Structural-, in-situ permeability- and K-Ar geochronological constraints from complex and heterogeneous fault zones: new perspectives on fluid circulation and seismogenesis in the upper crust

Long-lived, mature faults can be architecturally complex. The in-depth unravelling of their complexity, including understanding the dynamic and multiscalar evolution of their mechanical properties, requires detailed multidisciplinary studies. Indeed, an integrated approach is the only viable solution to the deconvolution of the at times very complex internal architecture of brittle fault zones, which represents a phenomenal archive of faulting history and conditions through time and in space. Fault zone architectures are commonly characterized by the spatial juxtaposition of “brittle structural facies” (BSFs), which progressively form and continuously evolve during faulting. The mutual spatial and temporal relationships of BSFs impact directly on the bulk static and dynamic permeability structure of fault zones. The permeability structure, in turn, plays a significant role on the distribution of georesources and on seismogenesis in the brittle upper crust, where both natural and induced seismicity are often associated with fluid migration and overpressure. Detailed models of the complexity of fault zones and of their static and dynamic permeability structure are thus necessary to refine our understanding of fluid pathways and of the mechanisms leading to fluid compartmentalization and possible overpressure in the crust. We present a multidisciplinary workflow to decipher exhumed complex fault zones by integrating detailed structural analyses with geochronological dating of the deformation events recorded by the constituent BSFs and systematic in-situ permeability measurements to connect deformation structures to deformation age and hydraulic properties. The absolute dating of BSFs constrains how fault hydraulic properties change not only through space but also in time (during seismic or orogenic cycles, for example) as the fault architecture (and permeability structure) progressively develop. As a case study, we apply this workflow to the Zuccale Fault in the northern Apennines (Italy), a major low-angle fault that formed and was repeatedly reactivated during a time interval spanning at least the last 22 Myr. A stark spatial heterogeneity of its present-day permeability (up to four orders of magnitude) emerges as a key structural and hydraulic feature, even for tightly juxtaposed BSFs. Results show how complex fault architectures steer the 3D hydraulic structure of the brittle upper crust with direct implications on styles and modes of fluid ingress and flow.