Crust and upper mantle resistivity structure in the southwestern end of the Kuril Arc as revealed by the joint analysis of conventional MT and network MT data

A joint analysis of data obtained by conventional magnetotellurics and netw ork magnetotellurics (band-width, 0.003-7,680 sec) revealed detailed resist ivity structure from the shallow crust to the upper mantle in the eastern p art of Hokkaido, Japan, situated in the southwestern end of the Kuril Arc. The results are summarized as follows: (1) A conductive layer (a few to 10 Omegam), having a basin structure, is distributed widely to a maximum depth of about 6 km in the upper crust. Considering other independent studies, s uch as seismic reflection, gravity and drill core analyses, the bottom of t his layer coincides with the boundary between the Tertiary and the Cretaceo us formations. (2) A conductive layer (10-40 Omegam) situated in the lower crust extends from the volcanic front toward the backarc side, and is simil ar to feature with the Northeastern Japan Arc. (3) A highly resistive layer (5,000-10,000 Omegam) is analyzed in the upper to middle crust of the fore arc side. Since the distribution of this layer is consistent with the high positive gravity anomaly region (227 mgal in maximum), the causative materi al may be common. A collisional tectonic event between the Eurasia plate an d the Okhotsk Paleoland in the Cretaceous period may possibly be related wi th the existence of the layer, although the detailed tectonic implications are left to be solved. (4) The resistivity of the upper mantle is 40-100 Om egam. (5) The resistivity of the Pacific plate is estimated as 700-1,000 Om egam, which is almost consistent with that of the Northeastern Japan Arc (5 00 Omegam).