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AGOSTA

Rend. online SGI, 1 (2008), Note Brevi, www.socgeol.it, 9-11, 4 figg.

Enucleazione e crescita di faglie trascorrenti nei grainstone carbonatici
pleistocenici dell’isola di Favignana (Isole Egadi, Sicilia)

FABRIZIO AGOSTA (*), EMANUELE TONDI (*), ANTONINO CILONA (*), ATILLA AYDIN (**),
MARINA ALAGNA (*), PIETRO RENDA (***) & GIUSEPPE GIUNTA (***)

ABSTRACT L’enucleazione delle faglie è avvenuta attraverso mec-
canismi di shear banding, con formazione di due set di ban-
Nucleation and development of strike-slip faults in Pleisto- de con porosità inferiore rispetto al protolite carbonatico
cene carbonate grainstones of the Favignana Island (Sicily, Italy). (fig. 1). L’evoluzione delle singole bande è avvenuta attra-

After detailed field and laboratory analyses of faulted carbonate Fig. 1 - L’immagine, realizzata al microscopio ottico a nicols incrociati,
grainstones cropping out in the Favignana Island, we document mostra una singola banda con cinematica sinistra avente una minore
interactive failure processes responsible for strike-slip fault nucle- porosità rispetto alla roccia incassante, dalla quale è separata da una
ation and development. First, these processes involved strain local- zona a porosità intermedia.
ization into narrow bands; than, pressure solution took place within – Photomicrograph of a compactive, left-lateral shear band obtained
the bands with formation of isolated stylolites that were subse- under a cross-polarized light. The band is flanked by a slightly compacted
quently sheared producing cataclastick rocks. The transitions from zone.
one deformation process to another, which were likely controlled by
changes in the material properties, are recorded by different ratios
and dissimilar distributions of the fault dimensional attributes.

In particular, the results of our study allow us to: (i) identify
two conjugate sets of right-lateral and left-lateral strike-slip faults
trending NW and NNE, respectively; (ii) document the progression
of deformation from single compactive shear bands, to zones of
compactive shear bands, to zones of compactive shear bands with
discontinuous cataclasis and slip surfaces, and finally to faults with
an inner cataclastic core surrounded by wider damage zones made
up of compactive shear bands, joints, and possible dilational bands;
(iii) decipher the linkage processes of lower-rank shear structures
responsible for fault development; and (iv) define the scaling rela-
tionships among length, thickness, and displacement of strike-slip
faults with different amounts of offset.

Based on their internal architecture and petrophysical proper-
ties, the study strike-slip faults behave as combined barrier-conduit
hydraulic structures to fluid flow. The single compactive shear
bands, the shear band zones, and the cataclastic cores of the faults
have a lower porosity relative to the surrounding carbonate host
rocks, and therefore form seals for cross-fault fluid flow. On the con-
trary, the discrete and undulated slip surfaces present either within
or at the edges of the fault cores enhance the along-fault fluid flow.

Sulla base di rilevamenti strutturali di campagna e
di analisi al microscopio ottico ed elettronico di campio-
ni di roccia integra e deformata, si documentano i pro-
cessi di enucleazione e crescita di faglie trascorrenti nei
grainstone carbonatici pleistocenici dell’isola di Favigna-
na (Isole Egadi, Sicilia). Queste analisi hanno permesso
di caratterizzare i meccanismi deformativi propri di
queste rocce porose e di definire i rapporti scalari tra gli
elementi dimensionali delle faglie affioranti. Tali faglie
sono sub-verticali, appartengono a due set orientati
NNE e NO ed hanno una cinematica rispettivamente si-
nistra e destra.

(*) Dipartimento di Scienze della Terra, Università di Came- Fig. 2 - Collage fotografico, a destra, e schema di dettaglio, a sinistra,
rino, Via Gentile III da Varano - 62032 Camerino (MC), Italy. dei due set di zone di bande che mostrano mutue relazioni di rigetto.
– Photo collage of the two sets of compactive shear band zones, on the
(**) Department of Geological and Environmental Sciences, right, and related structural map, on the left; the two sets show mutual
403 Serra Mall, Stanford University, Stanford, California, U.S.A. crosscutting relationships.

(***) Dipartimento di Geologia e Geodesia, Università di Palermo,
Via Archirafi, 20-22 - 90133 Palermo, Italy.

Agosta Fabrizio: Tel. 0737/402619; Fax 0737/402644; fabrizio.
agosta@unicam.it