Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece

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1 TECTONICS, VOL. 30,, doi: /2010tc002751, 2011 Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece Seth C. Kruckenberg, 1,2 Olivier Vanderhaeghe, 3 Eric C. Ferré, 4 Christian Teyssier, 1 and Donna L. Whitney 1 Received 11 July 2010; revised 16 November 2010; accepted 5 January 2011; published 13 May [1] Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces, folds, and fabrics while units mantling the dome display a simpler geometry, typically formed by transposition during crustal extension. We use field observations and magnetic fabrics in the Naxos dome (Greece) to quantify the complex flow of anatectic crust beneath an extensional detachment system. The internal structure of the Naxos dome is characterized by second order domes (subdomes), pinched synforms, and curved lineation trajectories, which suggest that buoyancy driven flow participated in dome evolution. Subdomes broadly occur within two compartments that are separated by a steep, N S oriented, high strain zone. This pattern has been recognized in domes formed by polydiapirism and in models of isostasy dominated flow. The preferred model involves a combination of buoyancy and isostasy driven processes: the Naxos dome may have been generated by regional N S extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome. This pattern is complicated by gravitational instabilities and/or overturning of the high melt fraction crust leading to the growth of subdomes. As the migmatites within the Naxos dome reached a higher structural level, they were affected by regional top to the NNE kinematics of the detachment system. Dome formation therefore occurred by a combination of coeval and coupled processes: upper crustal extension, deep crust contraction during convergent flow of anatectic crust, diapirism and/or density driven crustal convection forming subdomes, and north directed detachment kinematics. Citation: Kruckenberg, S. C., O. Vanderhaeghe, E. C. Ferré, C. Teyssier, and D. L. Whitney (2011), Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece, Tectonics, 30,, doi: /2010tc Introduction [2] Crystallized partially molten and magmatic crust (i.e., migmatite) is common in orogens and is exposed in many gneiss domes exhumed within metamorphic core complexes [Brun et al., 1994; Vanderhaeghe et al., 1999; Teyssier and Whitney, 2002; Whitney et al., 2004]. These gneiss domes typically consist of a core of migmatite that exhibits a wide range of melt fractions, from solid dominated (metatexite) to magma dominated (diatexite and magma). These domes record in their pressure temperature time and deformation histories the flow of orogenic crust under extension [e.g., 1 Department of Geology and Geophysics, University of Minnesota Twin Cities, Minneapolis, Minnesota, USA. 2 Now at Department of Geoscience, University of Wisconsin Madison, Madison, Wisconsin, USA. 3 G2R, Université Henri Poincaré Nancy 1, Vandoeuvre lès Nancy, France. 4 Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois, USA. Copyright 2011 by the American Geophysical Union /11/2010TC Tirel et al., 2004; Rey et al., 2009a, 2009b, 2011]. Flow of partially molten and magmatic crust in core complexes may be driven by buoyancy dominated and/or isostasydominated processes. Consequently, the deformation patterns of migmatite domes yield important insights on dome dynamics as well as heat and mass transfer in orogens through the movement of anatectic crust. [3] We investigated the internal dynamics of the Naxos migmatite dome (Cyclades, Greece) using a combination of field and magnetic fabric characterization that utilizes the anisotropy of magnetic susceptibility (AMS) in migmatites [Ferré et al., 2003, 2004; Kruckenberg et al., 2010] to document flow planes and flow directions. The Naxos migmatites have a complex infrastructure containing subdomes [cf. Ledru et al., 2001; Vanderhaeghe, 2004; Whitney et al., 2004] that are organized into elongate, en echelon compartments separated by a steeply dipping high strain zone. Structures developed in the migmatites were acquired during melt present deformation [Vanderhaeghe, 2004; Kruckenberg et al., 2010] and affected by top to NNE shear associated with the structurally overlying Naxos detachment fault during exhumation and dome development. The Naxos 1of24

2 Figure 1. Tectonic context, geologic map, and cross sections of the Naxos dome. (a) Schematic map of the Aegean domain showing Naxos in the frame of the Alpine orogen (ACM, Attic Cycladic Massif). (b) Geologic map of the island of Naxos, Greece, after Jansen [1973], Vanderhaeghe [2004], Siebenaller [2008], and Kruckenberg [2009]. Isograds are derived from Jansen and Schuiling [1976]. A more detailed version of the geologic map of Naxos is provided in Figure S1. (c) Cross sections are modified after Jansen [1973], Vanderhaeghe [2004], Siebenaller [2008], and Kruckenberg [2009]. Double arrows indicate sense of shear retrieved from kinematic analysis of the ductile fabric. dome provides a unique record of mobilization of the deep crust to higher structural levels and illustrates the relationship between extension in the upper crust contemporaneous with flow in the deep crust. 2. Geologic Context and Previous Work [4] The Aegean domain, in a back arc position relative to the Hellenic trench, is affected by extension related in part to retreat of the African plate, which is subducting beneath the southern margin of Eurasia [Spakman et al., 1988; Gautier et al., 1999; Jolivet et al., 1994]. The Aegean extensional domain is characterized by metamorphic core complexes that exhibit a range of recorded finite extension, with a maximum amount on the island of Naxos, in the middle of the Aegean domain (Figure 1a) [e.g., Lister et al., 1984; Jolivet et al., 1994]. Extension in Naxos and other Cycladic Islands has resulted in low angle normal fault systems with a consistent top to N or top to NE sense of shear over the past 30 Myr [e.g., Buick, 1991a, 1991b; Gautier and Brun, 1994; Jolivet et al., 1994]. Present day geodetic data document anticlockwise rotation of the Aegean domain relative to a fixed European plate with a divergent flow pattern implying radial spreading and vertical thinning [e.g., McClusky et al., 2000; Jiménez Munt et al., 2003]. [5] The island of Naxos consists primarily of interlayered marble and schist mantling a migmatite dome (Figures 1b and 1c and Figure S1) [Jansen, 1973; Jansen and Schuiling, 1976; Dürr et al., 1978; Vanderhaeghe, 2004]. 1 These metamorphic rocks on Naxos record an early phase of subduction and orogenic crustal thickening owing to convergence between Africa and Eurasia [Dewey and Şengör, 1979; Bonneau and Kienast, 1982], followed by extensional collapse caused by roll back of the subducting African slab along the Hellenic trench system [Gautier et al., 1999; Jolivet et al., 1994]. An early Alpine (M 1 ) event, dated at circa 50 Ma [Wijbrans and McDougall, 1988], is defined by blueschist facies relicts exposed on the southeastern side of Naxos [Andriessen et al., 1979; Wijbrans and McDougall, 1988; Avigad, 1998]. This high pressure low temperature metamorphic event ( 10 kbar, 350 C) was followed by near isothermal decompression [Schliestedt et al., 1987; Duchêne et al., 2006] involving rapid exhumation and greenschist facies metamorphism at circa 25 Ma [Wijbrans and McDougall, 1988]. 1 Auxiliary materials are available in the HTML. doi: / 2010TC of24

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4 A second metamorphic event (M 2 :upto 10 kbar, 750 C) is characterized by a greenschist to upper amphibolite facies metamorphic overprint followed by crustal anatexis, the formation of migmatites, and the development of a dome [Jansen and Schuiling, 1976; Buick and Holland, 1989; Keay et al., 2001; Duchêne et al., 2006; Martin et al., 2006, 2008]. The origin of the Naxos dome has been attributed to diapirism [Jansen and Schuiling, 1976; Vanderhaeghe, 2004], exhumation during regional extension [Gautier et al., 1993], and superposed folding resulting from E W regional shortening followed by top to the NNE shearing [Urai et al., 1990; Buick, 1991a; Avigad et al., 2001]. [6] The migmatite core forms a NNE SSW elongate structural dome ( 5 12 km) with complex trajectories of migmatitic foliation (Figures 2, 3, and 4 and Figures S1 and S2). Migmatites in the dome record U Pb crystallization ages of circa 17 Ma [Keay et al., 2001; Martin et al., 2006, 2008]. A granodiorite pluton dated at circa 12 Ma [Wijbrans and McDougall, 1988; Keay et al., 2001] is exposed on the western part of the island and has intrusive contacts with the marble and schist sequence [Jansen, 1973; Gautier et al., 1993]. Cooling of the dome accompanied regional extension and exhumation [Brichau et al., 2006; Seward et al., 2009]. [7] The metamorphic sequence that surrounds the migmatitic core consists of alternating layers of marble and schist and less abundant layers of amphibolite, metabauxite, and metavolcanic rocks (Figure 1b and Figure S1). These ductilely deformed units are characterized by greenschist to amphibolite facies assemblages that increase in grade toward the migmatite dome (Figure 1b and Figure S1) [Jansen and Schuiling, 1976; Duchêne et al., 2006]. A transposition foliation in the metasedimentary rocks typically has a gentle dip, although dip angles increase to >45 adjacent to the migmatite dome [Urai et al., 1990; Vanderhaeghe, 2004]. A prominent mineral lineation defined by elongate quartz rods or the alignment of metamorphic minerals, such as amphibole, is oriented NNE SSW and plunges gently [Buick, 1991b; Gautier and Brun, 1994; Vanderhaeghe, 2004]. Axes of multiple fold generations are consistently parallel to the stretching lineation and hook type interference patterns result from the superposition of the colinear fold axis orientations [Urai et al., 1990; Buick, 1991b; Vanderhaeghe, 2004]. Sheath folds with noses oriented parallel to this stretching direction are observed at a variety of scales [Urai et al., 1990; Vanderhaeghe, 2004]. These early formed fabrics and folds are locally overprinted by open to tight upright folds with steep axial surfaces and N S trending hinges [Jansen, 1973; Buick, 1991a; Avigad et al., 2001; Vanderhaeghe, 2004]. Within the mantling units, intensity of mylonitic deformation increases toward the contact of the migmatite core, and kinematic criteria document dominant top to the NNE sense of shear [Urai et al., 1990; Buick, 1991a, 1991b; Gautier et al., 1993; Vanderhaeghe, 2004]. 3. The Naxos Migmatites 3.1. Overview [8] In migmatites, the continuity of the gneissic foliation is taken as a proxy for the former solid framework: metatexites being characterized by gneisses and schists with a continuous foliation enclosing leucosome, and diatexites being dominated by granite enclosing enclaves, selvages, and/or crystals [e.g., Sawyer, 1996, 2008; Vanderhaeghe, 2009]. The core of the Naxos dome consists predominantly of leucocratic diatexites characterized by a heterogeneous granite with a well developed magmatic texture containing enclaves of metamorphic rocks (Figure 5). Diatexites are texturally and compositionally heterogeneous, including nebulitic, schollen, and schlieren structured varieties [Kruckenberg et al., 2010]. Metatexite is less abundant and typically occurs at the periphery of the dome or adjacent to metasedimentary septa, such as marble or schist layers. Metatexites are characterized by paragneisses and schists with a composite continuous gneissic foliation marked by alternating quartzofeldspathic and biotite layers with leucosomes. Migmatitic foliation is most commonly expressed by wispy schlieren of biotite (Figure 5b) or alignment of rafts (schollen) within diatexite (Figure 5a), and less commonly by stromatic layering in metatexite (Figure 5d). The orientation of migmatitic foliation is highly variable (Figure 6a) and lineation is largely lacking in the Naxos migmatites. However, in some localities lineation is defined by the preferred alignment of biotite schlieren structures that generally trend NNE SSW (Figure 6b). [9] Melt present deformation structures are abundant in the Naxos migmatites. For example, leucosome occurs in fold hinges with axial planes subparallel to foliation (Figures 5b and 5d). Hinge zones of flow folds (Figure 6c) contain a magmatic isotropic crystal structure (Figures 5b and 5d). Leucosomes are common in shear bands that crosscut the diatexite (Figure 5c) and within the necks of boudins, suggesting that melt accumulated within dilatant structural sites during melt present deformation. These observations, together with microstructures indicating the former presence of melt [e.g., Marchildon and Brown, 2002, 2003; Sawyer, 2001; Holness, 2008], suggest that most of the structural fabric was acquired during melt present deformation associated with bulk flow in the Naxos migmatites [Kruckenberg et al., 2010]. Consequently, migmatites within the Naxos dome are interpreted to comprise both former magmas (i.e., diatexites containing crystals and enclaves in suspension within a melt) and former partially molten rocks (i.e., metatexites) [cf. Vanderhaeghe, 2009]. Figure 2. Geologic map of the migmatitic core of the Naxos dome. Note the variability of the migmatitic foliation within the dome core as shown by foliation trajectories. Patterns of migmatitic foliation delineate second order domes (subdomes) defined by regions of concentric foliation, separated by steeply dipping zones of high strain or pinched synforms. Note also the contact relationships of the migmatitic foliation with mantling units, commonly concordant but discordant to the dome margin in some localities (e.g., NW of the town of Keramoti). Cross section lines correspond to those in Figures 3 and 4 and Figure S2. Lithologic units mantling the migmatitic core are modified after Vanderhaeghe [2004] and Siebenaller [2008]. A larger, more detailed version of this geologic map that also contains the results from anisotropy of magnetic susceptibility (AMS) analyses in dome migmatites is provided in Figure S1. 4of24

5 Figure 3. (a) North (left) to south (right) geologic cross sections of the migmatitic core of the Naxos dome. Note the overturning of migmatitic foliation within subdomes (south dipping limbs of migmatitic foliation in B B, C C ) and the presence of tightly pinched synforms between subdomes (i.e., between the northern and central subdomes: B B, C C ). The large region of schist shown in B B is associated with the infolding of mantling units and migmatitic fabrics that dip back into the core (see corresponding cross section line D D ). (b) West (left) to east (right) geological cross sections of the migmatitic core illustrate (1) the presence of subdomes separated by a median high strain zone (H H, I I ), (2) synforms between intervening subdomes (G G ), and (3) steeply dipping to fanning zones of migmatitic foliation (F F ). Locations of cross section lines are shown in Figure 2 and in Figure S1. High grade schists and amphibolites mantling the migmatitic core are grouped for structural clarity in the construction of cross sections. A larger, more detailed version of these cross sections can be found in Figure S2. 5 of 24

