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Tectonic significance of different block-in-matrix structures in exhumed convergent plate margins: examples from oceanic and continental HP rocks in Inner Western Alps (Northwest Italy)
Full text linkIn the Inner Western Alps, three different types of block-in-matrix structures (BIMs) formed sequentially through time at a convergent plate margin. These show the superposition of progressive deformation from (i) subduction to eclogite-facies depths, (ii) collision, accretion, and exhumation of oceanic crust, represented by the Monviso Meta-ophiolite Complex, to (iii) collision, accretion, and exhumation of the continental Dora Maira units. The Type 1 occurs in the metasedimentary cover of the Dora Maira Unit and consists of a map-scale broken formation with boudinaged 'native' blocks of marble (Early Jurassic) in a calcschist matrix. It results from the tectonic overprinting of exhumation-related folding (D2-stage) on an earlier subduction-related dismembered succession (D1-stage). Type 1 also includes 'non-mappable' BIMs with 'exotic' blocks, resulting from the gravitational collapse of the Triassic carbonate platform of European Continental Margin, triggered by the Early Jurassic rifting. In the Monviso Meta-ophiolite Complex, Types 2 and 3 represent tectonically induced broken and dismembered formations, respectively. They differ from each other in the degree of stratal disruption of primary interbedded horizons of mafic metabreccia (Type 3) and mafic metasandstone (Types 2 and 3) sourced by the Late Jurassic-Early Cretaceous denudation of an oceanic core complex. Dismembered interbeds (Type 2) and isolated blocks were mixed together (Type 3) by the overlap of D2 tectonics and late- to post-exhumation extensional shearing (D3-stage). Development of these types of BIMs may be common in many exhumed convergent plate margins, where severe tectonics and metamorphic recrystallization under high-pressure conditions normally prevent the reconstruction of BIMs or mélange-forming processes. Our findings show that documenting the mode and time of the processes forming BIMs is highly relevant in order to reconstruct the oceanic seafloor morphology and composition of associated stratigraphic successions, and their control in the evolution of those convergent plate margins
Mantle-cover sequence in the Western Alps metaophiolites : a key to recognize remnants of an exhumed Oceanic Core Complex (OCC)
No full textAlpine ophiolites represent the sutured Jurassic Tethys ocean interposed between the European and African plates. They mostly consist, similarly to modern oceanic lithosphere at slow spreading ridges, of mantle peridotites intruded by gabbros sealed by basaltic lavas and sediments (e.g., Lagabrielle & Cannat, 1990). A true sheeted dike complex has never been recognized. In modern slow spreading ridge systems, mantle rocks and gabbros are commonly exposed at the seafloor along detachment shear zones which exhume Oceanic Core Complexes (OCC). In the hangingwall of detachment faults, syn-tectonic sediments bearing mafic/ultramafic breccias unconformably rest atop mantle rocks or ophicarbonate breccias. In the southern Aosta valley, the Mount Avic serpentinite massif is interpreted as a fossil OCC (Fontana et al., 2013), as attested by the occurrence of ophicarbonate breccias (Tartarotti et al., 1998 and refs.). In the nearby Champorcher valley a primary mantle-cover sequence is tentatively reconstructed. Here, massive to mylonic serpentinite pass upward to ophicarbonate breccias covered by carbonate-rich serpentinitic metarenites and by a chaotic rock unit on which calcschists, embedding cm-sized clasts of actinolite/tremolite-schists, rest unconformably. The chaotic rock unit has a block-in-matrix fabric with rounded to irregularly-shaped blocks of serpentinite, cm-to several dm in size, randomly distributed within a matrix of foliated impure marbles and calschists. The internal fabric of this metasedimentary succession is well consistent with mass-transport processes related to an active tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. This mantle-cover sequence may represent part of the Mount Avic OCC documenting, as for Balestro et al. (2015) for the Monviso meta-ophiolite Complex, one of the first examples of remnants of OCCs preserved in exhumed eclogitic ophiolites. The correct interpretation of this mantle-cover sequence is crucial for better interpreting intra-oceanic processes that controlled the Tethys ocean floor evolution during the Jurassic extensional stage
Pre-Alpine extensional tectonics of a peridotite-localized oceanic core complex in the Late Jurassic, high pressure Monviso ophiolite (Western Alps)
Full text linkThe Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed, eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere. The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex, which developed during the initial stages of the tectonic evolution of the Alpine Tethys. The NNWstriking, 20-25-km-long shear zone (Baracun Shear Zone) contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni-Cr concentrations and high Cl contents. Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds, providing a critical age constraint for the timing of its formation. Metabasaltic-metasedimentary rocks in the hanging wall increase in thickness away from the shear zone, characteristic of syn-extensional rock sequences in supradetachment basins. A Lower Cretaceous post-extensional sedimentary sequence unconformably cover the syn-extensional strata, the detachment shear zone, and the ophiolitic footwall, establishing a strong structural evidence for the intraoceanic, seafloor spreading origin of the tectonic fabric of the Monviso ophiolite, prior to the onset of subduction zone tectonics in the Alpine Tethys. The Monviso ophiolite and the Baracun Shear Zone represent a peridotite-localized oceanic core complex, which survived both the subduction and continental collision tectonic stages of the Alpine orogeny. Intraoceanic detachment faults and oceanic core complexes may play a significant role in subduction initiation, and hence their recognition in orogenic belts is an important step in reconstructing the record of ocean basin collapse and closure
Mantle-cover sequence in the Western Alps metaophiolites: a key to recognize remnants of an exhumed Oceanic Core Complex (OCC).
