70 research outputs found
Tectonic evolution of the Karakoram metamorphic complex (NW Himalayas) reflected in the 3D structures of spiral garnets: Insights from X-ray computed micro-tomography
We thank Pentti HölttÀ and Syed Zahid Shah for many useful discussions.
We thank the referees, F. George and an anonymous reviewer, for
their constructive comments.Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs (Foliation Intersection Axis) formed normal to tectonic shortening directions. Thanks to technological advances in X-ray computed micro-tomography (XCMT), FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples. XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional (3D) quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs. We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex (KMC), NW Himalayas, containing numerous garnet porphyroblasts with spiral-shaped inclusion trails. The XCMT imaging reveals an E-Wtrending FIA within the sample, which is consistent with orthogonal N-S collision of the India-Kohistan Island Arc with Asia. Garnet long axes (X-GT) have variable plunges that define a broad sub-vertical maximumand a small sub-horizontalmaximum. The X-GT principle maxima lie at N-090 and N-120. Smaller maxima lie at N-020 and N- 340. Geometric relationships between X-GT axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow. The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite, sillimanite and kyanite in the KMC. Late staurolite growth in the sample reveals howthe rocks extruded to the surface via a significant role of roll-on tectonics, which can be correlated with the Central Himalayas
Modeling mass transfer in fracture flows with the time domain-random walk method
The time domain-random walk method was developed further for simulating mass transfer in fracture flows together with matrix diffusion in surrounding porous media. Specifically, a time domain-random walk scheme was developed for numerically approximating solutions of the advection-diffusion equation when the diffusion coefficient exhibits significant spatial variation or even discontinuities. The proposed scheme relies on second-order accurate, central-difference approximations of the advective and diffusive fluxes. The scheme was verified by comparing simulated results against analytical solutions in flow configurations involving a rectangular channel connected on one side with a porous matrix. Simulations with several flow rates, diffusion coefficients, and matrix porosities indicate good agreement between the numerical approximations and analytical solutions.Peer reviewe
Determining Crack Aperture Distribution in Rocks Using the C-14-PMMA Autoradiographic Method : Experiments and Simulations
Because cracks control the global mechanical and transport properties of crystalline rocks, it is of a crucial importance to suitably determine their aperture distribution, which evolves through alteration processes and rock weathering. Due to the high variability of crack networks in rocks, a multiscale approach is needed. The C-14-PMMA (polymethylmethacrylate) method was developed to determine crack apertures using a set of artificial crack samples with different controlled apertures and tilt angles and also using Monte Carlo simulations. The experiments and simulations show the same result: the estimation of apparent aperture w(A) was successful regardless of tilt angle, even if the estimates are less accurate for low tilt angles (Peer reviewe
Response of electrically coupled spiking neurons: a cellular automaton approach
Experimental data suggest that some classes of spiking neurons in the first
layers of sensory systems are electrically coupled via gap junctions or
ephaptic interactions. When the electrical coupling is removed, the response
function (firing rate {\it vs.} stimulus intensity) of the uncoupled neurons
typically shows a decrease in dynamic range and sensitivity. In order to assess
the effect of electrical coupling in the sensory periphery, we calculate the
response to a Poisson stimulus of a chain of excitable neurons modeled by
-state Greenberg-Hastings cellular automata in two approximation levels. The
single-site mean field approximation is shown to give poor results, failing to
predict the absorbing state of the lattice, while the results for the pair
approximation are in good agreement with computer simulations in the whole
stimulus range. In particular, the dynamic range is substantially enlarged due
to the propagation of excitable waves, which suggests a functional role for
lateral electrical coupling. For probabilistic spike propagation the Hill
exponent of the response function is , while for deterministic spike
propagation we obtain , which is close to the experimental values
of the psychophysical Stevens exponents for odor and light intensities. Our
calculations are in qualitative agreement with experimental response functions
of ganglion cells in the mammalian retina.Comment: 11 pages, 8 figures, to appear in the Phys. Rev.
PadrÔes de infestação de comunidades de plantas daninhas no agroecossistema de cana-crua
InfluĂȘncia do teor de ĂĄgua na superfĂcie do solo sobre a eficĂĄcia e seletividade do flazasulfuron, na cultura de cana-de-açĂșcar
Levantamento fitossociolĂłgico de plantas daninhas em ĂĄreas de produção de cana-de-açĂșcar
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