## 2 Geological Setting  
The Western Mediterranean region (Figure 1a) has experienced a complex tectonic history since the Mesozoic-Cenozoic, involving the opening and destruction of the Late Triassic/Jurassic Ligurian Ocean and west Alpine Tethys through curved-shaped subduction zones of various polarities [45; 46]. The Voltri Massif in the Ligurian Alps (Figure 1a) is an HP eclogite-bearing ophiolite domain consisting of a tectonically complex area between the Western Alps and the Northern Apennines. The Western Alps are characterized by a main westward tectonic nappe-stacking [47; 48; 49], and the Northern Apennines, by an east/northeastward vergent fold-and-thrust belt [50; 51]. In terms of the tectonost stratigraphic framework, the Voltri Massif consists of three main tectonic units [52; 53]. These units (Figure 1b), i.e., the Erro-Tobio serpentintized Iberzolite, the Beigua serpentinite, and the Piedmontese Nappe, are derived from (a) oceanic crust and mantle, (b) a continental margin, and (c) flysch units derived from the sedimentary cover of an oceanic basement [53]. The boundary between the Western Alps and Northern Apennines is marked by the Sestri-Voltaggio Zone [54], which lies east of the Voltri Massif (Figure 1b) and separates the ophiolitic domain of the Ligurian Alps from the Northern Apennines. This zone is 5-6 km wide, forming an N-S strip from Sestri Ponente in the Ligurian Sea up to Voltaggio in the north [55] (Figure 1b), and it is characterized by complex kinematics related to the main phases of the Alpine orogenesis [56]. It is also referred to as a Sestri-Voltaggio Fault [57] since it records Oligo-Moocene brittle kinematics [58; 59]. Figure 1: Geographical and geological information concerning the study area: (**a**) Satellite image of the Western-Central Mediterranean (modified from [57] and references therein). Explanation: LA: Ligurian Alps, VM: Voltri Massif, WA: Western Alps. (**b**) Simplified geological map of the Voltri Massif and location of the structural stations (modified after [32; 53; 55; 60]). Explanation: the numbered red squares are the structural stations (SS) with fault-slip data, as presented by [32], SVF: Sestri-Voltaggio Fault. The present structure of the Apennines resulted from the long-lasting interaction between the African plate or its indenter (i.e., Adriatic-Apulia foreland) and the European plate (Corsica-Sardinia foreland). This interaction involved processes like a Triassic to Early Cretaceous rifting and oceanic spreading, transtension and opening of a young oceanic basin of the Tethyan margin (i.e., the Ligurian-Piedmont ocean), as well as a Late Cretaceous-Early Tertiary eastward intraoceanic subduction that changed polarity to westward subduction in the Paleocene (from Alpine to Apennine) at the European-Adria plate margin [61; 62; 63; 64]. Due to the structural position of the Voltri Massif between the two orogens, the Tertiary late-orogenic processes have been described and interpreted differently amongst geoscientists, and the paleostress history remains under debate. This complex scenario is also shown in the evolution of the Tertiary Piedmont Basin (TPB), an Oligo-Miocene wedge-top basin located next to the Voltri Massif and between the two opposite verging orogens (Figure 1). The sedimentation in the TPB occurred in three main tectonic episodes: the exhumation of the Ligurian sector of the Western Alps, the opening of the Liguro-Provencal basin, and the formation of the Apennines thrust belt [57]. In particular, various data show that the brittle-ductile to brittle deformation of both the basement rocks and the TPB deposits was very complex from the Oligocene onwards, since folding and thrusting, as well as normal and strike-slip faulting, were documented, e.g., [58; 65; 66; 67; 68]. Tectonic reconstructions have interpreted the Voltri Massif as an extensional domain, accommodating lithospheric thinning since the Late Eocene-Early Oligocene [69], or as an exhumed terrain driven by means of polyphase compressional structures [70; 71].