Two-Stage Ridge Propagation and the Geological History of the Lau Backarc Basin

Geophysical and geological data for the Central Lau Basin have been interpreted to constrain the tectonic evolution of the Lau Basin since 6 Ma.Seafloor morphology, residual magnetic anomaly lineations, and variations in seafloor acoustic backscattering indicate that two ridge propagation events have been responsible for the formation of the eastern floor of the basin.The earlier propagator formed the Eastern Lau Spreading Center and progressed into a horst-and-graben-dominated extensional terrain derived from the fragmenting Lau-Tonga composite arc.According to our interpretation of recompiled magnetic data, this propagating ridge probably initiated around 5.5 Ma at the southeastern end of a structure that became the Peggy Ridge.At present, the propagator has a tectonic rift tip at the southern end of the Valu Fa Ridge (23°S).This tip has advanced southward at a rate of 120 mm/yr, more than twice the rate reported for most other "open ocean" propagating ridges.The later propagator, referred to as the Central Lau Spreading Center, began from the southeastern limit of the Peggy Ridge between 1.2 and 1.5 Ma and has its present rift tip at 19°22'S.The western floor of the Lau Basin is dominated by irregular ridges and troughs, and our interpretation of the topography is that this crust, originally adjacent to the waning Lau Ridge arc, experienced a protracted period of extensional rifting and local easterly migrating arc magmatism before the advance from north to south of the first propagator.Backarc extension then transferred largely to the new spreading axis.Basement recovered from only two of the backarc sites (836 and 837) has unequivocal (and unmixed?) affinities to the present Central and Eastern Lau axial lithologies; all of the other basin sites have complex parentages.Sidescan sonar data suggest the persistence of some minor present-day extension within the western-rifted portion of the basin; thus, off-axis magmatism and/or extension outside of the present axis is not precluded.