Koji Uno

Recent advances in the cretaceous stratigraphy of Korea

Abstract A subrounded, accidental, zircon grain from a rhyolite sample of the Oknyobong Formation has shown an U–Pb CHIME isochron age, 187 Ma, implying its derivation from a Jurassic felsic igneous rock. Such a lower limit of the geologic age of the Oknyobong Formation, combined with its pre-Kyongsang upper limit, constrains that the Oknyobong Formation belongs to the Jasong Synthem (Late Jurassic–early Early Cretaceous) typified in North Korea. The Jaeryonggang Movement terminated the deposition of the Jasong Synthem and caused a shift of the depocenter from North Korea to the Kyongsang Basin, Southeast Korea. The Cretaceous–Paleocene Kyongsang Supergroup of the Kyongsang Basin is the stratotype of the Kyongsang Synthem, an unconformity-bounded unit in the Korean Peninsula. The unconformity at the base of the Yuchon Volcanic Group is a local expression of the interregionally recognizable mid-Albian tectonism; it subdivides the Kyongsang Synthem into the Lower Kyongsang Subsynthem (Barremian–Early Albian) and the Upper Kyongsang Subsynthem (Late Albian–Paleocene). The latter is unconformably overlain by Eocene and younger strata. The Late Permian to Early Jurassic radiolarian fossils from the chert pebbles of the Kumidong and the Kisadong conglomerates of the Aptian–Early Albian Hayang Group of the Kyongsang Basin are equivalent with those of the cherts that constitute the Jurassic accretionary prisms in Japan, the provenance of the chert pebbles in the Kyongsang Basin. Bimodal volcanisms throughout the history of the Kyongsang Basin is exemplified by the felsic Kusandong Tuff erupted abruptly and briefly in the Late Aptian when semi-coeval volcanisms were of intermediate and mafic compositions. The mean paleomagnetic direction shown by the Kusandong Tuff is in good agreement with the Early Cretaceous directions known from North China, South China and Siberia Blocks.

Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites

Abstract Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Japan were studied using paleomagnetism, rock magnetism, optical microscopy and scanning electron microscopy (SEM) by comparison with unremagnetized granitic rocks. The natural remanent magnetization (NRM) of the remagnetized rocks is strong (0.3–1.7 A/m) and shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks preserve weak NRM (

Late Cretaceous paleomagnetic results from northeast Asian continental margin: The Sikhote Alin Mountain Range, eastern Russia

Welded tuffs in the Upper Cretaceous Kisin Group have been collected for paleomagnetic study from 24 sites in Sikhote Alin, eastern Russia. Normal paleomagnetic directions from 11 sites are regarded as primary directions, whereas reversed paleomagnetic directions from 13 sites are ascribed to secondary origin. The normal directions of the Kisin Group together with the characteristic directions of the Sijanov Group yield the Late Cretaceous characteristic direction of Sikhote Alin (D = 355.0°, I = 59.8° with α95 = 7.2°). A significant separation (15.4°±11.9°) of pole positions is observed between Sikhote Alin (85.9°N, 11.3°E with A95 = 10.1°) and the North China Block. Sikhote Alin has been subjected to counterclockwise rotation through 20.5°±17.7° with respect to North China Block. The rotation occurred during 66–51 Ma, and is ascribed to sinistral motion along the central Sikhote Alin fault.

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

Abstract We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D =345.8°, I =49.9°, α 95 =14.6° ( N =9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.

Paleomagnetic results of the Late Permian Gobangsan Formation, Korean Peninsula: Remagnetization in the southeastern periphery of the Bagjisan Syncline

Sedimentary rocks in the Late Permian Gobangsan Formation were collected at 7 sites for paleomagnetic study in the southeastern periphery of the Bagjisan Syncline, Korean Peninsula. The Gobangsan Formation revealed a stable secondary magnetization component with unblocking temperatures of 500–580°C and 650°C from two sites, while the other sites possessed only a present day viscous remanence. The secondary component resides in magnetite and hematite. The site-mean directions of the two sites before tilt correction (D = 355.9°, I = 50.2° with α95 = 4.3° and D = 355.7°, I = 53.3° with α95 = 6.1°) suggest that the remagnetization occurred after Early Cretaceous. The most plausible mechanism of the remagnetization is considered to be a chemical authigenesis because the other possible mechanisms of the remagnetization such as thermoviscous process and Recent weathering can be ruled out by rock magnetic experiments. The timing of the remagnetization is constrained during Tertiary time, because the observed directions are distinguishable from the Cretaceous directions and because Recent remagnetization is unlikely. This is ascertained by good agreement between the observed and the Tertiary directions.