Byoung-Hoon Hwang

Tectonic Evolution of the Gyeongsang Basin, Southeastern Korea from 140 Ma to the Present, Based on a Strike-Slip and Block Rotation Tectonic Model

The geometric model involving two conjugate strike-slip fault sets with opposite-sense block rotations synthesized by structural, petrological, geochronologic, and paleomagnetic data from the Gyeongsang Basin from Cretaceous to Tertiary time is placed in the tectonic framework of East Asia. As a result, the birth and evolution of the Gyeongsang Basin might reflect a regional continental strike-slip zone in a convergent plate boundary such as the Tan-Lu wrench tectonic system. According to this model, we suggest six major geotectonic stages in the Gyeongsang Basin since the Cretaceous—e.g., 140-120, 120-110, 110-99, 99-80, 80-50, and after 50 Ma—which include the collision of an accretionary plateau with proto-Japan, subduction of the Izanagi-Pacific ridge, collision of India-Eurasia, northward approach of the Philippine plate, and the East Sea opening. Over more than 90 m.y., the Gyeongsang Basin apparently underwent three events of block rotations in opposite directions, and two events of clockwise rotation of the whole basin or the Korean Peninsula. The first series of events rotated with respect to Eurasia and the second rotated together with Eurasia. We present the tectonic evolution of the Gyeongsang Basin as a model for the tectonic development of East Asia from the Cretaceous to the present.

Cenozoic Strike-Slip Displacement along the Yangsan Fault, Southeast Korean Peninsula

Granitic rocks astride the Yangsan fault on the southeast Korean Peninsula can be classified into five rock types in two groups, based on field relationships and petrographic features. Group I rocks are granodiorite, enclave-rich porphyritic granite, and enclave-poor porphyritic granite. Mafic microgranular enclaves (MME) and/or mafic clots in rocks of this group imply magma mixing. Group II rocks are equigranular and micrographic granite indicative of shallow emplacement. Group II rocks intrude Group I rocks without magma mingling at several outcrops. The granitic rock facies on both sides of the Yangsan fault are similar, suggesting post-50 Ma dextral motion of approximately 21 km in a N20°E direction.

Timing of granitic magma mixing in the southeastern Gyeongsang Basin, Korea: SHRIMP-RG zircon data

Late Cretaceous calc-alkaline granites in the Gyeongsang Basin evolved through the mixing of mafic and felsic magmas. The host granites contain numerous mafic magmatic/microgranular enclaves of various shapes and sizes. New SHRIMP-RG zircon U–Pb ages of both granite and mafic magmatic/microgranular enclaves are 75.0 ± 0.5 Ma and 74.9 ± 0.6 Ma, respectively, suggesting that they crystallized contemporaneously after magma mixing. The time of injection of mafic melt into the felsic magma chamber can be recognized as approximately 75 Ma by field relations, petrographic features, geochemical evolution, and SHRIMP-RG zircon dating. This Late Cretaceous magma mixing event in the Korean Peninsula was probably related to the onset of subduction of the Izanagi (Kula)–Pacific ridge.

Tectonic Implication of A-type Granites across the Yangsan Fault, Gigye and Gyeongju Areas, Southeast Korean Peninsula

Based on field relations and petrographic characteristics, alkali-feldspar granite in the Gigye area on the western side of the Yangsan fault is classified as an A-type granite. It is mainly composed of quartz, perthitic K-feldspar, and interstitial ferri-annite plus sodic amphiboles such as riebeckite and arfvedsonite, indicating crystallization under hypersolvus, anhydrous conditions. These petrographic features are the same as those of the A-type granite in the Gyeongju area on the east side of the Yangsan fault. These distinctive, pre-Eocene granites are separated by about 21.3 km, indicating Cenozoic dextral strike-slip of this amount along the Yangsan fault.

Petrogenesis of the Eonyang granitoids, SE Korea: new SHRIMP-RG zircon U–Pb age and whole-rock geochemical data

Petrogenesis of the Late Cretaceous Eonyang granitoids in the Gyeongsang Basin is considered in terms of field relations, petrography, geochemistry, Sr–Nd isotopes, and sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) geochronology. The plutons can be divided into two groups: Group I consists of quartz-monzodiorite (QMD), granodiorite (GD), enclave-rich porphyritic granite (ERPG), and enclave-poor porphyritic granite (EPPG), showing evidence of magma mixing/mingling. Group II comprises coarse-grained porphyritic granite (CPG), fine-grained micrographic granite (FMG), and equigranular granite (EG), displaying non-mixing and highly differentiated characteristics. The age of the EPPG, reflecting magma mixing, is 73.3 ± 2.0 Ma. The Eonyang granitoids show subalkaline, calc-alkaline, I-type, and high-K characteristics except for the mafic magmatic/microgranular enclaves (MMEs), which have medium-K calc-alkaline compositions. The mafic facies (QMD + GD + MME) are metaluminous, whereas the felsic facies (ERPG + EPPG + CPG + EG) are peraluminous. Two geochemical evolution trends are present, magma mixing/mingling in Group I and fractional crystallization in Group II. Based on the Sr–Nd isotope geochemistry, the various rock facies in Group I evidently resulted by hybridization of two magmas under different physical conditions. In contrast, the petrogenesis of Group II rock types involves two alternative possibilities: (1) mantle contamination after fractionation of a parental magma derived from the same source as Group I or (2) fractionation of a parental magma derived from a source different from that of Group I. The geochemistry of the Eonyang granitoids suggests mixed-origin magma generation in a Late Cretaceous subduction setting.

Two different magma series imply a Palaeogene tectonic transition from contraction to extension in the SE Korean Peninsula

Late Cretaceous–early Tertiary granites in the Gyeongsang Basin have distinctly different bulk-rock compositions. Calc-alkaline I-type metaluminous granites display petrographic features implying magma mixing, whereas A-type granites are hypersolvus and peralkaline. I-type plutons mainly consist of enclave-rich granodiorites and enclave-poor porphyritic granites typified by abundant plagioclase phenocrysts; these granitoids contain various mafic clots and magmatic/microgranular enclaves (MMEs). A-type bodies are perthitic alkali-feldspar granites characterized by interstitial annite + riebeckite-arfvedsonite. New SHRIMP-RG zircon U–Pb age dating of an I-type enclave-poor porphyritic granite and an A-type alkali-feldspar granite yielded ages of 65.7 ± 0.7 and 53.9 ± 0.3 million years, respectively. Based on prior geochronologic data and these contrasting ages of granitic magma genesis, SE Korea may have evolved tectonically from latest Cretaceous compression to late Palaeocene extension (i.e. orogenic collapse). The later part of the 66–54 Ma magmatic gap apparently includes the time of tectonic inversion in the SE Korean Peninsula, a far-field effect of the collision of the Indian subcontinent with Eurasia. This process is also reflected in the 69–52 Ma NNE-trending Eurasian apparent polar wandering path.

Double injection events of mafic magma into supersolidus Yucheon granites to produce two types of mafic enclaves in the Cretaceous Gyeongsang Basin, SE Korea

Petrographic and geochemical features of the Cretaceous Yucheon granites and their mafic microgranular/magmatic enclaves (MMEs), SE Korea, reveal that the MMEs originated from magma mixing. Mesoscopic and microscopic features indicate that mechanical mixing operated heterogeneously to produce the MMEs with a wide range of sizes and textures. Chemical compositions of amphibole, biotite, and plagioclase rims of both the MMEs and host granites are almost identical, indicating that chemical homogenization took place to some extent after the mechanical mixing. Plagioclase cores, however, have various compositions depending on the host rocks and/or sampling locations, suggesting their sluggish re-equilibration. The MMEs are divided into Type A (low TiO2, very fine-grained, chilled margins) and Type B (high TiO2, fine- to medium-grained, no chilled margins). The lower TiO2 MMEs cooled more rapidly and interacted with granitic magma for a shorter period of time than the higher TiO2 MMEs. Additionally, the former are less enriched in HREEs than the latter. Zoned plagioclase has two zones of increased An content. These features are indicative of double injection events of mafic magma. A previous model explains the magma mixing as resulting from the generation of a slab window due to Kula-Pacific ridge subduction. The model cannot, however, explain the eastward younging of the granites in Korea, necessitating a new, more elaborate model of Cretaceous geodynamics and magmatism in East Asia.

Cenozoic wrench tectonics and oroclinal bending in SE Korea

Based on fault geometry, petrography, and geochronology of granitic rocks as well as palaeomagnetic data from the Gyeongsang Basin, two conjugate fault sets are explained as a reflection of NNE-trending right-lateral wrench tectonics. According to this interpretation, the Gaum and Yangsan fault sets correspond to antithetic faulting by R′-shear and synthetic faulting by R-shear, respectively; they have rotated clockwise and counterclockwise, respectively, due to NE–SW compression (shortening), as a result of a NNE-trending wrenching force (simple shear). During progressive deformation, NS- or NNW-trending strike–slip faulting by P-shear occurred in the Yeongyang sub-basin, and finally the Yangsan fault formed as a wrench fault bisecting the P-shear and R-shear directions. Extension of the faults (R-shear, striking ∼N22°E) generated by block rotation on the east side of the Yangsan fault (wrench fault, striking ∼N13°E) resulted in convex eastward deflections. We suggest that this was caused by oroclinal bending of the existing faults generated by block rotations in opposite directions and is inferred to have been closely related to the East Sea (i.e. Sea of Japan) opening.