EPISODIC TRIPLE-JUNCTION MIGRATION BY RIFT PROPAGATION AND MICROPLATES

Abstract

We describe a model for episodic, open-ocean triple-junction migration based on observations of actual triple-junction evolutions. Migration progresses by repeated episodes of rift propagation, microplate formation, and microplate accretion to an adjacent, larger plate. These episodes may be highly variable in space and time depending on triple-junction geometry, velocity triangle, and other factors affecting local thermal and rheological conditions. Resulting tectonic features may include an abandoned transform fault, straight and potentially curving pseudofaults, sheared and potentially rotated abyssal-hill fabric, and a paleomicroplate with no associated failed rift. This model, developed mainly from the evolution of the Pacific-Antarctic-Nazca triple junction, may be relevant for other types of triple junctions such as the Bouvet and Azores triple junctions and ridge-ridge-ridge triple junctions in the Indian and Pacific Oceans. Episodic migration is found to occur even when the triple junction is kinematically stable. This model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history.