Brian Hahn, William E. Holt (SUNY Stony Brook), Paul Silver (DTM, Carnegie Institute Washington), Corne Kreemer (College de France, Aix en Provence)
Continental area is reduced during continental collision events and increased during rifting and extension. But what is the net effect: reduction, expansion, or neither? To address this question we calculated the present-day areal rate of change for the continents using a global strain-rate model. A finite-element approach with over 24,000 grid elements is used to define the velocity gradient tensor field within all continental plate boundary zones. Over 3,800 GPS velocity vectors, along with Quaternary fault-slip rates, are used to define the deformation field within the plate boundary zones and to define spherical cap motions. Regions near subduction margins, such as the west coast of South America, are corrected for recoverable elastic strain from large seismic events. We calculate an approximate worldwide rate for continental areal change equal to -0.19 +/- 0.02 km2/yr. The global RMS plate velocity inferred from space geodesy is in good agreement with RMS plate velocity estimates inferred from a three million year average of sea floor spreading rates (e.g., NUVEL-1A). Even allowing for RMS plate velocities to differ by as much as a factor of 2 or 3 over geologic time, our measurement of a net shortening of continental area has far reaching implications. The -0.19 km2/yr roughly corresponds to continental areal shrinkage of 10% per 100my, or about a factor of 2.5 in a billion years, and if incompressible, causes them to thicken by the same percentage. A negative global rate is not entirely surprising since most continental rifting events produce relatively short-lived continental extension, after which new oceanic crust is created. Thus, plate tectonics preferentially reduces continental area and increases the thickness of continental crust and lithosphere. Assuming that crustal area and crustal thickness have remained roughly constant over time, and that this shrinkage rate is representative, the the worldwide rate of -0.19 km2/yr implies that about 10 km3/yr of crustal volume must be added to the margins of continents, through erosion, to balance this shrinkage. This required erosion rate is similar to the worldwide river sediment load, and thus erosion may provide a means of maintaining this balance. An inevitable consequence of reducing continental area, as a fundamental process of plate tectonics, is that continental sediments that sit on top of oceanic lithosphere (apparent continents) must be continuously reincorporated into the continental volume over geologic time in order to maintain constant continental area. Thus the incorporation or addition of oceanic lithosphere into the continental lithosphere, over geologic time, may have a fundamental impact on the character and evolution of the continents. Such a fundamental process may explain why the seismic structure of most Phanerozoic continental lithosphere is essentially oceanic in character.