Structural evolution and Pressure-Temperature-path (P-T- path) of the Moglio-Testico Unit (Western Ligurian Alps): a re- appraisal

The Moglio-Testico Unit (Western Ligurian Alps) is classically regarded as a fragment of the Ligure-Piemontese oceanic crust deformed during the building of the Alpine belt (Vanossi et al., 1986) and recorded very low-grade metamorphic imprint (Bonazzi et al., 1987). However, a roboust and updated dataset about its tectono-metamorphic history is lacking. To face these uncertains we performed a geological survey on the Moglio-Testico Unit cropping out between the Tanaro and Arroscia Valleys (Western Ligurian Alps). Here the Moglio-Testico Unit is characterized by a Late Cretaceous-Paleogene(?) sedimentary succession characterized by Palombini Shale that pass upward to a turbiditic sequence. The latter consist of marls and shales intercalated with arenitic beds passing upward to a fine-grained turbidites related to deep-sea to basin-plain environment. We combined meso-to micro-scale structural analysis with Chlorite-Phengite multi-equilibrium thermo-barometry in order to constrain the tectono-metamorphic evolution of the Moglio-Testico Unit. Two overprinted fold systems (D1 and D2) confined to the unit were documented. D1 event produced a S1 slaty cleavage associated to F1 isoclinal folds with scattered fold axis and axial planes dipping toward SW and NE. D2 event is marked by S2 crenulation cleavage associated to F2 tight fold showing NW-SE fold axis dipping toward NW and SE and flat-lying to SW and/or NE-dipping axial planes. After the coupling of the units, the subsequent deformation events are represented by a fold system with sub-horizontal axial plane without metamorphic recrystallization and high-angle normal faults, both though to be developed at very shallower structural levels. During the D1 and D2 deformation events metamorphic re-crystallization, i.e. chlorite and phengite grains, along the S1 and S2 foliation planes is present. Thermo-barometric estimations suggest that the Chl-Phg couples along the S1 are in equilibrium at higher pressure and temperature than those growth along the S2, drawing a P-T path coherent with a subduction setting. Structural analysis and thermo-barometry allowed us to reconstruct the tectono-metamorphic evolution of the Moglio-Testico Unit. Our results indicate that the Moglio-Testico Unit recorded a polyphase deformation history under HP-LT metamorphic conditions probably during its subduction, accretion and subsequent exhumation into the Alpine accretionary wedge. Vanossi, M. (1986): Geologia delle Alpi Liguri. Memorie della Società Geologica Italiana, 28, 598 pp.