Fault-Zone Weakening Processes Along the Reactivated Outer Hebrides Fault Zone, Scotland

Journal of the Geological Society, London, Vol. 154, 1997, pp. 105–109, 3 ﬁgs, 1 table. Printed in Great Britain

Fault-zone weakening processes along the reactivated Outer Hebrides Fault Zone, Scotland

J. IMBER1, R. E. HOLDSWORTH1, C. A. BUTLER1,3 & G. E. LLOYD2 1Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK 2Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK 3Present address: Elf Caledonia Ltd, Bridge of Don, Aberdeen AB23 8GB, UK

Abstract: The Outer Hebrides Fault Zone is a major reactivated structure cutting amphibolite-grade Lewisian basement gneisses in NW Scotland. During a regionally important phase of sinistral strike-slip movements, the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist-facies phyllonitic shear zones. Later ESE-directed extensional strain was preferentially focused into these pre-existing zones of weakness. The syn-tectonic alteration of a relatively strong, feldspar/hornblende-dominated load-bearing framework microstructure to an interconnected weak layer microstructure of fine-grained, strongly aligned phyllosilicate aggregates leads to the long-term weakening in the fault zone. Comparison with experimental data suggests that this produces a shallowing of the frictional–viscous creep (‘brittle–ductile’) transition and a substantial reduction in total crustal strength. Similar processes may account for the apparent weakness of many long-lived fault zones.

Keywords: reactivation, phyllonites, fault zones, rheology.

Geological and geophysical investigations around currently fault zone (Table 1; see Butler 1995; Butler et al. 1995). The active long-lived fault zones within the crust have shown that examples discussed in this paper are from North Uist (Fig. 1b), kinematic strain partitioning is a common feature in continen- but are broadly representative of the fault zone as a whole. tal regions undergoing deformation (e.g. Tikoff & Teyssier In the Burrival–Eigneig Bheag region of North Uist ([NF 1994). It has been suggested that this occurs because fault 910 620 to NF 925 600]; Fig. 1b), a thick (>1 km), east-dipping zones are weak relative to surrounding country rocks. Most zone of brittle, thrust-related cataclasites, pseudotachylytes studies have focused on fault zones developed in the upper and variably brecciated blocks of parent gneiss (‘crush crust (<5 km depth), e.g. the Cajon Pass study (Zoback & melange’ of Sibson 1977) is progressively overprinted in an Lachenbruch 1992). The weakening effects, localized along eastward direction by an ESE- to SE-dipping phyllonitic major fault strands, have been attributed either to the presence fabric. The overprint is heterogeneous, producing a braided of anomalously low-friction clay gouges (Wang 1984); and/or to the development of elevated pore-fluid pressures (Rice 1992; Chester et al. 1993; Byerlee 1990). There is, however, little direct geological evidence to support these hypotheses (e.g. see Scholz 1990) and they do not explain why many continental faults and shear zones are reactivated after long periods of inactivity (>1 Ma). Such reactivation requires long term weakening which is more readily explained by considering the rheological effects of phase changes that occur during fault rock evolution, i.e. reaction–softening processes (White & Knipe 1978; Rubie 1990; Wintsch et al. 1995). In this paper, we present field and microscopical evidence for weakening processes associated with syn-tectonic retrogression and fluid flow along a well exposed reactivated basement fault zone in Scotland; the Outer Hebrides Fault Zone (Sibson 1977; White & Glasser 1987; Butler 1995; Butler et al. 1995).

Kinematic history of the Outer Hebrides Fault Zone and evidence for weakening The Outer Hebrides Fault Zone is a major ESE-dipping structure cross-cutting a pre-existing high-grade basement complex developed in Archaean to early Proterozoic Lewisian gneisses (Fig. 1a). Following an early thrusting event at upper Fig. 1. (a) Onshore outcrop of the Outer Hebrides Fault Zone greenschist facies, evidence for which is preserved only in the (OHFZ) and distribution of phyllonites. (b) Detailed geological map northern part of the fault zone (Lewis and Scalpay; Fig. 1a), a of the Burrival–Eigneig Bheag region, North Uist (after Butler et al. four phase kinematic history is recognized along the whole 1995).

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Table 1. Summary of the kinematic evolution of the Outer Hebrides epidote and quartz (Fig. 2a). Both the cataclasite matrix and Fault Zone clasts of host gneiss show variable degrees of post-tectonic retrogression. Older Protophyllonites and phyllonites little affected by later ex- Top-to-NW ductile thrusting (Lewis and Scalpay only) tensional reworking typically consist of quartz, feldspar, horn- Upper greenschist mylonites; ?Proterozoic blende, sericite, chlorite, actinolite, epidote, opaque minerals Top-to-NW brittle thrusting and calcite. In thin section, distinct mineralogical domains Cataclasites, pseudotachylytes; c. 430 Ma derived from the felsic, mafic and cataclastic protoliths are recognisable. These domains are typically flattened, folded and Sinistral strike-slip Lower greenschist phyllonites; ?Late Caledonian overprinted by a new foliation defined by fine-grained aggregates of aligned sericite and chlorite flakes (Fig. 2b). Thin Low angle dip-slip extension section observations suggest that the phyllosilicates were de- Folds and detachments in phyllonites; age uncertain rived from the chemical alteration of feldspar or hornblende; High angle dip-slip extension cataclasite has been altered to aggregates of chlorite and Steep brittle faults; Mesozoic epidote. In feldspars (the dominant mineral in the parent Younger gneisses) the early stages of alteration are characterized by the appearance of small (10–20 ìm), randomly oriented sericite needles within the feldspar host (Fig. 2c). Progressive alteration leads to an increase in the volume of sericite, and at network of NNE- to NE-trending phyllonitic shear zones that approximately 20% by volume phyllosilicate, individual nee- enclose augen of lower strain in which a protophyllonitic fabric dles appear to coalesce and become aligned parallel to the of varying intensity is developed (Fig. 1b; Sibson 1977). A foliation observed in the field. Little-altered regions of feldspar shallowly plunging to strike-parallel, NE-trending mineral are commonly cross-cut by arrays of epidote-, quartz and stretching lineation is associated with the protophyllonitic calcite-filled intragranular fractures which are absent in fabric, and may also be preserved in some phyllonitic shear adjacent sericite-rich foliated material (Fig. 2c). zones. Kinematic indicators viewed on surfaces parallel to this The most highly-altered fault rocks preserve shear sense lineation and perpendicular to the foliation (asymmetric por- criteria that indicate either sinistral strike-slip (e.g. Fig. 2d), or phyroclasts, shear bands, sigmoidal fabrics) consistently give a later ESE-directed extension. These well developed phyllonites sinistral, top-to-the-NE sense of shear. consist of isolated quartz porphyroclasts in a fine-grained The most highly sheared regions of phyllonite are commonly matrix of aligned sericite flakes, and quartz intergrown with bounded by a linked system of E- to SE-dipping brittle chlorite needles (Fig. 2d). Individual sericite grains are aligned detachments, forming fault-bounded units in which with their long axes parallel to the mineral lineation, and have centimetre- to metre-scale ESE-verging folds are developed, straight grain boundaries showing little evidence of kinking or together with an ESE-plunging mineral stretching lineation fracturing. Internally, the porphyroclasts are characterized by and top-to-the-ESE extensional shear bands. At well-exposed patchy undulose extinction and poorly developed subgrains, localities (e.g. [NF 920 600]), it is possible to record a progres- which are commonly observed to be cross-cut by intragranular sive clockwise reorientation of the earlier NE-trending mineral shear fractures infilled by fine-grained quartz–chlorite aggre- lineation into a down-dip, ESE-plunging attitude. These ob- gates (Fig. 2d). Fibrous quartz–chlorite aggregates also occur servations suggest that, following sinistral shear, top-to-the- as overgrowths in pressure shadows around porphyroclast ESE extension preferentially focused into the pre-existing grain boundaries. Arrays of chlorite inclusions locally occur strike-slip phyllonitic shear zones. We conclude that, during along porphyroclast margins oriented at low angles to the extension, deformation became progressively more brittle so foliation and may be analogous to the dissolution interfaces that macroscopically ductile reworking of shear zone fabrics described in sheared quartzites by Hippertt (1994). gave way initially to low-angle detachments and finally to higher angle faults (Table 1). The focusing of almost all later extension into pre-existing phyllosilicate-rich mylonites is Weakening processes associated with phyllonitisation thought to indicate that fault zone weakening has occurred. The microstructural studies demonstrate that the fundamen- tal process associated with phyllonitisation along the Outer Hebrides Fault Zone was the grain-scale alteration of chain Microstructural and mineralogical evolution of the and framework silicates to fine-grained aggregates of aligned, phyllonites intrinsically weaker phyllosilicate minerals. The grain-scale In North Uist, there are two main protoliths from which the alteration processes controlled the textural evolution of the phyllonites are derived: pristine Lewisian gneisses and catacla- fault rocks and probably determined the range and relative sites derived from brittle crushing of gneiss during earlier rates of the deformation mechanisms that operated as phyllo- thrusting. The gneisses comprise layered aggregates of nitisation progressed. In pristine gneiss, the interlocking K-feldspar, An-rich plagioclase, quartz and biotite in varying grains of feldspar or hornblende surround pockets of weaker, proportions, with distinct mafic lenses and bands of horn- plastically deformed minerals such as quartz. This texture blende, plagioclase and biotite. The rock fabric is dominated may be described as a load-bearing framework microstruc- by interlocking grains of feldspar or hornblende, apparent in ture (Handy 1990), and implies that the bulk rheological thin section, which surround isolated pockets of quartz and behaviour of the rock was controlled by the strength and biotite (Fig. 2a). Most quartz grains are flattened parallel to distribution of the feldspar and hornblende grains. In con- the gneissose banding and display a slight undulose extinction. trast, phyllonites (e.g. Fig. 2d) consist of a contiguous The cataclasites are composed of angular clasts of quartz and phyllosilicate-rich matrix containing isolated quartz por- feldspar enclosed in a pale green matrix of comminuted albite, phyroclasts, a texture analogous to the interconnected weak

