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14 F. NIGRO
Fig. 15. Simplitìed 3D neotectonic crustal features ofthe Centrai
Mediterranean. The picture shows the oblique movement of the
African-Ionian plates compared to Tyrrhenian spreading and the
main surface effects (crustal buried and/or emergent right-lateral
transpressional faults, associated en-echelon fold systems and the
main out-of-sequence thrusts), mostly related to the compressive
stress induced in the subducted hinge zone of African plate by the
"Tyrrhenian upper mantle". See text for explanation
tors") can be a resultant of coupling of motion vectors that
represent the northward movement of Africa and mantle
processes. These "motors" are influenced by minor impor-
tant factors, such as severa! structures resulting from older
intraplate stresses and the Coriolis drift, and a geometrie
system characterised by plates moving on a sphere. The
overall stretching ofthe Tyrrhenian area may be the result of
complex, repeated tectonic stages, being the predominant
effects of one ofthese main driving forces.
The recognised neotectonic lineaments in mainland Sic-
ily, as well as in the surrounding submerged areas (Figs. 7,
8, 13 and 14), are interpreted in this paper as important crus-
tal structures ofthe Centrai Mediterranean, being an expres-
sion of a regional shear system that controls the Tyrrhenian
basin evolution. Thus, the trends of these structures and
their regional patterns might be considered to be some ofthe
main effects of the Tyrrhenian dynamics during the last few
Ma. These effects (mostly represented by different scale
shear zones) are now combined in a kinematic mode! of
plate motion on a sphere affected by some mega-shear dis-
continuities. These discontinuities are related to the overall
northward motion of Africa during the last Ma. The sche-
matic structural setting presented in Figs. 13 and 16 summa-
rises the main structural lineaments related to the geody-
namic processes of the Tyrrhenian opening, reconstructed
with the use of field observation in mainland Sicily dis-
cussed above and with the published and unpublished seis-
mic data.
For a better understanding of the possible relations be-
tween the Tyrrhenian extension process and the coeva! sur-
rounding structures (extensional and!or compressional/tran-
spressional as described before), the oblique convergence of
the N-NNW Ionian subduction versus the SE Tyrrhenian
spreading direction has also to be considered. This might in-
duce strong extension in the centrai, as well as in peripheral
basinal areas and create intraplate stresses with prevalently
transpressional displacements in Sicily. A superimposed in-
itial thennal input from the uppennost mantle to the lower
crust may also be considered.
Fig. 16. Crustal structures of the Centrai Mediterranean related
to neotectonic evolution. The sketch shows the present-day main
structural features of the Tyrrhenian area, connected with rheg-
matic-like mechanisms. According to this mode!, the Mag-
naghi-Vavilov area is interpreted as a sphenochasm basin which
determines the easternmost transcurrent structures (e.g. Marsili
and Paola Basins). See text for explanation. l - thinned and/or
oceanic-type crust; 2- Sicily-Apennine Thrust Be!t; 3- superim-
posed extensional areas; 4 - extension vectors; 5 - compression
vectors
This mode! does not agree with the existing ones pro-
posed for the Tyrrhenian-Apennine system kinematic evo-
lution. These models involve:
a) Europe-Africa convergence, with latera! "extrusion"
of some continental microplates (plastic-rigid deformation)
and formatian of oroclines (Tapponier, 1977; Bocca!etti et
al., 1982);
b) lonian-Adriatic passive subduction, with extension
in the internai areas and compression in the external areas
(Boccaletti & Guazzane, 1972; Malinvemo & Ryan, 1986;
Patacca & Scandone, 1990); and
c) Tyrrhenian extension as a consequence of gravita-
tiana! collapse processes of a thickened lithosphere (Reutter
et al., 1980; Channel & Mareschal, 1989).
In the present author's opinion, the gravitational col-
lapse processes alone (c-models) cannot produce the recog-
nised rhegmatic basins configuration (Figs. 13 and 16). The
lonian-Adriatic passive subduction alone (b-models) may
produce E-W trending shear systems (as in the a-type mode!
of Boccaletti et al., 1982), relatable to the Africa-Europe
NNW-SSE convergence movement, but not a spheno-