The movement history and fault rock evolution of a reactivated crustal-scale strike-slip fault: the Walls Boundary Fault Zone, Shetland

Transcription

1 Journal of the Geological Society, London, Vol. 164, 2007, pp Printed in Great Britain. The movement history and fault rock evolution of a reactivated crustal-scale strike-slip fault: the Walls Boundary Fault Zone, Shetland L. M. WATTS 1,2,R.E.HOLDSWORTH 1,J.A.SLEIGHT 1,2,R.A.STRACHAN 3 &S.A.F.SMITH 1 1 Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK ( r.e.holdsworth@durham.ac.uk) 2 Present address: EPE/21 Brunei Shell Petroleum Co. Sdn. Bhd., Julan Utara, KB3534 Seria, Negara Brunei Darussalam 3 School of Earth & Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK Abstract: The Walls Boundary Fault Zone is a crustal-scale strike-slip fault that cuts Precambrian Caledonian basement terranes in Shetland and has been interpreted as the northern continuation of the Great Glen Fault Zone in Scotland. This paper presents the first detailed account of the kinematic history and fault rock assemblages associated with the onshore evolution of the Walls Boundary Fault Zone. These observations suggest that it initiated as a major late Caledonian (Silurian Devonian) sinistral strike-slip fault associated with the successive development of mylonites and cataclasites. These fault rocks are preserved only locally, and elsewhere are obscured by the effects of later brittle overprinting and dismemberment of the fault zone during dextral strike-slip reactivation, probably during late Carboniferous inversion of the Orcadian Basin. This led to the development of extensive cataclasite and fault gouge assemblages, which are widely preserved along the Walls Boundary Fault. Narrow zones of post-triassic dip-slip, and finally sinistral strike-slip, reactivation are localized within earlier-formed gouge-filled fault cores. There are some similarities to the kinematic history of the Great Glen Fault Zone, most notably the recognition of late Caledonian sinistral shear and post-devonian dextral reactivation, but the post-triassic reactivation histories appear to differ significantly. It is widely recognized that the reactivation of crustal-scale structures, especially strike-slip faults, plays a central role during the deformation of continental crust, and that such discontinuities also often control the location and geometric evolution of sedimentary basins during rifting (e.g. Doré et al. 1997; Holdsworth et al. 1997, 2001). Located approximately equidistant from the Scottish mainland and Norway, the Walls Boundary Fault Zone in Shetland occupies a key location in the Caledonides and is thought to have had an important role in controlling basin evolution in the United Kingdom Continental Shelf from the Late Palaeozoic to the present day. Following the pioneering study of Flinn (1977), the Walls Boundary Fault Zone is cited widely as an example of a major reactivated structure, but the detailed onshore geometry, kinematic history and fault rock assemblages have been only briefly described. In this paper, we document the kinematic, textural and metamorphic evolution of the Walls Boundary Fault Zone, including the fault rock distributions, field relationships, textures and microstructures. We then compare these findings with those made along major reactivated structures along-strike in both Scotland and Norway to better assess the importance of the Walls Boundary Fault Zone in the tectonic evolution of the NE Atlantic margin. Regional setting Country rocks The Shetland Islands (Fig. 1) form part of a Precambrian to Early Palaeozoic basement window surrounded by Devonian and, offshore, younger sedimentary rocks. The islands form part of a Mesozoic Cenozoic basement high, the Shetland Platform, which separates the Central and Viking grabens to the east from the West Shetland and Faeroe Shetland basins to the west (Fig. 1; e.g. Johnson et al. 1993). The Shetland Platform is transected along its length by a series of steeply dipping to subvertical predominantly transcurrent faults preserving several phases of reactivation (Flinn 1977, 1992). The largest and regionally most significant of these faults is the Walls Boundary Fault Zone. The Precambrian Caledonian basement units of Shetland are thought to be of Laurentian affinity, and to correlate with Lewisian, Moine and Dalradian rocks of mainland Scotland (Fig. 2; e.g. Pringle 1970; Flinn 1977, 1985, 1988; Harris et al. 1994; Holdsworth et al. 1994). In Unst and Fetlar, Dalradian-like rocks are overlain by an ophiolite thought to have been emplaced by thrusting during the early Ordovician (c. 470 Ma; Flinn 1985; Flinn et al. 1991; Spray & Dunning 1991). Later Caledonian deformation affected the Dalradian and older rocks of Shetland, with the supposed along-strike equivalent of the Moine Thrust (the Wester Keolka Shear Zone, Pringle 1970; Andrews 1985) preserved in the northernmost part of the Shetland mainland (Fig. 2). Regionally, an important change in kinematic regime occurred towards the end of the Caledonian orogeny with the onset of sinistral transcurrent movements on orogen-parallel faults and shear zones (e.g. Watson 1984; Soper et al. 1992; Dewey & Strachan 2003). On Shetland, this is believed to be recorded by displacements along the north south-striking Walls Boundary Fault Zone and associated structures, which are commonly linked along strike with the Great Glen Fault Zone in Scotland (Fig. 1; Flinn 1977, 1992; Stewart et al. 1999). Onshore, the Caledonian basement rocks west of the Walls Boundary Fault Zone are unconformably overlain by Lower and Middle Devonian sedimentary and volcanic rocks, intruded by Late Devonian (c. 350 Ma) granites (Northmaven and Sandsting complexes, Fig. 2). The equivalent basement rocks east of the 1037

2 1038 L. M. WATTS ET AL. Fig. 1. Map to show the location of the Shetland Islands, major faults and offshore basins of the NE Atlantic margin (adapted from Doré et al. 1977). CG, Central Graben; FSB, Faeroe Shetland Basin; GGF, Great Glen Fault; HBF, Highland Boundary Fault; HT, Halten terrace; JML, Jan Mayen Lineament; MB, Møre Basin; MT, Moine Thrust; MTFC, Møre Trøndelag Fault Complex; VG, Viking Graben; WBFZ, Walls Boundary Fault Zone; WSB, West Shetland Basin; ESB, East Shetland Basin; SSF, Shetland Spine Fault; WT, Westray Transfer. Walls Boundary Fault Zone are intruded by Early Devonian (c. 400 Ma) granites (Graven, Brae, Spiggie complexes), and are unconformably overlain by Middle Devonian sandstones. The Devonian granites collectively appear to be equivalents of the Newer Granite suite in Scotland (Atherton & Ghani 2002). The Devonian volcano-sedimentary sequences of Shetland (Mykura 1976; Mykura & Phemister 1976) are believed to be part of the Orcadian West Orkney basin system which extended northwards from NE Scotland. To the NW, the basin system consists of distinct half-graben sub-basins controlled by extensional faults dipping generally east to SE (Norton et al. 1987; Enfield & Coward 1987). In Shetland, Devonian sedimentation was partly controlled by sinistral strike-slip displacements (Séranne 1992). According to some workers, these events are then post-dated by Late Carboniferous Permian faulting and folding caused by brittle dextral movements along the Walls Boundary, Nesting and possibly Melby Faults (e.g. Coward et al. 1989; Séranne 1992). Others (e.g. Flinn 1969, 1977, 1992) consider these dextral movements to be predominantly Mesozoic. Offshore, the Shetland Platform incorporates a number of Permo-Triassic extensional basins that are bounded by later, mainly north south-trending faults (Fig. 1; Hitchen & Ritchie 1987; McGeary 1989; Andrews et al. 1990; Johnson et al. 1993). The platform is mostly free of Mesozoic sedimentary rocks. During the Late Jurassic, a major rifting episode affected the Viking Graben and East Shetland Basin, resulting in regionalscale footwall uplift and widespread erosion of the Shetland Platform (Johnson et al. 1993). Thick sequences of post-rift Cretaceous and Tertiary sedimentary rocks are also largely absent, possibly as a result of further uplift and erosion of the Shetland Platform during early Tertiary magmatic underplating of the NW Scottish lithosphere (e.g. Brodie & White 1994). Walls Boundary Fault Zone and associated structures The Shetland Platform and Islands are cut by three main faults, the Walls Boundary Fault Zone, the Nestings Fault and the Melby Fault (Fig. 2). Flinn (1977), Conroy (1996) and Watts Fig. 2. Generalized geological map of the Shetland Islands, showing the main faults and locations of the principal localities described in the text. WKSZ, Wester Keolka Shear Zone. (2001) have shown that the poorly exposed, steeply east-dipping Melby Fault preserves good indications of reverse movement associated with gouge development at its type locality (Melby; see Fig. 2), with some equivocal evidence for an earlier phase (or phases) of dextral strike-slip displacement.

3 REACTIVATED WALLS BOUNDARY FAULT 1039 The Nestings Fault (Fig. 2) splays off the Walls Boundary Fault Zone south of the islands and is a steep fault along which c. 16 km of dextral displacement has been suggested based on offsets observed on geological maps (Flinn 1977, 1992). Exposure is generally poor, but in the best exposed section at Wadbister Voe (Fig. 2) fault-related deformation affects a zone up to 600 m wide, although extensive cataclasis and gouge development are only seen a few tens of metres from the fault (Flinn 1977; Séranne 1992; Conroy 1996; Watts 2001). All studies of the Nestings Fault have documented evidence for a single phase of dextral strike-slip, with the development of shallowly plunging slickenlines, Riedel shears, shear bands and predominantly dextral-verging kink folds (e.g. Sérrane 1992, fig. 13). In his onshore study of the Walls Boundary Fault Zone, Flinn (1977) mapped a gouge-filled fracture containing subhorizontal slickensides, which he took to represent the latest movement on the central fault plane of the Walls Boundary Fault Zone, hereafter termed the Walls Boundary Fault. He also recognized that the Walls Boundary Fault Zone was associated with broad zones of cataclasis, subsidiary faulting and localized folding up to 2 km wide, with dextral senses of vergence dominant. Importantly, Flinn also recognized slices of mylonite exposed sporadically along the trace of the Walls Boundary Fault Zone, and related these to an earlier fault, possibly the northwards continuation of the Great Glen Fault Zone. Offshore, deep seismic profiles north and south of Shetland show that the steeply dipping Walls Boundary Fault Zone offsets the Moho (McGeary 1989; McBride 1994b). Flinn (1961) suggested that the Walls Boundary Fault Zone is linked to the Great Glen Fault Zone, although McBride (1994a) mapped a 35 km wide stepover structure between the Walls Boundary Fault Zone and Great Glen Fault Zone based on interpretation of seismic data. Permo-Triassic successions infill basins bounded by the Walls Boundary Fault Zone and show no evidence of thickening towards the fault, suggesting post-triassic movements (McGeary 1989). Farther south, movements on the Great Glen Fault Zone have affected sedimentary rocks as young as Early Cretaceous and possibly Tertiary (Underhill 1991), but an absence of rocks of this age on the Shetland Platform means that it is uncertain whether there have been any movements of this age along the Walls Boundary Fault Zone. In this paper, we focus on the Walls Boundary Fault Zone in Shetland as it is the best exposed regional structure, preserving the widest variety of fault rocks of several different ages, and it probably accommodates the largest overall displacements. Zones of rock intensely deformed as a result of movement along the Walls Boundary Fault Zone are exposed along several coastal sections and inlets (voes), mostly on the mainland (Fig. 2). We focus on the type section where the widest range of fault rock assemblages and movement phases is preserved: Sullom Ness of Haggrister. We then refer to other localities where additional key relationships are (better) preserved and follow this with a description of the textures and microstructures of the various fault rock assemblages preserved. Walls Boundary Fault Zone structure and fault rock assemblages Type section: Sullom Ness of Haggrister In a coastal section located along the west side of Sullom Voe (Figs 2 and 3a), the Walls Boundary Fault Zone enters at the Ness of Haggrister in the south with a trend of 0108 and bifurcates into several fault strands northwards (Fig. 3a). Inland exposures are poor. A more detailed account of the geological characteristics of the protoliths and fault rocks of the type locality for the Walls Boundary Fault Zone is available online at A hard copy can be obtained from the Society Library. Protoliths and pre-walls Boundary Fault Zone structures. Granodiorite of the Devonian Graven Complex is exposed east of the Walls Boundary Fault Zone at Sullom and the Ness of Haggrister (Fig. 3a). The porphyritic granodiorite is locally strongly altered, with hornblende replaced by chlorite, and feldspars by sericite aggregates. A magmatic fabric (Fig. 3b), is defined by weak roughly north south alignment of phenocrysts and xenoliths of psammitic and hornblende gneiss (but not mylonite), all of which are cross-cut by unfoliated pegmatites. On the southern side of the Ness of Haggrister, calc-schists, limestones and quartz mica schists of the Dalradian Queyfirth Group are exposed west of the Walls Boundary Fault Zone (Fig. 3a). Outside the fault zone, the regional foliation is steeply NEdipping and contains a shallowly SE-plunging mineral stretching lineation (Fig. 3c). In surfaces viewed parallel to the lineation, sinistrally sheared quartz veins lie within the schistosity. At least two orientations of open folds are recognized, both of which are thought to predate shearing along the Walls Boundary Fault Zone. Further north, banded gneisses of possible Lewisianoid origin are exposed west of the fault zone at Sullom and north of the Houb of Lunnister (Fig. 3a; Mykura & Phemister 1976). Fault rocks and relative age relationships. West of the Walls Boundary Fault, mylonites are exposed along the coast on the west side of Sullom Voe from South House to Gaza. They are interleaved with banded gneisses and collectively define a faultbounded block at least 300 m wide (Fig. 3a). The best preserved mylonites occur at Lunnister [HU ] where they comprise alternating layers of mylonite and ultramylonite 5 75 cm thick (Fig. 4a and b). Pink grey quartzo-feldspathic mylonites are interbanded with less abundant, blue grey hornblendic mylonites. Locally they grade from protomylonites to gneiss, suggesting that the protolith of the mylonites was the banded gneisses (Fig. 4c). Many contacts between ultramylonite and mylonite are marked either by green, micaceous foliated cataclasites 1 10 mm thick (Fig. 4a), or millimetre-thick, highly altered pseudotachylite veins. At Lunnister, cataclastically deformed rocks are ubiquitous in the mylonites and banded gneisses. Typically, an outer zone of intensely fractured mylonite and breccia grades into a central cataclasite unit 1 mm 75 cm thick (e.g. Fig. 4d). NNE SSW- to NE SW-oriented fractures containing millimetre-thick gouges cross-cut the cataclasites and mylonites. South and north of Lunnister, the intensely fractured mylonites are cut by networks of centimetre- to millimetre-thick veins of calcite, quartz, epidote, albite and scapolite (e.g. Fig. 4c e). Pale grey cataclasites, ranging from 1 mm to 50 cm in thickness and with a north south-oriented colour banding, cross-cut mylonites and earlier cataclasites (Fig. 4e). There are no cross-cutting relationships between the later cataclasites and gouge-filled faults preserved, but as both locally contain hematite cements and appear to have been formed by the same kinematic regime (see below), it is suggested that they are of broadly the same age. Sheared pelites, cataclasite and blue gouge derived from the Queyfirth Group are exposed west of the Walls Boundary Fault Zone core on the south side of the Ness of Haggrister (Figs 5a and 6a). Metre-scale ductile shear zones containing dextral S C9