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Fig. 2. Microstructural evolution during phyllonitisation. See Fig. 1b for location of fault rock samples. (a) Photomicrograph of quartzo-feldspathic Lewisian gneiss cross-cut by a seam of cataclasite (C) (image in ppl); Q, quartz; F, feldspar; B, biotite. Sample from west of Burrival, North Uist [NF 905 626]. Scale bar is 1 mm. (b) Photomicrograph of (strike-slip) protophyllonite showing early stages of foliation development due to syn-tectonic retrogression of quartzo-feldspathic gneiss and originally cross-cutting seam of cataclasite (dark layer) (image in ppl). Q, quartz; Ch, chlorite aggregates after cataclasite; S, sericite and albite aggregates after plagioclase. Scale bar is 1 mm. (c) Photomicrograph of feldspar in protophyllonite, partially retrogressed to a fine grained aggregate of sericite and albite (image in crossed polars). The less retrogressed part of the grain (F) has undergone brittle fracturing, with infills of quartz and epidote (Fr), whilst the more retrogressed region (S) has begun to deform in a ductile manner. Scale bar is 1 mm. (d) Backscatter scanning electron microscope photomontage of strike-slip phyllonite cut parallel to the NE-trending mineral lineation. Dark phases are quartz and feldspar, whilst lighter phases include sericite, chlorite, epidote and ore minerals. Note braided network of fine-grained, phyllosilicate-rich high strain shear zones and larger relict grains of quartz and feldspar. Fabric asymmetries indicate top-to-the-NE (sinistral) senses of shear. Scale bar is 300 µm.

