Abstract
Seafloor spreading magnetic anomalies of oceanic plates progressively decay and disappear when approaching and entering subduction. Previous studies focused on the Japan Trench showed that before subduction, the flexure of oceanic lithosphere opens normal faults and rejuvenates hydrothermal circulation which results in the alteration of magnetic minerals. After subduction, thermal demagnetization affects the magnetic minerals of the oceanic crust when the increasing temperature of the subducting slab passes their Curie temperature. No other subduction zone was investigated so far. Here we study the evolution of magnetic anomalies in five subduction zones. The major observed differences are controlled by the age of the lithosphere and related physical properties. Before subduction, magnetic anomalies of old oceanic lithosphere show a significant decay (20-30%) related to alteration whereas those of young lithosphere at the Cascadia subduction zone displays no such decay as a consequence of negligible flexure on the young Juan de Fuca plate. After subduction, the magnetic anomalies of the subducting plate show a rapid decay reflecting the thermal demagnetization of extrusive basalt, amounting for 30-40% in old oceanic lithosphere and more than 70% in the young lithosphere of Cascadia. Beyond complete thermal demagnetization of extrusive basalt, about 40% of the magnetization remains in the old lithosphere and less than 30% in the young lithosphere of Cascadia.
Published Version
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