Distinctive upper mantle anisotropy beneath the High Lava Plains and Eastern Snake River Plain, Pacific Northwest, USA
Distinctive upper mantle anisotropy beneath the High Lava Plains and Eastern Snake River Plain, Pacific Northwest, USA
复制标题
美国太平洋西北地区高熔岩平原和东蛇河平原下方独特的上地幔各向异性
DOI：
10.1002/ggge.20275
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发表时间：
2013
期刊：
影响因子：
3.7
通讯作者：
M. Long
中科院分区：
文献类型：
--
作者：
L. Wagner;M. Long
太平洋西北地区 (PNW) 在过去约 17 Ma 中经历了大量的板内火山活动，从斯廷斯/哥伦比亚河溢流玄武岩开始，一直到高熔岩平原 (HLP) 和东部蛇河平原 (SRP) 仍在持续的火山活动。在这里，我们提出了两个互补的数据集（SKS 分裂和瑞利波相速度各向异性），它们对 HLP 和 SRP 区域下方的上地幔的各向异性结构施加了约束。在 HLP 下方，SKS 相主要揭示了东西向快速分裂方向和大（高达 ∼2.7 s）的延迟时间，并且 δt 具有明显的横向变化。从瑞利波色散来看，HLP 下方各向异性强度的横向和深度变化也很明显。在SRP下方，SKS分裂延迟时间要小得多（~0.5秒），表面波观测表明上地幔各向异性区域（~50-150公里深度）的几何形状与在SRP外部观察到的通常平行的板块运动平行快速方向有很大偏差。在HLP之下，异常强的各向异性的几何形状与上地幔深层的各向异性相似，导致相长干涉和大的SKS分裂延迟时间。在SRP下方，浅地幔中的异常各向异性区域的几何形状不同，导致相消干涉并减少SKS分裂延迟时间。我们讨论了对这些观察结果的几种可能的解释，包括橄榄石晶格择优取向（LPO）强度的变化、橄榄石织物类型的转变以及对齐部分熔化的贡献。
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 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 ∼2.7 s) delay times, with pronounced lateral variations in δt. 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.