Magnetotelluric imaging of the SW Japan forearc - a lost paleoland revealed?

We investigated the crustal structure of the Kii peninsula, in the southwestern Japan forearc, using the magnetotelluric (MT) method. We found two upper-crustal anomalies: (1) a sub-horizontal conductive layer dipping northwest, at depths of 5-10 km beneath the Cretaceous accreted terrane; (2) a resistive block at depths of 2-15 km beneath the Tertiary accreted terrane, The former is interpreted as a flysh unit. The latter is consistent with the high-velocity structure detected by refraction experiments and may be a lost continental fragment of the Kuroshio paleoland. Such a continental fragment has been predicted by sedimentological studies, but there had been no geophysical supporting evidence. The mid-crustal seismicity clusters near the bottom of the resistive body. This suggests that the lost paleoland is beneath the current forearc. We also found a conductive layer in the deep crust, consistent with seismic reflectors at 15-25 km depth. This implies trapped free water reflecting a geotherm of 400 degrees C. (C) 1997 Elsevier Science B.V.