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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
573
DOI: 10.4408/IJEGE.2013-06.B-55
GEOLOGICAL STRUCTURES OF THE VAJONT LANDSLIDE
M
attteo
MASSIRONI
(*)
, D
ario
ZAMPIERI
(*)
, L
aura
SUPERCHI
(*)
,
a
nDrea
BISTACCHI
(**)
, r
iccarDo
RAVAGNAN
(*)
, a
LessanDro
BERGAMO
(*)
,
M
onica
GHIROTTI
(***)
& r
inaLDo
GENEVOIS
(*)
(*)
Università di Padova - Dipartimento di Geoscienze - Padova, Italy
(**)
Università degli Studi di Milano Bicocca - Dipartimento di Scienze dell'Ambiente e
del Territorio e di Scienze della Terra - Milano, Italy
(***)
Università di Bologna - Dipartimento di Scienze Biologiche, Geologiche e Ambientali - Bologna, Italy
K
ey
words
: Vajont, rockslide, folds, interference pattern,
flexural slip, thrusts
INTRODUCTION
The Vajont landslide history represents a dramatic
example of the incompleteness of the investigations car-
ried out before the 1963 event on some relevant geologi-
cal aspects. It was indeed a sequence of natural events
and misconceived technical operations leading to the
catastrophic landslide, an event for which the complex
combination of the geological factors involved have not
yet lead (50 years after the catastrophe) to an unambigu-
ous and complete explanation of the phenomenon.
The Vaiont landslide occurred on the southern limb
of the Erto syncline, which is dipping 30° to 50° to-
wards the north-northeast and north, deeply reworking
a former paleo-landslide covering the northern slope
of the Monte Toc (G
iuDici
& S
eMenza
, 1960; s
eMen
-
za
, 2010). The sliding surface is localized within the
middle-upper Jurassic Fonzaso formation, a sequence
of thin-stratified limestones with thin (0.1-5 cm) inter-
calations of clays (H
enDron
& P
atton
, 1985).
A focal point to explain the kinematics of the land-
slide is the adoption of a reliable geological model that
takes into account the highly three-dimensional charac-
ter of the geological structures of the northern slope of
Monte Toc. The knowledge of the geometry and shape
of the sliding surface at large scale, minor structures
such as folds and steps and the rock mass characteriza-
tion of all the involved lithological units, represents the
ABSTRACT
The Vajont rockslide is not the largest rockslide
in the Italian Alps, but it is a reference at a worldwide
scale due to its complex behaviour and catastrophic ef-
fects in terms of economic losses and human casualties.
An essential aspect to be considered in any kind
of approach to the Vajont landslide, whether hydrogeo-
logical, geomorphological or related to geomechanical
modelling, is the highly three-dimensional character
of the geological structures of the Monte Toc northern
slope. The knowledge of the geometry and shape of the
sliding surface as well as of minor structures, such as
folds and steps, represents the starting point for any
subsequent analysis and interpretation of the landslide.
In order to reach an in-depth evaluation of the
structural setting, remote sensing analyses (LIDAR
and photogrammetric DTMs) integrated by field data
surveys were applied. Our results have clarified the
structural relationships between major folds and faults
affecting the Monte Toc slope and shown at various
scales the primary importance of tectonic folds in
controlling the kinematics of the 1963 event. Two
pre-existing fold systems (ca. E-W and N-S trending),
deforming the sliding surface, and associated minor
structures (small scale faults and fractures) have most
likely conditioned the sliding process. Particularly the
concave shape of the sliding surface, reflecting a ma-
jor syncline (Massalezza syncline), leads to the sepa-
ration of the landslide into two distinct blocks with
different kinematics.
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M. MASSIRONI, D. ZAMPIERI, L. SUPERCHI, A. BISTACCHI, R. RAVAGNAN, A. BERGAMO, M. GHIROTTI & R. GENEVOIS
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
GEOLOGICAL FRAMEWORK
GEO-STRUCTURAL SETTING
The study area belongs to the eastern Southern
Alps domain, which represents the Neogene-Present
back-thrusted (S-vergent) part of the Alpine chain.
The Vajont valley coincides with the core of an Al-
pine syncline (Erto syncline) with an E-W to WNW-
ESE trending axis (F
erasin
, 1965; R
iva
et alii, 1990)
gently plunging towards the E (B
roiLi
, 1967) (Figs.
1, 2). The deformed rocks are Liassic to Eocene car-
bonates and marls (S
eMenza
, 1965; M
asetti
, 1986).
