Anticrack-associated faulting at very high pressure in natural olivine

Abstract

SHALLOW earthquakes are produced by brittle shear fracture of rock and/or fictional sliding on pre-existing fault surfaces¹. At very high pressures, however, brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress-relief processes accomodate strain at lower shear stresses than those necessary to activate faulting. Nevertheless, more than 20% of earthquakes with magnitudes greater than five occur at depths greater than 300km, where the pressure exceeds l0 GPa (ref. 2). Possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high ^3–8. One of these mechanisms is associated with anticrack development during the olivine → spinel (α → γ) phase transformation in Mg₂GeO₄ at pressures of 1–2 GPa (refs 4–6); it is particularly attractive because it operates between mineral structures known to occur in the mantle. We show here that this mechanism also can operate in natural silicate olivine, (Mg,Fe)₂SiO₄, during onset of the α → β transformation at the much higher pressures at which deep earthquakes occur. These results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes⁴.