Abstract The northernmost coastal sector of the Italian peninsula from the La Spezia Gulf to Monte Pisano, including the Alpi Apuane mountain range, represents a special morpho-structural domain of the inner northwestern Apennines. Described by naturalists since the Roman age, its location on the land and sea track of the Italian Grand Tour makes it a special zone in the Italian peninsula that was visited by some of the most eminent European geologists of the nineteenth century, including Brongniart, Buckland, De la Beche, Hoffman, Escher von der Linth, Murchison and Lyell. The area has been also the homeland of naturalists and scientists who played a significant role during the nineteenth century in the advancement of geological studies in Italy. Thanks to Capellini's ‘Geological map of the La Spezia Gulf and surroundings’ (1863) and Zaccagna's ‘Geological maps of the Alpi Apuane’ (1879–97), the area became central in the history of the foundations of modern Italian geological maps. However, the opportunities provided by early mapping and palaeontological discoveries for developing tectonic concepts were squandered by Italian Apennines geologists, who remained stuck on explanations of autochthonism, thus missing an early recognition of nappe tectonics that was only accepted in the middle of the twentieth century.

ABSTRACT The structural style at depth of the Umbria-Marche fold-and-thrust belt, which occupies the outer province of the Northern Apennines of peninsular Italy, has long been debated and interpreted in terms of thin-skinned or thick-skinned deformation models, respectively. Thin-skinned models predict that the Mesozoic–Tertiary sedimentary cover was detached along Upper Triassic evaporites and translated northeastward along stepped thrust faults above a relatively undeformed basement. On the other hand, thick-skinned models predict the direct involvement of conspicuous basement slices within thrust-related folds. A description of selected examples in the southeastern part of the Umbria-Marche belt reveals that some compressional structures are indeed thin-skinned, their style being controlled by rheological properties of a mechanically heterogeneous stratigraphy containing multiple décollements, whereas other structures are genuinely thick-skinned, their style being dominated by the reverse-reactivation of pre-orogenic normal faults deeply rooted within the basement. Therefore, the contrast of thin- versus thick-skinned structural styles, an issue that has generated a long-lasting debate, is only apparent, since both styles are documented to coexist and to have concurred in controlling the final compressional geometry of the fold-and-thrust belt.

ABSTRACT The Messinian Salinity Crisis (5.85–5.35 Ma) represents a nearly unprecedented unloading and loading event. During the Messinian Salinity Crisis, two important things happened in terms of surface load changes—the accumulation of thick evaporites represent a load addition, while the desiccation of the Mediterranean represents a load subtraction. The desiccation and evaporite deposition were followed by the rapid addition of water, refilling the Mediterranean during the Zanclean. The calculated flexural response to load changes imposed significant changes on the horizontal stresses in the upper crust of the Northern Apennines, an active orogenic system characterized by simultaneous zones of extension and compression that migrated eastward over time. We show that these flexural stresses (approaching ± 50 MPa), added to the preexisting stresses across the Northern Apennines, were large enough to have caused some areas in compressional stress regimes to flip to extensional regimes, and vice versa. Among other things, our model predicts that the Marches Apennines region, which was beginning to undergo compression at the leading edge of the orogen, should have experienced a brief interval of extensional deformation. Previous structural studies of this region have shown that there was, in fact, a brief period of extensional faulting near the onset of the Messinian Salinity Crisis, which was then followed by a return to compressional deformation, just as our model predicts. The hypothesis presented here provides a novel explanation for this extension occurring in an area that should have been undergoing compression, one in which the unique events of the Messinian Salinity Crisis generated significant flexural stresses that changed the deformational regime during the relatively brief time of the Messinian Salinity Crisis. We further suggest that this hypothesis may provide insight into similar unexpected tectonic episodes during this time period in other parts of the Apennines.