North American Plate Dynamics

Abstract

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 2o-3 ø. 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 1012 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, compressire stresses of the order of 5-10 MPa transmitted across the major transforms improve the fit to the data. Compressire stresses across convergent margins along the Aleutians and the Middle America trench are important. INTRODUCTION Several features of the North American plate make it ideal for a study of the forces acting on a plate. First, we have abundant information on the orientations of intraplate deformation. There are substantial regions over which the inferred stress orientations are consistent. The largest such region extends from the eastern front of the Rocky Mountains perhaps as far east as the mid-Atlantic ridge and indicates an ENE trend for the maximum horizontal stress direction [Zoback and Zoback, 1980, 1989]. Second, the boundaries of the North American plate are, for the most part, well known. There are ample lengths of ridge, transform fault, and convergent boundaries, although the plate itself is not attached to any significant length of subducted slab. Thus the plate provides an opportunity to constrain the roles of several possible tectonic forces associated with plate boundaries. Finally, the plate contains both continental and oceanic lithosphere, which will permit a study of the role of horizontal density contrasts associated with continental margins and crustal thickness variations.This study of the forces acting on the plate uses an elastic finite element analysis. The technique is appropriat...

Stress, seismicity and structure of shallow oil reservoirs of Clinton County, Kentucky. Final report

“…In this province, contemporary stress is characterized by a consistent ENE orientation of the maximum horizontal compressive stress. This orientation is consistent with that derived from an elastic single-plate finite-element tectonic model of the North American plate with "ridge push" forces, and is interpreted as a consequence of crustal extension at the Mid-Atlantic Ridge (Zoback and others, 1986;Zoback and Zoback, 1989; Richardson and Reding, 1991;Zoback, 1992a;1992b).…”

Why is Africa rifting?

“…The stresses resulting from the lithospheric structure (GPE) were calculate d by applying the pressure obtained from the calculation of the GPE to a depth of compensation of 100 km to each of the lateral faces of the elements in the shell, as in Richardson & Reding (1991).…”

Stress field constraints on intraplate seismicity in eastern North America

“…The source of this uniform broad-scale regional stress field in midplate North America is believed to be related to platedriving stresses [e.g., Adams and Bell, 1991;Richardson and Reding, 1991;Zoback and Zoback, 1991]. This orientation and its regional consistency were first noted by Sbar and Sykes [1973] in the northeast eastern United States and by Haimson [1977] in the Great Lakes region and by Zoback and Zoback [1980] [1987,1989], Adams and Bell [1991], and Zoback and Zoback [1989,1991] have expanded the data coverage and the extent of the region characterized by this relatively uniform maximum horizontal stress direction to include most of the United States east of the Rockies and nearly all of Canada.…”