Structural and climate drivers of the historic Masiere di Vedana rock avalanche (Belluno Dolomites, NE Italy)

The “Masiere di Vedana” rock avalanche, located in the Belluno Dolomites (NE Italy) at the foot of the Mt. Peron, is re-interpreted as Historic on the base of archeological information and cosmogenic 36Cl exposure dates. The deposit is 9 km2 wide, has a volume of ~170 Mm3 correspondings to a pre-detachment rock mass of ~130 Mm3, and a maximum runout distance of 6 km and an H/L ratio of ~0.2. Differential velocities of the rock avalanche moving radially over different topography and path-material lead to the formation of specific landforms (tomas and compressional ridges). In the Mt. Peron crown the bedding 5 is subvertical and includes carbonate lithologies from lower Jurassic (Calcari Grigi Group) to Cretaceous (Maiolica) in age. The proximal deposit is made of Calcari Grigi Group limestone, the distal deposit comprises upper Jurassic limestones (Fonzaso Formation, Rosso Ammonitico, and Maiolica), while the middle Jurassic Vajont Limestone dominates the central sector of the deposit. In the release area the bedding, the SSE-vergent frontal thrust planes, the NW-vergent backthrust planes, the NW-SE fracture planes, and the N-S Jurassic fault planes controlled the failure and enhanced the rock mass fragmentation. Cosmogenic 10 36Cl exposure ages, mean 1.90 ± 0.45 ka, indicate failure occurred between 340 BC and 560 AD. Although abundant Roman remains were found in sites surrounding the rock avalanche deposit, none was found within the deposit, and this is consistent with a Late Roman or early Middle Age failure. Seismic and climatic drivers are discussed. Over the last few hundred years, earthquakes up to Mw 6.3 including that at 365 AD, affected the Belluno area. Early in the first millennium, periods of climate worsening with increasing rainfall are known in the NE Alps. The combination of climate and earthquakes induced progressive 15 long-term damage to the rock. The present Mt. Peron crown shows hundreds of meters-high rock prisms bound by backwall trenches, suggesting a potential landslide hazard for the whole mountain belt north of Belluno affected by the same structural characteristics.

Orientations of bedrock discontinuities, such as bedding, foliation, joints, fractures and faults, were measured in the southern wall of Mt. Peron.

Cosmogenic 36 Cl exposure dating
Twelve different boulders located in topographically high positions with respect to the surroundings within the deposits were 90 sampled for dating with cosmogenic 36 Cl. For boulders VB13 (VB13a, VB13b) and VB14 (VB5 same boulder as VB14) two samples were taken. Samples were taken to cover the full extent of the deposit, from right near the source area to the distal sector.
For 36 Cl sample preparation we used the method of isotope dilution as described by Ivy-Ochs et al. (2004). Total Cl and 36 Cl were determined at the ETH AMS facility of the Laboratory for Ion Beam Physics (LIP) with the 6 MV tandem accelerator. The 95 36 Cl/Cl ratios of the samples were normalized to the ETH internal standard K382/4N with a value of 36 Cl/Cl = 17.36 x 10 -12 which is calibrated against the primary 36 Cl standard KNSTD5000 (Christl et al., 2013;Vockenhuber et al., 2019). Full process chemistry blanks (3.4 x 10 -15 ) were subtracted from measured sample ratios. All fourteen rock samples were processed. Only seven were measured successfully due to too high 36 S, also in relation to the very low 36 Cl concentrations in these samples.
All measured data are presented here. Major and trace element concentrations were determined with XRF (Supplementary 100 Material SM3) and ICP-MS (Supplementary Material SM4), respectively. We calculated surface exposure ages with the LIP ETH in-house MATLAB code based on the parameters presented in detail in Alfimov and Ivy-Ochs (2009, and references therein). A production rate of 54.0 ± 3.5 36 Cl atoms (g Ca) -1 yr -1 , which encompasses a muon contribution at the rock surface of 9.6%; and a value of 760 ± 150 neutrons (g air) -1 yr -1 . These values are in excellent agreement with production rates recently published by Marrero et al. (2016). Production from all major elements and through low energy neutron capture in 105 light of the trace elements (Supplementary Material Table SM4a) were fully considered. Production rates were scaled to the latitude, longitude, and altitude of the sites based on Stone (2000). No correction was made for karst weathering of the boulder surfaces (cf. Styllas et al., 2018). The extent of karst dissolution on the boulder surfaces varies significantly from boulder to boulder. Implementing a rate of 5 mm/ka would change the ages by less than 4%, which does not affect any of the conclusions drawn here. Stated errors of the exposure ages (Table 1) include both analytical uncertainties and those of the production rates 110 (Alfimov and Ivy-Ochs, 2009). Two different surfaces of boulder 13 were analyzed (VB13a, 1.45 ± 0.12; VB13b, 1.45 ± 0.12 ka); the weighted mean of 1.45 ± 0.08 ka is used for further discussion.

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The Peron sector includes the talus apron deposits at the foot of Mt. Peron, the rock avalanche deposits on the east side of the river and the terrace of the town of Peron (at about 380 m a.s.l.). Boulders at the foot of the slope range up to 20 m in diameter ( Fig. 5a). Three boulders in the Peron sector were dated with cosmogenic 36 Cl ( Fig. 3; Table 1): VB3a (Rosso Ammonitico; 1.83 ± 0.28 ka), VB3c (Fonzaso Fm.; 3.62 ± 0.41 ka) and VB14a (Calcari Grigi Group; 2.39 ± 0.39 ka). Based on the trend of all obtained ages, the age of VB3c is interpreted as an outlier, its age possibly reflecting the presence of inherited 36 Cl due 140 to pre-exposure. The town of Peron lies on a terrace made of rounded gravel layers with rare sand lenses that are interfingered with talus deposits (Caneve, 1985).
In the Vedana sector, the rock avalanche deposit displays an irregular forested topography with relief on the order of tens of meters (Fig. 5b). Huge blocks hundreds of cubic meters in size, mostly made of Calcari Grigi, dominate the carapace. This covers the main body of the deposit made of shattered rocks, which is comprised of very angular clasts (up to tens of cm 145 in diameter) in a matrix of silty sand. VB2 boulder (Calcari Grigi) gave an age of 1.49 ± 0.26 ka. In the Ponte Mas quarry (Figs. 3,5g), an open section showed glacial till (up to 3 m thick) incorporated into the base of the rock avalanche deposit, its original bedding completely obliterated. This sediment is composed of sub-rounded clasts (up to 20 cm in length), some of them striated, supported by a silty clay matrix. Clasts are sedimentary and volcanic, reflecting the catchment of the Cordevole paleoglacier (cf. Pellegrini et al., 2006). Several ENE-WSW trending incisions cut through the Vedana and Torbe sectors (main 150 ones highlighted on Fig. 3). Irregular patches of sandy-silty and fine gravel sediments are found in the Vedana low-lying areas between the blocky reliefs.
The Torbe sector encompasses the distal northern lobe of the rock avalanche, characterized by 10 to 20-m high isolated hills and hummocks (Fig. 5d), "toma" (Turnau, 1906;Abele, 1974) that emerge from a flat topography. They are roughly aligned ENE-WSW, circular at the base and have slope angles of 35°-40°. The toma are made of very angular Calcari Grigi boulders 155 and clasts, with many jig-saw puzzle structures in a sandy, gravelly matrix (Fig. 5e). Six cores taken in the flat area between the hills (see Fig. 3 and Supplementary Material SM2) show up to two meters of fining upward silty sand above the rock avalanche.
Torbe is crossed by the largest incision of the whole Masiere di Vedana (Fig. 3), ENE-WSW trending,~50 m wide and up to 20 m deep in respect to the mean topographic surface. A shallower incision, few m deep, is located at the base of Piz Vedana slope, still conveying a small amount of water coming from the Vedana Lake. This is in turn covered, with a sharp and undulated contact, by up to 20 m of rock avalanche debris decimetric in size, with boulders (~1 m diameter) on top. On the Roe Alte rocky upland, the rock avalanche is at most 2 meters thick, with rare boulders (1-2 m diameter). Two boulders made of Vajont Limestone (Fig. 3) have been dated with 36 Cl (VB12, 2.35 ± 0.21 ka; VB13, 1.45 ± 0.08 ka; Table 1). South of the town of Mas, the Cordevole River flows into narrow meanders entrenched~20 m into 180 rock avalanche deposits, alluvial material, glacial sediments and bedrock. The terrace of the Vignole village is almost totally made of rock avalanche debris, despite being remarkably flat.
