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ABSTRACT
The Alpine and Apennine belts come into contact in Central
Ligu ria, in an extremely complex structural arrangement; this junc-
tion is usually referred to as the Ligurian Knot. The geological map-
ping for the 213-230 “Genova” and 212 “Spigno Monferrato”
1:50,000 quadrangles necessitated the updating of the stratigraphic,
structural and metamorphic descriptions of this area, and a redefinition
of some of the units and geological ensembles already established in
Central Liguria, i.e. the Voltri Group, the Sestri-Voltaggio Zone and
the Val Polcevera Unit. We propose a new definition of tectonic and
tectono-metamorphic units, grouped in three main associations, as
follows: a) units derived from oceanic crust and mantle; b) units
derived from a continental margin; c) Flysch units derived from the
sedimentary cover of an oceanic basement. In group a), we propose to
keep the already established Palmaro-Caffarella, Cravasco-Voltaggio,
Varazze and Figogna units and to establish a new Voltri tectono-
metamorphic Unit, including all the rocks showing a metamorphic
climax in eclogite facies conditions, with a variably developed green-
schist facies overprint. In group b) we propose to keep the already
established Gazzo-Isoverde Unit and to define a new Angassino-
Terma tectono-metamorphic Unit, encompassing limited outcrops of
quartzite and dolomitic limestone associated with the Voltri rocks.
In group c) we propose to replace the former Val Polcevera Unit with
new Ronco, Montanesi and Mignanego tectonic units. We propose to
abandon the term “Voltri Group”, for the term “Voltri Massif” that
can be used to define a set of units, including the Voltri, Varazze,
Palmaro-Caffarella and Angassino-Terma units, irrespective of
their different lithology and/or paleogeographic derivation and/or
metamorphism. For the Sestri-Voltaggio Zone, we propose to use
this term only for the Cravasco-Voltaggio and Gazzo-Isoverde units
(and not the Figogna Unit), to emphasize their common tectono-
metamorphic evolution.
K
EY WORDS
: Voltri Group, Sestri-Voltaggio Zone, Val Pol-
cevera Unit, Ligurian Alps, Ligurian knot.
INTRODUCTION AND GEOLOGICAL SETTING
The Ligurian Alps represent the southeastern termi-
nation of the Western Alps (fig. 1). The Ligurian and the
Western Alps were created by the Cretaceous-Eocene
convergent evolution of the European and Apulian plates
(V
ANOSSI
et alii, 1984; P
OLINO
et alii, 1990; B
ELTRANDO
et
alii, 2010 and references therein) that involved their con-
tinental margins and the Jurassic Ligurian-Piedmontese
ocean lying in between; slices of subcontinental lithospheric
mantle were also involved (i.e. in the Lanzo Massif; N
ICO
-
LAS
et alii, 1972, and in the Voltri Massif; P
ICCARDO
et alii,
1988). In greater detail, four main paleogeographic
domains have been detected in the Ligu rian Alps (V
ANOS
-
SI
et alii, 1984; D
ECARLIS
et alii, 2013): the Provençal,
Briançonnais, Piedmontese/Pre-Piedmontese and Ligu -
rian-Piedmontese domains. The first three domains are
related to the European passive rifted margin, whereas
the Ligurian-Piedmontese domain is of oceanic nature.
Convergence was accommodated by E-SE dipping
subduction (e.g. P
OLINO
et alii, 1990; S
TAMPFLI
et alii,
1998), which created an orogenic prism, encompassing
rocks scraped from both the underriding and overriding
plate and trench-related flysch sediments. The subduction
system evolved until the middle Eocene continental
collision that stopped the E-SE dipping subduction. The
following exhumation occurred at the Eocene-Oligocene
boundary (C
ARRAPA
et alii, 2004), in different sectors of
the Ligurian Alps through different mechanisms, such as
tectonic denudation via detachment (e.g. for the Briançon-
nais units, M
AINO
et alii, 2012) or by mass transfer in a
low viscosity subduction channel (e.g. the Ligurian-
Piedmontese units, F
EDERICO
et alii, 2005, 2007). The
emplacement of the exhumed rock bodies with a vergence
towards paleo-Europe built up the Alpine belt, which
resulted from the structural stack of several units, heteroge-
neous from both the point of view of the paleogeographic
derivation and the structural-metamorphic evolution
(V
ANOSSI
et alii, 1984).
The ongoing convergence of the European and
Apulian plates was then accommodated by a W directed
subduction (i.e. “Apenninic” subduction) which later resulted
in the Apennine orogenic phase (E
LTER
& P
ERTUSATI
,
1973; C
ASTELLARIN
, 1992, 2001; M
OLLI
et alii, 2010). The
transition from an E-SE dipping subduction (i.e. “Alpine”
subduction) to a W directed subduction (i.e. “Apenninic”
subduction) is a matter of debate. A flip in the direction
of subduction (e.g. D
OGLIONI
et alii, 1998; H
ANDY
et alii,
2010) is the most usual explanation, but many other geo-
dynamic models have also been proposed for the Alps-
Apennine transition (e.g. H
OOGERDUIJN
S
TRATING
, 1994;
J
OLIVET
et alii, 1998; F
ACCENNA
et alii, 2004; V
IGNAROLI
et
alii, 2008); however, an in-depth discussion of this topic is
beyond the scope of this work.
The units of the Ligurian Alps are overlain by the late-
to post-orogenic clastic sediments of the (Upper Eocene)-
Oligocene to Miocene Tertiary Piedmont Basin (TPB) that
is a syn-tectonic NeoAlpine-Apennine Basin (L
ORENZ
,
1968; G
ELATI
& G
NACCOLINI
, 1988, 1998; M
UTTI
et alii,
1995; F
EDERICO
et alii, 2004; R
OSSI
et alii, 2009; M
AINO
et
alii, 2013).
