A Normal Faults System in the Monte Nerone area and its significance in the recent seismo-tectonic setting of the Northern Umbria-Marche Apennines (Italy).

. The faults system mapped in the northern Marche Apennines, the NW sector of Monte Nerone, Italy, shows many 10 indications of recent activity. This area has been affected by some strong historical earthquakes, such as the Cagli earthquake of 1781, similar to seismic events close by affecting the southern Marche, Umbria, Lazio, and Abruzzi areas in recent years, we focused our investigation on this sector. The original field mapping work integrated with seismic and subsurface data suggests very similar genesis and kinematics to those of the recent seismic events in the south of Marche region. In addition, this interpretation could also attest the 15 extensional tectonic activity affecting the whole Adriatic side of the watershed backbone of this sector of the Apennines, with probable inversion of involved previous compressional features, such as thrust ramps.


Introduction and Aims
The study of the recent tectonic evolution of the Umbrian-Marchean-Romagnan Apennines is essential for understanding seismicity and the related risk in a very active chain sector (ISIDe Working Group, 2007;Mantovani et al., 2014). The 20 knowledge of the areas related to the recent events in central Italy in L' Aquila (2009) and Amatrice (2016) has been considerably furthered thanks to the strong research activities of numerous researchers and institutions. Much progress has subsequently been made in these areas where surface and deep geological knowledge has been reviewed and reinterpreted (Buttinelli et al., 2018;Scognamiglio et al., 2018;Bignami et al., 2019;Improta et al., 2019;Ercoli et al., 2020).
Historical data also give indications of a high risk in the northern areas of the Umbrian-Marchean Apennines close to those 25 affected by the events in 2016 due to the possibility of a reactivation of the seismogenic structures with disastrous consequences.
According to historical sources (Presciutti et al., 2017), one of the main events known in this region is the Cagli earthquake of 1781, for which a magnitude of 6.5 Mw was estimated (Baratta, 1896;Monachesi, 1987;Rovida et al., 2019), very simila r to those measured in central Italy.

Geological Features of the Study Area
The study area is an internal sector of the Umbrian-Marchigiano Apennines (Province of Pesaro-Urbino, northern Marche) 60 which includes the north-western tip of the anticline of Monte Nerone and the neighboring areas of the Biscubio River valley and its tributaries. The stratigraphy can be summarized as a carbonate sequence of Jurassic to Oligocene ages; for a more detailed definition, refer to the literature (Centamore et al., 1991). This succession sedimented on the edge of the African platform, during the Liassic rifting became a passive margin (Alvarez, 2009). The gradual increase in the terrigenous content of the Scaglie Group indicates the transition to a turbidite complex of a Miocene foredeep, represented in this region by the 65 Marnoso-Arenacea Formation (Ricci Lucchi, 2003).
The main local tectonic structure consists of a NW-SE-oriented anticlinorium, consistent with the regional Apennine trend, with a longitudinal extension of about 30 km and a wavelength of 5-6 km. In the study area, this structure has a periclinal 5 The field-derived map and subsurface data were imported in GIS and 3D modeling environments, enabling us to synthesize a geological model.
The hardware used in the field were as follows: a tablet PC (Windows operating system) with a stylus, a GPS Bluetooth antenna (NMEA protocol), an Android smartphone and camera, and a commercial UAV equipped with a 12-megapixel 90 camera.

Ground-Based Evidence: Mesoscale Observation and Measurement Points
In the study area, two main faults were mapped: the Sassorotto and the Col Lungo faults. These faults consist of different segments, that can be mapped thanks to geomorphological features and investigated in depth when exposed in outcrops, allowing to capture structural measuraments (Fig. 3).

