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The Central Mediterranean region in the Tertiary was characterized by intermittent phases of subduction and collision, the opening of back-arc extensional basins (i.e., Liguro-Provençal, Alboran, and Tyrrhenian basins) and episodes of slab lateral tearing, segmentation and break-off that led to the currently complex geological setting. Although the shallow tectonic evolution of the region has been relatively well constrained by abundant geological data, several uncertainties persist about the mechanisms that generated the present-day surface morphology and deep slab geometry. In this study, we reproduce the recent large-scale evolution of the Central Mediterranean and the associated strain-induced upper mantle fabrics and seismic anisotropy combining geodynamic and seismological numerical modelling techniques. 3D petrological-thermo-mechanical numerical models have been carried out using the viscoplastic code I3MG (Gerya, 2019), to compute the LPO and the synthetic SKS splitting we use a modified version of D-REX (Kaminski et al., 2004) and the softwares package FSTRACK (Becker, 2006), respectively. The subduction models were designed and calibrated according to paleogeographic-tectonic reconstructions of 30 Ma and seismological observations from the Mediterranean region available in the literature. We test different initial configurations in order to optimize the fit between predicted and observed slabs position and obtain a final model configuration resembling the present-day surface and deeper structures. It is found that the opening of back-arc extensional basins in response to the south-eastward retreat of the Ionian slab is a common feature in all models and slab lateral tearing or break-off occurs when Adria continental margin enters the trench. More importantly, we show that structural heterogeneities within the Adria plate and/or the geometry of its Tyrrhenian passive margin have a profound impact on (i) the development of a slab window below the Central Apennines, (ii) the retreat of the Northern Apenninic trench till the Adriatic Sea, and (iii) the retreat of the Ionian slab till the present-day position. In general, this study highlights the importance of coupling macro-scale geodynamic modelling with micro-scale simulations of strain-induced upper mantle fabrics and seismological synthetics in order to better constrain the tectonic evolution of complex convergent margins such as the Central Mediterranean. 

SGI 2021

Geodynamic and seismological modelling of the recent dynamics of the Central Mediterranean region

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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.

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poster

A major shortcoming of current thermodynamic databases for phase equilibrium calculations at deep mantle conditions is the lack of reliable, comprehensive and physically-consistent thermodynamic data of the constituent mineral phases. Some of the most popular thermodynamic databases (e.g. Holland & Powell, 2011) are calibrated overthe results of selected HT-HP experiments, which may suffer from uncertainties related to technical problems, pressure scale calibration, metastability issues or the unquenchable nature of some mantle minerals. This may lead in turn to assessed thermodynamic properties for mantle phases which may be internally-consistent but clearly suffer from physical unsoundness, thus hindering a close link between thermodynamic modelling, seismological observations and mantle geodynamics. In that sense, ab initio calculations based on quantum-mechanical theory are one of the most reliable methods available to obtain information on thermodynamics, phase relations and seismic properties of mineral phases at deep mantle conditions. The striking feature of the so-called density functional theory (DFT) is that it can accurately predict physico-chemical and thermodynamic properties of crystalline solids in a wide range of P–T conditions, without the need of any experimental data. We performed ab initio B3LYP calculations to determine thermodynamic and seismic properties of ringwoodite (ideal formula Mg2SiO4), which represents the stable polymorph of olivine in the lower part of the Earth’s mantle transition zone. Thermodynamic properties of ringwoodite that depend on vibrational frequencies (i.e., heat capacities, thermal expansion, isothermal bulk modulus, entropy, enthalpy, Gibbs free energy) have been computed in the framework of quasi-harmonic approximation by a full phonon dispersion calculation or, alternatively, by a modified Kieffer’s model splitting acoustic and optic contributions. Both methods reproduce well the vibrational density of states, allowing an accurate determination of the equation of state parameters and thermodynamic properties as well. The calculated heat capacity in the low- to medium-T range is in excellent agreement with the few calorimetric investigations made so far on this phase (Akaogi et al., 2007). The obtained results are then used to predict relevant phase equilibria of ringwoodite in the Earth’s deep mantle and to discuss some implications for mantle seismic discontinuities.

Towards an ab initio physically-consistent thermodynamic database for deep mantle phases: the case of Mg2SiO4 ringwoodite

Investigating the texture and chemistry of mantle xenoliths allow us to constrain the nature of the upper mantle, the mechanism that generate melts and the evolution of the lithosphere beneath regions where no samples were exposed by tectonic activity. Texture, as well as chemical composition and mineralogical paragenesis, reflects the temperature, pressure, stress conditions, melting and/or contamination events undergone before and during the entrapment in the host magma (Pearson et al., 2003). In particular, the distribution of glass pockets (blebs) and veins inside xenoliths, as well as the texture of silicate, oxide and sulphide phases seem to be important in understanding some aspects of mantle metasomatism (Coltorti et al., 2000; Hughes et al., 2016; Lenaz et al., 2017). Classic 2D thin section observation might overlook some textural and mineralogical features, like oriented distribution of some mineral phases, possible interconnection of glass pockets and veins, distribution and morphological features of accessory minerals like sulphides. Extending the textural analysis of mantle xenolith to three-dimensional scale is therefore a potentially powerful tool in petrography and petrology. By using a multi-scale 3D textural analysis through X-ray computed microtomography (micro-CT), we characterized three mantle xenoliths from different geodynamic settings (i.e. mobile belt zone, pericraton, oceanic hotspot). We used an approach structured in increasing steps of resolution, starting with conventional X-ray micro-CT imaging (voxel size: 30 mm) and moving on to phase-contrast synchrotron-based X-ray micro-CT to reach a voxel size of 0.9 mm. We performed several exercises to test the effectiveness of micro-CT imaging on textural characterization of mantle xenoliths, comparing the results with the observation of conventional optical microscopy. One of these exercises involve a model to simulate the random sectioning of several thin sections and estimate the probability of accurate modal classification. Furthermore, 3D models obtained allow us to collect several textural information that were impossible to determine in a 2D classification. For example, we identified: spinel layering in one sample (MG10x), presence of gas vesicles in glass of xenolith Bi4, and silicic glass scattered through sample FN38. Moreover, high-density volumes were detected in nodules MG10x and Bi4, showing no relation with the spinel layering in the first one and a preferential concentration along fractures in the latter one.

A comparative study between X-ray computed microtomography and thin sections observation of mantle xenoliths

The Mediterranean is one of the most complex convergent margins on Earth. Its recent tectonic history was characterized by the coexistence of different compressional and extensional phases associated with episodes of orogenesis, slab rollback, slab tearing and oceanic spreading. Since  the  late  1990s, numerous seismological studies have been carried out in the region aiming at imagining the isotropic and anisotropic structures of the upper mantle. Among the various imaging techniques, isotropic P-wave travel-time tomography was largely used. However, seismic anistropy is widespread throughout the Mediterranean area and neglecting its effects could introduce significant artefacts in the inversions that could bias our understanding of the Earth’s interior structure and dynamics. Here we discard the isotropic approximation and invert for both P-wave isotropic velocity anomalies and seismic anisotropic by using the method proposed by VanderBeek & Faccenda (2021). We find that the magnitude of the isotropic P-wave velocity anomalies are substantially reduced when inverting for seismic anisotropy. This suggests that lateral variations in temperature and/or composition are smaller that what can be inferred from isotropic tomographies. P-wave fast azimuths orient mostly parallel to the trend of the Balcanic and the Alpine orogens in Eastern and Central Europe, respectively. In the Central Mediterranean the P-wave fast azimuths are sub-parallel to the Oligocene/Miocene-to-present retreating direction of the Ionian trench which led to the opening of the Liguro-Provençal and Thyrrenian basins and rotation of the Corsica-Sardinia block. In synthesis, the pattern of the P-wave fast azimuths is largely consistent with the S-wave fast azimuths determined from the splitting of SKS waves and from Rayleigh waves. This poses further constraints on the interpretation of the regional geodynamic evolution and on the accuracy of the employed inverse method.