4 1040 L. M. WATTS ET AL. Fig. 3. (a) Geological map of the Walls Boundary Fault Zone in the Sullom to Ness of Haggrister section (for location see Fig. 2); (b e) lower hemisphere stereographic projections of structural data. fabrics localize within pelitic protoliths and overprint regional structures and fabrics (Fig. 5a). Unfoliated pale grey cataclasites cross-cut the regional fabrics and ductile shear zones within the pelites. Blue incohesive fault gouge cross-cuts all the cataclastic, sheared pelite and regional fabrics west of the Walls Boundary Fault (Fig. 6a). Cataclastic rocks east of the Walls Boundary Fault occur at Sullom and the Ness of Haggrister (Fig. 3a) and are derived from granodiorite. Non-foliated (Fig. 5b) and foliated (Fig. 5c) varieties of uncertain relative age occur, both of which are associated with brittle dextral faulting. The intensity of cataclastic deformation generally increases towards the fault core, where a hard red gouge is preserved adjacent to the Walls Boundary Fault plane (Fig. 5d). The central core of the Walls Boundary Fault Zone is completely exposed in a narrow inlet on the southern side of the Ness of Haggrister (Fig. 6a). West of the Walls Boundary Fault, a 5 m wide zone of pale grey, calcite-rich unfoliated cataclasite

5 REACTIVATED WALLS BOUNDARY FAULT 1041 Fig. 4. (a) Structural log through the sequence of mylonites and cataclasites at Lunnister [HU ]; (b) plan view of typical finely banded mylonites at Lunnister [HU ]; (c) plan view of carbonate vein networks overprinting a gneissose fabric with the local development of cataclasis (C) [HU ]; (d) plan view of north south-oriented pale cataclasite containing bands of hematite, which locally form small-scale vein networks [HU ]; (e) plan view of brecciated mylonite cross-cut by dextral strike-slip R-type Riedel shears containing pale grey cataclasite and hematite mineralization [HU ].

6 1042 L. M. WATTS ET AL. Fig. 5. Fault rocks and associated minor structures along the Sullom to Ness of Haggrister section. (a) Plan view of shear zone 90 m west of the Walls Boundary Fault on the south side of the Ness of Haggrister [HU ]. The clockwise rotation of the foliation into the shear zone, indicating a dextral sense of shear, and the dextral shear bands within the 30 cm wide shear zone (split arrows), should be noted. Double-headed arrows mark the width of the shear zone. S, S-plane, C9, C9 plane. (b) Plan view of pegmatite dyke cross-cut by R- and R9-type Riedel shears with dextral and sinistral strike-slip offsets, respectively, suggesting an overall dextral sense of shear; dashed line is the trace of the Walls Boundary Fault [HU ]. (c) Oblique view of horizontal surface, showing a network of red foliated cataclasites, which birfurcate as indicated by arrows [HU ]. (d) Image looking north to show the subvertical contact (Walls Boundary Fault) between indurated red gouge (RG) to the right and incohesive blue gouge (BG) to the left; the enclave of red gouge within the soft blue gouge should be noted [HU ]. overprints calcareous schists and is cut by an anastomosing network of centimetre-thick blue gouges that carry fragments of cataclasite, calcite and analcime (zeolite) vein material. A 2 m thick unit of blue gouge occurs adjacent to the Walls Boundary Fault and contains a steep fabric defined by aligned clay particles with subhorizontal lineations. West-dipping shears cross-cut the main foliation within the blue gouge. The east side of the blue gouge comprises a 40 cm thick zone that contains north southoriented inclusions of red gouge (Fig. 5d), suggesting that the blue gouge is younger. The contact between the interleaved gouge zone and the indurated red gouge to the east is a sharp polished fault surface (004/87W), the Walls Boundary Fault (Figs 5d and 6a), on which quartz slickenfibres plunge shallowly to the north. The red gouge is 1 m thick, and in the east overprints a 10 m thick iron-stained and unfoliated cataclasite derived from the granite. The degree of cataclasis decreases to the east and grades into intensely fractured granite. Fault zone mesostructure. The mylonites preserved between Gaza and South House (Fig. 3a) are deformed by a series of millimetre- to tens of metre-scale, dextrally verging folds. Poles to the mylonitic foliation planes lie along a girdle (047/23S) with the pole to the girdle (â axis) plunging 678 towards 3178 (Fig. 3d). Centimetre- to metre-scale minor folds with this orientation are present, with sharply kinked to rounded closures and straight limbs (e.g. Fig. 4a). They consistently show dextral vergence, with steep axial planes striking variably NW SE to NE SW (Fig. 3e). Mylonite stretching lineations are subhorizontal or shallowly plunging, but vary in orientation as a result of the effects of later folding (Fig. 3d). Locally, the dextral folding has led to the development of a weak axial planar fabric that has produced intersection lineations that plunge steeply NW, parallel to fold hinges (Fig. 3e). The best-exposed mylonites and cataclasites crop out at Lunnister (Fig. 4a e), where they lie close to the hinge of a mesoscopic dextrally verging fold. Here, the mylonitic foliation dips steeply to the NW, with mineral stretching lineations plunging shallowly to the NE and SW (Fig. 4a). The mylonitic foliation wraps feldspar and hornblende porphyroclasts that display well-developed sinistral ó- and ä-type geometries when viewed normal to the foliation and parallel to the mineral lineation. In places, centimetre-scale intrafolial isoclines are preserved. Green, micaceous cataclasites up to 10 mm thick occur at contacts between ultramylonites and mylonites (Fig. 4a).

7 REACTIVATED WALLS BOUNDARY FAULT 1043 Fig. 6. (a) View of the Walls Boundary Fault Zone core on the southern side of the Ness of Haggrister [HU ] with inset depicting the geometric configuration of R- and P-type Riedel shears, consistent with an overall dextral sense of shear for the Walls Boundary Fault Zone; (b d) lower hemisphere stereographic projections of structural data collected on the south side of the Ness of Haggrister: (b) i and ii, east of the Walls Boundary Fault Zone core; (c) i v, west of the Walls Boundary Fault Zone core, (d) i v, in the Walls Boundary Fault Zone core. They are foliated and contain a range of lineation orientations from subhorizontal to dip-slip. Along the whole coast section, the main set of cataclasites form linked zones of foliation-parallel and cross-cutting structures (e.g. Fig. 4d and e). Surfaces containing slickenside lineations and other kinematic indicators within the early cataclasite are rare. North south-oriented fractures containing millimetre-thick pale grey late cataclasites and gouges cross-cut all other structures. Fracture surfaces contain subhorizontal slickenside lineations, along which dextral strike-slip offsets of up to 30 cm can be measured (e.g. Fig. 4e). In the well-exposed section through the Walls Boundary Fault Zone core on the south coast of the Ness of Haggrister (Fig. 6a), intense deformation associated with faulting extends c. 150 m west into the Queyfirth Group. In this zone, the subvertical regional foliation and folds are reoriented to strike almost north south (Fig. 6c, i), with the latter tightened to be locally isoclinal. Late-stage centimetre- to metre-scale, dextrally verging folds with sharply kinked to rounded closures exhibit axial planes striking NW SE with hinges plunging moderately north (Fig. 6a inset and c, ii). North south-trending ductile mica-rich shear zones occur within pelitic schists at distances,100 m west of the Walls Boundary Fault (e.g. Fig. 5a). In plan view, the regional foliation swings into the shear zone in a clockwise direction and dextral S C9 fabrics are well developed (Figs 5a and 6c, iv).

8 1044 L. M. WATTS ET AL. Subsidiary faults containing centimetre-thick soft blue gouge and incohesive breccia first appear 120 m west of the Walls Boundary Fault. The near-vertical gouge-filled faults strike north south and contain mainly subhorizontal slickenside lineations and fibres (Fig. 6c, iii). Subhorizontal slickenfibres indicate dextral movements defined by the stepping of quartz fibres. The faults form clusters or anastomosing networks that increase in frequency to the east and tend to localize within pelitic units where they cross-cut the mica-rich shear zones. At distances,50 m west of the Walls Boundary Fault, faulting and cataclasis is so intense that earlier shear zones and structures are largely unrecognizable. Foliation-parallel (north south) faults commonly branch into networks of subvertical fractures in various orientations, all of which contain subhorizontal slickenside lineations (Fig. 6c, iii). NE SW-striking fractures display dextral strike-slip offsets of 5 30 cm and are interpreted as R-type shears (Fig. 6a inset and c, iv). NW SE-trending fractures develop along the axial planes of dextral verging kink folds and display dextral strike-slip displacements of 5 30 cm. These fractures are interpreted as P-type Riedel shears (Fig. 6a inset and c, v). To the east of the Walls Boundary Fault Zone core, faultrelated deformation extends for at least 300 m into the granodiorite of the Devonian Graven Complex. At 250 m east of the Walls Boundary Fault [HU ], epidote, quartz and chlorite veins several millimetres thick cross-cut the weak north southstriking magmatic fabric. Subvertical NE SW-trending fractures with subhorizontal slickenside lineations display dextral strikeslip offsets of pegmatite sheets of 2 30 cm, and are interpreted as R-type Riedel shears (Figs 5b and 6b, i). Less common WNW ENE-trending fractures display sinistral strike-slip offsets of pegmatite veins of 2 10 cm, and are interpreted as R9-type Riedel shears (Fig. 5b). The fractures are infilled by millimetrethick crystalline aggregates of epidote and quartz. From 100 m east of the Walls Boundary Fault, unfoliated and foliated cataclasites occur ranging from 2 mm to 3 cm thick, striking north south and dipping steeply east; slickenside lineations plunge steeply east (Figs 5c and 6b, ii). Quartz and epidote slickenfibre steps indicate dip-slip normal movements. Where the cataclasites contain a foliation it trends north south and dips steeply east (Fig. 6b, ii). In the Walls Boundary Fault Zone core, cataclasites overprinting the Queyfirth Group schists west of the fault contain a well-developed shear fabric, with widespread dextrally offset, fault-parallel calcite veins 1 mm to 1 cm thick. Cross-cutting braided networks of subvertical fractures dipping steeply NW (Fig. 6d, i) contain subhorizontal slickenside lineations and display dextral offsets of 1 30 cm. The fractures lie clockwise of the Walls Boundary Fault and are interpreted as R-type Riedel shears. The cataclasites are cut by soft blue gouges that contain a subvertical north south-trending foliation with shallowly southplunging lineations defined by aligned clay particles (Fig. 6d, ii). In surfaces viewed perpendicular to the Walls Boundary Fault and parallel to the lineation, centimetre-scale shear bands within the blue gouge indicate dextral shear. The gouges are cross-cut by west-dipping shears containing dip-slip lineations (Fig. 6d, iii) defined by the alignment of clay particles. East of the Walls Boundary Fault, colour-banded red gouges locally carry subhorizontal lineations and millimetre-scale dextral shear bands that are associated with ENE WSW tensional fractures (filled with calcite) and discrete NW SE fractures showing an apparent sinistral shear (Fig. 6d, iv). The Walls Boundary Fault carries subhorizontal quartz slickenfibres, which plunge shallowly to the north (Fig. 6d, v), indicating sinistral strike-slip movements that appear to represent the last regionally significant movements along the fault. Summary. Figure 7 shows a summary of the geometric configuration, kinematic history, fault rock distribution and relative age relationships seen along the Walls Boundary Fault Zone at Sullom, Lunnister and the Ness of Haggrister. The structure is dominated by a kilometre-scale braided network of subvertical dextral faults associated with cataclasis and the development of fault gouge. It is proposed that an initial curve in the Walls Boundary Fault Zone trace led to the formation of a left-stepping restraining bend and a local region of dextral transpression (a positive flower structure ; Fig. 7 inset), leading to the uplift and exhumation of basement gneisses and mylonites that preserve evidence for an earlier phase of sinistral shear. The mylonites and early cataclasites are the earliest recognized fault rocks and are preserved only west of the Walls Boundary Fault. The shear sense and age relationship of the early cataclasites relative to the other cataclastic fault rocks are ambiguous; it is possible that they formed during the final stage of sinistral shearing responsible for the development of the mylonites. The rocks to the east of the Walls Boundary Fault record only dextral strike-slip and later events, suggesting that the granodiorite of the Graven Complex may not have lain adjacent to the Walls Boundary Fault Zone during sinistral shear or may not have been intruded at that time. The most recent movement on the Walls Boundary Fault Zone was sinistral strikeslip, post-dating a minor dip-slip event. Roddom et al. (1989) obtained K Ar whole-rock ages of Ma from the mylonites at Lunnister, and assumed that the protolith was a granodiorite of the Devonian Graven Complex. The present study suggests, however, that the mylonites were derived from banded basement gneisses and formed during an early phase of sinistral strike-slip movement. No evidence of sinistral strike-slip is preserved in the Graven Complex and it is suggested that the sinistral shearing event probably predates intrusion. Given the overprinted and highly fractured nature of the mylonites, it is suggested that the ages obtained by Roddom et al. (1989) reflect partial resetting of older ages that occurred during the main phase of dextral shearing along the Walls Boundary Fault Zone. Key highlights from other sections Similar fault rock assemblages and kinematic sequences are wholly or partially preserved at a number of localities along the Walls Boundary Fault Zone (Fig. 2). Here we highlight some key relationships preserved that are not displayed, or are ambiguous, in the type section. Early mylonites. Diffuse zones of early sinistral shearing (S C fabrics) are preserved in basement rocks adjacent to the Walls Boundary Fault at a number of localities (e.g. Ollaberry, Brae Isthmus, Papa Little, Aith Voe; Conroy 1996; Watts 2001). Mylonitic rocks that are unambiguously associated with sinistral shearing are preserved only at Red Ayre and Sand at the southernmost exposures of the Walls Boundary Fault Zone. The shoreline of Red Ayre on the coast of Seli Voe, 350 m west of the offshore Walls Boundary Fault trace (Fig. 2) exposes a 300 m 3 60 m xenolith within the granite of the Devonian Sandsting Complex (Fig. 8a and b) of mylonite and early cataclasites lithologically similar to those exposed at Lunnister. A subvertical foliation trends north south with a subhorizontal lineation defined by elongate quartz and feldspar grains. The

