NORTH AMERICA DYNAMICS AND WESTERN U.S. TECTONICS

The contributions of principal tectonic forces to observed intraplate stress in the North American (NA) plate are evaluated by comparing observed plate stress with stress modeled by scaling and summing 30 basis stress fields, each derived from a modeled load acting on NA. The stress field created by each load is calculated with the finite element method in a 2D spherical cap. Modeled loads represent: various portions of plate boundary, such as San Andreas tangential and normal loads; basal tractions including those obtained from 3D whole-Earth flow calculations [Becker and O'Connell, 2001; BO], and tractions concentrated on the NA cratonic root; and gravitational potential energy calculated over the plate.

Implications for NA plate dynamics include: (1) ridge push, which in the North Atlantic is unusually great, compresses NA against the western NA transform faults; (2) western NA potential energy is nearly sufficient to overcome this compression; (3) NE-oriented root drag equivalent to ~4 MPa acting on the root places western U.S. in a "stress shadow" that causes extension over most of this area; (4) basal tractions excited by global flow occur at ~20% the level estimated by BO; (5) shear load on transform boundaries averages 1-2 TN/m (1 TN/m=10^12 N/m, equivalent to 20 MPa on a 50 km-thick plate), with a San Andreas shear load of 1.6±0.4 TN/m which is largely responsible for NNW transport of the Sierra Nevada block and shear distributed across the western Great Basin; (6) a strong outward-directed pull on most subduction zones, which for Cascadia is important for northern Basin and Range extension and NNW transport of the Sierra Nevada block; (7) plate-normal stresses of ~1 TN/m are required along transform faults, which keeps the plate from extending in these areas; and (8) ocean plateau subduction at Yakutat (>6 TN/m) strongly compresses the continent. Of particular interest are the conclusions that forces concentrated on the cratonic root resist NA motion and that global-flow tractions are significantly less than those estimated by BO, which together imply a thin, relatively weak asthenosphere and stagnant deep Earth, and that fault shear stress levels are 20-140 MPa, which are low compared to lab-based expectations for frictional behavior, but are much larger than typical earthquake stress drops.