Full text linkA Jurassic oceanic core complex in the high-pressure Monviso ophiolite (western Alps, NW Italy)
Full text linkThe eclogite-facies Monviso ophiolite in the western Alps displays a complex record of Jurassic rift-drift, subduction zone, and Cenozoic collision tectonics in its evolutionary history. Serpentinized lherzolites intruded by 163 ± 2 Ma gabbros are exposed in the footwall of a thick shear zone (Baracun shear zone) and are overlain by basaltic lava flows and synextensional sedimentary rocks in the hanging wall. Mylonitic serpentinites with sheared ophicarbonate veins and talc-and-chlorite schist rocks within the Baracun shear zone represent a rock assemblage that formed from seawater-derived hydrothermal fluids percolating through it during intra-oceanic extensional exhumation. A Lower Cretaceous calc-schist, marble, and quartz-schist metasedimentary assemblage unconformably overlies the footwall and hanging-wall units, representing a postextensional sequence. The Monviso ophiolite, Baracun shear zone, and the associated structures and mineral phases represent core complex formation in an embryonic ocean (i.e., the Ligurian-Piedmont Ocean). The heterogeneous lithostratigraphy and the structural architecture of the Monviso ophiolite documented here are the products of rift-drift processes that were subsequently overprinted by subduction zone tectonics, and they may also be recognized in other (ultra)high-pressure belts worldwid
Fossil mantle-sediments interface recognized in the Western Alps metaophiolites : a key to unravel the accretion mechanism of the Jurassic Tethys ocean
Full text linkIn 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
Representation and transfer of geological knowledge in IT-supported projects
No full textInformation Technologies (IT) have the capability to improve
the clearness and the usefulness of scientific information, and related applications in earth sciences could allow to make geological data more sharable among different users. This paper illustrates
an approach to represent the knowledge paths followed by field geologists involved in assessment and description of complex structuralgeological settings and processes, through the use of specific IT applications. The proposed approach is based on three working steps: i) building of a conceptual map (cMap) that defines the project
approach to the study matter, drives the acquisition of field data and gives rules for GIS representation of interpreted geological features (prefieldwork
stage); ii) capturing of data directly in the
field by means of digital devices, in a way suitable to retrace the acquisition data steps and to separate the observed features from the interpreted ones (fieldwork stage); iii) management of information in relational GIS databases by means of «geological» metadata that could define the «weight» of data and explain the adopted
interpretations (postfieldwork
stage). An application of these working
steps is given in a case study of the stability evaluation of a quarry rock mass. In the example, different conceptualizations and investigation methods are combined so that they are sharable
among field geologists and engineering geologists in order to allow
crucial decision in characterization and modelling of the quarry slope. Besides, ITbased approaches should get retraceability of decisional processes possibl
Remnants of a Late Jurassic detachment shear zone in the Monviso meta-ophiolite complex (Western Alps)
Full text linkThe Monviso meta-ophiolite Complex is a major eclogitized remnant of the Ligurian–Piedmont oceanic lithosphere stacked in the Western Alps, and, despite the overprint of subduction- and collisional-related metamorphism and tectonics, displays exceptional records of its Jurassic rift-drift history.
In the Monviso meta-ophiolite Complex, serpentinized metaperidotite intruded by 163 ± 2 Ma metagabbros are exposed in the footwall of a major shear zone and are overlain by metabasalt and syn-extensional calcschist with ophiolite-derived detrital intercalations in the hanging wall. The shear zone consists of mylonitic serpentinite, sheared meta-ophicarbonate and talc-and-chlorite schist, representing a rock assemblage originally formed as a result of rock-fluid interactions between gabbros, serpentinite and seawater-derived hydrothermal fluids along an oceanic core complex–related detachment fault.
A Lower Cretaceous calcschist, marble, and quartz-schist metasedimentary assemblage unconformably overlies the footwall and hanging-wall units, representing a post-extensional sequence that do not display any evidence of deformation and metasomatic processes, as would be expected if shearing occurred during subduction or collisional Alpine-related stages.
The Monviso meta-ophiolite Complex represents an ancient oceanic core complex formed in an embryonic ocean (i.e., the Ligurian-Piedmont Ocean), and its heterogeneous lithostratigraphy and structural architecture may be a model for recognizing products of rift-drift processes in other (ultra)high-pressure belts worldwide
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
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