Ductile deformation Character of ductile structures

Volumetric proportions

Quantitative estimates (volume %) of the proportions of different rock types from close to the surface down, in some cases, to c. 1,000 m depth have been calculated on a borehole by borehole basis and presented in the form of histograms and summary tables in successive model versions and stages /SKB 2005a, 2006a, Appendix 4 in Stephens et al. 2007/. Only the data from cored boreholes, which have a varied bearing and inclination, have been used. However, no consideration of the orientation of rock contacts and, thereby, the true thickness of rock intersections has been carried out in this analysis.

On the basis of the pre-conditions that the borehole is greater than 200 m in length and that it is situated entirely in both the local model volume and the Forsmark tectonic lens, estimates of the proportions of different rock types have been determined for the volume that has been selected as a potential repository. Metagranite (101057 or 101058) with a range between 68 and 83%, a mean value of 75% and a standard deviation of 5%, dominates inside this volume (Table 5-3). Along three of these cored boreholes (KFM06A, KFM08C and KFM08D) as well as borehole KFM06C, which are all in the north-eastern part of the target volume, these granitic rocks are affected, in part, by the type of alteration referred to as albitization (see above). The volumetric proportions of the subordinate rock types in this volume are also provided in Table 5-3.

Thickness of amphibolite

Although the subordinate rock amphibolite is clearly affected by ductile deformation and is, by definition, metamorphic in character, this rock is inferred to have intruded originally as dykes.

Amphibolite occurs as narrow, dyke-like tabular bodies and irregular inclusions that are elongate in the direction of the mineral stretching lineation (see also section 5.2.4). Due to the low content or absence of quartz in this rock type, it requires special treatment in the thermal modelling work (see chapter 6).

On the basis of the borehole length and the orientation of the contacts of amphibolites in twenty-one cored boreholes, estimates of true thickness have been calculated (Appendix 5 in /Stephens et al.

2007/). Although some bodies are more than a few metres in thickness and, locally (e.g. KFM06C, KFM08D), are some tens of metres thick, most are inferred to be minor rock occurrences, i.e. thin geological entities. The thicker bodies in boreholes KFM06C and KFM08D occur in different parts of the fine-grained and partly albitized granitic rocks in the north-eastern part of the target volume. An amphibolite with similar thickness is also exposed at the ground surface in this area. Stochastic simula-tions of the subordinate rocks in the target volume, including amphibolite, are presented in chapter 6.

5.2.4 Ductile deformation

facies metamorphic conditions. Later ductile strain (section 3.1) occurred predominantly along more discrete deformation zones in the high-strain belts around the tectonic lenses.

Structures measured during the bedrock mapping work at the surface (Figure 5-1) include a planar grain-shape fabric (tectonic foliation), a tectonic banding, a linear grain-shape fabric, which is inferred to correspond to the direction of stretching (mineral stretching lineation), and minor fold axes. Breakdown and analysis of these data in the context of separate subareas were completed in model version 1.2 /SKB 2005a/. Laboratory measurements of the anisotropy of magnetic susceptibility (AMS) from surface samples were also evaluated in /SKB 2005a/. AMS data provide an independent assessment of the orientation of ductile structures in the bedrock. They also provide complementary information bearing on the degree and character of the ductile strain.

Only planar structures, including tectonic foliation and minor ductile and ductile-brittle shear zones, have been measured during the geological mapping of boreholes. The orientation data for these ductile structures as well as the contacts of mafic rock units (predominantly amphibolite) have been presented on a systematic, borehole by borehole basis in model stage 2.2 (Appendix 6 in /Stephens et al. 2007/).

Deformation inside the Forsmark tectonic lens

Folding inside the Forsmark tectonic lens is apparent at different scales. On a larger scale, the boundaries between the dominant metagranite and other rock units are folded (Figure 5-2) and it is inferred that the lens is affected by a major fold structure. These rock units include a metatonalite (Figure 5-7a) in the south-eastern part of the lens and a tectonically banded, heterogeneous unit with metagranite, felsic to intermediate metavolcanic rock and amphibolite, affected by high ductile strain (Figure 5-7b) and intruded by pegmatite, in the north-westernmost part, between the nuclear power plant and SFR.

Figure 5‑7. Character of the bedrock inside the Forsmark tectonic lens. a) Lineated and weakly foliated medium-grained metatonalite (101054), intruded by finer-grained metatonalite (101051), which, in turn, is intruded by pegmatite (101061). This rock unit forms a folded mega-xenolith inside the metagranite in the tectonic lens. b) Tectonically banded, foliated and lineated meta-igneous rocks, with intra-folial fold structures, in the coastal area between the nuclear power plant and SFR. The strongly deformed rocks in this outcrop are folded around the major synform inside the lens. c) Folded tectonic foliation with minor shear zone development in metagranite (101057) close to drill site 6. The tectonic foliation is discordant to a Group D granite dyke (111058). d) Folded tectonic foliation and rock contact between metagranite (101057) and amphibolite (102017) south-west of drill site 5, in the marginal part of the tectonic lens.