6 Figure 4. Composite isometric cross section array illustrating the three dimensional structure of the migmatitic core of the Naxos dome. Highlighted are many of the key characteristics in the migmatitic core defined by the structural pattern of migmatitic foliation including the presence of subdomes, steeply dipping N S oriented foliation of the median high strain zone, synforms, and folded domains (e.g., northern part of dome). Cross sections correspond to those in Figure 3, and the locations of cross section lines are shown in Figure 2 and Figure S1. A larger, more detailed version of this image is also provided in Figure S2. Despite a predominantly magmatic character, weak high temperature creep is also evident in some migmatites, particularly at the periphery of the dome or associated with entrained metasedimentary units, as is shown by the presence of undulose extinction, chessboard subgrains, and deformation bands in quartz (Figure 7) [Vanderhaeghe, 2004] Contact Relationships With Mantling Units [10] The nature of the mantling sequence migmatite contact zone is highly variable around the dome core (Figure 2 and Figure S1). Along the western and northern margin of the dome, amphibolite facies schist and intercalated amphibolite units form a gradational contact with the migmatites. Near the dome margins, schistose layers show an increase in leucosome abundance (i.e., metatexite), grading into diatexite in the core of the dome. In the northwest portion of the dome, the contact is locally intruded by granitic dikes and sills that are variably transposed and folded with schist and amphibolite (Figure 2 and Figure S1) [Vanderhaeghe, 2004]. Here, the fabric of the migmatite unit is largely concordant with the foliation in the surrounding schist and amphibolite layers of the dome (Figures 2 4 and Figures S1 and S2). [11] On the eastern side of the dome, a continuous layer of marble demarcates the contact with the migmatites (Figure 2 and Figure S1). Migmatitic foliation at the northern and southern limits of the enveloping marble is concordant to foliation in the mantling metamorphic units and to the enveloping surface of the dome. However, this migmatitic foliation is highly discordant in the central region of the dome (east of Mount Koronos; Figure 2 and Figure S1). Migmatitic foliation is locally overprinted by a zone of solid state deformation within a few tens to hundreds of meters of the eastern marble contact, particularly near the town of Keramoti (Figure 2 and Figure S1), where fabrics in the migmatites and mantling units steepen and are transposed. [12] In the southern half of the dome, a sheared leucogranitic gneiss layer marks the contact between the migmatites and the mantling marble and schists [Jansen and Schuiling, 1976; Buick, 1991a, 1991b] (Figures 2 and 8 and Figures S1 and S2). This leucogranitic gneiss is distinguished from the heterogeneous diatexites that make up 6 of 24

7 Figure 5. Representative field photos of migmatites in the core of the Naxos dome. (a) Diatexite with discontinuous mafic selvages aligned parallel to, and helping define, the migmatitic foliation. (b) Leucocratic diatexite with wispy biotite layers and schlieren characteristic of migmatitic foliation throughout much of the Naxos dome. (c) Shear band within diatexite showing the accumulation of leucosome in dilatant structural sites. These observations suggest that structural fabrics were acquired during melt present deformation in the migmatites. Note coin for scale. (d) Folded leucosome rich metatexite; folds are outlined by the biotite subfabric, and hinges of folds commonly contain a magmatic isotropic crystal structure, consistent with melt present deformation in the migmatites. the majority of the migmatite core by its homogeneous composition, lack of selvages or metasedimentary septa, and by a pervasive solid state deformation overprint (Figure 8a). Leucogranitic gneiss horizons deformed in the solid state are interleaved over a range of scales (centimeters to meters) with diatexitic migmatite of dominantly magmatic texture (Figures 2 and 8b and Figure S1). Contacts between diatexite and leucogranite are largely gradational. This may reflect increased melt segregation and/or migration from the site of melting (i.e., the diatexites) to form more homogeneous leucogranites at their emplacement site along the contact with the mantling metasedimentary units. Leucogranitic gneiss shows mylonitic fabrics with top to NNE kinematic criteria, in agreement with shear sense indicators in the mantling metasedimentary rocks. Layers of leucogranitic gneiss also occur within the schist sequence surrounding the southern portion of the dome (Figure 2 and Figure S1). 4. Structural Analysis of Migmatite [13] The internal geometry of the Naxos migmatite dome was mapped in detail (Figures 2 4 and Figures S1 and S2). Measurements of structural fabric were collected every 150 m throughout the dome core. The Naxos migmatites display obvious foliation in outcrop, but linear fabrics are not as clear. Consequently, field mapping was complemented by measurement of the anisotropy of magnetic susceptibility in 7of24

8 Figure 6. Stereonets of structural data for migmatites in the core of the Naxos dome. (a) Migmatitic foliation. (b) Lineation, commonly defined by the preferred alignment of biotite schlieren structures. (c) Axes of flow folds within dome migmatites. Kamb contours of the structural data are shown with 2 sigma contour intervals. Equal area stereographic projections were made with Stereonet version by Richard Allmendinger ( the Naxos migmatites [Kruckenberg et al., 2010]. The AMS analysis demonstrated a correlation between the AMS principal axes and the fabrics measured in the field, thereby providing a proxy for lineation and a measure of the fabric shape and degree of anisotropy from the AMS tensor [Kruckenberg et al., 2010]. These data are used here for testing ideas about the dominant dome forming processes Three Dimensional Geometry of the Migmatite Core [14] Trajectories of foliation measured in the field define the three dimensional form surfaces within the Naxos dome (Figures 2 4 and 9a and Figures S1 and S2). Concentric patterns of migmatitic foliation and layers of metasedimentary septa enclosed in the migmatite (Figures 9b and 9c) delineate second order kilometer scale domal structures (subdomes) that are separated by pinched synforms and high strain zones (Figures 2 4 and Figures S1 and S2). An approximately N S oriented zone of intensely deformed, steeply dipping, interleaved leucogranitic gneiss and diatexite affected by solid state deformation extends into the dome core for over a kilometer, forming a median zone of high strain in the southern half of the dome (Figure 3b, I I, Figures 4 and 8, and Figures S1 and S2). This steeply dipping median strain zone separates two en echelon subdomes in the southern and central portions of the dome, respectively (Figure 2 and Figure S1). This high strain zone does not extend outside of the dome core into the mantling sequence. Rather, it projects along its trend northward into the central portion of the migmatite core (south of Mount Koronos and east of the town of Kinidharos; Figure 2 and Figure S1) with a zone of intensely folded metasedimentary septa, steeply dipping migmatitic foliation, and a synform with a dominantly N S oriented axial trace and north plunging axis (Figure 2 and G G section in Figure 3b and Figure S2). [15] Both of the subdomes separated by the high strain zone (hereafter southern and central subdomes; Figures 2 4) are characterized by steeply dipping migmatitic foliation, which is overturned and southward dipping along the northern edge of the central subdome (B B, C C : Figure 3a and Figure S2). The overturning of units in this region is also reflected in the orientation of steeply dipping marble layers that wrap the central subdome along its western and northern edge (B B, G G : Figures 2 4 and Figures S1 and S2). A pinched synform, with a south dipping axial trace (at approximately latitude; Figure 2 and Figure S1), defines the boundary between the central subdome and another subdome exposed in the northern portion of the dome centered near Mount Koronos (B B, C C : Figures 3a and 4 and Figure S2). [16] The migmatitic foliation in the vicinity of Mount Koronos defines a broadly arching subdome (hereafter northern subdome) visible along a N S section (B B, C C : Figure 3a and Figure S2). Along the southern flank of this subdome, foliation defines mesoscale folds with WNW ESE fold axes (Figure 2 and Figure S1) that cascade into the pinched, E W oriented synform that separates this northern subdome from the central subdome (B B, C C : Figure 3a and Figure S2). These folds have axes at high angle to dominantly NNE SSW orientated folds in the units that mantle the dome. These cascading folds also occur in association with discordant migmatitic fabrics along the eastern contact of the dome (Figure 2 and Figure S1). An E W section through the Mount Koronos region (F F : Figure 3b and Figure S2) illustrates that the migmatitic fabric is partially overturned to the west in the northern subdome, near a pinched axial zone of vertical to fanning foliation. This zone separates the overturned western edge of the subdome from a western zone of steeply dipping migmatitic foliation containing a large structural panel of marble interfolded with the migmatites (Figures 2 and 9b and Figure S1). [17] Migmatitic foliation is characteristically steep in the northern portion of the dome. Along much of the northwestern schist migmatite contact, this foliation is overturned and dips eastward toward the core of the dome (Figure 2 and Figure S1; D D : Figure 3 and Figure S2). This deformation pattern is similar to that illustrated by the overturned limb on 8of24

