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One of the few near-complete continental crustal sections exposed on Earth&#39;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 (&lt;18 km depth) and pyroxene hornblendites (&gt;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&#39;s surface today.

SGI 2021

Joint Geophysical-Petrological Modeling on the Ivrea Geophysical Body Beneath Valsesia, Italy: Constraints onthe Continental Lower Crust

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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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Earth Sciences should be understood not only as necessary for a better understanding of the interior of the planet Earth, but also as a defense of society from dangerous geological events, for the understanding of climatic variations, for the planning and use of georesources in a ethical and respecting and protecting the environment.

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.

technical paper

The Adamello batholith is the largest Tertiary magmatic complex of the Alps, and it is mainly composed of tonalites, trondhjemites and granodiorites with minor mafic bodies. Adamello was emplaced at the end of the subduction of the Tethys ocean below the Adria continental margin, when the high pressure metamorphism was at the peak in the Lepontine Alps (Brouwer et al., 2005). The Adamello earlier felsic and mafic plutons are the Corno Alto (including Malga Campo and Monte Ospedale diorites) and the southern Re di Castello (with M. Mattoni, M. Frerone and Blumone mafic apophyses; Bianchi et al., 1970; Tiepolo et al., 2011; Schaltegger et al., 2019). All these magmatic bodies were emplaced along a NE-SW paleo-lineament characterized by extensional/transtensional conditions (Laubscher, 1985). This is supported, in proximity of the present Giudicarie lineament, by an extensional paleo - lineament bordering tectonic depressions holding late Cretaceous to Eocene flysch deposits between Upper Val di Non and Bergamo area (Bernoulli and Winkler, 1990). Extensional to transtensional conditions were typical of the upper Alpine crust including the Austroalpine and Southalpine domains between Late Cretaceous and Early Eocene (Schmid et al., 2004). In this extensional scenario the first magmatic unfoliated bodies of Adamello were passively intruded in the Southalpine sector. Contrariwise, tonalite plutons, characterized by a pervasive magmatic foliation, were emplaced since the Late Eocene – Early Oligocene when the alpine compressive – to – transpressive deformation was active in the Adria middle crust. This new tectonic phase is responsible for the onset of the Insubric/periadriatic dextral lineament, due to the westward-directed push of the Adria plate and the dominant southward/southeastward directed movement of the Alpine thrust pile including the Southalpine and Austroalpine units

Cenozoic subduction in the Central Alps and the intrusion of the Adamello batholith

Topography and drainage systems are powerful recorders of tectonic activity especially in regions with limited geological data. The Atlas-Meseta system of Morocco experienced a late Cenozoic topographic rejuvenation related to mantle dynamics. This recent, large-scale, mantle-driven uplift is documented by a few geological evidence such as uplifted, late Miocene, shallow-water marine deposits in the Middle Atlas Mountains. Conversely, the Anti-Atlas doesn’t have any stratigraphic records that may allow deciphering this uplift history. The presence of a high standing, relict landscape and the transient state of river networks, however, can be used to reconstruct the recent surface evolution of this less understood orogen. Furthermore, the Anti-Atlas belt experienced limited Cenozoic crustal shortening and this offers the possibility to investigate the magnitude, timing and rates of the dynamic contribution. In this study, we combine geomorphic analysis of topography, stream profiles, celerity of knickpoints and linear inverse landscape modelling with available geological evidence, to decipher the spatio-temporal pattern of surface uplift in the Anti-Atlas. Our results highlight the presence of a landscape and a river network in a transient state of adjustment, and document ~ 1000 m and ~ 900 m of surface uplift and topographic paleo-relief since at least the Miocene. Furthermore, our analysis indicates that this late Cenozoic, regional topographic resurgence occurred during the late Miocene and is also influenced by climate oscillation, which play a key role in shaping the new topography. This approach allows to quantitatively constrain the transient state of landscape and hence to unravel the contribution of regional, mantle-driven, surface uplift on mountain building processes.

Rejuvenation of a tectonically inactive mountain belt: insights from the Anti-Atlas (Morocco)

