Plate Tectonics and the Boundary between Alps and Apennines

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Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 128, No. 2 (2009), pp. 317-330, 7 figs. (DOI: 10.3301/IJG.2009.128.2.317)
Plate Tectonics and the Boundary between Alps and Apennines
ANDREA ARGNANI (*)
ABSTRACT
retroarco (REHAULT et alii, 1984; MALINVERNO & RYAN, 1986; KAet alii, 1988) richiedono l’esistenza di un’ampia area oceanica
ubicata a ovest del Promontorio Adriatico. Pertanto, la collisione
continentale che ha dato origine alle Alpi s.s. non poteva continuare
verso SO ma doveva passare lateralmente, lungo il margine di placca, ad una subduzione oceanica. L’inversione di polarità nella subduzione che si osserva attualmente andando dalle Alpi, dove la placca africana sovrascorre quella europea, agli Appennini, dove
avviene il contrario, è ritenuta una caratteristica originaria del sistema orogenico mediterraneo, attiva sin dall’inizio della convergenza. La ricostruzione cinematica del movimento delle placche
consente di ricostruire il tracciato del punto nel quale avviene il
passaggio da una polarità all’altra. Dal Cretaceo superiore all’Eocene il movimento relativo verso nord del Promontorio Adriatico ha
portato il punto che marca il passaggio di polarità a spostarsi nel
tempo lungo il margine. Pertanto, aree che avevano subito la collisione continentale alpina sono state successivamente interessate da
una subduzione oceanica a polarità opposta, appenninica. Questa
sequenza di eventi ha portato al collasso della catena alpina della
Corsica (e.g., BRUNET et alii, 2000) e successivamente all’apertura
del bacino di retroarco balearico, che sì è originato dall’Oligocene
superiore al Miocene inferiore al disopra di una subduzione oceanica arretrante verso est (REHAULT et alii, 1984). Il Tirreno settentrionale, invece, si è originato nel Miocene medio (MAUFFRET &
C ONTRUCCI , 1999) a causa della delaminazione della litosfera
continentale del margine adriatico che ha fatto seguito alla consunzione della litosfera oceanica durante la rotazione della microplacca
Sardo-Corsa (ARGNANI, 2002). La principale implicazione della ricostruzione proposta è che il limite Alpi-Appennino non va considerato come una linea tettonica ma piuttosto come un segmento di catena che è stato inizialmente interessato da una tettonica alpina e poi,
successivamente, da una tettonica appenninica.
STENS
A new proposal attempting to solve the long-debated issue of
the polarity of subduction in the Corsica-Northern Apennine system
is presented. Models adopting an original W-dipping subduction and
models preferring a flip in the polariy of subduction, from E-dipping
to W-dipping, encounter major difficulties at a regional scale. It is
considered here that the main inconsistencies faced by both models
are due to the two-dimensional approach of reconstructions. The
Late Cretaceous to Present-Day kinematics of the Central Mediterranean has been reconstructed using the magnetic anomalies in the
Atlantic Ocean and assuming a solid connection between Africa and
Adria. Oligocene to Present calcalkaline volcanism and backarc
extension in the Balearic and Tyrrhenian basins requires the presence of a wide oceanic embayment to the west of the Adriatic
Promontory. It follows that the continental collision that gave rise to
the Alps s.s. could not continue SW-ward of Adria. The flip of subduction polarity that can be currently observed, going from the Alps,
where Africa is overriding Europe, to the Apennines, where the
opposite occurs, was likely on original feature since the beginning of
convergence. Kinematic reconstructions allow the point along the
plate boundary where the flip of polarity occurs to be tracked back
in time. Following the N-ward motion of the colliding Adriatic
Promontory, the point of polarity flip moved along the plate boundary from Late Cretaceous to Eocene. As a result, areas that previously experienced continental collision were subsequently affected
by oceanic subduction. This sequence of events led to the collapse of
the Alpine belt of Corsica and to the opening of the Balearic backarc
basin above a retreating oceanic subduction. A similar kinematic
evolution is currently ongoing in Taiwan. Finally, the Northern
Tyrrhenian basin opened when delamination affected the Adriatic
continental margin, following the consumption of oceanic lithosphere at the end of Corsica-Sardinia rotation.
KEY WORDS: Alps-Apennines boundary, Alpine Corsica,
Northern Apennines, Plate kinematics, subduction
polarity.
TERMINI CHIAVE: Limite Alpi-Appennino, Corsica Alpina,
Appennino settentrionale, cinematica delle placche,
polarità di subduzione.
INTRODUCTION
RIASSUNTO
La Tettonica delle Placche e il limite tre Alpi e Appennino.
Viene presentata una nuova ipotesi interpretativa che cerca di
risolvere il dibattuto problema della polarità della subduzione nel
sistema Corsica–Appennno settentrionale. Sia i modelli che adottano un’unica subduzione immergente verso ovest (es. PRINCIPI &
TREVES, 1984), sia i modelli che seguono l’ipotesi di un flip di polarità, inizialmente est-immergente poi ovest-immergente (es. BOCCALETTI et alii, 1971), incontrano problemi a una scala regionale.
Si ritiene che buona parte di queste incongruenze sia legata
all’approccio sostanzialmente bidimensionale che viene adottato. La
cinematica del Mediterraneo centrale dal Cretaceo superiore all’attuale è stata ricostruita utilizzando i poli di rotazione derivati dalle
anomalie magnetiche dell’Oceano Atlantico (DEWEY et alii, 1989;
MAZZOLI & HELMAN, 1994) e considerando Adria come un promontorio africano (CHANNELL, 1996). Il vulcanismo calcalcalino dell’intervallo Oligocene-Attuale (SAVELLI, 1988) e l’apertura dei bacini di
(*) ISMAR-CNR. Via Gobetti, 101 - 40129 Bologna. E-mail:
[email protected].
The geology of the region encompassing Corsica, the
Northern Apennines and Western Alps is remarkably
puzzling as the opposite structural polarity observed in
Alpine Corsica and the Northern Apennines has to be
explained and related to the evolution of the Alps
(ALVAREZ, 1976). These issues have been addressed by
several Authors, who mostly focussed on the relationships between Corsica and the Northern Apennines; in
some occasions the relationships of both orogenic belts
to the Alps has been also attempted (e.g., ELTER & PERTUSATI, 1973; ALVAREZ, 1991). Interpretations concerning Corsica and the Northern Apennines can be broadly
framed within two groups. In the first group the initial
eastward (European) subduction was followed by a flip
of polarity, leading to the westward (Apennine) subduction of Adria (e.g., BOCCALETTI et alii, 1971a,b, 1980;
ALVAREZ, 1991; DOGLIONI et alii, 1998; MALAVIEILLE et
alii, 1998). In contrast, a single west-dipping subduction,
active since Late Cretaceous is required by the interpre-
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A. ARGNANI
tations of the second group (e.g., PRINCIPI & TREVES,
1984; JOLIVET et alii, 1998).
These interpretations, however, cannot fully account
for all the geological data. Models implying only a west
dipping subduction face some difficulties in explaining
the westward emplacement of high pressure/low temperature (HP/LT) ophiolites and European crystalline basement in Corsica (MOLLI & TRIBUZIO, 2004), moving further to the north the problem of the connection with the
E-SE-dipping Alpine subduction in the Western Alps.
