Using Incremental Elongation and Shearing to Unravel the Kinematics of a Complex Transpressional Zone T

Journal of Structural Geology 115 (2018) 64–81

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Using incremental elongation and shearing to unravel the kinematics of a complex transpressional zone T

∗ P. Xypoliasa, , N. Gerogiannisa, V. Chatzarasb, K. Papapavlouc, S.C. Kruckenbergd, E. Aravadinoua, Z. Michelse a Department of Geology, University of Patras, GR-26500, Patras, Greece b School of Geosciences, The University of Sydney, NSW 2006, Sydney, Australia c Geotop, Université du Québec à Montréal, H2X 3Y7, Montréal, Canada d Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467, USA e Department of Earth Sciences, University of Minnesota, Minnesota, USA

ARTICLE INFO ABSTRACT

Keywords: This study presents in-depth geometric and kinematic analyses of a complex transpressional shear zone (Fellos Ductile shear zone Shear Zone, FSZ) that integrates structural mapping with microstructural and quartz crystallographic texture Object lineation data. The FSZ strikes NE-SW and formed in the short limb of a map-scale antiform. The foliation pattern within Quartz fabrics the zone indicates dextral shearing whereas the macroscopic object lineation is dispersed over a half great-circle Triclinic deformation girdle along the mean mylonitic foliation. Based on this deformation pattern, the FSZ could be interpreted as a Cyclades dextral, NE-directed triclinic transpressional zone. However, the integration of field-based with microtectonic data reveal a more complicate kinematic history. We show that the elongation trend is dispersed along an entire great-circle girdle when we take into account the trends of incremental elongations, recorded by fabrics with different strain memories. Mapping of incremental shear directions implies that the FSZ initiated as a NE-directed dextral transpressional shear zone, and progressively evolved into a NW-directed dextral zone. The passage from NE-to NW-directed shearing was accompanied by transpression whilst local transtension likely occurred during the last stages of ductile deformation. Deformation in the FSZ ended up, at semi-ductile conditions, with localized NE-directed dextral shearing. Our study demonstrates that the integration of field observations and fabrics/microstructures that have different strain memories is a powerful tool for unravelling the complex kinematics of high-strain zones.

1. Introduction shear-normal is possible with increasing strain (Tikoff and Greene, 1997; Passchier, 1998; Fossen and Tikoff, 1998). In these zones, the Deformation patterns in ductile shear zones are attributed to high lineation is either hosted in the vorticity normal section (VNS; i.e., the symmetry monoclinic flow paths and low symmetry triclinic flow paths plane that contains shear sense indicators with maximum asymmetry; (Passchier, 1998; Jiang et al., 2001). The understanding of monoclinic Robin and Cruden, 1994) or lies parallel to the vorticity vector. Triclinic and triclinic flow paths, especially in thinning shear zones, has been the models of transpressional zones predict stretching lineations that form focus of many field, numerical, and strain modeling studies (Fossen and “J-shaped” or half great circle girdle patterns on stereographic projec- Tikoff, 1993; Tikoff and Greene, 1997; Law et al., 2004; Iacopini et al., tions with increasing strain whereas the vorticity vector is parallel with 2010; Xypolias, 2010). In thinning shear zones, the foliation plane the intersection of the foliation plane with the shear zone boundary displays only small variations in its orientation and develops small (Jiang and Williams, 1998; Lin et al., 1998; Jones et al., 2004; angles with the shear zone boundary. The behavior of linear-fabric Fernández and Díaz-Azpiroz, 2009). elements, in turn, is strongly dependent on the nature of the flow. In In some natural examples of transpressional shear zones, lineations thinning shear zones with monoclinic symmetry, the stretching linea- spread over most or all of the great circle which defines the average tion displays point maxima parallel or normal to the shear direction; mylonitic foliation (e.g., Czeck and Hudleston, 2003). Previous studies although switching of finite-elongation directions from shear-parallel to (e.g., Jiang, 2014) emphasized that existing models of monoclinic or

