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(a) Department of Earth Sciences , Sapienza University of Rome, Piazzale Aldo Moro, 5, 00185, Rome, Italy. E-mail: first.author@gmail.com
(b) Institute . ………
(c) School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK
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Manuscript history: received …………. 2013; accepted ………….. 2013; editorial responsibility and handling by Domenico Calcaterra.
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EXAMPLE OF CONTRIBUTION
ABSTRACT
In the Southern Apennines and Calabria outcrop metamorphic slices
of hercynian and/or alpine age. The reconstructions by Glauco Bonardi
and co-authors of an alpine subduction-related origin for the emplacement
of those rocks is still supported by a number of tectonic and geodynamic
constraints. Alps and Apennines are two end members of opposite
subduction zones (i.e., “E”-ward and “W”-ward), having different depth
and evolution of the accretionary prism basal decollement. The PTt
evolution of the metamorphic rocks of Calabria can be better reconciled
with an earlier alpine-type evolution, where the continental and oceanic
slices of crust have been subducted and re-exhumed by the deepening of
the decollement planes into both the upper and lower plates and accreting
the orogen. The early alpine emplacement of those metamorphic rocks is
testified by their high structural elevation, the HP/LT assemblages and by
stratigraphic constraints (e.g., Bonardi et al., 2005). Therefore it can be
interpreted that the earlier alpine subduction was gradually substituted by
the opposite W-directed Apennines subduction, having in its hangingwall
the boudinated relics of the alpine history.
KEY WORDS: Alps, Apennines, crystalline basement, subduction.
INTRODUCTION
Glauco Bonardi was not only a great geologist, but also a
fine gentleman and a good friend (Fig. 1). We had a number of
discussions on the tectonic evolution of the Southern
Apennines and the Calabrian arc. One of the statements that I
cannot forget is his aristotelian view of science, particularly
when referring to any geodynamic model of the Apennines:
“prima mostra e poi dimostra”, i.e., “first show, and then
demonstrate”, or in other words “first show the data, then you
can make a model based on those data”. The issue we were
trying to face is the existence or not of an “alpine” core in the
Apennines. There are a number of models depicting the
Southern Apennines and Calabria, including their internal
metamorphic thrust sheets, as a fold and thrust belt generated
by the Apennines subduction alone, without an earlier
subduction zone directed toward the opposite direction (e.g.,
Knott, 1987; Rossetti et al., 2001).
Alternatively, since historical papers such as Amodio
Morelli et al. (1976), until Bonardi et al. (2005), the
metamorphic slices outcropping in Calabria and Peloritani
mountains have been attributed to an earlier alpine phase,
followed by the later apenninic history (Alvarez et al., 1974).
Amodio Morelli et al. (1976) interpreted the metamorphic
slices of ophiolites and basement rocks in Calabria as
correlatable to the Pennidic and Austroalpine units of the
Western Alps. Bonardi et al. (2005) suggested that the lower
Miocene Paludi Formation seals the European vergent alpine
thrusts, whereas it is involved by the Africa (Adriatic)-vergent
Apennines thrusts. A number of papers updated the PTt and
structural history of those metamorphic and magmatic slices
(Caggianelli et al., 1991; 1999; Cello et al., 1996; Graessner et
al., 2001; Rossetti et al., 2001; Liberi et al., 2006; Heymes et
al., 2010; Langone et al., 2010; Appel et al., 2011) overlying
the sedimentary cover (Bonardi et al., 1994; Iannace et al.,
2007; Parente et al., 2007).
The motion of the subduction hinge toward or away relative
to a fixed upper plate has been shown to be the simple
kinematic indicator of the occurrence of two different
subduction styles (Doglioni et al., 2007). When the subduction
hinge converges toward the upper plate, the upper plate is
shortened and a double vergent belt, such as the Alps, forms.
On the contrary when the slab and the related hinge retreat
relative to the upper plate, the upper plate is stretched (and a
backarc basin opens), and a single vergent belt develops, such
as the in the Apennines-Tyrrhenian Sea system.
Alpine relics are boudinated in the hangingwall of the
Apennines, stretched by the Tyrrhenian backarc rifting
(Doglioni et al., 1998). Relative to the upper plate, the
subduction hinge moved toward it in the Alps (or alpine-type
belts), allowing deep thrusts involving both upper and lower
plates, with prograde and retrograde metamorphism. In the
Apennines, the subduction hinge migrated away with respect to
the upper plate (from late Eocene to Present, Carminati et al.,
2010; 2012). Therefore, in the Apennines there is not an upper
plate actively converging with respect to the lower plate,
although the transition between the two opposite subduction
systems may have lasted about 20 Ma, from the late Eocene to
the early Miocene. The Apennines kinematics and depth of the
decollement atop the lower plate precludes the possibility to
involve deep slices of the basement and the thrusting of both
plate margins.
GEOLOGICAL SETTING
The Alps and the Apennines (Beltrando et al., 2010, and
references therein) are associated to two opposite subductions,
which inverted the Permo-Mesozoic tethyan passive
continental margins bordering segments of southern Europe,
the Adriatic plate, northern Africa and possibly an intevening
microplate (Doglioni, 1992; Guerrera et al., 1993). The Alps
are related to the subduction of Europe ”eastwardsoutheastward” underneath the Adriatic plate (Panza &
Mueller, 1978; Dal Piaz et al., 2003; Dal Piaz, 2010;
Brandmayr et al., 2011), whereas the Apennines were
generated by the “westward” subduction of the Adriatic-Ionian
lithosphere (Scandone, 1980; Malinverno & Ryan, 1986;
Doglioni, 1991; Doglioni et al., 1999; Scrocca et al., 2005;
Chiarabba et al., 2008).
