Distribution of Quaternary volcanoes in the Northeast Japan arc : geologic and geophysical evidence of hot fingers in the mantle wedge

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Clustering of volcanic centers, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the rear of the volcanic arc are closely correlated in Northeast Japan. These observations may be best explained by the presence of inclined, finger-like, hot regions in the mantle wedge of the subduction zone. These fingers, perpendicular to the arc-trench system in the Northeast Japan arc, have an average separation of -80km and an average width of 50km. Each of ten fingers extends from the deep mantle (>150km) below the back-arc region toward the shallower mantle (-50km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. Volcanic basements are uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity on the surface. Although volcanic activity is rare along the Japan Sea coast, the hot mantle fingers exist with-in the mantle wedge as evidenced by tomographic results. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers, which have not yet been erupted, but may have accumulated at the Moho discontinuity.
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