United States

The morphology of back-arc basins is tightly linked to the dynamics of their associated subduction zone. Back-arc basins often present multiple spreading centres that form one after the other (e.g. Mariana subduction zone), propagate and rotate (e.g., Lau Basin) following trench retreat. In some cases, rift jumps can create continental fragments or microcontinents (e.g., Coral Sea, Central Mediterranean, Scotia Sea). The Tyrrhenian back-arc basin, in particular, is characterized by the presence of multiple sub-basins and a discontinuous production of new oceanic crust. Moreover, geophysical data show that the basin seafloor is partly made of exhumed serpentinized mantle and local intrusion of magma bodies. All these features reveal a picture of a back-arc basin that is much more complex than that of seafloor spreading at mid-ocean ridges. Importantly, the processes controlling rift jumps and mantle exhumation are still not fully understood, but they are certainly related to the dynamics of subduction and mantle flow. In this work, we show how episodic trench retreat shapes the morphology of back-arc basins and can produce rift jumps. We use the finite element code ASPECT to model the rifting of continental lithosphere in 2D with boundary conditions that simulate the asymmetric type of extension caused by the trench retreat. We perform a parametric study in which we systematically vary the duration of different extensional phases, simulating episodes of trench retreat. Our results show that when extension is continuous, continental break-up occurs and a spreading centre similar to a mid-ocean ridge develops. On the other hand, in models with multiple extensional phases, ridge jumps consistently occur and are often associated with mantle exhumation between the old and the new spreading ridge. This is in good agreement with what observed in Tyrrhenian basin. Timing and length of ridge jumps may depend on rheological properties of the lithosphere, but, importantly, we show that the resulting back-arc basin morphology is controlled by the duration of the different stages of extension.

SGI 2021

Ridge jumps and mantle exhumation in the Tyrrhenian back-arc basin

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L'Istituto Nazionale di Oceanografia e Geofisica Sperimentale - OGS e il Dipartimento di Matematica e Geoscienze dell'Università di Trieste sono orgogliosi di ospitare il 90° Congresso della Società Geologica Italiana a Trieste dal 14 al 16 settembre 2021. La modalità del congresso è interamente online.

Con il motto "Geologia senza frontiere" questo evento mira a promuovere le Scienze della Terra come disciplina scientifica proiettata nel futuro, oltre i confini geografici, politici e disciplinari, per comprendere meglio l'interno del Pianeta Terra e la struttura del sottosuolo, difendere la società da pericolosi eventi geologici, comprendere meglio la variabilità climatica, pianificare l'utilizzo delle georisorse in modo etico e nel rispetto dell'ambiente.


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The National Institute of Oceanography and Experimental Geophysics - OGS and the Department of Mathematics and Geosciences of the University of Trieste are proud to host the 90th Congress of the Italian Geological Society in Trieste on 14-16 September 2021.  The Congress will be held entirely online.

With the motto "Geology Without Borders" this event aims to promote Earth Sciences as a scientific discipline projected into the future, beyond geographical, political and disciplinary boundaries, to better understand the interior of Planet Earth and the structure of the subsurface, defend society from dangerous geological events, better understand climatic variability, plan the use of geo-resources in an ethical way and respecting the environment.


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technical paper

The Soana valley is located on the border between Piedmont and Aosta Valley regions; it offers countless opportunities for studying the lithological and structural features of the Alpine collisional chain. In particular, the Upper Soana valley displays, from west to east and from lower to higher structural levels, a natural, sectional profile across the Gran Paradiso Massif, the ophiolitic Piedmont Zone, the Santanel nappe and the Sesia-Lanzo Zone. These units represent transposed fragments of the paleo-European continental crust, the Ligurian-Piedmont ocean basin, and the paleo-African continental crust, respectively. The study area is located at Colle della Borra (2.578 m), within the Piedmont Zone, where new detailed observations on the geological context and on lithologies associated to massive sulphide mineralization, were made. This area pertains to the Colle della Rosa-Dondena Unit (CRD), which has previously been described as a greenschist-facies tectonic unit interposed between two eclogitic-facies ophiolitic units. The ophiolitic sequence exposed at Colle della Borra, consisting of dominant calcschists with subordinate metabasites layers, quartzites and marbles, has therefore been considered to be of Combin type. In this work, garnet-chloritoid-bearing leaden micaschists, equivalent to those described in previous works (Battiston, 1984) in the adjacent Rosa dei Banchi klippe (Monte Nero eclogitic unit) were found, suggesting the presence of a possible eclogitic-facies metamorphic imprint inside the CRD. The long lasting high-pressure metamorphic history of these micaschists is evidenced by the presence of chloritoid associated either with a main schistosity together with garnet, or with a primary schistosity marked by relict isoclinal folds. The lithological contact between the micaschists and the overlying calcschists is gradational, suggesting its stratigraphic nature. Furthermore, the finding in calcschists of numerous lawsonite relics completely replaced by epidote, white mica and quartz is clear evidence of the high-pressure metamorphic signature of the Colle della Borra sequence. This suggests that the entire CRD, in which the calcschist is the dominant lithotype, may have experienced, a metamorphic peak at a pressure at least exceeding the greenschist-facies. The certain presence of previously unreported high-pressure mineral assemblages in the metasediments of the Colle della Borra ophiolitic sequence calls for reconsidering the tectono-metamorphic evolution of the entire CRD and marks the starting point for further studies in this area.

