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_Journal of Structural Geology 32 (2010) 192–201_ _Hybrid veins from the southern margin of the Bristol Channel Basin, UK_ _Mandefro Belayneh*, John W. Cosgrove_ _Imperial College London, Department of Earth Science and Engineering, Royal School of Mines, Exhibition Road, London SW7 2AZ, United Kingdom_ _Article info_ _Article history: Received 9 April 2009; Received in revised form 29 October 2009; Accepted 10 November 2009; Available online 7 December 2009_ _Keywords: Hybrid veins Transtensional Bristol Channel Basin_ _Abstract_ _Sinistral and dextral en echelon hybrid calcite veins are exposed on the southern margin of the Bristol Channel Basin, North Somerset on a Liassic carbonate platform. These hybrid veins are transitional between pure extensional (mode I) and shear fractures (mode II, mode III) and experienced both extensional and shear displacements during their formation. In the study area they occur in conjugate sets with the conjugate angle ranging between 10 x14 and 50 x14. Based on the kinematic analysis of these veins a new model for the opening of the Bristol Channel is proposed._ _Crown Copyright ? 2009 Published by Elsevier Ltd. All rights reserved._ _1. Introduction_ _Pollard and Aydin (1988) define joints as the subset of fractures which show evidence of only opening mode displacement, i.e., mode I fracture formed by extension. Others, for example, Price (1966) and Hancock (1985), define joints as a surface along which there has been no appreciable displacement. The latter definition allows all fractures (mode I, mode II, mode III and mixed-mode) to be classified as joints as long as the displacements are small. In subsiding sedimentary basin where fluids enriched with dissolved minerals are abundant, barren fractures can often be filled with minerals such as calcite and quartz. Although Peacock (2004) proposed that joints and veins should be analysed separately, attributes of fractures and veins can be analysed in the same way. For example, the mechanical and statistical approaches proposed by Nelson (1985) for the analysis of fractures can also be applied to veins to determine the stress history linked to the formation of the various vein sets and to study their initiation, propagation, coalescence and mechanical interaction. Likewise, the statistical approach to fracture analysis which attempts to relate the different fracture distribution patterns to the mechanisms of their formation (Hudson and Priest, 1979; Priest and Hudson, 1976, 1981; Sen and Kazi, 1984; Huang and Angelier, 1989; Narr and Suppe, 1991; Rives et al., 1992; Odling, 1997; Odling et al., 1999; Gillespie et al., 1993, 2001; van Djik et al., 2000) can be used to analyse veins and dykes (see e.g. Pollard and Segall, 1987; Jolly and Sanderson, 1995; Vermilye and Scholz, 1995; Jolly et al., 1998; Ryan et al., 2000; Belayneh et al., 2006). The study of veins is important because in addition to providing the data usually used in fracture analysis (geometry, orientation and distribution) they often provide, in the form of fibrous mineral infill, information relating to the displacements associated with their formation. In addition, minerals crystallizing in the presence of fluids may trap fluid inclusions which can be used to determine the fluid composition and the pressure and temperature conditions at the time of vein formation._ _Experiments on brittle failure show two fundamentally different types of fractures: extension fractures (mode I) and shear fractures (modes II and III) (see e.g. Atkinson, 1987; Twiss and Moores, 1992). The combination of any of the above modes of crack displacement gives rise to mixed-mode failure. The brittle failure criterion can be graphically expressed as a composite Griffith and Navier–Coulomb criteria (Fig. 1a) and the type of brittle failure, whether extension or shear, is determined by the magnitude of differential stress (e.g., Anderson, 1951; Price and Cosgrove, 1990). The formation of shear fractures requires a differential stress greater than four times the tensile strength, T, and the formation of extensional fractures a differential stress less than four times the tensile strength. Joints form parallel to s1 (Fig. 1a(iii)) and normal to s3 and shear fractures form as conjugate sets approximately 30 x14 on either side of the s1 s2 plane, (Fig. 1a(v)). These geometric relationships between the different fracture types and their causative stress field, form the basis of fracture analysis. An examination of the combined_ _0191-8141 $ – see front matter Crown Copyright ? 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.11.010_ _M. Belayneh, J.W. Cosgrove Journal of Structural Geology 32 (2010) 192–201_ _193_ _Fig. 1. (a) Navier–Coulomb–Griffith combined extensional and shear brittle failure envelope showing five stress states all of which will cause failure, (i)-(iii) have a low differential stress (<4 T) and result in extensional failure, (v) has a large differential stress and results in shear failure, (iv) has a differential stress around 4 T and results in hybrid failure. (b) Summary diagram showing the relationship between the types of failure and the causative stress field (taken from Woodcock et al., 2007 modified from Cosgrove, 1995)._ _Griffith and Navier–Coulomb criterion indicates that there is a transition from shear to extensional fractures through an intermediate fracture type referred to in this work as ‘hybrid’ fractures. These fractures show both extensional and shear displacements (Fig. 1a(iv)). The hybrid fractures range in orientation between that of extensional fractures and that of shear fractures. Although it is clear from the Griffith Mohr–Coulomb criteria that they form when the differential stress is approximately 4 T, the factors leading to their formation remain largely unclear (see e.g. the discussion by Engelder, 1999 who questioned their existence)._ _Experiments conducted on Carrara Marble by Ramsay and Chester (2004) and Rodriguez (2005) show