c d

a b

101054

101051

101061

101057

111058

101057

102017 101054

101051

101061

101057

111058

101057

102017

a b

Pole to foliation (N=520) Trend/plunge of pole to best-fit great circle

Pole to best-fit great circle

Mineral lineation (N=368) Mean trend/plunge=139/37 Fisher κ value=4.9

Fold axis (N=20) Mean trend/plunge=126/38 Fisher κ value=3.8

Site mean, pole to magnetic foliation (N=102) Site mean, magnetic lineation (N=102)

On a smaller scale, folding of the penetrative tectonic foliation (Figure 5-7c) with an inferred fold axis that plunges moderately to the south-east is indicated from the outcrop data (Figure 5-8a).

Furthermore, data from individual boreholes that intersect the bedrock inside the tectonic lens con-firm the presence of large-scale folding (Figure 5-9a, b). The ductile structures along the fold hinge are also more steeply inclined in the north-western (Figure 5-9a, b) compared with the south-eastern (Figure 5-9a, c) part of the lens (see also /Stephens and Forssberg 2006/). Both the mineral stretching lineation and the measured fold axes (Figure 5-8a) plunge to the south-east. It is apparent that fold axes are sub-parallel to the stretching direction in the bedrock.

The smaller-scale features indicate that the major folding is synformal in character in the north-western part of the lens, where the target area is situated, and antiformal in character further to the south-east, i.e. the fold is tube-like in character (see also section 5.5). In general, the mineral stretching lineation is more prominent than the tectonic foliation (LS-tectonites). Outcrop observations indicate that the amphibolites that intrude the metagranite are folded (Figure 5-7d).

This is also apparent from the borehole data, where the amphibolites show different orientation distribution patterns close to the hinge (Figure 5-9a, b), along the hinge (Figure 5-9a, c) and on the south-western limb (Figure 5-9a, d) of the major fold structure inside the tectonic lens. It is clear that the amphibolites consistently follow the orientation of the folded planar ductile structures in the bedrock /Stephens et al. 2007/.

The AMS data measured in the laboratory (Figure 5-8b) and the ductile structural data measured at outcrop (Figure 5-8a) are in excellent agreement with each other /SKB 2005a/. The AMS data confirm both the conclusions drawn concerning the orientation of the major folding inside the tectonic lens and its more constrictive ductile strain. They also indicate a generally lower degree of ductile strain inside the tectonic lens compared with that outside it to the south-west.

Deformation along the margin to and outside the Forsmark tectonic lens

As the margins of the Forsmark tectonic lens are approached, the tectonic foliation in the metagranite increases in intensity. Minor ductile shear zones along the tectonic foliation in the south-western limb of the major synform show dextral strike-slip deformation /Nordgulen and Saintot 2006/.

Outside the tectonic lens, strongly deformed rocks are present, both to the south-west (Figure 5-10a) and to the north-east (Figure 5-10b–d) of the lens on both flanks of the major synform. The rocks are foliated, lineated and, in part, also heterogeneous and banded (SL- and BSL-tectonites). Even on the north-eastern limb of the major synform, a dextral strike-slip component of displacement along the high-strain fabric is present (Figure 5-10b). The intense tectonic banding is deformed by minor folds (Figure 5-10c) that are inferred to be parasitic and related to the major synform. Locally, eye-shaped, tubular folds are present (Figure 5-10d). The structural data from, for example, borehole KFM04A, which lies partly inside the highly strained rocks to the south-west of the tectonic lens and partly inside the marginal part of the lens, illustrate the consistent steep dips to the south-west at depth in this subarea (Figure 5-9a, d).

Implications for modelling work

The evaluation of the ductile deformation has important implications for the modelling work. This concerns both the establishment of a conceptual model for rock domains (see section 5.4) as well as an understanding of the spatial distribution of different types of brittle structures. In particular, the ductile deformation has contributed to the development of a strong bedrock anisotropy that has steered the development of younger brittle deformation at the site (see section 5.5).

Figure 5‑9. Orientation of planar ductile structures and mafic rock contacts (mostly amphibolite) in selected cored boreholes. An estimation of the degree of point, girdle or random distribution pattern (Vollmer fabric index, PGR) in the raw data is provided for each borehole. The pole to each planar structure is plotted on the lower hemisphere of an equal-area stereographic projection. No Terzaghi cor-rection has been applied. However, the orientation of each borehole is provided, in order to help judge the significance of this bias. (a) Location of boreholes in relation to the Forsmark tectonic lens. (b) Borehole KFM08A. (c) Borehole KFM03A. (d) Borehole KFM04A.

c Borehole bearing = 272º/86º d Borehole bearing = 045º/60º

a b Borehole bearing = 321º/61º

Fo rsm

ark te

ctonic le

ns Eckarfjärden DZ

KFM08A

KFM04A

Forsmar k DZ

Singö DZ

KFM03A

Rock unit with high ductile strain