Strain localization in pseudotachylyte veins at lower crustal conditions

Viscous shearing in the dry and strong lower crust often localizes in pseudotachylyte veins (i.e. quenched molten rocks formed by the frictional heat released during seismic slip), and it has been suggested that brittle (coseismic) grain-size reduction and fluid infiltration in the fractured domains are necessary to weaken the anhydrous granulitic lower crust. However, the deformation mechanisms responsible for the associated strain weakening and viscous shear localization in pseudotachylytes are yet to be explored. This study investigates the deformation microstructures of mylonitized pseudotachylytes in anorthosites from Nus- fjord, northern Norway, where ductile shear zones invariably nucleate in pseudotachylyte veins. Thus, pseudotachy- lytes are weaker than the host rock during superposed ductile deformation. Pristine pseudotachylytes contain microlites of plagioclase, clinopyroxene, amphibole and orthopyroxene, flow structures, and chilled margins. Some pseudotachylytes have lost the pristine microstructure and have recrystallized into a fine-grained ( < 10 \u3bc m) mixture of plagioclase, amphibole, clinopyroxene, biotite, quartz \ub1 K-feldspar \ub1 orthopyroxene. Thus, the fine grain size in the mylonites ( < 20 \u3bc m) is not the product of progressive grain-size reduction with increasing strain, but is an initial characteristic of the shear zone (pseudotachylyte) precursor. The stable mineral assemblage in the mylonitic foliation consists of plagioclase, hornblende, clinopyroxene \ub1 quartz \ub1 biotite \ub1 orthoclase. Geothermobarometry and thermodynamic modelling indicate that pristine pseudotachylytes and their mylonitized equivalents formed at ca. 700 \u30aC and 0.6-0.9 GPa. Diffusion creep and grain boundary sliding were identified as the main deformation mechanisms in the mylonite on the basis of the lack of crystallographic preferred orientations, the high degree of phase mixing, and the nucleation of hornblende in dilatant sites. In contrast with common observations that fluid infiltration is required to trigger viscous deformation, thermody- namic modelling indicates that a limited amount of fluid (0.4 wt%, similar to the bulk fluid content measured in the host rock) is sufficient to stabilize the mineral assemblage in the mylonite. This suggests that cosesimic grain size reduction resulted in fluid redistribution into the fractured domains and not necessarily in fluid infiltration. Recent experiments suggest that very small amount of water (tens of ppm) are effective in facilitating mineral reactions if sufficient porosity in present. Coseismic fracturing and creep cavitation in the mylonitized pseudotachylytes en- hance the porosity of the shear zone and result in nucleation of new phases in dilatant sites. This process keeps the grain size of the polymineralic aggregate in the grain-size sensitive creep field, thereby stabilizing strain localiza- tion in the mylonitized pseudotachylytes. This study highlights that pseudotachylytes caused by brittle faulting can be precursors of viscous, weak shear zones in the dry lower crust, indicating lower crustal earthquakes as agents of rheological change from strong, brittle lower crust, to strong lower crust with embedded fine grained, weak viscous shear zones

The Nusfjord exhumed earthquake source (Lofoten, Norway): deep crustal seismicity driven by bending of the lower plate during continental collision

The origin of earthquakes in the lower crust at depth of 20-40 km, where dominantly ductile deformation is expected, is highly debated. Exhumed networks of lower crustal coeval pseudotachylytes (quenched frictional melt produced during seismic slip) and mylonites (produced during the post- and interseismic viscous creep) provide a snapshot of the earthquake cycle at anomalously deep conditions in the crust. Such natural laboratories offer the opportunity to investigate the origin and the tectonic setting of lower crustal earthquakes.The Nusfjord East shear zone network (Lofoten, northern Norway) represents an exhumed lower crustal earthquake source, where mutually overprinting mylonites and pseudotachylytes record the interplay between coseismic slip and viscous creep (Menegon et al., 2017; Campbell and Menegon, 2019). The network is well exposed over an area of 4 km2 and consists of three main intersecting sets of ductile shear zones ranging in width from 1 cm to 1 m, which commonly nucleate on former pseudotachylyte veins. Mutual crosscutting relationships indicate that the three sets were active at the same time. Amphibole-plagioclase geothermobarometry yields consistent P-T estimates in all three sets (700-750 °C, 0.7-0.8 GPa). The shear zones separate relatively undeformed blocks of anorthosite that contain pristine pseudotachylyte fault veins. These pseudotachylytes link adjacent or intersecting shear zones, and are interpreted as fossil seismogenic faults representing earthquake nucleation as a transient consequence of ongoing, localised aseismic creep along the shear zones (Campbell et al., under review).The coeval activity of the three shear zone sets is consistent with a local extensional setting, with a bulk vertical shortening and a horizontal NNW-SSE extension. This extension direction is subparallel to the convergence direction between Baltica and Laurentia during the Caledonian Orogeny, and with the dominant direction of nappe thrusting in the Scandinavian Caledonides. 40Ar‐39Ar dating of localized upper amphibolite facies shear zones in the Nusfjord area with similar orientation to the Nusfjord East network yielded an age range of 433–413 Ma (Fournier et al., 2014; Steltenpohl et al., 2003), which indicates an origin during the collisional (Scandian) stage of the Caledonian Orogeny.We propose that the Nusfjord East brittle-viscous extensional shear zone network represents the rheological response of the lower crust to the bending of the lower plate during continental collision. (Micro)seismicity in the lower crust in collisional orogens is commonly localized in the lower plate and has extensional focal mechanisms. This has been tentatively correlated with slab rollback and bending of the lower plate (Singer et al., 2014). We interpret the Nusfjord East shear zone network as the geological record of this type of lower crustal seismicity

Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong Lower Crust

The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite,6garnet6K-feldspar. Amphibole-plagioclase geothermobarometry and thermodynamic modeling indicate that pristine and mylonitized pseudotachylytes formed at 650\u20137508C and 0.7\u20130.8 GPa. Thermodynamic modeling indicates that a limited amount of H2O infiltration (0.20\u20130.40 wt. %) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites

Una trayectoria docente comprometida con el cambio social, potenciada por el arte e impulsada por acciones de liderazgo

El propósito de este trabajo es abordar algunos aspectos de la historia de vida de Sandra, una profesora de Artes Plásticas que, iniciando su trayectoria en contextos desfavorecidos y complejos, se fue construyendo como docente comprometida socialmente, con gran arraigo y pertenencia a la escuela pública. Se trata de una trayectoria caracterizada por acciones de inventiva y liderazgo, a través de las cuales la docente logra posicionar un área tradicionalmente relegada en la escuela, hasta convertirse ella misma -una Profesora de Plástica- en la Directora de Escuela Secundaria. Desde ese cargo, aborda hoy el desafío de contribuir a la mejora educativa y hacer posible la nueva Escuela Secundaria para todos. (Párrafo extraído del texto a modo de resumen)Eje 2: Educación artística, investigación y sociedadFacultad de Bellas Arte

Tertiary pegmatite dikes of the Central Alps

The largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E–W direction and 15 km in a N–S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the beryl-columbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N–S to NNE–SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed pegmatites from this area may contain miarolitic pockets. There is no systematic difference in the mineral assemblage between the two structural types of pegmatites. However, the chemistry of pegmatite minerals, especially of garnet, in addition to field data suggests that the dikes of the Codera and Bodengo areas represent two distinct generations of pegmatites. Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time span between 29 and 25 Ma (and possibly as young as 20 Ma), with the youngest dikes postdating the ductile deformations of the Alpine nappes. The present work presents a first comprehensive field description and geochemical – mineralogical characterization of the Alpine pegmatite field of the Central Alps

Una trayectoria docente comprometida con el cambio social, potenciada por el arte e impulsada por acciones de liderazgo

El propósito de este trabajo es abordar algunos aspectos de la historia de vida de Sandra, una profesora de Artes Plásticas que, iniciando su trayectoria en contextos desfavorecidos y complejos, se fue construyendo como docente comprometida socialmente, con gran arraigo y pertenencia a la escuela pública. Se trata de una trayectoria caracterizada por acciones de inventiva y liderazgo, a través de las cuales la docente logra posicionar un área tradicionalmente relegada en la escuela, hasta convertirse ella misma -una Profesora de Plástica- en la Directora de Escuela Secundaria. Desde ese cargo, aborda hoy el desafío de contribuir a la mejora educativa y hacer posible la nueva Escuela Secundaria para todos. (Párrafo extraído del texto a modo de resumen)Eje 2: Educación artística, investigación y sociedadFacultad de Bellas Arte

Una trayectoria docente comprometida con el cambio social, potenciada por el arte e impulsada por acciones de liderazgo

El propósito de este trabajo es abordar algunos aspectos de la historia de vida de Sandra, una profesora de Artes Plásticas que, iniciando su trayectoria en contextos desfavorecidos y complejos, se fue construyendo como docente comprometida socialmente, con gran arraigo y pertenencia a la escuela pública. Se trata de una trayectoria caracterizada por acciones de inventiva y liderazgo, a través de las cuales la docente logra posicionar un área tradicionalmente relegada en la escuela, hasta convertirse ella misma -una Profesora de Plástica- en la Directora de Escuela Secundaria. Desde ese cargo, aborda hoy el desafío de contribuir a la mejora educativa y hacer posible la nueva Escuela Secundaria para todos. (Párrafo extraído del texto a modo de resumen)Eje 2: Educación artística, investigación y sociedadFacultad de Bellas Arte