Pressure fluctuation during uplift of the Northern Apennines (Italy): a fluid inclusions 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 Na ppe outcropping in the La Spezia area, were estimated on the basis of a det ailed microstructural, structural, petrographic and fluid inclusion study o f 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 H 2O-CH4 mixtures issued from water-organic matter interactions in temperatur es conditions that may have reached at least 260 degreesC or more (280 degr eesC) depending on the considered depth estimates and maximum pressures aro und 210-250 Wa. (.) Evidence of strong fluid pressure fluctuation between l ithostatic and hydrostatic within the metamorphic formations (up to 100-150 Wa), possibly linked to fault-valve activity at the beginning of the uplif t, 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 extens ional fissures formed perpendicular to the D2 folds axial foliation. (.) Ch anges 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 microfi ssures during retrograde fluid evolution. Such mixing processes are an indi cation of the connection between separate fluid reservoirs with different t emperature conditions. Consequently, mass and heat transfer have to be take n into account, as the downward percolation of cooler fluids probably accel erated the rate of cooling of the exhumed formations. These processes are c ertainly 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 in clusion studies can provide accurate quantitative estimates of fluid pressu re variations during the evolution of orogenic terrains and could, therefor e, constitute a useful tool in tectonic interpretations at the light of the models developed for pressure variations in the upper crust. (C) 2001 Else vier Science BN. All rights reserved.