Multiphase Boudinage: a case study of Amphibolites in Marble in the Naxos Migmatite core

In multiply deformed terrains, multiphase boudinage should be common, but identification and analysis of these is difficult. Here we present an analysis of multiphase boudinage and fold structures in deformed amphibolite layers in marble from the migmatitic center of the Naxos metamorphic core complex. Reconstruction of multiple boudinage generations is possible due to the exceptional 3D outcrop conditions. We identify five distinct generations of boudinage, reflecting the transition 10 from high-strain high-temperature ductile deformation to medium to low strain brittle boudins on the retrograde path during cooling and exhumation. All boudin generations indicate E-W horizontal shortening and variable directions of bedding parallel extension, evolving from subvertical extension in the earliest boudins to subhorizontal N-S extension during exhumation. Two phases of E-W shortening can be inferred, indicating E-W shortening in the Aegean before activity of the brittle North Anatolian Fault. This study highlights the wealth of information that can be gained from detailed analysis of 15 multiphase boudinage structures.


Introduction
Boudins are periodic deformation structures that form in mechanically layered rocks under layer parallel extension (Ramberg, 1955). Since their earliest description (Harker, 1889;Lohest, 1909;Ramsay, 1881) and initial confusion in 20 nomenclature of mullions and boudins, (c.f. Kenis et al., 2005;Kenis and Sintubin 2007) our understanding of these structures has greatly improved, and a taxonomy was established to describe the diversity of observed boudin structures (Goscombe et al., 2004). Although boudins are often depicted in 2-dimensions it has been recognized that in 3D boudins can be complex (Zulauf et al., 2011b). This complexity can be apparent when boudins are the result of more than one deformation event. Some multiphase structures such as mullions or bone boudins are indicative for a certain sequence of 25 deformation (Kenis et al., 2005;Maeder et al., 2009). Similarly, Chocolate Tablet Boudins form by two phases of extension of layers in different directions (Abe and Urai, 2012;Zulauf et al., 2011aZulauf et al., , 2011b, and have been used in some studies for the reconstruction of the deformation history of rocks (Casey et al., 1983;Reber et al., 2010). However, in the majority of studies of boudinage, examples of the different deformation events that could have resulted in a particular geometry are less obvious (Goscombe et al., 2004). Considering the number of studies that have utilized crosscutting shear zones, veins or 30 superimposed folds for the reconstruction of multiply deformed domains, the use of polyphase boudinage for this purpose is yet not well developed. We propose that their ubiquity and rich structure (Goscombe and Passchier, 2003;Ingram and Urai, 1999;Maeder et al., 2009;Schmalholz et al., 2008;Schmalholz and Maeder, 2012;Van Noten and Sintubin, 2010;Zulauf and Zulauf, 2005) with a large spectrum of deformation mechanisms, from brittle-ductile rupture to ductile localization provide the potential for a new toolbox in structural geology . 35 In this study we identify five different boudinage phases in outcrop, and each phase provides insights into the rheology and metamorphic grade of formation. Based on earlier work on boudinaged pegmatite dikes in high grade marble of the migmatite core from the Naxos metamorphic dome (Schenk et al., 2007), we focus in this study on polyphase deformation Solid Earth Discuss., https://doi.org /10.5194/se-2017-91 Manuscript under review for journal Solid Earth Discussion started: 23 August 2017 c Author(s) 2017. CC BY 4.0 License. structures in amphibolite layers in these marbles. Since the amphibolites are older than the pegmatites, they record a longer deformation history, which is reflected by more complex boudinage structures. Combining field data with microstructural analysis we link the different phases of deformation to metamorphic conditions on the retrograde path. In addition to insights into the rheological and deformational evolution of the lower metamorphic levels of the Aegean metamorphic core complex, this study may help to constrain the nature of East-West shortening of the domain, recognized by several authors (Urai et al., 5 1990; Ring and Layer, 2003;Jolivet and Brun, 2010;Hinsbergen and Schmid, 2012;Menant et al., 2013;Malandri et al., 2017) .

