Constraints on rift propagation history at the cobb offset, juan de Fuca Ridge, from numerical modeling of tectonic fabric

Abstract

Rift propagation has been proposed as the mechanism for the reorganization of the Juan de Fuca Ridge during the Cenozoic. In particular, magnetic anomalies in the Cobb Offset region of this ridge have been interpreted in terms of a complex “dueling propagator” history, in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments. Although the overall propagation history has been inferred from the magnetic data, these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant. We thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery. By modeling lineations as isochrons, synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically. For the Cobb, we used a propagation history model with a given timing and direction of propagation, and generated synthetic fabric for different propagation rates. The fabric is best fit for propagation rates between 400 and 900 mm/yr. Slower ( < 300 mm/yr) propagation rates yield significantly poorer fits, whereas faster ( > 900 mm/yr) propagation rates yield slightly poorer fits. The fabric results are thus in accord with the interpretation, based on magnetics alone, that the rates of rift propagation significantly exceeded the spreading rates. An interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale ( ~ 1 km) by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics.