Full waveform tomography of the upper mantle in the South Atlantic region: Imaging a westward fluxing shallow asthenosphere?
Full waveform tomography of the upper mantle in the South Atlantic region: Imaging a westward fluxing shallow asthenosphere?
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DOI：
10.1016/j.tecto.2013.06.015
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发表时间：
2013-09-24
期刊：
影响因子：
2.9
通讯作者：
Bunge, Hans-Peter
中科院分区：
文献类型：
--
作者：
Colli, Lorenzo;Fichtner, Andreas;Bunge, Hans-Peter
南大西洋区域的一个突出特点是整个洋盆的剩余水深极不对称。有人认为，剩余测深是动态的性质，所产生的大低速地震异常位于下地幔的非洲板块。遗憾的是，由于非洲和南美洲板块上地震台站稀少，以及存在大面积地震区，地幔不均匀性的格局，特别是上地幔不均匀性的格局并不为人所知。在这里，我们提出了一个新的南大西洋上地幔的地震层析成像研究。我们的模型是基于一个完整的地震波形反演的各向同性的3-D地震结构，使用一个强大的伴随方法，能够从每个地震图中提取最大的信息，约4000高品质的地震图。该理论需要模拟地震波在三维非均匀地球模型中的传播，该模型采用谱元方法计算，其中观测地震图和合成地震图之间的差异使用通过时频变换获得的相位失配来量化。该模型的图像显着的慢地震速度在浅亚岩石圈地幔约150和约1300公里的深度之间的分支在非洲和南美大陆的下地壳根之间的连续通道。在更大的深度，低于300-350公里，缓慢的异常不太明显，在对流平面形状的变化是由整体快速地幔孤立，圆形的补丁。低粘度软流层中热浮力物质从垂直平流到水平平流的对流平面形状的变化可能与该地区异常的剩余水深相一致，我们提出了一个简单的压力驱动流的流体动力学模型来评估这种情况的可行性。(C)2013爱思唯尔有限公司版权所有。
A prominent feature of the South Atlantic region is its strongly asymmetric residual bathymetry across the ocean basin. It has been suggested that the residual bathymetry is dynamic in nature, arising from the large slow velocity seismic anomaly located in the lower mantle beneath the African plate. Unfortunately, the pattern of mantle heterogeneity particularly in the upper mantle is not well known owing to the sparsity of seismic stations and the existence of large aseismic regions on the African and South American plates. Here we present a new seismic tomographic study of the South Atlantic upper mantle. Our model is based on a full seismic waveform inversion of approximate to 4000 high-quality seismograms for isotropic 3-D seismic structure using a powerful adjoint methodology capable of extracting maximum information from each seismogram. The theory requires simulation of seismic wave propagation in 3-D heterogeneous earth models computed with a spectral-element method where the differences between observed and synthetic seismograms are quantified using phase misfits obtained through a time-frequency transform. The model images a continuous channel of pronounced slow seismic velocity in the shallow sublithospheric mantle between approximate to 150 and approximate to 1300 km depth that branches in between the cratonic roots under the African and South American continents. At greater depth, below 300-350 km, the slow anomalies are less pronounced and a change in the convective planform is indicated by isolated, round shaped patches in an overall faster mantle. It is possible that the depthwise change of the convective planform from vertical to horizontal advection of hot buoyant material in a low viscosity asthenosphere can reconcile the anomalous residual bathymetry in the region, and we present a simple fluid dynamic model of pressure driven flow to assess the feasibility of this scenario. (C) 2013 Elsevier B.V. All rights reserved.