∗ Corresponding author. E-mail address: [email protected] (P. Xypolias). https://doi.org/10.1016/j.jsg.2018.07.004 Received 2 March 2018; Received in revised form 10 July 2018; Accepted 10 July 2018 Available online 17 July 2018 0191-8141/ © 2018 Elsevier Ltd. All rights reserved. P. Xypolias et al. Journal of Structural Geology 115 (2018) 64–81 triclinic deformation cannot explain these variations in lineation or- Blueschist unit was associated with a single deformation phase that was ientation. Explanations for the great-circle dispersion of lineations in contemporaneous with decompression of rocks from the stability field transpressional zones include: (a) spatial variation in the orientation of of blue amphibole to that of actinolite. This deformation phase is ex- the pure shear component coupled with a sub-horizontal and constantly pressed by a planar fabric that varies in intensity from a widely-spaced directed simple shearing (Czeck and Hudleston, 2003; Fernández et al., crenulation cleavage to a mylonitic foliation and a well-developed ENE- 2013); (b) radial distribution of stretching lineation caused by bulk to NE-trending stretching lineation, which is associated with top-to-(E) flattening strain (Xu et al., 2003); (c) spatial variability in strain state NE sense of shear (Fig. 1b) (Xypolias et al., 2003, 2013; Mehl et al., governed by the progressive migration of shear zone boundaries into 2007; Ziv et al., 2010; Xypolias and Alsop, 2014). In north Andros, the the undeformed wall rocks (sustainable transpression; Jiang, 2007); (d) mean direction of shearing is N50°E (Xypolias and Alsop, 2014). The mesoscale partitioning of the flow path governed by multiscale rheo- cleavage/foliation is axial planar to open-to-isoclinal, commonly logical heterogeneities (Jiang and Bentley, 2012; Jiang, 2014); and (e) gently-to-moderately inclined, cylindrical folds that trend at small- deformation overprinting (e.g., Toy et al., 2013). angle to the stretching lineation (Papanikolaou, 1978; Mukhin, 1996; Thus, the large dispersion of lineations in transpressional zones is a Avigad et al., 2001; Xypolias et al., 2012). These folds have wave- complex and controversial issue. A critical question that arises is how lengths that range from a few centimeters to several hundreds of me- frequently this fabric pattern appears and by which means we can ro- ters. Analysis of fold deformation patterns in south Evia and north bustly record it. It is emphasized that many studies of natural trans- Andros has shown that the map-scale folds are commonly overturned pressional zones that ascribe the observed lineation dispersion to tri- and form extensive trains of SE- or NW-verging folds. These folds define clinic flow, rely on the field-observed object/stretching lineation (e.g., two major synformal depressions that are separated from one another Goscombe and Gray, 2008; Gage et al., 2011; Massey and Moecher, by a major antiformal culmination centered on the strait between Evia 2013), yet rarely investigate potential misalignment of incremental and and Andros (Fig. 1b) (Xypolias and Alsop, 2014). In the culmination, finite elongation directions (c.f. Sullivan and Law, 2007; Little et al., the map-scale folds verge toward the outer edges of the structure and 2013). The latter can be recorded by fabrics with different degrees of define an overall convergent fold trace pattern in the transport direc- sensitivity to changes in the flow regime (i.e., quartz oblique-grain- tion, whereas within depressions they verge toward the inner center of shape fabric and crystallographic textures). Thus, the real extent of fi- the structure and display a fold trace pattern that diverges in the nite and incremental elongation directions dispersion in a ductile shear transport direction (Fig. 1b). This fold pattern has been interpreted to zone could be hidden at the microscale. Therefore, many shear zones indicate flow perturbation during top-to-the-NE shearing and is the could be associated with larger dispersions in the elongation direction result of wrench-dominated differential shearing on the flanks of the than what is recorded by mineral or object lineations visible in the field. culminations and depressions (Alsop and Holdsworth, 2007; Xypolias In this study, we combine detailed field, microstructural and quartz and Alsop, 2014). Thus, the SE-verging folds that define the south flank crystallographic texture analyses and present an in-depth geometric and of the culmination in north Andros initiated at small angles to the kinematic analysis of a ductile shear zone in which the macroscopic transport direction in response to dextral differential shearing. As object lineation defines half of a great-circle girdle along the mean pointed-out below, the FSZ structurally truncates such a map-scale SE- mylonitic foliation. Analyzing the orientations of incremental elonga- verging fold. tions, we explore the entire range of elongation plunges, their temporal relationship, and the main cause for the observed large lineation dis- 3. Geometry and structural elements persion. 3.1. Defining the Fellos Shear Zone (FSZ) 2. Geological and structural setting The FSZ strikes NE-SW and dips moderately toward northwest The studied shear zone, called hereinafter as Fellos Shear Zone (Fig. 2). The shear zone can be traced along its strike over a map length (FSZ), is located at the northwestern part of Andros Island, Greece, of at least 1.5 km and exhibits a constant thickness of about 250 m. The where the Blueschist unit rocks of the Cycladic Massif are exposed FSZ cuts out the inverted common limb of a NE-trending antiform- (Fig. 1a). The Blueschist unit is a metamorphosed late Paleozoic - Me- synform pair deforming both the Ochi-Makrotantalo and Styra subunits, sozoic volcanosedimentary sequence that comprises metapelites, mar- as well as their contact (Fig. 2; cross-section XX'). New geological bles, and metabasites with local lenses of meta-ultrabasic rocks (Dürr, mapping in the study area shows that the Ochi-Makrotantalo subunit 1986; Okrusch and Bröcker, 1990). In northwest Cyclades, the meta- consists of epidote-chlorite schists with metaophiolitic lenses that grade morphic path of the Blueschist unit reached epidote-blueschist-facies upwards to quartz-rich schists whereas the Styra subunit is dominated conditions (P > 11 kbar and T = 450–500 °C) in the Eocene followed by calcite schists (Fig. 2). The exposed part of the FSZ comprises rocks by greenschist-facies retrogression (P = 5–6 kbar and T = 350–450 °C) of the Ochi-Makrotantalo subunit and primarily consists of quartz in the Oligocene-Miocene boundary (Maluski et al., 1981; Katzir et al., schists (quartz > 75%) with local intercalations of mica schists, 2000; Bröcker and Franz, 2006). The Blueschist unit is thrust over the quartzofeldspathic schists, and cm-scale, foliation-parallel quartz veins. Basal unit, which is mainly composed of Mesozoic marbles (Shaked No spatial variation in the content of mica-rich layers within the zone is et al., 2000; Xypolias et al., 2010; Chatzaras et al., 2011). observed. Therefore, the zone is generally lithologically homogeneous. In Andros and Evia Islands, the metamorphic pile of the Blueschist Within the FSZ, the schists typically display a mylonitic foliation unit is divided into two subunits, named as the Ochi-Makrotantalo and (Sm), which is nearly parallel to the NW-dipping axial planes of map- Styra subunits (Fig. 1b) (Papanikolaou, 1978; Dürr, 1986; Xypolias and scale folds (Figs. 2 and 3a). Within thin bands of lower strain, which are

Alsop, 2014). The structurally higher Ochi-Makrotantalo subunit is locally observed at the marginal parts of the FSZ, the Sm transposes an chiefly made up of clastic metasediments and marbles as well as iso- earlier foliation (Fig. 3b). The upper boundary of the FSZ is sharp and is lated lens-shaped metaophiolitic bodies (Papanikolaou, 1978). The controlled by the occurrence of an approximately 50 m thick ribbon of underlying Styra subunit, also referred to as North Cyclades or Lower serpentinites, which separates mylonites of the shear zone from the subunit in the local literature, largely consists of metapelitic schists and hanging-wall epidote-chlorite schists (Fig. 2); the latter exhibit an axial calcite marbles. Structural data from south Evia indicate that the jux- planar crenulation cleavage of varying spacing that dips north- taposition of the Ochi with the Styra subunit was accomplished by ESE- westwards. The attitude of the upper boundary of the FSZ (N30°E/ directed thrusting during the subduction stage (Xypolias et al., 2012). 60°NW) is identical to that of the mylonites observed just beneath the Structural studies (Ziv et al., 2010; Xypolias et al., 2012) in Andros serpentinites. The lower boundary is more diffuse and was mapped and Evia have shown that the ductile-stage exhumation of the based on the foliation intensity gradient observed in the field. It has also

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Fig. 1. (a) Simplified geological map showing the major tectonic units in the Aegean region (PZ, Pelagonian Zone; SMRM, Serbomacedonian and Rhodope Massifs; SZ, Sakarya Zone; VZ, Vardar Zone). Box indicates the location of the map in Fig. 1b. (b) Simplified geological/structural map of south Evia and Andros Islands and schematic 3D diagram illustrating the variable orientation and geometry of large-scale folds across the area (modified after Xypolias and Alsop, 2014). Box indicates the location of the map in Fig. 2. the same attitude as the upper boundary and coincides with the axial display shallow, SW-plunging slickenside striae, while the sense of plane of the transposed synform (Fig. 2; cross-section XX'). In map- obliquity between the minor shear zones and Sm reveals NE-SW dextral view, thin ribbons of epidote schists with minor serpentinite lenses are shearing (Fig. 4f). Dextral shearing is also supported by the drag of axial lined up along the lower boundary of the FSZ (Fig. 2). Moving away planar foliation outside the shear zone, from NE-striking near the shear from the zone, the wall rocks display a drastic decrease in foliation zone boundaries to N-striking away from them (Figs. 2 and 4a, e). This intensity. relationship, in combination with the observed NW-plunging intersec- Our structural analysis and mapping was primarily focused on the tion between the axial planar foliation and the shear zone boundaries middle segment of the FSZ (Fig. 2) because the macroscopic object indicate NE-directed shearing (Figs. 1b and 4e). lineation, as described below, displays a complex orientation pattern Structural mapping of the FSZ reveals spatial variation in both the compared to the remaining parts of the zone. We use the term “object” angle-θ and dip of foliation. Specifically, in the north and south do- rather than “stretching” to characterize the lineation (see Piazolo and mains of the FSZ, the angle-θ is typically smaller than 20° and the fo- Passchier, 2002 for definition) because its orientation relative to the liation dips commonly 60°–80° towards WNW (Figs. 4a and 5a). In the maximum axis of the finite strain cannot be established in this segment intermediate domain, in turn, the angle-θ is greater (30°–35°) whereas of the FSZ. For the purpose of this analysis, we present detailed folia- the foliation strikes more northerly and dips with smaller angle tion, lineation and pitch variation maps (Fig. 4a, b, c), which are de- (45°–60°) than in the north and the south domains (Figs. 4a and 5b). scribed below. Detailed analysis across the intermediate domain also reveals that the angle-θ decreases from ca. 30° in the middle to ca. 10° in the marginal parts (Fig. 5b; i). The middle part of the intermediate domain con- 3.2. Foliation mapping taining the highest θ angles is exclusively characterized by mylonites. Mylonites are sporadically interleaved with thin bands that display In the FSZ, the S is characterized by a small dispersion in attitude m closely-spaced crenulation cleavage in the marginal parts of the inter- varying from steeply NW- to moderately W-dipping with the trajec- mediate domain, as well as throughout both the north and south do- tories displaying, in map-view, an overall dextral sigmoidal deflection mains. This spatial variation in foliation intensity implies that the strain (Fig. 4a, d). The intersection between the mean S and the shear zone m is higher in the middle part of the intermediate domain. Spaced clea- boundary indicates NE-directed dextral shearing (Fig. 4d). The strike vage within the shear zone is axial planar to tight to isoclinal folds. In angle (θ) as well as the dihedral angle between the foliation and the the intermediate domain, fold hinge lines display gently W to NW shear zone boundaries is systematically smaller than 45°. Locally, the plunges, whereas in the north and south domains they consistently S curves asymptotically into ductile to semi-ductile minor shear zones, m plunge gently towards the SW (Fig. 6). SW-plunging folds oriented which dip moderately towards NW and commonly range in length from parallel to map-scale folds are also recorded outside the shear zone few tens of meters up to 200 m (Figs. 3c and 4a, f). These shear zones

