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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities
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Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities

机译：地幔剪切速度模型和横向非均质性模式的不连续性

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Resolution of the pattern of large-scale shear velocity variations above and below the known and postulated mantle discontinuities could provide constraints on the nature of mineral phase transitions, changes in composition, and the scale of mantle convection. To achieve good resolution across a full range of depths, we use a diversified data set consisting of body and mantle wave waveforms, travel times, and surface wave phase velocities. Our main focus is on the 670-km discontinuity, long presumed to be an important barrier, or impediment, to whole mantle convection. Our data set has a relatively high radial resolution throughout the mantle; in the transition zone and some 200 km below it, the long period waveforms, dominated by multiple surface reflections, make a particularly important contribution. We use a local spline support to parameterize the model; this allows us to obtain a smooth model (twice differentiable) and simplifies calculation of the model and its derivatives in applications such as three-dimensional ray tracing. In one inversion we use a continuous radial representation throughout the mantle; in the other, a discontinuity is allowed across the 670-km boundary. Both models suggest that the long-wavelength anomalies of the transition zone and the mantle below 750 km are significantly different. Near 670 km these two models display notable differences in the peak amplitudes and lateral scales of major anomalies. The degree 2 spherical harmonic, which dominates the large-scale shear velocities in the transition zone, is strongly attenuated at the top of the lower mantle where the power spectrum is essentially white. Resolution tests show that these results are robust, which suggests a possible reorganization of the flow between the upper and lower mantle. At other depths the power spectra of our models as a function of depth indicate a modest change near 400 km, where the dominating effect of degree 5 (shields) is replaced by degree: 2 (slabs). The power of the heterogeneity in at mid-mantle depths is low and nearly flat as a function of spherical harmonic degree up to l = 12, with no detectable change near 1000 km. The increase of the power in the lowermost mantle is rather gradual, not characteristic of a discontinuous change. Cross sections of our models at major subduction zones indicate that major downwelling of cold slab material may occur at some locations. On the other hand, there are numerous examples of an abrupt change of the sign of the velocity anomalies across the 670-km discontinuity. [References: 81]
机译：在已知和假定的地幔不连续面之上和之下的大规模剪切速度变化模式的解析可能会限制矿物相变的性质，成分的变化以及地幔对流的规模。为了在整个深度范围内获得良好的分辨率，我们使用了由体波和地幔波波形，传播时间和表面波相速度组成的多样化数据集。我们的主要重点是670公里的不连续性，长期以来一直被认为是整个地幔对流的重要障碍或阻碍。我们的数据集在整个地幔中具有相对较高的径向分辨率。在过渡区及其下方约200 km处，由多次表面反射主导的长周期波形做出了特别重要的贡献。我们使用局部样条线支持对模型进行参数化;这使我们可以获得平滑的模型（可微分两次），并简化了在三维射线跟踪等应用中对模型及其导数的计算。在一次反转中，我们在整个地幔中使用了连续的径向表示。另一方面，在670公里的边界上允许不连续。这两个模型都表明，过渡带和750 km以下的地幔的长波异常明显不同。在670 km附近，这两个模型在主要异常的峰值振幅和横向尺度上显示出显着差异。在过渡带的大范围剪切速度中占主导地位的2级球谐谐波在功率谱基本上为白色的下地幔顶部被强烈衰减。分辨率测试表明，这些结果是可靠的，这表明上下地幔之间的流动可能会发生重组。在其他深度处，我们模型的功率谱作为深度的函数表明在400 km附近有适度的变化，其中5度（屏蔽）的主导作用被2度（平板）代替。地幔中部深度的非均质性函数较低，并且几乎是平坦的，它是直至l = 12的球谐度的函数，在1000 km附近没有可检测到的变化。最下层地幔的功率增加是逐渐的，不是不连续变化的特征。我们模型在主要俯冲带的横截面表明，冷板坯材料的主要下沉可能发生在某些位置。另一方面，有许多例子表明，在670公里的不连续处，速度异常的符号突然改变。 [参考：81]

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