The Ductile‐to‐Brittle Transition Recorded in the Balmuccia Peridotite Body, Italy: Ambient Temperature for the Onset of Seismic Rupture in Mantle Rocks

Abstract

AbstractTo constrain the conditions of the brittle‐ductile transition in peridotite, we studied the deformation sequence of a small‐displacement fault system developed in the Balmuccia peridotite body, northern Italy. The ambient pressure‐temperature conditions of the deformation were estimated using equilibrium mineral assemblages and mineral compositions. Fault‐related deformed rocks, including pseudotachylytes, cataclasites, and mylonites, were classified into three groups according to the metamorphic facies at the time of their formation: Group 1 consists of mylonites, cataclasites, and pseudotachylytes of the spinel‐lherzolite facies; Group 2 consists of cataclasite and pseudotachylyte, but no mylonite, of the plagioclase‐lherzolite facies; and Group 3 consists of cataclasite and pseudotachylyte of a lower‐grade hydrous facies. Cataclasite of the spinel‐lherzolite facies coeval with fault‐vein pseudotachylyte was identified for Group 1 and ambient temperature of the earthquake was determined by applying geothermometers to the mylonite and cataclasite. Among the deformed rocks of the spinel‐lherzolite facies (Group 1), mylonite is truncated by veins of pseudotachylyte. However, the mineral compositions of the mylonite and the cataclasite associated with the pseudotachylyte are essentially identical, yielding a temperature for the ductile‐to‐brittle transition in the peridotite of ~720 °C at a pressure of ~0.6–1.6 GPa. In the pressure range, low‐pressure side, which overlaps with conditions of surrounding crustal rocks, is more consistent with obtained fault structures and regional tectonics. This condition lies on a geotherm of an oceanic lithosphere of the age of ~10 Ma and is consistent with a predicted temperature and depth using published flow laws of olivine.