layer microstructure of Handy (1990). In such a case, the dislocation creep mechanisms as proposed by Goodwin & bulk strength of the rock was controlled by the rheology of Wenk (1995) in similar fault rocks. The presence of dissolution the fine-grained matrix. interfaces along the margins of quartz porphyroclasts, and the There is evidence that both quartz and relict feldspar grains occurrence of fine intergrowths of quartz and oriented chlorite deformed in a brittle manner during phyllonitisation (e.g. Fig. needles in the phyllonitic matrix suggest that pressure solution 2c and d). The fractures produced are commonly infilled by processes were also important deformation mechanisms new, strongly-aligned fibrous minerals precipitated from solu- (Hippertt 1994). tion. The nature of these infills, and the presence of pressure The operation of fluid-assisted diffusive mass transfer during shadows around quartz porphyroclasts strongly suggest that phyllonitization strongly suggests that the grain-scale altera- fluid-assisted diffusive mass transfer mechanisms were also tion of feldspar and hornblende to phyllosilicates in Outer operative. With increasing alteration of feldspar and horn- Hebrides fault zone rocks was controlled by the syn-tectonic blende, new phyllosilicate grains became strongly aligned and influx of hydrous fluids. Mass-balance calculations based upon coalesced to form an anastomosing network which accommo- geochemical analyses of fault rocks and parent gneisses from dated macroscopically ductile strains (Fig. 2c). The phyllosili- the fault zone led Walker (1990) to conclude that the devel- cate phases, in particular sericite, are very fine-grained making opment of the phyllonites was associated with the fluxing of it difficult to directly determine the operative deformation Fe- and Mg-rich fluids through the fault zone, leading mechanisms. However, the morphology and intense preferred to widespread silica dissolution and reprecipitation. The orientation of sericite grains is consistent with deformation by main metamorphic reactions which took place during

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10–30% of the total fault rock volume. We propose that this is sufficient to form a large-scale interconnected weak layer rheology analogous to the texture seen in thin section. If this comparison is correct, it suggests that the overall rheological behaviour of the fault zone may be controlled by the strength of the much weaker, volumetrically subordinate phyllonitic shear zones. Wintsch et al. (1995) have modelled the influence of aligned phyllosilicates on the rheology and strength of continental fault zones using data from laboratory deformation exper- iments (Fig. 3b). The upper strength bound represents the behaviour of typical quartzo-feldspathic continental crust with an upper brittle crust and a lower crust deforming by viscous creep with a strength maximum at the frictional-viscous creep transition. The lower limit is extrapolated from the experimental deformation of single mica crystals (Mares & Kronenberg 1993), and represents an ‘ideal’ monomineralic phyllonite with perfect alignment and contiguity of grains. Experimental deformation of mica schists has shown that as the volume, degree of alignment and contiguity of the phyllosilicates within a rock decrease, the bulk strength of the rock increases (Shea & Kronenberg 1993). Whilst the phyllosilicates in the phyllonites from the Outer Hebrides Fault Zone generally show very good alignment, their volume and contiguity is diminished by the presence of stronger phases in the matrix (e.g. quartz), so the strength of the fault zone might be expected to plot between the upper and lower strength bounds in Fig. 3b (e.g. around position ‘M’). However, theoretical work (e.g. Wheeler 1992) Fig. 3. (a) 3D sketch of phyllonite belts and low strain has shown that in polymineralic rocks where pressure solution protophyllonitic clast based on field observations (cf. horizontal view processes are also operative, the bulk strength of the rock can with Fig. 1b). Taken from Butler (1995). (b) Schematic strength v. be dramatically reduced. As fluid-assisted diffusive mass trans- depth profile of the continental crust. Upper bound represents fer processes were also important in the development of the frictional quartzo-feldspathic crust at shallow depths linked to curve Outer Hebrides Fault Zone phyllonites, then the strength may for wet quartzite at greater depths in viscous creep regime. Lower have approached that of experimentally deformed single mica strength bound extrapolated from experimental deformation of crystals (e.g. around ‘P’ in Fig. 3b). This would have led to a single muscovite crystal oriented favourably for slip (Mares & marked shallowing in depth of the frictional-viscous creep Kronenberg 1993). Strain rate c. 10"14 s"1. C, typical cataclasite; M, typical mylonite; P, typical phyllonite (after Wintsch et al. 1995). transition and also would significantly reduce the strength of the continental crust as a whole.

phyllonitisation essentially involved the hydration of feldspar and hornblende, and are envisaged to be of the form: Conclusions (1) K-feldspar+plagioclase+H+

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Received 16 May 1996; revised typescript accepted 16 August 1996. Scientiﬁc editing by Rob Butler and Alex Maltman.

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