Mélanges are abundant in both accretionary and collisional orogenic belts. Their chaotic, block-in-matrix structure can have different origins: sedimentary mélanges can be overprinted by later metamorphic and deformative events or, conversely, tectonic mélanges can form directly at the plate interface, at different tectonic levels and either in prograde (i.e. during underplating) or retrograde (i.e. during exhumation) conditions.The HP-metaophiolitic Voltri Massif (W Alps, Italy), considered as an exhumed piece of the plate interface of the Alpine orogen, includes various, well-preserved examples of tectonic mélanges at different scales (from m- to km-scale). Here, we investigate a 100 meters-thick tectonic mélange, where blocks of various metamorphic lithologies (e.g. metagabbro, eclogite, serpentinite, calchschist and qtz-micaschist) and sizes (0,1-m- to 10-m scale) are dispersed within an intensely foliated, lithologically heterogeneous matrix made of a mixture among serpentinite-schist, chlorite-actinolite schist and graphitic schist, predominantly equilibrated at grenschist facies conditions.Preliminary field investigations reveal a pronounced strain and metamorphic partitioning between the matrix and the blocks. These latter show internal metamorphic layering, shear zones and extensional veins discordant to the pervasive s-c-fabric and folding that characterize the enclosing matrix. Locally, eclogitic blocks show progressive internal fragmentation (e.g., fracturing/veining) up to pervasive brecciation. Petrographic/microanalytical investigations on the most preserved (Fe-Ti-bearing) metagabbro and metabasalt blocks indicate prograde peak metamorphism either in eclogite (grt + omp + rt ± Na-amp ± ph assemblage) or blueschist-facies (Na-amp + ttn + chl ± ep ± ph assemblage); some eclogites show either a retrograde syn-tectonic stage in blueschist facies or a static greenschist overprint. PT estimates on eclogitic blocks indicate a peak stage at P = 18,6 ± 1,0 Kbar (gnt-ph-cpx geobarometer) and T = 530 ± 10°C. The block-matrix transition is characterized by dm- to cm-thick metasomatic rinds rich in hydrous minerals, such as tremolitic amphiboles, biotite, chlorite and minor titanite, tourmaline, adularia and sulphides. Locally, tensile fractures filled by a polymineralic gouge material with the same mineral composition (±biotite) and syntectonic extensional veins with fibrous amphibole depart from the rinds and intrude the prisitne blocks. Abundant hydrothermal fluid circulation is suggested, among other, by peculiar microstructures, i.e. the growth of chlorite in vermicular form.The block-in-matrix structures and microstructures (shear zones and extensional cracks repeatedly crosscuting eachother) point to the occurrence of a cyclic deformation characterised by episodic switch between brittle and ductile regimes and changes in the rehological properties of blocks and matrix. The occurrence of (i) abundant mélange matrix, (ii) metasomatic rinds digesting blocks with (iii) sets of veins/cracks irradiating inside the intact rocks suggest the key-role played by fluids in the evolution of the Piota River mélange.The evidence recorded in the studied lithologies, such as episodic switch between deformation regimes assisted by transient exceed of the rock tensile strenght by pore fluids overpressure, would permit to better understand the mechanisms controlling slow earthquake generation at shallow plate interface. Morover, this study, combined with studies of other melange occurrences of the Voltri Massif, will help to better understand the complex geodynamic phenomena acting on collisional orogens.
Summary I nno r th eV ic o aL nd, bound yb w B sd Rob rrcharacterized by the Millen Schist (MS), a high strain belt, comprising two superposed elements. The upper element is made up of mafic metavolcanites that have a strong affinity to the Glasgow Volcanics. The underlying element comprises phyllitic schist and minor greywacke that bears a strong resemblance to those of the Robertson Bay arenaceous sequence. The two elements of the MS are in contact along a thrust surface where detailed structural analysis has revealed a consistent top to NE sense of shear. The southwestern boundary between Millen Schist and Bowers Terrane is known as the Leap Year Fault, but the fieldwork indicates that this boundary has transitional features and that the Leap Year Fault cut a pre-existing tectonic structure. Citation: Crispini L., Capponi G., and L. Federico (2007), Tectonics at the Bowers - Robertson Bay Terrane boundary, northern Victoria Land (Antarctica), in Antarctica: a Keystone in a changing World - Online Proceedings of the 10th ISAES, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1047, Extended Abstract 073, 4 p.
Summary Gold mineralization is associated with the paleo-Pacific margin of Gondwana and is related to processes of subduction/accretion and magmatism. Northern Victoria Land (Antarctica) was part of this margin during the Paleozoic, however no occurrence of gold has been reported up to now in the Transantarctic Mountains. Here we describe for the first time gold-bearing quartz veins in northern Victoria Land. The veins are hosted primarily by metabasalts and occur in a brittle-ductile high strain zone with curved geometry and reverse kinematic. Veins are extensional and shear veins; they often have ribbon/banded texture typical of crack and seal processes. Preliminary petrographic investigations point to strong hydrothermal alteration with growth of chlorite + quartz and then of sericite + carbonates + sulphides approaching the vein. Available data are discussed with the aim to understand the origin of the gold deposit and to frame it in the regional tectonic evolution.
Abstract A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals. The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia. The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P – T metamorphic conditions (blueschist-facies). A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input. Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface. We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.
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