INTEGRATED STUDIES OF THE MECHANICS OF WEAK LOW-ANGLE NORMAL FAULTS

In a stretching crust “standard” frictional fault reactivation predicts that normal faults should cease to slip at dips of 30°-40°. This mechanical prediction is consistent with the absence of moderate-to-large seismic ruptures on continental low-angle normal faults, LANF. In marked contrast, widespread geological data and seismic reflection profiles show that LANF have played a key-role in crustal extension. In the last ten years we have tried to address this LANF paradox by using a multidisciplinary approach that involves seismological data, field and microstructural studies and rock deformation experiments. In particular we have focused our attention to the Northern Apennines of Italy by comparing active with ancient-exhumed LANFs belonging to the same fault system.

In the present day extending area a regional LANF is characterized by a nearly constant rate of earthquake production (about 3 events per day, ML < 2.3), that cannot explain the 1-2 mm/yr of slip rate constrained by geodetic data. Along the fault we also observe the presence of clusters of earthquakes occurring with relatively short time delays and rupturing the same fault patch. Field and microstructural studies along an ancient exhumed LANF show that one of the main deformation mechanisms is fluid-assisted dissolution of the original protolith and precipitation of phyllosilicates (talc and smectite) to form foliation surfaces. These foliated surfaces (low friction and velocity strengthening) surround lenses of non foliated materials (high friction and potentially velocity weakening) with dimensions ranging from some centimeters to several meters. Within this fault zone structure, fault creep and aseismic slip along weak and phyllosilicate-rich foliated surfaces might tectonically load patches of stronger, potentially velocity-weakening material from which a seismic rupture could nucleate. This mechanical-structural model could be a viable explanation for fault weakness and aseismic slip associated with the repeated occurrence of microearthquakes documented along the microseismically active LANF of the active area.