Preliminary 1-D Electrical Conductivity Structure of Oldest Oceanic Mantle from Magnetotelluric Observation, Pacific Array Project

Abstract:

Pacific Array project is an international collaborative large-scale experiment, which aims to deploy “an array of arrays” covering a large portion of the Pacific basin using multiple leapfrog deployments of geophysical ocean bottom observation instruments such as ocean bottom seismometers (OBSs) and ocean bottom electro-magnetometers (OBEMs). Pacific Array project is expected to shed new light on the plate tectonics and its mechanism by providing essential data on the structures of the lithosphere/asthenosphere system (LAS) in the ocean environment; meanwhile, each unit array also contributes to solving regional scale problems.

As a part of the Pacific Array project, Korea-Japan joint research group conducted the “Oldest-1” array observation from 2018 to 2019. The Oldest-1 array was located on the oldest part of the Pacific plate between Guam and the Marshall Islands, with site ages range between 161 Ma and 184 Ma, and on average ~175 Ma. 12 OBSs and 7 OBEMs from Earthquake Research Institute, The University of Tokyo were deployed in October 2018 and retrieved nearly a year later, using research vessels Isabu (2018) and Onnuri (2019) of Korea Institute of Ocean Science and Technology.

The preliminary analysis of OBEM data shows a 1-D electrical conductivity structure beneath the Oldest-1 array with a resistive topmost layer (~0.1 S/m) and thick resistive lid structure, in comparison with electrical conductivity structures retrieved from other old oceanic mantle (130-140 Ma) in northwestern Pacific. We will present the preliminary 1-D resistive structure, possible biases/constraints to be considered for the interpretation, and systematic re-analysis of the data along with other observations on the northwestern Pacific region.