Geoscience Frontiers 10 (2019) 2101e2115 H O S T E D BY Contents lists available at ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research Paper Deciphering the presence of axial-planar veins in tectonites Elena Druguet Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain a r t i c l e i n f o a b s t r a c t Article history: Veins and dikes are often oriented subparallel to the axial surfaces of folds in the adjacent layered or Received 7 August 2018 foliated rocks. This implies an awkward situation, since veins would lay in planes close-to-parallel to the Received in revised form maximum stretching axis. A series of geometric models have been conceived in order to gain insight into 9 December 2018 the possible mechanisms for their formation. The models are based on the analysis of a varied selection Accepted 18 February 2019 of field structures and on the review of similar structures in the existing literature. A first categorization Available online 29 March 2019 consists on distinguishing between axial-planar veins achieved by either progressive or polyphase deformation. Five models of axial-planar veins resulting from progressive deformation are described and Keywords: Dike discussed: (1) fold-related veins associated with the standard folding mechanisms, (2) fracture arrays Fold localized along the short limbs of folds (asymmetric fold-related veins), (3) folds associated with rotation Fracture of extension veins (vein-related folds), (4) high strain and transposition of early veins, and (5) high strain Metamorphic rock and late veins parallel to axial planar foliations (axial planar foliation-related veins). The axial planar Migmatite geometry is achieved through variable amounts of progressive rotational strain, except in model 5, in Shear which the co-planarity is acquired at the time of vein intrusion. The possibility for axial-planar veins to have developed in two distinct phases in the context of polyphase or polyorogenic tectonics has also been explored and discussed. This study contributes to a better understanding of the intriguing in- terplays between deformation, metamorphic and magmatic processes in orogenic belts. Ó 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction mutual crosscutting relationships can be useful to determine regional strain and to reconstruct the relative and absolute timely Sheet-like bodies (veins and dikes) generated from melts or tectonic history of a region (e.g. Hanmer and Passchier, 1991; Carosi hydrothermal fluids commonly occur in close association with folds et al., 1999; Short and Johnson, 2006; Druguet et al., 2008; Sassier in naturally deformed rocks. This can be observed in many defor- et al., 2009; Bons et al., 2012; Maeder et al., 2014; Oriolo et al., mation zones under a wide range of metamorphic conditions, from 2018). very low-grade metamorphism under hydrothermal conditions (i.e. Veins are often oriented subparallel to the axial surfaces of folds quartz or carbonate veins) to anatexis (i.e. migmatitic leucosomes). in the adjacent layered or foliated materials (e.g. Edelman, 1973; Magmatism can also be associated with regional folding, producing Ramsay and Huber, 1987; Passchier et al., 1990; Rosenberg et al., melts that are commonly emplaced as sheet- or lens-shaped bodies 1995; Davidson et al., 1996; Brown et al., 2004; Craw et al., 2006; (Hutton and Reavy, 1992; Carreras and Druguet, 1994; Kisters, Skelton et al., 2015) or might even be restricted to fold hinge 2005; Brown, 2013). They can be subsequently folded and/or bou- zones or short limbs of asymmetric folds (paired hook folds of dinaged depending on their orientation. These planar intrusives Hudleston, 1989). Veins may also occur subparallel to a well- may be either concordant with the planar fabrics in the host rocks developed axial planar foliation in the host rocks, even if the foli- or discordant, crosscutting layering and other planar fabrics, such ation lies at a high angle to the instantaneous shortening axis (e.g. as a gneissic foliation, schistosity or crenulation cleavage Lucas and St-Onge, 1995). (Vanderhaeghe, 1999; Marchildon and Brown, 2003). The struc- In the context of a single finite deformation, fold axial surfaces tures developed in both veins and anisotropic host rocks and their and associated axial-planar foliations tend to track the XY plane of the finite strain ellipsoid (X  Y  Z), while extension veins such as hydrofractures would eventually form closely parallel to the E-mail address: elena.druguet@uab.cat. instantaneous shortening axis Z, i.e. at a high angle with coeval fold Peer-review under responsibility of China University of Geosciences (Beijing). https://doi.org/10.1016/j.gsf.2019.02.005 1674-9871/Ó 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2102 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 axial surfaces. Thus, an apparent contradiction arises between this Sawyer and Robin, 1986). The process can be extended to closed theoretical concept and the ubiquitous cases observed in nature of system partial melt segregation in stromatic migmatites (Oliver and veins that are subparallel to the fold axial surfaces. While in axial- Bons, 2001). This possibility will also be briefly considered. planar foliations the relationship of subparallelism indicates that A thoughtful question concerning the matter of this work and both, folds and foliations formed during the same deformation most cases of complex deformation structures is if the observed event and thus the term has a clear genetic connotation, this is not finite structure is the result of a single deformation event (formed the case with axial-planar veins in which the observed geometrical either simultaneously or progressively) or, contrarily, can only be relationship may have been developed by different processes. explained through different deformation phases (polyphase Research specifically addressed to the question “how do axial deformation). Both progressive and multiphase deformation his- surface-parallel veins develop?” has been performed since the early tories are plausible in the context of mid- to low-crustal orogenic 1970s, e.g. Edelman (1973), Gayer et al. (1978), McLellan (1988), belts, although attempts should be made to integrate the different Hudleston (1989), Hand and Dirks (1992), Vernon and Paterson structures of a region in a single structural-kinematic model (2001), Weinberg and Mark (2008), Weinberg et al. (2015). These (Fossen et al., 2019). The key for deciphering this question largely studies evidence the commonness of these structures in tectonites lies on the structural/geometrical analysis of veins and folds, sup- and that they are particularly abundant in high-grade metamorphic ported by a wide variety of additional evidences, such as the and migmatitic terrains. Almost all these works are based on field geometrical and kinematic analysis of other neighbouring regional observations and analysis of natural structures from theoretical structures (incorporated to this work) and petrological, geochem- considerations. Experimental studies are less numerous, pioneered ical and geochronological methods (outside the scope of this by Hudleston (1989) who performed some simple experimental study). A first categorization consists on distinguishing between models to explain the formation of a certain type of axial-planar axial-planar veins achieved by either progressive or polyphase veins (his paired hook folds). However, the mechanisms of forma- deformation. tion of axial surface-subparallel veins remain somewhat enigmatic (Vernon and Paterson, 2001; Kisters, 2005) and difficult to model, 3. Folds and axial-planar veins in progressive deformation largely because their inherent complexity, involving fluid flow, brittle and ductile deformation processes and generally significant According to interpretations in the existing literature, pene- amounts of stress and strain partitioning associated to rheological contemporaneous folding and axial-planar veining during a single contrasts. deformation phase is a commonly invoked scenario. The following The present paper focuses on veins that are approximately situations are compatible with the development of veins subpar- parallel to fold axial surfaces, with the aim of gaining insight into allel to fold axial surfaces during progressive deformation. the possible mechanisms for their formation. For the sake of simplicity, the term “axial-planar vein” will be used in the present 3.1. Veining associated with the standard folding mechanisms paper, as used in previous works addressed to this structural type of veins (e.g. Edelman, 1973; Hudleston, 1989; Weinberg et al., 2013, Tangential longitudinal strain (TLS) and flexural flow (FF) are 2015). standard folding mechanisms of single-layer folds (Ramsay, 1967; Fig. 1). TLS usually develops in homogeneous isotropic and FF in 2. Conceptual and methodological approach to axial-planar anisotropic layers. TLS is typical of competent layers within a veins multilayer sequence (Ramsay and Huber, 1987). A striking aspect is that, despite the two models have the same contour geometry (they In order to deepen the understanding of the mechanisms that are both parallel, class 1B folds), strain distribution in each model