GK:1.2.1 Tectonic and Seismic Shifts on the Korean Peninsula
Geography of Korea: I. Natural Environment > 2. Topography > 1) Tectonic and Seismic Shifts on the Korean Peninsula
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1) Tectonic and Seismic Shifts on the Korean Peninsula
(1) Peninsula Tectonics
About half the Korean Peninsula consists of stable metamorphic rock formed during the pre-Paleozoic Precambrian period. Metamorphic rock is widely distributed throughout various massifs, including the Hambuk, Gyeongi, and Mt. Seobaek massifs (Figure 1-1). The Korean Peninsula’s massifs (Kor. yukgoe 陸塊) began to develop about 600 million years ago from the start of the Paleozoic period. Among the peninsula’s metamorphic rocks, some have been found to date to about 2.7 to 2.9 million years ago. This fact would indicate that although its appearance would have been completely different than at present, the Korean Peninsula was already in existence during the Paleozoic period and before.
The Hambuk massif that partially encompasses Hamgyeongbuk-do province in North Korea and the Pyeongbuk massif that covers all of Pyeonganbuk-do and portions of Hamgyeongnam-do provinces, also in North Korea, comprise large areas of granite gneiss rock, part of the China’s Liaodong land block. Between the Hambuk and Pyeongbuk massifs is the Macheollyeongsanmaek mountain range. Because it forms a belt of sedimentary Precambrian rocks, in geological terms it is also called the Macheollyeong geosyncline. However, the term Nangnim land block is sometimes used to refer collectively to the area encompassing the Hamguk massif, Pyeongbuk massif, and Macheollyeong geosyncline.
The Gyeonggi massif that occupies much of the Korean Peninsula’s middle section is composed of metamorphic complex and granite, which is in contrast to the nearby Shandong massif of China. The Sobaeksan massif, which is distributed along a northeast-southwest direction following the Sobaek mountain range, is made up of the gneiss complex of Mt. Taebaek and Mt. Jiri, and is similar in geological makeup to the South China massif.
The basin sitting between these ancient massifs was, during the Paleozoic period, covered by a rising sea, leaving an extensive lake and forming a layer of sedimentary rock upon a metamorphic foundation. The Pyeongan Basin between the Gyeonggi and Pyeongbuk massifs is a sedimentary basin formed by a thick stacking of the metamorphic Sangwon Supergroup formed in the late Precambrian, the Joseon Supergroup from the early Paleozoic, and the Pyeongan Supergroup from the later Paleozoic/early Mesozoic. In its geological formation the Pyeongan Basin bears similarities to the Yellow River Basin in China.
The Okcheon Orogenic Belt and Okcheon Metamorphic Belt, which are sandwiched between the Sobaeksan and Gyeonggi massifs, were at one time considered collectively as the Okcheon geosyncline. However, from the 1970s they began to be recognized as two distinct geological features and each came be termed by its current name. The Okcheon Orogenic Belt is composed of the Joseon and Pyeongan Supergroups, while the Okcheon Metamorphic Belt is composed of low-grade metamorphic rock of the Okcheon Supergroup, whose dating is not clear.
Because the Joseon Supergroup lower sedimentary layer of the Pyeongan Basin and Okcheon Orogenic Belt consists of limestone, its area of distribution has given rise to both karst topography and cement industries. However, the Pyeongan Supergroup that makes up its upper sedimentary layer is comprised of sandstone and shale, along with buried anthracite that make it one of the peninsula’s most important coal deposits.
The Gyeongsang Supergroup dating to the Cretacious period is distributed throughout the Gyeongsang Basin, located in the southeastern Sobaeksan Massif. Formed by sedimentary deposits from an extensive lake, the Gyeongsang Supergroup is composed, from the bottom, of: the Sindong Group, a fluvial deposit layer with psephite core; followed by the Hayang Group, a lacustrine deposit with sandstone and shale core; atop this, the Yucheon Group, effusive rocks such as volcanic tuff and andesite layered by an aqueous sedimentary layer; and finally, interpenetrating all these strata are deposits of Bulguksa granite. However, the generally igneous Yucheon Group are not limited to the Gyeongsang Basin, but are scattered relatively widely through the southern peninsula in Jeolla-do and Chungcheong-do provinces, and are often found in mountains due to their resistance to erosion.
There are also smaller-scale sedimentary basins. The Tumen Basin and Yeongsan River Basin’s Yeongsan Depression, the northernmost basins on the peninsula, date to the end of the Paleozoic, the Okmasan Depression located in Korea’s western coastal region in Chungcheongnam-do province is a strata from the Mesozoic, while the Tumen Basin at the lower reaches of the Tumen River and the Pohang Basin are deposits from the Tertiary period of the Cenozoic.
(2)Cataclysmic Changes on the Korean Peninsula
Geologically, the Korean Peninsula has enjoyed relative stability and experienced few geological cataclysms, which have primarily been concentrated in the Mesozoic period (see Table 1-1). The early Mesozoic Triassic period’s orogeny, the so-called Songnim Disturbance, mainly affected the northern peninsula, creating the Pyeongan Basin and folding the strata there. The result was a severe disturbance in the Pyeongan Supergroup and the discontinuous appearance of anthracite.
The Daebo orogeny, coming at the end of the Jurassic period, was the most powerful cataclysmic disturbance in Korea’s geological history, diastrophically affecting the entire peninsula. This event brought about a major disturbance to the existing strata of the peninsula, to include the massive influx of so-called Daebo granite. There are fundamental differences between the geotectonic events on the Korean Peninsula prior to and after the late Jurassic period. The peninsula strata during and prior to the Jurassic period underwent severe geothermic changes—being folded are experiencing reverse faults for instance, whereas after the Jurassic these strata experienced almost no diastrophic disturbances. The Bulguksa Disturbance occurred from the end of the Cretaceous and extending to the early Tertiary period of the Cenozoic. The Bulguksa Disturbance was centered on the Yeongnam region in southeastern Korea and entailed some minor folding of strata and the influx of Bulkguksa granite. In contrast to the Daebo granite, which can be found widely distributed throughout the peninsula, Bulguksa granite is limited to the Yeongnam region in the peninsula’s southeast.
Following the Bulguksa Disturbance there were no major geotectonic upheavals to speak of. However, from the middle of the Cenozoic the asymmetrical Yogok Disturbance along the axes of the Taebaeksanmaek and Hamgyeongsanmaek ranges began its gradual movement, a disturbance that continues to this day. The Yogok Disturbance, which has worked to shift these ridge axes towards the East Sea, has had a profound influence on the topography and configuration of the Korean Peninsula.
From the Quaternary period of the Cenozoic, volcanic activity became widespread. This volcanic activity was centered around Mt. Baekdu (Paektu) and after forming a relatively expansive lava plateau the activity extended southward along the Macheollyeongsanmaek range, linking Mt. Chilbo, Ulleung-do Island, and Jeju Island in a sort of volcanic crescent, what is termed the “Korean crescent” (Hangukho). It also formed the Cheorwon-Pyeonggang (Gangwon-do province) and Singye-Goksan (Hwanghae-do province) lava plateaus.