9 9of24 an E W section through Mount Koronos (F F : Figure 3b and Figure S2). In the northern part of the dome, migmatitic foliation is typically folded about N S oriented fold axes and is interfolded with enveloping schist and amphibolite layers at the contact (Figure 2 and Figure S1). [18] In the southern half of the migmatite core, foliation has a dominantly N S orientation and is characterized by vertical, steeply dipping, or fanning migmatitic foliation (Figures 2 4 and Figures S1 and S2). The southern subdome is an exception where foliation has gentle to intermediate dip (Figure 2 and Figure S1). Panels of marble that are boudinaged and isoclinally folded within the migmatites are abundant in the southern half of the migmatite dome (Figures 2 and 9c and Figure S1). Marble layers interfolded with migmatite units are particularly evident in the central portion of the migmatite dome south of the town of Kinidharos (Figure 2 and Figure S1). In the southern half of the dome, the region of steeply dipping foliation and intense interfolding of marble and migmatite occurs along strike with the zone of pervasive solid state deformation recorded within interleaved diatexite and leucogranitic gneiss (i.e., high strain zone; Figures 2 and 8 and Figure S1). Discrete zones of solid state deformation also occur in thin schistose horizons associated with some folded marbles (Figure 2 and Figure S1) Anisotropy of Magnetic Susceptibility Analysis [19] AMS has been proven as a very efficient petrophysical tool for fabric quantification in magmatic rocks. Surveyed at the pluton scale, it provides a strain pattern that can be used to differentiate fabrics developed by emplacement dynamics (i.e., during magmatic flow) or that formed as a result of regional deformation [e.g., Benn et al., 1997, 2001; Talbot et al., 2004]. AMS has also proven to be a valuable analytical tool in migmatite studies because it allows for recovery of the kinematic axes of flow, inferred from straininduced fabrics that are acquired along a continuum from high temperature viscous or viscoplastic flow through hightemperature plastic flow [Ferré et al., 2003, 2004; Kruckenberg et al., 2010]; thereby aiding tectonic studies of crystallized partially molten crust [e.g., Teyssier et al., 2005; Schulmann et al., 2009; Charles et al., 2009]. A combination of structural and magnetic analysis is therefore well suited to test dome forming processes. [20] Magnetic characterization of the Naxos migmatites was conducted to test the AMS technique as a tool to recover strain induced fabrics reflecting flow in migmatites [Kruckenberg et al., 2010]. The initial study documented the Figure 7. (a and c) Photomicrographs and (b) backscattered electron images of select cubic specimen of the Naxos migmatites illustrating microstructure and textural characteristics. A well developed igneous fabric is characterized by interlocking crystals of zoned plagioclase, potassium feldspar, quartz (commonly interstitial), and biotite. In Figure 7c, deformation bands in some quartz grains (white arrows) suggest weak high temperature creep in a subset of the Naxos migmatites, particularly at the periphery of the dome or associated with entrained units within the median high strain zone. Mineral abbreviations from Kretz [1983].

10 Figure 8. (a) Outcrop photograph (view to the south) of sheared leucogranitic gneiss at the western margin of the Naxos migmatites near the town of Kournochori. Kinematic indicators in the leucogranitic gneiss record consistent top to the NNE sense of shear, as is observed in the metasedimentary sequence that mantles the migmatite dome. (b) Field photograph shows steeply dipping, interleaved structural panels of diatexite and leucogranitic gneiss in the southern portion of the migmatite dome. View of photograph is to the north from near the town of Tsikalario and illustrates the well developed solid state fabric that wraps northward into the dome core. The structural fabric in this part of the dome is part of a steeply dipping median high strain zone that extends well over a kilometer into the dome and separates two distinct subdome domains to the east and west of this high strain zone. See cross section line I I for region corresponding to photo in Figure 8b and H H cross section line for the extension of this median highstrain zone into dome core as it separates subdomes (Figures 3 and 4 and Figure S2). nature of the magnetic minerals that contribute to the AMS and demonstrated the correspondence of the magnetic fabric with the migmatitic fabric at a variety of scales. Here we build on this methodology to interpret the magnetic fabrics recorded in the migmatites in relation to the tectonic evolution of the Naxos dome. [21] A total of 155 samples (3058 cubic specimens) collected at 110 localities throughout the migmatitic core were analyzed and magnetically characterized (Figure 10 and Table 1). Bulk magnetic susceptibility (K m ) values of sample averages range from approximately 10 17, [SI] but are generally low (< [SI]) in all but 15 of the 155 samples analyzed (Figure 10). The majority (90%) of the analyzed migmatite samples are characterized by paramagnetic lithologies (K m < [SI]) distributed throughout the dome core (Figure 11a). In dominantly paramagnetic samples, the variations in bulk magnetic susceptibility are the result of the heterogeneous distribution of biotite. More highly magnetic regions (K m > [SI]) indicate that, with increasing K m, the AMS is controlled by the ferromagnetic contribution of magnetite. These regions occur largely at the periphery of the dome or in its northern part (Figure 11a) [Kruckenberg et al., 2010]. [22] The shape of the AMS ellipsoid is described by the T parameter [Jelínek, 1978] and varies from oblate (T > 0) to prolate (T < 0). The majority of migmatite samples yield T parameters characterized by oblate fabrics, although prolate fabrics are observed in some samples (Figures 10a and 10b and Table 1). In the northern portion of the migmatite dome (north of latitude), the shape fabric is highly variable, with prolate and oblate fabrics in close proximity (Figure 11b). In the southern half of the dome, the shape parameter, T, consistently indicates oblate fabrics with some intermediate values (Figure 11b). Dense sampling in the well defined central subdome shows dominantly oblate values of T (Figure 11b) associated with steeply dipping migmatitic foliation (Figure 2 and Figure S1). Synformal regions to the west and north of this subdome display intermediate values of T [Kruckenberg et al., 2010]. [23] The total degree of anisotropy (P ) describes the deviation of the AMS ellipsoid from a sphere and increases from a value of 1 with increasing anisotropy [Jelínek, 1978]. In the Naxos migmatites, values of P that are corrected for the diamagnetic contribution have values ranging from to (Figures 10a and 10c and Table 1) [Kruckenberg et al., 2010]. P shows a positive correlation with K m (Figure 10a) and, in weakly magnetic specimens, with T (Figure 10c). The general pattern that emerges from the distribution of P at the scale of the dome (Figure 11c) is that the most anisotropic samples (P > 1.20) occur in the north, in the south, and in regions of steeply dipping foliation. Migmatite samples from within subdomes generally show anisotropy values of (Figure 11c). [24] Comparison of K m and T (Figures 10b and 11) show that variations in T are not solely a function of the magnetic mineralogy (i.e., paramagnetic versus ferromagnetic domi- 10 of 24