is may give rise to different types of axial-planar veins, a series of totally different, since in TLS and FF folding the strain localizes in geometric models have been conceived. They are based on the the hinges and limbs respectively. analysis of a varied selection of structures in tectonites and on the Stress gradients and differences in strain distribution determine review of similar structures in the existing literature in terms of that the distribution of fracture and vein networks in each model theoretical considerations, structural analysis and modelling. will also be very different (Cosgrove, 2015). In TLS folding, exten- The majority of the selected examples are from field areas where sion veins are expected to form a radial array around the fold hinges the author has done either previous extensive regional geological from the neutral surface towards the outer arcs where layer- research (Variscan basement of NE Iberia, Rainy Lake zone in the parallel extension dominates, while veins would grow parallel to Archean Superior Province of Canada) or sporadic field research layers and concentrically in the compressed zone of the inner arc (Variscan basement of NE Sardinia, Western Gneiss region of the (Fig. 1a). The veins in the outer arc of TLS folds are typically wedge- Norwegian Caledonides). shaped (see e.g. Bastida et al., 2012; Jacques et al., 2014). Addi- It is not aimed to perform a systematic classification, but a tionally, vein-filled saddle reef structures may form in the hinge qualitative categorization of axial-planar veins and the mecha- regions of multilayer folds (Windh, 1995; Jacques et al., 2014). In nisms for their formation, based on the combination of (i) structural contrast, fractures associated with FF folds will be heterogeneously and geometrical relationships obtained from observations mostly distributed across the fold limbs, transverse to both the layering at the outcrop scale and (ii) theoretical constraints regarding strain, and the axial plane (Fig. 1b). Veins associated to both folding kinematics and rheological/mechanical parameters. Each model models usually have a low aspect ratio, since in TLS folds their represents a particular situation, but different models are not lengths are limited to the thickness of the competent layers and in mutually exclusive. They will be presented and illustrated with FF folds veins cannot cross from one limb to the other. natural examples in the successive sections. These fold-related veins can be deformed with subsequent Because of their close link to fractures, most veins are typically progressive deformation and, in some instances, this may favour regarded as tectonic structures. The majority of the veins described vein rotation into parallelism with the fold axial surface. A very in this paper are not an exception as they have likely been origi- common situation during fold evolution is the progressive change nated by brittle fracturing. There is evidence, however, of veins that from buckling to homogeneous shortening as illustrated in Fig. 1. result from metamorphic subsolidus segregation or from coupled TLS fold model is more efficient to develop patterns of veins sub- deformation and segregation (the so called “segregation veins” of parallel to the axial surface after progressive fold tightening (e.g. E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2103 Figure 1. (a) Hypothetical distribution of vein sets developed during pure tangential longitudinal strain folding. With superimposed homogeneous flattening, the radial set of veins stretches and rotates into parallelism with the fold axial surface. (b) Hypothetical distribution of vein sets in pure flexural flow folding model. The superimposed 50% homogeneous flattening is not sufficient to reorient the veins formed in the fold limb into parallelism with the fold axial surface (modified after Ramsay and Huber, 1987). Kruger and Kisters, 2016). However, examples of axial-planar veins shearing of the short limbs of the “S”-shaped folds is sometimes in the literature strictly interpreted as derived from these basic visible (Fig. 3c). folding mechanisms plus homogeneous strain are rather scarce. Occasionally, there is a continuity of layers and S1 across the neck “inter-bead” regions devoid of suture (Fig. 3b), telling that the 3.2. Complex fracture arrays localized along the short limbs of folds beaded shapes were already achieved at the time of pegmatite (asymmetric fold-related veins) intrusion. In this case, pegmatites could have intruded in en- echelon opening fractures. Strain partitioning and localization processes may enhance the In addition, some pegmatites have wall-rock rims that are development of stepped veins or dilatant Riedel-type shears along enriched in tourmaline (Fig. 3d,e). Tourmaline-rich rims (TRR) are the short limbs of regional asymmetric folds (Fig. 2). For this reason preferentially developed in the micaschists, due to transformation axial-planar veins formed by this process will be referred to as of mica into tourmaline (Druguet et al., 1997). The TRR exhibit a asymmetric fold-related veins. more competent behaviour than the adjacent non-tourmalinized The model is based on previous studies (Druguet et al., 1997; schists. The angular relationships between S1 and pegmatite walls Bons et al., 2004) conducted in a high-grade metamorphic zone at the time of intrusion are fixed relative to the dikes in the TRR formed by sillimanite schists and pegmatite intrusions at the due to tourmalinization. This angle varies between 50 and 90 Lighthouse area in the Cap de Creus peninsula (Variscan basement (Fig. 3d,e), indicating that pegmatites were emplaced at high angles of the Eastern Pyrenees). The area corresponds to the type locality to S1 during D2 folding and shearing. Bulk layering rotated dextrally for the “apparent boudins” of Bons et al. (2004). Pegmatite veins are up to 45 from the shortening field into the lengthening field aligned in form of strings of beads or lenses closely parallel to the (Druguet et al., 1997). axial surfaces of regional D2 folds and crosscut the previous S1 The orientation of the intrusive sites may have been primarily layer-parallel foliation in the alternating metagreywackes and controlled by the S1-parallel shear associated to flexural flow, metapelites (Fig. 3). D2 folds are associated to a complex trans- rather than by the bulk D2 transpressional flow. Folds in the schists, pressional shear zone contemporaneous with pegmatite emplace- either those near the contacts with pegmatites or those further ment (Druguet et al., 1997). away, would initiate by S1-parallel sinistral flexural flow and sub- The most characteristic pattern is the deflection of S1 in the sequent bulk oblique shortening, as shown in models of Fig. 2. pegmatite-schists interfaces, giving rise to asymmetric “S”-shaped Flow partitioning between fold limbs would result into ductile folds with the pegmatites occupying the short limbs. These folds do stretching localized in the long limbs and, associated to the short not differ much neither in orientation, nor in style from the D2 limbs sited in the shortening field, a complex pattern of brittle or asymmetric folds far from pegmatite dikes. Axial surfaces and semi-brittle fractures that would enable pegmatite emplacement pegmatite walls are oriented close to the average EeW trending D2 (Fig. 2). Such fractures could be composed of pull-aparts linked by folds (Fig. 3). In the rare cases where pegmatites are intruded at shear fractures such in the models of Peacock and Sanderson high angles to S2 foliation, they have been folded. The folds in the (1995). Veins would fill the dilatant pull-aparts to form an array dikes are coplanar with those in the country rocks but more open, of lenses separated by shear fractures (e.g. Sawyer and Robin, 1986; indicating that some deformation had occurred before pegmatite Tikoff and Teyssier, 1992; Bons and Druguet, 2007). Other struc- emplacement and that it continued afterwards (Carreras and tures related to a process similar to the one here proposed are Druguet, 1994). No penetrative solid-state fabric is present in described in the context of en-echelon extension quartz-vein arrays these pegmatites. associated to contractional kink bands (Ramsay and Huber, 1987; The isolated pegmatite segments are usually connected by sur- Sintubin et al., 2012). faces of suture lacking pegmatite material (Fig. 3b,c), which could Some thin axial-planar pegmatite veins of the Cap de Creus represent fracture segments collapsed just after pegmatite injec- study case are very thin and sub-planar instead of lensoid-shape tion or, most likely, former discontinuous fractures (Bons et al., (e.g. Fig. 3d). In these cases, the intrusive sites could correspond 2004; Bons and Druguet, 2007). S1 is disrupted and or deflected to dilatant zones in the R1-Riedel shear orientation (Fig. 2b). along the sutures as well. A sinistral offset accompanied with Dilatant Riedel-type shears have been reported and documented 2104 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 Figure 2. Models for the development of axial planar, low aspect ratio veins by fracture localization in the short limbs of asymmetric folds during progressive folding. The resulting