Deep seated gravitational slope deformation in an

Rend. online Soc. Geol. It., Vol. 22 (2012), pp. 83-85, 4 figg.
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Deep seated gravitational slope deformation in an alpine ophiolites
massif: the case of “Badia di Tiglieto” (Voltri Massif, northern Italy)
LAURA FEDERICO (*), FRANCESCO FACCINI (*), SILVIA TORCHIO (*), ANNA ROCCATI (*), LAURA CRISPINI (*),
ANDREA VIGO (*), EUGENIO POGGI (*), MARCO FIRPO (*) & GIOVANNI CAPPONI (*)
RIASSUNTO
Deformazioni gravitative profonde di versante in un massiccio
ofiolitico alpino: il caso della Badia di Tiglieto (Massiccio di Voltri, Italia
settentrionale)
Le deformazioni gravitative profonde di versante (DSGSD) rappresentano
un’importante forma del territorio, a scala intermedia tra evoluzione tettonica e
morfogenesi gravitativa. Nel corso del rilevamento geologico per il Foglio 212
“Spigno Monferrato” ne sono stati individuati diversi casi; questo contributo è
focalizzato sulla DGPV che starebbe all’origine della Piana della Badia in cui è
stata edificata nel X secolo l’Abbazia cistercense di Tiglieto, situata in alta val
d’Orba a circa 25 km a NW di Genova.
Sono stati riconosciuti numerosi indizi morfotettonici che consentono di
identificare una DSGSD che governa l’attuale assetto e dinamica di versante, e
probabilmente anche del crinale spartiacque.
Le superficie profonde di scorrimento possono rappresentare riattivazioni
di pre-esistenti discontinuità strutturali. Nel corso del rilevamento sono state
individuate diverse famiglie di faglie che, come indicano anche studi pregressi
sulla neotettonica dell’area, potrebbero rappresentare fattori predisponenti alla
tettonica-gravitativa. Altro fattore strutturale che può aver innescato il DSGSD
è rappresentato da una superficie di thrust nella parte alta del versante a monte
della DSGSD stessa, che porta peridotiti poco serpentinizzate a sovrascorrere
su serpentiniti e serpentinoscisti molto tettonizzati, che potrebbero aver
collassato a causa del forte contrasto di competenza.
KEY WORDS: deep seated gravitational slope deformation,
post-orogenic evolution, neotectonics, Voltri Massif,
Western Alps
INTRODUCTION
the low present deformation rate (mm/y, in the alpine range)
and iv) the occurrence of smaller landslides inside the
deformed mass and v) the presence of ancient collapses in the
lower part of the slope (AGLIARDI et alii, 2001).
THE “BADIA DI TIGLIETO” DSGSD
During fieldwork for the “Spigno Monferrato” (212)
quadrangle geological map (FEDERICO et alii, this congress), a
large number of DSGSD have been detected. They are more
frequently found close to the main watershed, because of the
complex geologic setting and of the neotectonic activity in this
segment of the alpine chain. Among the observed cases, we
focused on the DSGSD located in the Orba valley at about 25
km NW of Genoa (Fig. 1). This DSGSD probably originated
the plain where the Tiglieto Cistercian Abbey has been built in
the Xth century.
This area represents one of the most outstanding geological
sites of the Beigua Natural Park that belongs to the European
Geoparks network.
We identified for the first time this DSGSD: in the recent
investigations aimed at land planning and management and in
the italian landslides project some inactive landslides are
actually recognized along the slope (REGIONE LIGURIA, 2004),
but the sackung-type DSGSD that involved the whole ridge,
watershed, valley floor system has not been recognized.
Deep seated gravitational slope deformation (DSGSD)
represent an important landform, at an intermediate scale
between tectonic evolution and gravitative morphogenesis.
They have received growing attention only in the last few
decades, but their understanding bears important scientific and
applicative implications.
DSGSD genesis is usually associated to lithological
features, tectonic styles, geomorphological dynamics and
quaternary climatic variations. Their identification, not always
univoque, often relies on: i) morphological elements similar to
those present in fine soil landslides, but at a bigger scale; ii) the
large scale of the structure, of the same order of the slope; iii)
_________________________
(*) Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, C. so
Europa 26, 16133 - Genova (Italy)
Fig. 1 – Location of the studied area.
The Abbey plain is an almost flat area of about 0,3 km2
where “ancient terraced alluvial deposits” are reported in the
Lavoro eseguito nell’ambito del progetto "Cartografia Geologica del Foglio
Spigno Monferrato" della Regione Liguria.
83
84
2
F EDERICO ET AL .
Fig. 2 – Geological schema of the studied area.
Sheet 83 “Genova” of the Geological Map of Italy.
