NORTH-AMERICAN PLATE DYNAMICS

Deformation within the North American plate in response to various tectonic processes is modeled using an elastic finite element analysis. A coarse grid contains 328 elements and 190 nodes, while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2-degrees-3-degrees. Abundant information about the present-day state of intraplate stress constrains the modeling. The dominant pattern that must be fit by all acceptable models is an ENE trend for the maximum compressive stress for most of the plate east of the Rocky Mountains. The tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere, shear and normal stresses transmitted across transforms, normal stresses transmitted across convergent boundaries, stresses due to horizontal density contrasts within the continent, and shear tractions applied along the base of the plate. Model stresses are calculated with respect to a lithostatic reference stress state. Distributed ridge forces of magnitude 2 x 10(12) N/m predict deviatoric stresses of the order of 20-40 MPa that are capable of accounting for the dominant observed ENE stress trend. Driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate. Assuming that ridge forces account for the dominant ENE stress trend, bounds may be placed on other possible forces. For example, shear stresses transmitted across transform boundaries along the San Andreas and Caribbean are small, of the order of 5-10 MPa. Also, compressive stresses of the order of 5-10 MPa transmitted across the major transforms improve the fit to the data. Compressive stresses across convergent margins along the Aleutians and the Middle America trench are important.