Apennines Subduction Research Articles

The “underfilled trinity model” of foreland basins revisited: reality or myth?

The “underfilled trinity” model of foreland-basin stratigraphy was proposed based on the observation that the initial sedimentary stages along the western and northern front of the western and central Alps were represented by shallow-water carbonates (Calcaires à Nummulites) overlain by hemipelagic marls (Marnes Bleues) and capped by siliciclastic turbidites (Grès d’Annot). Subsequently, this model has been widely accepted and applied in foreland basins worldwide. We here re-investigated the Eocene-Oligocene sedimentary succession of the Western Alps to check its validity. Major geological features of this region include: i) the existence of a Cretaceous-Eocene unconformity spanning more than 25 Myr in the studied sections; ii) a virtually synchronous age of the Calcaires à Nummulites, Marnes Bleues, and Grès d’Annot formations across the Western Alps; iii) a long-term deepening-upward trend, from inner to outer ramp, documented by the Calcaires à Nummulites, followed by the pelagic Marnes Bleues and by the Grès d’Annot turbidites; iv) the provenance of the Grès d’Annot Formation from the Maures-Estérel Massif and/or Corsica-Sardinia block in the south, rather than from the Alpine orogen in the east. By integrating field observations, sedimentological, biostratigraphic, and provenance analyses, we found the Eocene “underfilled foreland” model too simplistic and inadequate to explain the basin evolution in the western Alpine region. Based on the alternative scenario proposed herein, the Annot and Barrême basins formed in the late Eocene (40-35 Ma) in an extensional/transtensional setting during a period of major change in tectonic stress fields across western Europe on the upper plate of the Apennine subduction.

Orogen-parallel discontinuity of the Apennines subduction zone in Southern Italy as seen from mantle wedge seismic structure

We investigate the seismic structure of the mantle wedge of the Apennines subduction zone (Central Mediterranean) using teleseismic receiver function (RF). We inverted RF for both isotropic and anisotropic properties of the mantle wedge, from below the overriding Moho to the “plate boundary”, i.e. the interface that separate the slab from the mantle wedge. Given the distribution of the seismic network, we are able to map out the change in the elastic properties at the transition between southern apennines and the Calabrian arc, given by the change in the subduction style (i.e from the subduction of continental materials to oceanic plate). We found that the anisotropy in the mantle wedge is similar between all seismic stations, generally highly anisotropic (> 10%), with a direction of the symmetry axis that rotates clockwise from North to South, following the Calabrian arc geometry and likely indicating the mantle flow driven by the slab retreat. The elastic properties of the subducted crust are more heterogeneous. To the North, the subducted crust shows a highly anisotropic (> 10%) behavior, and it occurs at larger depth (around 70 km depth), where to the South anisotropy is less intense (around 7%) and the subducted crust is shallower (around 60 km depth). These results point out a change in the subduction style that can be given by either a change in the metamorphic phase (more evolved blueschist facies stage to the North, initial greenschist facies stage to the South) or a different origin for the subducted materials (continental to the North and oceanic to the South). The differences in the anisotropic behavior of the subducted crust are reflected in the topography of the plate boundary, which becomes shallower from North to South, suggesting the existence of either a step in the slab topography or a more gentle ramp.

Anatomy of a km-scale fault zone controlling the Oligo-Miocene bending of the Ligurian Alps (NW Italy): integration of field and 3D high-resolution digital outcrop model data

