On velocity anomalies beneath southeastern China: An investigation combining mineral physics studies and seismic tomography observations

Abstract

Seismic tomography studies reveal distinct velocity and VP/VS anomalies in the mantle transition zone (MTZ) beneath the Yangtze Craton and Cathaysia Block in southeastern China. The anomalies under the Yangtze Craton are characterized by high velocity (both VP and VS) and low VP/VS ratio, while those beneath the Cathaysia Block are characterized by low velocity (especially VS) and high VP/VS ratio. Here, we conduct analyses of phase relations and thermoelasticity to model the effects of thermal and chemical homogeneities in the MTZ, by taking advantage of recent simultaneous VP and VS seismic tomography results under southeastern China. We attempt to quantify the seismic tomography results and examine the effects of temperature, chemical composition, and water (or protonization) on velocity anomalies in the deep mantle. We find VP/VS to be a powerful parameter in distinguishing the various effects of temperature, chemical composition, and protonization. We conclude that an ancient stagnated oceanic slab is most likely the main cause of the observed fast velocity and low VP/VS anomalies in the MTZ under the Yangtze Craton. This ancient slab material is most likely a product of paleo Pacific subduction around 100–125Ma ago, when the oceanic plate abruptly changed its direction of motion. Such an event has been shown to be closely related to the magmatic activities around eastern China, the ultrahigh-pressure metamorphism zone between the Yangtze Craton and the North China Craton, and the destruction of the lower crust of the North China Craton. The anomalies under the Cathaysia Block, on the other hand, are likely due to dehydration-induced partial melting of subducted Pacific slab materials. Here the large low VS anomaly in MTZ coincides with the extensive Mesozoic to Cenozoic igneous features on the surface, suggesting a state with lower viscosities in the upper mantle. Dehydration-induced partial melting in MTZ may have also promoted deformation of the South China fold belt. Our results suggest that these lithospheric processes are directly related to the tectonic interaction between the oceanic and continental plates in southeastern China and that a better understanding of past deep mantle dynamic processes may place important constraints on the evolution of the cratons in China.