Solenn Hertgen
email: solenn.hertgen@univ-rennes1.fr

Philippe Yamato

F G Luiz

Samuel Morales

Angiboust

Morales

Samuel Angiboust

Evidence for brittle deformation events at eclogite-facies P-T conditions (example of the Mt. Emilius klippe, Western Alps)

Keywords: brittle-ductile deformation, eclogite breccias, fluid-rock interaction, subduction interface

Eclogitic rocks are crucial for the understanding of tectonic processes as they provide key constraints on both the P-T-t evolutions and the deformation modes sustained by rocks in subduction zones. Here we focus on eclogitised and deformed mafic bodies that are exposed within granulites from the continental basement slice of the Mt. Emilius klippe (Western Alps, Italy). These eclogites exhibit highly deformed garnetite and clinopyroxenite layers. In some places, these deformed rocks (up to mylonitic grade) can be found as clasts within meter-thick brecciated fault rocks that formed close to the lawsonite-eclogite facies peak P-T conditions. Garnet-rich layers are dominated by brittle features, whereas deformation within clinopyroxene-rich layers is accommodated by both creep and fracturing. We present a petrostructural study of these eclogites, that allows to track the brittle deformation history associated with chemical evolution. Based on these data, we propose a new tectonometamorphic model for these rocks, related to the alpine eclogitic stage. This model is consistent with the coexistence of both ductile and brittle features that developed at similar P-T conditions (i.e., at P ~ 2.15-2.40 GPa and T ~ 500-550 °C), and closely associated with fluid circulations. Our study demonstrates that crustal material, buried along the subduction interface at HP-LT conditions, can record several successive brittle events in places where deformation is classically envisioned as ductile. We suggest, based on our observations, that strain-rate increase along plate interface shear zones may trigger fracturing and fluid infiltration which in turn enables brittle-ductile instabilities along these deformation networks.

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M A N U S C R I P T

Introduction

The rocks constituting Earth's crust are traditionally thought to deform in a brittle manner at shallow depths and by ductile flow at deeper levels (e.g., [START_REF] Brace | Limits on lithostatic stress imposed by laboratory experiments[END_REF][START_REF] Ranalli | Rheological stratification of the lithosphere[END_REF]. The transition between the brittle crust, affected by discontinuous deformation (i.e., fracture-dominated), and the ductile crust, affected by continuous deformation (i.e., flow-dominated), known as the brittle-ductile transition, is characterised by networks of shear zones where both modes of deformation coexist (e.g., [START_REF] Handy | Seismicity, structure and strength of the continental lithosphere[END_REF]Hobbs et al., 1986;[START_REF] Passchier | The generation of ductile and brittle shear bands in a low-angle mylonite zone[END_REF].

However, field observations, geophysical studies, and laboratory experiments indicate that brittle and ductile deformation mechanisms can actually affect rocks at all crustal levels (e.g., [START_REF] Abers | The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow[END_REF][START_REF] Angiboust | Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)[END_REF][START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF]Austrheim and Andersen, 2004;Mancktelow, 2009;[START_REF] Pennacchioni | Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary nappe, Italian Western Alps)[END_REF][START_REF] Simpson | Deformation of granitic rocks across the brittle-ductile transition[END_REF]. For instance, the formation of brittle features of various length scale, such as breccias, pseudotachylytes and fractured garnets, has been documented for pressure (P) and temperature (T) conditions up to eclogite-facies conditions, in both hydrated oceanic crust (e.g., Austrheim and Andersen, 2004;[START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF]John and Schenk, 2003) and dry continental crust (e.g., Austhreim et al., 1996). The origin of these brittle structures can be related to seismic faulting, intermediate-depth earthquakes, and/or fluid percolation (e.g., [START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF]Austrheim and Boundy, 1994;Austrheim et al., 1997;[START_REF] Engvik | Interaction between fluid flow, fracturing and mineral growth during eclogitization, an example from the Sunnfjord area, Western Gneiss Region, Norway[END_REF][START_REF] Hacker | Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?[END_REF]John and Schenk, 2006;Lund and Austrheim, 2003;[START_REF] Rietbrock | A narrowly spaced double-seismic zone in the subducting Nazca Plate[END_REF]. In many cases, these brittle structures are precursors on which ductile shear zones nucleated (e.g., [START_REF] Brodie | Deep crustal extensional faulting in the Ivrea Zone of northern Italy[END_REF]Lund and Austrheim, 2003;[START_REF] Pennacchioni | Control of the geometry of precursor brittle structures on the type of ductile shear zone in the Adamello tonalites, Southern Alps (Italy)[END_REF][START_REF] Pennacchioni | Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions[END_REF][START_REF] Segall | Nucleation of ductile shear zones on dilatant fractures[END_REF].

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In addition, field evidence, microstructural observations, and chemical data show that both brittle and ductile deformation modes can occur simultaneously up to eclogite-facies conditions (e.g., [START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF]Lund and Austrheim, 2003). However, there is a need to explain the coexistence (both in space and time) of these two deformation modes at identical P-T conditions. In order to address this issue, we present the results of a petrostructural study performed on eclogites exposed within the continental-derived tectonic slice of the Mt. Emilius klippe (Western Alps, Italy). These rocks have been highly deformed in a ductile manner, but also exhibit brecciated features [START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF]. Therefore, they constitute an excellent opportunity for improving our understanding of deformation mechanisms affecting rocks buried in subduction zones.

