Youngdo Park

Pretectonic and posttectonic emplacements of the granitoids in the South Central Okchon belt, South Korea: Implications for the timing of strike-slip shearing and thrusting

Structural analyses of three granitic plutons in the south central Okchon belt of the Korean peninsula reveal that the Baegnok and Cheongsan plutons are pretectonic with respect to right lateral strike-slip ductile shearing along the Cheongsan shear zone and later top to the ESE thrusting, whereas the Boeun pluton is posttectonic with respect to these two deformation events. U-Pb sphene age data from the three plutons indicate that the intrusion ages of the Baegnok, Cheongsan, and Boeun plutons are 222.7 ± 2.1 Ma, 216.9 ± 2.2 Ma, and 171.7 ± 1.4 Ma, respectively. Microstructural evidence preserved in deformed rocks from the NE striking Cheongsan shear zone and adjacent, NNE striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ESE directed thrusting. Since these thrusts crosscut the late Triassic - Early Jurassic Taedong Group, we estimate that the thrusts developed at ∼180 Ma during the Daebo tectonic event, just prior to the intrusion of the Boeun pluton. We also constrain the age of the Cheongsan shear zone at Middle - Late Triassic time, corresponding to the Songrim orogeny, so that the shear zone is not related to deformation along the Middle Jurassic Honam shear zone (∼176 Ma). The Honam shear zone is not a simple tectonic boundary between the Okchon belt and the Yongnam massif but an anastomosing ductile shear zone not only affecting the Okchon belt and the Yongnam massif but also crosscutting the boundary between them at a low angle. We suggest that the right lateral Cheongsan shear zone is a major boundary between the Okchon zone and the Yongnam massif with Paleozoic cover rocks. The Kyonggi massif and the Okchon zone (South China block) were juxtaposed against the Yongnam massif and Taebaeksan basin (North China block) along the Cheongsan shear zone during a late stage of the Middle Triassic Songrim orogeny, probably related to the late Paleozoic - early Mesozoic collision between the North and South China blocks.

The Kyonggi Shear Zone of the Central Korean Peninsula: Late Orogenic Imprint of the North and South China Collision

The crustal‐scale Kyonggi shear zone of central Korea has been identified as a major boundary between the Precambrian Kyonggi massif in the south and the Imjingang belt in the north. The latter is an eastward extension of the Qinling‐Dabie‐Sulu collisional belt of China. Field observations and microstructural analysis indicate that the extensional shear zone evolved from a deep crustal ductile regime to a shallow crustal brittle regime, associated with a rapid uplift of the Kyonggi massif following the Late Permian–Early Triassic collision between the Sino‐Korean and Yangtze cratons. A Rb‐Sr muscovite age ( \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

Experimental investigation of Ostwald-ripening rates of forsterite in the haplobasaltic system

226\pm 1.2

Evidence for Permo-Triassic collision in Far East Asia: The Korean collisional orogen

\end{document} Ma) of the mylonite suggests that the extensional ductile shearing occurred during the Late Triassic.

High P-T granulite relicts from the Imjingang belt, South Korea: Tectonic significance

Abstract We describe non-rotation of garnet porphyroblasts in non-coaxially deformed metapelites of the Imjingang orogenic belt in the middle Korean Peninsula. The three-dimensional geometry of inclusion trails within 1–2 mm garnet porphyroblasts was analyzed by serial polishing and reflected-light microscopy. Most of the garnet porphyroblasts have straight inclusion trails of quartz and some plagioclase (S i ) oblique to an external foliation (S e ). Serial polishing of two samples normal to mineral/elongation lineation (A plane), and normal to S e and parallel to the lineation (B plane), with twenty intervals of about 0.1 mm, reveals that the straight S i is a planar structure. Measurement of the angles between S i and S e on both planes from samples of sixteen outcrops shows that S i orientation of individual garnets is fairly constant in each sample. The attitude of S i in geographic space, determined by using its pitch angles on the A and B planes, is more or less constant throughout the area (over 6.5 km traversing normal to the strike of regional foliation). The sense of asymmetry of strain shadows at the margin of garnet porphyroblasts and the S i /S e relationship indicate rotation of S e rather than garnet porphyroblasts with respect to the geographic reference frame. Rotation of S e is also supported by a sense of shear inferred from other shear criteria, all indicating a top-up-to-the-south shearing.

Quaternary reactivation of Tertiary faults in the southeastern Korean Peninsula: Age constraint by optically stimulated luminescence dating

Abstract The rate of Ostwald ripening of forsterite is experimentally measured in a haplobasaltic melt (SiO 2 –Al 2 O 3 –CaO–MgO). The goal of the experiments is to determine if the rate of Ostwald ripening is sufficient to affect textural development in crystallizing magmas of a basaltic composition. With preequilibrated charges, experiments were run isothermally so that all textural changes could be ascribed to Ostwald ripening. An increase of crystal size from 3.5 μm to 14 μm (in average) has been observed during 10 days. Glass compositions were identical in all charges, indicating that the observed increase in average crystal size is not the results of chemical disequilibrium but the results of Ostwald ripening. Due to the constraints on experimental duration, the rate-limiting process (i.e., diffusion control or reaction control) could not be determined. Our experimental results, however, demonstrate that the rate of Ostwald ripening in basaltic rocks is sufficient to be of fundamental importance in the development of textures in igneous rocks. These results strongly suggest that Ostwald ripening, as well as nucleation and crystal growth, should be considered in analyses of textures in igneous rocks.

Fault slip analysis of Quaternary faults in southeastern Korea

Abstract Quartz inclusions within intertectonic garnet porphyroblasts of metapelites from the southern Imjingang belt have a lattice preferred orientation. c -Axis fabrics of the quartz inclusions tend to show a single girdle pattern at a high angle to an earlier foliation together with a submaximum subparallel to an earlier lineation. This lattice preferred orientation of the quartz inclusions might result from intracrystalline deformation together with solution creep prior to the garnet growth or from selective entrapment of quartz grains with a specific lattice orientation during the garnet growth.