|
Fault Geometry and Associated Processes |
Department of Geology, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK
Complex systems of mesoscopic brittle fractures striking transversley to orogenic margins are characteristic upper-crustal structures in forelands (and hinterlands) that have experienced extension as a consequence of continental collision. They are especially well developed in foreland peripheries and some forelands lacking major grabens and strike-slip faults, the best-known expressions of horizontal extension. Despite the contribution of the mesofractures to foreland strain being less than that achieved by macrofault zones they are important structures because: (i) they are widespread and pervasive at outcrop scale, (ii) they influence the location of some major topographic lineaments and (iii) they permit regionally significant extension directions to be determined where macrostructures are absent.
The principal styles of mesoscopic brittle structures are planar normal faults, vertical extension joints and veins, and steeply inclined, conjugate hybrid and shear joints. Commonly, the strike of fracture sets is uniform throughout large areas and although it is generally normal to a deformation front, this simple relationship is not present everywhere, particularly distant from some markedly arcuate fronts.
Two examples of parts of forelands containing brittle extensional structures approximately coeval with late-Alpide events are the European platform in S England and N France, and the Arabian platform neighbouring the Gulf coast. In both regions, the fractures are superimposed obliquely on older Tertiary structures and comprise dominant sets of extension fractures accompanied locally by normal faults and hybrid joints. The dominance of extension and hybrid fractures among the failure modes indicates that differential stresses remained low throughout large areas of the forelands and that the effective minimum stress was tensile.
* Present address: BP Petroleum Development Ltd, Farburn Industrial Estate, Aberdeen, AB2 0PB, UK.