New evidence of high-pressure metamorphism in an ophiolite sequence in the Upper Soana valley, Western Alps

Microstructural investigations of late-Variscan trondhjemites (c. 310 Ma) and granites (c. 300 Ma) from the upper-amphibolite facies Aspromonte Unit of the Peloritani Mountains (NE Sicily; Fiannacca et al., 2019, 2020) revealed for the first time shear-zone deformation affecting  both granitoids during their cooling, from submagmatic (T > 650  °C) to low temperature solid state conditions (T < 450  °C) (Fazio et al., 2020). The magmatic texture of the granitoids is usually largely preserved, but evidence for non-coaxial shearing is provided by sigmoidal porphyroclasts, mica fish and rare domino-torn boudins in tiny andalusite crystals. The occurrence of widespread chessboard quartz, together with rare submagmatic fractures, points to deformation in the presence of late-Variscan melt, thus constraining the activity of the shear zone during the post-collisional stages of the Variscan orogenic cycle. Submagmatic deformation microstructures were largely overprinted by progressive lower temperature microstructures during cooling of the granitoid bodies. Solid state-high temperature deformation (T > 450 °C) is documented by feldspar bulging, quartz grain boundary migration recrystallization, ribbon quartz and core-and-mantle structures. Finally, low-T subsolidus deformation microstructures consist of quartz bulging, subgrain rotation recrystallization, mica kinks and feldspar deformation twins. No systematic relationship between age or emplacement depth, and deformation mechanisms has been observed by comparing the microstructures developed in the older trondhjemites, emplaced at higher depth (c. 0.5 GPa), and in the younger granites, intruding the trondhjemites after their exhumation at relatively shallow crustal levels (c. 0.3 GPa).  The two continuous sequences of submagmatic to low-temperature subsolidus deformation, recorded in both the older and younger granitoids, suggest a duration of c. 20 Ma for shear-zone activity associated to middle crust exhumation in the South-European Variscan Belt.

Twenty million years of supra- to sub-solidus deformation during exhumation of middle crust from the South-European Variscan Belt (Peloritani Mountains, NE Sicily)

Northeastern Italy is an area characterized by an active tectonic regime with a convergence rate of about 2 mm/yr due to the NNW motion of the Adria plate and its counterclockwise rotation with respect to Eurasia.  The N-S shortening is accommodated by the Southeastern Alps and the External Dinarides, mainly dominated by active thrust-and-fold systems and strike-slip faults (D’Agostino et al., 2005; Serpelloni et al., 2016). In the present study, we exploited the potential of geodetic data to extract information about the surface deformation in response to active tectonics.   We used a Multi-Temporal InSAR approach called Stanford Method for Persistent Scatterers (StaMPS). The technique, based on the detection of coherent and temporally stable pixels in a stack of single-master differential interferograms, allows obtaining mean ground velocity maps. We used Sentinel-1 SAR images acquired in ascending and descending orbits, spanning 2015-2019 temporal interval, as inputs for SNAP2StaMPS and StaMPS software (Foumelis et al., 2018; Hooper et al., 2012). After the PSI processing, we applied spatial-temporal filters and we performed some post-processing operations to obtain reliable PS candidates. We also performed a calibration by using Adria-fixed GNSS velocities to remove orbital ramps and residual atmospheric contributions. Finally, we obtained Line-Of-Sight calibrated mean ground velocity maps for ascending and descending tracks and, after the decomposition, East-West and vertical velocity maps. According to the results, we identify some deformation patterns, potentially related to active tectonics. Towards the Alps, we notice a positive vertical signal reaching about 2 mm/yr. In contrast, the East-West map shows deformation patterns with 1-2 mm/yr rate on the northeastern sector and on the Friulian pedemountain and plain areas. The analysis and the modeling of these signals will clarify the relationship with active tectonics in the study area.

