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Tectonometamorphic evolution of the Massa Unit in Punta Bianca area (Northern Apennines, Italy)
No full textThe Northern Apennines (Italy) is characterized by the occurrence of low-grade metamorphic units derived from the Adria plate margin, which are the lowermost tectonic units of the apenninic nappe-pile. They are commonly referred to as the Tuscan Metamorphic Units and comprise the Apuane and Massa units. These units, including basement rocks of Hercynian age and a Mesozoic-Tertiary sequence, were affected by a polyphase tectonic history, during which they reached peak metamorphic conditions and were subsequently exhumed to the surface. New quantitative data about the structural and metamorphic evolution of the Massa Unit cropping out in the Punta Bianca area will be presented. Three main ductile tectonic phases, followed by two brittle and brittle-ductile tectonic events have been recognized. The first tectonic phase (D1) was related to continental collision and northeastward thrusting. The second tectonic phase (D2) was linked to the main exhumation stage of the metamorphic rocks, achieved during syn-orogenic extension. The later D3 phase was related to a renewal of the compressive tectonics. In addition an episode of ductile to brittle NE-thrusting (D4) is documented before the main event of high-angle normal faulting (D5). Finite strain analyses were performed on metabreccias and metabasalts with deformed strain-markers (clasts and vesicles), supporting that the D2 phase occurred under general flattening. A study of progressive deformation was performed on vein sets developed in different moments during the tectonic history, allowing to reconstruct a more detailed tectonic evolution and to understand the complex relationships between structural elements formed during different moments of the tectonometamorphic evolution of the area. Microstructural analyses were combined with petrographic studies to reconstruct e P-T path. Synkinematic recrystallization of white micas + chlorite + quartz + albite ± epidote in metapelites has been recognized along both S1 and S2 foliations developed parallel to F1 and F2 folds respectively.
Electron microprobe X-Ray maps and chemical points highlighted different compositions of white micas and chlorite recristallized along both S1 and S2 foliations. Temperature estimations constrained D1 phase at 330 ± 10 °C and D2 phase at 350 ± 5 °C while decreasing pressure values have been detected from D1 to D2 tectonic phase
Crystallographic orientation and grain size data obtained by Electron Back Scatter Diffraction (EBSD) on quartz analysed in mylonitic quartzite from the Island of Elba (Italy)
Get PDFRaw Electron Back Scatter Diffraction (EBSD) data on deformed quartz from a mylonitic quartzite sample of the Calamita Schists (Island of Elba, Italy) is available at https://doi.org/10.17632/8c937t6zs4.32. The investigated sample (IESP3SP78) was collected in quartz-rich outcrops exposed at the Praticciolo Cape and was used to realize an oriented thin section (cut parallel to lineation and perpendicular to foliation). Preliminary investigations were carried out by transmitted-light and scanning electron microscopy (SEM), in order to select key areas for EBSD analysis. EBSD mapping was performed on selected areas of deformed quartz, which was the only phase indexed and were processed to derive orientation maps, pole figures, inverse pole figures, misorientation axis distribution in sample and crystal coordinates. While the processed data is available on the original research article (“Fluid-assisted Strain Localization in Quartz at the Brittle/Ductile Transition”; https://doi.org/10.1029/2019GC008270), this contribution is devoted to supply the unprocessed EBSD data, together with a methodological description, aimed to allow the reproduction of the processed dataset. A brief statistical description of the investigated EBSD maps is also available. This data is valuable because it offers grain size and orientation analysis of deformed quartz investigated in a natural study case and the present publication makes it accessible to those working on naturally and experimentally deformed quartz
The brittle/ductile transition at upper crustal level: geometry, strain partitioning and fluid circulation. The case study of the Calamita Unit (Elba Island, Northern Apennines, Italy)
No full textTectonometamorphic evolution of the Massa Unit in the Punta Bianca area (Northern Apennines, Italy)
No full textAbstract
