Rock fabric refers to the complete spatial and geometrical configurations of all those components that make up a rock and includes features such as lineation, foliation, lattice-preferred orientation, grain size, etc. It provides a critical guide to understanding various geological process such as m...
Rock fabric refers to the complete spatial and geometrical configurations of all those components that make up a rock and includes features such as lineation, foliation, lattice-preferred orientation, grain size, etc. It provides a critical guide to understanding various geological process such as magma emplacement in plutonic rocks, paleo-current direction in sedimentary rocks and pyroclastic or lava flows, tectonic strain/stress in deformed rocks. Various methods, such as macroscopic analysis in rock slab, microscopic analysis in SEM and TEM, lattice-preferred orientation using U-stage and texture goniometer, have been used to reveal the rock fabrics. For decades, the AMS (anisotropy of magnetic susceptibility) as a new method has proved to be a useful and effective technique for determining the rock fabrics. The AMS method, therefore, is a well-established technique that can be used to measure rock fabrics and to interpret various geological processes. The main purpose of this article is to apply the AMS method to determine the petrofabrics of various geological settings (granite emplacement, crustal deformation and regional tectonic setting, injection and emplacment mechanism of magmatic and clastic dikes, fault movement and paleostress field, paleocurrent direction and source vent location and so on) and to interpret their causes. This new method and its results will decipher the major unresolved geological phenomena and give a chance to reinterpret the wellknown geological events in Korea through cooperating with the previous results.
Theme 1. Granite emplacement: Since last two decades, a great attention has been shown to the question of emplacement of granitic magma. The scientists have interested in problems of the relationships between magmatic processes and regional tectonic deformation, pluton architecture and magma emplacement, and the role played by regional or local structures. In this study, I attempted to reveal rock fabrics by using AMS method for the Bulguka Granites in and around the Yangsan Fault System, and to interpret movement history of the fault and emplacement of granitic magma. The NNE trending Yangsan Fault in southeast Korea is not only a significant tectonic line in Korean peninsula, but also an important vestige of the tectonic evolution of East Asia. The granitic rocks in the vicinity of the Yangsan Fault, known as the Bulguksa granite, are of the Late Cretaceous to Early Paleogene age. Based on microstructures in thin-section, geologic structures observed in field, weak PJ values, and relationship of the fabric in country rocks, the magnetic fabrics formed by regional tectonic stress during emplacement and crystallizing of the granitic magma. In late Cretaceous granites (70-60 Ma), sub-vertical NE-SW striking foliations and the sub-horizontal NE trending linaeation have been formed by a NW-SE compressive stress and NE-SW tensional stress, respectively, contemporaneous with the emplacement of the granitic rocks. This stress field must have caused a sinistral movement of the Yangsan Fault. This conclusion corresponds to a general tectonic view that NNE trending fault system in East Asia (e.g. Tan-Lu Fault and proto-MTL) did sinistral strike-slip movement by oblique subduction of the proto-Pacific Plate beneath the Eurasian Plate during this time. In early Paleogene granites (~50 Ma), sub- vertical lineation and sub-horizontal foliation suggest that granite emplacement is related to concordant with country rocks and granitic magma emplaced through ballooning or doming process without a tectonic deformation. These emplacement and tectonic settings could be related to regional stress propagated by the collision of India and Eurasia continents.
Theme 2. Clastic and magmatic dike: Clastic dikes are generally classified into neptunian and injected dikes. Using the magnetic fabrics (AMS), we attempt to classify the clastic dikes in the late Cretaceous Dadaepo Basin, SE Korea, and interpret their emplacement mechanisms. The neptunian dikes exhibit a typical oblate sedimentary fabric which makes a sharp contrast with the injected dikes. The fabrics of the injected dikes are greatly influenced by current conditions (flow directions, rheological properties, and rates) and transportation types (imbrication or rolling) of filling materials. Based on the AMS fabrics, they are classified into four types. (1) Type-IV is formed by grain imbrication in low- to moderate-energy vertical flow of a Newtonian fluid and characterized by a bilateral symmetry of fabrics across the dike. (2) Type-RV results from grain rolling in vertical high-energy flow and is characterized by subvertical k2 and subhorizontal k1 axes on the dike plane. (3) Type-IH is formed by grain imbrication in horizontal low- to moderate-energy flow, resulting in subvertical k3 and subhorizontal k1 and k2 axes. (4) Type-RH is formed by grain rolling in horizontal high-energy flow, resulting in streaked k2-k3 on the dike plane and horizontally clustered k1 axes. The AMS fabrics of each type can be a significant indicator for flow direction. Based on abundant AMS fabrics of high-energy current, coexistence of paleoseismic structures, and tectonic setting of the basin, earthquake-induced liquefaction is the most plausible trigger for the dike formation. The attitudes of injected dikes can be used as a powerful tool for reconstructing paleostress field.
The mafic dike swarms are extensively distributed in the anorthosites, Sancheong area. Based on dike attitude and relative chronology in field observation, they are classified into four groups: M-55, M-30, M-NW, F-NW Groups in ascending order. Using AMS method, I attempted to determine rock fabrics of dike and then interpreted the injection and emplacement mechanism of magma. The M-55, M-30, and M-10 Groups have consistently magnetic fabrics characterized by subhorizontal k3 axes perpendicular to the dike plane. Based on the k1 axes, they are classified into two types. The V-type is formed by vertical flow of magma and characterized by subvertical k1 axes on the dike plane. Meanwhile, the H-type is formed by horizontal flow of magma and characterized by subhorizontal k1 axes on the dike plane. The H-type and V-type fabrics are dominantly observed in tip and central part of dike, respectively. Spatial distribution of magnetic fabrics within each dike are not symmetrical but systematic. This suggested that fabrics of dike are developed by the Binghamian dike model. Also, fabrics of M-55 Group are influenced by shear strain along dike margin.
Theme 3. Fault rocks: The Quaternary faults are extensively observed along major inherited fault zones in SE Korea. Their geometry and kinematics provide a very useful piece of information about the Quaternary crustal deformation and stress field in and around Korean Peninsula. Using AMS method, I attempted to determine the slip senses of the Jinti, Mohwa, Suseongji2, and Wangsan faults and then interpreted the fabric development process of fault gouge and the characteristics of stress field during the Quaternary. All the magnetic fabrics of the faults, except the Wangsan Fault, consistently indicate a dominant reverse-slip sense with weak strike-slip component. Most of the oblate fabrics are nearly parallel to the fault surface and the anisotropy degrees generally increase in proportion to the oblatenesses. These results suggest that the fabrics of the fault gouges resulted from a progressive deformation due to continuous simple shear during the last re-activation stage as reverse faulting. It is also interpreted that the pre-existing fabrics were overwhelmed and obliterated by the re-activated faulting. Paleostress field calculated from the fault slip data indicates an ENE-WNW compressive stress, which is in accord with those determined from previous fault tectonic analysis, focal mechanism solution, and hydraulic fracturing test in and around Korean Peninsula.
Theme 4. Pyroclastic rocks and lava flows: The Eoil Basin, is on of Miocene basins in the SE Korea along the East Sea, is bounded by NE-ENE trending normal faults and NNW trending strike-slip faults. The basin-fills are composed of Gampo Conglomerate, Eoil Formation, and Songjeon Formation in ascending order. I performed to determine the magnetic fabrics of the Eoil Basalt and Paljori tuff. Concerning the AMS results, then, I discuss paleocurrent of pyroclastic flow and lava flow, location of the source, and the basin evolution. Based on the k1 axes, paleocurrent directions of pyroclastic flow and lava flow are NE-SW or ENE-WSW and source vent located in the southeast outside of the basin. The results suggest that the Eoil Basin is a kind of pull-apart basin origiated by the strike-slip faulting of NNW-SSE trending and extension of the basin is propagated from NE subbasin to SE subbasin.
