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Geochemical analyses of sediments, pore water and headspace gas of the piston cores taken from the eastern part of Ulleung Basin of the East Sea (Japan Sea) were carried out to assess the origin of the sedimentary organic matter and interstitial fluid. Several tephra layers within the core are identified as the Ulleung‐Oki (10.1 ka), the Aira‐Tanzawa (23 ka) and the Ulleung‐Yamato (30.9 ka) tephras. With the exception of these volcanic layers, the cores consist predominantly of muddy sediments that contain >0.5% total organic carbon. Atomic C/N ratios and δ13Corg values suggest that the organic matter originated from marine algae rather than from land vascular plants, whereas Rock‐Eval pyrolysis suggests that the organic matter is thermally immature and comes from a land vascular plant (Type III). These conflicting results seem to be caused by the heavy oxidization of the marine organic matter. Sulphate concentration profiles of pore waters show strongly linear depletion (r2 > 0.97) with sediment depth. The estimated sulphate–methane interface (SMI) depth using the sulphate concentration gradient was nearly 3.5 m below seafloor (mbsf) in the southern part of the study area, and deeper than 6 mbsf in the northern part of the area. The difference in SMI depths is likely associated with the amount of the methane flux. The methane concentration below the SMI in the two southern cores increases rapidly, implying the occurrence of methanogenesis and anaerobic methane oxidation (AMO). In contrast, the two northern cores have a low methane concentration below the SMI. values measured from all cores were in the range of −83.5 to −69.5‰, which suggests that the methane derives from a methanogenic microbe. values become decreased toward SMI, but increased below SMI; therefore, has eventually the minimum value near the SMI. The values are also decreased when the methane concentration is increased. These phenomena support the typical occurrence of AMO in the study area.

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Paleomagnetic samples were collected from late Neogene basalt flows from Thailand. All of these flows are horizontal and are relatively unaltered in thin section. These rocks possess a stable magnetization which is believed to be primary. Samples from 48 lava flows were collected from sites located within the Khorat Plateau, the Chao Phraya‐Phitsanulok Basin, and the mountainous terrane west of the Chao Phraya‐Phitsanulok Basin. These data were combined with previously reported late Neogene data from five flows from western Thailand. Although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination, the average declination has a net clockwise rotation of 13.5±5.8 from the geocentric dipole field. Furthermore, the mean declination values from the 29 flows from the Khorat Plateau are indistinguishable from the present dipole field direction (Dm = 4.3°±7.5°) and indistinguishable from the mean declination from 28 late Neogene volcanic flows from Vietnam. In contrast, the mean declinations from 24 flows collected from central and western Thailand are deflected significantly clockwise (Dm = 24.4°±7.7°) from the geocentric dipole field direction. The differential rotation between western and central Thailand versus the Khorat Plateau suggests that Indochina is composed of at least two structural blocks which underwent a different rotational history. These observations, when combined with geologic and geophysical data from the Chao Phraya‐Phitsanulok Basin, Gulf of Thailand, and the intermontane basins of western Thailand, suggest that the rotations are recording a late Neogene phase of E–W extension of these basins. We suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the Indian plate under western Burma. We envision the tectonics of this region is similar in style to the Basin and Range region of the western United States. Last, we have observed field relationships from some of the rhyolites located in the central basin. Although these rhyolites are reported to be Mesozoic or Paleozoic in age, our field observations and a K‐Ar age date show that at least some of these rhyolites are younger than the basalts. We suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension.