大地水准面异常和非洲下地幔异常体的结构
|
摘要
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。
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.
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.
引文
1 Morgan WJ.Convection plumes in the lower mantle[J].Nature,1971,230:42-43.
2 Zhao Dapeng.Seismic structure and origin of hotspots and mantle plumes[J].Earth and Planetary Science Let-ters,2001,192:251-265.
3 Samuel Henri and Farnetani Cinzia G.Thermochemical convection and helium concentrations in mantle plumes[J].Earth and Planetary Science Letters,2003,207:39-56.
4 Fitton J G,et al.Thermal and chemical structure of the ice land plume[J].Oceanographic Literature Review,1993,45:672.
5 Ritsema J,Ni S,Hel mberger D V and Crotwell H P.Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa[J].Geo-physical Research Letters,1998,25:4245-4248.
6 Ritsema J,van Heijst HJ and Woodhouse J H.Complex shear velocity structure beneath African and Iceland[J].Science,1999,286:1925-1928.
7 Zhao Dapeng.Global tomographic i mages of mantle plumes and subducting slabs:insight into deep Earth dynamics[J].Physics of the Earth and Planetary Interi-ors,2004,146:3-34.
8 Trampert Jeannot,et al.Sensitivities of seismic veloci-ties to temperature,pressure and compositioninthe low-er mantle[J].Physics of the Earth and Planetary Interi-ors,2001,124:255-267.
9 Ni Sidao,Tan Eh,Gurnis Michael and Hel mberger Don.Sharp sides to the African superplume[J].Science,2002,296:1850-1852.
10 Ni Sidao and Hel mberger Don V.Further constraints on the African superplume structure[J].Physics of the Earth and Planetary Interiors,2003,140:243-251.
11 http://cddisa.gsfc.nasa.gov/926/egm96/egm96.ht ml
12 Masters G,Jordan T H,Silver P Gand Gilbert F.As-pherical earth structure fromfoundamental spherical-mode data[J].Nature,1982,298:609-613.
13 北京大学地球物理系等.重力与固体潮教程[M].北京:地震出版社,1982.
13 Depart ment of Geophysics of Beijing University,et al.(ed.)Course of gravity and the earthquake tide[M].Beijing:Seismological Press,1982.(in Chinese)
14 傅容珊,黄建华.地球动力学[M].北京:高等教育出版社,2001.
14 Fu Rongshan and Huang Jianhua.Geodynamics[M].Beijing:High Enducation Press,2001.(in Chinese)
15 Gran,S P,van der Hilst R Dand Widiyantoro S.Glob-al seismic tomography:a snapshot of convection in the earth[J].GSA Today,1997,1-7.
16 Schubert G,Masters G,Olson P and Tackley P.Super-plumes or plume clusters[J].Physics of the Earth and Planetary Interiors,2004,146:147-162.
17 Ni Sidao and Hel mberger Don V.Seismological con-straints on the South African superplume;could be the oldest distinct structure on Earth[J].Earth and Plane-tary Science Letters,2003,206:119-131.
2 Zhao Dapeng.Seismic structure and origin of hotspots and mantle plumes[J].Earth and Planetary Science Let-ters,2001,192:251-265.
3 Samuel Henri and Farnetani Cinzia G.Thermochemical convection and helium concentrations in mantle plumes[J].Earth and Planetary Science Letters,2003,207:39-56.
4 Fitton J G,et al.Thermal and chemical structure of the ice land plume[J].Oceanographic Literature Review,1993,45:672.
5 Ritsema J,Ni S,Hel mberger D V and Crotwell H P.Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa[J].Geo-physical Research Letters,1998,25:4245-4248.
6 Ritsema J,van Heijst HJ and Woodhouse J H.Complex shear velocity structure beneath African and Iceland[J].Science,1999,286:1925-1928.
7 Zhao Dapeng.Global tomographic i mages of mantle plumes and subducting slabs:insight into deep Earth dynamics[J].Physics of the Earth and Planetary Interi-ors,2004,146:3-34.
8 Trampert Jeannot,et al.Sensitivities of seismic veloci-ties to temperature,pressure and compositioninthe low-er mantle[J].Physics of the Earth and Planetary Interi-ors,2001,124:255-267.
9 Ni Sidao,Tan Eh,Gurnis Michael and Hel mberger Don.Sharp sides to the African superplume[J].Science,2002,296:1850-1852.
10 Ni Sidao and Hel mberger Don V.Further constraints on the African superplume structure[J].Physics of the Earth and Planetary Interiors,2003,140:243-251.
11 http://cddisa.gsfc.nasa.gov/926/egm96/egm96.ht ml
12 Masters G,Jordan T H,Silver P Gand Gilbert F.As-pherical earth structure fromfoundamental spherical-mode data[J].Nature,1982,298:609-613.
13 北京大学地球物理系等.重力与固体潮教程[M].北京:地震出版社,1982.
13 Depart ment of Geophysics of Beijing University,et al.(ed.)Course of gravity and the earthquake tide[M].Beijing:Seismological Press,1982.(in Chinese)
14 傅容珊,黄建华.地球动力学[M].北京:高等教育出版社,2001.
14 Fu Rongshan and Huang Jianhua.Geodynamics[M].Beijing:High Enducation Press,2001.(in Chinese)
15 Gran,S P,van der Hilst R Dand Widiyantoro S.Glob-al seismic tomography:a snapshot of convection in the earth[J].GSA Today,1997,1-7.
16 Schubert G,Masters G,Olson P and Tackley P.Super-plumes or plume clusters[J].Physics of the Earth and Planetary Interiors,2004,146:147-162.
17 Ni Sidao and Hel mberger Don V.Seismological con-straints on the South African superplume;could be the oldest distinct structure on Earth[J].Earth and Plane-tary Science Letters,2003,206:119-131.
版权所有：© 2023 中国地质图书馆 中国地质调查局地学文献中心