*4.2. Event 2—Regimes 2 and 3*

In contrast to Regime 1, both Regimes 2 and 3 are dominated by normal fault extension (Figure 4). Most are dip-slip, which suggests a low level of structural inheritance and reactivation of earlier structures. Most fault surfaces are planar with relatively steep dips, which suggests that they developed at shallower crustal levels than the reverse faults of Regime 1. However, the gentle dips of some of the normal faults suggests the reverse faults of Regime 1 have been reactivated as normal slips. Note that the dominate trend of normal faults in Regime 3 are the same as for the reverse faults of Regime 1 (e.g., the fault poles are similar). Syntectonic quartz is common on the surfaces of these inherited normal faults, with the quartz probably inherited from Regime 1.

**Figure 4.** Regime 2 (**top**) and Regime 3 (**bottom**). Lower Hemisphere Equal Area projection of (**a**) grea<sup>t</sup> circles of fault planes; and (**b**) poles to fault planes (open circles) and of calculated trend and plunge (\*) of σ2. Symbols are as in Figure 3.

The two subsets of faults strike at right angles. The normal faults of Regime 2 strike NW-SE. and the normal faults of Regime 3 strike NE-SW. Regime 2 faults indicate NE-SW extension whereas Regime 3 faults indicate NW-SE extension.

The stress inversion of fault slip data for Regime 2 is stable (2 faults rejected at MIFL = 55%, no faults rejected at 20%, and only 4 of the 20 faults rejected at MIFL = 80%), but is only loosely constrained because of the limited number of data (18 acceptable faults). As with Regime 1, the numerical results are good, but the solution is highly dependent on the grouping of data. For example, removing the two nearly vertical faults results in a significantly different solution. The stress regime at MIFL = 55% indicates a 33◦ ± 10 degrees trending extension with oblique σ1, σ2 and σ3 axes. The σ3 axis plunges 33◦ NE, which is not surprising in light of the presence of nearly vertical faults with the downthrown side to the northeast. The Φ ratio of 0.46 indicates triaxial stress.

In contrast, the large number of fault slip data in Regime 3 provides a highly constrained stress tensor solution. The solution is stable (13 of 75 faults rejected at MIFL = 40%, 4 at 20% and 20 at 55%). The stress orientations and Φ are similar regardless of the MIFL, which indicates that the stress tensor is well constrained. On the other hand, the slip vectors have a large scatter (Figure 4). The number and geometrical variety of the data are more important than the average of parameter estimates and their standard deviations. Removal of fault slip data does not change the inversion results within the range of uncertainties, which confirms that geometrical constraints on the stress tensor exerted by the variety in fault slip attitudes is more important to a good interpretation. At MIFL = 40% the σ1 axis is nearly vertical and indicates a 110◦ azimuth of extension (the σ3 axis plunges only 3◦ to the west). The direction of extension is constrained within ±5 degrees. A Φ ratio of 0.49 indicates typical triaxial stress.

No clear chronological difference could be established between Regimes 2 and 3. The perpendicularity in fault trends and corresponding directions of extensions strongly sugges<sup>t</sup> that these two regimes are linked through a permutation (relative magnitude switch) between the intermediate and minimum principal stresses, σ2 and σ3. These two regimes thus probably belong to a single major extensional event which we identify as Event 2. Because Regime 3 is represented by a much larger number of brittle

structures than Regime 2, the dominating direction of extension is inferred to be WNW-ESE (azimuth 110◦). A tensor inversion with Regimes 2–3 taken together shows that the influence of the fault slip data from Regime 3 prevails, and the combined tensor solution resembles that of Regime 3. For a MIFL = 30% the Φ ratio is lower (0.35) but the direction of extension is similar (115◦). The stability of the solution is much less, which suggests the distinction between Regimes 2 and 3 is in fact significant in terms of stress states, even though both are produced by the same tectonic event. Brittle tectonic analyses have revealed significant changes in stress regimes within a single tectonic episode [4–6,13,15]. The duality of stress regimes (2 and 3) may simply result from a permutation between the σ2 and σ3 axes, a common phenomenon in fault tectonics.

Although there remains some indication of ductile-brittle transition for some faults with abundant quartz coating and slip-parallel quartz growth, most Event 2 faults are typically brittle, as shown by both the fault surface characteristics and their steep dips. Relative chronologies with respect to other events show that Regimes 2 (certainly) and 3 (probably) post-dated the Regime 1. Our data thus sugges<sup>t</sup> that the extension of Regimes 2–3 post-dated the compression of Regime 1 and suggests that Regimes 1–3 reflect the oldest two faulting events well represented at the site.
