Samuel Angiboust

Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes?

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.

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30–40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near-peak metamorphic conditions (400–500°C, 1.1–1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42–48 Ma). Some of these networks exhibit mutual crosscutting relationships between mylonites, foliated cataclasites, and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40Ar-39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics, we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (∼10−12 s−1) accompanied by fluctuation of supralithostatic and quasi-lithostatic fluid pressures (1 ≥ λ > 0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface.

Ilmenite breakdown and rutile-titanite stability in metagranitoids: Natural observations and experimental results

Abstract Rutile and titanite commonly form by replacement of ilmenite in metamorphic rocks. Exhumed orthogneiss from the Western Alps show that titanite is mostly stable below 1 GPa while rutile seems to dominate within rocks recrystallized under higher pressures. We herein investigate phase relationships for four granitic compositions with variable CaO content at medium to high-pressure conditions (0.7–1.6 GPa, 450–650 °C) with a focus on ilmenite breakdown and Ti-bearing species formation. Our piston-cylinder experiments show that, in the investigated P-T range, ilmenite reacts during metamorphism above 1.2–1.4 GPa to form rutile. Below this pressure, titanite is the dominant Ti-bearing species for most granitoid compositions. We also show that the position of this reaction curve is strongly influenced by the whole-rock Ca activity. For low-Ca activities, rutile may be stable down to 0.7 GPa (and below) within ilmenite pseudomorphs while the titanite stability field may extend to pressures >1.3 GPa for Ca-richer compositions. Both species may be stable in one single sample depending on the local Ca activity gradients. The finding of metamorphic rutile within metagranitoids with CaO contents >2 wt% can be considered, under certain conditions, as a reliable indicator of high-pressure metamorphism. This study also highlights the importance of improving our knowledge of the phase relationships between rutile and titanite as a function of P-T-X to better interpret the textural and tectonic history in natural samples as well as the meaning of age values yielded by rutile and titanite geochronometers.

Fluid pathways and high-P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

The Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed in granulite-facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite-facies remnants of the Tethyan seafloor (Zermatt-Saas zone). Near the base of the Mt. Emilius massif, a shear zone presents eclogite-facies hydrofracture systems associated with deformation-induced re-equilibration of granulites during high pressure metamorphism. We herein focus on a pluri-hectometer-length domain of sheared mafic boudins hosted in the granulitic paragneiss. These mafic boudins exhibit garnetites and garnet veins, clinopyroxenites, clinozoisite and calcite veins. We report multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid-rock interaction, metasomatism and volume change. Our integrated petrological, micro-textural and geochemical investigations illustrate the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite-eclogite-facies field (at ~ 2.15-2.4 GPa, 500-550°C). The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (c. +6.5 per mil) near those for marble layers (and nearby calc-silicates) embedded within the metasomatized granulites (+8 to +10 per mil). It is proposed that infiltration of externally-derived H2O-rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short-distance transport and re-precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate, and creating long-lasting mechanical instabilities during subduction zone deformation.

A 100-m.y.-long window onto mass-flow processes in the Patagonian Mesozoic subduction zone (Diego de Almagro Island, Chile)

Diego de Almagro Island was formed by the subduction and accretion of several seafloor-derived tectonic slices with very heterogeneous ages and pressure-temperature-time (P-T-t) paths. The highest element of the pile (the Lazaro unit) evidences subduction in the high-P granulite field (∼1.3 GPa, 750 °C) at ca. 163 Ma. Below it, a thin tectonic sliver (the Garnet Amphibolite unit) preserves eclogite-facies remnants (∼570 °C and ∼1.7 GPa) formed at ca. 131 Ma (in situ U-Pb zircon rim ages). Peak assemblages were nearly fully amphibolitized during decompression down to ∼1.2 GPa and ∼600 °C at 125–120 Ma (Rb-Sr multimineral dating). The underlying Blueschist unit has ∼50 m.y. younger metamorphic ages and exhibits slightly cooler peak burial conditions (∼520 °C, 1.7 GPa; ca. 80 Ma, in situ white mica Ar-Ar ages and multimineral Rb-Sr dating) and is devoid of amphibolitization. The mylonites from the sinistral strike-slip Seno Arcabuz shear zone bounding Diego de Almagro Island to the east also exhibit amphibolite-facies (∼620 °C and ∼0.9 GPa) deformation at ca. 117 Ma (multimineral Rb-Sr ages). In situ white mica Ar-Ar dating and multimineral Rb-Sr dating of low-T mylonites (∼450 °C) along the base of the Lazaro unit reveal partial resetting of high-T assemblages during tectonic displacement between 115 and 72 Ma and exhumation of the slice stack. Detrital zircon U-Th-Pb ages indicate that the material accreted on Diego de Almagro Island has been mostly recycled from a Permian–Triassic accretionary wedge (Madre de Dios accretionary complex) exposed along the subduction buttress. Geological and geochronological constraints suggest that the rocks of the Seno Arcabuz shear zone and the Lazaro unit were tectonically eroded from the buttress, while the underlying Garnet Amphibolite and Blueschist units instead derive from the subducted oceanic basin, with increasingly younger maximum depositional ages. The very long residence time of the rocks (∼90 m.y. for the Lazaro unit) along the hanging wall of the subduction interface recorded long-term cooling along the Patagonian subduction zone during the Mesozoic. Diego de Almagro Island therefore represents a unique window onto long-term tectonic processes such as subduction interface down-stepping, tectonic erosion, and episodic underplating near the base of an accretionary wedge (40–50 km).