The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations
The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations
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DOI：
10.1029/2023jb026667
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发表时间：
2023-05
期刊：
影响因子：
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通讯作者：
Shangqin Hao;P. Shearer;Tianze Liu
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文献类型：
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作者：
Shangqin Hao;P. Shearer;Tianze Liu
阿拉斯加是一个构造活跃的地区，有着悠久的俯冲和加积历史，但其深部地震结构的知识受到相对稀疏的台站分布的限制。通过结合来自EarthScope可移动阵列和其他区域地震网络的数据，我们使用地球仪SH波混响获得了阿拉斯加下方莫霍面和上地幔不连续面的高分辨率全州地图。地壳厚度通常与海拔高度相关，最深的莫霍面位于俯冲的雅库塔特板块基底增生的区域，这与其更高的密度一致，这是由于更多的镁铁质成分。布鲁克斯山脉的地壳厚度与基于艾里等厚线和弱自由空气重力异常的预测一致，表明该地区可能没有显著的密度异常。我们还解决了大部分地区的410，520和660不连续面，增厚的地幔过渡带（MTZ）和阿拉斯加中部520和660不连续面之间的正常深度差（d 660-d520），表明在MTZ上部存在俯冲的太平洋板块。在阿拉斯加东南部，一个接近正常的MTZ和一个明显较小的d 660-d520，表明MTZ下部可能存在地幔上涌。在阿拉斯加半岛下方，MTZ变薄意味着太平洋板块可能尚未到达该地区的MTZ，这也与最近的层析成像模型一致。总体而言，结果表明，弯曲或分段的太平洋板块与阿拉斯加中部和阿拉斯加半岛下的不同深度。
Alaska is a tectonically active region with a long history of subduction and terrane accretion, but knowledge of its deep seismic structure is limited by a relatively sparse station distribution. By combining data from the EarthScope Transportable Array and other regional seismic networks, we obtain a high‐resolution state‐wide map of the Moho and upper‐mantle discontinuities beneath Alaska using teleseismic SH‐wave reverberations. Crustal thickness is generally correlated with elevation and the deepest Moho is in the region with basal accretion of the subducted Yakutat plate, consistent with its higher density due to a more mafic composition. The crustal thickness in the Brooks Range agrees with the prediction based on Airy isostasy and the weak free‐air gravity anomaly, suggesting that this region probably does not have significant density anomalies. We also resolve the 410, 520, and 660 discontinuities in most regions, with a thickened mantle transition zone (MTZ) and a normal depth difference between the 520 and 660 discontinuities (d660‐d520) under central Alaska, indicating the presence of the subducted Pacific slab in the upper MTZ. A near‐normal MTZ and a significantly smaller d660‐d520 are resolved under southeastern Alaska, suggesting potential mantle upwelling in the lower MTZ. Beneath the Alaska Peninsula, the thinned MTZ implies that the Pacific slab may not have reached the MTZ in this region, which is also consistent with recent tomography models. Overall, the results demonstrate a bent or segmented Pacific slab with varying depths under central Alaska and the Alaska Peninsula.