string of beads have a strong resemblance to boudins, especially when progressive ductile deformation modifies their shape. Based on field observations in Cap de Creus and previous research (see Fig. 3; Druguet et al., 1997; Bons et al., 2004; Bons and Druguet, 2007; Carreras and Druguet, 2013). In (a) veins initiate as dilation jogs along a stepped fracture, while in (b) vein represents a dilatant R-type shear. ISA ¼ bulk instantaneous stretching axes. Dashed lines represent fold axial surfaces. experimentally by Rosenberg and Handy (2000), Holtzman et al. extension fractures (and the infilling vein) with respect to a foliation (2003), Holtzman (2005), Niemeijer et al. (2010) and J. Carrerast or layering during layer-parallel shearing, giving rise to a local (personal communication, 2018). deflection and folding of the planar fabric adjacent to the vein Vernon and Paterson (2001) proposed two similar mechanisms (Gayer et al., 1978; Hudleston, 1989; Swanson, 1992; Grasemann and for magma injection subparallel to fold axial planes in migmatites. Stüwe, 2001; Passchier, 2001; Carreras et al., 2005; Coelho et al., One consists on magma infilling dilational jogs in strongly asym- 2005; Exner and Dabrowski, 2010; Figs. 4 and 5). metric folds, and the other on injection of magma into extensional Drag folds related to rotation of veins are typically asymmetric, shear zones in fold limbs with an oblique extensional crenulation with the vein located in the short limb (Figs. 4 and 5). Other cleavage. geometrical features that usually accompany folds in the adjacent This model is also supported by analogue experiments per- host rock are contractional or extensional offsets due to slip along formed by Barraud et al. (2001) in which partially molten material the vein or fracture (Figs. 4 and 5b). Both folds and offsets in the localizes in shear bands along the short limbs of syn-anatectic folds. adjacent host rocks do not extend much beyond the vein or fracture tips. In those cases where the regional structure is characterized by 3.3. Folds associated with rotation of extension veins a folding pattern, the regional folds and the vein-related folds have (vein-related folds) no matching orientations. The resulting structure was referred to as paired hook folds by Despite bearing geometric similarities with the previous model Hudleston (1989). Passchier (2001) introduced the terminology of asymmetric fold-related veins (Section 3.2), the present model flanking structure, flanking fold and flanking shear band for a wider responds to a very different deformation process. Rotation of a set of structures. The associated slip can be synthetic or antithetic fracture (fault, join or vein) or a secondary shear zone towards the with regard to the overall shear sense, and drag folds can be normal fabric attractor or shear direction during progressive deformation or reverse with regard to the slip along the vein/fracture induces flow perturbations in the adjacent materials. In layered or (Grasemann et al., 2018). The sense of slip along the vein/fracture foliated rocks, such perturbations may materialize as deflections can switch during progressive deformation and also the sense of or folds with their axial surfaces parallel to the rotating vein drag can change from normal to reverse along the vein or fracture (Fig. 4). surface. The initial orientation, nature and mechanical behaviour of The process leading to vein-related folds occurs under general fractures and veins (frictional or viscous, stiff or soft) are among the flow conditions, either coaxial or non-coaxial deformation, although most important factors controlling the final geometry of vein- it is favoured in shear zones of prevailing rotational strain. The related folds. Hudleston (1989) pointed to the particular effect of resulting structures have been extensively investigated in previous competence contrast between vein and host. Moreover, although in works by means of analogue and numerical modelling. They have the examples shown and modelled by Gayer et al. (1978) and been mostly interpreted as a result of progressive rotation of Hudleston (1989) layering deflects passively, it may also behave E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2105 Figure 3. Pegmatite beads and veins aligned closely parallel to D2 folds in high-grade schists from the Lighthouse area in Cap de Creus (Variscan of the Eastern Pyrenees). This is the type locality for the “apparent boudins” of Bons et al. (2004). (a) Structural map of the area. (b) Detail map of two strings of pegmatite beads. (c) Two pegmatite beads separated by a sinistral shear fracture. (d, e) Examples of pegmatite veins and associated tourmaline-rich rims preserving the original high angle between the veins and the bedding-S1 foliation. Progressive ductile deformation of the schists outside the rigid tourmaline-rich rims gave rise to asymmetric “S”-shaped folds consistent with the overall D2 fold pattern in the area. mechanically active, giving rise to different types of folds. Pro- Folds tighten and become isoclinal, with thickened hinges and gressive deformation up to large strains gives rise to tight folds and strongly sheared fold limbs, but also veins or vein segments are collapse structures around the veins. This is illustrated in the transposed into parallelism with fold axial surfaces and, when example of Fig. 5e corresponding to a high strain domain of the developed, with axial planar foliations (Figs. 6a and 7a). Canadian Rainy Lake transpressional deformation zone (Druguet This is a main mechanism for veins to achieve axial-planarity et al., 2008). Flanking structures present in nearby domains of and can be observed in both non-metamorphic (Fig. 7a) and overall lower strain consistently indicate a lesser amount of rota- metamorphic environments, but are especially abundant in high- tion of competent veins (Fig. 5c,d). temperature, high-strain zones (e.g. Passchier et al., 1990), like in Kocher and Mancktelow (2005), Gómez-Rivas et al. (2007) and the boundaries of the Punta dels Farallons - Volt Andrau migmatitic Exner and Dabrowski (2010) developed methods and used them to complex at Cap de Creus (Druguet and Hutton, 1998; Fig. 7b,c, see estimate the finite strain and kinematics of deformation. In a recent the areas inside the white squares) or at the margins of shear zones review paper, Grasemann et al. (2018) catalogued flanking folds as in protomylonitic rocks such as in the Rainy Lake zone next to the complex structures and give some guidelines for their correct Quetico fault in Ontario, Canada (Fig. 7d; Poulsen, 1986; Druguet interpretation and use as kinematic indicators. et al., 2008). This type of veins whose axial-planarity is not an Finally, it is worth to note that vein-related folds are not original feature but is achieved after a transposition process are exclusive of progressive deformation but can also be the result of obvious if different vein segments are exposed at the outcrop, like polyphase deformation (see Section 4). in Fig. 7. Otherwise, however, they may be easy to recognize and interpret for their characteristic structures. For vein/host high 3.4. High strain and transposition of early veins competence contrast, the stretched vein segments develop into boudinage structures (Fig. 7b). In the cases of incompetent veins or High strains can be attained when progressive deformation af- vein/host low competence contrast, veins become thin and record fects folded sequences and contemporary veins that were originally internal deformation fabrics, usually in the form of a solid-state not necessarily axial-planar (e.g. the extension veins of Section 3.3). foliation (Fig. 7c,d). 2106 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 Cosgrove, 1997). The resulting axial planar foliation-related veins are represented in the sketches of Fig. 6b and some natural ex- amples from Cap de Creus and from the Central Pyrenees are shown in Fig. 8. This is the mechanism most invoked to explain axial-planar veins achieved directly, without necessity of any subsequent relative rotational strain of veins and fold axial sur- faces into mutual parallelism (Oliver et al., 1990; Simpson, 2000; Vernon and Paterson, 2001; Arnaud et al., 2004; Weinberg and Mark, 2008). Obviously, this mechanism implies a late intrusion or injection of veins during progressive deformation, once the axial planar foliation is already developed. It can be easy to distinguish from the transposed veins described in the previous section if there is little or none further deformation that might affect the veins, or in presence of additional structural or geometrical criteria, such in the example shown in Fig. 8b of syntectonic leucogranites from the Volt Andrau high-strain zone in Cap de Creus. These leucogranites form a sub-vertical network of foliation-parallel and foliation-oblique dikes (Candami et al., 2013). In the detail of Fig. 8b, an apophysis normal to the axial planar foliation-dike is folded, but in a much more open form than the enclosing transposed metasediments, indicating that the intrusion of the interconnected network of leucogranites took place at the latest stages of folding. Other models have postulated that the local stress and strain differences