Displacement rate measurement in Northeastern Italy by using PS-InSAR and GNSS data

The Apennines and Dinarides-Hellenides orogens share the same foreland in Apulia (Southern Italy), and in its submerged southeastern continuation in correspondence of the Salento offshore. In this last setting, the two thrust belt fronts of the opposite and converging orogens become very close (up to 150 km) giving rise, in this area, to one of the largest lithospheric anticlines in the world. Through the interpretation of seismic reflection profiles from CROP and ViDEPI projects, the interference between the Hellenides foreland basin and the southern Apennines one on the Apulian swell have been investigated. The results allow to unravel the tectono-sedimentary evolution of the southern segment of the Adria Plate, subducting both toward W-SW and E-NE and producing the two foredeeps that interfere during Quaternary. The deposits of the Hellenides foreland basin migrated towards W-SW at least from the Oligocene due to the E-NEward flexure of the Adria Plate in the northeastern part of the Ionian sea (i.e., on the area of the future Apulian swell). The same area, arching, was involved in the eastward roll-back of Adria Plate at least from the early Pleistocene, and the sedimentary wedge of the south Apennines foreland basin migrated towards E-NE beginning to occupy by the outermost part of the Hellenides foreland basin. From late Early and Middle Pleistocene the area coinciding with the culmination of the arc, where the two opposite foreland basin were interfering, began to uplift during arching, forming the Apulian swell. Starting from this moment, the Apulian swell became a threshold for sediments coming from the two opposite orogens, and two different narrow depocenters, well separated from each other, represent today the Apennines and Hellenides foredeeps, respectively.

Interference between Apennines and Hellenides foreland basins around the Apulian swell (Italy and Greece)

Mountain building is a paradigm still under debate in the greater Alpine area. Open questions remain regarding subduction polarity (was there a switch in the Alps?), slab attachment and detachment, the nature of low velocity domains and differences in the composition of the European and Adriatic Plates. Recent geophysical imaging and (seismo)tectonics studies are of great help in providing vastly improved resolution images of the subsurface structures in the crust and mantle. In particular, the European AlpArray project with its dense seismological network has provided 4-years of data from the entire greater Alpine area, leading us to examine the subsurface with renewed vigour. In the first part of our talk, we explore the pros and cons of the different geophysical methods that allow us to discern the lithospheric structure, with particular attention to the greater Alpine region and the Adriatic plate. In a second part, we focus on differences and similarities between models that have been proposed to explain the tectonic and geodynamic framework of the circum-Adriatic area, including the Alps, northern Apennines and Dinarides. We highlight first-order differences in the seismological characteristics of the European and Adriatic plates that govern along-strike variation of the orogenic structure of these mountain belts. Finally, we revisit models that have been proposed in the past and show how the new results change our view of how inherited structures have affected crustal and mantle evolution during mountain building.

