320 research outputs found
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
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
Deformation and ultrafine recrystallization of quartz in pseudotachylyte-bearing brittle 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 m)
aggregates along microshear zones (50-150 m 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 m) 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 (> 800C) 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.
Bestmann, M., Pennacchioni, G., Frank, G., Göken, M., de Wall, H., 2011. Pseudotachylyte in muscovitebearing
quartzite: coseismic friction-induced melting and plastic deformation of quartz. Journal of Structural
Geology 33, 169-186
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
Effects of recrystallization and strain on Ti re-equilibration in quartz in a cooling pluton
No full textSince a couple of years the trace amount of Ti in quartz (Ti-in-quartz or TitaniQ) has been used to constrain the deformation temperature in quartzitic rocks. Independently of how precise the estimate of deformation temperature could be, a basic question still remains controversial of how effective is dynamic recrystallization to reset the Ti in quartz in mylonites. The study of a heterogeneous ductile shear zone developed during post-magmatic cooling of a titanite-bearing granodiorite allows the effect of strain and recrystallization on Ti re-equilibration in quartz to be assessed. The different strain facies show a heterogeneous distribution of Ti content (measured by SIMS) which correlates well with cathodoluminescence (CL) intensity. In the granodiorite protolith CL-bright Ti-rich (20-38 ppm) quartz shows CL-dark Ti-poor haloes (Ti as low as 6-8 ppm) surrounding euhedral titanite. Grain-scale heterogeneities include Ti depleted (CL-darker) grain boundaries (Ti 4-6 ppm). In the protomylonite quartz shows a variable degree of recrystallization associated with strain gradients along S-C foliations anastomosing around feldspar porphyroclasts. Original CL-dark haloes surrounding titanite were passively stretched into the foliation; away from these haloes recrystallized quartz appears mainly bright in CL and retained high Ti contents as in the protolith. Quartz-filled pressure shadows, appended to disrupted feldspar porphyroclasts, show dark CL indicative of very low Ti content (1-3 ppm). In the mylonites and ultramylonites quartz forms totally recrystallized layers that are dominantly dark in CL but show internally a “subtle” CL layering subparallel to foliation reflecting variations of Ti in the range of 3 to 12 ppm. EBSD analysis of quartz indicates that prism was the dominant crystallographic slip system, associated with subgrain formation and subgrain rotation recrystallization, at all stages of deformation. This indicates together with dynamic recrystallization of K-feldspar and plagioclase (Oligoclase: An 16-20%) deformation conditions at ∼ 500 ◦C. We conclude that under, the dominant conditions of deformation at ∼ 500 ◦C: (i) Ti content is strongly dependent on microstructure; (ii) high strain and complete recrystallization by subgrain rotation produced only incomplete homogenization of Ti, (iii) water-assisted synkinematic precipitation of new quartz in pressure shadows dramatically changed the Ti content of quartz to very low values. These observations pose serious limitations to the use of the Ti-in-quartz thermo-barometer to constrain ambient conditions of ductile deformation
Uncoupling Sensation and Perception in Human Time Processing
No full textTiming emerges from a hierarchy of computations ranging from early encoding of physical duration (time sensation) to abstract time representations (time perception) suitable for storage and decisional processes.However, the neural basis of the perceptual experience of time remains elusive. To address this, we dissociate brain activity uniquely related to lower-level sensory and higher-order perceptual timing operations, using eventrelated fMRI. Participants compared subsecond (500 msec) sinusoidal gratings drifting with constant velocity (standard) against two probe stimuli: (1) control gratings drifting at constant velocity or (2) accelerating gratings, which induced illusory shortening of time. We tested two probe intervals: a 500-msec duration (Short) and a longer duration required for an accelerating probe to be perceived as long as the standard (Long—individually determined). On each trial, participants classified the probe as shorter or longer than the standard. This allowed for comparison of trials with an “Objective” (physical) or “Subjective” (perceived) difference in duration, based on participant classifications. Objective duration revealed responses in bilateral early extrastriate areas, extending to higher visual areas in the fusiform gyrus (at more lenient thresholds). By contrast, Subjective duration was reflected by distributed responses in a cortical/subcortical areas. This comprised the left superior frontal gyrus and the left cerebellum, and a wider set of common timing areas including the BG, parietal cortex, and posterior cingulate cortex. These results suggest two functionally independent timing stages: early extraction of duration information in sensory cortices and Subjective experience of duration in a higher-order cortical–subcortical timing areas
Correction: Use of the non-paretic arm reflects a habitual behaviour in chronic stroke
No full textCorrection to: Journal of NeuroEngineering and Rehabilitation (2025) 22:135 h t t p s : / / d o i. o r g / 1 0. 1 1 8 6 / s 1 2 9 8 4-0 2 5-0 1 6 6 1-5 In the sentence beginning 'Our results demonstrate that not using the ... in demanding (e.g. time-limited) situations. ' in the Conclusions heading under the Abstract section of this article [1], The word "not" needs to be deleted from the sentence. The corrected sentence is 'Our results demonstrate that using the non-paretic arm may reflect a habit response that is more readily triggered in demanding (e.g. time-limited) situations. ' The original article has been corrected. References 1. Sporn S, Bonyadi E, Fathana R, et al. Use of the non-paretic arm reflects a habitual behaviour in chronic stroke. J Neuroeng Rehabil. 2025;22:135. h t t p s : / / d o i. o r g / 1 0. 1 1 8 6 / s 1 2 9 8 4-0 2 5-0 1 6 6 1-5
Functional connectivity between the dorsolateral prefrontal cortex and the ipsilateral primary motor cortex
No full textIntroduction
The prefrontal cortex has a crucial role in higher cognitive functions and various line of evidence point to a tight connectivity network between the dorsolateral prefrontal cortex (DFLPC, BA46) and the ipsilateral primary motor cortex (M1).
