Two-phase contractional deformation of the Jurassic Daebo Orogeny, Chungnam Basin, Korea, and its correlation with the early Yanshanian movement of China

The Daebo Orogeny is a major orogenic event in Korea that affected the Early to Middle Jurassic Daedong Supergroup. Our investigation of the Nampo Group (part of the supergroup) in the Chungnam Basin reveals the occurrence of two phases of shortening. The majority of the Nampo Group and a basement slab were folded and displaced toward the WNW along the Ocheon thrust during the first phase (Middle Jurassic - early Late Jurassic). Field relationships suggest a ramp-flat geometry for the Ocheon thrust. Mechanical stratigraphy within the slice is consistent with a detachment fold model. Geometric modeling suggests 5.5 km and 3.3 km as the minimum amount of shortening accommodated by the Ocheon thrust and its slice for the ramp slopes of 15 and 30, respectively. The second phase (late Late Jurassic) was a N-S shortening which produced roughly E-W-trending reverse faults and folds to form a dome-and-basin structure. These shortening events occurred over the entire Chungnam Basin, and coincide in orientation and timing with those reported from the Ogcheon Belt and the Gyeonggi massif. The first and second phases of the Daebo Orogeny are respectively correlated with the shortening in eastern China and the sinistral transpression along the Tan-Lu Fault, both belonging to the early Yanshanian movement. Subsequently in the Early Cretaceous, a regional extension commenced over Korea and eastern China. These remarkable correlations suggest that two stages of crustal thickening occurred throughout the eastern Asia during the Middle to Late Jurassic, followed by regional thinning in the Early Cretaceous.