Diffuse Oceanic Plate Boundaries: Mapping the Relationship Between Angular Velocity and Torque as a Function of Assumed Rheology

Diffuse plate boundaries comprise about 15% of Earth's surface. The poles of rotation across diffuse oceanic plate boundaries, which are roughly 1,000 to 2,000 km in cross-strike width, tend to lie within the plate boundary themselves. Zatman et al. (2001) showed that this is a predictable consequence of plate geometry and simple physics if component plates are strongly coupled across the diffuse oceanic plate boundaries that separate them. Here we build on the work of Zatman et al. (2001) and of Zatman & Gordon (2003) to construct maps of torque orientation. On these maps we plot the boundary between torque orientations that produce poles of rotation in the diffuse plate boundary and torque orientations that produce poles of rotation outside the diffuse plate boundary. We assume that the upper lithosphere fails at a specific yield strength and that the lower lithosphere deforms by creeping flow. Acting in parallel, the integrated or averaged behavior of these two layers can be approximated as a power-law viscous fluid. The value of the power-law exponent, n, ranges from 3 (if the lower layer dominates) to infinity (if the upper layer dominates). We calculate maps for exponents of 3, 10, 30, and 100. All the maps closely resemble one another, providing further support to the conclusion that the mapping from pole of rotation to torque orientation is insensitive to rheology (Zatman & Gordon 2003). Moreover, the maps show that it is very unlikely (p < 2%) that the pole of rotation will lie outside of the boundary in the along-strike direction. There is a higher probability (p ≈20% assuming that all orientations of torque are equally likely) that the pole of rotation will lie outside the boundary in the across-strike direction. Implications will be discussed for the India-Capricorn, Capricorn-Australia, North America-South America, Nubia-Somalia, and Nubia-Eurasia diffuse plate boundaries.