The Northern Apennine (Italy) is characterized by the occurrence of low-grade metamorphic units derived from the Adria plate margin, which are the lowermost tectonic units of the apenninic nappe-pile. They are commonly referred to as the Tuscan Metamorphic Units and they comprise the Apuane Unit and the Massa Unit. These units, including basement rocks of Hercynian age and a Mesozoic-Tertiary sequence, were affected by a polyphase history of apenninic age, during which they reached their peak metamorphic conditions and were subsequently exhumed to the surface. This thesis provides new quantitative data about the structural evolution of the Massa Unit cropping out in the Punta Bianca area, derived from field mapping, progressive deformation studies and finite strain analysis and about the metamorphic conditions experienced by this unit. The low-grade rocks of Punta Bianca were affected by peak metamorphism characterized by white mica + chlorite + quartz + albite ± epidote assemblage in metapelites and new growth of Wm + Chl + Act + Ep in metabasalts. Structural-Geological mapping allowed recognizing three main ductile tectonic phases, followed by two brittle and brittle-ductile tectonic events. The first tectonic phase (D1) was related to continental collision and northeastward thrusting. The second tectonic phase (D2) was linked to the main exhumation stage of the metamorphic rocks, achieved during syn-orogenic extension. The later D3 phase was related to a renewal of the compressive tectonics. In addition an episode of brittle NE-thrusting (D4) is documented before the main event of high-angle normal faulting (D5). Finite strain analyses were performed on metabreccias and metabasalts with deformed strain-markers (clasts and vesicles), demonstrating that the D2 phase occurred under general flattening. In addition a study of progressive deformation was performed on vein sets developed in different moments during the tectonic history, allowing to reconstruct a more detailed tectonic evolution and to understand the complex relationships between structural elements formed during different moments of the tectonometamorphic evolution of the area. Furthermore samples were investigated at the microscale combining petrographic and structural approach. Preliminary Scanning Electron Microscope (SEM) studies were also performed on some samples. Observations at the microscale allowed to select a key area (230x450 μm) within a sample (CH8), which was chemically mapped (X-Ray maps) using the Electron Microprobe (EMP) of the University of Milan. This allowed recognizing two generations of white mica + chlorite, the first one growing on the S1 foliation and the second one on the S2 foliation. Point analyses were also performed on the same area on white micas and chlorites of these two generations, thus showing that they are chemically different. The application of chlorite geothermometry (Cathelineau, 1988; Jowett, 1991; Xie et al., 1997) allowed to constrain D1 phase at 330 ± 10 °C and D2 phase at 350 ± 5 °C. These data were combined with mica geobarometers (Massonne & Schreyer, 1987; Massonne & Szpurka, 1997), which give consistently lower P for sin-D2 white mica in respect to the earlier white mica (S1). The corollary of each estimate is briefly discussed in the frame of the geodynamic evolution of the Northern Apennines, where during the D1 phase peak of pressure was reached whereas the D2 was related to the first stages of exhumation and thermal peak.
Riassunto
L’Appennino Settentrionale presenta una struttura a falde in cui le Unità Toscane Metamorfiche (TMU), con metamorfismo di basso grado, occupano la posizione strutturalmente più profonda. Queste unità, che comprendono al loro interno un basamento ercinico e rocce di età Meso-Cenozoica, sono state interessate da un’evoluzione polifasica di età appenninica, durante la quale hanno raggiunto le condizioni di picco del metamorfismo e successivamente sono state esumate. Durante questa tesi è stato affrontato lo studio dell’evoluzione tettono-metamorfica dell’Unità di Massa che affiora nella zona di Punta Bianca, situata nel settore più interno dell’Appennino Settentrionale (La Spezia, Italia). Il rilevamento geologico e l’analisi mesostrutturale sono stati affiancati dall’analisi della deformazione progressiva di affioramenti chiave e dallo studio della deformazione finita. Sono stati riconosciuti tre principali eventi deformativi duttili seguiti da due eventi tettonici fragili e fragili-duttili. La prima fase (D1) è stata messa in relazione alla fase principale di impilamento delle unità tettoniche. La seconda fase (D2) è stata interpretata come legata al principale evento di esumazione dell’unità di Massa verso la superficie, avvenuta in un contesto estensionale sin-collisionale. Infine la terza fase (D3) è stata interpretata come legata ad una ripresa della compressione con sviluppo di pieghe dritte. Segue un evento deformativo fragile-duttile con sviluppo di thrusts vergenti verso NE (D4), precedente alla formazione di faglie dirette fragili ad alto angolo (D5). L’analisi della deformazione finita è stata eseguita su metabrecce e metabasalti contenenti strain-markers (clasti e variole rispettivamente) deformati essenzialmente durante le fasi D1 e D2, dimostrando condizioni di flattening generale, associate ad un ellissoide della deformazione oblato. Inoltre, lo studio della deformazione progressiva è stato attuato su diversi set di vene formatisi in differenti momenti della storia deformativa, permettendo quindi di riconoscere un’evoluzione tettonica più dettagliata. L’analisi