Examensarbete vid Institutionen för geovetenskaper
ISSN 1650-6553 Nr 237
Structural Model of the Lambarfjärden
Area from Surface and Subsurface
Data in Connection with
the E4 Stockholm Bypass
Structural Model of the Lambarfjärden
Area from Surface and Subsurface
Data in Connection with
the E4 Stockholm Bypass
Anna Vass
Anna Vass
Uppsala universitet, Institutionen för geovetenskaper Examensarbete E1, Berggrundsgeologi, 30 hp ISSN 1650-6553 Nr 237
Tryckt hos Institutionen för geovetenskaper, Geotryckeriet, Uppsala universitet, Uppsala, 2012.
The present Master thesis is written in connection with the E4 Stockholm bypass, which is a future motorway linking northern and southern Stockholm, and will mostly run through tunnels. The bypass will cross the Lake Mälaren in the Lambarfjärden area, where a fault is indicated on the geological maps, which would create risks for tunneling. Several geological and geophysical studies and measurements have been carried out in connection with the bypass project. These include surface mapping, drillings (core logging and water-loss measurements) and geophysical measurements (reflection and refraction seismics). The objectives of this thesis are to interpret the available geological and geophysical data, integrate the reports and the results of these studies and finally to create a structural model of the Lambarfjärden area affected by the tunnel.
The available data provided a great opportunity to examine the area. The core logging and geophysical measurements supported the existence of the fault indicated on the geological maps. The core logging revealed that this pre-existing deformation zone was reactivated, possibly as dextral strike-slip, and the orientations of the subsurface fractures corresponded well with the Riedel structures occurring in such shear zone. The field work has found evidence for ductile sinistral movement which, together with the results of the core logging, could indicate a conjugate deformation zone in the area. The water-loss measurements showed several intervals along the boreholes where significant water-loss took place. Furthermore, the thesis calls the attention to both opening and closure of differently oriented weaknesses, fractures.
Examensarbete vid Institutionen för geovetenskaper
ISSN 1650-6553 Nr 237
Structural Model of the Lambarfjärden
Area from Surface and Subsurface
Data in Connection with
the E4 Stockholm Bypass
1 Table of Contents ABSTRACT ... 3 1. INTRODUCTION ... 5 2. BACKGROUND ... 8 2.1. Surface mapping ... 8 2.2. Drillings ... 8 2.2.1. Drill cores ... 10
2.2.1.1. Drill core logging ... 10
2.2.1.2. Borehole Image Processing System ... 11
2.2.1.3. Limitations of the drill core logging ... 11
2.2.2. Water-loss measurements ... 12
2.3. Geophysical measurements on Lake Mälaren ... 13
3. GEOLOGICAL SETTING ... 14
3.1. Regional – Svecofennian orogen ... 14
3.2. Local ... 15
3.2.1. Bergslagen Province ... 15
3.2.2. Lambarfjärden ... 21
4. STRESS FIELD ... 24
4.1. Current regional stress field in Scandinavia ... 24
4.2. Current local stress field in eastern Bergslagen ... 26
4.3. Palaeostress field and deformation history in eastern Bergslagen ... 26
5. BACKGROUND DATA ... 29
5.1. Drill core data ... 29
5.1.1. 08F351K ... 29 5.1.2. 08F352K ... 29 5.1.3. 10F353K ... 29 5.2. Water-loss measurements ... 30 5.3. Geophysical measurements ... 31 6. METHODS ... 34
6.1. Surface fracture analysis ... 34
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6.3. Field work ... 35
6.4. Correlation of the water-loss with subsurface fracture data ... 35
7. RESULTS ... 36
7.1. Surface fracture analysis ... 36
7.2. Subsurface fracture analysis ... 37
7.2.1. Fracture description and mineral-fillings ... 37
7.2.1.1. Drill core 08F351K... 37
7.2.1.2. Drill core 08F352K... 40
7.2.1.3. Drill core 10F353K... 42
7.2.2. Comparison of fracture orientations in drill cores 08F351K and 08F352K ... 45
7.2.3. Description of the fracture orientations along the 10F353K ... 47
7.3. Field work ... 49
7.4. Correlation of the water-loss with subsurface fracture data ... 51
8. DISCUSSION ... 54
8.1. Surface fracture analysis ... 54
8.1.1. Field work ... 54
8.2. Mineral-fillings and alterations of the fracture surfaces ... 55
8.3. Deformation zone ... 55
8.3.1. Fault reactivation in dextral strike-slip mode ... 55
8.3.2. Conjugate deformation zone system ... 58
9. CONCLUSIONS ... 61
ACKNOWLEDGMENTS ... 62
3 ABSTRACT
The present Master thesis is written in connection with the E4 Stockholm bypass, which is a future motorway linking northern and southern Stockholm, and will mostly run through tunnels. The bypass will cross the Lake Mälaren in the Lambarfjärden area, where a fault is indicated on the geological maps, which would create risks for tunneling. Several geological and geophysical studies and measurements have been carried out in connection with the bypass project. These include surface mapping, drillings (core logging and water-loss measurements) and geophysical measurements (reflection and refraction seismics). The objectives of this thesis are to interpret the available geological and geophysical data, integrate the reports and the results of these studies and finally to create a structural model of the Lambarfjärden area affected by the tunnel.
The available data provided a great opportunity to examine the area. The core logging and geophysical measurements supported the existence of the fault indicated on the geological maps. The core logging revealed that this pre-existing deformation zone was reactivated, possibly as dextral strike-slip, and the orientations of the subsurface fractures corresponded well with the Riedel structures occurring in such shear zone. The field work has found evidence for ductile sinistral movement which, together with the results of the core logging, could indicate a conjugate deformation zone in the area. The water-loss measurements showed several intervals along the boreholes where significant water-loss took place. Furthermore, the thesis calls the attention to both opening and closure of differently oriented weaknesses, fractures.
5 1. INTRODUCTION
The E4 Stockholm bypass is a new motorway linking southern and northern Stockholm from the west (Fig. 1-1). Currently there is only one major link road, that is the Essingeleden (Fig. 1-1) opened in the late '60s and designed for 80 000 vehicles a day. On a normal working day, the traf-fic now reaches up to 160 000 vehicles. Due to this signitraf-ficant increase in traftraf-fic, the Es-singeleden is not only subjected to serious wear leading to a great need for maintenance and re-pair but also makes the transport system of the area very vulnerable.
The rapid growth of Stockholm's population and the increasing international potential of the re-gion have called attention to the crucial need for a new link between northern and southern Stockholm.
The Stockholm bypass will minimize the mentioned traffic vulnerability and relieve the over-loaded Essingeleden. It will provide a more efficient communication and create a faster connec-tion for both the private and public transport between the southern and northern parts of the city as well as the country. The importance of the E4 Stockholm bypass and the accompanied engi-neering geology challenges hav convinced me to get involved in this project.
The construction of the bypass is planned to start earliest in 2012 and will take approximately 8-10 years. The project is carried out by the Trafikverket (Swedish Transport Administration), and financed by the tax-payers (80%) and the state (20%). The total length of the motorway will be 21 km of which 18.3 km will go through tunnels to reduce the impact on the environment.
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The thesis focuses on the Lambarfjärden area (Fig. 1-2). The bypass here will cross the Lake Mä-laren and reach its maximum depth with 70 m underground.
After the conduction of a feasibility study and the creation of a preliminary design plan, different geological, geophysical and geotechnical studies and measurements have been carried out by several companies since 2007. As a consequence, a good database is provided. However, the re-sults of these studies have neither been interpreted nor integrated into a concise geological model yet. The objectives of this thesis are therefore to (1) interpret the available geological and geo-physical data, (2) integrate the reports and the results of these studies and finally to (3) create a structural model of the Lambarfjärden area affected by the tunnel.
