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_Journal of Structural Geology 33 (2011) 92-106_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _The development of cavities and clastic infills along fault-related fractures in Tertiary basalts on the NE Atlantic margin_ _Richard J. Walker a,*, Robert E. Holdsworth a, J. Imber a, D. Ellis b_ _a Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK_ _b Statoil (UK) Ltd., One Kingdom Street London, W2 6BD, UK_ _article info_ _Article history: Received 16 June 2010; Received in revised form 30 November 2010; Accepted 1 December 2010; Available online 8 December 2010_ _Keywords: NE Atlantic Passive margin Clastic intrusion Clastic infills Faults in basalt_ _abstract_ _Field-based geological observations have revealed the hitherto unrecognised development of post-magmatic, brittle deformation structures cutting Tertiary volcanic rocks in the Faroe Islands. These faults and fractures are characteristically associated with different styles of clastic sedimentary infills including: 1) 0.3-1.0 m thick clastic units infilling open fractures formed along pre-existing steeply-dipping to sub-vertical faults; 2) 0.1-0.6 m thick sub-horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts; 3) Anastomosing mm-scale and planar dm-scale clastic intrusion features mobilised and emplaced during transient, fault-related overpressuring events along pre-existing fractures cutting the surrounding volcanic units. The infill features provide evidence for the existence of sustained open cavities in the subsurface. The clastic materials are commonly internally affected by later fault-related deformation and lack mineralisation, unlike all preceding faulting episodes in the Faroes region, perhaps reflecting their near-surface development. We believe structures equivalent to these features may occur widely in other parts of the NE Atlantic margin, particularly along the outer arcs of gentle regional-scale fold hinges. The uncemented fracture-hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability Cenozoic volcanic sequences of the NE Atlantic region._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Many upper crustal fault zones contain significant volumes of brecciated wall rock, which can potentially form high permeability pathways for the migration of mineralising hydrothermal fluids or hydrocarbons (Sibson, 1986, 1989; Roberts, 1994; Caine et al., 1996; Cowan, 1999; Woodcock et al., 2006, 2007). These fault-related breccias are formed by a variety of processes that operate at different rates. For example, at depths below 2 km, fault-breccia formation is widely believed to occur due to two different mechanisms: gradual abrasion and wear during fault slip (e.g., Woodcock et al., 2006) and implosion due to large, geologically instantaneous changes in fluid pressure adjacent to dilational fault jogs (e.g., Sibson, 1986). At shallower crustal depths (0-2 km, as a conservative estimate), however, mechanically strong rocks (e.g., crystalline or carbonate rocks) may be able to support fault-related dilational_ _features as persistent, open subterranean cavities with fluid flow properties similar to karstic aquifers found in limestone terrains. These voids can be filled by sedimentary breccias that may be deposited gradually or injected rapidly due to overpressure events (e.g., Beacom et al., 1999). Understanding the development of these fault-related breccias is scientifically and economically important, since the different breccia types (abrasion vs. implosion vs. cavity-fill) have contrasting sealing and fluid flow histories. Breccias resulting from implosion or abrasion may become sealed relatively quickly after faulting, as the associated mineralising fluids enter and rapidly cement newly formed cavities between breccia blocks. On the other hand, cavities that are open for longer time periods, particularly those formed in shallow crustal settings (0-2 km), will be associated with much longer-lived and persistent fluid flow, operating prior to, during and after the development of breccia along the fault (e.g., Wright et al., 2009)._ _The present paper focuses on the nature and development of well-exposed examples of weakly or uncemented cavity infills associated with fractures cutting Palaeogene basaltic lava sequences in the Faroe Islands. It is shown that the formation of open cavities in the subsurface occurred with post-magmatic fault reactivation, probably_ _R.J. Walker et al. Journal of Structural Geology 33 (2011) 92-106_ _Fig. 1. (a) Simplified structural elements map of the Faroe-Shetland Basin, NE Atlantic margin with location of the Faroe Islands: EFH, East Faroe High; FS-B, Flett Sub-Basin; JB, Judd Basin; CR, Corona Ridge; FR, Flett Ridge; RR, Rona Ridge; BFZ, Brynhild Fault-Zone; CFZ, Clair