Geological Setting
The island of Naxos is located in the center of the Aegean Sea. This region has experienced back-arc-extension since the Eocene due to retreat of the Hellenic subduction zone towards the southwest (Gautier et al., 1999;Grasemann et al., 2011;10 Jolivet and Brun, 2010). This long-lived slab roll back makes the region one of the prime targets for understanding subduction dynamics and associated exhumation of HP and UHP rocks, as well as for understanding the evolution of metamorphic core complexes. The Cycladic islands have all undergone complex metamorphic evolution related to a lithosphere scale detachment system. The main detachment system (North Cycladic Detachment System) can be followed across the entire Aegean Sea, but is interrupted by the Mid-Cycladic Lineament north of Paros and Naxos (Walcott and 15 White, 1998b).
Naxos is the largest of the Aegean islands and a prominent example of a metamorphic core complex (Lister et al., 1984).
Situated in the footwall of the North Cycladic detachment system and exposing a rich variety in lithologies and metamorphic facies, Naxos has been the subject of multiple studies, and plays a key role for the understanding of the tectonic history of the Aegean domain (Buick, 1991;Gautier et al., 1993;Jansen, 1973;Jansen and Schuiling, 1976;Jolivet et al., 1994Jolivet et al., , 2004Jolivet et al., , 20 2010Keay et al., 2001;Urai et al., 1990).
The geology of Naxos is dominated by a N-S trending elliptical structural and metamorphic dome, comprised of marble, metavolcanics, metabauxites and metapelitic schists of Permo-Triassic age (Andriessen et al., 1979) (Fig. 1).
The present-day metamorphic facies distribution is the result of at least three distinct metamorphic phases associated with Alpine deformation (Urai et al., 1990). The first metamorphic event at high pressure-low-temperature conditions is 25 associated with top to the south shearing in the Hellenic subduction zone (D1), and formation of the Hellenides at approximately 50 Ma (Andriessen et al., 1979;Dürr et al., 1978;Urai et al., 1990;Wijbrans and McDougall, 1988).
Large scale isoclinal folds and other deformation structures formed during D1 were overprinted by top to the north shearing of D2, starting at the onset of crustal thinning and post-orogenic extension in the early Miocene (Mercier et al., 1976;Ring et al., 2010;Urai et al., 1990). As a consequence of crustal thinning and extension of the Aegaen domain the rocks of Naxos 30 were overprinted by regional scale greenschist metamorphism at ~25 Ma (M2 a ), and more localized high temperature -low pressure metamorphism (M2 b ) with thermal dome formation and partial anatexis at ~16 Ma (Duchene et al., 2006;Keay et al., 2001;Martin et al., 2006Martin et al., , 2008. The metamorphic grade of M2 b decreases from a high grade migmatitic core down to greenschist facies along its fringes (Jansen and Schuiling, 1976). The estimated 50-70km top-to-North shearing of the North Cycladic detachment (Brichau et al., 2006;Jolivet et al., 2004Jolivet et al., , 2010 is reflected by a pervasive N-S lineation and shallowly 35 dipping foliation warping around the metamorphic dome. On larger scale, N-S trending tight and isoclinal, coaxially refolded folds (B1&B2) with fold axes commonly parallelized to the N-S lineation during shearing. A late stage of upright open folds (B3) deform the axial plane foliation of B1 and B2 (Buick, 1991;Jansen, 1977;Urai et al., 1990), indicating E-W shortening. On the smaller scale D2 deformation manifests in a large variety of structures in the various metamorphic grades and lithologies. In recent years, several studies have addressed deformation structures in the high grade magmatic core, and 40 it could be shown that structures in the migmatite dome formed through viscoplastic flow in syn-migmatic M2b conditions (Kruckenberg et al., 2010Rey et al., 2011). In the later stages of the M2b metamorphism, pegmatite dikes intruded Solid Earth Discuss., https://doi.org /10.5194/se-2017-91 Manuscript under review for journal Solid Earth Discussion started: 23 August 2017 c Author(s) 2017. CC BY 4.0 License. the marble (e.g. Buick 1991). The complex tectonic history is reflected in U-Pb ages from zircon in amphibolites, which show inherited old cores and two phases of later overgrowth at 40 and 15-14 Ma, respectively (Bolhar et al., 2016). These ages may be interpreted either as early intrusion, during ocenazitation of the Aegean, or as late emplacement during largescale extension (Bolhar et al. 2016). Zircon and apatite fission track data from all tectonic units exposed on Naxos as well as U-Pb ages show a consistent cooling and exhumation scenario: cooling started at ~25 Ma, and continued until ~ 8 Ma 5 (Seward et al. 2009, Brichau et al., 2006, John and Howard, 1995. Structures in the central migmatite dome have been analyzed in the field and several syn-migmatic subdomes were identified (Vanderhaeghe, 2004) (Fig. 2) . Within the mantling sequence around the migmatite dome, mylonitic shear zones indicate relative upward movement of the migmatite while progressive transposition and folding of granitic dikes result from vertical flattening and top to-the-NNE shearing during dome formation (Vanderhaeghe, 2004). Outcrop-scale mapping and analysis 10 of the magnetic fabric of the internal structure of the migmatite revealed two compartments of subdomes separated by a N-S trending high strain zone, which contains km-scale rafts of marble folded into pinched synforms (Kruckenberg et al. (2010, Figure 1). The authors conclude that an interplay between buoyancy and isostasy-dominated flow is responsible for the complex internal structure of the migmatite, and dome formation was driven by upper crustal extension and shearing with coeval lower crustal E-W convergent flow. 15 This paper builds on a case study which focused on deformed pegmatite dikes within marbles from the high strain zone mentioned above (Schenk et al., 2007), showing how pegmatites are affected by brittle domino boudinage. Schenk et al. (2007) proposed that Mode-I interboudin fractures formed at lithostatic fluid pressure conditions after solidification of the pegmatite. Decreasing pore fluid pressures under continuing N-S extension of the pegmatite inhibited further opening mode deformation in the inter-boudin zones and consequently the boudin blocks rotated during ongoing deformation of the 20 surrounding marble. In this study we explore the polyphase deformation recorded in boudinaged amphibolite layers in these marbles. In addition to blocky boudinage which also occurs in the pegmatites, the amphibolite layers record higher temperature deformation, as witnessed by pinch and swell boudins. These structures have as of now not received much attention, but, as they record multiple deformation events, may provide insights into the tectonic evolution of Naxos and the Aegean. 25

