SHEAR-WAVE SPLITTING VARIATION OVER SHORT SPATIAL SCALES ON CONTINENTS

We have analysed broad-band SKS, SKKS, PKS and S data from 12 stations of the IRIS/IDA network for shear-wave splitting parameters, fast polarization direction phi and delay time delta t. These include several stations in the Former Soviet Union (FSU) (GAR, ARU, KIV, TLY, AAK, OBN), as well as SUR (Sutherland, South Africa), ESK (Eskdalemuir, UK), ALE (Alert, NWT, Canada), NNA (Nana, Peru), PFO (Pinon Flat, CA, USA), and ERM (Erimo, Japan). We compare observations from these stations with those from other nearby stations to gain insight into the mechanisms by which the anisotropy registered by split shear waves may arise. Intraplate comparisons exclude absolute plate motion as an orienting mechanism. Alternatively, the anisotropy may be linked to either fossil deformation within the continental lithosphere or to small-scale convection. The close correspondence with geologic fabric supports the former interpretation. Commonly, the fast polarization direction is oriented parallel to the deformational axes of compressional tectonic regions. In regions of presently active tectonics, we also see fast polarization directions orthogonal to the maximum horizontal stress in the crust. Deformation localized in the margins of tectonic blacks may account for the spatial splitting variability, and can be facilitated by volatile fluxing of the continental lithosphere during the preceding subduction event.