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

Abstract

[1] The Pacific Northwest (PNW) has experienced voluminous intraplate volcanism over the past $17 Ma, beginning with the Steens/Columbia River flood basalts and continuing with the stillongoing 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 $2.7 s) delay times, with pronounced lateral variations in 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 ($0.5 s), and surface wave observations suggest a region of upper mantle anisotropy ($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.Components: 12,914 words, 9 figures.

Magmatic arc structure around Mount Rainier, WA, from the joint inversion of receiver functions and surface wave dispersion

“…Consequently, if they correspond to faster and slower directions, this could cause an artificially low velocity layer [ Hacker and Abers , ]. All the ray paths came from the WNW and ESE, i.e., almost normal to the trench, and parallel to the fast direction inferred by Wagner and Long []. Therefore, anisotropy cannot explain the slow velocity in the mantle wedge.…”

A model space search approach to finite-frequency SKS splitting intensity tomography in a reduced parameter space

“…These authors documented striking variability in SKS splitting delay times along the HLP dense line (see fig. 6 of Wagner & Long 2013), with an increase in measured delay times from 1.5 to 2.5 s across a lateral distance of 200 km. This lateral variability in delay times was interpreted to represent small-scale lateral variability in the thickness of the anisotropic layer, the strength of olivine LPO in the upper mantle, or the presence of partial melt (in a shape preferred orientation, or SPO) in the shallowest upper mantle.…”

“…The results of the resolution tests for depth-dependent models presented in this study highlight the potential for constraining the depth distribution of anisotropy in the upper mantle, a particularly exciting avenue for future application of SKS splitting intensity tomography to actual data. We illustrate this potential by discussing results from the HLP SKS splitting data set, which was presented in Long et al (2009) and Wagner & Long (2013). These authors documented striking variability in SKS splitting delay times along the HLP dense line (see fig.…”

“…This is demonstrated by a similar test that has the same input model but a different (and much denser) station distribution. In addition to our tests with a random spatial distribution of 25 stations, we also carried out resolution tests using the actual station locations of the dense High Lava Plains seismic network (Long et al 2009;Wagner & Long 2013), which operated Oregon, Nevada and Idaho (USA) between 2006 and 2009. Specifically, we used the identical station configuration as Long et al (2009), which incorporated HLP stations as well as surrounding USArray Transportable Array stations and the nearby Wallowa Mountains array.…”

“…Favier & Chevrot 2003;Chevrot 2006;Sieminski et al 2008), the time is right for substantial progress on the application of SKS splitting tomography to real data. We are currently implementing our approach to data from the HLP SKS splitting data set (Long et al 2009;Wagner & Long 2013), and other recently deployed experiments that include dense linear arrays represent obvious candidates for future work (e.g. the PULSE and CAUGHT experiments in South America; Eakin et al 2015;Long et al 2016).…”