Using incremental elongation and shearing to unravel the kinematics of acomplex transpressional zone
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, Greeceb School of Geosciences, The University of Sydney, NSW 2006, Sydney, AustraliacGeotop, Université du Québec à Montréal, H2X 3Y7, Montréal, Canadad Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467, USAe Department of Earth Sciences, University of Minnesota, Minnesota, USA
This study presents in-depth geometric and kinematic analyses of a complex transpressional shear zone (FellosShear Zone, FSZ) that integrates structural mapping with microstructural and quartz crystallographic texturedata. The FSZ strikes NE-SW and formed in the short limb of a map-scale antiform. The foliation pattern withinthe zone indicates dextral shearing whereas the macroscopic object lineation is dispersed over a half great-circlegirdle along the mean mylonitic foliation. Based on this deformation pattern, the FSZ could be interpreted as adextral, NE-directed triclinic transpressional zone. However, the integration of field-based with microtectonicdata reveal a more complicate kinematic history. We show that the elongation trend is dispersed along an entiregreat-circle girdle when we take into account the trends of incremental elongations, recorded by fabrics withdifferent strain memories. Mapping of incremental shear directions implies that the FSZ initiated as a NE-di-rected dextral transpressional shear zone, and progressively evolved into a NW-directed dextral zone. Thepassage from NE-to NW-directed shearing was accompanied by transpression whilst local transtension likelyoccurred during the last stages of ductile deformation. Deformation in the FSZ ended up, at semi-ductile con-ditions, with localized NE-directed dextral shearing. Our study demonstrates that the integration of field ob-servations and fabrics/microstructures that have different strain memories is a powerful tool for unravelling thecomplex kinematics of high-strain zones.
1. Introduction
Deformation patterns in ductile shear zones are attributed to highsymmetry monoclinic flow paths and low symmetry triclinic flow paths(Passchier, 1998; Jiang et al., 2001). The understanding of monoclinicand triclinic flow paths, especially in thinning shear zones, has been thefocus of many field, numerical, and strain modeling studies (Fossen andTikoff, 1993; Tikoff and Greene, 1997; Law et al., 2004; Iacopini et al.,2010; Xypolias, 2010). In thinning shear zones, the foliation planedisplays only small variations in its orientation and develops smallangles with the shear zone boundary. The behavior of linear-fabricelements, in turn, is strongly dependent on the nature of the flow. Inthinning shear zones with monoclinic symmetry, the stretching linea-tion displays point maxima parallel or normal to the shear direction;although switching of finite-elongation directions from shear-parallel to
shear-normal is possible with increasing strain (Tikoff and Greene,1997; Passchier, 1998; Fossen and Tikoff, 1998). In these zones, thelineation is either hosted in the vorticity normal section (VNS; i.e., theplane that contains shear sense indicators with maximum asymmetry;Robin and Cruden, 1994) or lies parallel to the vorticity vector. Triclinicmodels of transpressional zones predict stretching lineations that form“J-shaped” or half great circle girdle patterns on stereographic projec-tions with increasing strain whereas the vorticity vector is parallel withthe intersection of the foliation plane with the shear zone boundary(Jiang and Williams, 1998; Lin et al., 1998; Jones et al., 2004;Fernández and Díaz-Azpiroz, 2009).
In some natural examples of transpressional shear zones, lineationsspread over most or all of the great circle which defines the averagemylonitic foliation (e.g., Czeck and Hudleston, 2003). Previous studies(e.g., Jiang, 2014) emphasized that existing models of monoclinic or
https://doi.org/10.1016/j.jsg.2018.07.004Received 2 March 2018; Received in revised form 10 July 2018; Accepted 10 July 2018
triclinic deformation cannot explain these variations in lineation or-ientation. Explanations for the great-circle dispersion of lineations intranspressional zones include: (a) spatial variation in the orientation ofthe pure shear component coupled with a sub-horizontal and constantlydirected simple shearing (Czeck and Hudleston, 2003; Fernández et al.,2013); (b) radial distribution of stretching lineation caused by bulkflattening strain (Xu et al., 2003); (c) spatial variability in strain stategoverned by the progressive migration of shear zone boundaries intothe undeformed wall rocks (sustainable transpression; Jiang, 2007); (d)mesoscale partitioning of the flow path governed by multiscale rheo-logical heterogeneities (Jiang and Bentley, 2012; Jiang, 2014); and (e)deformation overprinting (e.g., Toy et al., 2013).
Thus, the large dispersion of lineations in transpressional zones is acomplex and controversial issue. A critical question that arises is howfrequently this fabric pattern appears and by which means we can ro-bustly record it. It is emphasized that many studies of natural trans-pressional zones that ascribe the observed lineation dispersion to tri-clinic flow, rely on the field-observed object/stretching lineation (e.g.,Goscombe and Gray, 2008; Gage et al., 2011; Massey and Moecher,2013), yet rarely investigate potential misalignment of incremental andfinite elongation directions (c.f. Sullivan and Law, 2007; Little et al.,2013). The latter can be recorded by fabrics with different degrees ofsensitivity to changes in the flow regime (i.e., quartz oblique-grain-shape fabric and crystallographic textures). Thus, the real extent of fi-nite and incremental elongation directions dispersion in a ductile shearzone could be hidden at the microscale. Therefore, many shear zonescould be associated with larger dispersions in the elongation directionthan what is recorded by mineral or object lineations visible in the field.
In this study, we combine detailed field, microstructural and quartzcrystallographic texture analyses and present an in-depth geometric andkinematic analysis of a ductile shear zone in which the macroscopicobject lineation defines half of a great-circle girdle along the meanmylonitic foliation. Analyzing the orientations of incremental elonga-tions, we explore the entire range of elongation plunges, their temporalrelationship, and the main cause for the observed large lineation dis-persion.
2. Geological and structural setting
The studied shear zone, called hereinafter as Fellos Shear Zone(FSZ), is located at the northwestern part of Andros Island, Greece,where the Blueschist unit rocks of the Cycladic Massif are exposed(Fig. 1a). The Blueschist unit is a metamorphosed late Paleozoic - Me-sozoic volcanosedimentary sequence that comprises metapelites, mar-bles, and metabasites with local lenses of meta-ultrabasic rocks (Dürr,1986; Okrusch and Bröcker, 1990). In northwest Cyclades, the meta-morphic path of the Blueschist unit reached epidote-blueschist-faciesconditions (P > 11 kbar and T=450–500 °C) in the Eocene followedby greenschist-facies retrogression (P= 5–6 kbar and T=350–450 °C)in the Oligocene-Miocene boundary (Maluski et al., 1981; Katzir et al.,2000; Bröcker and Franz, 2006). The Blueschist unit is thrust over theBasal unit, which is mainly composed of Mesozoic marbles (Shakedet al., 2000; Xypolias et al., 2010; Chatzaras et al., 2011).
In Andros and Evia Islands, the metamorphic pile of the Blueschistunit is divided into two subunits, named as the Ochi-Makrotantalo andStyra subunits (Fig. 1b) (Papanikolaou, 1978; Dürr, 1986; Xypolias andAlsop, 2014). The structurally higher Ochi-Makrotantalo subunit ischiefly made up of clastic metasediments and marbles as well as iso-lated lens-shaped metaophiolitic bodies (Papanikolaou, 1978). Theunderlying Styra subunit, also referred to as North Cyclades or Lowersubunit in the local literature, largely consists of metapelitic schists andcalcite marbles. Structural data from south Evia indicate that the jux-taposition of the Ochi with the Styra subunit was accomplished by ESE-directed thrusting during the subduction stage (Xypolias et al., 2012).
Structural studies (Ziv et al., 2010; Xypolias et al., 2012) in Androsand Evia have shown that the ductile-stage exhumation of the
Blueschist unit was associated with a single deformation phase that wascontemporaneous with decompression of rocks from the stability fieldof blue amphibole to that of actinolite. This deformation phase is ex-pressed by a planar fabric that varies in intensity from a widely-spacedcrenulation cleavage to a mylonitic foliation and a well-developed ENE-to NE-trending stretching lineation, which is associated with top-to-(E)NE sense of shear (Fig. 1b) (Xypolias et al., 2003, 2013; Mehl et al.,2007; Ziv et al., 2010; Xypolias and Alsop, 2014). In north Andros, themean direction of shearing is N50°E (Xypolias and Alsop, 2014). Thecleavage/foliation is axial planar to open-to-isoclinal, commonlygently-to-moderately inclined, cylindrical folds that trend at small-angle to the stretching lineation (Papanikolaou, 1978; Mukhin, 1996;Avigad et al., 2001; Xypolias et al., 2012). These folds have wave-lengths that range from a few centimeters to several hundreds of me-ters. Analysis of fold deformation patterns in south Evia and northAndros has shown that the map-scale folds are commonly overturnedand form extensive trains of SE- or NW-verging folds. These folds definetwo major synformal depressions that are separated from one anotherby a major antiformal culmination centered on the strait between Eviaand Andros (Fig. 1b) (Xypolias and Alsop, 2014). In the culmination,the map-scale folds verge toward the outer edges of the structure anddefine an overall convergent fold trace pattern in the transport direc-tion, whereas within depressions they verge toward the inner center ofthe structure and display a fold trace pattern that diverges in thetransport direction (Fig. 1b). This fold pattern has been interpreted toindicate flow perturbation during top-to-the-NE shearing and is theresult of wrench-dominated differential shearing on the flanks of theculminations and depressions (Alsop and Holdsworth, 2007; Xypoliasand Alsop, 2014). Thus, the SE-verging folds that define the south flankof the culmination in north Andros initiated at small angles to thetransport direction in response to dextral differential shearing. Aspointed-out below, the FSZ structurally truncates such a map-scale SE-verging fold.
3. Geometry and structural elements
3.1. Defining the Fellos Shear Zone (FSZ)
The FSZ strikes NE-SW and dips moderately toward northwest(Fig. 2). The shear zone can be traced along its strike over a map lengthof at least 1.5 km and exhibits a constant thickness of about 250m. TheFSZ cuts out the inverted common limb of a NE-trending antiform-synform pair deforming both the Ochi-Makrotantalo and Styra subunits,as well as their contact (Fig. 2; cross-section XX'). New geologicalmapping in the study area shows that the Ochi-Makrotantalo subunitconsists of epidote-chlorite schists with metaophiolitic lenses that gradeupwards to quartz-rich schists whereas the Styra subunit is dominatedby calcite schists (Fig. 2). The exposed part of the FSZ comprises rocksof the Ochi-Makrotantalo subunit and primarily consists of quartzschists (quartz > 75%) with local intercalations of mica schists,quartzofeldspathic schists, and cm-scale, foliation-parallel quartz veins.No spatial variation in the content of mica-rich layers within the zone isobserved. Therefore, the zone is generally lithologically homogeneous.
Within the FSZ, the schists typically display a mylonitic foliation(Sm), which is nearly parallel to the NW-dipping axial planes of map-scale folds (Figs. 2 and 3a). Within thin bands of lower strain, which arelocally observed at the marginal parts of the FSZ, the Sm transposes anearlier foliation (Fig. 3b). The upper boundary of the FSZ is sharp and iscontrolled by the occurrence of an approximately 50m thick ribbon ofserpentinites, which separates mylonites of the shear zone from thehanging-wall epidote-chlorite schists (Fig. 2); the latter exhibit an axialplanar crenulation cleavage of varying spacing that dips north-westwards. The attitude of the upper boundary of the FSZ (N30°E/60°NW) is identical to that of the mylonites observed just beneath theserpentinites. The lower boundary is more diffuse and was mappedbased on the foliation intensity gradient observed in the field. It has also
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the same attitude as the upper boundary and coincides with the axialplane of the transposed synform (Fig. 2; cross-section XX'). In map-view, thin ribbons of epidote schists with minor serpentinite lenses arelined up along the lower boundary of the FSZ (Fig. 2). Moving awayfrom the zone, the wall rocks display a drastic decrease in foliationintensity.
Our structural analysis and mapping was primarily focused on themiddle segment of the FSZ (Fig. 2) because the macroscopic objectlineation, as described below, displays a complex orientation patterncompared to the remaining parts of the zone. We use the term “object”rather than “stretching” to characterize the lineation (see Piazolo andPasschier, 2002 for definition) because its orientation relative to themaximum axis of the finite strain cannot be established in this segmentof the FSZ. For the purpose of this analysis, we present detailed folia-tion, lineation and pitch variation maps (Fig. 4a, b, c), which are de-scribed below.
3.2. Foliation mapping
In the FSZ, the Sm is characterized by a small dispersion in attitudevarying from steeply NW- to moderately W-dipping with the trajec-tories displaying, in map-view, an overall dextral sigmoidal deflection(Fig. 4a, d). The intersection between the mean Sm and the shear zoneboundary indicates NE-directed dextral shearing (Fig. 4d). The strikeangle (θ) as well as the dihedral angle between the foliation and theshear zone boundaries is systematically smaller than 45°. Locally, theSm curves asymptotically into ductile to semi-ductile minor shear zones,which dip moderately towards NW and commonly range in length fromfew tens of meters up to 200m (Figs. 3c and 4a, f). These shear zones
display shallow, SW-plunging slickenside striae, while the sense ofobliquity between the minor shear zones and Sm reveals NE-SW dextralshearing (Fig. 4f). Dextral shearing is also supported by the drag of axialplanar foliation outside the shear zone, from NE-striking near the shearzone boundaries to N-striking away from them (Figs. 2 and 4a, e). Thisrelationship, in combination with the observed NW-plunging intersec-tion between the axial planar foliation and the shear zone boundariesindicate NE-directed shearing (Figs. 1b and 4e).
