Global patterns of azimuthal anisotropy and deformations in the continental mantle

Abstract

We present a summary of measurements of azimuthal anisotropy in the continental mantle based on the SKS technique and performed mostly with the active participation of the authors. The directions of polarization of the fast quasi-shear wave and the time delays between the quasi-shear waves are obtained at nearly 70 locations in all continents, except Antarctica. These data are interpreted in terms of lattice-preferred orientation of olivine which is caused by deformations in the mantle. The depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km. The fast directions in SKS do not show clear correlation with the fast directions of the teleseismic P at the same seismograph stations.In the regions of present-day convergence the fast direction of anisotropy usually aligns with the plate boundary. This correlation implies that the direction of shortening is the same in the crust and the upper mantle. In the regions of rifting, the inferred direction of mantle flow usually aligns with the direction of extension in the crust.Outside the regions of recent tectonic activity we, most likely, observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere. On a global scale, in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy. The correlation is especially strong in central and eastern parts of North America. A clear absence of any evidence of large-scale azimuthal anisotropy in the data of long-range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in SKS is generated mainly at greater depths, in the asthenosphere. Orientation of olivine at these depths reflects recent and present-day flow in the mantle rather than processes of a distant geologic past.

SKS splitting beneath Capital area of China

“…To a great extent, the plate motion determines the degree of the mantle anisotropy. Generally, the direction of absolute plate motion (APM) is consistent with the fast-wave polarization direction, which implies that mantle flow play significant role in upper mantle anisotropy (Vinnik et al, 1992). In Capital area, the WNW APM direction using HS3-NUVEL1a model (Gripp and Gordon, 2002) is consistent with the fast-wave direction, which images the upper mantle anisotropy is mainly caused by the mantle flow of the subduction of the Pacific plate to Eurasian plate ( Figure 5).…”

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

“…Thus shear-wave splitting patterns at cratons may potentially provide insight into the dynamics of the generation and stabilization of the continental lithosphere. On the other hand, in the absence of a more viscous and stable tectosphere, the fossil anisotropy at temperatures > 900øC could be reset by subsequent deformation [Vinnik et al, 1992], and splitting might reflect present-day asthenospheric shear associated with the absolute plate motion.…”

“…Because olivine is anisotropic and develops strain-induced lattice-preferred orientation, splitting results may be used to infer mantle strain fields [see review by Savage, 1998]. However, there is an ongoing dispute over whether •p at continental cratons is parallel to past geologic features and reflects the fossil lattice-preferred orientation (LPO) of olivine minerals within the Precambrian continental lithosphere [Silver and Chan, 1991;Silver, 1996], or if •p is directed parallel to the present-day absolute plate motion and reflects LPO associated with asthenospheric shear [Vinnik et al, 1992[Vinnik et al, , 1995. Silver [1996] presents an extensive compilation of the continental splitting results, showing that several continental shields (the Canadian Shield, Brazilian Craton, and Kaapval Craton) display •p parallel to ancient geologic structures, consistent with anisotropy caused by vertically coherent deformation of the crust and mantle during Precambrian orogenesis.…”

“…Shear-wave splitting measurements yield the direction of polarization •p of the fast shear wave and the delay time 6t between the fast and slow shear waves [Silver and Chan, 1991;Vinnik et al, 1992]. Because olivine is anisotropic and develops strain-induced lattice-preferred orientation, splitting results may be used to infer mantle strain fields [see review by Savage, 1998].…”

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

“…Transverse anisotropy with a horizontal symmetry axis is commonly derived from the shear birefringence of body phases [16,17]. The fast polarization directions obtained from such measurements have been compared to the direction of APM [9], to tectonic trends [13], or to predictions from forward models of ¢nite deformation [18^20] to argue the validity of either end-member case or advocate a combination of both.…”