Box Tomography: first application to the imaging of upper-mantle shear velocity and radial anisotropy structure beneath the North American continent
Box Tomography: first application to the imaging of upper-mantle shear velocity and radial anisotropy structure beneath the North American continent
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DOI：
10.1093/gji/ggy078
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发表时间：
2018-06-01
影响因子：
2.8
通讯作者：
Romanowicz, B.
中科院分区：
文献类型：
--
作者：
Clouzet, P.;Masson, Y.;Romanowicz, B.
EarthScope Transpotable Array（TA）部署在整个美国大陆提供密集的阵列覆盖，并有机会对北美（NA）上地幔的稳定部分进行高分辨率三维地震速度成像。基于我们以前的长周期波形层析成像建模，我们提出了一个更高分辨率的3-D各向同性和径向各向异性剪切波速度模型的NA岩石圈和软流圈。该模型是使用的组合，低至40秒的周期和波场的计算，使用谱元素方法进行区域和ECOMETERIALS数据的ECOMETERIALS和区域波形。我们的研究是第一个层析应用的“盒层析成像”，这使我们能够包括在我们的反演中的地震事件，而计算的地震波场只有一次，从而显着降低了数值计算成本的几个迭代的区域反演。我们确认了大陆太古代部分下方存在高速根，在某些地区可达200-250 km，但这些根的厚度并非处处与相应太古代省的地壳年龄相关。特别是，岩石圈在上级克拉通西部厚（约250 km），而在东部薄得多（约150 km）。这可能与200-110 Ma大西洋开放期间NA板块通过大流星热点导致的大流星根的热机械侵蚀有关。在岩石圈之下，上地幔低速带（LVZ）在NA大陆之下到处都存在，甚至在克拉通最厚的部分之下，尽管它在那里不太发达。剪切速度最小值的深度有很强的横向变化，而LVZ的底部在270-300 km深度附近到处都是相对平坦的，最大30 km的小起伏显示出最厚岩石圈下的翘曲和构造区域下的下挠，可能反映了残余温度异常。径向各向异性结构的分辨率较差，但在岩石圈高度变形的区域显示出明显的特征。
The EarthScope Transpotable Array (TA) deployment provides dense array coverage throughout the continental United States and with it, the opportunity for high-resolution 3-D seismic velocity imaging of the stable part of the North American (NA) upper mantle. Building upon our previous long-period waveform tomographic modeling, we present a higher resolution 3-D isotropic and radially anisotropic shear wave velocity model of the NA lithosphere and asthenosphere. The model is constructed using a combination of teleseismic and regional waveforms down to 40 s period and wavefield computations are performed using the spectral element method both for regional and teleseismic data. Our study is the first tomographic application of 'Box Tomography', which allows us to include teleseismic events in our inversion, while computing the teleseismic wavefield only once, thus significantly reducing the numerical computational cost of several iterations of the regional inversion. We confirm the presence of high-velocity roots beneath the Archean part of the continent, reaching 200-250 km in some areas, however the thickness of these roots is not everywhere correlated to the crustal age of the corresponding cratonic province. In particular, the lithosphere is thick (similar to 250 km) in the western part of the Superior craton, while it is much thinner (similar to 150 km) in its eastern part. This may be related to a thermomechanical erosion of the cratonic root due to the passage of the NA plate over the Great Meteor hotspot during the opening of the Atlantic ocean 200-110 Ma. Below the lithosphere, an upper-mantle low-velocity zone (LVZ) is present everywhere under the NA continent, even under the thickest parts of the craton, although it is less developed there. The depth of the minimum in shear velocity has strong lateral variations, whereas the bottom of the LVZ is everywhere relatively flat around 270-300 km depth, with minor undulations of maximum 30 km that show up warping under the thickest lithosphere and downwarping under tectonic regions, likely reflecting residual temperature anomalies. The radial anisotropy structure is less well resolved, but shows distinct signatures in highly deformed regions of the lithosphere.