Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling

A recent extensive seismic wide-angle experiment revealed a new image of cr ustal and uppermantle structure across Northern Honshu Arc, Japan. The west ern part of the arc recorded the crustal deformation by the Miocene back ar e spreading of the Sea of Japan. The crust is composed of highly deformed T ertiary sedimentary layers, a relatively low velocity (5.75-5.9 km/s) cryst alline basement and a 15-km thick lower crust with a velocity of 6.6-7.0 km /s. Clear westward crustal thinning from 32 to 27 km represents the extensi onal deformation by the backarc spreading. The crust attains the maximum th ickness (32-35km) east of the backbone range for which the magmatic intrusi on/underplating since 10-15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crus t, probably remaining a stable block since the time of the backarc spreadin g.