藏西北地幔过渡带地震波速度结构研究
摘要
本文利用三重震相波形模拟获得了藏西北地幔过渡带近东西向剖面的SH波和P波速度结构.震源深度偏移波形叠加的方法可验证数据的一致性,提高三重震相识别的可靠性.研究发现,藏西北过渡带的SH波和P波速度结构并不简单线性相关.过渡带内SH波速度梯度小,其660km间断面速度跳跃大;而P波速度结构则表现为过渡带内速度梯度大,且660km间断面速度跳跃小.与此同时,两种速度结构均显示660km间断面埋深无明显异常.上述结果与藏中和拉萨块体过渡带速度结构基本一致与印度地盾下方相比,研究区过渡带底部SH波速度相当,而P波略显高速,因而Vp/Vs比值稍大.结合矿物物理研究,我们认为地震波速度异常的差异可能主要是由过渡带内化学成分的横向不均匀性,例如铝含量的增多引起的.考虑到青藏高原的构造演化,研究区过渡带内铝含量的增多可能与高原形成过程中特堤斯洋板片的俯冲-拆沉有关.
引文
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28Jacobsen S D.Effect of water on the equation of state of nominally anhydrous minerals.Rev Mineral Geochem,2006,62:321-342
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30Estabrook C H,Kind R.The nature of the660-kilometer upper-mantle seismic discontinuity from precursors to the PP phase.Nature,1996,274:1179-1182
31Shearer P M,Flanagan M P.Seismic velocity and density jumps across the410-and660-kilometer discontinuities.Science,1999,285:1545-1548
32Higo Y,Inoue T,Irifune T,et al.Effect of water on the spinel-postspinel transformation in Mg2SiO4.Geophys Res Lett,2001,28:3505-3508
33Van der Voo R,Spakman W,Bijwaard H.Tethyan subducted slabs under India.Earth Planet Sci Lett,1999,171:7-20
2Ringwood A E.Phase transformation in the mantle.Earth Planet Sci Lett,1969,5:401-412
3Ito E,Takahashi E.Postspinel transformations in the system Mg2SiO4-Fe2SiO4and some geophysical implications.J Geophys Res,1989,94:10637-10646
4Niu F,Kawakatsu H.Direct evidence for the undulation of the660km discontinuity beneath Tonga:Comparison of Japan and California array data.Geophys Res Lett,1995,22:531-534
5Castle J C,Creager K C.Seismic evidence against a mantle chemical discontinuity near depth beneath Izu-Bonin.Geophys Res Lett,1997,24:241-244
6Clooier J D,Helffrich G R,Wood B J.Seismic discontinuities and subduction zones.Phys Earth Planet Inter,2001,127:35-49
7Tajima F,Grand S P.Variation of transition zone high-velocity anomalies and depression of discontinuity associated with subduction zone from the southern Kuriles to Izu-Bonin and Ryukyu.J Geophys Res,1998,103(B7):15015-15036
8Brudzinski M R,Chen W P.A petrologic anomaly accompanying outboard earthquakes beneath Fiji-Tonga:Corresponding evidence form broadband P and S waveforms.J Geophys Res,2003,108(B6):2299,doi:10.1029/2002JB002012
9Wang Y,Wen L X,Weidner D,et al.SH velocity and compositional models near the660km discontinuity beneath South America and northeast Asia.J Geophys Res,2006,111:B07305,doi:10.1029/2005.JB003849
10Gao W,Matzel E,Grand S P.Upper mantle seismic structure beneath Eastern Mexico determined from P and S waveform inversion and its implications.J Geophys Res2006,111:BO8307,doi:10.1029/2006JB004304
11Chen W P,Tseng T L.Small660km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere.J Geophys Res,2007,112:B05309,doi:10.1029/2006JB004607
12Tseng T L,Chen W P.Discordant contrast of P-and S-wave speeds across the660km discontinuity beneath Tibet:A case for hydrous remnant of sub-continental lithosphere.Earth Planet Sci Lett,2008,doi:10.106/j.epsl.2008.01.038
13Chu R S.Upper mantle velocity structure beneath the Tibetan Plateau from triplicated seismic P waveforms.Doctor Dissertation.Saint Louis University,2008
