Division into fracture domains

Already during the single-hole interpretation work (see section 5.2.2), it was recognised that the upper part of the bedrock inside the tectonic lens at Forsmark contains an increased frequency of sub-horizontal and gently dipping, open and partly open fractures. A systematic assessment of the variation in the frequency of particularly open and partly open fractures with depth along each borehole (p. 30–32 and Appendix 3 in /Olofsson et al. 2007/ and section 5.2.5 in this report) contrib-uted significantly to the division of the bedrock between deformation zones into fracture domains /Olofsson et al. 2007, p. 33–39/. The intervals along which each fracture domain is inferred to be present in each cored borehole were illustrated in borehole logs /Stephens et al. 2007, Appendix 13/.

The allocation of a borehole section to a particular fracture domain was carried out as a working hypothesis for the subsequent statistical modelling of fractures and minor deformation zones at the site. Furthermore, on the basis of these borehole data, a 3D geometric model for four of the six fracture domains (FFM01, FFM02, FFM03 and FFM06), inside the target volume, was constructed (/Olofsson et al. 2007, p. 40–43/ and Figure 5-34). Since few data are available outside this volume and the character of fracture domains FFM04 and FFM05 are poorly constrained, no model for these two fracture domains could be constructed /Olofsson et al. 2007/. Fracture domains FFM01, FFM02, FFM03 and FFM06 are addressed in the DFN modelling work /Fox et al. 2007/ and are described briefly in the following text.

Fracture domain FFM01

Fracture domain FFM01 (Figure 5-34) is situated within rock domain RFM029 inside the target volume. It lies north-west of the steeply dipping zone NE0065, beneath the gently dipping or sub-horizontal zones A2, A3 and F1, and beneath a depth that varies from c. 40 m (large distance from zone A2) to c. 200 m (close to zone A2).

Relative to the overlying fracture domain FFM02, the bedrock in this domain shows a lower frequency of open and partly open fractures. Furthermore, gently dipping or sub-horizontal fracture zones are also uncommon inside this part of rock domain RFM029. It is apparent that the character of this domain is favourable to a local accumulation of high in situ rock stresses (see chapter 7) at one or more times during geological history, in connection with, for example, sedimentary loading processes /SKB 2006a, Stephens et al. 2007, p. 155/.

Vertical or steeply dipping fractures that strike ENE to NNE and NNW, as well as gently dipping to sub-horizontal fractures are conspicuous in this domain (Figure 5-34). Fracture minerals are dominated by calcite, chlorite, laumontite, adularia, hematite and quartz (Figure 5-34). Both epidote (older generation) and laumontite are present along the different sets of fractures (Figure 5-35a) and reactivation of these fractures under different metamorphic conditions is apparent. However, laumontite is more conspicuous in the steeply dipping compared with the gently dipping to sub-horizontal fractures (Figure 5-35a). By contrast, clay minerals, which belong to a younger generation of minerals, are present most conspicuously along the gently dipping and sub-horizontal fractures (Figure 5-35a). All these features are similar to those observed in the ENE (NE) sub-set of fracture zones that is also prevalent in this part of the target volume (cf. Figure 5-29).

Fracture domain FFM02

Fracture domain FFM02 is situated close to the surface inside the target volume, directly above fracture domains FFM01 and FFM06 (Figure 5-34). It is characterised by a complex network of gently dipping and sub-horizontal, open and partly open fractures, which, conceptually, are considered to merge into minor fracture zones. The gently dipping and sub-horizontal fractures are oriented at a large angle to the present-day minimum principal stress in the bedrock (see chapter 7 and Figure 5-36). This relationship favours the reactivation of older, gently dipping fractures as extensional joints, the formation of new sub-horizontal sheet joints and the overall development of

Figure 5‑34. Three-dimensional model for fracture domains FFM01, FFM02, FFM03 and FFM06 in the north-western part of the Forsmark tectonic lens, viewed towards the ENE. The local model block is shown in pale grey. The gently dipping and sub-horizontal zones A2 and F1 as well as the steeply dipping deformation zones ENE0060A and ENE0062A are also shown (after /Olofsson et al. 2007/). The orientation of fractures inside the different fracture domains are plotted as poles to planes in stereographic projections (equal-area, lower hemisphere) and contoured. In order to limit the bias that is related to the orientation of boreholes, a Terzaghi correction has been applied in these plots. The fracture mineralogy in each domain is also shown and the order of mineral presentation reflects the order of abundance (based on /Sandström et al. 2008, section 6.1/).

