GNGTS – Atti del 17° Convegno Nazionale – 01.02
C. Doglioni (1), F. Innocenti (2) e G. Mariotti (2)
(1)
(2)
Dipartimento di Scienze della Terra, Università La Sapienza, Roma, Italy
Dipartimento di Scienze della Terra, Università di Pisa, Italy
ON THE GEODYNAMIC ORIGIN OF MT ETNA
Abstract. The Etna volcano is located in an anomalous position with respect to the
Apennines subduction, i.e., on the hinge zone of the slab. Moreover its alkaline geochemistry
does not favor a source from the deep slab as for the Eolian magmatism. The reduced
volume of erupted magmas are moreover not compatible with those of hot-spots.
The different dip of the regional foreland monocline at the front of the Apennines in the Ionian
sea and Sicily implies a larger rollback of the subduction hinge in the Ionian sea. The larger
subduction rollback of the Ionian lithosphere toward the southeast with respect to the Hyblean
plateau in Sicily might explain: i) the Plio-Pleistocene alkaline magmatism of eastern Sicily
(e.g. the Etna volcano); ii) the Late Pliocene to present right lateral transtensional tectonics
and seismicity of eastern Sicily. The area of transfer of different dip and rollback occurred
along the inherited Mesozoic passive continental margin between Sicily and the oceanic
Ionian sea, i.e., the Malta escarpment. The active seismicity along the Malta escarpment is
an indirect evidence that the subduction process is still active.
INTRODUCTION
Eastern Sicily in southern Italy is characterized by active destructive
seismicity and intense magmatism (e.g., Baratta, 1910; Hirn et al., 1997). The
tectonic activity and magmatism is concentrated along the Malta escarpment
(Fig. 1), which is an inherited Mesozoic continental margin (Cita et al., 1981;
Scandone et al., 1981; Casero et al., 1984). The Mesozoic rifting generated
the Ionian oceanic sea, which aborted during the Late Cretaceous or early
Tertiary, based on heat flow data and other geologic constraints (Della
Vedova and Pellis, 1992). The seismicity and the Etna volcano (Azzaro and
Barbano, 1996; Cocina et al., 1998) are at the front or in the foreland of the
Apennines accretionary wedge, in an anomalous external position with
respect to the classic magmatism of the volcanic arc associated with the
Apennines subduction zone (Serri et al., 1993). We propose that its location is
controlled by the different degrees of rollback of the subduction hinge due to
the different composition of the foreland lithosphere inherited from the
Mesozoic Tethyan rifting, i.e., oceanic in the Ionian sea and continental in the
Hyblean plateau.
.
Tyrrhenian sea
Bari
Palerm o
Sicily
ETNA
6° 5°
1°
A B
Hyblean
Plateau
MALTA
escarpm.
2°
18°br ia
la
Ca
7°
8°
D
IONIAN sea
21°
C
Front of the Apennines
100km
Fig. 1 - Schematic tectonic map of the area and location of the representative sections
where the foreland dip has been measured.
MT. ETNA BRIEF HISTORY
Mt. Etna is the largest European active volcano; it covers a surface of
30x40 km and has an elevation of about 3350 m. It is constituted by nested
strato-volcanoes, often characterized by summit calderas, the more important
of which is the Ellittico Caldera formed about 15 Ka ago (Condomine et. Al.,
1995). The strato-volcanoes and associated small scattered eruptive centers
grew on a plateau lava, produced by fissural flows of tholeiitic/transitional
composition, for which an age of about 0.5 Ma has been determined (Gillot et
al., 1994; Corsaro & Cristofolini, 1997). The plateau was covered by the
recent volcanics; however it is yet morphologically recognizable in the
southern part of the volcano (Favalli et al.). The eruptive axis of the largest
central volcanoes shifted westward through time; however their morphological
continuity has been interrupted as a consequence of the collapse of eastern
flank of the volcano system, which formed the Valle del Bove. The volcanic
activity has been essentially effusive even if several pyroclastic sequence
related to sub-Plinian and Plinian eruptions have been identified in the
Holocene sequence (Coltelli et al., 1995; Coltelli et al., 1998). The postplateau volcanic products show a composition ranging from picritic basalt and
alkali basalt to trachytes, being hawaiites the dominant rocks (D' Orazio,
1994). The predominance of hawaiitic products in the recent activity is
considered resulting mainly of fractionation/mixing processes of a picritic
primary magma, occurred in a relatively deep magma chamber (Armienti et al.
