705 research outputs found
A Media Converter Prototype for 10-Mb/s Ethernet Transmission Over 425 m of Large-Core Step-Index Polymer Optical Fiber
No full textPermian geodynamics of the central Southalpine by tectono-thermal record in post-Variscan conglomerates
No full textThe central Southern Alps consist of the pre-Alpine basement and Permian-Mesozoic covers, both affected by the Alpine fold-and-thrust belt. The pre-Alpine basement recorded heterogeneous structural and metamorphic evolutions and therefore consists of different tectono-metamorphic units related to different stages of the Variscan evolution. To the east, rocks recorded the effects of the Variscan tectonic burial and escaped the subsequent collision, whereas the units outcropping westward recorded both the effects of Variscan tectonic burial and collision and the westernmost basement rocks even host late-Variscan intrusives and recorded the effects of lithosphere thinning-related Triassic high-temperature (Spalla et al., 2014). Lower Permian volcanoclastic sequences infill intermontane basins and are the oldest sedimentary rocks uncoformably capping the basement (Berra et al., 2016; Zanoni & Spalla, 2018 and refs therein). These sequences consist of volcanites overlaid by lacustrine sandstone and alluvial fan conglomerates. According to radiometric constraints, the age of the conglomerates is more recent westward. These conglomerates contain pebble- to boulder-sized crystalline clasts. The metamorphic evolution recorded in clasts are related to the Variscan orogeny and revealed that the thermal maturity of orogenic traces increases westward, likewise the general record in the metamorphic basement, indicating that conglomerates were fed by the erosion of tectono-metamorphic units similar to those exposed today. In the westernmost conglomerate, clasts recorded high-temperature metamorphism and some derive even from late-Variscan intrusives and later tourmalinite-breccia. Since the conglomerates rejuvenate westward with the increase of orogenic maturity in clasts, we speculate that the post-Variscan lithosphere was affected by westward propagating extension, also responsible for intermontane wrenching. To test this hypothesis, we started 2D numerical simulations on the thermo-mechanical evolution of the lithosphere affected by westward propagating extension
Structural and metamorphic evolution of an ocean-continent transition (OCT) zone mélange deformed under HP conditions during Alpine subduction (Western Italian Alps)
No full textWe report on the structural architecture and metamorphic evolution of a
mélange, developed originally in an ocean-continent transition
(OCT) zone along the boundary between the continental crust of the
Sesia-Lanzo (SLZ) and the oceanic Piemonte Zones (PZ) in the axial part
of the Western Alps. All these units were deformed together under
high-pressure conditions. The mélange consists of thin layers of
calcschist, fine-grained gneiss, quartzite, minor metabasic rocks and
serpentinite, and occurs all along the western margin of southern SLZ,
extending from Santanel klippe to Lanzo Massif, over a distance of 50 km
(Spalla et al., 1983; Battiston et al., 1984). Calcschist rocks range
from phyllites to carbonatic schists and marbles; fine-grained gneisses
of continental origin (very similar to those of SLZ) include phengitic
white mica, chlorite, ± garnet ± albite and relict
allanite. Thinly layered quartzites are white mica- and garnet-bearing.
