THE PLATE TECTONIC APPROXIMATION: Plate Nonrigidity, Diffuse Plate Boundaries, and Global Plate Reconstructions

The central assumption of plate tectonics, that plate interiors are rigid, remains a useful but uncertain approximation. Strain rates of stable plate interiors are bounded between 10⁻¹²–10⁻¹¹ year⁻¹ and ∼4 × 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 oceans exceed dimensions of 1000 km on a side. Diffuse plate boundaries cover ∼15% of Earth's surface. The maximum speed of relative plate motion across any one diffuse plate boundary ranges from ∼2 to ∼15 mm/year, which is faster than some upper bounds on intraplate motion across stable plate interiors (≤2 mm year⁻¹). Strain rates in diffuse plate boundaries can be as high as ∼10⁻⁸ year⁻¹, ∼25 times higher than the upper bound on strain rates of stable plate interiors, but ∼600 times lower than the lowest strain rates across typical narrow plate boundaries. The poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary, which is a consequence of the strong coupling across the boundary.