In-Chang Ryu

Neoproterozoic Bimodal Volcanism in the Okcheon Belt, South Korea, and Its Comparison with the Nanhua Rift, South China: Implications for Rifting in Rodinia

A systematic geochemical examination and zircon U‐Pb dating of Neoproterozoic bimodal volcanic rocks from the Chungju area in the northeast segment of the Okcheon Metamorphic Belt (OMB), South Korea, allow a comparison with similar rocks from the Nanhua Rift, South China. The bimodal metavolcanics of the Chungju area comprise the subalkaline to alkaline basalts (hereafter “mafic metavolcanic rocks”) and trachytes (hereafter “felsic metavolcanic rocks”). The mafic metavolcanic rocks are characterized by light rare earth element (LREE)‐enriched and “humped” trace‐element patterns with moderate depletions in Sr and Ti and variable but reasonably low ϵNd(T) values between +0.83 and +2.99. These geochemical features are consistent with the areas origin in an ocean island basalt (OIB) mantle source with minor crustal contamination. The felsic metavolcanic rocks are characterized by LREE‐enriched patterns with a remarkable negative Eu anomaly. They display an overall enriched trace‐element pattern with significant depletions of Sr, P, Eu, and Ti. They have the geochemical characteristics typical of A1‐type granites and ϵNd(T) values between +1.30 and +2.54. The incompatible element versus incompatible element diagrams for both rocks exhibit nearly smooth positive trends. In the Y/Nb versus Yb/Ta diagram, all the felsic rocks plot within the OIB field. Hence, our data imply a genetic linkage between the mafic and the felsic rocks. The felsic rocks were most likely generated from basaltic protoliths through extensive fractional crystallization plus minor crustal contamination/assimilation. Igneous zircon from a felsic metavolcanic rock from the Munjuri Formation in the OMB gives a SHRIMP U‐Pb age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A Middle Ordovician Drowning Unconformity on the Northeastern Flank of the Okcheon (Ogcheon) Belt, South Korea

762\pm 7

Development of regional uplift and uplift-related strata in Gunsan Basin, Yellow Sea: implications for Cenozoic crustal extension

\end{document} Ma, providing a temporal link to the Neoproterozoic rift‐related volcanism in South China. Rare detrital zircons extracted from a felsic sample from the Gyemyeongsan Formation yield a mixture of ages: ∼1.9 Ga, ∼870 Ma, and ∼250 Ma. The presence of some Triassic metamorphic overgrowths reflects a high‐grade metamorphic event coeval with that recently identified in the Gyeonggi and Yeongnam massifs and linked to an Early Triassic collision event in South Korea. The lithological and geochemical data from the Neoproterozoic metavolcanic rocks in the OMB of South Korea show similarities with the Nanhua Rift in South China, pointing to a North Asian connection associated with the disruption of Rodinia.

Phanerozoic high-pressure eclogite and intermediate-pressure granulite facies metamorphism in the Gyeonggi Massif, South Korea: implications for the eastward extension of the Dabie-Sulu continental collision zone

The Okcheon metamorphic belt (OMB) in central South Korea is surrounded by Middle Jurassic granitoid batholiths that intruded South Korea extensively; the granitic bodies form a complex about 200 km long and 150 km wide as part of a Mesozoic granite belt along the East Asian continental margin. Middle Jurassic magmatism was related to ridge subduction that occurred around 200 to 166 Ma, with the main magmatic period between 175 and 166 Ma; main cooling ages range from 168 to 152 Ma. The magmatism was divided into two stages: (1) a deeper, earlier stage, which resulted in emplacement of diorite, granodiorite and granite as shown in the northeast OMB; and (2) a shallower, younger stage, which resulted in emplacement of granite and two-mica granite as shown in the southwest OMB. Most granitoids are peraluminous to metaluminous I-type granitoids that originated in a volcanic arc; an exception is an S-type two-mica granite. Inherited cores of 998 and 262 Ma U-Pb SHRIMP II zircon ages from the two-mica granite indicate that two-mica granite is reworked crustal material formed by earlier magmatism before the Middle Jurassic event. Together with previous studies on the Middle Jurassic granitoids, the present result indicates that subduction of the Farallon-Izanagi ridge beneath Asia caused widespread igneous activity throughout South Korea, especially during Middle Jurassic ridge subduction.

