Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States
Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States
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DOI：
10.1038/nature02231
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发表时间：
2004-02
期刊：
影响因子：
64.8
通讯作者：
T. Song;D. Helmberger;S. Grand
中科院分区：
文献类型：
--
作者：
T. Song;D. Helmberger;S. Grand
地幔410 km深度处的地震不连续性一般归因于（Mg，Fe）2SiO4（参考文献）从橄榄石结构到华兹斯利结构的相变。由于这种不连续性对热扰动的响应，它的深度变化常常被当作地幔温度的代表。例如，由于克拉珀龙斜率为正，冷异常会抬高410公里的不连续面，而暖异常会压低不连续面。但由于地震波速度不均匀性和不连续面地形之间的权衡，往往阻碍了对这些不连续面的详细分析，而且结构往往显得非常复杂。在这里，我们同时模拟地震折射波和散射波从410公里的不连续性在美国西部，以约束该地区的结构。我们发现一个低速区，剪切波速度下降5%，在美国西北部下方的410公里的不连续性，从西南部俄勒冈州延伸到北方盆地和山脉省的顶部。这个低速带的厚度从20公里到90公里不等，横向变化迅速。它的空间范围与上覆火山活动的异常组成和在该地区上方观察到的地震“接收器功能”观测相吻合。我们解释低速区的成分异常，可能是由于一个致密的部分熔融层，这可能与以前的俯冲的Farallon板块和弧后延伸。这一层的存在可能表明地球过渡带的含水量很高。
The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4(refs , ) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic ‘receiver-function’ observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.