On modelling the lithosphere in mantle convection with non-linear rheology

Numerical convection experiments were carried out with the aim of simulating the lithosphere as a strong mechanical boundary layer participating in the circulation, and to study its dynamical role and the governing parameters. The rheological model parameters were successively refined, effective viscosity depending on (1) depth, (2) temperature and pressure, and (3) temperature, pressure, and stress. In all cases a high-viscosity plate rested on a low-viscosity asthenosphere; in the two latter cases it could in principle subduct, but did so only if zones of weakness were built into it. It was possible to model active or inactive plates (moving faster or slower than the asthenosphere below). Because of a lack of numerical resolution it was however, not possible to simulate a narrow sinking slab; rather a broad zone of cooled and highly viscous material developed, often limiting the rate of descent and leading to non-steady convection. The circulation, including subduction, was stabilized by introduction of stress-dependence of viscosity (non-linearity), dissipation, and adiabatic heating. The parameter chiefly responsible for deciding the (active or passive) role of the plate is its decoupling from its neighbours, achieved in the models by assuming weakness zones. Another important result seems to be that the assumption of plausible mantle rheologies and heat input leads to equally plausible effective viscosities, plate velocities, and to upper-mantle temperatures which are relatively low by current ideas, but conforming to earlier estimates based on convection theory. Viscosity distribution and flow pattern are also in reasonable agreement with more detailed boundary layer computations. The main obstacles to our modelling are the numerical limitations, forcing upon us such artificialities as two-dimensionality, rectangular model boxes, coarse grids, and generalized weakness zones.           ARK: https://n2t.net/ark:/88439/y061130 Permalink: https://geophysicsjournal.com/article/176 &nbsp

Gravitational Radiation from Compact Binary Pulsars

An outstanding question in modern Physics is whether general relativity (GR) is a complete description of gravity among bodies at macroscopic scales. Currently, the best experiments supporting this hypothesis are based on high-precision timing of radio pulsars. This chapter reviews recent advances in the field with a focus on compact binary millisecond pulsars with white-dwarf (WD) companions. These systems - if modeled properly - provide an unparalleled test ground for physically motivated alternatives to GR that deviate significantly in the strong-field regime. Recent improvements in observational techniques and advances in our understanding of WD interiors have enabled a series of precise mass measurements in such systems. These masses, combined with high-precision radio timing of the pulsars, result to stringent constraints on the radiative properties of gravity, qualitatively very different from what was available in the past.Comment: Short review chapter to appear in "Gravitational Wave Astrophysics" by Springer-Verlag, edited by Carlos F. Sopuerta; v3: a few major corrections and updated references. Comments are welcome

Nature’s nations: the shared conservation history of Canada and the USA

Historians often study the history of conservation within the confines of national borders, concentrating on the bureaucratic and political manifestations of policy within individual governments. Even studies of the popular expression of conservationist ideas are generally limited to the national or sub-national (province, state, etc.) scale. This paper suggests that conservationist discourse, policy and practice in Canada and the USA were the products of a significant cross-border movement of ideas and initiatives derived from common European sources. In addition, the historical development of common approaches to conservation in North America suggests, contrary to common assumptions, that Canada did not always lag behind the USA in terms of policy innovation. The basic tenets of conservation (i.e. state control over resource, class-based disdain for subsistence hunters and utilitarian approaches to resource management) have instead developed at similar time periods and along parallel ideological paths in Canada and the USA

Non-linear rheology and return flow in the mantle

A simple model of mantle return flow in response to lithospheric plate motions is developed. Such a model is realistic if the buoyancy forces are concentrated in the plates. One-dimensionality is chosen as a simplification to study effects of mantle rheology in as much isolation as possible. Rheology is modelled as a combination of dislocation creep, diffusion creep and fluid phase transport; parameters are those appropriate for olivine. We have varied temperature, grain size, influence of partial melt, diffusivity and activation energy, grain deformation versus grain boundary sliding dominated creep, and surface plate velocity. A peculiar feature of non-linear rheology is the existence of low-stress high-viscosity regions, which, however, are of little dynamic importance because deformation there is very small. The main results are (1) that the model does not predict an excessive pressure gradient to be required by the return flow (which would be evident in a rise of the sea floor and strong increase in free air gravity anomalies toward the trenches); (2) that no excessive shear stresses at the plate bottom are predicted (which might result in observable heat flow effects and intra-plate seismicity and would require implausibly great driving forces at the plate ends); (3) that the model predicts the return flow to extend into the deeper mantle; this follows, however, from the simplifying assumption of olivine rheology below 400 km depth and would then argue for rather high temperatures, small grain sizes, possibly important fluid phase transport, and small activation volume. Recent work on the variation of activation volume with pressure and phase changes suggests a rather 'soft' lower mantle and thus supports the notion of 'deep' return flow. In interpreting the results one must, of course, keep in mind that the model is a purely mechanical one with a predetermined temperature profile (varied within plausible limits) and that the physics of the thermodynamic aspects of the flow problem is ignored.         ARK: https://n2t.net/ark:/88439/y009016 Permalink: https://geophysicsjournal.com/article/43 &nbsp