During the same Oligocene-early Miocene time inter-
val, the opening of the Ligurian-Provençal Basin caused
the counterclockwise rotation of the Corsica-Sardinia
block, and the related torsion of the Western Alps. The
result was that the southern part of the Western Alps (i.e.
(*) DISTAV - Dipartimento di Scienze della Terra, dell’Ambiente
e della Vita, Università di Genova, Corso Europa, 26 - 16132 Genova -
Italy. Corresponding author: Giovanni Capponi; e-mail: capponi@
dipteris.unige.it
Geology of the Eastern Ligurian Alps: a review of the tectonic units
G
IOVANNI
C
APPONI
(*), L
AURA
C
RISPINI
(*), L
AURA
F
EDERICO
(*) & C
RISTINA
M
ALATESTA
(*)
Ital. J. Geosci., Vol. 135, No. 1 (2016), pp. 157-169, 6 figs., 5 tabs. (doi: 10.3301/IJG.2015.06)
© Società Geologica Italiana, Roma 2016
Author’s personal copy
the Ligurian Alps) was rotated by tens of degrees to the
present WNW-ESE structural trend (e.g. V
ANOSSI
et alii,
1994; M
AFFIONE
et alii, 2008; D
ECARLIS
et alii, 2014), con-
trasting with the N-S to NNE-SSW trend of most of the
Western Alps. This tectonic event resulted in a top-to-the
E-NE backthrusting of the involved units, including the
TPB, which partially acted as a piggy-back basin (M
OSCA
et alii, 2009; M
AINO
et alii, 2013), and affected their inter-
nal structural architecture, with thrust faults, strike-slip
faults and folds that accommodated a SW-NE shortening
(C
RISPINI
et alii, 2009; F
EDERICO
et alii, 2009, 2014).
The change in the direction of subduction and the
resulting junction between the Alpine and Apennine
chains occurred in the Ligurian section, and this is the
reason why central Liguria is a key area for investigating
the link between the Alps and the Apennines. The coun-
terclockwise rotation with the connected NE-directed
backthrusting of the Ligurian Alps added complexity to
the area, which is known in literature as the “Ligurian
knot” (L
AUBSCHER
et alii, 1992).
The geological mapping for the 213-230 “Genova”
1:50,000 Sheet (C
APPONI
& C
RISPINI
, 2008a,b) in the
framework of the CARG Program (i.e. the Italian program
of geological cartography) and for the 212 “Spigno Mon-
ferrato” 1:50,000 Sheet (Ligurian part, Regione Liguria
cartographic project, C
APPONI
et alii, 2013a,b) required
the updating of the stratigraphic, structural and meta-
morphic observations, leading to a re-definition of some
of the units and geological ensembles established in cen-
tral Liguria (representing the eastern part of the Ligurian
Alps, fig. 1), i.e. the Voltri Group, the Sestri-Voltaggio
Zone and the Val Polcevera Unit.
The aim of this paper is to review such units and discuss
their definition. We will use paleogeography as the princi-
pal criterion to define the geological ensembles that are
consistent with the paleogeographic derivation; in such a
framework we will then use the criterion of the tectono-
metamorphic unit (S
PALLA
et alii, 2005).
As the pre-Oligocene basement is the object of this
study, the Oligocene-Pliocene deposits will not be treated
in this paper.
UNITS OF CENTRAL LIGURIA
IN THE PREVIOUS LITERATURE
T
HE
V
OLTRI
G
ROUP
(G
RUPPO DI
V
OLTRI
A
UCT
.)
Definition
The “Gruppo di Voltri” was defined by I
SSEL
(1892)
as including all rocks comprised in the quadrangle
defined by the villages of Albisola, Sestri Ponente, Voltag-
gio and Valosio (fig. 2). This definition was irrespective of
their different paleogeographic derivation or their meta-
morphic features; in the area identified by I
SSEL
(1892),
metaophiolites and related metasediments are prevalent, but
rocks derived from a continental margin (i.e. the crystalline
rocks of Arenzano and limited outcrops of dolomitic lime-
stone and quartzite) also occur.
Later authors widely used the term Voltri Group, with
a more precise geological meaning, i.e. to refer to the
metaophiolite and related metasediments, re-equilibrated
in High Pressure-Low Temperature (HP-LT) climax con-
158
G
.
CAPPONI ET ALII
Fig. 1 - A) Sketch map of the eastern
Ligurian Alps; B) Sketch map of the
Western Alps. The area of A) is red
squared.
ditions; such rocks were compared with those occurring
in the Western Alps (F
RANCHI
, 1915) that were considered
a continuation of this part of Liguria.
The definition of the Voltri Group was also retained
during the geological mapping for the 2nd edition of the
82 “Genova” Sheet of the Geological Map of Italy (A
LLASI
-
NAZ
et alii, 1971) and in the related papers of the 1970s
(C
HIESA
et alii, 1975, 1976; C
HIESA
& R
OSSI
, 1976; F
OR
-
CELLA
, 1976; M
ESSIGA
et alii, 1977).
Since the 1980s, an increasing number of authors
have used the term Voltri Massif as an interchangeable
term for the Voltri Group, although the earlier term still
continued to be used.
Paleogeographic origin
In terms of its paleogeographic origin, the age and the
lithostratigraphic features of the rocks of the Voltri Group
suggest derivation from the Ligurian-Piedmontese ocean,
which developed between the European and Apulian plates
in the Jurassic-Cretaceous. Peridotite represents subconti-
nental lithospheric mantle, which was tectonically denuded
during the rifting phase and coupled with the oceanic crust
in an early stage of the evolution; papers by H
OOGERDUIJN
S
TRATING
et alii (1993) and S
CAMBELLURI
et alii (1995,
1997, 2001) have described a phase of serpentinization
predating the HP metamorphism, testifying to an early
exposure of such rocks on the oceanic floor; their most
likely derivation is from the Apulian plate (H
OOGERDUIJN
S
TRA TING
et alii, 1990; E
RNST
, 1981; P
ICCARDO
, 2012).
C
HIESA
et alii (1975), as other authors before, included
minor sequences made of quartzite and dolomitic lime-
stone in the Voltri Group. The lithostratigraphic features
of such rocks suggest a derivation from a continental
margin and this makes it difficult to assess their meta-
morphic grade, which was mostly considered as equiva-
lent to the surrounding rocks of the Voltri Group.
The units of the Voltri Group
The paper by C
HIESA
et alii (1975) deals with petro-
graphic and metamorphic features of the rocks of the
Voltri Group and suggests a hypothesis for its structural
architecture, with staking of the peridotite-rich units on
top of the serpentinite and metagabbro units, in turn
overlapping the metasedimentary units. In this paper, the
authors have also established different units in the Voltri
Group (fig. 2), on the basis of their petrographic features
and tectonic relationships:
– the Beigua, Ponzema, San Luca-Colma, Erro-Tob-
bio and Varazze units, mainly composed of ultramafic
GEOLOGY OF THE EASTERN LIGURIAN ALPS
159
Fig. 2 - Sketch map of the eastern
Ligurian Alps, with the units of the
Voltri Group (after C
HIESA
et alii,
1975) and of the Sestri-Voltaggio
Zone (C
ORTESOGNO
& H
ACCARD
,
1984; M
ARINI
, 1976, 1989).
rocks and metagabbro, the Beigua Unit encompassing
most of the eclogite;
– the Voltri-Rossiglione, Ortiglieto, Alpicella and Pal-
maro-Caffarella units, mainly composed of metasediment
and metabasite.
The units defined by C
HIESA
et alii (1975) were exten-
sively quoted in the following papers (C
APPONI
et alii,
1986; C
HIESA
et alii, 1976; C
ORTESOGNO
et alii, 1977;
D
ESMONS
et alii, 1999; M
ESSIGA
et alii, 1977; P
ICCARDO
et
alii, 1979).
M
ESSIGA
et alii (1983) simplified the definition, using:
– Erro-Tobbio serpentinized lherzolite, to refer to the
peridotite;
– Beigua serpentinite, to refer to the serpentinite with
metagabbro (in this way grouping the Beigua, San Luca
Colma, Ponzema and Varazze units);
– Piedmontese Nappe (calc-schist), in which they
grouped the metasediment with the metabasite of the
Voltri-Rossiglione, Ortiglieto, Alpicella and Palmaro-Caf-
farella units.
A similar simplified approach was used by D
RURY
et alii (1990) and H
OOGERDUIJN
S
TRATING
et alii (1990)
who used:
– Erro-Tobbio Unit, to refer to the peridotite;
– Beigua Unit, to refer to the serpentinite with
metagabbro;
– Voltri-Rossiglione Unit, to refer to the metasedi-
ment with metabasite.
Such simplified definitions of the units can be found
in the following contributions, e.g. H
OOGERDUIJN
S
TRA
-
TING
(1991), S
CAMBELLURI
& R
AMPONE
(1999), V
IGNAROLI
et alii (2005).
Other authors abandoned the units defined by C
HIE
-
SA
et alii (1975), referring simply to the lithology (C
AP
-
PONI
, 1991; C
APPONI
& C
RISPINI
, 1997, 2002; S
PAGNOLO
et
alii, 2007).
A different approach can be found in the papers by
V
IGNAROLI
et alii (2005, 2008, 2009), who defined the
Upper and the Lower Tectonic Complex: the Lower
Tectonic Complex comprises the Voltri-Rossiglione and
the Beigua units, whereas the Upper Tectonic Complex
comprises the Erro-Tobbio Unit, the TPB and other units
not related to the Voltri Group.
Metamorphism of the Voltri Group
Most of the rocks of the Voltri Group were re-equili-
brated in eclogite facies metamorphic conditions at about
50 Ma (F
EDERICO
et alii, 2005), then overprinted by
greenschist facies conditions (C
HIESA
et alii, 1975; E
RNST
,
1976; C
IMMINO
et alii, 1981; M
ESSIGA
& S
CAMBELLURI
,
1991) at about 33 Ma (F
EDERICO
et alii, 2005). The green-
schist overprint is more or less pervasive, depending on
the reactivity of the rock chemistry and on the deforma-
tion. The eclogite facies assemblages are more extensively
and better preserved in the mafic and ultramafic rocks,
which are less retrogressed in greenschist facies, whereas
they are rare in the metasediment, where the greenschist
overprint is usually pervasive (C
APPONI
& C
RISPINI
, 2002).
Though most of the rocks of the Voltri Group show
eclogite facies metamorphic climax conditions, there are
two exceptions:
– the Varazze Unit shows a greenschist facies meta-
morphic imprint without eclogite facies relics, as already
observed by C
HIESA
et alii (1975). In this Unit, therefore,
the greenschist facies conditions represent the metamor-
phic climax (D
ESMONS
et alii, 1999);
– the Palmaro-Caffarella Unit shows a blueschist
facies metamorphic climax, as indicated by C
ORTESOGNO
et alii (1979) and C
ABELLA
et alii (1994). M
ALATESTA
et alii
(2012) recently supplied a pseudosection study for a Pal-
maro-Caffarella metagabbro, indicating blueschist facies
peak conditions of 10<P(kbar)<15 and 450<T(°C)<500; no
eclogite relics occur in the metagabbro.
For many years, the Erro-Tobbio Unit was also con-
sidered to display different metamorphic features from
most of the rocks of the Voltri Group. This unit encom-
passes peridotite (lherzolite, harzburgite, pyroxenite levels
and minor dunite) that extensively shows mantle assem-
blages, with no metamorphic re-activation in the Alpine
cycle. From a textural point of view, the Erro-Tobbio
peridotite shows a mantle granular texture, in places
grading to tectonite and mylonite texture, due to mantle
deformations (D
RURY
et alii, 1990; H
OOGERDUIJN
S
TRA
-
TING
, 1991). Such an apparent lack of metamorphic and
structural re-activation in the Alpine cycle drove C
HIESA
et alii (1975) to interpret such rocks as mantle slices that
had escaped from the Alpine subduction and then been
thrust onto the other rocks of the Voltri Group, in a late
Alpine phase.
However, later researchers (P
ICCARDO
et alii, 1988;
C
APPONI
& C
RISPINI
, 1990; D
RURY
et alii, 1990; H
OOGER
-
DUIJN
S
TRATING
, 1990; H
OOGERDUIJN
S
TRATING
et alii,
1990; S
CAMBELLURI
et alii, 1991; S
CAMBELLURI
et alii,
1995) also showed the occurrence of eclogite facies Alpine
metamorphic re-equilibration in the Erro-Tobbio rocks,
particularly well-developed in metagabbro dykes, emplaced
in peridotite. The bodies of peridotite (in places at the
km-scale) that preserve mantle features from both the
metamorphic and structural point of view are the result
of huge strain partitioning, with Alpine metamorphic
re-activation limited to shear zones, where structural
re-activation also occurs. This is shown also by the cross
section of fig. 3, where km-scale peridotite boudins are
deformed and wrapped in the greenschist facies foliation
that occurs in most of the rocks of the Voltri Group
(C
APPONI
& C
RISPINI
, 2008b).
T
HE
S
ESTRI
-V
OLTAGGIO
Z
ONE
(Z
ONA
S
ESTRI
-V
OLTAGGIO
AUCT
.)
The Sestri-Voltaggio Zone was defined by G
ÖRLER
(1962) and I
BBEKEN
(1962), to refer to a complex of
rocks, occurring from Sestri Ponente to Voltaggio (fig. 2)
in a 5-6 km-wide N-S strip (G
ÖRLER
& I
BBEKEN
, 1964).
The great complexity of the Sestri-Voltaggio Zone has
always captured the attention of the geologists (G
ÖRLER
& I
BBEKEN
, 1964; G
ELATI
& P
ASQUARÈ
, 1970; S
TURANI
,
1973), and its western contact with the Voltri Group
(usually referred to as the Sestri-Voltaggio Line) has
been considered as the boundary between Alps and
Apennines for a long time (R
OVERETO
, 1939). Other
authors have described the Sestri-Voltaggio Line as
a contact between units with different metamorphic
conditions, later tilted to the vertical by late-orogenic
tectonics (C
ORTESOGNO
& H
ACCARD
, 1984; C
APPONI
, 1991)
160
G
.
CAPPONI ET ALII
or as an extensional tectonic surface placing low pres-
sure on high pressure metamorphic units (H
OOGER
-
DUIJNG
S
TRATING
, 1991). C
RI SPINI
& C
APPONI
(2001)
highlighted the evolution of the role of the Sestri-Voltag-
gio Line, changing from a boundary between different
tectono-metamorphic units to a dextral transcurrent
system, in the different stages of the Alpine and late Alpine
evolution.
Units of the Sestri-Voltaggio Zone
Three units are usually grouped in the Sestri-Voltaggio
Zone (C
ORTESOGNO
& H
ACCARD
, 1984): the Figogna, the
Cravasco-Voltaggio and the Gazzo-Isoverde units (tab. 1).
C
ORTESOGNO
& H
ACCARD
(1984) included the Busalla
Flysch in the Figogna Unit, but it was later considered as
an independent Unit by M
ARINI
(1989) (see later).
H
OOGERDUIJNG
S
TRATING
(1991) proposed that the
Sestri-Voltaggio Zone was comprised of only two units,
i.e. the Cravasco-Voltaggio and the Gazzo- Isoverde units,
and the Figogna Unit was to be considered as the western
portion of the Lavagna Nappe (V
AN
W
AMEL
et alii, 1985),
which consists of a sequence of pelagic limestone, slate,
marl, siltstone and quartz-arenite, cropping out discon-
tinuously to the east of Genova as far as La Spezia (see
fig. 2 of V
AN
W
AMEL
, 1987).
The Cravasco-Voltaggio Unit comprises metaophio-
lite and related metasediment, equilibrated in LT
blueschist facies metamorphic peak conditions (C
ABELLA
et alii, 1994) with a following metamorphic imprint at
lower pressure.
Similarly, the Figogna Unit comprises metaophiolite
and related metasediment, but it is equilibrated in pumpel-
lyite-actinolite facies metamorphic peak conditions. The
ophiolitic association of the Figogna Unit was for a long
time considered devoid of metagabbro (C
ORTESOGNO
&
H
ACCARD
, 1984), but C
RISPINI
(1995) and the geological
mapping for the 213-230 “Genova” Sheet (C
APPONI
&
C
RISPINI
, 2008a) revealed that some metagabbro occurs,
albeit in very limited volumes.
The Gazzo-Isoverde Unit encompasses a metasedi-
mentary succession, made up of dolostone with horizons
of evaporite, limestone and schist, of Triassic-Liassic age;
for this reason this unit was referred to also as the Trias-
Lias Unit in the previous literature. The isolated outcrop
of Case Ferrere (C
APPONI
et alii, 1998; C
APPONI
& C
RI
-
SPINI
, 2008a) can also be ascribed to this Unit.
The rock association of the Gazzo-Isoverde Unit
suggests a deepening carbonatic platform environment
(a stratigraphic analysis can be found in L
UALDI
, 1991).
So far no metamorphic assemblages constraining the
PT evolution have been found, but it has been said that
the Gazzo-Isoverde rocks share the same syn-meta -
morphic deformations as the Cravasco-Voltaggio Unit,
and this suggests a blueschist facies equilibration
for the Gazzo-Isoverde Unit also; this is also supported
by deformations of calcite, that indicate a T>300°C
(C
RISPINI
, 1995).
The lithostratigraphic characters of the Figogna and
Cravasco-Voltaggio units suggest that they are derived
from the Ligurian-Piedmontese oceanic domain, of Juras-
sic-Cretaceous age, whereas the Gazzo-Isoverde Unit is
derived from a continental margin. Though there is no
conclusive evidence for its provenance either in the paleo-
European margin (pre-Piedmontese-Piedmontese Domain)
or the paleo-African (Austroalpine Domain), the similari-
ties in the lithostratigraphic features (L
UALDI
, 1991) sug-
gest that a derivation from the pre-Piedmontese domain
(C
ORTESOGNO
& H
ACCARD
, 1984) is more likely.
T
HE
V
AL
P
OLCEVERA
U
NIT
The Val Polcevera Unit (fig. 2) was established by
M
ARINI
(1976, 1989), and encompasses, from top to
bottom, the Ronco Formation (a marly-arenaceous flysch),
the Mignanego Shale Formation, the Montanesi Shale
GEOLOGY OF THE EASTERN LIGURIAN ALPS
161
Fig. 3 - Cross section, redrawn and modified after C
APPONI
& C
RISPINI
(2008a). Apart the Tertiary Piemontese Basin (TPB) breccia and
conglomerate, all lithologies pertain to the Voltri Massif. See fig. 5 for location.
T
ABLE
1
Units of the Sestri - Voltaggio Zone.
Formation and the Palombini dei Giovi Formation
(tab. 2); the stratigraphic age is known only for the
Ronco Formation, which contains nannofossils of late
Santonian-early Campanian age (C
APPONI
& C
RISPINI
,
2008b). All these rocks were previously grouped in the
Busalla Flysch (C
ORTESOGNO
& H
ACCARD
, 1984), consi -
dered as the sedimentary cover of the Figogna Unit.
The Val Polcevera Unit is affected by low-grade meta-
morphism and it is usually ascribed to the Internal Ligu -
rian Domain; M
ARINI
(1998) considered this unit as the
pro parte equivalent of the Monte Gottero Unit.
M
ARINI
(1989) proposed that the Val Polcevera Unit
encompasses two tectonic slices, i.e. the “Elemento di
Mignanego” and the “Elemento di Ronco”, with a com-
mon stratigraphic base represented by the Palombini dei
Giovi Formation and by the Montanesi Shale Formation;
that common stratigraphic base is overlain by the Migna -
nego Shale Formation in the “Elemento di Mignanego”
and by the Ronco Formation in the “Elemento di Ronco”.
In this interpretation (M
ARINI
, 1989), the deeper slice
(Elemento di Mignanego) is mostly overturned and the
higher slice (Elemento di Ronco) has normal polarity.
The two slices are in contact along a décollement surface
that corresponds to the axial surface of a km-scale recum-
bent fold.
A considerably different interpretation of this unit is
given by E
LLERO
(2000) and E
LLERO
et alii (2001), who
recognized tectonic contacts among the formations of
the Val Polcevera Unit and hence assigned the rank of
tectonic unit to each formation, incorporating the
Palombini dei Giovi Formation in the Montanesi Shale
Formation and splitting the Ronco Formation in two (the
Ronco and the Bric Caree formations). In this way
E
LLERO
(2000) and E
LLERO
et alii (2001) defined four
units (tab. 2), interposed between the Figogna and the
Antola units (fig. 2).
From the structural top to the structural bottom, the
four units are:
– the Ciaè Unit, comprising the Bric Carree Forma-
tion, a new formation established by E
LLERO
(2000);
– the Vallecalda Unit, comprising the Ronco Formation;
– the Serra Riccò Unit, comprising the Montanesi
Shale Formation;
– the Bric Montaldo Unit, comprising the Mignanego
Shale Formation.
The Montanesi Shale Formation includes the Palom-
bini dei Giovi Formation (after M
ARINI
, 1989), that
E
LLERO
(2001) considered as a lithofacies of the Mon-
tanesi Shale Formation.
Although the metamorphism of all four units is of
low-grade, E
LLERO
(2000) and E
LLERO
et alii (2001) also
support slightly different metamorphic conditions on the
basis of the mineralogical parameters of the phyllosilicates.
REVIEW AND DISCUSSION OF EXISTING DATA
T
HE
V
OLTRI
G
ROUP
Definition
The Voltri Group (fig. 2) was established by I
SSEL
(1892) but, as the word “Group” is a dedicated lithostrati-
graphic term, intended to refer to a set of formations, this
is not correct. Therefore, although defined some time ago
and in common usage since then, the use of the term
Voltri Group should be abandoned.
The term Voltri Massif appears to be more correct
and feasible, to define a group of tectono-metamorphic
units outcropping in central Liguria.
The units of the Voltri Group
The Voltri Group was divided into several units (C
HIESA
et alii, 1975), i.e. the Beigua, Ponzema, San Luca-Colma,
Erro-Tobbio, Varazze, Voltri-Rossiglione, Ortiglieto, Alpi-
cella and Palmaro-Caffarella units (fig. 2), on the basis of
their petrographic features and tectonic relationships
(C
HIESA
et alii, 1975). It is therefore conjectured that
there are metamorphic and/or structural differences
between the units; this indeed happens for the Varazze
and the Palmaro Caffarella units, with greenschist and
blueschist facies metamorphic climax conditions, respec-
tively, contrasting with the prevalent eclogite facies meta-
morphism.
Conversely, for the other units, the differences in the
metamorphic evolution are not obvious: although most of
the eclogite is hosted in the serpentinite of the Beigua
Unit, eclogites are also associated, however, with the
other units i.e. the Ponzema, San Luca-Colma, Erro-Tob-
bio, Voltri-Rossiglione, and Ortiglieto units. In particular,
eclogite bodies are associated with:
– the Ponzema serpentinite (M
ESSIGA
& P
ICCARDO
,
1974; C
APPONI
& C
RISPINI
, 2008a);
– the San Luca-Colma mafic and ultramafic rocks,
e.g. at Mt Colma (C
APPONI
& C
RISPINI
, 2008a,b and refer-
ences therein);
– the Voltri-Rossiglione calcschist, along the Masca
River (C
APPONI
& C
RISPINI
, 2008a) and at Cima di Mezzo
(M
ALATESTA
et alii, 2012);
– the Alpicella calcschist, along the Teiro River
(D’A
NTONIO
et alii, 1984; C
APPONI
et alii, 2013b);
162
G
.
CAPPONI ET ALII
T
ABLE
2
Val Polcevera Unit.
As defined by M
ARINI
(1976, 1989).
As divided by E
LLERO
(2000)
– the Ortiglieto calcschist, along the Meri River (C
AP
-
PONI
et alii, 2009);
– the Erro-Tobbio peridotite, as indicated by several
authors (e.g. C
APPONI
& C
RISPINI
, 1990).
Recent PT determinations and pseudosections (M
ALA
-
TESTA
et alii, 2012) for the eclogite hosted in the Ponzema
Unit and in the Voltri-Rossiglione Unit show PT condi-
tions of climax metamorphism in the eclogite facies
(460<T°C<500 and of 22<Pkbar<28), comparable to those
of the Beigua eclogite. Furthermore, although the green-
schist facies retrogression is more pervasive in the
metasediments, relics of eclogite metamorphism are
widespread, as HP phengite (F
EDERICO
et alii, 2005) in
calcschist and micaschist and spessartine-rich garnet in
quartzschist (D
ESMONS
et alii, 1999).
The occurrence of eclogite in different units, its
association with the metasediment and the relics of
eclogite assemblages in metasediment demonstrate that
all these rocks were involved in the Alpine subduction
and underwent eclogite facies metamorphic re-equili-
bration: this indicates that, apart from the Varazze and
the Palmaro-Caffarella units, there are no significant
differences in the metamorphic evolution of the units
defined by C
HIESA
et alii (1975).
In terms of the structural architecture, the geological
mapping and the structural analysis (C
APPONI
& C
RI
-
SPINI
, 2002, 2008a; C
APPONI
et alii, 2013a,b) indicate that
the rocks of the different units of the Voltri Group (apart
the Varazze and the Palmaro-Caffarella units) shared the
same structural evolution, which encompassed multiple
superposed, syn-metamorphic folding events. The papers
and maps by D’A
NTONIO
et alii (1984), C
APPONI
et alii
(1994), C
APPONI
& C
RISPINI
(2008a) and C
APPONI
et alii
(2013b) demonstrate that the boundaries between the
serpentinite of the Beigua and Ponzema units and the
metasediment of the Voltri-Rossiglione and Alpicella
units are mostly controlled by ductile folding.
In conclusion, with the exclusion of the Varazze and
Palmaro-Caffarella units, there are no factual differences
in the metamorphic and structural evolution of most of
the units of the Voltri Group, whose definition (C
HIESA
et
alii, 1975) appears consequently to be questionable.
With regard to the Upper and the Lower Tectonic
Complex, defined by V
IGNAROLI
et alii (2005, 2008), such
a scheme was already questioned by C
APPONI
et alii (2009)
and, most importantly, was abandoned by the same
authors in a later paper (V
IGNAROLI
et alii, 2010).
T
HE
S
ESTRI
-V
OLTAGGIO
Z
ONE
The Sestri-Voltaggio Zone encompasses three units
(C
ORTESOGNO
& H
ACCARD
, 1984), i.e. the Cravasco-Vol -
taggio, Figogna and Gazzo-Isoverde units, which are not
homogeneous in their metamorphic and paleogeographic
characters (tab. 1).
The Cravasco-Voltaggio and the Figogna Unit can be
correlated on the basis of their paleogeographic deriva-
tion, but their metamorphic signatures indicate different
evolutions in the orogenic prism. Conversely, the
Cravasco-Voltaggio and the Gazzo-Isoverde units can be
correlated on the basis of their metamorphic and struc-
tural imprints, but they derive from different paleogeo-
graphic domains: as a consequence, there is no feature
that is common to all three units and we do not see any
convincing geological reason to cluster them in a com-
mon zone.
A different approach was taken by H
OOGERDUIJN
S
TRATING
(1991), who grouped only the Cravasco-Voltag-
gio and the Gazzo-Isoverde units in the Sestri-Voltaggio
Zone, putting emphasis on their common metamorphic
and structural evolution, and considered the Figogna Unit
as the western part of the Lavagna Nappe; this approach
appears to better describe the arrangement of the zone.
T
HE
V
AL
P
OLCEVERA
U
NIT
In the older literature (M
ARINI
, 1989) it was assumed
that the Palombini dei Giovi Formation and the Mon-
tanesi Shale Formation formed a common stratigraphic
base overlain by the Mignanego Shale Formation in the
“Elemento di Mignanego” and by the Ronco Formation in
the “Elemento di Ronco”. Nevertheless, only the strati-
graphic age of the Ronco Formation has been established,
and the lack of fossil content in the other formations pre-
cludes determining their stratigraphic position with
respect to the Ronco Formation. Poor conditions and lack
of continuity in the rock outcrops prevent the direct
observation of the geometric relationships between the
formations and so both the stratigraphic succession and
the structural architecture remains uncertain; the inter-
pretation by M
ARINI
(1989) is hence rather speculative.
Conversely, E
LLERO
(2000) and E
LLERO
et alii (2001),
assigned the rank of tectonic unit to each formation of
the Val Polcevera Unit, incorporating the Palombini dei
Giovi Formation in the Montanesi Shale Formation and
splitting the Ronco Formation in two (the Ronco and the
Bric Caree formations), in this way defining four units
(tab. 2).
