Eclogite breccias in a subducted ophiolite: A record of intermediatedepth earthquakes? S. Angiboust1*, P. Agard1, P. Yamato2, and H. Raimbourg3 1

ISTEP, UMR CNRS 7193, UPMC Sorbonne Universités, F-75005 Paris, France Geosciences Rennes, Université Rennes1, F-35 042 Rennes, France 3 ISTO, Université d’Orléans, F-45071 Orléans, France 2

ABSTRACT Understanding processes acting along the subduction interface is crucial to assess lithospheric-scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing the subduction interface, we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones. We herein report the finding of eclogite breccias, formed at ~80 km depth during subduction, in an almost intact 10-km-scale fragment of exhumed oceanic lithosphere (Monviso ophiolite, Western Alps). These eclogite breccias correspond to meter-sized blocks made of 1–10 cm fragments of eclogite mylonite cemented by interclast omphacite, lawsonite, and garnet, and were later embedded in serpentinite in a 30–150-m-wide eclogite facies shear zone. At the mineral scale, omphacite crack-seal veins and garnet zoning patterns also show evidence for polyphased fracturing-healing events. Our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust, accompanied by the input of externally derived fluids. We also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab, intermediate-depth earthquakes at ~80 km depth. INTRODUCTION Most intermediate-depth (70–300 km) earthquakes worldwide concentrate in subduction zones along 2 or 3 distinct seismic layers separated vertically by a 10– 40-km-thick weakly seismic core (Yamasaki and Seno, 2003; Fig. 1A). Accurate relative relocations (Rietbrock and Waldhauser, 2004) suggest that the upper seismic layer may correlate with the top, crustal part of the slab, where massive dehydration of minerals formed by seawater

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GEOLOGICAL SETTING This several-kilometer-long, almost intact section (Fig. 1C) of the Tethyan ocean reached lawsonite-eclogite facies conditions at ~80 km depth between 50 and 40 Ma (Angiboust et al., 2012). The Lago Superiore Unit is crosscut by two major (kilometer scale) eclogite facies shear zones, located at the boundaries between basalts and gabbros and between gabbros and serpentinites (intermediate and lower shear zones; ISZ

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alteration takes place through metamorphic reactions (Green and Houston, 1995; Hacker et al., 2003), whereas the lower layer may correspond to the top, most hydrated part of the slab mantle (Yamasaki and Seno, 2003). Eclogitization of the basaltic crust (i.e., crystallization of garnet and omphacite) is known to dramatically affect the structure, density, mineralogy, fluid content, and stress state of the downgoing slab sinking into the Earth mantle (e.g., Rondenay et al., 2008).

The exact location (absolute depths are known within 3–5 km only; Rietbrock and Waldhauser, 2004) and mechanical process at the origin of this seismicity (e.g., hydraulic fracturing or dehydration embrittlement; Davies, 1999; Hacker et al., 2003) are still debated (Kuge et al., 2010). In addition, few studies have documented the brittle behavior of eclogitized oceanic crust in exhumed ophiolitic belts (e.g., Philippot and van Roermund, 1992; John and Schenk, 2006; Healy et al., 2009). We herein present a fossil example of oceanic crust brecciation under eclogite facies conditions from the Lago Superiore Unit of the Monviso ophiolite (Western Alps; Fig. 1B; Agard et al., 2009), that attests to brittle behavior of oceanic crust at intermediate depths.

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Figure 1. A: Schematic view of subduction zone featuring double seismic zone (after Hacker et al., 2003) and showing slicing of Lago Superiore Unit at ~80 km depth. B: Location of Monviso ophiolite within western Alpine belt. C: Section across LSU showing four main shear zones: upper (USZ), intermediate (ISZ), lower (LSZ), and basal (BSZ). D: Pressure-temperature (P-T) path of LSU and relative timing of shear zone activity (mapping and thermobarometric data from Angiboust et al., 2011, 2012, respectively). Lws—lawsonite; Grt—garnet; Lws-ECL— lawsonite-eclogite facies. *E-mail: [email protected]. GEOLOGY, August 2012; v. 40; no. 8; p. 707–710; doi:10.1130/G32925.1; 4 figures; Data Repository item 2012197.