9 REACTIVATED WALLS BOUNDARY FAULT 1045 Fig. 7. Schematic diagram to illustrate fault rock distribution, structure and geometry of the Walls Boundary Fault Zone between Sullom and Ness of Haggrister (not to scale in vertical dimension). Inset shows interpretation suggesting that the basement rocks and mylonites are preserved in an uplifted and exhumed dextral restraining bend of the Walls Boundary Fault Zone. surrounding granite lacks any fabric, but the granite, the mylonites and the early cataclasites are overprinted by later cataclasites and gouges associated with dextral shearing along the Walls Boundary Fault Zone (Watts 2001). Miller & Flinn (1966) recorded K Ar biotite ages of 334 Ma and 360 Ma from the granite, and Mykura & Phemister (1976) dated hornblendes extracted from diorite at 369 Ma (K Ar), which are interpreted to date intrusion of the Sandsting Complex. If the latter age is reliable, it provides a lower age limit for the formation of the mylonites and early cataclasites. The Walls Boundary Fault Zone at Sand is a 1.5 km wide zone of steep faults (Fig. 9a; Flinn 1977; Watts 2001). The Walls Boundary Fault is unexposed, being marked by a flat-bottomed valley that can be traced northwards within Garderhouse Voe to the south side of the Ness of Bixter (see below). Between [HU ] and [HU ], a 30 m wide fault-bounded sliver of blastomylonite, cataclasite and foliated cataclasite is preserved along the Aith Voe Fault (Fig. 9a; Flinn 1977) a branch of the Walls Boundary Fault Zone thought to have formed at a restraining bend during dextral shearing (see Fig. 7a inset). The Aith Voe Fault blastomylonite sliver separates probable Queyfirth Group metasediments (psammite, pelite, quartzites) from granodiorite of the Late Devonian Spiggie Complex, which locally displays a north south-trending magmatic fabric defined by aligned feldspar phenocrysts. Blastomomylonites derived from both acidic and mafic protoliths are present, with a strong north south-trending, subvertical foliation defined by stretched and flattened mineral aggregates, containing a subhorizontal stretching lineation defined by elongate feldspar, quartz, hornblende and muscovite grains (Fig. 10a and b). In sections perpendicular to the foliation and parallel to the stretching lineation, ó-type porphyroblasts consistently indicate sinistral senses of shear (Fig. 9b). Granite veins trending north south cross-cut the blastomylonites (Fig. 10b) and do not carry a mylonitic foliation. NE SW millimetre- to centimetrethick units of green cataclasite, with dextral offsets of up to 20 cm, cross-cut the mylonitic foliation and granite veins. These are interpreted as R-type Riedel shears indicating a later dextral sense of shear, whereas a subordinate set of ENE WSW-trending cataclasites displaying offsets of 1 10 cm in a sinistral sense are interpreted as R9-type Riedel shears. Centimetre-scale, dextrally verging kink folds overprint the blastomylonitic foliation and cross-cutting cataclasites. Immediately east of the main unit of blastomylonite, a 4 m wide fault-bounded sliver of green-coloured, unfoliated cataclasite (Fig. 10a) contains numerous fragments of blastomylonite and is cross-cut by a 2 m wide porphyritic granite. Still further east, a 4 m wide, fault-bounded sliver of foliated cataclasite (Fig. 10a) contains clasts of both blastomylonite and isotropic cataclasite, and displays a north south-trending subhorizontal lineation defined by aligned clasts and mica grains. In sections perpendicular to the foliation and parallel to the lineation, ó-type fragments consistently indicate dextral shear senses. To the east, the foliated cataclasites are cut by a north south-trending, 2 m

10 1046 L. M. WATTS ET AL. Fig. 8. (a) Geological map of the shoreline of Red Ayre on the coast of Seli Voe (for location see Fig. 2) with structural data presented as stereographic projections; (b) irregular contact (dashed line) separating granite to the east from mylonite to the west. wide soft blue gouge associated with the Aith Voe Fault, which in turn grades into incohesive breccia and intensely fractured granodiorite of the Devonian Spiggie Complex (Fig. 10a). The gouge contains a strong fault-parallel foliation with a dip-slip lineation defined by aligned clay particles. Slickenfibre lineation steps along the Aith Voe Fault plane suggest a reverse dip-slip sense of movement. These observations suggest that the Walls Boundary Fault Zone at Seli Voe and Sand comprises a kilometre-scale positive flower structure defined by faults containing gouge associated with broad zones of cataclasis. The Aith Voe Fault itself is a relatively late, large-scale reverse fault that carries a sliver of older blastomylonites formed during sinistral shear. Lithologically and kinematically, the blastomylonites are most obviously similar to the mylonites at Red Ayre and Sullom Ness of Haggrister. At Sand they are post-dated by granite veining, the formation of unfoliated cataclasite, the intrusion of porphyritic granite, the formation of foliated cataclasites and finally by reverse movement along the Aith Voe Fault. Both phases of cataclasite appear to be associated with dextral movements and it is possible that all the events post-dating the blastomylonites, including the intrusion of the granite sheets, formed during a single protracted phase of dextral shearing and positive flower structure development (see Watts 2001). Early cataclasites. Foliated and unfoliated early cataclasites are recognized at a number of localities, where they are often associated with mylonite slivers (e.g. Sullom Ness of Haggrister, Red Ayre, Papa Little; Fig. 2). In most cases, clear shear-sense indicators are not present, because the cataclasites are developed at low angles or subparallel to the pre-existing foliations, so that it is difficult to establish offsets. Asymmetric fabrics are rare or absent, but on the Brae Isthmus, immediately south of the Ness of Haggrister (Figs 2 and 11a), early cataclasites with good shear-sense indicators are preserved in deformed granodiorite of the Early Devonian Brae Complex. At this locality, the Walls Boundary Fault separates hornblende schists of the Queyfirth Group from a fault-bounded sliver of crystalline limestone of uncertain affinity and the Brae Complex granodiorite to the east (Fig. 11a). The granodiorite is coarsegrained with hornblende and feldspar phenocrysts up to 1 cm in length defining a weak north south-trending magmatic fabric that lies subparallel to aligned centimetre-scale xenoliths of dolerite and ultramafic rocks and enclaves of microdiorite. Randomly oriented, coarse-grained undeformed pegmatite dykes m thick cross-cut the granodiorite. Pale green cataclasites with no fabric cross-cut the granodiorite and appear to grade into dark green mica-rich cataclasites that contain a near-vertical north south-trending foliation defined by aligned mica aggre-

