Pressure fluctuation during uplift of the Northern Apennines (Italy): a fluid inclusion study

P-T conditions existing after the main syn-collision tectonic phase in the western part of the Northern Apenninic chain (Italy), e.g. in the Tuscan Nappe outcropping in the La Spezia area, were estimated on the basis of a detailed microstructural, structural, petrographic and fluid inclusion study of quartz of syn-tectonic (D2) veins developed in the Tertiary flysch at the top of the Tuscan Nappe (Macigno formation). Three main fluid events have been distinguished as follows. During retrograde metamorphism (D1 to D2 phase), fluids in equilibrium with turbidites from the Tuscan Nappe were H2O-CH4 mixtures issued from water-organic matter interactions in temperatures conditions that may have reached at least 260 °C or more (280 °C) depending on the considered depth estimates and maximum pressures around 210-250 MPa. • Evidence of strong fluid pressure fluctuation between lithostatic and hydrostatic within the metamorphic formations (up to 100-150 MPa), possibly linked to fault-valve activity at the beginning of the uplift, triggered phase separation of the water-methane fluids and production of methane-rich and water-rich fluids; fluctuations in pressure during these events played a crucial role in quartz crystallization especially in extensional fissures formed perpendicular to the D2 folds axial foliation. -bu Changes in the fluid regime and sources with time are evidenced by the input of brines, which mix to distinct degrees and are trapped in healing microfissures during retrograde fluid evolution. Such mixing processes are an indication of the connection between separate fluid reservoirs with different temperature conditions. Consequently, mass and heat transfer have to be taken into account, as the downward percolation of cooler fluids probably accelerated the rate of cooling of the exhumed formations. These processes are certainly common to most orogenic terrains and can be quantitatively studied through P-V-T-X reconstruction using fluid inclusion data on the drainage zones where the mixing processes occurred. This work confirms that fluid inclusion studies can provide accurate quantitative estimates of fluid pressure variations during the evolution of orogenic terrains and could, therefore, constitute a useful tool in tectonic interpretations at the light of the models developed for pressure variations in the upper crust.