5.3 Fracture Network Conceptual Models for Metamorphic and Intrusive Igneous Rocks

In general, the non-stratabound fracture network model is applicable for intrusive and metamorphic rocks. These rocks have highly variable fracture (and fracture zone) spacings. Fracture lengths are also very variable and with lognormal distributions (e.g., Rouleau & Gale, 1985). In metamorphic rocks, the hierarchical model can also be present because part of the fractures may abut against FPFs (foliation parallel fractures), and these generally comprise a prominent fracture set. This pattern and the throughgoing fractures may form a dense and connected fracture network. Zones of persistent and closely spaced fractures are common in metamorphic and igneous intrusive rocks and can impart high fracture connectivity and transmissivity (Paillet et al., 1987). Fractures that result from reactivation of foliation, veins and other previously existing structures can contribute significantly to groundwater flow. The importance of subhorizontal fractures for flow at shallow depths (i.e., a few tens of meters) has been described in several regions (Manda et al., 2008; Boutt et al., 2010; Fiume, 2013; Fernandes et al., 2016b). Reactivation of subhorizontal pre-existing structures at small normal stresses, as described in Section 3.5, is a likely explanation for this phenomenon. The subhorizontal sets become hydraulically non-active at depth, and fractures with dips greater than 40° support flow at greater depths (Boutt et al., 2010). Numerical modeling experiments conducted by Manda and others (2008) show that FPFs account for increases of 20 to 30 percent of flow in fracture networks. However, in the numerical simulations, the same aperture was assigned for all fracture sets and types, and the authors report that this was done because fracture apertures are subject to local variations in stress fields and rock properties. We suggest that a systematic field survey of features that indicate flow could be used to assign different apertures and different percentages of open fractures to each fracture set. Such a survey can be conducted along scanlines in large outcrops, as suggested by Fernandes and others (2016b) and Fiume and others (2020), as well as in well image records (Section 4.4). Wells in metamorphic rocks, when compared to those in massive rocks such as non-foliated granites, tend to present higher specific capacity values; this is likely due to the positive influence of FPFs in these aquifers (Fernandes et al., 2016b).

Granites may bear the same regional fracture sets that are present in metamorphic rocks. However, different fracture patterns, probably related to thermal contraction and tectonic stresses during uplift, are to be expected in these rocks. The practical implication is that the regional fracture sets observed in the host metamorphic rocks cannot be assumed to be present in the intrusive rocks, and vice versa.

Both plutonic and metamorphic rocks may fracture in sheet or exfoliation joints that form parallel to the Earth’s surface at depths generally less than 100 m. They are typically subhorizontal but may have dips depending on the variability of surface topography. Near-surface joints are often most significant for water storage and movement.