*4.1. Event 1—Regime 1*

This regime is characterized by reverse faults compatible with a NW-SE compression (Figure 3). The frequent presence of quartz veins and along-slip quartz growth indicate that slip probably occurred close to the ductile-brittle transition. The relatively deep and hot character of this tectonic deformation suggests that this event is the oldest event.

**Figure 3.** Regime 1. Lower Hemisphere Equal Area projection of (**a**) grea<sup>t</sup> circles of fault plane orientation, and (**b**) poles to fault planes (open circles) and (\*) the calculated trend and plunge of σ2. The arrows indicate the rake of slickenlines on the fault planes and point in the direction of slip. Arrows pointing toward the center of the projection indicate compression, those pointing away from the center indicate extension. Solid circles with two arrows pointing in opposite directions indicate the sense of strike-slip movement. Those with no arrowheads have an indeterminant sense of movement. M = magnetic north, N = true north.

The stress inversion of fault slip data for this data subset shows that for a reasonable threshold, MIFL = 40%, only 2 of the 19 fault slip data are considered unacceptable. Similar solutions were obtained for minimum individual fit levels of 20% (no data being eliminated) or 40% (4 data eliminated). The stress regime determined is thus stable.

The calculated stress regime indicates a nearly horizontal compression that trends 133◦ N. The stress axes σ2 and σ3 are oblique, with plunges of 45◦, in agreemen<sup>t</sup> with the low value, 0.13, for the ratio Φ = (<sup>σ</sup>2 − <sup>σ</sup>3)/(<sup>σ</sup>1 − σ3). This low Φ reveals σ2 and σ3 are closer in magnitude than either is to σ1. The solution cannot be considered very accurate because the number of acceptable fault planes inversions is small (17). The direction of compression is constrained within ±10 degrees, but the values of Φ and the attitudes of stress axes σ2 and σ3 may vary widely as a function of data removal within this set. In summary: the oldest brittle tectonic episode that we can recognize corresponds to a NW-SE compression that reactivated deep fractures in reverse faulting.
