Crustal structure, volcanism, and opening mode of the Ulleung Basin, East Sea (Sea of Japan)

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A study of the Ulleung Basin in the southwestern East Sea (Sea of Japan), based on single- and multi-channel seismic reflection profiles and ocean bottom seismometer (OBS) data, suggests that the basin formed largely by pull-apart opening and the deep, northern basin is underlain by thicker-than-normal oceanic crust (10 km thick), probably formed during the earliest stage of seafloor spreading. Volcanic activity in association with basin opening occurred over much of the basin but it ceased first in the south while it continued in the north until recently. The principal evidence for pull-apart opening includes: (1) an overall rhomboidal shape of the basin consisting of two basement lows and a median high, known to be characteristic of pull-apart basins; (2) the location and orientation of the major volcanic sources suggesting more-or-less N–S opening; and (3) the strike-slip fault system along the western basin margin, which appears to have guided the pull-apart opening. The lack of extensional deformation along the western basin margin, which would have been prevalent if SW Japan had rotated away from the Korean Peninsula as suggested by paleomagnetic data, is also not in favor of the alternative, rotational opening of the basin. The absence of graben-and-rift topography in the acoustic basement in the deep, northern Ulleung Basin is indicative of the non-brittle or ductile nature of the crust. Tau– p analysis of the OBS data reveals that the igneous crust consists of upper and lower layers with velocities typical of oceanic layers. The velocity–depth profiles with a two-gradient velocity structure is also characteristic of oceanic crust. However, the lower layer is much thicker than oceanic layer 3 and the change of velocity gradient, which occurs at around 6.5 km/s in typical oceanic crust, occurs at about 6 km/s, suggesting that the crust is not a typical oceanic crust. The meager basin size and short opening period may suggest that the basin opening did not lead to the formation of full-fledged oceanic crust but instead resulted in embryonic or incipient oceanic crust thickened by upper-mantle thermal perturbations evoked by rifting.