Please use this identifier to cite or link to this item: http://hdl.handle.net/2122/4719
AuthorsUeda, T.* 
Obata, M.* 
Di Toro, G.* 
Kanagawa, K.* 
Ozawa, K.* 
TitleMantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies.
Issue Date2008
Series/Report no.8/36(2008)
DOI10.1130/G24739A.1
URIhttp://hdl.handle.net/2122/4719
Keywordspseudotachylyte
peridotite
Subject Classification04. Solid Earth::04.01. Earth Interior::04.01.02. Geological and geophysical evidences of deep processes 
AbstractWe report a new type of ultramafi c 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 fi ne (<2 μm) 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 matrices of ultramylonite and pseudotachylyte may indicate that fl uid 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 ~800 °C and 0.7–1.1 GPa.
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