# Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)

Michele Locatelli, Anne Verlaguet, Philippe Agard, L. Federico, Samuel Angiboust

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Michele Locatelli, Anne Verlaguet, Philippe Agard, L. Federico, Samuel Angiboust. Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps). Lithos, 2018, 320, pp.378-402. 10.1016/j.lithos.2018.09.028. insu-03589314

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Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)

M. Locatelli, A. Verlaguet, P. Agard, L. Federico, S. Angiboust

<figure><img src="image_2.png" /></figure>

DOI: https://doi.org/10.1016/j.lithos.2018.09.028

Reference: LITHOS7248

To appear in: LITHOS

Received date: 16 June 2018

Revised date: 21 September 2018

Accepted date: 24 September 2018

Please cite this article as: M. Locatelli, A. Verlaguet, P. Agard, L. Federico & S. Angiboust, Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps). LITHOS (2018), https://doi.org/10.1016/j.lithos.2018.09.028

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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# Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)

Michele Locatelli<fnref n="1" />, A. Verlaguet<fnref n="1" />, P. Agard<fnref n="1" />, L. Federico<fnref n="2" />, S. Angiboust<fnref n="3" />

## Abstract

The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents an almost intact fragment of Tethyan oceanic lithosphere metamorphosed at ~80 km depth (~2.6GPa - 550°C) during the Alpine subduction. We focus our study on a major shear zone cutting across this slab fragment at low angle (the Lower Shear Zone; LSZ). Here, in its talc and tremolite-rich serpentinite matrix, are embedded (together with metasedimentary lenses) variously brecciated Fe-Ti and Mg-Al metagabbro blocks. The latter were either interpreted as eclogitic breccias resulting from intermediate-depth rupture or as inherited, overprinted oceanic core complex features. Our new field, structural and petrographic data testify the genesis of this metagabbro breccia blocks at eclogite-facies conditions. Three types of eclogitic blocks can be distinguished, with non-random distribution (and decreasing size from top to base) throughout the ~200-meter-thick and ~15 km-long LSZ: (1) Fe-Ti-metagabbros, brecciated and scattered in the upper to intermediate levels of the LSZ; (2) meter-size blocks and decameter-scale slivers of intact Mg-Al metagabbros, locally brecciated; (3) dm- to m-scale blocks of intact Fe-Ti metagabbros without breccia fabrics. Brecciation at eclogite facies conditions (at ~80 km depth) is documented by: i) the eclogitic foliation of intact Mg-Al-rich metagabbros (composed of omphacite + clinozoisite ± rutile and locally garnet) cut by breccia planes (cemented by omphacite + garnet ± lawsonite) and ii) the occurrence in breccia class of minerals that are fractured and offset along peak P-T omphacite-bearing planes. Rupture preferentially affected the Fe-Ti metagabbros, suggesting that rheological contrasts controlled the locus of brecciation. The occurrence of a first omphacite-rich matrix (M1, ~2.7GPa - 580°C) crosscut by omphacite + garnet-bearing matrix M2 (~2.4GPa - 560°C), witnesses multiple brittle rupture events, prior to a stage of eclogite facies fluid ingression marked by massive lawsonite recrystallization (matrix M3).

Keywords: eclogite-facies breccia, plate interface, Monviso, Western Alps

<footnote n="1">¹ Institut des Sciences de la Terre de Paris (ISTeP), Sorbonne Université</footnote>

<footnote n="2">² Dipartimento di Scienze della Terra, dell'Ambiente e della Vita (DISTAV), Università di Genova</footnote>

<footnote n="3">³Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005 Paris, France</footnote>
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# 1. Introduction

Documenting processes acting at the subduction plate interface is fundamental for understanding (and modeling) interplate mechanical coupling, fluid migration pathways and/or processes triggering seismicity. Most field studies, geophysical investigations and mechanical models have focused on processes acting within the seismogenic zone (15-30 km-depth, Toyoshima, 1990), where megathrusts nucleate (Bachmann et al., 2009; Ikesawa et al., 2003; Meneghini et al., 2010; Rowe et al., 2005; Vannucchi et al., 2012). By contrast, there is a general lack of data on the mechanical and geochemical processes acting deeper (> ~40 km).

Two distinct seismic layers are recognized beyond the seismicogenic zone (Fig. 1; Green and Houston, 1995; Hacker et al., 2003a, 2003b; Yamasaki and Ikeno, 2003); of these, geophysical and petrological data suggest that the upper one may correlate with the topmost, previously hydrothermalized crustal part of the slab ongoing massive metamorphic dehydration (Fig. 1; Bostock et al., 2002; Green and Houston, 1995; Hacker et al., 2003b; Rondenay et al., 2008). However, rheological processes associated with intermediate-depth seismicity (relocated between 40 to 300 km-depth; e.g., Nakajima et al., 2009; Preston et al., 2003; Shiina et al., 2013; Tsuji et al., 2008) and/or deep fluid circulation (e.g., hydraulic fracturing, dehydration embrittlement, mineral transformation induced faulting and pseudotachylites nucleation; Austrheim and Boundy, 1994; Davies, 1999; Hacker et al., 2003b; Yang and Green, 2004) are still largely debated due both to the lack of direct observations and to instrumental uncertainties (Kuge et al., 2010).

Although still limited, there is growing evidence that large scale, "fossil" exhumed portions of subducted lithosphere (Arago et al., 2009), may record both chemical and mechanical processes operating in the depth range of intermediate-depth seismicity (e.g., Angiboust et al., 2012b, 2011; John et al., 2009; Kirby et al., 1996; Scambelluri et al., 2017; Yang et al., 2014). The recent report of oceanic crust brecciation at eclogite facies conditions (in the Lago Superiore Unit of the Monviso meta-ophiolite, Western Alps; Angiboust et al., 2012a; Fig. 2) points to the possibility of retrieving patterns of intermediate-depth seismic rupture from a well-preserved slab fragment (Fig. 1). However, from structural and stratigraphic evidence, other authors interpreted these breccias as inherited pre-Alpine detachment fault rocks from an oceanic core complex or sedimentary-derived breccias (e.g., Balestro et al., 2018, 2015; Festa et al., 2015). Furthermore, pre-subduction and pre-collision breccias are widely described in the Alpine ophiolites from the Central Alps (e.g., Desmurs et al., 2001), Western Alps (e.g., Manatschal and Müntener, 2009) and Corsica (Marroni and Pandolfi, 2007). Considerable debate also exists in the literature on the feasibility of brittle
rock-failure at eclogite-facies conditions (e.g., Jin et al., 2001; Jung et al., 2004; Kirby et al., 1996; Raleigh, 1967; Raleigh and Paterson, 1965), leaving open the debate about their nature and origin.

The aims of this paper are therefore: i) to present evidence for eclogite-facies brecciation recorded by the metagabbro blocks disseminated in the Monviso shear zones (particularly in the Lower Shear Zone; LSZ); ii) to define their spatial extent and distribution across the LSZ; iii) to unravel the relative chronology of brecciation and associated mechanical/chemical changes and iv) to discuss their implications on intermediate-depth processes.

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## 2. Geological setting

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### 2.1 - Regional geological setting

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The Western Alps formed as a result of the eastward subduction of a branch of the Tethyan Ocean and of part of the thinned European continental margin beneath Adria, followed by collision from the Oligocene onwards (Coward and Dietrich, 1989; Lagabrielle and Lemoine, 1997; Laubscher, 1991; Schmid and Kissling, 2000). The reader is referred to more comprehensive publications for further details on the tectonic geodynamic evolution (e.g., Agard et al., 2009; Beltrando et al., 2010).

The study area belongs to the ~30 km long Monviso meta-ophiolite complex (Fig. 2), one of the largest slices of oceanic lithosphere among the Liguro-Piemontese domain, which extends from the Voltri complex (Liguria Alps, Italy) to the Zermatt-Saas zone (Valais, Switzerland and Valle D'Aosta, Italy). Notethorily, the structural and metamorphic characteristics of the Monviso are broadly comparable to those observed in the Zermatt-Saas and Voltri meta-ophiolites (e.g., Angiboust and Agard, 2010; Dal Piaz et al., 2003; Scambelluri et al., 1991; Schmid et al., 2004),

The Liguro-Piemontese domain is formed by the juxtaposition of km to 10 km-long exhumed eclogite-facies tectonic slices to lower grade (blueschist to eclogite facies) fragments of a fossil accretionary wedge, known as the Schistes Lustrés (e.g., Agard et al., 2009, 2002; Marthaler and Stampfli, 1989; Plunder et al., 2013).

A major westward-dipping normal shear zone system separates the Schistes Lustrés complex from the Monviso meta-ophiolite below (Fig. 2), exhibiting a metamorphic gap between the two units (respectively equilibrated at upper blueschist-facies and eclogite-facies peak conditions; e.g., Ballevre et al., 1990; Philippot, 1990). Below and to the east of the Monviso meta-ophiolite, separated by a ductile normal fault system (Blake and Jayko, 1990), the coesite-bearing Dora Maira
massif represents a sliver of the thinned continental European distal margin involved in the Alpine subduction and detached from the downgoing slab during the Late Eocene.

Age constraints indicate that peak burial for the Monviso meta-ophiolite was achieved around 45-46 Ma (Duchêne et al., 1997; Rubatto and Angiboust, 2015; Rubatto and Hermann, 2003), while younger ages are inferred for the metamorphic peak of the Dora Maira massif (~120 km at 39-33 Ma; Castelli et al., 2007; Chopin, 2003, 1984; Gebauer et al., 1997; Schertl et al., 1991).

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## 2.2 - Structure and P-T conditions of the Monviso meta-ophiolite

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The Monviso meta-ophiolite provides well-preserved fragments of Tethyan oceanic lithosphere, with recognizable lithostratigraphic successions. It used to be subdivided into several tectonometamorphic units (Lombardo, 1978) and more recently in two main units: the Monviso s.s. unit (MU) and the Lago Superiore Unit (LSU; Angiboust et al., 2011). For the Monviso Unit, peak conditions of 480°C/22 kbar were proposed by Angiboust et al., (2012a), whereas slightly different eclogitic peak temperatures were calculated for theago Superiore Unit: 580°C (and 19 kbar; Schwartz et al., 2000), 650°C (and 26 kbar; Messina et al., 1999), 545°C (and 20 kbar Castelli et al., 2002), 550°C (and 25 kbar; Groppo and Castelli, 2010) and 550°C (and 26 kbar; Angiboust et al., 2012a).

This study focuses on the LSU, which comprises, from bottom to top: serpentinized lherzolitic mantle intruded or capped by late Jurassic Mg-Al and/or Fe-Ti gabbros (152 Ma; Lombardo et al., 2002; 163 Ma, Rubatto and Hermann, 2003); banded tholeiitic basalts (with locally preserved pillow structures); diabases and mixed calcareous/pelitic Cretaceous metasediments (Balestro et al., 2014, 2013; Castelli et al., 2002; Lombardo, 1978). Strong lateral variations in lithostratigraphy, with one or more of the above horizons missing, were attributed to an irregular seafloor structure typical of slow spreading oceans (Lagabrielle and Lemoine, 1997).

The original stratigraphic sequence is partly disrupted by shear zones (Angiboust et al., 2012a, 2011; Festa et al., 2015; Lombardo, 1978; Philippot and Kienast, 1989). We particularly focus on two of them (Figs. 3a, b and SM1): the Lower Shear Zone (LSZ) and the Intermediate Shear Zone (ISZ), which locally mark the metagabbros-serpentinites and metabasalts-metagabbros boundaries, respectively.

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2.3 - Intra-slab shear zones within the Lago Superiore unit: the Lower and Intermediate shear zones (LSZ, ISZ)

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Two main km-scale shear zones partially disrupt the LSU unit of the Monviso meta-ophiolite, both made of a serpentinite-rich matrix embedding several kinds of blocks. The LSZ separates the main Mg-Al metagabbro body (to the west) from the underlying massive serpentinite sole constituting the eastern cliff of the meta-ophiolite Monviso massif (Figs. 3, 4a and SM1). In the study area, the shear zone has an extension along strike of over 15 km from the south (Colle di Luca pass) to the north (Rocce Fons) with a variable thickness ranging from ~300 m at Colle di Luca pass or Lago Superiore area to ~500 m at Pian Radice (Figs. 3a and SM1). The LSZ matrix is made of strongly deformed mylonitic serpentinite with locally interbedded dm-thick layers of strongly sheared talc-chlorite schists. Serpentinite mainly consists of fine-grained, strongly-deformed antigorite (+/- magnetite, talc, magnesite and secondary olivine), whereas talc- and chlorite-schists contain lawsonite pseudomorphs, clinozoisite, light-green amphibole, rutile, apatite and rare quartz. Foliation inside the LSZ generally dips 20° to 40° to the W and rotates to the W-SW in the area around Ghincia Pastour and Lago Superiore (SM1). The mylonites usually show internal chaotic geometries with dragged, disharmonic folds (particularly around blocks and slices) progressively turning into mylonitic shear bands near the shear zone boundaries. Blocks of eclogite-facies gabbros, eclogitic metasediments and massive serpentinite (the first type being the most abundant) are embedded in the talc and tremolite rich serpentinite matrix of the LSZ.

