Chapter 4 Fault Zone Structure and Deformation Processes along an Exhumed Low-Angle Normal Fault: Implications for Seismic Behavior

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Significant controversy exists as to whether low-angle normal faults (i.e., normal faults dipping less than 30°) can be seismically active in the brittle upper crust. In this chapter, we present a comprehensive review of our field studies on the exhumed Zuccale low-angle normal fault (central Italy) aimed at characterizing the fault zone structure, and we use geological evidence to make inferences about the possible seismic behavior of the detachment. The key component of the fault zone is a pervasively foliated fault core, crosscut by numerous carbonate mineral veins interpreted to be hydrofractures, which is sandwiched between hanging wall and footwall blocks where the deformation is entirely brittle. To explain this, we suggest that an initial phase of frictional brecciation and cataclasis increased fault zone permeability and facilitated the pervasive grain-scale influx of CO 2 -rich hydrous fluids into the fault zone. The fluids reacted with the fine-grained cataclasites and triggered low-grade alteration and the onset of stress-induced dissolution and precipitation processes (i.e., pressure solution). The resulting switch from frictional to pressure-solution accommodated deformation led to shear localization and the formation of a foliated fault core rich in phyllosilicates. We predict a fault slip-behavior in which aseismic creep occurs by pressure-solution-accommodated “frictional-viscous” slip along the phyllosilicate-rich foliate at low-friction coefficients (0.2 to 0.3), with transient small ruptures induced by local buildups of fluid overpressure trapped below the low-permeability fault core.