Gently dipping fracture zones
5.7 Integrated geological model
An attempt has been made in Figure 5-41 and Figure 5-42 to present rock domains, deformation zones and fracture domains, at the Forsmark site, in an integrated geological model. The upper part of Figure 5-41 shows the different rock domains in a regional perspective, and all the deformation zones with trace lengths at the ground surface longer than 10 km. The lower part of this figure shows two vertical profiles across the north-western part of the candidate volume, i.e. the target volume, Table 5‑8. DFN model for fracture domain FFM01. r0‑fixed model alternative.
Domain Set ID Trend Plunge Kappa r0 kr P32 [r0-∞]* P32 [1-564]*
FFM01 NE 314.9 1.3 20.94 0.039 2.72 1.74 0.17
NS 270.1 5.3 21.34 0.039 2.75 1.29 0.11
NW 230.1 4.6 15.70 0.039 2.61 0.95 0.13
SH 0.8 87.3 17.42 0.039 2.58 0.63 0.09
FFM06 NE 314.9 1.3 20.94 0.039 2.79 3.30 0.26
NS 270.1 5.6 21.34 0.039 2.78 2.15 0.17
NW 230.1 4.6 15.70 0.039 2.66 1.61 0.18
SH 0.8 87.3 17.42 0.039 2.58 0.64 0.09
* Match P32 represents the correlated size-intensity at which both the outcrop trace data and the borehole intensity data are matched, i.e. the size-intensity model will simultaneously fit the outcrop and the borehole data. The match P32 is valid from the minimum radius of the distribution (r0) to infinity. Note that in all calculations for Forsmark stage 2.2, infinity is assumed to be equal to 1·1031 m. Match P32 is equivalent to the arithmetic mean value of P32 for a given fracture domain, calculated from 6 m length intervals, outside all deformation zones, and excluding intervals marked as ‘Affected By DZ’.
Figure 5‑39. Realisation of the TCM alternative model inside a 50 m3 cubic volume in fracture domain FFM01. See text for further description.
Figure 5‑40. Realisation of the r0-fixed alternative model inside a 50 m3 cubic volume in fracture domain FFM01. See text for further description.
NE set NS set NW set SH set
±
c b
a
Kamb. Exponential
Start: 4, Cl=2 Total Data: 31622 Equal Area
Total Data: 45 Equal Area
4 sig.
16 sig.
28 sig.
40 sig.
52 sig.
Lower Hem.
Lower Hem.
N N
NE set NS set NW set SH set
±
c b
a
Kamb. Exponential
Start: 4, Cl=2 Total Data: 5192 Equal Area
Total Data: 10 Equal Area
4 sig.
12 sig.
20 sig.
28 sig.
36 sig.
Lower Hem.
Lower Hem.
N N
Figure 5‑41. Integrated geological model for rock domains and deformation zones at the Forsmark site in a regional model scale perspective. The reader is referred to the legend in Figure 5-24a for an explanation of the different colours in the 3D regional model in the upper part of the figure (view to north) and in the 2D profiles in the lower part of the figure. These colours represent the dominant rock type in each rock
RFM029R
0 m
-2100m
- 500 m elevation
RFM023
RFM025
RFM029R
RFM034
RFM020 K1
A1
J2 1203
WNW 0001
RFM026
NW 0003 NW 0017 RFM018 RFM012
B8
NW 1200
RFM043
240 (WSW)
(1633648/
6701896) (1629095/
6699147)
060 (ENE)
WNW 0835B RFM021
RFM021 NW 0805 (Zone 8) WNW 0836
871 (Zone H2) HFM23 HFM20
HFM21 KFM08B
KFM09B
KFM07A KFM07CKFM07B
KFM08A KFM08D
KFM08C
KFM11A
KFM09A
HFM38 HFM33
HFM34 HFM35 HFM28
HFM22
Eckarfjärden DZ
Singö DZ
RFM044
WNW 0809A
RFM032
RFM034
RFM017
RFM032
A1
A6 A5 A4 A7
A3 A2 A3
A8A2 B7
B1
B6 B5 B23
B4
E1
F1 F1
RFM029R KFM08A
KFM08B
KFM08D KFM08C
KFM06A KFM06C
KFM06B HFM04
-2100 m 0 m
ENE 0062A
NE 0065
WNW 0023 WNW 0123
ENE 0060A ENE 0810
866
NNW 0101
310 (NW)
(1636032/
6696650) (1630380/
6701267)
130 (SE)
- 500 m elevation
KFM03A KFM02A
KFM02B
HFM07 HFM06
KFM03B HFM08
HFM38
HFM22 HFM16 HFM05
WSW-ENE profil e
NW-SE profile
Forsmark nuclear power plant
Target volume
WSW-ENE profile through target volume
NW-SE profile along candidate volume
The rock domain marked as RFM029R in the regional model volume contains the potential
repository for the disposal of highly radioactive nuclear waste. This domain occurs within a tectonic lens with lower ductile strain, and is surrounded by rocks that show a strong anisotropy in a WNW to NW direction (Figure 5-41). The higher ductile strain associated with this anisotropy has influenced the orientation of the different rock domains outside the tectonic lens. A moderately to steeply plunging synform is conspicuous in the target volume in the north-western part of the lens, and both the boundaries between rock domains and the penetrative ductile fabric in the lens dip more gently in the south-eastern part of the lens. These structural features are conspicuous in the two profiles (Figure 5-41).
