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RJ-IEGMATIC-LIKE BASINS IN SICILY
chasm-like basin as in the present paper. Moreover, the in-
itial thennal input and the asymmetric upper mantle "intru-
sion" are being subsequently controlled by shear zones re-
sulting from a complicateci and changing regional stress
fiele!. The temporal change of stress fiele!, recorded in the
poliphase-deformed Plio-Pleistocene sequences, suggests a
rhythmical tectonics in the Centrai Mediterranean related to
the resultant of group of vectors in the space that express
forces (Fig. l 7). In Fig. 17A a reference system is shown in
which the velocity vectors that contro! the geodynamic evo-
lution of Centrai Mediterranean are drawn. It is possible to
decompose tbe velocity vectors as the resultants of African
subduction-Egadi Fault movements (Va) and mantle uplift-
Central Tyrrhenian opening (Vr). Besides, the Afiican sub-
duction rate is mucb lower with respect to the Egadi Fault
rate, wbile the mantle uplift, as well as the Magnaghi-Vavi-
lov basins opening, are considered to represent as angular
velocities. Tbe predominance intime ofVa or Vr establish a
compressional/transpressional or extensional tectonic re-
gime respectively, in the Tyrrhenian and peri-Tyrrbenian ar-
eas (Fig. 178).
Fig. !SA illustrates a possible kinematic evolution of a
non-rigid plate moving on a sphere dissected by crustal
sbear discontinuities. Fig. 178 l represents the initial setting, northward prolongation, Fig. 188 l). This crustal "line-
characterised by the N-S/NNE-SSW trending Apen-
nine-Sicilian-Maghrebian mobile belt, and by severa! in-
cipient mega-sbear zones formed due to the northward Af-
rica plate motion. Tbe development of the initial stage is
shown in Figs. 1882 and 1883, in which a progressive anti-
clockwise rotation and eastward shifhng of the "Tyrrhenian
microplate" producing extension and overlap areas is identi-
fied. In Figs. !SA and 188 the presence of the Ionian sub-
duction complex has not been considered.
The main structural features in Fig. 1883 are compara-
ble with tbe observed structures in mainland Sicily and in
the surrouncling submerged areas (see also Figs. 13, 14 and
16).
In spite of the fàct that the fìeld observations and the
geopbysical data pertain to a relatively small area, it is pos-
sible to hypothesise tbe Tyrrbenian dynamics as caused by
prevalent shear mechanisms, related to tbe Africa-Europe
convergence and to examine boundary conditions that char-
acterise the Centrai Mediterranean (the presence of a mobile
belt, an active subduction complex, etc.). These conditions
migbt bave played a part in the dynamic development of the
system.
Thus, the rhegmatic mode! proposed in this paper con-
siders:
- the presence of a thickened African lithosphere below
the more internai areas of the Sicily-Apennine Chain
(Caputo et al., 1972; Morelli et al., 1975; Cassinis et al.,
1979; Scandone 1979, see Fig. 15);
- the crustal delamination (back-stop-like processes)
occurrence, necessary to create the thickness of the sub-
ducted African lithosphere (Channel & Mareschal, 1989);
- the northward Neogene direction of the African plate
(Dercourt et al., 1986; Malinverno & Ryan, 1986; Dewey et
al., 1989);
- the Tortonian-Pleistocene roll-back of the African
subducted slab (Malinverno & Ryan, 1986);
15
- the roll-back rate, which is higher than the African
slab sinking rate.
This mode! also considers the oblique convergence vec-
tors between the African-Ionian subduction complex and
the Tyrrhenian opening during the late Pliocene-Pleisto-
cene.
According to these conditions, it is likely that in the
back area of the African-Europe collisional system, rheg-
matic-like structures might bave developed during the Plio-
Pleistocene.
During the late Miocene-early Pliocene, the rhegmatic
structures formed in the back area ofthe N-S trending thick-
ened African-Europe suture, were the result of crustal pure-
shear configuration.
In the early phase of Tyrrhenian stretching, a strong
lower-crust thinning and beat input (upper mantle uplift) oc-
curred. Viscous dissipation (low deviatoric stress) occurs
where thermal anomaly takes piace. In this stage, the exten-
sion in the upper crustal leve! started from an important
right-lateral transcurrent crustal fault (Egadi Fault and its
ament" bounds the Sardinia 8lock eastwards. Some mega-
Riedels were also activated (e.g. the "Selli Line").
Activation of a low angle norma! fàult system cutting
the upper (brittle) crustal leve! only might have tàcilitated
subsidence of the proto-Tyrrbenian basin. Tbe low stress
domain widened with tbe expansion ofthe tbermal anomaly,
but the lower crust and upper mantie became stronger as the
amplitude of the thermal anomaly has decreased.
The evolution ofthe "Selli Line" in a sphenochasm-like
structure (eastward and/or south-eastward extension associ-
ateci with the formation of oceanic-like crust in the Mag-
nagbi and in the Vavilov Basins (Fig. 1882) could be related
to tbe northward motion of the African plate, producing at
the same time an increasingly arched shape of the Sicil-
ian-Apenninic suture zone, with clockwise rotations in Sic-
ily (Fig. 1883). The consequent asymmetric mantie uplift
(Channel & Mareschal, 1989; mode! 4) is related to the
asymmetric stretching.
In the following stage, a rapid lithospheric thinning
could have been caused by convective thinning of the litho-
sphere, as a result of gravitational instability in the density
contrast between the mantle, the lithosphere and the asthe-
nosphere. Asymmetric mantle f1ow, and its south-eastward
migration, might have induced a subcrustal compressive
stress causing progressive verticalization of the previously
subducted African plate and of the Ioni an microplate (roll-
back-like processes), consistent with geophysical and pet-
rographic data fì·om the Aeolian area (Barberi et al., 1974;
Wang et al., 1989; Crisci et al., 1991 ). The asymmetric up-
per mantle "intrusion" (Channel & Mareschal, 1989) might
bave "delaminated" the Wadati-Benioff sur±àce toward tbe
S-SE. In the binge zone of the subducted plate, a strong
stress could bave produced a semi-brittle reverse crustal
movement and a furtber contraction of tbe Sicily belt. This
is even more likely as a zone ofweakness was createci by tbe
previous African crust delamination related to the Miocene
back-stop processes. The main effects of this contraction in
mainland Sicily are also expressed by some out-of-sequence
thrusting (Fig. 15) with major emplacement involving ofthe