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The Apennines comprise a Neogen—Quaternary accretionary prism that shows several anomalies with respect to classic alpine‐type mountain belts, namely (i) low elevation, (ii) a shallow new Moho below the core of the belt, (iii) high heat flow in the internal parts, (iv) mainly sedimentary cover involved in the prism, (v) a deep foredeep and (vi) a fully developed back‐arc basin. The suction exerted by a relatively eastward migrating mantle can determine the eastward retreat of the subduction zone and an asthenospheric wedging at the retreating subduction hinge. Heat flow, geochemical and seismological data support the presence of a hot mantle wedge underlying the western side of the Apenninic accretionary prism. A thermal model of the belt with foreland dipping isotherms fits with deepening of the seismicity toward the east. Mantle volatiles signatures are also widespread in springs along the Apennines.

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Molecular composition, CH4 isotopes and gas flux of all main terrestrial mud volcanoes and other methane seeps in Italy are being assessed for the first time. Whereas 74% of the Italian gas reservoirs are biogenic, about 80% of the seeps release thermogenic gas. Dry‐seep gas generally maintains the reservoir C1/(C2 + C3) “Bernard” ratio while mud volcanoes show molecular fractionation likely occurring during advective migration. Accordingly, a simple and direct use of the “Bernard” parameter might be misleading when applied to mud volcanoes as it could not always reflect the reservoir composition. Methane flux into the atmosphere from macro‐seep areas is in the order of 102–106 t km−2y−1. Microseepage is widespread throughout large areas and, on a regional scale, it provides the main methane output. A first emission estimate for the total hydrocarbon‐prone area of Italy suggests levels of 105 t y−1, comparable to national sources from fossil fuel industry.

Chemical and isotopic characteristics of natural gas and thermal water discharges from the western back-arc Tyrrhenian Sea across the Apennine thrust-belt to the Po Valley and Adriatic coast foredeep basins in the Northern Apennines (central-northern Italy) reveal a large-scale fluid motion in the upper crust, both vertically and horizontally. On the basis of gas compositions, two different domains of rising fluids have been distinguished: (1) CO 2 -rich, He-poor, 3He/4He-high domain in the western peri-Tyrrhenian extensional sector; (2) CH 4 -rich, He-rich, 3He/4He-low domain in the eastern Adriatic compressional sector. Such gases, rising from various depths, are crossed by a huge lateral N 2 -rich water flow, in the peri-Tyrrhenian sector, of Ca-SO 4 (HCO 3 ) meteoric-derived waters that move in a regional wide aquifer hosted in a quite thick Mesozoic carbonate series.Morphologically, the CO 2 vents consist of mud basins with high gas-rate emission, where the rising fluids move upwards through diatremes. On the other hand, CH 4 emissions seep out from typical mud volcanoes with a reduced gas flow-rate, where the fluid motion is likely related to saline diapir extrusions, and the methane is mostly carried to the surface by the associated mud. The two rising mechanisms described locate southwest and northeast of the Apennine watershed respectively.From a seismic point of view the CH 4 domain in the thrust-belt and foredeep areas is characterized by a large number of earthquakes, indirectly pointing to a different rheological behavior of the terrains with respect to the more internal peri-Tyrrhenian area. Owing to the quite high thermal gradient of the latter, the boundary of the brittleductile crust in the peri-Tyrrhenian sector can be located at a <10 km shallow depth.Although the presence of several post-orogenic basins would suggest widespread extensional tectonics in all areas west of the Apennine watershed, those located in the easternmost part display gas vents with typical crustal 3He/4He ratios. As this ratio is very sensitive to deep fluids rising from the mantle, we hypothesize that such basins at the foot of the Apennines are not due to tensive stress, as suggested by their morphological shape. They are piggy-back (thrust-top) basins developed and evolved in a still acting compressive tectonic regime.

[1] Samples rich in organic matter were collected from boreholes in the southern part of the Po Plain (Italy), a rapidly subsiding sedimentary basin. New 14 C dates, obtained from these samples, and 14 C data from the literature allowed for the evaluation of the rates of natural subsidence in the area. These range from 0 to 5 mm/yr. Most areas show rates around 1 and 2 mm/yr. The obtained subsidence rates are comparable with burial rates obtained from archaeological data. These data are interpreted in terms of long-term (related to tectonics and geodynamics) and short-term (glacial cycle) processes. It is concluded that approximately 50% of the subsidence rates are related to tectonics, geodynamics and sediment load/compaction while the other half is controlled by ice melting. The effects of the glacial cycle through time are discussed. It is shown that the southeastern Po Plain generally behaves as a far-field area in relation to the high-latitude ice caps. Deviations from this behavior observed in the northeastern part of the Po Plain are tentatively interpreted as the effects of ice formation and melting in the Alps during the last glacial cycle.Components: 6485 words, 7 figures, 1 table.

The compositional data collected at three thermal springs in the Umbria region over a sampling period lasting two and a half hydrological years included the entire seismic sequence striking the Central Apennines (Italy) in 1997–1998. Wide temporal variations in the chemical composition and temperature of thermal waters were observed at all the sampling sites. The widest were recorded at the Triponzo sampling site, where a temperature drop of 2 °C and 20 °C preceded, by 4 days and 2 days, respectively, the occurrence of the event characterized by the deepest hypocentre of the entire seismic sequence. Despite such a macroscopic preseismic anomaly, the recorded variations were not related to single seismic events or to the release rate of the seismic energy. They actually seem to have been induced by permeability variations related to crustal deformation in the absence of elastic energy release and were thus linked to the whole seismogenic process.