Basin Fills And Geological Structures Of The Miocene Yangpo Subbasin In The Janggi-Myeon, Pohang, Se Korea

Geological mapping, stratigraphic classification of the basin-fills, and analysis of various structural elements, including strata attitudes, geometries and kinematics of major and minor faults and shear fractures, and attitudes of dikes, were carried out in order to reveal the geometry and evolution of the Yangpo subbasin, which is located in the southeastern part of the Janggi Basin, SE Korea. The Yangpo subbasin has a NE-trending elongate shape, at least 5 km long and up to 2.5 km wide, and the geometry of an asymmetric graben tilting northwestward in section. The subbasin is bounded by ENE- and NE-trending normal faults in the northern and northwestern margins, respectively, and by ENE-trending normal faults connected by NW-trending dextral strike-slip faults in the southern margin. Based on the constituent volcanic materials, the basin-fills are classified into two distinctive stratigraphic units, i.e. the lower dacitic and the upper basaltic volcanogenic and associated volcaniclastic rocks, which indicate that a bimodal volcanism occurred during the basin formation. The subbasin is divided into the south and north structural domains, based on the variations of the attitudes of strata and dikes. The strata in the former dip consistently toward the NW or NNW, while those in the latter have convergent dip directions toward a diatreme formed by phreatomagmatic explosions of basaltic magma. Basaltic dikes are aligned consistently toward the NE or ENE in the former, while those in the latter show a roughly radial distribution. The Yangpo subbasin is interpreted to have been initially extended under WNW-ESE trending tensional stress. Afterwards, the resultant extensional structures were rotated about 30 degrees clockwise, because of the persistent dextral simple shear operating during the East Sea opening. These features probably indicate that the subbasin is a pull-apart basin similar to the nearby Early Miocene basins. In the earlier stage of the basin extension, a large amount of dacitic volcanic materials accumulated and the faulted blocks were rotated clockwise progressively and tilted toward the northwest. Afterwards, a diatreme was produced by basaltic volcanic activity together with pervasive intrusion and extrusion of basaltic magma. The formation of the diatreme caused significant disturbance of the surrounding dacitic volcaniclastic sediments.