Page 14 - Ragg_et_al-1999-Tectonics

682 RAC• ET AL.' TECTONIC STRESSIN SICILY

of the state of stress in the upper crust. No systematic
coherence between the brehkout orientation and the
stratigraphy(Figures4-8) was observed.

From the comparisonof observedstressesand the modeled
stressfield we summarizethe following conclusions:

1. In spite of the very complex stress pattern that is
indicatedthroughthe observeddata and the interpolatedstress

fieldof theMediterraneaitni,s possiblteo trace,backmostof

this patternto a simple model that includesonly the crustal
structure(elastic thickness)and the main tectonic featuresthat
are the PantelleriaRift and the Malta Escarpmentacting in a
far-field that showsa NNW-SSE orientation(Figure 13).

2. The influence of the opening of the Pantelleria Rift
appearsto be dominantin central Sicily. From the modelone
could predict that on the northern shoulderof the Pantelleria
Rift the SHmax orientation turns from NNW to NE, that is,
orthogonalto the spreadingaxis, whereason the rest of the
rift and adjacent sectors, the stress field does not rotate
significantly.

3. A possible hypothesis to explain that the NE
compressiondoesnot propagateon the Hyblean Plateauis the
occurrencealongthe westernmargin of the Hyblean Plateauof
a large transferzone (Acate, Comiso, and Scicli fault system,
seeFigure 1) which accommodatesdifferent ratesof spreading
of the Pantelleria Rift by dextral movement [Grasso and
Reuther, 1988]. Thus it inhibits the stress transfer to the
Hyblean Plateau.

4. The large scatterof the data in foredeep(Figure 9) could
be explainedby flexural tensile stresseswhich superimpose
the regional stressin sucha way that when tensile stressesare
predominant,the resulting stressis oriented NNE-SSW, that
is, parallel to the flexural downbendingof the underthrusting
African plate margin [Adam and Reuther, 1995]. Comparing
the modeled magnitudes of the horizontal stress in the
foredeep,it is obviousthat nearly all wells with a dataquality
of D are locatedin an area with modeledisotropicstressesT. he
reasonfor the isotropy could be the reduction of the NNW
compression by NNW tensile stresses, since geological
evidenceandthe regionalstructuralsettingof the areaallow us
to rule out activeENE compressionwith similar magnitudesin
comparisonwith the far-field stress.

Figure 12. Stress maps derived for different model
configurations(seeTable 4). The stresssymbolsof the models

show the maximum horizontal stress orientation and their

lengthindicatesthe stressmagnituderelative to the maximum

stress obtained for the individual models. Because of the

elastic material properties in the finite element (FE)
computationthe scaleof the absoutevaluesis determinedby
the displacementsat the model boundaries,for which there is
certain variability in the choice of the values. Thus we
consideredin our interpretations the relative stress values
only.