Le pieghe minori nelle Unita Imeresi e Trapanesi dei Monti di Palermo e il loro significato nell'evoluzione tettonica dell'area

The Palermo Mts. , in north-western Sicily, are part of the Maghreb-Apennine fold and thrust belt. This portion of the Sicilian belt consists of a stack of imbricated thrust sheets, made up of Mesozoic-Cenozoic sedimentary cover, originally belonging to the "Imerese" and "Trapanese" Domains (Caflisch, 1966; Catalano et alii, 1979): in particular, the Imerese rocks (Triassic Scillato Fm.) are superposed over the Trapanese sequence ("Scaglia") crops out in the southern part of the study area (M. Leardo-Marineo). These structures are locally overlain by remnants of the uppermost Sicilide Nappes. This paper deals with the mesoscopic folds, developed in rocks of the Imerese and Trapanese tectonic units, cropping out in the eastern portion of the Palermo Mts. The main objectives of the study are: - description of the geometry and the style of deformation of the minor folds; - analysis of the relationships between the minor folds and the other structures (major folds, joints, cleavages, etc.); - reconstruction of the orientation of the associated deformation field; - interpretation of the minor folds within the tectonic evolution of the area. The geometry and the style of deformation of the observed folds are typical of folding in sedimentary rocks, under conditions of brittle deformation (low pressure and temperature). Morphologically, most of the folds can be regarded as sine waves or chevron folds: in general, they are parallel or sub-parallel folds, with a more or less pronounced thickening of the hinge zone, falling into classes 1B and 1C of the Ramsay (1967) classification. The systematic occurrence of pressure solution cleavages, orthogonal to the bedding surface, suggests the existence of an early shortening phase, pre-dating the folding phase, and oriented parallel to bedding (i.e. horizontal). Secondary deformation mechanisms, such as flexural slip (competent, calcareous strata) and flexural flow (incompetent, marly strata) frequently accompany folding. Bulbous hinges and limb thrusts have also been observed in the hinge zones of the calcareous strata. The presence of a regular multilayer, made up of well bedded limestones and/or alternating competent and less competent strata, favours fold initiation. Most of the observed mesoscopic folds are developed in well-bedded pelagic limestones, with strata 10-40 cm thick; the wavelength of the folds is in the range 1-10 m, and their amplitude is 0.2-2 m. Smaller folds (wavelength in the order of few cm) are developed in marly intervals within calcareous successions. In order to reconstruct the deformation history of the region, the attitudes of the hinges and axial surfaces of the folds have been collected and statistically analysed. Although some previous papers deal with the orientation of minor folds in this part of the Sicilian belt (e.g. Ghisetti & Vezzani, 1984; Oldow et alii, 1990; Nigro & Renda, 2000), very few data about fold orientation within the Imerese Units have been collected and published until now. Two subsequent systems (H1 and H2) of tectonically generated folds have been recognised, superposed over pre-existing, ductile folds developed in still soft sediments (slumps). The interference pattern, resulting from the superposition of H1 and H2 folds, can be observed in well exposed sites, especially in the Imerese units. The orientation of H1 (older) folds depends on the tectonic unit affected by folding: in the Imerese rocks the hinges of the H1 folds trend NNW-SSE (N337 degrees ), with a WSW (N240 degrees ) vergence; whilst in the Trapanese units H1 folds trend about E-W (N088 degrees ), with southward (N190 degrees ) vergence. The orientation of H2 (younger) folds is independent of the tectonic unit in which they are nucleated. Fold hinges trend in the N45 degrees -N60 degrees range in both Imerese and Trapanese units. These orientation data can be compared with palaeomagnetic data available for this region, showing that the tectonic units were subjected to large clockwise rotation (up to 120 degrees ) during their emplacement (Oldow et alii, 1990; Channel et alii, 1990). In particular, Imerese units experienced rotations as large as 130 degrees , comparable with that of the Panomide units (90 degrees -140 degrees , with a mean value of about 120 degrees ); Trapanese units experienced significant lesser clockwise rotations (47 degrees -70 degrees , with a mean value of about 60 degrees ). The orientation data of the fold hinges within the Imerese units, confirm the common tectonic history of Panormide and Imerese units, suggested by paleomagnetic data (Oldow et alii, 1990). H1 folds could have been generated early, before the tectonic transport and clockwise rotation of the tectonic units. If this hypothesis is correct, the "original" orientation of the Imerese H1 folds is N330 degrees -120 degrees ; = N210 degrees ; and that of the Trapanese H1 folds N270 degrees -60 degrees = N210 degrees . The deformation field would then have been uniformly oriented, and the tectonic units rotated with respect to it through time, during their tectonic emplacement. The younger fold system (H2), showing the same orientation in both Imerese and Trapanese units, would be initiated after most of the rotation (i.e. emplacement) of the tectonic units is accomplished, reflecting only minor, local rotations, localised near the major transpressive faults.