11 Figure 9. Select large scale structural features within the migmatitic core of the Naxos dome. (a) View to the north of Mount Koronos showing the eastern contact of the migmatite core with mantling marble units. Dashed line shows generalized form of migmatitic foliation that defines a northern subdome. See corresponding cross section F F in Figure 3 and Figure S2. (b) Structural panels of marble septa interfolded with migmatites on the western side of the northern Mount Koronos subdome. To the west of this entrained marble panel, migmatitic foliation dips consistently and steeply to the west. This western marble panel corresponds to that in cross section F F in Figure 3 and Figure S2. (c) View to the north near the town of Kinidharos (foreground) of large scale boudinage in steeply dipping and folded marble layers entrained within dome migmatites. The marble unit in this image partially envelops the western margin of the central subdome (south of Mount Koronos). See corresponding G G cross section in Figure 3 and Figure S2. nated), since prolate values of T occur both in migmatites where the AMS is controlled by biotite (i.e., samples with K m < [SI]) and by magnetite (K m > [SI]) [Kruckenberg et al., 2010]. Comparison of T and P (Figures 10c and 11) suggests that oblate fabrics are more anisotropic, although highly anisotropic samples with prolate fabrics occur in a few locations (Figure 11). These data might reflect differences in the composition of the migmatite core between the northern portion, characterized by variable T and P parameters and higher values of K m, and the dominantly paramagnetic southern half. The variations in T provide a description of the organization of the migmatitic fabric that may not vary solely as a function of strain, which may be influenced by the heterogeneous distribution of the constituent magnetic mineralogy [Kruckenberg et al., 2010]. In the paramagnetic Naxos migmatites, oblate values of T correspond to subparallel alignment of biotite, which may attest to flattening. Prolate fabrics likely reflect the organization of biotite folia about a zone axes parallel to the direction of bulk viscoplastic flow [Kruckenberg et al., 2010]. [25] At the outcrop and smaller scales, the principal directions of the AMS are generally well clustered and the magnetic fabric orientations are in good agreement among closely spaced migmatite samples (Figure S1). At the scale of the dome, the magnetic foliation is generally N S oriented and steeply dipping, although regions of intermediate dip and variable orientation occur (Figure 12a). Magnetic foliation orientation is well correlated to variation in migmatitic foliation measured in the field over a range of scales [cf. Kruckenberg et al., 2010], and therefore similarly delineates the trend of the planar structural fabric internal to the migmatites (Figure 12a). [26] The trend of magnetic lineation (K 1 direction) is also well correlated to the field lineation, where preserved (Figures 6b and 12b and Figure S1). This further suggests that the AMS is a reliable predictor of the strain induced fabric in migmatites, and by inference the kinematic axes of viscoplastic flow in migmatites at a range of scales [Kruckenberg et al., 2010]. At the dome scale, the trend of K 1 displays less variation in orientation than magnetic foliation and is broadly subparallel to the direction of NNE SSW extension inferred from mineral lineation and structural fabrics in the units mantling the migmatite core, but of variable plunge (Figure 12b). However, in the central and eastern region of the migmatite dome, trends of K 1 are oblique to the orientation of mineral lineation in the mantling units (Figure 12b). The plunge of K 1 is also spatially variable in the Naxos migmatites. Gently plunging magnetic lineations are dominant throughout the migmatitic core (Figure 12b). However, steeply plunging magnetic lineations are abundant in the central portion of the migmatite dome, in particular within the central subdome and adjacent synforms (Figure 12b and Figure S1). [27] Comparison of the magnetic lineation, K 1, with the shape parameter, T, provides insight into the organization of the migmatitic fabric as a function of orientation within the migmatite core. Steeply plunging magnetic lineation (>50 60 ) is largely restricted to samples characterized by oblate values of T (Figure 13a). Consequently, no migmatites with steeply plunging lineations are associated with prolate fabrics, which are largely restricted to NNE SSW orientations of gentle to intermediate plunges of K 1 (i.e., parallel to the orientation of regional extension) (Figures 13b and 14). Oblate fabrics are dominant for all trends of the magnetic lineation and occur on foliations of all dip angles (Figure 13). 5. Discussion 5.1. Internal Structure of the Naxos Dome [28] Field mapping and magnetic fabric characterization define the three dimensional structure and deformation patterns within the migmatitic core (Figures 2 4 and Figures S1 and S2). Migmatitic foliation at the margin of the dome 11 of 24

12 is broadly concordant with the enveloping metasedimentary units, as is shown in the north and south of the dome where migmatite and mantling units are interfolded (Figures 2 4 and Figures S1 and S2). Structures at the contact between the core migmatites and the mantling marbles and schists are transposed into concordance, but elsewhere display distinct structural patterns, suggesting a decoupling between two units with distinct flow patterns and/or rheologic behavior. [29] Away from the contact zone, the migmatitic core is characterized by a highly variable foliation that reveals an infrastructure characterized by kilometer scale second order domes (i.e., the northern, central and southern subdomes; Figure 2) separated by regions of steeply dipping migmatitic foliation, synforms, and in places by the high strain zone (Figures 2 4 and Figures S1 and S2). Subdomes and synformal structures in the migmatite core do not project into the mantling metasedimentary sequence, and folds of the wavelength and amplitude of the subdomes do not exist in the enveloping units. Broadly, subdomes and intervening synforms are organized into two elongate, en echelon compartments within the Naxos dome that are separated by a steeply dipping median high strain zone (Figure 14). [30] In the southern portion of the dome, the median highstrain zone demarcates the boundary between the southern and central subdomes, which are separated by steeply dipping migmatitic fabric trending N S, deformed metasedimentary septa, and intervening layers of deformed leucogranitic gneiss in diatexite (Figures 2 4 and 14 and Figures S1 and S2). This high strain zone projects northward toward the NW margin of the dome, where migmatitic foliation shows a low angle discordance with the dome contact. In contrast, the central and northern subdomes are separated by an E W trending pinched overturned synform with folds that cascade down into the synform along the edges of the northern subdome (B B, C C : Figures 2 and 3a). Steeply dipping migmatitic foliation of dominantly N S orientation is common in regions lacking subdomes. These regions are moreover characterized by folding within the migmatite, or migmatite units interfolded with metasedimentary septa about generally N S trending, subhorizontal fold axes (Figure 6c). [31] AMS and structural analysis reveals the pattern of strain induced fabrics within the Naxos migmatites. Structural and magnetic fabrics record strain developed during the rheological transition from dominantly viscous flow at the magmatic stage (i.e., diatexite) to viscoplastic flow in a partially molten system (i.e., crystal mushes and metatexite) at decreasing temperature [cf. Vanderhaeghe, 2009]. Fabrics produced during viscous flow may be less well preserved in the Naxos migmatites, owing to their subsequent evolution during deformation associated with exhumation. Therefore, the preservation of migmatitic fabric, and particularly inferences about flow history, may be biased to emphasize the deformation acquired during later stages of dome development as strain markers get locked into two 12 of 24 Figure 10. Low field anisotropy of magnetic susceptibility (AMS) results for migmatite samples of the Naxos dome. Data are station averages for 155 localities (3058 cubic specimens) of the Naxos migmatites analyzed by Kruckenberg et al. [2010]. (a) Total degree of magnetic anisotropy, P, versus bulk magnetic susceptibility, K m. All values of P are corrected for the diamagnetic contribution (Table 1) [Kruckenberg et al., 2010]. The majority of samples (N = 140) have magnetic properties that fall within the field of paramagnetism, and biotite is the primary carrier of the AMS in the Naxos migmatites. A subset of samples (N = 15) have magnetic properties controlled by magnetite [Kruckenberg et al., 2010]. (b) Shape parameter, T, versus bulk magnetic susceptibility, K m, illustrating the large variation in the organization of the migmatitic fabric. The majority of samples are characterized by oblate fabrics. (c) Shape parameter, T, versus total degree of magnetic anisotropy, P, suggestingthatmoreoblatesam- ples are more anisotropic, with the most anisotropic samples having prolate shape parameters.

