117,343 research outputs found
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. Battiston P., Benciolini L., Dal Piaz G. V., De Vecchi G., Marchi G., Martin S., Polino R. & Tartarotti P. - (1984) - Mem. Soc. Geol. It. 29, 209-232. Benciolini L., Martin S. & Tartarotti P. - (1984) - Mem. Soc. Geol. Ital. 29, 127-151. Spalla M. I., De Maria L., Gosso G., Miletto M. & Pognante U. - (1983) - Mem. Soc. Geol. Ital. 26, 499-514
Thermobarometry of phengite-bearing eclogites in the Glacier-Rafray Austroalpine unit, Italian Western Alps. Looking for the geothermal gradient of a fossil accretionary prism.
No full textBrecce serpentinitiche nel massiccio ultrabasico del Monte Avic (Falda Ofiolitica Piemontese): possibili evidenze di erosione sottomarina
No full textIl vino più alto d'Europa": l'influenza della geologia e del clima nella coltura della vite in Valle d'Aosta
No full text«The highest wine of Europe»: influence of geology and climate on grape growing in the Aosta valley.
Climate, quaternary dynamics, vine cultivars and wine production contribute to define three different physiographic zones in the Aosta Valley. The Zone A (fig. 2), a NW-SE trending valley at the foot of the Mt. Bianco, is characteried by cold humid climate, with relatively short summer (fig. 3). fluvial and gravitational dynamics. The «Blanc de Morgex et de La Salle» wine from «Prié Blanc» grapes as well as the «Enfer d’Arvier» from «Petiti Rouge» grapes are here produced. Prié Blanc vines are cultivated on the better exposed (SW) alluvial and detrital fans where soil is characterized by cm-sized skeleton of schists and carbonate debris (figs. 4a, 4b e 5a). Petit Rouge vines are cultivated on a big landslide of late glacial age. The soil is characterized by chaotic m- to decameter-sized blocks with sandy matrix (figs. 4c and 5b). The Zone B is characterized by dry climate, Pleistocenic glacial and gravitative deposits. The Torrette and Chambave wines from «Petit Rouge» grapes, and the «Nus» from «Vien de Nus» grapes are obtained. Vines are cultivated on Pleistocenic tillites (figs. 4d and 5c). The Zone C is characterized by temperate humid climate, eluvial and gravitative dynamics. The «Arnad-Monjovet» and «Donnas» wines are here produced from Nebbiolo grapes. The vines are cultivated on steep terraced slope with eluvial deposits, thin humus-rich soil wich skeleton is characterized by cm-sized macaschists
Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)
No full textThe eclogite facies Zermatt-Saas ophiolite in the Western Alps includes a composite chaotic unit exposed in the Lake Miserin area, in the southern Aosta Valley region. The chaotic unit is characterized by a block-in-matrix texture consisting of ultramafic clasts and blocks embedded within a carbonate matrix. This unit overlies massive serpentinite and ophicarbonate rocks and is unconformably overlain by layered calcschist. Despite the effects of subduction and collision-related deformation and metamorphism, the internal stratigraphy and architecture of the chaotic unit are recognizable and are attributed to different types of mass transport processes in the Jurassic Ligurian-Piedmont Ocean. This finding represents an exceptional record of the preorogenic history of the Alpine ophiolites, marked by different pulses of extensional tectonics responsible for the rough seafloor topography characterized by structural highs exposed to submarine erosion. The Jurassic tectonostratigraphic setting envisioned is comparable to that observed in present-day magma-poor slow- and ultraslow-spreading ridges, characterized by mantle exposure along fault scarps that trigger mass transport deposits and turbiditic sedimentation. Our preorogenic reconstruction is significant in an eclogitized collisional orogenic belt in which chaotic rock units may be confused with the exclusive product of subduction-related tectonics, thus obscuring the record of an important preorogenic history. © 2017 Geological Society of America
Serpentinite mélanges in the Western Alps metaophiolites and their significance in detecting the primary physiography of oceanic mantle-sediment interface.
No full textSeveral orogenic belts around the world as the Alpine-Hymalayan, the circum-Mediterranean, and the Appalachians contain melanges that are key records of the tectono-stratigraphic evolution of orogens. The Alpine metaophiolites constitute the sutured Jurassic Tethys Ocean interposed between the European and African plates. Here, serpentinites are often directly covered by metasediments. In the Mount Avic serpentinite massif (Italian Western Alps), interpreted as a fossil oceanic core complex, the serpentinite-sediment contact is marked by a kilometer-scale and few meters-thick serpentinite melange. Upon massive to veined and brecciated serpentinite, the melange starts with white marble and carbonatic calcschist alternating with decimeters-thick horizons of serpentinite metabreccia and metagraywacke, including isolated metric blocks of serpentinite. The sequence continues with rounded to irregularly-shaped blocks of serpentinite, cm-to several dm in size, randomly distributed within a matrix of foliated impure marbles and calcschist. The serpentinite melange has been severely overprinted by Alpine tectonic deformation and metamorphic recrystallization through subduction-related stretching and boudinage and collision-related folding. Nevertheless, its internal fabric still retains records of a block-in-matrix structure well consistent with mass-transport processes related to an active tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. The products of mass-transport processes and faulting are unconformably sealed by flysch-type calcschists embedding cm-sized clasts of actinolite/tremolite-schists. This scenario has profound implications in the physiography of the Tethys Ocean. A correct reconstruction of products and processes occurred at the mantle-sediment interface is thus crucial for better interpreting the complete orogenic cycle, from intra-oceanic extensional processes to subduction-accretion and exhumation ones, affecting the Alpine ophiolites
Structural evolution and metasomatism of subducted metaophiolites in the Northwestern Alps
No full textA subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context
Fossil mantle-sediments interface recognized in the Western Alps metaophiolites : a key to unravel the accretion mechanism of the Jurassic Tethys ocean
No full textABSTRACT
In the southern Aosta Valley (Italian Northwestern Alps), meta-ophiolites are mainly composed of serpentinized mantle-derived peridotites intruded by gabbros and rodingitic dykes, well exposed in the Mount Avic area, and of smaller amounts of mafic rocks and metatrondhjemite. This rock assemblage recalls the "slow-spreading" lithosphere created at modern mid-ocean ridges. Meta-ophiolites show a dominant early Alpine subduction-related metamorphic imprint under eclogite/blueschist facies conditions, variously retogressed under greenschists facies conditions. In the high Champorcher Valley (SW of Mount Avic) serpentinites are directly covered by a serpentinite mélange followed by flysch-like calcschists with detrital ophiolitic interbeds. Despite the pervasive Alpine tectonic deformation and metamorphic recrystallization through subduction-related stretching and boudinage and collision-related folding, the mélange internal fabric still retains records of a block-in-matrix structure, well consistent with mass-transport processes related to an active oceanic tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. The products of mass-transport processes and faulting are unconformably sealed by flysch-type calcschists embedding cm-sized clasts of actinolite/tremolite-schists interpreted as detrital ophiolitic material. The serpentinite mélange is interpreted as syn-extensional sedimentary rocks produced at the mantle-sediments interface on the Jurassic Tethys ocean floor and subsequently overprinted by subduction zone tectonics
Geochemistry of eclogitised Fe-Ti gabbros from different lithological setting (Aosta valley ophiolites, italian western Alps). Protolith composition and eclogitic paragenis
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