*4.4. Event 3—Regime 5*

Regime 5 (Figure 6) is better represented than Regime 4. The stress tensor inversion indicates strike-slip faulting consistent with a nearly E-W compression and N-S extension. These strike-slip faults clearly postdate the reverse faults of Regime 1. The stress tensor inversion is stable (1 of 24 faults rejected at MIFL = 55%, none rejected at 25%, and 7 rejected at MIFL = 70%). Another indication of inversion stability is that the removal of dextral fault slips does not significantly modify the inversion results.

**Figure 6.** Regime 5. Lower Hemisphere Equal Area projection of (**a**) grea<sup>t</sup> circles of fault planes and (**b**) of poles to fault planes (open circles) and of calculated trend and plunge (\*) of σ2. Symbols are as in Figure 3.

The stress regime calculated at the MIFL = 55% level is characterized by a nearly horizontal σ3 axis that trends 0◦ and a gently plunging σ1 axis whose azimuth trends 83◦. The direction of extension is not more tightly constrained than ±10 degrees because only 2 left-lateral faults were measured. A typical triaxial stress is indicated by the Φ ratio of 0.50.

Contrary to the typical, most strike-slip faults of Regime 5 are far from vertical. Many of the right-lateral faults dip towards the NW or SE, which suggests that they were inherited from the normal fault planes of Regime 3. Two left-lateral faults have gentle SW dips, which suggests that they were inherited from normal fault planes of Regime 2.

Although the faults of Regime 4 are reverse and the faults of Regime 5 are strike-slip, the directions of compression are similar considering the large angular uncertainty in the trend of compression of Regime 4. For this reason, we combine Regimes 4 and 5 within a single Event 3 that is dominated by a roughly E-W compression and can generate both reverse and the strike-slip faulting. The stress tensor for this combination resembles that for Regime 5 because of the larger number of faults in Regime 5. Unlike the combination of Regimes 2 and 3, the combination of Regimes 4 and 5 shows good stability (1 of 30 faults eliminated for MIFL = 20%, 9 for 55%). But the rejected faults are 2 of the 5 reverse faults of Regime 4, and the individual misfits of the three remaining Regime 4 reverse faults are large. The simplest solution suggests mixing Regimes 4 and 5 are indeed distinct.

For a reasonable fit level of 35% (2 faults rejected), the Φ ratio is 0.43, indicating triaxial stress despite the mixture of strike-slip and reverse faults. The stress regime is characterized by a gently plunging σ3 axis with a trend azimuth of 351◦ and a nearly horizontal σ1 axis with a trend of 83◦. The data indicate stress regimes 4 and 5 belong to a single event dominated by WNW-ESE compression and that a permutation between σ2 and σ3 changes the faulting from reverse to strike slip.

### *4.5. Event 4—Regimes 6, 7 and 8*

The strike-slip faults of Regimes 6–7 are shown and analyzed together (Figure 7). These regimes are dominated by strike-slip faulting. Sixty-nine faults are observed, the largest ones forming typical strike-slip zones composed of two walls on either side of a 1–3 m wide deformed zone with numerous smaller faults, fractures, rotated blocks, and gouge. The strike-slip faults strike approximately NNW-SSE for right-lateral faults, and NNE- SSW for left-lateral ones, indicating N-S compression.

**Figure 7.** Regime 6–7. Lower Hemisphere Equal Area projection of grea<sup>t</sup> circles of fault planes (**a**); and (**b**) of poles to fault planes (open circles) and of calculated trend and plunge (\*) of σ2. Symbols are as in Figure 3.

The stress tensor solution is tightly constrained by the large number of the faults and the variety of their orientations (17 faults rejected at MIFL = 45%, 5 at 20%, and 23 at 55%). The stress orientations remain extremely stable as the MIFL increases. In addition, removal of fault slip data does not significantly a ffect the inversion. The geometrical constraints exerted by the variety in fault slip attitudes are strong. Because a significant overlap in stress trends is present between right-lateral and left-lateral faults, several data displayed incompatible senses of motion. This explains why faults were eliminated even for low levels of MIFL. Separation into two Regimes, 6 and 7, solved this problem and reduced the number of inconsistent senses to zero for each of the stress tensors, but was not retained because no independent qualitative evidence supported a separation of these Regimes.

The stress regime calculated at MIFL = 45% is characterized by gently plunging σ1 and σ3 axes (plunges of 14◦ and 17◦ degrees respectively), with a nearly N-S trending, azimuth 188◦, compression. This direction of compression is constrained within less than ± 5◦. The Φ ratio of 0.45 indicates typical triaxial stress.

Relative chronology data provides good evidence that this major strike-slip event postdated the normal faults of Event 3. Although some strike-slip faults of Regime 6 and 7 have relatively gentle dips suggesting that they were inherited from earlier regimes of reverse and normal faults, most of these strike-slip faults are vertical or steeply dipping, cutting through all pre-existing structures rather than reactivating them. It is likely that several NE-SW trending faults result from right-lateral reactivation of the left-lateral faults of Regime 5, but observation is speculative because of the right-lateral friction that generally destroyed the criteria supporting the evidence of an earlier left-lateral motion.

Regime 8 is represented by only a few dip-slip reverse faults (Figure 8). The relative chronology data indicate that this regime occurred before the Regimes 6 and 7. As with Regime 4, the stress inversion provides very stable solutions, but this stability is not significant because the number of faults is so small. The tensor solution is in fact poorly constrained. For MIFL = 50%, all data are acceptable, and the calculated stress regime indicates an azimuth 330◦ compression with a nearly horizontal σ1 axis, a steeply plunging σ3 axis and a Φ ratio of 0.23. The direction of compression is constrained within ±20◦.

Because the direction of compression suggested by this pattern of reverse faults is not far from N–S (with an azimuth of compression approximately 160), they may be related to regimes 6–7 through a relative magnitude shift between the intermediate and minimum stress axes. If Regime 8 is added to 6 and 7 the inversion rejects all 4 faults in Regime 8. As in the case of Regimes 2–5, this suggests a common tectonic event involving a stress permutation between σ2 axis and σ3 axes. There is no evidence that Regime 8 resulted from a separate tectonic event.

**Figure 8.** Regime 8. Lower Hemisphere Equal Area projection of (**a**) grea<sup>t</sup> circles of fault planes and (**b**) of poles to fault planes (open circles) and of calculated trend and plunge (\*) of σ2. Symbols are as in Figure 3.

Event 4 comprising Regimes 6–8 was certainly more recent than the reverse and normal faults of the ductile-brittle transition (compression of Regime 1, and the extension of Regime 2). The contacts of the diabase dikes of Jurassic age are reactivated as strike-slip faults of Regimes 6 and 7 indicating that the faulting and diabase dike intrusion in these regimes occurred 200–146 Ma or later. The NW-SE extension is compatible with the regional extension (based on local NE-SW diabase dike trends, [24]) affecting the study area during the initial opening of the north Atlantic Ocean 200–175 Ma [34], Figure 5.
