High Stress Deformation and Short‐Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway).

Earthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure. Plain Language Summary: Earthquake initiation within strong, dry rock types in the Earth's lower continental crust requires high driving stresses if fluids are absent. Several mechanisms have been proposed to generate these unusually high stresses deep in the lower crust, many of which imply very short‐term stress increases. High short‐term stresses are proposed by field‐based studies and numerical modeling, but the geological record for these stress increases occurring in the build‐up to earthquakes is not always obvious because the effects of the subsequent earthquake normally will overprint the evidence. However, in some cases a complete record of stress change before, during and after an earthquake may survive. In this study, detailed observations of the deformation of pyroxene (a silicate mineral) characterizes the short‐term changes in stress and temperature experienced before and during earthquakes recorded in the lower crust. Pyroxene crystals close to faults exhibiting ancient earthquake‐generated frictional melts (pseudotachylytes) are investigated from an exhumed fault zone in Lofoten, northern Norway. The microstructures imply high stress increases prior to the earthquake, followed by widespread fragmentation and grain growth linked to the passage of earthquake slip. These results support very high stresses localized within the lower crust and show that crystal deformation can capture such changes. Key Points: Microstructures capable of recording transient stress changes prior to and during earthquake rupture in the lower crustLocalized differential stresses exceeding 1 GPa during prerupture loading accommodated in dislocation glide of pyroxenesCoseismic deformation represented by pulverization style fragmentation and thermally activated grain growth within orthopyroxene [ABSTRACT FROM AUTHOR]