Jin Han Ree

Tectonic and sedimentary evolution of the Korean peninsula: a review and new view

Abstract This review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts (Late Proterozoic–Neogene) in the Korean peninsula. The Korean peninsula is part of the Amuria Plate and represents an important link between continental blocks of North and South China and the island arcs of Japan. The basement rocks, exposed in the Kyonggi and Yongnam massifs, consist of 2.7 to 1.1 Ga high-grade gneiss and schist. These massifs are separated by the Okchon Fold Belt which comprises metasedimentary rocks and bimodal meta-volcanic rocks. The stratigraphy of the Okchon Group is unclear at present. The Okchon Basin was probably initiated as an intraplate rift prior to the Late Proterozoic. The Hwanggangri Formation (clast-bearing phyllite) most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin. The stratigraphic relationship between the Okchon Group and the Choson Supergroup (Cambro-Ordovician) of the Taebaeksan Basin is poorly constrained. The Choson Supergroup unconformably overlies the Yongnam Massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments, reflecting numerous sea-level fluctuations. The sequence is disconformably overlain by siliciclastic sequence of Pyongan Supergroup (Carboniferous–?Triassic) which formed most likely in shallow marine, deltaic, and fluvial environments. The Imjingang Belt is an east-trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics (Devonian–Carboniferous), underlain unconformably by Proterozoic basement rocks. Late Proterozoic amphibolites of oceanic affinity were metamorphosed at about 8–13 kbar and 630–740°C during the late Permian to the early Triassic. The south-vergent contraction and top-down-to-the-north normal faulting are suggestive of a suture belt between the North China Block (Sino-Korea Craton) and the South China Block (Yangtze Craton), an extension of Sulu Belt across the Yellow Sea. Entire peninsula experienced strong deformation and metamorphism during the suturing event, namely Songrim orogeny. During this orogenic event, the Kyonggi Massif (and the Okchon Basin) accreted to the Yongnam Massif (and Taebaeksan Basin) along the South Korean Tectonic Line running northeast–southwest. A series of northward-trending thrust formed along the boundary zone to the east (Kaktong and Kongsuwon thrusts and others). Piggyback basins locally developed along the thrust faults, forming the Taedong Group. The crustal deformation resumed in the early to late Jurassic (Daebo event) under contractional setting. Dextral ductile shearing associated with thrusting and folding continued in the mid-southern part of the peninsula. It was due to orthogonal (northwestward) subduction of the Izanagi Plate under the Asian continent. The Yongnam Massif experienced continuous dextral offset along the Honam Shear Zone. In the early Cretaceous, the Izanagi Plate began to subduct northward and caused formation of strike–slip basins in retroarc setting, i.e., Kyongsang Basin in the southeastern part and a number of small-scale basins in the mid-southwestern part of the peninsula. Small-scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions. Extensive intrusion of granitoids occurred from Triassic to Early Tertiary with a gap between 160 and 100 Ma, representing continental magmatic arc. In the Tertiary, the southeastern margin of the Korean peninsula experienced back-arc opening. Pull-apart basins formed in the Miocene, bounded by the Yangsan and Hupo faults. The Yonil Group, sedimentary fill of the Pohang Basin, comprises more than 1-km-thick siliciclastic sequence which represents deposition in fan-delta systems on the hanging wall of the Yangsan fault. Thick (more than 10 km thick) sediments in the Ulleung Basin margin were deformed in the late Miocene due to the northward movement of Kyushu Block. Quarternary volcanic events in Cheju Island represent intraplate hot spots.

Possible eastward extension of Chinese collision belt in South Korea: The Imjingang belt

Structural, petrological, and geochronological data from the middle Korean peninsula indicate that the Qinling-Dabie-Sulu collisional belt of east-central China crosses the Yellow Sea and extends into the Imjingang belt. The Yeoncheon complex, first identified as the western Imjingang belt, comprises primarily north-dipping metamorphic sequences: (1) the northern Jingok unit, consisting of Barrovian-type metapelites, and (2) the southern Samgot unit, consisting of calc-silicate and amphibolitic rocks. South-vergent structures with reverse-sense shearing are dominant in the Jingok unit, whereas late normal-sense shearing is pervasive in the Samgot unit and the deformed granitoid to the south. These structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt. Pressure-temperature ( P-T ) estimates from the amphibolites suggest a high-P amphibolite-facies metamorphism (8–13 kbar and 630–790 °C), possibly evolving from eclogite facies conditions along a clockwise P-T path. Sm-Nd and Rb-Sr geochronological data suggest that the amphibolites emplaced in Late Proterozoic time were metamorphosed during Permian-Triassic time.

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.

Coseismic recrystallization during shallow earthquake slip

Solidified frictional melts, or pseudotachylytes, remain the only unambiguous indicator of seismic slip in the geological record. However, pseudotachylytes form at >5 km depth, and there are many rock types in which they do not form at all. We performed low- to high-velocity rock friction experiments designed to impose realistic coseismic slip pulses on calcite fault gouges, and report that localized dynamic recrystallization may be an easy-to-recognize microstructural indicator of seismic slip in shallow, otherwise brittle fault zones. Calcite gouges with starting grain size −1 , and total displacements between 1 and 4 m. At coseismic slip velocities ≥0.1 m s −1 , the gouges were cut by reflective principal slip surfaces lined by polygonal grains 2 + CaO. The recrystallized calcite aggregates resemble those found along the principal slip surface of the Garam thrust, South Korea, exhumed from

Seismic slip record in carbonate-bearing fault zones: An insight from high-velocity friction experiments on siderite gouge

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6–1.3 MPa and at seismic slip rates of 1.3–2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO 2 gas, as confirmed by CO 2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records.

An experimental steady-state foliation

The history of the intensity and orientation of a grain-shape foliation was investigated in octachloropropane deformed in simple shear at 80% of its absolute melting temperature and a shear strain-rate of 4 × 10−5S−1. Foliation orientation, developed from the beginning of the deformation, remains steady throughout the deformation. Foliation intensity becomes steady after a bulk shear strain of about 0.9 has been accumulated. The steadiness of foliation orientation and intensity is achieved by some balance between foliation-strengthening and foliation-weakening processes. The main foliation- -strengthening process is intragranular plastic deformation. Foliation-weakening processes include dynamic recrystallization by migration of straight or or slightly wavy grain boundaries, grain dissection, rotational recrystallization, grain amalgamation, relative rigidity of hard grains and grain boundary sliding. A small but definite angle, although not constant, is observed between the foliation and the long axis of the bulk total strain ellipse. The average aspect ratio of grains is lower than that of the bulk total strain ellipse by a factor of about 0.4 at total shear strain of 1.3. The average grain size also stabilizes, becoming steady from a shear strain of 0.5 onwards. A steady grain-shape foliation may be a possible paleostress or a paleostrain-rate indicator, but cannot be a paleostrain indicator.

Grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane

Abstract In situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure- and simple-sheared octachloropropane at 75–85% of its absolute melting temperature and at strain rates of 10 −5 -10 −6 s −1 are discussed. Discontinuities in the strain, rotation and/or translation components of deformation across the grain boundary induce grain boundary sliding. The influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding. Grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding. Grain boundary diffusion also causes grain boundary migration (Types II and III) that is different from conventional grain boundary migration (Type I) in being non-conservative, and in the details of boundary movement with respect to material points within grains. Grain boundary openings filled with fluid (octachloropropane vapor) are strongly associated with grain boundary sliding. Openings develop preferentially along grain boundaries at low angles to the shortening direction. Once openings grow, they are closed by thrusting of sliding grains and by diffusion in faster strain-rate experiments, and entirely by diffusion in slower strain-rate experiments. An approximately steady openings ratio of 0.5–3% of the sample volume persists without the development of any large-scale fracture. All grain boundary openings disappear during static readjustment of the microstructure after deformation.

Seven types of subgrain boundaries in octachloropropane

Abstract Subgrain boundaries in thin sheets of octachloropropane deformed at 0.7–0.8 T M on the stage of a microscope are seen to appear in the material in seven different ways. Type I boundaries show the classical evolution by polygonization of bent crystals. Type II are essentially kink boundaries, which migrate sideways during deformation to reach their present positions in the crystals. Type III develop at the sites of former grain boundaries, by reduction of misorientation of adjacent grains. Type IV and V originate by impingement of migrating grain boundaries or subgrain boundaries, respectively. Type VI propagate in their own planes behind migrating grain boundaries to which they are attached. Type VII develop statically from optically strain-free grains by a process probably otherwise similar to the Type I process. Two thirds of the boundaries are Types I or II. In view of the variety of subgrain boundary histories in OCP, interpretation of similar features in minerals ought to be undertaken cautiously. Criteria are needed for telling the different types of subgrain boundaries apart in situations where only a final view of the structure is available, as in optical and electron micrographs of rocks.

Experimental evidence for the simultaneous formation of pseudotachylyte and mylonite in the brittle regime

The coexistence of pseudotachylyte, a brittle fault rock representing seismic slip, and mylonite, a plastic or semiplastic fault rock associated with aseismic slip, has been debated for its origin in earthquake-generating faults. Deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt (pseudotachylyte) in the brittle regime, equivalent to upper crustal conditions. The plastic deformation of halite, resulting in mylonite, is facilitated by the conduction of heat from a slip-concentrated molten layer formed by frictional heating at seismic slip rates. Thus mylonites, which are (1) kinematically consistent and (2) just at the contact with pseudotachylyte, might have formed together with the pseudotachylyte in the brittle regime during seismic slip.

Non-rotation of garnet porphyroblasts and 3-D inclusion trail data: an example from the Imjingang belt, South Korea

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.