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502 E. GUEGUEN ET ALII

Palermo fault seems consistent with the local occurrence,
along this structure, of folds and thrusts (fig. 2): these
could be regarded as induced by a local transpressional
deviation from a purely strike-slip deformation.
In addition to offset calculations inferred from slip
data along the Palermo fault, independent criteria may be
used to attempt quantify the magnitude of displacement
accommodated by the entire fault network at the south-
ern Tyrrhenian Sea margin. Palaeogeographic reconstruc-
tions of the central Mediterranean area since Oligocene
time (GUEGUEN et alii, 1998) indicate that the Calabrian
Massif began to separate from the Corsica-Sardinia block
in Burdigalian time, and was hence translated eastwards
to its present-day position by at least 300 km. The strike-
slip faults of the southern Tyrrhenian Sea margin, docu-
Fig. 7 - 3D sketch diagram, not to scale, of the proposed interpretation mented in this study, appear as the most suitable candi-
of the southern Tyrrhenian Sea margin as an extensional strike-slip dates to have accommodated this estimated dextral
duplex (STSSD): the NW-SE trending faults connecting the main E-W displacement of the Calabrian Massif. Taken at face
trending boundary faults involve a minor component of extension,
consistent with the results of fault kinematic analysis. value, a 300 km motion covered during the last 10 Ma
– Interpretazione cinematica (illustrata attraverso l’ausilio di un blocco- may appear a great displacement indeed. This value,
diagramma tridimensionale) del margine tirrenico meridionale come however, seems consistent with the proposed amount of
un duplex trascorrente destro. I segmenti di faglia con direzione media 400 km for coeval orogenic contraction estimated for the
NW-SE, che si raccordano con le due principali zone di faglia orientate
W-E, mostrano una componente minore di deformazione distensiva, nearby southern Apennines fold-and-thrust belt (PATACCA
coerente con i risultati dell’analisi cinematica. et alii, 1990). Therefore, by simple analogy with the
amount of contraction proposed for the southern Apen-
nine belt, we believe that the 300 km of dextral strike-slip
tum, and calculated a mean stretching factor for the pos- represents a conservative estimate of the displacement
tulated passive margin. However, detailed analysis of taken up by faults of the southern Tyrrhenian Sea margin.
structures imaged in the NSic1 depth converted profile In summary, the results of our analysis point out to a
(PEPE et alii, 2000, plate 1) reveal that the sediments over- dextral strike-slip origin for the faults of the southern
lying the basement are affected by contractional struc- Tyrrhenian Sea margin. This view supports previous strike-
tures, namely folds, reverse faults and related thrusts (fig. 2), slip interpretations by RÉHAULT et alii (1984), GUEGUEN et
that are difficult to explain in terms of purely exten- alii (1998), GUARNIERI (2004) and LENTINI et alii (2006).
sional tectonics. The normal separation that PEPE et alii
(2000) inferred from their interpretation of the NSic1 THE SOUTHERN TYRRHENIAN SEA MARGIN: A LITHOSPHERIC-
depth converted profile (PEPE et alii, 2000, plate 1) indi-
cate a 1.8 km displacement accommodated by the 67° SCALE STRIKE-SLIP DUPLEX
NE-dipping Palermo fault. This interpretation, although The pattern of the main structures of the southern
consistent with the imaged extensional fault separation, Tyrrhenian Sea margin recognised on a regional tectonic
appears kinematically poorly constrained because of the map (fig. 1) defines a major, E-W trending deformation belt
lack of reliable fault slip data along the main structures bounded by two main, parallel fault zones, the UEL and
offshore northern Sicily. On the other hand, fault slip KAL, that are connected by the oblique Marettimo, Trapani,
data from the inland continuation of these structures in San Vito, Palermo, Gratteri-Mt. Mufara and Eolie faults.
the Egadi Islands and in northern Sicily, described in this The analysis of structural data described in the previous
study, point out to their dextral strike-slip nature, with sections indicates dominant dextral fault kinematics. The
only local, minor transtensional and transpressional devi- resulting picture closely reminds that of a brittle zone of
ations (e.g. see RENDA et alii, 2000; this study). In particu- simple shear, where the E-W trending segments represent
lar, the kinematic data from the Palermo fault make it the main boundary faults, while the NW-SE trending seg-
possible to calculate the offset accommodated by this ments represent the synthetic connecting faults, i.e. the R
structure. The transtensional character inferred for this Riedel Shears (i.e. see HANCOCK, 1985). However, when
fault, whose mean striation plunges 8° towards N129°E, seen in detail, the deformation zone appears to deviate from
may be regarded as resulting from the combination of the simple shear zone model, in that the latter requires
two separate slip components, namely a major dextral strict parallelism of the main boundary faults, with no
strike-slip component and a minor extensional dip slip thickness change of the shear zone throughout its length.
component. If we use this mean plunge value, and com- The fault map pattern of the southern Tyrrhenian Sea mar-
bine it with the 1.8 km of vertical separation along the 67° gin, in fact, exhibits a moderate thickening in its central
NE-dipping Palermo fault inferred from the NSic1 seis- zone, where the UEL and KAL boundary faults are con-
mic line (PEPE et alii, 2000; see their plate 1), we obtain a nected by the Palermo fault and Gratteri-Mt. Mufara Line
value of 11.9 km of total transtensional displacement, (see fig. 1). Therefore, as an alternative to the simple shear
with a value of 11.7 km of horizontal component of dex- zone model, we believe that the overall geometry of the
tral strike-slip. Therefore, it appears that the 1.8 km of southern Tyrrhenian Sea margin is best described by the
vertical displacement taken up by the Palermo fault just strike-slip duplex model by WOODCOCK & FISHER (1986).
represents the minor slip component, that is very little We indicate this structure as the Southern Tyrrhenian
with respect to the calculated, total transtensional offset. Strike-Slip Duplex (STSSD). A conceptual, highly schematic
Moreover, our dextral strike-slip interpretation of the 3D kinematic model of the STSSD is shown in fig. 7.