HR: 0800h
AN: GP11D-0874    [Abstracts]
TI: A Preliminary Paleomagnetic Result of the Lower Triassic Sedimentary Rocks in the Danyang Area, Korea: its Tectonic Implications
AU: * Park, Y
EM: aegis@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AU: Doh, S
EM: sjdoh@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AU: Suk, D
EM: dwsuk@hanyang.ac.kr
AF: Hanyang University, Dept. of Earth and Marine Sciences, Ansan, 425-791 Korea, Republic of
AB: Southern Korean Peninsula is tectonically divided into Gyeonggi Massif, Okcheon Belt and Yeongnam Massif from northwest to southeast. In recent decades, many geologists and paleomagnetists have endeavored to elucidate the tectonic affinity of the Gyeonggi and Yeongnam massifs with adjacent Chinese blocks, such as North China Block (NCB) or South China Block (SCB), before the collisional event of East China, but it is still controversial. A paleomagnetic study has been carried out for the Lower Triassic Donggo Formation, exposed in the Danyang area on the Yeongnam Massif, to provide reliable paleomagnetic constraints on the origin of the Yeongnam Massif. The characteristic remanent magnetization (ChRM) components were isolated from a total of 120 samples in 13 sites. The ChRMs in this study reveal both normal and reversed polarities and pass a reversal test at the 95% confidence level. Stepwise untilting test shows that the maximum clustering of ChRM directions occurs at 100% untilting, indicating that the ChRMs isolated in this study are of primary in origin. The paleomagnetic pole position calculated from the site mean directions of the primary remanence in the Danyang area is at $38.3\deg$N, $18.5\deg$E (A$_{95}$ = $6.5\deg$). This pole is displaced westward by $49.4\deg$$\pm$$11.5\deg$ with respect to the Early Triassic paleopole reported from the Taebaek area which is located at approximately 50 km distant to the northeast of the study area. The Donggo Formation in the Danyang area is distributed with NE-SE trend, parallel to the general trend of the Okcheon Belt, while that in the Taebaek area is distributed with E-W trend. Furthermore, Paleomagnetic direction (D/I = $354.9\deg$/$61.5\deg$, k = 58.0, \alpha$_{95}$ = $4.7\deg$) of the Cretaceous overprint component in the Taebaek area shows significant eastward declinational deflection (c.a. $60\deg$) with respect to that (D/I = $55.8\deg$/$54.8\deg$, k = 18.3, \alpha$_{95}$ = $9.5\deg$) of the Late Cretaceous-Early Tertiary remagnetization component in the northeastern Okcheon Belt (Jeongseon area) with an statistically insignificant inclinational difference. It is therefore interpreted that the Taebaek area rotated clockwise by about $50\deg$-$60\deg$ with respect to adjacent areas, such as Danyang and Jeongseon areas, after the Early Triassic. Korean paleomagnetic poles that satisfy more than four reliability criteria of Van der Voo (1990), since the Late Carboniferous, have been selected and synthesized for comparison with Chinese apparent polar wander (APW) paths. Comparisons of Korean paleopoles with the APW paths of the NCB and SCB show that the Yeongnam Massif belongs to the NCB, while the Gyeonggi Massif has been a part of the SCB before the completion of suturing between the NCB and SCB. This tectonic correlation based on paleomagnetic data agree well with that based on the paleontologic data. In addition, the relative tectonic movements of Korean blocks with respect to the NCB and SCB since the Late Carboniferous will be further discussed in the presentation.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting