Mantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies

Abstract

We report a new type of ultramafic pseudotachylyte that forms a fault- and injection-vein network hosted in the mantle-derived Balmuccia peridotite (Italy). In the fault vein the pseudotachylyte is now deformed and recrystallized into a spinel-lherzolite facies ultramylonite, made of a fine (< 2 mu m) aggregate of olivine, orthopyroxene, clinopyroxene, and spinel, with small amounts of amphibole and dolomite. Electron backscattered diffraction study of the ultramylonite shows a clear crystallographic preferred orientation (CPO) of olivine. The fault vein pseudotachylyte overprints a spinel-lherzolite facies amphibole-bearing mylonite, indicating that shear localization accompanying chemical reaction had taken place in the peridotite before seismic slip produced frictional melting. The occurrence of amphibole in the host mylonite and that of dolomite as well as amphibole in the matt-ices of ultramylonite and pseudotachylyte may indicate that fluid was present and had evolved in its composition from H2O-rich to CO2-rich during ductile deformation with metamorphic reactions, which may account for the observed rheological transition from ductile to brittle behavior. The spinel-lherzolite facies assemblage in mylonites, P-T estimations from pyroxene geothermometry and carbonate reactions, and the type of olivine CPO in deformed pseudotachylyte indicate that both the preseismic and the postseismic ductile deformations occurred at similar to 800 degrees C and 0.7-1.1 GPa

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

“…A weak LPO and lower dislocation density of olivine and the presence of four-grain junctions in the small-grained area are clear evidence that grain-boundary sliding influenced the deformation of olivine (Hirth and Kohlstedt 1995). A similar change in the deformation mechanism in the mylonite zone has been reported by Drury et al (1991), Jin et al (1998), Ueda et al (2008), Linckens et al (2011), andDrury et al (2011).…”

“…430 Ma) (Bingen et al 2001;Glodny et al 2008). Similar mylonite structures in ultramafic rocks are reported to exist at various localities, such as Balmuccia Peridotite, Italy (Ueda et al 2008), Hilti Massif, Josephine Peridotite, USA (Skemer et al 2010(Skemer et al , 2013, Oman (Linckens et al 2011;Michibayashi and Oohara 2013), and Étang de Lers, France (Drury et al 2011).…”

“…Exact timing of the pseudotachylyte generation is not clear, but during the Caledonian orogenic event, preexisting pseudotachylyte is reactivated. Increased pressure and temperature during the orogenic event and free fluid channeled through the pseudotachylyte vein-enhanced grain growth, and the mylonite may have evolved from a crystallized frictional melt or ultra-comminuted material (Spray 1995) as described by Ueda et al (2008). These newly formed small grains act as a weak layer, accelerating shear localization.…”

Seismic evidence for thermal runaway during intermediate‐depth earthquake rupture

“…Laboratory experiments of frictional properties at seismic slip velocities [Di Toro et al, 2006;Del Gaudio et al, 2009;Di Toro et al, 2011] and field observations of mafic and ultramafic rocks in subduction settings [Obata and Karato, 1995;Ueda et al, 2008] both show evidence of extremely high temperatures in the form of pseudotachylites, which are glassy veins formed by fast melting during seismic slip. The only seismological evidence we are aware of for high-temperature rise and inferred frictional melting is for the deep M w 8.2 earthquake in Bolivia [Kanamori et al, 1998;Bouchon and Ihmle, 1999].…”

Magnetic properties of fault pseudotachylytes in granites

“…These unusual fault rocks, regarded as earthquake fossils [Andersen and Austrheim, 2006;Lin, 2008], hold important clues on the physical processes of seismic slip [e.g., Austrheim and Boundy, 1994;Di Toro et al, 2005;Ueda et al, 2008;Del Gaudio et al, 2009;Janssen et al, 2010].…”