A deformation zone is indicated along the Lambarfjärden (Stålhös, 1968; Persson et al., 2001). It is worth to note, however, that no exact characteristics, descriptions or evidences can be found in literature which would either explain or support the existence of this deformation zone. The new-est map of the area (Persson et al., 2001) followed the Stålhös map with respect to this particular deformation zone (Lars Persson, personal communication, March 30, 2012). To our current knowledge, thus, no tangible evidence for the existence of the fault is available
This zone has been mapped as a normal fault dipping approximately 70-80° towards Southeast (Stålhös 1968; Persson et al., 2001; Trafikverket, 2010).
Understanding fault dimensions and internal structures is crucial and widely applied for geologi-cal and engineering purposes. In petroleum geology a fault can act as a structural trap, therefore the main focus is on the possible across-fault leakage, permeability, and thus the reservoir quality.
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Faults also play an important role in earthquake nucleation due to the great loss of cohesion with-in the rock suite. Estimatwith-ing the strawith-in distribution, modelwith-ing the pressured rock stability, predict-ing the fault growth or the sub-seismic fault nucleation are also important applications in which the exact understanding of fault characteristics are crucial.
8 2. BACKGROUND
A wide range of data, retrieved by a number of geological and geophysical methods, was availa-ble for this MSc thesis. This chapter outlines these background methods that have been used since 2007.
2.1. Surface mapping
Surface mapping of the entire area along the planned route was carried out by WSP. Aerial pho-tographs, existing geological and construction maps were used in order to locate the outcrops in the area. The surface mapping included mapping of outcrops, detailed geological description (rock types, color and texture), determination and orientation of the structures (e.g. foliation, lin-eation), as well as fracture mapping. Only those fractures that were longer than 1 m have been described and measured.
Orientation of weakness zones was also measured. WSP defined weakness zones as crushed, highly fractured zones which usually coincide with topographical lows, valleys.
The strike of the structures is accurate; however, caution must be taken regarding the dip values, since tight cracks made it difficult to carry out accurate measurements. In order to prevent any mishap, a simplification has been made. Fractures that seemed steep have been termed “steep”, whereas shallow dipping fractures have been noted as “flat”.
The surface mapping by WSP did not include kinematic data, which would have been important to detect possible displacement directions, gather information about the fault characteristics and thus model the framework in which the fault plays an important role.
The measurements (strike and dip of the weakness zones, fractures, foliations and lineations) ob-tained from the surface mapping, as well as the other field observations mentioned above (geo-logical descriptions) have been provided in excel spreadsheets. These spreadsheets comprise the mentioned information for the entire area of the planned tunnel route. The findings and measure-ments have not been either interpreted or plotted on stereographic projection plots.
2.2. Drillings
In order to gather crucial geological information of the underground rocks, several boreholes were drilled along the planned tunnel route. The drillings were carried out by the Züblin Company, whereas in certain boreholes, core drillings were performed for which the DrillCon Company was responsible.
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Table 2-1 Basic information about the boreholes in the Lambarfjärden area.
Borehole length (m) Hole diameter (mm) Core diameter (mm) BIP range (m) 08F351K 98,55 76 51 51.15 - 98.55 08F352K 98,80 76 51 48.22 - 98.63 10F353K 421 76 51 3.2 – 359
Fig. 2-1 Aerial photograph of the locations of the three boreholes drilled in the studied area. Yellow lines are indicating the planned route (satellite image: Google Earth, 2012).
10 2.2.1. Drill cores
2.2.1.1. Drill core logging
The drill core logging was carried out by GeoSigma, and comprises detailed geological descrip-tion of the drill cores (lithologies, structures) and fracture characterizadescrip-tion. Small (under 20 cm) lithological changes, such as veins, xenoliths and enclaves have not been recorded. Moreover, cross-cutting relationship of the fractures and veins, sealed or partially open fractures and possi-ble kinematic indicators have not been described.
Two geotechnical systems were used for rock characterization during the core logging, RMR and Q.
RMR refers to the Rock Mass Rating system which was published by Bieniawski in 1976. The system is based on six parameters: uniaxial compressive strength of the rock material, Rock Quality Designation (RQD), spacing, condition and orientation of discontinuities, and groundwa-ter conditions. Estimation of the values are based upon laboratory measurements or published tables (e.g. compressive strength), and visual impressions (e.g. condition of the discontinuities). The system aims to estimate the strength of rock masses.
Two parameters have been set in advance, the orientation of the discontinuities to 0 and the groundwater conditions to 15.
Q refers to the Rock Tunneling Quality Index published by Barton et al. (1974), and is defined by six parameters: RQD, joint set number (Jn), joint roughness number (Jr), joint alteration number
(Ja), joint water reduction factor (Jw) and the Stress Reduction Factor (SRF). The Q value aims to
determine the tunnel support requirements. Both the Jw and SRF values have been set to 1.
The fracture logs comprise the fracture density, strike and dip, mineral fillings, alteration, rough-ness and shape of the fracture surface, width, aperture, alpha angle, as well as the value of Q and the RMR system. These fracture logs have been provided in excel spreadsheets for this thesis. Depending on the mineral assemblage on the fracture surfaces and their relative quantity to each other, the minerals have been sorted into four groups, Mineral 1, 2, 3 and 4, where Mineral 1 is being the most frequent one on that specific surface. This grouping is included in the core logging as well.
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2.2.1.2. Borehole Image Processing System
The exact orientations of the different features on the core have been defined through the Bore-hole Image Processing System (BIPS) by GeoSigma. This imaging technique has made it possi-ble to continuously record the borehole wall with a precise azimuth orientation. The raw image of the wall is a cylinder, which afterwards gets unrolled for a better processing, resulting in a 2D “map”. Due to this transformation, the linear features and planes that cut the borehole appear as sinusoidal lines on the image (Fig. 2-3). The instrument attributes colors to every pixel (i.e. measured point), creating the final image, on which a quantitative and qualitative analysis can be carried out.
The software, through which the images have been processed, is called BoreMap, which is a non-commercial program developed by GeoSigma and SKB. The core logging and the borehole im-age analysis were carried out simultaneously in order to produce an extensive and integrated da-taset.
2.2.1.3. Limitations of the drill core logging
During the drill core logging by GeoSigma (1) no distinction has been made between natural and drilling-induced fractures, (2) only open fractures have been recorded and described, but not the par-tially open and healed (or sealed) fractures, and (3) the cross-cutting relationship of neither veins, nor fractures had been described.
Distinguishing the natural and induced-fractures within the core is essential on many levels. Natural fractures form due to the geological stress-field and its changes throughout geological history; whereas induced fractures initiate during or after coring and form along the in-situ principle stress trajectories and are controlled by pre-existing weaknesses. Knowledge of natural and drilling-induced fractures, and their orientation make is possible to reconstruct the stress-history within the bedrock. Moreover, forecasting nucleation or reactivation of the fractures is also possible.
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Open fractures act as fluid pathways within the rock, but may not be exposed to further displacement. Description of the healed and partially open fractures, on the other hand, is crucial since these act as tectonically weak zones where potential further displacement could occur if the fracture orientation is such that either the current stress-field or the induced stress (i.e. tunnel drilling) would reactivate the-se.
Cross-cutting relationship of the fractures and veins is crucial to identify relative age relation-ships of fractures. This also gives a key opportunity to reconstruct the deformation history or the timing of possible hydrothermal activity within the suite
2.2.2. Water-loss measurements
Water-loss measurements were performed in every borehole by DrillCon. These measurements allow us to acquire crucial information on the open fracture network and the permeability of the surrounding rocks.
