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Deformation and ultrafine recrystallization of quartz in pseudotachylyte-bearing faults: a matter of a few seconds
No full textTectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip
along faults and are mainly reported from the brittle crust in association with cataclasites. The
temporal and spatial association of fine-grained quartz with pseudotachylytes have been described
within the literature for several locations and seems to be an important feature characteristic for
seismic processes.
In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained
(grain size 1-2 μm) aggregates along microshear zones (50-150 μm thick) in the host rock adjacent to
pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern
Alps), and tonalite (Adamello fault, Southern Alps). The transition from the host quartz to microshear
zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3-0.5 μm) polygonization
to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic
aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz
towards the microshear zone shows a progressive misorientation from the host grain, by subgrain
rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. Thus
these ultrafine quartz aggregates appear to be typically associated with pseudotachylytes in nature.
Microshear zones localized on precursory fractures developed during the stages of earthquake
rupture propagation and the very initial stages of fault slip (Bestmann et al., 2011).
Based on thermal models we suggest that crystal plastic deformation of quartz accompanied by
dramatic grain size refinement occurs during seismic faulting at the base of the brittle crust as a
result of the high temperature transients (> 800°C) related to frictional heating in the host rock
selvages of the slip surface. These localised high deformation temperatures made possible that the
process of recrystallization, including recovery processes, could occur in a time lapse of a few tens of
seconds.
REFERENCES
Bestmann, M., Pennacchioni, G., Frank, G., Göken, M., de Wall, H., 2011. Pseudotachylyte in muscovite-bearing quartzite: coseismic
friction-induced melting and plastic deformation of quartz. Journal of Structural Geology 33, 169-186
Exploring the Ar isotope record of an early Miocene pseudotachylyte in an early Oligocene intrusion (Rieserferner pluton, eastern Alps)
No full textWe investigate the influence of unmelted clasts on the 40 Ar\\ 39 Ar age record of a Miocene pseudotachylyte within the early Oligocene Riesenferner pluton (eastern Alps). This case study is ideal to investigate the effect of inherited Ar retained in survivor clasts because, although seismic faulting was only slightly younger (b20 Ma) than the igneous source rock, the pseudotachylyte includes layers with significant variations in K content, of either dominant plagioclase and biotite microlites or dominant submicrometric muscovite matrix. Numerical modeling, assuming pure diffusive loss and a short-lived thermal pulse due to frictional heating, indicates that unmelted plagioclase clasts represent the most critical issue in interpreting Ar data. Plagioclase clasts of only a few to tens micrometers in size are expected to retain most of radiogenic Ar accumulated before coseismic faulting. A comparable behavior is only predicted for much larger (ten to fifty times larger) K-feldspar clasts. In-situ 40 Ar\\ 39 Ar laser analyses of low-K microlitic domains, free of discernable clasts, yield slightly, but significantly, older ages than those from the high-K matrix. Results suggest that older ages (up to ~4% on average) from low-K domains are due to contamination by plagioclase clasts and suggest an age of 17.12 ± 0.22 Ma for coseismic slip. This age dates the last stage of activity of the Giudicarie Fault during tectonic indentation of the Dolomites block of the southern Alps against the nappe stack of the eastern Alp
Ti distribution in quartz across a heterogeneous shear zone within a granodiorite: The effect of deformation mechanism and strain on Ti resetting
No full textThe study of a heterogeneous ductile shear zone that developed at ~ 500 °C and 0.2 GPa during post-magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on Ti re-equilibration of quartz to be assessed. Understanding this effect is critical for applying Ti-in-quartz thermobarometry to mylonites. Differently strained quartz across the shear zone shows a heterogeneous distribution of Ti concentrations ([Ti]) (measured by Secondary Ion Mass Spectrometry, SIMS) ranging between 2 and 45 ppm. Quartz cathodoluminescence (CL) is proven by spectral analysis to be correlated with [Ti], allowing CL images to be calibrated as Ti maps using SIMS measurements. Coarse-grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration (GBM) and mostly (65–75% area) contain 20–38 ppm Ti. Resetting to lower [Ti] occurred locally: (i) in haloes surrounding titanite and biotite inclusions ([Ti] as low as 6 