microstrutturale è stata finalizzata allo studio delle principali deformazioni duttili e le relative foliazioni. Per studiare l’evoluzione metamorfica dell’Unità in esame, su campioni selezionati sono state svolte analisi preliminari al microscopio elettronico a scansione (SEM), finalizzate alla selezione di aree chiave che presentassero relazioni microstrutturali tra le diverse fasi deformative molto chiare. Tali osservazioni hanno permesso di selezionare un’area rappresentativa (230x450 μm) in un campione (CH8), che è stata caratterizzata chimicamente (mappe elementari a Raggi X e punti analisi) mediante la microsonda elettronica situata presso l’Università di Milano. Questo ha permesso di riconoscere due diverse generazioni di associazioni di mica bianca + clorite, sincinematiche rispetto sia alla foliazione S1 che alla foliazione S2 ma con diversa composizione. L’applicazione di diverse calibrazioni di geotermometri su clorite (Cathelineau, 1988; Jowett, 1991; Xie et al., 1997) ha permesso di stimare valori di temperatura di circa 330±10 °C durante la fase D1 e valori di circa 350±5 °C durante la fase D2. Sono stati inoltre applicati alcuni geobarometri basati sul contenuto in Si4+ della mica chiara (Massonne & Schreyer, 1987; Massonne & Szpurka, 1997), che hanno fornito valori di pressione variabili comprese tra 0,6-0,8 GPa per la fase D1 e 0,5-0,6 GPa per la fase D2, indicando una generale diminuzione di pressione passando dalla prima alla seconda fase deformativa. I dati P-T ottenuti, per quanto da intendersi come preliminari, supportano un modello dove la fase deformativa D2 è legata al principale evento esumativo delle TMU di Punta Bianca
Evolution of shear zones through the brittle-ductile transition: The Calamita Schists (Elba Island, Italy)
Get PDFA network of shear zones that evolved through the brittle-ductile transition is exposed in the Calamita Schists, Elba Island, Italy. The shear zones formed during Late Miocene contractional deformation coeval with high grade contact metamorphism (∼650 °C) related to the emplacement of plutonic rocks at shallow crustal levels (∼7–10 Km). An early stage high metamorphic grade foliation was overprinted by mylonitic deformation that progressively localized on low-metamorphic grade shear bands producing S-C mylonites during cooling of contact aureole. Localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning between ‘hard’ high grade relics and ‘soft’ shear bands. Softening of shear bands occurred likely due to fluid influx and retrograde growth of fine-grained phyllosilicates. The interconnection of anastomosing shear bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing mylonites with a composite mylonitic foliation and C′ shear bands. An estimate of the vorticity number Wk of the flow of ∼0.3–0.5 was obtained from the orientation of C′ shear bands measured at the meso- and thin section-scale. Close to the brittle-ductile transition, the growth of soft phyllosilicates allowed C′ shear bands to act as precursory structures to brittle deformation localized into an array of low-angle faults and shear fractures
Heterogeneous brittle-ductile deformation at shallow crustal levels under high thermal conditions: The case of a synkinematic contact aureole in the inner northern Apennines, southeastern Elba Island, Italy
Get PDFWe present an example of interaction between magmatism and tectonics at shallow crustal levels. In the Late
Miocene the metamorphic units of the eastern Elba Island (northern Apennines) were intruded at very shallow
crustal levels by a large pluton (> 60 km2) with the development of an hectometre-sized contact aureole defined
by growth of low-pressure/high-temperature mineral assemblages (Pmax < 0.2 GPa, Tmax ~ 650 °C). Structural
data show that the contact aureole is associated with a km-sized antiform of the foliation and by several metreto
decametre-thick high-strain domains consisting of strongly foliated rocks containing synkinematic HT/LP
mineral assemblages and ductile shear zones of variable thickness. These shear zones are characterized by a
mylonitic foliation variably overprinted by cataclasis. Quartz microfabrics indicate that the dynamic crystallization
processes progressively changed from grain boundary migration, associated with the thermal peak of
contact metamorphism, to subgrain rotation and bulging recrystallization, the latter mostly associated with the
cataclastic overprint. These transitions of recrystallization mechanisms in quartz are related to a progressive
decrease of temperature during deformation. Deformation accompanied the development and cooling of the
contact aureole, which recorded the switch from high temperature ductile to low temperature brittle conditions.
The geometry of the studied deformation structures is consistent with the constraints of the regional tectonic
evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Deformation and Material Transfer in a Fossil Subduction Channel: Evidence From the Island of Elba (Italy)
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