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and LSZ in Fig. 1C; Angiboust et al., 2011). Mylonitic eclogite facies Fe-Ti metagabbros, which cap the top of the thick Mg-Al metagabbroic body along the ISZ (Fig. 1C), host omphacite-filled crack-seal veins previously interpreted as dehydration reaction products of the downgoing crust (Philippot and van Roermund, 1992). Similar eclogitized Fe-Ti metagabbros are found in the LSZ (Fig. 1C); they were torn off the ISZ by intense subduction-parallel shearing, and later embedded within serpentinite in the LSZ. Fe-Ti metagabbros from both the ISZ and LSZ fossilize deformation events that took place near peak burial of the Lago Superiore Unit, in a narrow pressure-temperature (P-T ) range within the lawsonite-eclogite facies (i.e., 500–550 °C and 22–26 kbar; Fig. 1D). Over this range of P-T conditions, estimated internal fluid release amounts to ~1 wt% in the upper mafic crust (with glaucophane-lawsonite-chlorite proportions shifting from 9, 11, and 5 wt% to 3, 8, and 0 wt%, respectively; Angiboust et al., 2012). ECLOGITIC BRECCIAS AND OTHER EVIDENCE OF BRITTLE FAILURE A specificity of the LSZ is the existence of eclogite blocks (cropping out >10 km along strike) consisting of rounded meter- to decameter-sized pods of highly strained Fe-Ti metagabbros embedded within a strongly foliated serpentinite and talc schist matrix.

Blocks are for the most part spectacularly fresh eclogitic breccias (Figs. 2A and 2B) made of coalesced fragments of mylonitic eclogite with discordant foliation and sharp edges, showing variable disruption (further mesoscopic and microscopic breccia images are shown in Fig. DR1 in the GSA Data Repository1). To our knowledge, this is the first report of such eclogitic breccias. Most fragments within the eclogite breccia are between 1 and 10 cm in length (see clast size distribution in Fig. DR2) and exhibit a marked mylonitic foliation identical to the fabric reported along the ISZ (Lago Superiore area; Philippot and van Roermund, 1992). In the rare outcrops where post-breccia deformation was limited, the breccia clasts are chaotically oriented (Woodcock and Mort, 2008) and the corresponding texture is among the “wear abrasion” and/or “fluid-assisted brecciation” types defined by Jébrak (1997, p. 115; see the Data Repository for further discussion). Note that many of these clasts were disseminated in the block vicinity within the weak serpentinite-rich matrix. The matrix cementing the mylonitic clasts is composed of as much as 30 vol% lawsonite (now pseudomorphed by epidote), omphacite, and garnet (100–1000 µm diameter; Fig. DR1F). Garnet from the interclast matrix is generally weakly zoned, in contrast to garnet derived from the mylonitic clasts, which preserves prograde

zoning, and systematically exhibits a marked increase in Mg content rimward (Fig. DR3). Most blocks also exhibit fractures dominantly filled by omphacite and lawsonite pseudomorphs (Fig. DR1C). These water-rich recrystallized domains (>2–3 wt% H2O, as opposed to Fe-Ti metagabbro mylonites containing <1 wt% H2O after eclogitization; Spandler et al., 2011) point to a significant fluid input during brecciation. No such eclogitic breccias were found in the ISZ. On the contrary, omphacite-filled crackseal veins reported in the ISZ (Philippot and van Roermund, 1992) are also found in the LSZ as folded veins intermingled with eclogite mylonite clasts on the block surface (Fig. DR1C). Fracturing processes are also observable at the mineral scale, where several brittle events are fossilized in an interclast eclogite sample (Figs. 2C and 2D) exhibiting a first garnet generation (garnet I, Fig. 2E) fractured and healed by a Mn-enriched garnet overgrowth. A second fracturing episode is attested to by the presence of a complex fracture pattern cemented by Mgenriched composition (garnet II, Fig. 2E). The final, outer garnet rim is devoid of fractures and fossilizes these two successive fracturing events (Fig. 2E). These fractures healed rapidly before the next deformation increment (as indicated by the preservation of the pre-fracturing garnet geometry), suggesting a post-fracturing stress drop, as for seismic cycle patterns (Sibson,

