Page 3 - Tavarnelli_et_al2003

COMPOSITE STRUCTURES RESULTING FROM NEGATIVE INVERSION
data ar·e derived from the southe1nrnost structures of the
Outer Units exposed in the Isle of Favignana, the major
island of the Egadi group (TAVARNELLI et a/;;, 2003) .
CONTRACTION Al"D EÀ'TENSION
IN THE ISLE OF FAVIGNANA
Early contractional and late extensional deformations
are well recognised in the Isle of Favignana. Its structure,
reconstlucted through detailed l: 12.500 scale mapping
(ABATE et alii, 1997), is characterised by the superposition
of two main tectonic slices, that were stacked along a
shallow thrust during the construction of the outennost
Apennine-Maghrebide belt: the Punta Faraglione Unit,
above, and the Mt. Santa Caterina Unit, below (fig. 2a;
lNCANDELA, 1995; ABATE et a/;;, 1997) . The stratigraphic
sections of both units consist of Upper Triassic-Lower
Jurassic p latform car·bonates grading upwar·ds into Mid-
dle Jurassic-Eocene pelagic limestones and marls (INCAN-
DELA, 1995). These sediments are unconfonnably overlain
by Miocene neritic limestones, which record the onset of
contt·actional defonnation, and were probably deposited
on top of an uplifted peripheral bulge in front of the
evolving fold-and-thrust belt (NIGRO & RENDA, 1999). The
youngest exposed deposits that experienced contr actional
defonnation ar·e marls of Tortonian age. These deposits,
along with Messinian gypsiferous clays encountered in a
deep bor ehole offshore (BENEO, 1961), constt·ain the age
of tluusting, and enable the fina! southward emplace-
ment of the Mt. Santa Caterina Unit onto the Punta
Faraglione Unit to be r eferred to a generic Late Miocene-
Early Pliocene inte1-va! (INCANDELA, 1995) .
The main contractional structure is the thn1st fault
along which the Mt. Santa Caterina Unit oven:ode the
Punta Faraglione Unit, producing a minimum horizontal
displacement of 3200 m (fig. 2b) . This stmcture, that we
indicate as the Favignana Thmst, parallels bedding in its
hanging-wall and footwall for most of its length, and has
t11erefore the features of a major flat. Additional contraction faults that tt·uncate the Favignana Thmst (fig . 2b) . We
within the Mt. Santa Caterina Unit is taken up by minor
reverse faults and related folds that emanate from the
Favignana Thrust and defme a km-scale imbricate fan out-
croppi.ng on both flanks of Mt. Santa Caterina (fig. :L b). The marls, ca!carenites and ca!cirudites of Pleistocene age,
occun:ence of calcite shear veins along the thrust surfaces
indicates that fluid circulation and pressure-solution mech-
anisms played an impo11ant role dming contraction. The
Favignana Thrust surface is defo1med to defme a broad
synfonn wit11 gently dipping limbs, that affects t11e stacked
Punta Faraglione and Mt. Santa Caterina units (fig. 2b).
This synf01m may either be cored by a deeper blind t1111.1st
related to t11e sequential migration of t11e Apennine defor-
mation front toward the foreland, or alternatively could
result from slip along a listric n01mal fault. We believe that
both explanations are realistic and viable, because there is
no evidence to rule out either mode!, and because both
solutions apperu· equally consistent with the regional evolu-
tion ofthe Apennine-Maghrebide fo!d-and-tlm.1st belt. How-
ever, since there is abundant evidence for tilted nonna!
fault sm·faces (e.g. see below), we favour the view that late
tilting was induced by post-orogenic extension.
The genera! fo!d-and-th1ust stluctm·e of Favignana is
dissected by younger strike-slip and extensional deforma-
tions. The main structures ar·e two N-S trending, right-lat-
eral strike-slip faults that are traced for over 2 km west
32 1
\\ ~ X' Punta Quaternary
Fara glio ne d e po sits
\ .- Fig. 3 Mt. Santa
Caterina Unit
.,•.:.--..~
Punta
··11\ Faraglione Unit
thrusts
strike-slip
faults
normal
faults
o 45
b Scindo Km
Passo
x Punta
500
o 500
m
Fig. 2 - Geology o f the l sle o f Favignana. a) S implifiecl geologica!
map (m oclifiecl after ABATE et a/i i, 1997) . b) The Punta Faraglione-
Scindo Pas so cross-sect.ion (trace X -X' in fig. 2a) .
- Geologia dell'Isola d i Favignana. a ) Carta geologica semplificata (da
ABATE et a/ii, 1997, con modifiche) . b) La se=ione Punta Faraglione-
S cindo Passo (h·accia X -X' di fig. 2a).
and east of Mt. Santa Caterina, respectively. These struc-
tures separ·ate the Punta Sottile, Scindo P asso and Cala
Azzm-ra b locks or compartments (fig . 2a). N -S trending
strike-slip faults are associated with E -W trending n01mal
interpret norma! and strike-slip faults as the extensional
and transfer segments of a linked extensional fault sys-
tem. Most n01mal and strike-slip faults are sealed by
indicating that the main def01mation episodes took p iace
during Pliocene time. However, Pleistocene deposits are,
in turn, locally tnmcated by some of these structures,
indicating that !oca! fault reactivation occurred during
the Quate1nary. Most n01mal faults dip towards the north
and produce offsets of severa! tens of metres. Kinematic
analysis of mechanical striations and fibres measured
along the thmsts and n01mal faults (fig. 3) indicate that
early (i.e. Miocene-P liocene) contt·actional and late (i.e.
Pliocene-Quaterna1y) extensional defo1mations were
roughly coaxial.
At Punta Faraglione, the Favignana Thmst (X in fig.
4a) dips southwards at 30°, as a consequence of late defor-
mation. The t1111.1st surface is truncated and offc;et by a
stepped n01mal fault, with gently north-dipping segments
connected by sub-horizontal flats (Y in fig. 4a), producing
a minimum offset of 300 m, with displacement toward the
n01th (fig . 2b) . The present att.itude of this structure does
not seem origina!, but rather probably results from tilting
of a moderately N-dipping n01mal fault by ymmger defor-