THE ROLE OF PRESSURE SOLUTION SEAM AND JOINT ASSEMBLAGES IN THE FORMATION OF STRIKE-SLIP AND THRUST FAULTS IN A COMPRESSIVE TECTONIC SETTING; THE VARISCAN OF SOUTHWESTERN IRELAND Filippo Nenna and Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected]

scale such as strike-slip faults and thrust-cored folds in Abstract various stages of their development. In this study we focus on the initiation and development of strike-slip The Ross Sandstone in County Clare, Ireland, was faults by shearing of the initial JVs and PSSs and deformed by an approximately north-south compression formation of thrust faults by exploiting weak shale during the end-Carboniferous Variscan orogeny. horizons and the strike-parallel PSSs in the adjacent Orthogonal sets of fundamental structures form the sandstone intervals. initial assemblage; mutually abutting arrays of 170˚ Development of faults from shearing of initial oriented set 1 joints/veins (JVs) and approximately 75˚ fundamental structural elements with either opening or pressure solution seams (PSSs) that formed under the closing modes in a wide range of structural settings has same stress conditions. Orientations of set 2 (splay) JVs been extensively reported. Segall and Pollard (1983), and PSSs suggest a clockwise remote stress rotation of Martel and Pollard (1989) and Martel (1990) have about 35˚ responsible for the contemporaneous described strike-slip faults formed by shearing of shearing of the set 1 arrays. Prominent strike-slip faults thermal fractures in granitic rocks. Myers and Aydin are sub-parallel to set 1 JVs and form by the linkage of (2004) and Flodin and Aydin (2004) reported strike-slip en-echelon segments with broad damage zones faulting formed by shearing of joints formed by an responsible for strike-slip offsets of hundreds of metres. earlier episode of deformation in sandstone. Willemse Thrust faults with up to 30 metres of offset initiate et al. (1997) and Graham Wall et al. (2006) elucidated within shale horizons and follow either the PSSs in the strike-slip faulting from a combination of veins and sandstones or high-angle shales within tilted sequences. pressure solution seams in carbonate rocks. Perhaps, the Within the large thrust fault zones, compartmentalized closest to the strike-slip and thrust faults development blocks of rocks are bounded by segments of thrust faults in the clastic rocks of the Irish Variscan setting are with various dip angles. Strike-slip and thrust faults are those by Florez et al. (2005) and Gonzales and Aydin contemporaneous and owe their existence to initial (2008) from the Andean foreland in Bolivia and Chile, weaknesses in the form of JVs and PSSs, as opposed to respectively. These authors reported thrust faults being switching relative stress magnitudes and orientations in confined within shale dominated horizons and strike- conjunction with the Andersonian models of faults and slip faults taking place within the stiffer sandstone and related stress orientations. conglomerate units. In both cases, the strike-slip faults formed by reactivation of the cross joints initially Keywords: perpendicular to structural trends of the fold and thrust Pressure solution seams; Joints; Veins; Fracture; belts. Strike-slip faults; Thrust faults In all these cases and a few others which dealt with normal faults (not listed here), the basic mechanism Introduction was determined to be shearing of the initial fundamental structural elements, JVs and PSSs (set 1 The coastal exposures of the Carboniferous Ross structures), formation of splay JVs and PSSs (set 2 Sandstone in western Ireland have been of great interest structures), and the shearing of these splays resulting in for the study of deep water successions and turbidite higher order of structures. Although splay fractures are channel architecture (Pulham, 1989; Martinsen et al., called by different names (wing crack, tail cracks, and 2003; Pringle et al., 2003; Strachan & Alsop, 2006; horsetail structures) the mechanism and the resulting Pyles, 2008). However, the detailed structural evolution fracture geometry is the same (de Joussineau et al., of the Irish Variscan deformation within this formation 2007). is relatively overlooked. This location provides Gonzales and Aydin (2008) reported that through- exceptionally well exposed arrays of opening mode going strike-slip fault zones with larger slip magnitudes fundamental structures such as joints and veins (JVs), are slightly oblique to the initial joint orientation due to and closing mode pressure solution seams (PSSs) as the linkage of stepping segments. The strike-slip faults well as more complex structures at the geological map can continue to grow to form a linked network of fault

