Global tectonics and eustasy for the past 2 billion years

Abstract

Continental freeboard and eustasy, as gauged by the relative position of the world shelf break with respect to sea level, have varied by ± 250 m from today's ice-free shelf break depth of ∼ 200 m, during the past 600 Ma. Assuming constant or uniformly accreting continental crust and ocean water volume in an ice-free world, sea level fluctuations can be attributed to variation in the world ocean basin volume caused by changes in either its area or its depth relative to the world shelf break. An increase in volume and lowering of sea level occur as: (1) the world ocean floor ages, cools and subsides; (2) accreting continents collide, thicken and decrease in area; and (3) poorly conductive continental platforms become thermally elevated due to a size-induced stasis over the mantle. Conversely, a decrease in the age of the world ocean floor, attenuation of continental crust during rifting, and an increase in continent number and mobility, will reduce the world ocean basin volume and raise sea level. Theoretical sea level calculated from these principles correlates well with calibrated, first-order cycles of eustatic sea level change for the Phanerozoic. The record closely fits a simple model of retardation and acceleration of terrestrial heat loss during alternating periods of supercontinent accretion and fragmentation. Calibrated to sea-level highstands, successive first-order marine transgressions and orogenic “Pangea” regressions characterize a self-sustaining, ∼ 440 Ma plate tectonic cycle for the late Precambrian and Phanerozoic. The cycle can be recognized as far back as 2 Ga from the tectonic evidence of continental collision and rifting recorded in global orogenic peaks and mafic dike swarms, and may be related to major episodes of glaciation and evolutional biogenesis.