Architecture and permeability structure of the Sibillini Mts. Thrust and influence upon recent, extension-related seismicity in the central Apennines (Italy) through fault-valve behavior

Abstract

The central Apennines are a fold-thrust belt currently affected by post-orogenic extensional seismicity. To constrain the influence that the inherited thrust-related structures exert on the present seismic behavior of the belt, we provide the high-resolution structural and hydraulic characterization of one of the most external exposed thrust fault systems of the central Apennines, the Sibillini Mts. Thrust Front (STF). We integrate structural mapping, multiscale structural analysis, and in situ air permeability on the brittle structural facies of the thrust zone. We also performed K-Ar dating of selected fault rocks to better constrain structural inheritance. The STF is defined by a complex, ∼300-m-thick deformation zone involving Meso-Cenozoic marl and limestone that results from the accommodation of both seismic and aseismic slip during shortening. Permeability measurements indicate that the low permeability (10−2 ÷ 10−3 D) of the marly rich host rock diminishes within the thrust zone, where the principal slip surfaces and associated S-C structures represent efficient hydraulic barriers (permeability down to ∼3 × 10−10 D) to sub-vertical fluid flow. Field data and K-Ar dating indicate that the STF began its evolution ca. 7 Ma (early Messinian). We suggest that the studied thrust zone may represent a barrier for the upward migration of deep fluids at the hypocentral depth of present-day extensional earthquakes. We also speculate on the influence that similar deformation zones may have at depth on the overall regional seismotectonic pattern by causing transient fluid overpressures and, possibly, triggering cyclic extensional earthquakes on normal faults prone to slip while crosscutting the earlier thrust zones (as per a classic fault valve behavior). This mechanism may have controlled the origin of the 2016−2017 central Apennines devastating earthquakes.