Protoplate Tectonics Along the Fastest Seafloor Spreading Center

R N Hey, P D Johnson, F Martinez

J Korenaga

The fastest present-day seafloor spreading occurs along the EPR between the Easter and Juan Fernandez microplates, 27-32S. Here GLORI-B, SeaBeam 2000 and magnetics data show the Pacific - Nazca plate boundary geometry is being reorganized by rift propagation, with the recent tectonic behavior predominantly as a huge duelling propagator system. The overlap zone is presently deforming by bookshelf faulting and has produced a zone of pervasively deformed transferred lithosphere (from the Pacific to the Nazca plate) characteristic of PR tectonics. Cross-cutting structural zones with relatively high reflectivity indicate about 6 major episodes of very fast propagation of the failing East rift followed almost immediately by complete cessation of spreading on these segments, as the dominant West rift continued to propagate steadily south at about 170 km/m.y. Shallow bathymetry forming a narrow linear southern boundary of the overlap zone suggests that West rift spreading has recently curved to connect with the East rift via a transform-type zone. An unusual NW-trending linear depression nearby may indicate the overlap zone has recently attempted to begin to rotate as a microplate, although the succesful evolution of this duelling propagator into a rollerbearing microplate would require the eventual formation of a northern boundary as well. The width of this overlap zone (about 120 km) is slightly smaller than those of the deformed cores of the Easter and Juan Fernandez microplates (about 130-140 km), suggesting that this is the scale at which lithosphere stops deforming by bookshelf faulting and begins to rotate as a rigid microplate, and thus that neoplate tectonics may be about to begin here.