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F i g u re 2 . M ult is c a le fracturing recorded by eclogite breccias. A: Eclogite breccia from lower shear zone (LSZ). Hammer head is 18 cm. B: Centimeter-sized mylonitic clasts cemented by lawsonite (Lws) eclogite facies paragenesis. Omp—omphacite; Grt— garnet. C, D: Mn and Mg content (mol%) X-ray chemical maps of garnet from LSZ eclogite showing several healed fossilized fracture networks. E: Interpretative sketch of same garnet showing presence of two main garnet generations: first one exhibits Mn-rich oscillatory patterns, and second is characterized by Mg-rich overgrowth sealing fractures. Successive fracturing events are tentatively related to seismic cycle (see text for details).

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1 GSA Data Repository item 2012197, Figure DR1 (additional eclogite breccia pictures), Figure DR2 (clast size distribution histogram), and Figure DR3 (additional petrological and P-T constrains on eclogite breccia formation), is available online at www.geosociety.org/pubs/ft2012.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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1986). The latest garnet overgrowth (in the outermost rim; Figs. 2D and 2E), which shows the same Mg-buffered composition in all LSZ Fe-Ti metagabbros, points to element transfer from externally (serpentinite) derived fluids (Angiboust et al., 2011). INTERPRETATION Structural, chemical (such as chemically buffered interclast garnet composition; Fig. DR3), and thermobarometric results strongly suggest that the three successive deformation increments observed occurred in a narrow P-T range, between 70 and 80 km depth (Figs. 1D and 3; Angiboust et al., 2012; see the Data Repository). Crack-seal vein formation accompanying mylonitization first occurred along the ISZ (Fig. 3A; Philippot and van Roermund, 1992). A later, larger-scale fracturing episode (Fig. 3B) led to the formation of eclogite breccias in the LSZ and to the rotation of the 1–10 cm mylonite clasts (Figs. 2A, 2B, and 3B) in the presence of externally derived fluids. This brecciation may have reactivated the rheologically weaker tensile omphacite-bearing veins (Fig. 3A, inset; Zhang and Green, 2007). After brecciation, ongoing deformation within the eclogite breccia was only limited, as evidenced by the sharp edges of mylonite fragments (Fig. 3B; Fig. DR1) and by the absence of mylonitization of the interclast material. By contrast, eclogite facies ductile deformation continued around the eclogite breccia (Fig. 3C), suggesting that brittle deformation was short lived and rapidly sealed (i.e., frozen in and taken over) by ductile deformation around the blocks. Subsequent shearing and migration along the LSZ led to significant block rotation and minor disaggregation within serpentinite by a ball bearing mechanism, and to an overall rheological weakening of the LSZ. Embedding within serpentinite also prevented the eclogite breccias from pervasive retrogression and allowed for their preservation. These observations lead us to infer important consequences. 1. This short-lived mesoscopic eclogite facies brecciation, as well as microscopic fracturing, was likely seismic and could correspond to a relict damage zone (as shown by fragmented and rotated clasts preserving an inherited structural fabric, and by pulverization as seen from relict garnet cores; see McGrath and Davison, 1995; Doan and Gary, 2009). These breccias probably formed within a few meters of the fault core, at the most, as indicated by the linear density of macrofractures (~100/m, as inferred from averaged field estimates; Mitchell and Faulkner, 2009). The fault core has not been observed in the field (or pseudotachylites, which preferentially form in water-poor protoliths; Andersen and Austrheim, 2006), but it could have recrystallized under eclogite facies conditions or been overprinted by later ductile deformation.

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Figure 3. Eclogite breccia-forming evolution. A: Top of gabbroic layer (Fig. 1C), lined up with Fe-Ti metagabbros, underwent pervasive eclogite facies mylonitization leading to marked grain size reduction and segregation of garnet (Grt) and omphacite (Omp) beds. Note omphacite veins cutting across mylonitic fabric (inset). Gray dotted line represents inferred inherited structural weakness. B: Intermediate-depth intraslab earthquake (star) produces faulting of Fe-Ti metagabbro mylonites, rotation of mylonitic fabrics, and infiltration of external H2O-rich fluids (white arrow). C: Present-day occurrence of preserved lower shear zone (LSZ) eclogite breccia showing serpentinite foliation deflected around block.

