Search Results - (Author, Cooperation:P. M. Shearer)

2014-07-12

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

Magnetotelluric and multichannel seismic reflection measurements indicate that the Phanerozoic lower continental crust is commonly electrically conductive and reflective, in contrast to a more resistive and transparent middle to upper crust. A few per cent free saline water can provide an explanation for both results along with the apparent requirement that neither the conductive nor the reflective properties are retained when lower crustal rocks are brought to the upper crust. Common 10 km thick and 20–30 Ωm resistivity layers can be explained with 0.5–3 per cent pore water, if there are equilibrium pore geometries and the salinity is close to that of sea-water as suggested by lower crust fluid inclusions. Seismic velocities and impedances must be affected if such porosity exists. Seismic reflectors with reflection coefficients of 5–10 per cent can be explained by layers or lamellae with porosity contrasts of 1–4 per cent and reasonable effective pore aspect ratios of 0.1–0.03. A minimum temperature of 350°C is estimated from a correlation between heat flow and depth to the top of conductive and reflective layers. The upward limit in the crust may occur at an impermeable boundary formed by hydration reactions at the top of greenschist facies conditions or by precipitation of silica. It also may be associated with the minimum temperature for ductile behaviour and equilibrium grain boundary pore configurations. The maximum temperature is about 700°C according to the evidence indicating that there is no free water in granulite facies conditions. Areas that have been subject to such high temperature conditions without the subsequent addition of water, i.e. the lower crust of shields, are generally non-reflective and electrically resistive.

Electronic Resource

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

Ray tracing through gradients in anisotropic materials is complicated by singularities where the two quasi-shear wave slowness sheets cross or touch. Difficulties associated with such points can be removed by explicitly including polarization in the ray tracing equations. Slowness sheet and wavefront plots show the polarization and velocity behavior of various anisotropy models of aligned cracks in the upper crust. A simple scaling of the elastic tensor with depth can be shown to be approximately correct for models of aligned cracks within an isotropic host matrix with a linear velocity gradient. Ray tracing examples for models of aligned cracks within a strong vertical velocity gradient in the uppermost crust demonstrate various features of azimuthal anisotropy, including amplitude and polarization anomalies and shear-wave splitting. Quasi-shear wave polarizations typically twist along ray paths, with stronger twisting near the symmetry axis in hexagonally symmetric media. Strong anisotropy can cause unusual effects, such as ray paths which have three turning points in laterally homogeneous models.

Electronic Resource

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

Quasi-shear wave polarizations typically twist along ray paths through gradient regions in anisotropic media, causing frequency dependent coupling between the qS-waves. This coupling is much stronger than the analogous coupling between P- and SV-waves in isotropic gradients because of the small difference between the qS-wave velocities. Geometrical ray theory is typically valid for qS-waves only at relatively high frequencies, and does not converge to the isotropic result in the limit of infinitely weak anisotropy. Using the plane-wave layered response, we show examples of this coupling and how it may cause frequency dependent shear-wave polarizations. We consider two special cases where the coupling is especially strong in hexagonally symmetric media: (i) intersection singularities where the slowness sheets cross, and (ii) kiss singularities where the slowness sheets touch at the symmetry axis. We show numerical and asymptotic solutions for the pulses generated in these situations. In some cases, far-field excitation of both quasi-shear waves (and shear-wave splitting) will result from an incident wave composed of only one of the quasi-shear waves.

Electronic Resource