Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System
Depth‐Dependent Azimuthal Anisotropy Beneath the Juan de Fuca Plate System
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DOI：
10.1029/2020jb019477
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发表时间：
2020-07
期刊：
影响因子：
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通讯作者：
Z. Eilon;D. Forsyth
中科院分区：
文献类型：
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作者：
Z. Eilon;D. Forsyth
我们使用表面波测量，以揭示各向异性的胡安德富卡和戈尔达板块系统内的深度的函数。使用双平面波方法，我们测量了基模瑞利波的相速度和方位各向异性，求解各向异性剪切速度。这些表面波测量联合反演与SKS分裂研究使用马尔可夫链方法的约束。我们发现，这两个数据集是一致的，目前的反演，提供了新的限制板下的应变的垂直分布和扩散中心的过程。Juan de Fuca板块内部的各向异性在~40 - 90 km深度之间的低速区最强（~2.4%），ENE方向由板块运动和卡斯卡迪亚俯冲带的地幔回流之间的相对剪切驱动。与Pn测量结果不一致的是，Juan de Fuca岩石圈的弱各向异性（~1.1%）与预期的脊垂直方向高度倾斜，可能意味着与熔融或离轴下降流相关的复杂的岩石圈内组构。在戈达微板块中，强的浅各向异性（~1.9%）与Pn反转一致，并与扩展一致，并且可能通过边缘驱动的内部应变增强。在低速带中具有模糊方向的弱各向异性可以通过Gorda的年轻和相对于太平洋的适度运动来解释。更深（≥90 km）的组构似乎受Farallon板块边缘调制的区域流场控制：在Gorda下方各向异性很强（~1.8%），但在Juan de Fuca下方不存在，该区域位于板块的应变阴影中。
We use surface wave measurements to reveal anisotropy as a function of depth within the Juan de Fuca and Gorda plate system. Using a two‐plane wave method, we measure phase velocity and azimuthal anisotropy of fundamental mode Rayleigh waves, solving for anisotropic shear velocity. These surface wave measurements are jointly inverted with constraints from SKS splitting studies using a Markov chain approach. We show that the two data sets are consistent and present inversions that offer new constraints on the vertical distribution of strain beneath the plates and the processes at spreading centers. Anisotropy of the Juan de Fuca plate interior is strongest (~2.4%) in the low‐velocity zone between ~40‐ to 90‐km depth, with ENE direction driven by relative shear between plate motion and mantle return flow from the Cascadia subduction zone. In disagreement with Pn measurements, weak (~1.1%) lithospheric anisotropy in Juan de Fuca is highly oblique to the expected ridge‐perpendicular direction, perhaps connoting complex intralithospheric fabrics associated with melt or off‐axis downwelling. In the Gorda microplate, strong shallow anisotropy (~1.9%) is consistent with Pn inversions and aligned with spreading and may be enhanced by edge‐driven internal strain. Weak anisotropy with ambiguous orientation in the low‐velocity zone can be explained by Gorda's youth and modest motion relative to the Pacific. Deeper (≥90 km) fabric appears controlled by regional flow fields modulated by the Farallon slab edge: anisotropy is strong (~1.8%) beneath Gorda, but absent beneath the Juan de Fuca, which is in the strain shadow of the slab.