The Korean peninsula, unlike nearby Japan, is underlain mainly by Precambrian terrane. Superimposed on these ancient rocks in South Korea is a northeasterly trending geosynclinal assemblage which forms the Ogcheon Zone. It is about 60 kilometres wide, and consists of both metamorphic and unmetamorphosed strata ranging in age from Precambrian to Cretaceous. The Precambrian crystalline terrane that emerges north of the geosynclinal belt is known as the Gyonggi Massif and that to the south is the Ryongnam Massif. Farther southeast is the Cretaceous Gyongsang sedimentary basin. It occupies most of the southeastern part of the Korean peninsula, but the southeastern shore is overlapped by the margin of a Tertiary basin of sedimentary and volcanic rocks.
Mesozoic plutonic rocks are widespread, forming major intrusions in all of the main geologic provinces of South Korea. The batholithic rocks are divided into two main groups, the Daebo granites of Early Jurassic to Early Cretaceous age, the Bulgugsa granites of Late Cretaceous to Early Tertiary age. Most of the older granites are aligned in the regionally dominant, northeasterly direction known as the Sinian trend. The younger granites show no clear pattern and seem to be related to later block movements. Most of both the Jurassic and Cretaceous plutonic rocks are granodiorite and granite, but small plutons of basic to intermediate composition exist in both groups. As in Japan, the ages of the granitic rocks are generally younger to the southeast, but in both countries there are numerous complications. In the Ogcheon Zone is a major thrust fault which separates the older and metamorphosed part of the Ogchean assemblage from the younger, unmetamorphosed strata. Aligned along the fault is a sequence of basic rocks (amphibolite, gabbro, basalt, andesite, and chlorite schist) that may indicate the fault is an ancient geosuture. Base metal deposits are associated with both the Daebo and Bulgugsa granites but not, as far as is known, with any of the volcanic rocks.