The nature of magnetic anomalies in subduction zones

Resumo

The analysis of the magnetic survey data suggests the presence of a frontal zone of intense magnetic anomalies in a number of the Pacific island-arc systems. These zones with amplitudes of 100–300 nT are observed within the Kuril–Kamchatka and Aleutian island arc systems, in Southern and Central America, and Alaska. As demonstrated by the solution of the inverse problem and petromagnetic investigation of the rocks, these zones are presumably related to the serpentinite bodies which form as a result of the hydration of the upper mantle peridotites by the oceanic water penetrating through a system of cracks and fractures into the subducting slab at its bend. The rock magnetic studies show that magnetite is the main carrier of magnetization in these serpentinite bodies. Hydration of the subducting slab also causes hydration of the mantle rocks of the overriding plate with the formation of the magnetized serpentinite wedge. The decompaction of ultrabasic rocks under hydration is marked by a decrease in the gravity field and velocities of elastic waves. As the subducting plate loses water, it becomes embrittled and becomes the localization region for the epicenters of the strongest earthquakes. Magnetic survey can be used for revealing the potential sources of catastrophic earthquakes.