A new model for brittle failure at depth involving high-pressure metamorphism

International audienceIntermediate-depth earthquakes are registered in convergence zones where crustal rocks are expected to deform by ductile flow. This paradox is also evidenced in exhumed crustal rocks where brittle structures (e.g., pseudotachylytes and breccias) associated to high-pressure metamorphism have been documented. If the link between brittle deformation and metamorphic reactions appears obvious today, the mechanism involved is still a burning issue. We propose that the initial heterogeneity of rocks, by itself, is sufficient to trigger both metamorphic reaction and brittle deformation. Based on a mechanically consistent dynamic model, we show that local pressure variations due to pre-existing heterogeneities can be high enough to reach the thermodynamic conditions required for reaction initiation. Brittle behaviour is then controlled by the strength difference between the untransformed host rock and its reaction product. This continuous process also explains the higher pressures recorded in eclogite facies rocks of ductile shear zones compared to their brittle host rock. Our results, constraint by natural data, have therefore significant implications for intermediate-depth seismicity

Transient weakening during the granulite to eclogitetransformation within hydrous shear zones (Holsnøy, Norway

International audienceIn Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partiallyeclogitised within hydrous shear zones, that have been interpreted as widening over time withfluid influx and strain. We here present a detailed petrological description of metre-scale shearzones from this area. The granulite protolith (originally plagioclase + garnet + two pyroxenes) istransformed into an albite + zoisite + garnet + clinopyroxene assemblage within a few tens ofcentimetres of the shear zones. The outer edge of the shear zones consists in a fine-grainedheterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz.An eclogite composed of coarser omphacite + kyanite + garnet + zoisite + phengite quartz formsthe core of the shear zones. As the shear zones widened over time, this lateral evolution from theedge to the core of the shear zones reflects the temporal evolution of the granulite from thebeginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite +garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite faciesassemblage. This transient assemblage appears to be mechanically weaker than both the startinggranulite and the final eclogite, based on field and petrological findings. We investigate the impactof transient weakening during syn-tectonic metamorphism using a one-dimensional numericalmodel of a fluid-fluxed, reacting shear zone. Our numerical model shows that transient weakeningis required to explain the field and petrological data. Furthermore, we show that, while fluidinfiltration was predominantly responsible for the widening of the shear zones, strain hardeningduring the end of the eclogitisation reactions sequence had a noticeable widening effect on theshear zones

Transient weakening during the granulite to eclogitetransformation within hydrous shear zones (Holsnøy, Norway

International audienceIn Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partiallyeclogitised within hydrous shear zones, that have been interpreted as widening over time withfluid influx and strain. We here present a detailed petrological description of metre-scale shearzones from this area. The granulite protolith (originally plagioclase + garnet + two pyroxenes) istransformed into an albite + zoisite + garnet + clinopyroxene assemblage within a few tens ofcentimetres of the shear zones. The outer edge of the shear zones consists in a fine-grainedheterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz.An eclogite composed of coarser omphacite + kyanite + garnet + zoisite + phengite quartz formsthe core of the shear zones. As the shear zones widened over time, this lateral evolution from theedge to the core of the shear zones reflects the temporal evolution of the granulite from thebeginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite +garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite faciesassemblage. This transient assemblage appears to be mechanically weaker than both the startinggranulite and the final eclogite, based on field and petrological findings. We investigate the impactof transient weakening during syn-tectonic metamorphism using a one-dimensional numericalmodel of a fluid-fluxed, reacting shear zone. Our numerical model shows that transient weakeningis required to explain the field and petrological data. Furthermore, we show that, while fluidinfiltration was predominantly responsible for the widening of the shear zones, strain hardeningduring the end of the eclogitisation reactions sequence had a noticeable widening effect on theshear zones

Brittle failure at high-pressure conditions: the key role of reactioninducedvolume changes

International audienceMetamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, duringsuch transformations, the nucleation of new phases with different strength, grain size and/ordensity compared to the primary phases is enhanced, and transient processes due to the ongoingreaction are then activated.Eclogitization of lower crustal rocks during continental subduction constitutes an emblematictransformation illustrating these processes. In such tectonic context, it has been shown thateclogitization seems to be closely associated with the occurrence of seismogenic events. However,the mechanisms that trigger brittle failure in such high pressure environments remain highlydebated. Indeed, whether the change in density or the change in rheology can lead toembrittlement is still enigmatic.By using 2D compressible mechanical numerical models we studied the impact of the strongnegative volume change of the eclogitization reaction on the rocks rheological behaviour. We showthat eclogitization-induced density change occurring out of equilibrium can, by itself, generatessufficient shear stress to fail the rocks at high-pressure conditions.Rupture initiation at depth in continental subduction zones could therefore be explained byvolume changes, even without considering the modifications of the rheological properties inducedby the transformation. Our results also indicate that the negative volume change associated withbrittle failure can enhance the propagation of the eclogitization process by a runaway mechanismas long as the reaction is not limited by the lack of reactants.Powered b

Brittle failure at high-pressure conditions: the key role of reactioninducedvolume changes

International audienceMetamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, duringsuch transformations, the nucleation of new phases with different strength, grain size and/ordensity compared to the primary phases is enhanced, and transient processes due to the ongoingreaction are then activated.Eclogitization of lower crustal rocks during continental subduction constitutes an emblematictransformation illustrating these processes. In such tectonic context, it has been shown thateclogitization seems to be closely associated with the occurrence of seismogenic events. However,the mechanisms that trigger brittle failure in such high pressure environments remain highlydebated. Indeed, whether the change in density or the change in rheology can lead toembrittlement is still enigmatic.By using 2D compressible mechanical numerical models we studied the impact of the strongnegative volume change of the eclogitization reaction on the rocks rheological behaviour. We showthat eclogitization-induced density change occurring out of equilibrium can, by itself, generatessufficient shear stress to fail the rocks at high-pressure conditions.Rupture initiation at depth in continental subduction zones could therefore be explained byvolume changes, even without considering the modifications of the rheological properties inducedby the transformation. Our results also indicate that the negative volume change associated withbrittle failure can enhance the propagation of the eclogitization process by a runaway mechanismas long as the reaction is not limited by the lack of reactants.Powered b