between the components of a rock can trigger meta- morphic differentiation processes, with development of a compo- sitional banding including subparallel segregation veins and mafic selvages. Such metamorphic or migmatitic banding could develop in an orientation perpendicular to the regional shortening direction (Robin, 1979; Sawyer and Robin, 1986). In this case, veins would form during continuous deformation, without brittle failure, by solution transfer of the soluble material that migrates to low pressure interfaces between layers of different competence. How- ever, there are few examples in the literature of axial-planar veins interpreted as developed only by segregation (subsolidus or ana- tectic) processes without invoking brittle failure (Sawyer and Figure 4. Model for the development of vein-related folds by progressive rotation of Robin, 1986; Hand and Dirks, 1992; Le Hébel et al., 2002). veins originally oriented within the instantaneous shortening field into parallelism with the shear plane. Based on experimental study under layer-parallel dextral shear Finally, it is worth to point out that vertical crenulations, conditions by Hudleston (1989). The final structure falls in the category of antithetic bandings and foliations represent ideal orientations for buoyancy- slip-reverse drag. ISA ¼ bulk instantaneous stretching axes. Dashed lines represent fold driven melt drainage (Hand and Dirks, 1992) or for tectonically axial surfaces. driven extrusion (Lehmann et al., 2017). 3.5. High strain and late veins parallel to axial planar foliations 4. Folds and axial-planar veins in polyphase or polyorogenic (axial planar foliation-related veins) deformation As described above, progressive deformation causes fold tight- In polyphase or polyorogenic tectonic settings, vein formation ening and transposition of pre-existing fabrics. Another well and fold development in the neighbouring layered or foliated rocks known effect of progressive deformation is the development of belong to two distinct deformation events separated by a switch in axial planar foliations and crenulation cleavages oriented parallel to orientation of the stress/strain fields. Thus, veins could either the XeY plane of the instantaneous strain ellipsoid. In folded se- predate folds or folds could predate veins. quences subjected to relative high strain, the new axial planar fabrics are well developed, displaying a variable degree of pene- 4.1. Veins predating folds trativeness depending on the nature of the lithologies involved. These planar anisotropies represent planes of weakness which will This setting consists on a superposed phase of shortening par- have a strong influence on the orientation of the rock stresses (Price allel to a pre-existing layering or foliation and at a high angle to also and Cosgrove, 1990). pre-existing veins. Such veins would have been intruded during a Veins or dikes can form by hydrofracture in orientations close- first deformation phase at a high angle to layering within the to-parallel to the axial planar foliation, that is, in the plane instantaneous shortening field (e.g., as an extension vein, Fig. 9). perpendicular to the instantaneous shortening axis if fluid (or These veins may be deformed (folded if they are more competent melt) pressure overcomes the maximum principal stress plus the than the host) or not (as in the example of Fig. 9) before the second tensile strength of the wall rock normal to the axial planar deformation phase. anisotropy (Wickham, 1987; Lucas and St-Onge, 1995; Vernon and Throughout the second deformation event, defined by an Paterson, 2001; Brown, 2013). This process may operate under important switch in the direction of regional shortening, the conditions of high fluid/magma pressure, a foliation surface of low anisotropic host rocks, now laying in the shortening field, become tensile strength and low differential stresses (Gratier, 1987; folded, while the vein, now laying in the extensional field, becomes E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2107 Figure 5. Examples of veins and associated drag folds (vein-related folds) that are likely formed by vein rotation during progressive deformation. All the structures fall in the category of synthetic slip normal drag. (a) Kink-band-like folds associated to quartz veins in slates (Coll de Jou, Variscan of the Central Pyrenees). (b) Rotated (anti-clockwise) cuspate-lobated pegmatite dike and associated drag folds affecting sheared mid-grade schists at Els Estanyols (Canigó massif, Variscan of the Eastern Pyrenees). Notice foliation- parallel sinistral shearing denoted by the thinner vein to the left. (cee) Rotated (anti-clockwise) veins of leucogranite (c and d) and quartz (e) and associated drag folds affecting metavolcanic rocks from Rainy Lake zone (Superior Province of the Canadian Shield). Figure 6. (a) Model of high strain deformation causing transposition of layers or foliations and a pre- or syn-folding vein in a case of progressive layer-parallel shortening. (b) Model for the formation of fold-related late veins in the same case of progressive layer-parallel shortening. Vein intrudes parallel to the axial planar foliation (black lines) developed after a considerable amount of deformation. ISA ¼ bulk instantaneous stretching axes. 2108 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 Figure 7. Field examples to illustrate the subparallelism between veins and fold axial surfaces due to intense folding and transposition. (a) Tight folds and associated axial planar cleavage (dashed lines) and axial-planar quartz veins affecting an anchimetamorphic sequence of slates and quartzitic sandstones (solid lines represent bedding surfaces). Collserola massif, 12 km NW from Barcelona (Catalan Coastal Ranges). (b) Folded and boudinaged quartz vein and crenulated sillimanite schists at Cala Prona (Cap de Creus). (c) Isoclinally folded leucogranite vein and sillimanite schists at Volt Andrau Area (Cap de Creus). (d) Tightly folded leucogranite veins and metavolcanic rocks at the margins of the Quetico Fault (Rainy Lake zone, Western Superior Province, Canada). White square areas in b, c and d indicate the areas of axial-planar vein segments. stretched. The way vein-parallel extension is accommodated will The first example consist in quartz veins embedded in low- depend on the competence contrast with the host rocks. Two end- grade, finely laminated metasedimentary rocks from the Central member situations are depicted in Fig. 9: (i) vein homogeneous Pyrenees affected by polyphase Variscan tectonics (see micro- stretching for incompetent veins or low competence contrast be- structures in Fig. 10). Vein morphology varies between single high tween vein and host, in which case the vein would likely develop a aspect ratio veinlets and arrays of stepped lenses. In both cases, the foliation, and (ii) boudinage for higher competence contrast. At veins crosscut the dominant layer-parallel cleavage (Sd) at a high intermediate rheological conditions, pinch-and-swell structures angle and are slightly folded with the axial planar of the folds. This could develop. Besides, as an intrinsic consequence of boudinage of feature evidences that the veins were likely injected during the competent veins, a precipitate or melt can fill the interboudin re- development of the Sd. Later, during a subsequent deformation gions, progressively developing into a new generation of axial- phase with shortening almost perpendicular to the veins, the planar vein (lower-right sketch in Fig. 9). dominant cleavage and parallel veins became respectively crenu- Moreover, the final structural patterns in the layered rocks will lated and folded. Notice in Fig. 10 that these late folds are not also reflect the effects of flow perturbations and strain localization confined to areas where late axial-planar veins are present, but due to the differences in mechanical behaviour between veins and regularly extend over the rock sample. This is contrary to what host. As observed in the model of Fig. 9 and natural examples (Figs. happens in vein-related folds (Figs. 2 and 3) where the folds die out 10, 11a and 12), more tighten folds, localized shears and kink-bands away from the vein. (e.g. Allan et al., 2017) or the typical foliation drag around boudins The second example has been taken from the classical paper by can develop in the host at the host-vein interfaces. This is a relevant Edelman (1973), and shows axial-planar quartz and granitic veins difference with the relatively simpler or less heterogeneous fold in high-grade gneisses and amphibolites from the Archipelago of pattern that usually characterizes the areas adjacent to axial planar SW Finland (Fig. 11). They were interpreted by Edelman (1973) as foliation-related veins (Figs. 6b and 8). On the other hand, the due to the release of elastic stresses perpendicular to the axial plane present case of veins predating folds in a context of polyphase of folds, which would cause opening of tension fissures and for- tectonics bears geometrical similarities with veins localized along mation of veins. Another plausible interpretation would be that the short limbs of folds (asymmetric fold-related veins of Figs. 2 and veins predate folds, given the