Stanford Rock Fracture Project Vol. 22, 2011 D-1 zones (Peacock, 1991) with deformation being Sandstone form potential hydrocarbon reservoirs in the eventually concentrated on the largest faults (de subsurface offshore from the west coast of Ireland Joussineau and Aydin, 2007). (Croker, 1995; Johnston et al., 2001; Martinsen et al., As with strike-slip faults, thrust faults can also be 2003). Studies on fold-thrust belt structural viewed as the linkage of initially isolated fault segments assemblages in carbonates emphasize that fracture and to form a larger, through-going, structures (Eisenstadt fault networks can act as a high permeability network and de Paor, 1987; Aydin, 1988; Ellis and Dunlap, through an otherwise low permeability host rock 1988; Nicol et al., 2002; Davis et al., 2005). Fault (Agosta and Aydin, 2006; Graham-Wall et al., 2006). segmentation can be enhanced in heterogeneous rock This information is relevant to fluid migration through sequences if differences in rock properties exist across the rock during the period of deformation as well as at layer interfaces. This acts to pin thrust tip-lines in the present time (Braithwaite, 1989; Bjorlykke and mechanically weak layers that are adjacent to strong Hoeg, 1997; Gibson, 1998; Aydin, 2000). layers (Nicol et al, 2002). Eventually thrust faults cut across the stiff layers above (Boyer and Elliot, 1982) Geological setting forming flats and ramps and producing thrust related It is commonly believed that the formation of folds (Suppe, 1985). However the details of how a Pangaea at the end of the Carboniferous is marked by a thrust propagates through the stiffer units and the roughly east-west oriented fold-thrust belt which spans architecture of thrust fault zones and related dip through Europe (where it is known as the Variscan domains remain to be less known. In this study, we orogenic zone), northern Africa and North America document the role of PSSs and JVs in incipient thrust (Fig. 1). In Ireland, Variscan nearly north-south faults which sole into shale horizons and transect stiffer directed compression was responsible for units. Through the associated folding, the resulting fault approximately east-west oriented structural fabrics zones have highly complex internal architectures which apparent in the surface geological map of the study area would be difficult to elucidate without understanding (Fig. 2). O’Reilly et al. (1996) stated that Variscan the underlying mechanisms. fabrics are not present in the deeper crust suggesting a This investigation is of the development of faults ‘thin-skinned’ mode of crustal deformation. These from PSS and JV assemblages within a region of fold authors note that the deeper structural lineaments trend and thrust tectonics. We describe the deformation northeast-southwest, which associates them to the mechanism and the evolution of damage network using Caledonian Orogeny, which is Devonian in age. This systematic documentation by detailed mapping of trend is also observed in the surface geology to the structures at their progressive stages of development. north of Ireland where the Caledonian fabrics form the The objectives of this study are (1) to determine the dominant structural grain. The lack of Variscan fabrics relative timing, orientation, distribution and interaction in this part of the country indicates that the of PSSs and JVs during the initial tectonic episode and approximately east-west regional structures present in extract information that these structures can provide County Clare represent the northern-most surface extent concerning the causative forces and the remote of the Variscan orogeny in Ireland. Using balanced 2D principle stress directions during the orogeny; (2) to cross sections, Cooper et al. (1986) proposed a Variscan produce a chronological reconstruction of the shearing deformation history for Ireland as a progression of or inversion of fundamental structures to elucidate the layer-parallel shortening, buckling and thrusting, evolution of the fracture patterns, relative timing, sense though a thorough mechanical assessment of structural and magnitude of shearing responsible for the evolution within their study is not evident. formation of strike-slip faults and (3) to conceptualize the role of shale horizons and PSSs and JVs in the initiation and growth of thrust faults and as part of their internal architecture. This provides a better insight into both the nature and the degree of deformation associated with the Irish Variscan orogeny within Ireland The understanding of the occurrence of coeval PSS and JV formation in naturally deformed sandstone/siltstone turbidites and their shearing leading to the initiation and development of faults has implications for fluid flow in analogous systems. For example, sandstones of similar age to the Ross Fig. 1. A global pre-Mesozoic reconstruction of the end-Carboniferous Variscan Orogenic belt adapted from Woodcock and Strachan (2000).

Stanford Rock Fracture Project Vol. 22, 2011 D-2 The structures of interest occur in the Ross Field observations Sandstone Formation composed predominantly of deep Large scale structures of the Loop Head penninsular water turbiditic sandstones and siltstones with such as large folds and faults were mapped at a scale of occasional shaly intervals located in County Clare, on 1 :10,000 onto topographic basemaps in order to the west coast of Ireland (Figs. 2 and 3). This formation establish the Variscan structural trends and to form a is part of the infill of the Shannon Basin which formed framework for subsequent detailed mapping. at the end of the Devonian crustal extension (Collinson The large scale fold and fault structures on a et al., 1991). The oldest sediments which form the base sandstone platform, which we will refer to as the ‘Ross of the Namurian succession are laterally equivalent platform’, was mapped at a scale of 1:5,000 (Fig. 4b; Clare Shale Formation and Ross Sandstone Formation location 1). This platform is located near an antiform (Fig. 3). These formations are overlain by the Gull eroded from the base known as the ‘Bridges of Ross’ Island siltstones/sandstones representative of slumped found on the coast near the village of Ross. It provides continental shelf and unstable slope deposits, and the Central Clare Group deltaic cyclothems representative of delta progradation (Rider, 1974; Gill, 1979). The Ross Sandstone is only exposed at its type section on the Loop Head peninsula as it thins rapidly both to the north and to the south (Rider, 1974). Dating of the unit using goniatite beds place the top of the Ross Sandstone at 318-317Ma (Rider, 1974). The broader area of southwest Ireland displays plenty of evidence in the form of approximately east- west trending thrust cored folds (Fig. 4) that indicate initially approximately north-south compression during the Variscan Orogeny at the end of the Carboniferous (O’Reilly et al., 1996).

Fig. 3. Geological column of the Devonian and Upper Carboniferous rocks found in County Clare, Ireland (adapted from Rider, 1974; Collinson, 1991; Sleeman & Pracht, 1999). The Fig. 2. Geological units with major fold axes and Ross Sandstone represents the base of the thrust fault traces within County Clare (modified Shannon Group on the Loop Head peninsula and from Sleeman & Pracht (1999). A geological is laterally equivalent to the Clare Shale column to accompany this map is given in Fig. 3. Formation, a sequence of dark grey shales Inset: location of the Loop Head peninsula within overlaying the Visean limestones (Sleeman & County Clare, Ireland. Pracht, 1999).

Stanford Rock Fracture Project Vol. 22, 2011 D-3 4a; location 2). We also present maps to show the orientation and distribution of larger offset en-echelon strike-slip faults taken from a platform to the south- west of the Ross platform (Fig. 4a; location 3). This particular location is only accessible at low tide. The outcrop scale structures were identified in the field and detailed maps were made directly onto photographs and photmosaics which were then digitally traced using Adobe Illustrator. These maps document structural arrays (of a single fracture type), assemblages (of multiple fracture types) and zones of structures found within the Ross Sandstone outcrops. Detailed maps of strike-slip fault arrays and thrust-cored folds were made using an array of measuring tapes and a compass to record distances between, and orientations of, structures and lithological boundaries.

