Conclusions
The stress orientations from cooling joints in the mid-Tertiary granite give the same ?3 direction as the striated faults along the detachment surface. Therefore, the stress orientation remained constant during the time the granite was emplaced through the time the detachment fault formed.
If ?1 is vertical and the fault is low angle, how did it move? The most likely reason for failure in this case was an increase in fluid pressure and/or a significant decrease in friction, perhaps due to fault gauge or clays along the fault zone.