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The Izu Arc has been colliding with the Honshu Arc in central Japan since ca. 15 Ma. In order to understand the provenance changes related to this collision, we studied lower to middle Miocene sandstones in and around the collision zone by analyzing their framework composition and the chemistries of detrital clinopyroxene, garnet, and chromian spinel.

Sandstone deposited in the trench and forearc basin of the Honshu Arc prior to collision includes grains of detrital garnet and chromian spinel, which originated mainly from granites and low pressure-temperature (P-T) metamorphic rocks, and forearc peridotite, respectively, parts of the Honshu Arc. The forearc and trench-fill sandstones differ in terms of their framework composition; sedimentary lithics are more abundant in the forearc sandstone than in the trench. The two groups of sediments were supplied from different parts of the Honshu Arc.

The lower part of the clastic sequence deposited within the Izu Arc is composed mainly of volcaniclastic rocks and yields detrital clinopyroxenes that originated from the Izu Arc. In contrast, the upper part is similar to the lower Miocene trench-fill deposits in terms of its framework composition and the chemistry of detrital garnet and chromian spinel. This reflects a change in provenance triggered by the initial contact of the Izu Arc and the trench between the Eurasian and Philippine Sea plates.

The lower part of the middle Miocene trench-fill that was deposited following initial contact is also similar to the lower Miocene trench-fill. The upper part, however, resembles lower Miocene sedimentary rocks of the forearc basin. This suggests that the transport path was changed by collision. During the initial stages of collision between the Honshu and Izu Arcs, the Honshu Arc was preferentially uplifted, and therefore supplied most of the detritus to the collision zone.

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