Rayleigh wave phase‐velocity heterogeneity and multilayered azimuthal anisotropy of the Superior Craton, Ontario
Rayleigh wave phase‐velocity heterogeneity and multilayered azimuthal anisotropy of the Superior Craton, Ontario
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安大略省上克拉通的瑞利波相速度异质性和多层方位各向异性
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发表时间：
2009
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通讯作者：
S. Lebedev
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作者：
F. Darbyshire;S. Lebedev
利用周期为40-160 s的瑞利波相速度资料，研究了加拿大安大略省上级威尔顿和格伦维尔地区上地幔的方位各向异性结构。152两站色散测量相结合的层析反演，同时解决各向同性和各向异性的条款，使用最小二乘技术。我们进行了一系列的测试，以获得最佳的正则化（平滑和阻尼），并评估各向同性和各向异性异常之间的分辨率和权衡。层析反演是能够解决各向同性相速度异常的规模为200-300公里，并区分不同的各向异性制度在500公里的规模在整个研究区域。层析成像模型中的各向同性相速度异常在区域平均值周围的范围高达± 2%，这与Brune和Dorman（1963年）的加拿大地盾色散曲线相似，相速度比全球参考模型高出3%。方位向相速度各向异性的幅度最大可达1.2%，根据各向同性相速度异常和方位向各向异性，研究区的东西向划分是明显的。在西部上级，各向同性相速度一般高于区域平均值。各向异性观察到在所有的时期，与ENE-WSW到NE-SW的快速传播方向。在≤120 s的周期，各向异性可能是由于冻结的岩石圈组构与构造边界对齐，而在较长的周期各向异性被解释为产生于现今的岩石圈下的流动。从公布的SKS测量的快速方向接近整个周期范围内采样的快速瑞利波传播方向，大SKS分裂时间可以占这种接近重合的快速传播方向。在安大略东部的大部分地区，相速度低于区域平均水平。快速传播方向在40-130 s周期内由西北-东南向旋转到140-160 s周期内的西北-东南向。结果表明，由于当前和最近的岩石圈下流动的各向异性织物冻结到岩石圈和织物之间的快速传播方向的差异。上级威尼托和格伦维尔省的特点是大规模的结构变化，反映了该地区复杂的构造历史。这项研究突出了东部和西部的安大略的特点之间的差异，并表明发生在亚热带上地幔的多层各向异性。
SUMMARY We study the azimuthally anisotropic upper-mantle structure of the Superior Craton and Grenville Province in Ontario, Canada, using Rayleigh wave phase-velocity data in the period range 40–160 s. 152 two-station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least-squares technique. We perform a series of tests to derive optimal regularization (smoothing and damping) and to assess the resolution of, and trade-offs between, isotropic and anisotropic anomalies. The tomographic inversion is able to resolve isotropic phase-velocity anomalies on a scale of 200-300 km and to distinguish between different anisotropic regimes on a 500-km scale across the study region. Isotropic phase-velocity anomalies in the tomographic model span a range of up to ±2 per cent around a regional average which is similar to the Canadian Shield dispersion curve of Brune & Dorman (1963), with phase velocities up to 3 per cent above global reference models. The amplitude of azimuthal phase-velocity anisotropy reaches a maximum of ∼1.2 per cent. A clear east–west division of the study area, based on both isotropic phasevelocity anomalies and azimuthal anisotropy, is apparent. In the western Superior, isotropic phase velocities are generally higher than the regional average. Anisotropy is observed at all periods, with ENE–WSW to NE–SW fast-propagation directions. At periods ≤120 s, the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries, whereas the anisotropy at longer periods is interpreted to arise from present-day sublithospheric flow. The fast directions from published SKS measurements are close to the fast Rayleigh wave propagation directions throughout the period range sampled, and the large SKS splitting times may be accounted for by this near-coincidence of fast-propagation directions. Across most of eastern Ontario, phase velocities are lower than the regional average. Fast-propagation directions rotate from ∼NW–SE at 40–130 s period to WNW–ESE at periods 140–160 s. The results suggest a difference in fast-propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow. The Superior Craton and Grenville Province are characterized by large-scale structural variations that reflect the complex tectonic history of the region. This study highlights differences between the characteristics of eastern and western Ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle.