Electronic Supplement 1: Field descriptions of faults and host rocks

Døsjeneset, Sotra (BG–045a and BG–045b)The fault is exposed in an old quarry (now used as a junk yard and storage area for construction material) opposite the marina at Døsjeneset, along Døsjevegen, approximately 1.3 km SSE of the bridge between Bildøy and Sotra (GPS 285450 E, 6695860 N, zone 32). Geologically, the outcrop is located in the Øygarden Complex. The host rocks are medium- to coarse-grained banded and migmatitic gneisses of granitic composition (Fig. A1.1A, C). K-feldspar shows a pronounced red colour throughout most of the outcrop. The host rocks are well foliated; the foliation strikes approximately E–W (085), with moderate dips (39°) to the S. The fault strikes NNE–SSW (022) and dips steeply (80°) to the ESE. No obvious kinematic indicators were observed. The fault, including both fault core and damage zone, is approximately 7 m wide (Fig. A1.1A). The damage zone, adjacent to the footwall, is characterised by extensively fractured host rock. It can be distinguished from the brecciated fault core by the still recognisable and mostly continuous foliation (Fig. A1.1D). The fault core, adjacent to the hanging wall, is 2 m wide at the foot of the outcrop (Fig. A1.1B) and comprises extensively fractured to completely brecciated host rock (Fig. A1.1E) and thin layers and pockets of clay gouge (Fig. A1.1F). Towards the top of the outcrop, the fault core becomes more focused, narrowing to approximately 30 cm width (Fig. A1.1B). In the lower part of the outcrop, three distinct fault gouge strands separate breccia and larger blocks. Two of these gouges were sampled; sample BG–045a was collected in the centre of the fault core and sample BG–045b closer to the hanging wall. The gouge is dark grey, sandy and non-foliated to very poorly foliated (Fig. A1.1F). In the same quarry, a second fault zone is exposed, though not easily accessible. It is approximately 10 m wide and includes two main subvertical and N–S striking faults. The fault core of the western fault is marked by an approximately 1 m-thick green cataclasite.

Figure A1.1. Field photographs of the fault at Døsjeneset. (A) The entire fault is approximately 7 m wide, with a damage zone adjacent to the footwall and a 0.3–2 m wide fault core adjacent to the hanging wall. Yellow dotted lines mark the trace of the foliation in the host rock. The damage zone is intensely fractured but the foliation is still mostly recognisable and continuous. (B) Fault core with sample locations. Red lines mark distinct clay gouge strands in the lower part of the outcrop. (C) Undisturbed host rock; banded and migmatitic granitic gneisses. (D) Fractured host rock in the damage zone. Yellow dotted lines mark still recognisable foliation. (E) Breccia in the fault core. (F) Dark grey, poorly foliated sandy fault gouge; close to the sample location of BG–045a.

Knappetunnelen (KN1 and KN2)Samples KN1 and KN2 were collected during the construction of Knappetunnelen, a tunnel forming part of the Ringvei Vest, western Bergen. The fault was indicated by Statens Vegvesen during reconnaissance as a potential fracture zone due to its linear expression at the surface (T. Kirkeby, chief geologist at Statens Vegvesen, pers. comm., 2013, 2015). The fault was encountered approximately 1100 m from the northern tunnel entrance at Liavatnet (approx. GPS 293700 E, 6697080 N, zone 32). The fault strikes NNE–SSW (200) and dips steeply (70–80°) towards the WNW. Slickenlines on a slip surface indicate an oblique strike-slip dominated movement (20→214). The host rock is part of the Øygarden Complex and includes alternating layers of quartzo-feldspathic gneiss and amphibolite (Fig. A1.2A). The hanging wall seems to be more granitic in composition than the footwall. Displacement is impossible to determine; however, the surface lineament is significant and can be followed for at least 5 km. The entire fault zone is at least 20 m wide, with an approximately 10 m-wide damage zone on both sides of the fault core. The latter is 1–2 m wide and bound by two distinct principal slip surfaces, PSS1 (towards the footwall) and PSS2 (towards the hanging wall; Fig. A1.2A). The fault core comprises lenses of brecciated granitic gneiss surrounded by layers and lenses of clay gouge. Based on two outcrops on the eastern and western tunnel walls, two distinct clay types could be observed, one green continuous layer at the hanging wall side (PSS2, sample KN2; Fig. A1.2C) and an alternating light to dark grey clay found as discontinuous lenses at the footwall side of the fault (sample KN1; Fig. A1.2B). The latter surrounds lenses of brecciated host rock. The entire fault zone was secured by a massive steel construction and covered by concrete in order to prevent rock fall.

Figure A1.2. Field photographs of the fault in the Knappetunnel. (A) View of the western tunnel wall. The fault core is 1–2 m wide and comprises lenses of host rock which are surrounded by layers and lenses of clay gouge. Principal slip surface PSS1 bounds the fault core towards the footwall and PSS2 towards the hanging wall. (B) Sample location for KN1, collected at the tunnel floor approximately 3 m east of the western tunnel wall. (C) View in the direction of tunnel drilling (towards S) when the fault was first encountered. The eastern tunnel wall is already covered in concrete on the left side of the picture. This is the characteristic green gouge of PSS2 close to the hanging wall. Sample KN2 was collected 1 m below this exposure. All photographs by Terje Kirkeby, Statens Vegvesen.

Ytrebygdsvegen (BG–115)The fault is exposed at the junction between Storrinden and Ytrebygdsvegen, approximately 350 m NNW of the Kokstadvegen/Ytrebygdsvegen roundabout (GPS 293780 E, 6691400 N, zone 32). The fault is hosted in white to greenish, fine- to medium-grained meta-anorthosites of the Lindås Nappe (Fig. A1.3A, E). The host rock is well foliated and lineated; the foliation strikes approximately E–W (087), with moderate dips (37°) to the south, and the lineation plunges 30° towards the SSE (159). Foliation planes are speckled with flakes of white mica up to 1 mm in size. The fault strikes NW–SE (138) and dips 54° to the SW. The outcrop is parallel to the fault plane, exposing the hanging wall and part of the fault core (Fig. A1.3A, B). The immediate footwall is not exposed at this location and the width of the fault can therefore not be determined. Coming from the hanging wall, two distinct fault rocks can be identified: (1) 20–30 cm of green, fine-grained, strongly sheared meta-anorthosite. The foliation of the host rock seems to bend into this zone that is characterised by many undulating shear planes, commonly with thin calcite coatings (Fig. A1.3B, C). Striations on shear planes and calcite coatings plunge gently (20°) towards the SSE (147). (2) Digging at the foot of the outcrop has exposed 10–20 cm of foliated clay gouge (Fig. A1.3D). The gouge is very clay-rich and mostly of a pale greenish-grey colour with dark grey and brownish layers towards the footwall. The dated sample is a sticky, plastic, greenish-grey clay collected close to the hanging wall.

Figure A1.3. Field photographs of the fault along Ytrebygdsvegen. (A) The outcrop is approximately parallel to the fault, exposing the hanging wall and part of the fault core. Clay gouge is exposed at the foot of the outcrop. (B) The slightly overhanging outcrop shows the fine-grained, green, strongly sheared variety of the anorthosite in the fault. (C) Detailed picture, position indicated in (B). Striations (yellow dashed lines) plunge gently towards the SSE. Cc marks calcite coatings. (D) Clay gouge, position indicated in (B). The gouge is foliated and very clay-rich. Sample was collected close to the hanging wall. (E) Meta-anorthosite host rock.

Terminalvegen (BG–116)The fault is exposed along Terminalvegen, just west of the junction with Hardangervegen, between Indre Arna and Espeland (GPS 305380 E, 6701115 N, zone 32). Geologically, the outcrop is part of the Lindås Nappe. The host rock is a fine- to medium-grained, thinly banded gneiss (metagabbro to meta-anorthosite) with alternating white plagioclase-rich layers and black to dark green amphibole-rich layers on the mm- to cm-scale (Fig. A1.4A, D). White mica flakes up to 1 mm in size can be observed on the foliation planes. The well-developed foliation strikes N–S to NNE–SSW (008–023) with steep dips (70–90°) both towards the east and the west, reflecting folding around generally N–S-trending fold axes. A pronounced lineation plunges shallowly (3–17°) towards the N to NNE (009–017). Two major and several minor faults are exposed in this outcrop, in addition to several calcite-filled fractures. The latter are 1–20 cm wide with mostly straight walls and filled by very coarse-grained calcite (Fig. A1.4A). They strike WNW–ESE to NW–SE (115–146) with moderate to step dips (54–80°) towards the SSW and SW. In the northern half of the outcrop, one calcite vein seems to be offset by one of the two main faults, suggesting that the calcite veins are either cogenetic with the faults or predate them. This fault is subvertical and strikes NW–SE (127) and is marked by an approximately 3 m-wide damage zone and strongly brecciated 0.5 m-wide fault core, including a very thin layer of fault gouge. Gently plunging striations (27→118) indicate an oblique strike-slip dominated movement. The sampled fault is exposed close to the southern end of the outcrop. It strikes ENE–WSW (068) and dips 60° towards the SSE. The fault core is 10–25 cm wide and comprises strongly brecciated wall rock and a light grey, crumbly, sandy fault gouge (Fig. A1.4B, C). Fracturing in the damage zone extends up to 1 m into the hanging wall. No obvious kinematic indicators were observed on this fault.

Figure A1.4. Field photographs of the fault along Terminalvegen. (A) Overview of the southern part of the outcrop, showing the sampled fault and one of the calcite-filled fractures (detailed view as inset). (B) Fault with sample location. (C) The sampled pale greenish-grey sandy fault gouge. (D) Host rock: banded gneiss of metagabbroic to meta-anorthositic composition.

Olsvik Båthavn (BG–110)The fault is exposed at the parking area of Olsvik Båthavn (marina), approximately 1 km west of Vindenes on Sotra (GPS: 278930 E, 6705210 N, zone 32). The host rock belongs to the Øygarden Complex and is a grey, medium-grained granitic gneiss (Fig. A1.5D) with rare amphibolite layers of several centimetres to decimetres thickness. The host rock is well foliated and lineated. The foliation strikes NE–SW (140) and dips shallowly (18°) to the SE. The lineation plunges shallowly (10°) towards the E (097). The sampled fault marks the western boundary of a 35 m-wide, NNW–SSE-striking fracture zone, which can be followed on aerial photographs for at least 2 km. In this larger fracture zone, 1–3 m-wide intensely fractured sections alternate with 2–6 m-wide sections with fewer fractures (Fig A1.5A). Most of the fractures observed in the outcrop show no or little sign of shear movement and have been classified as joints rather than faults. In the intensely fractured sections, joint spacing is on the mm- to cm-scale; in the more massive parts of the outcrop, joints are typically several decimetres to metres apart. Joint orientations range from NNW–SSE to N–S (330–353) with steep dips (60–86°) to the ENE and E. In the eastern part of the outcrop, one of the intensely fractured sections cuts an older NNE–SSW-striking fault associated with intense red feldspar alteration and epidote mineralisation. Rare amphibolite layers show intense alteration (weathering?) where they are intersected by joints. The sampled fault strikes NNW–SSE (335) and dips steeply (79°) to the ENE. It is approximately 0.5 m wide with a brecciated fault core that includes thin layers and lenses of a dark grey to brown, clay-rich fault gouge (Fig. A1.5B, C). The damage zone, characterised by dm-spaced jointing, extends approximately 0.5 m into the footwall and several metres into the hanging wall.

Figure A1.5. Field photographs of the fault at Olsvik Båthavn. (A) The sampled fault marks the western boundary of an approximately 35 m wide NNW–SSE striking fracture zone with alternating strongly fractured sections and more massive sections. (B) The fault core is approximately 0.5 m wide and comprises breccia and lenses of a dark grey to brown sandy clay gouge. (C) Detailed photograph of fault gouge. (D) Granitic gneiss host rock.

Espelandstunnelen (BG–048)The sample was collected by Harald Hauso (Statens Vegvesen) during construction of Espelandstunnelen, along county road no. 57, close to Herland, Lindås. The host rocks are banded and locally migmatitic gneisses and amphibolites of the Lindås Nappe. The fault was reported to be parallel to the foliation, which strikes NNW–SSE in the area with steep dips to the ESE. Statens Vegvesen encountered several faults with similar orientation during tunnel drilling (H. Hauso, pers. comm., 2007, 2016). The sample is from a 10 cm-wide zone of breccia and fault gouge close to the northern tunnel entrance.

Bjørsvik (BG–134)The fault is exposed along road E39 (Fig. A1.6A), at the neck of the Lindås peninsula, just east of the turnoff to Bjørsvik (GPS: 308065 E, 6727600 N, zone 32). Geologically, the outcrop is located within the Bergen Arc Shear Zone (BASZ), a 2–3 km-wide arcuate shear zone that separates, in the Lindås–Osterøy area, the Western Gneiss Region in the footwall from the Bergen Arc System and Devonian conglomerates of the Fensfjorden Basin in the hanging wall (e.g., Wennberg et al., 1998). At the neck of the Lindås peninsula, the BASZ reworks rocks of the Western Gneiss Region, the Kvalsida Gneiss (correlated with either the Lindås Nappe or the Jotun Nappe) and the Major Bergen Arc. In this area, the BASZ is an oblique (dextral-normal) shear zone with at least 16 km of displacement (Wennberg et al., 1998). The BASZ was reactivated by a major brittle fault, the Fensfjord Fault. This fault runs parallel to the BASZ along the Fensfjord and Austfjord and has a strong topographic expression where it is exposed on land between Bjørsvik and Stallen (Lindås peninsula), as well as on Osterøy. The fault that we sampled is located in the footwall of the Fensfjord Fault. The host rocks are grey, fine-grained granitic (mylonitic) gneisses of the Western Gneiss Region (Fig. A1.6D) and dark grey garnet-mica schists (Fig. A1.6E) that Wennberg et al. (1998) interpreted as slices of the Lower Allochthon. The foliation strikes NW–SE with moderate dips towards the SW. The fault seems to exploit one of the weaker mica schist layers and strikes NNW–SSE (158) with a moderate dip (54°) to the WSW. In the upper part of the outcrop, the fault core is well defined and 10–20 cm wide (Fig. A1.6B). It comprises a dark grey to brown, crumbly, completely brecciated variety of the mica schist, with 2–3 cm of a grey plastic clay gouge in the centre (Fig. A1.6C); the latter was sampled. The hanging wall shows little damage, but there is significant brecciation of the immediate footwall. In the lower part of the outcrop, the fault splays into a several metre wide, strongly brecciated zone (Fig. A1.6A). No obvious kinematic indicators were observed on the fault directly, but a number of minor slip surfaces at low angles with the sampled fault (strikes between 128–182, dips between 47–80° to the SW and W) show lineations plunging 28–76° towards the SE to S.

Grønnevikstunnelen (GT) and Bjorøytunnelen (BT)Field descriptions and photographs of the faults in Grønnevikstunnelen and Bjorøytunnelen can be found in Rasmussen (2013) and Fossen et al. (1997), respectively.

Figure A1.6. Field photographs of the fault in Bjørsvik. (A) The fault is well-defined in the upper part of the outcrop where the sample was collected but splays into a wide, strongly brecciated zone at in the lower part of the outcrop. (B) Close-up of sample location. Little deformation in the hanging wall, but considerable brecciated of the immediate footwall. (C) Incohesive, crumbly, completely brecciated micaschist in the fault core with a 2–3 cm wide plastic grey clay gouge that was sampled. (D) Fine-grained granitic (mylonitic) gneiss that is the dominant rock type in the outcrop. (E) Dark grey garnet micascshist; the fault seems to exploit one of the weaker micaschist layers.

References (Electronic Supplement 1)Fossen, H., Mangerud, G., Hesthammer, J., Bugge, T. & Gabrielsen, R.H. 1997: The Bjorøy Formation:

a newly discovered occurrence of Jurassic sediments in the Bergen Arc System. Norsk Geologisk Tidsskrift 77, 269–287.

Rasmussen, J.B.A. 2013: Detaljstudie av skyvekontakt mellom Vesle Bergensboge og Øygardskomplekset med tunneltekniske aspekt. MSc thesis, University of Bergen, 98 pp.

Wennberg, O.P., Milnes, A.G. & Winsvold, I. 1998: The northern Bergen Arc Shear Zone — an oblique-lateral ramp in the Devonian extensional detachment system of western Norway. Norsk Geologisk Tidsskrift 78, 169–184.