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Structural and petrographic map of the southwestern margin of the Biella pluton, Western Alps
No full textStructural and petrographic map of the north-eastern margin of the Oligocene Traversella pluton (Internal Western Alps)
No full textStructural and petrographic analysis at the north-eastern margin of the Oligocene Traversella pluton (Internal Western Alps, Italy)
Full text linkIn this work a new form surface map at 1:5000 scale of the
north-eastern margin of the Oligocene Traversella pluton, synthesised
from a field structural study assisted by optical petrography, is
presented. Relative chronology of superposed foliations and effects
of progress of contact metamorphic reactions in the contact aureole
are shown on the structural map for each of the country rock-types
of the Sesia-Lanzo Zone. The structural and metamorphic evolution
was reconstructed across the north-eastern margin of the pluton by
separating the pre- and the post-intrusive tectonic history and defining
the contact aureole outline. Contact metamorphic assemblages
and thermobarometric estimates indicate that this intrusive body
emplaced at shallow structural levels (contact metamorphism peak
conditions: T < 750°C; P < 0.2 GPa
Governance and knowledge, towards an innovation in sustainability oriented policies?
No full textThe aim of the paper is to identify and discuss the innovations in the relationship between governance and knowledge triggered by the introduction of SEA in the Province of Milan Master Plan 2003-2007. This general planning instrument, defined and approved by the Provincial Government in 2003, had to be significantly revised after the issue of the new Regional Planning Law (L.R. 12/2005, March 2005). The new Planning Law required, among other aspects, the implementation of SEA according to EU Directive 42/2001. The characterizing elements in this SEA process, compared to the planning procedures traditionally in place before, are an extensive stakeholder involvement process and a stricter relationship between the production of sound environmental research and planning decisions. Both elements may be able to introduce significant innovations in the planning procedures, and, in order to fully understand the real extent of such innovations it is important to precisely review some significant steps in the decision making process. Finally, the paper will consider how the inclusion of new governance practices in parallel to more traditional hierarchical structures may allow to re-frame the local planning policy, and to better integrate sustainability concerns and local knowledge into the policy making and consultation procedures, thus enhancing the effectiveness and legitimacy of the whole process
Permian thinning of the Southalpine Variscan crust : insights by metamorphic pebbles and cobbles from post-orogenic conglomerates
No full textTesting the thermal state of Biella pluton country rocks via numerical model of magma cooling
Full text linkThe Biella pluton (30-31 Ma) is a Periadriatic intrusive of the Alps, emplaced in the internal part of the HP continental Sesia Lanzo Zone (Berger et al., 2012). Pluton cooling involved contact metamorphism overprinting eclogitic to greenschist facies assemblages in country rocks. Multiscale structural analysis combined with thermos-barometric estimates suggest that magma intrusion took place at shallow crustal levels (Zanoni et al., 2008; Zanoni et al., 2010; Zanoni, 2015).
The emplacement history indicates that the intrusion of Biella body occurred when the Sesia Lanzo Zone had almost completed its exhumation under low thermal state consistent with an ongoing subduction (e.g. Roda et al., 2012 and refs therein). The mechanism proposed for triggering the Periadriatic magmatism are either subduction (Tiepolo et al., 2014) or slab break-off (e.g. Von Blankenburg & Davies, 1995). However recent numerical modeling (Freeburn et al., 2015) suggests that a slab break-off related melting does not result in the widespread magmatism characterizing many collisional belts.
In order to unravel the thermal state of the Biella stock country rocks at the intrusion time we develop a preliminary 2D thermal model of pluton cooling testing four different thermal gradients of the host rocks. We compare the extent and the variation in the thermal peak of the contact aureole recorded in the country rock (Fig. 1) with the results of the numerical simulations.