14Tilmann F,Ni J.INDEPTH III.Seismic team Seismic imaging of the downwelling Indian lithosphere beneath central Tibet.Science,2003,300:1424-1427
15Li C,Hilst R D,Toksoz M.Constraining P-wave velocity variations in the upper mantle beneath southern Asia.Phys Earth Planet Inter,2006,154:180-195
16Huang J L,Zhao D P.High-resolution mantle tomography of China and surrounding regions.J Geophys Res,2006,111:B09303,doi:10.1029/2005JB004066
17Kind R,Yuan X H,Saul J,et al.Seismic images of crust and upper mantle beneath Tibet:Evidence for Eurasian Plate subduction.Science,2002,298:1219-1221
18Sol S J,Meltzer A,Zurek B,et al.Insight into the lithospheric structure and deformation in Eastern Tibet from splitting and travel time variations of core phases.Eos Trans Amer Geophy Union,2004,85:Abstract T31A-1279
19Yao H,van der Hilst R D,De Hoop M V.Surface-wave array tomography in SE Tibet from ambient seismic noise and two-station analysis:I—Phase velocity maps.Geophys J Int,2006,166:732-744,doi:10.1111/j.1365-246X.2006.03028.x
20Pegler G,Das S.An enhanced image of the Pamir-Hindu Kush seismic zone from relocated earthquake hypocenters.Geophys J Int,1998,134:573-595
21Fuchs K,Muller G.Computation of synthetic seismograms with the reflectivity method and comparison with observations.Geophys J R Astron Soc,1971,23:417-433
22Schmid C,van der Lee S,Giardina D.Correlated shear and bulk moduli to1400km beneath the Mediterranean region.Phys Earth Planet Int,2006,159:213-224
23Boyd O S,Jones C H,Sheehan A F.Foundering Lithosphere Imaged Beneath the Southern Sierra Nevada,California,USA.Science,2004,305:660-662
24Cammarano F,Goes S,Vacher P,et al.Inferring upper mantle temperatures from seismic velocities.Phys Earth Planet Int,2003,138:197-222
25Wang J,Sinogeikin S V,Inoue T,et al.Elastic properties of hydrous ringwoodite.Am Mineral,2003,88:1608-1611
26Yusa H,Inoue T,Ohishi Y.Isothermal compressibility of hydrous ringwoodite and its relation to the mantle discontinuities.Geophys Res Lett,2000,27:413-416
27Jacobsen S D,Smyth J R,Spetzler H,et al.Sound velocities and elastic constants of ironbearing hydrous ringwoodite.Phys Earth Planet Int,2004,143-144:47-56
28Jacobsen S D.Effect of water on the equation of state of nominally anhydrous minerals.Rev Mineral Geochem,2006,62:321-342
29Wang Y,Wen L X,Weidner D.Upper mantle SH-and P-velocity structures and compositional models beneath southern Africa.Earth Planet Sci Lett,2008,267:596-608
30Estabrook C H,Kind R.The nature of the660-kilometer upper-mantle seismic discontinuity from precursors to the PP phase.Nature,1996,274:1179-1182
31Shearer P M,Flanagan M P.Seismic velocity and density jumps across the410-and660-kilometer discontinuities.Science,1999,285:1545-1548
32Higo Y,Inoue T,Irifune T,et al.Effect of water on the spinel-postspinel transformation in Mg2SiO4.Geophys Res Lett,2001,28:3505-3508
33Van der Voo R,Spakman W,Bijwaard H.Tethyan subducted slabs under India.Earth Planet Sci Lett,1999,171:7-20
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