Joint/vein and pressure solution seam assemblages Two types of fundamental structures are ubiquitously present forming arrays at high angle to bedding: opening mode joints and veins (JVs), and closing mode pressure solution seams (PSSs). This Fig. 4. Mapped regional scale fold and fault traces as observed on the Loop Head Peninsula (a) and the assemblage of what we will refer to as set 1 JVs (JV1s) Ross platform near the Bridges of Ross location (b) within County Clare. The geological boundaries are adapted from Pyles (2008). exceptionally exposed examples of pressure solution seams (PSSs), joints and quartz-filled veins (JVs), folds, strike-slip and thrust faults with offsets ranging from millimeters to tens of meters, which are the focus of the present study. This location provides a great opportunity to collect high quality data including the orientations, cross-cutting and abutting relationships, distributions, and the nature of younger shearing deformation and formation and development of various fault sets. Fracture damage networks around medium offset strike-slip faults were also mapped within the Ross Sandstone but found to the east of Kilbaha (Fig.

Fig. 6. Image of exposed north-south joint surfaces in the Ross Sandstone (a) with plumose structures indicating the opening mode origin Fig. 5. Orthogonal JVs and PSSs in the Ross for these fractures (pencil shown for scale) and Sandstone at the Kilbaha location. Inter-butting PSS traces (b) identified by their sinuous trace relationships suggest contemporaneous and colour contrast against the host rock formation of these structures. (hammer shown for scale).

Stanford Rock Fracture Project Vol. 22, 2011 D-4 and set 1 PSSs (PSS1s) forms an orthogonal network Although bed-parallel PSSs are also present in the (Fig. 5). Upon closer examination it can be seen that the Ross Sandstone, it is the high-angle PSSs (PSS1s) that JV1s frequently abut against the PSS1s and that there are of interest in this study as they are the most are also instances of the opposite abutting relationships pervasive. They are also easily distinguished from indicating their simultaneous development in a depositional structures such as thin clay lenses that are geological time frame even though PSS1s appear to not necessarily formed by the pressure solution process. initiate and terminate JV1s more than the other way The high-angle PSSs are characterized by the colour around from a statistical point of view. contrast of the residue with respect to the host rock The JVs in the Ross location can easily be identified (Fig. 6b), strike-parallel orientation and sinuous form. by quartz infilling and typical plumose morphology PSS1 trace orientation measurements were taken over a when their surfaces are exposed (Fig. 6a). JV1s are at length of seam to even out the small local changes high angle to bedding and have an average orientation produced by the sinuosity of the structure. An average of 170˚ (Fig. 7a). Although JVs are strata bound, the orientation of approximately 75˚ was measured (Fig. vertical continuity of the larger composite JVs crossing 7a), but given the wavy geometry, this average many layers as previously conceptualized by Helgeson orientation has a greater error associated with it than and Aydin (1991) are often on the scale of many that of the straighter JVs and as shown in the map in meters. Figure 5, the PSs1s and JV1s are almost orthogonal to the observer. Similar PSSs to those we describe here

can be found in clastic rocks exposed near the city of Cork in the south of Ireland were described in detail by Nenna and Aydin (2011) and reported by other workers who referred to the PSSs as cleavages (Trayner and Cooper, 1984; Cooper et al., 1986; Meere, 1995).

Strike-slip faults In various locations on the Ross platform (Fig. 4b, location 1), sheared versions of the fundamental structural elements along with unsheared ones are observed (Fig. 8). Some JV1s and PSS1s were determined to have right- and left- lateral shearing based on measured offsets and splays, which we refer to

Fig. 7. Equal area, lower hemisphere stereonets to show the poles-to-planes of the structures Fig. 8. Mapped photograph to show unsheared (PSSs, JVs, strike-slip and thrust faults) and JV1s and PSS1s, and sheared ones that have bedding in the Ross and Kilbaha locations. produced JV2s and PSS2s as splays.

Stanford Rock Fracture Project Vol. 22, 2011 D-5 deposited (Fig. 9b). Some of the individual PSS1s in this same location have also undergone left-lateral shearing to produce set 2 closing mode splays. In addition, a series of pull-aparts with multiple quartz veins occurs between overlapping sheared PSS1s along the extensions of the JV2 system. One of these shows 3.2 centimeters of left-lateral shear across this particular PSS1 as estimated from the combined widths of the quartz veins within the pull-apart (Fig. 10). The most intriguing Fig. 9. Photographs of vein surfaces in the Ross Sandstone phenomenon is that the shearing of JV1s and in the Ross location to show: quartz infill on fracture surface PSS1s was simultaneous based on the detailed with slickenlines (a) suggesting a sub-horizontal (strike-slip) maps showing clear evidence for the sense of shear (pencil oriented sub-parallel to slickenlines interaction of the two shear systems: shown for scale); multiple slip surfaces within a vein (b) occurrence of quartz veins along the extension suggesting slip continued to occur after vein deposition. of set 2 (splay) JVs within the pull-aparts located at stepovers between the sheared PSS1 as set 2 JVs (JV2s) and PSSs (PSS2s) respectively. The segments (Fig. 11). This indicates that the average orientation of the JV2s is 25˚, and PSS2s have an average orientation of 115˚ (Fig. 7b). These measurements were taken on straight sections away from the tips of the set 1 structures in order to avoid recording any less consistent intermediate orientations where the set 2 structures initially curve away from the set 1 ones. On the Ross platform we mapped an echelon array of JV1s with an associated set of JV2s at an acute angle to them (Fig. 8), indicating that right-lateral slip has taken place. A maximum shear displacement of about 40 centimeters was calculated for this particular sheared JV1 using the presence of sub-horizontal slickenlines on the exposed vein surfaces (Fig. 9a) and the apparent vertical offset of the inclined beds. Multiple slip surfaces with striations within the vein material show that episodic slip occurred after the vein material was

