Electrical conductivity of old oceanic mantle in the northwestern Pacific I: 1-D profiles suggesting differences in thermal structure not predictable from a plate cooling model

Kiyoshi Baba,Hisashi Utada,Noriko Tada,Masahiro Ichiki,Tetsuo Matsuno,Natsue Abe,Naoto Hirano,Luolei Zhang,Ruibai Li,Hisayoshi Shimizu,Pengfei Liang

doi:10.1186/s40623-017-0697-0

Abstract

Seafloor magnetotelluric (MT) experiments were recently conducted in two areas of the northwestern Pacific to investigate the nature of the old oceanic upper mantle. The areas are far from any tectonic activity, and “normal” mantle structure is therefore expected. The data were carefully analyzed to reduce the effects of coastlines and seafloor topographic changes, which are significant boundaries in electrical conductivity and thus distort seafloor MT data. An isotropic, one-dimensional electrical conductivity profile was estimated for each area. The profiles were compared with those obtained from two previous study areas in the northwestern Pacific. Between the four profiles, significant differences were observed in the thickness of the resistive layer beyond expectations based on cooling of homogeneous oceanic lithosphere over time. This surprising feature is now further clarified from what was suggested in a previous study. To explain the observed spatial variation, dynamic processes must be introduced, such as influence of the plume associated with the formation of the Shatsky Rise, or spatially non-uniform, small-scale convection in the asthenosphere. There is significant room of further investigation to determine a reasonable and comprehensive interpretation of the lithosphere–asthenosphere system beneath the northwestern Pacific. The present results demonstrate that electrical conductivity provides key information for such investigation.Graphical .

Highlights

The northwestern part of the Pacific Plate is composed of some of the oldest oceanic lithosphere on the planet
This change should mainly be attributed to relatively large-scale topographic effects, such as the coastlines west of the study areas
The collected data were analyzed based on MT methods, and 1-D electrical conductivity profiles that represent each array were estimated after topographic effect correction

Summary

Introduction

The northwestern part of the Pacific Plate is composed of some of the oldest oceanic lithosphere on the planet. Many global surface wave tomography studies have demonstrated clear age-dependence of shear-wave velocity structure (e.g., Maggi et al 2006; Nettless and Dziewonski 2008; Burgos et al 2014), and the high-velocity lid, which is interpreted as cool lithosphere, is imaged to be as thick as 100–150 km in the northwestern Pacific, the structure beneath the specific area of interest for this study has not been resolved in detail through such global studies. The age-dependent evolution of both the lithosphere and the asthenosphere should be regarded as a part of a system (e.g., Kawakatsu and Utada 2017) From this perspective, the nature of the oceanic lithosphere–asthenosphere system (LAS), for example, the relation between the evolving thermal structure and mechanical properties, is not yet fully understood based on seismic imaging methods

Methods

Results

Discussion

Conclusion