大地水准面异常和非洲下地幔异常体的结构

根据南部非洲CMB上存在一尖锐边界S波低速异常体(超级地幔柱)的最新研究成果,利用地震层析成像及重力场理论,对该异常体的结构进行了探讨。通过建立模型,计算了该模型S波低速异常体所对应的密度异常产生的区域大地水准面异常,并将其和观测大地水准面异常进行对比。通过调整模型的几何构架、物理参数和力学平衡效应(流体静力学平衡及对流动力学效应所引起的核幔边界地形的起伏),寻找能够符合实际观测大地水准面异常的模型参数,最终得到非洲底部异常体的结构。单一的异常体模型计算结果表明,只有在选取低S波速度高密度异常时,模型预测大地水准面异常才能较好地吻合实际观测(EGM96)分布,由此可以推断,该异常体可能是高密度、低S波速的化学成分不同的异常体。但是当考虑全部地幔地震层析成像的数据,将其引入模型大地水准面异常的预测并且尝试一系列模型之后发现如果用简单叠加地幔地震层析成像数据和模型异常体对大地水准面的影响来预测该地区大地水准面异常,无法获得与观测大地水准面相吻合的结果。因而考虑更为复杂的动力学模型将是很有必要的。

GEOID ANOMALY AND STRUCTURE OF ANOMALY BODY IN DEEP MANTLE UNDER SOUTH AFRICA

On the basis of the seismic tomography data we find that there is a low velocity body of S waveform on the CMB under south Africa. We have analyzed the structure of this anomaly body by using the seismic data and the geoid anomaly in this region. A number of models, which takes different geometry and parameters, is employed to fit the regional geoid anomaly pattern. The computed models show that when the model is taken as a high density and low velocity the predicted geoid patterns can consistent to the observed data(EGM96) very well. It infers that this anomaly body probably is a chemical anomaly body. However, if we take all seismic tomography data of the mantle to predict the geoid in this region it is impossible to get a model to fit the observed geoid. In this case it is necessary to develop more complicate dynamical model in the future.