Moreover, many models predict a continental collision
that is accomplished between Late Eocene and Late
Oligocene, independently from the initial polarity of subduction (BOCCALETTI et alii, 1980; MALAVIEILLE et alii,
1998; PRINCIPI & TREVES, 1984; JOLIVET et alii, 1998;
BRUNET et alii, 2000). This interpretation implies that the
post Oligocene evolution of the Northern Apennines
occurred by continental subduction, and contrasts with the
opening of the Balearic backarc basin behind the rotating
Corsica-Sardinia microcontinent, which requires the subduction of an oceanic lithosphere (REHAULT et alii, 1984).
This contribution aims to show that many of these
problems are due to the 2D approach that has been
adopted in the tectonic reconstructions. Moreover, the
geological evolution of the Mediterranean region in Late
Cretaceous-Tertiary has been largely controlled by the relative convergence between the African and European
plates (e.g., DEWEY et alii, 1973; LE PICHON et alii, 1988),
I therefore present a new interpretation that attempts to
reconcile most of the geological data within the kinematic
framework of Africa-Europe convergence, following an
earlier work (ARGNANI, 2002). A brief summary of the key
geological features of the region will be presented and
discussed, introducing the proposed evolutionary model.
ALPINE CORSICA AND NORTHERN APENNINES.
TERMS OF A CONTROVERSY
The gross geological features of the Corsica-Northern
Apennine system (fig. 1) are summarised below. However, since the geology of the Western Alps is not subject
to major controversy, I will only recall some relevant
features when necessary.
ALPINE CORSICA
The Europe-vergent Alpine belt of Corsica presents
HP/LT rocks assemblages (mostly middle Jurassic ophiolites from the Tethyan domain) and non metamorphic
ophiolites that are included in the upper nappe (MATTAUER & PROUST, 1976; WARBURTON, 1986). The stack of
nappes rests on the Hercynian basement of Corsica and
consists of, from top to bottom: i) an upper nappe (Macinaggio, Nebbio and Balagne units) composed of a mix of
unmetamorphosed ophiolites with their sedimentary
cover and of upper Triassic to Eocene carbonates resting
on pre-Carboniferous schists, belonging to Ligurian and
Adria domains, respectively; ii) a Schistes Lustrés nappe
with HP metamorphic ophiolites which represent remnants of the partly subducted crust and sediments of the
Tethys ocean; slices of HP metamorphosed crystalline
basement (Farinole and Serra di Pigno gneiss) are also
included in this unit; and iii) deformed Variscan basement with Alpine metamorphic overprint up to blueschist
facies (MOLLI & TRIBUZIO, 2004; MALASOMA et alii, 2006),
belonging to the former European continental margin
(Tenda Massif). A small flexural basin filled by about 500 m
of Eocene flysch sediments was developed over the
Variscan basement, in front of the thrust belt (Solaro
basin; WATERS, 1990), whereas early-middle Miocene
shallow water sediments were deposited unconformably
on top of the nappe stack in the S. Florent region (FERRANDINI et alii, 1998). The architecture of the Alpine Corsica
nappe piles and the inferred geological evolution suggest
a strong similarity with the Western Alps (DURANDDELGA, 1984).
The Schistes Lustrés unit (calcschists with metabasites and ophiolites) presents top-to-the-W shear and
HP/LT metamorphism of approximately Late Cretaceous
age which are related to obduction/collision (MATTAUER
& PROUST, 1976; WARBURTON, 1986; MALAVIEILLE et alii,
1998). Units belonging to the continental margin of Corsica were also involved in subduction during Late Cretaceous-Middle Eocene (MARRONI & PANDOLFI, 2003;
MOLLI & TRIBUZIO, 2004). Most authors agree that greenschist-facies E-dipping shear zones with top-to-the-E
sense of shear overprinted the HP-LT assemblages in the
Schistes Lustrés unit (BRUNET et alii, 2000; MARRONI &
PANDOLFI, 2003; MOLLI et alii, 2006; LEVI et alii, 2007)
during Oligocene/late Oligocene to early Miocene. This
deformation event could represent a collapse of the accretionary wedge. The tectonic regime affecting Alpine
Corsica during Late Eocene-?early Oligocene, instead, is
still controversial, varying from continental collisional
(MALAVIEILLE et alii, 1998), to continuous westward
nappe emplacement (BRUNET et alii, 2000), and to dominant strike slip or transpression (MARRONI & PANDOLFI,
2003; MOLLI et alii, 2006).
It is worth noting that the N-S-trending Central Corsica Fault Zone, representing the western boundary of
Alpine Corsica, was active from Oligocene to early
Miocene with left-lateral strike-slip motion (WATERS,
1990). This fault appears to be connected to the south
with the fault system that includes the Solenzara Fault
and the Aleria Fault (fig. 1) which bounds to the west the
deep (more than 8 km of sediments) Corsica basin for
which a pull-apart origin has been proposed (MAUFFRET
& CONTRUCCI, 1999; MAUFFRET et alii, 1999).
Palaeomagnetic data indicate that during early-middle
Miocene the Corsica-Sardinia block rotated counter-clockwise (CCW) (NAIRN & WESTPHAL, 1968; ZIJDERVELD et alii,
1070; TODISCO & VIGLIOTTI, 1993; VIGLIOTTI & LANGENHEIM, 1995; SPERANZA, 1999; GATTACCECA et alii, 2007),
and the solid rotation of Corsica and Sardinia (VIGLIOTTI et
alii, 1990) implies a common subduction system.
This rotation accompanied the Balearic backarc
opening that followed the late Oligocene rifting between
Corsica-Sardinia and Europe (REHAULT et alii, 1984;
FACCENNA et alii, 1997; CHAMOT-ROOKE et alii, 1997;
GUEGUEN et alii, 1998; ROLLET et alii, 2002). Subduction
of oceanic lithosphere under Sardinia is testified by continuous calcalkaline volcanism between 33 and 13 Ma
(SAVELLI, 1988), and a similar oceanic subduction has to
be assumed underneath Corsica (ROLLET et alii, 2002).
NORTHERN APENNINES
The Northern Apennines are composed of a tectonic
stack of mainly east-vergent thrust units. The Ligurian
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PLATE TECTONICS AND ALPS-APENNINES BOUNDARY
319
Molasse Basin
Jura Mnt.
Austroalpine
Southern Alps
Dinarides
Po Plain
TPB
Adriatic
Sea
A
Voltri
massif
Deformed units of European
continental margin and Valais
Crystalline massifs, Tenda massif
and Tuscan metamorphics
Units of Brianconnais
continental terrane
Sesia-Margna continental terrane
Alpine
Corsica
Central Corsica
Fault Zone
Solenzara F.
Balearic Sea
Aleria F.
Tyrrhenian
Sea
Sardinia
Units of Piemont-Ligurian ocean
Units with little metamorphism
deformed with W-ward vergence
Units with ophiolites and
HP metamorphism
Unmetamorphosed units with little
or no ophiolites
Naples
epi-Ligurian sediments, including
Corsica basin and TPB
Fig. 1 - Simplified gelogical map of the Western Alps, Corsica and Northern Apennines (adapted from various sources). The Piemont-Ligurian
units with little metamorphism and W-ward vergence include the Internal Ligurian units of the Northern Apennines also shown in figs. 5 and 7.
TBP stands for Tertiary Piedmont Basin. The approximate location of the Antola Unit within the Internal Ligurian units is marked with «A».
– Carta geologica semplificata delle Alpi occidentali, della Corsica e dell’Appennino settentrionale (adattata da varie fonti). Le unita del dominio
ligure-piemontese a basso metamorfismo e vergenza occidentale includono le unità Liguridi interne, che sono anche mostrate nelle figg. 5 e 7.
TBP: Bacino Terziario Piemontese. La posizione approssimata dell’unità di Antola all’interno delle Liguridi interne è indicata con «A».
terranes and their sedimentary cover are the uppermost
unit and overlie the deformed successions of the Adriatic
continental margin.
The Ligurian terranes represent a complex assemblage of highly deformed Jurassic to Paleogene sediments
deposited within an oceanic basin, with ophiolitic base-
ment rocks (ABBATE & SAGRI, 1970). These terranes are
conveniently subdivided into two groups, Internal and
External Ligurian, depending upon their stratigraphic
and structural position (e.g. MARRONI & TREVES, 1998;
MARRONI & PANDOLFI, 2007). The Internal Ligurian present a Jurassic oceanic basement overlain by a thin suc-
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A. ARGNANI
cession of Jurassic-lower Cretaceous pelagic sediments
and then by hemipelagic shales and turbiditic sandstones
of late Cretaceous-Paleogene age. The External Ligurian,
on the other hand, show a thick succession of calcareous
turbidites, ranging in age from late Cretaceous to middle
Eocene, that rests over a shaly melange. Altogether, the
Ligurian units represent an accretionary prism related to
the subduction of the Alpine Tethys oceanic domain.
Although the Ligurian units have the most internal provenance, and have been the earliest to be deformed during
the orogeny, they presently occur in an external structural
position, a character shared by many fold-and-thrust
belts (RODGERS, 1997).
In the Internal Ligurian an early west-vergent deformation appears to be related to underplating within a
east-dipping oceanic subduction context (HOOGERDUIJN
STRATING & VAN WAMEL, 1989; MARRONI & PANDOLFI,
1996; MARRONI et alii, 2004; LEVI et alii, 2007). These
structures were later overprinted by east-vergent deformation probably linked to vertical thinning and exhumation.
A comparable deformation history has been observed also
in the Antola Unit (LEVI et alii, 2006) that has presently an
uppermost position in the Apennine edifice, and is unconformably covered by the Tertiary Piedmont Basin succession. The Antola Unit is composed of Upper CretaceousPalaeocene deep water sediments, mostly turbidites, and
was deformed at a shallow structural level, recording both
the Alpine and the subsequent Apennine deformation.
Clasts of Internal Ligurian rocks, preserving this succession of deformational events, have been found in the
upper Eocene Portofino Conglomerates. As the youngest
sediments involved in the underplating are of early Paleocene age, the timing of this deformation can be constrained between Paleocene and late Eocene. Similarly,
clasts of eclogites dated about Middle Eocene have been
found in the Early Oligocene conglomerate of the Tertiary
Piedmont Basin (FEDERICO et alii, 2004, 2005) that rests
unconformably on the Voltri Massif, suturing the SestriVoltaggio tectonic line (e.g., SPAGNOLO et alii, 2007). The
history of exhumation for the Internal Ligurian and Voltri
Massif is therefore comparable to what is observed in
Alpine Corsica, where the Alpine units show a progressive
cooling since Late Eocene (FELLIN et alii, 2006). This complex deformational history has not been observed within
the External Ligurian which present only E-vergent folds
and thrusts (MARRONI & PANDOLFI, 1996).
The epi-Ligurian sedimentary succession crops out in
the outermost part of the Apennines and is characterized
by sediments ranging in age from late Eocene to Pliocene
that rest unconformably on the deformed Ligurian units
(SESTINI, 1970; CIBIN et alii, 2000). The stratigraphic succession shows a general shallowing upward trend with
relatively deep-marine sedimentation from Eocene to
early Burdigalian, followed, after a substantial hiatus, by
shelfal deposits of late Burdigalian age (CIBIN et alii,
2000; DI GIULIO et alii, 2000). A deepening upward trend
characterises the depositional environments of Langhian
to Tortonian sediments, with the shallow water Messinian
evaporites capping the sedimentary succession. The epiLigurian sediments of the Northern Apennines show a
pattern of post early Miocene CCW rotations, closely
resembling the rotation of Corsica-Sardinia (MUTTONI
et alii, 1998; 2000), suggesting that the Ligurian terranes
were emplaced on the Adriatic continental margin during
the opening of the Balearic backarc basin.
The system of foredeep basins progressively migrating north-eastward records the advancement of the
thrust front from late Oligocene to Present (RICCI LUCCHI, 1986; ARGNANI & RICCI LUCCHI, 2001). The turbidite
sediments of the Canetolo Unit, that were deposited in a
position intermediate between the continental and the
oceanic domains (ABBATE & SAGRI, 1970), are here taken
to represent the onset of the migrating foredeep system
(ARGNANI, 2002).
NORTHERN TYRRHENIAN AND TUSCAN PROVINCE MAGMATISM
The Northern Tyrrhenian Sea is characterized by an
Oligocene to Pliocene sedimentary cover resembling the
epi-Ligurian and Tertiary Piedmont basins overlying a
substrate made of Alpine and Ligurian units which has
been affected by extensional tectonics from Oligocene to
early Pliocene (see ARGNANI, 2002 for a summary).
Outcrops of Neogene magmatic rocks are found in
Corsica, in the Elba, Giglio, Montecristo and Capraia
islands, and in Tuscany. Magmatic rocks become progressively younger eastwards, from the 7.3 Ma Montecristo
granite to the 0.2 Ma Amiata volcanics (SERRI et alii,
1993; BARBERI et alii, 1994; SAVELLI, 2000); although the
onset of the Neogene magmatic cycle occurred in Corsica
as an isolated event (Sisco lamproites, 14.2 Ma; SERRI et
alii, 1993).
The magmatic evolution is indicative of delamination
of continental lithosphere (SERRI et alii, 1993), with the
first, and isolated, product (Sisco lamproites) occurring
just after the rotation of Corsica-Sardinia had been
accomplished. These pieces of evidence indicate that continental delamination of the Adriatic lithosphere could
not have started much earlier than middle Miocene
(ARGNANI, 2002).
KINEMATIC RECONSTRUCTIONS
Whereas most plate kinematic reconstructions pay
attention to the large scale picture (DEWEY et alii, 1973,
1989; LE PICHON et alii, 1988; DERCOURT et alii, 1993;
RICOU, 1996; STAMPFLI & BOREL, 2002) the aim of this
paper is to account for the geological complexity of a limited region, which in this case encompasses Corsica, the
Western Alps and Northern Apennines.
