Significance of systematic changes in crenulation asymmetries within metasediments across the Ogcheon Supergroup in the Goesan, southern Korea

Abstract

Garnet bearing metasediments from the Ogcheon Supergroup in the Goesan area, southern Korea, preserve evidence of several earlier deformation and metamorphic events, previously unrecognized. This study, based on a 3-D thin section observation of the rock, has revealed six distinctive planar structures including five foliations and micro-fold axial planes inside the porphyroblasts (S1 to S5 ) and five foliations in the matrix (S2 to S6), besides weak structures locally overprinting some of these matrix foliations. The rocks studied experienced early formation of a shallow foliation during D1, succeeded by NE-SW shortening (D2), vertical shortening towards the NE-SW (D3), NW-SE shortening (D4) that developed the regional NE strike, vertical shortening (D5) causing the variable dips of early planar structures, EW shortening (D6) causing local northwards rotation of the NE structures and some younger weak deformation. Asymmetric crenulations resulting from the intersection between these planar structures preserved within porphyroblasts and in the matrix were collected to see the rock movement especially regarding the NW-SE horizontal shortening (D4) and subsequent vertical collapse (D5), which produced the present NE-SW "Ogcheon trend". These asymmetries appear to switch repeatedly and systematically across the Goesan area. Garnet, staurolite and chlrorite porphyroblasts that overgrow these asymmetric crenulations provide evidence for progressive and episodic mineral growth closely related with each deformation event. This suggests that the NE-SW-striking foliation, with varying dips, resulted from NW-SE bulk coaxial shortening and subsequent vertical collapse rather than a series of SE directed progressive thrusting, which has been suggested.

Keywords

• 3-D analysis;
• crenulation asymmetries;
• porphyroblast;
• NE-SW shortening;
• Ogcheon Orogen