Cross-faults and differential stretching of hanging walls in regions of low-angle normal faulting: examples from western Turkey

Abstract

Summary Large oblique-throw hinge faults orientated at high angles to the strike of major breakaway faults in W Turkey are examined. They exhibit variable displacements which commonly increase towards the breakaway fault with which they seem to be associated and bound cross-blocks that in places have rotated around vertical axes by differing amounts and senses, and are interpreted in terms of the possibility that hanging walls of major low-angle breakaway faults (extensional allochthons), may exhibit differential internal extension accommodated by transverse faults that may or may not rotate around vertical axes. Such transverse faults, here called accommodation faults , are different from transfer faults in that they need not offset the main breakaway fault and may accomplish significant map-view distortions of extensional allochthons. Irrotational accommodation faults may give rise to horst and graben complexes or may create the image of domino-style, uniformly tilted normal and/or thrust faults at high angles to the strike of major breakaways, thus giving rise to the misleading image of extension in two directions in an extensional zone or shortening at high angles to regional extension. Although strike-slip offsets along irrotational accommodation faults need not bear any relation to the magnitude of regional extension, their strikes must at least be sub-parallel to the orientation of regional extension. Real shortening at high angles to regional extension may give rise to rotational accommodation faults. Neither the strike-slip offset along such faults nor their azimuth need bear any simple relationship to the magnitude and orientation of regional extension. Rotational accommodation faults bring about an equivalent rotation of all structures that form along them, such as pull-apart basins, and the hanging wall blocks they delimit, together with their pre-existing structures. All three kinds of cross-faults, i.e. transfer faults, irrotational and rotational accommodation faults may coexist in any one extensional region and may grade into one another both in space and in time creating a very complicated structural picture and causing irregular subsidence patterns. It is stressed that cross-faults have an extremely important bearing on our understanding of the geometry and evolution of extensional zones, here called taphrogens , and that only a three-dimensional consideration of the structure of taphrogens is likely to lead to a satisfactory understanding of their nature.