HR: 0800h
AN: T21B-0596    [Abstracts]
TI: Basal Shear Traction, not Slab-Pull, may be the Dominant Plate-Driving Force
AU: * Rucker, W K
EM: kurt.rucker@gmail.com
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095- 1567, United States
AU: Bird, P
EM: pbird@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095- 1567, United States
AB: We previously reported [2006 AGU T11F-05] that we used thin-shell finite-element program SHELLS [Kong & Bird, 1995, JGR] to model the 52 plates of the PB2002 plate model [Bird, 2003, G³], and adjusted fault friction to the value of 0.1, at which we obtained the best possible fit to global geodetic velocities, stress directions, seafloor spreading rates, and seismic anisotropy. Here we analyze the forces and torques in the best model, in an attempt to better understand the balance which drives and resists plate motions. All boundary tractions acting on any plate are divided into 3 types: lithostatic pressure, side-strength, and basal- strength. ("Strength" is a modified stress tensor from which lithostatic pressure has been subtracted.) Side- strength is dominated by fault friction. Basal-strength includes distributed basal shear tractions (for all plates) and net slab-pull (for subducting plates only). Computed basal strength tractions are very large for some small plates, but we show that these large values are probably dominated by errors resulting from locally-incorrect (global-mean) fault friction used to compute side-strengths. Any possible correlations with physical characteristics of plates is concealed by these large errors in basal-strength tractions for small plates. However, results for large non-subducting plates (whose mean side friction will more closely approximate the global mean) suggest basal shear tractions of 1 MPa or less. Since Forsyth & Uyeda [1975, GJRAS], many have considered net slab-pull to be the dominant driving force on subducting plates. To investigate such a model, we compute net slab-pull for 11 plates with large subducting slabs by assuming negligible distributed basal shear tractions, and ascribing all basal-strength forces to net slab-pull (as line forces along trenches, acting only on the subducting plate). These model-dependent net slab- pull forces are mostly of plausible magnitude (<4x10¹² N/m, except for outliers with large uncertainties). However, they show no clear correlation with: known sea floor age, relative or subduction velocity, trench depth, age-corrected trench depth, or trench length. While this lack of systematic relationships could be due to additional unknown errors in our model, we interpret these negative results as tending to disprove the initial assumption. That is, distributed basal shear traction forces are probably of comparable magnitude to, or greater than, net slab- pull forces, and the primary determinant of plate motion. Plate velocity, forward net slab-pull, and forward basal shear tractions are examined using Africa-fixed, no-net- rotation, and NUVEL1A-HS3 (hotspot) "absolute" reference frames. All analyses show basal tractions to be primarily forward, with the no-net-rotation reference frame leading to the most positive (forward) results for basal strength tractions. Our interpretation is that active mantle convection is typically driving plate motions with basal shear tractions, rather than resisting them.
UR: http://peterbird.name
DE: 4255 Numerical modeling (0545, 0560)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting