Backarc Oceanic Core Complexes Formed During Initial Spreading in the Southern Shikoku Basin

Seafloor spreading occurs in two distinct geodynamic environments, major ocean basins and backarc basins. Unusual magma-poor seafloor spreading has been identified at slow- and intermediate-rate spreading centers in major ocean basins, e.g., Mid-Atlantic Ridge, Southwest Indian Ridge, and Australia-Antarctica Discordance. Some of these spreading centers are characterized by corrugated bathymetry known as megamullions, and some by chaotic bathymetry. Serpentinized peridotite and altered gabbro have been sampled from megamullions, and the three-dimensional geological structures that form megamullions are known as oceanic core complexes. Oceanic core complexes have also been identified at extinct backarc spreading centers, e.g., Parece Vela Basin and Shikoku Basin. The Shikoku Basin formed in conjunction with subduction along the Izu- Bonin arc at the eastern edge of the Philippine Sea plate. Although the general spreading history of the basin is known from identification of magnetic lineations, the early tectonic history of Proto-Izu-Bonin arc breakup and subsequent initial backarc spreading is uncertain. We identify, describe, and interpret oceanic core complexes amid chaotic bathymetry of the southern Shikoku Basin just east of the Kyushu-Palau Ridge, the remnant arc of the Proto-Izu-Bonin arc, on the basis of marine geological and geophysical data including multichannel seismic reflection, seismic refraction, swath bathymetry, and gravity. Just west of the core complexes, the Kyushu-Palau Ridge has been dated as Oligocene in age (~25 Ma), and just to the east lies magnetic anomaly 6B (~23 Ma). Crustal structure derived from seismic and gravity data indicates that anomalously thin -less than 5 km thick- crust is located in the arc-ocean transition between the central Kyushu-Palau Ridge and southern Shikoku Basin, which suggests rift-related crustal thinning and low magma productivity during backarc spreading initiation. Near the core complexes, seamount fragments or small ridges strike perpendicular to the spreading axis, extending from the central Kyushu-Palau Ridge to the southern Shikoku Basin. These features suggest that magma-poor spreading forming the oceanic core complexes coexisted with the Proto-Izu-Bonin arc magmatism forming seamount fragments during initial, late Oligocene spreading in the southern Shikoku Basin. To explain the complicated initial backarc spreading of the southern Shikoku Basin, we compare and contrast the southern Shikoku Basin oceanic core complexes with those formed at other spreading systems, and discuss the implications of the oceanic core complexes for the tectonic evolution of the southern Shikoku Basin in the broad context of generic backarc spreading systems.