Contrasting seismic reflectivity of the lower crust and uppermost mantle between NE Japan and SW Japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths

摘要

Petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath Ichinomegata Crater in NE Japan and beneath Oki-Dogo Island in SW Japan. In contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath Ichinomegata Crater, relatively large contrasts in acoustic impedance are present beneath Oki-Dogo Island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase-bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks. Based on the existing petrologic models of the lower crust and uppermost mantle beneath Ichinomegata Crater and Oki-Dogo Island, we then constructed model structures of the lower crust and uppermost mantle beneath these two areas, composed of units with randomly arranged, 100 m thick, foliation-parallel horizontal layers of xenolith samples, and used these model structures to examine the seismic reflectivity beneath these two areas. The results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas. Although no other recognizable seismic reflector is expected beneath Ichinomegata Crater, many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath Oki-Dogo Island. Thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between NE Japan and SW Japan.