SR-Sassorotto Fault
This NW-SE trending fault was mapped from the Monte Forno stream to Casciaia Mochi. It affects the terrigenous turbidite sediments and the carbonatic succession almost along the anticlinal axis of the Monte Nerone. We discuss the features of 8 105 major outcrops along the fault trace. can be surveyed along the ridge to the other side up to the main road. Here, the main fault shows a thick breccia (30 cm) with different size blocks (millimeter to decimetric diameters) with embrication almost oriented toward the normal movement ( Fig. 4a ). The total displacement cannot be fully constrained in the visible outcrop but is estimated larger than 4 meters. Some minor planes with calcite striae suggest also strike slip movements, predating the more recent normal ones. In this outcrop, some synthetic faults show normal decimetric displacements. A minor antithetic fault is directly connected to the 120 main one (see Fig. 4a)

SR2. Monte Cardamagna Slope Near Ca' Rossara
To the SE of the previous point (SR1), an evident morphological change can be seen in the SW side of Ca' Rossara ( Fig. 4b).
While the forest does not allow direct observation of the lithological units, it is evident that in the steepest part towards the SW, the limestone layers of the Scaglia Bianca Fm outcrop, while in a gentler incline towards the NE, the Marne a Fucoidi 125 Fm outcrops. Due to the lateral relationship between Scaglia Bianca and Marne a Fucoidi, it is possible to deduce a relative displacement, with lowering of the SW part (Scaglia Bianca) compared to that of the NE part (Marne a Fucoidi), which, despite the morphology related to the valley erosion and the different characteristics of the units, could lead one to believe the contrary (relief inversion).
Along the path that climbs from Ca' Rossara towards Monte Cardamagna, in small gullies, minor normal faults are visible 130 and exposed. These can presumably be considered synthetic faults related to the main one.
Moving in the SE direction, towards Rio Vitoschio, the morphological evidence of the fault initially disappears (segmentation between the different sectors of the fault itself) and then reappears further to the SE.

SR3. At the confluence of the Pisciarello stream and Rio Vitoschio
Going up the Rio Vitoschio, on the right orographic side (500 m asl), the Calcari Diasprigni Fm and Maiolica Fm outcrop 135 with large slopes that continue towards the mountain.
Here, the fault is not evident, but the morphological/hydrographic set-up may have promoted the erosion of the fault surface, even with landslides, as evidenced by large deposits of boulders at the slope foot.
Along the first part of the Pisciarello stream, it was possible to measure a fault plane that is antithetic to the main one.

SR4. Prato del Conte 140
SW of Prato del Conte, a right tributary of the Pisciarello stream cuts through a valley. Images collected by the drone (the descent of the valley is unsafe) show a situation very similar to that of the previous site (A3). This incision is also well recognizable from plane photos and can be interpreted as a lineation. Its extension to the SE would allow a connection with an already mapped normal fault (Regione Marche, Carta geologica regionale 1:10000, 2020).
Although a fault plane does not crop out, the lithostratigraphic survey for the above-mentioned geological map clearly shows a normal fault with a lowering of the SW edge. The displacement as calculated from the cartography is considerable; Calcare Massiccio Fm is in contact with Maiolica in the upper Infernaccio valley and Maiolica with Marne a Fucoidi near Colonia Don Orione.
The surveys carried out in this study confirm this tectono-stratigraphic structure. In addition, they suggest that this already 150 mapped fault segment, even with limited displacement and map extension, is part of a more important one.
In the SE direction, the continuation of this already mapped fault (Regione Marche, Carta geologica regionale 1:10000, 2020) can be inferred from the geomorphology up to the Pian dell'Acqua valley. In detail, along the road that leads to Pianello village, near Casciaia Mochi, an escarpment mapped as a direct fault is well recognizable. It lowers towards the SW of the Marne a Fucoidi Fm with respect to the Maiolica Fm towards the NE. 155

SR6. The Biscubio River and the "Apecchiese" Main Road at km 32
This fault plane is well recognizable (Fig. 4c), and even mapped with the drone, on the outcrops near the road and the river bed, oriented in continuity with the Sassorotto fault (site SR1) as previously described.
In addition, a step can be observed in the riverbed, which creates a hydraulic jump just upstream of the fault itself. This step creates an anomaly in the hydrographic network, as noticed in Beatrice Smacchia's degree thesis (Smacchia, 2017) through 160 an analysis of the slope gradient.
The field survey highlights an anomalous accumulation of pluri-decimetric, rounded sandstone boulders as the cause of the hydraulic jump itself.