11 REACTIVATED WALLS BOUNDARY FAULT 1047 Fig. 9. (a) Geological map of the area west of Sand Voe (for location see Fig. 2); (b) plan view of blastomylonite with north south-oriented fabric and ó- type orthoclase porphyroclasts [HU ] with split arrows indicating shear sense parallel to the lineation; (c) cross-sectional view of green cataclasite containing angular fragments of blastomylonite [HU ]; (d) cross-sectional view of foliated cataclasite containing fragments of blastomylonite. gates. Lineations defined by elongate chlorite and muscovite grains are either subhorizontal or dip-slip in orientation (Fig. 11b). Foliated cataclasites with subhorizontal lineations are associated with centimetre- to millimetre-scale sinistral S C9 fabrics (Fig. 11c), whereas those with dip-slip lineations display millimetre-scale S C9 fabrics and shear bands consistent with normal (west-side-down) senses of shear. The foliated cataclasites vary in thickness from 1 to 50 cm and tend to bifurcate into several strands surrounding enclaves of altered granodiorite and unfoliated cataclasite, giving rise to a braided geometry. Later gouges with dextral offsets of up to 15 cm are often localized along the foliated cataclasites (Fig. 11c) and carry lineations defined by aligned clay particles, which plunge shallowly to the south. The expression of the Walls Boundary Fault Zone in Devonian rocks. An east west-oriented coastal section on the south coast of the Ness of Bixter (Fig. 2) exposes the Walls Boundary Fault Zone where it separates sandstones of the Middle Devonian Walls Formation to the west from a series of fault-bounded slivers of cataclasite, calcareous schist (?Queyfirth Group) and granite of the Late Devonian Spiggie Complex to the east (Fig. 12a; Watts 2001). All fault rocks are interpreted to be roughly the same age as no consistent cross-cutting relationships were observed in the field. Fault-related deformation is strongly asymmetric, extending 350 m west of the Walls Boundary Fault into the Devonian sandstones and,150 m east into calcareous schists and granite. The sandstones are grey, fine- to medium-grained and crossbedded, with minor mudstones. The steeply dipping bedding within the sandstones and mudstones changes orientation from NW SE outside the fault zone to north south within the fault zone (Fig. 12bi). Subvertical, NE SW-trending fracture networks exhibit centimetre-scale offsets of bedding and are interpreted as R-type Riedel shears formed as a result of dextral shear along the Walls Boundary Fault (Fig. 12b, ii). Subsidiary north southtrending subvertical faults containing centimetre-thick, calcitecemented gouge 2 30 cm thick first appear 350 m west of the

12 1048 L. M. WATTS ET AL. Fig. 10. (a) Structural log and cross-section through the Aith Voe Fault (for location see Fig. 9) with relevant structural data presented as stereographic projections above; (b) structural log of blastomylonites and overprinting structures [HU ]. Walls Boundary Fault [HU ] and are flanked by zones of incohesive breccia and cataclasite. These become increasingly abundant closer to the Walls Boundary Fault and at distances,100 m west of the fault core, broad zones of incohesive breccia up to 4 m wide are common preserving millimetre-scale calcite, scapolite and quartz vein networks. The gouge-filled faults preserve slickenside lineations in two main clusters: strike-slip and dip-slip (Fig. 12b, iii). The asymmetry of minor fold vergence, shear bands and quartz slickenfibre steps associated with these faults indicates either dextral or normal movements, with metre-scale domains dominated by either dip-slip or strikeslip displacements (e.g. Fig. 12c). The central fault (Walls Boundary Fault) is exposed in a narrow inlet on the south side of the Ness of Bixter oriented 002/85W and located within a fault core 50 m wide (Fig. 12c). The west side of the Walls Boundary Fault Zone core comprises 40 m of intensely fractured and locally incohesively brecciated sandstones with millimetre-thick calcite veins crosscut by north south-trending faults containing centimetre-thick gouges (Fig. 12c). A 3 m wide zone containing blue green fault gouge adjacent to the Walls Boundary Fault overprints incohesive fault breccia and is cross-cut by and included as xenoliths in a 1.5 m thick pegmatite (Fig. 12c). The gouge contains a strong fabric with subhorizontal lineations defined by aligned clay particles and is limited to the east by the Walls Boundary Fault, which carries subhorizontal slickenside lineations (08/003). The gouge preserves centimetre-scale dextral shear bands. To the east of the Walls Boundary Fault, a 4 m wide block of red coloured isotropic cataclasite is exposed that predates the blue green gouge immediately to the west and is succeeded to the east by a slice of intensely fractured and locally brecciated calcareous schist cross-cut by millimetre-thick gouge and calcite veins (Fig. 12c). In summary, the deformation zone associated with the Walls Boundary Fault Zone in the Devonian sandstones at the Ness of Bixter is more than two times wider than that preserved in the adjacent metasediments and granite. A single dextral transtensional phase of brittle faulting and associated deformation is recognized, with the faults appearing to define a negative flower structure possibly developed along a releasing bend in the Walls Boundary Fault, which would also account for the preservation here of Devonian cover rocks (Fig. 12c inset). Summary of field observations The Walls Boundary Fault Zone preserves a long record of movements at progressively lower temperatures and crustal depth (Fig. 13, left column). It is defined primarily by a zone of 500 m