Methods
The field data presented in this study was collected in active and recently abandoned marble quarries in the central hills of Naxos, in the vicinity of Kinidaros (Fig. 2). As a result of the mining activities outcrop conditions are exceptional, with large and unweathered vertical and horizontal saw cut surfaces (Fig. 3). As compared to the often heavily weathered natural outcrops of the island with only limited 3D exposure, the quarries allow for extensive high quality observations. 30 Orientation data was collected in situ using a Breithaupt structural compass. We use cross cutting and interaction relationships to establish a relative temporal order of the structural elements. Parts of the age relationship observations were conducted in mined blocks in the quarry. The exact origin of these blocks in the quarry is not known. However, the orientation of the blocks can be constrained, as the faces are always cut either subvertically or subhorizontally, and the orientation of the amphibolite layers and most structures was found to be consistent throughout the quarry. We used these 35 blocks for structural analysis on five sides of the block and for sampling for microstructural analysis in plane polarized and crossed polarized transmitted light using the Virtual Petrography (ViP) microscope (Virgo et al., 2016).

Results
We have identified six generations of outcrop scale deformation structures in the study area (Fig. 3, 4): two generations of pinch-and-swell boudins (Fig. 5), folds (Fig. 6) Figure 7). In addition to these, the marbles are affected by late deformation structures such as rare mylonitic shear zones and brittle normal-to oblique slip faults (Fig. 5c) that have been described in detail in the marble (Schenk et al., 2005) and surrounding lithologies (Buick, 1991;Cao et al., 2017;Vanderhaeghe, 2004).
In the following we give a short overview of the bulk properties and microstructure of the marble and amphibolite before we describe the different generations of boudins and folds in more detail. For the description of the boudin structural elements 5 and geometries we follow the terminology of Goscombe et al. (2004).

Marble
The marbles of the migmatite core are characterized by their mineralogical purity and large grain size. The bulk grain size of the marble in the study area is the largest observable on the island with grain diameters up to 20 mm (Covey-Crump and Rutter, 1989;Ebert et al., 2009). 10 The marbles are diffusely banded with band thicknesses of lighter and darker gray marble ranging from few centimeters up to 1.5 meters. White bands of high purity are often found adjacent to amphibolites. Around some amphibolites an orange to red iron staining of the marble from iron oxides can be observed predominantly along grain boundaries.
The bulk marble appears to have an isotropic texture; however, the quarrymen report a N-S trending vertical preferred fracturing orientation of the marble in mining operations that might indicate a grain scale anisotropy and a foliation normal 15 CPO was reported by Ebert et al. (2009).
The dominant marble microstructure is characteristic for dynamic recrystallization at high temperature ( Fig. 8): amoeboid grains with lobate grain boundaries and orientation families indicating extensive strain induced grain boundary migration. In triple junctions, angles between grain boundaries are typically at around 120 degrees. Subgrain boundaries are less common, indicating subgrain rotation. 20 The overall grain size distribution is heterogeneous and bimodal: The majority of grains are between 300 and 600 µm in 2D sections. Exceptionally large grains (>10mm) which occur locally in some layers are interpreted to have formed by exaggerated grain growth during dynamic recrystallization.
The vast majority of calcite grains in the marble show twinning, undulose extinction and local grain boundary migration recrystallization (in the boudin necks of domino boudins, see below) to much smaller grain size as a consequence of low-25 temperature deformation overprint.

