Solution Exercise 9
a) The image below (modified from Dunne & Hancock, 1994) shows the orientation of the principal stresses with regard to a joint that bears a plume with a horizontal axis. The N30W/90 fracture bears a plume on its surface, which indicates that it propagated through the opening mode; thus, it is a joint. Because joints are perpendicular to σ3, this stress is horizontal and strikes N60E. The axis of the plume is horizontal and, given that σ1 is parallel to it (as shown in the image below), this stress is horizontal and strikes N30W. Because both σ3 and σ1 are horizontal, these fractures were formed under a strike-slip regime.
b) The EW/42SW fracture has a dip that is closer to the one of the thrust faults. However, the typical dip of these faults is 30o or less. So, more data, such as surface features (plume or striae, for example) and/or fracture pattern (conjugate or parallel) are needed in order to know the propagation mode, the principal stresses and the tectonic regime that produced this fracture.
c) It is not possible that both fracture sets have been formed during the same tectonic event, as the N30W/90 fracture was formed in the strike-slip regime and fractures of intermediate dip do not fit into any type of fracture pattern that is generated by that regime (Figure 30).