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Fig. 2. New detailed geological-structural map of the Fellos Shear Zone (FSZ) and surroundings showing the major lithological subunits and the structural elements; the location of the map is shown in Fig. 1. Box indicates the location of maps in Fig. 4. Lettered section X-X' refers to the cross-section depicting the localization of deformation along the FSZ.

(Figs. 2 and 6). (Fig. 4b). It is worth noting that streaks defining SW-plunging lineations are generally composed of coarse-grained mica whereas fine-grained mica streaks define W to NW-plunging lineations. 3.3. Lineation mapping Map-scale patterns of lineation trajectories show that the object lineation in the intermediate domain of the FSZ forms a Z-shaped pat- Outside of the shear zone the foliation plane contains a well-de- tern; lineation is variable, changing orientation from gently/moderately veloped SW-plunging mineral/stretching lineation defined by mica SW-plunging to moderately (W)NW-plunging toward the middle part of streaks, quartz rods, elongated epidote aggregates, and occasionally, by the intermediate domain (Fig. 4b). These structural trends correspond the shape preferred orientations of actinolite and glaucophane needles to a ca. 100° clockwise rotation of the lineation within the foliation (Fig. 4b, e). In the mylonites of the FSZ, the object lineation is typically plane moving from the marginal parts to the middle part of the inter- defined by mica streaks and in some cases by both mica streaks and mediate domain (Fig. 5b; ii). This is also clearly depicted on both the quartz rods. The object lineation within the FSZ is not everywhere well- pitch variation map and diagram showing that the pitch of the lineation developed and shows a wide spread of orientations (angular range of on the Sm plane, measured clockwise from the south, increases pro- 110°) over the larger part of the great circle describing the average gressively toward the middle part of the intermediate domain (Figs. 4c fi plane of Sm (Fig. 4b, d). Speci cally, in the north and south domains, and 5b; iii). the object lineation is commonly well-developed and plunges gently In the next sections, we investigate the shear sense in individual toward SW, similar to the lineation outside of the shear zone (Fig. 4b, domains presenting a detailed microtectonic kinematic analysis of the e). In contrast, within the intermediate domain the object lineation is FSZ. It is noted that mesoscopic kinematic indicators are rarely typically weakly developed and varies from SW- to NW-plunging

Fig. 3. Representative outcrop-scale photographs from the FSZ. (a) Typical quartz and mica schist mylonites observed in the middle part of the intermediate domain;

(b) Tight to isoclinal folds with axial planes parallel to the mylonitic foliation; (c) Ductile to semi-ductile minor shear zone indicating NE-SW dextral shearing. Sm, mylonitic foliation.

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Fig. 4. (a) Map of the FSZ showing foliation trajectories and variation in foliation dip; (b) Map of the FSZ showing object lineation trajectories and variation in lineation plunge angle; (c) Map of the FSZ showing variation in object lineation pitch; (d) and (e) Equal-area/lower-hemisphere projections of object lineation and foliation in the FSZ and the wall rocks, respectively; (f) Equal-area/lower-hemisphere projections of minor shear zones and related slickenside striae in the FSZ. observed throughout the FSZ. Outside the shear zone, our structural representative samples across the north domain (Figs. 4a and 5a). The observations are in accordance with previous kinematic studies sug- precise geographic location of each sample is given in Appendix A gesting regionally consistent NE-directed shearing (e.g., Mehl et al., (Supplementary Geospatial Data). All samples are characterized by a 2007; Xypolias and Alsop, 2014). single mylonitic foliation whereas microfolds are not observed. In each sample, two perpendicular thin sections were cut - one parallel to the

object lineation and normal to Sm, and one normal to both the object 4. Samples description lineation and Sm. The 45 analyzed samples fall into three lithological groups: (a) Due to the structural complexity of the FSZ, we focused our sam- quartz veins (25 samples); (b) quartz schists (12 samples), and (c) mica pling on the intermediate domain, where we collected 39 oriented schists (8 samples). The quartz veins are oriented parallel to the Sm and samples distributed along a traverse oriented approximately perpendi- their thickness ranges from 3 to 10 cm. Typically, the quartz schists cular to the FSZ (Figs. 4a and 5b). Moreover, we collected 6

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epidote and opaque minerals. In all samples, quartz has been affected by extensive dynamic recrystallization (at least 80% recrystallization by area fraction), and by only limited static annealing. In few quartz veins, large relict ribbons (> 2 mm) are decorated by recrystallized grains forming typical “core-and-mantle” microstructures. Recrystallized quartz grains exhibit straight or slightly sutured/serrated boundaries and several of these grains display subgrains. These features reveal that the dynamic recrystallization occurred by progressive subgrain rotation with a contribution of grain boundary migration (Stipp et al., 2002). The most common kinematic indicators, in the quartz and mica schist samples, are mica fish, C- and C'-type shear bands, and occasionally, oblique-grain-shape fabric in pure quartz domains. Oblique- grain-shape fabrics are dominant in all quartz vein samples. To further constrain kinematics we performed quartz crystallographic texture analyses in the majority of the collected samples. In an attempt to investigate the behavior of individual kinematic indicators in different lithologies, we present the results of the kinematic analysis in the following order: (a) the results from the mica fish and shear bands, which are mainly observed in the schists; (b) the results from the oblique- grain-shape fabric, which is mainly observed in the quartz veins; and (c) the results of quartz crystallographic texture analysis.

