The Jinan basin is one of the Cretaceous pull-apart basins in South Korea locating along the Yongdong-Kwangju fault system. The Jinan basin consists of tuff, tuffaceous mudstone, sandstone, conglomerate and these sedimentary rocks in the Jinan basin can be classified into the Lower and Upper sedimentary units. The Lower unit does not contain felsic to intermediate volcanic rocks or volcano clastic rocks but the Upper unit contains them. The basalts rarely occur underlained by felsic volcanic rocks. The basalts are plotted either on the calc-alkaline basalt (CAB) or within plate basalts (WPB) field suggesting that they are formed from asthenospheric mantle contamined by crustal matrial during their uplift to the surface. The intermediate to felsic volcanic rocks are plotted on the volcanic arc granitoid (VAG) field in the tectonic discrimination diagrams. The rare earth elements (REE) and trace element patterns of them also show arc related character indicating their arc origin.
The sensitive high resolution ion microprobe (SHRIMP) age dating was carried out to get the weighted mean ages of the youngest detrital zircons from the sedimentary rocks at the lowest and top parts of the Lower sedimentary unit and at the top of the Upper sedimentary unit. The ages obtained from the three sedimentary rocks are 2048 Ma, 187.0 Ma and 87.9 Ma, respectively. The K/Ar age dating on the volcanic rocks in the Upper sedimentary unit, gives the extrusion ages from 90.1 to 75.7 Ma which can be correlated to Turoian to Campanian. The basalts, which expected to erupt between the sedimentation of the two sedimentary units, give K/Ar ages of 93.1 Ma.
The sedimentation in the Jinan basin may have started between ca. 130-100 Ma and continued until ca. 87.9 Ma at least. The Lower and Upper sedimentary units can be correlated to the Singdong and Yucheon groups in the Gyeongsang basin, respectively. In the Jinan basin, the sedimentary rocks, which can be correlated to the Hayang group in the Gyeongsang basin, are expected to exist based on the finding of basalt because basalt mainly occurs in the Hayang group in the Gyeongsang basin.
This study combined with the previous studies suggest the following tectonic evolution. At the time between ca. 130-100 Ma, the collision between the Siberia and Manchuria blocks occurred causing southwards compression in the Korean Peninsula, resulting in the first stage of sinistral strike-slip movement along the Yongdong-Kwangju fault system. During this stage, the first stage of the pull-apart basin occurred in the Jinan basin and the Lower sedimentary unit was deposited in the basin. At the later part of the first stage, regional extension occurred due to the roll-back of volcanic arc causing the extrusion of basaltic volcanic rocks in the Jinan basin. The later part of the first stage can be correlated to the time of the Hayang group sedimentation with basaltic layers in the Gyeongsang basin. From ca. 90 Ma, the northwards compression from the Pacific Ocean became important resulting in the second stage sinistral strike-slip movement along the Yongdong-Kwangju fault system. It caused the second stage pull-apart basin where the Upper sedimentary unit was deposited with extensive felsic and intermediate volcanisms.