Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere
Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere
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DOI：
10.1016/s0012-821x(03)00198-5
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发表时间：
2003-06
影响因子：
5.3
通讯作者：
F. Simons;R. Hilst
中科院分区：
文献类型：
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作者：
F. Simons;R. Hilst
我们解释的三维地震波速度结构的澳大利亚上地幔的方位各向异性的估计过去和现在的岩石圈变形进行比较。我们从重力异常相对于地形的方向来推断化石应变场，绕过了外推地壳测量的需要，并从现今板块运动中推导出地幔变形的当前方向。我们的观测提供了区分化石和同期变形所需的深度分辨率。方位各向异性的分布是由多模式面波传播确定的。机械各向异性，或均衡补偿的方向变化，是一个代理的化石应变场，并来自两个波长带的数字重力和地形数据的光谱相干分析。地震和构造数据的联合解释解决了澳大利亚上地幔的流变转变。在150-200 km的浅部，强烈的地震各向异性形成复杂的图案。在这个制度的地震快轴是在大角度的方向上的主要缩短，定义一个机械耦合壳幔盖变形的造山过程为主的挤压。在这里，地震各向异性可以被认为是“冻结”，这表明，过去的变形留下了一致的印记上的岩石圈深度剖面。方位地震各向异性低于10200公里是较弱的，并优先与印度-澳大利亚板块的快速运动的方向。快轴与现今板块绝对运动方向的一致性表明了干燥橄榄石地幔的简单剪切变形。在热点参考框架中表示的运动比无净旋转参考框架更好地匹配地震观测。因此，地震各向异性支持热点参考框架是物理上最合理的概念。独立于板块运动模型，地震各向异性可以用来推导出一个最适合的方向，整个地幔剪切。
We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than ∼150–200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust–mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered ‘frozen’, which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below ∼200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear.