Solution Exercise 17
a) Fracture sets 1 and 2 have the same average direction, N20E, and dip toward opposite senses, NW and SE, respectively. These characteristics alone resemble both the conjugate fractures of thrust and normal faults. However, these sets are approximately perpendicular (they make an angle of roughly 80° to 90°) to each other and this is an indication that they do not form a conjugate pair.
Set 1 is formed by parallel fractures as shown in the photographs of Exercise 17, although the dip varies by around 20o from line 1 to line 2. The parallelism is a pattern typical of joints formed by the opening mode, but the dips do not fit any of the Andersonian tectonic regime. Consequently, none of the stresses that generated set 1 are vertical. As we have seen, joints have to be perpendicular to σ3, so set 1 was generated perpendicularly to a σ3 that dips around 45° toward S70E.
Fracture set 2 is also formed by subparallel fractures, so we assume that they are also joints. At the time of the formation of set 2 (average orientation = 20E/40SE), σ3 dipped around 50o toward N70W.
The subvertical dips and subhorizontal striae lineation of set 3 (average orientation = N70W/88SW) are both consistent with strike-slip faults of the strike- slip tectonic regime where σ2 is vertical. The presence of gouge zones (the original rock was finely ground) indicates that the deformation was intense. The shear sense is not known, so we can only say that σ1 is horizontal and can be either ~N40W, in the case where the faults are dextral, or ~N80E, in the case where they are sinistral.
b) Sets 1 and 2 abut against each other, and the σ3 necessary to generate set 1 is perpendicular to the one that generated set 2. Both characteristics together suggest that they were formed by a single tectonic event in which the σ3 orientation probably alternated with the orientation of σ2. This is similar to the gridlock joint pattern, although in this pattern both joint sets are vertical and the tectonic regime is Andersonian.
The fact that sets 1 and 2 cut through set 3 is, by itself, an indication that they are younger. The deformation caused by the faults of set 3 is intense and forms a gouge. Thus, if sets 1 and 2 were older, they would have been strongly disturbed by the faults.
c) The three fracture sets provide a well-connected fracture network due to the following characteristics: set 1 is formed by closely spaced and persistent fractures (the fractures continue beyond the rock wall exposures), with frequent fracture zones; fractures of set 1 are well connected to sets 2 and 3, as they are approximately orthogonal; and all sets occur in both lines. Weathering along fracture set 1 as shown in photograph (a) of Exercise 17 is evidence of flow in present time. This suggests that flow could predominantly take place along set 1 fractures. Because of the low dips of the fractures of this set, it is likely that an important horizontal flow component toward WNW (dip sense of set 1) takes place. A thorough analysis of the flow evidence of all the fracture sets should be carried out in order to develop a more complete understanding on the preferential flow paths.
Regarding groundwater extraction, this location seems to have a favorable potential. The low dips of set 1 and 2 increase the probability that vertical wells will intercept a large number of fractures, mainly those of set 1 because of its density. This situation improves the chance of intersecting high-K fractures.