Transformational Faulting: The Case of Quartz‐Coesite

Abstract Transformational faulting in olivine has been widely invoked as a possible triggering mechanism for deep‐focus earthquakes, yet its dynamics remain debated. It is unclear whether it requires the combined effects of heat release, volume change and mechanical strength contrast, or whether it can be triggered by any one of these alone. Here, we tested if transformational faulting can be caused solely by a large volume change, through deformation experiments on novaculite during the quartz‐coesite transition. Under sluggish reaction kinetics, acoustic emissions and microstructural observations indicate strain localization and fault plane formation. Under fast kinetics, deformation was distributed and associated with fewer, lower‐magnitude events. b ‐values of ∼1 and ∼3 reflect these two regimes. Our results provide the first evidence of transformational faulting during the quartz‐coesite transition, suggesting that volume change is the primary driver of brittle failure at high pressure, broadening the range of mineral transitions generating deep and intermediate‐depth earthquakes.