Collision and subduction structure of the Izu–Bonin arc, central Japan, revealed by refraction/wide-angle reflection analysis

Abstract

Since the middle Miocene, the Izu–Bonin arc (IBA) has been colliding from the south with the Honshu arc in central Japan associated with subduction of the Philippine Sea plate (PSP). This process is responsible for forming a complex crustal structure called the Izu Collision Zone (ICZ). To obtain direct evidence of the deep structure dominated by collision and subduction, an intensive seismic experiment using explosive and vibroseis sources was conducted in the eastern part of the ICZ. CMP reflection and refraction/wide-angle reflection data were acquired on a 130-km-long seismic line crossing the Kanto Mountains (KM), the Tonoki–Aikawa Tectonic Line (TATL) and the Tanzawa Mountains (TM) from north to south. The TATL is considered to be a collision boundary separating the Honshu arc (KM) from the IBA (TM). The structural model constructed by refraction tomography and forward ray tracing shows remarkable lateral velocity variation across the TATL and some clear reflectors in the deep crust. A north dipping reflector beneath the KM was interpreted to be the deeper extension of the TATL. From the geometry of reflectors, we interpret the Tanzawa block is delaminated from the subducting slab due to the collision to form a wedge-like body thrusting between the upper and lower crust of Honshu. The TM itself shows a velocity structure almost consistent with the upper part of the crust of the Izu intraoceanic arc, south of our study area. The relocated hypocenter distribution using our velocity model shows high seismic activity concentrated around the collision boundary, which is in a marked contrast of low seismicity within the Tanzawa block. These features of seismicity are strongly dominated by the collision of the IBA with the Honshu arc.