When is a fault ‘extinct’?

Abstract

Abstnct: Existing regulatory definitions of time-periods over which evidence should be searched to demonstrate whether a fault is, or is not, active may be unconservative in slowly deforming continental regions where recurrence intervals of surface faulting are in excess of 100 OOO years. Selfevidently, no definition of an active fault can be satisfactory when founded on some extrinsic time-period. Instead, it must be derived from the properties of the system itself, preferably through knowledge of fault-movement recurrence intervals, or alternatively and conservatively from an understanding of the time-period over which the present conditions of regional stress and strain (the 'current tectonic regime') have prevailed. Borrowing from volcanological terminology, out of the definition of the 'active' fault emerges a n antithesis: the 'extinct' fault. Time-periods by which directly dated faults are designated as active or extinct have to be locally derived.

Constraining the timing of fault reactivation: Eocene coseismic slip along a Late Ordovician ductile shear zone (northern Victoria Land, Antarctica)

“…Results therefore again highlight the inherent diffi culty in investigating structures characterized by long periods of tectonic quiescence, particularly those located in intraplate regions, for which an accurate evaluation of recurrent periods of fault movement is precluded. Under these circumstances, the key issue in assessing seismic hazard is whether a fault is healed and extinct or just temporarily inactive and awaiting reactivation through an appropriate stress regime (Muir Wood and Mallard, 1992;Butler et al, 1997). The present study confi rms and reinforces previous conclusions by providing compelling evidence showing that when dealing with fault activity and reactivation processes in polydeformed rocks, faults should be classifi ed as active or extinct only after gaining a full under standing of the time period over which the current conditions of regional stress and strain have prevailed (the "current tectonic regime" of Muir Wood and Mallard, 1992).…”

Contribution of active faults in the intraplate area of Iberia to seismic hazard: The Alentejo-Plasencia Fault

“…In this study, we define active faults as faults that have moved within the "current tectonic regime" (Muir- Wood and Mallard, 1992). The current tectonic regime is the period in which the tectonic stresses have remained constant (stress axis orientation and type of tectonic regime) and deformation can be assumed to be constant.…”

Dextral strike-slip faulting history of the mid-cretaceous d'abbadie fault zone, Pelly mountains, South-Central Yukon, Canada

“…Reactivation is defined as the accommodation of geologically separable displacement events (intervals > 1 Ma) along pre-existing structures (Holdsworth et al 1997) or along structures that formed prior to the onset of the current tectonic regime (Muir Wood and Mallard 1992). Several criteria based on stratigraphic, structural, geochronological and neotectonic evidence (Holdsworth et al 1997) all constitute reliable geological evidence that may be used to recognize the potential for a fault to…”