Subduction-induced strain in the upper mantle east of the Mendocino triple junction, California
Subduction-induced strain in the upper mantle east of the Mendocino triple junction, California
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DOI：
10.1029/1999jb900422
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发表时间：
2000-04
影响因子：
--
通讯作者：
R. Hartog;S. Schwartz
中科院分区：
文献类型：
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作者：
R. Hartog;S. Schwartz
我们在加州东北部Mendocino三重交界处以东的伯克利数字地震台网的五个台站观察到远震横波的分裂，这取决于地震相位的到达方向。观测到的后方位角变化不能用水平对称轴的横向各向异性变化来解释，而是归因于具有倾斜对称轴的织物的存在。我们假设各向异性是由橄榄石晶体的首选排列引起的。对橄榄石A轴可能方向的网格搜索显示，在门多西诺三重交叉点以南，它们向东倾斜，而在三重交叉点以北，它们向东北倾斜。在将射线路径与上地幔快、慢速纵波速度异常的空间分布进行对比的基础上，我们得出结论：各向异性位于地震慢区，方向受急倾快速纵波速度异常的几何形状控制。假设快速P速度异常代表俯冲的板块物质，我们得出结论，在三重交界处以北的台站下面的结构很可能是由这个刚性的、强烈的下行板块和周围地幔之间的差速运动引起的。在三交界以南，该构造可能在Farallon板块的俯冲作用仍在进行时（6 Ma之前）发育。然而，我们更倾向于将观测结果归因于最近与北美岩石圈下无板窗打开有关的软流圈流动。流动受到北部和东部刚性岩石圈的调节。
We observe splitting of teleseismic shear waves at five stations of the Berkeley Digital Seismic Network located east of the Mendocino triple junction in northeastern California that is dependent on the arrival direction of the seismic phases. The observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis. We assume that the anisotropy is caused by the preferred alignment of olivine crystals. A grid search over possible orientations of the olivine a axes reveals that south of the Mendocino triple junction they dip to the east, whereas north of the triple junction they dip to the northeast. On the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow P wave velocity anomalies in the upper mantle, we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast P wave velocity anomaly. Assuming that the fast P velocity anomaly represents subducted slab material, we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid, strong down going plate and the surrounding mantle. South of the triple junction the fabric may have developed while subduction of the Farallon plate was still ongoing in this region (prior to 6 Ma). However, we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the North American lithosphere. The flow is modulated by the presence of rigid lithosphere to the north and east.