Miocene fault-controlled sedimentation and thrust propagation in the previously faulted external zones of the Umbria-Marche Apennines, Italy

Abstract

Thrust propagation through previously faulted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin archi-tecture. A detailed geological study has been carried out in the external zone of the Umbria-Marche Apennines, from Monte San Vicino to the north, to Montagna dei Fiori to the south. Stratigraphic and structural data, together with the construction of a series of balanced and restored geological sections, point out the fundamental role played by the pre-orogenic basin architecture in controlling the geometry and evolution of the fold and thrust belt. Pre-thrusting structures include not only those inherited from the Mesozoic rifted continental margin, but also synsedimentary faults associated with Miocene extension which occurred ahead of the advanc-ing thrust front. The latter structures produced important fa-cies and thickness variations in the units deposited during the late Burdigalian-early Messinian, pre-evaporitic stages of foredeep development. In the southern sector (Montagna dei Fiori), high values of Messinian regional subsidence, bathymetry and sedimentation rate overcome the effects of synsedimentary extensional tectonics, which is best recorded in pre-Messinian sequences. On the other hand, Messinian regional subsidence was significantly less in the northern sec-tor (Monte San Vicino). Here, several minor sub-basins de-veloped within the foredeep, generally reaching evaporitic conditions during the middle Messinian (marked by the de-position of the Gessoso-solfifera Fm). In this area, a ma-jor control by pre-thrusting normal faults on sedimentation is recorded in the foredeep siliciclastic sequences. Late Burdigalian-early Messinian extension, possibly associated with flexure of the foreland lithosphere, peripheral bulge up-lift and/or foreland tectonic activity, was followed by a late Messinian (post-evaporitic) contractional episode of regional extent. During shortening, inversion of preexisting Miocene Correspondence to: S. Mazzoli (s.mazzoli@geo.uniurb.it) extensional structures was quite limited. Hanging wall basin fills were retained and pre-thrusting faults show very lim-ited or no reversal of slip. However, these faults and their hanging wall basin fills are generally deformed by buttress-ing phenomena, and fault planes are often tilted and/or lo-cally reactivated in strike-slip. The present-day structure is dominated by newly-formed thrusts that cut across the pre-existing extensional architecture, which is in general quite well preserved within different thrust sheets.