Shattered Veins Elucidate Brittle Creep Processes in the Deep Slow Slip and Tremor Region

Deep Slow Slip and Tremors (SSTs) are a combination of transient clusters of tectonic tremors and slow slip associated with extremely elevated fluid pressures. SSTs are thought to reflect a transition from viscous to brittle plate interface rheology and likely exert a first-order control on megathrust seismicity. Nevertheless, the deformation mechanisms governing the source of SSTs remain elusive. We herein document the occurrence of vein networks precipitated and brecciated within the deep SST region under blueschist-facies conditions. These lawsonite-rich vein sets exhibit extensive evidence of brittle deformation and are spatially related to localized, finely milled (cataclastic) shear bands. Petro-geochemical data reveal that brittle deformation was accompanied by the injection of several ultramafic-, mafic- and metasedimentary-derived fluid pulses, imprinting characteristic Cr, high field strength elements, and light over heavy rare earth elements positive anomalies in the vein breccias while leaching light rare earth elements from the cataclastic blueschist host. Our results suggest that metamorphic veins represent zones of mechanical anisotropy within the rock volume prone to localized shearing, brittle deformation and episodic injection of externally derived fluids. These networks demonstrate the importance of former vein sets as structural heterogeneities in triggering fluid-controlled brittle creep events. The combined effects of high pore fluid pressures and rheological heterogeneities in the form of metamorphic veins could trigger the nucleation and propagation of SSTs at the margins of this mechanically anisotropic environment, and thus determine where slip will take place along deep subduction interfaces.INSU/CNRS Grant (Tellus program)IDEX research chairIDEX Universite de Paris ANR-18-IDEX-0001Eidgenossische Technische Hochschule Zuric

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

International audienceThe Monviso ophiolite Lago Superiore Unit constitutes a well-preserved, almost continuous upper fragment of oceanic lithosphere subducted at c. 80 km depth, thereby providing a unique opportunity to study mechanical coupling processes and meter-scale fluid-rock interactions occurring at such depths in present-day subduction zones. It is made of (i) a variably thick (50-500 m) section of eclogitized basaltic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body and of (ii) a c. 1 km thick serpentinite sole. We herein focus on the three major eclogite-facies shear zones found at the top of the unit, at the boundary between basalts and gabbros, and between gabbros and serpentinites, respectively. Strain localization occurred at lithological interfaces, irrespective of material strength. While ductile deformation dominates along the shear zones, local brittle behaviour is demonstrated by the existence of numerous eclogite breccias of Fe-Ti metagabbros and widespread garnet fractures, possibly linked with intermediate-depth eclogite-facies (micro)seismicity. These m- to hm-sized fragments of Fe-Ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro-serpentinite boundary (Lower Shear zone; LSZ). Pervasive and focused fluid flow is attested in the LSZ by significant alteration of bulk rock compositions, weakening of the rocks and widespread crystallization of hydrous parageneses. By contrast, the Intermediate Shear zone (ISZ) shows evidence for more restricted, short-range fluid flow. The activity of both the ISZ and LSZ ceased during early lawsonite eclogite-facies exhumation, when deformation localized deeper within the serpentinite sole, allowing for the detachment (and preservation) of this large ophiolitic fragment

Permeability of subducted oceanic crust revealed by eclogite-facies vugs

International audienceWe herein report the finding of rare eclogite-facies vugs forming millimeter-to centimeter-sized pockets in meta-ophiolites from the western Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements including Cr, Mn and rare earth elements. Thermodynamic modelling reveals that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite and chlorite between 505-525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geological and geochronological constrains indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 Myr. These observations can only be explained by the presence of extremely low permeability values (< 10-22 m²) to keep the fluid confined at the meter scale within vugs on such timescales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into the Earth's mantle, with implications for volatile recycling budgets

Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy)