Fig. 11. Detailed map of a sheared PSS1 and JV1 Fig. 10. Image (a) and map (b) to show PSS1s with assemblage in the Ross Sandstone. The splays pull-aparts as evidence of left-lateral shearing. The show that the PSS1s have undergone left-lateral JV2s within the pull-aparts have a quartz infill. JV1s slip, and the JV1s have undergone right-lateral are present in this locality, but are not present in this slip. PSS2s are much shorter than the other image. structural elements and are not clearly mapped at this scale.

Stanford Rock Fracture Project Vol. 22, 2011 D-6 right-lateral fault arrays can be observed at location 3, and were mapped using a measuring tape and compass technique (Fig. 13). The faults have left stepping en- echelon arrangements with individual segments each having maximum offsets in the range of 0.6 to 8.7 metres. Average orientations of strike-slip faults were made by taking an average orientation through each en- echelon set (Fig. 7c). For strike-slip faults with only a few metres of offset only closely spaced fault segments are linked by damaged rock produced by shearing of the JV1s and PSS1s so the average orientation of the sets of linked right-lateral strike-slip faults are close to that of the original JV1s. Mapped fold hinge lines have an apparent right- lateral offset of about 20 metres across the Ross Bay (Fig. 4b). This large offset is distributed amongst many sub-parallel fault segments within a zone that runs in a north-south orientation through the bay, some of which are observed on the beach wave platform exposures

Fig. 12. Array of approximately N-S trending strike- slip faults near Kilbaha (location 2) (a). Mapped photograph (b) of strike slip fault with about 90cm of slip. Most of the offset occurs along a single slip surface with no obvious fault rock present. shearing of the JVs and the formation of their splays coincide with the shearing of the PSSs and their pull- apart veins. We have already referred to small offset strike-slip faults with right- and left- lateral displacements along most JV1s and PSS1s, respectively. On the Ross platform the right-lateral shearing of JV1s is greater in magnitude and frequency than the left-lateral component along the PSS1s. In what follows we document progressively larger strike-slip faults and their internal architecture. A series of sub-parallel right-lateral faults each with offset in the order of 30 centimetres to 2.5 metres are observed at location 2 (Fig. 12a). Zones of fractured rock around the slip planes are formed from extensive splays (both opening and closing mode) between adjacent sheared PSS1s and JV1s, as seen on a mapped example that accommodates approximately 90 centimetres of offset (Fig. 12b). The majority of the offset occurs along one or more slip-surfaces that are sub-parallel to the JV1s, but no obvious fault core is observed by the naked eye. This is true of also of the Fig. 13. Map created at location 3 (see Fig. 4a) to strike-slip faults observed at location 3, where the show the en echelon pattern of strike-slip faults. observed offsets reach up to 8.7 metres. Larger offset Displayed offsets were found using slickenline and bedding orientation data.

Stanford Rock Fracture Project Vol. 22, 2011 D-7 Fig. 14. Distribution of offsets along a zone of strike-slip faults in ‘Ross Bay’ (Fig. 4b) using bedding and slickenline orientation data. The cumulative offset across the width of the bay is approximately 20m using the apparent strike-slip offset fold axes shown in Fig. 4b. Note that these faults segments are sub-parallel and that their apparent convergence is a perspective artifact of the photograph.

(Fig. 14). Maximum observed offsets of these faults are right-lateral and in the order of 0.1 to 12.6 metres. Fig. 15. The array of set 1 and 2 JVs and PSSs at a Though we have not observed fault zones location neat the large strike-slip fault marked in Fig. accommodating offsets larger than 20 metres in the 2. The similarity to the arrays observed around the strike-slip faults in the Ross location show that these Ross area, faults with similar orientations to those we larger strike-slip faults have a similar formational observe are located in similar lithologies south of the history. River Shannon and have apparent strike-slip offsets more than 100s of metres as shown in the geological map (Fig. 2). These faults appear to be formed by the Though both reverse and thrust faults are present in the shearing of earlier weaknesses and grew by linkage of area, for simplicity we will refer to them both as thrust progressively larger segments as described earlier but faults regardless of the dip-angle of the fault plane. unfortunately, many of these faults that we visited do One of the smallest thrust faults among many others not provide with good surface exposures of the fault observed on the Ross platform has an offset in the order core to characterize their internal architecture and of a centimeter and cuts through the apex of a small growth mechanism. However it is clear that the smaller fold (Fig. 16). The fault merges into or possibly size right-lateral faults in the vicinity of these large initiates from a thin shale layer. Displacement occurs mapped faults exposed on the wave platform (Fig. 15) along strike-parallel PSS1s in the sandstone units with are similar to those observed in the Ross location. The associated bed-parallel slip and crenulation in the orientations of these strike-slip faults on the geological adjacent shale unit. maps are in the region of 160˚ to 110˚ (Fig. 7c), though Faults with approximately 1 metre of displacement these map traces may not be completely accurate given are observed where bedding is folded and shale layers the poor quality of inland exposures. The angular are at a high-angle (Fig. 17). An example of such a fault difference between these large offset strike-slip faults was mapped both in cross section and along its trace in and the smaller ones described previously is reasonable map view. The fault is composed of several segments as we expect larger offset faults to have a larger damage that originate from the shale layers and continue network and incorporate more distal en-echelon fault through the sand layers along PSSs. Thick vein deposits segments than smaller offset faults. in the order of a few centimetres are located on the fault planes and have slickenlines on their surfaces indicating Thrust faults an approximately north-south direction of slip (Fig. 18a). The JV1s associated with a small scale strike-slip Thrust faults at different stages of development fault system truncate against the thrust fault plane. have been observed at the core of folds within the Ross A much larger thrust fault with associated tight Sandstone at location 1, though more, mostly folds is observed 25 meters to the south of the inaccessible, examples exist throughout the region. intermediate thrust-cored fold (Fig. 19). This fault