13 2.3. Geophysical measurements on Lake Mälaren
The geophysical measurements on Lake Mälaren were carried out by the GeoNova company in collaboration with the Hungarian ELGI.
In order to detect the depth to the bedrock and the fracture network, penetrating solar radar (resulting in a so-called subbottom profile), as well as reflection and refraction seismics were used.
The objectives for creating a subbottom profile were to detect the water depth, the sediment thickness and the depth to the bedrock. The subbottom profile systems are characterized by the ability to penetrate the upper sediment layers beneath the sea- or lakebed. The measurement is based on a chirping system, through which several kHz wide signals are emitted that can pene-trate up to 100 m. A broad range of frequencies are available (0.5 – 40 kHz), and the measure-ments can be carried out in both shallow (lakes) and deep (ocean) waters (Meridata, 2011). The equipment provides a high resolution; however, coarser sediments such as moraines limit the penetration depth and therefore the resolution as well. During this project, GeoNova used a chirp-ing system with a frequency of 2-7 kHz, resultchirp-ing a penetration depth up to 80 m.
Reflection and refraction seismics were used to detect the depth to the bedrock and the possible fracture networks. The advantages of this technique compared to the subbottom, is that even though the resolution is lower, the penetration is deeper, and it gives information about the frac-tures as well.
The seismic energy source for the measurements was provided by a nitrogen air gun. During the refraction measurements the hydrophones were spacing 3-5 m, and the sampling rate was 0.5 m/s; whereas during the reflection measurements the hydrophone spacing was decreased to 1-3 m, and the sampling rate to 0.125 m/s.
14 3. GEOLOGICAL SETTING
3.1. Regional – Svecofennian orogen
The research area, Lambarfjärden, is located within the Bergslagen Province, which formed dur-ing the Svecofennian orogen (Fig. 3-1). The 'Svecofennian' domain comprises a sequence of su-pracrustal and intrusive rocks formed in the time span of 2.0 - 1.75 Ga (Gaál and Gorbatschev, 1987). Nevertheless, the term 'Svecokarelian' is widely used for the igneous activity, ductile de-formation and metamorphism that were taking place between 1.9 and 1.8 Ga (Kositinen et al., 2001).
The Svecofennian orogen makes up the central part of the Fennoscandian Shield (Fig. 3-1). It is bordered to the northeast by the Archean domain and to the southwest by the Neoproterozoic Sveconorwegian orogen (1.10-0.92 Ga). The northwest border is defined by the Caledonian oro-gen, formed during the Mesosoic (0.5-0.4 Ga), whereas allochtonous Phanerozoic sediments can be found to the south and east (Kositinen et al., 2001).
The Svecofennian domain mainly comprises supracrustal and igneous rocks formed between 1.96-1.84 Ga. These rocks are to some extent affected by subsequent deformation and
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phism. Granitoid and migmatite provinces from a later stage (1.85-1.76 Ga) occur in different volumes within the domain, for instance along the western and northern margin. Gulf of Finland, Southern Finland and minor areas in central Sweden were subjected to rapakivi granite intrusions as well in the time span 1.65-1.47 Ga.
Several tectonic models for the evolution of the Svecofennian orogen have been presented. An early extensive model was proposed by Gaál and Gorbatschev (1987), in which they defined a WNW-directed subduction zone at the Archean cratonic border to which terranes of different siz-es were accreted after 1.91 Ga. The Svecofennides were inferred to be the result of these events. The increasing number of geological and geophysical investigations, such as the BABEL project (BABEL Working Group, 1990), Fennia Working Group (1998) or EUROBRIDGE Seismic Working Group (2001), provided the opportunity to analyze the area in detail for a better under-standing of its tectonic evolution. Gaál and Gorbatschev's (1987) basic concept was later refined and further developed by several authors (e.g. Gaál, 1990; Claesson et al., 1993; Korja et al., 1993; Gorbatschev and Bogdanova, 1993). Nironen (1997) and Lahtinen et al. (2005) provided so far the most detailed and presently most accepted model for the evolution of the Svecofennian orogen.
Andersson et al. (2006) used the term 'Svecofennian' for describing the different tectonic- and magmatic events leading to the final development of the Bergslagen. They divided the c. 2.1-1.75 Ga period into three main stages: (1) the 'proto-Svecofennian' (2.1-1.93 Ga), a major crust-forming period leading to the establishment of several microcontinents (Nironen, 1997), (2) the 'Early Svecofennian (1.91-1.86), in which significant reworking of the early crust occurred in conjuction with magmatism and the formation of new crust. The peak of the magmatic activity was reached by the final accretion of the terranes to the craton at 1.86 Ga (Allen et al., 1996; Ni-ronen, 1997), followed by the (3) 'Late Svecofennian' (1.85-1.75 Ga) with regional metamor-phism and granitoid magmatism related to the formation of the Transscandinavian Igenous Belt (TIB). The deformation was predominantly characterized by the migmatization of gneisses.
3.2. Local
3.2.1. Bergslagen Province
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The evolution of the domain involved several stages of magmatism, thermal doming and subsid-ence, ductile deformation and metamorphism, as well as tectonic switching (e.g. Gaál and Gor-batschev, 1987; Gaál, 1990; Allen et al., 1996; Hermansson et al., 2008). By integrating the con-cept of migratory tectonic switching (term introduced by Collins, 2002) and cyclic tectonic evo-lution (Allen et al., 1996), Hermansson et al. (2008) proposed a possible model to explain the development of the Bergslagen domain (Fig. 3-2).
The Bergslagen domain comprises mainly felsic and subordinate mafic metavolcanic and metasedimentary rocks, in addition to felsic and mafic intrusive rocks (Fig. 3-3). The oldest part of the supracrustal sequence is represented by turbiditic metagreywackes merging into quartzite and rhyolitic-dacitic metavolcanics. These metavolcanics are interbedded with metamorphosed conglomerates and sandstones. Skarns and carbonates dominate the upper sequence of the metavolcanics, which is overlain by clastic metasediments. The supracrustal rocks were subjected to felsic subvolcanic intrusions in the time span of 1.9-1.89 Ga, whereas the clastic metasedi-ments were intruded by mainly mafic dykes and sills (Stephens et al., 2009).
Stålhös (1976) defined two major fold phases in the studied area, F1 and F2. F1 was interpreted to be attributed to an E-W compression, whereas F2 was presumably a result of a secondary stress triggered by the competence contrast between the already solidified granitoid bodies and the less competent supracrustal matrix. Stålhös (1981, 1991) introduced the term 'cross-folding' to describe this second folding phase, which also resulted in a pervasive lineation and foliation with-in the early Svecofennian rocks. Stålhös (1991) proposed that F2 was related to a N-NW-directed
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thrusting. Sjöström and Bergman (1998) rejected the F2-triggering concept, the E-W compres-sion, as was suggested by other studies (e.g. Stephens and Wahlgren, 1996; Beunk et al., 1996). According to their view, an overall E-W compression would have caused sinistral shear, whereas most major shear zones indicate dextral movement. Instead they suggested N-S compression (or an E-W extension) during F2. However, they argue that an E-W compression is required at the early stage of the first deformation (F1) in order to form the overturned folds. Previous studies (Stålhös 1981 and 1991) interpreted the folding and metamorphism to be synchronous. However
it has been suggested that the peak metamorphism outlasted the shearing and folding (Allen et al., 1996; Sjöström and Bergman, 1998).
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Stephens et al. (2009) divided the Bergslagen region into four major structural and metamorphic zones (Fig. 3-4). These zones show a great variation in metamorphic grade, structural features and their orientations. The northern and southern domains display the most similarities with migmatitic metamorphism occurring in both. The fold axial surfaces (F1) show a WNW-ESE orientation with N-NW vergence. A gently to moderately, E to SE plunging mineral stretching lineation (L2) and fold axes have been described, in addition to a more open, upright folding (F2) with a N-S strike of fold axes. The planar grain-shape fabric strikes ENE-WSW – NE-SW. In contrast, the central, low-medium metamorphic grade domain is characterized by E-W – NE-SW oriented fold axial surfaces (F1) with a vergence towards N-NW. The fold axes and the lineation (L2) plunge moderately to steeply. A positive correlation between the metamorphic grade and the plunge of L2 has been suggested (Sjöström and Bergman, 1998). F2 axial surfaces have an
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tation varying between NW-SE – NNW-SSE. The western domain displays a complex structural pattern due to the c. 1.1 Ga Sveconorwegian overprint. The Sveconorwegian structures pre-dominantly strike WNW-ESE, whereas the Sveconorwegian structures are oriented N-S.
Major, steep dextral shear zones to the north of Bergslagen, dated to be late orogenic (Högdahl, 2000), are preferably striking WNW-ESE. The most significant deformation zone in the region is the Singö Shear Zone (SSZ) (Talbot and Sokoutis, 1995) with two southward splays, the Österbybruk-Skyttorp Zone and the Gimo Zone (e.g. Bergman et al., 1996; Persson and Sjöström, 2003). The SSZ is indicated here as the Singö Deformation Zone and the Forsmark Deformation Zone (Fig. 3-4). The Bergslagen domain was subjected to at least two phases of metamorphism, the Svecokarelian and the Sveconorwegian metamorphism events (Fig. 3-5). Penetrative ductile deformation took place between 1.87-1.86 Ga (Hermansson et al., 2008) mainly under amphibo-lite facies, although at some localities, granuamphibo-lite (western and south-eastern Bergslagen) and
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greenschist facies (northwest of the domain) do occur in connection with the Sveconorwegian orogen (e.g. Lundström, 1995). Geochronological studies carried out by Andersson (1997) indi-cated that most of the regional metamorphism and migmatization occured in the time span of 1.83-1.77 Ga. Prior to this, however, variable hydrothermal alterations affected the area to some extent. There is also contact metamorphism associated with an intrusive body yielding an age of 1.7 Ga (Stephens et al., 2009). In conjunction with the 1.87-1.86 Ga event, a younger, 1.8 Ga metamorphic event affected the southern structural domain.
It has been inferred that the isogrades (constant metamorphic grades) are likely to coincide with deformation zones and breaks in the form lines (general direction of the strike of a the ductile fabric) (Fig. 3.4) (Sjöström and Bergman, 1998). This has led to the conclusion that a structural control to some extent plays a significant role within the defined metamorphic domains.
Crustal thicknesses (Fig. 3-6) as well as observed gravity anomalies (Fig. 3-7) also show cou-pling to the metamorphic grade. It has been argued that higher-grade metamorphism occur in regions with distinctive crustal thicknesses indicated by gravity maxima (Stephens et al., 2009). These positive Bouguer anomalies correspond to excess of mass at depth within the crust, which
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indicates mafic underplating (Sjöström and Bergman, 1998; Stephens et al., 2009). Based on the gravity data, it has been inferred that southern Bergslagen was subjected to two separate phases of underplating (at or prior to 1.85 Ga and around 1.8 Ga) (Stephens et al., 2009). The gravity high in central Bergslagen has been attributed to granitic intrusions at shallow depth (Stephens et al., 2009).
3.2.2. Lambarfjärden
The studied area, Lambarfjärden and the surroundings, comprises metaigneous and metasedimen-tary rocks (Fig. 3-8). The former includes granodiorites and granites with gneissic transformation and frequent migmatization. These older rock segments (1.88 Ga, Persson and Persson, 1997) that occupy most of the studied area are locally intruded by granitic and gneissic veins (Persson et al., 2001). Large volumes of dolerite dykes which are often sub-parallel to the foliation and to the main fracture zones are present within these older metaigneous rocks. Grey or red colored, fine-to medium-grained, massive granites (the so-called Stockholm-granite) yielding a younger
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age (1.80 Ga, Ivarsson and Johansson, 1995) occur predominantly in lenses. The metasedimen-tary rocks include metagreywackes and mica schists with local migmatization and gneissic trans-formation. Both the metaigneous and metasedimentaries are intruded by granite, aplite and peg-matite dykes with either NE or NW orientation. NE-oriented metabasite xenoliths are present in small areas. Along the coast of Lambarjärden, mylonites and breccias have been observed.
Furthermore, a minor deformation zone (a normal fault) is exposed between Lovön and Hässelby.
The studied area has been subjected to a higher-grade metamorphism compared to other parts of the Bergslagen (Fig. 3-5). Almandin – cordierite – sillimanite – microcline occurrence indicates a low pressure (4 kbar) and high-temperature deformation (up to 700ºC locally) (Stålhös, 1969). Dominant geological directions show a good agreement with the regional structures: E-W, WNW and NW are the major trends of structures (Stålhös, 1969). Fracture sets within the Stockholm area were studied by Persson (1998), who described three major fracture sets: NW - WNW, ENE
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24 4. STRESS FIELD
4.1. Current regional stress field in Scandinavia
Focal mechanisms of the earthquakes and different in-situ stress measurements have provided an opportunity to identify the stress field in Scandinavia (e.g. Zoback et al., 1989; Gregersen et al., 1991; Bungum et al., 1991; Arvidsson and Kulhanek, 1994). The recent stress field is a result of the Atlantic ridge-push force (Stephansson et al., 1986; Slunga, 1989; Clauss et al., 1989) and the continent-continent collision between Europe and Africa (Gregersen, 1992; Müller et al., 1992). As a consequence, a NW-SE compressive horizontal stress direction can be observed in northern Europe (Fig. 4-1).
Both the maximum (σH) and minimum (σh) horizontal stress components exceed the vertical
stress (σv), which is considered to be the weight of the overburden rocks (Stephansson et al.,
1991).
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Even though the NW-SE maximum horizontal stress is dominating, small-scale variations do oc-cur (Stephansson, 1989; Clauss et al., 1989). Stephansson (1988) attributed these local deviations to three main aspects: (1) Existence of faults, (2) frictional strength and roughness discontinuities within the uppermost crust and (3) irregularities of faults and fracture patterns affecting the gravi-tational and tectonic stress field. However, below 300 m this scatter in the orientation of the max-imum horizontal stress is reduced, and displays mainly the NW-SE direction (Stephansson et al., 1991).
In addition to the far-field plate tectonic stresses, the post-glacial isostatic rebound also affects the crustal dynamics. The current rebound movements reflect a dome, centered in the northern Gulf of Bothnia with vertical velocity rates of 10-11 mm/yr. In the study area, these velocities are in the range of 5-7 mm/yr (Fig. 4-2). The horizontal movements are directed outwards of the up-lift center, with a rate of 0.7 mm/yr in the research area.
The seismotectonic pattern of Scandinavia consequently compiles the effects of the ridge-push force, continent-continent collision and the post-glacial rebound (e.g. Zoback, 1992; Marotta et al., 2004). This conclusion was also reflected by Arvidsson and Kulhanek (1994), who defined four main seismic zones within Scandinavia based on focal mechanism data constructed from earthquakes between 1976 and 1990 (Table 4-1). The zones differ in the occurring main sense of faulting and the orientation of compressional and extensional axes. The center of uplift is domi-nated by extension (i.e. normal faulting), whereas the brim shows increasing influence of com-pression (i.e. thrust faulting). In accord with this, the post-glacial rebound as an earthquake-generating mechanism plays an increasing role towards the uplift center.