In spite of a similar background, Alps and Apennines have
very different geologic signatures and represent two end
members of orogens related to subduction zones (Fig. 2). In
synthesis, the following characters can be recognised in Alps
and Apennines respectively: 1) Subduction hinge converging
TITLE OF THE CONTRIBUTION (STYLE: HEADING ODD PAGES)
3
Fig. 2 – The two end-members of subduction zones show distinct PTt evolution. The metamorphic rocks outcropping in Calabria and internal Apennines are
interpreted as related to the Alpine type evolution, which implies deeper decollement planes, affecting both the upper (U) and the lower (L) plates. In the
Alpine type to the left, the subduction hinge (H) converges relative to the upper plate, whereas it diverges in the Apennines type to the right. The Apennines
type section in reality has the lower plate presently diverging relative to the upper plate. Moreover, according to the model presented, the Apennines contains
the boudinated relicts of the earlier Alpine orogen. The Apennines subduction should have developed along the retrobelt of the Alpine belt.
and diverging relative to the upper plate; 2) Double vs. single
vergence; 3) High vs. low morphological and structural
elevation; 4) Deep vs. shallow rocks involved; 5) The
occurrence of higher metamorphic degree vs. lower
metamorphic degree; 6) The basal décollement involves the
crust and the LID of both upper and lower plates whereas only
the shallow crust of the lower plate contributes to the
accretionary prism; in the Apennines, the upper plate was not
structured as a contractional retrobelt (as a consequence the
term collision should be abandoned), but it was stretched and
dismemberd to form the backarc basin; 7) Alpine shallow vs.
apenninic deep foredeep; 8) Low vs. high dip of the foreland
monocline; 9) Thickened crust all over the alpine belt, whereas
it is thinner in the western side of the Apennines; 10) The Alps
have both in the upper and in the lower plate a pre-subduction
Moho, whereas the Apennines have in the footwall plate a presubduction Moho, but in the hangingwall they have a new
forming Moho; 11) Thickened lithosphere vs. a shallow
asthenosphere in the hangingwall; 12) No vs. well developed
backarc basin and related alkaline-tholeiitic magmatism; 13)
Scarce vs. larger abundance of subduction-related volcanism;
14) Smooth vs. high amplitude gravity and heath flow
anomalies (Carminati & Doglioni, 2012). The differences
between the two belts seem to be sensitive to the geographic
polarity of the subduction. In the hangingwall of the Apennines
subduction there occurs the boudinated and stretched relict of
the former double verging Alps, and only more externally in
the central-eastern side the real Apennines accretionary prism
was developed. The Apennines nucleated along the retrobelt of
the Alps, where there was oceanic or thinned continental
lithosphere to be consumed.
Corsica thrust sheets
The thrust sheets in northeast Corsica are usually attributed
to the Alps, due to their similarity with the Schists Lustrés with
meta-ophiolites cropping out in the Alps. The main
emplacement of those rocks occurred during the Eocene, well
before the Oligo-Miocene opening of the backarc Provençal
Basin. Therefore, once restored the Early Miocene
counterclockwise rotation of the Corsica-Sardinia microplate,
the Corsica thrust-sheets match the Maritime Alps. In addition,
4
FIRST AUTHOR ET AL.
(STYLE: HEADING EVEN PAGES)
the Alps front can be connected to the Betics front (southern
Spain) through the Balearic promontory. For these reasons
Alps, Alpine Corsica and the Betics can be reasonably
considered to be part of the same belt (Doglioni et al., 1997;
1998; Carminati et al., 2010).
The Alps and similar contractional belts are double vergent
and are typically 200-250 km wide. According to this
constraint, the front of the alpine retrobelt associated with the
Corsica front should have been at least 200 km more to the
east. The alpine thrust sheets outcropping in Corsica can be
followed in the western Tyrrhenian Sea both in seismic lines
and dredging. The metamorphic basement slices outcropping in
Tuscany, Calabria, and northeast Sicily could be interpreted as
relicts of that inherited orogen (e.g., Bonardi et al., 1994).
Alpine relicts, stretched and collapsed by the backarc extension
can be recognized in the Tyrrhenian basin (see sections by
Elter & Pandeli, 1990; Mauffret et al., 1999; Pascucci et al.,
1999; Sartori, 2005). Also magmatism supports the occurrence
of an alpine metasomatized mantle within the Apennines
(Peccerillo, 2005). Along their southern prolongation, the Alps
were probably still active (late Oligocene-early Miocene) while
the Apennines subduction initiated. This could explain the coexistence of early Miocene HP/LT metamorphism (Alpine
type) in the Tuscan archipelago synchronous with western
Sardinia calcalkaline magmatism and foredeep subsidence
suggesting the started activity of the Apennines subduction.
Similarly, in the Southern Apennines-Calabria, the transition
between the ealier Alpine subduction to the Apennines flip
may have persisted until the early Miocene (Bonardi et al.,
2005).
ACKNOWLEDGMENTS
I thank the organizing committee of the meeting to honorate
Glauco, a geologist who deeply contributed to the advancement of
geology in Italy. I also want to remember his wife Paola De Capoa
who always assisted Glauco, sharing his kindness, geological skill,
and widespread culture.
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