Upper-mantle Stracture beneath Antarctica and Evolution of Continental Lithosphere

在南极记录的新西兰南部浅层地震的宽带地震记录中，我们发现在距离29°-35°的SH波到达后有40-70个S震源显著到达，在28°-35°的距离有20-50个P波到达S有显著意义。这些横穿南极东部的波可以通过稍微修改加拿大屏蔽层的一维P波和S波的波速结构来模拟。在所得到的速度模型中，在岩石圈以下的速度降低形成了深低速带。低速带下方的S波解释了29°-55°观测到的SH波。然而，观测到的P波表明，P波速度的相应降低较小。这些P波和S波的波速变化可能是大陆岩石圈亏损(橄榄石和镁#含量较高)的表现。对于比下伏地幔更贫乏的岩石圈成分，根据我们的速度模型估计的温度剖面从岩石圈到下地幔是平滑的。岩石圈损耗也可以解释S的岩石圈速度异常大于P波的速度异常。如果整个上地幔的成分是一致的，岩石圈底部就需要一个很大的温度梯度，这意味着一个不太可能持续的热沉。因此，岩石圈和下伏地幔之间的成分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。

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.