Amphibolite
Amphibolite layers are by far the most common inclusion in the marbles; pegmatite dikes and bodies of felsic schists are less frequent. In the study area amphibolite layers consistently strike N-S with dips from close to vertical in some quarries and less steeply (~60°) to the East in others (Location 10-15 and 32-35, Figure 2). Spacing of the layers is in the range of tens of 30 centimeters to several meters and their thickness is usually less than 20 cm. The bulk mineralogy of the amphibolites consists of mostly hornblende (tschermakite) and plagioclase (andesine) with muscovite, biotite, and accessorial titanite (Schenk et al., 2007). The interface of marble and amphibolite is well defined. Some layers show internal layering at the scale of a few centimeters caused by variation of plagioclase content. The foliation which is sometimes present in the amphibolites (for example in the hinges of folds) results from compositional variation and not from preferred orientation. In 35 all thin sections the amphibolite has an isotropic texture, even in zones where high strains can be inferred (e.g. in fold hinges and in the pinches of pinch-and-swell boudins). We interpret this as an indication for static recrystallization of the amphibolite after a high strain deformation, erasing the fabric of earlier dynamic recrystallization by static grain growth (C. Passchier, personal communication). Indicators for some low strain deformation of the bulk amphibolite are minor undulouse extinction in feldspars and amphibole grains fractured along their cleavage planes. Nearly all amphibolite layers are boudinaged and at least five different types of boudins can be distinguished based on outcrop and thin section which are described in detail below. It is interesting to note that very thin layers of amphibolite (<5mm) only show discernible boudinage if they are very close to a thicker amphibolite layer to form harmonic boudins. If the distance is high enough, very thin amphibolite layers do not boudinage but act as passive deformation marker horizons in the marble (Fig. 7e). 5

Long wavelength pinch-and-swell boudins
The generation that is overprinted by all other structures is long wavelength pinch and swell boudinage (lλ). lλ can only be found in layers thicker than ~5 cm and due to the long wavelength they can best be observed and measured at large N-S trending layer parallel cuts that expose the amphibolite layer across a large area (Fig. 3c, 9). The thickness of the amphibolite is reduced to locally less than a millimeter in the pinches, but a full disruption of the amphibolite (tapering boudins) was 10 never observed. The amphibolite exhibits an isotropic texture in both pinches and swells despite considerable strain (this indicates static recrystallization of the amphibolite after this deformation phase). The trace of the boudins (the lineation of the boudin necks (L b ), Goscombe et al., 2004) is consistently sub horizontal throughout the study area, indicating vertical extension of the layers. Wavelength λ of the boudinage correlates with amphibolite thickness and can reach several meters for amphibolites with a thickness of a few decimeters. 15 Estimated from the length of the pinches (M') and the thickness difference between pinches and swells (W/W'), the amount of extensional strain is at least 100%, probably several hundred percent.

Short wavelength pinch-and-swell boudins
Pinch and swell boudins with a much shorter wavelength (sλ) overprint lλ . In many layers sλ is only developed in the lλ 20 pinches. However, clear age relationships can be observed where sλ boudins are present and well exposed in the swells as well as the high-strain pinches of lλ: in both sλ boudin necks have the same orientation, showing that sλ is clearly younger ( Fig. 9). Within the same layer the wavelength of this generation is commonly less than half the wavelength of the previous lλ pinch and swell boudins. The lineation of boudin necks (L b ) is dipping shallowly to the south (20°-45°) and intersects with the local lλ boudin necks at an angle of >20°. 25 The swells of sλ are mostly symmetric with convex exterior planes and can be classified as drawn boudins and tapering boudins following the classification scheme by Goscombe et al. (2004). Occasionally fish mouth terminations are found ( Fig. 10c). Neither in thinned out pinches nor in strongly deformed parts of the swells we were able to identify markers for dynamic recrystallization in the amphibolite, indicating that this generation also predates the static recrystallization of the amphibolite. sλ boudins are in many cases associated with biotite that can often be found enriched in the pinches, wrapping 30 around the outer perimeter of the boudins. The amount of extensional strain by short wavelength pinch-and-swell boudinage is with an estimated minimum of 50% considerably lower than that of the previous generation.
The general appearance of sλ boudins suggest a ductile deformation of both amphibolite and marble. However, associated with some boudins of sλ we found structures which may indicate deformation at lower temperatures: Some highly thinned out pinches show a higher fraction of calcite grains in the amphibolite, up to the point that grains of 35 amphibole and plagioclase are fully isolated in the marble matrix. The isolated grains have a grainsize comparable with the statically recrystallized amphibolite in the swells, which is interpreted as a granular flow deformation of the amphibolite postdating static recrystallization (Fig. 8d). This granular flow could have occurred in the final stages of sλ, or, during the domino boudin phase (see below) when amphibolite layers already thinned in the lλ, and sλ, were more susceptible to Furthermore, some boudins of this generation that appear as symmetric drawn boudins on first sight, exhibit localized low angle shear zones in the interboudin zones. The shear zones extend into the surrounding marble at ~30° to the layer with a microstructure analogous to domino boudins (see below).
Whether these structures have formed in the late stage of sλ or are a much younger overprint could not be resolved.