This interpretation appears to be more conservative,
and better able to describe the geological situation. How-
ever the institution of the Bric Carree Formation (and the
Ciaè Unit) is questionable, as the Bric Carree Formation
is actually only a facies of the Ronco Formation, charac-
terized by thicker strata, as indicated by the same authors
(E
LLERO
, 2000; E
LLERO
et alii, 2001).
THE REVISED UNITS
All the units studied can be considered as part of the
same orogenic prism, built up during the subduction-
collision and exhumation events of the Alpine orogenesis.
Terrains from different paleogeographic domains were
involved in the same geodynamic process at different
structural levels.
The contrasting structural and metamorphic evolu-
tions (ranging from eclogite to pumpellyite-actinolite
facies) of the units derived from the Ligurian-Piedmon-
tese oceanic domain indicate that they were involved in
different ways and at different levels in the orogenic
prism though derived from the same paleogeographic
domain. Recent works suggest that the eastern sector of
the Voltri Massif could represent a “fossil” serpentinite
subduction channel (fig. 4; F
EDERICO
et alii, 2007;
M
ALATESTA
et alii, 2012) whereas the tectonic units to the
east (i.e. the Palmaro-Caffarella, Cravasco-Voltaggio,
Gazzo-Isoverde and Figogna units) could represent slices
subducted at different depths and then accreted to the
orogenic prism. In this framework, as already outlined by
GEOLOGY OF THE EASTERN LIGURIAN ALPS
163
M
OSCA
et alii (2009) and M
OLLI
et alii (2010), the Sestri-
Voltaggio Zone no longer has the geological meaning as
the Alps-Apennine junction as it was considered in the past.
The result of such processes is of great geological
complexity, and requires some schematization to be cor-
rectly understood. We propose a new definition for the
tectono-metamorphic and tectonic units of the pre-
Oligocene basement of the study area (i.e. the Voltri
Group Auct., the Sestri-Voltaggio Zone and the Val Pol-
cevera unit); such a definition is driven by the lithostrati-
graphic features, which are, in turn, linked to the paleo-
geographic domains, and groups the units in three main
associations, as follows (fig. 5):
a) units derived from oceanic crust and mantle;
b) units derived from a continental margin;
c) flysch units derived from the sedimentary cover of
an oceanic basement.
In this way we can define three ensembles that are con-
sistent from the paleogeographic point of view; in such a
framework we have distinguished several tectono-metamor-
phic units. By “tectono-metamorphic unit” we mean a rock
volume that can be distinguished from the adjoining rocks
on the basis of the metamorphic features and/or the inter-
nal structural features and/or its structural position with
respect to other units (S
PALLA
et alii, 2005). For the units
that are not metamorphic or have only very low-grade
metamorphism we use the simpler term “tectonic unit”.
U
NITS DERIVED FROM OCEANIC CRUST AND MANTLE
The units derived from oceanic crust and mantle
lithologically comprise metaophiolite, metasediment and
mantle peridotite, that can be ascribed to the Ligurian-
Piedmontese oceanic domain (V
ANOSSI
et alii, 1984) that
developed between the European and Apulian blocks
after the Jurassic, and to subcontinental lithospheric
mantle which was tectonically denuded during the rifting
phase and coupled with the oceanic crust. Such units
were involved in the Alpine subduction.
With regard to the Voltri Group and its units (C
HIESA
et alii, 1975), apart from the Varazze and the Palmaro-
Caffarella units, all the other units of the Voltri Group
underwent a metamorphic imprint with eclogite facies
climax conditions, followed by a more or less pervasive
retrogressive metamorphism. We therefore propose to
maintain the Varazze and the Palmaro-Caffarella units,
and to establish the “Voltri tectono-metamorphic Unit”
(fig. 5), that encompasses all the rocks showing a meta-
morphic climax in eclogite facies conditions, with a
variably developed greenschist facies overprint (tab. 3).
The other units (i.e. the Beigua, Ponzema, San Luca-Colma,
Erro-Tobbio, Voltri-Rossiglione, Ortiglieto and Alpicella
units) should be cancelled. Furthermore, we also suggest
the abandonment of the term “Voltri Group”.
The Cravasco-Voltaggio and Figogna units (classically
included in the Sestri-Voltaggio Zone) are also included
in the units derived from oceanic crust (tab. 3) and are
kept, as they show peculiar features in their metamorphic
re-equilibration.
U
NITS DERIVED FROM A CONTINENTAL MARGIN
The units derived from a continental margin lithologi-
cally comprise quartzite, dolostone with evaporite hori-
zons, limestone and shale and were involved in the Alpine
subduction events. We have included in such units the
Gazzo-Isoverde Unit (tab. 4) and the Angassino-Terma
Unit that is a new unit that we propose to establish (fig. 5)
that encompasses the limited outcrops of quartzite and
dolomitic limestone associated with the Voltri Massif.
The Gazzo-Isoverde Unit probably comes from the
pre-Piedmontese domain, whereas the provenance of
the Angassino-Terma Unit is more uncertain, due to its
limited extent that precludes a complete study of its
lithostratigraphic features. As an alternative interpreta-
tion such minor bodies can represent extensional relics of
the adjoining disrupted continental margins.
F
LYSCH UNITS
The flysch units derive from the Internal Ligurian
Domain and lithologically comprise arenaceous turbidite
and shale, i.e. the sedimentary cover of the Ligurian
oceanic basement, developed during its trenchward
motion in the Alpine subduction (M
ARRONI
& P
ANDOLFI
,
2001). They show low- to very low-grade metamorphism
(L
EONI
et alii, 1996) achieved at shallow depth in the
Alpine accretionary prism. Such units differ from the
Helminthoid Flysch (e.g. the Antola Unit), belonging to
the External Ligurian Domain, considered as derived
from a supra-subduction basin (M
ARRONI
et alii, 2010).
We suggest that the flysch units comprise three
tectonic units, interposed between the Antola and the
164
G
.