Metabasic rocks consist of metagabbros and metabasalts with minor
mylonitic serpentinites. All these lithologies of the mélange
unit and the rocks of SLZ and PZ together underwent four episodes of
deformation, giving rise to a complex regional tectonostratigraphy. The
earliest deformational structures are represented by up to ten
meter-scale isoclinal rootless folds. The metamorphic mineral
assemblages marking successive foliations indicate that all rock units
in the mélange, SLZ and PZ (Spalla et al., 1983; Benciolini et
al., 1984) experienced an early eclogite facies imprint, followed by
re-equilibration under blueschist facies conditions, and that they were
finally widely retrogressed under greenschist facies conditions during
the last two deformational episodes (D3 and D4 structures). The strong
synmetamorphic deformation of this mélange prevents an
unequivocal interpretation of its origin; hence, we envisage two
possible scenarios: i) the present day configuration of these thin,
intermingled layers, including rootless refolded isoclinal folds, is
entirely due to transposition that occurred in a mantle wedge at the
early stages of deformation under eclogite facies conditions during
active subduction; ii) a detrital origin of these alternating layers of
terrigeneous and carbonaceous rocks corresponds to a primary sequence of an extensionally-thinned continental margin near an OCT that was
reworked in the Alpine subduction system
What drives alpine tethys opening: suggestions from numerical modelling
No full textContinental crustal slices, preserving pre-Alpine metamorphism, are widely described in Alps and Apennine realms (Fig. 1). Variscan-age eclogites (430-326 Ma) generated from continental, oceanic and mantle rocks occur within these slices and suggest a pre-Alpine burial of continental crust at convergent plate margins, in a context of oceanic lithosphere subduction underneath continental upper plate, characterized by a low thermal regime, and followed by continental collision (e.g. Marotta and Spalla, 2007; von Raumer et al., 2013; Spalla et al., 2014). Permian-Triassic remnants (300-220 Ma) of high-temperature metamorphism, mainly occurring within Austroalpine and Southalpine domains (belonging to Adria plate) and associated with widespread basic to acidic igneous activity testi ed by large gabbro bodies (Fig. 1), indicate an increase of the lithospheric thermal regime (e.g. Lardeaux and Spalla, 1991; Schuster and Stüwe, 2008; Marotta et al., 2009; Spalla et al., 2014) related to asthenospheric upwelling and lithospheric thinning (e.g. Thompson, 1981; Sandiford and Powell, 1986; Beardsmore and Cull, 2001). During Late Triassic-Early Jurassic an important extensional stage leads to the break-up of the Pangaea continental lithosphere and the opening of the Alpine Tethys Ocean, accounted by the occurrence of ophiolitic sequences in the western Alps and Apennines (Fig. 1). The geodynamic signi cance of the Permian-Triassic high temperature and low pressure metamorphic event has been widely debated and recent numerical models suggest an origin consequent to successive lithospheric extension and thinning events eading to the Mesozoic continental rifting (e.g. Marotta and Spalla, 2007; Marotta et al., 2009; Spalla et al., 2014), whereas on the basis of recent paleogeographic reconstructions it has also been interpreted as engaged by the neo-Variscan late-orogenic collapse (e.g. Spiess et al., 2010; von Raumer et al., 2013). In the northern Atlantic region for instance, a sequence of rift basins from Permian to Cretaceous has been described occurring before the opening of the ocean (e.g. Doré and Steward, 2002) making the rifting of the North Atlantic Ocean a long lasting process with several extensional events associated with a migration of eulerian poles as testi ed by the anticlockwise and successive clockwise rotation of superposed rift axes.
Based on this idea, we test whether the lithospheric extension can lead the rifting of the Alpine Tethys by comparing numerical modelling of post-collisional extension and successive rifting and oceanization with Permian-Triassic to Jurassic natural data from the Alps and northern Apennines (Fig. 1). In particular, we focus our attention on the thermal state of the pre-rifting (Permian-Triassic in age) lithosphere in order to explore if the opening of the Alpine Tethys started on a stable continental lithosphere or rather developed on a thermally perturbed one. We here discuss the results obtained for two subsequent numerical models that simulate the evolution of the European lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys. The rst model accounts for the evolution of the crustal lithosphere after the Variscan subduction and collision (300 Ma) up to 220 Ma (Marotta et al., 2009). The second model accounts for the rifting of the continental lithosphere from 220 Ma up to reach the crustal breakup and the formation of the oceanic crust (Marotta et al., 2016). For both models different initial geodynamic con gurations have been tested and we compare the results with natural data of Permian-Triassic metamorphic rocks and Jurassic gabbros and peridotites (Fig. 1), in order to evaluate which con guration best matches the observations. Natural data belong to different structural Alpine domains. Continental rocks are collected from Helvetic and Penninic domains (European paleomargin) and from Austroalpine and Southalpine domains (Adriatic paleomargin) and oceanic rocks are collected from Alpine and Apennine ophiolites (Fig. 1). The comparison is made in terms of contemporaneous agreement to lithology, pressure and temperature values, and ages. The differences between model predictions and natural P-T-age data are synthesized in Fig. 2, where the ages estimate for the rocks are shown using light grey bars for radiometric ages and dark grey for geologically determined ages.
