The first crustal‐scale controlled source seismic refraction experiment in the southern Korean Peninsula, KCRUST2002, was carried out along a 300‐km long profile across this peninsula in December 2002. Iterative processing and modeling produced a laterally varying layered crustal velocity model. The crust is thickest (34 km) below the Okcheon fold belt in the middle of the transect and thinnest (28 km) at the eastern end where the Cretaceous Gyeongsang basin is characterized by 5 km of low velocity material that constitutes the upper crust. The P velocities in upper and lower crust range from 5.4 to 6.0 km/s and from 6.4 to 6.7 km/s, respectively. The average crustal Poisson's ratio is found to be 0.25–0.27 (Vp/Vs = 1.73−1.78) along the profile. A mid‐crustal velocity discontinuity is recognized in the northwestern part of the transect. The underlying mantle has velocities in the range of 7.9–8.1 km/s.

No abstract

No abstract

The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). Two‐dimensional crustal structure of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling. The crustal structure shows the emplacement of high‐velocity (>7 km/s) lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust. The high‐velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting. Magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the Korean Peninsula. We suggest that the rifting and subsequent seafloor spreading at the Korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature.

Multi‐channel seismic reflection profiles from the trough areas of the southern South Korea Plateau, East Sea (Sea of Japan), reveal high‐amplitude, positive‐polarity bottom‐simulating reflectors (BSRs) at about 500 ms two‐way travel time (ca. 450 m) below the seafloor. The positive polarity and no noticeable drop of frequency (low‐frequency shadow) below the BSR strongly suggest that the BSR is due to an opal‐A/opal‐CT phase boundary. The BSR often marks a sharp interface between low‐amplitude reflections above and high‐amplitude reflections below. The weak, but distinct reverse‐polarity event, seen 70–150 ms two‐way travel time below the BSR, may represent the base of the silica diagenetic zone. The geothermal gradients (78–111°C/km) estimated from the sub‐bottom depth of the BSR are comparable to that (98°C/km) measured at the nearby Kita‐Yamato Trough, where the opal‐A/opal‐CT boundary has been penetrated by the Ocean Drilling Program (ODP) drilling.