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Abstract Thermal runaway has been proposed as a potential mechanism to initiate intermediate-depth earthquakes as well as lithospheric-scale shear zones at depths of 50–300 km where brittle failure is inhibited due to the confining pressure. Microstructural analyses of mantle shear zones and pseudotachylites, experimental results as well as numerical models suggest that grain size reduction plays a significant role in this process and facilitates the initiation of thermal runaway and thus also the nucleation of intermediate-depth earthquakes as well as the formation of lithosphere-scale shear zones. Here I investigate the impact of complex composite rheologies on grain size assisted thermal runaway using numerical models. Results indicate that diffusion creep, dislocation accommodated grain boundary sliding as well as low-temperature plasticity do have a significant impact on the deformation of mantle rocks under lithospheric conditions and that peak stresses as well as the timing of grain size assisted thermal runaway. Ductile deformation in the shear zone leads to weakening of the rock due to grain size reduction and shear heating. Due to this weakening, the rheological contrasts between the shear zone and the surrounding rock are increased and the critical stress required to initiate thermal runaway is progressively decreased until the rock fails by self-localizing thermal runaway. However, dislocation accommodated grain boundary sliding reduces the temperatures at which this process is feasible, as it weakens the rock to such an extent that critical runaway stresses are never reached. Application of these numerical models to the 2013 Wind River earthquake (Wyoming) suggests that grain size assisted thermal runaway is a feasible mechanism for the nucleation of this earthquake. Highlights • Investigation of grain size evolution and shear heating in simple shear. • Fully coupled thermomechanical 1D models with complex composite rheologies. • Thermal runaway more likely when grain growth is slow. • Additional creep deformation mechanisms slightly inhibit grain size assisted thermal runaway. • 2013 Wind River earthquake is explained with grain size assisted thermal runaway. [ABSTRACT FROM AUTHOR]

We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction (‘dehydroxylation’) from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1·0 to 0·5 GPa. [ABSTRACT FROM AUTHOR]

Abstract EGU 2019Grain size reduction as a weakening mechanism in 3D numerical models:Is it sufficient to initiate and stabilise transform faults?Transform faults are conservative plate boundaries since they do not produce or consume any crust. Instead, they accommodate motion via strike-slip movement. In nature, these highly localised, ductile shear zones show a great amount of grain size reduction up to ultra-mylonites. Various studies concluded that grain size reduction can lead to a progressive localisation of strain (Jaroslow et al., 1996, Jin et al., 1998, Warren and Hirth, 2006).In numerical models, transform faults are difficult to reproduce, as mantle convection simulations tend to produce poloidal rather than toroidal motion. In order to obtain toroidal motion in numerical models a strain weakening mechanism is necessary. So far, in transform fault modelling ad hoc strain weakening has been applied to reproduce the observed weakening phenomena (e.g. Hieronymus, 2004, Choi et al., 2008, Gerya, 2010a), but without including a physics-based weakening mechanism. However, also numerical studies suggest grain size reduction to be a potential candidate for weakening (e.g. Kameyama et al., 1997). In order to utilise a more realistic weakening we incorporate grain size reduction due to dynamic recrystallisation in the ductile regime. We investigate the effect of grain size reduction occurring at transform faults using a 3D-numerical model with a composite rheology (diffusion and dislocation creep). Viscosity is therefore pressure-, temperature- and grain size-dependent. Instead of imposing ad hoc strain weakening, we let the model evolve self-consistently and allow for a gradually developing strain weakening and strain localisation due to grain size reduction.Our results show that weakening purely by grain size reduction can localise strain strong sufficiently to initiate a transform fault. However the stability and longevity of the transform faults highly depends on the amount of grain damage, the thickness of the brittle crust and to a smaller extent on the velocity of the spreading centres. Our most stable transform faults arise when the crust is thick and when we use a temperature-dependent grain damage parameter. Yet, so far all transform faults are only stable for a maximum of around 1.5 Myr. [ABSTRACT FROM AUTHOR]

• Transient high stress precedes pseudotachylyte in amphibolite-facies ultramylonites. • Quartz deformation evolves from dislocation to diffusion creep before seismic slip. • Thermal runaway is physically possible, but not supported by microstructures. • Pseudotachylytes are explained by stress transfer from a shallow seismogenic source. Coeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550 °C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition. [ABSTRACT FROM AUTHOR]