Based on a multi-scale (i.e., from outcrop-to grain-scale) and a multi-method analytical approach (i.e., optical and electron microscopy, electron microprobe and electron backscatter diffraction analyses), we propose a new tectono-metamorphic model for the formation of the eclogites located within a major shear zone (Arbolle shear zone) at the base of the Mt. Emilius klippe, which involves concomitant brittle and ductile deformation at the metamorphic peak, in the eclogite-facies P-T conditions, and associated with multiple fluid circulation events. We finally discuss possible implications for deformation mechanisms, fluid circulations and fluid/rock interactions taking place in subduction zones under high pressure (HP) conditions.

Geological setting of the Mt. Emilius klippe

Origin and affinity

The Mt. Emilius klippe is exposed in the southern Aosta valley (Piemonte units, Italy)

A C C E P T E D M A N U S C R I P T 5 and overlies the Piemonte meta-ophiolites (Fig. 1a). This klippe corresponds to a continental basement unit, belonging to the austro-alpine nappe-stack (Ballèvre et al., 1986) and connected to the Jurassic margin of the Adriatic plate. The continental basement slices (e.g., Mt. Emilius klippe, Glacier-Raffray unit, Etirol-Levaz slice, Theodul Glacier unit) constitute the "inferior austro-alpine units" (Ballèvre et al., 1986;Weber and Bucher, 2015) or "eclogitic lower austro-alpine outliers" [START_REF] Dal Piaz | The Austroalpine-Piedmont nappe stack and the puzzle of Alpine Tethys[END_REF]. They crop out between the lower grade Combin unit, and the higher grade Zermatt-Saas unit [START_REF] Dal Piaz | Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas ophiolite, western Alps[END_REF]. These continental slices contain both mafic and felsic intrusions (e.g., Beltrando et al., 2010;[START_REF] Dal Piaz | Il lembo di ricoprimento del Glacier-Rafray (Dent Blanche ls)[END_REF], and record a multi-stage metamorphic history, pervasive deformation, and numerous reactived pre-alpine contacts during alpine subduction and exhumation [START_REF] Compagnoni | The Sesia-Lanzo Zone: high pressure-low temperature metamorphism in the Austroalpine continental margin[END_REF][START_REF] Beltrando | Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study[END_REF]. Hence, these continental basement slices could correspond to extensional allochtons of the hyper-extended continental margin that surrounded the Piemonte-Ligurian ocean (Beltrando et al., 2010;[START_REF] Beltrando | Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study[END_REF][START_REF] Dal Piaz | Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas ophiolite, western Alps[END_REF].

A polymetamorphic history

The protoliths of the Mt. Emilius klippe rocks are pre-alpine in age, and mostly correspond to granitoids and sillimanite-biotite-garnet-plagioclase paragneisses, which include dykes and pegmatitic quartz-plagioclase veins, calc-sillicates, marbles and mafic rocks. The mafic rocks correspond to garnet-clinopyroxene-plagioclase-hornblende granulites and banded gabbros [START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF][START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF]. The Mt. Emilius klippe rocks underwent a first stage of metamorphism at medium pressure-high temperature (MP-HT) conditions (i.e., 0.6-0.8 GPa and 700 °C; Dal [START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF]Fig. 1b, point 1). The MP-HT assemblages are preserved as relics similar to those found in the Sesia-Lanzo unit (Dal Piaz et , 1983). This was followed by a first stage of retrogression under amphibolite-facies P-T conditions (Fig. 1b, point 2), prior to the high pressure-low temperature (HP-LT) alpine metamorphism [START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF].

A C C E P T E D M A N U S C R I P T 6 al.
Rocks of the Mt. Emilius klippe suffered a later alpine tectono-metamorphic overprint, that essentially occurred under HP conditions [START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF][START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF]. The P-T conditions of this eclogitic event (Fig. 1b, point 3a) were initially estimated at 1.1-1.3

GPa and 460-470 °C by Dal Piaz et al. (1983). However, the P-T conditions for the Mt.

Emilius eclogites have been re-estimated at 2.15-2.40 GPa and 500-550 °C [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF]; see dark star in Fig. 1b). These conditions are consistent with peak pressures of 2.2-2.4

GPa [START_REF] Angiboust | The Zermatt-Saas ophiolite: the largest (60-km wide) and deepest (c. 70-80 km) continuous slice of oceanic lithosphere detached from a subduction zone?[END_REF]Fig. 1b, point 3b), 2.1-2.3 GPa (Weber and Bucher, 2015; see dark circle in Fig. 1b) and 2.0-2.5 GPa (Fassner et al., 2016; see square in Fig. 1b) estimated for the underlying Zermatt-Saas ophiolitic unit, the Theodul Glacier unit and the Etirol-Levaz slice, respectively.

The alpine metamorphic evolution was accompanied by polyphased deformations, as attested by various mineral relics and structures [START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF][START_REF] Pennacchioni | Evoluzione strutturale del M. Emilius (Austroalpino, Alpi occidentali)[END_REF][START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF]. Two main deformation episodes affected the mafic lithologies at eclogite-facies conditions. The first one induced development of foliation parallel to the protolith layering, incomplete transformation of the granulites and ended up by a brittle deformation event that produced veins at eclogite-facies P-T conditions. The second deformation event led to the formation of S-L tectonites and mylonites (according to Pennacchionni, 1996). Salt-rich aqueous fluids infiltrated the rocks during deformation [START_REF] Scambelluri | Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)[END_REF]. These fluids seem to have played a key role during eclogitization of the granulite protoliths, deformation localization and enhanced metamorphic re-equilibration [START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF][START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF].

Finally, the eclogitic facies assemblages underwent a retrograde overprint, first under

A C C E P T E D M A N U S C R I P T 7
blueschist conditions and later under greenschist-facies P-T conditions, associated with late alpine exhumation (Bearth et al., 1980;[START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF][START_REF] Pennacchioni | Progressive eclogitization under fluid-present conditions of pre-Alpine mafic granulites in the Austroalpine Mt Emilius Klippe (Italian Western Alps)[END_REF]Fig.1b, point 4). The late greenschist-facies overprint, of relatively moderate intensity inside the Mt.

