tropy in the
Federica Marone and Barbara Romanowicz
"On the depth distribution of azimuthal anisotropy in the continental upper mantle", Nature, 447, 198-201, 2007
The most likely cause of seismic anisotropy in the Earth's upper mantle is lattice preferred orientation (LPO) of anisotropic minerals such as olivine. Its presence reflects dynamic processes related to the formation of the lithosphere as well as to present-day tectonic motions. A powerful tool to detect and characterize upper mantle anisotropy is the analysis of shear wave (SKS) splitting measurements. Because of the poor vertical resolution afforded by this type of data, a long lasting controversy has opposed proponents of a lithospheric origin for the observed splitting under stable continents, "frozen-in" at the time of the formation of the craton, to those4 who argue that the anisotropy originates primarily in the asthenosphere, and is induced by shear due to present-day absolute plate motions (APM). In addition, predictions from surface wave derived models are largely incompatible with shear wave splitting observations. Here we show that this disagreement can be resolved by simultaneously inverting surface waveforms and SKS splitting data. We present evidence for the presence of two layers of anisotropy with different fast axis orientations in the cratonic part of the North American upper mantle. At asthenospheric depths (200-400 km) the fast axis is sub-parallel to the absolute plate motion (APM), confirming the presence of shear related to current tectonic processes, whereas in the lithosphere (80-200 km), the orientation is significantly more northerly. In the western, tectonically active, part of North America, the fast axis direction is consistent with APM throughout the depth range considered, in agreement with a much thinner lithosphere.
See also related papers:
Federica Marone, Yuancheng Gung and Barbara Romanowicz
"High resolution 3D radial anisotropic
structure of the North American upper mantle from inversion of surface waveform data,
Geophys. J. Int.
171, 206-222, doi: 10.1111/j.1365-246X.2007.03465.x
Federica Marone and Barbara Romanowicz "Non-linear crustal corrections in high-resolution waveform seismic tomography ", Geophys. J. Int., 170, 460-467.