Reconstructing the localized transient high stress state of seismogenic faults in the lower crust, Lofoten, Norway

Pseudotachylytes (frictional melts produced during seismic slip) in the metamorphosed anorthosites of the Lofoten archipelago preserve a record of seismic rupture in the dry lower crust at 650–750 °C, 0.8 GPa. Pyroxene deformation microstructures associated with preseismic loading and coseismic fragmentation reveal strongly localized transient stresses that presumably reached GPa-level magnitude. However, such transient high stresses have never been measured in exhumed seismogenic faults. In this work, we use high-angular resolution electron backscatter diffraction (HR-EBSD) on pyroxene grains to obtain spatial datasets of residual stresses retained in the crystal lattice. With these data, we aim to reconstruct the progressive build-up and release of transient high stress as it is recorded in the pyroxenes, and whether this recorded stress is related to preseismic loading or coseismic fragmentation.

The analysed anorthosite wall rock of a pseudotachylyte at the Nusfjord locality consists mostly of plagioclase, diopside, and enstatite, with diopside forming the main target minerals of this study. HR-EBSD maps were obtained in the wall rock at various distances from the pseudotachylyte interface along a 10 mm long transect, and on survivor clasts within the pseudotachylyte.

EBSD reveals that most diopside in the wall rock contains micron-scale deformation twins, except within 50 microns of the pseudotachylyte where it is fragmented. Residual stresses obtained via HR-EBSD vary along the transect, and are generally lower at greater distance from the pseudotachylyte. The highest values approximately coincide with the lithostatic pressure. The residual stresses are not in agreement with the very high transient stresses (>1 GPa) expected during the rupture propagation. Rather, the analysed diopside recorded the progressive build-up of stress during preseismic loading.