26 Main sense of faulting Orientation of compressional axes Orientation of extensional axes
Triggering earthquake mech-anisms
Kattegat (offshore of southern
Swe-den)
strike-slip N-S E-W Ridge-push
South-central Sweden
mixed
(strike-slip to normal) NW-SE NE-SW
Ridge-push ≈ post-glacial re-bound
Eastern Sweden and Gulf of
Both-inia
normal N-S E-W (Ridge-push) « post-glacial rebound
Northern Lapland mixed WSW-ENE to
NW-SE N-S and NW-SE
Ridge-push ≈ post-glacial re-bound
Table 4-1 Defined seismic zones in Scandinavia with faulting characteristics, direction of compressional- and extensional axes and the major earthquake-generating mechanisms (based on Arvidsson and Kulhanek, 1994). The sedimentation at Kattegat overprint the otherwise dominating ridge-push force giv-ing rise to deviations in the axes, while the other localities display no influences other than the ridge-push and post-glacial rebound (Arvidsson and Kulhanek, 1994).
4.2. Current local stress field in eastern Bergslagen
The Bergslagen province is characterized by a thrust fault regime with a predominant NW-SE orientation of maximum horizontal stress (Fig. 4-1).
A detailed study of the local stress field has been carried out in view to the construction of the Forsmark waste disposal (Sjöberg et al., 2005; SKB, 2005; SKB, 2007). Even though Forsmark lies 140 km north of Stockholm, the stress observations and measurements could provide a good information for the Lambarfjärden (Fig. 4-1).
The stress measurements at Forsmark (overcoring, hydraulic fracturing, hydraulic testing of pexisting fractures and the study of borehole breakouts) have shown a dominant thrust fault re-gime. The principal stresses (σ1, σ2, σ3) in such a regime are identified as σH, σh and σv,
respec-tively. The orientation of σH is 145°, which is in agreement with the dominant regional NW-SE
direction.
In Forsmark, three main depth ranges (0-150 m, 150-400 m and 400-600 m) have been distin-guished in respect to the stress magnitudes. Since the Förbifart tunneling will only affect the up-per approximately 80 m of rocks, the deeup-per intervals will not be considered further. At this shal-low depth, a magnitude of 20 MPa for σ1 and 12 MPa for σ2 have been established (SKB, 2007).
The vertical stress increases with depth up to 4 MPa at 150 m.
A detailed description and further information on the stress field at Forsmark can be found in SKB (2007).
4.3. Palaeostress field and deformation history in eastern Bergslagen
27
the regional tectonic framework have made it possible to outline the development of different stress fields. Their study has shown that during the brittle-ductile transition at about 1.8 Ga, the area was characterized by transpression with NW-SE to N-S bulk shortening triggered by a σ1
with similar direction. In conjunction with these, a transition from ductile to a more brittle deformation occurred in shallower depths. Development of vertical and steep (dipping towards SW) deformation zones striking dominantly NW-SE to NNW-SSE were taken place during this period (Stephens et al., 2007) (Fig. 4-3 A).
28
During the latest Svecokarelian deformation (after 1.8 Ga), a N-S compression caused displacement along steep NW - SE, ENE - WSW and NNE – SSW-striking discontinuities (Stephens et al., 2007) (Fig. 4-3 B).
A transpressional stress field with NE-SW striking σ1 developed at 1.7 - 1.6 Ga, which resulted
reactivation of pre-existing steep, ENE - WSW and NNW –SSE-striking conjugate fractures (Stephens et al., 2007; Saintot et al., 2011) (Fig. 4-3 C). Compression occurred along flat discontinuities. These flat zones have been interpreted to both end at the steep ENE-NNE discontinuities and to be displaced by them (Stephens et al., 2007; SKB, 2011).
Several stages of rifting with accompanied sediment loading and subsequent erosion had occurred after 1.6 Ga (e.g. Stephens et al., 2007; Saintot et al., 2011) (Fig. 4-4).
During the Sveconorwegian event (1.1 – 0.9 Ga), the regional stress-field was again characterized by WNW-ESE σ1, which caused reactivation along pre-existing fracture surfaces (Fig. 4-3 D).
After this event, permutation of stress tensors has been suggested with uncertain triggering mechanisms. Scenarios for far-field tectonic influences (such as the Caledonide or Alpine events), as well as active regional palaeostress fields in northern Europe have been suggested as likely triggering mechanisms (Saintot et al., 2011).
29 5. BACKGROUND DATA
This chapter outlines the geological and geophysical data that was available for this MSc thesis, and have been used in order to gather crucial surface and subsurface information about the geo-logical framework of the Lambarfjärden.
5.1. Drill core data 5.1.1. 08F351K
The drill core comprises red - grey, fine - to medium-grained granite with occasional pegmatites. Within the interval of 82-83 m amphibolites have been found.
At the core length of 76 m, a 5 mm wide, NE-striking, shallow dipping gouge occur, whereas between 54 and 55 m, a brittle-ductile zone with NE strike and steep dip is present.
The most frequent mineral-fillings are calcite, chlorite and clay minerals. Red feldspar, hematite, quartz, talc and silty-sandy fracture surfaces occur less frequently in the core.
5.1.2. 08F352K
The core comprises grey-red, fine- to medium-grained granite with occasional pegmatites. Within the interval of 83-84 m amphibolite has been observed.
The core has revealed two breccia and one gauge occurrences. Breccias occur at core lengths of 56.3 m and 93.5 m with NW and NE-strikes, respectively. Both are moderately dipping. At a core length of 55 m, a 23 mm wide sub-horizontal, northerly-striking gouge has been found.
The most frequent mineral-fillings are calcite, chlorite and clay minerals, and many surfaces dis-play no mineral fillings. In conjunction with these, hematite, quartz, laumontite, red feldspar, prehnite, biotite, epidote and silt-sand occur on the surfaces less frequently.
5.1.3. 10F353K
The core dominantly comprises genisses of granitic origin, with occasional pegmatites and am-phibolites. The latter occur frequently in the interval of 124-146 m.
Breccias striking NW with moderate-steep dips are present at core length 205 m, 275 m and 358 m. At core length 358 m gouge occur with NW strike and moderate dip. Cataclastic and mylonitic structures occur more frequently towards the centre of Lambarfjärden. Both structures strike dominantly NW and have medium dips.
Calcite, chlorite, lamumontite, prehnite and clay minerals are the most frequent mineral-fillings, whereas dull and oxidized surfaces, pyrite, zeolite, feldspar, hematite, silt-sand and talk occur less frequently.
30 5.2. Water-loss measurements
Table 5-1 shows the amount of water-loss in 10 minutes in the boreholes 08F351K and 08F351K.
Well Borehole length interval (m) Water loss (l) per 10 minutes 08F351K 64.66 – 67.66 7.4 67.66 – 70.66 17.4 82.66 – 85.66 17.9 08F352K 53.80 – 56.80 208.6 71.80 – 74.80 10.6 77.80 – 80.80 19.9 80.80 – 83.80 33.7 83.80 – 86.80 11.5 86.80 – 89.90 6.4 95.80 – 98.80 4.1
Table 5-1 Water-loss in the 08F351K and 08F352K boreholes
There are two main intervals within the 08F351K, which display considerable amounts of water-loss: 67.66 - 70.66 m and 82.66 – 85.66 m. In the 08F352K, on the other hand, several intervals can be distinguished based on the amount of water-loss. The most prominent water-loss (yielding more than 200 liters) is observed between 53.8 and 56.8 m. Along a longer interval (77.80 – 89.90 m), different intensities of water-loss are observed, resulting in 71.5 liters of infiltration. In front of and behind this interval, two, 3 meters long intervals are observed with relatively less amount of water-loss.
