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GEODYNAMICS OF THE SOUTHERN TYRRHENIAN 83

INTEGRATING SPACE GEODESY, STRUCTURAL whose southern boundary is currently interpreted as a
AND GEOPHYSICAL DATA: A MODEL true passive margin (e.g. see KASTENS et alii, 1988; KAS-
FOR THE SOUTHERN TYRRHENIAN SEA MARGIN
TENS & MASCLE, 1990; SARTORI, 1989, 1990; PEPE et alii,
1998, 2000, among many others). According to the pas-
The original geodetic evidence presented in this paper sive margin model, the main structures inferred from
indicates that there has been NW-SE directed shortening seismic reflection profiles offshore northern Sicily are
during the last 10 years within the area defined by the steep E-W trending, mainly N-dipping normal faults. This
measurment sites of Cagliari, Matera and Noto (fig. 3). interpretation, although consistent with the imaged
NW-SE directed shortening reflects active deformations, extensional fault separation, is poorly constrained due to
and poses the question of where these deformations are the lack of fault kinematic data. On the other hand, struc-
accommodated. Since active compression is believed to tural analysis on the fault segments exposed onshore
generate deformations on the frontal part of the thrust northern Sicily and the Egadi Islands, that represent the
wedge of the Apennines-Maghrebides belt (TORELLI et alii, inland continuation of offshore structures, has revealed a
1998), one possibility is that the geodetically measured dominant dextral strike-slip character, with only a local,
NW-SE shortening is accommodated by active thrusting minor extensional component (e.g. see RENDA et alii,
and related south-verging folding at the Apennines belt- 2000). In the light of this kinematic evidence, we propose
Hyblean foreland transition zone. However, the Apennines that the extensional structures imaged in seismic profiles
thrust front in southeastern Sicily appears dissected and offshore northern Sicily (e.g. see PEPE et alii, 1998, 2000)
offset by more recent strike-slip and/or transtensional are, in fact, transtensional structures whose development,
fault zones, namely the NNE-SSW trending, dextral Sci- in addition to the minor recognised extensional separa-
cli fault and the NNW-SSE trending, sinistral Malta tion, also involved a major component of dextral strike-
Escarpment (GRASSO & REUTHER, 1988; GARDINER et slip. This interpretation is broadly consistent with the
alii, 1995). Therefore, based on these overprinting rela- NW-SE directed shortening field inferred from original
tionships, we believe that the dominant character of geodetic measurements, and with the STSSD model pro-
active deformations is strike-slip, and that the geodeti- posed by RENDA et alii (2000).
cally measured NW-SE shortening reflects active dextral As already outlined in the previous sections, the
deformations in the southern Tyrrhenian area, rather results of integrated space geodesy and structural analy-
than thrusting in southeastern Sicily. ses reveal a NW-SE (fig. 3) shortening, and the results of
A comparison of the results deriving from available focal mechanism analysis indicate a shortening field
structural (fig. 1) and geophysical evidence (fig. 2), inte- ranging from WNW-ESE, in western Sicily, to NNW-SSE,
grated with original space geodesy data (fig. 3), shows in the Eolian Islands (FREPOLI & AMATO, 2000). This
that the main features of the southern Tyrrhenian Sea broad range of orientations could result from the paucity
margin may result from a sub-horizontal shortening field, of available data, or, alternatively, from the fact that the
whose trend ranges from NW-SE (fig. 3) to WNW-ESE earthquakes used for focal mechanism analysis are local-
(fig. 2). This broad consistency among independent data ised in narrow, seismogenic belts rather than over a wide
derived from different techniques, supports the deforma- area, like the sector considered for geodetic strain calcu-
tion model proposed by RENDA et alii (2000), where the lation. The latter hypothesis seems likely in that, although
southern Tyrrhenian Sea margin is interpreted as a major there is a high variability in the directions of shortening
E-W trending strike-slip fault zone. The tectonic map of inferred from focal mechanism analysis in western Sicily
fig. 1 shows the distribution of the main faults, where the and in the Eolian Islands, the calculated mean N130°E
dominant structures correspond to the Ustica-Eolie Line, orientation, obtained by averaging the N99°E and N162°E
to the North, and to the Mt. Kumeta-Alcantara Line, to directions inferred from 47 events (fig. 2), is broadly con-
the South. The NW-SE trending Marettimo, Trapani, San sistent with the N137°E direction of shortening inferred
Vito, Palermo and Etna faults, and the NNW-SSE trend- from geodetic measurements.
ing Gratteri-Mt. Mufara Line, are confined within a def- There appears to be a general consistency between
ormation belt bounded by the Ustica-Eolie and the Mt. the data obtained from independent geological, geophys-
Kumeta-Alcantara lines. These faults consistently display ical and geodetic techniques. The slight discrepancy in
dextral kinematics, with NW-SE trending faults also the shortening directions inferred from different sources
showing a minor extensional component, and were all of data could account for the minor component of exten-
active in the Pliocene-Quaternary interval. The combined sion recorded by fault kinematic indicators. We believe
arrangement of E-W and NW-SE trending faults (fig. 4a) that the local oblique-slip character of some faults reflects
reminds that of a brittle zone of simple shear (fig. 4b), a deviation from a purely strike-slip deformation to a
where the E-W segments represent the main boundary transtensional deformation, induced by fluctuations from
faults, while the NW-SE trending segments represent the a N130°E to N137°E directed strain fields, inferred from
synthetic connecting faults, i.e. the R Riedel Shears (e.g. geophysic and geodetic analyses, respectively.
see HANCOCK, 1985). The investigated fault system, how- In summary, our investigation further supports the
ever, deviates from an ideal zone of simple shear, in that preliminary interpretation of the southern Tyrrhenian
it exhibit thickness variations along its lenght, and is thus Sea margin as a major dextral strike-slip duplex (STSSD:
best defined as a strike-slip duplex sensu WOODCOCK & RENDA et alii, 2000). In addition, and more important,
FISCHER (1986). This structure has been first described our study demonstrates the effectiveness of integrating
and indicated as the Southern Tyrrhenian Strike-Slip independent results from different analythical techniques.
Duplex (SSTSD; fig. 4c) by RENDA et alii (2000). We feel that this multidisciplinal approach will ultimately
The results of our analysis, and the deriving strike- result in a better understanding of the processes respon-
slip duplex interpretation, have important implications sible for the deformation of lithospheric plate boundaries,
for the geodynamic evolution of the Tyrrhenian Sea, as well as of other mobile zones of the Earth’s crust.