Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: The tellaro detachment, Italy

Classical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid-late Miocene low-angle normal fault that was responsible for ~500 m of tectonic vertical thinning in the carbonatedominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ~8 and 4 km depths and peak temperature ~190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes.