On the dynamics of the Juan de Fuca plate

摘要

The Juan de Fuca plate is currently fragmenting along the Nootka fault zone in the north, while the Gorda region in the south shows no evidence of fragmentation. This difference is surprising, as both the northern and southern regions are young relative to the central Juan de Fuca plate. We develop stress models for the Juan de Fuca plate to understand this pattern of breakup. Another objective of our study follows from our hypothesis that small plates are partially driven by larger neighbor plates. The transform push force has been proposed to such a dynamic interaction between the small Juan de Fuca plate and the Pacific plate. We aim to establish the relative importance of transform push for Juan de Fuca dynamics. Balancing torques from plate tectonic forces like slab pull, ridge push and various resistive forces, we first derive two groups of force models: models which require transform push across the Mendocino Transform fault and models which do not. Intraplate stress orientations computed on the basis of the force models are all close to identical. Orientations of predicted stresses are in agreement with observations. Stress magnitudes are sensitive to the force model we use, but as we have no stress magnitude observations we have no means of discriminating between force models. Model stresses in the north and the south of the Juan de Fuca plate are similar. Breakup along the Nootka fault zone, and not in the Gorda region, can be explained if the local strength of a lithosphere column depends on lithospheric age. Stresses are invariant to whether or not transform push is acting on the Mendocino Transform fault. We therefore conclude that the introduction of this extra plate driving force is not required by observations for the Juan de Fuca plate.