Stanford Rock Fracture Project Vol. 22, 2011 D-8 Folds The regional overview map (Fig. 2) highlights the presence of broad northeast to east-west trending folds which on the Loop Head peninsula typically have half- wavelengths between 0.9 and 2.6 kilometers. The trends of the folds vary from approximately 70˚ to 90˚ even along the same fold; inclined in the middle or southern parts becoming east-west at the periphery. In areas such as the Loop Head coastline around Ross, tight low- amplitude kink folds exist and are concentrated around thrust faults which also have atrace orientation range similar to those of the fold trends (Fig. 4). Open folds are also present in this area, but they do not seem to be Fig. 16. Mapped photograph of a small scale thrust associated with noticeable thrust faulting. fault present on a vertical rock face. The maximum apparent offset of this thrust is about 1cm. Discussion and conceptual model for fracture assemblages and fault penetrates the entire cliff face and no offset markers can evolution be observed on both the hanging wall and footwall due to the restricted exposed intervals. Though given the large amplitude of the folding (in the order of 15 to 20 Joint and PSS assemblages metres) and the abrupt change in bedding orientation on Most rock units within the crust are under either side of the fault plane one may estimate an offset compression rather than tension due in part to the that is at least equal to the height of the cliff face which weight of overlying rock, and in part due to limited is approximately 25-30 metres. processes in the Earth’s crust generating regional The fault strands include bedding plane faults tensile stresses. Compressive stress conditions generally following low-angle and upright shale units on the northern and southern limb of the large scale fold. Highly deformed and extensively veined sandstone lenses are compartmentalized between the sheared shale beds (Fig. 20). The majority of these veins within the thrust fault zone appear to be strike-parallel and sub- horizontal in some places in contrast to the JV1s or JV2s described earlier (Fig. 18b). The geometry of the fault strands and fractures (JVs and PSSs) and the distribution and variation of the dip domains of fault bounded blocks are extremely complex especially in map view.

Fig. 17. Map of an intermediate scale thrust fault zone (a). Fault surfaces shown as lighter colours. Fig. 18. Slickenlines on a fracture surface that North-south oriented cross sections showing thrust indicate a high component of dip-slip on a thrust faults within shale layers in original position and fault (a) and steeply dipping beds containing sub- their ascent by following PSSs within the sand layers horizontal veins associated with thrust faulting (b). (b). The maximum apparent offset of this thrust is The location of (b) is given in Fig. 4b. about 1m. Stanford Rock Fracture Project Vol. 22, 2011 D-9 favor the formation of PSS, but the occurrence of spherical limestone pebbles over time in which a cyclic assemblages of both PSS and JV in orogenic belts is pattern of elastic behavior / creep behavior / failure was also common (Ferket et al., 2000; Graham Wall et al., envisioned under a constant displacement rate. This 2006). Of these, the JVs perpendicular to the strike of model would suggest that grain scale micro-cracks the tilted beds or the PSSs and the trend of the would form at each grain contact within and adjacent to contraction belts (sometimes called strike-perpendicular a PSS. At the outcrop scale in the Ross Sandstone the or cross-fold joints) are often referred to as JV1s have spacings in the order of 10s of centimeters accommodating orogen-parallel extension (Engelder and lengths in the order of meters but their relations to and Geiser, 1980; Zhao and Jacobi, 1997). However, the pressure solution appears to be similar. they do not necessarily indicate remote tensile stress Contemporaneous formation of the JV1s and PSS1s is parallel to the structure belts but rather the smaller further supported by their mutually abutting component of the horizontal compressive principle relationship, indicating that the same stress conditions stress as simulated in triaxial tests by Griggs and are responsible for the formation of these two opposite Handlin (1960). Pollard and Aydin (1988) reviewed the modes of structures. literature on jointing and concluded that joints initiate at The vein material found in many of the exposed flaws which perturb the stress field. These perturbations fracture surfaces in the studied locations consist of large may convert a remote compressive stress into a local well formed prismatic crystals which we deduce to be tensile stress large enough to initiate fracturing. Further quartz. This material may be sourced from the quartz examples of local tensile stress generation caused by dissolved from the formation of the pressure solution compressive remote stresses include areas on the crests seams. Though we have not tested this hypothesis in of folds and in rock units with high pore-fluid pressures this study, the notion is well supported by experimental (Bessinger et al., 2003). It is also possible that a large studies (Gratz, 1991; Gratier et al., 1999; Gratier et al., degree of volume reduction along the PSSs may 2005). These two processes have different time scales; produce a local tension that may cause tensile fracturing pressure solution creep is slower than brittle fracturing. (Fletcher and Pollard, 1981; Katsman, 2010; Zhou and However, apparently they still interact and enhance Aydin, submitted) which is further discussed in the each other. following paragraph. The association of pressure dissolution with high- Loading conditions required for PSS angle opening mode fractures has been recognized formation since the early days of geology (Sorby, 1879). More recently, Gratier et al. (1999) have numerically Based on the presence of bedding-parallel PSSs in modeled the stress variation at a contact between two the Ross Sandstone, which have not been the focus of this study, the overburden stresses are of sufficient magnitude for quartz dissolution to occur. The geological column for the Shannon basin as observed on the Loop Head peninsula (Fig. 3) suggests that at

Fig. 19. Map of large scale thrust fault zone (a) with fault surfaces shown as lighter colours. North-south oriented cross sections (b); a photograph showing Fig. 20. Annotated photograph to show the the details of cross section bii is show in Fig. 20. relationship between low- and high-angle thrusts Mapped photograph of the Ross outcrop to show 25- and the various dip domains between them. This 30m apparent vertical offset location corresponds with Fig. 19bii.