1996) whose existence has been suggested on the base of sismological data.
According with Sharp et al. (1980) the chamber is relatively large, shows an
ellipsoid shape with semi-axis of 22 and 31 km and 4 km of vertical extension
and is probably constituted by a complex network of dikes. This configuration
may explain the geochemical and isotopic variability observed in the youngest
lavas which is consider to reflect mainly the geochemical heterogeneity of the
source area (Tonarini et al., 1995; Armienti et al., 1996).
GEODYNAMIC SETTING OF MOUNT ETNA
The Etna volcano is located in the hangingwall of the Apennines
accretionary wedge (Fig. 1), and therefore its deep sources had and have to
compete with the advancing basal decollement of the prism. This would favor
a coeval compressional, but not unique, tectonic setting for the Etna volcano
(Lanzafame et al., 1997).
Several authors proposed alternative settings for the development of the
volcanic edifice, spanning from an hot spot origin (Tanguy et al., 1996) to the
an asymmetric rifting process (Continisio et al. 1997) and the dislocation
between the "Malta-Sicilian block" and the Ionian basin (Gillot et al., 1994), or
other several tectonic features intersecting in the Etna area (Di Geronimo et
al. 1978; Lanzafame et al., 1997; McGuire et al. 1997). Many authors
recognize the most important feature being the Malta Escarpment and the
Messina-Giardini fault zone (Lo Giudice and Rasà, 1986); McGuire et al.
1997; Hirn et al., 1997; Lanzafame and Bousquet, 1997). Monaco et al.
(1997) propose a dilatation strain on the footwall of an east-facing normal fault
in the "Siculo-Calabrian rift zone" where WNW-ESE-directed regional
extension takes place.
DIP OF THE REGIONAL FORELAND MONOCLINE
The foreland regional monocline is a typical structure of any front of
thrust belt or accretionary wedge (e.g. Bally, 1983; De Celles and Giles,
1996). The dip of the monocline records the subsidence rates associated to
the flexure of the foreland and the related foredeep formation. We measured
the dip of the monocline on seismic reflection profiles at the front of the
Apennines in eastern Sicily and in the Ionian sea (Bello et al., 1998;
Cernobori et al., 1996). The dip of the monocline at the front of the Apennines
in eastern Sicily is in the order of about 1°-2° in the Hyblean plateau,
increasing to about 4° to 6° below the thrust sheets of northeast Sicily (Bello
et al., 1998). Moving eastward into the Ionian sea, in the Calabria southern
offshore the monocline becomes rapidly steeper, with an inclination of more
than 20° (Cernobori et al., 1996). The separation between the two different
degrees of foreland flexure occurs along the inherited Mesozoic passive
continental margin of the Malta escarpment (Fig. 2).
Tears allowing different dips of the subduction zone have been
proposed in the Andes by Gutscher et al. (1999) in order to explain lateral
variations of the seismicity due to the interaction of the Carnegie ridge with
the trench.
Front of the Apennines
7°
18°
D
8° Ionian sea
21°
5°
6°
2°
C
Malta escarpment
B
A
1°
80
Hyblean plateau
N
160 km
Fig. 2 - Profiles of four (A-B-C-D) foreland monocline dip, two in the Hyblean plateau,
and two in the Ionian sea. Note the larger dips in the Ionian area moving along strike with
respect to the Hyblean-Sicilian area.
Hyblean plateau
0
km
1
2
Abyssal Ionian sea
L Pliocene Trubi
Messinian
0
km
Tertiary s.l.
Cretaceous
Jurassic
pelagic
Malta
escarpment
3
4
1
2
water
3
Norian
Tidal flat
Dolomite
4
Plio-Pleistocene
Messinian
5
5 M-U Miocene
6
6
7
7
Tertiary s.l.
K/U Jurassic
8
continental
basement
9
8
LJ/Trias
?
9
oceanic
basement
Fig. 3 - Representative stratigraphic columns of the Hyblean plateau and the Ionian
sea.
