Along-Arc Variations in the Location of Frontal Volcanoes and the Orientation of Volcanic Cross-Chains in Subduction Zones: 3-D Mantle Wedge Flow and Sub-Arc Mantle Temperatures in the southern Kuril-NE Japan subduction zone

Abstract:

We develop a 3-D thermal model for a margin segment that extends from the southern Kuril Islands to NE Japan, using a realistic geometry for the subducting Pacific plate, and investigate the effect of 3-D mantle wedge flow on the mantle wedge temperature and its relation to the distribution of arc volcanoes. Mantle wedge flow is driven largely by viscous coupling between the subducting slab and the overlying mantle, and its flow pattern is influenced by the geometry of the subducting slab and the subduction direction relative to the trench. Along the southern Kuril-NE Japan margin, the slab takes a complex geometry with varying subduction obliquity. The 3-D modeling results show that in NE Japan, the directions of the mantle wedge in-flow and out-flow are E-W, nearly parallel to the subduction direction. However, in southern Kuril, due to oblique subduction, the mantle flows in from NE, obliquely to the subduction direction, and flows out parallel to the subduction direction. These mantle wedge flow patterns are consistent with those inferred from the measured seismic anisotropy of the mantle wedge. The predicted inflow directions in both NE Japan and southern Kuril correlate well with the E-W and NE-SW orientations of cross-arc chains, respectively, indicating that the cross-chain orientation may be guided by the mantle inflow direction. In southern Kuril, obliquity subduction and steeper slab dip results in slightly cooler mantle wedge than in Tohoku at shallow depths (<100 km depth). Further, the northerly mantle inflow in southern Kuril and the westerly inflow in NE Japan converge in the hinge zone where the slab bends to accommodate a dip direction change between southern Kuril and NE Japan, discouraging mantle inflow within the hinge zone and causing the mantle wedge to be relatively cold. The slab surface depths beneath the frontal volcanoes in southern Kuril and NE Japan are about 120 km and 100 km, respectively. The along-arc variation in the mantle wedge temperature likely affects the locations of melt generation and frontal volcanoes. We qualitatively examine whether the correlations between arc location, cross-chain orientation, slab geometry, and subduction obliquity that are found in southern Kuril and NE Japan are present elsewhere.