Abstract

Structural analysis of the S. Croce Camerina-Comiso-Chiaramonte fault system has been carried out along the western border of the Hyblean Plateau (SE Sicily), between the villages of Comiso and Chiaramonte Gulfi. This system extends as far as the village of Monterosso Almo and separates an uplifted sector to the south-east, where Oligo-Miocene limestones outcrop, from a downfaulted sector to the north-west, where large alluvial fans and Pleistocene marine sediments lie unconformably on the Oligo-Miocene succession. This latter represent the top of the Hyblean Mesozoic-Cenozoic sedimentary succession which is flexured and downfaulted to the north-west towards the front of the Siculo-Maghrebian Chain. The structural setting is characterised by the occurrence of a SSW-NNE oriented shear zone, formed by three major SW-NE trending, left-stepping en-echelon, dextral strike-slip faults, named the Comiso, Chiaramonte and Monterosso faults. The shear zone is characterised by contractional (small positive flowers) and minor extensional structures (normal faults and fractures), roughly oriented SW-NE and WSW-ENE, respectively. In the overlapping sectors between the en-echelon fault segments, push-up structures occur, represented by NNW-SSE trending thrusts and folds. The age of deformation of the Comiso-Chiaramonte-Monterosso fault zone is predated by Pliocene volcanics and Lower Pleistocene calcarenites and postdated by Middle Pleistocene alluvial deposits. The fault system developed contemporaneously with the well-known Scicli-Ragusa dextral wrench zone (Ghisetti & Vezzani, 1980; Grasso et alii, 1986; Grasso & Reuther, 1988), also oriented SSW-NNE, as a response to the same stress field which caused the partial reactivation of Neogene normal faults with right-lateral strike-slip displacements and the neoformation of associated structures, characterised by the over-stepping geometry of the main faults, in accordance with the simple shear model. The stress field, characterised by the SW-NE direction of the P-axis, is apparently incompatible with the Africa-Europe convergence direction during the Pleistocene. For this reason, the Comiso-Chiaramonte-Monterosso fault zone can be considered, together with Scicli-Ragusa system, to be the onshore continuation of the SSW-NNE oriented transform zone which in the Strait of Sicily accommodated the spreading of the Pantelleria Rift during Plio-Pleistocene times (Ghisetti & Vezzani, 1980; Reuther & Eisbacher, 1985; Grasso et alii, 1986; Reuther, 1987; Reuther et alii, 1993).

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