Tectonic switching in ductile shear zones can explain hydrothermal ore deposits

Switching between extension and shortening on tectonic plate boundaries has been linked to the formation of hydrothermal ore deposits but a mechanism to explain this relationship remains elusive. Many ore deposits that form during tectonic switches occur within shear zones. Prior to tectonic switches, shear zone structures commonly develop a configuration optimised to accommodate deformation, which creates permeable pathways for ore fluid migration. Here we present numerical models showing that shear zone structures that formed before the tectonic switch are geometrically modified to accommodate the new shearing direction during switching. This reduces strain rate and mean stress in the shear zone, causing fluid influx. To investigate this process in a naturally deformed shear zone, we examined rocks from the Bergen Arc shear zone in Norway, where sinistral shearing is progressively overprinted by dextral shearing. We find that during tectonic switching veins formed in dilatational spaces that opened as the sinistral structures were reconfigured to accommodate dextral shearing, comprising up to 10% of the rock volume. These data indicate that microstructural reconfiguration during tectonic switching drives fluid influx to shear zones, potentially introducing ore fluids and forming hydrothermal ore deposits.