Synthetic tomography of plume clusters and thermochemical piles
Synthetic tomography of plume clusters and thermochemical piles
复制标题
羽流簇和热化学堆的合成断层扫描
DOI：
10.1016/j.epsl.2008.11.018
复制
发表时间：
2009
影响因子：
5.3
通讯作者：
J. Ritsema
中科院分区：
文献类型：
--
作者：
A. Bull;A. Mcnamara;J. Ritsema
地震层析成像阐明了非洲和中太平洋下地幔中广泛的低剪切波速度结构，其物理和成分来源尚不确定。一种假设认为，这些异常是由相对较热和本质上致密的物质造成的，这些物质被地幔流动席卷到大型热化学物质堆中。另一种假说认为，它们是成像不佳的窄热羽状星团。地球动力学计算预测了羽流团和热化学堆的温度场本质上不同的特征。然而，层析模型的非均匀分辨率使得地球动力学温度场与层析横波异常之间的直接对比充其量是微乎其微的。在这里，我们计算了3D球状地幔对流模型的合成层析图像，并评估了热柱和热化学堆与实际地震层析图像的一致性程度。利用实验和理论矿物物理约束，将地球动力学温度场转换为横波速度。所得到的横波速度场随后与来自地震模型S20RTS的分辨率算符进行卷积，以模拟与地幔非均匀地震采样相关的衰减和变形。与S20RTS和其他全球地震模型一致，我们证明了羽状星团在层析图像上被模糊成两个宽广的对极速度异常。较大的热化学堆积被层析滤光器微弱地扭曲。与S20RTS的2阶最大值不同，速度不均匀功率谱在球谐3阶处达到峰值，但与S20RTS和许多其他地震模型相似，它的衰减很快。考虑到数值模拟参数的不确定性，从热化学堆和羽状团模型预测的层析成像与S20RTS具有同样好的相关性。然而，热化学桩在视觉对比和谐波谱的整体特征方面比层析成像更好地匹配。
Seismic tomography elucidates broad, low shear-wave velocity structures in the lower mantle beneath Africa and the central Pacific with uncertain physical and compositional origins. One hypothesis suggests that these anomalies are caused by relatively hot and intrinsically dense material that has been swept into large thermochemical piles by mantle flow. An alternative hypothesis suggests that they are instead poorly imaged clusters of narrow thermal plumes. Geodynamical calculations predict fundamentally different characters of the temperature fields of plume clusters and thermochemical piles. However the heterogeneous resolution of tomographic models makes direct comparison between geodynamical temperature fields and tomographic shear-wave anomalies tenuous at best. Here, we compute synthetic tomographic images for 3D spherical mantle convection models and evaluate how well thermal plumes and thermochemical piles can be reconciled with actual seismic tomography images. Geodynamical temperature fields are converted to shear-wave velocity using experimental and theoretical mineral physics constraints. The resultant shear-wave velocity fields are subsequently convolved with the resolution operator from seismic model S20RTS to mimic the damping and distortion associated with heterogeneous seismic sampling of the mantle. We demonstrate that plume clusters are tomographically blurred into two broad, antipodal velocity anomalies in agreement with S20RTS and other global seismic models. Large, thermochemical piles are weakly distorted by the tomographic filter. The power spectrum of velocity heterogeneity peaks at spherical harmonic degree 3, unlike the degree-2 maximum in S20RTS, but decays rapidly similar to S20RTS and many other seismic models. Predicted tomography from thermochemical pile and plume cluster models correlate equally well with S20RTS given the uncertainties in the numerical modeling parameters. However, thermochemical piles match tomography better in visual comparison and in the overall character of the harmonic spectrum.