Abstract
Deep well core samples provide significant clues to clarifying the location of the Median Tectoic Line (MTL) in the Kanto Plain which is covered by thick Cenozoic sediments. Pre-Miocene basement cores from the Matsubushi SK-1 drilled in 1957 by Japan Petroleum Exploration Co., Ltd. were re-examined petrologically and geochronologically. Eight core samples collected from 1,922 m to 1,948 m are mainly composed of weakly mylonitized hornblende-biotite tonalite and fine-grained mafic rock. A K-Ar age of hornblende from weakly mylonitized tonalite is about 67 Ma. The core samples are correlative geochronologically with the plutonic rocks of the Ryoke belt. The mineral composition and microstructures of core samples are quite similar to those of the Hiji gneissose tonalite and associated basic rocks distributed along the MTL in the Chubu Region and Hiki Hills in the Kanto Mountains. Degree of mylonitization estimated by grain size of recrystallized quartz suggests that the MTL extends within 500-1,500 m to the south of the borehole site, and a possible superficial extension of the estimated MTL position is also about 1-3 km south if the MTL has a dip of moderate angle.