The Erto syncline lies on the hanging wall of a main
structure of the Venetian Alps (Belluno thrust, D
og
-
Lioni
, 1992) and it is paired with the trailing limb of
a main frontal asymmetric anticline located south of
the study area. The shape of the Erto syncline is very
asymmetric, because the northern limb is reversed
and stretched, lying just at the foot of the Mt. Borgà
and Spesse thrusts (Mt. Salta thrust in R
iva
et alii,
1990), two older thrusts passively transported on the
back by the Belluno thrust. Therefore, the Erto syn-
cline can be referred to as a recumbent fold with a
gently dipping axial plane (F
Leuty
, 1964).
The sliding layered sequence was laterally con-
strained by a system of subvertical faults (Croda Bianca
and Col Tramontin Lines to the east and west branch
of the Col delle Erghene Line to the west), while the
rockslide crown was constrained by E-W structures
(Col delle Erghene Line; R
iva
et alii, 1990) (Figs. 1, 2).
STRUCTURAL CHARACTERIZATION
In order to define the relationships among tec-
tonic setting, rockslide, and topography of the Va-
jont landslide, a morpho-structural investigation of
the whole area has been performed. The analysis
was carried out through the integration of traditional
structural field investigations with remote sensing
techniques. In particular, the remote sensing analy-
sis was carried out on air-photos and DTMs derived
from photogrammetry and recent LIDAR acquisi-
tions, whereas the conventional structural field sur-
vey was aimed to describe and interpret brittle and
folding deformations. The application of remote
sensing techniques was particularly suitable to the
Vajont case study due to the steep and inaccessible
rock face at the head scarp and sliding surface.
The information collected in the field concerns dips
and dip-directions of bedding planes, faults and frac-
starting point for any subsequent analyses and interpre-
tation of the landslide.
Indeed many authors have illustrated the impor-
tance of considering the influence of structural features
like faults and fractures on the development of large
rock slope instabilities (A
gLiarDi
et alii, 2001; A
Mbrosi
& C
rosta
, 2006; b
riDeau
et alii, 2009; J
aboyeDoff
et
alii, 2009). M
assironi
et alii (2003, 2011) and Di L
uzio
et alii (2004) have also demonstrated the importance of
the interplay between the orientation of folds and faults
with respect to the slope in controlling large gravitation-
al mass movements.
In this paper we show how the applied remote sens-
ing techniques (analyses of LIDAR and photogrammetric
DTMs) along with the geo-structural field investigation,
allowed to characterize in detail the structure of key areas
inside and outside the landslide and to clarify relevant
aspects concerning the structural setting of the Vajont
valley with particular regard to northern slope of Monte
Toc. All of the identified faults and folds systems in the
Vajont area reflect the regional tectonic pattern, here in-
terpreted as the result of a superposition pattern of differ-
ent deformation stages. Hence, the location and geometry
of the Vajont rockslide are controlled by regional tectonic
features. In particular, the interference between two fold
systems strongly affects the Vajont sliding surface and is
fundamental to evaluate the rockslide kinematics.
Fig. 1 - Structural sketch of the Vajont area (modified after
R
iva
et alii, 1990)
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GEOLOGICAL STRUCTURES OF THE VAJONT LANDSLIDE
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
575
ticline where it merges into the E-W to WNW-ESE
oriented Erto syncline (R
iva
et alii, 1990; B
roiLi
,
1967; H
enDron
& P
atton
, 1985; D
ogLioni
& C
ar
-
Minati
, 2008; g
enevois
& g
hirotti
, 2005). At the
scale of the entire Monte Toc northern slope another
major fold has been revealed by our field analysis,
photogrammetric data (W
oLter
et alii, submitted)
and LIDAR DTMs, that have highlighted an appre-
ciable difference in the average attitude of the bed-
tures and trends and plunges of fold axes. Due to the
openness of most of the fold hinges, the latter meas-
urements have been mainly derived by fold limbs at-
titudes. All structural data were plotted stereographic
projections (Fig. 3).
