The deformation history of syn-orogenic foredeep basins and its bearings on orogenic dynamics : A view from SW Tuscany, Northern Apennines, Italy

Foredeep basins flanking mountain belts provide a clue for the interpretation of orogenic dynamics. When studying recent or active mountain belts, whose history is still well preserved in the foredeep sedimentary record, the kinematics of folding and thrust emplacement may be effectively unravelled through the analysis of deformation fabrics and their overprinting relationships observed within syn-orogenic deposits.
The Northern Apennines of Italy represent a classical ground for the study of orogenic processes through investigation of associated foredeep basins. Much information has been inferred by observations of stratigraphic and sedimentological features of syn-orogenic deposits, yet with relatively little attention to the superimposed deformation structures.
The Upper Oligocene Macigno Formation that crops out in the coastal section of SW Tuscany represents the sedimentary fill of a foredeep basin developed during the collisional stages that led to the construction of the Apennine mountain belt. The stratigraphic sequence consists of alternating sandstones and siltstones, that are affected by Km-scale contractional structures, namely SW-dipping thrusts and related NE-verging folds. An original detailed field survey carried out at the 1:5000 scale along a superb coastal exposure, integrated with analysis of mesoscopic fabrics and of their overprinting relationships, makes it possible to unravel a complex deformation history. Three main deformation stages are recognised: i) top-to-the-foreland shearing; ii) folding and iii) thusting. Individual stages are correlated with the main tectonic phases related to the progresive deformation history of the Macigno Fm. basin and to incorporation of its sedimentary fill within the evolving Apennine orogenic system.
The kinematic history inferred from deformation fabrics and their overprinting relationships within Macigno Fm. deposits in SW Tuscany shows remarkable analogies with the structural evolution of foredeep sediments from other fold-and-thrust belts, yet with little deviations from current kinematic models. A close comparison of the deformation history recorded by syn-orogenic sediments with that experienced by their pre-orogenic substrata higlights indeed critical differences. These provide original information on the modes of accretion of foredeep deposits within evolving orogenic belts, thus contributing to an enhanced understanding of orogenic dynamics during mountain building.