楡川盆地 北東部 白堊紀 火山岩類의 火山岩石學 및 地質構造

Petrology and structural geology of the late cretaceous volcanic rocks in the northeastern part of Yucheon basin

The studied area is largely occupied by thick piles of the late Cretaceous volcanic rocks of the Yucheon group, which is northeastern border part of the vast volcanic region in the Yucheon basin. The Yucheon group overlies the Geoncheonri Formation and is intruded by granitic and dioritic stocks and dykes. The group can be devided into two parts ; the lower is Jusasan andesitic rocks which was called as Jusasan Porphyrite Formation by Tadeiwa in 1929, and the upper is Unmunsa rhyolitic rocks. The volcanic pile consists mainly of various tuffs such as tuff breccia, lapilli tuff, coarse to fine tuff and tuffaceous sediments, and interlayered flows, which range from basaltic andesite to rhyolite in their lithology. The results of petrochemical and volcanostratigraphic studies on the Jusasan andesitic socks suggest that the volcanic rocks were derived from two cyclic evolutions of magmatic fractionation. Systematic study of 5226 joints from the area reveals two sets of steep joints striking N20˚-40˚E and N40˚-70˚W, are dominant and coincide with the fault pattern developed in the area. Three defferent maximum principal stress axes were recognized from conjugate shear joints, which are trending east-west, north-northwest, and north-northeast.