FOLDS
The shape of the Monte Toc northern slope is the
direct expression of the back-limb of the Belluno an-
F
ig. 2 - Geological map modified after R
ossi
& s
emenza
(1965) and R
iva
et alii (1990). Stratigraphic sequence redrawn after
R
ossi
& s
emenza
(1965) and P
aRonuzzi
& B
olla
(2012)
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M. MASSIRONI, D. ZAMPIERI, L. SUPERCHI, A. BISTACCHI, R. RAVAGNAN, A. BERGAMO, M. GHIROTTI & R. GENEVOIS
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
The folds that can be easily recognized, even from a
great distance (Fig. 4), on the entire sliding surface con-
sist in a series of E-W to WNW-ESE structural terraces
(sensu t
Wiss
& M
oore
, 1992, Fig. 11.13) and mono-
clines (see also b
roiLi
, 1967 and h
enDron
& P
atton
,
1985), and of frequent N-S to NNW striking undula-
tions (axes average trend/plunge: N010°/40°), often
controlling gulley incisions, particularly in the western
lobe of the sliding surface (see also h
enDron
& P
at
-
ton
, 1985). The E-W to WNW-ESE folds affecting the
Fonzaso beds frequently display flexural slip processes
generating meso-scale flat-ramp thrusts verging toward
the south (Fig. 4). This kinematics, developing in con-
trast to the local gravitational gradient, unambiguously
demonstrates the tectonic origin of these folds, ruling
out other interpretations that would explain their nucle-
ation uniquely as a consequence of the shear stresses at
the base of the sliding mass (either the paleo-landslide
or the 1963 event; see for example c
arLoni
& M
azza
-
nti
, 1964 a, b and P
aronuzzi
& b
oLLa
, 2012).
planes at the eastern and western parts of the sliding
surface (Fig. 3). Indeed strata have an average dip of
45° and dip direction N 340° in the eastern side, 35°/
N360° in the middle and 35°/N020° in the western
sides delineating an open syncline with a hinge in
correspondence to the Massalezza Stream. This fold,
from here on named Massalezza syncline, accounts
for the overall concave shape of the sliding surface
and the different average attitudes of the strata on the
sliding surface already pointed out also by s
eMenza
(2010) and P
aronuzzi
& b
oLLa
(2012). The structur-
al complications related to the interference between
the parasitic minor folds of the Erto and Massalezza
synclines are responsible of the undulation and fold-
ing of the sliding surface which was generally at-
tributed to gravitational processes (e.g. h
enDron
&
P
atton
, 1985; P
aronuzzi
& b
oLLa
, 2012) or even
neglected. Only b
roiLi
(1967) recognized the tec-
tonic origin of these features, although he did not
provide any structural explanation of them.
Fig. 3 - Stereo-plots (equal-angle, lower-hemisphere) showing the average attitudes of strata and joints in the different do-
mains (1 to 5) of the Vajont landslide and surroundings
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Editrice
577
Hence, the meso-scale structural pattern is dominated
by superposed folding in the lower-central area of the
The most striking E-W structural terrace (Fig.4 a)
runs through the eastern lobe, where detrital talus rests
at the middle of the slope. The photogrammetric analy-
sis and field survey have shown that there the strata
change their dip from 45° in the upper part, to 30° at
the terrace and 50° in the lower flank. As well as the
meso-scale structures with a similar asymmetry, also
this structural terrace is likely the product of a hidden
south-verging flat-ramp-flat thrust induced by flexural
processes (Fig. 4 b, c, d). Indeed, the terrace is the up-
hill boundary of an evident step on the sliding surface
where a stratigraphic jump is recorded. This suggests a
gravitational reactivation of the pre-existent flat-ramp-
flat thrust system (Fig. 5). To the west the E-W struc-
tural terrace interferes with a N-S trending antiform
plunging to the north with a dip angle of about 40°.
This represents a spectacular large scale example of a
refolded fold which, in our opinion, is responsible of
a major N-S trending step separating the western lobe
of the sliding surface from the eastern one east of the
Massalezza stream (Fig. 4 a).
According to the usual parasitic folding behavior,
the N-S fold system increases its frequency approaching
to the hinge zone of Massalezza syncline and similarly
the E-W to WNW-ESE monoclinal folds increase at
the base of the slope close to the Erto syncline hinge.
