Page 12 - 05 albarello

Dario Albarello, Enzo Mantovani and Marcello Viti

negligible in the study area. This seems the Table II. Elastic parameters of weak zones (indi-
case in most of the zones considered (Viti cated by numbers in fig. 4) corresponding to inter-
et al., 1997). Furthermore, as suggested by plate boundaries. For transform boundaries, the val-
Grunthal and Stromeyer (1992), this assump- ues of Young modulus parallel (Ev) and perpendicu-
tion could be justified by the ductile behaviour lar (Ex) to the shear zone are repòrted. The azimuth
of the lower crust in the zones considered (Viti
et al., 1997), which may allow the decoupling of the shear zone is indicated by e.
of the upper crustal layer from the underlying
lithospheric mantle. Zone Ex Ey e
(1010 Pa) (10 10 Pa)
The third major simplification of the model n
concerns the fact that deformation belts have
been simulated by using elastic elements. One 2 10 0.01 -23
must be aware that this simplification does not 3 0.01 -10
simulate the complex tectonic processes which 4 10 0.01
take place in this kind of zones. However, it 5 10 0.01 19
seems reasonable to assume that, when short 6 0.01 -51
time intervals are considered, average forces 7 lO 0.01
resisting deformations at the boundaries of the 8,10,16 IO 0.01 -33
belts, being the resultant of all forces (friction 9 0.01
on major faults, buoyancy and viscous re- 11,15 10 0.1 -70
sistence to subduction, etc.) which actually ex- 12,13,14 0.01 0.001
ist within the belt, linearly depend on the aver- 0.001 90
age deformation of the whole tectonic struc- O.l
ture. On this assumption, the use of elastic ele- *
ments (isotropic or orthotropic) to simulate 0.001 *
short-term tectonics may represent a first order 0.001 *
approximation of the average mechanical be- *
haviour of true deformation belts. In this con-
text, numerica! values of elastic constants used shallow brittle structures and deeper parts of
to characterize these structures (table Il) should the lithosphere) in geodynamic processes has
only be considered numerica! artifacts useful to been stressed by several authors, on the basis
reproduce realistic strain rates. of observational evidence and mechanical ex-
periments (Tapponier and Molnar, 1976;
Different modelling approaches in terms McKenzie and Jackson, 1983, 1986; Kirby,
of viscous/viscoelastic deformation of litho- 1985; Scotti and Nur, 1990; Ratschbacher
spheric sheets subjected to horizontal tectonic et al., 1991; Jackson, 1993; Twiss et al., 1993;
loads and buoyancy forces have been proposed Somette et al., 1993; Avouac and Tapponier,
by other authors (England and McKenzie, 1993; Vissers et al., 1995; Thatcher, 1995).
1982; England and Houseman, 1989). More re- The importance of this problem in the area
cently, Bird (1989) proposed a more sophisti- here considered is underlined by a large
cated version of this kind of approach by amount of geophysical-geological data, as
assuming a rheological stratification of the mentioned earlier in this work.
tectonosphere, which includes the elastic-brittle
behaviour of the shallow crust. This approach 6. Conclusions
has been used by Bassi and Sabadini (1994) to
model the Centrai Mediterranean deformation lt has been tentatively shown by finite ele-
pattern. This kind of modelling could neglect a ment computations that the main features of
basic aspect of the real tectonic processes, i.e., the observed displacement and strain fields in
the presence of major discontinuities where the Calabrian are and surrounding regions can
most deformation is accommodated. The basic be satisfactorily reproduced as effects of hori-
role played by tectonic discontinuities (in both zontal forces induced by the convergence of
the major confining blocks i.e., Africa, Adriatic

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