DIP VARIATIONS VS. SEISMICITY
There is a consistent seismicity around the Apennines which is located
in the foreland of the belt (Amato et al., 1993). In particular the most seismic
zones are the Tremiti and Gargano alignments, the eastern Sicily or western
Ionian, and some limited areas of the Po basin. The Tremiti alignment has
been interpreted as the right-lateral transfer zone of the larger slab retreat of
the central-northern Adriatic lithosphere with respect the Puglia area (Doglioni
et al., 1994). This differential retreat was interpreted as generated by the
thicker and lighter Puglia lithosphere that penetrated the mantle at lower rates
for the higher buoyancy during the Pleistocene, with respect to the northern
Adriatic counterpart. There are also earthquakes in the Po Basin, e.g., the
Reggio Emilia (October 1996) which developed well below (16 km) the basal
decollement of the accretionary wedge (4-5 km). This quake had a strike-slip
focal mechanism and it is located along a change in dip of the foreland
monocline (7°-9° to the west and 15°-20° to the east, Pieri and Groppi, 1981).
We speculate that the differential subsidence rates in both sides of the
monocline require a transfer zone which should be active during the general
roll-back of the subduction zone. A similar case appear to be the Malta
escarpment; the inherited Mesozoic margin between the continental crust in
eastern Sicily and the oceanic Ionian basin, respectively west and east of the
escarpment clearly rolled and roll-back at different rates, due to the higher
density of the oceanic Ionian basin. The larger retreat of the subduction hinge
in the Ionian sea and consequently of the foreland monocline implies that the
Mesozoic Malta escarpment has been reactivated as a right-lateral transfer
zone for the differential flexure of the foreland. This could kinematically
explain the eastern Sicily seismicity which is characterized by right-lateral
strike-slip and tensional focal mechanisms (Azzaro and Barbano, 1996).
Moreover the seismicity is an indication that the differential rollback is still
active, in other words it indirectly indicates that the subduction is still alive.
The differential rollback should die out toward the foreland where the tip
line of the rupture propagated (Fig. 4). This is consistent with seismicity which
decreases moving southward into the lower Ionian sea, north of Libya. We
expect that the tip line of the differential rollback will continue to migrate
southward at a few cm/yr, with accompanying seismicity and magmatism.
If we would make a few rheological profiles along the differential motion
between the Ionian and Hyblean lithospheres, the northern part would have a
smaller and shallower crustal brittle peak with respect to the southern part;
this is expected for the asthenosphere wedge which is shallower beneath the
belt, and the foreland is rather cooler and crustal brittle peak deeper and
larger (Dragoni et al., 1995). Therefore along the Malta escarpment, that is
the area of transfer, there likely is a change in the rheological properties due
to lateral geodynamic and thermal variations. This could explain the larger
historical seismicity not in the northern part of the Malta escarpment where
the highest slip rates are expected, but in its central and cooler part.
In summary, the lateral variations in dip of the foreland monocline could
be a key in the interpretation of some of the enigmatic seismic provinces in
full foreland or below the foredeep which do not always follow the outcropping
thrust-belt features.
CONCLUSIONS
The Malta escarpment was the Mesozoic boundary between the
inherited Mesozoic Ionian ocean to the east and the continental Sicily to the
west. During the Quaternary this alignment was used for differential rollback
of the Apennines subduction: the larger retreat of the Ionian lithosphere can
account for the right-lateral transfer zone along the Malta escarpment which is
seismically active. The transtension also allowed the upraise of the Etna
volcano magmas. These kinematics could explain the anomalous external
position and alkaline geochemistry of the volcano, which is not soured by the
slab, but it is in some way related to it. Similar ruptures due to differential rollback of the subduction hinge in the foreland of other orogens could explain
intraplate 'anorogenic' deformation and seismicity.
MaltaEscarpment
West
East
tip line of the
differential
rollback
Etna
Mesozoicrifting
steeper
forelandmonocline
Quaternary larger roll-back
Fig. 4 - Cartoon of the tectonic evolution of eastern Sicily - Ionian sea transition, and
related emplacement of the Etna volcano along the active right-lateral transtension due to the
differential rollback in the footwall of the accretionary wedge (omitted for sake of simplicity).
ACKNOWLEDGMENTS
Many thanks to A. Bally, M. Bello, R. Catalano, S. Merlini, F. Mongelli
and our dear lost friend G. Pialli for helpful discussions. The Italian MURST
(Cofin 97) and CNR supported this study (grants 97.00246.CT05, and
98.00228.CT05).
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