Objectives
The main objective of this study was to determine the precise timing and the spatial specificity of this functional connectivity during the performance of a choice-reaction task.
Materials and methods
Twin coil, neuronavigated transcranial magnetic stimulation (TMS) was used during the performance of a choice-reaction task. By varying the time of stimulation (ISI 6, 8, 12 ms) after a cue (stimulus-onset asynchronies 75, 100, 125 ms) which signalled either a free selection or specified finger movement, the interactions between BA46 and M1 could be investigated. Furthermore, we tested whether the influence of a BA46 stimulation is specific to muscles involved in the task or not by investigating task involved and not-involved muscles.
Results
Our results indicate that in unselected muscles during trials with externally specified responses, stimulation of BA46 increases excitability of M1 at a SOA of 75 ms. In freely selected trials, stimulation of BA46 at a SOA of 100 ms facilitates M1 excitability. Our data suggested that the main effects occured at a ISI of 12 ms pointing to an indirect connectivity. In selected muscles this differences disappeared and the M1 output to these muscles was influenced by whether or not the muscle was selected or not. No effects could be observed when BA9 was stimulated.
Conclusion
The present results suggest that there is anatomically specific functional connectivity between left BA46 and left M1 during free and specified selection of a movement. This is the first study allowing to draw conclusions about the precise timing of this important functional connectivity
Motor cortex excitability following short trains of repetitive magnetic stimuli
No full textTrains of repetitive transcranial magnetic stimuli (rTMS) appear to have effects on corticospinal excitability that outlast the duration of the train. In order to investigate the mechanism of this effect in more detail we applied short periods of rTMS consisting of up to 20 stimuli at 5 Hz, 10 Hz or 20 Hz (rTMS) to the motor cortex at an intensity equal to resting threshold in 11 healthy, relaxed subjects. Spinal excitability, as judged by effects on the H-reflex or on transcranial anodal facilitation of the H-reflex, was not affected by the rTMS. However, cortical excitability, as judged by the effect on the size of EMG responses evoked by a suprathreshold TMS pulse, was decreased for up to 1 s after the end of rTMS. Post-train suppression was more powerful following longer trains or higher frequencies of rTMS. The predominant suppression contrasts with previous reports of facilitation, particularly after high-frequency rTMS. A second set of experiments, however, showed that this could be converted into facilitation if the intensity of rTMS was increased. We conclude that the after-effects of rTMS depend on its frequency, intensity and duration. The results are consistent with a model in which inhibition and facilitation build up gradually during the course of a conditioning train. Inhibition reaches its maximum effect after only a small number of stimuli, whereas facilitation takes longer. The threshold for evoking inhibition is lower than that for facilitation. Thus if moderate intensities of conditioning train are applied, inhibition is predominant after short trains, whereas facilitation dominates after long trains
Consensus paper: Combining trascranial stimulation with neuroimaging (vol 2, pg 58, 2009)
No full textErratum to ‘Consensus paper: Combining trascranial
stimulation with neuroimaging’ [Brain Stimulation 2(2):58-
80]
Hartwig R. Siebner Til O. Bergmann, Sven Bestmann,
Marcello Massimini, Heidi Johansen-Berg, Hitoshi Mochizuki,
Daryl E. Bohning, Erie D. Boorman, Sergiu Groppa,
Carlo Miniussi, Alvaro Pascual-Leone, Reto Huber, Paul
C.J. Taylor, Risto J. Ilmoniemi, Luigi De Gennaro, Antonio
P. Strafella, Seppo Ka ̈hko ̈nen, Stefan Klo ̈ppel, Giovanni B.
Frisoni, Mark S. George, Mark Hallett, Stephan A. Brandt,
Matthew F. Rushworth, Ulf Ziemann, John C. Rothwell,
Nick Ward, Leonardo G. Cohen, Ju ̈rgen Baudewig, Toma ́sˇ
Paus, Yoshikazu Ugawa, Paolo M. Rossini
The publisher regrets that some of the authors’ degrees
were listed incorrectly in the above mentioned paper.
Please see corrected list below:
Hartwig R. Siebner, MD, Til O. Bergmann, MSc, Sven
Bestmann, PhD, Marcello Massimini, MD, PhD, Heidi
Johansen-Berg, PhD, Hitoshi Mochizuki, MD, PhD, Daryl
E. Bohning, PhD, Erie D. Boorman, MSc, Sergiu Groppa,
MD, Carlo Miniussi, PhD, Alvaro Pascual-Leone, MD,
PhD, Reto Huber, PhD, Paul C.J. Taylor, PhD, Risto J.
Ilmoniemi, PhD, Luigi De Gennaro, PhD, Antonio P.
Strafella, MD, PhD, Seppo Ka ̈hko ̈nen, MD, PhD, Stefan
Klo ̈ppel, MD, Giovanni B. Frisoni, MD, Mark S. George,
MD, Mark Hallett, MD, Stephan A. Brandt, MD, PhD,
Matthew F. Rushworth, PhD, Ulf Ziemann, MD, John C.
Rothwell, PhD, Nick Ward, MD, PhD, Leonardo G. Cohen,
MD, Ju ̈ rgen Baudewig, PhD, Toma ́sˇ Paus, MD, PhD
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