Emilius unit, is mainly visible in the 100-200 meters basal part of the klippe, i.e., at the contact with the underlying HP ophiolites that belong to the Zermatt-Saas unit.

Sampling strategy and analytical methods

Sampling strategy

150 samples were collected in the Mt. Emilius klippe (Fig. 1a), from which 75 polished thin sections were made. The samples come from different localities (e.g., Arbolle, Menabreaz, Laures, Bon Plan) and are distributed over the whole klippe (Fig. 1c). In this study, we focus on the Arbolle area, located along the western side of the Mt. Emilius klippe, above the Echo lake (N45°40'6.2''-E007°21'43.8''; Fig. 1c). Indeed, although brittle structures have been observed in other localities, the Arbolle area shows the largest amount of these deformation features and the most complete record of cross-cutting relationships.

Scanning electron microscopy and microprobe

Mineral characterisation, study of their spatial distribution and semi-quantitative chemical analyses were carried out with an Ultra 55 Plus Carl Zeiss Scanning Electron Microscope (SEM) equipped with an energy-dispersive spectrometer and operating at an accelerating voltage of 20 kV at the German Research Centre for Geosciences (GFZ) of Potsdam. The quantification of Si, Ti, Al, Fe, Mn, Mg, Ca, Na and K was performed with a

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8 JEOL JXA 8230 Electron Probe Micro-Analyser (EPMA) at the GFZ, using 15 kV acceleration voltage, a beam current of 20 nA and a beam diameter of 10 μm and the following standards: orthoclase (Al, Si, K), rutile (Ti), wollastonite (Ca), albite (Na), periclase (Mg), hematite (Fe) and rhodonite (Mn). X-ray element maps were aquired using a 15 kV acceleration voltage, a beam current of 30 nA and a beam size of 2 µm.

Electron backscatter diffraction

Crystallographic-preferred orientation (CPO) of HP clinopyroxene was determined via automatic indexation of Electron BackScatter Diffraction (EBSD) patterns [START_REF] Adams | Orientation imaging: the emergence of a new microscopy[END_REF][START_REF] Prior | The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks[END_REF]. Measurements were performed in two samples sectioned normal to (Hielscher and Schaeben, 2008;Mainprice et al., 2011), considering only measurements with CI > 0.2. All the pole figures were plotted in the tectonic reference frame, with vertical foliations oriented E-W and horizontal lineations in the same direction.

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Field observations and main rock-types

In the Arbolle area, we observed a large shear zone within the granulites (Fig. 2a), which comprises i) granulite clasts (Fig. 2b) representing undeformed host rock slices, ii) eclogitised and foliated mafic granulites (Fig. 2c), iii) mafic boudins (Fig. 2d), comprising garnet, clinopyroxene, epidote, white micas, quartz, pyrite and calcite, and iv) eclogite breccias (Fig. 2e), composed of garnet, clinopyroxene, and/or epidote broken fragments. The clasts within breccias have variable sizes, are randomly oriented, and generally present a mylonitic foliation. A fine-grained eclogitic matrix formed by clinopyroxene, epidote and garnet envelops the clasts. Breccias crop out as fractured metric boudins, sometimes surrounded by 10-15 centimeters thick layers formed by almost 100% garnet. These garnetites are fractured, cemented by a first clinopyroxene foliation and embedded, in some places, in a second eclogitic foliation containing clinopyroxene, epidote and garnet (Fig. 2f). All rocktypes are crosscut by different types of HP veins (e.g., garnet or clinopyroxene; Fig. 2g).

Deformation styles and petro-chemical constraints

The main rock-types and structures observed in the field are here described in more detail, in relationship with the chemical composition of the rock-forming minerals they host (mainly garnet and clinopyroxene, which are ubiquitous in these rocks). Mineral abbreviations are after Whitney and Evans (2010).

Eclogitised granulites

The eclogitised granulites (Fig. 2c) are composed of garnet, clinopyroxene, EPMA analyses carried out on garnet reveal two generations marked by distinct compositions (Fig. 3d). The fractured garnet cores (Grt0) are richer in magnesium (X Py = 0.43-0.44; Fig. 3d). Their high X Mg ratios (X Mg = 0.52-0.54; see Tab. A in supplementary material) indicate that garnet grew at high temperatures (~750 °C, calculated using the methods of [START_REF] Ai | A revision of the garnet-clinopyroxene Fe 2+ -Mg exchange metamorphism[END_REF] and [START_REF] Ravna | The garnet-clinopyroxene Fe2+-Mg geothermometer: an updated calibration[END_REF]; see Tab. C in supplementary material). Garnet rims (Grt1), also found as smaller idiomorphic garnet grains in the clinopyroxene-rich matrix, present a lower Mg content (X Py = 0.06-0.39; Fig. 3d). Such a low X Mg ratio (X Mg = 0.10-0.48; see Tab. A in supplementary material) suggests growth at lower temperature than Grt0 (~550 °C; see Tab. C in supplementary material). From Grt0 to Grt1, the concentration changes are sharp, with decreasing Mg content while the Fe, Ca and Mn contents increase (Fig. 3d; see also [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF].

Two different types of clinopyroxene can be identified microscopically, according to the occurrence of titanite inclusions. They have a composition ranging between diopside (Di 100-Jd 0) and omphacite (Di 50-Jd 50, see Fig. 3d and Tab. B in the supplementary material).

