Structural styles and regional tectonic setting of the "Gela Nappe" and frontal part of the Maghrebian thrust belt in Sicily

The Gela Nappe of south central Sicily provides an example of a curved segm ent of an orogenic front that can be examined both onshore and offshore for deformational style and amount of shortening. Synorogenic sediments allow the deformation to be dated. Two distinct structural styles are observed in the Gela Nappe: The central salient part of the nappe (Caltanissetta basin ), consists of a single thrust sheet containing a train of continuously tig htening folds and the reentrant margins of the nappe (Sciacca and Monte Jud ica) consist of a stack of several thrust sheets. These different structura l styles correspond to the pretectonic Mesozoic stratigraphy of the forelan d plate. Carbonate platforms exist on the Adventure bank and Hyblean Platea u ahead of Sciacca and Monte Judica, respectively, while the Caltanissetta basin region appears to have accumulated basinal clay facies. Where the res istant carbonate stratigraphy provides a buttress to the propagation of the thrust front, deformation is taken up by imbrication on Steep ramps throug h the carbonates generating a relatively thick orogenic wedge. In the basin al setting, where no strong rheology exists, the low angle of friction on t he clay detachment levels requires the growing thrust wedge to be much thin ner with a very low foreland dip. Hence the thrust front propagates much fa rther forward into the basin than it does in the adjacent platformal areas, producing a nonlinear thrust front. In the basinal region, accretion of fo reland material to the nappe by imbrication was only prominent during the M essinian when subaerial exposure prevented low-friction transport of the na ppe across the highest levels of the stratigraphy. A steady thickening of t he nappe by internal folding suggests an increase in friction along the bas al detachment, possibly due to progressive compaction of the clays.