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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps
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Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

机译：瑞利波相速度图对大盆地方位各向异性的深度约束

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摘要

We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semicircular pattern of fast seismic directions. Away from central Nevada the fast directions are similar to N-S in the eastern Great Basin, NW-SE in the Walker Lane region, and they transition from E-W to N-S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28-102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an similar to E-W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere-asthenosphere boundary at depths of similar to 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain similar to 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.
机译：我们提出了大盆地地区在16 s和102 s之间的周期获得的基模瑞利波方位角各向异性相速度图。这些地图为在内华达州中部一个弱的各向异性各向异性带附近观测到的半圆形剪切波分裂模式的起源提供了第一深度限制。提出了多种解释来解释该信号的解释，包括上升流，围绕平板的环形地幔流，岩石圈滴落和巨大脱离，但尚未达成共识。我们的相速度研究有助于限制该地区上地幔的三维各向异性结构，并有助于更好地了解美国西部下方发生的变形机制。使用来自USArray便携式阵列的数据进行色散测量。在16 s和18 s的周期中（主要是对地壳进行采样），我们在内华达州中部发现了一个各向异性较低的区域，该区域的相速度局部减小，并被快速地震方向的半圆形模式包围。远离内华达中部的快速方向类似于大盆地东部的N-S，沃克巷地区的NW-SE，并且它们从大盆地西北部的E-W过渡到N-S。我们的短时相速度图与最近的地壳接收器功能结果相结合，与岩石圈中局部厚壳附近存在半圆形各向异性信号是一致的。在更长的时期（28-102 s）（从最上层地幔中取样），各研究区的各向同性相速度显着降低，并且与E-W快速轴相似，快速方向更加均匀。相速度和各向异性的过渡可以归因于深度约为60 km的岩石圈-软流圈边界。我们用当今的软流圈流动驱动形变解释在较长时期内观察到的快速地震方向，这可能与Juan de Fuca平板回滚和太平洋软流圈向东驱动的地幔流的组合有关。我们的结果也为区域SKS分裂观测提供了背景。我们发现，尽管在快速方向上有相似的模式，但我们的短周期相速度各向异性只能解释大约30％的SKS分裂时间。这意味着区域剪切波分裂信号的起源是复杂的，并且还必须具有显着的亚岩石圈以下分量。

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