1,720,998 research outputs found
Study of exhumed paleo-seismic fault as a gauge to estimate earthquake source parameters
No full textExhumed faults decorated by pseudotachylyte, solidified friction induced melt recording a seismic rupture (Sibson, 1975), might give information about the earthquake source. Two cases, representative of different seismogenic environments have been investigated: the pseudodotachylyte-mylonite association in the lower crust, in the Ivrea Zone metagabbros, and the “upper crust” pseudotachylytes in the Tertiary Adamello granitoid batholith.
In metagabbros, a cyclical and coeval production of frictional melts and high temperature localized ultramylonites, during amphibolite metamorphic conditions, has been documented.
From a selected pseudotachylyte in the Adamello batholith the earthquake energy budget has been estimated, concluding that the most energy was dissipated as frictional heat. The thermal evolution of a frictional melt was modeled, demonstrating that the pristine cataclastic structure used for the component of the surface energy estimation might locally have been preserved.Faglie esumate sigillate da pseuotachiliti, fusi di frizione solidificati che registrano una rottura sismica (Sibson, 1975), possono fornire informazioni sulla sorgente del terremoto. Sono stati studiati due casi rappresentativi di ambienti simogenetici differenti: l’associazione pseudotachiliti-miloniti nella corsta inferiore, nei metagabbri della Zona d’Ivrea e le pseudotachiliti della “crosta superiore” nel batolite granitoide terziario dell’Adamello.
Nei metagabbri è stata documentata la produzione ciclica e contemporanea di fusi di frizione e ultramiloniti localizzate di alta temperatura, entrambe prodotte in facies anfibolitica.
Studiando una pseudotachilite prescelta dal batolite dell’Adamello, si è stimato il bilancio energetico di un terremoto, concludendo che la maggior parte dell’energia si è dissipata sotto forma di calore di frizione. Infine si è modellizzata l’evoluzione termica di un fuso di frizione, dimostrando che l’originaria struttura cataclastica usata per stimare il contributo dell’energia di superficie può potenzialmente essersi localmente preservata
Amphibolite-facies pseudotachylytes in Premosello metagabbro and felsic mylonites (Ivrea Zone, Italy)
No full textPseudotachylytes (solidified friction-induced melts produced during seismic slip) that formed under high
temperature ambient conditions might retain information on earthquake mechanics beneath the brittle–
ductile (elasto-frictional/viscous-plastic) transition in the crust. Thin, sharply bounded pseudotachylytes
and ultramylonites occur together inside the granulite-facies Premosello metagabbro and inside country
felsic mylonites of the Ivrea Zone (Italian Southern Alps). Petrographic and microstructural data indicate
that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies
(500–650 °C) metamorphic conditions. The occurrence of foliated pseudotachylytes and of local remnants
of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented
planar heterogeneities for nucleation of ultramylonites. The association of coeval pseudotachylyte and
mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients, related to downward
propagation of seismic ruptures from the upper brittle crust, followed by post-seismic stress relaxation
by ductile flow
Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla unit, Western Alps)
No full textWe have studied quartz microstructures and the c-axis crystallographic preferred orientations (CPOs) in four granitoid samples representative of increasing ductile shear deformation, from a weakly deformed granitoid (stage 1) to a mylonitic granitoid (stage 4). The quartz c-axis CPO measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area. All the samples belong to the Austroalpine Arolla unit (Western Alps) and were deformed at greenschist facies conditions. The quartz c-axis CPO was analyzed using a U-stage and the optical orientation imaging technique. The magmatic plagioclase, forming more than 50% of the volume of the granitoid, is extensively replaced by a mica-rich aggregate even in weakly deformed samples of stage 1. These aggregates flow to form an interconnected weak matrix with increasing deformation, wrapping relatively less strained quartz grains that undergo dominantly coaxial strain. Recrystallization of quartz ranges from less than 1% in the weakly deformed granitoid to up to 85% in the mylonitic granitoid, with average grain strain of 41% and 64%, respectively. With increasing strain and recrystallization, quartz grains in the granitoids show a sequence of transient microstructures and CPOs. Crystal plastic deformation is initially accomplished by dislocation glide with limited recovery, and at 50% grain strain it results in a CPO consistent with dominantly basal 〈a〉 slip. At 60% grain strain, recrystallization is preferentially localized along shear bands, which appear to develop along former intragranular cracks, and the recrystallized grains develop a strong c-axis CPO with maxima orthogonal to the shear band boundaries and independent of the host grain orientation. Within the granitoid mylonite, at an average quartz grain strain of 64%, recrystallization is extensive and the c-axis CPO of new grains displays maxima overlapping the host c-axis orientation and, therefore, unrelated to the bulk sense of shear. The host-controlled CPO is inferred to reflect pervasive recrystallization by progressive subgrain rotation. The switch from ‘shear band-control’ to ‘host-control’ on c-axis CPO occurred between 40% and 70% of recrystallization. In the quartz mylonite, the quartz c-axis CPO develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions. This study indicates that the CPO evolution of quartz may significantly differ in cases of polymineralic vs. monomineralic rocks under the same deformation conditions, if quartz in the polymineralic rock behaves as a ‘strong’ phase