Structural mapping of the FSZ reveals spatial variation in both theangle-θ and dip of foliation. Specifically, in the north and south do-mains of the FSZ, the angle-θ is typically smaller than 20° and the fo-liation dips commonly 60°–80° towards WNW (Figs. 4a and 5a). In theintermediate domain, in turn, the angle-θ is greater (30°–35°) whereasthe foliation strikes more northerly and dips with smaller angle(45°–60°) than in the north and the south domains (Figs. 4a and 5b).Detailed analysis across the intermediate domain also reveals that theangle-θ decreases from ca. 30° in the middle to ca. 10° in the marginalparts (Fig. 5b; i). The middle part of the intermediate domain con-taining the highest θ angles is exclusively characterized by mylonites.Mylonites are sporadically interleaved with thin bands that displayclosely-spaced crenulation cleavage in the marginal parts of the inter-mediate domain, as well as throughout both the north and south do-mains. This spatial variation in foliation intensity implies that the strainis higher in the middle part of the intermediate domain. Spaced clea-vage within the shear zone is axial planar to tight to isoclinal folds. Inthe intermediate domain, fold hinge lines display gently W to NWplunges, whereas in the north and south domains they consistentlyplunge gently towards the SW (Fig. 6). SW-plunging folds orientedparallel to map-scale folds are also recorded outside the shear zone
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 andschematic 3D diagram illustrating the variable orientation and geometry of large-scale folds across the area (modified after Xypolias and Alsop, 2014). Box indicatesthe location of the map in Fig. 2.
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(Figs. 2 and 6).
3.3. Lineation mapping
Outside of the shear zone the foliation plane contains a well-de-veloped SW-plunging mineral/stretching lineation defined by micastreaks, quartz rods, elongated epidote aggregates, and occasionally, bythe shape preferred orientations of actinolite and glaucophane needles(Fig. 4b, e). In the mylonites of the FSZ, the object lineation is typicallydefined by mica streaks and in some cases by both mica streaks andquartz rods. The object lineation within the FSZ is not everywhere well-developed and shows a wide spread of orientations (angular range of110°) over the larger part of the great circle describing the averageplane of Sm (Fig. 4b, d). Specifically, in the north and south domains,the object lineation is commonly well-developed and plunges gentlytoward SW, similar to the lineation outside of the shear zone (Fig. 4b,e). In contrast, within the intermediate domain the object lineation istypically weakly developed and varies from SW- to NW-plunging
(Fig. 4b). It is worth noting that streaks defining SW-plunging lineationsare generally composed of coarse-grained mica whereas fine-grainedmica streaks define W to NW-plunging lineations.
Map-scale patterns of lineation trajectories show that the objectlineation in the intermediate domain of the FSZ forms a Z-shaped pat-tern; lineation is variable, changing orientation from gently/moderatelySW-plunging to moderately (W)NW-plunging toward the middle part ofthe intermediate domain (Fig. 4b). These structural trends correspondto a ca. 100° clockwise rotation of the lineation within the foliationplane moving from the marginal parts to the middle part of the inter-mediate domain (Fig. 5b; ii). This is also clearly depicted on both thepitch variation map and diagram showing that the pitch of the lineationon the Sm plane, measured clockwise from the south, increases pro-gressively toward the middle part of the intermediate domain (Figs. 4cand 5b; iii).
In the next sections, we investigate the shear sense in individualdomains presenting a detailed microtectonic kinematic analysis of theFSZ. It is noted that mesoscopic kinematic indicators are rarely
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 ofdeformation along the FSZ.
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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observed throughout the FSZ. Outside the shear zone, our structuralobservations are in accordance with previous kinematic studies sug-gesting regionally consistent NE-directed shearing (e.g., Mehl et al.,2007; Xypolias and Alsop, 2014).
4. Samples description
Due to the structural complexity of the FSZ, we focused our sam-pling on the intermediate domain, where we collected 39 orientedsamples distributed along a traverse oriented approximately perpendi-cular to the FSZ (Figs. 4a and 5b). Moreover, we collected 6
representative samples across the north domain (Figs. 4a and 5a). Theprecise geographic location of each sample is given in Appendix A(Supplementary Geospatial Data). All samples are characterized by asingle mylonitic foliation whereas microfolds are not observed. In eachsample, two perpendicular thin sections were cut - one parallel to theobject lineation and normal to Sm, and one normal to both the objectlineation and Sm.
The 45 analyzed samples fall into three lithological groups: (a)quartz veins (25 samples); (b) quartz schists (12 samples), and (c) micaschists (8 samples). The quartz veins are oriented parallel to the Sm andtheir thickness ranges from 3 to 10 cm. Typically, the quartz schists
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 inlineation plunge angle; (c) Map of the FSZ showing variation in object lineation pitch; (d) and (e) Equal-area/lower-hemisphere projections of object lineation andfoliation 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.
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consist of more than 75% quartz whereas the quartz percentage in themica schists ranges between 40% and 60%. In addition to white mica,the schists include garnet, chlorite, biotite, glaucophane, actinolite,
epidote and opaque minerals. In all samples, quartz has been affectedby extensive dynamic recrystallization (at least 80% recrystallization byarea fraction), and by only limited static annealing. In few quartz veins,large relict ribbons (> 2mm) are decorated by recrystallized grainsforming typical “core-and-mantle” microstructures. Recrystallizedquartz grains exhibit straight or slightly sutured/serrated boundariesand several of these grains display subgrains. These features reveal thatthe dynamic recrystallization occurred by progressive subgrain rotationwith a contribution of grain boundary migration (Stipp et al., 2002).
The most common kinematic indicators, in the quartz and micaschist samples, are mica fish, C- and C'-type shear bands, and occa-sionally, oblique-grain-shape fabric in pure quartz domains. Oblique-grain-shape fabrics are dominant in all quartz vein samples. To furtherconstrain kinematics we performed quartz crystallographic textureanalyses in the majority of the collected samples. In an attempt to in-vestigate the behavior of individual kinematic indicators in differentlithologies, we present the results of the kinematic analysis in the fol-lowing order: (a) the results from the mica fish and shear bands, whichare 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 intermediatedomain, mica fish and shear bands display clear asymmetry both inlineation-parallel and lineation-normal thin sections, consistent withtriclinic deformation geometry (Fig. 7a and b; Table 1). Six samplesindicate apparent monoclinic deformation denoted by asymmetric ki-nematic indicators either in lineation-parallel (2 samples) or lineation-normal (4 samples) thin sections (Fig. 7c and d; Table 1). Ambiguousasymmetry in both thin sections was observed in 5 samples (Table 1).
The 6 samples that are consistent with monoclinic deformation in-dicate either top-down-to-the-W sense of shear (lineation-parallelasymmetry) 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 inthe FSZ, we combine the kinematic data from these 6 samples with the9 samples that suggest triclinic deformation; these 15 samples are dis-tributed in different structural depths within the FSZ (Table 1). For thispurpose, we adopted the methodological approach proposed by Toyet al. (2012) called hereinafter as the Multiple Sections Technique.
A prerequisite for the application of the Multiple Sections Techniqueis the existence of a series of sections perpendicular to the foliation andat varying angles with each other. Taking into account the wide spreadin the lineation orientations observed in the intermediate domain, alarge variety of thin section orientations was achieved fulfilling theaforementioned prerequisite. The aim of this method is to identify anapparent reversal of shear sense when all sections are viewed from thesame direction. The application of this method requires rotation of in-dividual thin section planes about the strike of the mean Sm until thelatter becomes horizontal, so the kinematic data obtained from eachthin 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 aplane where a reversal of the shear sense is observed. This plane isperpendicular to the VNS. Finally, rotating back onto the mean Sm weobtain a mean shear direction towards NW (318°/36°) (Fig. 8a).
6. Oblique-grain-shape fabric
A preferred alignment of recrystallized quartz grains that is obliqueto the macroscopic Sm was observed in 19 samples distributedthroughout the FSZ; 14 are from the intermediate domain and 5 fromthe north domain (Table 2). These samples are either foliation-parallelquartz veins (16 samples), or quartz schists (3 samples) that containpure quartz domains. In microscale, the Sm is defined either by thin,
Fig. 5. Simplified cross-sections across the (a) north and (b) intermediate do-main of the FSZ; the location of cross-sections is shown in Fig. 4a. The structuralposition of the samples used for microtectonic analysis is also shown; the exactlocations of samples are given in Appendix A (Supplementary Geospatial Data).Lettered diagrams show the spatial variations in (i) the strike angle (θ) betweenthe shear zone boundary (SZB) and the mylonitic foliation (Sm), (ii) the objectlineation plunge direction, and (iii) object lineation south pitch across the FSZ.
Fig. 6. Equal-area/lower-hemisphere projections of fold axis and pole to axialplane measured in the FSZ and the wall rocks.
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often discontinuous mica layers that bound the pure quartz domains inthe schists, or by mica impurities that mainly occur as laths in thequartz veins (Fig. 9). Mica grains aligned with the quartz oblique-grain-shape fabric were not observed. Recrystallized quartz grains definingthe oblique shape fabric display a strong shape preferred orientation,
irrespective of the thin section orientation (Fig. 9). The largest re-crystallized grains may display subgrains with boundaries that are (sub-) parallel to the grain boundaries (Fig. 9a–c).
6.1. Kinematic analysis
Ten out of the 14 samples from the intermediate domain record anoblique alignment of recrystallized quartz grains in both the lineation-parallel and lineation-normal thin sections (Figs. 9c–f and 10a). In these10 samples, which are consistent with triclinic deformation geometry,the observed grain alignments represent the traces of an oblique-grain-shape fabric (Sq) on sections, which are not orthogonal to its attitude.These traces will be called as Sq' and Sq″ for lineation-parallel andlineation-normal sections, respectively (Fig. 10a). In an attempt toevaluate the shear sense in each one of these 10 samples, we applied asimple geometric method that combines the apparent angles δ′ and δ″between the Sq and the Sm recorded in lineation-parallel and lineation-normal sections, respectively (Fig. 10a). The aim of this method is toestimate the true attitude of the Sq. To determine the angles δ′ and δ″,we measured in each corresponding thin section the orientations of thelong axes of 150–300 quartz grains recrystallized oblique to the Sm. Thefrequency distribution histograms, which were constructed for thestatistical analysis of the orientation data, generally display continuouspopulations of readings (Fig. 10b; Appendix A: Supplementary Micro-tectonic Data). From each histogram, we assigned the mean and max-imum apparent angle between the grain shape fabric and the Sm;namely the δ′mean and δ′max for lineation-parallel sections and, δ″mean
and δ″max for lineation-normal sections (Fig. 10b; Table 2).Using the mean apparent angles δ′mean and δ″mean we projected the
mean grain long axis orientations Sq'(mean) and Sq″(mean) on lineation-parallel and lineation-normal section planes, respectively (Fig. 10c).
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) showingcoaxial 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 1Structural depth (d), deformation symmetry (DS) and asymmetry of mica fishand shear bands.
Sample d (m) DS Azimuth of apparent shearsense in LPS
Azimuth of apparent shearsense in LNS
S6B 21 O – –S6C 21 T 252 348S7 37 M 272 –S8 68 T 252 001S10 72 M 272 –S11 86 M – 359S12B 92 M – 009S14 109 T 315 02615 115 M – 21416 120 T 310 21417 137 T 312 02321 163 O – –S22 163 O – –S23 170 T 066 359S24 189 M – 35225 195 T 240 001S28 205 O – –S31 213 T 233 331S33 222 O – –34 230 T 250 344
Structural depths are measured perpendicular to the FSZ upper boundary; M,Monoclinic; T, Triclinic; O, Orthorhombic; LPS, lineation-parallel section; LNS,lineation-normal section.
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The plane containing the Sq'(mean) and Sq″(mean) defines the mean obliquefoliation Sq(mean). Therefore, the shear orientation is normal to the in-tersection between the Sq(mean) and the Sm while the sense of shear isinferred 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,
we applied the same procedure using the angles δ′max and δ″max. Bothangle sets gave consistent results for all the analyzed samples (Fig. 10c;Appendix A: Supplementary Microtectonic Data). The results of thisanalysis reveal that the vast majority of samples consistent with triclinicdeformation are associated with top-down-to-the-WNW shear sense toNNW-SSE dextral shearing with a small normal-sense component(Fig. 10d; Table 2).
The remaining 4 samples from the intermediate domain indicateapparent monoclinic deformation and preserve an oblique foliationvisible in either lineation-parallel (3 samples) or lineation-normal (1sample) thin sections (Fig. 9a and b; Table 2). The sense of obliquitybetween Sm and Sq in these samples indicates shear sense ranging fromtop-down-to-the-WSW to NNW-SSE dextral shearing with a smallnormal-sense component (Fig. 10d). All 5 samples from the north do-main indicate apparent monoclinic deformation. Two of these samplesthat display oblique foliation in lineation-parallel sections indicate NE-SW dextral or sinistral shearing (Fig. 10d). The remaining 3 samplesdisplay oblique foliation in lineation-normal sections and consistentlyindicate 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, definesa domainal elongation lineation resulting from the development of thequartz oblique shape fabric. This assumption is supported by the factthat the most elongated quartz grains in an oblique-grain-shape fabricform small angles with the Sm and are sub-parallel to the shear or-ientation. In samples displaying triclinic deformation, the angular de-viation, ω, of the domainal elongation from the macroscopic objectlineation, measured on the foliation plane, varies between 11° and 78°(Fig. 10a; Table 2).