The regional deformation zones are located entirely outside the tectonic lens in the bedrock which contains a strong anisotropy generated as a result of ductile deformation. Furthermore, the gently dipping zones are concentrated in the south-eastern part of the lens, where the ductile structures and rock contacts, in particular the contacts between amphibolite and metagranite, also dip more gently. These features are conspicuous in the 3D regional model and the NW-SE profile, respectively (Figure 5-41).
In the local model, domain RFM029R has been divided into domains RFM029 and RFM045, on the basis of the degree of alteration referred to as albitization. Only three steeply dipping fracture zones (ENE0060A and ENE0062A with their attached branches, and WNW0123), with a trace length longer than 3,000 m, intersect these domains in the targeted part of the local model volume (Figure 5-42). As indicated on the map at –500 m elevation (Figure 5-42) and in the NW-SE profile (Figure 5-41), no gently dipping zones intersect this depth in the volume north-west of zone ENE0062A. The division of the north-western part of the tectonic lens into fracture domains FFM01, FFM02 and FFM06 (Figure 5-42), and the spatial distribution of these domains (Figure 5-34), have been determined by three factors; proximity to the ground surface, the distance to the gently dipping zone A2 and lithology. Fracture domains FFM01 and FFM06 occur at the depth of a potential repository. The overlying fracture domain FFM02 extends to greater depths in the vicinity of zone A2, where it is also present above this zone. Fracture domain FFM03 is located solely above zone A2, i.e. in its hanging wall.
Figure 5‑42. Integrated geological model for rock domains, deformation zones and fracture domains at the Forsmark site in a local model scale perspective. The figure shows two dimensional horizontal surface models at different elevations, –150 m and –500 m (RHB 70), inside the local model volume. In each figure, zones marked in red are steeply dipping or vertical and have a trace length at the surface longer than 3,000 m. Zones marked in blue-green are steeply dipping or vertical and are less than 3,000 m in length.
Zones marked in green are gently dipping. Other features are labelled directly on the figures.
The following ten bullets provide a summary of the key geological aspects of the Forsmark site.
• The bedrock in rock domains RFM029 and RFM045 is dominated by a rock type with high quartz content (metagranite). Mean values of the quartz content in two different varieties of this rock type are 36 and 40%. The subordinate rock type referred to as amphibolite contains little or no quartz (range 0 to 6%). This rock type is a more conspicuous component in the more altered rock domain RFM045. Field relationships suggest that the increased degree of alteration in this domain is, at least partly, related to the increased frequency of occurrence of amphibolite. These features have important implications for the thermal and rock mechanical modelling work.
• The bedrock anisotropy, which was established in the high-temperature ductile regime at 1.87 to 1.85 Ga, has influenced the variable spatial distribution of the different sets of deformation zones at the site: Steep WNW-NW and NNW sets lie predominantly outside the tectonic lens in the high-strain belts; the steep ENE to NE set lies inside the tectonic lens, in particular in the targeted north-western part; gently dipping zones are predominantly present where the ductile structures and rock contacts are more gently dipping in the south-eastern part of the candidate area. Only the WNW-NW set shows clear evidence of a combined ductile and brittle deformational history.