13 REACTIVATED WALLS BOUNDARY FAULT 1049 Ness of Haggrister, Red Ayre, Sand) within fault-bounded blocks that were apparently uplifted and exhumed in a series of positive flower structures associated with restraining bends developed during dextral strike-slip movements along the Walls Boundary Fault Zone (e.g. Fig. 7). An early set of foliationparallel cataclasites, with local psuedotachylite development, is often associated with the mylonites and appears to have formed during a brittle sinistral shearing event. The sinistral events probably predate or possibly overlap intrusion of the Devonian granites and Middle Devonian sedimentation, whereas the dextral events are younger. Devonian rocks preserved adjacent to the Walls Boundary Fault Zone (e.g. Ness of Bixter) appear to be located in negative flower structures associated with releasing bends formed during dextral shear along the Walls Boundary Fault Zone (e.g. Fig. 12c inset). Walls Boundary Fault Zone fault rock microstructures and evolution The textural and mineralogical features of the fault rocks are now analysed briefly in the order of their age (oldest to youngest). The earliest fault rock assemblages formed during sinistral strike-slip are referred to here as primary fault rocks. Later fault rocks formed during dextral strike-slip and younger reactivation events are referred to as secondary fault rocks. A more detailed description of the microstructures of the country rock protoliths has been given by Watts (2001). Fig. 11. (a) Geological map of Brae Isthmus (for location see Fig. 2); (b) stereographic projection showing structural data from foliated cataclasite at Brae Isthmus; (c) plan view of foliated cataclasite at Brae Isthmus [HU ]; split arrows indicate C9 planes of sinistral S C9 fabric within cataclasite (arrows parallel to lineation). Arrow labelled G indicates foliation-parallel gouge vein with a dextral strike-slip displacement of 2 cm. to 2 km width of braided, mostly subvertical faults associated with broad zones of cataclasis and the development of fault gouge formed during a possibly protracted phase (or phases) of dextral strike-slip movement. Subsequent minor phases of dipslip and sinistral strike-slip reactivation are almost entirely localized within pre-existing fault gouges in the central core of the Walls Boundary Fault Zone which is exposed at only a few localities (Ness of Haggrister, Brae Isthmus, Papa Little). The earliest recognized fault-related deformation comprises blastomylonitic and mylonitic fault rocks formed during sinistral shear. They are well preserved only at three localities (Sullom Primary fault rock assemblages Blastomylonites. These comprise feldspar porphyroclasts, which are wrapped by aggregates of polygonal feldspar and quartz (Fig. 14a). Porphyroclasts are wrapped by strain-free, equigranular (typically mm) polygonal feldspar grains that display no crystallographic preferred orientation and commonly form tails and continuous bands parallel to the macroscopic foliation. The feldspar porphyroclasts display sweeping undulose extinction and commonly contain arrays of healed microcracks (Fig. 14a). Quartz ribbons interbanded with layers of feldspar also display equant-polygonal, strain-free grains of similar grain sizes to the feldspars. Within finer-grained blastomylonites, layers of white mica grains (,0.1 mm) occur between quartz and feldspar layers defining a relict sinistral S C9 fabric (Fig. 14b). Polygonal quartz and feldspar grains (c. 0.1 mm in diameter) are elongate with long axes parallel to the macroscopic foliation and feldspar porphyroclasts are absent. Isolated quartz ribbons (c. 0.5 mm thick) comprise coarser-grained, polygonal, strain-free quartz grains (0.5 1 mm) compared with the surrounding matrix. These textures are consistent with secondary recrystallization (grain growth) in which the distribution and size of other phases such as mica strongly controlled the grain size and shape of the quartz as a result of grain-boundary pinning (e.g. Vernon 2004, p. 184). The polygonal annealed appearance of quartz and feldspar and the preservation of secondary recrystallization textures are typical of amphibolite-facies blastomylonites deformed at temperatures.500 8C (e.g. Simpson 1985; Passchier & Trouw 1996); that is, at depths.16 km (assuming an average geothermal gradient of 30 8C km 1 ). Mylonites. Acidic types comprise feldspar porphyroclasts wrapped by polycrystalline quartz ribbons and bands of ultrafinegrained feldspar, sericite and biotite (Fig. 14c). Quartz aggregates display strong crystallographic preferred orientations. Polycrystal-

14 1050 L. M. WATTS ET AL. Fig. 12. (a) Geological map of the Ness of Bixter (for location see Fig. 2); (b) stereographic projections to show (i) sandstone bedding, (ii) R-type Riedel shears and (iii) gouge-filled faults west of the Walls Boundary Fault Zone core; (c) structural log and cross-section through the Walls Boundary Fault Zone core on the south side of the Ness of Bixter [HU ], with inset showing interpretation suggesting that the Devonian cover rocks are preserved in a downfaulted dextral releasing bend of the Walls Boundary Fault Zone. line quartz ribbons with aspect ratios of between 20:1 and 100:1 typically comprise flattened quartz grains (c ìm, aspect ratios between 3:1 and 10:1) with interlobate grain boundaries. The long axes of the lobate grains are oriented either parallel to, or, in more highly strained quartz ribbons, at angles up to 508 in a clockwise direction to the trace of the macroscopic foliation, which is consistent with sinistral shear (Fig. 14d). Larger quartz grains (10 20 ìm) are mantled by equigranular interlobate quartz grains (c ìm), giving rise to core and mantle structures, and display strong sweeping undulose extinction, deformation lamellae and well-developed subgrains. Subgrain boundaries generally pass laterally into lobate grain boundaries. The size of the subgrains (c. 1 ìm) is similar to that of the adjacent lobate quartz grains, indicating subgrain rotation as a mechanism of recrystallization (Urai et al. 1986). Feldspar (albite, K-feldspar) porphyroclasts are subrounded to well rounded, (0.5 5 mm across) and display ä- and ó-type (e.g. Fig. 14c) geometries that are consistent with sinistral shear. The feldspar clasts commonly display partial breakdown to sericite, with larger feldspar clasts (typically.1 mm) flattened in the

15 REACTIVATED WALLS BOUNDARY FAULT 1051 Fig. 13. Deformation depth time diagram summarizing the kinematic and fault rock evolution of the Walls Boundary Fault Zone in Shetland, the Great Glen Fault Zone in Scotland and the Møre Trøndelag Fault Complex in western Norway (for locations see Fig. 1). VF, Verran Fault; HSF, Hitra Snåsa Fault. foliation as a result of arrays of shear and tensile intergranular fractures, infilled by aggregates of ultrafine-grained feldspar and indicating an overall sinistral sense of shear (e.g. Fig. 14e). Many feldspar clasts display strong patchy extinction interpreted to be due to submicroscopic cataclasis. Both albite and K-feldspar porphyroclasts are mantled by aggregates of ultrafine, equigranular interlobate feldspar grains (diameters c ìm), which are drawn out to form laterally continuous tails oriented parallel to the macroscopic foliation. Ultramylonite layers contain fewer, smaller porphyroclasts than adjacent mylonites. White mica grains commonly display mica-fish geometries consistent with sinistral shear (Fig. 14f). Sub-millimetre-scale S C fabrics consistent with sinistral shear are locally developed (Fig. 14g), with C-planes defined by ultrafine-grained aggregates of biotite, sericite and chlorite. Occasional dark, semi-opaque bands of ultrafine-grained ultramylonite with sharp, straight boundaries are developed parallel to the macroscopic mylonite foliation (Fig. 14h) and contain rounded clasts of quartz. Injection structures locally nucleate from the dark material and cut the mylonite. These dark-coloured units are thought to be derived from foliation-parallel pseudotachylite veins formed during localized high strain rate events, which have then been partially reworked during mylonitization (see Sibson 1980; White 1996). Mafic protomylonites comprise hornblende and feldspar porphyroclasts wrapped by polycrystalline quartz ribbons and bands of ultrafine-grained feldspar, sericite, chlorite and biotite. The quartz ribbons and semi-brittle feldspars display similar microstructures to those observed in felsic mylonite. Locally, the margins of hornblende porphyroclasts are replaced by retrograde biotite, actinolite and less commonly chlorite. The semi-brittle behaviour of feldspar porphyroclasts, the albitic composition of the plagioclase and the coexistence of retrograde biotite, actinolite and hornblende in preference to chlorite within mafic mylonites suggest mylonitization under mid- to upper greenschist-facies conditions. This is consistent with the widespread evidence for dynamic recrystallization of quartz and semi-brittle microstructures associated with feldspars, suggesting that deformation took place between 400 8C and 500 8C (Simpson 1985; Hirth & Tullis 1992); that is, at depths of km (assuming an average geothermal gradient of 30 8C km 1 ). Secondary fault rock assemblages Early cataclasites. At Lunnister, foliation-parallel to gently cross-cutting early cataclasites derived from the mylonites are typically bounded by sharp fractures or fault surfaces. In thinsection, injection veins nucleate from the sharp surfaces and cross-cut both the cataclasite and the adjacent mylonite (Fig.

16 1052 L. M. WATTS ET AL.

17 REACTIVATED WALLS BOUNDARY FAULT a). The injection veins comprise a brown isotropic material with aphanitic or glassy texture commonly altered to a clay-rich isotropic paste; they are interpreted to be devitrified pseudotachylites. The cataclasites comprise finely comminuted fragments of feldspar, quartz, hornblende, mica and mylonite set within a fine-grained matrix (Fig. 15b). The fragments are randomly oriented, angular to subangular and range from 1 cm to c. 10 ìm in size. Both quartz and feldspar clasts are cut by extensive arrays of intergranular fractures generally lacking fibrous vein infills, suggesting that fracture opening was not associated with localized high pore-fluid pressures. A later phase of randomly oriented, sub-millimetre-thick scapolite veins cross-cut both the mylonites and the cataclasites. The foliated cataclasites exposed at Brae Isthmus are derived from granodiorite of the Graven Complex. Subangular to rounded clasts ( mm) of quartz, feldspar, epidote and cataclasite are set within a brown-coloured, ultrafine-grained matrix with a foliation defined by aligned fragments and colour variations (Fig. 15c). The foliation displays an anastomosing geometry that encloses foliation-parallel lenses of cataclasite with aspect ratios of up to 10:1, which are locally boudinaged. The foliation is cut by sub-millimetre-scale sinistral shear bands (Fig. 15c). Backscatter SEM studies show that the foliation is formed by aligned sericite and chlorite fibres (1 5 ìm in length), which surround quartz and albite clasts to form strain shadows parallel to the macroscopic foliation (Fig. 15d). Quartz clasts display quartz overgrowths parallel to the macroscopic foliation. A stable assemblage of quartz, albite, chlorite, sericite and epidote is associated with foliation development, whereas original feldspar and hornblende grains record intense alteration to aggregates of phyllosilicates and epidote. This implies lower greenschist-facies metamorphism and hydration, consistent with the ubiquitous preservation of cataclastic deformation textures for both quartz and feldspar suggesting temperatures of between 250 8C and 300 8C (Simpson 1985; Hirth & Tullis 1992) or depths of km (assuming an average geothermal gradient of 30 8C km 1 ). Sericitization and chloritization reactions are widespread and indicate the involvement of a chemically active fluid phase leading to both reaction softening and the onset of fluid-assisted diffusive mass transfer mechanisms. Late cataclasites. These include the main set of cataclasites developed in all wall rock types during dextral shear. Where overprinting mylonites and early cataclasites, they comprise randomly oriented, subangular to subrounded clasts ( mm) of feldspar, quartz, mylonite, cataclasite and gneiss set within a fine-grained cataclastic matrix that has been altered to a clay-rich isotropic paste (Fig. 15e). Within the finer-grained clayrich layers, an apparent foliation is present, defined by alignment of clay particles. Fault-parallel hematite veins, mm thick, form braided networks within the finer-grained cataclasites. In places, the cataclasites contain fragments ( mm) of scapolite veins probably related to those that cross-cut earlier formed mylonites and cataclasites (Fig. 15e). The cataclasites derived from granitic protoliths comprise randomly oriented, subangular to angular fragments of feldspar and quartz, typically set within a fine-grained cataclastic matrix of chlorite, sericite, epidote, feldspar and quartz (Fig. 15f). Partially sericitized feldspar clasts are cut by extensive arrays of intergranular fractures, which are filled with chlorite and green biotite fibres oriented normal to the fracture surfaces (Fig. 15g). Millimetre-thick epidote and quartz veins both cross-cut the cataclasites and occur locally as clasts within the cataclasite matrix, suggesting that veining was coeval with cataclasis. Cataclasites derived from Devonian sandstones at the Ness of Bixter comprise randomly oriented, finely comminuted fragments of quartz ( mm) and minor feldspar set within a finegrained matrix of quartz, calcite and chlorite, which often appears to be altered to a clay-rich isotropic paste. Extensive networks of millimetre-thick calcite veins commonly cut the sandstones in the wall rocks adjacent to the cataclasite. All the late cataclasites show evidence for cataclastic flow (mechanical disaggregation, grain-size diminution) with local development of pressure solution in foliated cataclasites. Feldspar and quartz are deformed by cataclasis, indicating temperatures of less than 250 8C (Simpson 1985) corresponding to depths shallower than km (assuming an average geothermal gradient of 30 8C km 1 ). Elevated pore-fluid pressures are inferred locally with the development of grain- and vein-scale hydro-fractures and the precipitation of fibrous chlorite, green biotite or calcite (e.g. Fig. 15g). The presence of an active fluid phase is also consistent with the widespread retrogression of feldspars and other minerals to fine-grained aggregates of phyllosilicate. The presence of syntectonic clay alteration in some cataclasites (e.g. Ness of Bixter) indicates temperatures of less than 200 8C or depths less than 6 km (assuming an average geothermal gradient of less than 30 8C km 1 ) (Frey 1988; Warr & Cox 2001). Gouges. Cohesive red- to brown-coloured fault gouge is common within the core of the Walls Boundary Fault (e.g. Ness of Haggrister). In thin-section, it appears as a red brown, clay-rich, isotropic paste that has been cemented by hematite and contains randomly oriented subrounded to rounded clasts ( mm) of cataclasite, quartz and feldspar (Fig. 15h). It commonly displays a foliation defined by aligned clay grains. In places, the gouge is cut by fractures, which display sinistral strike-slip offsets. Randomly oriented calcite and analcime (zeolite) veins ( mm thick) cross-cut and are present as fragments within the gouge. The presence of analcime indicates temperatures of less than 150 8C or depths shallower than 5 km (assuming an average geothermal gradient of 30 8C km 1 ) (Frey 1988). Fig. 14. Photomicrographs of primary fault rock assemblages; scale bars represent 1 mm. (a) Relict core and mantle structure within blastomylonite; the feldspar porphyroclast (f) contains healed microcracks (mc) and is wrapped by strain-free equigranular feldspar grains; (b) fine-grained blastomylonite with no preferred orientation of feldspar and quartz grains and a relict sinistral S C9 fabric: the S-planes (s) are defined by elongate quartz grains and the C-planes (c) are defined by white mica; (c) mylonite containing relatively undeformed but altered asymmetric feldspar (f) ó-type porphyroclasts surrounded by polycrystalline quartz ribbons (q) interlayered with fine-grained feldspar, sericite and biotite; (d) close-up of highly strained quartz ribbons; the long axes of the lobate quartz grains (black dashed lines) are oriented up to 508 clockwise from the macroscopic foliation (white dotted lines), consistent with sinistral shear; (e) cataclastically deformed feldspar grains (f) surrounded by quartz ribbons (q) and fine-grained dynamically recrystallized feldspar (rf); angular fragments derived from the feldspar grain are drawn out parallel to the macroscopic foliation (dashed line) and are cut by sinistral R- type Riedel shears (split arrows); (f) ultramylonite with newly formed muscovite grain (m), which displays mica-fish geometry consistent with sinistral shear; (g) sinistral S C9 fabric (c, c-planes; s, s-planes); (h) mylonitic pseudotachylite vein (mps) within felsic mylonite (fm) developed parallel to the macroscopic foliation (dashed line).

18 1054 L. M. WATTS ET AL.