Eclogitic breccias

The eclogitic breccias are composed of garnetite clasts cemented by clinopyroxene (Fig. 4a). At the microscopic scale, the garnetite clasts show an internal foliation formed by clinopyroxene (HP Cpx1) and clinozoisite (Czo1), and are embedded in an external foliation (Fig. 4b) also composed of clinopyroxene (HP Cpx2) and clinozoisite (Czo2), and characterised by a different orientation compared to the older foliation (Fig. 4b-c).

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11 Chemical analyses carried out on garnet (see Tab. 1 and Tab. A in supplementary material) from garnetite clasts reveal a single generation of garnet (Grt1) with homogeneous X Mg ratios, but highly variable Ca abundances (X Gro = 0.43-0.94; Fig. 4d). Garnet crystals from garnetite clasts are sometimes fractured and cemented by a newer garnet generation characterised by slightly higher Fe, Mn and Mg contents, and lower Ca contents (Fig. 4d).

The chemical analyses performed in clinopyroxene from garnetite clasts reveal two clusters with different composition (Tab. 1). The first one corresponds to diopside compositions (X Di = 0.76-1, X Jd = 0-0.24, X Ae = 0-0.13; left Fig. 4d, Tab. B in supplementary content) and the second one corresponds to omphacite compositions in the Morimoto ( 1988) classification (X Di = 0.52-0.57, X Jd = 0.39-0.48, X Ae = 0-0.09; right Fig. 4d, Tab. B in supplementary material).

We carried out EBSD crystallographic orientation measurements on clinopyroxene occurring as an interstitial phase in a garnetite clast (Fig. 4e). The poles (100) of the clinopyroxene are distributed along a discontinuous band parallel to the foliation, with two maxima parallel to X and Y, whereas the poles of ( 010) and ( 001) are concentrated at low angles from the lineation and pole of the foliation, respectively (Fig. 4f). The misorientation angle distribution shows a major peak for low misorientation angles from 2 to 5°, followed by minor peaks with higher misorientation angles (Fig. 4g). Although only determined on a few clinopyroxene grains in this sample (clinopyroxene is a minor phase here), the CPO appears to be very strong. The major peaks for low misorientation angles either indicate that clinopyroxene crystallised with a slightly distorted lattice, or that the grains were reworked, since they display evidence of minor amounts of crystal plasticity after crystallisation. Note that similarly to CPO, the occurrence of most peaks at low misorientation angles could also be related to the limited number of pyroxene crystals analysed in this sample. EBSD crystallographic orientation measurements have been conducted along this pervasive eclogitic foliation, where clinopyroxene occurs as discontinuous bands interleaved with garnet and/or clinozoisite-rich bands (Fig. 5e). The intensity of the CPO is moderate.

The poles of (100) are distributed along a broad band normal to the foliation and with a maximum concentration at low angles to the pole of the foliation. The poles of (001) are also spread along a continuous band, this time sub parallel to the foliation and with maxima lying at low angles to the lineation of reference. The poles of (010) are poorly aligned (Fig. 5f).

The misorientation angle between clinopyroxene neighbour crystals displays a small peak for low misorientation angles (i.e., 2-5°) and otherwise an almost normal distribution of misorientation angles that tend to follow the uncorrelated and random distribution lines (Fig. 5g).

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13

Vein types

Different veins types (i.e., garnet-, clinopyroxene-, clinozoisite-chlorite-and calciterich veins) have been observed at various spatial scales (i.e., from outcrop to thin section scale). Macroscopic clinopyroxene veins crosscut the eclogitic foliation (Fig. 6a) and are composed of clinopyroxene, clinozoisite and garnet. Clinopyroxene filling fractures does not show any growing preferential orientation (minerals can be parallel, perpendicular or randomly oriented compared to the edges of the fractures). Clinozoisite veins, in which white micas can be also found, cut through the eclogitised granulite (Fig. 3a). In these clinozoisite veins, undeformed, perfectly rhombic sections are well identifiable (Fig. 3b). These sections are now replaced by clinozoisite and white micas, suggesting that these sections correspond to lawsonite pseudormorphs. In garnetite clasts, clinopyroxene veins are crosscut by both clinozoisite veins and chlorite veins (Fig. 6b). Garnetite clasts are also crosscut by calcite veins (Fig. 6c; [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF]. These various vein families do not exhibit any preferential orientation.

Garnet veins cut through clinopyroxene crystals (Fig. 6d). Clinopyroxene veins have also been identified (Fig. 6e

Age of the eclogite deformation

Before going further into the description of the deformation, it is important to first clarify its age: i) in the eclogitic structures, pre-alpine garnet crystals have not been clearly identified. Our chemical analyses (section 5) show that all garnet crystals growing in breccias, along the foliation and in veins, are Grt1 type, which thus crystallised during the alpine stage.

ii) Most of clinopyroxene crystals are HP clinopyroxene crystals (see [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF], and, therefore, also formed during the alpine stage. The composition of both diopside and omphacite can be analysed within a single grain without any real structural difference: some grains have diopside-rich cores and omphacite-rich rims but the opposite is also observed.

The presence of diopside instead of omphacite at HP could be due to the high calcium concentrations present in these rocks [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF]. iii) Associated diopside-and omphacite are indifferently observed in fractures cutting through alpine garnetite clasts (see section 5.2; Fig. 4a), parallel to the alpine foliation embedding these clasts (Fig. 5a-b-c), or even, in fractures cutting through this latter foliation (Fig. 6 a). iv) The CPO obtained for HP clinopyroxene, which does not require HT conditions [START_REF] Piepenbreier | Plastic flow of omphacite in eclogites at temperatures below 500 C-implications for interplate coupling in subduction zones[END_REF], is caused by ductile deformation in eclogite-facies P-T conditions. v) Finally, brittle events implying HP clinopyroxene also occurred during this stage. This is evidenced by clasts of HP clinopyroxene, which present the same chemical compositions than those from the eclogitic [START_REF] Angiboust | Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)[END_REF]. Therefore, even if prealpine deformation and mineral relics (i.e., garnet and clinopyroxene) may be preserved in the host granulite (Fig. 3), the brittle and ductile deformations observed in the eclogites from the Arbolle region, and summarised in figure 7, are undoubtedly related to the alpine orogeny.

