Distinctive upper mantle anisotropy beneath the High Lava Plains and Eastern Snake River Plain, Pacific Northwest, USA

作者：Wagner Lara S^*; Long Maureen D

来源：Geochemistry Geophysics Geosystems, 2013, 14(10): 4647-4666.

DOI：10.1002/ggge.20275

• anisotropy
• Rayleigh waves
• shear wave splitting
• high lava plains
• Snake River Plain
• Yellowstone

The Pacific Northwest (PNW) has experienced voluminous intraplate volcanism over the past similar to 17 Ma, beginning with the Steens/Columbia River flood basalts and continuing with the still-ongoing volcanism in the High Lava Plains (HLP) and eastern Snake River Plain (SRP). Here we present two complementary datasets (SKS splitting and Rayleigh wave phase velocity anisotropy) that place constraints on the anisotropic structure of the upper mantle beneath the HLP and SRP regions. Beneath the HLP, SKS phases reveal dominantly E-W fast splitting directions and large (up to similar to 2.7 s) delay times, with pronounced lateral variations in delta t. Lateral and depth variability in the strength of anisotropy beneath the HLP is also evident from Rayleigh wave dispersion. Beneath the SRP, SKS splitting delay times are much smaller (similar to 0.5 s), and surface wave observations suggest a region of upper mantle anisotropy (similar to 50-150 km depth) with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the SRP. Beneath the HLP, the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle, resulting in constructive interference and large SKS splitting delay times. Beneath the SRP, the geometry of the anomalous anisotropic region in the shallow mantle is different, resulting in destructive interference and reduced SKS splitting delay times. We discuss several possible explanations for these observations, including variations in olivine lattice-preferred orientation (LPO) strength, transitions in olivine fabric type, and a contribution from aligned partial melt.

• 出版日期2013-10