Folds 5
In many locations amphibolite layers are isoclinally folded, in asymmetric folds that occur in both S and Z chirality (both looking down or horizontally) (Fig. 6). Their age relationship with the pinch and swell structures is not fully resolved, but they are crosscut and displaced by domino boudins, pegmatite dikes (compare Figure 3A  The fold axes have variable plunge to north or south. Normal to the fold axis the asymmetric folds rarely have limbs longer 10 than 2 m. In contrast, parallel to the fold axis they extend for several tens of meters, often over multiple quarry soles. The thickness of the amphibolite in the fold hinges is strongly increased relative to the limbs. The hinges are in some locations characterized by a weak foliation fan with statically recrystallized amphibolite and thin calcite veins (Fig. 10). The foliation is often highlighted by cuspate-lobate amphibolite -marble interfaces. The cuspate-lobate structures may be the result of interface buckling during folding (Biot, 1964), however calcite inclusions in the extension of cusps in some folds suggest 15 that they formed by shortening of a preexisting boudin generation subnormal to the shortening axis of the fold (Fig. 10d). In the field, fold hinges are often associated with an increased occurrence of biotite. In the thin sections, cuspate-lobate structures sometimes show actinolitic rims with a shape preferred orientation parallel to the interface, while the amphibolite in the cusps has an, statically recrystallized texture (Fig. 10b).
A rare variety of small scale folds are concentric open folds, these were only encountered in a single non-in situ block of 20 marble close to Location 2 (Fig. 6a).

Brittle boudins
Brittle boudins are characterized by sharply localized fractures in the amphibolite. Due to the generally steep orientation of the brittle fractures they are much more obvious on the quarry floor then on vertical sections (Fig. 4, 7).
We differentiate three types, based on the failure mode, relative age and characteristics of the fractures: domino boudins, 25 torn boudins and hairline veins.

Domino Boudins
Domino boudins occur in many amphibolite layers of the study area but by far not in all. They are characterized by asymmetric, rotated boudin blocks and sharply defined shear fractures. The shear fractures cut the amphibolite layers at an angle θranging from 60°-90° (defined as the angle between the fracture and boudin interface, see Goscombe et al., 2004). 30 On both sides of the shear fractures in the amphibolite, thin shear zones extend into the surrounding marble as zones with sharp slip planes and dramatically reduced grain size (Fig. 11, 12). With increasing distance from the amphibolite these shear zones splay and curve towards parallel to the layer which indicates a layer parallel shear component that is in line with forward vergent rotation of the boudin blocks.
The largest amount of displacement is parallel to the shear fractures (D) while dilation between the block faces (N) is 35 generally low. The mineralogy of interboudin zones regularly contain, besides calcite, also an opaque mineral (presumably pyrite) and sheared biotite. On exposed shear fractures, a slickenside lineation is commonly present.
A full detachment of boudin blocks by large extension into asymmetric tapering boudins (Goscombe 2004) is rare in the study area; in most amphibolite layers the domino blocks are still in contact to each other. The length to width ratio (L/W) of this generation can be as low as 1 but more commonly it is in the order of 3-5 and in extreme cases >8.
Looking down on the quarry floor, over distances up to 30 m, within one amphibolite layer the sense of block rotation and shear displacement is commonly consistent. Along one amphibolite layer, antithetic shear fractures and block rotations in the opposite sense are rare. Adjacent amphibolite layers usually show the same sense of block rotation and shear displacement. 5 However, both senses of block rotation and shear displacement are common and these bundles of layers with the same sense of block rotation and shear displacement are usually separated by layers without domino boudins.
Domino boudin necks are highly variable in strike, due to the variation discussed above, and also due to variance in θ.
Domino boudin necks have a rake between vertical and ~60° to the South.
Domino boudins consistently crosscut and displace sλ, as well as (very rarely) small-scale isoclinal fold hinges. Domino 10 boudins are the one boudin type that can be found with similar orientation to those in the amphibolite in nearly all pegmatites (compare Schenk et al., 2007). Their formation thus postdates both Pinch-and swell generations (sλ and lλ), small-scale folding and pegmatite intrusion.

Torn boudins
Torn boudins are the second class of brittle boudins in the amphibolite. In contrast to the domino boudins, torn boudin 15 always form in a Mode-I fracture normal to the Amphibolite layer with sharp, intra-and intergranular fractures. The main component of displacement is dilatational/normal to the fracture, however in some layers minor systematic rotation (< ~15°) of the torn boudin blocks can be observed.
In amphibolite layers that contain both torn boudins and domino boudins, the torn boudins show a lower spacing of the necks (L/W=1.5-3) than the domino boudins. 20 In some amphibolite layers at least 2 generations of torn boudins are present, each with a stable direction, that intersect at a low angle or form wing cracks (Virgo et al., 2013(Virgo et al., , 2014 (Fig. 9e). The rake of the older set is usually oriented subvertical while the younger set dips steeply to the south. The often times similar orientation makes age relationships of torn and domino boudins rare in outcrop. Where a clear relationship was observable the domino boudins are older than the torn boudins. 25 Torn boudin necks are filled with calcite and less commonly chlorite (Fig. 11). The calcite fill is blocky with grains often larger than the fracture aperture and include solid inclusion bands, indicating a cementation by precipitation. Features that would indicate an invasive flow of the surrounding marble into the opening fracture like a systematic grain size reduction in the surrounding marble or abrasion along the neck faces was never observed in thin section. However, deformation of passive markers indicate that flow of the surrounding marble into the boudin neck does play a role when the segregation of 30 torn boudin becomes larger than a few millimeters (Fig. 7e).

Hairline veins
Hairline veins form (analog to boudins) by fracturing in layer parallel extension. The decision to call this generation of structures hairline veins rather than boudins is due to the fact that they are not strictly confined to the amphibolite but can also occur in the marble. surrounding marble above and below (Fig. 7 f). The orientation of the veins is parallel to the local chlorite filled hairline veins in the amphibolite.
Both of these veins in marble and amphibolite are parallel to the orientation of the most prominent set of large scale E-W joints and faults in the marble. The faults (Fig. 5c) can have a displacement of a few m, and they have a several m wide damage zone and a thin fault core in which open fractures are filled with fault breccia, fine grained sediment and karst 5 deposits.

Discussion
Integrating the observations in several quarries allows us to interpret five distinct generations of boudinage in N-S trending subvertical amphibolite layers that record the internal deformation of the marbles from high grade M2b metamorphic conditions (lλ pinch and swell boudins) over the pro-and retrograde path (sλ pinch-and-swell boudins, domino boudins and 10 torn boudins) to recent shallow deformation (hairline veins, joints, faults). The decrease in temperature and pressure is reflected by the embrittlement of the amphibolite and marble through the stages of deformation and by the structurally associated minerals that change from biotite to chlorite and secondary calcite.
All boudin generations indicate E-W shortening with different orientations of layer parallel extension, rotating in the plane of layering from vertical (lλ) over south vergent (sλ, Domino boudins) and horizontal (Domino boudins, torn boudins) to 15 North vergent (Hairline veins, joints). The strains involved decrease from >>100% elongation in lλ over about 10% in domino boudins to ~1% in the hairline veins.
A compilation of our results is given in Fig. 13, relating the meso-and microstructural observations and our interpretation of the strain and temperature conditions to other structures and events. Before we discuss these results in the context of the geodynamic evolution of the Aegean, we examine aspects on our interpretation in more detail. 20 The origin of the amphibolite layers in the marbles is not conclusively known, but it is widely accepted that they have already existed during the high pressure metamorphism of M1 (Avigad, 1998;Bolhar et al., 2016;Buick and Holland, 1989;Martin et al., 2006). Their present day N-S trending subvertical orientation in the study area is interpreted to be a result of the high strain viscoplastic deformation during the partial melting stage of the migmatite in M2b (Kruckenberg et al., 2010Vanderhaeghe, 2004). The marbles of our study area belong to the pinched synforms that formed in the high strain 25 zone between the subdomes of the migmatite dome. The orientation of the amphibolites is in good correspondence to the limbs of the interpreted large scale structure in this zone. Furthermore, the long wavelength pinch and swell lλ, (the oldest boudinage structure) is consistently parallel to the axis of this pinched synform. The boudins are interpreted to have formed by (only) vertical elongation of the largescale limbs (c.f. Ramsay, 1967) during E-W shortening in M2b conditions.
Considering that boudinage of the limbs first requires the formation of isoclinal folds leads to the conclusion that the amount 30 of E-W shortening has to be much higher than what is reflected by the lλ boudins.

