Slowly Deforming Megathrusts within the Continental Lithosphere: A Case from Italy

The Late Pliocene–Quaternary Outer Thrust System of the Apennine-Maghrebides fold-and-thrust belt extends ~2000 km from northern Italy to Sicily. Its northernmost arc is seismically active and represents a test case to study geometries and seismogenesis of slowly deforming megathrusts developed within the continental lithosphere. Two distinct SW-dipping reverse shear zones (T1 and T2) in the Outer Thrust System of eastern Central Italy have been recently unveiled thanks to geological and integrated seismological information. These shear zones penetrate the Adria continental lithosphere to a maximum depth of ~60 km, with an outward convex shape associated with an outwarddiverging radial pattern. This paper presents new constraints on the megathrusts’ geometry in light of a novel microseismic catalog (from 2009–2022) specifically focused on the compressional volume. Further details in the reconstruction of T1 and T2 are derived from a recent compressional seismic sequence (November 2022, MW 5.5) located in the Adriatic offshore. It activated the outermost T1 upper crustal segment with pure compressional kinematics and illuminated T2 at lower crustal depths. We integrate geological sections, seismic lines, serial hypocentral cross sections, and focal mechanisms to build a detailed nonplanar 3-D model of the thrusts involved. In addition, we build Coulomb stress scenarios for analyzing the possibility of the static interplay between the upper crust T1 segment activated by the 2022 sequence and the underlying T2 crust segment. The overall results may be relevant for assessing seismic hazards in areas with multi-depth active structures and for gaining insights into plate tectonic dynamics.