Magnetotelluric exploration of the North Pacific: progress report and preliminary soundings near a spreading ridge

Abstract

Intensive sea floor magnetotelluric field work has been carried out over the North Pacific for the past four years including a total of six experiments, each involving the deployment of up to eighteen instruments, in clusters or arrays. Because sea floor magnetotelluric exploration today is still subject to a broad range of often stringent limitations, an important objective of this program is to assess the potential contribution of this technique to submarine geophysics by checking the consistency of its predictions with more generally accepted knowledge. A progress report is presented on this subject. In one of the recent experiments which took place on the Cocos plate in the immediate vicinity of the Pacific Rise crest at 12°N, the electrical conductivity is shown to rise rapidly at a depth as shallow as 20 km and to remain relatively high for at least another 150 km. This finding is in harmony with earlier electrical conductivity models of the upper mantle proposed for two widely different provinces of the North Pacific. The experiment reported here includes two stations located 35 and 120 km to the east of the axis of the spreading center, and with crustal ages of 0.8 and 2.7 My respectively. The conductivity profiles are remarkably similar at both stations, with only a slight decrease in the total conductance in the upper 200 km for the eastern station, the one most distant from the ridge crest. This lateral conductivity evolution is consistent with the progressive cooling of the new lithosphere and with the increased distance from ascending magma. Another feature common to both soundings is the presence of a significant, though small, anisotropy of the sea floor impedance that does not seem explainable in terms of the coastal effect. A similarity of origin with seismic velocity anisotropy observed on the Cocos plate is not excluded. The conductivity structure proposed for this Pacific Rise area appears remarkably similar to that of the Atlantic Ridge crest beneath northeastern Iceland.