Rock fabric refers to the complete spatial and geometrical configurations of all those components that make up a rock and includes features such as lineation, foliation, lattice-preferred orientation, grain size, etc. It provides a critical guide to understanding various geological process such as magma emplacement in plutonic rocks, paleo-current direction in sedimentary rocks and pyroclastic or lava flows, tectonic strain/stress in deformed rocks. Various methods, such as macroscopic analysis in rock slab, microscopic analysis in SEM and TEM, lattice-preferred orientation using U-stage and texture goniometer, have been used to reveal the rock fabrics. For decades, the AMS (anisotropy of magnetic susceptibility) as a new method has proved to be a useful and effective technique for determining the rock fabrics. The AMS method, therefore, is a well-established technique that can be used to measure rock fabrics and to interpret various geological processes. The main purpose of this article is to apply the AMS method to determine the petrofabrics of various geological settings (granite emplacement, crustal deformation and regional tectonic setting, injection and emplacment mechanism of magmatic and clastic dikes, fault movement and paleostress field, paleocurrent direction and source vent location and so on) and to interpret their causes. This new method and its results will decipher the major unresolved geological phenomena and give a chance to reinterpret the wellknown geological events in Korea through cooperating with the previous results.
Theme 1. Granite emplacement: Since last two decades, a great attention has been shown to the question of emplacement of granitic magma. The scientists have interested in problems of the relationships between magmatic processes and regional tectonic deformation, pluton architecture and magma emplacement, and the role played by regional or local structures. In this study, I attempted to reveal rock fabrics by using AMS method for the Bulguka Granites in and around the Yangsan Fault System, and to interpret movement history of the fault and emplacement of granitic magma. The NNE trending Yangsan Fault in southeast Korea is not only a significant tectonic line in Korean peninsula, but also an important vestige of the tectonic evolution of East Asia. The granitic rocks in the vicinity of the Yangsan Fault, known as the Bulguksa granite, are of the Late Cretaceous to Early Paleogene age. Based on microstructures in thin-section, geologic structures observed in field, weak PJ values, and relationship of the fabric in country rocks, the magnetic fabrics formed by regional tectonic stress during emplacement and crystallizing of the granitic magma. In late Cretaceous granites (70-60 Ma), sub-vertical NE-SW striking foliations and the sub-horizontal NE trending linaeation have been formed by a NW-SE compressive stress and NE-SW tensional stress, respectively, contemporaneous with the emplacement of the granitic rocks. This stress field must have caused a sinistral movement of the Yangsan Fault. This conclusion corresponds to a general tectonic view that NNE trending fault system in East Asia (e.g. Tan-Lu Fault and proto-MTL) did sinistral strike-slip movement by oblique subduction of the proto-Pacific Plate beneath the Eurasian Plate during this time. In early Paleogene granites (~50 Ma), sub- vertical lineation and sub-horizontal foliation suggest that granite emplacement is related to concordant with country rocks and granitic magma emplaced through ballooning or doming process without a tectonic deformation. These emplacement and tectonic settings could be related to regional stress propagated by the collision of India and Eurasia continents.
