Frictional properties and healing in sedimentary subduction fault zones: insights from the Sestola-Vidiciatico unit, Northern Apennines.

Subduction zone thrust fault systems accommodate deformation and guide the dynamics of converging tectonic plates. Investigating the frictional, healing, and deformation mechanisms at shallow depths within these zones is important for enhancing models of fault zone mechanics, slip behavior, and material transfer processes, as well as for understanding the nucleation and propagation of megathrust earthquakes. The Gova Fault Zone (GFZ), located within the basal portion of the Sestola-Vidiciatico Unit (SVU) in the Northern Apennines (Italy), offers a unique opportunity to examine the frictional properties, healing dynamics, and deformation processes within an exhumed erosive subduction channel.

The GFZ consists of multiple thrust surfaces that juxtapose highly folded and strained phyllosilicate-rich sediments derived from the upper plate (Marmoreto Marls, Fiumalbo Shales, FIU, and Civago Marls Fms., CIV) onto foredeep turbidites (Gova Sandstone Fm, GOV) of the lower plate.

We sampled the rocks in across the GFZ transect from the footwall GOV sandstones into the SVU shear zone, a sheared Civago marls and Fiumalbo shales.

Fault rocks were reduced to gouge and sheared in a rotary shear apparatus equipped with a hydrothermal vessel (RoSA+HYDROS, University of Padova) under an effective normal stress of 20 MPa and fluid pressure of 6 MPa, at both 25°C, and 200°C. We measured the friction and the time-dependent recovery of frictional strength during simulated interseismic periods (i.e., the healing).

The footwall GOV sandstone gouges are dominated by cataclastic processes, are frictionally strong and their friction increase from μ = 0.49 at 25°C to µ = 0.57 at 200° C. They are also characterized by positive healing and a transition from stable slip to stick-slip at 200° C.

The hangingwall rocks are frictionally weaker: the CIV marls show μ = 0.27 (25°C)-0.31 (200° C) and the FIU shales µ = 0.2 (25°C)-0.25 (200° C), display null/negative frictional healing rates. Both rocks maintained stable sliding, facilitated by phyllosilicate lamellae reorientation and buckling. The CAT, show intermediate friction µ = 0.37 (25°C)-0.42 (200° C) but negative healing rate slip behavior dominated by stable sliding.

The measured in the experiments of the footwall sandstones would, in nature, force strain to migrate into the weaker lithologies of the hangingwall. This “migration” of strain favors (1) the development of thrust surfaces within the hangingwall, such as the FIU on CIV contact, and contributes (2) to erosion, enabling the transfer of material from the upper plate to the lower plate. The positive frictional healing and stick-slip behavior at 200° C in the siliciclastic fault rocks (GOV) suggest a role of the footwall in favoring seismic slip nucleation. Conversely, the weakness and lack of frictional healing suggest a predominantly aseismic or slow slip behavior in the phyllosilicate-rich lithologies. Collectively, we found that frictional and microstructural heterogeneities between subducting sediments and the tectonic mélange at shallow depths may control the erosional vs. accretional character of subduction zones and be responsible for complex slip behavior within the frontal thrusts of subduction zones.