## 2 Geological Setting  
The Luco dei Marsi carbonate ridge extends in an NNW-SSE direction and borders the western side of the Fucino Basin (Figure 1), the main Quaternary continental basin of the central Apennine Belt, e.g., [40; 41]. The geological structure corresponds to a wide, symmetric anticline (Figure 2), which originated during the Late Miocene orogenesis by folding a sedimentary multilayer made by Lower to Upper Cretaceous, shallow-water carbonates, and Middle Miocene carbonate ramp deposits. These last ones are scattered in a few outcrops along the eastern edge of the slope, where they unconformably overlie the Upper Cretaceous sequence. Figure 2: Geological map of the Luco dei Marsi area, modified after [PERSON] et al. [39]. Legend: Quaternary deposits: (1) landslides; (2) residual clay deposits; (3) talus slope deposits; (4) elevial-colluvial deposits; (5) alluvial and lacustrine deposits of the Fucino Basin; (6) middle Miocene (mM) ramp deposits (_Calcari a Briozoi and Lithatami Auct._); (7) Upper Cretaceous limestones; (8) middle-Upper Cretaceous limestones including both the BSZ (basal shear zone) and the HJL (highly-jointed level). T1–T5 and F1–F7 indicate the number of strike-slip and normal faults, as reported in the multiple-step, conceptual model. Borehole B1–B3 data were taken from [[https://www.isprambiente.govit](https://www.isprambiente.govit)]([https://www.isprambiente.govit](https://www.isprambiente.govit)) (accessed on 21 January 2021). The anticline setting is characterized by parallel layering with a constant thickness and a cylindrical geometry. This folding style is typical of most of the fault-bend folds developed into the thick carbonate sequences of the central Apennines, which are composed of homogeneous material. The fold is affected by syn- and post-orogenic brittle deformation. The HJL was observed within the lowermost section of the Upper Cretaceous sequence (Cenomanian in age). This level, found right above the BSZ, is characterized by closely-spaced, open fractures mainly perpendicular to bedding planes and parallel to the fold axial surface (Figure 3a). These features can be interpreted as longitudinal joints developed in the fold extrados, near the hinge zone, as a response to tensile strain in thinly-layered beds. NNW-SSE-oriented normal faults dissect the entire anticline, whereas later WNW-ESE trending strike-slip faults--mainly with left-lateral kinematics--are concentrated in the southern sectors. The first system marks the eastern boundary of the carbonate ridge and is composed of several faults (F1-F4 in Figure 2 and F1 in Figure 3b) disposed in a cascade-like structure; the vertical down-throw is limited to a few tens of meters along each fault, but the whole system accommodates a cumulative displacement of more than 100 m.  
A multiple-step model of the slope's morphotectonic evolution was drawn in [PERSON] et al. (2019), which included the main erosive, depositional, and tectonic events since the Early Pleistocene to the historical times (Figure 4). According to the model, apex of normal faulting, linked to the opening of the Fucino Basin, occurred in the Late Pleistocene. Such a tectonic process determined the slope lateral unconfinement necessary to trigger the DSGSD. Nowadays, the fault activity is shifted basin-ward along the Luco dei Marsi structural line (LMF, F0 in Figure 2). Figure 3: Stratigraphic sequence and main tectonic elements: (**a**) Upper Cretaceous, thinly-layered, biodetritic limestones in the southern sector of the ridge affected by severe jointing; (**b**) left hand resting on a normal fault plane along the eastern edge of the Luco dei Marsi slope (F1 in Figure 2). Figure 4: Multiple-step, conceptual model of the Quaternary morphotectonic evolution of the Luco dei Marsi ridge and the Fucino Basin western edge, modified after [PERSON] et al. [39].