IDENTIFICATION OF BASIN INVERSION BASED ON SOFT-SEDIMENT DEFORMATION STRUCTURES (SSDS): AN EXAMPLE FROM SE KOREA

Tectonic inversion of sedimentary basins, which is one of the major mechanisms causing uplift and erosion after deposition, can lead to expose deep buried sediments in thrust-fold belt systems. In sedimentary basins influenced by faulting, water-saturated sediments can lead to the formation of soft-sediment deformation structures (SSDS). Deformed SSDS can indicate polyphase deformation events, which can provide valuable insights into the inversion process of deformation history in the basin. This study focuses on a well-exposed Miocene sedimentary outcrop from the Bomun sub-basin, situated in the Gyeongju area of the SE Korean Peninsula. The Bomun sub-basin is a narrow N-S trending small basin, which is included within the Pohang basin. It was reported that the basin was formed associated with the right-lateral fault termination of the Yeonil Tectonic Line (a boundary fault of Ulsan Fault Zone). Various SSDSs such as load structures, clastic dikes, neptunian dikes, and convolute beddings are developed indicating different deformation events. Extension fractures including neptunian dikes and normal faults show a distinct NNE~NE orientation indicating σ_Hmax of NNE~NE (basin opening stage). However, the trend of fold axes (E-W) is not consistent with the strike (N-S) of the basin boundary fault and the stress condition. It indicates that the stress condition has changed after the extension stage. Furthermore, several SSDSs were sharply displaced by reverse faults. Paleostress analysis of these reverse faults indicates ENE direction of compression (σ_Hmax), similar to the current stress field. The observed clear liquefaction and widespread development of SSDSs, and faults indicate repetition of substantial seismic activity around this basin (Yangsan-Ulsan Fault System).

Acknowledgement: This research was supported by a grant(2022-MOIS62-001(RS-2022-ND640011)) of National Disaster Risk Analysis and Management Technology in Earthquake funded by Ministry of Interior and Safety(MOIS, Korea).