The data clearly shows that the 08F352K well was subjected to higher amount of water-loss compared to the 08F351K.
31 Core length (m) Hydraulic conductivity [m/s] 18 - 21 4,74E-005 21 - 24 6,33E-006 24 - 27 7,36E-006 189 - 192 1,31E-005 273 - 276 1,21E-005
Table 5-2 The highest hydraulic conductivities in the borehole 10F353K.
5.3. Geophysical measurements
The subbottom profile method did not give any information about the depth to the bedrock, which was explained by the possibility that the sediments comprise gas (Nilsson, 2008).
The measured p-wave velocities through reflection and refraction seismics can be seen on Figs. 5-1, 5-2 and 5-3.
32
Velocities at the sediment-bedrock interface yield the lowest values (3500 m/s) where the greatest depths to bedrock occur. Shallow depths to the bedrock are usually represented by high veloci-ties.
The observed velocities in the bedrock generally fall within the expected gneiss velocity interval that is 4400-5200 m/s (Bourbie et al., 1987). Low-velocity zones (4000 m/s), similarly, occur in the deepest parts of the Lambarfjärden with the exception of the northwestern part. High veloci-ties can be observed mainly at shallower depth to the bedrock.
Velocity rates in the bedrock and at the sediment-bedrock interface do not necessarily coincide.
33
34 6. METHODS
This chapter outlines the methods that have been used, and which results contribute to the conclusions of this thesis.
Two main softwares have been used to acquire information of the background data; these are the Stereo32 and MOVE.
Stereo32 is a structural geological software through which orientation data can be displayed and evaluated. All of the stereographic projections that are included in this thesis correspond to equal area projections of the lower hemisphere. Moreover, all of the measurements in rose diagrams have been grouped into class sizes of 10.
MOVE is a commercial software developed for structural modeling and analysis. It has been used as supplementary software to visualize the study area, wells, and the fractures within them. The version MOVE 2011 has been used.
6.1. Surface fracture analysis
As it has been mentioned in the Background chapter, the surface mapping carried out by WSP comprised information of the fractures for the entire bypass area. In order to obtain the infor-mation and measurements for the Lambarfjärden, the relevant outcrops had to be extracted from the excel spreadsheets. Approximately three hundred outcrops were mapped on which several hundreds of measurements in total were made. After locating the outcrops and extracting the ac-companied measurements, the orientations (i.e. strikes and dips) of the weakness zones and the surface fractures were plot on stereographic projection plots using Stereo32.
6.2. Subsurface fracture analysis
In order to characterize the subsurface fracture sets and thus obtain crucial information about the underground geological framework, the orientation data of the fractures retrieved by the core log-ging has been used. More than one thousand fractures have been under examination.
Three main analyses were carried out. Firstly, an attempt has been made to see if there are any correlations between the mineral-fillings and the fracture orientations in the cores. This analysis has been done for all of the fractures, mineral-fillings and for every drill cores (08F351K, 08F352K and 10F353K). The results of this analysis, however, are provided only for the most frequent mineral-fillings.
In order to carry out such analysis, separation has been made based on the Mineral 1 occurrence, i.e. fractures with the same Mineral 1 have been sorted into one group. Within each group, the fractures have been plotted in one stereographic projection plot, and subsequently the fracture surfaces, alterations, apertures (that are the opening and are normal to the fracture surface) have been described. In the case when there are more minerals on the surface (i.e. Mineral 2, 3 and 4), it is described in such a manner that e.g. the Mineral 1 is likely to be accompanied by Mineral 2 and 3.
35
of the fracture surfaces, different colors have been attributed to the poleplots (black pole – fresh surface, green – slight alteration, yellow – moderate alteration, red - highly altered fracture sur-face, whereas light blue indicates the poles to the gouges). This differentiation has been made in order to detect possible correlation between the fracture orientations and alterations of the frac-ture surfaces.
Secondly, in attempt to detect possible changes in the fracture orientations along the 08F351K and 08F352K drill cores, four main depth-intervals (35 – 44 m, 44 – 53 m, 53 – 62 m and below 62 m) have been set. Within each interval, dip and strike values have been plotted in separate stereographic projection plots using Stereo32.
Thirdly, the 10F353K drill core has been divided into five, 70 meter long intervals (core length 3-70 m, 71-140 m, 141-210 m, 211-280 m, 281-358 m and 359-420 m) in order to detect possible changes in the fracture orientations and characteristics along the core length. Within each inter-val, the fractures have been plotted in a stereographic projection plot using Stereo32. Fracture frequencies, surfaces and alterations have also been described within each interval to have a bet-ter understanding of the subsurface framework.
6.3. Field work
In order to complement fracture mappings by WSP with field observations of kinematic indicators, one outcrop has been studied in detail, which was chosen after careful examination of the geological map (Fig. 3-8). That area was located close to the main fault. My objective was to find any kind of shear-sense indicators. The outcrop provided a great opportunity to understand the fracture orientations in 3D as both horizontal planes and vertical walls were exposed.
6.4. Correlation of the water-loss with subsurface fracture data
An attempt has been made to interpret the results of the water-loss measurements. This interpretation involved correlation of the occurring and the amount of water-loss with the subsurface fracture data (fracture orientations, frequencies and apertures).
For the 08F351K and 08F352K boreholes, this analysis has been based on the examination of the fracture frequency and aperture plots with regard to the occurring water-loss.
The 10F353K borehole, on the other hand, has been investigated after plotting the calculated hydraulic conductivity along the borehole. The hydraulic conductivity is the rate of flow through a unit medium, and has been calculated by GeoSigma within 3 meters of intervals along the 10F353K borehole. The plotted conductivities afterwards have been compared with the fracture frequencies and aperture values.
36 7. RESULTS
7.1. Surface fracture analysis
Surface fractures both in Hässelby and on Lovön show the same pattern (Figs. 7-1 A and 7-1 B). Both strikes and dips of the fractures show limited scatter, which results a distinct NE-SW striking subvertical fracture set, whereas subordinately, a NW-SE subvertical set occurs.
Fig. 7-1 Rose diagram and density contour lines showing the fracture orientations in A) Hässelby and on B) Lovön.
Weakness zones display a dominant NW-SE strike direction (Fig. 7-2).
37 7.2. Subsurface fracture analysis
7.2.1. Fracture description and mineral-fillings
The following section gives a short description of the fractures with regard to the fracture orienta-tions, number of fractures, most frequent mineral-fillings and fracture surface alterations in every drill cores (08F351K, 08F352K and 10F353K). The number in brackets after each mineral-filling indicates the number of fracture surfaces on which that certain mineral occurs as Mineral 1.
7.2.1.1. Drill core 08F351K
The fractures of drill core 08F351K show a dominant WNW-ESE and NW-SE strike with a mod-erate-steep dip; however, a certain amount of shallow dipping fractures also occur (Fig. 7-3).
The number of fractures along the core shows frequent alternations of highly and low-fractured zones (Fig. 7-4). The most fractured zone is present within the core length 68-78 m, whereas be-low this interval, the number of fractures decreases significantly.
38 Calcite (n=78)
Calcite is the most common fracture-filling mineral. Calcite-filled fractures dominantly show undulating and rough surfaces with slight or no alterations. Even though in most cases only cal-cite can be observed on the fracture planes, it is likely to be accompanied by other minerals as well; hematite, clay minerals and chlorite are the major minerals that occur together with calcite. The aperture values range between 0.5 – 1 mm, and have an average of 0.51 mm.