Fracture mineralogy

calcite-chlorite-laumontite-adularia-hematite-quartz

Fracture mineralogy

calcite-chlorite-laumontite-adularia-hematite-(clay minerals-pyrite-quartz-asphaltite-goethite) and “no mineral”

Fracture mineralogy

calcite-chlorite-(adularia-prehnite-hematite-laumontite-quartz-clay minerals-pyrite) and “no mineral”

Fracture mineralogy

chlorite-calcite-adularia-hematite-laumontite-quartz

Fracture domains

(Figure 5-34). However, clay minerals, pyrite, asphaltite and goethite, which all belong to the younger generations of minerals, are more conspicuous in fracture domain FFM02 relative to all the other domains. Furthermore, fractures without any mineral coating or filling (“no mineral”) are also prominent in FFM02. However, there remain some uncertainties concerning the mapping of these fractures (see section 5.2.5 and /Stephens et al. 2007, p. 90/), and a complementary investigation has been initiated to shed more light on these structures. Once again, the older minerals epidote and laumontite, which formed at different times in the geological evolution, occur along the three different sets of fractures in FFM02 (Figure 5-35b), while clay minerals (Figure 5-35b), pyrite and goethite in the younger generations and “no mineral” are most conspicuous along the gently dipping to sub-horizontal fractures.

Figure 5‑35. Orientation of fractures with epidote, laumontite and clay minerals, i.e. from three different mineral generations, inside the fracture domains. a) FFM01. b) FFM02. c) FFM03. d) FFM06. The orien-tation of the fractures in each figure is shown as poles to planes in a stereographic projection (equal-area, lower hemisphere). In order to limit the bias that is related to the orientation of boreholes in the contoured plot, a Terzaghi correction has been applied in each of these plots.

b

c

d a

These observations provide support to the conceptual understanding of the evolution of the site in the brittle regime presented earlier (section 5.5.2). Whereas the effects of ancient tectonic processes

Fracture domain FFM03

Fracture domain FFM03 (Figure 5-34) is situated within rock domains RFM029 and RFM017, south-east of and outside the target volume. In particular, it is inferred to be present above zone A2 in borehole KFM02A and along the whole length of the boreholes KFM03A and KFM03B to the south-east of the local model volume. The rock domains in this volume are characterised by a high frequency of gently dipping fracture zones containing both open and sealed fractures. This structural feature inhibited the local build-up of rock stresses in connection with, for example, sedimentary loading processes /SKB 2006a, p. 121–126/. By corollary, the development of a significant stress-release fracture domain close to the surface, with the characteristics of domain FFM02, is also not favoured.

Vertical or steeply dipping fractures that strike ENE to N-S and NW, as well as gently dipping to sub-horizontal fractures are present in this domain (Figure 5-34). Calcite and chlorite dominate along the fractures; adularia, prehnite, hematite, laumontite, quartz, clay minerals and pyrite are also present (Figure 5-34). The subordinate mineral epidote occurs along the steeply dipping NW set of fractures and the gently dipping to sub-horizontal set, while laumontite is common along the steeply dipping ENE to N-S set of fractures (Figure 5-35c). It is inferred that at least these two generations of fracture minerals occur in different sets of fractures in this domain. Once again, clay minerals in the younger generation of minerals occur predominantly along gently dipping fractures (Figure 5-35c). Apart from some difference in the relative importance of steeply and gently dipping fractures, these features are strongly reminiscent of the orientation and mineralogy of the fractures in the contiguous, gently dipping fracture zones (cf. Figure 5-30a).

Figure 5‑36. Two-dimensional cartoon that illustrates the conceptual model for the possible reactivation of fractures with different orientations as extensional joints in the current stress regime (Quaternary). The orientations chosen correspond to the different sets of deformation zones (see section 5.7). The black line along the gently dipping structures indicates a considerable change in aperture; the dark grey line indicates a moderate change, and the pale grey line restricted change. These effects are most conspicuous close to the surface in fracture domain FFM02 and, particularly close to ancient gently dipping fracture zones.

Sub-horizontal sheet joints may also have formed during the Quaternary, especially in this fracture domain.

Quaternary (after c. 2 Ma)

Minimum principal stress=vertical

Maximum principal stress=140°, horizontal

Generation 4 minerals Clay minerals-calcite Also formation of open, sheet joints without minerals or filled by glacial sediment close to surface

Gently dipping Gently dipping

Fracture domain FFM06

Fracture domain FFM06 is situated within rock domain RFM045, inside the target volume. It resembles fracture domain FFM01 in the sense that it lies beneath both zone A2 and fracture domain FFM02 (Figure 5-34). It is distinguished from domain FFM01 by the widespread occurrence of fine-grained, altered (albitized) granitic rock, with slightly higher contents of quartz compared with unaltered granitic rock, i.e. on the basis of lithological characteristics.

Two sets of vertical or steeply dipping fractures that strike NE to N-S and NW to WNW, as well as gently dipping to sub-horizontal fractures are conspicuous in this domain (Figure 5-34). Fracture minerals are dominated by chlorite, calcite, adularia, hematite, laumontite and quartz (Figure 5-34).

Epidote is relatively uncommon along the fractures in this domain (Figure 5-35d). The more common mineral laumontite is predominantly found along the steeply dipping NE to N-S fractures (Figure 5-35d). However, some occurrences are also present along the fractures with other orienta-tions. Clay minerals, in the youngest generation of minerals, are present in both the steeply and gently dipping fractures (Figure 5-35d). These features are very similar to those observed in the NNE sub-set of fracture zones that is also prevalent in this part of the target volume (cf. Figure 5-29).