The Umbria-Marche Apennines (UMA) are an arc-shaped fold-thrust belt, with eastward convexity and vergence, representing the outer part of the Northern Apennines. In the late 1980â€™s, the UMA was interpreted by several authors as a typical thin-skinned thrust belt, whose thrusts were emplaced in an in-sequence, piggy-back mode, over a main, basal detachment, corresponding to the Triassic evaporites.
In the present paper we re-examine the tectono-sedimentary evolution of the UMA, mainly on the basis of seismic reflection data, stratigraphic data, and geomorphic observations. We compare it with the classic model, which describes the steady-state growth of a critical-taper fold-thrust belt, advancing over an undeformed basement. Considering the geomorphology and the subsurface geology, the UMA can be divided into two major regions, here named the Eastern UMA and Western UMA, with a boundary at the western margin of the main Apennine ridge, passing through Cantiano and Foligno. The Eastern UMA, comprising the main Apennine ridge and the outer (eastern) ridges and foothills, generally fits the critical taper model â€" moving from the foreland (NE) towards the hinterland (SW), both the structural and topographic elevations increase, while the top of the basement deepens from about 5 to about 12 km. In contrast, the Western UMA is a topographically and structurally low region, where Tertiary turbidites extensively crop out and the basement is much shallower (about 6 km) than below the adjacent mountain ridge.
Considering stratigraphic data in detail, we find that a major discontinuity in the evolution and eastward migration of the foredeep basins, related to the rise of the Apennines ridge, occurred during the Tortonian-Messinian time interval. Summarizing, the departures of the Umbria-Marche fold-thrust belt from the classic model are largely due to the presence of discontinuities in the depth to basement, the migration history of the foredeep, and the elevation of the topographic surface. All these discontinuities occur at the same geographic location (at the western edge of the Umbria-Marche main ridge) and they possibly reflect the same event, i.e. the rise of an anticlinorial ridge that was higher than might have been expected, based on the previously occurred deformation of the Western UMA.
By considering the UMA in the broader context of the Northern Apennines, we suggest two alternative scenarios to explain the described discontinuities: (1) The UMA depart only in detail from a classic fold-thrust belt, and the main ridge represents the effect of propagation of a deep master thrust into a more external position. (2) The Western and Eastern UMA are generated by two distinct major thrust systems (Etruscan and Umbrian). In this latter case, the two major thrusts could be related to different stages of the Northern Apennines evolution, characterized by different driving mechanisms, perhaps earlier collision vs. later slab roll-back.