Orientation of the calcite fractures shows a dominant NW-SE and a less dominant E-W direction, dipping moderate-steep (Fig. 7-5).
No correlation can be observed between the orientations of the fresh and slightly altered frac-tures.
Clay minerals (n=66)
Clay minerals are the second most abundant fillings in this core. The fractures filled with clay minerals dominantly show rough and smooth, undulating surfaces. Although the slightly altered fractures dominate, moderate and high alterations, as well as gouge can be observed. Clays min-erals are mainly accompanied by other mineral assemblages, like hematite, chlorite or calcite. The aperture values range from 0.5 to 1 mm, with an average of 0.51 mm.
Even though orientations of the fractures show a great scatter, a dominant NW-SE, as well as a minor WSW-ENE direction can be observed, in conjunction with moderate-steep dips (Fig. 7-5).The gouge and the two highly altered fractures dip shallowly; otherwise no correlation can be observed either in the orientations or dips of the differently altered fractures.
0 2 4 6 8 10 12 14 48 58 68 78 88 98 Num be r of f rac tur es Core length (m)
39
40 Chlorite (n=43)
The chlorite-filled fractures have dominantly undulating, slightly altered surfaces that are either rough or smooth. Most of these fracture surfaces display several mineral occurrences other than chlorite. Clay minerals, calcite and hematite are likely to appear as secondary minerals.
The aperture values range from 0.5 to 9.5 mm, with an average of 0.71 mm,
The fractures show medium-steep dips with orientations of WNW-ENE (Fig. 7-5).The moderate-ly altered fractures are likemoderate-ly to have a shallow-moderate dip.
7.2.1.2. Drill core 08F352K
Orientation of the fractures in the well shows a dominant WNW-ENE and N-S direction (Fig. 7-6). The majority of the fractures have a shallow or steep dip, but a large amount of moderately dipping fractures also occur.
Alternations of highly fractured and less-fractured zones along the core are more prominent com-pared to the 08F351K drill core (Fig. 7-7).
Calcite (n=64)
Fractures with calcite coating are the most abundant in this core. These fractures dominantly have rough and undulating surfaces either with no or slight alterations. A considerable amount of frac-ture surfaces display additional minerals, such as chlorite, hematite and clay minerals.
Fracture apertures range between 0.5 and 1.5 mm, and have an average of 0.54 mm.
A predominant WNW strike can be observed, although fractures with NE-SW orientation are also frequent (Fig. 7-5). Moderate-steeply dipping fractures comprise most of the calcite-filled frac-tures; however, shallow dips also appear. The two slightly altered fractures display shallow dips as well.
41 Chlorite (n=43)
The second most abundant fracture-filling mineral in this core is chlorite. These fractures have dominantly undulating, slightly altered surfaces that are either rough or smooth. Occasionally moderately altered surfaces can be found.
Chlorite is likely to be accompanied by other minerals, such as hematite, clay minerals and cal-cite.
The aperture values range from 0.5 mm to 1.5 mm, and have an average of 0.53 mm.
Orientation of the fractures shows a predominant E-W strike, whereas they dip mainly shallowly or steeply (Fig. 7-5).
Non-coated fracture surfaces (n=30)
A great amount of fracture surfaces are non-coated. These generally fresh fractures mainly have undulating and rough surfaces. In several cases, however, slight alterations occur.
The fractures have a constant aperture of 0.5 mm, and strike predominantly E-W and NNW-SSE (Fig. 7-8). The fractures mainly have shallow or moderate dips.
0 2 4 6 8 10 12 14 48 58 68 78 88 98 N um ber of fr act ure s Core length (m)
Fig. 7-7 Fracture frequencies in the 08F352K drill core.
42 Clay minerals (n=16)
Most of the fractures filled with clay minerals have smooth surfaces, but rough surfaces also oc-cur. Even though these surfaces are dominantly undulating, stepped and planar surfaces appear as well. The fractures are primarily slightly altered, although a considerable amount of moderately and highly altered fractures, as well as gouge can be observed.
Clay minerals are frequently associated with other mineral assemblages, such as chlorite, hema-tite and calcite.
Aperture values range within the interval of 0.5 – 5 mm, and have an average of 0.78 mm.
The fractures most frequently strike NE-SW, and they display various but mainly shallow dips (Fig. 7-5).
7.2.1.3. Drill core 10F353K
The 10F353K drill core is characterized mainly by three sets of fractures: a NW-SE-oriented set, which is the most frequent; a NE-SW and an E-W-oriented set. The fractures are dominantly horizontal and steep-vertical (Fig. 7-9 A and B).
Fig. 7-9 Rose diagram and density contour lines showing the fracture orientations in the 10F353K drill core between core lengths A) 3-207 meters and B) 208-358 meters.
Calcite (n=448)
Calcite is the most common fracture-filling mineral in the core. These fractures dominantly show slight alterations, although to some extent fresh and moderately altered fractures also occur. Only two fracture surfaces display high alterations.
43
The average aperture of the fractures is 0.63 mm, but the values are varying between 0.5 and 15 mm.
The calcite fractures display mainly steep dips and show two dominant strikes, NW-SE and NE-SW (Fig. 7-5). No correlation can be observed in the orientations of the differently altered frac-tures.
Chlorite (n=206)
Chlorite is the second most abundant fracture-filling mineral in the core. These fractures domi-nantly show slight alterations, however, a large number of moderately altered surfaces can be found. Nevertheless, four fracture surfaces display high alterations.
Chlorite is likely to be accompanied mainly by calcite, but minerals such as biotite, pyrite, lau-montite and clay minerals are also very common to occur together with it. However, a small number of fracture surfaces host more than two minerals (i.e. display Mineral 3 as well). In these cases calcite, pyrite, clay minerals, laumontite and zeolite can be observed.
The average aperture of the fractures is 0.58 mm, but the values are varying between 0.5 and 3 mm.
The chlorite-filled fractures show dominant NW-SE and E-W strikes (Fig. 7-5). Dip values range mainly in the shallow-moderate interval with a few fractures showing steep dips.
Laumontite (n=79)
Fractures dominated by laumontite have mainly slightly altered surfaces with a few showing moderate alterations. Laumontite is likely to be accompanied by other minerals, mainly by chlo-rite and calcite. In a few cases, however, Mineral 3 also occur, such as chlochlo-rite or pychlo-rite, occur together with laumontite.
The average aperture of the fractures is 0.51 mm, but the values are varying between 0.5 and 1.5 mm.
The laumontite fractures show two main sets of strikes, NW-SE and WNW-ESE (Fig. 7-10).The fractures display mainly moderate-steep dips.
Non-coated fracture surfaces (n=55)
A great number of fracture surfaces are non-coated. These fractures are either slightly altered or fresh.
The average aperture of the fractures is 0.89 mm, but the values are varying between 0.5 and 9 mm; the latter corresponds to the only moderately altered fracture.
44
A B C
45 Prehnite (n=44)
Fracture surfaces displaying prehnite are dominantly slightly altered. Nevertheless, a few moder-ately altered, as well as one highly altered fracture also occur.
Even though prehnite occurs solely on a number of surfaces, it is likely to be accompanied by several other minerals, such as chlorite, calcite, laumontite and clay minerals. In a few cases, however, calcite, clay minerals or hematite, occur as Mineral 3 on the prehnite-coated fracture surfaces.
The fracture aperture values are ranging between 0.5 and 6 mm. The average aperture is 0.81 mm.