Teleseismic Anisotropic P-wave tomography of the Central Mediterranean

The origin of the intracratonic basins (ICBs) is still highly debated and several hypotheses have put forward to explain their long-lasting subsidence, characterized by prolonged intervals of low rate subsidence alternating with episodic accelerations in subsidence rate (e.g., Hartley & Allen, 1994). The Congo basin (CB) is a natural laboratory for investigating the processes that govern the long–term evolution of the ICBs, due to its long tectonic history, during which a large thickness of sediments deposited (up to about 9 km). Its subsidence initiated very probably as a failed rift in late Mesoproterozoic and evolved during the Neoproterozoic and Phanerozoic under the influence of far-field compressional tectonic events, global climate fluctuation between icehouse and greenhouse conditions and drifting of Central Africa through the South Pole then towards its present-day equatorial position (Delvaux et al., 2021). We reconstructed the depth of the basement and of main sedimentary layers of the CB, by integrating the interpretations of almost 3000 km of seismic reflection profiles with the analysis of the gravity field. The obtained results show a very heterogeneous basement depth, characterized by a series of topographic highs and lows NW-SE oriented (Delvaux et al., 2021; Maddaloni et al., 2021). We further observed the migration of the sedimentary depocenters from the Proterozoic to Jurassic times and lateral thickness variations of the sedimentary layers. Both types of observations reflect a different behavior of the CB during the stages of its evolution, with a progressive decrease in the influence of the initial rift structure. The rift phase that gave origin to the CB has been simulated applying multidirectional slow divergent velocities to a cratonic block having a central weak zone, representing the suture area between the cratonic pieces composing the Congo craton. The numerical models, after a time lapse of 200 Myr, show the formation of a central circular subsided area, as effect of the radial extension, induced by the asthenosphere upwelling. The main structures, formed within the depressed topographic area, resemble the present-day basement depth of the CB, supporting the hypothesis that the origin of some ICBs can be due to the effect of multi-extensional stress applied on a cratonic area.

What does it link the surface to the deep Earth’s processes? The tectonic evolution of the Congo basin: an example of intracratonic basin

The Main Ethiopian Rift (MER), at the northern segment of the East African Rift System (EARS), offers an excellent tectonic setting to study earthquakes and their controlling mechanisms. Here, we use high resolution, 2D-lithospheric scale numerical modeling experiment combined with analysis of seismic moment release using EAGLE dataset to elucidate the mechanism for deep crustal seimsicity in the MER. The modeling experiment is caliberated by approporiate rheology and deformation history for the region. Our modeling results clearly indicate the migration of deformation from the Miocene border faults to 30 km wide, 60 km long Pliocene to recent rift floor faults (magmatic segments). While the strain rate is locaized at/beneath the magmatic segments, the brittle strain is distributed in wider region. We also estimate the variation of the magnitude of deviatoric stress with depth in the rift. The deviatoric stress has peak values of 80 and 160 MPa at about 6 and 18 km depth, respectively, which shows a good correlation with the very high seismic moment release. We conclude that the depth distribution of  crustal seimsicity in the MER is controlled by the rheology of the crust.  

Geodynamic modelling of lithospheric extension constrained the mechanism for deep crustal earthquakes in the Main Ethiopian Rift

The Western African Rift System forms an ensemble of narrow and deep basins, inserted into the context of the much broader oval shaped Chad basin, which though is only a few hundred meters deep, up to 450 m. The system includes the Termit, Tenéré, the Tefidet and the Kafra basins. The rift hosts up to 12 km thick sediments, accumulated starting from the first rift cycle active throughout Cretaceous extending from 145.5 Ma to 99.6 Ma. The second cycle of sedimentation continued with the second rift phase from 65.5 Ma to 23 Ma, and continued during the post-rift phase. Both rift phases produced documented volcanism intercalated in the sediment pile. The sedimentation of the Chad basin is described as that of a sag basin, with deposits starting from the end of the first western rift phase. Our interest lies in defining to which extent the documented volcanism affected the crustal column of the rift basins, and in particular if the crustal densities have been altered. The presence of increased density in the crustal column has implications on the isostatic equilibrium, and the general comprehension of the amount of magmatic rocks substituting the original or stretched crustal column. The input data that sustain our modeling are the gravity field, a digital terrain model, and a model of the thickness of the sedimentary cover. Such data are much more easily available than the deep seismic profiling reaching the lower crust, available only rarely. The methodology we use demonstrates that the information on the superficial crustal structure together with the gravity field, allows to define the lower crustal structure and density. We had found the densification of the lower crust below a Large Igneous Province (Parana’ basin, Mariani et al. (2013)), which had been interpreted as underplating, and below the Volcanic province in the Southern Alps (Veneto Volcanic province, Tadiello and Braitenberg (2021)). The timing of the magmatism of the West African rift is presumably tied to two pulses of volcanism documented in the rift, associated with the first post-rift phase from 96 to 88 Ma and the second post-rift phase from 23 Ma up to the Quaternary.

The traces of magmatic pulses in the crustal density structure of the Western African Rift System

The southern Patagonian Andes (~46-56°S) are well suited to investigate the tectonic vs. climatic interactions during mountain building. Oceanic subduction underneath the South America continent occurs jointly with the opening of an asthenospheric window and, during the late Cenozoic, the building and melting of the Patagonian Icefields. Although the asthenospheric window caused regional dynamic uplift estimated in the order of the tenths of mm per year during the last 3 Ma, the present-day uplift rates in the orogenic domain subject to glaciation are measured between ~10-40 mm/yr. These uplift rates are to a large extent related to the glacial rebound since the Little Ice Age (~ AD 1630), but the role of rheological mantle and lithospheric weakening due to the asthenospheric window is currently unconstrained. Here we use numerical thermo-mechanical modeling to estimate the uplift induced by deglaciation of an ice sheet accounting for the rheological effects of asthenospheric thermal anomalies.  Our results show two main phases of rock uplift: 1) a rapid increase in the uplift rates below the ice sheet when deglaciation starts, and 2) stable positive uplift rates during the deglaciation. For any tested, plausible rheological setting, the maximum uplift rates is <10 mm/yr in the absence of an asthenospheric thermal anomaly, and >30 mm/yr with thermal anomalies higher than 100°C. The higher the asthenospheric thermal anomaly, the higher and wider the uplift rates, which may also involve initially ice-free regions. Uplift rates similar to those observed today require an asthenospheric thermal anomaly of 150-200°C. We conclude that, although the driver of the present-day uplift rates is the deglaciation, the asthenospheric window largely controls its outstanding magnitude.

The role of the asthenospheric window and deglaciation on the present-day uplift of the southern Patagonian Andes

We used the results of seismic anisotropy as inferred from shear wave splitting analyses of the core-refracted phases and combined them with the quasi-Love wave observations and geodetic measurements to propose a geodynamical model of the Arabia-Eurasia collision zone. A detailed analysis of the non-null splitting and null splitting measurements obtained from a dense temporary network is utilized to investigate the possibility of lateral and vertical variations in the anisotropic parameters and the hypothesis of a dipping anisotropic layer (Sadeghi-Bagherabadi, 2018a; 2018b). The preferred 2-D geodynamical model of the western part of the collision zone suggests that the belt-parallel orientation of fast axes in the western Zagros originates from a lithospheric transpressional deformation. The plate motion parallel pattern in central Iran and western Alborz coincides with the decrease in the lithospheric thickness. Thus, we believe this trend has its origin in the asthenosphere. A combination of the keel effect of the thickened Zagros lithosphere, the asthenospheric edge-driven convection flow and the lithospheric deformation in the shear zones can cause the NW-SE-oriented splitting pattern reported in some parts of central Iran. The asthenospheric flow beneath the thinner lithosphere to the south of the Bitlis suture in northern Iraq is likely the causative mechanism for our observed plate motion-parallel splittings there. The variation of the convergence obliquity along the Alborz and Zagros inferred from analysis of geodetic data implies that a change in the pattern of lithospheric deformation and the consequent anisotropy is expected. The quasi-Love wave observations also indicate that the fast-axis orientations are highly affected by the lateral variations of the LAB depth in the region. 