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The ISZ cuts across the topmost part of the gabbroic sequence and extends for over 4 km from the Lago Superiore area to the north tip of Viso Mozzo, where it connects to the LSZ and pinches-out under the cover of Quaternary glacier deposits (Figs. 3a and SM1). Its thickness varies between 1 and 10 m and contains much less serpentinite matrix than the LSZ. Fe-Ti metagabbros outcropping along the ISZ directly at the top of Mg-Al metagabbros (notably in the Lago Superiore area, Figs. 3a, b and SM1) underwent mylonitization during eclogite-facies deformation, leading to the formation of a characteristic planar fabric dominated by an omphacitic clinopyroxene, garnet and rutile assemblage (Philippot and van Roermund, 1992). Eclogitic omphacite crack-seal veins, which occasionally crosscut these mylonitized gabbros, are believed to form by incremental opening associated with mylonitization (Philippot, 1987; Philippot and Kienast, 1989).

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Structural characteristics, petrography and distribution of the blocks inside the LSZ (and partly the ISZ) are described and discussed in the next paragraphs.

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### 3. Field mapping, sampling and methodology

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To perform an exhaustive characterization of eclogite-facies blocks disseminated in the shear zones developed inside the LSU of Monviso meta-ophiolite we adopted a "multiscale" approach combining detailed fieldwork and meticulous structural and petrographic analysis of collected samples, as described below.

## 3.1 Field mapping of Monviso meta-ophiolites

A new geological map of the area was compiled (Map SM1, with 3D model in Fig. 3a), with a particularly detailed mapping of shear zones crosscutting the LSU (in particular the LSZ) and of the different block types disseminated inside. All field mapping was based on the 1:10.000-scale maps from the Regional Geographic Office of Piedmont (Italian CTR map).

Four specific lithostratigraphic sequences analyzed along the strike of the LSU are presented in Fig. 3b to illustrate the important lithostratigraphic lateral variations inside the Monviso meta-ophiolite slice. Detailed logs and structural study were also performed on selected metasedimentary outcrops (West of Alpetto lake and Pra Fiorito Valley; SM2) in order to compare structures and compositions to those observed in mafic blocks.

## 3.2 Mapping of blocks: block nature and distribution

We describe as "blocks" all the solid, straight-sided bodies made of coherent rocks which appear to be still composed by their original lithology (e.g., non-metasomatized metagabbros, metasediments, etc.) and that are disseminated in the uncoherent mylonitic antigorite-schists composing the bulk of the LSZ and ISZ (Fig. 3c). A detailed field mapping of the different kinds of blocks, in particular Fe-Ti metagabbro blocks (more detail in section 4), was performed to determine their distribution and representativeness inside the LSZ (in particular) and ISZ. To overcome misinterpretation in block position due to erosional problems (i.e., blocks displaced by landslides; e.g., in the upper Bulé valley or north of Rocca del Lu; Fig. 3a and SM1), we used trigonometric calculations to re-locate the 3D position of blocks inside the LSZ across 5 preserved cross-sections (only for blocks whose position in the shear zone was preserved, as attested by intact primary contacts with the serpentinite foliation; Fig. 3a and SM1; see SM3 for details).

On SM4 are provided the detailed location, dimensions and structural characteristics of more than 180 blocks (for mapping, we considered a minimum block diameter of 1 m). Most of these are brecciated blocks, for which clast and matrix composition and relative proportions were also characterized.
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### 3.3 Structural and image analysis of eclogitic brecciated blocks

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A structural study at the outcrop scale was performed on brecciated blocks to characterize the rupture planes and was completed by image analysis run on ImageJ1.x software (Rueden et al., 2017) with dedicated windows distribution FiJi (Schindelin et al., 2012). On fresh eclogite-facies brecciated blocks, we measured (1) block length, width and height, (2) the clasts vs. matrix relative volumetric abundances, (3) the dimension and shape ratio of clasts, (4) the angle of misorientation between clast elongation axes or, where possible, between different clast foliations and (5) the number of clasts of different lithological composition.

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Careful in-loco drawings and photographs were then analyzed using FiJi® image-analysis software (Schindelin et al., 2012) to retrieve clast areas, perimeters, angles of maximum elongation axis, aspect ratio and roundness. The last parameter was recalculated as adaption of the Waddel's sphericity, as suggested by Mort and Woodcock (2008). Automated clast detection was not feasible due to the poor color contrast between different clasts and matrices, so all clast contours were first manually outlined (paint software) and photographs converted to grey-scale images.

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### 3.4 Microstructural and chemical analysis of clast and matrix: analytical techniques

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55 samples of brecciated blocks were drilled inside clasts, matrix, and at clast/matrix contacts for subsequent structural and chemical analysis. The microstructural study at the thin section scale was performed using both optical microscope and SEM (Zeiss Supra 55VP associated to SSD detector PTG Sahara for EDS analysis; iDTEl, UPMC). Mineral chemical compositions were then analysed by EPMA (CAMECA F5V and S100) at CAMPARIS (UPMC, Paris), with analytical conditions for spot analysis of 15kV 10 nA, wavelength-dispersive spectroscopy (WDS) mode. Used standards were FeO₃ (Fe), MnTiO₃ (Mn, Ti), diopside (Mg, Si), CaF₂ (F), orthoclase (Al, K), anorthite (Ca) and albite (Na). In microprobe data, mineral FeO was assumed to be FeO-total, with Fe⁺³ recalculated based on stoichiometry and charge balance. Major elements bulk-rock compositions used for pseudosection modelling were analysed by inductively coupled plasma-optical emission spectroscopy after fusion with LiBO₃ and dissolution in HNO₃. Li₂O was analysed by flame atomic absorption after fusion with Na₂CO₃ and dissolution in H₃BO₃. Fe₂⁺ was analysed by K₂Cr₂O₇ titration after acid attack (HF-H₂SO₄). The H₂O content was determined by loss on ignition.

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4. Block types and distribution within shear zones

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4.1 - Eclogitic metagabbros in the LSZ

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4.1.1 Block types

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Two end-member lithologies make up the metagabbro blocks distributed in the LSZ: the Fe-Ti
metagabbros, with the classical omphacite + garnet (+ rutile +/- pseudomorphs after lawsonite)
paragenesis, and the lighter coloured Mg-Al metagabbros with (generally chromium-rich)
omphacite, clinozoisite but no garnet. Mm- to cm-thick bands of garnet-bearing Mg-Al metagabbro
are also present, mainly at the contact between Fe-Ti and Mg-Al metagabbros (e.g., around Fe-Ti-
rich boudins or breccia layers).

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Their morphology, structure and mineralogy allow to classify the blocks into three different
groups:

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(1) Type 1 blocks are made of completely brecciated metagabbros and are the most representative and spectacular of the eclogitic bodies dispersed in the LSZ (62% of mapped blocks, Min Vol. 0.05 m³, Max Vol. 2613.81 m³; see SM3, 4, 5). They present a peculiar clast-in-matrix structure Fig. 4b, c, d), with clasts almost exclusively made of mylonitic Fe-Ti metagabbros with rare Mg-Al gabbros (~10%), cemented by an omphacite-rich matrix (+/- garnet and pseudomorphs after lawsonite). Matrix proportions vary, from block to block, from less than 10 vol.% to up to more than 50 vol.%. Dimension and extension of the clasts are also variable (for details SM3, 4, 5).

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(2) Type 2 blocks (22% of mapped blocks, Min Vol. 1.51 m³, Max Vol. 125663.71 m³) are made of unrecciated, foliated Mg-Al metagabbros showing strong internal mylonitization and complex folding. They locally embed up to 5 m thick unrecciated boudins of Fe-Ti metagabbros Fig. 4e, f, g). Although relatively rare (only 3% of the mapped blocks; see SM3, 4 and 5), there are some large hm-scale slivers of Type 2 blocks (Colle Di Luca: B196; Punta Murel: B169; Lago dell'Alpetto: B7; SM6). In both cases, Type 2 blocks foliation is in places abruptly truncated by planes of brecciated Type 1 Fe-Ti metagabbros (Fig. 4e, f, g, h). These Fe-Ti breccia planes always bound the hm-scale Type 2 blocks on their western flank Fig. 5a, b; i.e., facing the Mg-Al metagabbro cliff, dipping 20° to 60° to the W; Fig. 3a, SM6a, b, c, d) and are absent on the other side.

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At the outcrop-scale, the clast-in-matrix structure of eclogite breccias of Type 1 and Type 2
blocks is made of mylonitic clasts rotated to variable degrees and cemented by interstitial matrix
(Fig. 4c, d), with structures comparable to those observed at the micro-scale (i.e., discordant
mylonitic foliation underlined by rutile beds; see below).

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(3) Type 3 blocks are characterized by finely-foliated to massive, unrecciated Fe-Ti metagabbro blocks (16% of mapped blocks, Min Vol. 1.56 m³, Max Vol. 6.00 m³). Coarse grained fabrics are preserved in the core of the largest blocks (> 10 m³ e.g., Pian Radice area) and consist in almost undeformed coronitic garnet (< 6 mm) crystallizing around megacrystals of omphacite (< 8 mm), which were interpreted as pseudomorphs after magmatic pyroxene or plagioclase (Angiboust et al., 2011; Groppo and Castelli, 2010; Lombardo, 1978; Philippot and van Roermund, 1992; Pognante and Kienast, 1987).

All blocks are coated by late, variably thick metasomatic rinds (Figs. 3c, 4b).

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### 4.1.2 Metagabbro block distribution within the LSZ

Detailed mapping along the 15 km-long LSZ suggests a non-chaotic distribution of the metagabbro blocks (Fig. 5a and SM1): Type 1 and Type 2 blocks preferentially crop out in the intermediate-to-upper part of the shear zone, whereas unrecciated Type 3 blocks are restricted to the lower part of the LSZ (Fig. 5b).

Block distribution in the shear zone also depends on the volume of the blocks (Fig. 5b): the largest Type 1 breccia blocks (~80% of mapped Type 1 blocks, average volume > 50 m³) are restricted to the upper part of the shear zone, whereas smaller blocks (~20%, with average volume of 10 m³) are spread in the lower part of the LSZ. Most Type 2 blocks (~76%) and all Type 1 blocks are located stratigraphically above the big slivers of retrogressed Mg-Al metagabbro of Punta Murel, Colle di Luca and Alpetto Lake (Type 2). Unbrecciated Type 3 blocks, which are restricted to a 30-100 m thick-band at the base of the LSZ, show a general volume increase towards the serpentinite sole, with eclogite bodies reaching volumes of 20 m³ SW of Monte Granet (Fig. 5b; more details in SM4).

## 4.2 Serpentinite, metasedimentary and jadeitite blocks in the LSZ

Several massive serpentinite blocks and slivers are disseminated in the lower portion of the shear zone (Fig. 3a, b and SM1), associated to rounded, unrecciated Fe-Ti metagabbro blocks (Type 3 blocks) and rare meta-rodingite boudins.

Meter-scale lenses of metasediments are also dispersed within the LSZ, decreasing in abundance from N to S. Large slivers (up to 90 m thick) were only observed in Lago Superiore, Pra Fiorito and Alpetto area (Fig. 3a, b and SM1). Metasediments are calcschists and marbles metamorphosed under eclogite facies P-T conditions. Large slivers preserve evidence for alternations of metadolostones, quartz-mica-rich sandstones, garnet-lawsonite-chloritoid-paragonite-rich micaschists and meta-conglomerate strata. The latter, frequently clast-supported, are composed of
308 mm to dm (< 50 cm) clasts of gabbro, basalt and peridotite mixed with a strongly deformed
309 sedimentary matrix (Fig. SM2a, b). Clasts are well rounded and elongated (aspect ratio up to 4),
310 often boudinaged (Fig SM2c, d) and indicative of strong deformation (e.g., bodies West of Alpetto
311 Lake or in the upper Pra Fiorito Valley). For example, the W-dipping metasedimentary sliver
312 outcropping in the Pra Fiorito Valley (Fig. 3a and SM1a) shows three clast-supported and four
313 matrix-supported fining-upward conglomerate layers (Fig SM2e), with a progressive decrease of
314 serpentinite and relative increase of (meta) basaltic clasts from base to top of the outcrop.

