Page 3 - Ragg_et_al-1999-Tectonics

RAGGET AL.:TECTONICSTRESSIN SICILY 671
[Cogan et al., 1989; Yellin-Dror et al., 1997]. The uplift
could be explainedas the result of the compressionarlegime
causedby the continuing collision of the African foreland
with Europeanunits (e.g. the CalabrianArc; seeFigure 1). The
uplift of the bulged area associatedwith mafic volcanism
occurredprior to and during the rapid tectonic subsidence
which followed from middle Pliocene onwards [Grasso and
Ben-Avraharn, 1992; Schrnincke et al., 1997]. The
southwesternsegment of the foredeep is flanked by NE
trending faults which transect the western margin of the
HybleanPlateau(Figure 1). Thesefaults separatethe Hyblean
Plateau from areas to the west where flexural downbending
developsallowingunderthrustingof the African crustbeneath
the Maghrebianthrustbelt [Grassoand Reuther,1988; Grasso
et al., 1990].
In the easternoffshore, the Hyblean foreland is terminated migrated to the SSE with a regional vergence toward the
by the Malta Escarpment, one of the most prominent
physiographicfeatures of the central Mediterranean.The
scarp,with a regionaldip of -30 ø, is formedby a setof NNW
trending faults, most of them being recent and seismically
active [Amaro et al., 1995; Torelli et al., 1998]. The scarp
separatesthe Hyblean-Malta continentalshelf, of which the
HybleanPlateauis the emergedpart, from the Ionian Abyssal
Plain, which largely consistsof oceaniccrust [Ben-Avraharn
et al., 1995].
2.2. The Pantelleria Rift and the Western
Hyblean Margin
Horizontal movements govern the evolution of NW-SE
trending pull-apart grabens (Malta, Linosa, and Pantelleria
basins) forming the Pantelleria Rift System. The grabensof
the rift systemsubsidedabout 2 km in the last 1 Myr, with a
horizontal displacementof 1 mm/yr along the master fault
[Reuther and Eisbacher, 1985]. The present graben
configurationin the Strait of Sicily indicatesa compositeof
pull-apart and transfer basins [Jongsrna et al., 1985]
controlledby E-W dextralstrike-slipfaultsand NW-SE normal
faults. The resulting overall extension due to the incipient
crustalstretchingis directedNE [Reutheret al., 1993; Gardiner zone between the crustal units of the Calabrian Arc and those
et al., 1995 ].
The crustalthicknessdecreasesfrom 35 km below SE Sicily
to about 20 km below the island of Pantelleria. The active NE
orientedspreadingof the PantelleriaRift Systemhasled to the
activationof NE-NNE trendingtransferfaults which form a 30
km large dextral shearzone along the westernmargin of the
Hyblean-Malta Plateau [Ghisetti and Vezzani, 1981; Grasso
and Reuther,1988]. Major structureswithin the fault zone are
the Scicli, Comiso, and Acate faults. This shear zone starts
from the southern end of the rift area and connects the
PantelleriaRift with the northeasternend of the Maghrebian
thrust belt. Its activity started contemporaneouslywith the
rifting in the Strait of Sicily about 5 Myr ago [Reuther and
Eisbacher, 1985].
2.3. The Orogenic Belt: Gela Nappe, Maghrebian,
and Calabrian Arcs
The AdventurePlateauis locatedsouthof the westerntip of
Sicily and may have the geometryof the plate boundaryin
this region:sameorigin as the Hyblean-MaltaPlateaujudging
from the similarities in the stratigraphiccolumns (Figure 1,
see stratigraphic detail on the well logs of Figure 8). Its
crustalstructureis probablyalso different from that of the rest
of the African plate margin (the Pelagian Block) as arguedby
Ben-Avraharn and Grasso [1990]. The Adventure Plateau is
more deformed by a thin-skinnedthrusting [Catalano et al.,
1985] than the Hyblean sector. On a regional scale it lies on
the footwall of a wider structure whose hanging wall is
representedby the frontal zone of an inner orogenicsegment,
the Egadi Thrust Belt (ETF; see Figure 1). The migration of
the orogenic system occurs from the northwest to the
southeast progressively involving foreland areas. The
Adventure shelf thust belt passesto the east, in the Strait of
Sicily, to a late Miocene-Pliocene foredeep [Argnani et al.,
1989; Antonelli et al., 1992].
The Gela Nappe consists of a thrust wedge which has
forelandduring Neogene times [Butler et al., 1992; Grassoet
al., 1995]. In SE Sicily it representsthe latest compressional
episodeat the front of the Maghrebian thrust belt. The Gela
thrustwedgeliesjust aheadof a thruststack,the Mount Judica
imbricates [Bianchi et al., 1989], made of Triassic to Miocene
carbonateand marl deposits (see Figure 1 for location and
Figure 7 for stratigraphy).The deformationof the Gela Nappe
involves late Miocene to early Pleistocene sediments
depositedon the outer edge of the Caltanissettaforelandbasin
(Figure 1). The front of the Gela Nappe can be traced for
severalkilometers offshore southernSicily [Antonelli et al.,
1992].
The MaghrebianArc is composedof tectonicunits thrusted
onto the African foreland becauseof the Neogene collision
betweenthe African foreland and buoyant continentalcrustal
units north of it. These latter units form the Calabrian Arc
which extends offshore northern Sicily into the Tyrrhenian
Sea (Figure 1). The Maghrebian thrust belt was thus formed
between the African foreland and the Calabrian Arc. The suture
of the African forelandis not yet located.
3. BreakoutAnalysisand Data Acquisition
Boreholebreakoutanalysishas becomea widely used
methodto determinteheorientationof themaximumprincipal
horizontalstress(SHmax) [Bell and Gough,1979; Zobacket
al., 1985]. A breakoutarisesowing to stressconcentrationat
the boreholewall causedby an anisotropichorizontalstress
field. If the stressconcentratioenxceedsthe rock strength,
therockfails andbreakoutsform at an angleof 90ø to SHmax.
It is assumed that in the drillable crustal sections the vertical
stresscorrespondingto the weight of the overburdenis a
principalstress.This assumptionis justified by Zobacket al.
[1989], who showthat the majority of crustalearthquakes
have P or T axes within 10ø-15 ø from the horizontal or
vertical.
The boreholebreakoutsof this study are recordedwith a
four-armdipmeter(caliper),a standardloggingtool usedin the
oil exploration.The caliperarmsmeasuretwo perpendicular
diametersof the borehole and their orientation.At depth
intervals where borehole breakouts occur, the two diameter
recordingswill be different:one will be closeto the original
boreholediameter(bit size), andthe otherone will be larger