Fault-Zone; EFZ, Erlend Fault-Zone; GKFZ, Grimur Kamban Fault-Zone; JFZ, Judd Fault-Zone; VFZ, Victory Fault-Zone; WFZ, Westray Fault-Zone. (After Stoker et al., 1993; Rumph et al., 1993; Lundin and Dore, 1997; Sørensen, 2003; White et al., 2003; Jolley and Morton, 2007; Ellis et al., 2009). (b) Simplified geological map of the Faroe Islands and gross stratigraphic column for the Faroe Island Basalt Group (after Passey, 2009). Red dots indicate locations of clastic-filled fault cavities. (cee) Photographs of the Beinisvord (c), Malinstindur (d) and Enni (e) Formations with block diagrams displaying the typical characteristics and stacking styles of the lava units (after Passey and Bell, 2007)._ _93_ _94_ _R.J. Walker et al. Journal of Structural Geology 33 (2011) 92-106_ _during regional uplift. The implications for regional tectonic models and subsurface fluid flow are then briefly discussed._ _2. The Faroe Islands: geological setting_ _2.1 Regional context_ _Much of the NE Atlantic passive margin is covered in a thick pile of trap-style crystalline volcanics (Fig. 1a), as part of the North Atlantic Igneous Province (NAIP; emplaced ~62-54 Ma; Saunders et al., 1997), of which the Faroe Islands Basalt Group (FIBG; Passey and Bell, 2007) is a constituent. From a petroleum industry perspective, basins along the margin are relatively underexplored, in part due to the presence of this volcanic cover. In the past decade, the Faroes sector of the margin has opened for licensing, and based on the presence of several large hydrocarbon-producing fields in the nearby UK sector (e.g., the Clair, Foinaven and Schiehallion fields) exploration activity increased rapidly._ _The FIBG was emplaced at or around sea-level during the Palaeocene. The lavas display a progradational stacking geometry, with a true vertical thickness of about 2-3 km, requiring therefore a comparable magnitude of subsidence during the eruption period. To date, most structures preserved on the Faroe Islands are attributed to subsidence-related deformation (Geoffroy et al., 1994; Ellis et al., 2009; Passey, 2009). A progressive anticlockwise rotation in the regional extension vector, from NEeSW to NWeSE, is recorded by analysis of cross-cutting fault, fracture and dyke sets (see below) which is related to changes in the location and kinematics of ocean spreading in the North Atlantic region (Walker, 2010; Walker et al., 2011). No onshore structures have been related to the subsequent uplift that must have occurred to bring the Faroe Islands up to their current elevation (the highest peak, Slættaratindur, lies at 882 m A.S.L.). The principal aim of this study is to highlight the roles of post-magmatic to recent fault reactivation in forming open, subterranean cavities, fissures and caves that subsequently become infilled by clastic sediments. Unlike other earlier faulting episodes, these infills lack widespread mineralisation and may therefore have acted as preferential channel ways for the migration of hydrocarbon accumulations developed beneath or within the volcanic cover sequences of the NE Atlantic region._ _2.2 Stratigraphy_ _The FIBG is a prograding sequence creating a gross stratigraphic thickness in excess of 6.6 km, dominated by tholeiitic basalt lavas and divided into seven formations based on lithology and the presence of regionally recognised disconformity surfaces (see Rasmussen and Noe-Nygaard, 1969, 1970; Passey and Jolley, 2009) and geochemistry (Waagstein, 1988). The formations relevant to the present onshore study are from oldest to youngest: the Beinisvørð; Malinstindur; and Enni Formations (Fig. 1bee)._ _The Beinisvørð Formation (BF) is stratigraphically ca.3.3 km thick with only the upper 900 m exposed above sea level on the Islands (Fig. 1b and c). The BF comprises aphyric, laterally extensive sheet lobes, commonly separated by minor volcaniclastic horizons (Passey and Bell, 2007). The sheet lobes display well-developed columnar joints that are commonly observed to be exploited during faulting, and can result in greatly steeper fault-plane dips compared to faults cutting clastic horizons located between lava flow units._ _The overlying Malinstindur Formation (MF) is stratigraphically ca.1.4 km thick (Fig. 1b and d) and comprises subaerially emplaced, compound basalt lavas that are initially olivine-phyric evolving to aphyric, and then plagioclase-phyric types. Again, lavas are commonly separated by minor clastic horizons, typically volcaniclastic sandstones and siltstones, which were deposited during periods of volcanic quiescence (Ellis et al., 2002)._ _The lowermost 900 m of the Enni Formation (EF), is exposed on the islands (Fig. 1b and e), and comprises interbedded simple (sheet lobes) and compound tholeiitic basalt lavas._ Ключевые слова: e, r, o