315 In the LSZ rare meter-size blocks of jadeitite are also found (especially in the upper Bulé valley
316 and Colle di Luca; see Castelli et al. (2002) and Compagnoni et al. (2012) for details. Nevertheless,
317 their origin is unclear: the lack of primary contacts with the shear zone mylonite and their uneven
318 distribution in the LSZ may suggest quaternary erosion of the upper Mg-Al metagabbro cliff and
319 the subsequent dissemination of this blocks in the LSZ.

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321 4.3 Comparison with block types and distribution in the LSZ

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323 Rare, m-scale Type 1 brecciated blocks crop out chiefly on the western and eastern side of Lago
324 Superiore (Fig. 3a and SM1). Interestingly, these blocks stratigraphically overlay the Fe-Ti gabbro
325 sills intruded in the eclogitic Mg-Al metagabbros outcropping south-east of the Lausetto Lake,
326 following a stratigraphic scheme also observed in the USZ (Fig. 3b). Moreover, meter-scale
327 metabasaltic blocks (hanging wall of the ISZ) are found in the ISZ together with scarce
328 metasedimentary blocks. In the southern part of the shear zone, for example North-East of Chiaretto
329 Lake (Fig. SM1), are scattered big slivers (up to 60 m long) of massive serpentinite and
330 metasediments. The latter shows a strong deformation with N/S to NW/SE-trending major fold axis
331 and W-verging fold limbs describing clockwise Z-shaped W-verging parasitic folds. The presence
332 of allocthonous serpentinized metaperidotite and metasediment slivers in the ISZ suggests the
333 disruption of the preexisting ophiolitic sequence along this shear zone (Fig. 3a, b and SM1 ).

334 5. Brecciation patterns

335

336 5.1 Outcrop scale structural patterns

337

338 The structural study of breccia horizons was limited by the small size (80% < 5 m) of most blocks
339 that do not preserve the full width of breccia planes and by the widespread talc + amphibole +
340 chlorite-rich metasomatic coating that hinders the block surfaces. Nevertheless, several blocks from
341 the LSZ exhibit the complete transition from intact metagabbros to completely brecciated horizons
342 (Fig. 6a, b): five Type 2 blocks were chosen among them to conduct a detailed structural analysis
343 (in Prà Fiorito valley, north of Alpetto Lake and in the upper Bulè Valley; Fig. SM1). Type 2
344 blocks usually present only one single brecciation horizon, more rarely multiple brecciation planes
345 (Fig. 6c, SM6e, f), sharply truncating the preexisting eclogite-facies mylonitic foliation[Fig. 4e, f, g]. More than 80% of analyzed clasts have an area smaller than 10 cm² and 90% of the clasts in
346 breccia horizons are < 30 cm², with power-law distribution (Fig. SM5c).
347

348 In complete breccia planes, the rim to core sequence is characterized by a strong decrease in clast
349 size and increase in clast rotation angle, clast roundness and matrix vs clast amount[Fig. 6a]. The
350 progressive transition from intact rock to crackle, mosaic and finally chaotic-breccia zones in the
351 Fe-Ti breccia planes (Fig. 6a, b) is similar to the classic rim-to-core sequence observed in tectonic
352 breccias (Jébrak, 1997; Mort and Woodcock, 2008; Sibson, 1986):

353 (1) near the rupture surfaces, crackle breccia layers (10-10 cm thick) are characterized by very
354 angular clasts (Fig. 6b) detached from the fault walls by a complex array of fractures (Fig. 6a, b),
355 with limited displacement (≤ 2 cm) and almost no rotation. Eclogite clasts, locally with diameters
356 up-to 40 cm, exhibit jigsaw texture (Sibson, 1986); matrix represents only 10 vol%.
357

358 (2) textures progressively shift to mosaic breccia textures (20-50 cm thick Fig. 6b), with a higher
359 matrix content (< 45% of total volume, with up to 1 cm thick matrix layers Fig. 6a, b), smaller
360 clasts (< 20 cm) and slight clast rotation (< 10°).

361 (3) Breccia core portions (20-50 cm thick) show chaotic breccia s.l.) are matrix supported (> 55% of
362 total volume). The clasts are smaller (more that 80% of clast < 5 cm²), have aspect ratio < 2 and are
363 randomly oriented (Fig. 6b), giving rise to chaotic breccia fabrics. The transition from crackle to
364 chaotic breccia shows also a progressive increase in the modal amount of Fe-Ti metagabbro clasts
365 vs Mg-Al metagabbro clasts (Fig. 6a), the latter absent from breccia cores.
366

367 The analysis of the orientation of the maximum-elongation axis of the clasts with respect to the
368 inferred rupture planes (Fig. 6d and, for comparison, SM6f) reveals that in each single breccia
369 horizon, the clast orientations are not totally random: there are preferred orientations, particularly
370 around 20°±10° and 40°±10° (Fig. 6d). On the contrary, taking into account all the breccia planes
371 present in one block (when multiple Fig. 6c), clast distribution is random, with angles ranging from
372 0° to 180° (Fig. SM7f).

373 In most cases, rotated clasts were cemented after brecciation by a newly-formed matrix. Textural
374 and compositional characteristics of the clasts and the four successive matrix types observed are
375 described below.

376 Note that in the ISZ, only 4 small brecciated metagabbro blocks were observed (Type1) and thus
377 none of them record a full brecciation plane.
376

377

## 5.2 Different types of clast and matrix at the decimeter to millimeter scale

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In brecciated eclogite-facies blocks (Type1), the clasts are constituted by Fe-Ti metagabbro (90% of modal amount) and rarer Mg-rich metagabbros (mostly close to the unrecciated rock, 10% of modal amount). Mg-Al metagabbro clasts show the same fabrics and composition than Mg-Al metagabbros constituting the western cliff of the LSZ (Fig. 3a) and forming most of Type 2 blocks, whereas mylonitized Fe-Ti metagabbro clasts are comparable to the Fe-Ti metagabbro boudins embedded in Type 2 blocks. The mylonitic foliation of the Fe-Ti gabbro clasts is locally crosscut by omphacite-rich veins (Figs. 7a) up-to 1 cm wide, which locally include host-rock fragments. Opening geometries vary from crack-seal to tension gashes, similar to geometries of veins crosscutting the Lago-Superiore Fe-Ti metagabbros (Philipot, 1987; Spandler et al., 2011). Some of these veins are crosscut by the matrix sealing the clasts, and thus predate the brecciation event (Veins I; Fig. 7a), while others (Veins II, mainly observed in the ISZ) locally crosscut both clasts and the omphacite-rich matrix (or M1, hereafter), indicating the first brecciation event.

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In these brecciated metagabbro blocks, clasts are partly sealed by a matrix whose amount increases from rims to cores of breccia levels (Figs. 4c, 6a, 6b); crosscutting and textural relationships allow distinguishing four successive matrix types (Fig. 7):

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(1) At the transition between intact rock and crackle brecciated layers (Fig. 7e), slightly rotated clasts are surrounded by anastomosed, sub-millimetric domains (called "microbreccia" hereafter) made of omphacite and garnet grains showing strong grain-size reduction (< 100 µm) and re-orientation parallel to the clasts borders.

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(2) Such domains are sealed by small amounts of a light green omphacitic cement (hereafter called M1, Figs. 4c and 7c), whose amount increases (up-to 1 cm-thick mantles around clasts) while the microbreccia progressively disappears from the inner to outer levels of crackle breccias.

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(3) Along the crackle-mosaic breccia transition (Fig. 6a, b, c), M1 matrix progressively turns into another omphacite-matrix type (M2) which also contains garnet visible in hand-specimen (Fig. 7a).

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(4) In the inner portion of brecciated layers (i.e., chaotic breccia fabrics), crosscutting relationships show that a late matrix M3 postdates both M1 and M2 (Figs. 6a, b, c, 7b). M3 is the most abundant matrix type (Fig. 7e) and is characterized by impressive euhedral lozenge-shape pseudomorphs after lawsonite (< 2 cm; Figs. 7b). Noteworthily, all these matrices lack any evidence of pervasive mylonitic deformation (e.g. Fig. 7c) and/or foliation.

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At the contact with the antigorite-rich matrix embedding the blocks in the LSZ, blocks are rimmed by a hydrated metasomatic rind (composed by talc, chlorite, garnet, acicular amphibole, diopside

410
and rare phengite) that can locally be up to 0.5 m-thick (e.g., in Lago Superiore, Pta. Forcion and Colle di Luca; Fig. 4b). This metasomatic replacement affects both clasts and matrix, and thus postdates brecciation. Matrix zones are more strongly affected, however, and a complex set of sub-millimeter scale veins filled with hydrous minerals nucleate in the retrogressed matrix and radially crosscut the surrounding clasts (e.g., Fig. 7a, b, c, d). The degree of metasomatism rapidly decreases from block rims to cores, clearly suggesting an external fluid-assisted metasomatic event postdating both brecciation and block dismembering in the LSZ.

Note that the scarce brecciated metagabbro blocks sampled in the ISZ show similar structures (i.e., clasts locally crosscut by veins and embedded in matrix) but only small amount M2 matrix (Fig. 7a). Indeed, no extensive M3 lawsonite-rich matrix was observed in the poorly developed brecciated layers of these rocks.

All the structural relationships between clasts and matrices described above are summarized in figure 7e.

## 6. Clast and matrix: mineralogy and microstructure

### 6.1 - Clast mineralogy and microstructure

Fe-Ti metagabbro clasts are mainly composed of omphacite (up to 60 vol.%) garnet (35 vol.%), and rutile (Fig. 8a, b, c; Table 1). F FUNEDral omphacite and rutile crystals underline a well-developed mylonitic foliation. Here, rutile aggregates, grown at the expense of magmatic ilmenite, locally form polycrystalline ribbons up-to 70 µm-thick aligned along the mylonitic foliation (Fig. 8a). The dynamic recrystallization of omphacite results in grains with an aspect-ratio up to 1:15 elongated parallel to the foliation (Fig. 8b). Their irregular grain boundaries (both in the mylonitic foliation plane and in the pressure shadows Fig. 8a, b) indicate grain-boundary migration processes during the ductile mylonitic deformation. Sub-idioblastic garnets (200-500 µm) show conspicuous zoning under the microscope, with rims poorer in omphacite and rutile inclusions with respect to the cores (Fig. 8a). In the latter, folded omphacite crystals and rutile trails (Fig. 8c) underline garnet syn-kinematic growth. Local occurrence of randomly distributed square or lozenge-shape patches filled with fine grained (< 20 µm size, SM7a) clinozoisite and white micas are interpreted as pseudomorphs after lawsonite crystals. Foliation planes folded around these aggregates suggest that lawsonite crystallization was pre- to syn-kinematic with respect to Fe-Ti metagabbro ductile deformation, and thus coeval with the P-T peak eclogitic paragenesis. Other accessory minerals are sub-idiomorphic apatite, randomly distributed acicular glaucophane crystals (< 1 vol. % in these
rocks), rare quartz (included in garnet cores) and small paragonite crystals (< 2 vol. %, < 50µm long) parallel to the main foliation (Table 1). Locally, sub-mm-sized deep-green porphyroclasts embedded in the foliation are interpreted as relic crystals of magmatic clinopyroxene completely re-equilibrated as omphacite. Similarly, mm-scale porphyroclasts associated to coronitic garnet and interstitial rutile (Fig. 8d) are observed in the rare, low-strained Type 3 blocks of Fe-Ti metagabbros found at the base of the LSZ (Figs. 3a, 5 and SM1). Here, contrary to the above described mylonitic clasts, omphacite porphyroclasts show only rare undulose extinction and kink bands (fig. 8d), which attests to minor ductile deformation. Such rock fabric is thus interpreted as the result of the static, topotactic recrystallization of omphacite after clinopyroxene, garnet after plagioclase and rutile after titanite (or ilmenite) on the pre-existing magmatic texture of Fe-Ti rich gabbros (e.g., Messiga et al., 1999; Philippot and van Roermund, 1992).

The rare Mg-Al metagabbro clasts are made of mm-thick bands of deep-green omphacite alternating with whitish clinozoisite horizons (up to 1.5 cm; Fig. 4c). Glaucophane and pseudomorphs after lawsonite are occasionally found in the eclogitic foliation of these clasts (Table 1). Although garnet is absent in the Mg-richest lithologies, mm- to cm-thick garnet-rich layers are frequently observed inside gabbro layers showing a composition intermediate between Fe-Ti and Mg-Al metagabbros.

The mylonitic foliation of clasts is locally crosscut by mm- to cm-scale veins (Fig. 7a). They consist mainly of (i) acicular omphacite and secondary apatite crystals (Veins I), locally deflected at the contact with the clast host-rock (Fig. 8e) or (ii) tabular omphacite grains associated to mm-scale garnet and rutile (Veins II). Veins I formation is postdated by the crystallization of the matrix M1, while the crystallization of Veins II appear to be earlier to that of matrix M2 and M3, which seal all the structures developed in metagabbro blocks before the brecciation (e.g., Fig. 6a-b, 7a-c and 8f).

Table 1 summarizes the mineralogy of the four successive matrix types wrapping the eclogitic Fe-Ti and Mg-Al clasts of brecciated metagabbros (Fig. 7e).

In rare cases, the absence of matrix between clasts in the levels of crackle breccia nearest to the intact rocks (Fig. 6b) results in micrometric garnet offset (Fig. 8g) and pressure-solution-like fabrics at sharp clast-clast contacts, with progressive deflection of clast mylonitic foliation, and strong grain-size reduction and reorientation of mylonitic omphacite.
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In the outer part of breccia planes (i.e., at the contact with intact rock Fig. 7e), clasts are separated by sub-millimetric microbreccia domains, composed of sharply-fractured crystals of omphacite, garnet, apatite and rutile, i.e., the mineral assemblage present in the surrounding clasts. There, garnet crystals are fragmented and rounded, whereas rutile and omphacite are crudely reoriented parallel to clast boundaries, with a strong grain size reduction for omphacite down to 10 µm (by comparison: 200 µm in clasts) and progressive deflection of clast mylonitic foliation Figs. 9a, b, c).

M1 matrix consists of almost pure deep green omphacite (euhedral crystals up to 200 µm-long; Fig. 7c), with few euhedral apatite crystals and subordinate lawsonite pseudomorphs and talc. Scarce sub-millimetric corroded rutile and pyrite overgrown by iron oxides are dispersed close to clast-matrix borders. Contacts between clasts and matrix are usually sharp Fig. 7c and 8f), but FEG-SEM quantified maps exhibit complex radial infiltration patterns of M1-like omphacite inside clasts (Fig. 9d). This fabric, characterized by clasts irregularly fractured and cemented by M1, is also well visible at the outcrop scale Fig. 4c).

M2 matrix assemblage is omphacite-dominated, but contains additional garnet and rare rutile and lawsonite pseudomorphs (for LSZ and ISZ; Fig. 9e). The tabular omphacite crystals are locally up-to 300 µm-long, with straight grain boundaries and common triple-points junction Fig. 9f). The grains shows only minor undulose extinction Fig. 9f), without development of any foliation. Euhedral garnet (Fig. 9e) is clearly less structured than in mylonitic foliation of the clasts, but also shows zoning, with inclusion-rich cores (omphacite ± rutile) rimmed by inclusion-poor mantles. Accessory minerals are titanite and rare deep-blue acicular glaucophane, the latter crystallizing as interstitial phase between omphacite grains.

In breccia cores, matrix M3 is characterized by cm-sized euhedral lozenge-shape pseudomorphs after lawsonite (Figs. 7b and 9g). Lawsonite pseudomorphs are filled with a mesh of microcrystalline clinozoisite, paragonite and chlorite (up to 10 µm), which likely results from late epidote-blueschist facies retrogression. These lawsonite pseudomorphs are indeed surrounded by the following greenschist-facies assemblage: tremolite-actinolite (constituting more than 40% of M3 matrix) + chlorite + calcite + albite + epidote. However, textural arguments show that this greenschist assemblage replaced a former eclogite-facies paragenesis: occurrence of rare relicts of sub-mm omphacite inside tremolite-actinolite crystals, mm-thick tabular rutile in textural equilibrium with lawsonite crystals, and chlorite filling large (up to 0.2 mm) rounded domains with preserved rutile trails (Fig. SM7b, c) are suggestive of garnet pseudomorphs. Therefore, the mineralogy of the M3 matrix may have been very close to that of M2 (i.e., classical eclogitic paragenesis), but with a much higher proportion of lawsonite.
Contrary to the sharp contacts observed between clasts and M1 or M2 matrix, clasts contacts with M3 matrix are smoothed by the overgrowth of the greenschist-facies assemblage (Fig. 9g).

This late greenschist retrogression affected all matrix types, but was much more pervasive in M3, (M3 > M2 > M1), indicating that retrogression was limited in matrices with less hydrous phases (i.e., M1). External fluid ingression is nevertheless attested locally by late veins (Fig.7c) and interstitial domains (up-to 5 millimeter-thick) between clasts filled with greenschist fan-shaped chlorite aggregates (Fig. 8f). Fe-rich veins (up to 50 wt.% FeO, thickness up-to 50 µm) nucleate in these domains and crosscut the samples, preferentially following the clast-clast or clast-matrix boundaries, again clearly postdating brecciation. In several blocks from LSZ, especially in Type 2, a complex array of fibrous glaucophane veins (thickness up-to 3 cm) crosscut both clasts and matrices, postdating the brecciation.

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6.3 - Metasomatic rinds
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525

At their contact with the surrounding antigorite schist forming the matrix of LSZ, the mafic eclogitic breccias are strongly affected by a widespread metasomatism (Figs. 3c, 4b), which affects preferentially matrices (Figs. 7c and SM 7d). The pre-existing eclogite-facies assemblages are replaced by a mesh of talc and chlorite, a new-generation of brown metasomatic garnet, acicular amphibole and diopside plus very rare plagiogite. Lawsonite pseudomorphs (now replaced by epidote + paragonite) in textural equilibrium with the surrounding talc and metasomatic-stage chlorite are also observed, together with traces of omphacite crystals and dispersed rutile. A complex set of sub-millimeter scale veins of syngenetic talc, diopside and chlorite radiates from the hydrated metasomatic domains into the clasts (Figs. 7c-d). Similarly to block matrices, metasomatic rinds were locally replaced by later, cross-cutting greenschist facies assemblage (chlorite, tremolite-actinolite, calcite, rare quartz).

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7. Mineral chemistry
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539

Both clast and matrix mineral compositions are given in Tables 2 and 3.

541

7.1 - Clast mineral chemistry

543

In the mylonitic Fe-Ti metagabbro clasts, the first generation of omphacite (Omp1a, Dild30Ae25; Fig. 10a, b) forms the bright cores of large, partially dismembered porphyroclasts. These are
rimmed by a second omphacite generation (Omp1b, DdJd38Ae22; Fig. 10a, b) crystallizing also as
newly formed crystals aligned in the mylonitic foliation. Garnet is also zoned (Fig. 8a), with dark-
pink coloured cores (Grt1a, Grs24Prp5Alm71; Fig 10c) and lighter rims (Grt1b, Grs17Prp13Alm70;
Fig. 10c); garnet cores are locally affected by complex radial fractures (Fig. SM8b) filled and
sealed by Grt1b compositions. Smaller garnets (< 100 µm) with Grt1b composition are also
scattered in the foliation. Interestingly, garnet cores (Grt1a) are full of Omp1a inclusions, whereas
garnet rims (Grt1b) only contain few inclusions of Omp1b, showing that omphacite and garnet
compositions evolved jointly during mylonitization.

In the outer-most clast domains, a third generation of interstitial Al-rich omphacite (Omp2a, peak
composition Di₄₆Jd₄₈Ae₆; Fig. 10a, b) crystallized as rims around the previous omphacite crystals
with complex, asymmetric patterns (Fig. 9d). Interestingly, omphacite crystallizing in the pressure-
solution planes promoting micrometric garnet offset (Fig. 8c) is in equilibrium with Omp2a (Fig.
10a, b). Finally, a later generation of omphacite (Omp2b, Di₆Jd₃₀Ae₈; Fig. 10a, b), locally
replacing all previous omphacite generations, is restricted to less than 1mm at the clast-matrix
contact; Omp2b crystals seem to follow radial inflation patterns from the surrounding matrix into
the clasts (Fig. 9d, SM8d). At the clast borders, garnet (Grt1a-b) is also overgrown, on the matrix
side, by a later garnet generation (up-to 10 µm-thick mantles; Fig. 9d, SM8e), richer in FeO and
MnO (Grt2a: Grs₁₈Prp₂₈Alm₅₄, Fig. 10a).

Veins I (i.e., crosscutting clasts but predating brecciation; Fig. 7a) are filled with Omp1b omphacite, whereas omphacite in Veins II presents an Omp2b composition.

## 7.2 - Matrix mineral chemistry

In the microbreccia domains, fragmented omphacite and garnet crystals show compositions
similar to clast minerals (i.e., Omp1a cores rimmed by Omp1b compositions and Grt1a rimmed by
Grt1b). These minerals are locally sealed by interstitial Omp2a omphacite (Fig. 9c). Here,
fragmented Grt1 garnet in contact with Omp2a layers are rimmed by thin (<20 µm) Mn-rich
mantles with Grt2a compositions (Fig. 9d). Locally, anastomosed layers (< 70 µm thick) of Omp2b
seal these microbreccia domains (Fig SM8a).

The M1 matrix (omphacite only) is composed by about 60 vol.% of Omp2b (Figs. 9d and 10a, b)
crystals surrounding flake-shaped remnants of corroded, Al-richer Omp2a crystals (35 vol.%),
resulting in an extremely intricate mesh of both generations. The remaining 5 vol.% is composed by
crystals of rutiles and rare flakes of Omp1 omphacites close to the clast borders.
M2 matrix is largely made of tabular omphacite crystals of Omp2b composition, with subordinate Omp2a (mainly as crystal cores) and subordinate relics of Omp1a-b (Fig. 10a, b); composition of Omp2b is similar to the one analyzed in clasts and M1 matrix but locally noticeably enriched in Cr₂O₃ (up to 0.44wt.% vs < 0.1 wt.% in the mylonitic clasts; Table 2). Garnet cores (Grt2a composition, Fig. 10c) are rimmed by almandine-poorer garnet (Grt2b; Gr₂Prp₂₅Alm₄₅). Locally, the latter, are strongly enriched in Cr₂O₃ (up to ~1 wt%), with cores showing peculiar sectorial enrichment pattern (concentrations up to 0.3 wt% M7e). In Grt2a, the included omphacite have Omp2a compositions, while Grt2b appear to be equilibrated with inclusions of Omp2b (Fig. 10b).

Analysis of M3 matrix eclogite-facies assemblage is difficult due to the strong greenschist retrogression (Fig. 9g). The rare omphacite relics dispersed in the eclogite-actinolite mesh have an Omp3 composition (Di₇₀Jd₁₅Ae₁₅, Fig. 10a, b). No analysis could be made on lawsonite and garnet which are now completely pseudomorphosed by clinozoisic and chlorite, respectively.

In the metasomatic rinds, the newly-formed garnet is almandine-rich (Grt3: Gr₂Prp₂₁Alm₆₁, Fig. 10c).

For comparison, in the brecciated Fe-Ti metagabbros of ISZ, mylonitic clasts display the same Omp1a-Omp1b zonation found in the LSZ, with veins crystallizing Omp1b. In contrast to the LSZ, M2 matrix is composed by Omp2b in equilibrium with garnet, whose composition is comparable to Grt2a (cores) and Grt2b (rims) but slightly depleted in grossular and almandine (Fig. 10d). Locally, Cr₂O₃ content of M2 omphacite and garnets is enriched respect to those in M1, clasts and veins (1.02 wt.% vs 0.35 wt.% respectively).