Folds
More work is needed to fully clarify the relative age relations of the small scale isoclinal folds. Although we know from clear overprinting relationships that they predate the pegmatite intrusion and the brittle boudinage phases, their relation to the two types of pinch-and-swell structures is not fully resolved. In some cases the folds seem to overprint the short 35 wavelength boudins and in other cases the fold axis is parallel to the L b of this generation which would suggest that the folds and sλ might be structurally related. Most of the observed folds are asymmetric, with changing s or z chirality and a strong variation in the dip of the fold axis. Due to this strong variation in the orientation, these folds are not directly relatable to the large pinched synform discussed above. A structural relation between folds and boudins is not uncommon: One example are the above mentioned boudins that form in the stretching limbs of isoclinal folds. In analogue studies, Zulauf and Zulauf 40 Solid Very similar folds to those of our study area were described in boudinaged aplitic dikes in marble by (Poisel et al., 2016).
These folds and boudins show mutual age relationships and are interpreted as coeval structures that formed in a general 5 strain regime with components of layer parallel stretching and shear (B. Grasemann, pers. comm. 2017). The asymmetric folds in the amphibolite are generally in good correspondence with non-coaxial folds forming in simple shear and general strain regimes Holdsworth, 2006, 2007;Cobbold and Quinquis, 1980;Ridley, 1986). General strain conditions with pure shear and a layer parallel simple shear component may have prevailed in the marbles during some deformation stages. However, both the asymmetric folds and systematically rotated domino boudins occur in both polarities, implying 10 that layer parallel shear is a local phenomenon and does not reflect a regionally imposed shear deformation. The possibility that s-folds are an overturned variation of z-folds (compare Llorens et al. (2013)) is considered unlikely since no indication for such high shear strains was found.

Static recrystallization
The microstructure of the amphibolite in folds and pinch and swell boudins with a lack of indicators for dynamic 15 recrystallization suggests it has statically recrystallized. The foliation (e.g. in fold limbs) is inherited from a deformation fabric but overprinted by static grain growth so that it is at present reflected only by changes in the amphibolite composition (e.g. plagioclase content).
The static recrystallization is possibly coeval with the pegmatite intrusion in the marbles. Firstly, all statically recrystallized structures are older than the pegmatites and none is younger. Secondly, static recrystallization requires a phase of 20 deformation quiescence which was very likely the case in this phase: The pegmatites have intruded the marble in various direction, often subparallel to the amphibolite and reactivating the heterogeneity imposed by the amphibolite (Schenk et al., 2007). This high variability in orientation requires low differential stresses, and sigma 3 approximately normal to the amphibolite layers. Low differential stress also means that there is no substantial deformation of the amphibolite which is a prerequisite for the static recrystallization. Here we speculate that this may be related to a rheological inversion in the 25 migmatite core, when partially molten migmatite became weaker than marble.
The pegmatite intrusion and solidification and static recrystallization of the amphibolite mark the beginning of the transition from ductile deformation towards more localized brittle deformation of both amphibolites (and later also marble). This is reflected by overprinting of the short wavelength pinch-and-swell boudins by lower temperature deformation structures. Granular flow of the recrystallized amphibolite in the necks of sλ and shear zones in the marble with mechanical twins and 30 small recrystallized grains suggest temperatures below 300ºC during deformation (Burkhard, 1993). This part of the initial retrogression was suggested to occur from 20Ma to 16.6Ma (Siebenaller et al., 2013).

