SPECULATIONS ON EARTH′S RHYTHMS AND CONTINENTAL DYNAMICS
Wang Hongzhen(China University of Geosciences, Beijing, 100083)
Rhythms in earth history are probably a common feature and are recognizable in various geological processes, often with different ranks of time duration. Rhythms are generally characterized by episodicity, cyclicity or periodicity, and make up synthetically differtent development stages. They might be ultimately controlled by astronomic factors and backgrounds. Owing to the effect of superposition and disturbance through local conditions and possibly some non linear factors in geology, the rhythmic phenomenon may be complicated and shows in various degrees coupling and divergence. Long term cycles in sedimentation and tectono magmatism are emphasized, especially the Precambrian tectonic and magmatic stages, which are correlatable with each other and comparable with the major stratigraphic sequences. Stratigraphic sequences longer than the Mesosequence (30～40 Ma) on the platforms in China are briefly discussed. The controversial time limit of the Orthosequence (3rd order sequence) is reviewed on the basis of new results obtained in China. Continental geology is the main data source for earth dynamic studies. The formation of the continental crust and the assembling and dispersal of continents in earth′s history are discussed, including the possible driving force connected with different scaled mantle convections. Pangaea cycles with a time interval of 550～600 Ma forming the five pangaeas with alternating longitudinal hemisphere pattern and circum equater pattern are outlined. Possible earth expansion is addressed on the basis of increased length of solar days, recognition of ever more extensional structures on the earth, and the discrepancy between the global reconstruction of palaeocontinents and the pattern required by palaeobiogeographical interpretation. A proposal of limited earth expansion with radius reduction of no more than 25% since 2.5 Ga is presented. Expansion may have occurred through each Pangaea disintegration at ca 5% radius increase. Reconstruction of palaeocontinents on the reduced earth surface was designed and effected, and a test of the supposed earth expansion rates is provided by the method of the least dispersal of the palaeopoles of the same continent in the same period.
【CateGory Index】： P53