Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone

Abstract Fluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of seismogenic crust. However, stress conditions geological environment fluid-pressure cycling unclear. We address this problem by detailed structural investigation a vein-bearing shear zone Sagelvvatn, northern Norwegian Caledonides. In dominantly zone, synkinematic quartz veins locally crosscut mylonitic fabric high angle rotated folded with same sense as mylonite. Chlorite thermometry indicates that both veining mylonitization occurred ?315–400 °C. The vein-filled fractures interpreted episodically triggered creep in mylonite, where piezometry brittle failure modes consistent low (18–44 MPa) differential stress. Sagelvvatn is stretching elevated pressure drives hydraulic gradient expels fluids from host rocks. low-permeability zones, facilitates build-up pore-fluid until hydrofracture criterion reached tensile open. propose gradients established local cyclic variations during can drive episodic fluid escape result brittle-viscous fault slip