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

摘要

To 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 similar to 720 degrees C at a pressure of similar to 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 similar to 10 Ma and is consistent with a predicted temperature and depth using published flow laws of olivine.