THE NATURE AND ORIGIN OF DETACHMENT FAULTS

The overall sense of uncertainty associated with the nature and origin of detachment faults points to a need to re-examine the basic characteristics of detachment faults. They are typically structures that are not associated with fault gouge or vastly disrupted breccia. The detachment surface itself is always one of the youngest structural features observed in the extensional sequence, and they cut cleanly through all previously formed structures. This raises the question as to how the fracture might propagate. The answer is that there is abundant evidence that detachment faults involve subcritical propagation of fractures, in particular at the microscale where the detachment surface itself is often “knife-sharp” transecting all previously formed structures. This observation allows rationalization of many enigmatic aspects of the detachment fault problem. Some detachment faults are rotated high-angle normal faults, and as is evident in the Gulf of Corinth, these may continue to be seismically active down to low dip angles (~20°). Such structures are rotated LANFs and sensu stricto, they are not detachment faults. Detachment faults form at gently dipping, even subhorizontal orientations as the result of sub-critical failure in ductile shear zones as they begin to pass through the ductile-brittle transition. These faults appear to propagate at low-dip angles, and continue to operate at low dip-angles. This may be assisted by lenses of over-pressured fluid that create horizontal fractures, and then later movement. Similarly there may be reactivation of segments of existing structures such as thrusts, and the propagation of LANFs governed by these pre-existing weaknesses. Lithospheric dislocations that reverse their shear sense appear to be an important aspect of tectonic environments in which LANFs are typically discovered. Overall, however, detachment faults, particularly those of continental extent, may simply be the result of sub-critical propagation of fractures along crustal-scale shear zones as they pass through the ductile-brittle transition. Large-scale ductile structures allow rotation of the axes of deviatoric stress though large extants of the crust, and under these circumstances LANFs of large areal extent (detachment faults) may form during continental extension.