2. Fluids played a key role, as shown by the abundance of lawsonite, chemical trends in garnet, and the fact that fractures must have been rapidly healed by efficient, fluid-mediated element transfer. This is further supported by the lack of any such brecciation in the ISZ, where fluid ingression was much more limited (Angiboust et al., 2011; Spandler et al., 2011). 3. Brittle deformation leading to eclogite brecciation is found only in the LSZ and in resistant lithologies (Fe-Ti metagabbros) formerly located within the oceanic crustal section along the ISZ. Brecciation thus necessarily took place at the junction of the LSZ and ISZ (Fig. 4A), which we infer to be the locus of the relict damage zone. We propose that an

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intermediate-depth earthquake fractured a large part of the crust, cutting through the Mg-Al gabbros and the ISZ Fe-Ti gabbros (Fig. 4A); further fluid ingression and subsequent rheological weakening localized deformation along this fault plane, which became the LSZ (and along which eclogite breccias were dispersed by later deformation; Figs. 3C and 4B). IMPLICATIONS Our data provide a refined view, at the meter to kilometer scale, of possibly earthquake-related deformation processes in a subducting slab and the coexistence of viscous and frictional mechanisms at these depths (see also Handy et al., 2007). These processes can be set back precisely

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Figure 4. Tectonic scenario for formation and dissemination of eclogite breccias along lower shear zone (LSZ). A: Faulting (possibly reactivating inherited fault) allowed formation of eclogite facies breccias, infiltration of external fluids, and connection of intermediate shear zone (ISZ) and LSZ. Stars represent intermediate-depth intraslab seismicity, causing seismic events to Mw = 4. B: Ongoing shearing along eclogite facies shear zones and injection of serpentinite along fault breccia account for dissemination of breccia fragments along LSZ. Jump of deformation along basal shear zone permitted preservation of complete upper lithospheric section.

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within the oceanic crust of the slab (Fig. 4A) and correspond to: (1) short-lived, most likely seismic, mesoscopic brecciation of eclogites along the LSZ accompanied by significant fluid input from the hydrated slab mantle beneath (i.e., from antigorite breakdown at ~90–100 km; van Keken et al., 2011), and (2) deformation through shearing in the ISZ before (and possibly after) brecciation with evidence of brittle failure at the mineral scale (garnet fractures and crack-seal veins), but only limited fluid input. The former type can be related to the largest magnitude intermediate-depth earthquakes observed in active subduction zones (i.e., relatively few earthquakes with steep fault plane solutions and Mw ~ 3–4.5; e.g., Cassidy and Waldhauser, 2003), which were interpreted as fault planes cutting across the entire crust. Magnitudes recorded for these largest intermediate-depth earthquakes match displacements required to produce the eclogitic breccia (Mw ~ 3.5–4.5, considering several centimeter-scale displacements along a kilometer-scale fault zone). Although this shear zone has an overall reverse offset at present, we cannot determine whether initial faulting was normal or reverse (Rietbrock and Waldhauser, 2004; Igarashi et al., 2001, respectively), or whether faulting may have reworked inherited structures (Ranero et al., 2003). By contrast, the second type of process (shearing and microcracks along the ISZ, probably too small to be detected seismically) could correspond to minor deformation accompanying the somewhat smaller, yet more frequent, dominantly along-plane microseismicity reported for the upper seismic layer (i.e., Mw < 2.5; Rietbrock and Waldhauser, 2004). We herein document the complex interplay between ductile shearing, brittle failure, and fluid ingression accompanying intermediate-depth intraslab seismicity (Figs. 3 and 4). Most authors envision brittle failure at depth as triggered by in situ fluid release and dehydration embrittlement (Hacker et al., 2003) or by hydrofracturing (Davies, 1999). In principle, crustal dehydration embrittlement could explain the crack-seal veins of the ISZ. It cannot account for eclogitic brecciation, however, as shown by (1) the very low H2O content of the Fe-Ti metagabbros at such depths (<0.5 wt%), which precludes a massive release of in situ fluids, and (2) the clear evidence for externally derived fluids (Spandler et al., 2011). From field evidence, we thus conclude that crustal dehydration embrittlement is insufficient to break the entire crust and suggest, instead, that a large faulting event (and possibly coeval hydrofracturing by fluids released by antigorite breakdown occurring ~10 km deeper) is a more likely mechanism. ACKNOWLEDGMENTS We thank S. Guillot, L. Labrousse, and A. Schubnel for insightful discussions, assistance, and/or com-

