Cocos intraslab earthquakes were used to make a 3-D tomographic inversion to define a crystallographic orientation model for the mantle wedge beneath southeastern Mexico. This model provided insights regarding the pattern of the mantle wedge flow and its relationship to the geometry of the subducting slab. The mantle wedge was parametrized as a 3-D block model of crystallographic orientations assuming the elastic constants of olivine and orthopyroxene with orthorhombic symmetry (hexagonal symmetry was also tested). A linearized, damped, and iterative least-squares approach was used to account for the nonlinear behavior of the shear-wave splitting, numerically recalculating partial derivatives after each iteration. The best-fitting model is consistent with two main flow regimes: (1) 2-D corner flow in a mantle wedge core made up of A-type olivine fabric northwest of the Tehuantepec Ridge extension, and (2) 3-D trench-parallel mantle flow in a mantle wedge core made up of A-, C-, or E-type olivine fabric southeast of this geological feature. Around the Tehuantepec Ridge extension, a partially serpentinized mantle wedge tip is inferred since olivine axis orientations are trench-parallel regardless of whether a 2-D corner flow or a 3-D trench-parallel flow prevails. Right above the Tehuantepec Ridge extension (beyond the 100 km isodepth contour of the subducting slab), a change of well-resolved olivine axis orientations from trench-normal to trench-parallel while going from northwest to southeast is observed. It signals an abrupt change in the mantle flow pattern possibly through a vertical tear in the Cocos slab. 3-D toroidal flow could be driving subslab mantle material into the mantle wedge around the deepest slab segment. Lastly, approximately trench-normal olivine axis orientations are observed in the mantle wedge tip near the Mexico and Guatemala border region, and they could be explained by assuming the presence of B-type olivine fabric.

中文翻译：

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剪切波分裂断层扫描显示墨西哥东南部地幔楔的各向异性结构

Cocos 板内地震用于进行 3D 层析成像反演，以定义墨西哥东南部下方地幔楔的晶体取向模型。该模型提供了有关地幔楔流模式及其与俯冲板片几何形状关系的见解。地幔楔被参数化为晶体取向的 3-D 块模型，假设橄榄石和斜方辉石的弹性常数具有斜方对称性（还测试了六方对称性）。使用线性化、阻尼和迭代最小二乘法来解释剪切波分裂的非线性行为，并在每次迭代后以数值方式重新计算偏导数。最佳拟合模型与两种主要流态一致：(1) 地幔楔核心中的 2-D 角流，该地幔楔核心由特万特佩克海脊延伸部西北部的 A 型橄榄石组构组成，以及 (2) 3-D 海沟-地幔楔核心中的平行地幔流，该地幔楔核心由该地质特征东南部的 A、C 或 E 型橄榄石结构组成。在特万特佩克海脊延伸部分周围，由于橄榄石轴方向与海沟平行，无论是 2-D 角流还是 3-D 海沟平行流占主导地位，都推断出部分蛇纹石化的地幔楔尖。在特万特佩克海岭延伸部分的正上方（在俯冲板片的 100 公里等深线之外），观察到从西北向东南移动时，良好解析的橄榄石轴方向从海沟法向变为海沟平行。它标志着地幔流动模式的突然变化，可能是通过科科斯板块的垂直撕裂而发生的。3-D 环形流可能会将底板地幔物质驱动到最深板片部分周围的地幔楔中。最后，在墨西哥和危地马拉边境地区附近的地幔楔尖中观察到近似海沟正向橄榄石轴方向，并且可以通过假设存在 B 型橄榄石组构来解释它们。