convergence. The NW-trending extensional structures in S England and N France are considered to be of about the same age and related to the same stress regime as the neotectonic normal faults of the Lower Rhine embayment. However, because fractures of the NW-system cutting Palaeogene sediments in SE England are truncated by the erosion surface beneath the Plio-Pleistocene Red Crag deposits, it is possible that in England fracture initiation did not continue into Quaternary times.