## 7.3 Phase equilibrium modelling

To constrain more precisely the P-T conditions of crystallization of the Fe-Ti metagabbro mylonite and the eclogite-facies matrices, four pseudosections were modelled for samples I31-15 (mylonitic clast), L20-15A (matrix M1) and L63-15D (matrix M2) in the system NCKFMASHTO ( Table 4). TiO₂ was considered due to the abundant presence of rutile in the clast of the mylonite (e.g. 8a, b, c and d). The fluid phase was assumed to be pure H₂O and was set in excess as suggested by (i) the occurrence of pseudomorphs after lawsonite (up-to 11.5 wt% M7e; e.g., Pawley, 1994) both in clasts and (subordinate) in matrices (ii) the presence of atoll-shaped garnet (especially in the ISZ) whose crystallization may be facilitated by the presence of aqueous phases (e.g. Cheng et al., 2007) and (iii) the occurrence in metagabbro clasts of healed garnet fractures (e.g. M9a-b), which at HP-LT is suggested to be promoted by fluid phase (Angiboust et al., 2012a; Spandler et al., 2011). CO₂ was neglected, as only small amounts of calcite occur as secondary phase in the late-stage,
613 greenschist-facies cracks and surrounding selvages. From ICP-OS data (Table 4) molar XFe₂O₃
614 ratio was set to 0.20 for samples I31-15 (mylonitic clast) and L63-15D (matrix M2). Differently,
615 ICP-OS data for sample L20-15a gave a higher molar XFe₂O₃ ratio (up-to 0.40, Table 4).
616 Nevertheless, combining mineral modes, microprobe data and phase-equilibria modelling (M9)
617 the XFe₂O₃ value adopted for the P-T pseudosection of matrix M1 modelling was set again to 0.20.

618 For the modelling of peak mylonitic conditions (Step B in Fig. 11, sample I31-15), it was
619 necessary to consider the sequestration of elements induced by the growth zonations of garnet
620 porphyroblasts during the prograde path (Step A; e.g., Carson et al., 1999). At Step B (composition:
621 I31-15-Gt; Table 4), the effective bulk composition was adjusted from XRF composition by
622 removing 25% of the garnet modal abundance, following the methods of Evans (2004). This choice
623 was supported by the small size of garnet in the sample (maximum diameter: 200µm) and by the
624 cross-relation existing between the observed garnet cores modal amount (~30 vol.%) and the
625 modelled amount of garnet crystallized at StepA (~24 vol.%). Such subtraction was not performed
626 for matrices M1 (L20-15A) and M2 (L63-15D), as the absence of any remnants of prograde
627 mylonitic garnet Grt1a and Grt1b from the matrices suggests a complete dissolution/reaction of the
628 pre-existing minerals, thus excluding element sequestration and/or local bulk-effects.

629 The P-T pseudosections were calculated with Perple_X (software version 7.7.5; Connolly, 2005,
630 1990) with the internally consistent thermodynamic dataset hp04ver.dat (Holland and Powell, 2003,
631 1998). Mineral solid-solution models were Omph(GHP) for omphacite (Green et al., 2007), Ep(HP)
632 for epidote/clinozoisite (Holland and Powell, 1998), Gt(HP) for garnet (Holland and Powell, 1998),
633 cAmph (DP) for amphibole (Diener et al., 2007), Chl(HP) for chlorite (Holland and Powell, 1998),
634 feldspar for ternary feldspar (Furnam and Linsley, 1988) and the ideal solutions T for talc. H₂O-
635 CO₂ fluid solution model was from Holland and Powell (1991, 1998).

636 The pseudosections for sample I31-15 (step A) and recalculated bulk I31-15-Grt (Step B) are
637 dominated by tri- and quadrivariant fields, with a few di- and quini-variant fields, and are shown in
638 Figs. 11a-b. The P-T conditions were constrained by comparing predicted garnet and omphacite
639 isopleths with selected mineral compositions (Tables 2 and 3). All the boxes on Figs. 11a-d
640 incorporate typical uncertainties on EMPA analyses, ~3% to ~5% (Lifshin and Gauvin, 2001).
641 StepA, as defined by mylonitic garnet core composition Grt1a and included omphacite Omp1a
642 (Tables 2 and 3), is marked by thermodynamic equilibrium at ~2.3Gpa and ~530°C (Figs. 11a and
643 11e). The mylonitic peak conditions (StepB: ~2.6 Gpa, ~550°C; Figs. 11b, e) is constrained by
644 using the mylonitic garnet rim compositions (corresponding to Grt1b Table 3) which have the
645 lowest XGrs content and coincide with the highest XJad content observed for omphacite Omp1b
646 (Table 2). The PT conditions for M1 matrix (~2.7 GPa, ~580°C; Figs. 11c, e) were constrained by
the compositions of the Mn-rich garnet rims (Grt2a, Fig. 9d) crystalizing at equilibrium with the omphacite Omp2a at the sharp transition clast-M1 matrix (e.g. Fig. 9d; compositions in Tables 2 and 3). Remarkable agreement exists between the modelled modal volume of garnet (~1-2% vol) and the one observed in the rocks (~1% vol, Fig. 9d), supporting the accuracy of the pseudosection modelling. The later-stage M2 matrix (Figs. 11d, e) is marked by pressure of ~2.4 GPa and temperature of ~560°C (Fig. 11d); considering the analytical errors and the modelling uncertainty, such metamorphic conditions are totally comparable to those of the mylonitic steps A-B and M1 matrix (Fig. 11e). Mineral compositions used to constrain M2 matrix were garnet Grt2b and included omphacite Omp2b, the latter marked by a decrease in XJad and the relative enrichment in XDiop (Fig. 10b).

## 8. Discussion

### 8.1 Evidence for brecciation at eclogite-facies conditions

Based on (i) petrographic observations on Monviso Fe-Ti metagabbros, (ii) P-T estimates for the Lago Superiore Unit (~550°C, 2.6-2.7 GPa) and (iii) macrotextural relationships at the block scale, Angiboust et al. (2012a) interpreted the Monviso breccias as marking eclogitic brittle rupture, thereby potentially reflecting intermediate-depth seismicity. Nevertheless, from structural and stratigraphic observations, (Balestrieri, 2015a, 2014) and (Festa et al., 2015) interpreted some breccia blocks dispersed in the LSZ as relict sedimentary and tectonic breccias inherited from a Tethyan Oceanic Core Complex, as well as several Alpine ophiolites (e.g., Central Alps: Desmurs et al., 2001; Western Alps: Manaschal and Müntener, 2009; Corsica: Marroni and Pandolfi, 2007). Polymictic breccia can indeed be observed as conglomeratic horizons containing well-rounded fragments and boulders of metagabbro, metabasalt and serpentinitic (SM2) embedded in a strongly deformed, sedimentary-derived mylonitic matrix (SM2b). However, they are always intercalated with the metasedimentary lenses scattered in both the LSZ and LSZs (Fig. 3a, b) and their mafic fragments are never brecciated.

The following observations confirm that the ('eclogitic') breccia dispersed in the LSZ are of a different sort and were formed during eclogite facies conditions:

1. Mylonitization of the Fe-Ti gabbros predating brecciation takes place under eclogite facies. Evidence that foliation of the clasts forms at eclogite facies conditions is shown by i) the growth of syn-kinematic garnet (Fig. 8a, b) marked by ubiquitous pressure shadows (Fig. 8a), ii) the folding of rutile crystals, iii) the irregular boundaries of omphacite crystals indicating grain-boundary migration processes, iv) their large aspect ratio (up to 1:15) and v) the occurrence of CPO
for omphacite grains Figs. 8a, b). All these observations testify to processes of dynamic, ductile deformation accompanying eclogite-facies mylonitization (steps A and B in Fig. 11) and rule out topotactic, strain-free recrystallization of a foliation inherited from oceanic stages (except in rare un-brecciated Fe-Ti blocks; e.g., LSZ 28-15; Fig. 8d). This is in agreement with earlier conclusions by (Philippot and Kienast, 1989; Philippot and van Roermund, 1992).

(2) All eclogitic breccia planes preserved in the metagabbro blocks for ~15 km along strike of the LSZ cut across mylonitic eclogite facies foliation. At the outcrop scale, breccia intersects abruptly, at various angles, the eclogitic foliation made of intact Mg-Al metagabbros and Fe-Ti metagabbro boudins (e.g., Type 2 blocks; Figs. 4e, f, g, h and 6a, b). At the thin-section scale, brecciation cuts across syn-kinematic garnet (Grt1a-1b; Figs. 8g and 8a, b) and rutile trails aligned along the mylonitic foliation Figs. 8f, 9a-b).

(3) Within breccia planes, the matrices of the un-foliated clast-in-matrix structure (with straight grain boundaries, common triple-point junctions, and only minor evidence for undulose extinction in omphacite grains), contain typical eclogite facies paragenesis (e.g., Figs. 8f and 9). Their P-T estimates (Fig. 11) testifies to brecciation at eclogite facies conditions (M1: ~2.7 GPa, ~580°C; M2: ~2.4 GPa, ~560°C). The existence of successive, discrete brittle events at eclogite-facies conditions is demonstrated by (i) the clear crosscutting relationships between the mylonitic metagabbros and the matrices (e.g., Figs. 4c, d and 7a, b, c, d) as well as (ii) the many tension gashes and dilatant veins filled with omphacite (Omp1b) ± apatite found in clasts and metagabbro wall rocks (Figs. 4g and 7a). These later cut across the pre-existing mylonitic foliation and are post-dated by eclogite-facies matrices crystallization (Fig. 7a).

The sedimentary origin of these eclogitic breccias can be refuted for the following reasons:
(1) eclogite breccias lack elastic material (e.g., quartz, phengite, serpentinite or talc grains; Figs. 4a, 7a, b, c, d, 8c and 9b, c, d) and are essentially monomictic (omphacite ± garnet ± lawsonite), by contrast to what is observed in the polymictic Monviso metasedimentary conglomerates (e.g., SM2a-b; see also Balestro et al., 2015a, 2015b);

(2) clast analysis (Fig. 6c, d) and the progressive dismantlement from the fresh rock to the breccia cores (Fig. 6a, b) indicate that part of eclogitic breccias are tectonic. Preserved breccia planes indeed advocate for dynamic fracturing, as they record the full classical rim-to-core sequence (Jébrak, 1997; Mort and Woodcock, 2008) from intact metagabbro to fractured rock, crackle breccia, mosaic breccia and chaotic breccia (i.e., blocks B154, B1 and B77; Figs. 6a, b, c);

(3) eclogite breccias are essentially monogenic (see above; e.g., Figs. 4c, 6a, b and 7a, b, c, d).
Clasts of Mg-Al metagabbros are indeed restricted to the rims (20-50 cm zones) of breccia planes
cutting across the contact between Fe-Ti-gabbro and Mg-Al-gabbro (which is observed in < 10% of the blocks) and rapidly disappear towards the core of breccia planes. By contrast, breccias formed by oceanic detachment faults exhuming oceanic core complexes may exhibit a larger compositional variety in the fragments (metabasalts, metagabbros, metaperidotites, sometimes embedded in metasediments; e.g. Escartín et al., 2003, 2001). Whenever monogenic breccias are observed, as in hyperextended margins (e.g., Desmurs et al., 2001), their clasts preserve magmatic fabrics and/or foliation develops in the surrounding matrix, which is never observed in Monviso eclogitic breccias (e.g. Figs. 7c, 8f, and 9d, e, f ).

It is therefore possible to distinguish two types of breccias in the Lago superiore unit:

— the Fe-Ti metagabbro breccia blocks formed at eclogite-facies conditions (as first proposed by Angiboust et al., 2012b, 2011).

— the conglomeratic/breccia layers found associated to metasedimentary slivers (which can be ascribed to pre-Alpine, pre-subduction tectonic and/or sedimentary processes, as first proposed by Balestro et al., 2014).

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8.2 Reconstructing the discrete steps of eclogitic brecciation

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Structural/microstructural relationships, mineral chemistry data on breccia matrix and clasts and phase equilibrium modelling together with previous observations from the literature (Angiboust et al., 2012a; Philippot, 1987; Philippot and Kienast, 1989; Philippot and Selverstone, 1991; Schwartz et al., 2000; Spandler et al., 2011) allow to recognize several episodes in the brecciation process at eclogite facies conditions. Based on the P-T estimations by pseudosection modelling (Fig. 11) and the successive omphacite and garnet compositions (Fig. 10), these steps can be ascribed to prograde to peak mylonitization (Fig. 12; StepA-C) and early retrograde stage linked to exhumation (Fig. 12; steps D to E).

