Magnetic fabric and deformation of the metasediments in the southwestern Ogcheon Metamorphic Belt, Korea

Abstract

We have reanalyzed magnetic fabric data of the low- to medium-grade metasedimentary rocks in the southwestern part of the Ogcheon belt, and synthesized magnetic fabrics, regional structure and metamorphic history of the study area in order to reveal their tectonic implications. The Ogcheon belt is a fold-and-thrust belt running diagonally (SW-NE) across the Korean Peninsula, and divides the Gyeonggi massif and the Ryeongnam massif. In recent years the Ogcheon belt has been highlighted in the tectonic development of East Asia, because the Ogcheon belt is regarded a candidate for possible eastern extension of the Qinling-Dabie-Sulu belt, the suture zone between the North China (Sino-Korean) and the South China (Yangtze) blocks. It is controversial whether the Ogcheon belt was a suture zone or a transform fault during collisional event. Magnetic fabrics of the study area represent well petrofabrics observed in the field. The shapes of the magnetic fabric ellipsoids move from the oblate field in the biotite zone toward the prolate field in the garnet and garnet+staurolite zones as the metamorphic grade increases. Synthesis of magnetic fabrics and previously reported metamorphic evolution indicate the deformation history of the study area: (1) Penetrative NW dipping cleavage in the study area might form by prevailing NW-SE compressive force in the Ogcheon metamorphic belt (OMB) during the M1 regional metamorphism. (2) SE dipping foliations in the garnet+staurolite zone are due possibly to the intrusion of Jurassic granite in the northeastern area. (3) NE-striking ductile shearing occurred in the garnet+staurolite zone accomplished the development of petrofabrics of the study area during the middle Jurassic. This interpretation for the deformation history of the study area suggests the OMB was a suture boundary between the Gyeonggi and Ryeongnam massifs. The collision between the Gyeonggi (including the OMB) and Ryeongnam massifs occurred by clockwise rotation of the Ryeongnam massif in the late Triassic-early Jurassic. It was followed by the intrusion of granite and then the development of the Honam shear zone related to the subduction of the Izanagi plate beneath the Eurasia during the middle Jurassic.