5. Mica ﬁsh and shear bands

In 9 out of 20 quartz and mica schist samples from the intermediate domain, mica fish and shear bands display clear asymmetry both in lineation-parallel and lineation-normal thin sections, consistent with triclinic deformation geometry (Fig. 7a and b; Table 1). Six samples indicate apparent monoclinic deformation denoted by asymmetric kinematic indicators either in lineation-parallel (2 samples) or lineation- normal (4 samples) thin sections (Fig. 7c and d; Table 1). Ambiguous asymmetry in both thin sections was observed in 5 samples (Table 1). The 6 samples that are consistent with monoclinic deformation indicate either top-down-to-the-W sense of shear (lineation-parallel asymmetry) or NNW-SSE to NNE-SSW dextral shearing (lineation- normal asymmetry) failing to infer a consistent shear direction (Table 1). In an attempt to constrain the true (overall) shear direction in the FSZ, we combine the kinematic data from these 6 samples with the 9 samples that suggest triclinic deformation; these 15 samples are distributed in different structural depths within the FSZ (Table 1). For this purpose, we adopted the methodological approach proposed by Toy Fig. 5. Simplified cross-sections across the (a) north and (b) intermediate do- et al. (2012) called hereinafter as the Multiple Sections Technique. main of the FSZ; the location of cross-sections is shown in Fig. 4a. The structural A prerequisite for the application of the Multiple Sections Technique position of the samples used for microtectonic analysis is also shown; the exact locations of samples are given in Appendix A (Supplementary Geospatial Data). is the existence of a series of sections perpendicular to the foliation and Lettered diagrams show the spatial variations in (i) the strike angle (θ) between at varying angles with each other. Taking into account the wide spread the shear zone boundary (SZB) and the mylonitic foliation (Sm), (ii) the object in the lineation orientations observed in the intermediate domain, a lineation plunge direction, and (iii) object lineation south pitch across the FSZ. large variety of thin section orientations was achieved fulfilling the aforementioned prerequisite. The aim of this method is to identify an apparent reversal of shear sense when all sections are viewed from the same direction. The application of this method requires rotation of in-

dividual thin section planes about the strike of the mean Sm until the latter becomes horizontal, so the kinematic data obtained from each thin section to be projected onto a common reference frame (Fig. 8a) (see Toy et al., 2012 for details). As illustrated in Fig. 8a, there is a plane where a reversal of the shear sense is observed. This plane is

perpendicular to the VNS. Finally, rotating back onto the mean Sm we obtain a mean shear direction towards NW (318°/36°) (Fig. 8a).

6. Oblique-grain-shape fabric

Fig. 6. Equal-area/lower-hemisphere projections of fold axis and pole to axial A preferred alignment of recrystallized quartz grains that is oblique plane measured in the FSZ and the wall rocks. to the macroscopic Sm was observed in 19 samples distributed throughout the FSZ; 14 are from the intermediate domain and 5 from consist of more than 75% quartz whereas the quartz percentage in the the north domain (Table 2). These samples are either foliation-parallel mica schists ranges between 40% and 60%. In addition to white mica, quartz veins (16 samples), or quartz schists (3 samples) that contain the schists include garnet, chlorite, biotite, glaucophane, actinolite, pure quartz domains. In microscale, the Sm is deﬁned either by thin,

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Fig. 7. Representative photomicrographs (crossed-polarized light) of mica fish and shear bands observed in the intermediate domain of the FSZ. (a) and (b) Asymmetric mica fish in quartz schist (sample 17) in lineation-parallel and lineation-normal thin sections, respectively; (c) Quartz schist (sample S11) showing coaxial deformation in lineation-parallel thin section; (d) Asymmetric mica fish and shear bands in quartz schist (sample S11) in lineation-normal thin section. Sm, mylonitic foliation.

Table 1 irrespective of the thin section orientation (Fig. 9). The largest re- Structural depth (d), deformation symmetry (DS) and asymmetry of mica ﬁsh crystallized grains may display subgrains with boundaries that are (sub- and shear bands. ) parallel to the grain boundaries (Fig. 9a–c).

Sample d (m) DS Azimuth of apparent shear Azimuth of apparent shear sense in LPS sense in LNS 6.1. Kinematic analysis S6B 21 O –– S6C 21 T 252 348 Ten out of the 14 samples from the intermediate domain record an S7 37 M 272 – S8 68 T 252 001 oblique alignment of recrystallized quartz grains in both the lineation- S10 72 M 272 – parallel and lineation-normal thin sections (Figs. 9c–f and 10a). In these S11 86 M – 359 10 samples, which are consistent with triclinic deformation geometry, – S12B 92 M 009 the observed grain alignments represent the traces of an oblique-grain- S14 109 T 315 026 15 115 M – 214 shape fabric (Sq) on sections, which are not orthogonal to its attitude. 16 120 T 310 214 These traces will be called as Sq' and Sq″ for lineation-parallel and 17 137 T 312 023 lineation-normal sections, respectively (Fig. 10a). In an attempt to 21 163 O –– evaluate the shear sense in each one of these 10 samples, we applied a –– S22 163 O simple geometric method that combines the apparent angles δ′ and δ″ S23 170 T 066 359 S24 189 M – 352 between the Sq and the Sm recorded in lineation-parallel and lineation- 25 195 T 240 001 normal sections, respectively (Fig. 10a). The aim of this method is to –– S28 205 O estimate the true attitude of the Sq. To determine the angles δ′ and δ″, S31 213 T 233 331 we measured in each corresponding thin section the orientations of the S33 222 O –– long axes of 150–300 quartz grains recrystallized oblique to the S . The 34 230 T 250 344 m frequency distribution histograms, which were constructed for the Structural depths are measured perpendicular to the FSZ upper boundary; M, statistical analysis of the orientation data, generally display continuous Monoclinic; T, Triclinic; O, Orthorhombic; LPS, lineation-parallel section; LNS, populations of readings (Fig. 10b; Appendix A: Supplementary Micro- lineation-normal section. tectonic Data). From each histogram, we assigned the mean and max-

imum apparent angle between the grain shape fabric and the Sm; often discontinuous mica layers that bound the pure quartz domains in namely the δ′mean and δ′max for lineation-parallel sections and, δ″mean the schists, or by mica impurities that mainly occur as laths in the and δ″max for lineation-normal sections (Fig. 10b; Table 2). quartz veins (Fig. 9). Mica grains aligned with the quartz oblique-grain- Using the mean apparent angles δ′mean and δ″mean we projected the ﬁ shape fabric were not observed. Recrystallized quartz grains de ning mean grain long axis orientations Sq'(mean) and Sq″(mean) on lineation- the oblique shape fabric display a strong shape preferred orientation, parallel and lineation-normal section planes, respectively (Fig. 10c).

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we applied the same procedure using the angles δ′max and δ″max. Both angle sets gave consistent results for all the analyzed samples (Fig. 10c; Appendix A: Supplementary Microtectonic Data). The results of this analysis reveal that the vast majority of samples consistent with triclinic deformation are associated with top-down-to-the-WNW shear sense to NNW-SSE dextral shearing with a small normal-sense component (Fig. 10d; Table 2). The remaining 4 samples from the intermediate domain indicate apparent monoclinic deformation and preserve an oblique foliation visible in either lineation-parallel (3 samples) or lineation-normal (1 sample) thin sections (Fig. 9a and b; Table 2). The sense of obliquity

between Sm and Sq in these samples indicates shear sense ranging from top-down-to-the-WSW to NNW-SSE dextral shearing with a small normal-sense component (Fig. 10d). All 5 samples from the north domain indicate apparent monoclinic deformation. Two of these samples that display oblique foliation in lineation-parallel sections indicate NE- SW dextral or sinistral shearing (Fig. 10d). The remaining 3 samples display oblique foliation in lineation-normal sections and consistently indicate NNW-directed oblique-normal shearing (Fig. 10d; Table 2). We assume that the inferred shear orientation in individual samples, displaying either apparent triclinic or monoclinic deformations, deﬁnes a domainal elongation lineation resulting from the development of the quartz oblique shape fabric. This assumption is supported by the fact that the most elongated quartz grains in an oblique-grain-shape fabric