SR7. Villa Maria
At higher altitudes, moving in the NW direction on the ridge, only a minor fault crops out in correspondence with a 165 Pleistocene fluvial terrace near Villa Maria. Approaching the road to the Monte Forno village, a dextral strike-slip fault, a WSW-ENE oriented dextral strike-slip fault is evident putting in contact the most recent and terrigenous Marnoso-Arenacea Fm with the last stratigraphic terms of the carbonatic succession. This fault, displacing the thrusts, could have formed during the shortening in the tip area of the Monte Nerone anticline. The type of faulting behavior, located NW with respect to this one, is very different from those previously described, which involve the carbonatic succession. 170

SR8. Monte Forno stream
Along the ridge on the orographic right of the Monte Forno stream, terrigenous sediments of the Miocene foredeep crop out a lmost continuously and are mostly turbiditic and with lithological characteristics totally different from those of the carbonatic succession. This terrigenous succession consists of, at the base, a fully marly formation, the Schlier Fm, giving way, through the insertion of arenite levels, to The Marnoso-Arenacea. The latter unit is divided into two lithozones in this 175 area: the lower consists of prevailing marls with thin beds of sandstone, while the upper one is characterized by medium and thick arenite beds (Campbell, 1967).
Along the entire extension of the outcrop, this lithostratigraphy makes it possible to recognize a set of normal faults (at lea st three) which lower the SE sectors. This trend was mapped in 3D thanks to drone images (Fig. 4d). Surveying the ridge, an abrupt change in lithozones is clearly visible. While it is difficult to collect fault structural data due to the marl lithology 180 itself, the minor faults are however clearly recognizable. In some sectors of the ridge, in particular the lower one, the fragmentation of the series of arenite and marl strata is evident, with very intricate fault systems.
Along the stream, there are numerous nick points with jumps of up to 3 meters, not related to lithology. Unfortunately, the vegetation and debris do not allow to check the fault surfaces directly. The whole stream is affected by a strong erosion with over-excavation, which is much more pronounced than the creeks in the area. 185 The ridge also shows an uneven, jagged morphology with abrupt non-eroded counter-slopes. These observations lead to the hypothesis that this morphology is very recent and influenced by fragile tectonics.

CL -Col Lungo Fault
This fault looks arcuated on maps. It is well exposed from La Valle Country House in the west to the top of Monte Nerone in the east. Its eastern continuation can be inferred by morphology features up to La Montagnola. 190 In the Marche Region geological map (scale of 1:10,000), a normal fault that displaces a thrust in the western area is present, and in the east, it connects Maiolica with Scaglia Rossa. In addition, in this case, the cartographic extent of the fault is somewhat small (around 600 m), even with considerable displacement. 205

CL2. Col Lungo
Driving up the winding road that climbs to the top of Monte Nerone, the fault is well exposed (Fig. 5a). At the bend at 998.2 asl (Regione Marche, Carta tecnica numerica 1:10000, 2020), it is possible to see a fault plane that extends for more than 300 meters with an almost W-E direction. On the 1:10,000 scale geological map of the Marche Region, this outcrop is mapped as the Bugarone Fm, even though this formation should not be in this stratigraphic position. From a careful observation, it is 210 evident that the is not of sedimentary origin (making the official map surveyors interpret as Bugarone outcrop) but tectonic one. This is a typical fault breccia, exposed in several outcrops along a well-developed fault plane.

CL3. i Ranchi
From the sharp bend of the previous site (CL2), the outcrops of Scaglia Bianca and Scaglia Rossa are clearly visible in the upper wall towards the east under the "i Ranchi" site (Fig. 5b). Moving in the NE direction, the outcrop ends in a narrow 215 gorge in which a normal fault is visible (see the geological map), which lowers the Scaglia Bianca to the S compared to the Marne a Fucoidi to the N. Due to the danger of rockfall, the drone was used to map this zone.

CL4. Monte Nerone Road Pass
Along the road at the pass between Monte Nerone and Monte del Pantano, a normal fault clearly outcrops and displaces Majolica Fm layers (Fig. 5c). A thick fault breccia is exposed and there are some drag folds on both walls. The displacement 220 is not evaluable.

CL5. La Montagnola
Beyond the sites described above, there is no clear evidence of the fault in the outcrops. Towards Montagnola, there is a clear double ridge (Fig. 5d), which is difficult to explain with the lithological characteristics since this area is made up of limestone: the Maiolica Fm. 225 Additionally, even on the SW flank, there are some NW-SE-oriented slopes that do not seem to represent landslide ridges.
Beyond that, our understanding of the fault framework becomes quite speculative, and we did not find any clear evidence due to the lack of outcrops. The creek tributary to the left of the Biscubio River, downstream of Pian di Mulino, shows a sharp incision along its entire short length from the ridge near Monte Forno until reaching the main river. The terrigenous lithologies (marls and sandstones) crop out, often showing small waterfalls related to local nick points. Field and drone surveys showed that these jumps are not strictly related to lithological differences. Indeed, they are located in the most faulted areas or where faults a re inferred from the displacement of the stratigraphic lithozones, as described above. 235 This localized framework therefore seems to be the direct consequence of a very recent tectonics. The streams in neighboring areas do not show similar characteristics.

GM2. Biscubio river whitewater
Along the Biscubio river in the Sassorotto site, where the fault outcrops clearly point to the orographic left, a hydraulic jump is present a few meters upstream of the fault itself. Upstream, an accumulation of huge sandstone blocks transported and 240 deposited here, coming from the Marnoso-Arenacea Fm, is evident. This crops out a few kilometers upstream. This deposit is an anomaly in the river profile (Smacchia, 2017;Piacentini et al., 2020) and is difficult to explain with the present-day conditions of the riverbed. Not being an accumulation of landslides both for the lithology and for the rounded shape of the blocks, we must assume that some sort of physical barrier blocked the transport of the sandstones boulders, a slowdown that led to a decrease in the solid flow of the river itself. 245 There are two hypotheses: (i) a local landslide of which there is no evidence or (ii) a fault step that created a dam. Although there is no strong evidence, both could be caused by a seismic event such as the Cagli earthquake of 1781.
In the first hypothesis of a landslide dam, it should be possible to find part of the accumulation, even among the transported sandstone blocks; however, there is no such evidence of erosion by the river, in nearby areas (towards the road).
The second hypothesis regards the fault that would have created a lowering of the hanging wall upstream with a consequent 250 small dam basin. This is also not proven because the subsequent erosion due to the flow has dismantled this dam and part of its accumulation. The hydraulic jump, indeed, is recognizable about ten meters from the sub-outcropping fault. Figure 6 shows a possible model of the genesis and evolution of this anomalous riverbed profile.

GM3. Fosso del Molino 260
The right tributary of the Biscubio river, which flows from SSW to NNE, near Sassorotto has a large riverbed where recent alluvials together with blocks collapsed by steep slopes are gathered. The stream is now eroding these deposits, reaching the carbonatic substrate.
Right where the fault cut the riverbed, the excavation shows a transversal trend with respect to the small alluvial pla in. No clear evidence of the fault has been found, but this local trend is anomalous and could be easily attributed to recent tectonic 265 activity.

Geophysical data (CROP-03 reinterpreted)
The CROP-03 seismic section at the crustal scale crosses the deeper structures a few kilometers north-westwa rd from the study area (Fig. 1). The first geological interpretation of this seismic section was proposed by Barchi et al. (1998)  Since the aim of this seismic line was to investigate the deep crust characteristics and geometries (CROP), its spatial resolution was not adequate to highlight the superficial structures related to recent normal faulting, whose displacement is much lower than that of the deep thrusting structures. 280 Some minor discontinuities could be interpreted as evidence of normal faults surveyed during fieldwork (Fig. 7).
Some of these normal faults seems to connect with the main thrust ramps; however, the displacement in the Quaternary does not overbear the Mio-Pliocene thrusting shortening and, therefore, is not so evident in this section.

Seismicity
To identify the historical events of Umbria, Marche, Tuscany and Romagna, data from the Historical Archives Macroseismic 290 Italian (Rovida et al., 2017) and parametric catalogs of Italian earthquakes (Rovida et al., 2019(Rovida et al., , 2020 were merged. These catalogs (Camassi et al., 2011) report the Cagli earthquake (Baratta, 1896;Monachesi, 1987) and other events from less reliable sources, such as that of Monte Cardamagna on April 17, 1725, which suggests important seismic activity in the study area (Fig. 8). For the current instrumental earthquakes, we used data available on the INGV platform (ISIDe Working Group, 2007), 300 reporting them in a three-dimensional model (Fig. 9) and reprojecting them in a geological section that shows a correspondence with the interpreted structures (De Donatis et al., 2020).
In this work we also consider the data from the TABOO network (Chiaraluce et al., 2014;Latorre et al., 2016) arranged ad hoc for monitoring the ATF and correlated structures. In particular, in this paper we consider the data used by Valoroso et al. (2017). As reported by those authors,90% of the recorded seismic events correspond to hypocenters that can be correlated to 305 faults positioned in the ATF hanging wall.
Furthermore, some focal mechanisms are available (Chiaraluce et al., 2017;Valoroso et al., 2017) that can be clearly interpreted as normal faults.

Comparison with Southern Sectors of the Marche-Abruzzi Apennines
The latest studies on the close sectors of the recent earthquakes (L' Aquila, 2009;central Italy, 2016), provided a huge amount of data, which allowed us to formulate new seismotectonic models in contexts very similar to this study area.
In addition, the Retrace-3D project (RETRACE-3D Project, 2021) for the creation of a three-dimensional geological model highlights a congruity between earthquakes and tectonic structures. In detail, the Mio-Pliocene thrust ramps were reactivated as normal faults, with tectonic inversion (Scrocca, 2021). 320 As in the southern areas of the Umbrian-Marchigiano Apennines, and even in the northern areas, in comparable chain sectors, there are signs of extensional tectonics, as some authors have already evidenced (Chiaraluce et al., 2017). The structure is very similar to that of an NE-deepening LANF, with high angle-antithetic normal faults at the hanging wall, cutting or joining the Mio-Pliocene thrusts.

Interpretation and Discussion 325
From the field survey, the fault system identified in the Monte Nerone sector, which continues to the NW where foredeep turbidites outcrop, shows clear evidence of extensional tectonics.
The Sassorotto fault on the surface consists of a system of fractures typical of the areas where normal faults crop out (EMERGEO et al., 2016;Iezzi et al., 2019) despite being obliterated by anthropic and natural reworking over the last 230 years. 330 In carbonate units, it is more evident, and it is certainly the sum of many events that allow remarkable final displacements. In the siliciclastic turbidite unit, the geometric characteristics are different, most likely related to the different rock mechanics of marls (prevalence) and sandstones. In fact, in the Monte Forno stream area, faulting is more widespread with at least three branches pointing out by stratigraphic reconstruction (Fig. 2). Here, the articulated morphology indicates that these faults were formed in very recent times. Although geomorphology studies are still in progress, the morphology of the ridge, as well 335 as that of the impluvium, is still "fresh". This indicates that they did not undergo erosion phenomena during the most recent glacial phase, which eroded and "planed" the exposed surfaces in neighboring slope areas.
The Col Lungo fault plane also shows sharp fresh surfaces with articulated morphology that seem to suggest very recent movement.
Both in the Sassorotto and in the Col Lungo faults, kinematic indicators (Striae) can be found, which testify significant 340 transcurrent movements (DX or SN). These movements are likely to be older; even if identifying the specific timing is not feasible, they are assumed to be related to the previous shortening phases during orogenic thrusting. In fact, these faults are oblique to thrust planes and could be seen simply as being mechanical-lateral junctions.
These discontinuities were then reactivated as normal faults in the later phases, both for their position with respect to the structures and for evidently higher brittleness. 345 Such superficial evidence, as previous reported, represents nothing more than the sum of several recent events. Each of them may have undergone movements of centimeters to meters, very similar to those measured in the Sibillini Mountain faults after the 2016 earthquake. From the historical and instrumental data, a real and evident similarity can therefore be inferred among the seismogenic and tectonic situation of all these districts. It is assumed that these faults can reach the region above the ATF, where earthquakes form throughout the reactivation by inversion of the thrust ramps, interpreted by previous 350 authors with the use of commercial seismic lines (Barchi et al., 1998) and the CROP-03, which crosses this sector of the Apennines just to the NW.
In order to investigate the relationships among faults and earthquakes, we drew two schematic cross-sections from available data (Barchi et al., 1998;Mirabella et al., 2011) and this original study, on which earthquakes foci were projected (Fig. 10).  shows concentration of instrumental earthquakes very close to the intersection between ATF and thrust ramp inverted as normal faults. Section B was drawn following a similar section trace of (Chiaraluce et al., 2017), T1 in Figg. 2 and 3) and (Valoroso et al., 2017), 1 in Fig. 2 and 3) and re-projecting their earthquakes data from TABOO network.
As mentioned in (Chiaraluce et al., 2017), 90% of earthquake records are concentrated at the hanging wall of the low-angle 365 ATF fault. The evident alignment of earthquakes along sections crossing the Città di Castello and Pietralunga area, beyond Monte Nerone, shows faults antithetical to the ATF, such as those under examination, although an exact correspondence has not been found.
The earthquake distribution, however, is related to the extensional tectonics of this sector. Field evidence has to be thoroughly investigated, such as the inversion of thrusts separating the tectonic units of Pietralunga and Borgo Pace of the 370 Ma rnoso-Arenacea Fm. Although there are no good outcrop expositions, the anomalous stratigraphic position of some key beds at the hanging wall of these thrusts, compared to the main marker bed of the Marnoso-Arenacea Fm (Contessa key bed) at the footwall, suggests an overlap of younger and older stratigraphic units.

Conclusions
The tectonics of this sector of the Apennines is reinterpreted in light of new field observations and deep data overviews as 375 seismic lines and instrumental earthquakes hypocenters.
From these data, it is clear that recent tectonic activity here is extensive, as indicated recently by other authors (Chiaraluce et al., 2017;Valoroso et al., 2017) simply on the basis of focal mechanisms.
In the Marnoso-Arenacea innermost thrusts, it can be observed that these are nowadays affected by extensional movements, as evidenced in the field by the stratigraphic structure and by the distribution of the deep hypocenters. 380 On the surface, the faults that dissect the Monte Nerone anticlinorium proceed to the NW in the turbidite units of Marnoso-Arenacea Fm.
Although they are incontestable in some outcrops, the faults under study are not always so evident since they are recent and the last great earthquake could have activated them (historical documents dates back to 1781). Over the last two centuries, the erosion has hidden fault surfaces, if not apparently reversing the displacement (e.g., Sassorotto fault on Monte 385 Cardamagna). Indeed, the rapid erosion of the morphologies due to faulting is also evident from the observations in the soil breaks that formed during the Castelluccio plain earthquake in 2016. Here, fault surfaces with displacements of less than 7 cm, clearly evident in the days surrounding the occurrence of the earthquake, are now erased due to soil erodibility and by agricultural activity.
The fresh and segmented morphology of the ridges and impluvium of the Marnoso-Arenacea Fm, near Monte Forno, would 390 suggest that faults were active after the last glacial phase, which would have eroded and smoothed shapes due to fault movements. Therefore, here, the slopes and counter-slopes are clearly recognizable.
This work is just the beginning of the study of this part of the Apennines, which will have to foresee the contribution of researchers with different skills, such as quaternary geologists and geomorphologists, stratigraphers, structural geologists, and seismologists. 395 This contribution intends to start a debate on a seismically active sector of the Apennines, similar to that involved in the 2016 earthquake, where communities and human activities are at risk.

Author contribution.
MDD contributed to conceptualization, funding acquisition, investigation (field mapping), data curation, data analysis, supervision and validation, writing original draft preparation. GFP contributed to data curation, validation and visualization. 400 SS contribute to investigation, data analysis and visualization. MA contribute to investigation (field mapping), data curation and data analysis. NMG contribute to data curation and validation. FO contribute to investigation (field mapping), visualization.