Kinematic analysis was also performed applying the MultipleSections Technique (Toy et al., 2012) in order to evaluate the meansense of shear in the intermediate domain. The procedure is similar tothat for the mica fish and shear bands, described in the previous section.The method was applied using the thin sections from all samples dis-playing either monoclinic or triclinic deformation. The analysis re-vealed top-down-to-the-NW shear sense and is in full agreement withour kinematic analysis in individual samples (Figs. 8b and 10d). Due tothe limited dataset, the Multiple Sections Technique was not applied tothe north domain.
Fig. 8. Equal-area/lower-hemisphere projections showing the application pro-cedure and the results of the Multiple Sections Technique using kinematic dataof (a) mica fish/shear bands and (b) quartz oblique-grain-shape fabrics fromsamples of the intermediate domain. The inferred mean shear direction is alsoillustrated. See text for details.
Table 2Data 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
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 andδmax, 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 ofsamples 6B and 12B, respectively. (c) and (d) Oblique foliation in lineation-parallel and lineation-normal thin sections of sample 9, respectively. (e) and (f) Obliquefoliation in lineation-parallel and lineation-normal thin sections of sample 32, respectively. (g) and (h) Lineation-parallel and lineation-normal thin sections of sample16-9, respectively; oblique foliation is observed only in lineation-normal thin section. Sq' and Sq″ mark the grain long axis orientation with respect to the myloniticfoliation (Sm), in lineation-parallel and lineation-normal thin section, respectively.
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6.2. True angle of obliquity
Our analysis in samples displaying apparent triclinic deformationalso enabled us to evaluate the mean (δmean) and maximum (δmax) trueangle between the oblique-grain-shape fabric and the Sm (Fig. 10c;Table 2). Statistical analysis was also performed to assign δmean andδmax angles in samples displaying apparent monoclinic deformation(Table 2; Appendix A: Supplementary Microtectonic Data). The resultsof the 19 analyzed samples showed that δmean ranges between 26° and59° while δmax between 40° and 78° (Table 2). It is worth noting that in18 out of 19 samples, δmax is greater than 45°.
Typically, the angle δmax is controlled by the orientation of the ex-tensional instantaneous stretching axis (ISA) (e.g., Wallis, 1995;Xypolias, 2009). In this case, the recrystallized grains within an ob-lique-grain-shape fabric nucleate with their long axes parallel to the ISAand progressively rotate towards the shear plane. Alternatively, whenthe angle of obliquity is greater than 45° the fabric can be crystal-lographically controlled. Specifically, as pointed out in several studies(Lister and Snoke, 1984; Herwegh and Handy, 1998; Little et al., 2013),
oblique foliations can form at high angles (up to 70°) to the myloniticfoliation and the extension lineation if the grain boundary alignment iscontrolled by subgrain boundaries of quartz grains. In this case, thegrain boundaries should also rotate synthetically to the shear directionduring progressive deformation (Herwegh and Handy, 1998).
7. Quartz crystallographic textures
Quartz crystallographic preferred orientation (CPO) analysis wascarried out in 33 samples distributed across the intermediate domain(27 samples) and the north domain (6 samples) of the FSZ. The 25samples are foliation-parallel quartz veins whereas the remaining 8 arequartz schists. Quartz CPO analysis was performed with the followingtwo methods: (1) using a Leitz universal stage mounted on a Zeiss op-tical microscope, and (2) by means of electron backscatter diffraction(EBSD). Quartz CPOs were optically obtained from each sample onlineation-parallel thin sections; in totality 600 c-axis were measured ineach section. In 11 out of the 33 samples, quartz CPOs were also ana-lyzed by means of EBSD on polished thin sections. EBSD data were
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 domainalelongation from the macroscopic object lineation; Sq' and Sq″ mark the grain long axis orientation with respect to the mylonitic foliation (Sm), in lineation-parallel andlineation-normal section, respectively; δ′ and δ″ are the apparent angles between the oblique-grain-shape fabric and the Sm in lineation-parallel and lineation-normalsection, respectively; (b) Representative frequency histograms used to estimate the mean (δ′mean, δ″mean) and the maximum (δ′max, δ″max) apparent angles betweenthe oblique-grain-shape fabric and the Sm in lineation-parallel and lineation-normal thin-sections in sample 9; (c) Equal-area/lower-hemisphere projections sum-marizing the proposed procedure to estimate the shear direction in sample 9 using the mean (left) and maximum (right) apparent angles between the obliquefoliation and the Sm; (d) Equal-area/lower-hemisphere projections showing the inferred shear sense.
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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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acquired at the Department of Earth and Environmental Sciences atBoston College on a Tescan Vega 3 LMU scanning electron microscopeequipped with a LaB6 source, an Oxford Instruments Nordyls Max2
EBSD detector and the Aztec acquisition and analysis software suite.The c-axis data are presented on lower hemisphere, equal area projec-tions, with the plane of projection being perpendicular to the Sm(Fig. 11). The projection and contouring of the optically measured c-axis data were performed using Stereo32 software (by K. Röller andC.A. Trepmann; Ruhn-Univerität Bochum).
7.1. Quartz CPO patterns
The quartz c-axis orientations in samples that were analyzed by bothoptical and EBSD methods, show consistent patterns (Appendix A:
Supplementary Microtectonic Data). All of the obtained quartz c-axispatterns are characterized by strong c-axis maxima near or at the per-iphery of the equal-area projection diagrams. Of the 33 CPOs measured,14 define conventional patterns. With the term “conventional pattern”we refer to Type-I/II crossed-girdles, small-circle girdles and cleft-gir-dles patterns displaying orthorhombic or monoclinic symmetry (e.g.,Lister, 1977; Schmid and Casey, 1986; Law, 1990). The remaining 19display “unusual” patterns that are not symmetrical with respect to theobject lineation and are consistent with triclinic deformation (e.g.,Llana-Fúnez, 2002). Specifically, in 11 of these 19 c-axis plots, themaxima spread over the entire length or the larger part of a great circlegirdle 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“unusual” patterns resemble rotated conventional ones, as if they have
Fig. 11. (continued)
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been obtained from sections erroneously cut oblique to the shear or-ientation (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 maximadefine a great circle girdle that does not meet the center of the fabricdiagram. In the vast majority of these studies, the CPO plots were ro-tated following MacCready (1996) on interpreting such discrepancy asrecord of an incremental elongation, which does not coincide with thefield-observed object lineation. Adopting this interpretation, we rotatedthe “unusual” CPO plots about the foliation pole, in order to obtain apattern resembling a conventional one, and assuming that the vorticityvector lies in the foliation plane. We rotated the projection planeclockwise or anticlockwise until either the great circle girdle to passthrough 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 ofthe 19 samples in Fig. 11.
The 14 conventional c-axis CPO plots can be grouped into two maintypes 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-Icrossed-girdles (6B, 12B, 27, 16-6, 16-7, 16-8) (Fig. 11). A pattern re-sembling cleft-girdles was observed in one sample (14; Fig. 11). The 19rotated CPO plots are also classified into: (a) single- or variable kinkedsingle-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 patternsthat are interpreted as cleft-girdles (8, S10) (Fig. 11).
Concerning the a-axis plots, 8 out of 11 patterns were rotated fol-lowing the rotation angles from the corresponding c-axis CPO plots. In 7out of 11 CPO plots, the a-axis patterns display maxima that are spreadalong a continuous great circle normal to the c-axis maxima (Figs. 11and 12). These a-axes distributions also display a pronounced offseteither 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-tribution of partially linked maxima (15, 17; Fig. 12). In these plots,maxima form small circles, centered about the pole to the foliation, andare inclined clockwise about the object lineation. Two samples (8, 29)display a pattern of a-axes, which is transitional between a great-circleand a bimodal distribution, with weak offset anticlockwise from theobject lineation.
7.2. Sense of shear
Quartz c-axis patterns display asymmetry with respect to both theskeletal outline and density distribution in 28 out of 33 samples; 22patterns 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 ofasymmetry is variable, ranging mainly from NNE-SSW strike-slipshearing 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. 11and 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 isalso supported by the asymmetry observed in the distributions of a-axes, in the samples that the latter are available (Fig. 12).
The large variation in shear sense is spatially distributed within theFSZ. The 3D block diagram in Fig. 14 summarizes the spatial distribu-tion of shear sense in the intermediate and north domains. Specifically,the north domain is typically characterized by NE-SW dextral strike-slipshearing with a small normal or reverse component (Fig. 14). Sinistralstrike-slip shearing is restricted to the uppermost part of the northdomain (Fig. 14). In the intermediate domain, in turn, the inferredshear sense varies across the FSZ (Fig. 14). The upper part of the in-termediate domain is characterized by W-directed normal shearing toNW-directed dextral-normal shearing whereas the middle part is
dominated by SE-directed sinistral-reverse shearing. Localized NNE-SSW sinistral strike-slip shearing is recorded at the transitional zonebetween the middle and lower part of the intermediate domain. Basedon the foliation map, we infer the presence of a zone with sinistralshearing that extends northward to the uppermost part of the northdomain where similar sense of shear is also observed (Fig. 14). Thelower part of the intermediate domain is primarily characterized byNNW-SSE dextral strike-slip shearing with a small normal-sense
Fig. 12. Contoured quartz a-axis CPO plots (equal-area/lower-hemisphereprojections) obtained using electron backscatter diffraction (EBSD); r, rotatedCPO plots. The structural position of samples is shown in Fig. 5.
Fig. 13. Equal-area/lower-hemisphere projections showing the shear sense in-ferred from quartz crystallographic textures.
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component. The W-directed normal shearing is locally observed in thelower part of the intermediate domain, similar to the upper inter-mediate domain.
7.3. Strain symmetry
A correlation between the 3D strain symmetry and the pattern ofquartz c- and a-axis distributions is suggested by theoretical studies(Lister and Hobbs, 1980; Schmid and Casey, 1986) and supported bypetrofabric studies coupled with strain analyses in naturally deformedrocks (e.g., Law et al., 2010; Xypolias et al., 2010). By analogy to thesestudies, the small-circle girdles to transitional Type-I crossed-girdlespatterns recorded in 12 samples are interpreted to signify generalflattening (k < 1) to plane-strain (k=1) conditions. Single or variablekinked single-girdle patterns recorded in 18 samples indicate approx-imate plane-strain deformation, although the tendency in some samples(11, S11, 15, 22, 30, 16-9) toward a transitional pattern between Type-Icrossed-girdles and small-girdles could indicate slightly flatteningstrain. Evidence for constrictional strain is restricted to three samplesthat are characterized by cleft-girdle patterns (Sullivan and Beane,2010; Xypolias et al., 2013).
8. Grain shape analysis
Throughout the FSZ, quartz has been affected by extensive dynamicrecrystallization and, thus, the shape of the most deformed grains hasbeen restored to a more equant form (e.g., Law, 1986). Consequently,finite-strain analysis using quartz as strain marker is expected, by de-finition, to underestimate significantly the principal strain ratios.However, several studies (e.g., Tagami and Takeshita, 1998; Strine andWojtal, 2004; Xypolias et al., 2013) have shown that 3D shape analysisof recrystallized grains can record the true strain symmetry, in terms ofthe Flinn parameter (k), at lower strain magnitude (Nadai octahedralshear strain). Furthermore, microscopic analysis shows that the extentof dynamic recrystallization appears to decrease in quartz grains that
are embedded in mica-rich layers (Fig. 15a and b) (see Xypolias et al.,2007, 2013 for similar cases). Therefore, we performed 3D quartz grainshape analysis on 13 mica and quartz schist samples from the inter-mediate domain, assuming that the analysis of these samples may yieldmore accurate 3D strain estimates than the analysis of pure quartzsamples. 3D grain shape analysis was also carried out in pure quartzdomains of 6 samples, in order to compare the strain symmetry esti-mates in mica-free and mica-rich domains. Note that the analysis wasperformed in domains where the quartz grains do not display oblique-grain-shape fabric.
Grain shape analysis was conducted using Rf-φ data collected fromlineation-parallel and lineation-normal thin sections of the 19 samples.In each section the traces of at least 70–80 grain outlines were inputinto the software SAPE (Mulchrone et al., 2005) that automaticallyapproximates grain shapes as ellipses and extracts Rf-φ data. The ex-tracted data for each section were analyzed using the theta-curvemethod of Lisle (1985) while the calculation was made utilizing thecomputer-based approaches of Mulchrone and Meere (2001). The re-sults are presented in Appendix A (Supplementary Microtectonic Data)and are plotted on a Flinn diagram (Fig. 15c). 3D quartz grain shapeanalysis in both mica-rich and mica-free domains yielded similar strainsymmetry estimates (Fig. 15c). Specifically, as illustrated in the Flinndiagram, most of the data points (18 samples) fall within the field ofapparent flattening with k-values ranging between 0.11 and 0.93. Onesample falls within the field of apparent constrictional (k= 1.72)(Fig. 15c).