We report the first description of a ~ 15 km long NE-SW-striking transtensive fault network crosscutting the metamorphic units of the Ligurian Alps. The main fault zone, hereby named Horse Head Fault Zone, is up to 250 m thick, involves quarzite, metarhyolite, marble and alternation of dolostone and limestone and minor pelite. Relatively narrow (~ 1–3 m-thick) fault cores are characterized by gouge and cataclasites, surrounded by brecciated damage zones as thick as tens to hundreds of meters. Damage zones show widespread evidence for dilation in the form of dilation breccia, large calcite crystals and aggregates, and centimeter- to meter-thick veins. Moreover, the fault zone contains a multitude of polished slip surfaces with multiple sets of slickensides and slickenfibers. Oblique to strike-slip kinematics dominates over the large part of the fault mirrors and both overprint and are overprinted by down-dip slip surfaces. The fault network includes dominant NE-SW right-lateral faults with a minor normal component and NW–SE left-lateral steep faults with a negligible reverse component, consistent with a km-size dextral NE-SW-striking Riedel shear zone, in turn representing an antithetic R’ of the regional sinistral shear zone constituted by the Ligurian Alps after the nappe stacking. The Horse Head Fault Zone accommodated km-scale displacement before the Early Miocene, as it is sealed by the sedimentary deposits of the Finale Ligure Basin, thus predating the Corsica-Sardinia drifting. Results of this work constraint the bending of the Ligurian Alps as part of the Western Alpine arc as accomplished through two consecutive, late Oligocene and Early Miocene, stages driven by the combination of Adria rotation and the rollback of the Apennine subduction.

Late extension of a passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides files

The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence was little studied compared to their evolution during the post-Pangea rifting and oceanic expansion,i.e., from the Early Jurassic to the early Late Cretaceous. The present work firstly aims to make up for this shortcoming in the case of the distal European margin of the Alpine Tethys, namely the Briançonnais domain of the Western Alps. We show that this margin was affected by strong post-rifting extension mainly in Late Cretaceous-Paleocene times and propose to make it the type of the (rare) “Late Extension Passive Margins”. Remarkably, this extension shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate. Secondly, we assess the post-rifting evolution of the north-Tethyan paleomargin in the Maghrebides transects,i.e., south-west of the Briançonnais transect along the same European-Iberian paleomargin. For this purpose, we consider the Triassic-Eocene series of the “Dorsale Calcaire” in the Alkapeca Blocks located along southeastern Iberia until the Eocene then transported onto the North African margin. Examination of the literature shows that the Tethyan margin of the Alboran block was strongly affected by normal faulting as early as Late Jurassic-Early Cretaceous times whereas post-rifting extension of the Kabylian blocks mainly occurred in the Late Cretaceous-Paleocene like in the Briançonnais. We propose that post-rifting extension of the Alboran block southern margin resulted from the sinistral movement of Africa relative to Iberia while the later extension of the Kabylian blocks can be related to the further convergence kinematics. Subduction of the Ligurian-Maghrebian slab under the North African margin would have occurred at that time in the southward continuation of the Alpine subduction. The overriding Adria and North African margins did not experience significant compression at that time. During the Eocene, a subduction polarity reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca block. This was the beginning of the Apenninic subduction, which triggered the back-arc opening of the Mediterranean basins.

Depositional record of confined turbidites in syn-subduction intraslope basin: insight from the Tufiti di Tusa Formation (Southern Apennines, Italy)

A detailed lithostratigraphy and facies analysis of a type section of the Tufiti di Tusa Formation, including deep-marine clastic successions with syn-orogenic volcanic detritus and deposited in the late Eocene - early Miocene basin system at the front of the growing Maghrebian - Southern Apennines orogen, is discussed in the paper. Based on facies analysis and composition, the study section was subdivided into the following units, from bottom to top: Unit I, mostly formed by contained-reflected beds (including a bed similar to the Contessa megabed of the Marnoso-arenacea Formation in the Northern Apennines), with the ratio of sandstone intervals to mudstone intervals (S/M) of 0.6 and with mostly-calciclastic sandstone to siltstone fraction; Unit II, recording a moderate decrease in contained-reflected beds and a moderate increase in slurry beds, with S/M ratio of 0.9, and with mostly-siliciclastic sandstone to siltstone fraction; Unit III, recording a further decrease in contained-reflected beds and an evident increase in slurry beds and very-thick beds with a basal massive very coarse to coarse-grained sandstone, with S/M ratio of 2.5, and with mostly-volcaniclastic sandstone to siltstone fraction.In accordance with the depositional models for the infill of confined turbidite basins, Units I and II are here interpreted as representing a flow ponding depositional phase, while Unit III as iconic of a flow stripping depositional phase. The compositional variation from Unit I to Unit II records cutoff of calciclastic supply from underplate sources, possibly tied to tectonic uplift of the external basin margin; while that from Unit II to Unit III records sudden availability of volcaniclastic sediment possible due to burial of morphological high(s) between the internal volcanic arc (source of the volcaniclastic sediment) and the depositional basin, and/or establishment of tectonically-controlled conduits cutting the above high(s). This study may improve the knowledge not only of infilling evolution of confined turbidite basins, but also of the depositional setting of the late Paleogene Southern Apennines subduction margin in the Central Mediterranean.

New SKS splitting anisotropy measurements point to tearing beneath Central Italy

In the middle of the Mediterranean, the partly still active Apennines subduction system has been usually defined using tomographic images and available shear wave splitting measurements. In this paper we describe the new seismic anisotropy dataset for Central Italy, the region where the transition between Northern and Southern Apennines occurs. The new measurements show NW-SE fast polarisation directions beneath the belt, due to the retreat of the slab, NNE-SSW orientations from proper Adriatic mantle sources, and E-W directed anisotropy, attributed to mantle convection flow at the Thyrranian side. Additionally, the new data suggest the presence of a toroidal mantle flow through a tear in the Apenninic slab, from the Adria to the Tyrrhenian side. However, mantle circulation and flows, identified by the pattern of shear wave splitting results, seem different from what was proposed in previous geodynamic models. Indeed, our results support the presence of a vertical slab tear with limited dimension. In the geodynamic model we propose, the tear acts to accommodate a differential slab retreat. The slab partitioning results in a different pattern and strength of seismic anisotropy traced from the Central Apennines with respect to the adjacent Northern and Southern Apennines.

Deep External Fluid Source Along the Gubbio Normal Fault (Italy): Implications for Slip Along the Altotiberina Active Low-Angle Normal Fault System

Fluids play a critical role in fault slip, fault and damage zone development, and the distribution of seismicity in regional fault systems; however, determining the source of fluids within fault damage zones is complicated by superposition of fluid-related fabrics. Clumped isotopes applied to tectonic studies offer an opportunity to distinguish between fluid sources based on temperature and stable isotopic composition. Here we use the clumped isotope geothermometer and field observations and compare them with microstructural analysis to investigate the Gubbio normal-fault (GuF) core, a major SW-dipping epidetachment fault conjugate to the active Altotiberina low-angle normal fault in central Italy. Several distinct calcite vein sets reveal the incorporation of multiple generations of fluids during development of the damage zone. Clumped isotope results from these various precipitation phases record crystallization temperatures ranging from 87–154°C. These temperatures are broadly consistent with the presence of calcite deformation twin types II and III and are higher than estimates of regional peak burial temperatures. Additionally, stable isotope compositions within vein calcite are distinct from published isotopic values of the Apennine sedimentary succession, which constitutes the local bedrock. We propose that these observations suggest hydrothermal fluids migrated from depths greater than 6 km, which requires hydraulic connectivity along structural pathways between the shallow and deep crust, and fluid overpressures. These fluids reach the GuF via migration along the Altotiberina low-angle normal fault plane ​​and they may be either of diagenetic or of deeper subduction origin. We suggest they possibly originated from the proximal retreating Apennine subduction system, implying that subduction processes exert spatial control on the distribution of fluid-assisted normal faulting and related seismicity which is consistent with the co-migration of closely coupled subduction and hinterland extension in the Apennines from Miocene to Present.

3D crustal structure of the Ligurian Basin revealed by surface wave tomography using ocean bottom seismometer data

Abstract. The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-process the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. In onshore France, we observe a high-velocity area beneath the Argentera Massif, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The maps also indicate that the south-western and north-eastern Ligurian Basin are structurally separate. The lack of high crustal vP/vS ratios beneath the south-western part of the Ligurian Basin preclude mantle serpentinisation there.

The Curinga\u2013Girifalco Line in the framework of the tectonic evolution of the remnant Alpine chain in Calabria (southern Italy)

In the peri-Mediterranean metamorphic belts, the tectonic evolution of the Calabria–Peloritani terrane during the dominant compressive tectonics of the Eocene represents one of the most problematic points in palinspastic restorations. A matter of particular debate is its shortening, which could have occurred during the Alpine or the Apennine subduction. In this regard, a crucial joint is provided by the kinematics of one of the most relevant shear zones such as the Curinga–Girifalco Line, cropping out in central Calabria. This shear zone juxtaposed a nearly complete Hercynian crustal section (i.e. the Sila and Serre Unit) onto the remnants of the Castagna Unit. The data in the available literature on ductile kinematics from the south-eastern branch of the Curinga–Girifalco Line indicate a downward movement of the hanging wall. In the present paper we show new, ductile kinematic data and petrographic evidence from outcrops in the north-western and south-eastern branches of the Curinga–Girifalco Line. Our results highlight the coherent kinematics of the Eocene shortening during the Alpine subduction system, followed by (late Eocene?)Oligocene to early Miocene, dominantly ductile extensional reworking, relating to the Apennines subduction system.

Recent inversion of the Tyrrhenian Basin

Abstract The Tyrrhenian Basin is a region created by Neogene extensional tectonics related to slab rollback of the east-southeast–migrating Apennine subduction system, commonly believed to be actively underthrusting the Calabrian arc. A compilation of >12,000 km of multichannel seismic profiles, much of them recently collected or reprocessed, provided closer scrutiny and the mapping of previously undetected large compressive structures along the Tyrrhenian margin. This new finding suggests that Tyrrhenian Basin extension recently ceased. The ongoing compressional reorganization of the basin indicates a change of the regional stress field in the area, confirming that slab rollback is no longer a driving mechanism for regional kinematics, now dominated by the Africa-Eurasia lithospheric collision

Mapping the progressive geologic history at the junction of the Alpine Mountain Belt and the Western Mediterranean Ocean

This contribution illustrates a new type of map, the “geological event map” (1:250,000 scale), which highlights the progressive steps of the geologic history recorded in the polycyclic orogenic belt of the European Alps, at their southwestern termination facing the western Mediterranean Sea. The formula of historical progression simplifies comprehension of significant phenomenal visions and reorients public curiosity towards geologic processes. This part of the Alpine belt records a geologic history starting with the Variscan convergence during Devonian, throughout the opening of the Tethys Ocean, Alpine convergence, followed by Apennine subduction driving the opening of the Ligurian-Provencal Ocean. Finally, from late Miocene to Present a progressive tectonic inversion characterizes the Ligurian-Provencal continental margin. This geologic history, over a time period of 400 million years, which includes three successive Wilson Cycles, is displayed in 8 plates composed of maps and illustrations. Each map explains the effects of successive tectonic events by adding geologic changes that modify the petrogenetic and structural configurations. The last plate shows the finite state of the geologic history at present-time. The map legend is based on ten major geodynamic events of which rock associations, and their genetic environment, are described in simple divulgation terms, to stimulate interest of neophytes. Illustration of the evolution of rocks and structures side of each map aims at conveying to non specialists in tectonics and petrology the impact of mechanisms associated with the Earth’s deep engine upon surface changes.

3-D Pn tomography reveals continental subduction at the boundaries of the Adriatic microplate in the absence of a precursor oceanic slab

Slab pull generated by subducting oceanic lithosphere is generally considered as a major trigger for the onset of continental subduction. However, this may be in conflict with the occurrence of UHP terranes bearing no evidence of oceanic lithospheric rocks involved in the exhumation cycle. Here, we image the uppermost mantle P velocity structure beneath the Central Mediterranean, suggesting the possibility that the initiation of continental subduction may not require a precursor oceanic slab. We combine (i) a three-step inverted 3-D Pn tomography model of the Adriatic microplate with (ii) available geologic constraints and palinspastic reconstructions of the Africa–Eurasia plate-boundary zone. Our Pn tomography model reveals elongated regions with Vp < 7.6 km/s around the Adriatic microplate, clearly connected with the slab structure inferred from teleseismic P wave tomography and supportive of continental subduction along the Dinaric, Alpine and Apenninic subduction zones. Contrasting styles of subduction are observed on the opposite sides of the Adriatic microplate: a laterally variable SW-dipping subduction is documented beneath the Apennines, continental to the north and oceanic to the south, where rollback is faster; a laterally continuous NE-dipping continental subduction is documented under the Dinarides. The lack of a precursor oceanic slab under the Dinarides demonstrates that the onset of continental subduction, in complex plate-boundary zones, can be controlled by plate-tectonic processes far away from the subduction initiation site, and may take place without the contribution of the negative buoyancy of an old oceanic lithosphere.

Deep Structure of Northern Apennines Subduction Orogen (Italy) as Revealed by a Joint Interpretation of Passive and Active Seismic Data

AbstractThe Apennines is a well‐studied orogeny formed by the accretion of continental slivers during the subduction of the Adriatic plate, but its deep structure is still a topic of controversy. Here we illuminated the deep structure of the Northern Apennines belt by combining results from the analysis of active seismic (CROP03) and receiver function data. The result from combining these two approaches provides a new robust view of the structure of the deep crust/upper mantle, from the back‐arc region to the Adriatic subduction zone. Our analysis confirms the shallow Moho depth beneath the back‐arc region and defines the top of the downgoing plate, showing that the two plates separate at depth about 40 km closer to the trench than reported in previous reconstructions. This spatial relationship has profound implications for the geometry of the shallow subduction zone and of the mantle wedge, by the amount of crustal material consumed at trench.

Alps to Apennines zircon roller coaster along the Adria microplate margin

We have traced the particle path of high-pressure metasedimentary rocks on Elba Island, Northern Apennines, with the help of a U-Pb-Hf detrital zircon study. One quarter of the analysed zircons are surprisingly young, 41-30 Ma, with a main age peak at ca. 32 Ma, indicating an unexpected early Oligocene maximum deposition age. These Oligocene ages with negative εHf indicate a volcanic source region in the central-southern Alps. Though young by geological means, these zircons record an extraordinary geodynamic history. They originated in a volcanic arc, during the convergence/collision of the the Adria microplate with Europe from ca. 65 to 30 Ma. Thereafter, the Oligocene zircons travelled ca. 400 km southward along the Adria margin and the accretionary prism to present-day Tuscany, where they were subducted to depths of at least 40 km. Shortly thereafter, they were brought to the surface again in the wake of hinge roll back of the Apennine subduction zone and the resulting rapid extensional exhumation. Such a zircon roller coaster requires a microplate that has back-to-back subduction zones with opposing polarities on two sides.

Transfer zones in an oblique back-arc basin setting: Insights from the Latium-Campania segmented margin (Tyrrhenian Sea)

New multichannel seismic reflection profiles were acquired to unravel the structure of a portion of the eastern margin of the Tyrrhenian basin. This extensional feature is part of an Oligocene to Present back-arc basin in the hangingwall of the west directed Apennines subduction system. The basin provides excellent conditions to investigate the early stage processes leading to the development of rifted passive margins and to the emplacement of oceanic crust in an oblique setting. The interpreted post-stack-migrated seismic profiles highlight the geometry and kinematics of the Pontine escarpment that connects the Latium-Campania continental margin to the Vavilov basin. The latter is the main feature of the area, related to the early Pliocene extension of the Tyrrhenian Sea. Several morphological variations are pointed out along strike, mirroring different structural settings of the margin itself: a steeper margin to the north corresponds to high-angle possibly transtensional faults, whereas a smooth slope in the southern portion corresponds to several more distributed listric faults. This work contributes to the understanding of the interplay between extensional and transtensional tectonics along the margins of an oblique back-arc basin.

Multiple subduction imprints in the mantle below Italy detected in a single lava flow

Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western–Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow.

The Upper Miocene magmatism of the Island of Elba (Central Italy): compositional characteristics, petrogenesis and implications for the origin of the Tuscany Magmatic Province

Late Miocene intrusive magmatism of the Island of Elba, Tuscany (central Italy), consists of stocks, laccoliths, sills, and dikes showing dominant monzogranite and granodiorite compositions, with minor leucogranitic dike-sill complexes, aplites and pegmatites. A few mafic rocks occur as dikes, and as microgranular enclaves hosted inside the main intrusions. The Elba magmatism belongs to the Tuscan Magmatic Province, an 8.5 to 0.3 Ma old association of mafic to felsic rocks, of mantle and crustal origin, cropping out in Tuscany and northern Latium. Major and trace element abundances of Elba rocks are extremely variable, testifying to complex origin and evolutionary history for magmas. 87Sr/86Sr (~ 0.708-0.723) and 143Nd/144Nd (~0.5121-0.5124) are close or within the field of upper continental crust, with mafic dikes showing the lowest Sr- and the highest Nd-isotope ratios. Petrological, geochemical and textural data of Elba igneous rocks are better explained by invoking a leading role for multiple mixing processes between crust-derived felsic magmas and mafic-intermediate melts of ultimate mantle origin, accompanied by fractional crystallisation. Proxies of crustal anatectic melts are represented by some highly radiogenic-Sr rocks from northern Monte Capanne pluton. Crustal magmas were formed by melting of sedimentary rocks, likely metagreywakes, at pressures exceeding 0.3 GPa. Mafic-intermediate magmas have calcalkaline to shoshonitic compositions and originated in an anomalous mantle, moderately contaminated by siliceous sediments. Selective enrichments in Sr, Ba and LREE are shown by some intermediate rocks (Orano dikes), revealing the occurrence of a distinct magma type at Elba. Similar compositions are also observed at Capraia island, San Vincenzo and Campiglia (southern Tuscany), suggesting a regional relevance for this magma type. Sr-Ba-LREE-rich rocks do not show obvious genetic relationships with other Tuscany magmas and may represent a distinct end-member whose origin deserves further studies to be understood. Petrogenetic processes occurred at Elba allow a better understanding of magma genesis at the regional scale. Some of the first-order petrological characteristics of the Tuscany Magmatic Province are comprised inside the space defined by three main end-member compositions, represented by acidic peraluminous rocks, calcalkaline-shoshonitic basalts, and mafic ultrapotassic lamproites. Mixing of various proportions of end-members generated a wide spectrum of hybrid compositions. These, in turn, were affected by evolution processes, such as fractional crystallisation and crustal assimilation, further complicating the petrological puzzle of the Tuscany Magmatic Province. The lamproitic and calcalkaline magmas were respectively generated by melting of phlogopite-bearing harzburgitic and lherzolitic mantle rocks, which were contaminated by subducted upper crustal siliceous materials. Lithospheric mantle contamination occurred during the Eocene, forming harzburgite-sediment melange bodies. These were dismembered by ascent of lherzolitic asthenosphere behind the eastward migrating Northern Apennine subduction front, during Miocene to Quaternary backarc extension. Asthenospheric upwelling generated an increase in temperature, dehydration and melting of dispersed harburgite-sediment melange megaliths, with release of fluids that contaminated the surrounding newly emplaced lherzolites. Melting of such a heterogeneous mantle gave the wide range of mafic magmas observed in Tuscany. Backarc mantle doming and uprise of hot mafic magmas prompted anatexis within the continental crust of the overriding plate.

Geophysical mapping of Vercelli Seamount: Implications for Miocene evolution of the Tyrrhenian back arc basin

