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TECTONICS, VOL. 18,NO. 4, PAGES669-685, AUGUST 1999

Patterns of tectonic stress in Sicily from borehole breakout
observations and finite element modeling

SteffenRagg

GeophysicIanlstituteU, niversitoyf KarlsruheK,arlsruheG,ermany

Mario Grasso
IstitutodiGeologieaGeofisicaU,niversitdhiCataniaC, ataniaIt,aly

Birgit Mtiller

GeophysicInalstituteU,niversitoyf KarlsruhKe,arlsruhGe,ermany

Abstract. The orientation of in situ tectonic stress was              Africa. In this paper we analyze borehole breakoutsfrom 22
                                                                      wellswhichwere drilled on the HybleanPlateauandimmediate
deducedfrom boreholebreakout analysisof 22 wells from on-             surroundingsin SE Sicily, and on the AdventurePlateauin the
shoreSicily. The resultsallow us to distinguishthe stressfield        west(Figure 1). The breakoutdata provideinformationabout
of different geological units: (1) A nearly NNW (148 ΓΈ)               the orientation of contemporaryhorizontal compressionin
orientation is detected in the Hyblean Plateau. (2) ANNE              the upper crust. Furthermore,breakoutdata fill the gap of
SHmax direction characterizes the Gela area. To the north,            observationsbetween the shallow overcoring and surface
within the thrust belt (around Mount Judica), the SHmax               investigations of recent and active structures and the fault
direction swings to NE. In the northeasternsegmentof the              plane solutionsat depth. Other stressindicatorsare rare in our
foredeep,the CataniaPlain, the directionof SHmaxis roughly            studyareaof SE and W Sicily. Despitesignificantseismicity
parallel to the NE trending grabens that mark the northern            there are only a few fault plane solutionsavailable [Pondrelli
marginof the HybleanPlateau. (3) The southwesternsegment              et al., 1995]. Fault slip data of Quaternarysedimentsare not
of the foredeephasno preferentialSHmax orientationand may             available,and only a few in situ measurementsexist in eastern
act as a transitionzone with isotropic horizontal stresses.(4)        Sicilybecauseof unsuitablerock typesapartfrom the Hyblean
In westernSicily a SHmax orientation from N-S to NW-SE is             Plateau[Grassoet al., 1986;Bousqueet t al., 1989] (Figure 1).
observed.This fits well the kinematicsof the SE migrating
Egadi and Adventure thrust belts and the direction of                    The few stressdata ahead of our study did not allow a
shorteninginferredfrom the modernseismicityof the area. A             detailedassignmentof stressorientationor stressregime to
three-dimensionalfinite element modeling, including the               the area of investigation. The fault plane solutions of
most important tectonic features, was performed.Modeling              northern Sicily indicate NNE compression[Anderson and
resultsof the stressfield indicate that the NNW trending              Jackson, 1987] whereasin southernmostSicily and Malta a
SHmax is locally influenced by the variation of crustal               NW-SE compression is indicated from in situ stress
thickness and local zones of weakness. In addition, the               measurementsand fault plane solutions [Riuscetti and Schick,
PantelleriaRift causesa rotationof the regionalNW-SE stress           1973; Baumann and Reuther, 1985; Grasso et al., 1986;
orientationto NNE along the northernrim of the rift system,           Farrugia et al., 1989]. These data fit the general Western
including large on-shore areas in the adjacent central south          Europeanstresstrend [Mt;iller, 1995]. Overcoringdata in the
Sicily.                                                               area surroundingEtna [Bousquetet al., 1988; 1989] mirror a

1. Introduction                                                       N-S to NNE orientation of the maximum horizontal stress in

   The knowledgeof the orientationand magnitudesof the                good agreementwith adjacent shallow fault plane solution
stressfield is important for understandingthe sourcesof               data. These data were interpretedas resultsof an active N-S
lithosphericstressesand the nature of intraplate tectonic             compression in the northernmost part of Etna. From
processe[sZobackandZoback,1980]. The primaryobjectives                comparisonof observed stressdata with the results of finite
of this paperare to increasethe availability of reliable stress       element modeling the sources of tectonic stress and their
indicatorsandto interpretstressdataof an areawhichis part
of thecomplexcontinentacl ollisionzonebetweenEuropeand                relative influence are identified and discussed.

Copyrigh1t 999bytheAmericanGeophysicaUlnion.                          2. GeologicaSletting

Papernumber1999TC900010.                                              2.1. Hyblean Plateau, Gela-CataniaForedeep,
0278-7407/99/1999TC900010512.00
                                                                      and the Malta Escarpment

                                                                         The HybleanPlateaulies in the southeasterpnart of the
                                                                      island.It is partof the northernmarginof the Africanplate
                                                                      andhasremaineda relativelyundeformedforelandduringthe

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