Solution Exercise 12
a) The dip of set 1 and the presence of striae lineation parallel to its dip are consistent with faults of a compressive regime; thus, they propagated through shear mode. In a compressive regime the maximum principal stress (σ1) is horizontal and strikes perpendicularly to the direction of the faults; this corresponds to an average direction of N70W for σ1. Also, σ3 is vertical and σ2 is parallel to the direction of fracture set 1.
b) Fracture set 2 bears a plume, which indicates a propagation by the opening mode, so the fracture is a joint; it is subvertical and, consequently, σ3 is horizontal and strikes N45E (perpendicular to the direction of the fracture set). Subvertical joints can be formed by both extensional and strike-slip regimes. The axis of the plume, although irregular, is approximately horizontal, so σ1 is likely horizontal and in the same direction as the fracture. Therefore, the associated tectonic regime is of the strike-slip type.
c) The existence of active fluid flow along the fractures of set 1 is indicated by the presence of vegetation, oxidation and, mostly, by the weathering of the rock. The surfaces of fracture set 2 are oxidized but, considering that the oxidation process can take place on any surface that is exposed to the atmosphere, this flow evidence is likely not as reliable as weathering of the rock. Based on the flow evidence (weathering, vegetation, and oxidation), it is likely that set 1 has higher K when compared to set 2; thus, set 1 is probably a preferential pathway for groundwater flow. Set 2 crosscuts set 1 (upper part of photograph (a) in Exercise 11), providing a connected network. Subhorizontal fractures of set 1 could form aquifer intervals that may be vertically connected by set 2. The extent of these connections depends on the persistence and density of set 2 fractures.