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