G13A-0989
Intraplate Deformation Due to Motion of Plates over a Nonspherical Earth
Monday, 14 December 2015
Poster Hall (Moscone South)
Daniel Woodworth and Richard G Gordon, Rice University, Houston, TX, United States
Abstract:
The central tenet of plate tectonics is that the plates are rigid. Not long after the acceptance of plate tectonics, however, it was recognized that the motion of plates over a non-spherical Earth should cause intraplate deformation [McKenzie, 1972; Turcotte & Oxburgh, 1974]. Even so, no firm connection between hypothesized deformation and observed deformation has yet been made. An alternative cause of intraplate deformation is the horizontal contraction of lithosphere as it cools with age [Collette, 1974]. The rate of horizontal thermal contraction decreases as ~1/age and the resulting intraplate deformation should be large enough to cause observed plate circuit non-closures [Kumar & Gordon, 2009]. Strain rates thus obtained for 0 Ma-old, 0.1 Ma-old, 1 Ma-old, and 10 Ma-old oceanic lithosphere respectively are 2 × 10-2 Ma-1 (5 × 10-16 s-1), 8 × 10-3 s-1 (3 × 10-16 s-1), 1.5 × 10-3 Ma-1 (5 × 10-17 s-1), 2 × 10-4 Ma-1 (5 × 10-18 s-1) [Mishra & Gordon, 2015]. Across the Pacific Plate, such strains sum to intraplate relative velocities of up to ≈2 mm yr-1 [Kreemer & Gordon, 2014].Here we attempt to quantify rates of intraplate strain due to motion of plates over a nonspherical Earth to compare with strain rates due to horizontal contraction and due to observed intraplate deformation. We determine rates of northward motion of lithosphere using the SKS-MORVEL set of plate angular velocities relative to the deep mantle [Zheng et al., 2014]. Following Turcotte [1974], we use the approximation of a spherical Earth whose radius of curvature changes with the latitudinal motion of the plate. We considered two end-member cases—no radial strain and no change in thickness—in our calculations. We estimate average strain rates for the twenty-five major plates ranging from ~10-11 to 10-4 Ma-1 (3 × 10-25 to 3 × 10-18 s-1). For the Pacific Plate, we estimate strain rates that approach or exceed those due to thermal contraction only in the oldest lithosphere, where strain rates from thermal contraction are at a minimum. Further, for the Australia Plate, where anomalously high strain rates of ~10-7 to 10-3 Ma-1 (3 × 10-21 to 3 × 10-17 s-1) have been estimated from seismic moment release [Braun et al., 2009], we estimate strain rates due to nonsphericity of ~10-3 Ma-1 (3 × 10-17 s-1), consistent with the upper limits to values found from seismic strain rate.
