The stresses that drive the plates from below: Inferences from finite-element
models

Peter Bird 
UCLA

We divide the torques on each surface plate into 3 components:
lithostatic-pressure, side-strength, and basal-strength. We compute each
component for each of 52 plates using a thin-shell finite-element model of the
lithosphere with: topography, variable heat-flow, variable crust and
lithosphere thicknesses from seismic data, transient geotherms, nonlinear
rheology, and weak faults. We present an iterative method of adjusting boundary
conditions that results in correct plate velocities without the need for models
of deep mantle flow. Uncertainty remains because side-strength torques, and
therefore inferred basal-strength torques, depend on the effective friction of
faults. Therefore, we compute a two-parameter suite of models with differing
trench resistance and differing fault friction, and evaluate their misfits
relative to: seafloor spreading rates, geodetic velocities, intraplate stress
directions, and azimuths of seismic anisotropy. The minimum misfit occurs at
effective fault friction of 0.1 and trench resistance 2x10^12 N/m. In this
preferred model, computed values of mean basal-strength traction systematically
increase for smaller plates. We analyze error sources and find that the largest
source is unmodeled variation in effective friction of plate-boundary faults.
Discounting highly uncertain results, we find mean basal shear tractions of no
more than 1 MPa for the 6 largest slabless plates: AF 0.2 MPa; AN 0.1 MPa; NA
0.6 MPa; EU 1.0 MPa; SA 1.0 MPa; SO 0.9 MPa. The directions of basal shear
traction on these plates are generally forward, meaning subparallel to absolute
velocity. Basal-strength torques on plates with subducting slabs represent the
sum of net slab-pull and distributed basal shear traction; if these torques are
attributed to net slab-pull alone, net slab-pull is generally toward the trench
and of order 5x10^12 N/m. Thus, present plate motions on Earth appear to be
driven primarily by deep mantle convection, rather than by topography and
associated lithostatic pressures.