Page 11 - 05 albarello
P. 11
Finite element modelling of the recent-present deformation pattern in the Calabrian are and surrounding regions
rates deduced by geodetic measurements pression and a E-W to WNW-ESE extension,
(Baldi et al., 1983) and with the results of the which is in good agreement with the seismo-
moment tensor analysis carried out in this tectonic information described earlier.
work (see table 1). The transcurrent motions re-
sulting along the lateral guides of Calabria, 5. Discussion
characterized by sinistra! shear along the Pali-
nuro fault and dextral shear along the Vul- The displacement and strain fields shown in
cano fault (see fig. 5), agree with neotectonic figs. 5 and 6 are the final results of a series of
observations (Finetti and Del Ben, 1986; Van numerica! experiments we carried out by
Dijk and Okkes, 1991). changing a number of model parameters con-
ceming boundary conditions and discontinu-
The southem part of the Iblean microplate ities in arder to best fit observations. Thus, the
tends to move roughly NNE, parallel to the good experimental-theoretical fit finally ob-
African displacement, while in the northem tained cannot be used as a demonstration of the
part this block moves roughly northward. This fact that the adopted tectonic interpretation is
deviation is most probably due to a series of better than the others previously proposed. The
factors, such as the low compressibility of the usefulness of the results obtained is that they
Calabrian wedge, the presence of the Volcano prove that the deformation pattem in the Cen-
transform fault and the «Soft» border lying trai Mediterranean area may be explained as an
north of the lblean block. The kinematics of effect of horizontal forces induced by the
the Iblean zone shown in fig. 5 is consistent «Africa-Adriatic» convergence without invok-
with the results of the first VLBI measure- ing «local» driving forces, such as, e.g., those
ments in the station of Noto, located in the connected with «slab pull» or «active rifting»
southemmost edge of Sicily (Zarraoa et al., mechanisms.
1994; Lanotte et al., 1995).
The reliability of the results obtained rnight
The displacement of the Iblean block cre- be undermined by the fact that the adopted
ates compressional stresses in the Gela crustal modelling approach involves some simplifica-
wedge, which, consequently, tends to escape tions with respect to the real behaviour of the
roughly NNE. This escape is accommodated Earth. The first important limitation concems
by shortening of the «soft» boundary in Cen- the use of elastic elements. This choice would
trai Sicily (Caltanissetta nappe), which simu- be scarcely reliable if one tried to model long-
lates the underthrusting process occurring at term tectonic processes. However, the very
this border. short time interval here used implies that the
strain is quite small (of the arder of 10-5).
In the wake of the Gela block, extensional Thus, the difference between the real kinematic
strains occur along the boundary with the field (out of deformatian belts) and the one we
African foreland, so reproducing the exten- obtained can reasonably be considered negli-
sional tectonics observed in the Sicily Channel. gible.
The extensional rate resulting from modelling
in this zone (= 1-2 mm/yr) is compatible with Another simplification of the model is given
the indications of neotectonic data (see, e.g., by the «plane stress» approximation. The relia-
Reuther et al., 1993). bility of this choice depends on whether the
model can be considered very thin with respect
The sinistra! and dextral shear resulting in to its lateral extension. In our case, this condi-
the Sciacca and Comiso-Scicli faults respec- tion seems to be fulfilled given the small thick-
tively agree with the direction of transcurrent ness of elastic blocks (= 20 km) with respect to
motion observed in these zones (Argnani et al., their average horizontal dimensions (of the or-
1987; Finetti and Del Ben, 1986; Reuther,
1993; Ben Avraham and Grasso, 1990). In der of = 100 km). This hypothesis implies that
fig. 6 it is possible to note that, apart from the
Sicily Channel and the Ionian border of Cala- the mechanical contribution of the resistant
bria, the whole region is affected by a strain mantle layer to the total lithospheric strength is
field characterized by a SN to SSW-NNE com-
843