The computed thermal boundary of the aureole is between 300 and 400°C and is wider than the mapped one due to the difficulty to distinguish between contact and greenschists regional metamorphism at such low temperatures. The best fit occurs for the simulation accounting for temperatures between 427 and 527°C (700 and 800 K) at 8 km depth (Fig. 2), assuming conduction as the only effective heat transfer mechanism. This indicates that the emplacement occurred under thermal gradients between 55 and 65°C/km that would exclude a syn-subductive magmatism. The suggested thermal gradient for the country rock of the Biella pluton would represent the constraint for testing different scenarios responsible for the generation of Periadriatic magmatism
Structure and petrography of the southwestern margin of the Biella pluton, Western Alps
Full text link<p>This work presents a new form surface map of the southwestern margin of the Biella pluton at the scale 1:10,000. The Biella pluton is part of the Periadriatic intrusives of the Alps and is emplaced in the continental Sesia-Lanzo Zone of the western Austroalpine domain. The country rocks consist of metapelites and different metagranitoids. Pre-intrusive HP (high pressure) mineral assemblages are dominant in country rocks with the exception of metagranitoids dominated by HT (high temperature) assemblages. The plutonic rocks consist of monzonite with minor syenite. The ductile polyphasic deformation of the country rocks predates the pluton emplacement, with the exception of syn-intrusive folding and shearing, which were recorded in the country rocks of the deeper part of the pluton. Syn-intrusive deformation may be represented by brittle structures that bear mineralisation; however, the majority of the brittle deformation postdates the final emplacement of the pluton. The orientation of the pervasive foliation in the country rocks controls the space available for magma intrusion and possible magmatic flow during emplacement. The inferred diffusion of the thermal aureole in the country rocks is based on the variation in contact metamorphic minerals, which is described by microscopic analysis. The extent of the aureole appears to be controlled by the type of dominant mineral assemblages, rock permeability, and the orientation of the regional foliation in country rocks with respect to the pluton margin. The multiscale structural analysis reveals that the Biella pluton emplaced at a depth as shallow as the greenschist facies conditions or shallower.</p
From G-FORS to EPIGOV: which governance modes for environmental policy integration? The case of Strategic Environmental Assessment in spatial planning
No full textProceedings di convegn
The thermal state of Biella pluton country rocks as a tool to unravel the late orogenic tectonics of the Western Alps
Full text linkCrustal level and timing of late-orogenic plutons represent powerful tools for constraining the late exhumation history of tectonic units, within collisional belts. This work aims to contribute to this topic by quantitatively determining the thermal state of Biella Periadriatic pluton country rocks during its emplacement within the continental HP metamorphic rocks of the Sesia-Lanzo Zone (SLZ), in the western Austroalpine domain (Zanoni et al., 2008, 2010; Zanoni, 2016 and refs. therein). During the Alpine subduction and exhumation, the Sesia-Lanzo Zone records a polyphasic tectono-metamorphic evolution. Pluton cooling involved contact metamorphism overprinting eclogitic to greenschist facies assemblages in the country rocks. On the basis of the amount and type of contact metamorphic assemblages the aureole extent was mapped and the variation of recorded thermal peak validated by comparison with numerical modelling of the heat transfer during magma crystallisation. Thus, this work aims constraining the late exhumation history of the internal western Alps by determining the crustal level of pluton emplacement and the thermal gradient of its country rocks at the time of intrusion.
The Alps developed during subduction and closure of the Mesozoic Tethys and subsequent continental collision over Cretaceous–Oligocene times (e.g. Dal Piaz, 2010; Handy et al., 2010; Spalla et al., 2010). The Alpine convergent system involved the subduction of the European lithosphere underneath the Adria plate. The Periadriatic line is a main crustal break of the Alpine bounding the Southalpine continental crust acting as backstop wall of the orogenic wedge during the convergence (Polino et al., 1990). This lineament (Fig. 1) separates tectonic units that during the Alpine convergence experimented intense deformation and metamorphism (i.e. Austroalpine and Penninic domains) from rocks that recorded only shallow structural level deformation (i.e. Southalpine domain). The Periadriatic line is interpreted as the ascent path for the late collisional Oligocene magmas, which emplaced to form the Periadriatic plutons (Rosenberg, 2004). The Periadriatic plutons are traditionally considered as derived from the Alpine slab breakoff (Von Blanckenburg and Davies, 1995). However recently the Tertiary magmatic rocks south of the Periadriatic line, such as the southern Adamello pluton, Veneto Volcanic province, and dykes in the central Southalpine, are supposed to be generated during Alpine subduction (Tiepolo et al., 2011; Bartoli et al., 2013; Bergomi et al., 2015). The Austroalpine domain of the western Alps consists of continental rocks tectonically sampled by the margin of Adria plate during subduction (e.g. Roda et al., 2012) and is actually divided into two main tectonic units, namely the Sesia-Lanzo Zone (SLZ) and Dent Blanche nappe. Both tectonic units were subducted and exhumed during the Alpine cycle when the oceanic subduction was still active (Spalla et al., 1996; Babist et al., 2006; Roda et al., 2012). Between 60 and 80 Ma the SLZ reached the P-peak during subduction (Bussy et al., 1998; Cenki-Tok et al., 2011), corresponding to eclogite conditions at about 550°C and ≥ 2 GPa (Zucali et al., 2002; Zucali and Spalla, 2011). Eclogite metamorphism is followed by decompression blueschist and greenschist re-equilibrations (Pognante et al., 1980; Zucali et al., 2002), related to later exhumation stages (e.g. Spalla et al., 1991). The shallower levels of the exhumation path took place between 45 and 30 Ma (Inger et al., 1996; Cortiana et al., 1998; Babist et al., 2006; Zanoni et al., 2010). To the east the SLZ is delimited by the Periadriatic line and in its inner part hosts the Biella and Traversella plutons (Zanoni et al., 2008; Zanoni 2010, 2016). The Biella pluton shows concentric zoning with the outer part constituted by monzonite and the inner part by syenite and granite and a calc- alkaline composition (Bigioggero et al., 1994). The pluton has been dated at about 30 Ma (Romer et al., 1996; Berger et al., 2012) and its country rocks belong to the Eclogitic Micaschists Complex (Fig. 2). Along the northeaster and southwester margins of the
Biella pluton country rocks consist of metapelites, meta-aplites and metagranitods with minor metabasites. During the Alpine convergence country rocks recorded up to six ductile deformation stages that predated the intrusion of the pluton. Only locally syn-intrusive folding and faulting are recorded in the country rocks. Generally brittle structures post-date pluton emplacement. The igneous rocks record a magmatic foliation that in place is parallel to the pluton margin. During the emplacement the country rocks recorded different type of contact metamorphism assemblages that vary with the country rock type and the distance from the pluton margin. The contact metamorphism parageneses form ne-grained coronitic structures that overprinted eclogite parageneses and local greenschist parageneses. With the distance from the pluton the amount of contact metamorphism parageneses decrease. Close to the pluton margin the contact metamorphic mineral assemblages are characterised by plagioclase, K-feldspar, cordierite, biotite, spinel, Al-silicate (sillimanite closer and andalusite farer from the pluton), and locally corundum, orthopyroxene and garnet. Locally, up to about 10 m far from the pluton margin, partial melting is recorded. With the distance from the pluton corundum, orthopyroxene, garnet, spinel, and sillimanite disappear. The contact metamorphic minerals are detected up to about 900 m far from the pluton in plain view (Fig. 2). In igneous rocks interstitial amphibole has an Al content compatible with intrusion depth variable between 4 and 7 km. Ti content in amphibole and equilibrium between amphibole and plagioclase are consistent with intrusion temperature between 670 and 720°C (Zanoni et al., 2010). In the country rocks temperature peak, reached during pluton crystallisation, vary between about 700°C at the pluton margin, and 550°C at about 600 m far from the pluton (Zanoni et al., 2010). The comparison of thermal estimates in the country rocks with a 2D conductive thermal model for pluton cooling shows that the best t is for initial temperature in the country rocks between 430 and 530°C at 8 km depth (Fig. 3). That involves a thermal gradient of the country rock at time of intrusion ranging between 55 and 65°C/km. The crustal level of Biella pluton emplacement is as shallow as a few kilometres and this is consistent with magmatic rocks intersecting all ductile structures in the country rocks and with contact metamorphism assemblages replacing greenschist facies assemblages. The computed extent of the aureole for the simulations is wider than the mapped one because the computed thermal boundary of the aureole is between 300 and 400°C. Finally, the thermal gradient of the country rocks at the time of pluton intrusion is suf ciently high to justify the emplacement of Biella pluton during the accomplishment of the break-off of the Alpine slab
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