9. Discussion
The formation of the FSZ is the result of localization of deformationin the short limb of an inclined map-scale antiform, which was gener-ated by dextral differential shearing induced by flow perturbationduring NE-directed shearing (Xypolias and Alsop, 2014). Several stu-dies (e.g., Alsop and Holdsworth, 2007), in areas where models of flowperturbations have been applied, have shown that differential shear isbroadly distributed rather than localized along discrete strike-slipdominated shears. Therefore, the FSZ represents a natural exampleshowing that flow perturbation folding can be associated with strainlocalization on the flanks of culmination and depression structures. Apotential mechanism to account for this strain localization could be therheological contrast induced by various bounding lithologies to the FSZ(e.g., serpentinites and epidotites versus the quartz-mica metapeliticrocks).
9.1. Kinematic analysis based on field data – telling part of the story
Outside the FSZ, the recorded NE-to N-striking axial planar foliationbears a stretching lineation that consistently plunges shallowly towardsSW and is associated with NE-directed shearing and, thus, with a NNW-plunging vorticity vector (Figs. 4e and 16a: 1). Strain localization in theFSZ was associated with progressive intensification of this initial axialplanar foliation. Therefore, the observed deformation pattern outsidethe FSZ should reflect the structural configuration before the formationof the shear zone. Within the FSZ, the macroscopic object lineationdisplays a large orientation dispersion along the mean foliation whereasthe object lineation trajectories show a Z-shaped pattern in map-view(Figs. 4a and 16a: 2i). This pattern is attributed to the operation of theFSZ. Based exclusively on field observations, we could interpret the FSZto have formed in (N)NE-directed dextral transpression, as indicated by(a) dextral sigmoidal pattern of the foliation trajectories in map-view;(b) smaller than 35° obliquity between the shear zone boundaries andthe Sm; and (c) steeply NW-plunging intersection between the mean Smand the shear zone boundaries. This intersection should approximatethe vorticity vector in the FSZ and lies at a small angle to the vectorrecorded in the wall rocks. On the basis of the observed foliation andlineation pattern, the FSZ could be interpreted as a triclinic
Fig. 14. 3D block diagram summarizing the spatial variation of the shear senseinferred from quartz crystallographic textures in the FSZ. Equal-area/lower-hemisphere projections showing the mean shear sense in the north domain aswell as in different structural parts of the intermediate domain. SZB, Shear ZoneBoundary.
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transpressional zone with shear direction slightly oblique to the initialmonoclinic fabrics. The NE-directed dextral shearing localized alongminor, semi-ductile shear zones, which formed during the very latestages of the FSZ operation (Figs. 4f and 16a: 3).
9.2. Kinematic analysis based on both field data and microstructures/fabrics with different strain memories – the full story
The integration of field-based observations with microstructural andCPO data reveals a more complex kinematic history of the FSZ, com-pared to the one described above by taking into account the field dataonly (compare the upper and lower row of Fig. 16a). Analysis of quartzc-axis textures and oblique foliations reveal the occurrence of incre-mental elongations, which vary in orientation from SW to NNE andfrom W to NNW, respectively. These observations in combination withthe large dispersion in orientation of the macroscopic object lineation,which ranges from SSW to NW, indicate that the true dispersion inelongation directions is over an entire great circle girdle (Fig. 16b).
The macroscopic object lineation is primarily defined by micastreaks and secondarily by quartz rods. In sites where the objectlineation was measured, the grain size of mica is sufficiently large to bevisible to the naked eye, revealing that the mica has not been affectedby extensive recrystallization. In turn, the incremental maximumelongations obtained from both oblique-grain- shape fabric and quartzc-axis textures are based on recrystallized quartz grains and, hence,these elongations should reflect younger increment(s) of deformationcompared to that recorded from the object lineation. Therefore, it isreasonable to assume that the maximum elongation is rotated from aninitial SW (regional object lineation trend) to a more W to NW trend.This rotation should be accompanied by recrystallization of micaforming a more NW-plunging lineation, which is likely visible only atthe microscopic scale (for a similar case see Toy et al., 2012, 2013).This assumption is supported by the fact that the SW-plunging linea-tions are more clearly visible than the WSW to NW-plunging objectlineations.
Incremental elongations inferred from quartz c-axis textures displaylarger scattering in orientation and larger overlap with the objectlineation trend compared to the incremental elongations inferred fromoblique-grain-shape fabrics (Fig. 16b). This difference likely reflects thedifferent degrees of sensitivity of fabrics in response to temporalchanges to the imposed kinematic framework. This is in accord withmany studies (Law, 1986; Hippertt and Borba, 1992; Hongn andHippertt, 2001; Wallis, 1995; Herwegh and Handy, 1998; Xypolias,2010) suggesting that the oblique-grain-shape fabric is an in-stantaneous sensitive feature, whereas the bulk quartz c-axis textures
retain information for a relatively longer part of the deformation his-tory and survive late-stage changes. On the basis of this suggestion, weobserve an overall rotation of the incremental elongations inferred fromquartz c-axis textures and oblique-grain-shape fabrics toward a NWtrend. In individual samples, we observe either a clockwise rotation ofthe incremental elongations from a mean WNW to a mean NW trend, oran anticlockwise rotation from a mean SSW to a mean NW trend(Fig. 16b).
Fig. 16a (stages 2i-iii) illustrates structural maps based on macro-scopic object lineation, as well as on incremental lineation and shearingrecorded by quartz c-axis textures and oblique foliation analyses. Forthe above-mentioned reasons, these maps likely depict successive in-crements of the FSZ deformation. The incremental elongation mapobtained from quartz c-axis texture data indicates that the Z-shapedpattern recorded by object lineation progressively tightens and ismodified by the zone of localized sinistral shearing, which extendedfrom the uppermost part of the north domain to the bottom of themiddle part of the intermediate domain (Figs. 14 and 16a: 2ii). Struc-turally above this zone, the trajectories are subjected to clockwise ro-tation whereas beneath it, anticlockwise rotation occurs. Based on theincremental elongation map obtained from oblique-grain-shape fabrics,it seems that the trajectories are further rotated anticlockwise in thelower part of the intermediate domain and north domain, whereas theyare slightly rotated clockwise in the remaining part of the intermediatedomain. These rotations form a fan-shaped trajectory pattern definedby west (intermediate domain) to NNE (north domain) elongationtrends with a mean NW-SE trend (Fig. 16a: 2iii).