5.1 Age of the Mt. Peron rock avalanche 36 Cl surface exposure ages from boulders all across the deposit range from 1.45 ± 0.08 ka to 2.39 ± 0.38 ka (Table 1). All 185 ages show a good overlapping within uncertainties. A single sample (VB3c) gave a result markedly different from the others: 3.62 ± 0.41 ka. Although this age may point to pre-exposure of the sampled boulder surface (cf. Sewell et al., 2006;Merchel et al., 2013), as for example seen at Lavini di Marco (Martin et al., 2014), the possibility exists that this boulder is part of a partially buried older deposit located right at the foot of Mt. Peron. The poorly developed karst dissolution features (0.5-1 cm deep karren) on the tops of many boulders suggest as well that the deposit is relatively young. The average of 36 Cl 190 ages, excluding Vb3c as an outlier, is 1.90 ± 0.45 ka. The uncertainty of the mean is based on the cumulative probability of uncertainties for all samples based on a Gaussian probability distribution (one sigma level). Such a value indicates that the rock avalanche, considering the error range, occurred during historical times, between 340 BC and 560 AD. These results are in stark contrast to previous reconstructions, which pointed to a Lateglacial age (Mazzuoli, 1875;Hoernes, 1892;Squinabol, 1902;Dal Piaz, 1912;Venzo, 1939;Genevois et al., 2006;Pellegrini et al., 2006). The date 1113, 1114, 1117 AD proposed for 195 the main landslide event suggested by some authors (Piloni, 1607;Miari, 1830) may be associated to the Verona earthquake at 1117 AD. That event was clearly felt in the Belluno area, where it did cause several rockfalls (Guidoboni et al., 2005), but its age is not consistent with the cosmogenic dates on the Masiere di Vedana. To search for independent constraints for the age of the main landslide event, a detailed research in numerous archives and chronicles was undertaken. This area during Roman time was largely and uniformly inhabited by "incolae" for agricultural aims, being the area located next to the Claudia Augusta

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Altinate road connecting Feltre with Belluno ( Fig. 1) (Alpago-Novello, 1957, 1988. The presence of a Roman bridge crossing the Cordevole River north of the Mt. Peron indicates there was a connection to the main Claudia Augusta Altinate road. While numerous archeological Neolithic and Roman sites are reported around the Masiere di Vedana (Capuis et al., 1988;Frassine et al., 2016), no Roman or pre-Roman archaeological remains have been found within the rock avalanche deposits (Fig. 3). If the Masiere di Vedana deposit was settled after Roman times, previous settlements eventually located in the area would have been 205 buried by the event. The oldest record for the post-event human presence is a hospice built in the 12 th century AD (1155 AD; Magoga and Marin, 1998) on the fluvial terrace near the village of S. Gottardo (Fig. 3). Therefore, historical data indicate a time frame between Roman times and the Middle Ages.
The age uncertainties do not allow to directly determine if the Masiere di Vedana deposit was due to a single failure or multiple events. The distribution of the available dates has no spatial pattern across the deposit, no physical boundaries occur, no 210 buried soil layers have been found within the deposits. Moreover, the runout of the landslide indicates a single huge catastrophic event. Therefore, a single rock avalanche occurred in historical time contradicting previous interpretations (e.g., Genevois et al., 2006;Pellegrini et al., 2006) and asking for a re-evaluation of the landslide hazard.  (Figs. 3, 7a).
Topographic highs, like Castel Cuch and Roe Alte, were at that time mantled with glacial sediments attributable to the last glaciation.
The rock avalanche involved the detachment of about 130 Mm 3 from the southern face of Mt. Peron. Initial movement was 220 a sliding along the NW vergent backthrust-related planes (Fig. 2). En bloc movement may have been only briefly sustained as the pervasive network of fractures favored a massive collapse. The rock mass immediately evolved into a rock avalanche whose volume grown by fragmentation up to 170 Mm 3 and spread out onto the flat plain below. The H/L of~0.2 (apparent friction angle of 11°) marks the Mt. Peron event as extremely mobile, for example in comparison to the Fernpass rock avalanche which has a H/L of 0.9, a volume 1 km 3 and a significantly longer runout distance of 15.5 km (Prager et al., 2009). It may be possible 225 to glean information about the failure style from the distribution of boulder lithologies, which follows the stratigraphic order of the bedrock exposed in the source area and has been as well noted at the Tschirgant rock avalanche deposits in Austria (Dufresne et al., 2016) and the Frank slide in Canada (Charrière et al., 2016). In the Mt. Peron bedrock, the lithologic sequence from west to east is: Calcari Grigi Group, Vajont Limestone, Fonzaso Fm., Rosso Ammonitico and Maiolica. This pattern is and Roe by Fonzaso Fm., Rosso Ammonitico and Maiolica. Experiments and modelling suggest that this kind of zonation is likely to occur when the sliding mass propagates as a flexible sheet, with laminar flow (Friedmann et al., 2006). Several landforms within the Masiere di Vedana provide further clues on the processes of propagation and emplacement.
The tomas in the Torbe sector suggest differential velocities in the moving mass propagating on a water-saturated substrate Prager et al., 2009;Dufresne, 2012;Dufresne et al., 2016;Aaron et al., 2017). Tomas, with likely similar origin, Recently, More and Wolkersdorfer (2019) proposed for the Toma Hills at Fernpass an alternative origin from internal erosion by suffusion. However, at Masiere di Vedana the fluvial deposition above the rock avalanche suggests that suffusion process can be ruled out. In contrast to the increased mobility seen in the Torbe sector, in the central Masiere area, landforms indicative of stalling are present (Fig. 6). The stacked sub-parallel transverse ridges, much like those noted at Tschirgant (Patzelt, 2012;240 Dufresne et al., 2016;Ostermann et al., 2017), with slight overrunning of the ridges in front by those behind, indicate slowing down of the moving mass due to longitudinal compression (Nicoletti and Sorriso-Valvo, 1991;Dunning et al., 2005;Dufresne et al., 2016). Outcrop relationships (Fig. 6) suggest that the Pleistocene conglomerate inhibited the rock mass flow, in combination with the slight uphill gradient. The ridges at Masiere were previously interpreted as neotectonic lineaments by Baggio and Marcolongo (1984). 245 After the event, the Cordevole River changed its channel several times. The rock avalanche blocked the river, creating accommodation space to the north, where possibly a temporary lake formed. The river was then forced to flow westward across the deposit as indicated by the paleochannels in the Vedana and Torbe sectors (black arrows, Fig. 3), taking different paths at different times. Low-lying areas were progressively filled, as shown by the fining upwards sequence recorded in core TB1 (Supplementary Material Figure SM2a). The Torbe, Vedana and Peron terrace are flat surfaces at~380 m a.s.l. (Fig.   250 7b). Afterwards, a further sedimentation was hindered by the trenching in Torbe. The Cordevole river finally breached the landslide deposit to the southeast, through the Castel Cuch ridge made of Cenozoic rocks (Fig. 7a). The river initially flowed from Mas (about 375 m a.s.l.; green line in Fig. 7a) to the southern flank of Castel Cuch as suggested by the still recognizable paleochannel (Fig. 3) filled with well sorted, medium-to coarse-grained sand (Caneve, 1985). Subsequently, the Cordevole moved to the eastern side of the Pleistocene conglomerate cliff (Fig. 3). The final diversion of the river formed the Peron, Mas 255 and Vignole terraces and currently flows some meters below with upstream migration of the knickpoint (Fig. 7).

Driving factors and possible future hazards
In the Cordevole and Piave Valleys many landslides have been recorded (Fig. 8) and have caused a great deal of damage and casualties (Rossato et al., 2018). Moreover, rock avalanches such as Masiere di Vedana are difficult to predict (Hungr, 2006) and may be very destructive due to their huge volume and extreme runout (Guzzetti, 2000;Hungr, 2004;Geertsema et al., 2006;260 Evans et al., 2007;Sosio et al., 2008;Cui et al., 2011;Hermanns and Longva, 2012). In the light of the results we obtained, the search for the drivers of the Masiere di Vedana rock avalanche is both timely and imperative. Even if what determines the moment of failure may be difficult to pinpoint, increased pore pressure and seismic ground shaking are primary candidates in such cases (Wieczorek, 1996;Schuster and Wieczorek, 2002;Takahashi, 2001). However, rock avalanches may start without a definite trigger, as for example the Tsatichhu landslide (10 th September 2003) in Bhutan (Dunning et al., 2006) and the several 265 Randa events (total of 30 Mm 3 ) in 1991 in Switzerland (Loew et al., 2012). Failure normally occurs when in the rock mass resisting forces weaken till the driving forces overcome them (factor of safety ≤1; Glade and Crozier, 2005).
The Belluno area has a high mean annual rainfall (1643 mm in the time interval 1994-2018 at https://www.arpa.veneto.it/datiambientali/open-data) and is prone to extreme rainfall events (e.g., >300 mm of rain at Sospirolo during a single event: 27 th October -1 st November 2018; ARPAV, 2018). Moreover, at the time of the Masiere di Vedana rock avalanche, soon after the 270 beginning of the Christian Era, the eastern European Alps and NE Italy were affected by various periods of climate degradation during which several extreme meteorological events occurred (Wirth et al., 2013;Rossato et al., 2015). One of these extreme events had an impact all over Europe between 50 and 250 AD, with marked intensity and widespread flooding recognizable in the stratigraphic records (Macklin et al., 2006;Benito et al., 2015;Rossato et al., 2015). This event of severe rainfall could be a possible trigger for the Masiere di Vedana rock avalanche or, at least may have acted as a driving and destabilizing factor.

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Likewise, the Val Pola rock avalanche in the central Alps was triggered by a period of exceptional rainfall (Crosta et al., 2004).
The Veneto region is prone to earthquake activity and is categorized as level 2 seismic hazard ("possible strong earthquakes" in Ordinanza del PCM n. 3519/2006), as the historical record testifies (up to~Mw=6.5;Vigano`et al., 2013Vigano`et al., , 2015Rovida et al., 2016). Continuous instrumental monitoring of the Belluno area dates back only to 1977 (Sandron et al., 2014), preceding cataloged major seismic events in the region are based on either historical chronicles, dated building damages and/or observed 280 rockfalls (Piloni, 1607;Taramelli, 1883;Guidoboni et al., 2005Guidoboni et al., , 2018. Within a radius of 30 km from Mt. Peron, eight earthquakes with Mw greater than 5.0, and one exceeding 6.0, are listed (Fig. 1). The strongest (Mw = 6.3) in the nearby area, occurred just 20 km to the east of Mt. Peron on 29 th June 1873 (Rovida et al., 2016, and references therein). Severe damages to Belluno city were reported during the Asolo (25 th February 1695 AD; Mw=6.4) and Verona (3 rd January 1117; Mw=6.5) earthquakes whose epicenters were located, respectively, 60 and 140 km away. As for the time frame suggested by 285 our chronology, historical records report an important seismic event at July 365 AD with damages to the city of Belluno (Piloni, 1607). These data suggest that the Belluno area is sensitive to seismic shakings originating even hundreds of km away. In the Alps, earthquakes have been suggested as triggers for several rock avalanches (e.g., Grämiger et al., 2016;Ivy-Ochs et al., 2017;Köpfli et al., 2018). The most important effect of the frequent seismic activity is the progressive increase in the rock fatigue, with the formation and subsequent weathering of failure planes (Friedmann et al., 2003;Brideau et al., 2009;Parker 290 et al., 2013;Preisig et al., 2015;Gischig et al., 2016). Where these intersect, rock dissolution and the formation of caves is favored (Filipponi et al., 2009;Sauro et al., 2013), further weakening the mechanical properties of rocks (Pánek et al., 2009;Gutierrez et al., 2014). The Mt. Peron southern wall is known locally as the "weeping rock" due to the numerous caves and karst springs along the steep rock face (Fig. 2b).