E-W shortening
In the Aegean Sea, N-S to NE-SW trending fold axes indicative for E-W shortening have been found on various islands in the central Aegean, including Andros, Tinos, Mykonos, Syros, Naxos, and Paros (e.g. Angelier, 1977;Avigad et al., 2001;35 Gautier et al., 1993;Lecomte et al., 2010;Menant et al., 2013;Papanikolaou, 1980;Philippon et al., 2011Philippon et al., , 2012Urai et al., 1990). However, driving mechanisms, timing and amount of shortening in E-W direction in the Aegean remain an ongoing matter of debate, and different hypotheses have been suggested. The controversy is partly related to interpretation of the nature of the Mid Cycladic Lineament, where orientation of lineations change abruptly across a narrow zone (Walcott and White, 1998a). This may be either interpreted as strike-slip faulting associated with crustal scale shortening and block 40 rotations (e.g. Jolivet et al., 2013;Philippon et al., 2011), or normal faulting in an overall extensional regime (Gautier et al., Solid Earth Discuss., https://doi.org/10.5194/se-2017-91 Manuscript under review for journal Solid Earth Discussion started: 23 August 2017 c Author(s) 2017. CC BY 4.0 License. 1999;Hinsbergen and Schmid, 2012) including vertical axis rotations in the footwall (Malandri et al., 2017). Field evidence is controversial, as there seems neither support for a major detachment (Brun et al., 2016;Jolivet et al., 2015;Philippon et al., 2014), nor is strike-slip faulting observed on Paros, where the Mid Cycladic Lineament might be exposed (Malandri et al., 2017;Walcott and White, 1998a). Additional complexity is added due to interaction of slab roll-back with westward migration of the Anatolian Plate since ~11 Ma (Jolivet et al., 2013;Ring et al., 2010). It has been suggested the Anatolian 5 Plate is a main driver for observed E-W shortening, rendering the structures dominantly young if considering activity of the North Anatolian Fault (e.g. Menant et al., 2013). Earlier E-W shortening in the Aegean may be explained by the proto-North Anatolian Fault, as witnessed by ductile shearing in Turkey, that might have been linked to activity along the Mid Cycladic Lineament (Philippon et al., 2014 and references therein). Alternatively, E-W shortening may be partly independent of the North Anatolian Fault and related to crustal attenuation and viscoplastic deformation during upward flow and partial melting 10 of the crust and mantle in an overall extensional regime (Kruckenberg et al., 2010Malandri et al., 2017). This scenario does not exclude late-stage shortening related to movement of the North Anatolian Fault, but most strain is accommodated earlier, locally constrained to the thermal domes.
On Naxos, there is evidence for two phases of E-W shortening. The oldest boudinage structures (lλ pinch and swell 15 structures), which take up most of the strain show vertical extension direction. This fits with the model of a pinched synform in between subdomes in the migmatite . In this model, E-W shortening is related to upper crustal extension and lower crustal convergent flow, and has been associated with M2b metamorphism. This so-called viscous collision does not require plate convergence, when domes form by upward rotation of horizontal deep crustal flows during crustal extension Rey et al., 2011). This model implies that most of E-W shortening observed in 20 the area is relatively old, much older than activity of the brittle North Anatolian Fault. During this deformation phase, viscoplastic flow is the dominant deformation process, driven by buoyancy and isostatic equilibration, so that no high differential stress is required to form the observed structures. Short wavelength pinch and swell boudinage (sλ) accommodate less strain that lλ, and it is not clear whether they formed during or past peak metamorphism. However, they also formed during dominant viscoplastic deformation, i.e. during an early phase of E-W-shortening. 25 Younger structures mapped in the amphibolite, in particular all brittle boudin types as well as the open B3 folds outside the migmatite described in Urai et al. (1990) show N-S stretching and shortening in E-W direction. All these structures formed post peak M2, thus on the retrograde path (see above). These structures are possibly linked with compression due to the North Anatolian Fault. This hypothesis could be tested for instance by dating minerals in the respective boudin necks.
However, we note that the brittle structures only accommodate a small fraction of total strain in the amphibolite. 30 In summary, independent of local or regional importance of the signal the E-W shortening history of Naxos requires two distinct and temporally separated E-W shortening phases. The first phase between 16 and 20 Ma is dominated by viscoplastic flow and accommodates most of the total strain. After this deformation phase the amphibolite shows static recrystallization, implying little or no deformation. At a late stage, possibly related to movement of the North Anatolian  (Forster and Lister, 2009).

Conclusions
The internal deformation history of high grade marble bodies in the migmatitic center of the Naxos metamorphic dome is highlights the immense wealth of information that can be gained from detailed analysis of boudinage structures. Geological map of Naxos, modified after Jansen (1993), Vanderhage (2004 and Kruckenberg et al. (2011). The study area (red box) is located in the central high grade migmatic core of the Naxos metamorphic core complex.

12:
Neighbouring thin sections showing the localized deformation of a domino boudin (see Fig. 11 for location). a) In them 15 marble ~2 cm away from the amphibolite numerous parallel shear zones can be found. Grains along the shear zones show intense twinning and undulouse extinction. b) At the interface of the amphibolite the shear zone have merged into a single slip zone; the fine grained calcite fill easily breaks out during thin section preparation. c) The shear zone is partly filled with biotite and an opaque mineral (presumably pyrite) along the boudin face. The calcite in the zone between the separated blocks is intensely twinned and shows undulouse extinction. Amphibole grains are heavily fractured. 20 Figure 13: Overview of the results of this study in temporal relation to each other and to regional events. Note that only the relative order of events is considered and the time axis and temperature curve are not to scale. Results are grouped in rows, overlapping boxes are interpreted as contemporaneous. Data on temperature and regional events and structures are compiled 25 from Buick and Holland (1991), Katzir (1999), Schenk et al. (2007), Kruckenberg et al. (2010), and Cao et al. (2016.         Long wavelength p+s (lλ) rise of dome and subdomes in migmatite (Kruckenberg et al. 2010 (Schenk et al. 2007) boudinage of pegmatites (Schenk et al. 2007) retrograde metamorphosis, exhumation (Cao et al. 2016) deformation structures in Amphibolites regional events and structures secondary minerals amphibolite microstructure marble microstructure temperature principal strain in the marble