For the rst model we compare the results of two different con gurations. The rst one is characterized by a purely gravitational evolution of the lithosphere in order to simulate a late- orogenic collapse. The second con guration instead, is characterized by a forced extension of the lithosphere of 2 cm/yr. With respect to the purely gravitational simulation, for which the t between predictions and observations is obtained for few data only (Fig. 2), the forced extension simulation agrees well with all collected natural data (Fig. 2). The most peculiar character of the Permian–Triassic igneous activity is the widespread emplacement of gabbro stocks at the base of the crust and the occurrence of basaltic products in the volcanics. Therefore, we verify whether the P-T conditions predicted for the lithospheric and asthenospheric mantle by different con gurations cross the solidus of peridotite. Although predictions from all con gurations satisfy the thermal state for mantle partial melting, the latter is attained at 75 km depth for the purely gravitational con guration and at 50 km depth for the simulation with forced extension. Basaltic melt production is thus compatible with all the simulated tectonic settings but, to allow the partial melting of the continental crust, the thermal state must be similar to that suggested by simulation with forced extension. The nal thermo-mechanical setting is very different between the two con gurations. In the purely gravitational simulation both the crustal thickness and the lithospheric thermal state are similar to the initial conditions, while in the forced extension simulation a strong lithospheric thinning occurs together with a hot thermal state.
The second model simulates the extension of the continental lithosphere up to reach the crustal breakup and the formation of the oceanic crust. The model also includes the hydration of the uprising mantle peridotite and the extension rate is constant and xed to 1.25 cm/yr on the both sides of the domain (total extension rate of 2.5 cm/yr). Accounting for two different thermal con gurations of the lithosphere allows to constrain two different pre-rifting settings of the Alpine lithosphere (hot and cold simulations with 1600 K isotherm at 80 and 220 km depth respectively). The model results in a symmetric rifting of the continental lithosphere and shows the exhumation of a serpentinized lithospheric mantle (ocean-continent transition zone – OCTZ). The onset of the lithospheric thinning strongly depends on the initial lithospheric thermal state: for a cold and strong lithosphere, the thinning is very rapid (4.4 Ma) with respect to a hot and weak lithosphere (15.4 Ma). Similarly, the occurrence of the crustal breakup is shorter for a cold lithosphere (7.4 Ma) than for a hot lithosphere (approximately 31.4 Ma). For both the chosen initial thermal con gurations of the lithosphere, the exhumation of the serpentinized mantle starts before the oceanic spreading and the mantle partial melting, making the model compatible with a magma-poor rifting, as suggested for the Alpine case (e.g., Manatschal et al., 2015). In the hot con guration the continental crust thickness sensibly decreases during the extension from 30 km to approximately 5 km close to the OCTZ. In the cold model instead, the crustal thickness decreases from 30 km to approximately 20 km. The comparison between the natural data and the model predictions shows a good agreement with all of the oceanic data for both hot and cold con gurations. Taking into account that a hyperextended system has been proposed for the Alpine Tethys rifting (e.g. Manatschal et al., 2015) and a time span of approximately 30-40 Ma is considered between the rst extensional structures related to the rifting (200 Ma, Mohn et al., 2012) and the oceanic gabbros emplacement (170-160 Ma, see review in Marotta et al., 2009, 2016), a rifting developed on thermally perturbed lithosphere better agrees the natural data available in ophiolites. The comparison between Permian-Triassic to Jurassic natural data from the Alps and the northern Apennines and two subsequent numerical models simulating the evolution of the lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys suggests that: i) a forced extension of the lithosphere results in a thermal state that better agrees the Permian-Triassic high temperature event(s) than a solely late-orogenic collapse; ii) a rifting developed on a thermally perturbed lithosphere agrees with a hyperextended con guration of the Alpine Tethys rifting and with the duration of the extension up to the oceanization. These results suggest that the Alpine Tethys rifting and oceanization developed on a lithosphere characterized by a thermo-mechanical con guration consequent to a post-Variscan extension affecting the European realm during Permian and Triassic. Therefore, a long lasting period of continuous active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts (300 Ma, Fig. 3a), followed by the Permian-Triassic thermal peak highlighted by HT-LP metamorphism and gabbros emplacement (Fig. 3b), and ending with the crustal breakup and the formation of the Alpine Tethys ocean (170-160 Ma, Fig. 3c). This process could be characterized by alternated period of active extension and stasis, as proposed for the Northern Atlantic rifting or as envisaged for the Ivrea-Verbano Zone on the basis of three metamorphic ages (Permian, Triassic and Jurassic; Langone and Tiepolo, 2015). In order to explore this issue a continuous and polycyclic numerical model is necessary to record the thermo-mechanical inheritance of different events during the entire extensional process, and use ages and P-T-t paths of natural data as constraints
Numerical modelling of an ocean/continent subduction and comparison with Variscan orogeny real data
No full textThe effects of the viscous heating and the hydration of the mantle wedge on the continental crust recycling during the evolution of an ocean/continent subduction system are analysed by using a 2D finite element thermo-mechanical model. The dehydration of the oceanic slab, and the consequent hydration of the mantle wedge, is accomplished by lawsonite breakdown (Roda et al., 2010; Roda et al., 2011). The model shows the activation of convective cells in the mantle wedge that determine the recycling of subducted continental crust. Moreover, with respect to Marotta and Spalla (2007), in which the hydration of the mantle wedge was not taken into account, much more correspondences between P-T predictions and the natural P-T estimates of the Alpine Variscan metamorphism are obtained
Autovalutazione di spalla secondo Dawson nei pazienti con sindrome d’attrito subacromiale o rottura della cuffia. Variazione nei 2 sessi del punteggio e riproducibilità a medio termine.
