248 research outputs found
Heat flow increase following the rise of mantle isotherms and crustal thinning
Heat flow measurements in the western United States define a zone of high heat flow which coincides with the Basin and Range Province where extension has taken place recently. In this region, the average reduced heat flow is approx 30 mW sq. meters higher than in stable continental provinces; locally (e.g., Battle Mountain High), the heat flow anomaly can be more than 100 mW/sq meters above average. Estimates of the amount of extension range between 30% and 100% for the past 30 Ma. In the Colorado Plateau, which has been uplifted without major tectonic deformation, the heat flow is only slightly above average. Analytical calculations show that an abrupt change in heat flow at the base of the lithosphere 30 Ma ago would not affect the surface significantly. Uplift would proceed at a slow rate. A thermal perturbation at the base of a 40 km thick crust, however, would reach the surface faster and, after 30 Ma, the increase in surface heat flow would be about 75% of the amplitude of the heat flow anomaly. The number of volcanic rocks in the Basin and Range suggests that magma intrusions may provide an effective heat transfer mechanism. It can be show that if the source of the intrusions is at the base of the lithosphere, the response time will be much longer than 30 Ma, and most ot the heat transferred from the asthenosphere will be absorbed in the lithosphere
Selection of borehole temperature depth profiles for regional climate reconstructions
International audienceBorehole temperature depth profiles are commonly used to infer time variations in the ground surface temperature on centennial time scales. We compare different procedures to obtain a regional ground surface temperature history (GSTH) from an ensemble of borehole temperature depth profiles. We address in particular the question of selecting profiles that are not contaminated by non climatic surface perturbations and we compare the joint inversion of all the profiles with the average of individual inversions. We show that the resolution and the stability of the inversion of selected profiles are much improved over those for a complete data set. When profiles have been selected, the average GSTH of individual inversions and the GSTH of the joint inversion are almost identical. This is not observed when the entire data set is inverted: the average of individual inversions is different from the joint inversion. We also show that the joint inversion of very noisy data sets does not improve the resolution but, on the contrary, causes strong instabilities in the inversion. When the profiles that are affected by noise can not be eliminated, averaging of the individual inversions yields the most stable result, but with very poor resolution
Crustal heat production in the Superior Province, Canadian Shield, and in North America inferred from heat flow data
International audienceMeasurements of heat flow and U, Th, K concentrations are used to determine the amount of heat generated in various belts of the Superior Province, the largest Archean craton on Earth. These data allow estimates of the average crustal heat production and indicate compositional differences between upper and lower crustal assemblages. The bulk average heat production of the Superior Province crust is 0.64 ”W m-3 and is almost the same in different belts of slightly different ages, illustrating the remarkable uniformity of crust-building mechanisms. In the wider context of the North American continent, the bulk crustal heat production decreases from 1.0 ”W m-3 in the oldest Slave Province to a minimum of 0.55 ”W m-3 in the Paleo-Proterozoic Trans-Hudson Orogen. It increases in younger provinces, culminating with a high value of 1.05 ”W m-3 in the Phanerozoic Appalachian Province. In all provinces, U and Th enrichment is systematically associated with sedimentary accumulations. A crustal differentiation index is obtained by calculating the ratio between the average values of heat production at the surface and in the bulk crust. The differentiation index is correlated with the bulk average heat production, which suggests that crustal differentiation processes are largely driven by internal radiogenic heat
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Perceived pattern regularity computed as a summary statistic: implications for camouflage
Why do the equally spaced dots in figure 1 appear regularly spaced? The answer âbecause they areâ is naive and ignores the existence of sensory noise, which is known to limit the accuracy of positional localization. Actually, all the dots in figure 1 have been physically perturbed, but in the case of the apparently regular patterns to an extent that is below threshold for reliable detection. Only when retinal pathology causes severe distortions do regular grids appear perturbed. Here, we present evidence that low-level sensory noise does indeed corrupt the encoding of relative spatial position, and limits the accuracy with which observers can detect real distortions. The noise is equivalent to a Gaussian random variable with a standard deviation of approximately 5 per cent of the inter-element spacing. The just-noticeable difference in positional distortion between two patterns is smallest when neither of them is perfectly regular. The computation of variance is statistically inefficient, typically using only five or six of the available dots
