Solution Exercise 10
The N62E/74SE fracture bears a plume which indicates that it propagated through the opening mode; therefore, the fracture is a joint (extension fracture) and σ3 is perpendicular to it, close to horizontal and striking at around N30W. Exercise 8 shows that σ1 is parallel to the axis of a plume, so σ1 should approach vertical. This implies that the tectonic regime is extensional (σ1 is vertical and σ3 is horizontal). However, the typical dip of joints of the Andersonian extensional regime is vertical. Thus, assuming that σ1 is vertical, the fracture makes an angle of ~15° with this stress. This implies that it is a hybrid fracture, formed by both opening and shear modes.