Conceptual model

Influence of anisotropy in the ductile regime on deformation in the brittle regime The ductile deformation between 1.87 and 1.85 Ga at Forsmark contributed to the development of strong bedrock anisotropy at the site. The subsequent, 1.85 Ga and younger tectonic evolution gave rise to sets (and sub-sets) of deformation zones with different orientations, properties and spatial distributions, guided by the older bedrock anisotropy /Stephens et al. 2007/.

Vertical and steeply dipping deformation zones that strike WNW-ESE and NW-SE are restricted to the margins of and to the high-strain belts outside the Forsmark tectonic lens. Several of these zones show both ductile and polyphase brittle strain. The zones with a dip less than 90° dip to the south or south-west. For purposes of simplicity in the text that follows, these zones are referred to as the steep (or steeply dipping) WNW and NW sub-sets, respectively, (see /Stephens et al. 2007, section 2.4/ for nomenclatural considerations).

By contrast, the target volume at 400 to 500 m depth inside the lens is intersected by vertical and steeply dipping brittle deformation zones (fracture zones) that strike ENE-WSW to NNE-SSW.

The zones with a dip less than 90° dip in both directions but predominantly to the NNW to WNW.

These zones are labelled steep (or steeply dipping) ENE (NE) and NNE sub-sets in the following text. Since there are relatively few zones in this group that strike strictly NE-SW, the direction NE has been enclosed in parentheses. Although some gently, south- and SE-dipping fracture zones occur in this volume above 500 m depth, such zones are more conspicuous in the south-eastern part of the tectonic lens, outside the target volume. In this part of the bedrock, the ductile deformational structures and rock contacts are also gently dipping. Finally, a few vertical and steeply dipping deformation zones with NNW-SSE strike are present both inside and outside the Forsmark tectonic lens. The zones with a dip less than 90° dip in both directions, mostly to the WSW but also to the ENE. Once again, for purposes of simplicity, these zones are referred to as the steep (or steeply dipping) NNW set.

Conceptually, it is proposed that the gently dipping zones in the south-eastern part of the Forsmark lens follow boudinaged layers of amphibolite /Juhlin and Stephens 2006, Stephens et al. 2007, p.

158–159/, a subordinate rock type that is oriented parallel to the ductile fabric in the bedrock (see section 5.2.4). This spatial relationship steers the increased frequency of such structures in the south-eastern compared with the targeted, north-western part of the tectonic lens. The termination of the gently dipping zones in the geological model is based on the concept that they formed after the steeply dipping WNW and NW zones, and more or less at the same time as the steeply dipping ENE to NNE zones and at least some of the steeply dipping NNW zones. Moderately to steeply dipping and folded contacts and ductile structures inside the targeted, north-western part of the lens mitigates against any simple correlation between the occurrence of fracture zones and the occurrence of subordinate rock types in this volume.

Geodynamic evolution

Two different types of geological process have provided a profound impact on the geodynamic evolution in south-eastern Sweden, including the Forsmark area (section 3.1 and Figure 3-3), and thereby influence the conceptual thinking. These types of process comprise the following.

• Igneous activity and crustal deformation along an active continental margin at different time intervals mostly during Proterozoic time.

• Loading and unloading cycles around and after c. 1.45 Ga that are related to the deposition and denudation, respectively, of sedimentary rocks or glacial material. The sedimentary rocks formed in response to far-field tectonic activity, and glaciations occurred due to radical change in climate.

The loading and unloading cycles are coupled with the burial and exhumation, respectively, of the older crystalline bedrock.

As the effects of regional tectonic activity waned in south-eastern Sweden and shifted to a far-field realm, so the effects of loading with stress build-up and unloading with release of stress, increased in significance (see Figure 3-3 in chapter 3). Naturally, the variation in the build up of stress at any single place will depend on other factors. The inferred bulk crustal shortening during different regional tectonic events are shown in Figure 3-3 in chapter 3.

