Tomographic evidence for the mantle upwelling beneath southwestern Japan and its implications for arc magmatism
Tomographic evidence for the mantle upwelling beneath southwestern Japan and its implications for arc magmatism
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DOI：
10.1016/j.epsl.2006.11.024
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发表时间：
2007-02-15
影响因子：
5.3
通讯作者：
Hasegawa, Akira
中科院分区：
文献类型：
--
作者：
Nakajima, Junichi;Hasegawa, Akira
日本西南部的地震构造和火山构造受古太平洋板块和年轻的菲律宾海板块俯冲的影响。为了了解日本西南部的构造格局，我们利用日本岛上密集的地震台网记录的大量高质量的到时资料，试图确定日本西南部上地幔的三维P波和S波速度结构。我们的结果表明，在菲律宾海板之下，初国和四国地区至少在300公里以下存在着一个大型的低速异常。我们推断，低速区是一股从深部向上涌流。探测到两个由该低速带衍生的较浅低速带。一种是朝向初国地区的第四纪火山，另一种是朝向纪伊半岛下面的最上面的地幔。前者表明，初谷地区第四纪火山的岩浆来源于上地幔的上升流。这一观点支持了化学和岩石学的观点，即中部地区喷发的碱性玄武岩应该起源于更原始或更富集的地幔。后者是Kii半岛之下没有与俯冲的菲律宾海板块相对应的高速异常的原因。由于与地幔上涌的相互作用，菲律宾海板块可能在基伊半岛下方局部比周围地区更具水化作用。因此，在伴随着强烈的台内地震活动的递进变质过程中，可能会发生脱水反应，从而降低了板块内的地震速度。这样释放到盖层板块的水溶液可能是地幔类型氦的来源，并在盖层板块的地壳中触发了长期的群体活动。我们的结果表明，菲律宾海板块之下的不均匀结构对日本西南部的构造具有重要意义。(C)2006爱思唯尔B.V.保留所有权利。
The seismotectonics and volcanotectonics of southwestern (SW) Japan have been affected by the subduction of the old Pacific and the young Philippine Sea slabs. We have attempted to determine the three-dimensional P- and S-wave velocity structures of the upper mantle in SW Japan by using a large amount of high-quality arrival-time data recorded by the dense seismograph network on the Japanese Islands to understand the tectonic framework of SW Japan. Our results show the existence of a large low-velocity anomaly below the Philippine Sea slab in the Chugoku and Shikoku districts at least down to depths of 300 km. We infer that the low-velocity zone is an upwelling flow from depth. Two shallower low-velocity zones derived from this low-velocity zone are detected. One is toward the Quaternary volcanoes in the Chugoku district and the other is toward the uppermost mantle beneath the Kii Peninsula. The former suggests that the magma for the Quaternary volcanoes in the Chugoku district is attributable to the upwelling flow in the upper mantle. This idea supports the geochemical and petrological observations that the erupted alkalic basalt in the Chugoku district should have originated from a more primitive or enriched mantle. The latter is responsible for the absence of a high-velocity anomaly corresponding to the subducted Philippine Sea slab beneath the Kii Peninsula. The Philippine Sea slab could have been more hydrated locally beneath the Kii Peninsula than that in the surrounding regions by the interaction with the mantle upwelling. Consequently, dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity, lowering the seismic velocity in the slab. Aqueous fluids thus released to the overriding plate might be a possible source of mantle-type helium and trigger a long-term swarm activity in the crust of the overriding plate. Our results indicate the importance of the heterogeneous structure below the Philippine Sea slab with regard to the tectonics of SW Japan. (c) 2006 Elsevier B.V. All rights reserved.