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The Mt. Massico ridge (central southern Apennines, Italy) is characterized by a ~150‐m‐thick tectonic mélange located at the base of a Tortonian‐lower Messinian heterogeneous clastic succession consisting of layered sandstones, limestones, marls, and claystones with intercalated mass wasting deposits and isolated olistoliths, which deposited above Meso‐Cenozoic limestones. Geological mapping and structural analyses, integrated with illite‐smectite paleothermal indicators and U‐Pb dating of syntectonic calcite veins and slickenfibers, allowed us to unravel (1) the tectonic evolution of the Mt. Massico ridge and (2) the development of the intrawedge tectonic mélange in the framework of the Apennine accretionary wedge evolution. Results show that after thrusting and folding of Meso‐Cenozoic limestones during late Tortonian times (7.0 ± 1.6 Ma), late Messinian‐early Pliocene out‐of‐sequence thrusting (5.1 ± 3.7 Ma) juxtaposed ~3,300‐m‐thick, imbricate thrust sheets above the Tortonian‐lower Messinian clastic succession. During out‐of‐sequence thrusting, the base of the weak clastic deposits acted as a décollement horizon due to the rheological contrast and mechanical buttress with the underlying competent Mesozoic‐Cenozoic limestones. Heterogeneous deformation along the base of the clastic succession was accommodated by ductile pressure solution of claystones and marls, by brittle stratal disruption and fracturing/veining of competent olistoliths and primary foliation (i.e., sandstones and limestones strata), thus leading to the development of a tectonic mélange. The compressional phase was followed by extensional tectonics after the late Pliocene (minimum age 2.9 ± 0.5 Ma). We conclude that out‐of‐sequence thrusting, buttressing, and intraformational rheological contrast can be fundamental factors for the development of intrawedge tectonic mélange.

The inner Apennines (Italy) are characterized by scattered outcrops of continent-derived orogenic metamorphic units exposed along the Tyrrhenian coast from northern to southern Apennines. At least since the 1970s, some peculiar rocks exposed on Zannone Island (central Italy) have been described as the only Paleozoic-Triassic metamorphic complex linking those exposed in the northern-with those in the southern Apennines. Assessing the protolith nature, thermobaric conditions, and structural features of what is accepted to be the metamorphic unit of Zannone is, therefore, crucial to elucidate the early paleotectonic evolution of the Apennines-Tyrrhenian orogenic system. To that end, we interpreted seismic reflection profiles offshore Zannone, we carried out mesoscale and microscale structural investigations of representative outcrops on the island, performed X-ray diffraction analysis, and K-Ar and U-Pb geochronology of representative clay gouge and syntectonic carbonate veins. Results show that the metamorphic rocks of Zannone can actually be reinterpreted as belonging to nonmetamorphic siliciclastic turbidites, likely deposited in foredeep settings, and coeval to the Oligocene-Miocene Macigno Fm. of the northern Apennines. The turbiditic sequence was overthrust by Triassic dolostone in the early Miocene (~22 Ma), weakly deformed at <200°C, and downfaulted by postorogenic extensional faults starting~7 Ma ago. Hence, Zannone represents a unique geological location in central Italy where to study the innermost (i.e. oldest) thrust sheet of the central-northern Apennines, thereby shedding new light onto the early tectonics of the Apennines. Based on this evidence, a new evolutionary scheme for the early stage of the Apennines tectonic evolution is proposed.