A conceptual model for the formation and reactivation of the different sets of deformation zones has successively grown in confidence as progressively more data have been acquired /SKB 2005a, 2006a, Juhlin and Stephens 2006, Stephens et al. 2007/. The current model for the site is shown in Figure 5-26. Bearing in mind the character of the zones, this model predominantly concerns the brittle deformational history. The model has emerged with the use of low-temperature geochronological data that has shed light on the exhumation and cooling history, the relative time relationships and absolute ages of fracture minerals, and a comparison of kinematic data from brittle structures along deformation zones with the tectonic evolution in a regional perspective. For a more detailed overview and evaluation of these data, the reader is referred to /Söderbäck (ed) 2008/ and section 5.2.6.

General character of a fracture zone at potential repository depth at Forsmark

The conceptual thinking also penetrates upwards in scale, to an individual, steeply dipping fracture zone at 400 to 500 m depth inside the north-western, targeted part of the tectonic lens at Forsmark (Figure 5-27). This is necessary as a prerequisite to the modelling work, since such zones are rel-evant in this volume and since data bearing on their character arise, in general, from a highly limited number of borehole intersections and even more limited surface data. The fracture zone concept makes use of the generally accepted division of zones into undeformed host rock, transition or damage zone, and fault core, e.g. /Caine et al. 1996, Gudmundsson et al. 2001, Munier et al. 2003/, as well as the site-specific characterisation in the single-hole interpretation work (section 5.2.6).

hematite dissemination (red staining) of the minerals along and the wall rock adjacent to fractures.

Both sealed and open fractures usually increase in abundance inside the zones. However, the transition zone can also contain segments of bedrock that resemble the unaffected host rock outside the fracture zone (Figure 5-27).

In the cases where a fault core has been recognised along a zone (55% of the zones studied in boreholes), it is composed of a high frequency, especially of sealed fractures, commonly in the form of a complex sealed fracture network, in combination with rock alteration. Cohesive breccia or cataclasite are also conspicuous along some fault cores at Forsmark. The thickness of the fault core may vary from a few centimetres up to a few metres. Fault gouge has not been recognised along the fracture zones.

Figure 5‑26. Two-dimensional cartoons illustrating the regional scale geodynamics during the formation and reactivation of the different sets of deformation zones at the Forsmark site. This includes late Svecokarelian, low-T ductile and brittle deformation (stage 1), late Svecokarelian brittle deformation (stage 2), Gothian brittle deformation (stage 3), and a major phase of brittle reactivation during the Sveconorwegian orogeny (stage 4).

Formation of fractures and fracture zones during stage 4 cannot be ruled out. The different colour shadings along the zones indicate an inferred variable degree of response (strongest is black, intermediate is grey, weakest is pale grey) in each tectonic regime (modified after /Stephens et al. 2007/).

Bulk crustal shortening

Gently dipping zones follow orientation of contacts to rock units in more gently dipping parts Brittle deformation Generation 1 minerals Epidote-quartz-chlorite -hematite dissemination Formation and reactivation of zones – stage 3

Gothian (1.7–1.6 Ga)?

Gently dipping Gently dipping

Reactivation and formation of zones – stage 4 Sveconorwegian (1.1–0.9 Ga)

Bulkcrustal shortening

Generation 2 minerals Adularia-prehnite-laumontite-calcite-hematite dissemination

(growth of adularia is Sveconorwegian and/or pre-Sveconorwegian) Brittle deformation

Gently dippingGently dipping Formation and reactivation of zones – stage 2 Late Svecokarelian (after 1.8 Ga)?

Bulk crustal shortening

Gently dipping zones follow orientation of contacts to rock units in more gently dipping parts Brittle deformation Generation 1 minerals Epidote-quartz-chlorite -hematite dissemination

Gently dippingGently dipping

Bulk crustal shortening Formation of zones – stage 1 Late Svecokarelian (after 1.85 Ga)

Mylonite

Mylonite Mylonite

Epidote-quartz-chlorite-hematite dissemination Generation 1 minerals Low-T ductile and brittle deformation

Zones follow anisotropy in bedrock related to high-T ductile deformation