analogies with the previously 3) and with folds associated with rotation of extension veins (vein- described case from the Pyrenees. The vein segment encircled in related folds of Figs. 4 and 5) during progressive deformation, from Fig. 11a is folded with the gneissose foliation axial planar. A further which they are not always easy to discriminate. evidence that polyphase tectonics may have operated in the SW Three individual examples of axial-planar veins that could likely Finland case is shown in Fig. 11b. The vein system associated to a belong to this category will be now discussed. They are depicted in folded amphibolite layer very much resembles a series of folds Figs. 10e12. overprinting a previously boudinaged layer which contained thin E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2109 Figure 8. Examples of veins subparallel to an axial planar foliation (dashed traces). (a) Quartz vein from Cala Serena in Cap de Creus (Variscan of the Eastern Pyrenees). (b, c) Leucogranite dikes from Volt Andrau area in Cap de Creus. Notice in (b) the presence of a slightly folded apophysis normal to the main dike, indicating that leucogranite emplacement took place at the latest stages of regional folding. (d) Calcite veins parallel to a strongly developed crenulation cleavage in carbonatic slates from Son’s road near Esterri d’Àneu (Variscan basement of the Central Pyrenees). phase of folding. The field area is located in the Cap de Creus peninsula (Eastern Pyrenees), on a beach slope north of El Port de la Selva (Fig. 12; Carreras and Druguet, 2013). There, orthogneisses with a prominent gneissic foliation are cut at a high angle by a set of quartz veins. Whilst at the same time the veins are oriented sub- parallel to the axial surfaces of late folds and crenulations (Fig. 12a), and display signs of vein-parallel shearing and stretching (Fig. 12b,c). Structural correlations at the regional scale reveals that the dominant gneissic foliation is a composite S1/2 axial planar transposition foliation to which quartz veins are probably syntec- tonic, and that the late folds and crenulations (D3) are widespread at the regional scale (not restricted to sites of vein presence). The overprinting D3 folds locally produced type 3 fold interference patterns (Fig. 12d). Finally, vein-related folds can also develop in polyphase defor- mation in those cases where vein formation and rotation corre- spond to separate tectonic phases. An example could be the formation of veins in interboudin partings during a first phase of extension parallel to competent layers, followed by a second phase of layer folding/rotation. These could be the case of Fig. 5e. Figure 9. Model for the development of axial planar veins throughout polyphase deformation. Extension veins intrude at a high angle to the foliation associated to a first deformation phase and are subsequently stretched homogeneously or boudinaged 4.2. Folds predating veins (depending of competence contrasts) during a second phase of folding. Dashed lines represent fold axial surfaces. This setting consists on the injection of veins parallel to the axial surfaces of pre-existing folds or crenulations. A common occur- interboudin veins. This example is strikingly similar to natural and rence is that an axial-planar cleavage or foliation is already devel- experimental examples of folded boudins in Sengupta (1983). oped associated to the previous folding event, so it acts as a The third example also meets the conditions described for a weakness surface along which hydrofractures may form irre- setting in which veins achieve their axial planarity in a second spective of the orientation of the regional stress/strain field, just as 2110 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 Figure 10. Quartz veins injected into finely foliated low-grade metarhythmite (Pica d’Estats summit, Variscan of the Central Pyrenees). (a) Polished hand specimen. (b) Micro- photograph of the en-echelon vein array indicated in (a). The veins are at a low angle to the axial surfaces of the widespread crenulation. The open folds present in the vein segments indicate syntectonic injection with the deformation event that gave rise to the closely spaced dominant cleavage (Sd). Thus, the veins likely predate the crenulation event. it occurs with late veins in progressive high strain deformation crenulations. Pervasive melting would result into an inter- (Section 3.5; Fig. 6b). Distinction between the two scenarios (late connected vein network. Partial melt segregation processes that veins in progressive deformation vs. a first event of folding and a lead to the formation of stromatic migmatites consisting on layers second of axial-planar vein formation) is not structurally straight- of leucosome and melanosome is comparable to metamorphic forward (with limited exceptions such as in the field example segregation referred to in Section 3.5 (Oliver and Bons, 2001). shown in Fig. 8b) and may require performing petrological corre- Rosenberg and Handy (2000, 2001) performed a series of ex- lations or applying geochronological criteria. periments on syntectonic partial melting with very interesting re- sults. Under simple shear conditions, melt pathways were 5. The particular case of migmatites distributed along interconnected dilatant shear bands at low angles to the shear zone boundary. Under pure shear, transient melt Migmatitic terrains represent a paradigm for the study of the pockets developed parallel to the stretching direction. The results interplay between deformation, metamorphism, anatexis and from the experiments by Barraud et al. (2001) and Holtzman et al. magmatism and thus for the recognition and interpretation of the (2003) also confirmed the reliability of leucosomes to localize in structures that are object of this study. The presence of inter- shear bands along the short limbs of folds (model of asymmetric connected systems of leucosomes subparallel to the migmatitic fold-related veins; see Section 3.2). foliation or banding, melt-bearing shear bands and axial planar Most of the models suggested in the present paper match the leucosomes is a very common feature in many folded migmatitic mechanisms proposed by Vernon and Paterson (2001) for the for- domains (e.g. Sawyer, 2008; Pawley et al., 2015). This has entailed a mation of axial planar veins in migmatites. Six of their mechanisms large number of studies in which aspects related to this issue have apply to magma injection and the other three to in situ formation of been explored on the macro- to micro-scale by means of field leucosomes. analysis and experimental modelling (McLellan, 1988; Hand and Weinberg and Mark (2008) argued that synchronous folding Dirks, 1992; Brown, 1994, 2013; Druguet and Hutton, 1998; and melt migration enhances transposition, and Weinberg et al. Vanderhaeghe, 1999; Barraud et al., 2001, 2004; Vernon and (2013) explained the common occurrence of layer-parallel and Paterson, 2001; Weinberg and Mark, 2008; Weinberg et al., 2013; axial planar leucosomes in migmatites as the result of melt Lehmann et al., 2017). extraction along funnel-shaped channels subparallel to fold hinges, The low cooling rates prevailing at anatectic conditions allow driven either by magma buoyancy or by pressure gradients arisen interaction between the partially crystallized veins and the hot wall during deformation. In a more recent paper, Weinberg et al. (2015) rocks during folding. Deformation tends to localize around melts, emphasized the role of stress-driven ductile compaction in- producing a range of structures from melt bearing to solid-state stabilities in the formation of such axial-planar melt channels. stages, separated by the reversal of competence contrast between A deduction of the studies discussed above is that migmatitic leucosome and host rock (Druguet and Carreras, 2006). terrains are frequently characterized by their structural Based on the analysis of natural migmatites, Hand and Dirks complexity. Many of the resulting structures are interpreted as a (1992) proposed a model in which melt reactions initially take consequence of the coupling between polyphase deformation and place in high strain zones where the melting reaction activation multiple melting pulses (e.g. Talbot, 2008; Weinberg et al., 2013; energy is first overstepped. The generated leucosomes, as incom- Barbey et al., 2015). Thus, it is reasonable to consider the abun- petent material, would elongate in a plane normal to the direction dance of axial planar veins as an inherent phenomenon in ana- of maximum compression and parallel to the axial surfaces of tectic domains, and that they can likely form by various or E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2111 6. Final discussion and conclusions Axial-planar veins are common in tectonites and respond to a large variety of metamorphic, deformational and tectonic settings. Interpretation of the finite structures observed in the field has lead to previous and present studies to establish certain correla- tion between the resulting finite structures and formation mechanisms, that is, a correspondence between geometrical types and genetic models of axial-planar veins. Some particular geometrical and overprinting relationships are attributable to specific development mechanisms. Other structural relationships are not as straightforward, either because they are the compli- cated result of coupled processes, or because similar final geom- etries can be achieved by different mechanisms. Nevertheless, we have seen that coplanar folds and veins can be grouped into two main settings depending on the sequence of deformation processes that lead to their formation, that is, progressive and polyphase deformation settings (Fig. 14), Whilst polyphase deformation is the apparent best explanation for the occurrence of axial-planar veins (provided a switch in orientation of the kine- matic framework), it has been demonstrated that many of these finite structures can be better explained as a consequence of progressive deformation. Also, there are specific complex cases whose analysis transcends the progressive or polyphase defor- mation histories. This is the case of some migmatitic terrains dominated by a complex coupling of interacting deformation, metamorphic and fluid/melt flow phenomena (e.g. model 8 in Fig. 14). 