Yong Hee Park

Paleomagnetic evidence for local oroclinal bending in the eastern Korean Peninsula

A paleomagnetic investigation has been carried out for the Lower Triassic Donggo Formation exposed in the Danyang area on the Yeongnam Massif to constrain local and regional tectonic history of the Taebaeksan Zone, Korean Peninsula. Among a total of 201 samples from 18 sites of the Donggo Formation, the characteristic remanent magnetization (ChRM) components were obtained from 166 samples. Field tests such as tilt test and reversal test reveal that the ChRM of the Donggo Formation is of primary in origin. The Early Triassic paleomagnetic pole position calculated from the site-mean directions of the primary magnetization is at 40.7°N, 17.8°E (A95 = 5.3°). Comparison of the reliable paleomagnetic poles from the Danyang (this study), Taebaek, and Yemi areas in the Taebaeksan Zone reveals a regional division of the pole position, indicating the clockwise vertical-axis rotation of the Taebaek area by 47.2 ± 11.1° with respect to the Danyang and Yemi areas after the formation of the Pyeongan Supergroup. In addition, there is a significant relation between strike deviations and declination deviations with some flattening of the regression line in the southern Taebaeksan Zone. Thus, it is interpreted that the Pyeongan Supergroup in this region had a primary curvature at the timing of sedimentation and subsequently experienced post-Early Triassic oroclinal bending of approximately 47°.

Magnetic fabric and rock magnetic studies of metasedimentary rocks in the central Okcheon Metamorphic Belt, Korea

Anisotropy of magnetic susceptibility (AMS) and rock magnetic studies have been carried out for the metasedimentary rocks in the central Okcheon Metamorphic Belt. The study area is divided into three metamorphic zones: the biotite zone, the garnet zone, and the sillimanite+andalusite zone from southeast to northwest. Magnetic foliation dipping to the northwest is the dominant magnetic fabric in the biotite zone. Magnetic lineation plunging down-dip of the vertical cleavage plane is appeared in the southeastern part of the garnet zone, while magnetic lineation plunging to southeast prevails in the middle and northwestern parts of the garnet zone. It is interpreted that this apparent synform structure, defined by AMS fabric data, was formed by successive top-to-the-southeast vergent thrusting followed by back-steepening process during the regional metamorphism in the Late Paleozoic. In the sillimanite + andalusite zone, AMS fabric is clearly defined and may reflect a tectonic fabric, recorded during the thermal metamorphism in the middle Jurassic, despite the scarcity of rock fabric in the field. The spatial distribution of magnetic mineralogy, defined by the rock magnetic results, implies the 500°C isotherm at the boundary between the biotite and garnet zones, which is associated with the transformation of pyrrhotite into magnetite above 500°C under the oxidizing condition.