Journal Article Electrical resistivity structure under the western Cosmonauts Sea at the continental margin of East Antarctica inferred via a marine magnetotelluric experiment

Tetsuo Matsuno  ,  Yoshifumi Nogi  ,  Nobukazu Seama

9 ( 2 )  , pp.221 - 234 , 2015-06
The western Cosmonauts Sea, off the coast of East Antarctica, was a site of rifting of the Gondwana supercontinent and subsequent early seafloor spreading. To improve our understanding of the breakup of Gondwana, we conducted a marine magnetotelluric experiment to determine the electrical resistivity structure within the uppermost several hundred kilometers beneath the western Cosmonauts Sea. Magnetotelluric response functions at two sites, obtained after considering possible influences of non-plane magnetic field sources, suggest that these responses include distortions by topographic variations and conductive anomalies around the observation sites. Three-dimensional forward modeling confirmed that these distortions due to topographic variations and a thin (∼2-km thick) conductive layer immediately under the sites (mostly sediments) are severe. Furthermore, three-dimensional forward modeling to investigate the resistivity structure at deeper depths revealed an upper resistive layer (≥300 Ω-m), with a thickness of <100 km, and an underlying conductive half-space (∼10 Ω-m). The upper resistive layer and the underlying conductive structure most likely represent dry and water/melt-rich oceanic upper mantle, respectively. The upper resistive layer may be thinner than anticipated under the old seafloor of the study area (likely >90 Ma), and may suggest a conductive anomaly in the upper mantle produced by mantle convection and/or upwelling.
ゴンドワナ超大陸の分裂過程を理解する上で鍵となる東南極沖の西コスモノート海で海底電磁気探査を行い、海底下の比抵抗構造を明らかにした。二つの観測点でMT応答関数を推定した。推定した応答関数には、地形の起伏と海底堆積物層が原因のゆがみがあることを数値計算により確認した。それらゆがみの原因を考慮した上で推定した比抵抗構造は、深さ100 km未満の高比抵抗層とその下の低比抵抗の半無限領域である。推定したこの構造は、海底下の上部マントルの温度状態や含水量・部分溶融量に対応している。高比抵抗層の厚さは観測点の海洋底年代から予測されるものよりも薄い可能性があり、その原因としてマントル対流や上昇流にともなう低比抵抗異常域が考えられる。

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