Effects of “Hot Fingers” on the Thermal Structure and Mantle Wedge Flow Pattern in Subduction Zones: Implications for Seismic Anisotropy and Volcanic Spacing in Northeast Japan

Abstract:

Geophysical observations in Northeast Japan indicate its complex mantle wedge dynamics: in particular, 1) narrow low-seismic-velocity regions extending from the back-arc into the sub-arc mantle, so called “hot fingers” [Tamura et al., 2002], and 2) abrupt rotation of the seismically fast polarization direction from trench-normal to trench-parallel beneath the arc [Nakajima and Hasegawa, 2004]. Although the origin of hot fingers is not well understood, its strong spatial correlation with volcanic clustering at surface indicates that they play an important role in controlling arc volcanism. The cause of trench-parallel fast direction in the fore-arc mantle is also unclear; it has been attributed to a range of mechanisms, such as trench-normal mantle wedge flow, the presence of B-type olivine and trench-parallel alignment of melt pockets in the mantle wedge, and hydration along trench-parallel deep-cutting faults in the subducting slab. In this study, we examine the effects of hot fingers on the mantle wedge flow pattern and thermal structure beneath the arc and forearc through three-dimensional numerical experiments and investigate its implications for volcanic spacing and seismic anisotropy in Northeast Japan. In the model, the effects of hot fingers are implemented by imposing thermal anomalies on the back-arc-side vertical boundary. We found that the presence of hot fingers results in 3-D dynamic pressure gradients, which induce local along-arc mantle flow between hot fingers beneath the fore-arc, consistent with the observed pattern of seismic anisotropy in the mantle wedge. Between hot fingers, the along-arc mantle flow in the fore-arc suppresses corner flow, causing lower temperatures in the mantle wedge and in the upper portion of the subducting slab. The cooler condition is likely to hinder the dehydration of the subducting slab and flux melting in the mantle wedge, and this can explain the paucity of Quaternary arc volcanism between hot fingers in Northeast Japan.