Free Thermal Convection Driven by Nonlocal Effects
We report and explain a convective phenomenon observed in molecular dynamics
simulations that cannot be classified either as a hydrodynamics instability nor
as a macroscopically forced convection. Two complementary arguments show that
the velocity field by a thermalizing wall is proportional to the ratio between
the heat flux and the pressure. This prediction is quantitatively corroborated
by our simulations.Comment: RevTex, figures is eps, submited for publicatio
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
A discretized integral hydrodynamics
Using an interpolant form for the gradient of a function of position, we
write an integral version of the conservation equations for a fluid. In the
appropriate limit, these become the usual conservation laws of mass, momentum
and energy. We also discuss the special cases of the Navier-Stokes equations
for viscous flow and the Fourier law for thermal conduction in the presence of
hydrodynamic fluctuations. By means of a discretization procedure, we show how
these equations can give rise to the so-called "particle dynamics" of Smoothed
Particle Hydrodynamics and Dissipative Particle Dynamics.Comment: 10 pages, RevTex, submitted to Phys. Rev.
Labels direct infantsâ attention to commonalities during novel category learning
Recent studies have provided evidence that labeling can influence the outcome of infantsâ visual categorization. However, what exactly happens during learning remains unclear. Using eye-tracking, we examined infantsâ attention to object parts during learning. Our analysis of looking behaviors during learning provide insights going beyond merely observing the learning outcome. Both labeling and non-labeling phrases facilitated category formation in 12-month-olds but not 8-month-olds (Experiment 1). Non-linguistic sounds did not produce this effect (Experiment 2). Detailed analyses of infantsâ looking patterns during learning revealed that only infants who heard labels exhibited a rapid focus on the object part successive exemplars had in common. Although other linguistic stimuli may also be beneficial for learning, it is therefore concluded that labels have a unique impact on categorization
Thermal convection in fluidized granular systems
Thermal convection is observed in molecular dynamic simulation of a fluidized
granular system of nearly elastic hard disks moving under gravity, inside a
rectangular box. Boundaries introduce no shearing or time dependence, but the
energy injection comes from a slip (shear-free) thermalizing base. The top wall
is perfectly elastic and lateral boundaries are either elastic or periodic. The
observed convection comes from the effect of gravity and the spontaneous
granular temperature gradient that the system dynamically develops.Comment: 4 pages, 5 figure
Upper mantle velocity-temperature conversion and composition determined from seismic refraction and heat flow
International audience[1] We compile upper mantle P n velocities from seismic refraction/wide-angle reflection surveys in the southern Superior Province of the Canadian Shield and compare them with temperatures at the Moho deduced from heat flow data. Calculated Moho temperatures and P n velocities correlate well, showing that in this area, P n depends primarily on temperature. The obtained values of @V(P n)/@T depend weakly on the assumed value of Moho heat flow and are on the order of Ă6.0 Ă 10 Ă4 ± 10% km s Ă1 K Ă1 , within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks. Comparison between observed P n velocities and predicted values for several mineralogical models at Moho temperatures allows constraints on both the Moho heat flow and the shallow mantle composition. For all Moho heat flows, undepleted (clinopyroxene-rich) mantle compositions do not allow a good fit to the data. For depleted mantle compositions, temperatures consistent with the observed P n velocities correspond to values of Moho heat flow larger than 12 mW m Ă2. For our preferred Moho heat flow of 15 mW m Ă2 , the best fit mantle composition is slightly less depleted than models for average Archean subcontinental lithospheric mantle. This may be due to rejuvenation by melt-related metasomatism during the Keweenawan rifting event. The similarity in P n Ă T conversion factors estimated from this empirical large-scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies. Citation: Perry, H. K. C., C. Jaupart, J.-C. Mareschal, and N. M. Shapiro (2006), Upper mantle velocity-temperature conversion and composition determined from seismic refraction and heat flow
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