Crustal structure of the Izu collision zone in central Japan from seismic refraction data

Abstract

AbstractThe Izu‐Bonin arc (IBA) has been colliding with the more northerly Honshu arc since the middle Miocene, forming a globally unique active Izu arc‐arc collision zone (ICZ) in central Japan. To determine crustal structure of the ICZ, we constrain P and S wave velocities from active source refraction data. The velocity structures reveal that the style of collisions of the Misaka, Tanzawa, and Izu blocks, which were derived from the IBA, has been controlled by subduction of the IBA. High‐velocity rocks (Vp > 6.2 km/s) observed in the ICZ continuously extend to deeper crust, suggesting that plutonic rocks, formed by partial melting of the subducted IBA crust, intruded into the Honshu arc crust and the accreted crustal blocks of the IBA. The northernmost major fault in the ICZ dips southeastward, which is in clear contrast with other major northwestward dipping faults farther southeast. In the northernmost part of the ICZ, the upper 5 km crust of the Misaka block was delaminated from its lower part and obducted onto the Honshu arc. The Tanzawa block, southeast of the Misaka block, is bounded by northwestward dipping faults and is characterized by crustal stacking. A low‐velocity sedimentary layer between the Tanzawa and Izu blocks dips northwestward along the collisional boundary and reaches ~5 km depth, suggesting that the Izu block underthrust beneath the Tanzawa block. The overall geometry of the collisional boundaries exhibits a doubly vergent system, which is characterized by inward‐dipping thrust faults on both sides.