13 Table 1. Low Field Anisotropy of Magnetic Susceptibility (AMS) Measurements for Naxos Migmatite Stations a Locality n Latitude ( N) Longitude ( E) K m K 1 Dec (10 6 [SI]) T P P DC (deg) K 1 Inc (deg) K 1 s K 2 Dec (max/min) (deg) K 2 Inc (deg) K 2 s K 3 Dec (max/min) (deg) K 3 Inc (deg) K 3 s (max/min) NX / / /7.6 NX / / /1.7 NX / / /2.5 NX / / /2.5 NX / / /3.2 NX / / /6.4 NX / / /4.0 NX / / /4.3 NX / / /1.9 NX / / /1.4 NX / / /4.5 NX / / /0.8 NX / / /2.8 NX / / /2.2 NX / / /1.2 NX / / /3.5 NX / / /1.2 NX / / /2.9 NX / / /3.1 NX / / /6.2 NX / / /1.6 NX / / /1.9 NX / / /2.4 NX / / /4.9 NX / / /6.1 NX / / /12.1 NX / / /2.2 NX / / /2.6 NX / / /2.3 NX / / /4.8 NX / / /9.5 NX / / /4.9 NX662A / / /5.9 NX662B / / /2.9 NX663A / / /6.9 NX663B / / /7.1 NX664A / / /3.7 NX664B / / /4.3 NX665A / / /4.3 NX665B / / /3.6 NX666A / / /7.0 NX666B / / /7.9 NX667A / / /3.7 NX667B / / /5.5 NX668A / / /1.3 NX668B / / /1.9 NX669A / / /2.1 NX669B / / /1.5 NX670A / / /4.0 NX670B / / /2.7 NX671A / / /1.7 NX671B / / /1.5 NX672A / / /1.4 NX672B / / /2.3 NX673A / / /1.7 NX673B / / /1.8 NX674A / / /1.5 NX674B / / /2.2 NX675A / / /0.9 NX675B / / /1.0 NX676A / / /0.9 NX676B / / /1.4 NX677A / / /1.0 NX677B / / /2.1 NX678A / / /3.0 NX678B / / /3.2 NX679A / / /5.0 NX679B / / /2.9 NX680A / / /5.2 NX680B / / /6.7 NX681A / / / of 24

14 Table 1. (continued) Locality n Latitude ( N) Longitude ( E) K m K 1 Dec (10 6 [SI]) T P P DC (deg) K 1 Inc (deg) K 1 s K 2 Dec (max/min) (deg) K 2 Inc (deg) K 2 s K 3 Dec (max/min) (deg) K 3 Inc (deg) K 3 s (max/min) NX681B / / /2.3 NX682A / / /4.7 NX682B / / /8.9 NX683A / / /2.3 NX683B / / /3.3 NX684A / / /11.2 NX684B / / /13.0 NX686A / / /7.9 NX686B / / /4.0 NX688A / / /4.4 NX688B / / /4.4 NX690A / / /3.7 NX690B / / /3.0 NX / / /2.6 NX / / /7.1 NX / / /3.7 NX / / /5.3 NX / / /2.7 NX / / /1.4 NX701A / / /4.7 NX701B / / /1.3 NX / / /4.6 NX703A / / /7.4 NX703B / / /4.8 NX703C / / /5.4 NX703D / / /6.2 NX / / /0.8 NX / / /5.0 NX / / /2.8 NX / / /8.2 NX / / /7.7 NX / / /2.3 NX710A / / /1.6 NX710B / / /1.6 NX711A / / /3.7 NX711B / / /2.2 NX712A / / /2.8 NX712B / / /2.7 NX713A / / /4.4 NX713B / / /4.1 NX714A / / /7.0 NX715A / / /4.5 NX715B / / /5.2 NX716A / / /4.3 NX716B / / /3.2 NX717A / / /2.6 NX718A / / /4.5 NX718B / / /5.1 NX719A / / /2.6 NX719B / / /4.6 NX720A / / /5.2 NX720B / / /3.7 NX721A / / /8.0 NX721B / / /8.5 NX722A / / /2.6 NX722B / / /3.6 NX723A / / /1.9 NX723B / / /3.6 NX724A / / /14.7 NX724B / / /7.2 NX725A / / /6.6 NX725B / / /9.8 NX726A / / /4.2 NX726B / / /6.3 NX727A / / /1.9 NX / / /12.7 NX / / /18.6 NX / / /2.0 NX / / /2.9 NX / / /6.2 NX / / / of 24