Theme 2. Clastic and magmatic dike: Clastic dikes are generally classified into neptunian and injected dikes. Using the magnetic fabrics (AMS), we attempt to classify the clastic dikes in the late Cretaceous Dadaepo Basin, SE Korea, and interpret their emplacement mechanisms. The neptunian dikes exhibit a typical oblate sedimentary fabric which makes a sharp contrast with the injected dikes. The fabrics of the injected dikes are greatly influenced by current conditions (flow directions, rheological properties, and rates) and transportation types (imbrication or rolling) of filling materials. Based on the AMS fabrics, they are classified into four types. (1) Type-IV is formed by grain imbrication in low- to moderate-energy vertical flow of a Newtonian fluid and characterized by a bilateral symmetry of fabrics across the dike. (2) Type-RV results from grain rolling in vertical high-energy flow and is characterized by subvertical k2 and subhorizontal k1 axes on the dike plane. (3) Type-IH is formed by grain imbrication in horizontal low- to moderate-energy flow, resulting in subvertical k3 and subhorizontal k1 and k2 axes. (4) Type-RH is formed by grain rolling in horizontal high-energy flow, resulting in streaked k2-k3 on the dike plane and horizontally clustered k1 axes. The AMS fabrics of each type can be a significant indicator for flow direction. Based on abundant AMS fabrics of high-energy current, coexistence of paleoseismic structures, and tectonic setting of the basin, earthquake-induced liquefaction is the most plausible trigger for the dike formation. The attitudes of injected dikes can be used as a powerful tool for reconstructing paleostress field.
The mafic dike swarms are extensively distributed in the anorthosites, Sancheong area. Based on dike attitude and relative chronology in field observation, they are classified into four groups: M-55, M-30, M-NW, F-NW Groups in ascending order. Using AMS method, I attempted to determine rock fabrics of dike and then interpreted the injection and emplacement mechanism of magma. The M-55, M-30, and M-10 Groups have consistently magnetic fabrics characterized by subhorizontal k3 axes perpendicular to the dike plane. Based on the k1 axes, they are classified into two types. The V-type is formed by vertical flow of magma and characterized by subvertical k1 axes on the dike plane. Meanwhile, the H-type is formed by horizontal flow of magma and characterized by subhorizontal k1 axes on the dike plane. The H-type and V-type fabrics are dominantly observed in tip and central part of dike, respectively. Spatial distribution of magnetic fabrics within each dike are not symmetrical but systematic. This suggested that fabrics of dike are developed by the Binghamian dike model. Also, fabrics of M-55 Group are influenced by shear strain along dike margin.
Theme 3. Fault rocks: The Quaternary faults are extensively observed along major inherited fault zones in SE Korea. Their geometry and kinematics provide a very useful piece of information about the Quaternary crustal deformation and stress field in and around Korean Peninsula. Using AMS method, I attempted to determine the slip senses of the Jinti, Mohwa, Suseongji2, and Wangsan faults and then interpreted the fabric development process of fault gouge and the characteristics of stress field during the Quaternary. All the magnetic fabrics of the faults, except the Wangsan Fault, consistently indicate a dominant reverse-slip sense with weak strike-slip component. Most of the oblate fabrics are nearly parallel to the fault surface and the anisotropy degrees generally increase in proportion to the oblatenesses. These results suggest that the fabrics of the fault gouges resulted from a progressive deformation due to continuous simple shear during the last re-activation stage as reverse faulting. It is also interpreted that the pre-existing fabrics were overwhelmed and obliterated by the re-activated faulting. Paleostress field calculated from the fault slip data indicates an ENE-WNW compressive stress, which is in accord with those determined from previous fault tectonic analysis, focal mechanism solution, and hydraulic fracturing test in and around Korean Peninsula.
Theme 4. Pyroclastic rocks and lava flows: The Eoil Basin, is on of Miocene basins in the SE Korea along the East Sea, is bounded by NE-ENE trending normal faults and NNW trending strike-slip faults. The basin-fills are composed of Gampo Conglomerate, Eoil Formation, and Songjeon Formation in ascending order. I performed to determine the magnetic fabrics of the Eoil Basalt and Paljori tuff. Concerning the AMS results, then, I discuss paleocurrent of pyroclastic flow and lava flow, location of the source, and the basin evolution. Based on the k1 axes, paleocurrent directions of pyroclastic flow and lava flow are NE-SW or ENE-WSW and source vent located in the southeast outside of the basin. The results suggest that the Eoil Basin is a kind of pull-apart basin origiated by the strike-slip faulting of NNW-SSE trending and extension of the basin is propagated from NE subbasin to SE subbasin.
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