The NW-trending elongation is associated with NW-directed dextralshearing. NW-directed shearing is penetrative and mainly supported bymica fish and shear bands in addition to the oblique-grain-shape fabricswhile is associated with the formation of minor NW-plunging folds. Theprogressive rotation from the initial SW-trending to NW-trendingelongation is characterized by a complex kinematic picture includinglocalized sinistral shearing, which fades significantly at the last stage(Figs. 14 and 16a: 2ii, 2iii). This complex kinematic pattern is also re-corded in the quartz c-axis textures that show large dispersion in theshear direction from NE to NW (Figs. 13 and 16a: 2ii).
Summarizing, our analysis shows that the FSZ displays character-istics for (a) NE-directed dextral ductile shearing inferred by field-baseddata and partly by CPO data, (b) NW-directed dextral ductile shearingsupported by both microstructural and CPO data, and (c) NE-directedshearing at semi-ductile conditions as indicated by minor shear zones(Fig. 16a). Therefore, we posit that the FSZ commenced as a NE-di-rected dextral ductile shear zone, continued as NW-directed dextralductile shear zone and ended up, at semi-ductile conditions, with
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 theresults of quartz grain shape analysis. EFP, foliation-parallel ellipticity; ELN, lineation-normal ellipticity.
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localized NE-directed dextral shearing. Consequently, the FSZ is char-acterized by complex kinematics rather than by exclusively NE-directedshearing as it is inferred when only field observations are taken intoaccount. We also suggest that the observed lineation orientation dis-persion should be attributed to the progressive shift from NE-to NW-directed dextral shearing.
9.3. Thinning or thickening deformation?
The NE-directed dextral shearing is associated with transpression,however, the important question for understanding the FSZ evolution iswhether progressive shift from NE-to NW-directed dextral ductileshearing is associated with thinning or thickening of the zone. Thisambiguity stems from the fact that some of the recorded deformationfabrics and textures display conflicting characteristics that could be
Fig. 16. (a) Structural maps based on (1) pre-shear-zone elements, (2i) the macroscopic object lineation, (2ii) the incremental elongation and shearing inferred fromquartz 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 elongationdispersion and the clockwise or anticlockwise rotation from the incremental elongation inferred from quartz c-axis textures to the elongation inferred from quartzoblique-grain-shape fabrics.
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interpreted either by transpressional or transtensional deformation.Specifically, in a high-strain transpressional zone, the mylonitic folia-tion is expected to be sub-parallel to the shear zone boundaries as ob-served in the north domain rather than at an angle of 30° or higher asobserved in the intermediate domain of the FSZ (Figs. 4a and 5b). Apossible explanation could be that local (intermediate domain) trans-tensional shearing increased the obliquity between the Sm and the shearzone boundary. The oblique-grain-shape fabrics also display conflictingcharacteristics. Specifically, if the recrystallized quartz grains nucleatein the direction of ISA, as commonly assumed (Xypolias, 2009 and re-ferences therein), then the observed unusually high angles between theoblique-grain-shape fabric and the shear zone boundaries are consistentwith NW-directed transtensional shearing. However, oblique foliation isat high angle also with the Sm, a feature that is not expected in atranstensional zone. These conflicting indications could be interpretedwith a shift from transpressional to transtensional deformation duringthe last stages of deformation. Alternatively, oblique foliation at highangles with Sm can be formed in a thinning zone if grain boundaryalignment is controlled by the subgrain boundaries of quartz grains(Lister and Snoke, 1984). In several samples displaying oblique folia-tion, the boundaries of the recrystallized grains are sub-parallel tosubgrain boundaries and, hence, this alternative mechanism cannot beexcluded (Fig. 9a–c). Moreover, apparent flattening strain inferred byboth quartz crystallographic texture and grain-shape analyses is domi-nant in the FSZ revealing transpressional deformation. In contrast, thelocally observed constrictional strain, which is expressed by quartz c-axis cleft-girdles pattern associated with NW-directed shearing, iscompatible with transtensional deformation.
Taking into account all the aforementioned observations, we assumethat the progressive shift from NE-to NW-directed dextral ductileshearing was accompanied by thinning of the FSZ whilst local thick-ening likely occurred during the last stages of the ductile deformation.At semi-ductile conditions, the FSZ passed to a stage of localized thin-ning as indicated by minor NE-directed shear zones. A possible ex-planation for this complex kinematic history is that the FSZ served as anoblique ramp during flow perturbation folding. If so, dextral differentialshearing induced by flow perturbation should be accommodated byoverall transpressional deformation alternating with brief periods oflocalized transtension.
10. Conclusions
The Fellos Shear Zone (FSZ) is a NE-striking, dextral transpressionalzone that was formed in the short limb of a map-scale antiform. It re-presents the first reported natural example showing that flow pertur-bation folding can be associated with strain localization on the flanks ofculmination and depression structures. Within the FSZ, the macroscopicobject lineation displays a large dispersion in orientation defining ap-proximately a half great circle girdle (angular range of 110°) along themean mylonitic foliation. Based only on field-based data (foliation andlineation pattern), the FSZ could be interpreted as a NE-directed tri-clinic transpressional zone. However, the integration of field-based withmicrotectonic data reveal a much more complex kinematic evolution.Our analysis reveals a misalignment of the macroscopic object lineationwith the incremental elongation directions inferred from quartz ob-lique-grain-shape fabrics and quartz crystallographic textures, whichare recorded in fully recrystallized aggregates rather than reorientedpre-shear-zone elements. The combination of object lineation and in-cremental elongation data reveals that the true elongation scattering inthe FSZ defines the entire length of a great circle girdle. Based on theobserved variation in the incremental shear direction, which is parallelwith the maximum incremental elongation, we posit that the FSZcommenced as a NE-directed dextral transpressional zone, and pro-gressively evolved into a NW-directed dextral zone. The transition fromNE-to NW-directed shearing was accompanied by transpression whilstlocal transtension likely occurred during the last stages of the ductile
deformation. Deformation in the FSZ ended up, at semi-ductile condi-tions, with localized NE-directed dextral shearing.
The natural example of the FSZ shows that the integration of fieldobservations with fabrics/microstructures that have different strainmemories is a powerful tool to unravel kinematics of complex high-strain zones.
Acknowledgements
Constructive review comments by the journal editor Toru Takeshitaas well as by Bill Sullivan and an anonymous reviewer were helpful inclarifying important aspects of this manuscript and are gratefully ac-knowledged. We greatly appreciated insightful discussions with BasilTikoff in the field. Fieldwork for this study was supported by GrantC.924 (awarded to P. Xypolias) from the Research Committee of theUniversity of Patras (Programme K. Karatheodori). Research funds fromBoston College to S.C. Kruckenberg are acknowledged for supportingthe EBSD analytical results presented in this study.
Appendix A. Supplementary data
Supplementary data related to this article can be found at https://doi.org/10.1016/j.jsg.2018.07.004.
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