Solution Exercise 15
The image below schematically synthesizes the traces of the three subvertical fracture sets on the horizontal rock exposure described in Exercise 15.
Fracture sets 1 and 2 occur in a consistent association forming a conjugate pattern of vertical fractures typical of the strike-slip tectonic regime. The 60o angle between them indicates they are conjugate fractures propagated by shear.
In the strike-slip regime the maximum and minimum principal stresses, σ1 and σ3, are both horizontal; the former is parallel to the bisector of the conjugate fractures, which strikes N45E, and the latter is perpendicular to σ1 and its direction is N45W.
Fracture set 3 is comprised of persistent parallel fractures, which indicates that they are probably joints (opening mode fractures) that propagated perpendicular to σ3. Thus, the direction of this stress at the time the joints were formed was N45W. This orientation is consistent with the stress field that generated sets 1 and 2. Thus, it is reasonable to think that all three sets were generated under the same tectonic event (same tectonic regime and stress orientation). However, conjugate fractures and joints cannot be formed precisely at the same time because σ3 must be positive to form the conjugate sets 1 and 2 and negative to form set 3. This implies that the stress magnitude was variable during that tectonic event.
Fracture set 3 is persistent; and this coupled with the variable directions of sets 1 and 2 favors connectivity. Although set 3 is comprised of joints, presumably of greater aperture at the time they were formed, data on flow evidence would need to be collected in order to determine if this set currently bears significant groundwater flow. Systematic collection on flow evidence also needs to be performed for the other fracture sets.