Tectono‐sedimentary development of early synrift half‐graben subbasins in the Miocene Pohang Basin, southeastern Korea

Abstract

Early synrift half‐grabens, herein named the Chilpo and Sinheung subbasins, have been newly recognised in the north‐central area of the Miocene Pohang Basin, Korea. The subbasins are closely aligned along an array of NNW‐trending strike‐slip faults and are bounded by ENE‐striking normal faults. The sediment infill of the subbasins consists of a deepening‐ and fining‐upward sequence of alluvial cobble to boulder conglomerate (FA1), alluvial to nearshore granule to pebble conglomerate and sandstone (FA2) and hemipelagic mudstone (FA3). The younger strata sequentially onlap the older strata in the hangingwall basement or transfer zones, whereas the footwall basement is directly onlapped by the younger hemipelagic mudstone. These patterns may have resulted from a series of domino‐style block rotations, in which subsidence along normal faults caused the differential creation of accommodation space, whilst simultaneous uplift in the upslope of rotated hangingwall blocks controlled denudation, the shedding of sediments to downslope areas and asymmetric stacking of sediments in both subbasins. In contrast to the western margin of the Pohang Basin, where large footwall‐derived fan‐delta systems developed along the bounding faults, in the Chilpo and Sinheung subbasins, small hangingwall‐derived alluvial‐fan deltas formed during rifting. The development of these domino‐style half‐grabens would have resulted from the detachment and simultaneous transrotation of the crustal block away from the northwest during accelerated extension in the Pohang Basin. Recent geophysical studies have detected very thin basin‐fill sediments in the north‐central Pohang Basin and faults that are similar in orientation to those identified in this study, thereby supporting our observations. These findings confirm the early opening of the Pohang Basin, supporting the hypothesis that all of the Miocene basins in southeastern Korea formed in a setting of NW–SE extension, and further suggest the development of the wedge‐shaped transtensional basin, in response to the opening of the East Sea (Sea of Japan) after 17 Ma.