The kinematic reconstruction of Africa-Europe convergence has been performed using the rotational poles
described by DEWEY et alii (1989) and MAZZOLI & HELMAN (1994) who derived the rotational parametres from
the study of the magnetic anomalies of the Atlantic ocean,
computing the finite difference solution between individual anomalies. Adria has been considered an African
promontory since Mesozoic time, according to the recent
reviews of Mediterranean palaeomagnetic data (CHANNELL, 1996; MUTTONI et alii, 2001). For a given magnetic
anomaly (i.e. time) the positions of Africa, inclusive of
the Adriatic promontory, have been mapped respect
to Europe, kept fixed in its present position (fig. 2). As
Africa moved several hundreds of km towards Europe
from Cenomanian to Chattian, and 200-300 km of convergence occurred in Eocene-Oligocene time, any reconstruction that does not take into account this relative
motion is bound to face problems in explaining the geological data. Following the Alpine collision average N-S
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PLATE TECTONICS AND ALPS-APENNINES BOUNDARY
Europe
European margin
units
321
Alpine Molasse
Basin
Apulian plate
units
Chattian
Rupelian
Santonian
area of large CCW
rotation and
orogen-perpendicular
extension
trace of inferred
trench-trench transform
separating the opposite Alpine
and Apennine subductions
Maastrichtian
Fig. 2 - Africa-Europe relative plate motion from Santonian to Chattian.
Africa, identified by present-day coastline, is moving w.r.t. fixed Europe.
– Movimento relativo delle placche africana ed europea fra il Santoniano
e il Cattiano. Africa, identificata dalla linea di costa attuale, si muove
rispetto all’Europa, tenuta fissa.
convergence between Africa and Europe slowed down
since the Chattian, promoting the rollback of subducted
lithosphere that initiated the opening of the Western
Mediterranean backarc basins.
An additional constraint comes from the NW-ward
subduction of oceanic lithosphere that is testified by calcalkaline volcanism along the European margin (Sardinia, offshore western Corsica, Provence). The Balearic
backarc basin opened from late Oligocene to early-middle
Miocene (REHAULT et alii, 1984; CHAMOT-ROOKE et alii,
1997; FACCENNA et alii, 1997; ROLLET et alii, 2002)
whereas the Tyrrhenian backarc basin opened from Late
Miocene to Pleistocene (MALINVERNO & RYAN, 1986;
KASTENS et alii, 1988; ARGNANI & SAVELLI, 1999). Following these chronological constraints the subducted oceanic
domains have been restored within the plate kinematic
reconstructions.
At Present the polarity of subduction changes along
strike at the Mediterranean plate boundary. The European lithosphere is subducted under the Alps whereas the
African lithosphere is subducted under the Apennines, the
Dinarides and the Hellenic arc. In the Alps and in the
Eastern Mediterranean the present polarity of subduction
has been active since the beginning of plate convergence.
For the Western Mediterranean, the current polarity of
subduction can be traced back till the Oligocene, as indicated by the age of arc volcanism. In the kinematic reconstructions presented here, the current polarity of subduction of the Western Mediterranean is extended back to
the onset of subduction.
Two adjacent subductions having opposite polarity
can operate if they are separated by a trench-trench transform boundary (WILSON, 1965). Moreover, in order to
avoid the locking as convergence progresses, the trenchtrench transform has to propagate outward from the
plate boundary (fig. 3, inset). Major tectonic discontinu-
Fig. 3 - Geological hints supporting the occurrence of a lineament in
Adria which acted as a trench-trench transform (see text for further
details). The lineament is well imaged by satellite Free Air gravity
anomalies (ARGNANI, 2002). The sketch in the inset illustrates that a
propagating trench-trench transform (dashed line) is required for
two opposite subductions to operate without locking. Large open
arrow is plate convergence. The panel on the right shows that the
trench-trench transform grows in length when trench rollbak (black
arrow) occurs. By analogy with this last example, it is expected that
present-day continent-continent collision in the Alps is occurring
only to the north of the transform fault.
– Indizi geologici che supportano la presenza di un lineamento nella
placca Adriatica che ha agito da trasforme fra le due subduzioni, alpina
e appeninica (ulteriori dettagli nel testo). Il lineamento è evidente dalle
anamalie gravimetriche in aria libera (ARGNANI, 2002). Lo schema nel
riquadro mostra una trasforme fra due subduzioni opposte e illustra la
necessità che questa trasforme si propaghi in una delle due placche
(linea tratteggiata) affinché la subduzione non si blocchi. La freccia
grande indica la convergenza fra le placche. Il pannello a destra mostra
che la lunghezza della trasforme aumenta se è presente un arretramento
della placca subdotta (freccia nera). Per analogia con quest’ultimo
esempio, si ritiene che la collisione continente-continente nelle Alpi
avvenga attualmente soltanto a nord della trasforme.
ities inherited from the extensional evolution, such as
large-scale oceanic transforms, can possibly become
trench-trench transforms during the subsequent plate
convergence; in this event, the trace of these discontinuites can be followed out from the plate margin. Following
ARGNANI (2002) it is here assumed that the lineament in
fig. 3 represents the trace of the discontinuity through
which the polarity flip occurs in present-day configuration; in pre-convergence time it was likely a large scale
transform zone along the Adriatic continental margin
(ARGNANI et alii, 2004, 2006). The following lines of evidence support the interpretation that the lineament acted
as a trench-trench transform between the Alpine and
Apennine subduction.
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SANTONIAN
CHATTIAN
EURASIA
EURASIA
Alps
Dinarides
inactive
subduction
collapsed
Alpine belt
Golija-Pelagonian
Alpine Tethys
Neo-Tethis
AFRICA
AFRICA
Neo-Tethis
Shallow water carbonate platform
Basin on continental crust
(A)
1000 km
Deep water basin (oceanic domain)
1000 km
(B)
Fig. 4 - Tentative palaeogeography at Santonian (83 Ma) and Chattian (25.5 Ma) time (panels A and B, respectively) after kinematic repositioning. Note that the Apennine-vergent oceanic subduction is progressively taking over in Alpine Corsica as Adria moves northward, leaving an
inactive subduction in Corsica. The sector of the Alpine belt that likely collapsed following the progressive substitution from Alpine to Apennine
subduction is indicated with a gray pattern in the Chattian panel.
– Ipotesi di ricostruzioni paleogeografiche al Santoniano (83 Ma) e al Cattiano (25,5 Ma) a seguito del riposizionamento delle placche (riquadri
A e B, rispettivamente). Si noti che la subduzione a vergenza appenninica prende il sopravvento nella Corsica alpina durante il movimento verso
nord di Adria, lasciando la subduzione inattiva in Corsica. Il settore di catena alpina che collassa a seguito della progressiva sostituzione della
subduzione alpina con quella appenninica è indicato col pattern grigio nella ricostruzione del Cattiano.
i) the lineament is currently bounding the Southern
Alps crustal-scale backthrust and the western extent of
the foreland Molasse basin on the northern side of the
Alps (fig. 3; BIGI et alii, 1990), suggesting that the continent-continent Alpine collision is passing to a less tight
collision in the south-western Alps (e.g., FORD et alii, 2006).
ii) the Moho isobaths in the Alpine region (SCHMID &
KISSLING, 2000) show that the base of the European crust
is deeper than the base of the Adriatic crust underneath
the orogen, as expected with a European subduction;
however, the Adriatic Moho is deeper than the European
Moho in the Western Alps, to the SW of the intersection
between the Alpine belt and the above mentioned lineament, indicating a possible change in subduction polarity.
iii) in the Western Alps focal mechanisms are dominantly extensional, with a directon of extension that is
perpendicular to the the trend of the mountain belt (EVA
et alii, 1997; SUE et alii, 1999); this evidence contrasts
with the compression normal to the mountain belt that is
observed in the rest of the Alps (KIRATZI & PAPAZACHOS,
1995; BECKER, 2000) and may be related to the upperplate extensional regime that is occurring above the
retreating Apennine subduction.
iv) flexural subsidence in the western Alps foredeep
ended in early Oligocene time (FORD et alii, 1999) and
only a limited thickness of sediments was deposited subsequently (LICKORISH & FORD, 1998); on the contrary, a
succession of up to 5000 m of Oligo-Miocene sediments
were deposited in the Molasse basin, west of the Jura
Montains (KUHLEMANN & KEMPF, 2002).
v) palaeomagnetic work shows a post-Oligocene large
CCW vertical axis rotation in the southern Western Alps
and in Liguria (fig. 3; COLLOMBET et alii, 2002), suggest-
ing a pre-existing linear trend of the Alpine belt; the large
CCW rotation of this segment of the Alps may be related
to the retreating Apennine subduction that is expected to
occur south of the lineament.
From the constraints and assumptions mentioned
above, and using stratigraphic data for the African foreland together with the re-located oceanic areas, a Santonian palaeogeographic reconstruction is proposed as a
starting point (fig. 4a). The main features are the continental promontory of Adria bounded on its western side
by a wide oceanic embayment, and the polarity flip of subduction along the plate boundary. The ensuing geological
reconstruction for c.a. Palaeocene time (fig. 5a) shows
that the collision in the Alps, including Alpine Corsica, is
passing to oceanic subduction, with opposite polarity, to
the SW where the Ligurian ocean embayment is located.
The implication is that the Alps as a collisional orogen did
not continue all along the plate margin but terminated
somewhere in Corsica (see also, FORD et alii, 2006).
Continuing plate motion causes a lateral shift along
the plate boundary and terminates the collision in Alpine
Corsica as Alpine continental subduction is laterally substituted by an oceanic Apennnine subduction (figs. 4b
and 5b). It follows that the polarity flip, that changed the
setting from continental collision to oceanic subduction
is caused by lateral plate motion and, therefore, cannot
be reproduced by reconstructions based on a 2D cross
section without facing geological incongruities. As a consequence of polarity flip the Alpine belt of Corsica suffered an extensional collapse that is represented by the
top-to-East shear zones in greenschist facies observed in
Corsica (BRUNET et alii, 2000). The collapse of the Alpine
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PLATE TECTONICS AND ALPS-APENNINES BOUNDARY
(A)
orogenic belt
(km)
0
A
A
A'
15
30
European
passive margin
45
A'
(km)
proto-Apennines
0
B
Adria margin
B'
15
A'
B
trace of Trench-Trench-transform
subduction accretionary prism
External
Ligurian
domain
200
B'
A'
(km)
(km)
100
A
trace of
Trench-Trench-transform
N
Ext. Ligurian
domain
0
Adriatic passive margin
collapsed Alpine belt
(including Internal Ligurian)
30
Alpine Corsica
(B)
Alpine orogenic belt
Alpine Corsica
N
323
0
B
Ext. Ligurian domain
B'
100
200
Apennine subduction
accretionary prism
Fig. 5 - A) Map-view tectonic reconstruction of the Corsica-Apennine region for Santonian-Paleocene time. Note that continental collision is
occurring in Alpine Corsica, while the External Ligurian domains are undergoing deformation by oceanic subduction. Lithospheric- and crustalscale cross sections illustrate the two different tectonic settings north and south of the inferred trench-trench transform (modified after
ARGNANI, 2002); B) Tectonic reconstruction for Late Eocene-Early Oligocene time, after the roughly north-ward motion of the Adriatic
promontory (see fig. 2). Note that the Alpine orogenic belt of panel A has collapsed on top of the Apennine accretionary prism, following the
substitution of Alpine subduction with Apennine subduction.
– A) Ricostruzione paleotettonica in pianta della regione compresa fra Corsica e Appennino settentrionale per l’intervallo Santoniano-Paleocene.
Si noti che la collisione continentale si realizza nella Corsica alpina, mentre il dominio delle Liguridi esterne era sottoposto a deformazione in
regime di subduzione oceanica. Le sezioni a scala litosferica e crostale illustrano i diversi assetti tettonici presenti a nord e a sud della ipotizzata
trasforme che collega le due subduzioni (modificata da ARGNANI, 2002); B) Ricostruzione paleotettonica riferita all’Eocene superiore-Oligocene
inferiore, dopo lo spostamento verso nord del promontorio adriatico (vedi fig. 2). Si noti che la catena alpina del riquadro A è collassata sopra al
prisma di accrezione appenninico, a seguito della sostituzione della subduzione alpina con quella appenninica.
belt is interpreted as occurring within an actively convergent system, and it is mainly related to the large topographic contrast between the collisional orogenic belt and
the adjacent subuctive accretionary prism (see cross sections in fig. 5a). As oceanic subduction laterally substituted the continental collision, the orogen adjusted to a
new equilibrium by collapsing to a lower taper (figs 5b, 7).
Extension due to gravitational collapse likely affected
the upper parts of the orogen, leading to exhumation of
deep seated rocks. In the external parts of the accretionary wedge, as well as at deep levels, compression continued to operate. The evolution described above is illustrated with a geological cross section between Corsica
and the Northern Apennines (fig. 7) that takes into
account the large lateral plate motion that is accomplished in Middle-Late Eocene-Early Oligocene (fig. 4). As
discussed below close similarity exists between the late
Cretaceous-Palaeogene evolution of this area and the
recent evolution of the northern part of Taiwan.
After the polarity flip, westward-dipping oceanic subduction continued until Langhian in the Northern Apennines, accommodating the CCW rotation of Corsica-Sardinia. Following the consumption of oceanic lithosphere,
delamination affected the Adriatic continental margin,
leading to extensional tectonics and magmatism in the
Northern Tyrrhenian basin (ARGNANI, 2002). It might be
interesting to note that the character of the metasomatism in the mantle presently under Tuscany and its
inferred Eocene age (PECCERILLO, 1999), support the
occurrence of a former Alpine subduction which can be
related to the pre-flip evolution (fig. 5).
The palaeogeographic reconstructions presented here
follow ARGNANI (2002) and match pretty well those of
FORD et alii (2006) for the Eocene-early Miocene, which
are based on works carried out in the Western Alps. In
fact, the left-lateral strike slip fault that has been inferred
by FORD et alii (2006) at the SW end of the Western Alps
would find a better plate kinematic location if intepreted
as a trench-trench-tranform fault separating two subductions with opposite polarity, as in fig. 4.
TAIWAN: A «MIRROR ANALOGUE»
TO ALPINE CORSICA-NORTHERN APENNINES
The evolution of subduction for the last 10 Ma that
has been reconstructed in Taiwan (e.g., TENG, 1996;
SIBUET & HSU, 2004) offers a useful analogue that can
help understanding of the latest Cretaceous-Paleogene
evolution of the Alpine Corsica-Apennine system (fig. 6).