The deformation occurred close to the HP-LT metamorphic peak in the lawsonite-eclogite facies P-T conditions and in successive steps summarised as:

Eclogitisation and fluid-rock interaction (t1)

In the eclogite-facies P-T conditions, garnetite layers formed at the interface between mafic boudins and granulites. Considering the presence of heterogeneous chemical zoning in garnet (e.g., Fig. 4d) as well as the entrapment of oriented HP clinopyroxene inclusions (Fig. 4b-c), garnetite layers probably resulted from fluid-rock interaction processes during deformation. The rheological contrast existing at the interface between mafic boudins and the host granulite could be responsible for a preferential localisation of deformation, which could have been initiated by a first fracturing event that then allowed pumping of fluids.

In some mafic boudins, eclogitic banding (alternation of garnetite and clinopyroxenite layers; Fig. 2a-g) attests to a rather intense ductile deformation leading to the formation of mylonitic eclogites. A foliation is also present and defined by the HP clinopyroxene in garnetite layers. The CPO patterns observed in the HP clinopyroxene (Fig. 4f), together with predominant low misorientation angles (Fig. 4g), probably results from orientated growth under tectonic stress in HP-LT conditions.

Formation of eclogite clasts by fracturing event (t2)

A C C E P T E D M A N U S C R I P T 16

The more or less layered eclogitised mafic boudins and the associated garnetite levels (when present) are locally fractured as evidenced by the presence of HP clinopyroxene and/or garnet fractures (Fig. 2f-g and 4a). Such brittle behaviour is also observed at the microscopic scale where fractured garnet (Fig. 4d) and fractured HP clinopyroxene (Fig. 6d) are sealed by alpine minerals.

Ductile deformation and eclogitic breccias evolution (t3)

The eclogite clasts, which are in some places layered (Fig. 2a-g), form boudins embedded in an eclogitic foliation defined by HP clinopyroxene, clinozoisite and garnet (Fig. 1972), with foliation and lineation relatively well developed (Fig. 5f). This is in agreement with field evidence and microstructures (e.g., Fig. 5b-c-e). The fabric is also slightly oblique with respect to the main reference frame (foliation/lineation), suggesting a component of noncoaxial deformation during the CPO development (e.g., [START_REF] Abalos | Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain): relationships with strain symmetry during high-pressure deformation[END_REF]. The presence of undulose extinction and subgrain boundaries in the HP clinopyroxene, coupled with the low misorientation angles observed in the histogram (Fig. 5g), both suggest that deformation was mainly accommodated by intra-crystalline plasticity. Assuming a predominant slip system

A C C E P T E D M A N U S C R I P T

17 where the slip plane aligns with the flow plane and the slip direction with the flow direction (e.g., [START_REF] Nicolas | Crystalline Plasticity and Solid State Flow in Metamorphic Rocks[END_REF], then the main slip system in the HP clinopyroxene during ductile deformation was [001] (100).

New fracturing events and formation of HP veins (t4)

Both the elements within the shear zone (i.e., boudins and clasts) and the eclogitic foliation (presented above; t3) are crosscut by HP clinopyroxene veins (Fig. 6a), attesting to a brittle event after ductile deformation (t3). Furthermore, the co-stability of garnet and clinozoisite in these veins (t4) indicates that these two minerals grew under the same P-T conditions as the ductile deformation event (t3). Clinozoisite veins cut through all the structures described above (Fig. 2c, 3a and 6b). These veins include brown rhombic mm-sized crystals of clinozoisite associated with white micas, interpreted as lawsonite pseudomorphs (Fig. 3b). This would imply that these veins, corresponding to former lawsonite veins, were formed in the lawsonite-bearing eclogite field close to P-T conditions of the metamorphic peak. Finally, calcite is present either as veins (Fig. 6c) or in the matrix in direct contact with garnetite boudins and clasts. The fact that all these veins have different chemical compositions (Fig. 6), and that they include garnet and HP clinopyroxene zoning patterns (Fig. 4d and6f), point to the existence of multiple fracturing/healing events in the presence of a fluid phase during deformation at HP-LT conditions that involved a cyclic repetition of the phases t3 and t4.

Implications for the deformation of rocks in subduction zones and the role of fluids

This study confirms the importance of brittle deformation processes during

A C C E P T E D M A N U S C R I P T

18 metamorphism at HP-LT conditions. The tectono-metamorphic history proposed here shows that the rocks from the Mt. Emilius klippe underwent successive switches between brittle and ductile deformation at (or close to) the metamorphic peak pressure, in the lawsonite-eclogite facies P-T conditions. This example, close to peak metamorphic conditions, allows us to discuss i) possible processes leading to such a dual deformation mode at depth, along the subduction interface as well as ii) the role of fluids, which are indubitably present in this case.

Brittle behaviour during eclogite-facies metamorphism

In subduction zones, brecciation and formation of HP fractures could be explained via different processes that could be either external (i.e., related to the tectonic stresses) or internal (i.e., linked to the intrinsic properties of eclogites). When buried under HP-LT conditions, rocks undergo high shear stresses linked to subduction dynamics. At the onset of exhumation, when the metamorphic units detach from the subducting slab, the state of stresses sustained by the rocks changes (Yamato and Brun, 2017). This switch could be responsible for fracturing at depth and could possibly lead to breccias formation (e.g., [START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF][START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF][START_REF] Van Roermund | The dislocation substructures of naturally[END_REF].

