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Abstract
The Eoil Basin is one of Miocene basins in the SE Korea developed along the East Sea. The basin is bounded on the northwestern and southeastern margins by a series of normal faults trending NE or ENE, and on the northeastern and south-western margins by a series of dextral strike-slip faults trending NNW. Erosional truncation has well-exposed the most part of basin-fills of the Eoil Basin with conspicuous stratigraphic key markers and major structures, which enabled us to construct detailed geologic and structural maps. We collected stratigraphic and structural data to get an insight into the basin genesis. We have found that the Eoil Basin has an elongate shape and consists of a series of wedge-shaped half grabens all with NE axes: the northeastern Eoil subbasin (NE subbasin) and the southwestern Eoil subbasin (SW subbasin). Based on variations in lithofacies, thickness, and age of basin-fills, it is concluded that the two subbasins experienced different evolutional history. The NE subbasin can be further divided into two fault blocks by NE-trending intrabasinal normal faults. The two blocks in the NE subbasin also experienced different subsidence rate from each other, even though almost contemporaneous. Two types of folds are recognized in the NE subbasin: simple drag fold and transverse fold. Most of our field observations indicate that the main structures (i.e., border faults, intrabasinal faults and folds) of the Eoil Basin are syndepositional, even if some of them were at least sporadically active. The sequence of the main structural development is as follows: firstly, the northwestern border faults of the NE subbasin (possibly with the southeastern border faults of the subbasin); secondly, the intrabasinal normal faults in the NE subbasin with the intrabasinal folds; thirdly, the southeastern border faults of the SW subbasin; finally, the northwestern border faults of the SW subbasin. All structural data indicate that the direction of maximum horizontal extension is NW-SE in the study area. Based on these observations and other results of previous studies, it seems that the major strike-slip faults in the study area have acted as master faults of pull-apart basin, and that the NE-trending normal faults were generated by the NW-SE tensional stress which were induced secondarily by a NNW-SSE dextral shearing acted from early Burdigalian to 15 Ma during the opening of the East Sea. We, thus, concluded that the Eoil Basin is a kind of pull-apart basin (rhombochasm) originated by the strike-slip faulting of NNW-SSE trend acted in this time span.[PUBLICATION ABSTRACT]