Structural and lithological analysis of a foreland fold-and-thrust belt in the Umbria-Marche Apennines

Abstract

The geology in and around the Fiume Fiastrone valley (Umbria-Marche region, Italy) is controlled by a NW-SE striking fold and thrust belt of Neogene age. During a different stage SW-NE striking folds were created. The lithology in the area consists mainly of carbonates and carbonate shales of Mesozoic to Cenozoic age. The system has been analysed by means of morphological and structural analysis of exposed rock in the area around the Fiume Fiastrone valley. The geological situation before thrusting is proposed to be a SW-NE directed 8-10 km long anticline. Indications for this proposition are anticline-indicating orientations in the footwall at different locations with a maximum dip of 40 degrees. On the hanging wall the two nearly perpendicular folds created a 6 by 4 km dome structure that is bounded by the Sibillini thrust. This thrust accommodates 1800 m of lateral shortening; the Anidriti del Burano Fm. acts as its décollement. A literature study shows that NW-SE shortening started during the Quaternary; this would correspond to a shortening rate of roughly 0.1 mm/y in this direction. The implication of this theory is that the Sibillini thrust should show NW-SE compressional deformation. However, the outcrop of this thrust, at the hinge of the anticline, shows no considerable curvature over a length of about 1 km. Hence this observation does not confirm that theory. It may be possible though that the thrust fault exhibits curvature on a larger scale.