TRANSITIONS AND THE AMALGAMATION AND FRAGMENTATION OF SUPERCONTINENTS

Supercontinental amalgamation and fragmentation is perhaps the most dramatic manifestation of the dynamic nature of the Earth. Pangea is known to have amalgamated at the end of the Paleozoic Era and its fragmentation commenced early in Mesozoic times. Gondwanaland amalgamated in the latest Precambrian to Early Cambrian time interval, when it may have been combined with Laurentia to make the 'all southern' supercontinent Pannotia. Rodinia, the supercontinent that 'begat all other continents,' appears to have amalgamated at the end of the Mesoproterozoic, and fragmented with the opening of the Pacific Ocean basin. Thus the formation of supercontinents over the past one billion years appears to have broadly coincided with the close of the Mesoproterozoic, Neoproterozoic, and Paleozoic eras, yet the causes of both amalgamation and fragmentation remain obscure.

Detailed paleogeographic analysis supports the suggestion that deep mantle plumes played an active and critical role in the breakup of Pangea, and associations of large igneous provinces and dike swarms with older continental margins hint that this process may have operated back to Archean times. The sites of present subduction of oceanic lithospheric slabs, which provides the main driving force for the motion of the rigid surface plates, are controlled by Neoproterozoic rifted continental margins. Deep mantle plumes may therefore have provided the basic control not only on supercontinental breakup and plate growth, but also on the location of eventual plate destruction and subsequent suturing events. Hence the lower and upper thermal boundary layers of the planet, the core-mantle boundary and the lithospheric surface, may have been functionally linked since early in Earth history and have played an endogenic role in the major transitions identified by geologists. A major puzzle lies in the role of apparently unrelated extraterrestrial impacts at or close to the times of these transitions.