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 inter- connected 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 weak- ening 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 pro- cesses 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). 105 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/105/4887764/gsjgs.154.1.0105.pdf by guest on 30 September 2021 106 J. IMBER ET AL. 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 aggre- gates 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 altera- tion 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