6.1. Models of progressive deformation Five particular settings have been investigated, here summa- rized with some of their characteristic features (Fig. 14): Figure 11. Sketches of two field photographs from the Archipelago of SW Finland from (1) Veining associated with the standard folding mechanisms. Edelman (1973). The structures suggest formation of axial-planar veins by two su- Tangential longitudinal strain (TLS) folding is more efficient perposed deformation phases. (a) Line drawing of Fig. 3 of Edelman (1973). Pegmatite than pure flexural flow folding as a mechanism to develop veins (pink) are subparallel to the axial surfaces of folds in the layered gneisses (grey). axial-planar veins. However, in the case of TLS, the process The tip of the central vein (white circle) is folded with the main gneissose foliation as requires a multilayer with high competence contrast and veins axial surface. Like in the example showed in Fig. 10 of this article, this would indicate that the veins are pre- or syn-tectonic with the foliation that was later folded during a that form are usually very discontinuous and limited to the fold superimposed tectonic event. (b) Sketch of Fig. 5 of Edelman (1973). A layer or dike of outer arc. amphibolite (black) is folded within a gneissose matrix (pale grey) and includes a set of (2) Complex fracture arrays localized along the short limbs of folds granite veins (pink) parallel to the axial surface of folds (dashed lines). The structure (asymmetric fold-related veins). Veins are localized in the short resembles folded boudins originated by two deformation phases, a first phase of layer- parallel extension and granite vein formation and a second phase of layer-parallel limbs of asymmetric folds, generally as stepped dilatant pull- shortening that would have produced the folds. aparts between shears or along Riedel-type shears. The vein- bearing folds are co-relatable with regional folds. (3) Folds associated with rotation of extension veins (vein-related folds). Complex flanking folds develop as a result of vein rota- different combinations of the predicted mechanisms. A variety of tion. Generally, these local folds do not match with regional patterns of axial-planar or quasi-axial-planar leucosomes from folds in orientation and they die out progressively away from different world regions are shown in Fig. 13. In Fig. 13a, an isolated the vein. The veins themselves may record a rather complex diffuse melt patch seems to have migrated from the fold limbs and deformation history, especially if they are more competent than hinges to the axial surface, while in Fig. 13b,c leucosomes are more the host, as they can be progressively folded and boudinaged. elongated and occupy the sheared limbs of the asymmetric folds. (4) High strain and transposition of early veins. In this case, the Fold transposition and the development of leucosomes along the coplanarity of veins with folds is achieved after a transposition axial planar migmatitic fabric characterize the examples shown in process which is usually discernible in the rocks by the tight Fig. 13d and e. The last example of straight leucocratic veins par- folding of the layered/foliated host and the presence of bou- allel to the axial planes of the folds in migmatitic schists from Loch dinage or internal foliation in the veins (depending on Monar (Scotland) is particularly intringuing (Fig. 13f). These veins competence contrast). have been interpreted by Ramsay and Huber (1987) and (5) High strain and late veins parallel to axial planar foliations Hudleston (1989) as the product of local melt segregation during (axial planar foliation-related veins). In this case, an axial folding, which appears difficult to explain by any of the proposed planar foliation is already developed and this controls the simple mechanism. Most likely, fold transposition after melt infill orientation of late-tectonic veins. Veins are expected to record is needed to account for the extreme parallelism of the veins with little deformation compared to the high strain fabrics in the fold axial surfaces. host. 2112 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 Figure 12. (a) Field example of quartz veins subparallel to axial surfaces of folds and crenulations (S3) affecting El Port de la Selva orthogneiss (Les Clisques beach, NW Cap de Creus). Veins that vary in shape between planar and continuous (b) and irregular and segmented (c) are located along sheared fold limbs. See main text for discussion. (d) Refolded folds in quartz veins of an earlier generation are evidence that the region was affected by polyphase tectonics. Figure 13. Field examples of folded migmatitic rocks with leucosomes closely parallel or at a low angle to sheared fold limbs and fold axial surfaces. (a) Migmatitic schists from Volt Andrau migmatitic complex at Cap de Creus. Irregular leucosome is roughly parallel to the axial surface of the folds. (b) Migmatitic orthogneiss with leucosome-filled sheared limb (Porto Ottiolu, Variscan basement of NE Sardinia, Elter et al., 1990). (c) Folded stromatic migmatites with cross-cutting axial planar leucosome veins (Osvat- net, Western gneiss region, Norway). (d) Highly transposed mesocratic migmatitic gneiss with axial-planar granitic leucosomes (Porto Ainu, Variscan basement of NE Sardinia, Carosi et al., 2006). (e) Veins of leucotonalite parallel to the axial surfaces of folds affecting banded tonalites from the floor of the Bergell pluton in the Central Alps (Rosenberg et al., 1995; Davidson et al., 1996). Photograph courtesy of C. Rosenberg. (f) Multilayer folds produced under near-migmatitic conditions in the Moine Series rocks at Loch Monar (NW Scotland), with pegmatite veins parallel to the folds axial surface (analogue to Fig. 20.12, p. 416 of Ramsay and Huber, 1987; and to Fig. 2d of Hudleston, 1989). Photograph courtesy of J. Carreras. Within this category of progressive deformation, distinction can processes. Strain partitioning across lithological/rheological be made between those mechanisms by which the axial planar boundaries (banding or layering with competence contrast) dom- geometry is achieved at the time of vein intrusion (model 5), and inates the setting associated to tangential longitudinal strain those by which the geometry is achieved throughout the relative folding, whereas partitioning between different limbs of asym- rotation (by different degrees) of the veins and the anisotropy in metric folds, either by deformation with a shear component or by the host rocks (all the other models, 1 to 4). pure shear oblique to layering/foliation, dominates other fold- In models 1 and 2, axial-planar veins are already intruded into related axial planar veins. the regional instantaneous extensional field, responding to mech- In models 3 and 4, veins that are originally emplaced as exten- anisms which are strongly controlled by strain partitioning sion fractures into the regional instantaneous shortening field E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2113 Figure 14. Scheme summarizing the diagnostic characteristics of models leading to the axial-planar veins considered in this study. The drawings represent finite structures. Model 8 represents a specific case based on Weinberg et al. (2013) model for axial-planar leucosomes in migmatites. progressively rotate into the extensional field. This can be achieved requires to be contrasted with independent indicators. Some by different ways. In the case of model 3, folds are the consequence useful evidence for mechanisms associated to polyphasic tec- of vein rotation, whereas in model 4 the coplanarity is the result of tonics, although not always univocal, are the presence of fold high strain and transposition. superposition structures such as folded boudins or fold interfer- ence patterns, or the record of two or more generations of 6.2. Models of polyphase deformation crenulations, foliations and mineral assemblages. Where regional polyphase tectonics is in evidence, the possibility for axial-planar Discriminating between axial-planar veins formed by poly- veins to have developed in two distinct phases should not be phase or progressive