ppm); (ii) along grain boundaries; and (iii) towards the interface of quartz domains with other mineral domains. With increasing strain, quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains (> 100's μm long) surrounded by mantles of new grains (10–30 μm in size) recrystallized by subgrain rotation (SGR). Dynamic recrystallization by SGR, associated with prism slip, became increasingly dominant over GBM as strain increased towards the shear zone core. Significant resetting of Ti in quartz only occurred in high strain domains (at shear strain γ probably >> 10) in the shear zone core where fine recrystallization amounts to 50–60% by area and coarser cores are strongly sub-structured. These domains are not compositionally homogeneous and still show a range of [Ti] mostly between 2 and 10 ppm. In all strain facies of the shear zone quartz-filled pressure shadows associated with feldspar show an almost constant [Ti] of ~ 2 ppm. The pristine Ti content of the magmatic quartz mylonitized in the shear zone core is therefore significantly reset and converges “asymptotically” towards the “equilibrium” 2 ppm [Ti] shown by new quartz precipitated in pressure shadows. It is inferred that extensive recrystallization by SGR and repeated cycles of dislocation creep and rearrangement provided fluid access to quartz grain interiors, promoting chemical buffering and leading to partial re-equilibration to low [Ti]. These observations imply limitations on the use of the Ti-in-quartz thermobarometry to constrain ambient conditions of ductile deformation
Ti-in-qtz signatures of pseudotachylyte-bearing crystalline rocks
No full textTectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults.
Bestmann et al. (2012) have shown that transient high temperature conditions related to frictional heating during
coseismic faulting in the brittle crust promoted the dynamical recrystallization of quartz to ultrafine-grained (grain
size 1-2 m) aggregates along microshear zones in the host rock adjacent to pseudotachylyte veins. In this study
we investigate if there is any geochemical signature associated with this transient high temperature event. With
this aim we used Ti-in-quartz trace element data, which can be used as a thermometer (Wark and Watson, 2006).
Models of the temperature evolution of the host rock following coseismic slip and production of frictional
melts show that temperatures >800 C only last for a few minutes close to the vein walls (Bestmann et al., 2012).
The experimental data on diffusion of Ti in quartz (Cherniak et al., 2007) seems to exclude that any detectable Ti
diffusion could occur during the short-lived thermal event. However, the Ti-in-quartz investigation is motivated by
the fact that Ti diffusion could be enhanced in the recrystallized quartz aggregates by pervasive lattice damage and
by the percolation of melt along grain boundaries (Bestmann et al., 2012).
Micro-mapping of Ti trace amounts in quartz were carried out by using a nanoSIMS on two different
pseudotachylyte-bearing samples already used in the study of Bestmann et al. (2012): (1) the Schneeberg Normal
Fault Zone (SNFZ, Eastern Alps) within a muscovite-bearing quartzite, and (2) the Gole Larghe Fault Zone
(Southern Alps) within tonalites of the Adamello pluton.
In the Schneeberg NFZ, the metamorphic (amphibolite facies) host quartz grains and the ultrafine grained
recrystallized aggregates within microshear zones adjacent to pseudotachylytes both have an identical Ti signature
of 4-6 ppm. In the Adamello tonalite the magmatic quartz host grains are fractured and show a sharp decrease in
Ti from 40-55 ppm (magmatic host) to 11-15 ppm (healed fractures). This gives evidence of an extensive phase
of fluid-rock interaction along the Adamello faults. Similar to the SNFZ, the ultrafine grained quartz aggregate
along microshear zones mainly inherited the pre-seismic Ti signal from the fractured host quartz grains. There
are, however, steep Ti gradients surrounding very small («1 m) Ti-bearing 2nd phase particles present along the
grain boundary of ultrafine grained aggregates as a result of melt infiltration. These haloes (1-2 m) could reflect
enhanced Ti diffusion in highly deformed quartz during the coseismic thermal transient.
References
Bestmann, M., Pennacchioni, S., Nielsen, G., Göken, M., de Wall, H., 2012. Deformation and ultrafine recrystallization
of quartz in pseudotachylyte-bearing faults: a matter of a few seconds. Journal of Structural
Geology, 38, 21-38.
Cherniak, D.J., Watson, E.B., Wark, D.A., 2007. Ti diffusion in quartz. Chemical Geology, 236, 65–74.
Wark, D.A., Watson, E.B., 2006. TitaniQ: a titanium-in-quartz geothermometer. Contribution to Mineralogy
and Petrology, 152, 743-754
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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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Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
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