Reply to the comment by G. Capponi et al. on ‘Subduction polarity reversal at the junction
between the Western Alps and the Northern Apennines, Italy", by G. Vignaroli et al.
(Tectonophysics, 2008, 450, 34-50).
Gianluca Vignaroli a, Claudio Faccenna a, Laurent Jolivet b, Claudia Piromallo c, Federico Rossetti a
a
Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S.L. Murialdo 1, Rome, Italy
b
Laboratoire de Tectonique, Université Pierre et Marie Curie, 4 Place Jussieu, T 26-0 E1, Case 129, UMR CNRS 7072,
75252 Paris Cedex 05, France
c
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 65, 00143 Rome, Italy
1 - Introduction
We first would like to thank Capponi et al. (2008) for their comments and criticisms on our
paper, offering us the opportunity to discuss the data and the model presented in Vignaroli et al.
(2008a) and clarify the geological rationale behind our manuscript. Vignaroli et al. (2008a)
presented a large-scale reconstruction on the evolution of the Western Alpine-Northern Apennine
junction, based on shallow geological information derived from the Northern Apennines, the
Western and Ligurian Alps coupled with deep mantle structures from seismic tomography and
tectonic reconstructions. The aim of this paper is then to give an alternative, though simplified,
tectonic solution to the long-standing debate concerning the polarity of the subduction zone in the
central Mediterranean and its linkage with the Alpine orogeny and the formation of the arcs belt.
We condensed and simplified the huge wealth of geological information using cross-sections along
the three orogenic segments. One of the main points of the paper is that the Voltri Massif of the
Ligurian Alps is reinterpreted as an eclogitic-bearing domain exhumed by means of ductile-tobrittle extensional detachment tectonics with a top-to-the-W sense of shear. In this view, the
orogenic architecture and evolution of the Ligurian Alps presents affinities (both for geometry and
timing of deformation) with the widely accepted extensional structures recognized in the Western
Alps, in the Northern Apennines and, in general, in Alpine-type orogenic belts of the
Mediterranean. The detailed comment made by Capponi et al. (2008) is indeed centred on the
tectonic structure of the Voltri Massif (probably this comment should have been addressed to our
companion paper, Vignaroli et al., 2008b, focused on the Voltri Massif structures and available online on March 2008). The main point of the comment is that the exhumation of High-Pressure (HP)
metamorphic units exposed in the Voltri Massif was produced by thrusts rather than by synorogenic extensional detachments. In this reply, we would first like to make some general
considerations on the criteria/concepts adopted for the interpretation of the exhumation-related
structures and we will then discuss point-by-point the criticism of Capponi et al. (2008).
1
2 - Extension as a major mechanism of rock exhumation
Extension is widely recognised as a major mechanism for the unroofing of the HP-units
during (syn-orogenic) and after (post-orogenic) the build up of a mountain belt (e.g. Dewey, 1988;
Platt, 1993). In exhumed orogenic domain, extensional tectonics operates by means of retrogressive,
syn-greenschist mylonitic shears that rework previous compressional structures, with a typical
progression from ductile to brittle deformative conditions and shear localisation along brittle
detachment (e.g. Miller et al., 1983; Platt, 1986; Selverstone, 1988; Lister & Davis, 1989; Gautier
et al., 1993; Fletcher et al., 1995; Butler & Freeman, 1996; Balanyà et al., 1997; Jolivet et al., 1998;
Bucher et al., 2003). Distinctive characters of the extensional detachment in the field have been
described extensively and are mainly based on: (i) the upward progressive increases in non-coaxial
strain moving in the lower-plate units towards the shear zone; (ii) the presence of a normal-sense
metamorphic gap between upper-plate and lower-plate units. Although lower-plate units are always
represented by polymetamorphic rocks, upper-plate units comprise both non-metamorphic (e.g. in
the Cyclades Islands; Gautier et al., 1993) and/or metamorphic rocks (e.g. in Corsica: Daniel et al.,
1996; in Betic Cordillera: Galindo-Zaldívar et al., 1989; Martinez-Martinez et al., 2002). Then, synorogenic extensional detachments can be sandwiched inside an overall nappe stack where HP-units
are positioned at the bottom, but also on top, of the nappe pile. In these areas, detailed radiometric
analysis shows that upper-plate HP-units exhumed early in the orogenic history (or, at least, were
located at upper structural levels in the orogenic wedge), predating the onset of extensional
tectonics. Also in this case, the normal-sense metamorphic gap persists moving upward across the
detachment boundary.