- Eclogitization and mylonitization prior to brecciation (steps A and B):

The Fe-Ti and Mg-Al gabbros (either within the crustal sequence or intruded in the peridotitic mantle) underwent prograde eclogitization (step A, ~23 kbar/~530°C, Fig. 12), as inferred by phase equilibria on garnet and omphacite cores (respectively Grt1 and Omph1) from both mylonitic clasts (e.g. Figs. 8a-c) and low-strain, unreCCIciated Fe-Ti metagabbros (type 3 blocks Fig. 8d). In the latter, topotactic replacement of pre-existing magmatic clinopyroxene crystals by omphacite (Omp1a) associated to rutile (growing after magmatic ilmenite) and coronitic garnet Grt1a after plagioclase can be observed (Figs. 8d).

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Progressive ductile deformation (step B, ~26 kbar/~550°C, Fig. 12) is attested by the formation of a mylonitic foliation, with newly grown omphacite (Omp1b) as rims around Omp1a and coeval growth of Grt1b around Grt1a garnet cores (with Omp1b inclusions in Grt1b), grain size reduction, fracturing and dismembering of the omphacite pseudomorphs after magmatic clinopyroxene. This is accompanied by the progressive flattening of rutile ribbons in extremely elongated trails parallel to the foliation. Garnet underwent fracturing, especially where segregated into bands or lensing (Fig. 8g), possibly as a result of indentation between garnet crystal borders and/or hydrofracturing (as suggested by Spandler et al., 2011). This step was accompanied by the (probably transient) development of tension gashes and dilatant veins with Omp1b omphacite ± apatite.

### - Brecciation (steps C to E):

The first brecciation event (step C), coincident with the relative rotation of clasts of Fe-Ti (and rare Mg-Al) metagabbros, is recorded by thin (< 200 µm-thick Fig. 8f) inter-clast domains filled with either dismembered and re-oriented grains from clasts ("microbreccia" domains) or newly crystallized, ipidiomorphic omphacite-rich (Omp2a) M1 matrix (Fig. 9d). In both matrix types, this stage is associated to volumetrically limited Grt2a overgrowths around mylonitic garnets (Figs. 9d and 11c). In the microbreccia domains, fractured and angular omphacite and garnet exhibit strong grain size reduction, obliteration of foliation, and compositions similar to minerals in clasts: all these features show that they correspond to clast pieces brittlely-deformed during brecciation and subsequently sealed by interstitial omphacite Omp2a and associated Grt2a rims (Fig. 9c and SM8a). Pseudosection modeling suggests equilibrium of matrix M1 at ~27 kbar/~580°C, so that the onset of brecciation took place at the peak P-T conditions reached by the LSU metagabbros (Fig. 12).

The crystallization of Omp2b in both the microbreccia and M1 matrix domains (developing complex, radial-shape infiltration patterns overgrowing the pre-existing omphacite grains) (Fig. 9d and SM8b) testifies a later stage fluid infiltration, tentatively ascribed to the crystallization of the M2 matrix. In this latter stage the same assemblage is recorded, with euhedral Omp2a rimmed by Omp2b (locally Cr-rich) associated to garnets showing Grt2a cores and Grt2b rims (the latter including Omp2b crystals) with sectorial Cr-enrichment patterns. From structural relationships (Figs. 7a, b) it is possible to infer that the crystallization of M2 matrix (~24 kbar/~560°C: Step D in Fig. 12) took place after the development of both microbreccia and M1 matrix domains.

A third, volumetrically dominant (80 vol%) lawsonite-rich M3 matrix crosscuts all preexisting structures (Step E, Fig. 12; Fig. 7b). The abundance of lawsonite pseudomorphs in M3 (up to ~40 vol% in places), compared to the rather dry assemblages of M1 and even M2, advocates for late-stage massive fluid infiltration, most probably externally-derived (e.g., from serpentinite and metasediment dehydration at deeper levels, in agreement to what observed on HP-veins by Spandler
et al., 2011 or on blocks metasomatic rinds by Rubatto and Angiboust, 2015), since no significant
dehydration is predicted for Fe-Ti eclogitic metagabbros at such PT conditions (i.e., not until
lawsonite breakdown). We speculate that the infiltration of fluids may have been promoted by the
first steps of brecciation (steps C and D) by mechanical weakening and porosity increase. Due to
the impossibility to retrieve the original chemical composition of the M3 matrix, P-T constraints for
this stage are assessed using relative chronology (e.g., M3 postdating M2; Fig. 7b) and lawsonite
stability.

789 - Post-brecciation evolution(step F):

790 Step F corresponds to the alteration visible in the rims of brecciated blocks, i.e. subsequent rind
791 formation (up to 0.5 m thick) at their contact with the antigorite-rich matrix embedding them in the
792 LSZ (Figs. 4b, 7d). Rind formation affects both clasts and matrix, and thus postdates brecciation. In
793 metagabbro block rims, the preexisting eclogitic facies assemblage is replaced by talc, chlorite, Grt3
garnet, sodic amphibole, diopside, rare phengite and lawsonite pseudomorphs indicative of
794 (externally derived) fluids. Chemical alteration rapidly decreases from block rims to cores. These
795 rinds form in the LSZ after peak burial and brecciation, during exhumation(Fig. 12a; see also
796 Angiboust et al., 2014).
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798 Late-stage retrogression of the eclogite-facies blocks into a greenschist facies assemblage (i.e.,
799 tremolite-actinolite, chlorite, epidote, and mm-thick veins of calcite + albite + quartz + epidote; Fig.
800 9g and SM7b, c, d) is widespread in metabreccia blocks. The dryer matrices (M1 and M2) and less
801 permeable clasts underwent only partial retrogression at their rims and better preserved their
802 pristine eclogite-facies assemblages.

803
804 8.3 Locus of brecciation and progressive strain localization along the slab interface

805
806 8.3.1 Strain localization along Fe-Ti gabbro horizons

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808 In the Monviso metaophiolite brittle deformation and brecciation at eclogite-facies conditions
809 took place preferentially along Fe-Ti metagabbro horizons and in their vicinity (as shown by the
810 largely monogenic nature of breccia blocks), hence in the most resistant rocks, considering field
811 observations (i.e., strain partitioned in folded Mg-Al metagabbros and serpentinites) and available
812 flow laws. At metamorphic peak conditions reached by the Lago Superiore Unit (~27 kbar/580°C,
813 brecciation step C; Fig. 11e), the strength of eclogitic Fe-Ti metagabbros may indeed be up to 7
814 GPa (see Jin et al., 2001 for a comparable assemblage made of 50% garnet, 40% omphacite and
815 10% quartz) while no available data exist to describe the behaviour of Mg-Al metagabbros.
816 Nevertheless, taking in account the strong dependency between garnet volume fraction and the
strength of the rock (Jin et al., 2001), we can approximate that of the garnet-free Mg-Al metagabbros up-to one order of magnitude lower than those of Fe-Ti metagabbros. Thus, the strength of Fe-Ti metagabbros exceeds the one of both Mg-Al metagabbros and serpentinite (whose effective viscosity is ~4 orders of magnitude lower than metagabbros; e.g., Hilairet et al., 2007).

Is a matter of fact that the Fe-Ti metagabbro boudins embedded inside the in Mg-Al gabbros represent a strong rheological contrast that may explain brittle rupture localization, as seen at the boundaries between multi-layered lithologies/media with different competency (e.g., Strömgård, 1973; Treagus, 1981). Nevertheless, the behaviour of multi-phase material is not very well-known (e.g., Schüler et al., 2013) and that of monomineralic layers may deviate from flow laws: the strength of pure omphacite is for example twice lower than that of eclogite and four-time lower than garnetite (e.g., the "omphacitite" of Jin et al., 2001). Thus, we calculate that the almost pure Omp (+/-Ap) bearing veins developed during prograde mylonitization (Fig 3a, 8e, f and SM8a; see also Philippot, 1987, Philippot & Kienast, 1989 and Philippot & Van Roermund, 1992) may have acted as weaker layers that localized brecciation, as suggested by the preferential development of microbreccia domains at vein-clast boundaries (e.g. Fig. 8f). Moreover, "semi-brittle" modes of deformation, involving mechanical failure at the contact between monomineralic and polymineralic media, as observed in experiments on chalcopyroxene and garnet (Kirby and Kronenberg, 1984; Philippot and van Roermund, 1992; Rutter, 1986), might explain the rheological processes leading to brecciation. Ongoing reactions are also likely to modify these rheologies - i.e., the growth of garnet (along with fluid release) in Mg-Al metagabbros - likely increased the strength (and brecciation potential) of this lithology.

### 8.3.2 Progressive strain localization within the slab

Field mapping (Figs. 3a, b, SM1) shows that most of the breccia blocks (88% of Type 2 blocks and all Type 1) are concentrated in the upper part of the LSZ, below the Mg-Al gabbro horizon but above the large slivers of Mg-Al metagabbros (Figs. 5a, b). Their structural position is the same all along strike of the LSZ, from Colle di Luca to Ghincia Pastour (~15 km; Fig. SM1). Brecciation therefore occurred predominantly (i) along dykes and sills of Fe-Ti metagabbros emplaced within and towards the base of the Mg-Al metagabbro sequence (i.e. in the future LSZ; Figs. 3b and 13a, b) and to a lesser extent (ii) within Fe-Ti metagabbros sandwiched between the Mg-Al metagabbro and metabasalts layers (i.e., along the future ISZ). This advocates for brecciation in both the LSZ and ISZ, rather than a single brittle event in the ISZ with subsequent incorporation into the LSZ (as initially envisioned by Angiboust et al., 2012b).
Progressive deformation across the LSZ and ISZ is shown in figure 13. Location of brecciation was controlled by the strength contrast across the lithological sequence (§ 8.3.1) and assisted by further strain localization and fluid circulation into the Fe-Ti metagabbros (Fig. 13a). The eclogitic breccias, at least during the early stages of LSZ development, thus acted as a major mechanical discontinuity. Formation of the M1 matrix (Fig. 13a) marks the onset of brecciation at peak P-T conditions (~27 kbar/580°C) and testifies to changes in porosity/permeability, progressively enhanced in M2 and (mostly) in M3 matrices (Figs. 12 and 13a). The presence of the Lws-rich M3 matrix assemblage (Figs. 7b and 9g), which requires extensive fluid infiltration, indicates that eclogite breccias formed near the base of the Mg-Al metagabbro horizon became connected at that stage.

Strain localization in metagabbros (testified by their pervasive mylonitization and following brecciation) indicates that serpentinization of the top of the mantle section was initially insufficient to localize all ductile strain. It is likely, however, that serpentinization progressively extended into the peridotite sole and finally localized most of the strain in the LSZ, as suggested by (i) the partly rodingitized, unreccciated Fe-Ti gabbrons intrusions found in the basal peridotite (Figs. 3a-b and SM1; as in active slow-spreading oceans, e.g., the SW Indian Ocean and Mid-Atlantic ridges; Dick et al., 2008; Lagabrielle and Lempine, 1997; Lissenberg et al., 2009; Lagabrielle and Cannat, 1990) resemble Type 3 unreccciated Fe-Ti metagabbro blocks, which are restricted to the lower part of the LSZ (Figs. 5b and SM1) and which size increases towards the serpentinite sole; these blocks thus likely correspond to slabs/dykes initially emplaced into the peridotite sole and later incorporated into the LSZ during its progressive thickening (Fig. 13a).

(ii) incorporation of unreccciated Type 2 and Type 3 metagabbro slivers/blocks into the LSZ (and to a minor extent in the ISZ; Figs. 4a, 5 and 13a), suggestive of progressive, fluid-assisted network widening of the shear zone (e.g., Means, 1995) and local tectonic reworking of the crustal organization (Fig. 13a, b).

