" The geology of a lower-crustal earthquake source (Lofoten, Norway)" - Prof. Luca Menegon (Njord Centre, Department of Geosciences, University of Oslo, Norway)

Riassunto

Earthquakes are key agents of metamorphic and structural transformations of the lower crust. Lower-crustal earthquakes induce modifications of the fluid flow, mineralogy, density, and rheology of the rocks that have direct effects on the lithospheric response to major geodynamic processes, including continental collision and rifting, and on the stability of deep mountain roots. Although approximately 20% of intracontinental earthquakes of magnitude  5 nucleate in the lower crust at focal depths of 20-40 km, their origins remain controversial. Thus, understanding the nature of lower-crustal earthquakes is a fundamental goal in geoscience.
Several contrasting mechanisms have been proposed as the main triggers of lower-crustal earthquakes. These mechanisms include ductile shear instabilities, fluid overpressure, reaction-induced rock embrittlement, and brittle failure of dry, strong rocks under fluid-absent conditions. Field-based studies have established that earthquake sources exhumed from the dry lower crust commonly display evidence for a cyclic interplay between viscous, aseismic creep (producing mylonites in ductile shear zones) and brittle, seismic slip associated with the formation of pseudotachylytes (quenched frictional melt produced during coseismic slip). The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate during the activity of lower crustal shear zones and faults. This seminar will present results from the study of an exhumed network of lower crustal mylonites and pseudotachylytes from Lofoten, Norway, and discuss how we can use the observations to derive quantitative parameters related to lower-crustal seismicity.