In a general view, active tectonics and related accumulated fatigue have been suggested to contribute to intensification of 295 slope instability registered in the Belluno Dolomites during the last 1500 yr (Galadini et al., 2005). The Belluno Dolomites experienced a long deformation history since the Miocene, related to regional-scale stress connected to the counter-clockwise rotation of the Adria plate, indented with the Alpine orogeny (Márton et al., 2003;D'Agostino et al., 2008). Such forces overturned the bedding, formed the thrusts and backthrusts (WSW-ENE oriented), the two conjugate fracture sets (NW-SE oriented) and led to re-activation of the Jurassic faults (N-S oriented). The belt characterized by these deformations lies between 300 the Belluno thrust and the Val Carpenada -Val di Vido -Val Madonuta backthrust (Fig. 1), and extends from the Piave Valley to the east to the Caorame Valley to the west (Bosellini et al., 1981;Masetti and Bianchin, 1987;Bigi et al., 1990;Costa et al., 1996, Fig (Figs. 1, 9), and deserve focused hazard evaluation. The Piz Vedana (1324 m a.s.l.) should be mentioned as well, because even if lower than the other peaks, it looms over the artificial Lake Mis (Fig. 3). A massive rock failure that would hit the lake or damage the dam may pose a serious threat, possibly triggering a tsunami, as happened for instance at Vajont (e.g., Borgatti et al., 2004). Peron. The deposit extends over an area of 9 km 2 , with a total volume of~170 Mm 3 . A H/L ratiõ 0.2 is calculated, marking it as extremely mobile, which is also shown by the maximum runout of 6 km. Geomorphological, stratigraphic and historical evidence when combined with cosmogenic 36 Cl exposure ages, mean age 1.90 ± 0.45 ka, point to a single event that occurred in or after late Roman times but before the Middle Ages.
The steep rock wall on the south face of Mt. Peron shows a pervasive deformation; numerous fractures and faults cross-cut the sub-vertical to slightly overturned carbonate Mesozoic bedrock. The WSW-ENE directed backthrust planes, which are the most continuous ones, constituted the planes along which the rock mass initially slid, rapidly breaking-up and evolving into a rock avalanche.

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The stratigraphic sequence is preserved in the rock avalanche deposit. Lithologies that presently constitute the western part of the source area, were deposited in the proximal sectors (Vedana, Torbe), while the more easterly outcropping ones reached the distal areas (Masiere, Roe Alte). Landforms of the deposit suggest differential velocities during emplacement. In the NW sector (Torbe) enhanced mobility likely due to interaction with water-saturated path material is evidenced by the numerous ENE-WSW aligned tomas. In contrast, in the middle sector (Masiere) stacked transverse ridges point to stalling, perhaps 325 related to the gentle uphill gradient and impeded propagation over Pleistocene conglomerates. Post-event evolution comprises formation of backwater alluvial terraces and the wandering of the Cordevole River in the rock avalanche deposits, with incision and aggradation phases.
Identified pivotal drivers are the overall structural setting, exceptional rainfall events and seismic shakings. Their combination produced a pervasive fracturation and weathering of the rock mass, with progressive increase of rock fatigue. No 330 exceptional event may actually be required for such rock avalanches to occur, as accumulation of damage markedly lowers the energy needed to trigger failure.
Today, in the area between the Belluno thrust and its backthrusts from the Caorame to the Piave Valleys, the hazard for the failures of large blocks, prisms or larger rock volumes needs re-evaluation. The occurrence of a huge event like the Masiere di Vedana rock avalanche has to be considered.

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Data availability. All data are in the paper or in the supplemental material.
Author contributions. All authors contributed to discussion, field survey, data collection and improving the text, that has been written mostly Alfimov, V., and Ivy-Ochs, S.: How well do we understand production of 36Cl in limestone and dolomite?, Quat. Geochronol., 4 (6)