No full textDei 48 pz (27 F e 21 M), 28 (58.3%) e 20 (41.7%) avevano, rispettivamente, una sindrome
d'attrito subacromiale e RCR. L'eta media dei 28 pz con attrito (12 M e 16 F) e dei 20 con rottura (9 M e 11
F) b stata, rispettivamente di 55 e 65. Tale differenza h risultata statisticamente significativa (p<0.001). 11
punteggio medio ottenuto dai pz con attrito 6 stato 35.8 (maschi 30 e femmine 40), mentre quello realizzato dai
pz con rottura tendinea t stato 41.4 (maschi 39 e femmine 43). Per ciascuno dei precedenti valori il t-Test e
risultato statisticamente significative (p<0.01). Le femmine hanno totalizzato in assoluto punteggi piu elevati
(p<0.01). Dei 27 pz ai quali era stato ripetuto il questionario, 13 erano maschi e 14 femmine. La differenza
media di punteggio tra la prima e la seconda somministrazione 6 stata di 3.8 punti (p>0.05; coefficiente di
correlazione di Pearson=0,8). Non sono emerse associazioni statisticamente rilevanti tra classe lavorativa e,
rispettivamente, tipo di patologia e punteggio del questionario (p>0.05).
Discussione. Dati della letteratura indicano che il questionario di Dawson 6 attendibile nel determinare il
livello di intensità del dolore, riproducibile e sensibile. Dal nostro studio 6 emerso che il punteggio del
questionario non si 6 modificato sostanzialmente a 3 settimane di distanza dalla prima compilazione. Tale dato
indica che a medio temine la riproducibilità del questionario b conservata e che l’intensità del dolore, nei pz
attrito subacromiale e RCR, non si modifica. I pz con RCR hanno, mediamente, un dolore di spalla piu
intenso di quelli con sindrome d'attrito. Inoltre, dallo studio emerge come il punteggio medio di Dawson, nelle
Patologie considerate, sia più elevato nelle femmine. Ci6 potrebbe dipendere da una maggiore apprensione nei
Confronti del dolore o al maggiore riferimento, delle domande del questionario, ad attività domestiche
Prograde lawsonite during the flow of continental crust in the Alpine subduction : Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps)
No full textDetailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic re-equilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp0 + Cpx0). The prograde associations begin with S1a marked by AmpI + ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S1c AmpII + ZoI + GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI + Lws + GrtII + AmpII) within lawsonite-bearing eclogites, while S1e is associated with the break-down of lawsonite, producing the association OmpI + Ky + ZoII + GrtII + AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII + ZoI + GrtII + OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII + GrtII + Pg + ZoII) and S2b (Chl + AmpIII + Pg + Ab). The comparison between the reconstructed evolution of the Ivozio Complex and P-T paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction
Corrigendum to "Prograde lawsonite during the flow of continental crust in the Alpine subduction: Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps)" [J. Struct. Geol. 33 (2011) 381-398]
No full textDeformation vs. metamorphic re-equilibration heterogeneities in polymetamorphic rocks : a key to infer quality P-T-d-t path
No full textThe interaction between fabric gradients and reaction rate as a tool for individuating volumes carrying the longest «rock memory» is discussed through some examples from continental units of the Alpine chain. Here quality P-T-d-t paths have been inferred using a sampling strategy based on reconstruction of the metamorphic evolution, supported by a regionally valid deformation history and on the choice of sites for investigations on compositional variations, where mineral growth and sequences of overprinting fabrics are known. The examples show that correlation between degree of fabric evolution and progress of metamorphic transformation is positive and influence of strain partitioning on tectono-thermal rock memory must be taken into account during P-T-d-t reconstruction to avoid errors in determining the sequence of P-T re-equilibration steps and to obtain clustered P-T estimates relative to each step
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