ROTATION AND PRESERVATION OF EARLY-DEVELOPED STRUCTURES AROUND A COMPETENT HETEROGENEITY

Anomalous NW-SE strikes of bedding and foliation exist at the northeastern end of a northeast-trending Orogen, northern South Korea. This particular region has been significantly less affected by the deformation that produced the dominant NE-SW-trend of this orogen than the rest of the belt. The anomalous trend curves into the NE-SW regional trend to the south. They were previously interpreted to have been formed by (1) overprinting by a much younger fold; (2) passive rotation of the pre-existing NE trends by a sheath-like upheaval of a basement gneiss; (3) NE-wards extrusion of sediments at the belt margin during NW-SE shortening.

This study, based on combination of Macro-/Meso-structural and 3-D micro-structural analysis, has identified 7 generations of planar structures as well as 5 sets of foliation inflexion/intersection axes resulting from the first 6 planar structures preserved within garnet porphyroblasts. This reveals that the NW-SE-trending structures and several sets of crenulations in the study area formed before a major change in the direction of orogenic bulk shortening to NW-SE. NE-SW shortening during D₂ produced the NW-SE trend of the region. This was followed by gravitational collapse towards the NE-SW during D₃. These early fabrics, which generally appear as a composite foliation S_1,2,3, bend throughout the area around NE-trending D₄ and D₅ structures, and are locally folded around N-S-trending D₆ folds. Pre-Cambrian gneiss bodies in the northeast of the study area have created a strain shadow region that was protected from the effects of D₄/D₅ and preserved the NW trend of bedding and early foliations. However, in the rest of the Ogcheon Orogenic Belt the overall trend of bedding and foliations have been rotated into a NE-SW trend by the effects of D₄, leaving relics of the NW trends only within porphyroblasts and strain shadow quartz seams.