Comparison between tomographic structures and models of convection in the upper mantle
Comparison between tomographic structures and models of convection in the upper mantle
复制标题
上地幔层析结构与对流模型的比较
DOI：
10.1111/j.1365-246x.1996.tb06351.x
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
C. Sotin
中科院分区：
文献类型：
--
作者：
P. Vacher;A. Mocquet;C. Sotin
总结
Gruneisen和三阶有限应变理论被用来计算矿物的密度和地震波速度。采用Ito & Takahashi（1987）的矿物学模型，用Hashin-Shtrikman平均法计算了上地幔的地震速度。首先获得沿沿着线的1-D剖面，并与IASP 91模型进行比较。考虑了相变的热效应，考虑了不同的相变温度。对于α-橄榄石、β-尖晶石和γ-尖晶石，分别在1473、1573和1613 K引发的焙烧反应得到最好的结果。相变产生的热效应的合并，使速度跳跃在不连续性接近IASP 91。
其次，利用Dupeyrat，Sotin和Parmentier（1995）建立的对流模型，结合板块构造，计算了上地幔地震异常的一维剖面和二维场。平均剖面显示的地震速度梯度非常接近IASP 91，但个别值太高，表明对流模型的平均温度剖面太冷了400 K。当低通滤波到目前可用的层析成像模型的分辨率尺度时，计算的地震异常的幅度和形状与层析成像研究的结果一致。P波慢度的异常幅度在-2.7%至3.8%之间，S波慢度的异常幅度在-3.3%至4.5%之间。这些异常对应于-465至520 K的横向温度变化。这些计算用于（1）帮助解释全球层析成像模型，例如通过计算横向不均匀性的谱，（2）测试地幔对流数值模型计算中使用的基本假设的适当性，并建立一个理论温度剖面图，该剖面图将给出与IASP 91的最佳拟合。在最上层地幔中，该理论模型的形状接近于对流模型和1473 K温度，但在过渡区，剖面高度亚绝热。合成地震异常的频谱与层析成像研究的频谱相似，大部分能量包含在最严重的角阶数l内，并且能量随l的增加而快速下降。空间滤波对异质性具有明显不同的影响，这取决于它们各自的波长。有人建议，在层析模型中观察到的下降率的变化在l= 7是密切相关的过滤器波长，并可能在较小程度上对应于地幔非均质性的特征波长。
SUMMARY
Gruneisen's and third-order finite-strain theories are used to compute the density and seismic-wave velocities of minerals. Assuming the minealogical model of Ito & Takahashi (1987), seismic velocities of the upper mantle are calculated using the Hashin–Shtrikman averaging procedure. 1-D profiles are first obtained along adiabats, and compared to the IASP91 model. Different adiabats are considered in order to take into account the thermal effect of phase transitions. The best results are found with adiabats initiated at 1473, 1573 and 1613 K for α-olivine, β- and γ-spinel, respectively. The incorporation of thermal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of IASP91.
Next, a model of convection constructed by Dupeyrat, Sotin & Parmentier (1995), incorporating plate tectonics, is used to compute 1-D profiles and 2-D fields of seismic anomalies in the upper mantle. Averaged profiles show seismic-velocity gradients very close to those of IASP91, but individual values are much too high, suggesting that the mean temperature profile of the convection model is too cold by 400 K. When low-pass filtered to the resolution scale of presently available tomogrphic models, both the amplitude and shape of the computed seismic anomalies are consistent with the results of tomographic studies. The amplitude of the anomalies ranges between – 2.7 and 3.8% for P-wave slownesses, and from – 3.3 to 4.5% for S-wave slownesses. These anomalies correspond to lateral temperature variations of –465 to 520 K. These calculations are used (1) as an aid to the interpretation of global tomographic models, for instance by computing spectra of lateral heterogeneities, and (2) to test the adequacy of the basic assumptions used in the computation of numerical models of mantle convection, and to build a theoretical temperature profile that would give the best fit to IASP91. In the uppermost mantle this theoretical model has a shape close to both the convection model and the 1473 K adiabat, but in the transition zone the profile is highly subadiabatic. The spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies, with most of the energy contained within gravest angular orders l, and a fast decrease of energy with increasing l. The spatial filtering has clearly different effects on heterogeneities, depending on their respective wavelengths. It is suggested that the change of decreasing rate observed in tomographic models at l= 7 is closely related to the filter wavelength and may correspond at a lesser extent to a characteristic wavelength to mantle heterogeneities.