However, these clasts formed during brecciation do not systematically show large displacements (Fig. 2f), and the HP clinopyroxene, lawsonite and calcite-filled fractures present random orientations (Fig. 6). Yet, the clasts may have been quickly sealed by matrix garnet or HP clinopyroxene that would have prevented their mutual displacements and frozen the post-brecciation texture. If the evolution and the amount of shearing remain difficult to estimate, differential stress was high enough for failure criteria to be met. Furthermore, shearing appears to have been non-negligible as evidenced by differential rotation of the mafic clasts and development of the eclogitic foliation both within the clasts and wrapping around the clusters of clasts.

A C C E P T E D M A N U S C R I P T 19

To trigger fracturing, the eclogitisation process itself can also be invoked (e.g., Bjornerud et al., 2002[START_REF] Philippot | Deformation processes in eclogitic rocks: evidence for the rheological delamination of the oceanic crust in deeper levels of subduction A C C E P T E D M A N U S C R I P T 29 zones[END_REF]. The modification of chemical and physical properties of the buried material significantly affects how rocks respond to stress at depth. In particular, the progressive eclogitisation, which is assisted by deformation and fluid infiltration leads to mineralogical changes. The consequence of this could affect subduction zone dynamics by modifying both the density and the rheology of the progressively subducting crustal material (e.g., Austrheim and Engvik, 1997). The associated volume change can then be large enough to generate stresses allowing fracturing (e.g., Jamtveit et al., 2016). However, even if local stresses during metamorphic reactions may be high and cause localised fracturing, the amount of eclogitic mafic rocks observed in this area is very low (see Fig. 1c). Consequently, even though the amount of stress variations due to volume changes should not be neglected, especially during formation of the thick garnetite layer (i.e., at the onset of the eclogitisation, stage t1), it does not seem to be the key process at the scale of the entire Mt. Emilius klippe.

Role of fluids at eclogite-facies conditions

Fluid circulation along the subduction interface need to be considered because it enhances strain localisation and critically controls the rheology of rocks (e.g., [START_REF] Angiboust | Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface[END_REF]Austrheim and Engvik, 1997;Bjornerud et al., 2002;[START_REF] Hacker | Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?[END_REF]. On the one hand, it has been shown that fluid release during prograde breakdown of hydrous minerals with increasing temperature may generate brittle features such as veins (e.g., [START_REF] Gao | Mobilization of Ti-Nb-Ta during subduction: evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China[END_REF][START_REF] Philippot | Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction[END_REF] and tectonic breccias (e.g., [START_REF] Angiboust | Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes?[END_REF]. In particular, fluids can be responsible for an increase of internal fluid pressure, leading to brittle deformation by hydro-fracturing processes (e.g., [START_REF] Davies | The role of hydraulic fractures and intermediate-depth earthquakes in generating subduction-zone magmatism[END_REF][START_REF] Healy | Trench-parallel fast axes of seismic anisotropy due to fluid-filled cracks in subducting slabs[END_REF]. Fluids can thus be responsible for the switch from ductile to brittle behaviour as we

A C C E P T E D M A N U S C R I P T

20 observed in the Mt. Emilius area. On the other hand, fracturing allows the pumping and the localisation of fluid circulation (e.g., [START_REF] Sibson | Fluid flow accompanying faulting: field evidence and models[END_REF], Mancktelow, 2006). This process could then help for the re-equilibration of the pre-alpine minerals during the alpine tectonometamorphic events (Austrheim et al., 1996) and facilitate, by strain weakening, ductile deformation between two brittle events. Hence, a fundamental problem that remains to be solved, and that is common to all subduction zones interfaces worldwide, is how to decipher if fluids induced fracturing or, alternatively, if fracturing is responsible for fluid circulation.

The Mt. Emilius case study provide important insights to discuss this point because, whether they are a cause or a consequence of the deformation, fluids play an important role in this system.

Fracturing triggered by high fluid overpressure requires the production or arrival of fluids in a volume of rocks that is sealed by a sufficiently impermeable cap. In our case, fluids were probably initially absent because the Mt. Emilius unit is dry (mainly made of granulites).

The main source that can provide an important amount of fluid is the oceanic Zermatt-Saas unit located just below the Mt. Emilius tectonic slice (Fig. 1). The main question that arises is the significance of the numerous vein systems that are ubiquitous within the Mt. Emilius klippe. While they can reflect the presence of lithostatic to transient supra-lithostatic pore fluid pressures, it remains unsure how these fluids percolated through the Mt. Emilius klippe massif. Hydro-fracturing of the hanging wall [START_REF] Davies | The role of hydraulic fractures and intermediate-depth earthquakes in generating subduction-zone magmatism[END_REF] or propagation of porosity waves (e.g., Connolly, 2010) could be envisioned as possible processes. Alternatively, breccias formation and creation of porosity along the Arbolle shear zone may have enabled fluid pumping, channelizing various chemical contributions from the subduction interface region.

By considering that the influx of fluid is, initially, linked to a fracturing event triggered by an increase of differential stresses, it is possible to reconcile all our observations. The composition of the fluids pumped during each fracturing event could vary, depending on the

A C C E P T E D M A N U S C R I P T

21 pathways followed by fluids through the rocks. All the processes mentioned above (i.e., metamorphic reactions and their rheological impact) can then be activated afterwards and self-maintained by the introduction of fluids in the system.