Fig. 5 - Sketch showing the development of a structural
terrace by tectonic deformation and the subse-
quent inversion of the displacement along the
thrust surface by gravitational sliding. Fw ramp:
footwall ramp
Fig. 6 - a) Meso-scale refolded folds in the sliding surface
of domain 2; b) Meso-scale refolded folds in the
sliding surface of domain 3 (continuous line =
hinges of the Massalezza syncline fold set; dashed
line = hinge of The Erto syncline fold set); c) Top:
Interference pattern after R
amsey
(1967) (left) and
T
hiessen
& m
iens
(1980) obtained by considering
the average axes trends and axial planes of the
Massalezza syncline and Erto syncline sets. Bot-
tom: Contour plot (equal-area lower-hemisphere)
of the fold axes on the sliding surface
Fig. 4 - a) Eastern lobe of the Vajont sliding surface show-
ing a structural terrace with a syncline-anticline
couple; b) close view of the Fonzaso
Fm. layers
showing flexural slip thrusts with ramp-flat ge-
ometries and associated folds with a southern
vergence (anti-gravitational); c) interpretation
of the structural terrace of the Vajont eastern
lobe; d) flexural slip mechanism operating
within a syncline
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
sliding surface where both the parasitic folds of the
Massalezza and Erto synclines increase (Fig. 6a, b). The
interference among folds is also testified by the local
dispersion of fold axes revealed by the structural meas-
urements acquired in the field (Fig. 6c). Refolding proc-
esses becomes particularly relevant at the base of the
Massalezza stream. Despite the considerable dispersion
of the data, two dominant axial trends are well recogniz-
able: E-W to WNW-ESE and N-S to NNW, perfectly
consistent with the two major folds (i.e. Massalezza and
Erto synclines). Considering the representative orienta-
tions and geometries of the two fold sets and taking into
account the tilting process due to the Belluno ramp on
the N-S to NNW-SSE folds plunges, we have derived
the interference pattern according to the r
aMsey
(1967)
and t
hiessen
& M
iens
(1980) classifications obtaining
type 1-2 and type K interference patterns , respectively
(Fig 6c). This means “domes and basins” to “crescents
and mushrooms” type refolded folds deforming the slid-
ing surface at the meso-scale (Fig. 6c). These structures
gave rise to the observed fold axes dispersion which
have led several authors (e.g. b
roiLi
, 1967; P
aronuzzi
& b
oLLa
, 2012) to the misleading interpretation of a co-
existence of many fold sets, being actually two (N-S to
NNW-SSE and E-W to WNW-ESE).
The significant wideness of the two major syn-
clines as well as the north dipping orientation of the
Massalezza one exclude any gravitational genesis. The
north dipping folds and undulations must be unrelated
to any gravitational phenomena on the north facing
Monte Toc slope by definition, whereas the consider-
able scale of several E-W to WNW-ESE monocline
folds and the anti-gravitational vergence associate to
several of them similarly rule out a direct relation with
gravitational sliding processes. The strict consistency
between the meso-scale folds with the two major syn-
clines and the resulting interference structures confirm
the tectonic origin of many plicative deformations of
the area. Hence, most of the meso-scale folding, affect-
ing in particular the weakest and clay-rich horizons of
the succession (Fonzaso formation), is no doubt due to
the regional tectonic deformations and therefore pre-
existed any sliding event. Some E-W to WNW-ESE
folds are reworked by gravitational sliding (Fig. 5),
but the recent suggestion that all the meso-scale folds
should represent remnants of a ductile deformation
along a supposed thick shear zone at the base of the
paleo-landslide (P
aronuzzi
& b
oLLa
, 2012) does not
find an adequate support from the field evidence.
Looking at the pre-1963 geological map by r
ossi
& s
eMenz
a (1965), it appears clear that the accumula-
tion of the paleo-landslide is deformed onto two open
synclines E-W oriented and with a core made up of
the upper members of the Calcare del Soccher unit
and separated by a narrower anticline. This setting
does not change much in the nowadays framework as
testified by the post-1963 geological map of r
ossi
&
s
eMenza
(1965) (Fig. 2). It is most probable that the
southern syncline finds its eastern prosecution onto
the small syncline enclosed between the Col di Tra-
montin and Croda Bianca faults (Fig. 2). If this is the
case, the related axial trace is a good marker to obtain
a rough estimate of displacement of the upper portion
of the paleo-landslide that, in agreement with P
aro
-
nuzzi
& b
oLLa
(2012), but with different motivations,
could be estimated at about 500 m.
The N-S to NNW folds as well as the E-W to
WNW-ESE ones, are detectable in both the paleolan-
dslide and 1963 gravitational accumulations (Fig. 1).
In particular, meridian folds have been revealed by
undulation of some geological limits in the pre-1963
geological map of r
ossi
& s
eMenza
(1965), histori-
cal photographs (r64-36, r63-15, r63-11
in
M
asè
et alii, 2004), meso-strucutural data (b
roiLi
, 1967
and this work) and
o
n recent geophysical surveys
(f
rancese
et alii, 2013).