(iii) dissemination of eclogitic metagabbro blocks in the LSZ and ISZ occurred after the metamorphic peak (i.e., during early exhumation stages and after the eclogite-facies brecciation) likely at the eclogite to blueschist facies transition (Angiboust et al., 2014). We speculate that the source for the fluids driving the progressive serpentinization (and weakening) of peridotites derived from deserpentinization processes at deeper slab levels (as already stated by Spandler et al., 2011 or Angiboust et al., 2014). The interaction between these fluids and metagabbros led also to the post-brecciation rind formation (SM7d), with massive recrystallization of talc, chlorite, subordinate Cr-rich garnet (Gt3), serpentinite (plus sodic amphibole and accessory phengite).
Interestingly, the presence along the LSZ of isolated lenses of metasediments requires hm-km offsets along an initially discontinuous oceanic crust (Fig. 13a). We propose that such lenses may be inherited from the initial seafloor structure (with little lateral continuity compared to other shear zones interpreted as remnants of oceanic core complexes; e.g., Corsica; Lagabrielle et al., 2015) or correspond to extreme boudinage. The amplitude of the offsets is nevertheless limited by (i) the fact that eclogitic breccia planes are always found on the upper/western side of the Mg-Al metagabbro Type 2 slivers, broadly parallel to the LSZ shear zone walls (e.g., Punta Murel and Colle di Luca; Figs. 3b, 4a, 5a, SM1) and (ii) the ordered distribution of eclogitic breccia blocks in the upper part of the LSZ (Fig. 5a, b and SM1), which both advocate for only small displacements after their dissemination.

Final detachment of the Lago Superiore Unit took place in a shear zone located further below (i.e., the Basal Shear Zone; Fig. 13a, b).

### 8.3.3 Brecciation mechanism: triggered by earthquake or hydrofracturation?

Angiboust et al. (2012a) argued that the switch from ductile to brittle regime documented by eclogite brecciation (crosscutting the eclogitic mylonitic foliation) could result from either a change in strain rate (i.e., following an earthquake) or, and in fluid pressure (whether or not by dehydration embrittlement; Hacker et al., 2003b). The fluid ingression recorded by the M3 lawsonite-rich matrix (Lws~40 vol%) could thus be seen either as a consequence or as the trigger for strain localization. Evidence for instantaneous deformation includes the sharp breccia planes crosscutting the eclogitic mylonitic foliation, the formation of crackle-breccia with relative displacement (and the full transition from crackle to eclogitic breccia), the formation of microbreccia domains and the existence of minerals fractured and offset along omphacite-bearing planes (e.g., garnet; Fig. 8g).

It is impossible, at this stage, to assess whether eclogitic brecciation was coseismic: the lack of seismically-derived structures/microstructures (e.g., pseudotachylites), the limited extension of the breccia planes preserved in the blocks (< ~3-4 m) and the unknown duration of brecciation call for caution in relating breccias to earthquakes. It is interesting, however, to note that in Corsica, pseudotachylites are found in a similar structural position as the eclogite breccias, i.e. in metagabbros and peridotites at and above the Moho (Deseta et al., 2014; Magott et al., 2016). Elucidating the nature of infiltrating fluids and the deformation mechanism acting in the Fe-Ti metagabbros (work in progress) may provide further information and help decide which of the two processes was most effective.
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9. Conclusions

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This study documents progressive strain localization within the slab into the gabbroic crust during
and prior to the detachment of the large Monviso slab fragment:

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(1) In metagabbro blocks outcropping inside the LSZ, the majority of breccias formed by brittle rupture under eclogite-facies conditions. By the occurrence of eclogite-facies mylonitic Fe-Ti and Mg-Al metagabbro clasts we exclude their origin by sedimentary deposition in paleo-oceanic basins or high-pressure overprinting of breccia formed at shallow crustal depths. The lack of deformation in the matrix between eclogite clasts contrasts with the reworked matrix of other (pre-Alpine) metasedimentary slivers dispersed in the shear zone.

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(2) Brecciation controlled the initial stages of strain localization within the LSZ. It preferentially occurred in Fe-Ti gabbros embedded in Mg-Al metagabbros, suggesting that the rheological contrast between the two metagabbros controlled the onset of brecciation.

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(3) A complex succession of events accompanied brecciation. The existence of three successive types of matrix, with a sharp increase in fluid-content for the latest, lawsonite-rich one (M3), points to embrittlement then fluid ingression. Observation on preserved breccia planes of the full transition from intact metagabbro to crackle, mosaic and chaotic breccia, together with the presence of microbreccia domains and the occurrence of minerals fractured and offset along omphacite-bearing planes support the view that Monviso eclogite breccias were generated by instantaneous brittle rupture.

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(4) Further strain localization into the LSZ, marked by the incorporation of unrecciated mafic and ultramafic blocks and by network widening, was promoted by fluids and metasomatism.

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Acknowledgment

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This study was funded by the project "Zooming in between plates" (Marie Curie International Training Network no. 604713). We thank M. Fialin, N. Rividi (CAMPARIS), O. Boudouma (ISTeP) for analytical support, E. Delairis (ISTeP) for the preparation of thin sections and the Parco del Monviso for bureaucratic help. We also thank Tan Z., Bonnet G. and Soret M. for interesting discussions and the friends of Rifugio Alpetto for the gastronomic and moral support. Othmar Müntener and an anonymous reviewer are warmly thanked for their constructive reviews and suggestions, and Marco Scambelluri for his editorial handling leading to a significant improvement of the manuscript.

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## 10. List of Figures

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Fig. 1 Correlation between seismicity and mineral phase transformations in subduction zones (sketch refers to the Tohoku subduction zone, modified after Hacker et al., 2003) In the inset, the position of the Lago Superiore Unit (LSU) and its shear zones respect to the inferred FT condition (This study and previous works: Schwartz et al., 2000; Groppo & Castelli, 2011; Angiboust et al., 2012a). In the box, the estimated water content (wt%). Isotherms from Peacock and Wang, 1999

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Fig. 2 Tectonic sketch of the Western Alps. The Eocene eclogitic belt (green) is exposed on the upper-plate side of the orogen, between the Frontal wedge (light blue) and the remnants of a Late Cretaceous doubly- vergent wedge (pink). In the inset, a detail of the eclogite belt and the frontal wedge with the localization of the Monviso metaaphiolite. Maps modified after Malusá et al. (2011) and Agard et al. (2009).

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Fig. 3 (a) 3D rendering of the geology of the southern portion of the Monviso metaaphiolite. White, full-lines represent the traces of the geologic crosssection AA', BB'; black lines are the sections across the Lower shear zone (LSZ) where block distribution inside the shear zone was analyzed (more details in Fig. 5b).

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(b) Detailed stratigraphic logs across the Lago Superiore Unit (LSU) and the Monviso Unit (MU). From South to North the analyzed sectors are Gallarino, Viso Mozzo, Lago Chiaretto and Lago Superiore. For further details refer to the geological map presented in SM1.

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(c) Typical appearance of a m-scale metagabbro block from the LSZ. In evidence the pressure-shadows made by coherent (if compared to the bulk of the LSZ and ISZ) antigorite-rich schists with disseminated dm-to cm-scale clasts made by strongly-metasomatized metagabbros. Block B61, NE of Punta Murel.

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Fig. 4 (a) Panoramic view from Pian Radice to the North (Viso Mozzo). In evidence the stratigraphy of the Lago Superiore Unit with the position of the hanging wall of the Lower Shear Zone (LSZ; purple) and the Upper Shear Zone (USZ; blue).

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(b) Detail of a Fe-Ti metagabbro outcrop at the base of the Mg-Al metagabbros constituting the Bianco cliff. Notably, the intact Fe-Ti metagabbro is surrounded by a breccia layer with thickness up-to 50 cm, locally strongly metasomatized. Sampling location of specimen LSZ 63-15 (Fig.7d).
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(c) Typical clast-in-matrix appearance of eclogite-facies breccia on fresh surface. Here the rotated metagabbro clasts are cemented by omphacite-rich matrix (M1). Veins in the clasts are filled by fibrous omphacite ± apatite. Block B37, NW of Alpetto.

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(d) Typical appearance of eclogite breccia with strong weathering patina. The clast-in-matrix structure is still visible, with appreciable clasts rotation highlighted by foliation planes. Block B69, Punta Murel.

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(e-f) Example of a folded, mylonitic, eclogite-facies Mg-Al metagabbro unconformably cut by eclogite-facies breccia planes. Serpentinite-talcschists metasomatization is focused on the breccia matrix. Block B120, north of Punta Murel.

992

993

994

(g) Typical m-scale Type2 block. The eclogitic breccia develops crosscutting (here at low angle) the pre-existing mylonitic foliation of the intact Mg-Al metagabbro. To be noted the Fe-Ti boudins in the intact metagabbro. Block B73, Prá Fiorito valley.

995

996

997

(h) Transition from intact to brecciated rock recorded in a simple Fe-Ti metagabbro block. Here the breccia thickness reach-up 80 cm. B192, south of Colle di Luca.

998

999

Fig. 5 (a) Geological cross-section across the Lower Shear Zone (Punta Murel area). We enlightened the ordered distribution (dip towards West) of the breccia planes in the detached Mg-Al metagabbros slices dispersed in the LSZ. Unbrecciated Fe-Ti metagabbro blocks are concentrated at the base of LSZ. Mg: metabasalts; Rg: inactive rock glacier; Rf: rock-fall deposits; Eb: eclogite breccias; Fg: Fe-Ti metagabbros; Rg: retrogressed Mg-Al metagabbros; Mp: mafic peridotites (serpentinized); Rd: metarodingite dykes; Bs: basal serpentinites; Df: debris flow.

1000

1001

1002

1003

1004

1005

(b) Diagram showing the distribution of metagabbro blocks inside the LSZ. The larger part of eclogite breccia blocks (Type 1) outcrops in the upper part of LSZ, above the Mg-Al metagabbro slices, with the unbrecciated Fe-Ti metagabbro blocks (Type 3) located at the base. On the X-axis the normalized horizontal distance of blocks from the base of LSZ and on the Y-axis the normalized vertical distance of blocks from the base of LSZ.

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1008

1009

1010

Fig. 6 (a) A fully preserved eclogite-breccia fault-plane. Breccia fabrics show the transition from crinkle breccia (at the top) to chaotic breccia (at the core), according to the definitions Mort and Woodcock (2007). Block B154, Pra Fiorito valley.

1011

1012

1013

(b) schematic drawing of the clast-in-matrix structures observed in preserved eclogite-facies breccia planes. Used color-code: Fe-Ti metagabbro clast (white), Mg-Al metagabbro clasts (light gray), Omphacite-rich matrix (dark gray), Lws-rich matrix (light green). Block B154, Pra Fiorito valley.

1014

1015

1016

(c) Sketch of an eclogite-facies breccia block characterized by multiple rupture surfaces. The black box locates Fig. 6e. Original photo of the block in supplementary material SM6e. Block B77, Pra Fiorito Valley.

1017
1018 (d) Schematic sketch of the minor breccia plane in the block B77 with the evaluated volumetric percentage
1019 of clasts (black) vs matrix (white) on 5 equal-area sectors. In the inset, the orientation of the maximum
1020 elongation axis of the clasts dispersed in the matrix with respect to the single minor breccia plane. Only
1021 clasts with aspect ratio > 2 where analyzed.

1022 Fig. 7 Meso-structures of eclogite breccias. (a) Eclogitic vein with fibrous omphacite + apatite crystals
1023 developed in a mylonitic, eclogite-facies Fe-Ti metagabbro clast. Coarse-grain omphacite + garnet matrix
1024 (deep-green color) seals all the previous structures. Block B110, Intermediate Shear Zone (West of Lago
1025 Superiore).

1026 (b) Eclogitic metagabbro clasts (evident the mylonitic foliations) sealed by (ex)lawsonite-rich matrix.
1027 Noteworthy, in the upper-center part of the picture, the occurrence of a clast composed by reworked eclogite-
1028 facies breccia (mylonitic metagabbro clasts cemented by omphacite-rich matrix M1). Block B45, South of
1029 Punta Murel.

1030 (c) Mylonitic, eclogite-facies fragments of finely-grained and folded Fe-Ti metagabbro sealed by
1031 omphacite-rich M1 matrix; in evidence the mm-scale, euhectic omphacite grains of M1 matrix. This sample
1032 clearly shows the development of the talc-, chlorite-, amphibole- and garnet-rich metasomatism (and
1033 associated selvages - dashed arrows-) affecting all the metagabbro breccias found in the ISZ and LSZ.
1034 Sample LSZ01-17, E of P.ta Forcion.

1035 (d) Polished sample of eclogite breccia evidencing the crosscutting relationship between mylonitic clasts,
1036 the omphacite + garnet matrix (M2) and a metasomatic rind mineralization (talc + chlorite + garnet +
1037 amphibole ± Ca-rich diopside). The darker areas (mainly developed in the matrix portions) are
1038 chlorite/amphibole-rich recrystallization linked to late-stage retrogression under green-schist condition.
1039 Sample LSZ 63-15, from the base of Truc Bianco cliff (Fig. 4b).