The other sets are solely brittle in character, i.e. they are fracture zones. Ductile deformation along the steep WNW-NW set of deformation zones occurred after 1.85 Ga. The bedrock at the site was first able to respond to deformation in a brittle manner some time between 1.8 and 1.7 Ga.
• The different sets of fractures inside the deformation zones are genetically related to the zones that were established prior to 1.1 Ga, probably during the Palaeoproterozoic between 1.8 and 1.6 Ga. However, the influence of different compressive tectonic regimes for the development of the deformation zones in the brittle regime is apparent from the kinematic data along shear fractures, i.e. minor faults. Strike-slip displacement, both dextral and sinistral, is conspicuous along minor faults in each steeply dipping set, while both reverse dip-slip and subordinate strike-slip senses of movement have occurred along the gently dipping structures. Both the kinematic data and the mineralogy along fractures suggest that all four sets of deformation zones were affected by reactivation under brittle conditions, not least around 1.1 to 0.9 Ga. New fractures may also have formed during progressively younger geological events. On the basis of these considerations, it is uncertain exactly how the different sets of fractures along a zone are related both to each other and to the zone.
• The different sets of fractures at the site responded differently to fluid flow at different times during the geological evolution. In particular, the apparently simple mineralogy along the gently dipping fracture zones is an artefact obscuring a long and complex hydrothermal history. In the current stress regime, it is apparent that changes in aperture can be expected most significantly along the gently dipping fractures and to a less extent along the steeply dipping fractures that strike WNW to NNW.
• The orientation and mineralogy of the fractures inside domains FFM01, FFM03 and FFM06 resemble the orientation and mineralogy of fractures in the contiguous fracture zones, i.e. the steep ENE (NE), gentle and steep NNE sets of fracture zones, respectively. However, this correlation breaks down in domain FFM02. In this domain, there is a significant occurrence of gently dipping to sub-horizontal fractures but relatively few gently dipping zones, and there is a relatively high level of occurrence of fracture minerals from the younger mineral generations as well as fractures without any mineral coating or filling.
• It is suggested that a long geological history of tectonic processes during Proterozoic time has influenced the fracture pattern at deeper crustal levels in fracture domains FFM01, FFM03 and FFM06. However, a combination of these tectonic processes and a second geological process, related to unloading and release of stress, has influenced the fracture pattern in the near-surface realm, close to the sub-Cambrian unconformity and current ground surface in fracture domain FFM02. It is envisaged that reactivation of ancient fractures and even formation of new fractures (sheet joints) in connection with exhumation of the sub-Cambrian unconformity during the
There are also uncertainties in the continuity of tectonic processes at different size scales. These difficulties have been addressed using alternative geological DFN models. These are referred to as the TCM, TCMF, OSM+TFM and r0-fixed models.
• The alternative geological DFN models differ in method (e.g. TCM and r0-fixed) and even in concept (TCM, TCMF and r0-fixed versus OSM+TFM). However, none of the models considered have taken into account the heterogeneity in the density of larger structures at the Forsmark site when the scaling exponent (shape parameter) kr values have been calculated.
• The TCM, TCMF and r0-fixed models are all based on the assumption of a single power law function (tectonic continuum) to encompass a size-intensity relationship for all fracture radii up to 564 m. However, they use different methodologies to derive size-intensity relationships in fracture domains FFM01 and FFM06 at depth. The TCM and TCMF models for these domains use surface trace length data on all size scales. Account is taken of differences in the influence of different geological processes close to the surface and at depth only in the accompanying spatial model. In contrast, the r0-fixed model combines fracture frequency data from boreholes with surface trace length data for lineaments and deformation zones. It avoids the use of fracture size data from outcrop.
• The OSM+TFM model combines the size distributions derived from outcrop with the size distributions derived independently from lineaments and deformation zone traces, with a change in slope of the power law function at r = 50 m, i.e. two power-law functions. Although this model, to some degree, better honours data derived from both small and large scales simultaneously, there is, to our knowledge, little support for such a model in the literature. Furthermore, the model does not explain the similarity between the orientation and mineralogy of fractures inside domains FFM01, FFM06 and even FFM03 and the respective, contiguous zones.