Stanford Rock Fracture Project Vol. 22, 2011 D-10 3 least 1km of Namurian sandstones and siltstones (σv) assuming an average density (ρ) of 2500kg/m for overlaid the Ross sandstone at the peak of the Variscan the overburden and a minimum burial depth (h) of 2km orogeny approximately 300Ma. This is a minimum is about 50MPa (using σv = ρgh). For a burial depth of value as it does not take into account eroded material, 4km, the vertical stress is approximately 100MPa. but only units which are present in situ and are The PSSs oriented at high-angle to bedding are accessible at outcrop. Thermal maturation data attributed to the horizontal approximately north-south compiled by Corcoran and Clayton (2001) show that compression of the Variscan orogeny. Assuming they use of vitrinite reflectance is not sufficient to estimate require the same minimum stresses to form as with the maximum burial depth of rocks in County Clare due to bedding parallel PSSs, then the most compressive anomalously high recorded peak temperatures horizontal stress is at least 50-100MPa and possibly indicative of advective hydrothermal circulation that much greater than that based on the intensity of has overprinted the original geothermal gradients. deformation associated with the high-angle PSS array. However, Goodhue and Clayton (1999) suggested the maximum burial depth of late Carboniferous cover Strike-slip faulting units can be constrained between 2 and 4 kilometres The JV1s and PSS1s acted as pre-existing based on the extrapolation of the plotted borehole discontinuities along which slip occurred and their gradients to `zero coalifcation' as described by Dow subsequent shearing is indicative of a rotation of (1977). An approximate measure of vertical loading relative remote stress orientation to produce coeval right- and left-lateral strike-slip faults respectively. Given that the JV1s show a greater amount of shear displacement discontinuity relative to the PSS1s, the secondary deformation is likely due to a rotation of the greatest compressive stress to form an acute angle with the orientation of the JV1s. JV2 orientations relative to those of the original JV1s provide some constraint on the angle of rotation of the remote principle stresses responsible for the shearing (Dyer, 1988; Cruickshank and Aydin, 1995). The initial path deviation of the splay from the original fracture plane just beyond the fracture tip is a function of both remote stress orientation and the coefficient of friction (Du and Aydin, 1995; Willemse and Pollard, 1998; Davatzes and Aydin, 2003; Mutlu and Pollard, 2008). Work done by de Joussineau et al. (2007) suggests that the relative length of cohesive end zones of the initial fractures make little difference to the splay angles. However, fracture overlap and fracture separation can induce variations in splay angles, explaining the spread of set 2 structure orientations as seen in the stereonet projections (Fig. 7b). Though we tried to avoid these configurations by taking JV2 measurements on their straightest parts away from the original tip of a sheared isolated fracture where the splays ultimately curve into a direction parallel to the remote greatest compressive stress (Cotterelle and Rice, 1980; Nemat-Nasser and Horii, 1982). Based on the average JV1 and JV2 orientations on the Ross platform, a clockwise rotation of the principle Fig. 21. Photographs to show JV2s connecting stresses in the order of 35˚ can be inferred. The to a vein along a thrust fault plane from a difference in average orientation between the PSS1s location shown in Fig. 17 (a) and folding and and PSS2s is about 40˚, but the rotation inferred from strike-slip faulting where the orientation the JVs is more reliable because their straighter traces relationships between JV1s, JV2s and bedding are maintained even where beds are steeply allow for measurements with greater certainty. The dipping (b). JV2s in the pull-aparts of the sheared PSS1s are in the path of, and in a similar orientation to the splays of the