In central-southern Taiwan, where an orogenic belt is
actively deforming, the continental Eurasian plate is currently being subducted E-ward, underneath the oceanic
lithosphere of the Philippine Sea plate (fig. 6a). The orogenic belt in Taiwan is created because the Luzon volcanic arc is colliding with the Eurasian continental margin. To the north of the island, instead, the oceanic
lithosphere of the Philippine Sea plate is being subducted
N-ward underneath Eurasia, where the Okinawa backarc
basin is opening behind the Ryukyu trench. The two subductions with opposite polarity encounter in the northern
part of Taiwan, where the orogenic belt is currently
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A. ARGNANI
30 N
continental plate
Eurasian Plate
collision
3 Ma
Okinawa basin
collapsed
orogen
oceanic plate
trench-trench
transform
orogenic
collapse
2 Ma
Ryukyu trench
Taiwan
FTB
oceanic plate
Gagua Ridge
slab sinking
Luzon arc
Philippine
Sea Plate
Manila trench
BAB opening
trench
retreat
1 Ma
7 cm/yr
trench
retreat
Present
Luzon Arc
a
10 N
115 N
Okinawa
basin
135 E
7 cm/yr
Ryukyu
Islands
b
Fig. 6 - a) Taiwan Present-day plate tectonic setting. Lines with solid triangles mark oceanic subduction; line with open triangles marks soft
collision (volcanic arc-continent) in Taiwan. FTB: fold-and-thrust belt. The Luzon volcanic arc is interpreted to represent the trench-trenchtransform fault depicted in (b); b) Tectonic reconstruction of Taiwan for the last 3 My (adapted after TENG, 1996). The maps represent a
conveniently rotated mirror image that shows analogies with the Late Cretaceous-Early Miocene evolution of the central Mediterranean
(see fig. 4). The coastline of Taiwan is indicated in the maps with a dashed line, for comparison to the present-day setting shown in (a). The
schematic lithospheric cross sections illustrate the flip of subduction polarity driven by plate motion. BAB: backarc basin.
– a) Assetto tettonico attuale di Taiwan. Le linee con i triangoli pieni marcano la subduzione oceanica; la linea col triangolo aperto marca la collisione arco vulcanico-continente a Taiwan. FTB: catena e pieghe e sovrascorrimenti. L’arco vulcanico di Luzon è interpretato come la trasforme
fossa-fossa indicata in (b); b) Ricostruzione tettonica per Taiwan negli ultimi 3 Ma (adattata da TENG, 1996). Le mappe sono state appositamente
riflesse e ruotate per evidenziare l’analogia con l’evoluzione del Mediterraneo centrale dal Cretaceo superiore al Miocene inferiore (vedi fig. 4). La
costa di Taiwan è indicata nelle mappe per facilitare la comparazione con la situazione attuale, illustrata in (a). Le sezioni litosferiche schematiche illustrano il flip di polarità della subduzione dovuto al movimento delle placche. BAB: bacino di retroarco.
undergoing an extensional collapse (fig. 6b). The plate
reconstructions for the last 5 Ma show that the Rukyu
oceanic subduction has progressively moved S-ward
along the plate boundary (TENG, 1996; SIBUET & HSU,
2004), causing the orogen collapse in northern Taiwan.
If displayed in a conveniently rotated mirror image, the
recent evolution of Taiwan resembles closely what might
have happened in the central Mediterranean during the latest Cretaceous-Eocene (fig. 6). The obvious difference being
that the role played by the colliding Luzon Arc, in Taiwan,
is taken by the Adriatic Promontory in the central Mediterranean. In order for the two adjacent subductions with
opposite polarity to operate a trench-trench transform
boundary is required (WILSON, 1965). In Taiwan the most
obvious discontinuiy that seems to act as a transform boundary is the Luzon Arc. In the Adriatic Promontory this discontinuity has been interpreted as a former continental transform that separated Adria from the Alpine Tethys (fig. 4a,
Santonian; ARGNANI, 2002; ARGNANI et alii, 2004, 2006).
DISCUSSION
The geological reconstruction of the region encompassing Corsica and the Northern Apennines has been
tackled by a large number of papers, often resulting in
substantially different interpretations. Several of these
interpretations cannot fully account for all the observed
geological evidence. In many instances, as discussed
below, these interpretations suffer from the 2D approach
that they have adopted.
The interpretations that attempt to reconcile the
opposite polarity of Alpine Corsica and the Northern
Apennines can be broadly framed within two groups of
hypotheses, as previously mentioned. A group that envisages an initial eastward European subduction, followed
by a flip of polarity leading to the westward subduction of
Adria, and a group that proposes a single west-dipping
subduction since the onset of convergence.
Models that prefer a flip of subduction polarity
assume an initial intraoceanic subduction and the subsequent collision of a volcanic arc (BOCCALETTI et alii, 1971;
MALAVIEILLE et alii, 1998) or a microcontinent (BOCCALETTI et alii, 1980; ALVAREZ, 1991; MOLLI & TRIBUZIO,
2004). It is worth noting that whereas these models
assume a close similarity between Western Alps and
Alpine Corsica for the early subduction stage, the evolution of the Alps suggests that subduction occurred near
the Adria continental margin since the inception (POLINO
et alii, 1990; ROSENBAUM & LISTER, 2005). Moreover, it is
worth noting that the solution of placing a volcanic arc or
a microcontinent (Nebbio) in the ocean located between
Adria and Europe is highly speculative, as only some
small slices of continental crust are present within the
Nebbio unit of Corsica (WARBURTON, 1986) and no
remains of volcanic arc are present.
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In order to account for the geological complexity of
Corsica and the Northern Apennines a complicated evolution was initially envisaged (BOCCALETTI et alii, 1971).
The attempt to explain all the geological data along a
cross section, however, has led to a not too sound «ad
hoc» sequence of events to be inferred, consisting of: i) a
subduction flip (Late Oligocene) followed by ii) two coexisting west-dipping subductions, and by iii) a volcanic arc
collision (with Adria in Langhian), before reaching the
final continental collision in Tortonian.
A model assuming a Late Cretaceuos volcanic arc collision and based on the tectonic evolutions reconstructed
for the Cretaceous in Oman has also been applied to Corsica (MALAVIEILLE et alii, 1998). Unlike in Oman, however, no large slice of oceanic lithosphere is present in
Corsica. Moreover, the Miocene rollback of Apennine
subduction is interpreted to follow the Eocene continental
collision, when no oceanic domain is left for subduction.
Analogy between Corsica and Taiwan, which represents a modern analogue of Continent-Arc collision, has
been attempted (HUANG et alii, 2000). However, remnants
of the colliding Luzon volcanic arc are preserved in the
Coastal Range of Taiwan, and no HP/LT ophiolites occur
in the mountain belt (SUPPE, 1987). The comparison with
Alpine Corsica, therefore seems rather poor, as processes
that totally remove the volcanic arc have to be inferred in
the evolution step from Taiwan to Corsica.
