Supercontinent Tectonics, Mantle Dynamics and Response of Magmatism and Metallogeny

Supercontinents are assemblies of all or nearly all (> 90%) of the earth's continental blocks. The oldest supercontinent speculated is the one at 3.0 Ga termed as Ur. It is difficult to test the existence of this supercontinent due to its old age. There appear to have been twice in earth history when all of the continents were fused into one supercontinent. The first truly coherent supercontinent in earth history was probably Columbia, which formed between 1.85 and 1.90 Ga, began to fragment at ~1.6 Ga, and finally broke up at ~1.3-1.2 Ga. Columbia was followed by the second supercontinent Rodinia, which lasted from ~1100 Ma to 540 Ma. The Pangea (0.25 Ga) was not a true supercontinent, but an unusually large assembly of continents making a semi-supercontinent. The southern half (Gondwanaland) of this semi-supercontinent has a dispersion history, and the northern half (Laurasia, i.e. Paleo-Asia) has an amalgamation history. At present, the third supercontinent has not formed yet, and our planet is in midway to make a true supercontinent (Amasia) in the future. The mechanisms of formation and disruption of supercontinents have been two controversial topics. A synthesis of some of the recent conceptual models suggests that mantle dynamics exerted a significant control on the assembly and breakup of supercontinents through the history of the Earth. The formation process of supercontinents is controlled by super downwelling that develops through double-sided subduction zones as seen in present-day western Pacific, and also endorsed by both geologic history and P-wave whole mantle tomography. The super downwelling swallows all material like a black hole in the outer space, pulling together continents into a tight assembly. The fate of supercontinents is managed by superplumes (super-upwelling) which break apart the continental assemblies. With the advancement in numerical modeling techniques as well as the enhancement in computational power and resource, the numerical studies of mantle dynamics have markedly progressed toward the realization of seismic tomography images of mantle structure and a better understanding of geodynamic mechanisms. The solid Earth can be considered to comprise a plate tectonics domain with broadly horizontal motion in the upper mantle, plume tectonics dominated by vertical movements in the lower mantle region and an "anti-plate tectonics" zone characterized by horizontal movements at the bottom of the mantle. Although mantle tomography opens windows into the deep Earth, the imbricated remnants of "ocean plate stratigraphy" preserved in accretionary orogens still constitute useful geological tools to study subduction-accretion-collision history, particularly in relation to the assembly of older supercontinents on the surface of the globe. The dynamics of supercontinents also impact the origin and extinction of life, surface environmental changes as well as magmatism and metallogeny. Massive flow of material and energy was induced by mantle downwelling and upwelling accompanied by supercontinent assembly and breakup, which would also lead to large-scale magmatism and metallogeny, catastrophic environmental changes, sometimes even triggering mass extinction. When a rising mantle plume impinges the base of a supercontinent, the consequent continental rifting, formation of large igneous provinces, large scale metallogeny and volcanic emissions might lead to the initiation of a plume winter, the aftermath of which would be mass extinction and long-term oceanic anoxia. Supercontinent tectonics in relation to mantle dynamics thus provides a key to evaluate the history of evolution and destruction of the continental crust, to carry out the resource assessment, to understand the history of life, and to trace the major surface environmental changes of our planet.