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ments on the ideas. We benefited from the thoughtful comments from J.G. Barreiro, C. Balica, and an anonymous reviewer. REFERENCES CITED Agard, P., Yamato, P., Jolivet, L., and Burov, E., 2009, Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms: Earth-Science Reviews, v. 92, p. 53–79, doi:10.1016/j.earscirev.2008.11.002. Andersen, T.B., and Austrheim, H., 2006, Fossil earthquakes recorded by pseudotachylytes in mantle peridotite from the Alpine subduction complex of Corsica: Earth and Planetary Science Letters, v. 242, p. 58–72, doi:10.1016/j.epsl.2005.11.058. Angiboust, S., Agard, P., Raimbourg, H., and Yamato, P., 2011, Subduction interface processes recorded by eclogite facies shear zones: Lithos, v. 127, p. 222–238, doi:10.1016/j.lithos .2011.09.004. Angiboust, S., Langdon, R., Agard, P., Waters, D., and Chopin, C., 2012, Eclogitization of the Monviso ophiolite and implications on subduction dynamics: Journal of Metamorphic Geology, v. 30, p. 37–61, doi:10.1111/j.1525 -1314.2011.00951.x. Cassidy, J.F., and Waldhauser, F., 2003, Evidence for both crustal and mantle earthquakes in the subducting Juan de Fuca plate: Geophysical Research Letters, v. 30, 1095, doi:10.1029 /2002GL015511. Davies, J.H., 1999, The role of hydraulic fractures and intermediate-depth earthquakes in generating subduction-zone magmatism: Nature, v. 398, p. 142–145, doi:10.1038/18202. Doan, M.-L., and Gary, G., 2009, Rock pulverization at high strain rate near the San Andreas fault: Nature Geoscience, v. 2, p. 709–712, doi:10.1038/ngeo640. Green, H.W.I., and Houston, H., 1995, The mechanics of deep earthquakes: Annual Review of Earth and Planetary Sciences, v. 23, p. 169–213, doi:10.1146/annurev.ea.23.050195.001125. Hacker, B.R., Peacock, S.M., Abers, G.A., and Holloway, S.D., 2003, Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?: Journal of Geophysical Research, v. 108, 2030, doi:10.1029/2001JB001129. Handy, M.R., Hirth, G., and Bürgmann, R., 2007, Fault structure and rheology from the frictional-viscous transition downward, in Handy, M.R., et al., eds., Tectonic faults—Agents of change on a dynamic Earth: Dahlem Workshop Report 95: Cambridge, Massachusetts, MIT Press, p. 139–181. Healy, D., Reddy, S.M., Timms, N.E., Gray, E.M., and Brovarone, A.V., 2009, Trench-parallel fast axes of seismic anisotropy due to fluid-filled cracks in subducting slabs: Earth and Planetary Science Letters, v. 283, p. 75–86, doi:10.1016/j .epsl.2009.03.037. Igarashi, T., Matsuzawa, T., Umino, N., and Hasegawa, A., 2001, Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple-planed deep seismic zone: Journal of Geophysical Research, v. 106, p. 2177–2191, doi:10.1029/2000JB900386. Jébrak, M., 1997, Hydrothermal breccias in veintype ore deposits: A review of mechanisms, morphology and size distribution: Ore Geology Reviews, v. 12, p. 111–134, doi:10.1016/ S0169-1368(97)00009-7. John, T., and Schenk, V., 2006, Interrelations between intermediate-depth earthquakes and fluid flow

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Manuscript received 17 October 2011 Revised manuscript received 17 February 2012 Manuscript accepted 26 February 2012 Printed in USA

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