brittle structures ranging from joints (opening mode) via hybrid fractures to shear fractures. The experiments show that at low maximum compressive stress (7.5–60 MPa), the extension fractures display somewhat undulating, discrete, highly reflective calcite cleavage planes at sample scale and show roughness at grain scale. At intermediate compressive stress (70–120 MPa), the fractures are inclined between 3 x14 and 10 x14 to s1 and they exhibit patches of discrete, reflective and cleaved crystals between fault gouges showing slip lineation. At high maximum compressive stress (130–170 MPa), the fractures are inclined between 13 x14 and 22 x14 to s1. Gouges with grooves and striations formed declaring the down-dip slip direction on these fractures. This experimental work revealed a progressive increase of fracture angle, q (Fig. 1), with differential stress and generated a complete range of fractures from extension to shear fractures via hybrid fractures. Bobich (2005) carried out a similar triaxial experiment on Berea Sandstone and reported a transition from tensile to shear fractures via hybrid fractures when s1 is approximately 50 MPa._ _A variety of terms have been used by structural geologists to describe these structures including hybrid extension shear fractures (Price and Cosgrove, 1990), extensional-shear (Sibson, 1996, 1998, 2003), hybrid joints (Bahat, 1991; Hancock, 1994; MarínLechado et al., 2004), hybrid fracture (Ferrill et al., 2004), mixed-mode fractures (Twiss and Moores, 1992), transitional tensile (Suppe, 1985) and transitional fractures (van der Pluijm and Marshak, 1997; Engelder, 1999)._ _In a study of naturally occurring conjugate sets of en echelon tension gashes, Smith (1996) recognised that these systems could be classified in terms of two parameters. These are: (i) the angle between the two arrays of en echelon tension gashes, 2q in Fig. 2a i.e., the conjugate angle, and (ii) the angle between the individual tension gashes and the array of en echelon tension gashes of which it is a part, a (Fig. 2b) i.e., the vein array angle._ _As briefly noted above, within the context of fracture analysis the most useful indicators of stress orientation are the fractures themselves. Joints form normal to the least principal compressive stress, s3, i.e., parallel to the plane containing the maximum and intermediate principal stresses and conjugate shear fractures form symmetrically about the maximum principal compressive stress s1. These geometric relationships enable the orientation of the principal stresses to be determined. In addition, the mineral infill of some veins is fibrous and the fibers show the direction of opening. Conjugate shear fractures are often initiated as two sets of en echelon extensional fractures (Fig. 2a) filled with vein material (see e.g., Ramsay, 1967; Ramsay and Huber, 1987). Theoretically the extension veins in both sets should be parallel to each other and to the applied maximum principal compressive stress which bisects the acute angle between the sets (see e.g. array (ii) Fig. 2b). However, sets are often found in which the extensional veins in the conjugate sets either converge or diverge, Fig. 2b. In addition, these veins can have straight, pinnate, or sigmoidal geometries. Pinnate veins are a set of en echelon veins linked to and propagating away from a shear fracture at a small angle (Wilson, 1961). Sigmoidal vein shapes indicate concurrent fracture propagation and shear strain (Durney and Ramsay, 1973) and the veins can display an S- or Z-geometry (Fig. 2a). Veins can form by repeated opening and sealing, i.e., the mechanism of crack-seal proposed by Ramsay (1980) (see also Ramsay and Huber, 1987; Urai et al., 1991; Nicholson, 1991) or by the_ Ключевые слова: shear zone, fracture mechanics, evolution, sibson, differential, sinistral transtension, channel basin, woodcock, nemc?ok gayer, basin opening, formation, tectonophysics, mode, margin, fracture spacing, acute bisector, dextral transtension, analysis, vein set, kinematic indicator, formed, parallel, priest, shear fracture, buchanan, scale, opening mode, ?eld, inversion, geology, mahruqi, result, journal structural, repeated opening, extensional failure, calcite, drawing, bristol, anderson, naviercoulomb criterion, company, left-stepping, bristol channel, shear, fracture wall, initiation propagation, cosgrove, spacing, hydraulic fracturing, magnitude, peacock, remarkable change, basin inversion, ferrill, association, ramsay, shear displacement, basin, ?bres, orientation, conjugate, van djik, location, dextral sense, normal, form, journal, author, set, study, suppe, vertical, stress regime, bisector, echelon vein, atkinson, complex history, sinistral sense, odling, wa, structure, london, mineral, hybrid, gash, southern, bres, joint, rawnsley, zone, camera lens, jolly, smith, mpa, nemc?ok, engelder, upper, mar?, upper vein, york, displacement, anderson price, extensional, distribution, wnw–ese, brittle failure, sanderson, hybrid vein, wilson, calcite ?bres, conjugate angle, tension, tension gash, geological, nelson, angelier, belayneh cosgrove, principal, hancock, sinistral, eds, paola, study area, echelon, en, channel, conjugate set, area, hudson, structural geology, price, petroleum, history, fault, proposed, vein array, model, array, opening, doi, geological society, mechanism, early, fracture analysis, striking, bristol channel basin, southern margin, dextral, sediment, hybrid fracture, brittle, crack-seal mechanism, tensile strength, vein, extension fracture, sub-parallel, ha, rodriguez, failure, gayer, belayneh, pinnate geometry, fracture, discontinuity spacing, hybrid failure, shear failure, calcite bres, transtension, extensional fracture, strike, journal structural geology, bobich, sense, data, geometry, acute angle, showing, rock, single event, society, maximum, fracturing, e–w, kamerling, n–s, press, van, vein considered, stress eld, extension, indicator, angle, davison, huber, deformation, structural, differential stress, stress, proposed model