Recent and active tectonics of the external zone of the Northern Apennines (Italy)

We present a comprehensive study of the recent and active tectonics of the external part of the Northern Apennines (Italy) by using morphotectonic, geological–structural, and stratigraphic analysis, compared with the current seismicity of the region. This analysis suggests that the external part of the Northern Apennines is characterised by presence of three major systems of Quaternary compressive structures corresponding to (1) the Apenninic watershed, (2) the Apennines–Po Plain margin (pede-Apenninic thrust front), and (3) the Emilia, Ferrara, and Adriatic Fold systems buried below the Po Plain. Geological data and interpreted seismic sections indicate a roughly N–S Quaternary deformation direction, with rates <2.5 mm/year. The shortening decreased since the Pliocene, when our data indicate compression in a NNW–SSE direction and rates up to 7 mm/year. The trend and kinematics of the structures affecting the Apennines–Po Plain margin and the Po Plain subsoil fit well the pattern of the current seismicity of the area, as well as recent GPS and geodetic levelling data, pointing to a current activity of these thrust systems controlled by an overall compressive stress field. Close to the Apenninic watershed, earthquake focal mechanisms indicate that shallow extension is associated to deep compression. The extensional events may be related to a secondary extensional stress field developing on the hangingwall of the thrust system affecting the Apenninic watershed; alternatively, this thrust system may have been recently deactivated and overprinted by active normal faulting. Deeper compressive events are related to the activity of both a major basement thrust that connects at surface with the pede-Apenninic thrust front and a major Moho structure.     

Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong regional anisotropy. In this paper, we present the first petrophysical results on the only mantle xenoliths from Central Italy, which place new constraints on the upper mantle structures of this region. The Torre Alfina mantle xenoliths are very small in size, from few millimetres to about 1.5 cm. They are mainly dunites and harzburgites, with subordinate lherzolites and wehrlites. Since olivine and spinel are always present, they should have crystallised in the spinel-bearing lherzolite field. Their mineralogical composition is ol+spl±opx±cpx. Both olivines and pyroxenes are present as porphyroclasts and as neoblasts. The xenoliths show different degrees of recrystallization. Geothermobarometry on these xenoliths give a temperature range of 1040±40 °C and a pressure estimate of about 1.5 GPa, corresponding to 50 to 60 km depth. Previous seismic studies have estimated the Moho to be at 20 to 25 km in this region, hence the xenoliths come from a hot mantle, probably asthenospheric, below a lithosphere of about 25 to 40 km in thickness below the Moho. We measure the crystallographic preferred orientation (CPO) of olivines and pyroxenes using a SEM and the Electron Back Scattered Diffraction (EBSD) technique. The CPO shows all three axes of olivine are tightly clustered: [100] axis is typically more tightly clustered than [010] and [001] is the most widely distributed axis. The fabric strength expressed by the integral J index, varies from 4.5 to 25.9, and decreases with the degree of recrystallization. We use CPO data to calculate anisotropic seismic properties of the xenoliths. They are very homogenous and probably statistically representative of the mantle below the Torre Alfina area. Vp ranges from 8.4 to 9.1 km/s, Vs₁ from 4.8 to 5.0 km/s. The seismic anisotropy is more variable; AVp ranges from 9.8% to 19.3% and AVs from 7.3% to 13.4%. The majority of the xenoliths display an orthorhombic seismic symmetry, but xenoliths with a transverse isotropic behaviour have also been observed.

We consider four geodynamic models for the source region of the xenoliths (extension, shear, upwelling, slab tilted), defined by different orientations of the structural reference frame, and we calculated for each model the variation of the seismic properties with temperature, pressure and volume fraction of orthopyroxene. After comparing this variation of calculated seismic parameters with seismic observations from the region, we form the hypothesis that the xenoliths come from either an extensional tectonic zone (lineation X and foliation plane XY horizontal) or transcurrent shear zone (lineation X horizontal and foliation plane XY vertical) and that the mantle beneath Torre Alfina is composed by 70% olivine and 30% orthopyroxene forming an anisotropic layer of about 160 or 110 km in thickness, respectively.     