Conclusion

This study provides new field evidence shedding light on deformation processes that occur along the subduction interface in HP-LT conditions. The structural record observed here is of key importance because it witnesses of processes associated with slicing of metamorphic units at the metamorphic peak from a downgoing slab in a subduction zone. Understanding the significance of HP brittle deformation at eclogite-facies metamorphic conditions thus paves the way for further rheological and geophysical studies focused on mechanical processes rooted in the plate interface region of subduction zones. We have shown here, that ductile behaviour is not the only rule at eclogite-facies conditions as multiple successive brittle events can exist. These findings highlight the fact that lithological heterogeneities (such as mafic dykes or sills, for instance) are important since they facilitate the localisation of the deformation as well as fluid channelling. We suggest that embrittlement of the Arbolle shear zone was triggered by strain rate increase along the subduction interface possibly under lithostatic pore fluid pressure conditions. Enhanced fluid channelization enables then metamorphic reaction to proceed, leading to volume change and further fracturing.

Nevertheless, all these observations raise the more general question of how pervasive fluid circulations initiated in an initially dry rocks, pervasively metamorphosed, when clear localised drains connecting the different garnetite-and breccia-bearing mafic lenses are not observed. This therefore raise the need to i) better understand the complex link between fluid circulation, eclogitisation and deformation localization under HP-LT conditions and to ii)

A C C E P T E D M A N U S C

  figures were calculated and plotted using the MTEX toolbox for Matlab, version 4.2.1

  . 1) and titanite, with lower abundance of calcic amphibole, white mica, chlorite, quartz, calcite, apatite, rutile and pyrite. They are often crosscut by clinozoisite veins (Fig.3a-b; see section 5.4 for more details).

  A C C E P T E D M A N U S C R I P T 12 5.3. Eclogitic foliation surrounding eclogite breccias The eclogite clasts are embedded in a metamorphic foliation composed of clinopyroxene, clinozoisite and garnet (Fig. 5a, Tab. 1) and developed in the eclogite-facies P-T conditions. At the microscopic scale, the clinopyroxene grains defining this foliation are stretched and rotated (Fig. 5b-c-e). Chemical analyses performed on garnet crystals that are part of this foliation reveal only one type of garnet composition (Grt1; Fig. 4d, Tab. A in supplementary material). Analyses performed on clinopyroxene display two clusters with different chemical compositions. The first one corresponds to diopside compositions (X Di = 0.82-1, X Jd = 0-0.18; left Fig.4d, Tab. B in supplementary material) and the second one to omphacite compositions (X Di = 0.53-0.56, X Jd = 0.44-0.47; right Fig.4d, Tab. B in supplementary material).

  ), sometimes associated with clinozoisite. The chemical analyses carried out on garnet and clinopyroxene veins reveal homogeneous garnet compositions (Grt1; Fig. 6f, Tab. A in supplementary material) and clinopyroxene compositions ranging from diopside to omphacite (Fig. 6f, Tab. B in supplementary material). Vein-type garnet shows minor internal chemical variations, in Fe content for example (Fig. 6f). 6. Implications for the tectono-metamorphic history of the Mt. Emilius eclogites A C C E P T E D M A N U S C R I P T 14 Based on our petro-structural analyses associated with both micro-chemical and EBSD data, we propose a model for the tectono-metamorphic history of the eclogites from the Arbolle shear zone located at the base of the Mt. Emilius klippe. This model highlights the link between the P-T history, the deformation processes and the fluid circulations that possibly led to the formation of the brecciated eclogites of the Arbolle region. This model is presented in figure 7.

  section 5.2, Fig. 5d-6d-f, Tab. 1,

  2f and 5a). The foliation around the clasts (Fig.5b-c) indicates a ductile deformation process also described by Dal[START_REF] Dal Piaz | Metamorphic evolution of the Mt. Emilius klippe[END_REF]), Pennacchionni (1996) and[START_REF] Scambelluri | Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)[END_REF].The CPO patterns observed in HP clinopyroxene crystals (Fig.5f) are very close in shape and intensity to the omphacite CPO patterns from other alpine eclogites (e.g.,Bascou et al., 2001).As demonstrated byBascou et al. (2001) and[START_REF] Godard | Deformation-induced clinopyroxene from eclogites[END_REF], omphacite CPO patterns are normally related to the strain regime. Considering the above alignment and comparison between natural examples and numerical modelling results(Bascou et al,. 2002), our HP clinopyroxene CPO probably results from LS-type strain regime(Helmstaedt et al.,

FiguresFigure 1 :Figure 2 :

 12 Figures

Figure 3 :

 3 Figure 3: a) Field photograph of eclogitised granulite crosscut by a clinozoisite vein. b) Zoom

Figure 4 :

 4 Figure 4: a) Polished rock section showing fractured garnetite clasts cemented by Cpx. b)

Figure 5 :Figure 6 :

 56 Figure 5: a) Polished rock section showing an eclogite clast embedded in an eclogitic

Figure 7 :

 7 Figure 7: t1) Shear deformation triggered by a first fracturing event and associated with fluid

  R I P T 22 decipher whether the brecciation reported here occurred at seismic strain rates. Further documentation of similar natural examples will help better imaging deep fault zones and improve our understanding of the origin of intermediate-depth seismicity in subduction zones.