To overcome these problems, some Authors have
adopted the solution of placing a microcontinent (Nebbio)
that separates two oceanic domains located between Adria
and Europe (e.g., BOCCALETTI et alii, 1980; ALVAREZ, 1991;
MOLLI & TRIBUZIO, 2004). The occurrence of a colliding
microcontinent causing the flip of subduction, however,
poses other questions, like, for instance:
i) it is expected that the stratigraphic evolution of the
two oceans separated by a microcontinent should be
somewhat different; in the Southern Apennines, for
instance, the Lagonegro-Ionian basin (mid Triassic rifting) is separated from the Alpine Tethys (late Triassic rifting) by the microcontinent onto which the Apennine platform were deposited (e.g., ARGNANI, 2005). However, the
sedimentary successions of the oceanic domains of the
Western Alps, Alpine Corsica and the Northern Apennines
are all very similar, in age and stratigraphic evolution
(PRINCIPI et alii, 2004; MARRONI & PANDOLFI, 2007) suggesting deposition within a single oceanic domain.
ii) only some small slices of continental crust are present within the Nebbio unit of Corsica, although it seems
unlikely that a microcontinent that was large enough to
cause a collision should disappear almost completely.
Think, for instance, of Corsica-Sardinia that collided with
Adria to form the Apennines, or of the Sesia-Margna unit
of the Western Alps, that has been interpreted as a continental block involved in Alpine subduction (ROSENBAUM
& LISTER, 2005) and that presents a significant dimension on the map (fig. 1).
iii) the inferred flip of subduction affected the whole
of the western Mediterranean subduction and, therefore,
the Nebbio microcontinent was likely connected to a
larger continental terrane that was supposed to extend
over most of the western Alpine Tethys (Alkapeca
domain; e.g., MICHARD et alii, 2002), implying that a Corsica-like Alpine belt is extending further to the south (e.g.,
ALVAREZ, 1976), but again not much evidence of the rem-
325
nants of such a belt can be found in the Tyrrhenian basin
and further to the west, as the only comparable «Alpine»
units appear in the Betics (MICHARD et alii, 2002). In fact,
the Alpine units of Calabria, after palinspastic restoration, can be easily located near Corsica, and the HP/LT
metamorphism in the Kabylie is younger (30-25 Ma) than
that of Alpine Corsica and appears related to an African
subduction (MICHARD et alii, 2006).
The occurrence of the Nebbio microcontinent in
between Africa and Eurasia has implications on the evolution of northern Calabria, where Alpine units are cropping out, as recognized by ALVAREZ (1991). A discussion
of the palaeogeography of Calabria is outside the scope of
this work, although the arguments presented above, that
lead to rejection of the occurrence of a microcontinent in
Alpine Corsica, can be also applied to northern Calabria.
In a somewhat different approach an initial Alpine
subduction was followed by the development of a large
backthrust, responsible for the onset of the Apennine
foredeep during early Oligocene, and leading to a flip of
subduction polarity in late Oligocene (DOGLIONI et alii,
1998). The two opposite subductions are thought to coexist during the late Oligocene-early Miocene time interval.
However, the activity of the Alpine front in Corsica ended
by middle-late Eocene, and the Oligo-Miocene calcalkaline volcanic belt encompassing Provence, offshore Western Corsica, and Sardinia requires a mature west-dipping
subduction underneath Alpine Corsica which would
impede any E-dipping subduction.
Models implying a single west-dipping subduction, on
the other hand, have some difficulty in accounting for the
Middle Eocene HP metamorphism of units belonging to
the Corsica continental margin and the subsequent westward emplacement of ophiolites and crystalline basement
rocks onto the European margin (MARRONI & PANDOLFI,
2003; MOLLI & TRIBUZIO, 2005; MOLLI et alii, 2006; LEVI
et alii, 2007). In addition, these models cannot easily
account for the structural features observed in the Internal Ligurian units that indicate a deformation by underthrusting within a west-vergent accretionary prism
(HOOGERDUIJN STRATING & VAN WAMEL, 1989; MARRONI
& PANDOLFI, 1996; MARRONI et alii, 2004; LEVI et alii,
2006). Moreover, by adopting an Appenine subduction,
active since late Cretaceous underneath Corsica, these
models implicitly assume that the (transform) boundary
between Apennine and Alpine subduction is located north
of Corsica since Late Cretaceous; a look at plate motion
from Maastrichtian to Present (fig. 2) shows that within
that assumption the Western Alps would be now part of
the Apennines, as the transform boundary moves to the
north with the plates.
Recent attempts to relate a westward subduction in
Corsica with the easward subduction in the Alps (VIGNAROLI et alii, 2008) face some kinematic inconsistencies as
the authors, in order to explain a middle-late Eocene
metamorphic peak in the Voltri Massif, infer that oceanic
lithosphere was consumed at about 50 Ma, in the region
encompassing Corsica and Liguria. The absence of
oceanic lithosphere makes it difficult to open the Balearic
backarc basin by a slab rollback process, and cannot help
in explaining the calcalkaline volcanism in Provence.
Moreover, it is assumed that the change from Alpine to
Apennine subduction occurred in Liguria with a two-
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A. ARGNANI
sided (ablative) subduction, a process which is not currently observed along the present plate boundaries worldwide and which seems unlikely to occur in plate tectonics
on Earth (GERYA et alii, 2008).
Finally, many models predict a continental collision
that is accomplished between Late Eocene and Late
Oligocene, independently from the initial polarity of subduction (BOCCALETTI et alii, 1980; MALAVIEILLE et alii,
1998; PRINCIPI & TREVES, 1984; JOLIVET et alii, 1998;
MARRONI & TREVES, 1998; BRUNET et alii, 2000). These
models assume that the post-Oligocene evolution of the
Northern Apennines occurred by continental subduction,
and face the problem that such a process cannot be
mechanically sustained for too long because of the buoyancy of continental lithosphere (CLOOS, 1993; RANALLI et
alii, 2000). Moreover, a continental subduction contrasts
with the opening of the Balearic backarc basin, with
related calcalkaline volcanisms, behind the rotating Corsica-Sardinia microcontinent which requires the subduction of an oceanic lithosphere (REHAULT et alii, 1984;
ROLLET et alii, 2002).
In summary, models that assume a west-dipping subduction active since Late Cretaceous can hardly account
for the Middle Eocene HP/LT metamorphism of units
belonging to the Corsica continental margin, whereas
most of the reconstructions that imply a polarity flip are
based on a 2D approach and face some mechanical incongruities, besides requiring, in some instances, a great deal
of geological complexity.
CONCLUSIONS
Plate motion played a major control in the evolution
of the Alpine orogen s.l. as large-scale horizontal plate
motions, in the order of several hundreds of km,
occurred. For this reason the reconstructions presented
in the literature and based on a 2D approach are often
not adequate to account for the geological observables.