Understanding the European and the Adriatic plates from the top down

We present a review of geological and seismological data on the long-lived Schio-Vicenza Fault System (SVFS) at the northern Adria plate margin and provide its geodynamic and seismotectonic interpretation. The SVFS is a NW-SE trending, high angle complex structure transverse to the Southern Alps fold-and-thrust belt, and has been the object of geological and structural studies for more than a century and a half. The main segment of the SVFS is the Schio-Vicenza Fault (SVF), which has a significant imprint in the landscape across the Eastern Southern Alps and the Veneto-Friuli foreland. The SVFS is divided into a northern segment, extending across the chain between the town of Schio and the Adige Valley, and a southern one, coinciding with the SVF proper. The SVF borders to the east a foreland structural and morphological high formed by the Lessini and Berici Mts., where the Mesozoic bedrock is exposed, and by the volcanic Euganei Hills, separating it from the Veneto-Friuli foreland, and continuing southeastward up to the Adriatic coastline along the blind Conselve-Pomposa Fault. The segments of the SVFS have been active during several tectonic phases with different styles of faulting at least since the Mesozoic, with a well defined long-term dip-slip component of faulting and, conversely, a horizontal one not well constrained. The SVFS interrupts the continuity of the Southern Alps thrust fronts in the Veneto sector, suggesting that at least it played a passive role in controlling the geometry of the active thrust belt, and possibly also the current distribution of seismic release. In this context, the SVFS, and specifically its SVF segment, has been interpreted as the sinistral strike-slip boundary of the northeastern Adriatic indenter. The recent activity of the SVFS is highlighted by the moderate seismicity along the northern segment and few geological observations along the southern one. The review of the historical and instrumental seismicity along with active deformation data, shows that the SVFS does not appear to have generated large earthquakes during the last few hundred years, while the focal mechanisms of moderate seismicity points to a dextral strike-slip activity, driven by a N-S oriented compressional regime. These observations are corroborated by the field analysis of antithetic Riedel structures of the fault cropping out along the northern segment. Conversely, the southern segment shows geological evidence of recent sinistral strike-slip activity. The geological and seismological apparently conflicting data can be reconciled considering the sinistral strike-slip faulting style of the southern segment as driven by the indentation of the Adriatic plate, while the dextral strike-slip style along the northern segment can be explained as a sinistral opening "zipper" type shear zone junction, where intersecting pairs of simultaneously active faults with different sense of shear merge into a single fault system through a zippered section.

Role of the long-lived Schio-Vicenza Fault System at the northern Adria plate margin

The recent tectonic activity of the Susans Tricesimo Thrust System (STTS) was investigated through a multidisciplinary approach. The study area is located in the northernmost portion of the Friuli eastern prealpine border where the SSE verging front of the Southalpine Chain meets the NW-SE trending strike slip structures of the eastern Friuli-western Slovenia domain (Vrabec & Fodor, 2006; Poli & Zanferrari, 2018). At present, the 2-3 mm/yr accumulating deformation (Serpelloni et al., 2016) is released through the interaction among reverse, transpressive and strike-slip faults. The study area corresponds to the epicentral zone of two of the strongest earthquakes of NE Friuli: the 1511 Mw 6.3 and the 1976-77 (Mw ≤ 6.4) sequence (Rovida et al., 2021). In more recent times, two seismic sequences were registered in western Slovenia in 1998 (Mw 5.6) and 2004 (Mw 5.2). In order to further explore the involvement of the STTS during the strongest seismic events occurred in the area, this work is focused on the recent tectonic activity of the structure. Based on a multidisciplinary approach, the deep 3D-geometry of the STTS was reconstructed by combining ENI seismic lines interpretation and hypocentral distribution analysis. The morphotectonic survey conducted in the study area allowed to detect many surficial anomalies at the hanging-wall of the ST thrust. The collected morphotectonic hints clearly depict a NW-SE elongated zone of surficial deformation, highlighting the recent activity of the Colle Villano-W inner thrust. Lastly, the realization of a paleoseismological trench across the Colle Villano-W thrust revealed that the excavated glacial units of the Canodusso Subsynthem (18.000 - 19.000 yr 14C BP) experienced at last two deformative episodes. Particularly, the occurrence of the latest event in historical times seems likely, and the possible correlation with the 1511 or 1976 events must be considered.

New evidence of recent tectonic activity of the Susans-Tricesimo thrust-system (NE Italy)

Slovenia and Italy share a number of cross-border active faults. Cross-border faults include the continuation of the Dinaric fault system into Friuli and the continuation of the Adriatic foreland thrust system into the SE Friuli plain and south Alpine thrusts (e.g., Moulin et al., 2016; Atanackov et al., 2021). Seismotectonic characterization, past earthquake activity, and seismic hazard assessment require a holistic approach, encompassing entire faults and fault systems. Constraining past seismicity and linking historic events with causative faults has proven exceedingly difficult (e.g., Bavec et al., 2013; Patricelli & Poli, 2020; Grützner et al., 2021). This is a challenge across various fields of geology, as faults produce a variety of geological expressions, crossing different settings: marine – Gulf of Trieste, alluvial plain in Friuli, Karst and Slovenian Istria. The need for significantly improved cross-border harmonization has been obvious in most recent efforts, including HIKE, ESHM20, and the new seismic hazard map of Slovenia. We will discuss open questions and challenges in joint characterization of cross-border faults and open an invitation for collaboration in future work.