deformation is not always clear and usually underestimated. In this context, and depending on the sequence 2114 E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 of the events, veins would either predate folds (model 6, Fig. 14) Brown, M., 1994. The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened or folds would predate veins (model 7, structurally equivalent to orogens. Earth-Science Reviews 36, 83e130. model 5). Brown, M., 2013. Granite: from genesis to emplacement. The Geological Society of America Bulletin 125, 079e1113. Brown, C.M., Oliver, N., Dickens, G., 2004. Veins and hydrothermal fluid flow in the 6.3. Final remarks Mt. Whaleback Iron Ore District, eastern Hamersley Province, Western Australia. Precambrian Research 128, 441e474. Candami, D., Druguet, E., Enrique, P., 2013. Measuring the effects of rheology and Axial-planar veins are specific but quite abundant structures in regional tectonics on the syntectonic rocks of a migmatitic complex from Cap metamorphic and magmatic terrains. Deciphering the feasible de Creus. Geogaceta 54, 91e94. mechanism for their development has been determinant for gain- Carosi, R., Frassi, C., Montomoli, C., Iacopini, D., 2006. Excursion in the Variscan basement of northern Sardinia (Italy): field Guide. In: Köhn, D., De Paor, D. ing insight into the interplays between deformation, metamorphic (Eds.), General Contributions 2006, Journal of the Virtual Explorer, Electronic and magmatic processes during the evolution of orogenic belts. For Edition, vol. 22. Paper 3. instance, the recognition that certain mechanism has operated to Carosi, R., Lombardo, B., Musumeci, G., Pertusati, P.C., 1999. Geology of the higher Himalayan Crystallines in Khumbu Himal (eastern Nepal). Journal of Asian form one of these structures can be useful to formulate hypothesis Earth Sciences 17, 785e803. about the relative age of veins and the associated structures and, Carreras, J., Druguet, E., 1994. Structural zonation as a result of inhomogeneous non- once this is done, proceed with dating veins by geochronology coaxial deformation and its control on syntectonic intrusions: an example from techniques (e.g. Kontak and Smith, 1990; Fletcher et al., 2002; the Cap de Creus area (eastern-Pyrenees). Journal of Structural Geology 16, 1525e1534. Cottle et al., 2009). The present work provides some guidelines Carreras, J., Druguet, E., 2013. Illustrated Field Guide to the Geology of Cap de Creus. for the recognition of such mechanisms. Careful structural analysis Servei de Publicacions de la Universitat Autònoma de Barcelona, Bellaterra, may provide useful insight into the processes leading to the Barcelona, p. 123. Carreras, J., Druguet, E., Griera, A., 2005. Shear zone-related folds. Journal of development of these still somewhat intriguing structures. Some of Structural Geology 27, 1229e1251. these processes are rather obvious; while others are so complex Coelho, S., Passchier, C.W., Grasemann, B., 2005. Geometric description of flanking that demand the use of additional criteria, including tectonic and structures. Journal of Structural Geology 27, 597e606. Cosgrove, J.W., 1997. The influence of mechanical anisotropy on the behaviour of the kinematic correlations at the regional scale, microstructural anal- lower crust. Tectonophysics 280, 1e14. ysis and petrological and geochemical studies. Different models are Cosgrove, J.W., 2015. The association of folds and fractures and the link between not mutually exclusive, as they represent end-member situations folding, fracturing and fluid flow during the evolution of a fold-thrust belt: a brief review. In: Richards, F.L., Richardson, N.J., Rippington, S.J., Wilson, R.W., that can coexist in a single region and even in a single vein system. Bond, C.E. (Eds.), Industrial Structural Geology: Principles, Techniques and This is particularly manifest in complexly deformed hydrothermal, Integration, vol. 421. Geological Society, London, Special Publications, migmatitic and magmatic vein networks. pp. 41e68. Cottle, J.M., Searle, M.P., Horstwood, M.S.A., Waters, D.J., 2009. Timing of midcrustal Finally, this study serves as a basic 2D context from which to metamorphism, melting, and deformation in the mount Everest region of develop more complex and systematic physical and numerical southern Tibet revealed by U(-Th)-Pb geochronology. The Journal of Geology models of the interactions between deformation, fluid or magma 117, 643e664. flow and anatexis that lead to coplanar folds and veins in crustal Craw, D., Begbie, M., MacKenzie, D., 2006. Structural controls on Tertiary orogenic gold mineralization during initiation of a mountain belt, New Zealand. Mineral rocks. Deposita 41, 645e659. Davidson, C., Rosenberg, C., Schmid, S.M., 1996. Synmagmatic folding of the base of the Bergell pluton, central Alps. Tectonophysics 265, 213e238. Acknowledgements Druguet, E., Carreras, J., 2006. Analogue modelling of syntectonic leucosomes in migmatitic schists. Journal of Structural Geology 28, 1734e1747. Druguet, E., Czeck, D.M., Carreras, J., Castaño, L.M., 2008. Emplacement and defor- Thanks to Jordi Carreras for giving insightful advice during mation features of syntectonic leucocratic veins from Rainy Lake zone (Western article preparation and to Claudio Rosenberg for sharing his picture Superior Province, Canada). Precambrian Research 163, 384e400. in Fig. 13e. I am also particularly grateful to two anonymous re- Druguet, E., Hutton, D.H.W., 1998. Syntectonic anatexis and magmatism in a mid- crustal transpressional shear zone: an example from the Hercynian rocks of viewers for very constructive reviews that have greatly improved the eastern Pyrenees. Journal of Structural Geology 20, 905e916. the paper. Druguet, E., Passchier, C.W., Carreras, J., Victor, P., den Brok, S., 1997. Analysis of a complex high-strain zone at Cap de Creus, Spain. Tectonophysics 280, 31e45. Edelman, N., 1973. Tension cracks parallel with the axial plane. Bulletin of the References Geological Society of Finland 45, 61e65. Elter, F., Musumeci, G., Pertusati, P.C., 1990. Late Hercynian shear zones in Sardinia. Allan, M.M., Rhys, D.A., Hart, C.J.R., 2017. Orogenic Gold Mineralization of the Tectonophysics 176, 387e404. Eastern Cordilleran Gold Belt, British Columbia. Structural ore controls in the Exner, U., Dabrowski, M., 2010. Monoclinic and Triclinic 3D Flanking Structures Cariboo (093A/H), Cassiar (104P) and Sheep Creek (082F) mining districts: Around Elliptical Cracks, vol. 32, pp. 2009e2021. Geoscience BC Report 2017-15, p. 108. Fletcher, J.M., Miller, J.S., Martin, M.W., Boettcher, S.S., Glazner, A.F., Bartley, J.M., Arnaud, F., Boullier, A.M., Burg, J.P., 2004. Shear structures and microstructures in 2002. Cretaceous arc tectonism in the Mojave block: profound crustal modifi- micaschists: the Variscan Cévennes duplex (French Massif Central). Journal of cation that controlled subsequent tectonic regimes. In: Glazner, A.F., Structural Geology 26, 855e868. Walker, J.D., Bartley, J.M. (Eds.), Geologic Evolution of the Mojave Desert and Barbey, P., Villaros, A., Marignac, C., Montel, J.M., 2015. Multiphase melting, magma Southwestern Basin and Range, vol. 195. Memoir of the Geological Society of emplacement and PT-time path in late-collisional context: the Velay example America, pp. 131e149. (Massif Central, France). Bulletin de la Societe Geologique de France 186, Fossen, H., Cavalcante, G.C.G., Vizeu, R., Pinheiroc, L., Archanjo, C.J., 2019. 93e116. Deformation e progressive or multiphase? Journal of Structure Geology Barraud, J., Gardien, V., Allemand, P., Grandjean, P., 2001. Analog modelling of melt 125, 82e99. segregation and migration during deformation. Physics and Chemistry of the Gayer, R.A., Powell, D.B., Rhodes, S., 1978. Deformation against metadolerite dykes Earth (A) 26, 317e323. in the Caledonides of Finnmark, Norway. Tectonophysics 46, 99e115. Barraud, J., Gardien, V., Allemand, P., Grandjean, P., 2004. Analogue models of melt Gomez-Rivas, E., Bons, P.D., Griera, A., Carreras, J., Druguet, E., Evans, L., 2007. Strain flow networks in folding migmatites. Journal of Structural Geology 26, and vorticity analysis using small-scale faults and associated drag folds. Journal 307e324. of Structural Geology 29, 1882e1899. Bastida, F., Aller, J., Lisle, R.J., Bobillo-Ares, N.C., Menéndez, C.O., 2012. Saw-tooth Grasemann, B., Stüwe, K., 2001. The development of flanking folds during simple structures and curved veins related to folds in the south-central Pyrenees shear and their use as kinematic indicators. Journal of Structural Geology 23, (Spain). Journal of Structural Geology 34, 43e53. 