form small angles with the Sm and are sub-parallel to the shear orientation. In samples displaying triclinic deformation, the angular deviation, ω, of the domainal elongation from the macroscopic object lineation, measured on the foliation plane, varies between 11° and 78° Fig. 8. Equal-area/lower-hemisphere projections showing the application pro- (Fig. 10a; Table 2). cedure and the results of the Multiple Sections Technique using kinematic data Kinematic analysis was also performed applying the Multiple of (a) mica fish/shear bands and (b) quartz oblique-grain-shape fabrics from Sections Technique (Toy et al., 2012) in order to evaluate the mean samples of the intermediate domain. The inferred mean shear direction is also sense of shear in the intermediate domain. The procedure is similar to illustrated. See text for details. that for the mica fish and shear bands, described in the previous section. The method was applied using the thin sections from all samples displaying either monoclinic or triclinic deformation. The analysis re- The plane containing the Sq'(mean) and Sq″(mean) defines the mean oblique vealed top-down-to-the-NW shear sense and is in full agreement with foliation Sq(mean). Therefore, the shear orientation is normal to the in- our kinematic analysis in individual samples (Figs. 8b and 10d). Due to tersection between the Sq(mean) and the Sm while the sense of shear is the limited dataset, the Multiple Sections Technique was not applied to inferred based on the sense of obliquity between the Sq(mean) and the Sm (Fig. 10c). In order to test the accuracy of the shear direction estimates, the north domain.

Table 2 Data from oblique-grain-shape analysis in 19 samples. The locations of samples are illustrated in Fig. 5.

Sample Structural depth (m) Deformation symmetry δ′mean δ″mean δmean δ′max δ″max δmax Azimuth of shear sense Vorticity axis orientation ω

Intermediate domain 6B 21 Monoclinic - LPS 53° – 53° 78° – 78° 252 348–07 0° 7 37 Triclinic 40° 19° 42° 65° 41° 67° 305 196–17 22° 8 68 Triclinic 31° 24° 36° 52° 45° 58° 306 207–08 39° 9 70 Triclinic 45° 24° 48° 71° 54° 70° 316 200–25 28° 10 72 Triclinic 25° 31° 36° 45° 65° 68° 337 233–34 58° 12B 92 Monoclinic- LPS 28° – 28° 64° – 64° 285 189–07 0° 13 109 Triclinic 38° 16° 38° 62° 40° 64° 339 229–36 24° 15 115 Monoclinic - LPS 26° – 26° 51° – 51° 315 214–24 0° 16 120 Triclinic 28° 21° 33° 56° 40° 60° 345 245–26 33° 17 137 Triclinic 35° 10° 36° 63° 22° 63° 326 211–25 11° 23 170 Triclinic 39° 37° 49° 67° 64° 72° 126 205–09 40° 26 200 Triclinic 21° 59° 59° 48° 76° 77° 324 221–34 78° 29 205 Monoclinic - LNS – 35° 35° – 69° 69° 338 230–25 90° 32 222 Triclinic 21° 38° 41° 46° 62° 65° 294 193–25 62° North domain 16–10 6 Monoclinic - LPS 30° – 30° 46° – 46° 218 026–61 0° 16–9 53 Monoclinic - LNS – 57° 57° – 74° 74° 352 212–27 90° 16–7 105 Monoclinic - LPS 24° – 24° 40° – 40° 030 335–46 0° 16–6 157 Monoclinic - LNS – 52° 52° – 74° 74° 341 222–27 90° 16–5 206 Monoclinic - LNS – 55° 55° – 78° 78° 353 192–39 90°

Structural depths are measured perpendicular to the FSZ upper boundary; LPS, lineation-parallel shearing; LNS, lineation-normal shearing; δ′mean, δ″mean and δ′max,

δ″max, mean and maximum apparent angles between the grain shape fabric and mylonitic foliation in lineation-parallel and normal sections, respectively; δmean and δmax, mean and maximum true angle between the oblique and the mylonitic foliation; ω, angular deviation of shear direction from the macroscopic object lineation, measured on the foliation plane.

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Fig. 9. Representative photomicrographs (crossed-polarized light) of foliation parallel quartz veins. (a) and (b) Oblique foliation in lineation-parallel thin sections of samples 6B and 12B, respectively. (c) and (d) Oblique foliation in lineation-parallel and lineation-normal thin sections of sample 9, respectively. (e) and (f) Oblique foliation in lineation-parallel and lineation-normal thin sections of sample 32, respectively. (g) and (h) Lineation-parallel and lineation-normal thin sections of sample

16-9, respectively; oblique foliation is observed only in lineation-normal thin section. Sq' and Sq″ mark the grain long axis orientation with respect to the mylonitic foliation (Sm), in lineation-parallel and lineation-normal thin section, respectively.

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Fig. 10. (a) 3D sketch showing quartz oblique-grain-shape fabric in both lineation-parallel and lineation-normal sections; ω is the angular deviation of the domainal elongation from the macroscopic object lineation; Sq' and Sq″ mark the grain long axis orientation with respect to the mylonitic foliation (Sm), in lineation-parallel and lineation-normal section, respectively; δ′ and δ″ are the apparent angles between the oblique-grain-shape fabric and the Sm in lineation-parallel and lineation-normal section, respectively; (b) Representative frequency histograms used to estimate the mean (δ′mean, δ″mean) and the maximum (δ′max, δ″max) apparent angles between the oblique-grain-shape fabric and the Sm in lineation-parallel and lineation-normal thin-sections in sample 9; (c) Equal-area/lower-hemisphere projections summarizing the proposed procedure to estimate the shear direction in sample 9 using the mean (left) and maximum (right) apparent angles between the oblique foliation and the Sm; (d) Equal-area/lower-hemisphere projections showing the inferred shear sense.

6.2. True angle of obliquity oblique foliations can form at high angles (up to 70°) to the mylonitic foliation and the extension lineation if the grain boundary alignment is Our analysis in samples displaying apparent triclinic deformation controlled by subgrain boundaries of quartz grains. In this case, the also enabled us to evaluate the mean (δmean) and maximum (δmax) true grain boundaries should also rotate synthetically to the shear direction angle between the oblique-grain-shape fabric and the Sm (Fig. 10c; during progressive deformation (Herwegh and Handy, 1998). Table 2). Statistical analysis was also performed to assign δmean and δmax angles in samples displaying apparent monoclinic deformation (Table 2; Appendix A: Supplementary Microtectonic Data). The results 7. Quartz crystallographic textures of the 19 analyzed samples showed that δmean ranges between 26° and 59° while δmax between 40° and 78° (Table 2). It is worth noting that in Quartz crystallographic preferred orientation (CPO) analysis was 18 out of 19 samples, δmax is greater than 45°. carried out in 33 samples distributed across the intermediate domain Typically, the angle δmax is controlled by the orientation of the ex- (27 samples) and the north domain (6 samples) of the FSZ. The 25 tensional instantaneous stretching axis (ISA) (e.g., Wallis, 1995; samples are foliation-parallel quartz veins whereas the remaining 8 are Xypolias, 2009). In this case, the recrystallized grains within an ob- quartz schists. Quartz CPO analysis was performed with the following lique-grain-shape fabric nucleate with their long axes parallel to the ISA two methods: (1) using a Leitz universal stage mounted on a Zeiss op- and progressively rotate towards the shear plane. Alternatively, when tical microscope, and (2) by means of electron backscatter diﬀraction the angle of obliquity is greater than 45° the fabric can be crystal- (EBSD). Quartz CPOs were optically obtained from each sample on lographically controlled. Speciﬁcally, as pointed out in several studies lineation-parallel thin sections; in totality 600 c-axis were measured in (Lister and Snoke, 1984; Herwegh and Handy, 1998; Little et al., 2013), each section. In 11 out of the 33 samples, quartz CPOs were also analyzed by means of EBSD on polished thin sections. EBSD data were

73 P. Xypolias et al. Journal of Structural Geology 115 (2018) 64–81

Fig. 11. Contoured quartz c-axis CPO plots (equal-area/lower-hemisphere projections) obtained using electron backscatter diﬀraction (EBSD) and universal stage; r, rotated CPO plots. The structural position of samples is shown in Fig. 5.

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Fig. 11. (continued) acquired at the Department of Earth and Environmental Sciences at Supplementary Microtectonic Data). All of the obtained quartz c-axis Boston College on a Tescan Vega 3 LMU scanning electron microscope patterns are characterized by strong c-axis maxima near or at the per- equipped with a LaB6 source, an Oxford Instruments Nordyls Max2 iphery of the equal-area projection diagrams. Of the 33 CPOs measured, EBSD detector and the Aztec acquisition and analysis software suite. 14 define conventional patterns. With the term “conventional pattern” The c-axis data are presented on lower hemisphere, equal area projec- we refer to Type-I/II crossed-girdles, small-circle girdles and cleft-gir- tions, with the plane of projection being perpendicular to the Sm dles patterns displaying orthorhombic or monoclinic symmetry (e.g., (Fig. 11). The projection and contouring of the optically measured c- Lister, 1977; Schmid and Casey, 1986; Law, 1990). The remaining 19 axis data were performed using Stereo32 software (by K. Röller and display “unusual” patterns that are not symmetrical with respect to the C.A. Trepmann; Ruhn-Univerität Bochum). object lineation and are consistent with triclinic deformation (e.g., Llana-Fúnez, 2002). Specifically, in 11 of these 19 c-axis plots, the maxima spread over the entire length or the larger part of a great circle 7.1. Quartz CPO patterns girdle that does not pass through the center of the pole figure plots (6C, 7, S8, 10, S11, 22, S23, 30, 32, S33, 16-9; Fig. 11). The remaining 8 The quartz c-axis orientations in samples that were analyzed by both “unusual” patterns resemble rotated conventional ones, as if they have optical and EBSD methods, show consistent patterns (Appendix A:

75 P. Xypolias et al. Journal of Structural Geology 115 (2018) 64–81 been obtained from sections erroneously cut oblique to the shear orientation (8, S10, 12A, 17, 23, 25, 26, 29; Fig. 11). Several studies (Klaper, 1988; MacCready, 1996; Llana-Fúnez, 2002; Lebit et al., 2002; Pleuger et al., 2007; Olesen, 2008; Toy et al., 2008; Little et al., 2013; Rodrigues et al., 2016) have also reported “unusual” quartz c-axis CPO patterns where, for example, the maxima define a great circle girdle that does not meet the center of the fabric diagram. In the vast majority of these studies, the CPO plots were rotated following MacCready (1996) on interpreting such discrepancy as record of an incremental elongation, which does not coincide with the field-observed object lineation. Adopting this interpretation, we rotated the “unusual” CPO plots about the foliation pole, in order to obtain a pattern resembling a conventional one, and assuming that the vorticity vector lies in the foliation plane. We rotated the projection plane clockwise or anticlockwise until either the great circle girdle to pass through the center of the plot or a conventional pattern to be observed. The rotation angles vary from 20° to 85°and are illustrated for each of the 19 samples in Fig. 11. The 14 conventional c-axis CPO plots can be grouped into two main types of patterns: (a) single- or variable kinked single-girdle (9, 11, 15, 16, 18, 16-5, 16-10) and (b) small-circle girdles to transitional Type-I crossed-girdles (6B, 12B, 27, 16-6, 16-7, 16-8) (Fig. 11). A pattern resembling cleft-girdles was observed in one sample (14; Fig. 11). The 19 rotated CPO plots are also classified into: (a) single- or variable kinked single-girdle pattern (6C, 7, S8, 10, S11, 22, S23, 30, 32, S33, 16-9) or (b) small-circle girdles to transitional Type-I crossed-girdles pattern (12A, 17, 23, 25, 26, 29) (Fig. 11). Two samples yield c-axis patterns that are interpreted as cleft-girdles (8, S10) (Fig. 11). Concerning the a-axis plots, 8 out of 11 patterns were rotated following the rotation angles from the corresponding c-axis CPO plots. In 7 out of 11 CPO plots, the a-axis patterns display maxima that are spread along a continuous great circle normal to the c-axis maxima (Figs. 11 and 12). These a-axes distributions also display a pronounced offset either clockwise (16, 26; Fig. 12) or anticlockwise (6B, 7, 10, 23, 32; Fig. 12) with respect to the lineation. In two of the remaining samples, the a-axis CPO patterns are characterized by unevenly bimodal dis- Fig. 12. Contoured quartz a-axis CPO plots (equal-area/lower-hemisphere tribution of partially linked maxima (15, 17; Fig. 12). In these plots, projections) obtained using electron backscatter diffraction (EBSD); r, rotated maxima form small circles, centered about the pole to the foliation, and CPO plots. The structural position of samples is shown in Fig. 5. are inclined clockwise about the object lineation. Two samples (8, 29) display a pattern of a-axes, which is transitional between a great-circle and a bimodal distribution, with weak offset anticlockwise from the object lineation.

7.2. Sense of shear

Quartz c-axis patterns display asymmetry with respect to both the skeletal outline and density distribution in 28 out of 33 samples; 22 patterns are clearly asymmetric whereas 6 (12A, 14, S23, 23, 26, 16-5) are slightly asymmetric (Fig. 11). In these 28 c-axis plots, the sense of asymmetry is variable, ranging mainly from NNE-SSW strike-slip shearing to top-down-to-the-WNW shear sense; although four of these c- axis plots (14, 15, 16, 18) indicate top-up-to-the-SE shearing (Figs. 11 and 13). The remaining 5 plots display symmetric patterns (S10, 17, 27, 16-6, 16-7; Fig. 11). The sense of shear inferred from c-axis patterns is also supported by the asymmetry observed in the distributions of a- Fig. 13. Equal-area/lower-hemisphere projections showing the shear sense in- axes, in the samples that the latter are available (Fig. 12). ferred from quartz crystallographic textures. The large variation in shear sense is spatially distributed within the FSZ. The 3D block diagram in Fig. 14 summarizes the spatial distribu- dominated by SE-directed sinistral-reverse shearing. Localized NNE- ﬁ tion of shear sense in the intermediate and north domains. Speci cally, SSW sinistral strike-slip shearing is recorded at the transitional zone the north domain is typically characterized by NE-SW dextral strike-slip between the middle and lower part of the intermediate domain. Based shearing with a small normal or reverse component (Fig. 14). Sinistral on the foliation map, we infer the presence of a zone with sinistral strike-slip shearing is restricted to the uppermost part of the north shearing that extends northward to the uppermost part of the north domain (Fig. 14). In the intermediate domain, in turn, the inferred domain where similar sense of shear is also observed (Fig. 14). The shear sense varies across the FSZ (Fig. 14). The upper part of the in- lower part of the intermediate domain is primarily characterized by termediate domain is characterized by W-directed normal shearing to NNW-SSE dextral strike-slip shearing with a small normal-sense NW-directed dextral-normal shearing whereas the middle part is

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are embedded in mica-rich layers (Fig. 15a and b) (see Xypolias et al., 2007, 2013 for similar cases). Therefore, we performed 3D quartz grain shape analysis on 13 mica and quartz schist samples from the intermediate domain, assuming that the analysis of these samples may yield more accurate 3D strain estimates than the analysis of pure quartz samples. 3D grain shape analysis was also carried out in pure quartz domains of 6 samples, in order to compare the strain symmetry estimates in mica-free and mica-rich domains. Note that the analysis was performed in domains where the quartz grains do not display oblique- grain-shape fabric. Grain shape analysis was conducted using Rf-φ data collected from lineation-parallel and lineation-normal thin sections of the 19 samples. In each section the traces of at least 70–80 grain outlines were input into the software SAPE (Mulchrone et al., 2005) that automatically approximates grain shapes as ellipses and extracts Rf-φ data. The ex- tracted data for each section were analyzed using the theta-curve method of Lisle (1985) while the calculation was made utilizing the computer-based approaches of Mulchrone and Meere (2001). The results are presented in Appendix A (Supplementary Microtectonic Data) and are plotted on a Flinn diagram (Fig. 15c). 3D quartz grain shape analysis in both mica-rich and mica-free domains yielded similar strain symmetry estimates (Fig. 15c). Specifically, as illustrated in the Flinn diagram, most of the data points (18 samples) fall within the field of apparent flattening with k-values ranging between 0.11 and 0.93. One sample falls within the field of apparent constrictional (k = 1.72) Fig. 14. 3D block diagram summarizing the spatial variation of the shear sense (Fig. 15c). inferred from quartz crystallographic textures in the FSZ. Equal-area/lower- hemisphere projections showing the mean shear sense in the north domain as 9. Discussion well as in different structural parts of the intermediate domain. SZB, Shear Zone Boundary. The formation of the FSZ is the result of localization of deformation in the short limb of an inclined map-scale antiform, which was gener- component. The W-directed normal shearing is locally observed in the ated by dextral differential shearing induced by flow perturbation lower part of the intermediate domain, similar to the upper inter- during NE-directed shearing (Xypolias and Alsop, 2014). Several stu- mediate domain. dies (e.g., Alsop and Holdsworth, 2007), in areas where models of flow perturbations have been applied, have shown that differential shear is 7.3. Strain symmetry broadly distributed rather than localized along discrete strike-slip dominated shears. Therefore, the FSZ represents a natural example A correlation between the 3D strain symmetry and the pattern of showing that flow perturbation folding can be associated with strain quartz c- and a-axis distributions is suggested by theoretical studies localization on the flanks of culmination and depression structures. A (Lister and Hobbs, 1980; Schmid and Casey, 1986) and supported by potential mechanism to account for this strain localization could be the petrofabric studies coupled with strain analyses in naturally deformed rheological contrast induced by various bounding lithologies to the FSZ rocks (e.g., Law et al., 2010; Xypolias et al., 2010). By analogy to these (e.g., serpentinites and epidotites versus the quartz-mica metapelitic studies, the small-circle girdles to transitional Type-I crossed-girdles rocks). patterns recorded in 12 samples are interpreted to signify general flattening (k < 1) to plane-strain (k = 1) conditions. Single or variable 9.1. Kinematic analysis based on field data – telling part of the story kinked single-girdle patterns recorded in 18 samples indicate approximate plane-strain deformation, although the tendency in some samples Outside the FSZ, the recorded NE-to N-striking axial planar foliation (11, S11, 15, 22, 30, 16-9) toward a transitional pattern between Type-I bears a stretching lineation that consistently plunges shallowly towards crossed-girdles and small-girdles could indicate slightly flattening SW and is associated with NE-directed shearing and, thus, with a NNW- strain. Evidence for constrictional strain is restricted to three samples plunging vorticity vector (Figs. 4e and 16a: 1). Strain localization in the that are characterized by cleft-girdle patterns (Sullivan and Beane, FSZ was associated with progressive intensification of this initial axial 2010; Xypolias et al., 2013). planar foliation. Therefore, the observed deformation pattern outside the FSZ should reflect the structural configuration before the formation 8. Grain shape analysis of the shear zone. Within the FSZ, the macroscopic object lineation displays a large orientation dispersion along the mean foliation whereas Throughout the FSZ, quartz has been affected by extensive dynamic the object lineation trajectories show a Z-shaped pattern in map-view recrystallization and, thus, the shape of the most deformed grains has (Figs. 4a and 16a: 2i). This pattern is attributed to the operation of the been restored to a more equant form (e.g., Law, 1986). Consequently, FSZ. Based exclusively on field observations, we could interpret the FSZ finite-strain analysis using quartz as strain marker is expected, by de- to have formed in (N)NE-directed dextral transpression, as indicated by finition, to underestimate significantly the principal strain ratios. (a) dextral sigmoidal pattern of the foliation trajectories in map-view; However, several studies (e.g., Tagami and Takeshita, 1998; Strine and (b) smaller than 35° obliquity between the shear zone boundaries and

Wojtal, 2004; Xypolias et al., 2013) have shown that 3D shape analysis the Sm; and (c) steeply NW-plunging intersection between the mean Sm of recrystallized grains can record the true strain symmetry, in terms of and the shear zone boundaries. This intersection should approximate the Flinn parameter (k), at lower strain magnitude (Nadai octahedral the vorticity vector in the FSZ and lies at a small angle to the vector shear strain). Furthermore, microscopic analysis shows that the extent recorded in the wall rocks. On the basis of the observed foliation and of dynamic recrystallization appears to decrease in quartz grains that lineation pattern, the FSZ could be interpreted as a triclinic

