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Econ. Environ. Geol. 2024; 57(2): 253-261

Published online April 30, 2024

https://doi.org/10.9719/EEG.2024.57.2.253

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Relationship between Olivine Fabrics and Seismic Anisotropy in the Yugu Peridotites, Gyeonggi Massif, South Korea

Munjae Park*

Department of Earth and Environmental Sciences, Chungbuk National University, Cheongju 28644, Republic of Korea

Correspondence to : *mpark@cbnu.ac.kr

Received: February 19, 2024; Revised: March 4, 2024; Accepted: March 5, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Olivine, a major mineral in the upper mantle with strong intrinsic elastic anisotropy, plays a crucial role in seismic anisotropy in the mantle, primarily through its lattice preferred orientation (LPO). Despite this, the influence of the microstructure of mylonitic rocks on seismic anisotropy remains inadequately understood. Notably, there is a current research gap concerning seismic anisotropy directly inferred from mylonitic peridotite massifs in Korea. In this study, we introduce the deformation microstructure and LPO of olivine in the mantle shear zone. We calculate the characteristics of seismic anisotropy based on the degree of deformation (proto-mylonite, mylonite, ultra-mylonite) and establish correlations between these characteristics. Our findings reveal that the seismic anisotropy resulting from the olivine LPO in the ultra-mylonitic rock appears to be the weakest, whereas the seismic anisotropy resulting from the olivine LPO in the proto-mylonitic rock appears to be the strongest. The results demonstrate a gradual decrease in seismic anisotropy as the fabric strength (J-index) of olivine LPO diminishes, irrespective of the specific pattern of olivine's LPO. Moreover, all samples exhibit a polarization direction of the fast S-wave aligned subparallel to the lineation. This suggests that seismic anisotropy originating from olivine in mylonitic peridotites is primarily influenced by fabric strength rather than LPO type. Considering these distinctive characteristics of seismic anisotropy is expected to facilitate comparisons and interpretations of the internal mantle structure and seismic data in the Yugu area, Gyeonggi Massif.

Keywords Yugu peridotite, olivine, lattice preferred orientation, mylonite, seismic anisotropy

경기육괴 유구 페리도타이트의 감람석 미구조와 지진파 비등방성의 관계

박문재*

충북대학교 지구환경과학과

요 약

감람석은 강한 고유 탄성 이방성을 가진 상부 맨틀의 주요 광물이기 때문에 맨틀에서 발생하는 대부분의 지진파 비등방성은 감람석의 격자선호방향에서 직접적으로 기인한다. 그러나 압쇄암의 미구조가 지진파 비등방성에 미치는 영향에 대한 이해는 아직 잘 알려져 있지 않으며, 국내의 압쇄암화된 페리도타이트 암괴에서 직접적으로 추론한 지진파 비등방성 연구는 전무하다. 따라서 본 연구에서는 맨틀 전단대 내의 감람석의 변형 미구조와 격자선호방향을 소개하고, 이들의 변형 정도(원압쇄암,압쇄암, 초압쇄암)와 격자선호방향 결과에 따른 지진파 비등방성의 특성을 계산하고 이들의 상관관계를 파악해보았다. 그 결과, 초압쇄암의 감람석 격자선호방향에서 기인한 지진파 비등방성은 가장 약하게 나타났고, 원압쇄암의 감람석 격자선호방향에서 기인한 지진파 비등방성은 가장 강하게 나타났다. 이러한 결과는 감람석의 격자선호방향의 패턴과는 관계없이 격자선호방향의 배열 강도(J-index)가 감소함에 따라 지진파 비등방성이 점차 감소하는 것으로 나타났다. 또한, 모든 샘플에서 공통적으로 빠른 S파의 편파방향이 선구조와 아평행하게 배열하는 것을 보여 주었다. 따라서 유구 페리도타이트 내 감람석에서 기인한 지진파 비등방성은 격자선호방향의 타입보다 배열 강도에 의해 지배된다고 설명할 수 있다. 이러한 서로 대조되는 지진파 비등방성의 특성을 잘 고려하면, 경기육괴 유구 지역의 맨틀의 내부 구조와 지진파 자료를 비교하고 해석하는데 도움을 줄 수 있을 것으로 기대한다.

