Clayey sediments analysis as a useful tool to assessing the geodynamic evolution of fold-and-thrust belts: The case study of the Monte Alpi area (southern Apennines, Italy)

This study aims to contributing to the reconstruction of the geodynamic evolution of the Southern Apennines fold-and-thrust belts (SA-ftb). The main tectonic units across the regional decollement, high-strain zone of the SA-ftb outcropping at the Monte Alpi key area are investigated by using a multi-tool approaches. The shales of Late Triassic-Jurassic Zia Santa Fm. and the Upper Messinian terrigeneous deposits were analyzed by X-ray diffraction, X-ray fluorescence, and both optical and scanning electron microscopies. The results indicate that the two study rock units reflect their dissimilar tectonic evolution until the Lower Pliocene. In fact, data show that the Upper Messinian rocks include mafic-derived grains not observed in the Zia Santa Fm. rocks, which derived from dismantling of Liguride-derived ophiolitic units. Moreover, the measured chemical indices of alteration suggest a degree of paleoweathering processes ranging from moderate (Zia Santa Fm.) to intense (Messinian), in accordance with the amount of kaolinite. The mixed-layers illite/smectite features indicate a variable prograde alteration relatable with the overall structural architecture of the high strain zone documented at both meso and microscales. In particular, the Zia Santa Fm., which pertains to a shallower structural position, shows a slightly higher degree of diagenesis than the studied Messinian shales. The major degree of diagenesis/deformation is interpreted as due to the complex tectonic history and deformation processes of the Zia Santa Fm., which was previously labelled as part of the tectonic mélange decoupling the allochthons of the SA-ftb from the autochthon Inner Apulian carbonates of the Monte Alpi Unit. In particular, the Messinian shales show a variable diagenesis/deformation depending upon their distance from the regional decollement; samples collected in its close vicinity are characterized by a higher degree of illitization of the mixed layers I/S, and by total absence of kaolinite. This data suggests that kaolinite minerals took part in the smectite illitization process, which occurred within a compressive stress field associated to the migrating SA-ftb.