SOUTHEASTERN BOUNDARY OF THE JUAN-FERNANDEZ MICROPLATE - BRAKING MICROPLATE ROTATION AND DEFORMING THE ANTARCTIC PLATE

Data from a 1991 multibeam, sidescan sonar, and geophysical survey of the Juan Fernandez microplate at the Pacific-Nazca-Antarctic triple ju nction strongly support a model of edge-driven microplate rotation. Th ey reveal the nature of the microplate's southeastern boundary with th e Antarctic plate and show that plate boundary interactions are capabl e of significantly altering plate motions. Microplate rotation slowed progressively from >30-degrees/m.y. to <10-degrees/m.y. between Anomal y 2 time (approximately 1.9 Ma) and the early Brunhes (approximately 0 .6 Ma). Magnetics, bathymetry, and morphology suggest that this decele ration resulted from coupling across a rapidly evolving Juan Fernandez -Antarctic (JF-A) plate boundary. Over the past approximately 2 m.y., the JF-A plate boundary has lengthened, changed orientation, and migra ted to the southeast, into the Antarctic plate. This process deformed and transferred to the Juan Fernandez plate about 3000 km2 of the Anta rctic plate. Kinematic and morphologic observations support interpreti ng curved lineaments within this deformed zone as originally N-S abyss al hills generated at the Pacific-Antarctic Ridge that were later shea red by dextral slip bookshelf faulting within the migrating sinistral shear zone of the JF-A boundary. Deformation within this system docume nts that shear zone migration, not rift propagation, is the key proces s associated with development of curved lineaments and transferred lit hosphere. The JF-A plate-boundary shear-zone may have been quite narro w (approximately 10-20 km) despite its migration through the Antarctic plate. Narrow migrating plate boundaries are also observed at other s ystems, such as the Galapagos 95.5-degrees-W migrating offset which al so has a width of approximately 20 km. Such narrow deformation zones m ay be common along migrating plate boundaries in various tectonic envi ronments. Triple junction systems in this area appear to represent a s uite of plates and triple junctions crossing a range of scales. The ob servations here support models for triple junctions having complex evo lutionary histories involving rapid changes in plate boundary configur ation and stress field.