The hilly area upstream has low to moderate slope (less than
25%) and falls inside serpentinites of the Voltri
tectonometamorphic Unit (CAPPONI AND CRISPINI, 2008), with
minor occurrences of metabasites and eclogitic metagabbros
(Fig. 2). Recent detailed fieldwork accomplished in the area
(about 3 km2) between the ridge at about 800 m s.l.m. and the
Orba riverbed (about 360 m s.l.m.) revealed the occurrence of
very small outcrops of serpentinites, metabasites and eclogites
along the slope, whereas peridotite and serpentinite crop out on
top of the ridge. Locally ultramafites are overlaid with Upper
Eocene – Lower Oligocene (?) continental breccias (Brecce di
Costa Cravara).
Many morphotectonic elements which belong to the
following main groups (PEROTTI et alii, 1988), have been
recognized: i) ridges (straight ridges, plano-altimetric ridge
discontinuities, peaks alignment), ii) scarps (edge degradation
and/or landslide scarps, edge of erosion scarp or alluvial
terrace), iii) streams (fluvial elbow, straight valleys, upriver
confluence, straight riverbed stretch, anomalies in the stream
longitudinal profile), vi) high erosion, vii) general character
(saddles, poligenetic fans, landslides, reverse slopes, close
depressions and many alignments).
From all these morphological elements we identified a
DSGSD that controls the present slope setting and slope
dynamics; it likely involves also the ridge watershed thus
falling in the rock-flow type (DIKAU et alii, 1996) or in the
rotational sagging, possibly double sided (HUTCHINSON, 1988).
Deep sliding surfaces can represent the reactivation of preexisting structural discontinuities. Available literature on
neotectonic evolution of this area points out a general uplift in
the last 700 ky and suggests two main fault sets, either N-S or
NW-SE striking, which may guide the gravitational tectonics
(Fig. 3; FANUCCI et alii, 1982).
We identified in the field three main fault sets (Fig. 4)
either E-W, NW-SE or NE-SW-striking and steeply-dipping.
We moreover observed, on the ridge at the back of the
DSGSD, low-dipping thrust surfaces that bring very hard
peridotites on top of pervasively fractured, low-strenght
serpentinites. This thrust tectonics is widespread in the studied
area and involves both the bedrock and the sedimentary cover
(Brecce di Costa Cravara). This tectonics can be related to the
late-to-post orogenic east vergent thrust tectonics related to the
Corsica-Sardinia block rotation (CAPPONI et alii, 2001;
CRISPINI et alii, 2009).
The fluvial pattern appears to be controlled by some
fracture sets, either NW-SE or NE-SW-striking, and by less
frequent sets either N-S or E-W striking, which produced a
rock blocks evolution of the slope and consequent step profile.
In the contest of researches aimed at renovation and
strengthening of the Abbey some drilling has been performed
in the southern sector of the Abbey plain. The stratigraphy is
3
D EEP SEATED GRAVITATIONAL SLOPE DEFORMATION IN AN A LPINE OPHIOLITES MASSIF
characterized by fine-grained and coarse-grained soils and by
rock blocks up to 0,5 m wide; a very altered and fractured
serpentinites bedrock was found at 6-7 m below ground level.
85
presence of fine-grained deposits, the evident N diversion of
the Orba river (Fig. 1) and the tectono-stratigraphic slope
setting described above.
To sum up the “Badia di Tiglieto” DSGSD appears to be
originated by the concurrence of different geological elements:
first of all the presence of structural discontinuities and fault
sets influencing the slope setting and its dynamics, then the
high difference in rock strength between peridotites and
serpentinites producing an important geomechanical contrasts.
REFERENCES
AGLIARDI F., CROSTA G. & ZANCHI A. (2001) - Structural
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CAPPONI G., CRISPINI L., PIAZZA M. & AMANDOLA L. (2001) Field constraints to the mid-tertiary kinematics of the
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213 - 230 "Genova" della Carta Geologica d'Italia alla
scala 1:50.000. Apat - Regione Liguria, Selca, Firenze.
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Knot". Ital. J. Geosci. (Boll. Soc. Geol. It.), 128(2), 433 441.
Fig. 3 – Neotectonic map of central Liguria (modified after FANUCCI et alii,
1982). 1) area affected by relative uplift, 2) area affected by absolute uplift, 3)
area affected by different degree of uplift, 4) mainly vertical fault, 5) fault.
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Fig. 4 – Rose diagram of moderate-to-steeply-dipping (> 60°) faults measured
in the study area.
The origin of the Abbey plain appears to be related to a weir
lake produced by a landslide: this can be inferred by the
REGIONE LIGURIA (2004) – Progetto inventario dei fenomeni
franosi in Italia IFFI: primi risultati in Liguria, a cura di
Bottero D., Cavallo C., De Stefanis E., Gorziglia G. &
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