ABSTRACT
Assembling of palaeocontinents into Pangaea in earth history may have occurred periodically at 2500, 1900, 1450, 850 and 250 Ma, with a time interval of 500–600 Ma. These Pangaea occurrences are partly in accord with the Important orogenic phases in earth history, and are also comparable with the tectonic megastage boundaries marked by the formation of continental nuclei (2.8 Ga), continental paraplatform (1.8 Ga), continental platform (850 Ma) and Pangaea-250 Ma. Except Pangaea-250, the assembly of earlier pangaeas are all not quite certain. The Piper assembly of Proterozoic continents is used as a reference for Pangaea-1900 and possibly also for Pangaea-2500. Reconstruction has been made of Pangaea-850, which is different from that of Dalziel (1992) in the position of Laurentia.
Rhythms in earth’s history are probably a common feature and are notable in many geological processes. Different ranks of rhythms may discemed in sedimentation, organic evolution, magmatism and geomagnetic reversal. They show a cyclicity and periodicity that are more or less correlatable with each other. They may have reflected a common factor—the different kinds of cosmic cycles. Pangaea cycles of 500–600 Ma may represent the longest cycle known and may be related with the double cosmic year (2700 Ma). A correlation between the different ranks of stratigraphical sequences and those of cosmic cycles is preliminarily assumed.
The increasing length of solar days with time, the recognition of ever more crustal extensional structures on earth, the huge outpour of basalt flow, and the discrepancy in global reconstruction of palaeocontinents in relation to biogeographical interpretation, point to the possibility of an expanding earth, probably limited, asymmetric and punctuated. Following a non-uniformitarian viewpoint we have assumed episodic expansion of the earth after each break-up of the pangaeas with earth radius values of 80 percent after Pangaea-2500, 85 after Pangaea-1900, 90 after Pangaea-1450,95 after Pangaea-850, and approximate to the present after Pangaea-250. The expansion event probably took place along with the disintegration of each Pangaea, and was followed by a long, comparatively static period. A method of computer reconstruction of palaeocontinents on a smaller globe was designed on the basis of palaeomagnetic data and unchanged area and decreased curvature of the continents. For evaluation of the radius reduction adopted, we have applied the Egyed-Ward model, by use of the available palaeomagnetic data and the method of least diversion of pole values for the same period, to find out the optimum value of the expansion rate. The results for Meso-Neoproterozoic and for the Cambrian are on the whole consistent with data adopted in this paper Earth expansion is a controversial topic and the present work is preliminary and speculative. The main theme is to investigate the problem with a viewpoint of Neocatastrophism and an idea of punctuated progression in the history of the earth.