The Late Cretaceous-Pliocene geological evolution of
the Corsica-Northern Apennine system can be summarized
in a series of steps (fig. 7), based on reconstructed plate
kinematics and on a critical review of the geological and
geophysical data. Each step is characterized by a distinct
structural, stratigraphic, and magmatic pattern. A map
view of the tectonic evolution concerning the crucial timing when continental collision was substituted by oceanic
subduction (fig. 7, upper two panels) is shown in fig. 5.
a) late Cretaceous – middle-late Eocene: Consumption of oceanic lithosphere and incipient continental collision of Adria promontory with Europe led to the building
of the Alpine belt of Corsica with W-ward emplacement
of HP/LT metamorphic rocks belonging to the European
continental margin. The oceanic sediments of the Internal
Ligurian units were deformed during this stage within a
west-vergent accretionary prism. Oceanic subduction
with opposite polarity continued further to the south,
underneath Sardina (and Calabria), as a wide oceanic
embayment was present in the African plate west of the
Adriatic promontory; the sediments of the External Ligurian units were deposited in this deep water domain. The
Alpine belt s.s. (continent-continent collision), therefore,
did not continue further SW along the plate boundary.
b) middle-late Eocene-early Oligocene: Continuing
convergence of Africa towards Europe led to lateral
motion of the Adriatic promontory, causing left-lateral
strike slip and a polarity flip in the Corsica sector of the
plate boundary. The onset of orogenic collapse of Alpine
Corsica is promoted by this lateral polarity flip as oceanic
subduction took the place of continental collision and,
therefore, is not directly related to the Balearic extension.
Because of the polarity flip and ensuing orogenic collapse, the Internal and External Ligurian domains were
tectonicaly juxtaposed. It is worth noting that deep water
sedimentation still occurred in the External Ligurian
domain. Sediments in the epi-Ligurian basins record a
provenance from ophiolites and other rocks of the Ligurian domain that were possibly exposed along extensional faults as a consequence of the orogenic collapse.
The gravitational collapse here envisaged is comparable
to the extensional collapse that has been inferred to affect
the Internal Ligurian domain and the Ligurian Alps in
Late Paleocene-Early Eocene (HOOGERDUIJN STRATING &
VAN WAMEL, 1989; HOOGERDUIJN STRATING, 1994). The
role of Paleogene extension in the evolution of the Voltri
Massif, however, is still debated (cfr. SPAGNOLO et alii,
2007 with VIGNAROLI et alii, 2008). In this respect, it is
worth noting that 40Ar/39Ar ages in the clasts of high pressure rocks, sampled in the basal succession of the Tertiary Piedmont Basin, document a pre-Late Eocene fast
exhumation in the Ligurian Alps, followed by very slow
exhumation (CARRAPA et alii, 2003). The episode of fast
exhumation may correspond to the extensional collapse
of the SW part of the Alpine belt (fig. 5b). As convergence
continued, compressional tectonics resumed soon after,
as shown by the late Oligocene-early Miocene thrusting
involving both the Voltri basement and the overlying Tertiary Piedmont Basin sediments (CAPPONI et alii, 2001;
CAPPONI & CRISPINI, 2002).
It follows, therefore, that the boundary between the
Alps and the Apennines, often considered as a single tectonic line, should be regarded as a segment of the orogenic belt that has been first affected by Alpine tectonics
and later by Apennine deformation. The area extending
from the Tertiary Piedmont Basin to the southern
Tyrrhenian-Northern Apennines can be considered as
such an overlapping segment and is characterized by
units showing mixed Alpine and Apennine features. The
Corsica basin may have originated at this time because of
the concomitant collapse of the Alpine belt and left-lateral
motion along the plate boundary (ARGNANI, 2002). Calcalkaline volcanism in Sardinia and Provence further support the W-dipping subduction of oceanic lithosphere.
c) late Oligocene-middle Miocene: The gravitational
instability of the subducted oceanic lithosphere, together
with the reduced convergence rates, caused the sinking
and rolling back of the Adriatic plate and the ensuing
opening of the Balearic backarc basin. The opening
occurred in the early-middle Miocene and was preceded
by late Oligocene rifting. The CCW rotation of the Corsica-Sardinian block suggests that the rollback increased
from north Corsica to south Sardinia, according to the
availability of oceanic lithosphere. The Ligurian terranes
were emplaced on the Adriatic continental margin during
the Corsica-Sardinia CCW rotation (MUTTONI et alii,
1998; 2000). Deformation of the Adriatic margin, on the
other hand, started at this time, as indicated by the
Apuane Alps metamorphism (KLIGFIELD, 1979; COLI,
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PLATE TECTONICS AND ALPS-APENNINES BOUNDARY
Alpine Corsica
X
HP/LT
Eocene
subduction polarity flip
orogen collapse
IL
EL
early
Oligocene
X
rollback
Balearic basin
rifting
Macigno foredeep
Apennines
Apuane
rollback
late
Oligocene
late Burdigalian Langhian
delamination
Marnoso-arenacea
foredeep
Sisco
Langhian
asthenosphere
wedging
delamination
Extensional Tectonics
late Miocene Pliocene
Plutons
+
+
Fig. 7 - Schematic cross sections showing the evolution from Eocene
to Pliocene along a Corsica - Northern Apennines transect. IL and
EL indicate the Internal and External Ligurian, respectively. The
Apuane deformation is depicted at its early stage, before subsequent
deeper burial. Note that a large left-lateral motion is occurring along
the plate boundary from Eocene to Early Oligocene. Modified after
ARGNANI, 2002. The pins in the last three cross sections indicate that
there is no convergence between Corsica-Sardinia and Adria.
– Sezioni geologiche schematiche che illustrano l’evoluzione dall’Eocene
al Pliocene lungo un transetto dalla Corsica all’Appenino settentrionale.
IL ed EL indicano, rispettivamente, le unità Ligurdi interne ed externe.
La deformazione delle Alpi Apuane è indicata nella sua fase iniziale,
prima del successivo seppellimento. Si noti l’importante movimento
sinistro che avviene lungo il margine di placca fra l’Eocene e l’Oligocene
inferiore. Modificata da ARGNANI, 2002. I chiodi nelle ultime tre sezioni
indicano l’assenza di convergenza fra il blocco sardo-corso e Adria.
1989; CARMIGNANI & KLIEGFIELD, 1990; MOLLI & VASELLI,
2006). During this stage calcalkaline volcanic activity continued in Sardinia.
d) Langhian-early Pliocene: Following the end of
oceanic subduction the Corsica-Sardinia microcontinent
docked onto the Adriatic continental margin without giving rise to a fully developed collision (soft collision, e.g.,
BURCHFIEL & ROYDEN, 1991). The Adriatic continental
lithosphere was then affected by delamination with associated magmatism, with the Sisco lamproite (c.a. 14 Ma)
recording the onset of continental delamination. The
asthenospheric inflow caused by delamination heated
the crust and originated the granitic magmas that were
then emplaced as pluton in an extensional tectonic
regime. Altogether, crustal melts and melts from the
mechanical boundary layer characterize the magmatism
of this time interval (SERRI et alii, 1993). The progressive
327
deformation of the Adriatic continental margin and its
sedimentary cover is reflected in the eastward migration
of foredeep basins (RICCI LUCCHI, 1986; ARGNANI &
RICCI LUCCHI, 2001).
In summary, the kinematic reconstructions presented
here suggest that a flip in subduction polarity did occur
along the Corsica-Northern Apennines transect, but this
flip was simply due to the 3D nature of plate motions; it
can be explained once the relative Africa-Europe convergence is taken into account, and is not related to collision
of buoyant objects like a microcontinent or a volcanic arc.
ACKNOWLEDGEMENTS
Laura Federico and Walter Alvarez are gratefully thanked for
their careful and constructive reviews that have greatly improved the
quality of the paper.
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Received 3 April 2008; revised version accepted 11 October 2008.