## 6 A New Paleostress History and Tectonic Implications
Moreover, [90] suggested such an extension as a post-orogenic one that truncated the Pyrenean thrusting and formed the Liguro-Provencal and Tyrrhenian basins [95], showing a major change in subduction dynamics [51]. T3\({}_{\text{TRM}}\) defines a TRN stress regime that cannot correlate with [32]'s events. On the other hand, it can be correlated with some N-S tensional stresses detected in several parts of the Tertiary Piedmont Basin, especially in its SE portion, though its timing relation with the other stress regimes is unclear [66]. These N-S tensional stresses are compatible with relatively small ENE-WSW to WNW-ESE trending normal faults mapped in the SE parts of the Tertiary Piedmont Basin [65; 96; 97], similar in strike to those activated by the T3\({}_{\text{TRM}}\) stress tensor (Figure 2d). [57] also mentioned a transtentional stress regime in the Late Chattian representing the transition from Early Oligocene rifting-related extension to the Early Miocene rotation-related transpression. In particular, [55] stated that this transtenion marks the time when the Apennines start to 'pull' the southern Western Alps northeastward, producing a large shear zone (i.e., the Ligurian Alps-TPB) between the two chains. In this setting, the Tertiary Piedmont Basin acted as a strongly subsiding piggyback basin above rotating thrust sheets associated with the regional rotation caused by the oceanic spreading of the Liguro-Provencal basin, the Corsica-Sardinia drifting, and the eastward retreat of the northern Apennines slab. T4\({}_{\text{TRM}}\) (ENE-WSW, TRP) and T5\({}_{\text{TRM}}\) (NNE-ESE, TRP) can be correlated with event B of [32], dated in Early Miocene (Aquitanian-early Burdigalian (?)) by them in the Late Oligocene-Early Miocene in the Tertiary Piedmont Basin by [66], and in Aquitanian-Serravallian (\(\sim\)23-12 Ma) by [57]. It is worth noticing that both T4\({}_{\text{TRM}}\) and T5\({}_{\text{TRM}}\) stress tensors reveal transpressive deformation. In addition, T4\({}_{\text{TRM}}\) and T5\({}_{\text{TRM}}\) stress tensors, with the former preceding the latter, fit well with the \(\sim\)40\({}^{\circ}\) counterclockwise rotation of the \(\sigma_{1}\) axes from ENE-WSW to NNE-SSW trends [73]. Similarly, the T4\({}_{\text{TRM}}\) and T5\({}_{\text{TRM}}\) stress tensors agree with the paleomagnetic data results that reveal \(\sim\)50\({}^{\circ}\) of counterclockwise rotation for the Tertiary Piedmont Basin and the underlying Ligurian basement in the Aquitanian-Serravallian times [69]. Field studies and seismic profile interpretations in the area provide strong evidence for transgressional structures and strike-slip flower structure geometries along E-W to ENE-striking faults that truncate and offset several unconformities bounding the Oligocene to lower Miocene sedimentary sequence, which in turn are sealed by Upper Miocene sediments [58; 98]. The five (5) bulk stress tensors, T1 TRM through T5 TRM (Table 3), are well distinguished from each other and can be well correlated with the TRM site stress tensors (Table 2). Most TRM bulk stress tensors define TRP and TRN stress regimes instead of a (pure) strike-slip regime (SS). These stress regimes suggest that the deformation in the study area, which was part of the boundary between the orogenic belts of the Alps and Apennines, was not controlled by any large-scale (pure) strike-slip fault zone like the Sestri-Voltaggio Zone (Figure 1b), and was distributed along numerous and smaller strike-slip faults. Therefore, we consider the area to be a well-faulted relay ramp at a contractional overstep between larger-scale right-lateral strike-slip faults [99]. We have to note that estimated stress tensors of [32] are determined in some SSs from \(<\) 10 fault-slip data, and without any prior restoration or rotation of the recorded fault-slip data, although a ca. 50\({}^{\circ}\) of counterclockwise rotation for the Tertiary Piedmont Basin and the underlying Ligurian basement has been suggested in the Aquitanian-Serravallian times [69].