科研之友
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摘要
Events from the South Pacific, recorded in the Tien Shan region are studied with a migration method, to measure discontinuity depth along a profile between the Mariana and Molucca Sea subduction zones. Deflections of the upper-mantle discontinuities within and between the subduction zones are investigated using PP precursors filtered to periods of 3 to 10 s. We find P-wave reflections near the 410, 520 and 660 km discontinuities. The 410 km discontinuity is elevated in the subduction region and near the predicted depth (410 km) in the average velocity region. In some cases, we find negative polarities of the reflection from the 410 km discontinuity, which may indicate water-induced melting above the olivine to wadsleyite transition. The 520 km discontinuity is shallow between the two subduction zones, and deepens towards the Mariana backarc region: this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling (elevated region) and the formation of calcium-perovskite in a more fertile upwelling (depressed region). The 660 km discontinuity is split in the Halmahera slab with one reflection each near 600 and 700 km depth, consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite, respectively. In the region between the subduction areas, we find a downward deflection of the 660 km discontinuity with short length-scale (100 300 km) undulations that may reflect compositional variation.
- 出版日期2009-1