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Fig. 15. Photomicrographs of (a) lineation-parallel and (b) lineation-normal thin section of sample S31 used for 3D-shape-analysis. (c) Flinn diagram showing the results of quartz grain shape analysis. EFP, foliation-parallel ellipticity; ELN, lineation-normal ellipticity. transpressional zone with shear direction slightly oblique to the initial retain information for a relatively longer part of the deformation his- monoclinic fabrics. The NE-directed dextral shearing localized along tory and survive late-stage changes. On the basis of this suggestion, we minor, semi-ductile shear zones, which formed during the very late observe an overall rotation of the incremental elongations inferred from stages of the FSZ operation (Figs. 4f and 16a: 3). quartz c-axis textures and oblique-grain-shape fabrics toward a NW trend. In individual samples, we observe either a clockwise rotation of the incremental elongations from a mean WNW to a mean NW trend, or 9.2. Kinematic analysis based on both field data and microstructures/ an anticlockwise rotation from a mean SSW to a mean NW trend fabrics with different strain memories – the full story (Fig. 16b). Fig. 16a (stages 2i-iii) illustrates structural maps based on macro- The integration of field-based observations with microstructural and scopic object lineation, as well as on incremental lineation and shearing CPO data reveals a more complex kinematic history of the FSZ, com- recorded by quartz c-axis textures and oblique foliation analyses. For pared to the one described above by taking into account the field data the above-mentioned reasons, these maps likely depict successive in- only (compare the upper and lower row of Fig. 16a). Analysis of quartz crements of the FSZ deformation. The incremental elongation map c-axis textures and oblique foliations reveal the occurrence of incre- obtained from quartz c-axis texture data indicates that the Z-shaped mental elongations, which vary in orientation from SW to NNE and pattern recorded by object lineation progressively tightens and is from W to NNW, respectively. These observations in combination with modified by the zone of localized sinistral shearing, which extended the large dispersion in orientation of the macroscopic object lineation, from the uppermost part of the north domain to the bottom of the which ranges from SSW to NW, indicate that the true dispersion in middle part of the intermediate domain (Figs. 14 and 16a: 2ii). Struc- elongation directions is over an entire great circle girdle (Fig. 16b). turally above this zone, the trajectories are subjected to clockwise ro- The macroscopic object lineation is primarily defined by mica tation whereas beneath it, anticlockwise rotation occurs. Based on the streaks and secondarily by quartz rods. In sites where the object incremental elongation map obtained from oblique-grain-shape fabrics, lineation was measured, the grain size of mica is sufficiently large to be it seems that the trajectories are further rotated anticlockwise in the visible to the naked eye, revealing that the mica has not been affected lower part of the intermediate domain and north domain, whereas they by extensive recrystallization. In turn, the incremental maximum are slightly rotated clockwise in the remaining part of the intermediate elongations obtained from both oblique-grain- shape fabric and quartz domain. These rotations form a fan-shaped trajectory pattern defined c-axis textures are based on recrystallized quartz grains and, hence, by west (intermediate domain) to NNE (north domain) elongation these elongations should reflect younger increment(s) of deformation trends with a mean NW-SE trend (Fig. 16a: 2iii). compared to that recorded from the object lineation. Therefore, it is The NW-trending elongation is associated with NW-directed dextral reasonable to assume that the maximum elongation is rotated from an shearing. NW-directed shearing is penetrative and mainly supported by initial SW (regional object lineation trend) to a more W to NW trend. mica fish and shear bands in addition to the oblique-grain-shape fabrics This rotation should be accompanied by recrystallization of mica while is associated with the formation of minor NW-plunging folds. The forming a more NW-plunging lineation, which is likely visible only at progressive rotation from the initial SW-trending to NW-trending the microscopic scale (for a similar case see Toy et al., 2012, 2013). elongation is characterized by a complex kinematic picture including This assumption is supported by the fact that the SW-plunging linea- localized sinistral shearing, which fades significantly at the last stage tions are more clearly visible than the WSW to NW-plunging object (Figs. 14 and 16a: 2ii, 2iii). This complex kinematic pattern is also re- lineations. corded in the quartz c-axis textures that show large dispersion in the Incremental elongations inferred from quartz c-axis textures display shear direction from NE to NW (Figs. 13 and 16a: 2ii). larger scattering in orientation and larger overlap with the object Summarizing, our analysis shows that the FSZ displays character- lineation trend compared to the incremental elongations inferred from istics for (a) NE-directed dextral ductile shearing inferred by field-based oblique-grain-shape fabrics (Fig. 16b). This difference likely reflects the data and partly by CPO data, (b) NW-directed dextral ductile shearing different degrees of sensitivity of fabrics in response to temporal supported by both microstructural and CPO data, and (c) NE-directed changes to the imposed kinematic framework. This is in accord with shearing at semi-ductile conditions as indicated by minor shear zones many studies (Law, 1986; Hippertt and Borba, 1992; Hongn and (Fig. 16a). Therefore, we posit that the FSZ commenced as a NE-di- Hippertt, 2001; Wallis, 1995; Herwegh and Handy, 1998; Xypolias, rected dextral ductile shear zone, continued as NW-directed dextral 2010) suggesting that the oblique-grain-shape fabric is an in- ductile shear zone and ended up, at semi-ductile conditions, with stantaneous sensitive feature, whereas the bulk quartz c-axis textures

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Fig. 16. (a) Structural maps based on (1) pre-shear-zone elements, (2i) the macroscopic object lineation, (2ii) the incremental elongation and shearing inferred from quartz c-axis textures, (2iii) the incremental elongation and shearing inferred from quartz oblique-grain-shape fabrics, and (3) semi-ductile shearing. Equal-area/ lower-hemisphere projections summarize the mean shear sense for each stage. (b) Equal-area/lower-hemisphere projections showing the true maximum elongation dispersion and the clockwise or anticlockwise rotation from the incremental elongation inferred from quartz c-axis textures to the elongation inferred from quartz oblique-grain-shape fabrics. localized NE-directed dextral shearing. Consequently, the FSZ is char- 9.3. Thinning or thickening deformation? acterized by complex kinematics rather than by exclusively NE-directed shearing as it is inferred when only ﬁeld observations are taken into The NE-directed dextral shearing is associated with transpression, account. We also suggest that the observed lineation orientation dis- however, the important question for understanding the FSZ evolution is persion should be attributed to the progressive shift from NE-to NW- whether progressive shift from NE-to NW-directed dextral ductile directed dextral shearing. shearing is associated with thinning or thickening of the zone. This ambiguity stems from the fact that some of the recorded deformation fabrics and textures display conﬂicting characteristics that could be

79 P. Xypolias et al. Journal of Structural Geology 115 (2018) 64–81 interpreted either by transpressional or transtensional deformation. deformation. Deformation in the FSZ ended up, at semi-ductile condi- Specifically, in a high-strain transpressional zone, the mylonitic foliations, with localized NE-directed dextral shearing. tion is expected to be sub-parallel to the shear zone boundaries as ob- The natural example of the FSZ shows that the integration of field served in the north domain rather than at an angle of 30° or higher as observations with fabrics/microstructures that have different strain observed in the intermediate domain of the FSZ (Figs. 4a and 5b). A memories is a powerful tool to unravel kinematics of complex high- possible explanation could be that local (intermediate domain) trans- strain zones. tensional shearing increased the obliquity between the Sm and the shear zone boundary. The oblique-grain-shape fabrics also display conflicting Acknowledgements characteristics. Specifically, if the recrystallized quartz grains nucleate in the direction of ISA, as commonly assumed (Xypolias, 2009 and re- Constructive review comments by the journal editor Toru Takeshita ferences therein), then the observed unusually high angles between the as well as by Bill Sullivan and an anonymous reviewer were helpful in oblique-grain-shape fabric and the shear zone boundaries are consistent clarifying important aspects of this manuscript and are gratefully ac- with NW-directed transtensional shearing. However, oblique foliation is knowledged. We greatly appreciated insightful discussions with Basil at high angle also with the Sm, a feature that is not expected in a Tikoff in the field. Fieldwork for this study was supported by Grant transtensional zone. These conflicting indications could be interpreted C.924 (awarded to P. Xypolias) from the Research Committee of the with a shift from transpressional to transtensional deformation during University of Patras (Programme K. Karatheodori). Research funds from the last stages of deformation. Alternatively, oblique foliation at high Boston College to S.C. Kruckenberg are acknowledged for supporting angles with Sm can be formed in a thinning zone if grain boundary the EBSD analytical results presented in this study. alignment is controlled by the subgrain boundaries of quartz grains (Lister and Snoke, 1984). In several samples displaying oblique folia- Appendix A. Supplementary data tion, the boundaries of the recrystallized grains are sub-parallel to subgrain boundaries and, hence, this alternative mechanism cannot be Supplementary data related to this article can be found at https:// excluded (Fig. 9a–c). 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