Constraining the onset of flexural subsidence and the rate of forebulge-foredeep migration in the foreland basin system of the central-southern Apennine belt (Italy) by Sr-isotope stratigraphy

In fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep-water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The Apennines are a retreating collisional belt where the foreland basin system, in large domains, is floored by a subaerial forebulge unconformity developed due to bulge uplift and erosion. This unconformity is overlain by a diachronous sequence of three lithostratigraphic units made of: (i) shallow-water carbonates, (ii) hemipelagic marls and shales, and (iii) siliciclastic turbidites. Typically, the latter have been interpreted regionally as the onset of syn-orogenic deposition in the foredeep depozone, while little attention has been given to the underlying units. Accordingly, the rate of migration of the southern Apennine foreland basin-belt system has been constrained, so far, exclusively considering the age of the turbidites, which largely postdate the onset of sedimentation in the foredeep depozone. This thesis provides new high-resolution ages obtained by strontium isotope stratigraphy applied to the low-Mg calcite of bivalve shells sampled at the base of the first syn-orogenic deposits overlying the Eocene-Cretaceous pre-orogenic substratum. This new regional dataset of high-resolution ages obtained from the detailed analysis of 203 samples collected from 15 sites (37 sub-sites) across the central-southern Apennines, integrated with previously published data, provide a comprehensive spatial-temporal evolutive model of the Apennine belt and foreland basin system from the early Miocene to the Recent. In particular, this dataset indicates progressive rejuvenation of the strata sealing the forebulge unconformity toward the outer portions of the belt. Plotting the data on a restored section of the pre-orogenic Adria passive margin reveals that the age of the forebulge unconformity linearly scales with the position of the analyzed sites in their pre-orogenic position, pointing to an overall constant velocity of migration of the forebulge wave in the last 25 Myr. A comparative analysis of previously used datasets reveals that dating the base of the post-bulging carbonates represents the best tool to constrain the style and rate of the foreland flexuring