Orogens of continental collisional zone may include high-standing plateaus, both internally and externally drained. These morphotectonic features contain deposits and landforms essential to reconstruct their topographic evolution. In particular, when endorheic basins are integrated into external drainage networks, the rivers could retain first order information on the modes of capture as well as on the interplay between climate and tectonic processes. The Eastern Anatolian Plateau (EAP) of the Arabia-Eurasia collision zone, is one of the most representative examples of collisional plateau. It has a mean elevation of ~2000 m, presents three main endorheic basins (Van, Sevan and Urmia lakes), and is mostly drained by two river networks: the Kura-Arax drainage system to the NE and the Euphrates-Tigris to the SW. Seismic data indicate the presence of a thinned or totally removed lithospheric mantle beneath the plateau explaining the high heat flow and the late Cenozoic volcanic activity in the area. Despite the great number of studies on the EAP, its uplift history is still debated. In this study we quantitatively investigated the drainage systems (river longitudinal profiles and chi-plots) and the general topographic features (swath profiles, slope, local relief, filtered topography) of the EAP. The results speak to a topographic configuration characterized by a high-standing, low-relief plateau centered in the area of Lake Van, but strongly disrupted by tectonic structures with the formation of local topographic highs and lows that include endorheic basins. The hydrographic pattern is strongly disorganized and controlled by active tectonic structures. The irregular river longitudinal profiles indicate that they are in a transient state of disequilibrium as a consequence of regional uplift, capture events, or more localized tectonic activity. The presence of an uppermost segment characterized by low channel steepness suggests that the plateau interior has not been reached by the erosive wave produced by the uplift. In the chi-plots of the rivers draining the northern portion of the plateau two distinct segments record an increase in uplift rate. Conversely, the chi-plots of the rivers draining the southern sector and the Van and Urmia lakes show a single pattern that could be associated with a constant uplift. This difference in uplift pattern is partially confirmed by the stratigraphic record documenting the passage from a marine to a continental depositional environment. Indeed, according to the age of the most recent marine sediments exposed on the plateau, the northern portion emerged from the sea level around 40 Ma while the central and southern ones around 18 Ma. Among these latter, their elevation decreases to the south. In conclusion, the EAP is a high-standing plateau where the integration of hydrography is ruled by regional differential uplift and active tectonic structures.

The uplift of a collisional plateau unraveled by river network analysis: the case of the Eastern Anatolian Plateau

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 Cenozoic deformation of SE Asia has been commonly related to the India-Eurasia collision and to the Tibetan Plateau growth. The huge amount of geological data acquired in Tibet highlighted that the central Plateau is undergoing a NW-SE extension, accommodated by a quasi-continuous SE-ward crust drifting toward Indochina, between the Sichuan and the EHS rigid buttresses (Clark and Royden, 2000). Conversely, some authors described the collision as accommodated by the extrusion of several hundreds of km rigid mega-blocks, bounded by strike-slip faults with hundreds of km of lateral offsets (Tapponnier et al., 1982). Paleomagnetism has proven to be a reliable method to evaluate the displacement of the major strike-slip faults and to describe the crust behaviour. Several authors proposed that the Oligo-Miocene E-SE-ward crust extrusion has been accommodated by the rotation of few major semi-rigid crustal blocks (Li et al., 2017), otherwise, others related the scattered paleomagnetic rotation pattern to the presence of 2-5 km wide blocks, which underwent to independent rotation (e.g., Todrani et al., 2020). We synthesized and re-evaluated all paleomagnetic data collected in Tibet-Indochina assessing with modern and homogenous criteria, estimating the magnitude of the vertical-axis rotation with the coeval Eurasia paleopoles. We focused mainly on SE Tibet and northern Indochina, comparing the paleomagnetic rotation pattern with the present-day geodetic observations. The GPS velocity field shows a large-scale CW rotation of ~4°/Ma around the EHS, whereas the scattered paleomagnetic rotations display a non-consistent pattern. The Jurassic-to-Oligocene paleomagnetic sites W of the EHS display a prevailing of CCW rotations, while on SE the scattered CW rotations agree with the current kinematics. Moreover, it was suggested that the widespread CW paleomagnetic rotations from northern Indochina have been acquired since Oligocene, while the Pliocene-to-Holocene sites display slight-to-null rotations. Accordingly, we concluded that the revised paleomagnetic rotation pattern agrees with a diffused deformation, mainly active between ~25-13 Ma, is at odds with the present-day geodetic large-scale CW rotation and drifting of the Tibetan crust around the EHS.

Evolution of the Tibet-Indochina orogenic system since 60 Ma: from NE India corner indentation to collapse and eastward flow of Tibet crust

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

Joint Geophysical-Petrological Modeling on the Ivrea Geophysical Body Beneath Valsesia, Italy: Constraints onthe Continental Lower Crust

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Cenozoic subduction in the Central Alps and the intrusion of the Adamello batholith

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Joint Geophysical-Petrological Modeling on the Ivrea Geophysical Body Beneath Valsesia, Italy: Constraints onthe Continental Lower Crust

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Next from SGI 2021

Cenozoic subduction in the Central Alps and the intrusion of the Adamello batholith

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

Stay up to date with the latest Underline news!

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RESOURCES

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