15 Table 1. (continued) Locality n Latitude ( N) Longitude ( E) K m K 1 Dec (10 6 [SI]) T P P DC (deg) K 1 Inc (deg) K 1 s K 2 Dec (max/min) (deg) K 2 Inc (deg) K 2 s K 3 Dec (max/min) (deg) K 3 Inc (deg) K 3 s (max/min) NX / / /3.6 NX / / /4.7 NX / / /7.2 NX / / /4.6 NX / / /4.0 NX / / /1.9 NX / / /5.1 NX / / /3.5 NX / / /1.7 NX / / /2.8 NX / / /11.1 NX / / /2.0 NX / / /1.5 a Data are sample averages of multiple cubic specimens (3058 total cubic specimens) for 155 migmatite localities throughout the core of the Naxos dome. Complete data set, methodology, and identification of magnetic carriers are described by Kruckenberg et al. [2010]. The number of cubic specimen (n) at a given locality is provided with the geographic coordinates (latitude and longitude) of the sample locality. Magnetic parameters are based on the formulation of Jelínek [1978]; K m, bulk magnetic susceptibility; T, shape parameter of the AMS; and P, the total degree of magnetic anisotropy. P DC is the recalculated total degree of magnetic anisotropy that is corrected for the diamagnetic contribution in the Naxos migmatites [Kruckenberg et al., 2010]. The declination and inclination of the principal directions of the AMS are reported as Dec and Inc, respectively. The s values reported for each of the principal susceptibility axes are the angular values of the corresponding 95% confidence ellipse semiaxes. phase partially molten rocks upon crystallization. Interpreting the fabric in migmatites, which is the result of the integrated deformation history, therefore requires considering the transient rheology of the migmatites as they participated in dome evolution. [32] Structural fabrics revealed by AMS were dominantly acquired upon crystallization. Consequently, the patterns of the strain induced fabric are interpreted to reflect the kinematic axes of bulk viscoplastic flow (i.e., flow planes and lines) in the migmatites during development of the dome [Kruckenberg et al., 2010]. The pattern of lineation (field and magnetic) suggests that bulk flow recorded in the migmatites was dominantly oriented parallel to the long axis of the dome (Figure 14), and therefore in the direction of regional extension and top to the NNE shear. Magnetic lineation, K 1, varies throughout the dome, with the majority of localities having intermediate to gently plunging lineation (Figure 12b). However, regions of steeply plunging lineation are also observed in the central portion of the dome (Figure 12b), associated with steeply dipping fabrics in the central subdome and the pinched E W trending synform (Figures 2 4 and Figures S1 and S2). This pattern of highly variable lineation in the migmatitic core contrasts gently plunging, dominantly N S oriented lineation in the mantling units. The contrast between highly variable structural fabrics in the migmatitic core relative to the mantling sequence, suggest that flow in the migmatites and the mantling units responded to distinct forces during the development of the Naxos dome Origin of the Migmatite Dome [33] A model that successfully explains the dynamics of the Naxos migmatite dome must account for the nature of contacts between dome and mantling units, the presence of subdomes, pinched synforms, the high strain zone, and the pattern of migmatitic foliation and lineation. Any model must also account for the record of folding and detachment tectonics in the marble and schist units that mantle the migmatite, and contrast structures in the mantling metasedimentary sequence with those in the migmatite core. [34] Several models have been proposed for the development of the Naxos dome. Jolivet et al. [2004] proposed that A type domes (long axis of dome parallel to extension direction) in the Aegean region, and at Naxos in particular, form as a result of the amplification of displacement along early detachments, producing constrictional strain and folding of high temperature units in the flowing lower crust. However, the internal structure of the dome is not addressed in detail in this model. Other models suggest the combined effects of E W regional shortening and top to the NNE shearing [Urai et al., 1990; Buick, 1991a, 1991b; Avigad et al., 2001], or call on the role of buoyancy forces (e.g., diapirism, density driven convection) [Jansen and Schuiling, 1976; Vanderhaeghe, 2004]. Since the firstorder structure on Naxos has been attributed to interference between distinct fold generations [Urai et al., 1990; Buick, 1991a, 1991b; Avigad et al., 2001], a mechanism commonly invoked for dome formation [e.g., Myers and Watkins, 1985; Brown et al., 1992; Yin, 2004], we briefly Figure 11. Comparison and distribution of magnetic parameters obtained by anisotropy of magnetic susceptibility (AMS) analysis in migmatites of the Naxos dome. (a) Bulk magnetic susceptibility (K m ), (b) shape parameter (T), and (c) total degree of magnetic anisotropy (P ). Parameters are plotted at their respective localities along with a simplified geological map of the migmatitic core (Figure 2 and Figure S1) for comparison. Note that the colors in Figures 11b and 11c for T and P correspond to the scale of K m as in Figure 11a. The patterns of the magnetic parameters at the scale of the dome illustrate (1) the dominantly paramagnetic nature of the Naxos migmatites (biotite magnetic carrier; K m < SI), (2) that regions of dominantly oblate fabrics are associated with steeply dipping fabrics in the cores and margins of subdomes, and (3) that more highly anisotropic samples correspond either to intensely oblate fabrics or to samples in which the magnetic mineralogy is controlled by magnetite (K m > SI; N = 15 samples). 15 of 24

16 Figure of 24

17 Figure 12. (a) Fabric pattern of the magnetic foliation in the core of the Naxos dome. Note the close correspondence of the orientation of the AMS foliation and the migmatitic foliation measured in the field as indicated by thin contour lines. Stereonet inset shows the orientation of K 3 (pole to magnetic foliation) obtained from the AMS analyses. (b) Fabric pattern of the magnetic lineation (K 1 ) in the migmatitic core. Note the variation of the plunge of the magnetic lineation with steeply plunging K 1 occurring in the cores of some subdomes and associated synforms in the central portion of the dome. Magnetic lineation is dominantly N S oriented and of gentle plunge in other regions of the migmatitic core. Stereonet inset shows the orientation of K 1 (magnetic lineation) obtained from the AMS analyses. The AMS magnetic fabric attitudes (foliation and lineation) are given in greater detail in Figure S1 along with the numerical values of dip or plunge at respective stations sampled for AMS. 17 of 24

18 the migmatitic infrastructure, since the style, wavelength, and amplitude of subdomes and synforms are distinct from the regional structures that characterize the mantling units. Moreover, structures and fabrics within the core do not project into the enveloping units and are locally discordant at the migmatite mantling sequence contact. [36] While polyphase folding on Naxos was important in developing the structures observed in the metamorphic sequence that mantles the dome [Urai et al., 1990; Buick, 1991a, 1991b; Avigad et al., 2001], it is insufficient to Figure 13. (a) Shape parameter, T, of the anisotropy of magnetic susceptibility (AMS) plotted as a function of the plunge of magnetic lineation, K 1, on corresponding gentle, intermediate, and steeply dipping migmatitic foliation. Plot shows that oblate AMS fabrics occur on foliation planes of all dip. Steeply plunging magnetic lineation (>50 60 ) is restricted largely to the field of oblate fabric. (b) T as a function of the trend of magnetic lineation, K 1, on corresponding migmatitic foliation of variable dip. Plot demonstrates that oblate fabrics dominate for all orientations of the migmatitic fabric, whereas prolate fabrics are largely restricted to NNE SSW oriented layers (parallel to the stretching direction in mantling units). There is a strong concentration of lineations plunging NNE SSW, and other orientations are primarily associated with oblate fabrics. Symbols correspond to those in Figure 12a. review criteria by which to assess its role in developing the complex migmatitic infrastructure preserved on Naxos. [35] Doming by fold interference implies either a succession of strain ellipsoids with shortening axes at high angle to one other, or complex three dimensional deformation owing to a transpressional or transtensional kinematic regime [e.g., Tikoff and Fossen, 1999]. Fold interference is expected to result in two sets of subvertical schistosity planes at high angle (Figure 15a), which is not observed in the Naxos migmatites. Additionally, the structural fabrics recorded within the core of domes should be kinematically consistent with those in mantling units, implying a mechanical coupling between the dome core and the mantling units (Figure 15a). This is not demonstrated by the strain pattern in Figure 14. Synoptic figure illustrating the simplified largescale structural pattern in the migmatitic core of the Naxos dome. Primary features, inferred from combined structural and magnetic analysis of the Naxos migmatites, include subdomes separated by either a steeply dipping high strain zone or pinched synforms. In this simplified conceptual view, the median high strain zone can be viewed as subdividing the elongate migmatite dome into a western, and a composite eastern, subdome compartment. Magnetic lineations suggest that flow in the migmatite core was dominantly parallel to NNE SSW extension but variable in plunge (see Figure 12 and Figure S1). 18 of 24