The Alpine belts of the Mediterranean offer spectacular examples of these structures because
complex paleogeography produced punctuated accretion of both oceanic and continental blocks at
trench inducing pulse of exhumation in the inner portion of wedge (e.g. Platt, 1986; Jolivet et al.,
2003; Rosenbaum & Lister, 2004). In the inner sectors of both the Western Alps and the Northern
Apennines, the post-orogenic extension is described by greenschist facies mylonitic fabric, showing
both top-to-the-W and top-to-the-E tectonic vergences, with a continuous evolution from ductile to
brittle conditions (e.g. Philippot, 1990; Wheeler & Butler, 1993; Daniel et al., 1996; Jolivet et al.,
1998; Agard et al., 2001; Reddy et al., 2003). These extensional features have been described as
first-order structures driving the unroofing of the deep-seated rocks and controlling the present-day
boundaries between the different tectonic units. In the Voltri Massif, where the two orogenic
segments merge, only Hoogerduijn Strating (1994) describes the presence of extensional tectonics
2
within the orogenic nappe pile, while the common tectonic models postulate the exhumation of the
HP-rocks by means of polyphase compressional structures (e.g. Chiesa et al., 1975; Capponi et al.,
1999a; Vissers et al., 2001; Capponi & Crispini, 2002).
3 - The tectonic nappe pile and the metamorphic imprints
Figure 1a of Capponi et al. (2008) shows the geological sketch map of the Voltri Massif and
the surrounding areas in which the metamorphic units are grouped based on the peak metamorphic.
In our opinion, this way to distinguish the tectonic units is not sufficient alone to describe their
kinematic paths during the subduction-exhumation cycle, because it does not take into account (i)
the retrograde exhumation-related textural-metamorphic evolution and (ii) the geometric-structural
relationships of the different tectonic units. Our revised structural and kinematic scenario for the
Voltri Massif is instead based on the deformation and metamorphic characteristics of the major
tectonic boundaries separating the different units. This approach, integrated with geological
mapping, allows distinguishing the Lower Tectonic Complex (LTC), where the development of the
D2 syn-greenschist shear fabric is penetrative and constitutes the regional deformation feature, from
the Upper Tectonic Complex (UTC), where both metamorphic (pre- and Alpine) rocks and
sedimentary units are piled up without any evidence for the D2 ductile shearing. Then, based on the
evidence that (i) an increase of the D2 deformation occurs moving upward from the LTC towards
the contact with the UTC; (ii) a progressive brittle overprint operated by extensional structures is
documented in the LTC, attesting for the exhumation of the ductile shear zone; (iii) a kinematic
compatibility exists between mylonitic shearing in the LTC and the normal fault systems affecting
the LTC-UTC contact (same orientation of the maximum extension direction); (iv) a normal-sense
metamorphic gap occurs between sedimentary units (UTC) on top of lower-plate metamorphic ones
(LTC) (e.g. the Case Ferrere area; Cortesogno and Haccard, 1984; Hoogerduijn Strating, 1994; see
Fig. 9b in Vignaroli et al., 2008b); and (v) a tectonic contact controls the juxtaposition of the
lowermost levels of the Tertiary Piedmont Basin (the Costa Cravara Breccias; Charrier et al., 1964)
on top of the eclogite-bearing metaophiolites (see Fig. 3b in Vignaroli et al., 2008a), we argue that
the structural architecture of the Voltri region and the final exhumation of the deep-seated eclogitic
rocks can be better explained in a scenario dominated by nappe excision rather than nappe
construction (e.g. Wheeler and Butler, 1993).
a) The Erro-Tobbio peridotites
3
Published works on the Erro-Tobbio Unit (e.g. Hoogerduijn Strating et al., 1993;
Scambelluri et al., 1991; 1995; Hermann et al., 2000), have demonstrated that the peridotites
outcropping in the eastern part of the Voltri area (near the Tobbio Mount) preserve an eclogitic
metamorphism (close to the ultra-high pressure conditions) related to the Alpine subduction and
ascribed to the Late Cretaceous (Scambelluri et al., 1991). Hermann et al. (2000) stated that “the
Alpine assemblage (consisting of antigorite, titanian clinohumite, diopside, chlorite and magnetite)
is restricted to a few olivine and titanian clinohumite veins coeval with mylonitization of mafic
dykes under eclogitic facies conditions”. On the other hand, mylonitization and blastesis are
pervasive in the serpentinite rocks enveloping the peridotites. We agree with Capponi et al. (2008)
that this deformation partitioning process can occur during subduction metamorphism and, then,
fully serpentinized peridotites and less intensely serpentinized peridotites can occur. The structural
arrangement proposed by Capponi et al. (2008), largely based on the recent geological mapping
(Capponi and Crispini 2005, 2006a,b,c), considers the lherzolites of the Erro-Tobbio Unit as a
whole part of the Voltri ophiolitic mélange, questioning attribution to a distinct tectonic unit. This
interpretation poses some problems. First, as far as the tectonic interpretation proposed, it is
irrefutable that rootless lherzolitic bodies occur on top of the Beigua Unit being bounded by semifragile to cataclastic contacts (Chiesa et al., 1975; Hoogerduijn Strating, 1994; Vissers et al., 2001).
Also works by Capponi and co-workers (1986, 1999a,b) describe these contacts as sub-horizontal
thrusts surfaces. Furthermore, (i) also in the cited geological maps (Capponi and Crispini 2005,
2006a,c) most of the boundaries between the lherzolites and the Beigua serpentinites are tectonic,
being systematically marked by high-angle fault systems; and (ii) steeply-dipping, greenschistfacies shear fabric controls contact between lherzolites and Voltri units in the eastern sector of the
massif (Capponi & Crispini, 2002). Second, the age of the eclogitic metamorphism is
unconstrained. Considering a Late Cretaceous age as proposed in Scambelluri et al. (1991), it is
hard to explain how such rocks can be incorporated in a polymetamorphic Eocene mélange that
records a continuous exhumation stage from peak eclogitic condition to greenschist re-equilibration.
Third, our geological-structural analysis shows that there is incompatibility between the structuralmetamorphic features recognised in the Beigua and the Voltri-Rossiglione units with those detected
in the peridotite bodies. In particular, structural observations show that the syn-greenschist
mylonitic deformation does not penetrate the lherzolite bodies; rather, these ultra-mafic bodies
juxtapose as allochthonous units above the LTC by sub-horizontal, top-to-the-W ductile-to-brittle
shear surfaces. Summing up, we cannot exclude that some of the lherzolite bodies experienced a
similar burial-exhumation cycle coherent with the one of the Beigua Unit, but the structural
arrangement is more compatible with the interpretation considering the Erro-Tobbio Unit as a
4
rootless body resting on top of the Voltri tectonic edifice. Tectonic juxtaposition between these
groups of units likely occurred during the late-stage, syn-greenschist exhumation of the LTC, when
the Erro-Tobbio Unit had already reached nearly surficial conditions. In this view, a metamorphic
gap exists between the two complexes and extensional denudation of the LTC, rather than thrusting,
better accounts for this tectonic setting.
b) The Cravasco - Voltaggio Unit and c) Very low grade Alpine tectonic mélange
Following geological mapping by Cortesogno & Haccard (1984), our structural setting
revised the Sestri-Voltaggio Zone as a tectonic mélange, which includes Triassic-Liassic limestones
and dolomites, radiolarites, diabase, serpentinites associated with metagabbros and prasinites
showing blueschist peak metamorphism. In particular, the presence of anchimetamorphic
metasediments (named as sf and scL by Cortesogno & Haccard, 1984) in which mega-block of
metamorphic and non-metamorphic units are embedded, justifies the formation of a mélange in very
low-grade metamorphic conditions. In our tectonic scenario, the contact between the LTC and the
Sestri-Voltaggio Zone is controlled by the retrogressive syn-greenschist detachment system
evolving, in time and space, toward more brittle conditions. As for the Erro-Tobbio Unit, these
features are not recorded by the units of the Sestri-Voltaggio Zone and, consequently, a structuralmetamorphic break persists between the low-grade metamorphics of the Sestri-Voltaggio Zone and
the underlying LTC.
d) Low-grade metabasites
Although we omitted to specify the HP-metamorphism experienced by these rocks, the same
structural criteria used for the lherzolites were applied to the relationships between the metabasites
of the Mt Colma and Ortiglieto Lake with respect to the syn-greenschist main foliation in the LTC.
We observed that the mylonitic foliation in the LTC is brutally cut by sub-horizontal, ductile-tobrittle shear surfaces on which the metabasite blocks rest. The presence of a structural break is
evident from the inconsistency between attitude of S2 in the Voltri metasediments and the tectonic
contacts bounding the metabasites (Capponi et al., 2005; Vignaroli et al., 2008b).
e) The Oligocene-Miocene contractional structures and the Costa Cravara Breccias
A vast literature considers the kinematic structures in the Voltri units and surrounding
regions as contractional. Structures related to the early nappe emplacement have been considered as
showing top-to-the-W/NW sense of shear (e.g. Hoogerduijn Strating, 1994; Vissers et al., 2001;
Capponi & Crispini, 2002), while late-orogenic ones are top-to-the-E/NE (e.g. Capponi et al., 1999;
5
Capponi & Crispini, 2002; Piana et al., 2006), with the exception of the Case Ferrere area where
shear senses are top-to-the-W (Capponi et al., 1998). Based on this, the NE-directed back-thrusting
of the early formed Alpine belt is advocated for the formation of the Apennine belt. In most of these
papers, it is shown that brittle thrusting is the consequence of progressive ductile-to-brittle
contractional shearing (Capponi et al., 1999; Piana et al., 2006). Capponi et al. (2008) also
emphasize the contractional nature of the Tertiary Piedmontese Basin (TPB) on the base of the
geophysical modelling by Miletto and Polino (1992), although N-vergent thrusting of
metaophiolitic units onto TPB sediments has not been actually described in the Voltri Massif.
As reported in the literature, the elusive knowledge of the original nappe stack and the
superimposition of deformation events do not permit an univocal interpretation of structures
accommodating crustal-scale extension or shortening in metamorphic domains (e.g. Butler and
Freeman, 1996; Bucher et al., 2003). For example, kinematics of flat-lying shear zones cannot be
determined by simple correlation of homologous level at the hangingwall and the footwall.
Additional criteria should be considered, such as (i) the evolution of the style (coaxial or noncoaxial) and the regime (ductile to brittle) of the deformation moving through the rocks at the
footwall, and (ii) the presence of metamorphic gaps inside the nappe stack. A common assumption
done in the literature of the Voltri Massif is the interpretation of plicative structures as pertinent
only to contractional setting (Capponi, 1991; Capponi & Crispini, 2002; Capponi et al., 1986,
1999a; Vissers et al., 2001). However, it is widely accepted that folds at the footwall of major
extensional detachment system trend both parallel and orthogonal to the main stretching direction
(e.g. Spencer, 1984; Fletcher & Bartley, 1994; Mancktelow & Pavlis, 1994; Axen & Bartley, 1997;
Martínez-Martínez et al., 2002). Furthermore, our dataset does not corroborate the early
compressive interpretations for the ductile-to-brittle evolution of the Voltri units. The kinematic
analysis of brittle structures is compatible with the general E-W direction of ductile non-coaxial
(top-to-the-W) shearing in the LTC (Vignaroli et al., 2008b). Within this context, the sub-horizontal
shear planes described in the Bandita (Capponi et al., 1999a) and Case Ferrere areas (Capponi et al.,
1998) have been revised as semi-brittle top-to-the-W structures controlling the juxtaposition of
lherzolites and Triassic limestones on top of the Beigua Unit (D3 phase in Vignaroli et al., 2008b).