Tectono-metamorphic evolution of the Okcheon Metamorphic Belt, South Korea: Tectonic implications in East Asia

Abstract The Maggol Limestone of Ordovician age was deposited in the Taebaeksan (Taebacksan) Basin which occupies the northeastern flank of the Okcheon (Ogcheon) Belt of South Korea. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates in the early Middle Ordovician (earliest Darriwilian). Elsewhere this subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk-Tippecanoe sequence boundary beneath the Middle Ordovician succession and its equivalents, most in notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sea level fall during the early Middle Ordovician. The Sauk-Tippecanoe sequence boundary in South Korea, however, appears to be a discrete marine-flooding surface in the upper Maggol Limestone. Strata beneath this surface represent by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution-collapse breccias. This marine-flooding surface is onlapped by a thick succession of thin-bedded micritic limestone that is eventually overlain by a Middle Ordovician condensed section. This physical stratigraphic relationship suggest that second- and third-order eustatic sea level fall may have been significantly tempered by regional tectonic subsidence near the end of Maggol deposition. The tectonic subsidence is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e., the Yemi Breccia) in the basin. With continued tectonic subsidence, a subsequent rise in the eustatic cycle caused drowning and deep flooding of the carbonate platform, forming a discrete marine-flooding surface that may be referred to as a drowning unconformity. This tectonic interpretation contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously suggested. Thus, it is proposed that the Taebaeksan Basin in the northeastern flank on the Okcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician.

Geochronological constraints on multiple deformations of the Honam Shear Zone, South Korea and its tectonic implication

1000 AD 경에 매우 강력한 화산 폭발을 하였던 백두산은 아직도 폭발 가능성을 가지고 있다. 따라서 백두산의 폭발 가능성을 사전에 확인할 수 있는 정기적인 감시가 필요하다. 하지만 지정학적인 이유로 백두산 지역에 정기적인 감시 시설을 설치하거나, 정기적인 관측 자료를 얻는 것이 매우 힘든 상황이다. 따라서 인공위성을 이용한 백두산 화산 폭발 가능성에 대한 정기적인 감시 기술 개발이 필요한 실정이다. GRACE 인공위성에서 획득한 백두산 지역의 중력 자료로부터 계산된 지오이드 자료의 지난 10년간의 변화를 보면, 백두산 지역에서 지진 발생을 포함한 마그마 활동 가능성이 인지된 2002년부터 2005년 사이에 매우 뚜렷한 지오이드 감소가 확인되었다. 그리고 동일한 시기에 백두산 지역의 강수량이 줄어들지 않음을 고려할 때 이러한 변화는 백두산 하부의 마그마 활동에 연관되어있을 가능성을 지시한다. 백두산 지역에서 CHAMP 인공위성을 이용하여 측정된 지난 2000년도부터 2010년까지의 자력 변화를 살펴보면, 마그마 활동이 활발했던 2000년에서 2005년도 사이에 자력이 감소하고 그 이후 다시 증가하는 현상을 보여준다. 이러한 자력의 감소는 백두산 하부 마그마 온도의 증가에 따른 주변 암석의 비자성화(demagnetization)에 의한 것일 가능성이 있다. 이러한 자료는 인공위성을 이용하여 관측한 지오이드와 자력의 변화가 백두산 하부 마그마 활동을 감시 하는데 사용될 수 있는 가능성이 있음을 지시한다.

Middle Paleozoic metamorphism in the Hongseong area, South Korea, and tectonic significance for Paleozoic orogeny in northeast Asia

ABSTRACT The Mesozoic–Cenozoic Gunsan Basin is the northeastern part of the Northern South Yellow Sea Basin between eastern China and the Korean Peninsula. On the basis of seismic interpretation, this study presents and interprets geologic features of regionally uplifted structures, the Haema Arch, located in the central western part of the basin. The Haema Arch is defined as dome-shaped uplift complexes, 95 km long and 60 km wide. It is characterized by prominent basement uplifts along its margin and plunging syncline inside the arch. The marginal large-scale uplifts are bounded by outward-dipping faults. The uplift-related strata are identified on the hanging wall block of the bounding faults and within the Haema Arch, which can be divided into pre-, syn-, and post-uplift units. The pre-uplift unit rests on the acoustic basement and shows an upturned stratal pattern near the marginal large-scale uplift. The syn-uplift unit locally occurs on the hanging wall block of the bounding faults along the northern and southern margins. The uplift of the Haema Arch and its coeval fault-controlled subsidence possibly occurred during the late Oligocene. The post-uplift unit initially formed on remnant topographic lows during the early Miocene and subsequently covered the overall area of the Haema Arch and the Gunsan Basin. The late Oligocene uplifting of the Haema Arch can be interpreted as an isostatic response to tectonic unloading by the arch-bounding faults that possibly extend to detachment faults. We suggest that the Gunsan Basin underwent crustal thinning and extensional deformation during the late Oligocene, which accounts for the coeval uplifting and fault-controlled subsidence in the study area.