The fractures show a dominant NW-SE strike and mainly moderate dip (Fig. 7-10). Clay minerals (n=31)
Fractures surfaces displaying clay minerals are mainly moderately altered, but slight alterations, as well as a few high alterations also occur.
Clay minerals are mainly accompanied by other minerals, such as calcite, hematite and chlorite. In a few cases, however, more than one mineral, such as hematite and calcite, occurs together with clay.
The fracture aperture values are ranging between 0.5 and 1 mm, whereas the average aperture is 0.61 mm.
A dominant E-W strike can be observed, in addition to moderate-steep dips (Fig. 7-5).
7.2.2. Comparison of fracture orientations in drill cores 08F351K and 08F352K
Four main depth-intervals (35 – 44 m, 44 – 53 m, 53 – 62 m and below 62 m) have been set in attempt to detect possible changes in fracture orientations with vertical depth along the 08F351K and 08F352K drill cores. Within each interval, dip and strike values have been plotted in separate stereographic projection plots regardless of the mineral-fillings (Fig. 7-11).
The fractures in 08F351K drill core are characterized by steep dips along the entire core, whereas a slight change in the fracture strikes can be observed. In the first interval the fractures show a dominant WNW-ESE strike. This strike direction is still present in the depth interval of 44 – 53 meters; however, another peak appears in the direction of NW-SE. Below 53 m, this NW-SE strike is dominating. At the same time, the WNW-ESE striking fractures become less abundant. The fractures transecting the 08F352K drill core display a different pattern. Between 35 and 44 meters, the fractures mainly have WNW-ESE and NW-SE strikes. At the interval of 44 and 55 meters, two main fractures sets, striking N-S and E-W, can be observed, whereas between the depth of 53 and 62 meters, the fractures dominantly strike E-W. Even though below 62 m the NW-SE striking set is still frequent, a NE-SW-striking set appears and dominates.
46 08F351K 08F352K 35 – 44 m 44 – 53 m 53 – 62 m 62 – 75 m n = 49 Maximum = 5.0 Contour intervals = 10 Maximum density = 9.01 n = 100 Maximum = 7.0 Contour intervals = 10 Maximum density = 20.3 n = 72 Maximum = 7.0 Contour intervals = 10 Maximum density = 10.0 n = 26 Maximum = 3.0 Contour intervals = 10 Maximum density = 6.85 n = 69 Maximum = 3.5 Contour intervals = 10 Maximum density = 11.4 n = 37 Maximum = 3.0 Contour intervals = 10 Maximum density = 5.89 n = 59 Maximum = 4.0 Contour intervals = 10 Maximum density = 9.96 n = 70 Maximum = 4.0 Contour intervals = 10 Maximum density = 14.6
Fig. 7-11 a) N-S cross section showing the boreholes location relative to each other, and the two cores (08F351K and 08F352K) where the b) frac-ture orientations are shown in four depth intervals.
Depth intervals a)
47
7.2.3. Description of the fracture orientations along the 10F353K
The drill core 10F353K has been divided arbitrarily into five, 70 meter long intervals (core length 3-70 m, 71-140 m, 141-210 m, 211-280 m, 281-358 m and 359-420 m) in order to detect possible changes in the fracture orientations and characteristics along the core length. Within each interval the fractures have been plotted in a stereographic projection plot (regardless of the mineral-fillings) (Fig. 7-12), and subsequently the fracture frequencies, surfaces and alterations have been described.
3 - 70 m (n = 153)
Strikes of the fractures in this interval show a relatively big scatter, nevertheless a dominant NE-SW and E-W direction can be observed. The fractures mainly display horizontal-shallow dips. The first half of the interval indicates several, highly fractured zones with 7-8 fractures per meter. The highest number of fractures (n=16) is found at 24 m; however, the average number of frac-tures in this entire core interval is 2.24 per meter.
The interval is characterized by several non-fractured core segments that are becoming more fre-quent in the second half of the interval. Simultaneously, the number of fractures is decreasing. Slight alterations are the most common, but many surfaces are fresh. Only a few surfaces show moderate alterations.
71 - 140 m (n = 101)
The fractures predominately strike NW-SE and NE-SW, and mainly display horizontal-shallow dips. However, considerable amount of fractures have steep dips.
The total length of the fracture-free segments, that are most frequent between 98-119 m, reaches 27 m, making this interval the most unfractured in the entire 10F353K drill core. The average number of fractures within this interval is 1.46 per meter.
The fracture surfaces are mainly characterized by slight alterations, but fresh surfaces also occur occasionally.
141 - 210 m (n = 91)
The fracture orientation in this interval shows two distinct strikes, NW-SE and NE-SW. The frac-tures mainly display steep-vertical dips.
The interval is characterized by a total of 26 meters of non-fractured segments. At 143 m a highly fractured interval with 11 fractures is detected; most of them have moderately altered surfaces. The average number of fractures is 1.27 per meter, which gives the least amount of fractures within one interval.
Slight alterations are the most frequent, however, compared to the previous two intervals the number of moderately altered surfaces has increased, whereas simultaneously, the number of fresh surfaces has decreased.
211 - 280 m (n = 117)
Strikes of the fractures are NW-SE and NE-SW, whereas the dips are mainly steep-vertical. However, a considerable amount of shallow dipping fractures also occurs.
48
49
The number of fresh surfaces has significantly decreased, whereas highly altered surfaces have appeared. Most of the fracture surfaces, however, display slight alterations.
281 - 358 m (n = 286)
In this interval, fractures with WNW-ESE strikes are dominant with mainly moderate dips.
Segments without fractures are rare and the number of fractures is simultaneously increasing along the core length. This is also mirrored in the average number of fractures, which reaches 3.67 per meter.
Fresh surfaces have significantly decreased, whereas the moderately and highly altered surfaces have become more frequent compared to the previous interval. Slight alterations still dominate. 359 – 420 m (n = 284)
In this interval, no information regarding the orientations of the fractures is available due to the failure of the BIP sond passing through this section. The reason for this was that the borehole wall was uneven, which was probably as a result of rocks fallen down from the wall.
Only one, non-fractured meter has been described at 368 m. The average number of fractures has increased significantly to 4.55 per meter.
Slightly altered surfaces are the most common; however, moderately altered surfaces have notably increased compared to the previous intervals.
7.3. Field work
The examined outcrop consists of metagreywacke and metagranitoid with granitic to granodiorit-ic composition. Both suits showed strong deformation and quartz-veining.
Conjugate fracture sets with highly consistent strikes were found; one conjugate set was observed both on a vertical wall (Fig. 7-13) and on a horizontal outcrop surface (Fig. 7-14). On the latter, it has a strike of ENE - WSW. The other conjugate set is vertical and has a strike of NNE - SSW and NW - SE (Fig. 7-14).
The NNE – SSW-striking fractures locally display slight bending (dashed lines on Fig. 7-14). It is uncertain, if the E - W-striking fracture ends in the NNE - SSW fracture, or it continues (dashed line) and becomes parallel to the NNE - SSW direction.
The outcrop was examined for kinematic indicators that would provide information about a possible shearing. No brittle kinematic indicator has been found. However, ductile kinematic indicator has been observed. The outcrop displayed one strongly deformed gneiss part with observable foliation (Fig. 7-15). Even though the surface was highly weathered, the foliation and the veining made it possible to observe that along a quartz-feldspatic vein (oriented NNE) the foliation and small veins are bending with an apparent top to the north direction. As the surface plane, on which the foliation bending has been observed, is parallel to the stretching lineation, the plane is sufficient to determine possible sense of movements. The top to the left direction, thus, attributes to sinistral strike-slip.
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Fig. 7-14 Vertical wall showing conjugate fractures in the granitic suite.