Cross-border challenges in active fault definitions across the Slovenia-Italy border

The northernmost edge of the convergent margin between Eurasia and the Adria microplate is in the area of transition between Eastern Alps and External Dinarides. This area is the most seismically active area of the Alps, and several destructive earthquakes hit this region in historical times. The stress and strain tensor inversions from the focal mechanisms show a complex stress field with a resulting heterogeneous deformation pattern. Here, we present NAC (Northern Adria Crust): a 3D model of the geophysical crustal properties in this area (Magrin, Rossi, 2020). NAC has been built by critically choosing and integrating all available information about the depth of the main interfaces and the physical properties of the crust. We started from a VP dataset, and we converted it in VS and density by using empirical relations, tuned through the comparison with the available data from local tomographic inversion, and considering the lithologies of the area. Uncertainties and reliability of the model are quantified, taking into account the data quality and the interpolation procedure. NAC has two versions, with a different structure of the Moho: the first version considers one continuous surface for the whole area, while the second one implies three separate surfaces for the Adria microplate, Eurasia, and the Pannonian fragment. The differences between the two models are minimal, but the available data better sustain the solution of the fragmented crust. For its characteristics of multiparametric information and resolution, NAC can be precious for any purpose and use, where a detailed knowledge of the crustal structure of this area is required. Moreover, it is easy to improve NAC, including new data on structures or crustal properties. In the last years, new information on the crustal structures became available (e.g., Stipčević et al., 2020; Sadeghi-Bagherabadi et al., 2021). We compare NAC to some of them, in order to discuss its strengths,  limitations, and possible developments.

Deriving a new crustal model of Northern Adria: the Northern Adria Crust (NAC) model

The European Marine Observation and Data Network (EMODnet) is a Project aimed at the collection and harmonization in European Seas of existing data, to be represented on digital maps freely available through a dedicated portal (www.emodnet.eu), in order to facilitate their exchange and re-use. It is subdivided into thematic Lots concerning Bathymetry, Geology, Biology, Chemistry, Physics, Seabed Habitats and Human activities. Each Lot is carried out by a consortium of Institutions collating data for its thematic portal also by means of machine to machine communication. The Geology Lot (www.emodnet-geology.eu) is realized by a Consortium of European Geological Surveys. Information includes data at multiple scale on seafloor sediments grainsize, sedimentation rates, Quaternary geology, pre-Quaternary geology and stratigraphy, coastal behaviour, geological events, mineral resources, submerged landscapes. Metadata and available information collected during the Project are displayed on the Portal. The spatial data, organized in layers, are represented by the geometric primitives lines, points and polygons and the alphanumeric data are stored in harmonized and comprehensive databases, which can be consulted online through Attribute tables. Within the Project a subgroup of Geological Surveys of circum-Adriatic countries (Slovenia, Croatia, Montenegro, Albania and Italy) has been established in order to work together and provide already harmonized products to the Project. Elaboration has been carried out concerning pre-Quaternary units, as required by the Project, as well as pre-Pliocene units, considered by common agreement more meaningful in the Adriatic setting. In addition, other features to be represented were taken into account within the task “Geological events and probabilities”, coordinated by the Geological Survey of Italy – ISPRA. They include submarine landslides, earthquakes, volcanic centres, tsunamis, fluid emissions and Quaternary tectonics. However, the best achievement of the Adriatic group was the realization of a map of sediment grainsize at 1:250,000 scale, obtained thanks to the collaboration of CNR-ISMAR as well. Products elaborated for the entire Adriatic Sea highlight the continuity of geological units, as shown by the map of pre-Quaternary deposits, contribute to understand the geological evolution, as is the case of the map of pre-Pliocene deposits, and to outline sedimentary dynamics, as evidenced by the map of seafloor sediments. Moreover, collation of data across national boundaries allows to analyse in their entirety specific features, such as submarine landslides.

Ridge jumps and mantle exhumation in the Tyrrhenian back-arc basin

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New evidence of high-pressure metamorphism in an ophiolite sequence in the Upper Soana valley, Western Alps

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Ridge jumps and mantle exhumation in the Tyrrhenian back-arc basin

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New evidence of high-pressure metamorphism in an ophiolite sequence in the Upper Soana valley, Western Alps

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A look at the deep structure and dynamic of the circum-Mediterranean orogens

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