Development of an Intrawedge Tectonic Mélange by Out-of-Sequence Thrusting, Buttressing, and Intraformational Rheological Contrast, Mt. Massico Ridge, Apennines, Italy

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