Electrical Conductivity Structure Beneath the Ryukyu Trench-Arc System and Its Relation to the Subduction of the Philippine Sea Plate

Abstract

Seafloor measurements of geomagnetic variations were undertaken at six sites in the Ryukyu trench-arc system in order to investigate the electrical conductivity structure of the crust and upper mantle with special reference to the subduction of the Philippine Sea plate. In addition to the conventional analysis in which transfer functions for the vertical component are derived, we show the effectiveness of another type of transfer functions through which seafloor horizontal fields relate to horizontal fields simultaneously recorded on an island near seafloor sites. Because of the high attenuation of geomagnetic variations at periods shorter than 30min at seafloor sites, analysis was possible only for the periods longer than 30min. We estimated both types of transfer functions for the period range of 30 to 240min. We attempted a two-dimensional modeling for a profile orthogonal to the strike of the Ryukyu trench-arc system, using a finite element method. The comparison of transfer functions derived from observations and calculations yields a model showing two columnar conductors in the mantle wedge; one at the depth range from 20 to 60km in the forearc region and the other at the depth range from 60 to 130km or more beneath the northwestern margin of the Okinawa Trough behind the arc. According to recent mineralogical studies, dehydration of some minerals in hydrated peridotite at depths of about 100km and 150km plays an important role on magma genesis in subduction zones. The columnar conductor found below the forearc region is likely to be a zone containing water released through dehydration of some minerals in the hydrated peridotite, whereas the columnar conductor beneath the northwestern margin of the Okinawa Trough seems to correspond to a zone of partial melting which may be triggered by water released through dehydration of some other minerals.