Dapeng Zhao

Evolution of late Cenozoic magmatism and the crust–mantle structure in the NE Japan Arc

Abstract We review the evolution of late Cenozoic magmatism in the NE Japan arc, and examine the relationship between the magmatism and the crust–mantle structure. Recent studies reveal secular changes in the mode of magmatic activity, the magma plumbing system, erupted volumes and magmatic composition associated with the evolution of crust–mantle structures related to the tectonic evolution of the arc. The evolution of Cenozoic magmatism in the arc can be divided into three periods: the continental margin (66–21 Ma), the back-arc basin (21–13.5 Ma) and the island-arc period (13.5–0 Ma). Magmatic evolution in the back-arc basin and the island-arc periods appears to be related to the 2D to 3D change in the convection pattern of the mantle wedge related to the asthenosphere upwelling and subsequent cooling of the mantle. Geodynamic changes in the mantle caused back-arc basin basalt eruptions during the back-arc basin opening (basalt phase) followed by crustal heating and re-melting, which generated many felsic plutons and calderas (rhyolite/granite phase) in the early stage of the island-arc period. This was followed by crustal cooling and strong compression, which ensured vent connections and mixing between deeper mafic and shallower felsic magmas, erupting large volumes of Quaternary andesites (andesite phase).

Seismic anisotropy evidence for dehydration embrittlement triggering intermediate-depth earthquakes

It has been proposed that dehydration embrittlement of hydrous materials can trigger intermediate-depth earthquakes and form a double seismic zone in a subducting slab. Seismic anisotropy may provide a possible insight into intermediate-depth intraslab seismicity, because anisotropic properties of minerals change with varying water distribution, temperature and pressure. Here we present a high-resolution model of P-wave radial anisotropy tomography of the Japan subduction zone down to ~400u2009km depth, which is obtained using a large number of arrival-time data of local earthquakes and teleseismic events. Our results reveal a close correlation between the pattern of intermediate-depth seismicity and anisotropic structures. The seismicity occurs in portions of the Pacific and Philippine Sea slabs where positive radial anisotropy (i.e., horizontal velocity being faster than vertical one) dominates due to dehydration, whereas the inferred anhydrous parts of the slabs are found to be aseismic where negative radial anisotropy (i.e., vertical velocity being faster than horizontal one) dominates. Our anisotropic results suggest that intermediate-depth earthquakes in Japan could be triggered by dehydration embrittlement of hydrous minerals in the subducting slabs.