Co-evolution of continental lithosphere and deep mantle dynamics
Our knowledge about the continental lithosphere,over which human beings flourish,is still limited.According to the traditional plate tectonics theory,the movement of continental plates is driven by that of adjacent oceanic plates,during which the continental lithosphere only passively drifts around the Earth's surface while it exchanges mass with the underlying convective mantle only at plate boundaries or hotspots;the continental interiors,especially craton regions,mostly respond to mantle convection passively with its underlying lithosphere remaining intact over geological time.The above understanding was partly based on the traditional tectosphere model,stating that the cratonic mantle lithosphere has low density and high viscosity due to repetitive melt extraction from the ancient process of crustal formation,thus allowing for its longevity.However,recent studies show that the cratonic lithosphere is not as stable as previously thought,with many suffering various degrees of structural deformation,alteration,or even destruction.Meanwhile,reexamination of continental topography,gravity anomaly and global geoid based on updated seismic constraints reveals that the geodynamic properties of the cratonic lithosphere are actually quite different from traditional views.For example,the density of the cratonic mantle lithosphere likely increases with depth,with its mean value being notably higher than that of the ambient mantle;the continental lithosphere bears many weak zones/weak layers,especially the middle lithosphere discontinuity(MLD)at 70-120 km depth that could rheologically decouple the upper and lower cratonic mantle lithosphere during deformation;these properties could collectively promote gravitational instability including large-scale delamination or removal of the dense mantle lithosphere under adequate dynamic perturbations.Geodynamic processes that could trigger prominent lithospheric deformation,especially within the lower mantle lithosphere,include abnormal subduction behaviors of oceanic plates,and plume-lithosphere interaction during continental rifting.The former scenario,such as flat subduction,could dislocate or remove the lower continental lithosphere,causing permanent lithospheric thinning and facilitating craton destruction.This could account for the observed thinning and destruction of cratons within East Asia and western North America.The latter scenario may trigger episodic lower lithosphere delamination followed by gradual restoration,where the associated surface uplift and subsidence cause long-term thinning of the craton crusts and formation of intracratonic basins,respectively.The implied recycling of continental lithosphere into the convective mantle could be further supported by the large volumes of seismically fast anomalies at upper-mantle depths below the Atlantic Ocean,a region that has not experienced subduction during the past 200 million years,thus implying a compositional origin of these structures.In the above cases,the middle lithospheric discontinuity(MLD)may play a crucial role in separating the upper cratonic lithosphere from that below,such that the cratonic crusts remain largely unaffected or less deformed.This style of craton evolution differs significantly from the traditional wisdom,and could better explain many properties of the continental lithosphere.By summarizing these recent findings,we suggest that the continental lithosphere and the convective mantle co-evolve through supercontinent cycles,where the former could not only passively respond to the underlying dynamic forces,but also actively participate in the latter through lithospheric delamination/removal and relamination,processes that significantly enhance the efficiency of energy and mass exchange between Earth's surface and its interior.This transition in understanding demonstrates that evolution of the continental lithosphere may play a primary role in controlling the Earth's system behaviors ranging from deep mantle dynamics to environmental and climate changes.Like any new research direction,this topic still needs additional observation,experimentation,and geodynamic validation before it could be widely accepted and assimilated into the main theory of Earth sciences.
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