Stanford Rock Fracture Project Vol. 22, 2011 D-11 sheared JV1s. This evidence indicates that the shearing Thrust faulting of the JV1s, PSS1s and weak shale horizons (as will be The observations from small to intermediate and to addressed in the ‘thrust faulting’ section) was likely to large offset thrust faults show that the faults are have occurred simultaneously under the same remote associated with some folds and shales with intervening stress conditions. sandstone layers. We infer that the thrust fault segments Linkage of extensive set 1 and set 2 structures originate in the shale layers from either bed-parallel slip create a zone of damaged rock. The mapped fracture in the main transport direction. It has been well assemblages with increasing amounts of offset allow us established that thrust faults localize within shale units to conceptualize the evolution of these faults and their in fold and thrust belts and they cut across the damage zones. The small offset strike-slip faults on the intervening stiff units to the next shale horizon (Rich, Ross platform show the early stages of rock 1934; Boyer and Elliott, 1982) forming flats and ramps fragmentation and weakening where both opening and sections which eventually leads to duplexes and ramp- closing mode set 2 structures originate from, or abut related fault bend folds (Suppe, 1985) The flexural slip against, the set 1 structures. across the shale layers or laminas relative to the With increased offset, as seen in location 2 in Figure overlying sandstone is also well known (Behzadi and 4a, the fragmentation is localized in zones where JV1s Dubey, 1980; Ohlmacher and Aydin, 1995; Cooke and and PSS1s have significantly small spacings than those Pollard, 1997; Couples and Lewis, 1999). The slip outside of the zone (Fig. 12). In other places propagates upwards through the sandstone layers along fragmentation is due to the intersection of set 1 and set PSS1s (Fig. 16). The thrust faults with progressively 2 structure arrays (Fig. 11). Set 2 structures are larger offsets occur where folding is more pronounced pervasive within these zones and abut against the set 1 and the shales at steeply dipping southern limbs of the structures to isolate small rock blocks. The patterns of folds are noticeably sheared, indicating that the fragmentation resulting from these processes are quite development of folding and thrusting are regular and appear to be different from those reported interconnected. The presence of thick vein material on from other fault zones (for example, Mort and the slip planes with slickenlines indicates that the vein Woodcock, 2008). We infer that only a small amount of material was deposited before the cessation of the thrust slip is distributed amongst the many JV1s within this faulting (Fig. 18a). The total offset is distributed among zone due to the lack of visible offset markers or rotated several fault segments or strands that are related to slip blocks between fractures. The majority of the slip along bedding in the shale layers, detachment of occurs along a few slip surfaces if not a single one with sandstone layer interfaces or reactivation and linkage of no obvious fault core observed by the naked eye. This fold-axis parallel PSSs in the sandstone layers. These is in contrast to those faults with comparable offsets in dip-slip faults with dip angles from 0˚ to 90˚ other types of sandstones reported in the literature compartmentalise rocks into various dip domains within (Myers and Aydin, 2004; Flodin et al., 2005) but is the fold and thrust zone. With increasing offset, in-situ similar to those in turbidites reported by Gonzales and high-angle shales in the folds are entrained into the Aydin (2008). The strike-slip faults seen at larger anastomozing and discontinuous fault segments and scales at location 3 (Fig. 4a) contain many segments compartmentalise heavily veined sandstone bodies that consist of the described fragmented zones adjacent between them (Fig. 20). Note that these veins to a main slip surface or segment. The segments are in concentrated within the fold-thrust zone are primarily in en echelon patterns each with an offset that contributes a sub-horizontal orientation and most of them are likely to the total offset of the fault zone. With an increasing to have formed after the tilting of the beds (Fig. 18b). number of faults within the fault zone, larger total The JV2s we described earlier are observed to connect offsets can be accommodated by the segments. The veins along thrust fault planes (Fig. 21a) from which we fault zone in the Ross Bay (location 1) accommodates a infer that the thrust and strike-slip events overlapped in total slip of about 20 metres, and we infer that the time and space. largest apparent strike-slip offsets seen on the published Johnson (1977) suggests that thrust faults may act as geological map in the same orientation as the faults we triggers for the initiation of kink-folding by introducing observe in our studied locations south of the Shannon an initial localized slope to a horizontal multilayered River (Fig. 2; lower part of the map) are indeed zones rock unit. Under horizontal compression, the multilayer of strike-slip fault segments formed from yields at this irregularity provided the interfaces contemporaneous shearing of the original set 1 JV and between the layers have a relatively low strength so that PSS assemblage. slip between layers can occur. This suggests strain is largely confined to the kink-folds during deformation and is evidenced in the shales of southwestern Ireland where slickenlines are pervasive in foliations on the

Stanford Rock Fracture Project Vol. 22, 2011 D-12 short, steeply dipping limbs but not as much on the contemporaneously (Figs. 21a and 22d). The presence long, gently dipping limbs of the folds (Dewey, 1965). of strike-slip faults in a fold and thrust belt environment The remote stress ratio required to initiate slippage is a has been generally rationalised in terms of a major function of the frictional and cohesive strength of reorientation of the principle stresses in such a way that interfaces and the initial slope of the layers locally. The the minimum and intermediate principle stresses switch slippage allows for the growth of the kink-fold and in either in space locally or in time (Fig. 23) according to this case it will be unstable as an increase in angle the Andersonian model of three major faults and the requires a lower remote stress ratio for slip to occur corresponding stress states based on the Coulomb (Johnson, 1977). failure criterion for a homogenous medium (Anderson, 1951; Guiton et al., 2003). The data presented in this Structural element age relationships and study supports the contemporaneous evolution of strike- conceptual model slip and thrust faults from initial arrays of JV and PSSs as well as weak shaley stratigraphic horizons in the A conceptual model illustrating the evolution of Irish Variscan orogenic belt. Although a major stress observed structures from initial to advanced stages is rotation has been inferred based on the shearing of the shown in Figure 22. We have presented evidence that JV1s and PSS1s, this stress reorientation is more suggests contemporaneous formation of the JV1 and PSS1 assemblage (Fig. 22a). PSS1s generally remain at high angle to bedding and strike-parallel on all parts of folds suggesting that they at least began to form before the initiation of folding, and that the folding is caused by a remote compression in a similar orientation to that which caused the formation of the PSSs (Fig. 22b). However, there are younger PSSs within steeply dipping limbs of large folds, which are not bed- perpendicular indicating that the PSS formation continued locally during the late stages of folding. The contemporaneous shearing of this initial assemblage is determined by the relationships between JV2s and PSS2s (Fig. 22c). The orientation relationship between JV1s, JV2s (splays) and bedding is maintained in some cases even when bedding has a high dip angle (Fig. 21b). In Figure 21b the bedding has an orientation of approximately 070/70/S and the JV2 has an approximate orientation of 005/55/W, whereas the JV2s (splays) have an orientation of approximately 25/85/W where the bedding is sub-horizontal. The orientations of the JV1s are relatively unchanged relative to bedding attitude because they are sub-perpendicular to the fold axis. This rotation of the JV2s would suggest that at least some strike-slip faulting occurred before the peak of folding. However, fold axes with apparent offset by strike-slip faults as seen on the larger scale maps (Figs 2 and 4) indicate that the activity across some large strike-slip faults has likely occurred either after or during the advanced stage of folding. The variation in trend of the folds (about 70˚ to 90˚) may reflect the change in greatest principle stress orientation responsible for the strike-slip faulting but the fold Fig. 22. Conceptual block model to show the structures were not studied in detail. The presence of initial PSS1 and JV1 assemblage (a); the onset of folding (b); the rotation of the remote greatest multiple slip surfaces in the quartz vein infill shows that compressive stress that caused shearing of the strike-slip motion continued after the deposition of the initial assemblage to form PSS2s and JV2s with infill (Fig. 9b). large strike-slip offsets along the sheared JV1s Sheared JV1s and their splays are observed to join (c) and thrust faults that initiate from the shale the veins along thrust fault planes suggesting that horizons and propagate via PSSs in the strike-slip and thrust faulting occurred more or less sandstone beds, bedding planes or high angle shale beds (d).

Stanford Rock Fracture Project Vol. 22, 2011 D-13 pressure solution seams (PSSs) sub-parallel to fold axes and thrust fault traces. The mutually abutting relationship between these sets of structures suggests contemporaneous formation under the same stress conditions. Subsequently, set 1 PSSs and JVs have undergone left-lateral and right-lateral shear respectively evidenced by measurable offsets, pull-aparts and splay PSSs and JVs. The orientations of the set 2 (splay) structures suggest a relative clockwise remote greatest compressive stress rotation in the order of 35˚ during the Variscan orogeny that is responsible for the contemporaneous shearing of both set 1 arrays. As a higher proportion of shear is resolved on the set 1 JV system than that of the set 1 PSS system, the more prominent strike-slip faults are sub-parallel to or slightly inclined to the pre-existing JV set and have a right-lateral sense of slip. The zone of concentrated sheared set 1 and set 2 PSSs and JVs provide weak zones of fragmented rock along which larger faults develop. Evidently, there is not any significant fault Fig. 23. Conceptual model to show how the rock development along these faults comparing with switching of the least and intermediate those faults with comparable offsets formed in compressive remote stress can form strike-slip (a) sandstones of different types reported in the literature. and thrust faults (b) according to the Andersonian Zones of en echelon strike-slip faults are responsible for model based on Coulomb failure criteria for a homogenous medium. apparent strike-slip offsets up to hundreds of metres as seen at the geological map scale. relevant to resolving shear stresses on the previous Thrust faults initiate within shale horizons and principal planes which were then merely weak zones follow either the PSSs in the sandstone units or in some prone to slip in a strike slip sense. Note that according other cases, high-angle shale units within the steeper to the Andersonian model, the two conjugate strike-slip limbs of the folds that are entrained into fault zones. fault orientations make a dihedral angle bisected by the Offsets of these thrust faults range from the sub- direction of the greatest compression (Fig. 22a). centimetre scale to over 30 metres of apparent vertical However, in our study area (Fig. 22c) the two sets of offset. Within the large thrust fault zones, blocks of strike-slip faults are nearly orthogonal. The discrepancy rock bodies with varying dip domains are bounded by between our field observations and the Andersonian strands or segments of thrust faults with various dip model appears to be because of the presence of the angles thereby compartmentalising the deformed rock, initial assemblage of fundamental structures weakening the smallest size of these compartments are sandstone the rock in certain orientations. layers between sheared high-angle shale units. It is interesting to note that from a regional Strike-slip and thrust faulting are inferred to be perspective, the right-lateral strike-slip faults are almost contemporaneous and this coexistence may be due to orthogonal to the fold and thrust belts. Because of the the initial weaknesses in the form of JVs and PSSs, rotation of the greatest principle stress to form a small rather than by changing relative stress magnitudes and angle to the cross-fold orientation, the right-lateral orientations in conjunction with the Andersonian strike-slip faults along with the fold and thrust faults are relationships between the major fault types and the most prominent regional structures in the study principal stress orientations. A greater amount of shear area (see the geological maps in Figs 2 and 4). is resolved on the JV1s in cross-fold orientation than the PSS1s due to the clockwise rotation of the greatest principle stress, which explains the prominance of Conclusions roughly north-south right-lateral strike slip-faults at a Two orthogonal pervasive sets of fundamental regional level. structures form the background structures within the Ross Sandstone, which was subject to approximately 6 Acknowledgements north-south compressive deformation at the end of the Carboniferous: arrays of 170˚ oriented set 1 joints and Thanks go to David Pyles for his helpful comments veins (JVs) and approximately 75˚ oriented set 1 on the geology and structures present on the Loop Head

Stanford Rock Fracture Project Vol. 22, 2011 D-14 Peninsula. Funding was provided by the Stanford Rock Cruikshank, K.M., Aydin, A., 1995. Unweaving the joints in Fracture Project and a McGee grant from the Stanford Entrada Sandstone, Arches National Park, Utah, U.S.A. Department of Geological and Environmental Sciences. Journal of Structural Geology 17, 409-421. Davatzes, N.C., Aydin, A., 2003. Overprinting faulting mechanisms in high porosity sandstones of SE Utah. References Journal of Structural Geology 25, 1795-1813. Agosta, F., Aydin, A., 2006. Architecture and deformation Davis, K., Burbank, D.W., Fisher, D., Wallace, S., Nobes, D., mechanism of a basin-bounding normal fault in 2005. Thrust-fault growth and segment linkage in the Mesozoic platform carbonates, central Italy. Journal of active Ostler fault zone, New Zealand. Journal of Structural Geology 28, 1445-1467. Structural Geology 27, 1528-1546. Anderson, E.M., 1951. The dynamics of faulting. 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