Converted waves reveal a thick and layered tectosphere beneath the Kalahari super-craton
Converted waves reveal a thick and layered tectosphere beneath the Kalahari super-craton
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DOI：
10.1016/j.epsl.2006.11.048
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发表时间：
2007-02
影响因子：
5.3
通讯作者：
G. Wittlinger;V. Farra
中科院分区：
文献类型：
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作者：
G. Wittlinger;V. Farra
在太古宙克拉通之下，通常可以观察到厚而高速的树根。在这些高速龙骨内部，速度对比既不能通过表面也不能通过体波层析成像来检测。本文根据S-P-P和P-S转换波，给出了卡拉哈里超克拉通下存在不规则层状的厚龙骨的证据。为了满足面波资料和S-P转换的要求，速度模型在莫霍面之下应该有一个厚160公里的∼各向异性结构，该结构具有垂直的慢轴和随深度递减的各向异性参数。这种结构可能源于剪切变形下橄榄石[100]轴在水平面上的择优取向。这一结构与其下方的∼100公里厚的层一起，在克拉通下方形成了300公里厚的∼大陆根。在这个根部内，P速度和S速度应该分别比AK135速度模型平均高6%和4%。在∼300-350公里深之下，清楚地检测到一个低速带，可能是与克拉通溢流玄武岩有关的大型岩浆库的残余，或者是由瓦兹利石组合到橄榄石组合的熔融硅酸盐层，就在410公里深的不连续面上方。
Thick and high-velocity roots are generally observed beneath the Archean cratons. Inside these high-velocity keels, velocity contrasts are detected neither by surface nor by body waves tomographies. We present here evidences based on the S-to-P and P-to-S converted waves for the existence of an irregularly stratified and thick keel beneath the Kalahari super-craton. To satisfy surface wave data and S-to-P conversions, the velocity model should have beneath the Moho a ∼160 km thick anisotropic structure with vertical slow axis and decreasing anisotropic parameters with depth. Such a structure may stem from the preferred orientation of olivine [100] axis in the horizontal plane under shearing deformation. This structure, together with the ∼100 km thick layer underlying it, forms a ∼300 km thick continental root beneath the cratons. Inside this root, the P and S velocities should be higher on average respectively by an amount of 6% and 4% than the AK135 velocity model. Beneath ∼300–350 km depth, a low velocity zone is clearly detected that may be either the remainder of large magma reservoirs related to cratonic flood basalts or a melted silicate layer created by the transformation, just above the 410-km deep discontinuity, of wadsleyite assembly to olivine assembly.