Our extensional interpretation also accounts for: (i) the top-to-the-W detachment described by
Hoogerduijn Strating (1994); (ii) the occurrence of structures recording extension parallel to the
regional foliation (Capponi & Crispini, 1997); (iii) the evidence of progressive ductile-to-brittle
evolution during shearing in the LTC; and (iv) the presence of E-W-trending folds described for the
metasedimentary sequences of the Voltri Massif (e.g. the F1/F2 folds of Capponi, 1991). In this
view, compressive structures are eventually limited to the late stage evolution of the exhumed deep6
seated units of the Voltri Massif, in the frame of the Late Miocene to Pleistocene Apennine
overthrusting involving the Adriatic plate (e.g. Castellarin, 2001 and references therein).
The hypothesis of an extensional regime in the Voltri Massif is also supported by the
Oligocene-Early Miocene tectono-sedimentary evolution of the TPB. We agree with Capponi et al.
(2008) that sedimentary deposits are primarily trasgressive onto the metamorphic units, but
evidence of cataclastic deformation is reported in the Costa Cravara Breccias (e.g. Franceschetti,
1967; Allasinaz et al., 1971; Capponi and Crispini, 2002). In particular, we found (e.g. near the Mt.
Tobbio, near the Tiglieto village, in Case Ferrere area, near Piani di Praglia), evidences of flat-lying
metre-scale tectonic breccias marking contacts between the Costa Cravara Breccias and the
underlying serpentinites of the Beigua Unit. In the breccias, extensional and strike-slip fault
systems, often producing metre-scale fault damage zones, have been detected (Vignaroli et al.,
2008b). In addition, the recent analysis of the anisotropy of magnetic susceptibility data gathered
from the TPB samples (Maffione et al., 2008) supports an extensional tectonic setting for the synsedimentary evolution of the basin and suggests that compressive features may be related to postMiocene episodes.
f) The Lower Tectonic Complex and the Palmaro-Caffarella Unit
As mentioned above, the LTC consists of the Voltri-Rossiglione Unit (including the
Palmaro-Caffarella Unit) and the Beigua Unit, based on the coherent D2 structural pattern that we
recognised throughout the complex. Our observations carried out in rocks pertaining to the
Palmaro-Caffarella Unit of Capponi et al. (2008) document a main retrogressive, syn-greenschist
SL-fabric, with the general E-W stretching lineation. The D2 fabric is transpositive and
mineralogical assemblages of the early HP-metamorphic stage can be observed as relicts preserved
in D2 low-strain domains. Kinematic indicators coherently point to top-to-the-W (from WSW to
WNW). In these terms, the metasedimentary sequences of Palmaro-Caffarella Unit of Capponi et al.
(2008) reached indeed blueschist facies conditions, but their way back to the surface share similar
evolution to the one described for the metasedimentary units of the LTC. Eventually, the geological
map in Fig. 3a of Vignaroli et al. (2008a), is a simplified version of the more detailed one presented
in Vignaroli et al. (2008b).
4 - The tectonic scenario for exhumation in the Voltri Massif
The metamorphic peak conditions reported in Fig. 2 of Capponi et al. (2008) have been
interpreted by the Authors as indicative of a continuous cycle of subduction-exhumation attained in
7
a channel flow mechanism (Federico et al., 2007). However, the incomplete Alpine P-T-t
trajectories described for the Erro-Tobbio Unit, the allochtonous metabasites (MB), the Savona and
the Figogna units hamper to the same subduction-exhumation cycle model to these units. This
ambiguity is also evident from the available geochronological data of the Voltri units (Rubatto &
Scambelluri, 2003; Federico et al., 2005, 2007), where radiometric HT and LT closure systems
provide contrasting exhumation rates. In particular, a major age cluster occurs concurrently with the
onset of the TPB sedimentation at about 34 Ma. As far as the interpretation proposed for this
metamorphic stage (either peak or retrogressive metamorphism), this implies moderate (Federico et
al., 2005) or ultrafast exhumation rates (Rubatto & Scambelluri, 2003) that are at odds with the
steady-state channel flow model presented by Federico et al. (2007) in which exhumation should be
instead assisted by erosion.
We presented a new tectonic scenario for the Voltri Massif characterised by a change in
tectonic regime passing from lithospheric convergence and transpression (stages a and b in Figs. 6
and 7 of Vignaroli et al., 2008a) to extensional setting (stage c in both Figs. 6 and 7 of Vignaroli et
al., 2008a). In particular, we propose a two-stage exhumation for the Voltri Massif eclogites that
includes (i) an early non-coaxial (top-to-the-N/NNW) syn-blueschist deformation concomitant with
forced circulation in subduction channel (Vignaroli et al., 2005), and (ii) a subsequent fluid-assisted
syn-greenschist deformation controlled by extensional detachment tectonics (Vignaroli et al.,
2008a,b). Extensional tectonics was responsible for rock exhumation only from the greenschist PTconditions, leaving unconstrained the early exhumation path. Neither the text nor the figures of
Vignaroli et al. (2008a) support the hypothesis that the extensional setting determined the
exhumation of HP-rocks from approximately 200 km depths as stated in Capponi et al. (2008).
If our interpretation of the Voltri Massif as an extensional domain is correct, then the
linkage between the syn-greenschist extensional settings recognised in the Western Alps (e.g. Platt,
1986; Dewey, 1988; Philippot, 1990; Wheeler and Butler, 1993; Agard et al., 2002; Reddy et al.,
2003) and the Northern Apennines (Carmignani and Kligfield, 1990; Daniel et al., 1996; Jolivet et
al., 1998; Rossetti et al., 1999; Brunet et al., 2000) sorts out. This supports a unitary tectonic
scenario where post-orogenic extensional tectonics affected the Alpine-Apennine inner domains and
favoured the exhumation of deep-seated rocks (e.g. Platt, 1986; Dewey, 1988; Faccenna et al.,
2001; Jolivet et al., 2003; Rosenbaum & Lister, 2004). This tectonic scenario is supported by the
paleomagnetic data collected in the TPB (Maffione et al., 2008) advocating the formation of the
basin in an extensional regime synchronous with the bending of the Alpine chain. These data first
demonstrate that the arcuation of the western Alpine arcs occurred during the drifting of the
Corsica-Sardinia block. This implies that the two subduction systems, the one active below the Alps
8
and the one below the Apennines, are strictly linked each other and that the curvature of the
Western Alps is genetically related to the rollback of the Apennine slab. This also implies that
mantle material below the slab is forced to circulate and to migrate laterally, attaining a toroidal
flow pattern during slab rollback. We can then speculate that the arching of the Western Alps could
have been connected with this local pattern of mantle circulation favouring the arching process. In
the light of the comment raised by Capponi et al. (2008), we wish to point out that an adiabatic
upper mantle circulation is not expected to produce any heating. A heating pulse is expected only if
the mantle lithosphere is stretched in the back-arc domain or delaminated during convergence (e.g.
McKenzie, 1978, Bird, 1979; Brun and Faccenna, 2008). Despite the Voltri eclogites record an
increase of temperature during their retrogressive path (Messiga & Scambelluri, 1991; Vignaroli et
al., 2005), geological evidence for large-scale heating as claimed by Capponi et al. (2008) is not
contemplated in our model, since most of the exhumation (from eclogitic to greenschist conditions)
predated the onset of the extensional tectonics. Similar tectonic setting of HP/LT domains early
exhumed in the orogenic history has been described in the Mediterranean region (Aegean region,
Gautier et al., 1993; Parra et al., 2001; Alpine Corsica, Jolivet et al., 1990; Daniel et al., 1996;
Betic Cordillera, Galindo-Zaldívar et al., 1989; Platt et al., 2003).
5 - Conclusions
In our paper, we try to frame the evolution of the Voltri Massif into the regional tectonic
scenario of the Alps-Apennines system, where the Late Eocene-Early Oligocene extensional
tectonics reshaped the former orogenic construction, concomitantly with the retreating of the
Apennine slab (Jolivet et al., 2003). Data from the Voltri Massif document that its structural
arrangement is the result of a late-stage, top-to-the-W retrogressive (syn-greenschist) mylonitic
shears, evolving from ductile to brittle deformative conditions and reworking the early
compressional boundaries (Vignaroli et al., 2008b). Our extensional view provides a unitary, more
feasible, tectonic scenario for the post-orogenic evolution of the Alps-Apennine internal domains.
This tectonic framework supports the interpretation of the Apennines belt as an independent
orogenic chain, evolved during constant northwest-dipping retreating subduction (Principi and
Treves, 1984; Faccenna et al., 2001; Rosenbaum & Lister, 2004).
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9
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