The coincidence between the fold trends on the
slid mass with the ones of the sliding surface supports
the tectonic nucleation of most of the folding in the
area as well as the “en-mass” displacement of the
paleolandslide and 1963 gravitational events
FAULTS
The structural analysis of brittle deformations
has been focussed along the two major fault systems
enclosing the Monte Toc slope: the Col delle Tosatte
fault and the Croda Bianca-Col Tramontin system.
The Col delle Tosatte fault lies on the Piave valley left
slope cropping out in the Vajont gorge downstream of
the dam and is classically interpreted as an high angle
normal fault bounding the eastern margin of what has
been supposed a graben and named after Longarone
(s
eMenza
, 1960; r
iva
et alii, 1990). By contrast, our
field observations, historical photographs (A-11 and
A-25 in M
asé
et alii, 2004) and new kinematic data
have unambiguously revealed that the Tosatte fault is
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GEOLOGICAL STRUCTURES OF THE VAJONT LANDSLIDE
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
579
cal reconstructions suggest a reverse kinematics of the
Croda Bianca fault associated to steep or folded strata
beds at its footwall and open folds at its hanging-wall.
Therefore the Col Tramontin fault can be interpreted as
a high angle hanging-wall splay of the Croda Bianca
reverse fault, later reactivated as the eastern boundary
of the Vajont landslide.
In summary, the slope interested by the rockslide,
i.e. the southern limb of the Erto syncline, is enclosed
between two downward converging faults with a high-
ly reverse component of displacement (Croda Bianca-
Col Tramontin system to the east and Col delle Tosatte
fault to the west). The deformation along these two
conjugate fault systems plus the reverse activity along
the Belluno flat-ramp-flat thrust accounts for the two
main fold systems, showing a characteristic interfer-
ence pattern, recorded on the sliding surface.
JOINTS AND FRACTURES
On the basis of the structural framework recog-
nized in the Vajont area, the area was divided into five
main structural domains (Fig. 3). Domains 1 and 3,
which are respectively the eastern and western flanks
of the Massalezza syncline and correspond to the two
lobes of the sliding surface, are characterised by rela-
tively undulated bedding planes with some steps and
discontinuities. Domain 2, located near the centre of
the sliding surface, where the slope is carved by the
Massalezza gully, is the most complex since it is lo-
cated where the fold hinge of the Massalezza syncline
interferes with the Erto syncline. Domain 4 includes
the deposit area and Domain 5 is constituted by the
surroundings not affected by the 1963 event. In turn
Domain 5 has been subdivided into 5a and 5b. Do-
main 5a is characterized by bedding planes dipping
to the East, reflecting the eastward plunge of the Erto
syncline. Domain 5b is finally the one affected by the
Croda Bianca and Col Tramontin faults.
In the whole study area, 9 discontinuity sets (both
joints and fractures), have been recognized based on
their orientation (Fig. 3). The domains in the bedrock
are generally characterized by 4 or 5 systems (Fig. 3).
Among these the steep N-S striking ones (K3 and/or
K5) are common to all the domains and are related
either to the folding process responsible of the N-S
striking folds or to fracturing accompanying the major
N-S fault systems (Croda-Bianca, Col di Tramontin
and Col delle Tosatte faults). The conjugate NW-SE
actually dipping toward the east and is a westward-
directed reverse fault. This fault is associated to a
ramp anticline deforming the Liassic Igne formation,
at present overlying the Cretaceous Soccher sequenc-
es (Fig. 7). At the Col delle Tosatte fault footwall a
minor splay has been also found; it is associated to a
ramp anticline in the hanging wall and an asymmetric
syncline in its footwall, both involving the Soccher
sequence and Scaglia Rossa formation (Fig. 7). Pos-
sibly, the frontal part of this secondary ramp anticline
was previously misinterpreted as the morphological
evidence of a normal fault dipping toward the Piave
valley (r
iva
et alii, 1990).
Both the Col Tramontin and Croda Bianca faults
are traditionally reported as vertical in the geologi-
cal sections (b
esio
& s
eMenza
in r
iva
et alii, 1990)
and much more attention has been always paid to the
former one, because it bounds to the east the 1963
Vajont landslide mass (Semenza, 2010; P
aronuzzi
& b
oLLa
, 2012). However, as already pointed out by
P
aronuzzi
& b
oLLa
, 2012) (2012), the steep Col Tra-
montin fault is only a subsidiary element of the much
more relevant Croda Bianca fault which, on the other
hand, clearly dips towards the west as can be easily
recognized in the r
ossi
& s
eMenza
(1965) and Besio
and Semenza geological maps (in r
iva
et alii, 1990).
The subordinate relevance of the Col di Tramontin
fault with respect to the Croda Bianca one is evidenced
by its limited extension in the same maps, an by the
absence of any displacement in the Vajont Gorge as it
was mapped in the pre-1963 landslide map by r
ossi
&
s
eMenza
(1965). Field investigations and 3D geologi-
Fig. 7 - a) Structural sketch of the right side of the Va-
jont gorge downstream of the dam; b) stereoplot
(equal-angle lower-hemisphere) and stress inver-
sion (circle: σ1; triangle: σ2; square: σ3; red ar-
rows: maximum horizontal compressional axis,
WinTENSOR by D
elvaux
& s
PeRneR
, 2003) de-
rived from kinematic data of meso-faults associ-
ated to the Tosatte line
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
and Erto synclines. The structural analysis has gov-
erned the 3D geological reconstruction of the Vajont
landslide (see b
istacchi
et alii, 2013) that, once popu-
lated with the geomechanical data, will constitute the
base of 2D and 3D geomechanical models (e.g. c
as
-
teLLanza
et alii, 2013; h
ungr
, 2013). The structural
setting summarized above lead to the following major
consequences for the Monte Toc slope evolution.
1) The Massalezza syncline and related concave
shape of the sliding surface played a major role in the
evolution of the 1963 Vajont landslide, favouring the
collapse of the two lobes (eastern and western) that
followed two slightly different and northward con-
verging sliding paths.
2) Most of the folds recorded on the sliding sur-
face pre-existed the landslide events and may have
affected the gravitational processes in different con-
current modes:
a) as meso-scale roughness/waviness, unevenly dis-
tributed on the sliding surface (since the fold fre-
quency and interference patterns increase toward
the lower Massalezza ditch);
b) through reactivation of the fold-associated flat-ramp-
flat thrusts that are primary elements favouring the
interconnectivity between the clay rich layers and
seams as well as stratigraphic jumps of the sliding
surface within the Fonzaso beds;
c) controlling the distribution of the joint sets charac-
terizing the in situ rock mass.
ACKNOWLEDGMENTS
This research was financially supported by the
University of Padova, research project GEO-RISKS
(n. STPD08RWBY_001, principal investigator R. g
e
-
nevois
). The fault inversion results were obtained us-
ing (Dos or Win)TENSOR, a software developed by
D. Delvaux of the Royal Museum for Central Africa,
Tervuren, Belgium.
systems (K1 or K4 and K7) as well as the E-W striking
ones (K6 and secondarily K1) are extremely recurrent
and primarily related to fractures and joints associated
to the folds with the same trends (compare figures 2
and 5) and faults governing the flexural slip, most of
which probably reactivated by the gravitational phe-
nomena (in particular the northward dipping ones). It
is noteworthy that the more intensively folded domain
2 is also the one with the higher number of joint sets.
In the domain 4 are still recognizable all the major sets
of the other domains (plus several others related to
the gravitational phenomena), proving once again the
“en-mass” sliding behavior of both the paleolandslide
and the 1963 events.
DISCUSSION AND CONCLUSIONS
The structural investigation has revealed that the
Monte Toc northern slope, structurally located at the
back-limb of the asymmetric Belluno anticline (south-
ern flank of the Erto syncline), is enclosed between two
N-S to NNW-SSE striking and downward converging
reverse fault systems (Croda Bianca-Col Tramontin
system and Col delle Tosatte fault). This peculiar
structural setting has led to the N-S trending Mas-
salezza syncline, which accounts for the two distinct
lobes of the Vajont landslide (eastern and western)
and is associated to a series of poly-harmonic folds.
These folds interfere with similar meso- to large-scale
folds related to the Erto-syncline and striking E-W to
WNW-ESE. Despite some gravitational reactivation,
many of these folds are associated at various scales to
south-vergent anti-gravitational flat-ramp-flat thrust
systems generated by flexural slip processes. Both
E-W and N-S folds are still visible at various scales
also on the slid mass. Most of the joints and fractures
revealed by the structural analysis on the whole area
seem strictly related to the folding and faulting proc-
esses accompanying the formations of the Massalezza
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