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Upper-mantle Stracture beneath Antarctica and Evolution of Continental Lithosphere

南极洲下上地幔结构与大陆岩石圈演化

基本信息

批准号：
14540395
负责人：
KUGE Keiko
金额：
$ 1.47万
依托单位：
KYOTO UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2003
项目状态：
已结题

项目摘要

In broadband seismograms from shallow earthquakes occurring south of New Zealand recorded at the South Pole, we found significant arrivals 40-70 s after the first SH arrivals at distances of 29°-35°and 20-50 s after the first P arrivals at distances of 28°-35°. These waves traversing beneath East Antarctica can be modeled by slightly modifying one-dimensional P and S wave velocity structures of the Canadian shield. In the obtained velocity models, a velocity reduction below the lithosphere forms a deep low-velocity zone. S waves turning below the low-velocity zone explain the SH waveforms observed at 29°-55°. The observed P waveforms, however, suggest that a corresponding reduction in P velocity is smaller. These P and S wave velocity variations are likely to be a manifestation of a depleted continental lithosphere (high concentrations of olivine and Mg#). For a lithospheric composition more depleted than that of the underlying mantle, the temperature profile estimated from our velocity models is smooth from the lithosphere to the underlying mantle. Lithospheric depletion can also explain that the velocity anomaly in the lithosphere is larger for S than for P waves. Were the composition to be assumed uniform throughout the upper mantle, a large temperature gradient would be required at the bottom of the lithosphere, implying a heat sink which is unlikely to be sustainable. Thus, a difference in composition between the lithosphere and underlying mantle leads to a reasonable estimate of temperature profile that is consistent with the observed seismic waveforms.

在南极记录的新西兰南部发生的浅层地震的宽带地震记录中，我们发现在距离29°-35°的第一次SH到达后40-70 s和距离28°-35°的第一次P到达后20-50 s有显著的到达。通过稍微修改加拿大地盾的一维P波和S波速度结构，可以模拟穿过东南极洲的这些波。在得到的速度模型中，岩石圈以下的速度降低形成了一个深低速带。低速带以下的S波解释了在29°-55°观测到的SH波形。然而，观测到的P波波形表明，相应的P速度降低幅度较小。这些P波和S波速度变化可能是大陆岩石圈枯竭的表现（橄榄石和镁#的高浓度）。对于岩石圈成分比下地幔成分更枯竭的岩石圈，我们的速度模型估计的岩石圈到下地幔的温度剖面是平滑的。岩石圈耗竭也可以解释岩石圈中S波的速度异常大于P波的速度异常。如果假设整个上地幔的成分是均匀的，那么岩石圈底部就需要一个很大的温度梯度，这意味着不太可能持续存在一个热汇。因此，岩石圈和下地幔组成的差异导致了与观测到的地震波形相一致的温度剖面的合理估计。