The Umbria-Marche Apennines as a Double Orogen: Observations and hypotheses

The Umbria-Marche Apennines, an arc-shaped fold and thrust belt with eastward convexity and vergence, form the external part of the Northern Apennines. In the middle 1980s, the Umbria-Marche Apennines were interpreted by some as a classical thin-skinned foldthrust belt, with thrust sheets emplaced in an in-sequence, piggyback mode, from the interior to the exterior of the orogen, over a main, basal detachment in the Triassic evaporites. In the mechanics of this kind of edifice, the folds and thrusts would develop within a prism or tapered wedge, bounded at the base by an undeformed basement with a regional monocline dipping toward the hinterland, and at the top by a topographic and structural slope generally dipping toward the foreland. Other authors saw the Umbria-Marche Apennines as a more complex orogen, with basement involvement, possible tectonic inversions, and out-of-sequence thrusts. In the present paper, the geometry, structure, and tectono-sedimentary evolution of the Umbria-Marche Apennines are compared with the classical thin-skinned model. We suggest that this fold-thrust belt can be divided longitudinally into two sectors. The eastern part of the chain, comprising the high Mesozoic carbonate anticlinal ridges of the axial zone together with the Marche external folds, fits the classical model well, but the western part, comprising the Umbrian Pre-Apennines, shows striking differences. The boundary between the eastern and western parts of the Umbria-Marche Apennines is here termed the Scheggia-Foligno Line (SFL). East of this line, the Eastern Umbria-Marche Apennines show an eastward taper between the undeformed basement, dipping gently west, and an upper surface in which both topographic and structural elevation decrease toward the east. West of the Scheggia-Foligno Line, by contrast, seismic reflection profiles and subsurface data show basement involvement in the thrusting at shallow depths, while both the topographic and structural elevations are anomalously low compared to the more easterly parts of the chain. There is also a notable discontinuity in foredeep-basin evolution at the Scheggia-Foligno Line, with well developed foredeep basins in the Umbrian Pre-Apennines and the external Marche belt (the Marnoso-arenacea and Marche Plio-Pleistocene, respectively), whereas only thrust-top basins developed in the axial zone, during the Tortonian-Messinian interval. Various mechanisms, not all mutually exclusive, might be invoked to explain the discontinuity at the Scheggia-Foligno Line. These possible explanations include causes of local character, linked to the sedimentary and tectonic evolution of the region, involving episodic departures from steady-state conditions, interrupting the regular growth of the accretionary wedge. However, it also possible that the Western and Eastern Umbria-Marche Apennines represent completely different orogenic systems, with different causes - possibly with the former related to Corsica-Adria collision and the latter due to slab rollback of Adriatic lithosphere. In either case, this study demonstrates the complexity of evolution of the Northern Apennines, with adjacent zones showing abrupt variations in their history and style of deformation, which are difficult to incorporate in a single, unified geodynamic model.