Paleostress analysis of the Pohang-Ulsan area, Southeast Korea: tectonic sequence and timing of block rotation

In order to reconstruct the tectonic sequence in the Pohang-Ulsan area, eastern block of the Yangsan Fault, Southeast Korea, we analyzed the paleostresses for the Tertiary formations using fault-slip and tension gash data from 29 sites. Approximately ten episodic faulting events were recognized from fault populations classified from this study. Principal stress axes of the relatively younger faulting events are well correlated with those of the western block of the Yangsan Fault. However, this is not the case for the older events. A horizontal rotation for the principal stress axes of these older events at each site of the study area has produced a good correlation of the older principal stress axes between the two blocks. Consequently, this probably makes it possible to establish a regional tectonic sequence in the Pohang-Ulsan area. Because of the partial coexistence of neighboring coaxial events, the ten faulting events can be regrouped into five major tectonic events in a chronological order: (1) NW-SE compression and NE-SW extension; (2) NE-SW compression and NW-SE extension; (3) NNE-SSW extension and WNW-ESE compression; (4) E-W compression and N-S extension; and (5) NNW-SSE extension and ENE-WSW compression. Timing of the rotation of the principal stress axes at the sites is mainly related to the third tectonic event, NNE-SSW extension or WNW-ESE compression. The rotation found in the study area appears to have occurred in relation to a regional-scale block rotation. The timing falls on the moment when the SW Japanese Block converged into SE Korea (ca. 12 Ma). Orientations of syndepositional faults and sandstone dikes in the Chunbuk strata, and the dated ages of NE-SW trending dikes around the Guryongpo area support that the tectonic event with a NE-SW compression was active at least during the early-middle Miocene.