Absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries. Plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere. Intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress. The absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar. While the global pattern of absolute velocity is probably close to those predicted by such models, the absolute motions do not therefore provide a strong test of the driving mechanism. Comparison of predicted intraplate stress with the long‐wavelength features of the global stress field, however, as determined by in situ measurements, earthquake mechanisms, and stress‐induced geological structures, does prove to be a powerful test of possible driving forces. All absolute velocity models have several interesting properties. Lithosphere in the equatorial half of the Earth is moving significantly faster than lithosphere in the polar half. Some connection to the Earth's rotation is implied since this statement is demonstrably untrue for co‐ordinate poles much different from the geographic pole. Subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate, confounding attempts to explain the dip angles of Benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate. Ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere; such rapid migration may be a necessary but is not a sufficient condition for ridge jumps. Force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere. From the rms absolute velocities of individual plates, there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere. The predicted intraplate deviatoric stress cannot match the well‐determined stress fields in North America and Europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system. The mid‐plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities.
|Original language||English (US)|
|Number of pages||33|
|Journal||Geophysical Journal of the Royal Astronomical Society|
|State||Published - Aug 1975|
ASJC Scopus subject areas
- Geochemistry and Petrology