THE PLATE TECTONIC APPROXIMATION - PLATE NONRIGIDITY, DIFFUSE PLATE BOUNDARIES, AND GLOBAL PLATE RECONSTRUCTIONS

The central assumption of plate tectonics, that plate interiors are ri gid, remains a useful but uncertain approximation. Strain rates of sta ble plate interiors are bounded between 10(-12)-10(-11) year(-1) and s imilar to 4 x 10(-10) year(-1). The narrowness of all plate boundaries , the other main assumption of plate tectonics as originally conceived , is contradicted by many observations, both in the continents and in the oceans. Some diffuse plate boundaries in both continents and ocean s exceed dimensions of 1000 km on a side. Diffuse plate boundaries cov er similar to 15% of Earth's surface. The maximum speed of relative pl ate motion across any one diffuse plate boundary ranges from similar t o 2 to similar to 15 mm/year, which is faster than some upper bounds o n intraplate motion across stable plate interiors (less than or equal to 2 mm year(-1)). Strain rates in diffuse plate boundaries can be as high as similar to 10(-8) year(-1), similar to 25 times higher than th e upper bound on strain rates of stable plate interiors, but similar t o 600 times lower than the lowest strain rates across typical narrow p late boundaries. The poles of rotation of the plates flanking a diffus e oceanic plate boundary tend to be located in the diffuse boundary, w hich is a consequence of the strong coupling across the boundary.