Non-equilibrium melting of partially differentiated asteroids: insights from partial melting experiments on L6 chondrite DAV01001
Full text linkPlanetary differentiation in small bodies is believed to be ruled by several partial end-states that were dominated by low degrees of partial melting and melt segregation, before arriving at the formation of rocky planets. Having a better understanding of non-equilibrium melting processes in undifferentiated chondritic materials is critical to characterize planetary differentiation processes and the formation of rocky planets and differentiated asteroids. In this context, partial melting experiments of natural chondrites can provide unique insights into the petrological evolution associated with early planetary differentiation of planetesimals. For this study, we performed partial melting experiments using fragments from the ordinary chondrite DAV01001. Experiments were performed in a piston-cylinder at 1 GPa pressure, at temperatures from 1100 to 1300 °C and for 24 hours run duration. Reducing conditions were imposed by the use of graphite capsules. The experimental products were analysed using electron microprobe and synchrotron radiation computed microtomography (SR-μCT)
Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry
No full textMelting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure, Brazil
No full textAbstract–Shatter cones are one of the most widely recognized pieces of evidence for
meteorite impact events on Earth, but the process responsible for their formation is still
debated. Evidence of melting on shatter cone surfaces has been rarely reported in the
literature from terrestrial impact craters but has been recently observed in impact
experiments. Although several models for shatter cones formation have been proposed, so
far, no one can explain all the observed features. Shatter cones’ from the Vista Alegre
impact structure, Brazil, formed in fine-grained basalt of the Jurassic-Cretaceous Serra
Geral Formation (Parana large igneous province). A continuous quenched melt film,
consisting of a crystalline phase, mica, and amorphous material, decorates the striated
surface. Ultracataclasites, containing subrounded pyroxene clasts in an ultrafine-grained
matrix, occur subparallel to the striated surface. Several techniques were applied to
characterize the crystalline phase in the melt, including Raman spectroscopy and
transmission electron microscopy. Results are not consistent with any known mineral, but
they do suggest a possible rare or new type of clinopyroxene. This peculiar evidence of
melting and cataclasis in relation with shatter cone surfaces is interpreted as the result of
tensile fracturing at the tip of a fast propagating shock-induced rupture, which led to the
formation of shatter cones at the tail of the shock front, likely during the early stage of the
impact events
Shock‐heated graphite in three IAB iron meteorites—Implications on the formation of diamond
No full textIron meteorites, originating from the deepest parts of their parent bodies and separated during major break-up events, surprisingly rarely contain diamonds despite experiencing similar pressure–temperature conditions as diamond-bearing ureilites. In this study, graphite from three non-magmatic IAB iron meteorites Canyon Diablo, Campo del Cielo, and Yardymly was analyzed using micro-Raman spectroscopy, revealing the presence of the graphite G-band, the disorder-induced D-band, and occasionally the D′-band. Temperature estimates based on the G-band full width at half maximum (ranging from 1155 to 1339°C) are consistent with those found in ureilites. However, unlike in ureilites, no diamond bands were detected, as confirmed by μ-X-ray diffraction. The absence of diamonds is interpreted to be related to the thermal and mechanical properties of the iron meteorite matrix. Its high thermal diffusivity results in similar temperatures to ureilites, but its ductility dissipates shock-wave energy through plastic deformation, unlike the brittle ureilite matrix, which more effectively transmits the energy. Consequently, graphite in iron meteorites was heated but did not experience the high-pressure conditions required for diamond formation. Thus, we propose that impacts must either involve substantial energy or that graphite must be located close to the impact site, where it can experience high energies before these dissipate
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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