	geothermometer. Contributions to Mineralogy and Petrology, 115(4), 467-473.
	Austrheim, H., Boundy, T.M., 1994. Pseudotachylytes generated during seismic faulting and
	eclogitization of the deep crust. Science, 265(5168), 82-83.
	Austrheim, H., Erambert, M., Boundy, T.M., 1996. Garnets recording deep crustal
	earthquakes. Earth and Planetary Science Letters, 139(1), 223-238.
	Austrheim, H., Engvik, A.K., 1997. Fluid transport, deformation and metamorphism at depth in a collision zone. In: Fluid flow and transport in rocks: mechanisms and effects. Ed: B. Jamtveit & B.W.D. Yardley. Chapman & Hall, 123-138 Austrheim, H., Erambert, M., Engvik, A.K., 1997. Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization. Tectonophysics, 273(1), 129-153. Austrheim, H., Andersen, T.B., 2004. Pseudotachylytes from Corsica: fossil earthquakes from a subduction complex. Terra nova, 16(4), 193-197. M A N U S C R I P T Ballèvre, M., Kienast, J.R., Vuichard, J.P., 1986. La «nappe de la Dent-Blanche» (Alpes
	occidentales): deux unités austroalpines indépendantes. Eclogae Geologicae Helvetiae, 79(1), 57-74. Bascou, J., Barruol, G., Vauchez, A., Mainprice, D., Egydio-Silva, M., 2001. EBSD-measured lattice-preferred orientations and seismic properties of eclogites. Tectonophysics, 342(1), 61-80. A C C E P T E D Bascou, J., Tommasi, A., Mainprice, D., 2002. Plastic deformation and development of
	clinopyroxene lattice preferred orientations in eclogites. Journal of Structural
	Geology, 24(8), 1357-1368.
	Bearth, P., Dal Piaz, G.V., Elter, G., Gosso, G., Nervo, R., 1980. Il lembo di ricoprimento del
	Monte Emilius, Dent Blanche s.l. Osservazioni preliminari, Acad. Sci. Torino Mem.,
	144, 227-241.
	24

Table mineral

 mineral

				Garnet					HP Clinopyroxene		Clinozoisite
	location	granulite	clast	foliation vein	granulite	clast		foliation vein	granulite foliation
	sample	AR1414aa AR15cg AR2g AR7g	AR1414aa AR2g AR6	AR6	AR2a	AR1414aa	AR6
	%SiO2	40.07 38.46 38.49	38.30	39.30	55.48 53.71 55.62 54.19 54.80	55.66	40.08	39.61
	%TiO2	0.07 0.12	0.09	0.25	0.15	0.04 0.01 0.00 0.00	0.00	0.00	0.33	0.12
	%Al2O3	22.22 21.21 20.58	21.28	20.29	3.30 2.91 10.51 0.97	2.18	8.41	29.08	28.07
	%FeO	19.09 23.44	8.78	17.87	11.00	7.10 8.29 6.99 5.36	4.62	5.35	5.25	6.94
	%MnO	0.65 1.08	1.13	1.52	1.64	0.06 0.04 0.00 0.00	0.00	0.00	0.13	0.05
	%MgO	11.74 3.48	0.25	1.00	0.23	19.78 12.14 5.89 14.94 14.52	8.70	0.04	0.06
	%CaO	5.89 11.86 30.62	18.05	26.18	12.55 20.50 13.19 22.87 21.65	15.41	23.74	23.33
	%Na2O	0.00 0.02	0.05	0.00	0.00	1.28 2.37 6.93 1.07	1.71	5.88	0.00	0.02
	%K2O	0.00 0.00	0.00	0.00	0.00	0.00 0.00 0.00 0.00	0.00	0.00	0.02	0.00
	sum	99.73 99.67 99.98	98.27	98.79	99.59 99.97 99.13 99.40 99.48	99.41	98.67	98.21
	oxygens	12	12	12	12	12	4	4	4	4	4	4	12	12
	Si	3.01 3.02	2.95	3.04	3.07	2.00 1.98 2.02 2.03	2.07	2.01	3.08	3.04
	Ti	0.00 0.01	0.01	0.01	0.01	0.00 0.00 0.00 0.00	0.00	0.00	0.02	0.01
	Al	1.96 1.96	1.86	1.99	1.87	0.14 0.13 0.45 0.04	0.10	0.36	2.63	2.54
	Fe	1.20 1.54	0.56	1.19	0.72	0.21 0.25 0.21 0.17	0.15	0.16	0.34	0.45
	Mn	0.04 0.07	0.07	0.10	0.11	0.00 0.00 0.00 0.00	0.00	0.00	0.01	0.00
	Mg	1.31 0.41	0.03	0.12	0.03	1.06 0.67 0.32 0.84	0.82	0.47	0.00	0.01
	Ca	0.47 1.00	2.51	1.54	2.19	0.49 0.81 0.51 0.92	0.87	0.60	1.95	1.92
	Na	0.00 0.00	0.01	0.00	0.00	0.09 0.17 0.49 0.00	0.00	0.41	0.00	0.00
	K	0.00 0.00	0.00	0.00	0.00	0.00 0.00 0.00 0.00	0.00	0.00	0.00	0.00

Table 1 :

 1 Representative analyses of garnet, HP clinopyroxene and clinozoisite from granulites, clasts, foliations and veins (Mt. Emilius klippe).

Acknowledgements

M.A. Smit and an anonymous reviewer are highly thanked for their thoughtful comments and suggestions that greatly improved the quality of the manuscript. K. Joy, J. Lehmann and R.

Tartèse are also highly thanked for style checking. The authors thank the INSU SYSTER program that funded a part of this work. LFGM and SA thank the GFZ for the financial support for fieldwork. This is IPGP contribution #38XX.

Highlight

We present a structural and petro-chemical analysis of eclogites from the Mt. Emilius

We show that both brittle and ductile behaviour alternate during HP deformation Brittle deformation events occur close to the peak of metamorphism (~2.3GPa/525°C) HP fracturing events lead to breccias and veins associated to fluid circulation

We discuss the causes and consequences of such switches on subduction dynamics