715e724. Bons, P.D., Druguet, E., 2007. Some misleading boudin-like structures. Geogaceta 41, Grasemann, B., Dabrowski, M., Schöpfer, M.P.J., 2018. Sense and non-sense of shear 31e34. reloaded. Journal of Structural Geology 27, 249e264. Bons, P.D., Elburg, M.A., Gomez-Rivas, E., 2012. A review of the formation of tectonic Gratier, J.P., 1987. Pressure solution-deposition creep and associated tectonic dif- veins and their microstructures. Journal of Structural Geology 43, 33e62. ferentiation in sedimentary rocks. Geological Society of London Special Publi- Bons, P.D., Druguet, E., Hamann, I., Carreras, J., Passchier, C.W., 2004. Apparent cation 29, 25e38. boudinage in dykes. Journal of Structural Geology 26, 625e636. E. Druguet / Geoscience Frontiers 10 (2019) 2101e2115 2115 Hand, M., Dirks, P.H.G.M., 1992. The influence of deformation on the formation of Peacock, D.C.P., Sanderson, D.J., 1995. Pull-aparts, shear fractures and pressure so- axial-planar leucosomes and the segregation of small melt bodies within the lution. Tectonophysics 241, 1e13. migmatitic Napperby Gneiss, central Australia. Journal of Structural Geology 14, Poulsen, K.H., 1986. Rainy Lake Wrench zone: an example of an Archean Sub- 591e604. province boundary in Northwestern Ontario. In: de Wit, M.J., Ashwal, L.D. (Eds.), Hanmer, S., Passchier, C.W., 1991. Shear-sense indicators: a review. Geological Sur- Tectonic Evolution of Greenstone Belts, Technical Report 86-10, pp. 177e179. vey of Canada 72. Paper 90-17. Price, N.J., Cosgrove, J.V., 1990. Analysis of Geological Structures. Cambridge Uni- Holtzman, B.K., Groebner, N.J., Zimmerman, M.E., Ginsberg, S.B., Kohlstedt, D.L., versity Press, New York, p. 502. 2003. Stress-driven melt segregation in partially molten rocks. Geochemistry, Ramsay, J.G., 1967. The Folding and Fracturing of Rocks. McGraw-Hill, New York, Geophysics, Geosystems 4, 8607. p. 568. Holtzman, B.K., 2005. Viscous energy dissipation and strain partitioning in partially Ramsay, J.G., Huber, M.I., 1987. The Techniques of Modern Structural Geology. In: molten rocks. Journal of Petrology 46, 2569e2592. Folds and Fractures, vol. 2. Academic Press, London, p. 391. Hudleston, P.J., 1989. The association of folds and veins in shear zones. Journal of Robin, P.-Y.F., 1979. Theory of metamorphic segregation and related processes. Structural Geology 11, 949e957. Geochimica et Cosmochimica Acta 43, 1587e1600. Hutton, D.H.W., Reavy, R.J., 1992. Strike-slip tectonics and granite petrogenesis. Rosenberg, C.L., Berger, A., Schmid, S.M., 1995. Observations from the floor of a Tectonics 11, 960e967. granitoid pluton: Inferences on the driving force of final emplacement. Geology Jacques, D., Derez, T., Muchez, P., Sintubin, M., 2014. Syn- to late-orogenic quartz 23, 443e446. veins marking a retrograde deformation path in a slate belt: examples from the Rosenberg, C.L., Handy, M.R., 2000. Syntectonic melt pathways during simple High-Ardenne slate belt (Belgium). Journal of Structural Geology 58, 43e58. shearing of a partially molten rock analogue (norcamphorebenzamide). Journal Kisters, A.F.M., 2005. Controls of gold-quartz vein formation during regional folding of Geophysical Research 105, 3135e3149. in amphibolite-facies, marble-dominated metasediments of the Navachab Gold Rosenberg, C.L., Handy, M.R., 2001. Mechanisms and orientation of melt segregation Mine in the Pan-African Damara Belt, Namibia. South African Journal of Geology paths during pure shearing of a partially molten rock analog (norcam- 108, 365e380. phorebenzamide). Journal of Structural Geology 23, 1917e1932. Kontak, D.J., Smith, P.K., 1990. Geological and 40Ar/39Ar geochronological constraints Sassier, C., Leloup, P.H., Rubatto, D., Galland, O., Yue, Y., Lin, D., 2009. Direct mea- on the timing of quartz vein formation in Meguma Group lode-gold deposits, surement of strain rates in ductile shear zones: a new method based on syn- Nova Scotia. Atlantic Geology 26, 201e227. tectonic dikes. Journal of Geophysical Research-Solid Earth 114, B01406. Kocher, T., Mancktelow, N.S., 2005. Dynamic reverse modelling of flanking struc- Sawyer, E.W., Robin, P.-Y.F., 1986. The subsolidus segregation of layer-parallel veins tures: a source of quantitative kinematic information. Journal of Structural in greenschist to upper amphibolite facies metasediments. Journal of Meta- Geology 27, 1346e1354. morphic Geology 4, 237e260. Kruger, T., Kisters, A., 2016. Magma accumulation and segregation during regional- Sawyer, E.W., 2008. Atlas of migmatites. In: The Canadian Mineralogist, Special scale folding: the Holland’s dome granite injection complex, Damara belt, Publication, vol. 9. NRC Research Press, Ottawa, Ontario. Namibia. Journal of Structural Geology 89, 1e18. Sengupta, S., 1983. Folding of boudinaged layers. Journal of Structural Geology 5, Le Hébel, F., Gapais, D., Fourcade, S., Capdevila, R., 2002. Fluid-assisted large strains 197e214. in a crustal-scale décollement (Hercynian belt of south Brittany, France). In: De Short, H.A., Johnson, S.E., 2006. Estimating vorticity from fibrous calcite veins, Meer, S., Drury, M.R., De Bresser, J.H.P., Pennock, G.M. (Eds.), Deformation central Maine, USA. Journal of Structural Geology 28, 1167e1182. Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives, Simpson, G.D.H., 2000. Synmetamorphic vein spacing distributions: characterisa- vol. 200. Geological Society, London, Special Publications, pp. 85e101. tion and origin of a distribution of veins from NW Sardinia, Italy. Journal of Lehmann, J., Schulmann, K., Lexa, O., Závada, P., Stípská, P., Hasalová, P., Belyanin, G., Structural Geology 22, 335e348. Corsini, M., 2017. Detachment folding of partially molten crust in accretionary Sintubin, M., Debacker, T.N., Van Baelen, H., 2012. Kink band and associated en- orogens: a new magma-enhanced vertical mass and heat transfer mechanism. echelon extensional vein array. Journal of Structural Geology 35, 1. Lithosphere 9, 889e909. Skelton, A., Lewerentz, A., Kleine, B., Webster, D., Pitcairn, I., 2015. Structural Lucas, S.B., St-Onge, M.R., 1995. Syn-tectonic magmatism and the development of Channelling of metamorphic fluids on Islay, Scotland: implications for Paleo- compositional layering, Ungava Orogen (northern Quebec, Canada). Journal of climatic reconstruction. Journal of Petrology 56, 2145e2172. Structural Geology 17, 475e491. Swanson, M.T., 1992. Late Acadian-Alleghenian transpressional deformation: evi- Maeder, X., Passchier, C., Rudolph, R.A.J., 2014. Complex vein systems as a data dence from asymmetric boudinage in the Casco Bay area, coastal Maine. Journal source in tectonics: an example from the Ugab Valley, NW Namibia. Journal of of Structural Geology 14, 323e341. Structural Geology 62, 125e140. Talbot, C., 2008. Palaeoproterozoic crustal building in NE Utö, southern Sveco- Marchildon, N., Brown, M., 2003. Distribution of melt bearing structures in ana- fennides, Sweden. Gff-Uppsala 130, 49e70. tectic rocks from Southern Brittany, France: Implications for melt transfer at Tikoff, B., Teyssier, C., 1992. Crustal-scale, en echelon P-shear tensional bridges: a grain to orogen-scale. Tectonophysics 364, 215e235. possible solution to the batholithic room problem. Geology 20, 927e930. McLellan, E.L., 1988. Migmatite structures in the central gneiss complex, Boca de Vanderhaeghe, O., 1999. Pervasive melt migration from migmatites to leucogranite Quadra, Alaska. Journal of Metamorphic Geology 6, 517e542. in the Shuswap metamorphic core complex, Canada: control of regional Niemeijer, A., Marone, C., Elsworth, D., 2010. Frictional strength and strain weak- deformation. Tectonophysics 312, 35e55. ening in simulated fault gouge: competition between geometrical weakening Vernon, R.H., Paterson, S.R., 2001. Axial-surface leucosomes in anatectic migmatites. and chemical strengthening. Journal of Geophysical Research 115, B10207. In: Boland, J.N., Ord, A. (Eds.), Deformation Processes in the Earth’s Crust (Hobbs Oliver, N.H.S., Bons, P.D., 2001. Mechanisms of fluid flow and fluid-rock interaction Volume), Tectonophysics, vol. 335, pp. 183e192. in fossil metamorphic hydrothermal systems inferred from vein-wallrock pat- Weinberg, R.F., Mark, G., 2008. Magma migration, folding, and disaggregation of terns, geometry and microstructure. Geofluids 1, 137e162. migmatites in the karakoram shear zone, Ladakh, NW India. Geological Society Oliver, N.H.S., Valenta, R.K., Wall, V.J., 1990. The effect of heterogeneous stress and American Bulletin 120, 994e1009. strain on metamorphic fluid flow, Mary Kathleen, Australia, and a model for Weinberg, R.F., Hasalová, P., Ward, L., Fanning, C.M., 2013. Interaction between large-scale fluid circulation. Journal of Metamorphic Geology 8, 311e331. deformation and magma extraction in migmatites: examples from Kangaroo Is- Oriolo, S., Wemmer, K., Oyhantçabal, P., Fossen, H., Schultz, B., Siegesmund, S., 2018. land, south Australia. The Geological Society of America Bulletin 125, 1282e1300. Geochronology of shear zones ea review. Earth-Science Reviews 185, 665e683. Weinberg, R.F., Veveakis, E., Regenauer-Lieb, K., 2015. Compaction-driven melt ̌ Passchier, C.W., 2001. Flanking structures. Journal of Structural Geology 23, segregation in migmatites. Geology 43, 471e474. 951e962. Wickham, S.M., 1987. The segregation and emplacement of granitic magma. Journal Passchier, C.W., Myers, J.S., Kröner, A., 1990. Field Geology of High-Grade Gneiss of the Geological Society of London 144, 281e297. Terrains. Springer-Verlag, Berlin/Heidelberg/New York, p. 150. Windh, J., 1995. Saddle reef and related gold mineralization, Hill End gold field, Pawley, M., Reid, A., Dutch, R., Preiss, W., 2015. Demystifying migmatites: intro- Australia: evolution of an auriferous vein system during progressive deforma- duction for field-based geologist. Applied Earth Science 124, 147e174. tion. Economic Geology 90, 1764e1774.