HR: 11:00h
AN: U52A-03    [Abstracts]
TI: North America Dynamics and Western U.S. Tectonics
AU: Coblentz, D
EM: coblentz@lanl.gov
AF: Earth and Environmental Sciences Division, Los Alamos National Lab, Los Alamos, NM 87545
AU: * Humphreys, E
EM: genehumphreys@gmail.org
AF: Dept. of Geological Scoences, University of Oregon, Eugene, OR 97403 United States
AB: The contributions of principal tectonic forces to the observed intraplate stress field in the North American plate have been evaluated through a two-dimensional finite element analysis. The loads acting on the plate represent plate boundary segments, basal tractions (from whole-Earth flow calculations [from Becker and O'Connell, 2001] and concentrated on the cratonic root), and gravitational potential energy (GPE). Modeling constraint is provided by an observed regional stress field based on observations throughout the plate. A weighted ''basis-set'' method is employed to expedite evaluation of misfit between the observed and predicted intraplate stresses, providing an efficient means to evaluate a very large number (several million) of boundary and GPE force combinations acting on the plate, and refine our quantitative assessment of the best fitting models. Model results allow us to draw the following conclusions: (1) high oceanic ridge GPE compresses North America; high western North America GPE nearly overcomes this compression; (2) most subduction zones apply a strong outward-directed pull relative to a reference ridge; (3) transform-boundary shear and normal loads average ~10¹² N/m; (4) strong compression is concentrated at Yakutat; and (5) basal tractions are concentrated on the cratonic root at ~4 MPa to the northeast and elsewhere are significantly smaller than estimated by current global-flow calculations. The last conclusion implies a thin, relatively weak asthenosphere and stagnant deep Earth mantle. Fault shear stress levels are 20-140 MPa, which is low compared to lab-based expectations for frictional behavior but much larger than typical earthquake stress drops. In the western U.S., San Andreas shear load of ~1.5 10¹² N/m drags the Sierra Nevada NNW and creates shear across the western Great Basin; this load is balanced largely by north-south compression in Washington and western Oregon. Important to western U.S. extension are three factors: (1) high GPE; (2) root drag, which "shadows" western U.S. from ridge-push compression; and (3) an outward pull of 3-4 10¹² N/m at southern Cascadia, which directs tension toward southern Cascadia.
DE: 8110 Continental tectonics: general (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Union [U]
MN: Fall Meeting 2005