Page 13 - Ragg_et_al-1999-Tectonics

RAGG ET AL.: TECTONIC STRESS IN SICILY
Table 3. Material Propertiesof the Reck Types Used for the Modeling
Material Young'sModulus,GPa
Sediments 40
Carbonates 65
65
Granite 116
Gabbro
75
Metamorphic
From Landolt-Boernstein[1982]
opening strongly affects central south Sicily where SHmax
rotates to ENE and NE-SW orientations, which are roughly
perpendicularto the rift axis. This model may give the correct
trend for the stressorientations in western Sicily and in the
Catania Plain, but there is a big discrepancy between the
modeledand observeddata on the Hyblean Plateau.
As final model, we consider model Siz40 (Figure 13 and
Table 4), which representsthe trend of the observed stress
orientations, namely, NW-SE in western Sicily, NNE in the
Gela Nappe, NE in the Catania Plain, and NW-SE in the
HybleanPlateau.We want to emphasizethat owing to the grid
of the finite element model, it is only possibleto give the
trend of the stressorientation and not exactly the observed
values. It is worth highlighting that the area where the
observeddata show a rather strong scatter coincides with a
region, in the model Siz40, where the horizontal stressesare
nearly isotropic (Figure 14). In this area the deformation at
the boundaryand the opening of the Pantelleria Rift leads to
nearlyequalmagnitudesof SHmax and Shmin (Figure 14).
The main resultsderived from the numerical modeling are
the following:
(1) The observed stress field can be modeled under the
assumption of a NNW-SSE regional stress field, which is
modulated by the variation of crustal thickness as one
important parameter. The rotation resulting from thickness
variation is clockwise in the northeasternpart of Sicily and
Calabria and counterclockwisein westernSicily.
(2) Material propertiesdue to different rock types do not
cause significant stress reorientations. This is in accordance
Table 4. Boundary Conditionsfor the Models Describedin the Text
Name MohoShape DifferentMaterials
Siz36 yes no
Siz50 yes yes
Siz35 yes no
Siz40 yes yes
All modelscontainthevariationof the Moho depthin comparisonto a plainreferencemodel."Yes"in the differentmaterialscolumnindicatesthat
differentYoung'smoduliandPoisson'sratioshavebeentakeninto accountM. odelswith "yes"in the columnof Malta Escarpmenatccountfor the
Malta Escarpmenats a weak zone."Yes"in the rift openingcolumnmeansthat the PantelleriaRift was modeledby consideringdisplacements
perpendiculatro the rift axis.
681
Poisson'sRatio
0.28
0.33
0.25
0.34
0.18
with the resultsshown by the World StressMap [Zoback,
1992].
(3) The deflection of the SHmax orientation connected with
the opening of the Pantelleria Rift is toward NE-SW and
affects mostly central Sicily.
(4) The modelingof the Malta Escarpmentas a weak fault
zone overstripsthe effect of the PantelleriaRift openingin
the area of the Hyblean Plateau and leads to the NW-SE
orientationobservedon the plateau itself.
6. Discussion and Conclusions
The orientation of the maximum horizontal stressobserved
from breakout data in 22 wells in south-eastern and western
Sicilytogetherwith previouslyavailablestressmeasurements
[M•illeret al., 1992]allowsus to distinguishseveraltectonic
unitswith characteristicstresspatterns.
The resultsof finite element modeling show that the
diversityof observedstressorientationscan be explained
with a singleNW to NNW orientedfar-field stress,modulated
by crustal thickness variations and local sources.The far-field
stress is linked to plate tectonic processes,such as the
northwardmotionof major crustalunits locatedalongthe
Africanplatemarginandthe spreadingon the Sicily Strait
Rift Zone.Thelocalstressfieldis modulatedby weakfracture
zones and varying crustal thicknesses.
Applyingthe globalquality assignmentable of the World
StressMap, ourbreakoudt atacanberegardedasreliablestress
indicatorsa,ndthuswe contributteo a betterlocalknowledge
Rift Opening Malta Escarpment Figures
no no 12
no no 12
yes no 12
yes yes
13, 14