• Journal of the Korean Geophysical Society
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• v.2 no.1
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• pp.9-26
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• 1999

The Ulleung Basin (Tsushima Basin) in the southwestern East Sea (Japan Sea) is floored by a crust whose affinity is not known whether oceanic or thinned continental. This ambiguity resulted in unconstrained mechanisms of basin evolution. The present work attempts to define the nature of the crust of the Ulleung Basin and its tectonic evolution using seismic wide-angle reflection and refraction data recorded on ocean bottom seismometers (OBSs). Although the thickness of (10 km) of the crust is greater than typical oceanic crust, tau-p analysis of OBS data and forward modeling by 2-D ray tracing suggest that it is oceanic in character: (1) the crust consists of laterally consistent upper and lower layers that are typical of oceanic layers 2 and 3 in seismic velocity and gradient distribution and (2) layer 2C, the transition between layer 2 and layer 3 in oceanic crust, is manifested by a continuous velocity increase from 5.7 to 6.3 km/s over the thickness interval of about 1 km between the upper and lower layers. Therefore it is not likely that the Ulleung Basin was formed by the crustal extension of the southwestern Japan Arc where crustal structure is typically continental. Instead, the thickness of the crust and its velocity structure suggest that the Ulleung Basin was formed by seafloor spreading in a region of hotter than normal mantle surrounding a distant mantle plume, not directly above the core of the plume. It seems that the mantle plume was located in northeast China. This suggestion is consistent with geochemical data that indicate the influence of a mantle plume on the production of volcanic rocks in and around the Ulleung Basin. Thus we propose that the opening models of the southwestern East Sea should incorporate seafloor spreading and the influence of a mantle plume rather than the extension of the crust of the Japan Arc.

• Journal of the Korean Geophysical Society
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• v.7 no.2
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• pp.99-112
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• 2004

The Basin, a marginal basin located between the Antarctic Peninsula and the South Shetland Islands, is consist of three small basins, the Central, Eastern, Western Basins. Seismic data obtained on December 1995 show well-defined spreading ridges, basement highs, faults, morphology of the basin, distribution of sediments, crustal and sedimentary deformation, diapirs, and contourites. The main spreading axis of the Central Bransfield Basin connecting Deception and Bridgeman Islands continues up to the central part of the Eastern Basin, whereas deep basin covered by thick sediments without any spreading structures develops in the northeastern part. This indicates that back-arc spreading along the axis of the Bransfield Basin has been taken place in the southwestern part of the Eastern Basin, not in the northeastern part. Many NW-SE trending faults perpendicular to the axis of the basin would be related with strike-slip movement of the Shackleton Fracture. Zone. Extensinal strutures like deep basin without any spreading structures in the northeastern part, normal faults and diapirs on both continental slopes of the Eastern Basin would be formed by extension as a consequence of the sinistral movement between Antarctic and the Scotia plates.