Southeastern boundary of the Juan Fernandez microplate: Braking microplate rotation and deforming the Antarctic plate

Abstract

Data from a 1991 multibeam, sidescan sonar, and geophysical survey of the Juan Fernandez microplate at the Pacific‐Nazca‐Antarctic triple junction strongly support a model of edge‐driven microplate rotation. They reveal the nature of the microplate's southeastern boundary with the Antarctic plate and show that plate boundary interactions are capable of significantly altering plate motions. Microplate rotation slowed progressively from >30°/m.y. to <10°/m.y. between Anomaly 2 time (∼1.9 Ma) and the early Brunhes (∼0.6 Ma). Magnetics, bathymetry, and morphology suggest that this deceleration resulted from coupling across a rapidly evolving Juan Fernandez‐Antarctic (JF‐A) plate boundary. Over the past ∼2 m.y., the JF‐A plate boundary has lengthened, changed orientation, and migrated to the southeast, into the Antarctic plate. This process deformed and transferred to the Juan Fernandez plate about 3000 km2 of the Antarctic plate. Kinematic and morphologic observations support interpreting curved lineaments within this deformed zone as originally N‐S abyssal hills generated at the Pacific‐Antarctic Ridge that were later sheared by dextral slip bookshelf faulting within the migrating sinistral shear zone of the JF‐A boundary. Deformation within this system documents that shear zone migration, not rift propagation, is the key process associated with development of curved lineaments and transferred lithosphere. The JF‐A plate‐boundary shear‐zone may have been quite narrow (∼10–20 km) despite its migration through the Antarctic plate. Narrow migrating plate boundaries are also observed at other systems, such as the Galapagos 95.5°W migrating offset which also has a width of ∼20 km. Such narrow deformation zones may be common along migrating plate boundaries in various tectonic environments. Triple junction systems in this area appear to represent a suite of plates and triple junctions crossing a range of scales. The observations here support models for triple junctions having complex evolutionary histories involving rapid changes in plate boundary configuration and stress field.