Ductile shearing of Apennine allochtonous units and Messinian terrigenous deposits on top of the Inner Apulian Platform (Monte Alpi, Southern Italy)

Mylonites are common structural elements in basement complexes. There, strain localization within shear zones occurs at amphibolite to greenschist facieses. More rarely, it also takes place at low-grade to anchizonal conditions in the external portions of orogenic belts. In the present contribution, we document the large-scale architecture, micro-structure, and mineralogy of a prominent shear zone exposed along the southern flank of the Monte Alpi Unit, southern Apennines, Italy. Deformation localized within the Messinian sedimentary protolith topping the carbonates of the Apulian Platform, and in the lowermost tectonic units of the Apennine allochton. Integration of results achieved after field geological mapping, outcrop structural analyses, optical and SEM micropscopy, and X-Ray diffrattometry permits to assess the time-space evolution of the main deformation mechanisms in the aforementioned shear zone. The shear zone involved Messinian shale, sandstones and conglomerates originally deposited in a foreland basin system, and Mesozoic claystones, limestones, and marls that formed in deep basinal environments. Now days, the mylonitic foliation is sub-parallel to the tectonic contact between the Messinian sedimentary cover of the Apulian carbonates and the overlying allochton. Shear-related deformation produced a foliated mylonitic fabric dipping ca. 20° S, and a well-developed, east-trending stretching lineation defined by aligned quartz and/or calcite grains. The conglomeratic levels were boudinaged, and the individual elongated pebbles re-oriented along slip direction. The microstructure of mylonites is characterized by a fine-grained calcite matrix, which shows an intense foliation due to dark bands made up of oxides, organic matter, and minor phyllosilicates. X-ray diffraction data performed on the Messinian shales and Mesozoic claystones, indicate the presence of mixed layer illite/smectite with 80-90% of illite and R1/R3 ordering thus suggesting an high digenetic grade (temperature: 120-140 °C). The two analyzed lithologies mainly differ in the presence of kaolinite, which occurs in the more proximal Messinian facies. Altogether, outcrop-scale kinematic markers such as shear bands, rootles folds and asymmetric porphyroclasts show a consistent top-to-the-east shear sense. Mineralogical and microstructural data indicate that shearing took place at a depth of 6-7 km during the Early Pliocene emplacement of the Apennine allochton on the Apulian Platform, and then exhumed by Late Pliocene low-angle normal faulting, Lower Pleistocene transpression, and Middle-Pleistocene-Holocene high-angle extensional faulting. In summary, the eastward motion of the allochton produced intense and localized low-temperature shearing in sediments on top of the Apulian Platform and in the overlying allochton. A subsequent reactivation of this shear zones as low-angle normal fault during late Pliocene exhumation is envisioned.