DATA REPOSITORY/ G32925 Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes? Samuel Angiboust et al.

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1 DATA REPOSITORY/ G32925 Eclogite breccias in a subducted ophiolite: a record of intermediate-depth earthquakes? Samuel Angiboust et al. Additional description of the eclogite breccias (Figs. DR1-3)

3 Figure DR1: Photographs showing typical textures of eclogite breccias. A. Lower shear zone heterometric eclogite breccia showing numerous cm-sized eclogite mylonite fragments. B. Photograph of a complex intermingling of heterometric foliated mylonitic eclogite clasts within a talc-chlorite rich matrix (the latter being derived from the metasomatic reaction between the block and the surrounding serpentinite); coin for scale (25mm diameter). C. Photography of a polished section of a hand specimen from Punta Forcione area (sample 05; exact location and GPS coordinates are given in Angiboust et al., 2011). D. Interpretative sketch of figure C localizing mylonitic clasts (and their relative rotation attested by differences in strike and dip of the mylonitic foliation), the eclogite-facies fracture filling material (named inter-clast on the figure) and a primary Omp-Tlc vein formed during mylonitization (as reported elsewhere by Spandler et al., 2011). E. Microphotograph of a sharp clast-matrix interface (plane polarized light). F. Microphotograph of a typical inter-clast, recrystallized matrix containing numerous large (mm-sized) garnet (Grt) and abundant pseudomorphs after lawsonite (Lws ps.) within omphacite (Omp); plane polarized light. Figure DR2: Histogram showing the clast distribution in eclogite breccias for six different outcrops along the Lower shear zone. Note that the distribution is plurimodal, both dominated by small, mm long fragments. Although the inter-clast matrix contains some smaller, sub-millimetric porphyroclasts (such as garnet fragments), it was impossible to determine accurately their relative amount because the matrix has been partially dynamically recrystallized during eclogite-facies metamorphism. Eclogite breccias presented here dramatically differ from classical breccias in terms of P-T conditions of formation. The comparison with classifications from the literature must therefore be taken with caution since physico-chemical processes acting under eclogite-facies conditions possibly significantly differ from those prevailing in "typical" breccia localities worldwide. Microtextural observations suggest that eclogite breccias correspond to a protocataclastic chaotic breccia according to the classification of Woodcock and Mort (2008; i.e. between 30 and 60% of large clasts > 2 mm, with less than 50% matrix material; Fig.DR-1A,C). The relatively heterogeneous particle size distribution (Fig.DR-2) and the existence of markedly rotated rounded clasts (Fig.DR1-A)

4 point to wear abrasion processes, which is a typical physical process acting along shear zones (according to the classification from Jébrak, 1997). The strong rotation of the breccia fragments observed in the field particularly suggests that eclogite breccias did not solely form by hydraulic fracturing (because this mechanism does not usually lead to strong clast rotation). Pressure-driven dissolution-recrystallization features associated with healed fractures, which are commonly observed in several eclogite breccia garnet crystals (Fig.2C,D,E), are also typical of wear abrasion mechanisms (Jébrak, 1997).

5 On the other hand, the short-lived cementation of clasts (as seen from P-T constraints; see below) and the sharp lawsonite-filled fractures cutting across the mylonitic foliation (Angiboust et al., 2012; Fig.3i) witness the presence of a free fluid phase, possibly infiltrating the eclogites during the wear abrasion process. Although observations hint towards abrasive tectonic brecciation along the paleo-fault zone, we therefore suggest an additional contribution from hydraulic fracturing processes to explain some of the complex textural relationships between fragments and inter-clast material.

Figure DR3: A. Normalized core to rim garnet transects for four samples from the Lower shear zone eclogite breccias showing relative variations in almandine (Fe) and pyrope contents (Mg).

6 Figure DR3: A. Normalized core to rim garnet transects for four samples from the Lower shear zone eclogite breccias showing relative variations in almandine (Fe) and pyrope contents (Mg). Note that garnet radius may strongly differ from one to another (garnet from inter-clast is much larger than garnet from the mylonitic matrix). Garnet from the mylonitic clast (sample 05) exhibits a classical zoning showing a marked increase in Mg content rimwards (Angiboust et al., 2012; Fig.3g; Angiboust et al., 2011; Fig.5e,f). Occasionally, a relictual Fe-rich nucleus from the mylonitic fabric is preserved in the core of the fracture-filling garnets (sample 23). Note that a sharp compositional contrast exists between the (relict) garnet core and the overgrowth (which grew in equilibrium with the breccia matrix). By contrast, the two garnet crystals (samples 18-2 and 06b) that grew within the fracture-filling material (and are devoid of any nucleus) exhibit strikingly flat compositional profiles. B. Relative location of the four analysed garnet crystals with respect to the eclogite breccia texture. C. P-T paths for these four samples based on thermobarometric results (grey ellipses from Angiboust et al., 2012) showing that the three successive stages (a: mylonitization, b: brecciation, c: boudinage) occurred in a very narrow P-T range (see below for further justification).

7 Constraining the P-T conditions of eclogite breccia formation (Figs. 1D, DR3) Brecciation In a previous study, Angiboust et al. (2012) demonstrated that P-T conditions for peak metamorphism were relatively similar (between 540 and 560 C and kbar) for both the mylonitic breccia clasts (Vi17, Vi05) and for a foliated inter-clast material sample (Vi21). This suggests that the brecciation event, bracketed by these two stages, occurred close to peak conditions (within the errorbar of current thermobarometric methods: i.e. +/- 25 C and +/-2 kbar). Garnet fracturing Determining the P-T conditions of garnet fracturing (as shown in Fig.2C,D) is important to understand the timing of deformation within eclogite breccias. It is unfortunately impossible in our samples because there is no unambiguous textural relationship between the tiny healed garnet fracture and the adjacent (but not, strictly speaking, in contact) omphacite, phengite and lawsonite (i.e. the phases required to get a P-T point with the program THERMOCALC). We noted, however, that the fractures healed by a Mg-enriched garnet composition are always connected with the garnet rim (Fig.2C; Angiboust et al., 2011; their Fig.5d). Given that we systematically used a garnet rim composition to obtain our peak metamorphism estimates (Angiboust et al., 2012), it is therefore proposed that these fractures formed contemporaneously with the growth of garnet rims, i.e. close to peak metamorphic conditions. Post-brecciation deformation and boudinage Some samples (mainly observed on the block rims) exhibit a marked post-brecciation deformation where mylonitic clasts are embedded within a foliation consisting of omphacite, garnet, rutile, lawsonite +/- glaucophane +/- chlorite +/- talc (Angiboust et al., 2011). First, we can state that this stage of deformation (stage 3) occurred within the lawsonite eclogite stability field (Fig.1D; i.e. between 18 and 26 kbar at 550 C; Fig.DR3-C). Secondly, P-T estimates on sample Vi21 (slightly boudinaged breccia; Stage 2) provided similar P-T conditions to those obtained for the mylonitic clasts within the breccia (Stage 1; Angiboust et al., 2012). We therefore conclude that these three successive stages occurred in a relatively narrow P-T range in the lawsonite eclogite stability field close to peak conditions (incidentally advocating for complex, short-lived switches between brittle and ductile deformation patterns).

Subduction interface processes. recorded by eclogite-facies shear zones (Monviso, W. Alps)

Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps) Angiboust, S., Agard, P. ISTEP, Université Paris 06-UPMC, UMR CNRS 7193, 75252 Cedex 05 PARIS, France Raimbourg,