Strong seismic anisotropy in the deep upper mantle beneath the Cascadia backarc: Constraints from probabilistic finite-frequency SKS splitting intensity tomography
Strong seismic anisotropy in the deep upper mantle beneath the Cascadia backarc: Constraints from probabilistic finite-frequency SKS splitting intensity tomography
复制标题
DOI：
10.1016/j.epsl.2020.116172
复制
发表时间：
2020-06
影响因子：
5.3
通讯作者：
P. Mondal;M. Long
中科院分区：
文献类型：
--
作者：
P. Mondal;M. Long
火山活跃的高熔岩平原（HLP）地区是俄勒冈州东部卡斯卡迪亚俯冲带弧后的一个引人注目的构造岩浆特征。它的特点是年轻（<12 Ma），双峰式火山活动;流纹岩火山活动的时空趋势是倾斜的，从北美板块的绝对运动的预期。人们提出了几种模型来解释卡斯卡迪亚弧后的构造演化以及上地幔过程与火山活动之间的关系;然而，共识仍然难以捉摸。由于上地幔的地震各向异性反映了地幔流动和部分熔融等过程，因此对各向异性结构的限制可以揭示卡斯卡迪亚弧后地幔动力学和构造岩浆活动之间的联系。各向异性通常通过SKS分裂测量来约束;然而，它们的解释通常是模糊的，因为它们缺乏深度分辨率。在这里，我们提出了新的约束下的HLP地区的概率有限频率SKS分裂强度层析成像，它提供了横向和深度约束的各向异性结构的上地幔各向异性。我们的技术是基于马尔可夫链蒙特卡罗方法来搜索参数空间，我们使用有限频率灵敏度内核模型扰动分裂强度观测。我们使用的数据从宽带站的密集高熔岩平原实验，提供了良好的分辨率上地幔各向异性结构，通过分辨率测试证明。我们发现在卡斯卡迪亚弧后下的上地幔深部（200-400公里深）的地震各向异性特别强的证据，这表明在上地幔深部的流动，而不是在浅地幔的部分熔融对齐，提供了一阶控制剪切波分裂延迟时间。我们的模型提供了额外的支持的想法，卡斯卡迪亚弧后的地幔流是由回滚俯冲控制。我们的研究结果表明，在深上地幔的各向异性可能是更重要的解释SKS分裂测量在某些设置比通常赞赏，并提供了一个途径，调和明显矛盾的约束各向异性结构从表面波和SKS分裂。
The volcanically active High Lava Plains (HLP) region is a striking tectonomagmatic feature of eastern Oregon, in the backarc of the Cascadia subduction zone. It features young (<12 Ma), bimodal volcanic activity; the rhyolitic volcanism has a spatiotemporal trend that is oblique to that expected from the absolute motion of the North American plate. Several models have been proposed to explain the tectonic evolution of Cascadia backarc and the relationships between upper mantle processes and volcanic activity; however, consensus remains elusive. Because seismic anisotropy in the upper mantle reflects processes such as mantle flow and partial melting, constraints on anisotropic structure can shed light on the connections between mantle dynamics and tectonomagmatic activity in the Cascadia backarc. Anisotropy is often constrained via SKS splitting measurements; however, their interpretation is typically ambiguous because they lack depth resolution. Here we present new constraints on upper mantle anisotropy beneath the HLP region from probabilistic finite-frequency SKS splitting intensity tomography, which provides both lateral and depth constraints on anisotropic structure. Our technique is based on a Markov chain Monte Carlo approach to searching parameter space, and we use finite-frequency sensitivity kernels to relate model perturbations to splitting intensity observations. We use data from broadband stations of the dense High Lava Plains experiment, which provide good resolution of upper mantle anisotropic structure, as demonstrated via resolution tests. We find evidence for particularly strong seismic anisotropy in the deep upper mantle (200-400 km depth) beneath the Cascadia backarc, suggesting that flow in the deep upper mantle, rather than alignment of partial melt in the shallow mantle, provides the first-order control on shear wave splitting delay times. Our model provides additional support for the idea that mantle flow beneath the Cascadia backarc is controlled by rollback subduction. Our results suggest that anisotropy in the deep upper mantle may be more important to the interpretation of SKS splitting measurements in some settings than commonly appreciated, and provides an avenue for reconciling apparently contradictory constraints on anisotropic structure from surface waves and SKS splitting.