PROVENANCE OF SANDSTONES FROM NANKAI ACCRETIONARY PRISM: COMPARISON TO BEACH SANDS AROUND IZU ARC COLLISION ZONE

The present Nankai accretionary prism of 0-3 Ma in age is characterized by offscraped trench and overflowed sediments including semi-consolidated sandstones. The origin of sands cored during DSDP-ODP offshore Shikoku has been studied by petrographic analysis as the sands were derived from Honshu continental crust with Izu volcanic arc provenances. In order to understand the rate of contribution and to know the details of sand provenances among present river-beach systems, three representative provenances were classified from the beach sands the Tenryu River area to Sagami Bay area through the Izu Peninsula area; detritus 1) from granite-metamorphic belts and Cretaceous accritionary prism, 2) from Tertiary volcanic-orogen, and 3) from Tertiary-Quternary Izu volcanic arc. We counted 300 mineral or lithic fragment grains from 10 sandstones of accretionary prism collected by submersible SHINKAI 6500 from the Tenryu and Shionomisaki Canyons, and compared with 58 samples of beach sands to the west and east of the Izu arc collision zone. We also analyzed major element chemistry of detrital clinopyroxene from the accretionary prism samples, and compared with those of beach sands. Finally, we can conclude that there are at least two kinds of sandstones; one is quartz dominant, derived from granitic rocks, the other is volcanic fragments and clinopyroxene dominant. The former resembles the beach sands to the west of the collision zone, being rich in quartz, chert fragments, etc., where the Cretaceous accretionary prism occurs. On the other hand, the latter resembles the beach sands close to the Izu volcanic arc collision zone, being rich in feldspar, volcanic rock fragments and clinopyroxene. The chemistry of clinopyroxene belongs to the augite group in Wo-En-Fs diagram. The clinopyroxenes in the beach sands and those from the volcaniclastic sediments of the accretionary prism are very similar each other. As a result, the provenances for the prism sandstones of 0-3 Ma are classified into two different areas, the Honshu continental arc and Izu volcanic arc.