Stratigraphic and structural features of the Sibillini Mountains (Umbria-Marche Apennines, Italy)

Pietro Pierantoni(*), Giovanni Deiana(*) & Sandro Galdenzi(**)
(*) University of Camerino - School of Science and Technology -Geology Division. Via Gentile III da Varano, Camerino (MC) 62032 Italy. Tel. +39(0)737402601; fax +39(0)737402644; e-mail pietropaolo.pierantoni@unicam.it (**) Via Verdi, 10 - Iesi (AN) 60035 Italy.

In this paper we illustrate the stratigraphic and structural features of the Sibillini Mountains on the basis of a 1:40.000 geological map. Following the "Foglio 132 Norcia" (Geological Map of Italy at 1:100.000 scale; SCARSELLA, 1941), this new geological map is the first cartographic document that covers the whole area of the Sibillini Mts. This area is key for understanding the geological evolution of the external zones of the Apennine orogen, mostly owing to the pronounced structural elevation of the Apennine ridge at the Sibillini Mts. area. This allowed us to: a) carry out stratigraphic and structural analyses for the Umbria-Marche sedimentary cover to the oldest units; b) analyze the paleo-tectonic setting of this sector of the Afro-Adriatic continental margin and the behavior of pre-existing structures during the subsequent deformation events; c) investigate in depth the major Apennine thrust front ("Sibillini Mountains Thrust") exposed in several sites (Fiastrone, Ambro, Tenna and Tronto valley) and its relationship with the units of the adjacent Messinian foredeep. Inherited (pre-thrusting) structures played an important role on the tectonic evolution of the study area. In addition to those of Jurassic age, associated to thinning of the Adriatic continental margin (the most frequent), there are also some Cretaceous-Eocene extensional faults and others probably related to Miocene foreland deformation. These extensional faults show very limited, or none, reversal of slip during contractional neogenic deformation. Usually they were tilted and, subsequently, displaced by thrust faults; locally they were deformed by buttressing processes. The paleotectonic setting also influenced the development of minor folds, as is evident in the Mt. Bove anticlinorium. Shortening-related structures are mainly represented by asymmetric, northeast-verging thrust-related anticlines involving Mesozoic-Tertiary sedimentary successions. Among these structures, the most important and well-known is the Sibillini Mts. Thrust, which bounds the Apennine mountain front, separating it from the Marche-Abruzzi foothills. This major fault affects the eastern, vertical to overturned, limb of the arcuate shaped Mt. Fiegni-Mt. Vettore anticline, which shows several complications, mostly due to the occurrence of minor thrust splays forming isolated lens-shaped tectonic slices up to several kilometers-long. The geological cross sections show that in the southernmost part of the Sibillini Mts. Thrust the displacement is partly buried and forms a detachment located at the base of the Laga Fm. Much of the Neogene compressional structures has been dissected by NNW-SSE trending Quaternary normal and oblique-slip faults, some of which reactivated older extensional structures. These faults do not seem to crosscut the Sibillini Mts. Thrust. Some of them show fresh fault scarps in the substrate and/or affect recent continental deposits and are considered responsible for the intense seismic activity of the area.

Sibillini Mountains, Central Italy, Stratigraphy, Tectonics.