CAPPONI ET ALII
Fig. 4 - The eastern sector of the Voltri Massif, interpreted as a fossil
subduction channel. A sheared matrix of serpentinite and calcschist
encloses bodies of different lithologies, with different metamorphic
evolution. Redrawn and modified after M
ALATESTA
et alii (2012).
Figogna units (figs. 5, 6), that are from the structural top
to the structural bottom (tab. 5):
– the Ronco tectonic Unit, comprising the Ronco
Formation;
– the Montanesi tectonic Unit, comprising the Mon-
tanesi Shale Formation;
– the Mignanego tectonic Unit, comprising the
Mignanego Shale Formation.
In this interpretation we split the Val Polcevera Unit
of M
ARINI
(1989) into three tectonic units, partially in
agreement with E
LLERO
(2000) and E
LLERO
et alii (2001);
similarly we interpret the Palombini dei Giovi Formation
(after M
ARINI
, 1989) as a lithofacies of the Montanesi
Shale Formation. Nevertheless we do not agree with the
definition of the Ciaè Unit (and the related Bric Caree
Formation, E
LLERO
, 2000), which we consider pertains to
the Ronco Unit.
CONCLUSIONS
The aim of this paper is to review the geological units
of the eastern part of the Ligurian Alps, on the basis of
the recent fieldwork and analyses made for the geological
mapping related to the 213-230 “Genova” and the 212
“Spigno Monferrato” 1/50,000 sheets (C
APPONI
&
C
RISPINI
, 2008a,b; C
APPONI
et alii, 2013a,b).
The updating observations of the stratigraphic,
structural and metamorphic characteristics of the area,
has led to a re-definition of some of the units and
geological ensembles. All the studied units derive from
different paleogeographic domains, but have been involved
in the same geodynamic process at different structural
levels. They can be considered as part of the same
orogenic accretionary prism, built up during the Alpine
subduction-collision and exhumation events; driven by
the paleogeography, we grouped these units in three
main sets:
a) units derived from oceanic crust and mantle;
b) units derived from a continental margin;
c) flysch units derived from the sedimentary cover of
an oceanic basement.
In such a framework we have made a further distinc-
tion, following the definition of the tectono-metamorphic
units and tectonic units (sensu S
PALLA
et alii, 2005). The
result is a schematization that hopefully provides a better
understanding of the regional geology and an easier com-
parison with other sectors of the Western Alps.
GEOLOGY OF THE EASTERN LIGURIAN ALPS
165
Fig. 5 - Sketch map of the eastern
Ligurian Alps, showing the revised
units and the location of the Cross
Sections.
The units derived from oceanic crust and mantle per-
tain to the Ligurian-Piedmontese oceanic domain; among
them we propose to establish the tectono-metamorphic
Voltri Unit, encompassing all the rocks characterized by a
metamorphic climax in eclogite facies conditions, with a
variable greenschist overprint. The terms Varazze, Pal-
maro-Caffarella, Cravasco-Voltaggio and Figogna units,
already established, can still be used as they display a
different tectono-metamorphic evolution. Conversely, the
terms Beigua, Ponzema, San Luca-Colma, Erro-Tobbio,
Voltri-Rossiglione, Ortiglieto and Alpicella units (C
HIESA
et alii, 1975) should be abolished. We also propose to
abandon the term “Voltri Group”, still used in literature.
There is no conclusive evidence that the units derived
from a continental margin, have their provenance in
either the paleo-European margin (pre-Piedmontese-Pied-
montese domain) or the paleo-African (Austroalpine
Domain); nevertheless a derivation from the pre-Pied-
montese domain (C
ORTESOGNO
& H
ACCARD
, 1984) is
more likely. Among these units we propose to establish
the tectono-metamorphic Angassino-Terma Unit, encom-
passing the limited outcrops of quartzite and dolomitic
limestone associated with the Voltri Unit, and to keep the
term Gazzo-Isoverde Unit.
We suggest that the term Voltri Massif can be used to
describe a set of units, including the Voltri, Varazze, Pal-
maro-Caffarella and Angassino-Terma units, irrespec-
tively of different lithology and/or paleogeographic
derivation and/or metamorphism.
We support the suggestion of H
OOGERDUIJN
S
TRA
-
TING
(1991), to use the term Sestri Voltaggio Zone to
group only the Cravasco-Voltaggio and Gazzo-Isoverde
units, to emphasize their common tectono-metamorphic
evolution.
With regard to the flysch units derived from the Inter-
nal Ligurian Domain, we suggest dividing of the Val Pol-
cevera Unit into three tectonic units, that are, from the
structural top to the structural bottom, the tectonic
Ronco Unit, the tectonic Montanesi Unit and the tectonic
Mignanego Unit.
The structural stack of this part of the Ligurian Alps
is shown in the cross-section of fig. 6.
166
G
.
CAPPONI ET ALII
T
ABLE
4
Units derived from a continental margin.
T
ABLE
3
Units derived from oceanic crust and mantle.
T
ABLE
5
Flysch units.
Fig. 6 - Cross section, redrawn and modified after C
APPONI
& C
RISPINI
(2008a), showing the structural stack of the units. See fig. 5 for location.
A
CKNOWLEDGEMENTS
We would like to thank M. Maino and P. Mosca for their detailed
reviews that enabled us to greatly improve this manuscript. This
work was conducted with the financial support of the PRIN 2010
project “Nascita e morte dei bacini oceanici: processi geodinamici
dal rifting alla collisione continentale negli Orogeni mediterranei e
circum-mediterranei”, national Coordinator G. Capponi, and was
made possible by the geological mapping for the 213-230 “Genova”
Sheet, CARG Program (ISPRA and Regione Liguria) and for the 212
“Spigno Monferrato” Sheet (Regione Liguria).
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Manuscript received 8 November 2014; accepted 22 February 2015; published online 3 November 2015; editorial responsability and handling by C. Monaco.
