Tectonic regime and slip orientation of reactivated faults

The slip orientation of reactivated faults is predicted by assuming that one principal stress direction is vertical, that faulting occurs on pre‐existing planes of weakness with a slip direction along the applied shear stress, and by extending the original concept of tectonic regime so as to include the stress tensor aspect ratio. The predicted rake of slip is then analysed so as to reveal the respective influence of the fault‐plane orientation (strike and dip) and of the tectonic regime. The analysis of this direct problem allows one to describe the geometry of the slip directions within any given tectonic regime, its evolution as the tectonic regime varies, and the constraints provided by slip‐direction data on the tectonic regime. However, the question of what fault orientations would satisfy a failure criterion and thus be preferentially reactivated is not addressed. In any tectonic regime, pure strike‐slip occurs on vertical planes and only on them, except in wrench regimes, where it also occurs on all fault planes along four strikes that are determined by the tectonic regime; pure dip‐slip occurs on planes striking along one of the horizontal principal stress directions and only on them, except in radial compression and radial extension regimes, where it occurs on all planes except vertical ones; at fixed strike but varying dip, the slip direction is closest to dip‐slip for zero dip, and pure strike‐slip for vertical dip; at fixed dip and varying strike, the slip direction is pure dip‐slip for strikes along one of the horizontal principal stress directions and closest to strike‐slip for four strikes that are determined by the tectonic regime; the horizontal principal stress directions separate four quadrants: two where the slip direction is dextral and two where it is sinistral. The slip directions of all fault planes of the same dip can be easily deduced from those of subhorizontal planes and can be constructed geometrically. As the tectonic regime varies continuously from radial compression to radial extension, the domain of fault‐plane orientations where steep reverse slip directions are possible shrinks and disappears when the extensional regime is entered; conversely, the domain where steep normal slip directions are possible appears when the wrench regime is entered and grows thereafter. The strike‐slip domain first grows towards shallow dip until the wrench regime is entered, and then shrinks after the extensional regime is entered. Whereas in the model of failure in isotropic rocks, dip‐slip or strike‐slip data fully determine the original tectonic regime, in the model of failure on pre‐existing planes of weakness, constraints on the extended tectonic regime require additional information. The sense of shallow slip direction data, which are to be found on steeply dipping planes, constrains the horizontal principal stress directions, and the steepness of slip‐direction data on shallow dipping planes striking away from the principal stress directions constraints the extended tectonic regime. In the case of wrench regimes, an extra constraint arises from the fact that the extended tectonic regime also controls the strike at which the dip component of slip changes from normal to reverse. These constraints can be quantified by superposing a simple plot of the data rake versus strike with an abacus that is called Breddin's graph for tectonic regimes; while this graph does not replace inverse techniques, it may help in detecting tectonic phases in complex data sets. Without using this graph or resorting to inverse techniques, the transition from wrench to either compressional or extensional regime is difficult to infer from data that display a limited range of fault‐plane strikes and the same sense of dip component of the slip direction.