The structure of the Monte Amiata volcano-geothermal area (Northern Apennines, Italy): Neogene-Quaternary compression versus extension

The tectonic evolution of the Mt Amiata volcano-geothermal area is under discussion. Some authors state that this region, as well as the hinterland of the Northern Apennines, were affected by compression from the Cretaceous to the Quaternary. In contrast, most authors believe that extension drove the tectonic evolution of the Northern Apennines from the Early Miocene to the Quaternary. Field data, seismic analyses and borehole logs have been integrated in order to better define the structural features of the continental crust in the Mt Amiata geothermal area. In this paper I propose the hypothesis that the structure of the crust in the Mt Amiata volcano-geothermal area derives from two main geological processes: (1) contractional tectonics related to the stacking of the Northern Apennines (Cretaceous–Early Miocene), (2) subsequent extensional collapse of the hinterland of the mountain chain, and related opening of the Northern Tyrrhenian Sea (Early Miocene–Quaternary). Compressional and extensional structures characterise the Mt Amiata region, although extensional structures dominate its geological framework. In particular the extension produced: (a) Middle-Late Miocene boudinage of the previously stacked tectonic units; (b) Pliocene–Quaternary normal faulting which favoured the emplacement of a magmatic body in the middle-upper crust; and (c) the eruption of the Mt Amiata volcano, which gave rise to an acid and intermediate volcanic complex (0.3–0.19 Ma). The extension produced the space necessary to accommodate the Middle-Late Miocene marine and continental sediments. Pliocene and Quaternary normal and transtensional faults dissected the previous structures and influenced the Early Middle Pliocene marine sedimentation within the structural depressions neighbouring the Mt Amiata volcano. The magmatic body was emplaced at depth (about 6–7 km) during the Pliocene extension, and produced the eruption of the Mt Amiata volcano during the Late Pleistocene. This gave rise to local uplift, presently reaching about 3,000 m, as well as a negative Bouguer anomaly (−16 mgal), both centred on the Mt Amiata area. The crustal dome shows a good correspondence with the convex shape of the regional seismic marker known as the K-horizon, which corresponds to the 450°C isotherm, and the areas with greatest heat flow. This is probably a consequence of the above-cited magmatic body presently in the process of solidification. A Late Pleistocene eruption occurred along a crustal fissure striking N50° (Mt Amiata Fault), which crosscuts the crustal dome. Hydrothermal circulation, proven by the occurrence of thermal springs and gas vents (mainly CO₂ and H₂S), mainly occurs along the Mt Amiata Fault both in the northeastern ans southwestern sides of the volcano.     

Miocene low-angle detachments and upper crust megaboudinage in the Mt. Amiata geothermal area (Northern Apennines,Italy)

Information from surface and subsurface geology (boreholes and seismic reflection lines) are used to depict the geometry of the extensional structures (low-angle normal faults and related Tuscan Nappe megaboudins) affecting the Mt. Amiata geothermal area and developed during the early stage of the extensional tectonics which affected the inner Northern Apennines and Tyrrhenian Sea from the Early-Middle Miocene. Normal faulting involved the thickened middle-upper crust after the collisional stage and, in the Mt. Amiata region, took place over relatively short periods (5-7 Ma) characterised by rapid extensional strain rates. Normal faults showing articulated geometry (flat-ramp-flat) characterised by subhorizontal detachments (flats) and synthetic ramps, caused widespread megaboudinage mainly in the sedimentary tectonic units and particularly in the Tuscan Nappe. Evaporites occurring at the base of the Tuscan Nappe, the deepest sedimentary tectonic unit of the Northern Apennines, controlled the geometry of the faults, and rift-raft tectonics may be the style of this first extensional phase. Three Tuscan Nappe extensional horses (megaboudins) have been recognised in the subsurface of the Mt. Amiata area. They are characterised, in map view, by elliptical shapes and show a mean NNW-SSE lengthening. They are delimited at the base and at the top by east-dipping flats, while their western and eastern margins coincide with east-dipping ramps. On the whole, considering their geometrical features, these megaboudins correspond to extensional horses belonging to an asymmetrical east-dipping extensional duplex system.

Rollover anticlines deformed the western ramp of the megaboudins and rotated the uppermost flat as well as all the structures previously developed, which became steeply-dipping to the west.     

Topography and palaeogeographic evolution of a middle Miocene foredeep basin plain (Northern Apennines, Italy)

The Marnoso–arenacea basin was a narrow, northwest–southeast trending, foredeep of Middle–Late Miocene age bounded to the southwest by the Apennine thrust front. The basin configuration and evolution were strongly controlled by tectonics.

Geometrical and sedimentological analysis of Serravallian turbidites deposited within the Marnoso–arenacea foredeep, combined with palaeocurrent data (turbidite flow provenance, reflection and deflection), identify topographic irregularities in a basin plain setting in the form of confined troughs (the more internal Mandrioli sub-basin and the external S. Sofia sub-basin) separated by an intrabasinal structural high. This basin configuration was generated by the propagation of a blind thrust striking northwest to southeast, parallel to the main trend of the Apennines thrust belt.

Ongoing thrust-induced sea bed deformation, marked by the emplacement of large submarine landslides, drove the evolution of the two sub-basins. In an early stage, the growth and lateral propagation of a fault-related anticline promoted the development of open foredeep sub-basins that were replaced progressively by wedge-top or piggy-back basins, partially or completely isolated from the main foredeep. Meanwhile, the depocenter shifted to a more external position and the sub-basins were incorporated within an accretionary thrust belt.     

Tectonics and quaternary evolution of the Northern Apennines watershed area (upper course of Arno and Tiber rivers,Italy)

This work examines the connection between Quaternary tectonics and erosion/incision processes in the primary Tuscan‐Romagna watershed of the Northern Apennines, which essentially coincides with the topographic culmination of the Nero Unit structural ridge. Tectonic and geomorphic information were collected in the area where this ridge is crossed by the upper Tiber River course forming a deep gorge. Structural analysis and field mapping have revealed that the region experienced polyphase tectonics with superposed thrust folding events identifiable both at the map and mesoscopic scales. Hinterland‐SSW‐verging thrusts and thrust‐related folds deformed the whole thrust pile during the latest deformation phase. Backthrusts/backfolds controlled the development of intermountain basins nearby the main watershed during the Early Pleistocene and seemingly deformed, in the Tiber gorge, a low‐relief landscape developed in the Early Pleistocene (ca. 1.1 Ma). Successively, the upper Tiber River course area and Apennines axial zone underwent a generalized uplift, which is manifested by the deep incision of palaeo‐morphologies. This proposed sequence of events correlates well with the major geodynamic change of the Apennines revealed by an acceleration of uplift rates in the Middle–Late Pleistocene. This latter event may also correlate with increased rates of river incision recorded in Europe as a consequence of uplift and/or climate change. Copyright © 2008 John Wiley & Sons, Ltd.     

