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

Abstract

[1] The 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.Citation: Arai, R., T. Iwasaki, H. Sato, S. Abe, and N. Hirata (2013), Crustal structure of the Izu collision zone in central Japan from seismic refraction data,

Migration of earthquakes with a small stress drop in the Tanzawa Mountains, Japan

“…The cluster activity of 2012 seems to have occurred on a plane dipping to the SE with a NE-SW strike (Fig. 5), and the source area was located in the deeper extension of the SE-dipping Sone Hills faults (Arai et al 2013). This is also consistent with the tectonic characteristics of this region.…”

Transition from collision to subduction and its relation to slab seismicity and plate coupling

“…Numerical values in the plot are P wave velocities estimated from first arrival times. For deeper parts (in parentheses), we assumed velocities using the surrounding experiments (Arai et al 2009(Arai et al , 2013. Note that this reflective zone is located at the down-dip end of the strongly coupled plate boundary (see also Fig.…”

“…In this study, we used active-source data whose shot points are shown by red stars. Previous studies in the collision zone include seismic profiles whose shot points are indicated by yellow stars Arai et al 2009), green stars (Arai et al 2013), and blue stars . Record sections of SP-W1, SP-W5, and SP-E4 are shown in Figs.…”

Crustal structure in the northwestern part of the Izu collision zone in central Japan

“…From reflection analysis of the western profile, Sato et al (2006) found an aseismic slab beneath the Misaka and Tanzawa blocks at a depth of 30 to 35 km, along which the IBA middle/lower crust subducts (Arai et al 2009). Refraction analysis using the same data set (Arai et al 2013) revealed that the overall geometry of the collisional boundaries is that of a doubly vergent system (e.g., Willett et al 1993), which is characterized by inward-dipping reverse faults on both sides. Arai et al (2013) also showed that Miocene granitic plutons with high seismic velocities that intrude into the Honshu arc crust and accreted crustal blocks of the IBA (Tanzawa block) form the Kofu granitic complex (KGC) and the Tanzawa plutonic complex (TPC), respectively.…”

“…Refraction analysis using the same data set (Arai et al 2013) revealed that the overall geometry of the collisional boundaries is that of a doubly vergent system (e.g., Willett et al 1993), which is characterized by inward-dipping reverse faults on both sides. Arai et al (2013) also showed that Miocene granitic plutons with high seismic velocities that intrude into the Honshu arc crust and accreted crustal blocks of the IBA (Tanzawa block) form the Kofu granitic complex (KGC) and the Tanzawa plutonic complex (TPC), respectively. Petrological investigations have suggested that these intrusive rocks formed by syncollisional magmatism of the IBA crust that subducted beneath the Honshu arc (Saito et al 2007;Tamura et al 2010;Tani et al 2010).…”