33 research outputs found
Diffusion of Pt dimers on Pt(111)
We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to
Physical Review B; other papers of interest can be found at url
http://www.centrcn.umontreal.ca/~lewi
Response of a multi-domain continental margin to compression: study from seismic reflection-refraction and numerical modelling in the Tagus Abyssal Plain
The effects of the Miocene through Present compression in the Tagus Abyssal Plain are mapped using the most up to date available to scientific community multi-channel seismic reflection and refraction data. Correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line IAM-5. Four structural domains were recognized. In the oceanic realm mild deformation concentrates in Domain I adjacent to the Tore-Madeira Rise. Domain 2 is characterized by the absence of shortening structures, except near the ocean-continent transition (OCT), implying that Miocene deformation did not propagate into the Abyssal Plain, In Domain 3 we distinguish three sub-domains: Sub-domain 3A which coincides with the OCT, Sub-domain 3B which is a highly deformed adjacent continental segment, and Sub-domain 3C. The Miocene tectonic inversion is mainly accommodated in Domain 3 by oceanwards directed thrusting at the ocean-continent transition and continentwards on the continental slope. Domain 4 corresponds to the non-rifted continental margin where only minor extensional and shortening deformation structures are observed. Finite element numerical models address the response of the various domains to the Miocene compression, emphasizing the long-wavelength differential vertical movements and the role of possible rheologic contrasts. The concentration of the Miocene deformation in the transitional zone (TC), which is the addition of Sub-domain 3A and part of 3B, is a result of two main factors: (1) focusing of compression in an already stressed region due to plate curvature and sediment loading; and (2) theological weakening. We estimate that the frictional strength in the TC is reduced in 30% relative to the surrounding regions. A model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented. This model is consistent with both the geological interpretation of seismic data and the results of numerical modelling. (C) 2008 Elsevier B.V. All rights reserved.Instituto Nacional de Engenharia, Tecnologia e Inovacao(INETI); Landmark Graphics Corporation; Landmark University Grant Program; LATTEX/IDL [ISLF-5-32]; FEDERinfo:eu-repo/semantics/publishedVersio
3D MHD Simulations of Laboratory Plasma Jets
Jets and outflows are thought to be an integral part of accretion phenomena
and are associated with a large variety of objects. In these systems, the
interaction of magnetic fields with an accretion disk and/or a magnetized
central object is thought to be responsible for the acceleration and
collimation of plasma into jets and wider angle flows. In this paper we present
three-dimensional MHD simulations of magnetically driven, radiatively cooled
laboratory jets that are produced on the MAGPIE experimental facility. The
general outflow structure comprises an expanding magnetic cavity which is
collimated by the pressure of an extended plasma background medium, and a
magnetically confined jet which develops within the magnetic cavity. Although
this structure is intrinsically transient and instabilities in the jet and
disruption of the magnetic cavity ultimately lead to its break-up, a well
collimated, knotty jet still emerges from the system; such clumpy morphology is
reminiscent of that observed in many astrophysical jets. The possible
introduction in the experiments of angular momentum and axial magnetic field
will also be discussed.Comment: 15 pages, 4 figures, accepted by Astrophysics and Space Science for
Special Issue High Energy Density Laboratory Astrophysics Conferenc
Easter microplate dynamics
We use two-dimensional elastic finite element analysis, supplemented by strength estimates, to investigate the driving mechanism of the Easter microplate. Modeled stresses are compared with the stress indicators compiled from earthquake focal mechanisms and structural observations. The objective is to constrain the tectonic forces that govern the Easter microplate rotation and to test the microplate driving hypothesis proposed by Schouten et al. [1993] . We infer that the mantle basal drag cannot drive the microplate rotation but opposes it, and that the asthenospheric viscosity is no more than about 1 Ă 1018 Pa s. At most, the basal drag comprises 20% of the force resisting microplate rotation. The outward pull of the main plates can drive the rotation by shear drag applied along the northern and southern boundaries of the microplate. However, we propose an additional driving force which arises from the strong variation of the ridge resistance force along the east and west rifts, so that the main driving torques come from the pull of the major plates acting across the narrowing and slowing rifts. This requires the strength to increase substantially toward the rift tips due to thickening of the brittle lithosphere as the spreading rate slows
Patterns of stress at midocean ridges and their offsets due to seafloor subsidence
The effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two-dimensional (2D) horizontal stresses arising from isostatically compensated vertical loads. The topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge-parallel tensile stresses. These may predominate over the ridge-perpendicular stresses and explain observations at midocean ridge offsets such as (1) oblique normal faulting at ridge-transform intersections trending up to 60° relative to the ridge axis, and (2) nontransform offsets consisting of structures oriented at 45° relative to the ridge trend. At midocean ridge overlaps, rotation of the ridge-parallel tensile stresses favours rift propagation at more than 45° relative to the ridge trend. It is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets, and as a result large overlaps may eventually start to rotate and evolve into a microplate