5:15 PM - 6:30 PM
[SSS07-P02] Study of the delamination structure at the southern end of the Hidaka collision zone-Examination using numerical calculation-
Keywords:collision zone, Hidaka, reflection survey
Hidaka Collision Zone (HCZ) is located in the central and southern part of Hokkaido where the Kuril forearc dragged by oblique subduction of the Pacific plate has collided with the Northeastern Japan arc. From upper to upper part of the Kuril’s arc lower crust have been thrusted up along the Hidaka Main Thrust (HMT) and are exposed to the surface. Various seismic surveys have been conducted for the purpose of investigating the subsurface structure of this region.
Reflection survey was carried out across the collision zone at the south end of the HCZ. From obtained depth section, Ito et al. (2013) showed that the lower crust of the Kuril arc is divided into a northeast-dipping ascending layer and a southwest-dipping descending layer at a depth of 23km. Meanwhile, Kita et al. (2012) estimated the three-dimensional P wave velocity structure in the same region by travel time tomography. They interpreted that crustal material of the Northeastern Japan arc has subdected under the thrusting Kuril arc.
In order to understand that how mass excess due to the collision between island arcs will be resolved, it is necessary to know the structure of the deep part of the collision zone. In this study, based on the two interpretations, we prepared two kind of structure models and compared the features of the actual seismic profiles and synthetic seismic profile, and examined which model is appropriate. One of models is a Delami model in which delamination occurs and delaminated lower part of the Kuril arc’s lower crust is continuously subducting to the Pacific plate and the other model is a Non-delami model in which the crustal material of the Northeastern Japan arc is subducting under the thrusting Kuril arc. Numerical calculation was performed using the two models under the similar conditions as in the actual survey, and a synthetic seismic profiles was created by applying the reflection method to the calculated waveforms. As a result, synthetic seismic profile for the Delami model, the features of the descending and thrusting reflectors are almost the same as the actual seismic profile. However, the depth of the reflector that can be traced by the reflected wave from the upper surface of the lower crust below the Kuril arc in the synthetic seismic profile appears deeper than the actual seismic profile. On the other hand, synthetic seismic profile for the Non-Delami model, it was found that only the feature of the thrusting reflector is matched. From this result, it is considered that the delaminated structure of the Kuril arc is needed to explain the result of the actual seismic profile. However, the reflected wave from the deeper extension of the Kuril’s arc subducting lower crust does not reach the receivers within the recording length and can not be imaged on the synthetic seismic profile.
Next, we performed another numerical calculation to know the location where we shot to confirm it if the deeper extension of the subducting Kuril’s arc crust is present. As a result, if we shot at the southwestern point of 45 km from the southwestern end of the actual reflection survey line, the reflected wave from the deeper extension of the Kuril’s arc subducting lower crust will reach about 24 seconds after the shooting to the recievers of the previous reflection survey settled on the land area.
Reflection survey was carried out across the collision zone at the south end of the HCZ. From obtained depth section, Ito et al. (2013) showed that the lower crust of the Kuril arc is divided into a northeast-dipping ascending layer and a southwest-dipping descending layer at a depth of 23km. Meanwhile, Kita et al. (2012) estimated the three-dimensional P wave velocity structure in the same region by travel time tomography. They interpreted that crustal material of the Northeastern Japan arc has subdected under the thrusting Kuril arc.
In order to understand that how mass excess due to the collision between island arcs will be resolved, it is necessary to know the structure of the deep part of the collision zone. In this study, based on the two interpretations, we prepared two kind of structure models and compared the features of the actual seismic profiles and synthetic seismic profile, and examined which model is appropriate. One of models is a Delami model in which delamination occurs and delaminated lower part of the Kuril arc’s lower crust is continuously subducting to the Pacific plate and the other model is a Non-delami model in which the crustal material of the Northeastern Japan arc is subducting under the thrusting Kuril arc. Numerical calculation was performed using the two models under the similar conditions as in the actual survey, and a synthetic seismic profiles was created by applying the reflection method to the calculated waveforms. As a result, synthetic seismic profile for the Delami model, the features of the descending and thrusting reflectors are almost the same as the actual seismic profile. However, the depth of the reflector that can be traced by the reflected wave from the upper surface of the lower crust below the Kuril arc in the synthetic seismic profile appears deeper than the actual seismic profile. On the other hand, synthetic seismic profile for the Non-Delami model, it was found that only the feature of the thrusting reflector is matched. From this result, it is considered that the delaminated structure of the Kuril arc is needed to explain the result of the actual seismic profile. However, the reflected wave from the deeper extension of the Kuril’s arc subducting lower crust does not reach the receivers within the recording length and can not be imaged on the synthetic seismic profile.
Next, we performed another numerical calculation to know the location where we shot to confirm it if the deeper extension of the subducting Kuril’s arc crust is present. As a result, if we shot at the southwestern point of 45 km from the southwestern end of the actual reflection survey line, the reflected wave from the deeper extension of the Kuril’s arc subducting lower crust will reach about 24 seconds after the shooting to the recievers of the previous reflection survey settled on the land area.