EVOLUTION OF PACIFIC BASIN PLATES AND BOUNDARIES: CURRENT KNOWLEDGE AND REMAINING MYSTERIES

The Pacific basin is a major laboratory for the development of plate tectonic theory. The basin’s large size and diverse plate boundaries include prime examples of active spreading, subduction, and transform faulting. During the plate tectonics revolution, the active boundaries of these plates were identified from the earthquake distribution and interpretation of marine geophysical data including bathymetry, gravity, and magnetic anomalies. Fundamental plate tectonic discoveries based on data from the Pacific basin include: recognition of symmetric magnetic anomaly sequences at mid ocean ridges, recognition of vanished subducted plates, such as Farallon and Kula, based on one-sided sets of magnetic anomalies; behavior of oceanic and continental plates at triple junctions; motion of plates over hotspots. The magnetic anomaly sequences in the north and south Pacific were quickly recognized with early data sets, and matched to magnetic reversal time scales to yield rates of motion. However, advances in the details of the kinematic histories, past plate motions, and boundary dynamics are ongoing, using a combination of new technology, ocean drilling, and additional surveys.

Multibeam swath bathymetry, sidescan sonar, and deep-tow instruments provide detailed seafloor morphology beyond earlier capabilities. In areas lacking ship surveys, satellite radar altimetry and gravity reveal fracture zone trends and fossil plate boundaries. GPS-based locations reduce the noise in survey positioning and help to constrain rates of current plate motion. Satellite magnetic anomalies indicate areas needing future study.

Fragmentation of the former Farallon plate into the modern Explorer, Juan de Fuca, Rivera, Cocos, and Nazca plates, and earlier fossil microplates, provides an excellent example for evolution and demise of subduction zones. The microplates along modern spreading centers (Galapagos, Juan Fernandez, Easter) illustrate a style of complexity that may be preserved elsewhere in older seafloor. Major questions remain regarding past plate boundaries in the western Pacific, particularly between the Ontong Java and Manihiki plateaus; the nature of plate motion changes during the Cretaceous Normal Superchron; and the age and origin of crust currently being subducted at the NW corner of the Pacific plate.