ABSTRACT The Sibillini Mountains, which make up the southern part of the Umbria-Marche Apennines, were struck by a series of earthquakes in 2016, including five with magnitudes greater than 5. The largest event, M w 6.5, occurred on 30 October 2016. A M w 5.9 earthquake on 26 October ruptured several faults in the northern third of the Vettore–Bove fault system, and the M w 6.5 event produced surface ruptures along the entire 30-km length. Ground surveys conducted shortly after these earthquakes showed that many, but not all, of the surface ruptures corresponded to previously mapped faults. Also, some faults that had been mapped as Quaternary did not produce surface ruptures during the earthquakes. In this study, we present the results of detailed field mapping that was conducted prior to the 2016 earthquakes and provide evidence that all of the surface ruptures in the northern part of the Vettore–Bove fault system occurred along preexisting faults. Paleostress analysis shows that the reactivated faults had been active prior to 2016 in stress fields with similar orientations to the modern-day stress field. In addition, we show that one fault segment, which is the southern continuation of a major fault that slipped during the 2016 earthquakes, was not reactivated because it was unfavorably oriented.

ABSTRACT The formation of the “expansion breccia” observed in the Lower Cretaceous Maiolica limestone in the Umbria-Marches region of Italy is attributable to a fluid-assisted brecciation process that occurred during the late Miocene exhumation of the Northern Apennines. The hydrothermal fluids probably originated as brine solutions trapped in the Burano anhydrite while it was in a plastic state. The migration of the Burano from the plastic to the brittle domain during unroofing resulted in liberation and injection of over-pressured hydrothermal fluids into the overlying limestone, causing hydraulic fracturing. Mapping of breccia morphology along a 400-m transect showed structures produced by different flow regimes, with chaotic and mosaic breccia characterizing the core parts of the section and mineral-filled fractures and veins in the margins. Based on the clast size in the chaotic breccia, the estimated velocities for fluidizing the aggregates of clasts and sustaining the clasts in suspension are, respectively, 15 cm/s and 65 cm/s. Crack growth was probably the main mechanism for the fragmentation of the limestone. Explosion fracturing patterns were only sporadically observed in the breccia, indicating substantial heat loss of the over-pressured fluids during their ascent to the Earth’s surface.

ABSTRACT The reduced Jurassic sedimentary sequences deposited on a structural high in the Umbria-Marche Apennines, as well their relationships with adjacent expanded basinal sequences, have been reconstructed through detailed, interdisciplinary study of the Sasso di Pale and Monte Serrone areas near Foligno, Italy. The physiographic features of the basin originated in the Early Jurassic (latest early Pliensbachian), when extensional tectonic activity broke up a shallow water platform where the Calcare Massiccio had been deposited, and the area evolved from an edge-stepped structural high to a distally steepened ramp. The biostratigraphic framework of this paper is mainly based on calcareous nannofossils, which are a useful tool for dating condensed Jurassic successions. Although the sections studied have limited thickness and much lateral facies variation, the sedimentary evolution can be traced and interpreted within a wider Jurassic environmental perspective. In the upper Pliensbachian–lower Bajocian interval, local sea-level variations are compatible with the global sea-level curve. Furthermore, some of the characteristic events—such as the Pliensbachian–Toarcian crisis, the Early Toarcian Jenkyns Event, and the Middle Jurassic carbonate crisis—can be recognized. The present study shows how the reconstruction of local paleogeography can fit into a more general framework and how regional and global signals can be recognized even in a small structural high such as the one we have investigated.

ABSTRACT Stratigraphic analysis of two sections of a fluvial strath terrace exposed on the left bank of the Esino River near the village of Trocchetti (province of Ancona, Marche region of central Italy), and the study of a large landslide located near the village of San Cristoforo, a few kilometers down valley from the Trocchetti fluvial terrace, provide evidence for two catastrophic environmental events, namely: (1) the aggradation on the riverbed of coarse, chaotic gravel due to a violent flashflood; and (2) the formation of a large ephemeral lake as the consequence of the landslide that barred the river channel at San Cristoforo. Archaeological and historical information about the lost Roman city of Tuficum , which was located just a kilometer upriver from the Trocchetti terrace, and ceramic artifacts found in the chaotic gravel unit, led us to the hypothesis that both the flashflood and the landslide were induced by the sudden, severe climate change of the Late Antique Little Ice Age (mid-sixth century to mid-seventh century CE).

ABSTRACT The structural style at depth of the Umbria-Marche fold-and-thrust belt, which occupies the outer province of the Northern Apennines of peninsular Italy, has long been debated and interpreted in terms of thin-skinned or thick-skinned deformation models, respectively. Thin-skinned models predict that the Mesozoic–Tertiary sedimentary cover was detached along Upper Triassic evaporites and translated northeastward along stepped thrust faults above a relatively undeformed basement. On the other hand, thick-skinned models predict the direct involvement of conspicuous basement slices within thrust-related folds. A description of selected examples in the southeastern part of the Umbria-Marche belt reveals that some compressional structures are indeed thin-skinned, their style being controlled by rheological properties of a mechanically heterogeneous stratigraphy containing multiple décollements, whereas other structures are genuinely thick-skinned, their style being dominated by the reverse-reactivation of pre-orogenic normal faults deeply rooted within the basement. Therefore, the contrast of thin- versus thick-skinned structural styles, an issue that has generated a long-lasting debate, is only apparent, since both styles are documented to coexist and to have concurred in controlling the final compressional geometry of the fold-and-thrust belt.