Paleomagnetic study of the Mesozoic sedimentary rocks in the Gyeonggi massif: Implications for tectonics of Korea

Abstract

A paleomgnetic and an anisotropy of magnetic susceptibility (AMS) studies for the Mesozoic sedimentary rocks in the Gyeonggi massif, the late Triassic Daedong Supergroup in Chungnam province, were carried out to find out the characteristic remanent magnetization (ChRM) direction and an affinity of the strata in the tectonic frame for the Korean Peninsula. The Daedong Supergroup consists of the Wolmyungsan, Amisan, Jogyeri, and Baegunsa Formations in an ascending order. A total of 204 specimens from 26 sites were collected, among which 105 specimens from 14 sites and 108 samples from 15 sites reveal the characteristic directions and the meaningful directions of magnetic foliation/lineation, respectively. The mean ChRM direction of the Daedong Supergroup, except the Wolmyungsan Formation, after tilt correction is better clustered and is D/I=74.5o/36.7o (k=60.7, α95=5.1o). The corresponding paleomagnetic pole is at 208.2oE/24.1oN (n=14, dp/dm=3.5o/6.0o) and it is statistically indistinguishable from the late Triassic pole of the South China Block (SCB), indicating that the Daedong Supergroup had been a part of the SCB during the late Triassic Period. The AMS study of the strata shows well defined magnetic foliations and the attitude of the magnetic foliation varies 0.1o∼63.5o/37.4o∼69.1o with an average of 26.9o/52.8o (k=20.8, α95=8.6o) in geographic coordinates. The magnetic fabrics defined by planes of kmax and kint of anisotropy ellipsoids are coincident with the prevailing trend of regional structure in the study area, such as the Baegunsa Fault. It is interpreted that the ESE dipping foliations are possibly due to WNW-ESE compressive force related to the collisional event between the North China (Sino-Korean) and the South China (Yangtze) Blocks after the formation of the Daedong Supergroup, presumably during the Jurassic.