Zircon deformation features reveal sequence of transient high stress, tension and shearing during seismic faulting: A case study from the Ivrea-Verbano Zone, Italy

The mechanisms associated with the propagation of fault ruptures remain debated in terms of sequence of events, processes and magnitude of stresses involved. Microstructures of zircon grains located within and in the immediate vicinity of pseudotachylyte veins reveal a sequence of events transient in time and space and allow recognition of different processes during rupture. The dynamic rupture causes, at its propagating tip, a damage zone of several centimetres thickness. In this damage zone, zircon grains exhibit crystal-plastic deformation signatures ranging from crystal lattice bending continuous throughout whole grains, to distinct planar deformation bands and {112} twin lamellae. Presence of planar deformation bands and {112} twin lamellae suggest locally high stresses, based on similar features reported from meteorite impacts. Absence of well-developed subgrains indicate dominance of low temperature plasticity at the rupture tip. Subsequently, those grains with highest dislocation densities undergo in-situ grain fragmentation. The observed correlation of grains with very high dislocation densities and in-situ grain fragmentation suggests that the effective tensile strength of these grains is sufficiently decreased by the high stored elastic energy to cause their fragmentation when subject to tensile stresses in the wake of the propagating rupture tip. Subsequent displacement along connected damage zone fracture surfaces results in pseudotachylytes formation. Our data shows that dynamic rupture initiation and propagation results in stresses heterogeneously distributed in space, magnitude and sign causing both ductile and brittle deformation. Our study highlights the value of the accessory mineral zircon in deciphering the nature of rupture zone dynamics.