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to ~80km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U-Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high-pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain δ18O values of 4.5-6‰, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low δ18O values between 0.2 and 3.8‰, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by δ18O values of 3.0 and 3.6‰ in antigorite from surrounding serpentinites. In metasomatized eclogites within the lower shear zone, garnet rim formed at the metamorphic peak shows a shift to higher δ18O up to 6‰. The age of zircons in high-pressure veins and metasomatized eclogites constrains the timing of fluid flow at high pressure at around 45-46Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the lower shear zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals

Blueschist-facies paleo-earthquakes in a serpentinite channel (Zagros suture, Iran) enlighten seismogenesis in Mariana-type subduction margins

Zeynab Gharamohammadi, Ali Kananian, Philippe Agard and Jafar Omrani are acknowledged for logistical assistance. Olga Cazalla, Laura Crespo and Miguel Angel Hidalgo Laguna are acknowledged for their technical support at Centre of Scientific Instrumentation, University of Granada. We are very grateful to Sophie Nowak for further support during XRD analytical sessions. Onno Oncken is warmly thanked for insightful discussions and support regarding mechanical earthquake-related processes. Hugues Raimbourg and Isabelle Martinez are also acknowledged for exciting discussions and suggestions regarding fluid-rock interaction processes. The editorial handling by Alexander Webb is very much appreciated. Whitney Behr and Volker Schenk are warmly acknowledged for detailed and constructive reviews, but also for their stimulating words that helped improving this work. This project has been funded by an Initiative D'EXcellence (IDEX) grant 16C538 and the TelluS Program of CNRS/INSU to S.A. Partial funding was also provided by the University of Granada at CIC.The architecture and pressure-temperature conditions reached by a Cretaceous block-in-matrix serpentinite melange exposed in the Zagros suture resemble those imaged in the active Mariana subduction zone. There, large magnitude-earthquakes (M-w> 9) have never been recorded but smaller events - of poorly-constrained physical origin - in the range M-w similar to 3-6 are widespread. Field and petro-structural constraints led to a first report of blueschist-facies seismic fault-related rocks in the Zagros serpentinite melange, including breccias, foliated cataclasites and ultracataclasites; all observed within a foliated mafic metatuffaceous block embedded in serpentinite schists. Fine-scale petrological characterization of ultrafine-grained, fluidized cataclastic material reveals the presence of newly-formed glaucophane, lawsonite, phengite, albite and pumpellyite, an assemblage inferred (based on thermodynamic modelling) to have crystallized in the lower lawsonite-blueschist facies at similar to 0.6-1.0 GPa and 230-300 degrees C. Extensional veins containing similar mineral assemblages are observed crosscutting the aforementioned rocks but are also identified as comminuted fragments in all fault-related lithologies. Crosscutting relationships among the multiple generations of fluidized ultracataclasites and brecciated blueschists suggest that episodic faulting and hydrofracturing were contemporaneous processes at similar to 20-35 km depth, i.e., at similar conditions as reported for metabasalts expelled by Mariana serpentinite mud volcanoes. Mechanical modelling confirms that the studied fault-related features can only have formed under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promoted recurrent seismic faulting, as monitored in the Mariana seismogenic zone. These fluids are likely associated with externally and deeply-generated fluid pulses that may have reached the seismogenic window, imprinting a Ta-Th-Nb-HREEs-enriched trace element signature. This new faulted blueschist occurrence highlights the physical nature and the mechanical processes operating within fluid-saturated fault zones in the serpentinized subduction channel.Initiative D'EXcellence (IDEX) 16C538TelluS Program of CNRS/INSUUniversity of Granada at CI

Oxygen isotope record of oceanic and high-pressure metasomatism: a P-T-time-fluid path for the Monviso eclogites (Italy)

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to ~80 km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U-Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain δ18O values of 4.5–6 ‰, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low δ18O values between 0.2 and 3.8 ‰, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by δ18O values of 3.0 and 3.6 ‰ in antigorite from surrounding serpentinites. In metasomatised eclogites within the Lower Shear Zone, garnet rim formed at the metamorphic peak shows a shift to higher δ18O up to 6‰. The age of zircons in high-pressure veins and metasomatised eclogites constrains the timing of fluid flow at high pressure at around 45–46 Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the Lower Shear Zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals

Field report: Sailing around the exhumed roots of the Mesozoic Patagonian paleo-accretionary wedge (Diego de Almagro Island, Chile)

We undertook a boat expedition to explore the geological framework of a very remote, lesser-known island, in the Chilean Patagonia: the Diego de Almagro Island (latitude S51°33′). This uninhabited, ca. 400 km2 Island is one of the very rare exposures of the Mesozoic accretionary subduction complex along the Chilean margin. Unstable weather, strong winds, steep topography, and very dense vegetation make an on-land mission difficult. Careful preparation based on high-resolution satellite images is advised to optimize shore access and minimize risks of injury. Despite a relatively important degree of regional re-equilibration of metamorphic assemblages due to sluggish exhumation through the forearc crust, our results have shown that the island is composed of a nappe stack of ocean-floor derived slivers of meta-sedimentary units that exhibit very different pressure-temperature-time paths during burial by subduction under the Chilean margin and subsequent exhumation. These rocks are witness to a complex thermal evolution of the subduction zone between Jurassic and Cretaceous times from granulite facies to blueschist facies conditions as well as multiple episodes of accretion at ca. 35–40 km in depth for almost 100 Ma over the Mesozoic era. Keywords: Patagonia, Accretionary wedge, Blueschists, Subduction, Chil

Brittle/ductile deformation of eclogites: insights from numerical models

International audienceHow rocks deform at depth during lithospheric convergence and what are the magnitudes of stresses they experience during burial/exhumation processes constitute fundamental questions for refining our vision of short‐term (i.e. seismicity) and long‐term tectonic processes in the Earth's lithosphere. Field evidence showing the coexistence of both brittle and ductile deformation at high pressure – low temperature (HP‐LT) conditions particularly fuels this questioning. We here present 2D numerical models of eclogitic rock deformation by simple shear performed at cm‐scale. To approximate the eclogite paragenesis we considered the deformed medium as composed of two mineral phases: omphacite and garnet. We run a series of models at 2.0 GPa and 550°C for different background strain rates (from 10‐14 s‐1 to 10‐8 s‐1) and for different garnet proportions (from 0% to 55%). Results show that whole rock fracturing can occur under HP‐LT conditions for strain rates larger than ~10‐10 s‐1. This suggests that observation of brittle features in eclogites does not necessarily mean that they underwent extreme strain rate. Care should therefore be taken when linking failure of eclogitic rocks to seismic deformation. We also explore the ranges of parameters where garnet and omphacite are deforming with a different deformation style (i.e. frictional vs. viscous) and discuss our results in the light of naturally deformed eclogitic samples. This study illustrates that effective stresses sustained by rocks can be high at these P‐T conditions. They reach up to ~1 GPa for an entirely fractured eclogite and up to ~500 MPa for rocks that contain fractured garnet

Stress-driven fluid flow controls long-term megathrust strength and deep accretionary dynamics

The heterogeneity of frictional strength along the megathrust earthquake zone critically controls plate coupling and long-term subduction dynamics. However, the persistence and distribution of high-friction segments through space and time remain poorly constrained. Here, we show that accretion processes, such as tectonic underplating (i.e., basal accretion of material below the fore-arc region), can be used as a proxy to characterize the long-term frictional zonation of the subduction interface. We carry out numerical thermo-mechanical experiments, which predict a first-order control of tectonic-stress variations on fluid transport in deep fore-arc regions. Accordingly, positive feedback between fluid distribution and effective stress favours the stability of the interface frictional properties at Myr-scale which, in turn, controls the deep accretionary dynamics. We propose that the recognition of thick duplex structures resulting from successive underplating events over tens of Myr, allows for tracking subduction segments exhibiting an increasing frictional behaviour. Our numerical results help ascertain the long-term hydro-mechanical properties and distribution of coupling/decoupling segments of megathrust earthquake zones worldwide where active tectonic underplating is recognized.ISSN:2045-232