1040 (e) Schematic reconstruction of the structures developed in blocks with fully-preserved transition from
1041 intact rock to breccia layers. Such type of blocks correspond to the "Type 2" described in the text (e.g. the
1042 big sliver of Punta Murel B69- or the smaller blocks B154 in Pra Fiorito valley).

1043 Fig. 8 Microstructures of eclogite breccias as seen under optical polarizing microscope, if not otherwise
1044 indicated. Pictures (d, e, f) plane polarized light; (a, b,c) cross polarized light.

1045 (a) Mylonitic, eclogite-facies foliation of a Fe-Ti metagabbro clasts with the omphacite-rich, rutile-free
1046 pressure shadows developed around mylonitic garnet porphyroblasts. In the latter the cores are extremely-
1047 rich in inclusions of omphacite ± glaucophane (sample L14-52, LSZ).

1048 (b) Detail of the mylonitic, eclogite-facies foliation of a Fe-Ti metagabbro clasts. Omphacite grains
1049 crystallize with an aspect-ratio up to 1:15, elongated parallel to the foliation. Their irregular grain boundaries
1050 (detail in the box) indicate that grain-boundary migration processes were active during ductile deformation.
1051 (sample L50-15, LSZ)
1052 (c) Syn-kinematic garnet, with preserved omphacite trails inclusions, growth in a mylonitic (partially
1053 annealed) Fe-Ti metagabbro clasts (sample L14-48, LSZ).

1054 (d) In rare, low-strained metagabbros blocks (type 3 blocks), evidences for topotactic replacement of the
1055 pre-existing magmatic fabric is preserved. Here mm-scale omphacite porphyroclasts (crystalizing after
1056 magmatic clinopyroxene) are associated to coronitic garnet (after plagioclase) and interstitial rutile (after
titanite). In the box: a rare kink-band developed inside an omphacite porphyroclast, attesting minor ductile
1057 deformation. Sample L28-15, LSZ.
1058

1059 (e) Photomicrograph of a sheared omphacite-rich vein grown inside a mylonitic Fe-Ti metagabbro clast.
1060 The top-to-the right sense of shear is derived by the deflection direction of the acicular omphacite
1061 megacrystals. Sample I36-15, ISZ.

1062 (f) Upper box: clast-in-matrix fabric developed at mm-scale in the crenkle breccia horizon of sample L14-
1063 53 (LSZ). Clasts rotation and eclogite-facies matrix crystallization are evident and are totally comparable to
1064 those observed at the meso-scale. Lower-box: interpretation of the structures and mineralogy observed. To
1065 be noted (*): thin-section of Fig.9d is from the same rock sample but different thin-section (cut on the same
1066 microstructural location).

1067 (g) Micrometric garnet and omphacite offset along omphacite-bearing microfractures, evidencing an
1068 apparent displacement of ~250 µm. Color-code for the FEG-SEM quantified map: Ca (Green), Na (Red) and
1069 Mg (Blue). Sample L14-53, LSZ.

1070 Fig. 9 Microstructures of eclogite breccia are seen under optical polarizing microscope, if not otherwise
1071 indicated. Picture (a, e, g) plane polarized light; (f) cross polarized light.

1072 (a) Microfractured omphacite, garnet and rutile crystals crudely oriented in the "microbreccia" domains
1073 locally preserved between clasts. Notably, garnet shapes are sharp-edged. Later-stage Fe-rich veins
1074 preferentially infiltrate in these domains. Sample L14-53, LSZ.

1075 (b) SEM image evidencing the strong grain-size reduction and the omphacite and rutile crystals re-
1076 orientation at the sharp transition between a mylonitic-clast and microbreccia domain. Sample L14-53, LSZ.

1077 (c) SEM image showing the complex zonation of omphacite crystals in the "microbreccia" domains, with
1078 mylonitic Omp1a and Omp1b sealed by a third generation of omphacite Omp2a. As observed for garnets,
1079 crystals have sharp-edged shapes. Quantified EPMA map of these domains are presented in Fig. SM8a.

1080 (d) FEG-SEM quantified maps of the transition between a mylonitic Fe-Ti metagabbro clast (at the
1081 bottom) and the omphacite-rich matrix M1 (at the top). In evidence the infiltration-like geometries of Omp2b
1082 inside the clast departing from M1. Color-scale in the figure. In the upper box: EPMA quantified map
1083 showing the MnO enrichment of garnet generation Grt2a. Sample L14-53, LSZ.
1084 (e) Photomicrograph of the big hypidiomorphic omphacite crystals and inclusion-poor garnets constituting
1085 the bulk of M2 matrix. Note the sharp contact clast-M2 matrix. Sample L63-15b, LSZ.

1086 (f) M2 matrix omphacite grains: detail photomicrograph of the tabular grain boundaries crystallizing at
1087 triple-points junctions (120) and straight borders. Only here minor undulose extinction is observed, whereas
1088 no evidence for mylonitization is shown. Sample L63-15b, LSZ.

1089 (g) Photomicrograph of the transition between an Mg-Al metagabbro clast and ex-Lws rich matrix M3.
1090 The un-sharp contact between clast and matrix is linked to the massive green-schist recrystallization that
1091 partly obliterated the pre-existing structures. Sample L14-11, LSZ. See Fig. SM7a-b for more detail on
1092 matrix M3 paragenesis.

1093 Fig. 10 EPMA composition of selected minerals: a) Mg# [MgO/(MgO:FeO)*100] vs. CaO (wt.%) in LSZ
1094 + ISZ omphacites; b) Ternary-plot (Jad-Diop-Aeg) for selected omphacite from the LSZ and ISZ; c)
1095 Ternary-plot (Grs-Alm-Prp) for selected garnets from the LSZ; d) Ternary-plot (Grs-Alm-Prp) for selected
garnets from the LSZ (shaded) vs selected garnets from the ISZ Comparison with previous data from
1096 Groppo & Castelli (2011); Spandler et al. (2011); Angiboust et al. (2012a, 2014) is shown in Fig. 10 b and
1097 c; e) Evolution of the observed paragenesis with respect to microstructural domains observed in the studied
1098 samples from both LSZ and ISZ. The stars define the onset of the first brecciation event.
1099

1100 Fig. 11 P-T estimates for the studied eclogites, inferred from PerpleX pseudosection modelling in the
1101 system NCKFMASHTO; Step A-B, M1 and M2 boxes consider ca. 5% calculated uncertainties
1102 bulk composition in Table 4. Abbreviations: Omph, omphacite; Grt, garnet; Jad, jadeite; Aeg, aegirine; Grs,
1103 grossular; M1, matrix M1; M2, matrix M2, A and B, mylonitic steps.

1104 (a-b) P-T estimates for the prograde mylonitization of Fe-Ti metagabbro. Step A: crystallization of Grt1a
1105 and Omp1a; Step B: crystallization of Grt1b and Omp1b. Bulk and mineral compositions obtained from
1106 sample ISZ 31-15 (Tables 2, 3 and 1).

1107 (c) P-T estimates for the crystallization of M1 matrix. Bulk and mineral compositions obtained from
1108 sample LSZ 20-15a (Tables 2, 3 and 4). In the box: modelled modal volumes of omphacite (black lines) and
garnet (white, dotted lines).

1110 (d) P-T estimates for the crystallization of M2 matrix. Bulk and mineral compositions obtained from
1111 sample LSZ 63-15D (Tables 2, 3 and 4).

1112 (e) Estimated P-T trajectory from pseudosection modelling, integrated with the estimations from previous
1113 works on the Monviso, Schistes Lustrés and Dora Maira areas. The red, dotted-line box represents the PT
1114 estimates considering uncertainties on bulk composition; the thin, gray arrows suggest the relative
1115 chronology of crystallization obtained by structural evidences. SL-W(Ag), SL-E(Ag): West and East
1116 Schistes lustrés: Agard et al. (2009). VISO(GC): prograde eclogitization P-T path of Monviso metagabbros:
1117 Groppo & Castelli (2011). VISO(An): Lago Superiore Unit, Monviso: Angiboust et al. (2012a) for peak and
retrograde conditions; VISO(Me); Monviso metagabbros: Messiga et al. (1999). VISO(Sw): Monviso metagabbros and metabasites: Schwartz et al. (2000) for peak and retrograde conditions. Dotted line: inferred retrograde P-T path for the Lago Superiore Unit (Angiboust et al., 2012a). Blue, dotted lines represent the experimental dehydration curves for antigorite; BP03, Bromiley & Pawley (2003); UT95, Ulmer & Trommsdorff (1995) UT95); PN10, Padrón-Navarta et al. (2010).

Fig. 12 Relative chronology of events (with associated P-T position) leading to the brecciation of eclogite-facies Fe-Ti metagabbros from the LSZ. Eclogitized igneous texture (step A) are progressively obliterated by mylonitization (step B), with related opening of Omp1b± apatite veins I. Various amount of hydrous minerals grew at this stage (e.g., lawsonite). Progressive deformation culminated in the first brecciation event (Step C), with brittle disruption of metagabbros (e.g. formation of crackle breccia domains and micro-displacement of garnets), clasts rotation and crystalization of the omphacite-rich matrix M1. On-going deformation and increasing water-content of circulating fluids led to the crystallization of matrices M2 (Step D, omphacite + garnet) and M3 (Step E, lawsonite + omphacite + garnet). Localized metasomatism (with crystallization of newly-formed garnet + talc + chlorite + amphibole + Ca-rich diopside) occurs after all the previous events (Step F). The red, dotted-line box represents the PT estimates considering uncertainties on bulk composition; the thin, gray arrows suggest the relative chronology of crystallization obtained by structural evidences.

Fig. 13 (a) Localization of the brecciation events inside subducted oceanic crust (corresponding to the Monviso LSU) and its implication. At pre-subduction conditions (Stage1) the slow-spread oceanic crust is characterized by a complex intrusion pattern of Fe-Ti gabbros (as suggested by Lagabrielle et al., 1999). During subduction it reached eclogite-facies conditions (Stage2) and the increasing deformation coupled to the rheological contrast between Fe-Ti and Mg-Al metagabbros culminate at peak-burial conditions in the brittle failure (brecciation) mainly in the Fe-Ti metagabbros. As suggested by the crystallization of matrices progressively richer in water (M1: pure omphacite. M2: omphacite + garnet. M3, 70% of volume composed by lawsonite) and the subsequent development of metasomatic rinds and veins, brecciation likely promote the generation of porosity supporting the external fluid infiltration (Stage3). The interplay between on-going deformation and the fluid-assisted metasomatism likely decreased the strength of the rock, promoting the widening of the shear zones and the detachment/dissemination of blocks and slivers into the serpentinized LSZ and ISZ (Stage 3). (b) Schematic 3D tectonic restoration of a plate interface at intermediate depth inferred from the structures observed in the Monviso metaaphiolite, with the locus of the eclogite-facies brecciation (yellow stars) and the localization of the main shear zones.

1149

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1483
<figure><img src="image_3.png" /><figcaption>Figure1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure4</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure5</figcaption></figure>
<figure><img src="image_8.png" /></figure>

B154

Fig. 6b

b

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

B77

Figure 6e

<figure><img src="image_11.png" /></figure>

Figure 6
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure7</figcaption></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>Figure9</figcaption></figure>
a Mg# Vs CaO in the LSZ + ISZ omphacite

<figure><img src="image_32.png" /></figure>

b LSZ and ISZ omphacite generations

<figure><img src="image_33.png" /></figure>

c LSZ: garnet generations

<figure><img src="image_34.png" /></figure>

d LSZ: garnet generations

<figure><img src="image_35.png" /></figure>

e Minerals appearance by generation and structural domain

<figure><img src="image_36.png" /><figcaption>Figure10</figcaption></figure>
<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /><figcaption>Figure 11</figcaption></figure>
Step A

<figure><img src="image_43.png" /></figure>

Step B

<figure><img src="image_44.png" /></figure>

Onset of Brecciation

<figure><img src="image_45.png" /></figure>

Step F

<figure><img src="image_46.png" /></figure>

Step E

<figure><img src="image_47.png" /></figure>

Eclogitic breccia development

<figure><img src="image_48.png" /><figcaption>Figure12</figcaption></figure>

<figure><img src="image_49.png" /></figure>
<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /><figcaption>Figure13</figcaption></figure>