DATING FAULT ROCKS FROM THE ULJIN AREA, EASTERN KOREAN PENINSULA

We report Rb-Sr, K-Ar and ESR dating results for fault gouges from the Uljin area, Gyeongsangbuk-do, Korea. Three sets of faults are developed in country rocks of Precambrian basement gneisses. The fault 1 of NW strike is cut by the NE-trending fault 2. The fault gouges are composed of quartz, feldspar, dolomite, siderite and clay minerals of smectite, illite and kaolinite. The 10- angstrom illite peak is clearly identified in XRD pattern of <1µm fraction of the fault gouges. The Rb-Sr isotope data of 1N HCl leachates and residues for the <1µm fraction of the fault gouges yield 65.0+/-4.9 Ma for the fault 1, 54.2+/-0.4 Ma and 53.3+/-0.2 Ma for the fault 2 and 93.0+/-0.7 Ma and 54.0+/-2.8 Ma for the fault 3, which are reproducible and concordant with the field relationship. On the basis of Rb-Sr data, the following history of recurrent fault activity can be suggested: The fault 3 and 1 were developed in late Cretaceous (93 Ma) and early Paleocene (65 Ma), respectively and the fault 1 was reactivated in early Eocene (54 Ma) concurrently with the development of the fault 2. The dyke cut by the fault 2 has a concordant K-Ar age of 70.6+/-2.1 Ma. K-Ar ages of the fault gouges, however, do not always match with the field relationship and tend to be apparently older than the Rb-Sr ages, indicating the effect of inherited radiogenic Ar from the Precambrian country rocks. All the ESR signals of the fault gouges are saturated, denying the probability of Quaternary reactivation of the faults.