THE STYLES OF EXTENSION AND MAGMATISM AROUND THE ULLEUNG BASIN, THE EAST SEA (SEA OF JAPAN)

We present our new interpretations on the lithospheric extension and magmatism of the Ulleung Basin in the southwestern East Sea (Sea of Japan) after examining various geological and geophysical data including closely-spaced multichannel seismic reflection profiles acquired from the basin and its vicinity. The basin is considered as a classical example of back-arc basin where the lithosphere was stretched along NNW-SSE direction by dextral pull-apart tectonic motion with three stages of vigorous volcanic activities since the Early Miocene . The deep seismic data show that in N-S cross-sections the basin is asymmetric in basement topography, sediment thickness, structural pattern and volcanism, suggesting that the stretching was accommodated by a northward dipping low-angle detachment fault system. It appears that the southern part of the basin evolved into a non-volcanic passive rifted margin with single curvilinear listric bounding fault and splaying collateral normal faults. On the other hand, the northern margin experienced progressive block faulting with vigorous volcanic activities lasted until the end of the back-arc extension. The subsidence rate was more rapid in the southern part of the basin compared with the northern counterpart perhaps due to the removal of the decoupled lithospheric hanging wall, which allowed a thicker deposition of basin-fill sediments to the south. It is thought that the decoupling of the lithospheric foot wall under the northern part of the basin gave rise to decompressional melting in association with the upwelling of asthenosphere, which fed a volcanism of OIB-like petrochemical character. Considering the geological evidence of widespread and incessant volcanic activities in the East Sea before and during the back-arc opening as well as the present state of elevated heat flow of the sea, we argue that strain localization and/or crustal flow inside the abnormally hot lithosphere was one of the most important control of the lithospheric detachment under the basin. It is also possible that the inherited structural weakness related with the Paleo-Mesozoic Quiling-Dabie orogeny or Mesozoic arc accretion in this region was another important factor controlling the lithospheric detachment.