Seismic Anisotropy and Its Geodynamic Implications in Iran

Two different levels of magmatic differentiation have been found in the plumbing system of Mount Amiata geothermal field. With the aim to decipher the geometry and structure of the magmatic system feeding the geothermal field, we carried out a micro-textural petrography study, mineral chemistry analysis and thermobarometric cpx-liquid calculation (Masotta et al., 2013) of the Pleistocene lavas, domes and enclaves. The Pleistocene Mount Amiata volcano is part of the Radicofani – Mount Amiata volcanic system (Conticelli et al., 2011, 2015). The presence of rounded magmatic enclaves testifies the occurrence of a process in which the fresh mafic hot magma was injected into a cooler and mushy crystallised differentiated one, mixing and mingling with the latter (Ferrari et al., 1996; Conticelli et al., 2015; Marroni et al., 2015). The first arrival of mafic magma within the differentiated magma reservoir triggered the chemical mixing with the viscous and extremely differentiated trachydacitic resident magma to form magma batches with intermediate compositions. A reverse differentiation pathway is observed with time of magma emplacement, which is accompanied by decrease of silica contents and increase of MgO and compatible elements passing from early trachydacites to potassic trachybasalt (absarokite) (Conticelli et al., 2015). Our results support a scenario characterised by an initially intrusion of mafic silica-saturated potassic to calc-alkaline magmas that cumulated at depth and differentiated through crystal fractionation and minor crustal contamination. The magmatic system evolved from basaltic andesite to trachydacite, started from an intrusive event reconciled with the composition of the Radicofani calc-alkaline magma. The changes occurred in the source of magmas brought newly arrival mafic silca-undersaturated magmas which mixing and mingling. In depth basanite undergoes light processes of differentiation testified by the presence of Cr-diopside and fassaite (Huckenholz, 1973; Duda & Schmincke, 1985). 

Polybaric crystallisation and equilibrium conditions of Mount Amiata volcanic rocks, and their significance in the frame of magma evolution: insight from igneous mineral chemistry and microtexture petrography

One of the few near-complete continental crustal sections exposed on Earth's surface is the Ivrea-Verbano Zone (Western Alps, Italy), which is considered as a petro-geophysical reference of the continental lithosphere. Exposed peridotite slivers embedded in lower crustal rocks at the surface and large density, seismic velocity anomalies of the Ivrea Geophysical Body in the subsurface suggest that mantle-like rocks are located as shallow as a few kilometers depth, but the actual composition of the rocks producing these anomalies is unknown. Here we investigate how the published seismological and new gravimetric data in the location of Valsesia could be reconciled with petrologic data and models of the Ivrea-Verbano Zone. We use the Perple_X software to calculate densities and compressional wave velocities for a range of possible deep crustal rock types. We argue that amphibole gabbros (<18 km depth) and pyroxene hornblendites (>18 km depth) provide the best fit to the joint geophysical and petrologic constraints, whereas residual ultramafic rocks and anhydrous gabbros are inconsistent with the existing data. This indicates that the Ivrea Geophysical Body beneath the Valsesia area in the Ivrea- Verbano Zone preserves the structure of an igneous complex formed during magmatic underplating from the crystallization of hydrous mafic magmas. This would imply melting of a damp mantle source that produced a continental crust of an original thickness of up to ∼48 km in the Permian, of which ∼30 km are exposed at Earth's surface today.

Geodynamic and seismological modelling of the recent dynamics of the Central Mediterranean region

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Towards an ab initio physically-consistent thermodynamic database for deep mantle phases: the case of Mg2SiO4 ringwoodite

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