19 Figure 15. Conceptual figure illustrating dome forming processes. (a) Doming by fold interference. Note the continuity of structures between migmatites and mantling rocks and the presence of foliations at high angle to each other. This pattern is not observed in the Naxos dome. (b, c) Doming driven by buoyancy dominated flow, including diapirism/ polydiapirism (Figure 15b) and intradome density driven crustal convection during diapirism. Both of these processes are capable of generating the subdomes that characterize the migmatitic core of the Naxos dome. (d, e) Doming driven by isostasy dominated flow. Convergent flow of the partially molten and magmatic crust converges at depth, upwelling beneath the dome, and forming a zone of high strain that separates domains of upward flow. This flow pattern could be symmetric (Figure 15d) or asymmetric depending on the relative contributions of horizontal and vertical flow. account for the complex structural characteristics of the Naxos migmatites, which are indicative of flow under a magmatic or partially molten state. Similarly, models that best account for the record of deformation in the mantling units and that emphasize the exhumation of the middle crust during top to the NNE shearing [Gautier et al., 1993; Gautier and Brun, 1994] are not able to account for the three dimensional shape and internal structures of the migmatite dome [Vanderhaeghe, 2004]. [37] Consequently, important questions about dome formation on Naxos involve the relative role of buoyancy and isostasy in controlling the first and second order structures of the migmatite dome. In contrast to models involving fold interference or strong shearing with sheath folds, models of buoyancy and isostasy invoke motion between the core magmas and partially molten rocks relative to the mantling rocks. Buoyancy dominated flow emphasizes the role of density variations in the formation of Rayleigh Taylor gravitational instabilities (i.e., diapirism) [Dixon, 1975; Talbot, 1979; Ramberg, 1981; Cruden, 1988, 1990; Jackson and Talbot, 1989]. Isostasy dominated flow involves upward motion of low viscosity crust in response to extension, as in a metamorphic core complex [e.g., Brun et al., 1994; Axen et al., 1998] Buoyancy Dominated Flow [38] In the case of diapirism, the driving force for doming is the buoyancy of the migmatites forming the core of the domes [Ramberg, 1981]. The rise of high melt fraction crust may occur if the buoyancy contrast is favorable for diapirism (Figures 15b and 15c) since silicic rocks containing partial melt are less dense and less viscous than the material from which they segregate [Jurewicz and Watson, 1984; Burg and Vanderhaeghe, 1993; Soula et al., 2001; Vanderhaeghe, 2009]. A conceptual framework in which to understand deformation patterns associated with buoyancydriven flow is well established [e.g., Dixon, 1975; Talbot, 1979; Ramberg, 1981; Cruden, 1988, 1990; Jackson and Talbot, 1989]. Criteria that would be consistent with buoyancy driven flow are listed below, together with an evaluation of their validity for the Naxos dome. [39] 1. In diapirism, the internal fabric of the dome core should be acquired under the magmatic state and reflect a dominantly upward en masse motion of the low density material relative to denser mantling rocks as the diapir rises (Figures 15b) [Cruden, 1988; Jackson and Talbot, 1989]. The upward flow of dome migmatites on Naxos is consistent with (1) fabrics in the migmatites and mantling units that steepen and are sheared along the contact with the dome; (2) the overturning and infolding mantling units that wrap overturned migmatitic foliation in the northwest portion of the dome, consistent with core mantle relationships predicted by diapirism; and (3) dikes that are structurally rooted in the dome migmatites and intrude the mantling metasedimentary sequence are variably deformed, indicating outward rotation during relative upward motion of the dome core prior to transposition associated with top to the NNE directed shear [Vanderhaeghe, 2004]. [40] 2. In diapirism, the core fabric of the migmatite dome is also expected to be distinct from that of the mantling units since the magmatic flow is solely controlled by buoyancy and thus is independent from the regional tectonics. In the Naxos dome, migmatites are affected by complex form surfaces in the mantling units that can be described geometrically as complex 3 D folds that developed during coeval E W contraction and top to the NNE shear [Urai et al., 1990; Buick, 1991a; Avigad et al., 2001]. However, 19 of 24

20 Figure 16. Isometric block diagram emphasizing the three dimensional geometry of the Naxos migmatite dome, subdome infrastructure, and dome dynamics. The strain pattern within the migmatitic core, as revealed by the structural and magnetic fabric study, suggests that flow in migmatites occurred by a combination of buoyancy and isostasy driven flow during NNE SSW regional extension. East west contraction, possibly enhanced by convergent flow of the partially molten crustal layer, is consistent with the presence of steeply dipping migmatitic foliation throughout much of the dome and the formation of the median high strain zone. The variability of the internal structure in the migmatite core contrasts that of the mantling units, attesting to the importance of dynamics internal to the migmatites during dome formation. For simplicity, the metasedimentary sequence that mantles the dome is only shown schematically; it would extend both around and over the dome fabrics highlighted in this diagram. Moreover, not shown are fabrics developed in the mantling units that reflect the dominant effects of shearing during extension. the Naxos dome is characterized by a complex migmatitic infrastructure containing kilometer scale subdomes and synforms that are distinct from the regional structures that characterize the mantling units (Figures 1 4 and Figures S1 and S2). The northern and central subdomes also display discordant structural fabrics (foliation and lineation) at the contact with enveloping units, and have curvilinear fold axes at high angle to the overall NNE trend of the dome (Figure 2 and Figure S1). Moreover, the results of the AMS analysis reveal the presence of variably oriented, steeply plunging lineations in the migmatitic core (Figures 12b and 13). Lineations in the core contrast gently plunging, NNE SSW oriented lineation in the mantling units, and suggest a combination of vertical and horizontal flow within the Naxos migmatites (Figure 16). These observations attest to different deformational processes in the mantling units and those related to the internal dynamics of the migmatites, consistent with buoyancy dominated flow. [41] 3. In buoyancy dominated flow, subdome formation is expected to be driven by density variations and therefore may correspond to multiple diapiric intrusions (i.e., polydiapirism/nested diapirs; Figure 15b) [Stephansson, 1975; Collins, 1989; Bouchez and Diot, 1990; Weinberg and Schmeling, 1992] or regions of convective overturning during diapirism (Figure 15c) [e.g., Talbot, 1979; Weinberg, 1997]. Compositional density differences between more buoyant leucogranite rich zones and denser biotite rich, restitic zones (as is observed in the heterogeneous Naxos migmatites) may have favored diapirism and/or facilitated convective overturning during diapirism resulting in the formation of subdomes (Figures 15 and 16). On Naxos, subdomes represent second order dome structures (kilometer scale) that broadly occur within two en echelon compartments separated by a median high strain zone (Figure 14). The geometry of subdomes within the migmatitic core is consistent with the formation of multiwavelength gravita- 20 of 24