Geological confirmation for water-effected incipient melt origin of seismic low velocity zone (LVZ) beneath ocean basins

It is accepted that if there were no seismic low velocity zone (LVZ) beneath ocean basins, there would be no seafloor spreading and plate tectonics, but the origin of the LVZ has been under debate for over 50 years as if it is forever unsolvable with certainty. In this condensed paper, researcher briefly discuss the debate and conclude with elaborations that the issue has long been resolved, but the answer has been concealed in lengthy reviews with “pros” and “cons”. It is the water that causes incipient melting beneath oceanic lithosphere that gives rise to the seismic energy attenuation, reduced S-wave velocity, and enhanced electrical conductivity, forming the LVZ as simulated experimentally and inferred petrologically. The lack of LVZ beneath continental shield and cratonic regions is consistent with the construct of compositionally depleted and physically buoyant Archean mantle lithosphere of up to 300 km thick. The presence of LVZ beneath eastern continental China, especially beneath the North China Craton (NCC), is thus unexpected, but is readily understood as a straightforward consequence of basal hydration weakening that converted the cratonic basal lithosphere into the asthenosphere with LVZ properties with the water ultimately coming from dehydration of the paleo-Pacific slab stagnant in the mantle transition zone beneath the vast region. That is, the eastern China lithosphere thinning or NCC destruction since the Mesozoic is a geological manifestation for the process of LVZ formation, confirming geologically in simple clarity that the LVZ beneath ocean basins results from water-effected incipient melt.

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