Large reactivated landslides in weak rock masses: a case study from the Northern Apennines (Italy)

This case study paper is about a large rotational rock and earth slide—earth flow located in the Secchia River Valley, in the Northern Apennines of Italy, that has displayed multiple reactivation phases between 2002 and 2004. The main geological constraints of the mass movement are related to the overlap of flysch rock masses over clayey complexes that allows rock slides to take place in the source area. The disarrangement and weathering of rock masses following slope movements causes large amount of fine-grained debris to be accumulated on the slope and mobilised by earth sliding and flowing. Analysis of rainfall data at the onset of reactivation events has proved that they occurred after periods with cumulated values higher than the averages of the last 30 years. The quantification of the morphological modifications induced by these reactivations has been made possible by comparing pre- and post-event digital elevation models. Depletion and accumulation has been in the range of 30 m in different parts of the slope. In particular, an advancement of the landslide toe of more than 400 m, which caused a 30-m thick landslide tip to deposit, has been clearly seen. Monitoring data regarding subsurface movements and surface tension crack widening (tension cracks so large as to be properly described at trenches) has shown that sliding surfaces as deep as 43 m exist in the upper part of the landslide, while the accumulation lobe has moved by sliding and flowing over surfaces as deep as some 10 m. Velocities of cm/day have been recorded in the deep surfaces and in widening trenches of the source area, while the advancement of the accumulation lobe has been estimated as having velocities of up to 10 m/day. Groundwater in the landslide body has been observed at depths of 5–15 m in the upper areas, while it is estimated as being at the ground level in the toe. On this basis, it is concluded that the landslide still has a high potential for further development, both in the upper landslide zone and in the toe area.     

Risk assessment strategies for the reactivation of earth flows in the Northern Apennines (Italy)

The aim of this paper is to analyze the reactivation mechanism of ancient earth flows, with a view to gleaning information that can subsequently be utilized to formulate a risk-reduction strategy. All considerations made herein are the result of direct experience and observation of actual events which have occurred over the past few decades in the Northern Apennines. Particular attention has been paid to the analysis of the evolution of landslides during actual reactivation, acknowledging a typical, recurring succession of events that precede the failure of the slope. The hazard assessment of these large landslide bodies, which are of slope scale, constitutes a thorny problem, especially in view of the inapplicability of traditional deterministic models such as limit equilibrium stability analysis. Nevertheless, a site-specific assessment of probability of reactivation of these large and ancient earth flows is fundamental to effective land-use planning.     

Landslides types controlled by tectonics-induced evolution of valley slopes (Northern Apennines,Italy)

This paper investigates the role played by geomorphological and tectonic processes affecting a portion of an active mountain belt in causing the occurrence of different types of landslides developed in flysch bedrock. The adopted multidisciplinary approach (geomorphology, geology and geophysics) allowed to recognize in a portion of the Northern Apennines of Italy different types of landslides that developed in response to slope dynamics, in turn dependent on broader regional-scale tectonic processes. Sedimentary bed attitude, local tectonic discontinuities and lithology only partially influenced the type of landslides, which have been deeply affected by the activity of regional-scale antiform that controlled the hillslope geomorphic evolution in different ways. The growth of this structure and the tilting of its forelimb produced gently dipping slopes that approached the threshold angle that can cause the occurrence of (mainly) translational rockslides. Conversely, high-angle normal faulting parallel to the antiform axis (related to a later stage of activity of the antiform itself) strongly controlled the stream network evolution and caused the watercourses to deeply incise portions of their valleys. This incision produced younger steep valley slopes and caused the development of complex landslides (roto-translational slides-earth/debris flow). The results of the integrated study presented in this paper allowed to distinguish two main types of landslides whose development reflects the events that led to the geomorphological and geological evolution of the area. In this perspective, within the study area, landslides can be regarded and used as indicators of broader-scale recent tectonic processes.     

The Mesozoic continental rifting in the Mediterranean area: insights from the Verrucano tectofacies of southern Tuscany (Northern Apennines,Italy)

In the Alpine-Mediterranean region, the continental redbeds and shallow-marine siliciclastics related to the early depositional phases of the Late Permian-Mesozoic continental rifting are referred to as the most common representative of the “Verrucano tectofacies”. The Verrucano-type successions exposed in southern Tuscany are diachronous, spanning from Triassic to earliest Jurassic in age, and accumulated within the Tuscan domain, a paleogeographic region of continental crust that due to the opening of the Piedmont–Ligurian ocean formed part of the Adria passive-margin. They belong to the metamorphic Verrucano Group and the non-metamorphic Pseudoverrucano fm. Viewed overall, these Verrucano-type successions appear to manifest five episodes or pulses of an ongoing continental rifting. With the exception of the first episode that developed entirely within a terrestrial setting, each one is represented by basal Verrucano-type continental siliciclastics overlain by compositionally mixed marine deposits, which resulted from four diachronous, post-Middle Triassic transgressions. This suite of tectonic pulses produced the progressive westward widening (backstepping) of the Tuscan domain in the rifting south-Tuscany area.     

Sudoite,a rock-forming mineral in Verrucano of the Northern Apennines (Italy) and the sudoite-chloritoid-pyrophyllite assemblage in prograde metamorphism

Sudoite, the di-trioctahedral chlorite with ideal composition (Mg₂Al)(Al₂)(Si₃Al)O₁₀(OH)₈ is a widespread rock-forming mineral in meta-siltstones and psammites of the Verrucano sequence of the Northern Apennines. Sub-ellipsoidal aggregates, probably derived from muscovite clasts, consisting of sudoite, pyrophyllite and muscovite, are common; sudoite may also occur as thin blades in the rock matrix. The co-existence of sudoite, Ferich chloritoid and pyrophyllite, reported here for the first time, has been observed in specimens from the M. Argentario and Monticiano-Roccastrada areas. This three-phase assemblage, diagnostic of a specific metamorphic facies, may be a tool for detailed zonation of low-grade terranes.     

Petrology, Mineral and Isotope Geochemistry of the External Liguride Peridotites (Northern Apennines, Italy)

Mantle peridotites of the External Liguride (EL) units (NorthernApennines) represent slices of subcontinental lithospheric mantleemplaced at the surface during early stages of rifting of theJurassic Ligurian Piemontese basin. Petrological, ion probeand isotopic investigations have been used to unravel the natureof their mantle protolith and to constrain the timing and mechanismsof their evolution. EL peridotites are dominantly fertile spinelIherzolites partly recrystallizfd in the plagiodase Iherzplitestability field Clinopyroxenes stable in thespinel-facies assemblagehave nearly fiat REE patterns (Ce_N/Sm_N=06–08) at (10–16)C1and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasiticamphiboles also occur in the spinel-facies assemblage. TheirLREE-depleted REE spectra and very low Sr, Zr and Ba contentsindicate that they crystallized from hydrous fluids with lowconcentrations of incompatible elements. Thermometric estimateson the spinelfacies parageneses yield lithospheric equilibriumtemperatures in the range 1000–1100C, in agreement withthe stability of amphibole, which implies T<1100C. Sr andNd isotopic compositions, determined on carefully handpickedclinopyroxene separates, plot within the depleted end of theMORB field (⁸⁷Sr/⁸⁶Sr=070222–070263; ¹⁴³Nd/¹⁴⁴Nd=0513047–0513205)similar to many subcontinental orogenic spinel Iherzolites fromthe western Mediterranean area (e.g. Ivrea Zpne and Lanzfl N).The interpretation of the EL Iherzolites as subcontinental lithosphericmantle is reinforced by the occurrence of one extremely depletedisotopic composition (⁸⁷Sr/⁸⁶Sr=0701736; ¹⁴³Nd/¹⁴⁴Nd=0513543).Sr and Nd model ages, calculated assuming both CHUR and DM mantlesources, range between 24 Ga and 780 Ma. In particular, the12-Ga Sr age and the 780-Ma Nd age can be regarded as minimumages of differentiation. The transition from spinel-to plagioclase-faciesassemblage, accompanied by progressive deformation (from granularto tectonite-mylonite textures), indicate that the EL Iherzolitesexperienced a later, subsolidus decompressional evolution, startingfrom subcontinental lithospheric levels. Sm/Nd isochrons onplagioclase-clinopyroxene pairs furnish ages of 165 Ma. Thisearly Jurassic subsolidus decompressional history is consistentwith uplift by means of denudation in response to passive andasymmetric lithospheric extension. This is considered to bethe most suitable geodynamic mechanism to account for the exposureof huge bodies of subcontinental lithospheric mantle duringearly stages of opening of an oceanic basin. ^*Corresponding author. Present address: Dipartimento di Stienze della Terra, Univenit di Geneva, Corso Europa 26,16132 Genova, Italy     

Duplex architecture and late‐orogenic backthrusting in Foredeep Units of the Northern Apennines (Italy)

The Northern Apennines of Italy is a fold and thrust belt that resulted from the NE‐ward progressive overthrusting of a Mesoalpine stacking (the ocean‐derived Ligurian Units) onto the detached sedimentary cover of the Adria plate continental margin (Foredeep Units). The Futa Pass area represents a key sector for the reconstruction of the deformation history of two Foredeep Units (Acquerino and Carigiola Units). The tectonic evolution of this sector is characterized by the superposition of three main deformation stages, with a constant NNE–SSW compression direction. The oldest structure is represented by the NNE‐verging Acquerino Unit duplex structure, the roof thrust of which is represented by the Ligurian stacking basal thrust. The interpretation of this structure as a large‐scale duplex is supported by the presence in the outer sectors of the Northern Apennines belt of Ligurian Units directly overthrust on younger Foredeep Units. In the second deformation stage the NNE‐verging Tavaiano Thrust developed. This regionally significant tectonic surface juxtaposes the Acquerino Unit (already developed as a duplex) and the overlying Ligurian Units, onto the Carigiola Unit. During this stage the fault pattern of the Carigiola Unit was also developed, characterized by two conjugate fault systems, coherent with a NNE–SSW maximum compression direction. During the last deformation stage, a backthrusting with a top‐to‐the SSW sense of movement (the Marcoiano Backthrust) brings the Carigiola Unit and its tectonic cover over the Acquerino and Ligurian Units, with the development of a large footwall syncline. The deformation history presented here differs from previous studies, and so provides a contribution to the debate on Northern Apennines tectonic evolution. Copyright © 2008 John Wiley & Sons, Ltd.     

Convective support of long-wavelength topography in the Apennines (Italy)

Bio-alteration of basaltic glass in the oceanic crust has lately attained much attention. One of the many questions related to this topic is the depth at which bio-alteration presently takes place in the oceanic crust. For this purpose we have investigated samples from the deepest drill hole, i.e. Hole 540B at the Costa Rica Rift in the eastern equatorial Pacific. The glassy rim of pillow lava samples show alteration textures and δ¹³C values compatible with microbial activity throughout the upper 500 m part of the volcanic succession. The concentration and distribution patterns of carbon and potassium within the microbially altered parts, however, indicate that microbes presently interact with the fresh glass to depths of about 380 m into the volcanic basement, at temperatures up to ≈100 °C.     

Space/time tectono-sedimentary evolution of the Umbria-Romagna-Marche Miocene Basin (Northern Apennines, Italy): a foredeep model

The space/time evolution of the Umbria-Romagna-Marche domains of the northern Apennine Miocene foredeep is proposed. In this period, the turbidite siliciclastic sedimentation is represented mainly by the Miocene Marnoso-Arenacea Formation, which generally ends with mainly marly deposits. From the internal Apennine sectors (Umbria-Romagna domain) to the external Adriatic Margin (Marche domain) the siliciclastic succession overlies hemipelagic marly deposits (Schlier Formation). The whole depositional area can be considered as a single wide basin with depocenter or main sedimentation areas progressively migrating eastwards. This basin is characterized by some morphological highs which did not constitute real dams for the sedimentary flows (turbidity currents). Multiple feeding (arkose, litharenites, calcarenites) from different sources is related to palaeogeographical and palaeotectonic reorganization of the most internal, previously deformed, Apennine areas. The activation of the foredeep stage is marked by the beginning of the siliciclastic sedimentation (Late Burdigalian in the most internal sector). This sedimentation ends in the most external sector in the Early Messinian, pointing to a depositional cycle of about 9?C10?Ma. The diachronism of the base of the siliciclastic deposition proves to be almost 5?Ma. The syn-depositional compressional deformation, which shows a marked diachronism, affected the internal area of the foredeep in the Early-Middle Serravallian, and progressively migrated up to Late Miocene, involving more and more external sectors. The deformed siliciclastic sedimentary wedge constitutes an orogenic pile incorporated in the Apennine Chain, represented by different tectonic elements superimposed by means of NE-vergent thrusts. The main stratigraphic and tectonic events of the Toscana-Romagna-Marche Apennines are presented in a general framework, resulting also in a terminological revision.     

Geochemical mapping based on geological units: a case study from the Marnoso-arenacea formation (Northern Apennines,Italy)

Geochemical maps can provide us with much information on geology, earth surface processes and anthropogenic pressure and are valuable tools for ore prospecting and land management. Stream sediments represent an integral of the various possible sources of sediments upstream from the sampling point therefore there can be multiple signal sources but generally the prevailing signal source is the one related to bedrock geology. Stream sediments collected from active second-order channels including singular geological units, were selected in order to determine the geochemical characteristics of each unit. The aim of this study was to analyze their potential for using them to integrate geological interpretation and produce a geologically-oriented geochemical map. From the 770 samples collected for a regional geochemical mapping program, we selected 149 samples whose catchment basin included only one of the members recognized within the Marnoso-arenacea formation. This middle–upper Miocene (Langhian–Tortonian) turbiditic unit forms the backbone of the Romagna Apennines and has been subdivided into 14 members according to age and lithostratigraphic criteria. The results indicate that there are marked differences in the composition of the members of the Marnoso arenecea formation which indicate the provenance of the sediment and the palaeogeographic evolution of the units. By means of univariate and multivariate statistical analyses (Factor analyzes) two main types of sediment compositions are identified: Tortonian members are characterized by sialic coarse grain-sediments while the Langhian–Serravallian members are richer in carbonate fraction, slightly enriched in a mafic contribution. This study elaborated the geochemical data from a geological point of view by integrating the information available in literature to spatially extend the interpretation based on limited site observation as for petrographic studies. In general, the geochemical map based on a geological unit could be a useful tool for carrying out the geological reconstruction of a complex area.     

Sediment instability related to fluid venting in Miocene authigenic carbonate deposits of the northern Apennines (Italy)

Chemosynthetic carbonates, identified by isotopic, palaeoecological and sedimentological features, are concentrated in middle-late Miocene satellite and foredeep deposits of the northern Apennines. Chemoherms in the foredeep are hosted in thick pelitic intervals, probably deposited in intrabasinal structural highs, which are entirely or partly involved in large slumps, in many cases associated with extrabasinal slides. Sediment textures in carbonates and in the enclosing foredeep pelitic sediments indicate a link between hydrocarbon-fluid venting, sediment deformation and mobilisation, and tectonics. The intensity and style of fluid release phases directly influenced chemoherm typology, and also determined overpressure conditions in low shear strength pelitic sediments, favouring sediment mobilisation and influencing slope instability, which widely affected the Apennine foredeep. Chemosynthetic carbonates are associated with sites of tectonically fractured and compressed sediments in the Apennine foredeep-thrust belt system, thus indicating a relation with the tectonic loading of the Apennine thrust-sheets, which favoured fluid expulsion along forerunner faults. Possible gas hydrate contributions to fluid expulsion processes are discussed, based on sediment textures compared with modern vent areas. Finally, sediment instability may have facilitated a large amount of fluid escape, thus stopping carbonate precipitation.