Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors
Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors
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DOI：
10.1093/gji/ggt511
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发表时间：
2014-04
影响因子：
2.8
通讯作者：
S. Lessing;C. Thomas;S. Rost;L. Cobden;D. Dobson
中科院分区：
文献类型：
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作者：
S. Lessing;C. Thomas;S. Rost;L. Cobden;D. Dobson
我们调查的地震结构的上地幔和地幔过渡带下印度和中国西部使用PP和SS底面反射地震不连续面，到达的前兆PP和SS的到来。我们使用高分辨率阵列地震技术来识别地震能量，并绘制不连续深度的横向变化。我们发现在西藏、中国西部和印度下方410-440 km深度处的410 km不连续面（P410 P和S410 S）的深反射，以及在天山地区和东喜马拉雅山脉下方370-390 km深度处的410 km不连续面的升高的底面反射。这些反射很可能对应于橄榄石到华德士来石的相变。410 km的不连续面在西藏中部和北方有加深的趋势。我们还发现了在北方中国地下660 km和700 km深度之间的660 km不连续面（P660 P和S660 S）的反射，这可能是由于矿物转化的ringwoodite镁铁矿和钙钛矿。这些观测结果可能与该区地幔过渡带中下部存在冷物质相一致。我们还发现了一个更深的反射层之间的700和740公里的深度在西藏，这不能解释一个凹陷的660公里的不连续。然而，这种结构可以解释为分离的洋壳和形成的中性浮力富石榴石层下地幔过渡带，由于俯冲的特提斯洋洋的洋壳。对于源和接收器的几种组合，我们没有检测到P660 P和S660 S的到达，尽管源和接收器的类似组合给出了发育良好的P660 P和S660 S到达。我们的热力学模型的地震结构的范围内的组合物和地幔地热表明，非观测P660 P和S660 S的到来可能是由底面反射系数的入射角的入射地震波的依赖性。除了410公里和660公里的不连续反射，我们观察到间歇性反射在300公里和520公里的深度。研究区域的不连续结构可能反映了与过去和现在的俯冲和地幔对流过程有关的上地幔和地幔过渡带的横向热化学变化。
We investigate the seismic structure of the upper-mantle and mantle transition zone beneath India and Western China using PP and SS underside reflections off seismic discontinuities, which arrive as precursors to the PP and SS arrival. We use high-resolution array seismic techniques to identify precursory energy and to map lateral variations of discontinuity depths. We find deep reflections off the 410 km discontinuity (P410P and S410S) beneath Tibet, Western China and India at depths of 410–440 km and elevated underside reflections of the 410 km discontinuity at 370–390 km depth beneath the Tien Shan region and Eastern Himalayas. These reflections likely correspond to the olivine to wadsleyite phase transition. The 410 km discontinuity appears to deepen in Central and Northern Tibet. We also find reflections off the 660 km discontinuity beneath Northern China at depths between 660 and 700 km (P660P and S660S) which could be attributed to the mineral transformation of ringwoodite to magnesiowuestite and perovskite. These observations could be consistent with the presence of cold material in the middle and lower part of the mantle transition zone in this region. We also find a deeper reflector between 700 and 740 km depth beneath Tibet which cannot be explained by a depressed 660 km discontinuity. This structure could, however, be explained by the segregation of oceanic crust and the formation of a neutrally buoyant garnet-rich layer beneath the mantle transition zone, due to subduction of oceanic crust of the Tethys Ocean. For several combinations of sources and receivers we do not detect arrivals of P660P and S660S although similar combinations of sources and receivers give well-developed P660P and S660S arrivals. Our thermodynamic modelling of seismic structure for a range of compositions and mantle geotherms shows that non-observations of P660P and S660S arrivals could be caused by the dependence of underside reflection coefficients on the incidence angle of the incoming seismic waves. Apart from reflections off the 410 and 660 km discontinuities, we observe intermittent reflectors at 300 and 520 km depth. The discontinuity structure of the study region likely reflects lateral thermal and chemical variations in the upper-mantle and mantle transition zone connected to past and present subduction and mantle convection processes.