Since the Tortonian, the geodynamic evolution of the Tyrrhenian Sea has been driven by an eastward roll-back of the entire Apennine subduction system, triggering distinct episodes of back-arc basin formation with spots of oceanic crust. Major structural differences are observed between northern and southern portions of the Tyrrhenian Sea, reflecting two distinct evolution stages of the Ionian slab retreat. In the central portion of the Tyrrhenian Sea, the seafloor morphology is characterized by a set of magmatic intrusions and structural highs associated to an E–W magnetic lineament along the 41st geographical parallel. The Vercelli seamount represents one example of structural highs correlated to Miocene magmatic episodes along the 41st parallel zone. In this study, we discuss the results of new high resolution magnetic data and morphological mapping of the Vercelli seamount acquired during the VER2010 cruise. The seamount represents the relict part of a granitic intrusion emplaced during the Tortonian phase of Tyrrhenian rifting. Tectonic and deep-sea erosive processes have jointly modified the seamount structure that can be observed nowadays. Cumulative gradient analysis highlights an asymmetric morphology of the flanks as a result of erosive action of opposite water mass gyres which modelled the southern portion of the seamount. The joint interpretation of magnetic and bathymetric datasets identifies a high magnetized source laying close to the base of the seamount and located in correspondence to a small basin. This structure has been modelled as a post-Tortonian lava sequence emplaced between structural highs in correspondence of N–S elongated flat sedimentary basin. Modelling of new geophysical data highlights the relationship between crustal setting and magnetic evidences of the central Tyrrhenian Sea, providing a new interpretation of the 41st magnetic lineament.

Graviquakes in Italy

We discuss the mechanics of crustal normal fault-related earthquakes, and show that they represent dissipation of gravitational potential energy (graviquakes) and their magnitude increases with the involved volume (delimited by the seismogenic fault and an antithetic dilated wedge in its hangingwall), and the fault dip. The magnitude increases with the deepening of the brittle–ductile transition (BDT), which in turn enlarges the involved volume. The fault dip seems rather controlled by the static friction of the involved crustal layers. We apply the model to the extensional area of the Italian peninsula, whose geodynamics is controlled by the Alpine and Apennines subduction zones. The latter has a well-developed backarc basin and a large part of the accretionary prism is affected by on-going extensional tectonics, which is responsible for most of peninsular Italy seismicity. Analyzing the seismic record of the Apennines, the length of seismogenic normal faults tends to be at most about 3 times the hypocenter depth. We compile a map of the brittle–ductile transition depth and, assuming a fixed 45° or 60° fault dip and a dilated wedge developed during the interseismic period almost perpendicular to the fault plane, we compute the maximum volume of the hangingwall collapsing at the coseismic stage, and estimate the maximum expected magnitude. Lower magnitude values are obtained in areas with thinner brittle layer and higher heat flow. Moreover, lower magnitude relative to those theoretically expected may occur in areas of higher strain rate where faults may creep faster due to lower frictional values.

Pressure–temperature–deformation–time of the ductile Alpine shearing in Corsica: From orogenic construction to collapse

Definition of the Tertiary tectono-metamorphic history of Alpine Corsica is a key task to decipher the space–time linkage between the Alpine and Apennine subduction systems in the Mediterranean region. Alpine Corsica exposes a nappe stack of oceanic- and continental-derived units, structurally juxtaposed onto the former European continental margin (Hercynian Corsica). Still uncertain is the timing of involvement of the continental-derived units in orogenic construction and shift to regional extension. This paper focuses on reconstruction of the pressure–temperature–deformation–time evolution of selected ductile shear zones activated during transition from the tectonic underplating to the extensional reworking stages. New Rb–Sr mineral age data, integrated with structural and thermobarometric investigations constrain the waning stages of the high-pressure (from blueschist to greenschist facies metamorphic conditions) top-to-the-W thrusting of the HP, oceanic-derived realm (Schistes Lustrés Complex) onto the Hercynian Corsica along the East Tenda Shear Zone in the early Oligocene (from ~32 to ~27Ma). This early compressional evolution is overprinted by a major phase of retrogressive, syn-greenschist top-to-the-E extensional shearing in the Schistes Lustrés Complex with the last episode of deformation-related ductile recrystallization recorded during the early Miocene at ~20–21Ma, in a continuum transition from ductile to brittle shearing. The same early Miocene Rb–Sr deformation ages are recovered from the ductile-to-brittle top-to-the-E reactivation domains within the East Tenda Shear Zone, documenting that transition from compression to extension in Alpine Corsica occurred during the late Oligocene–early Miocene time lapse. Implications of these data are discussed in the broader context of the Tertiary geodynamic evolution of the Central Mediterranean region.