주요어 유구 페리도타이트, 감람석, 격자선호방향, 압쇄암, 지진파 비등방성

Article

Research Paper

Econ. Environ. Geol. 2024; 57(2): 253-261

Published online April 30, 2024 https://doi.org/10.9719/EEG.2024.57.2.253

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Relationship between Olivine Fabrics and Seismic Anisotropy in the Yugu Peridotites, Gyeonggi Massif, South Korea

Munjae Park*

Department of Earth and Environmental Sciences, Chungbuk National University, Cheongju 28644, Republic of Korea

Correspondence to:*mpark@cbnu.ac.kr

Received: February 19, 2024; Revised: March 4, 2024; Accepted: March 5, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Olivine, a major mineral in the upper mantle with strong intrinsic elastic anisotropy, plays a crucial role in seismic anisotropy in the mantle, primarily through its lattice preferred orientation (LPO). Despite this, the influence of the microstructure of mylonitic rocks on seismic anisotropy remains inadequately understood. Notably, there is a current research gap concerning seismic anisotropy directly inferred from mylonitic peridotite massifs in Korea. In this study, we introduce the deformation microstructure and LPO of olivine in the mantle shear zone. We calculate the characteristics of seismic anisotropy based on the degree of deformation (proto-mylonite, mylonite, ultra-mylonite) and establish correlations between these characteristics. Our findings reveal that the seismic anisotropy resulting from the olivine LPO in the ultra-mylonitic rock appears to be the weakest, whereas the seismic anisotropy resulting from the olivine LPO in the proto-mylonitic rock appears to be the strongest. The results demonstrate a gradual decrease in seismic anisotropy as the fabric strength (J-index) of olivine LPO diminishes, irrespective of the specific pattern of olivine's LPO. Moreover, all samples exhibit a polarization direction of the fast S-wave aligned subparallel to the lineation. This suggests that seismic anisotropy originating from olivine in mylonitic peridotites is primarily influenced by fabric strength rather than LPO type. Considering these distinctive characteristics of seismic anisotropy is expected to facilitate comparisons and interpretations of the internal mantle structure and seismic data in the Yugu area, Gyeonggi Massif.

Keywords Yugu peridotite, olivine, lattice preferred orientation, mylonite, seismic anisotropy

경기육괴 유구 페리도타이트의 감람석 미구조와 지진파 비등방성의 관계

박문재*

충북대학교 지구환경과학과

Received: February 19, 2024; Revised: March 4, 2024; Accepted: March 5, 2024

요 약

감람석은 강한 고유 탄성 이방성을 가진 상부 맨틀의 주요 광물이기 때문에 맨틀에서 발생하는 대부분의 지진파 비등방성은 감람석의 격자선호방향에서 직접적으로 기인한다. 그러나 압쇄암의 미구조가 지진파 비등방성에 미치는 영향에 대한 이해는 아직 잘 알려져 있지 않으며, 국내의 압쇄암화된 페리도타이트 암괴에서 직접적으로 추론한 지진파 비등방성 연구는 전무하다. 따라서 본 연구에서는 맨틀 전단대 내의 감람석의 변형 미구조와 격자선호방향을 소개하고, 이들의 변형 정도(원압쇄암,압쇄암, 초압쇄암)와 격자선호방향 결과에 따른 지진파 비등방성의 특성을 계산하고 이들의 상관관계를 파악해보았다. 그 결과, 초압쇄암의 감람석 격자선호방향에서 기인한 지진파 비등방성은 가장 약하게 나타났고, 원압쇄암의 감람석 격자선호방향에서 기인한 지진파 비등방성은 가장 강하게 나타났다. 이러한 결과는 감람석의 격자선호방향의 패턴과는 관계없이 격자선호방향의 배열 강도(J-index)가 감소함에 따라 지진파 비등방성이 점차 감소하는 것으로 나타났다. 또한, 모든 샘플에서 공통적으로 빠른 S파의 편파방향이 선구조와 아평행하게 배열하는 것을 보여 주었다. 따라서 유구 페리도타이트 내 감람석에서 기인한 지진파 비등방성은 격자선호방향의 타입보다 배열 강도에 의해 지배된다고 설명할 수 있다. 이러한 서로 대조되는 지진파 비등방성의 특성을 잘 고려하면, 경기육괴 유구 지역의 맨틀의 내부 구조와 지진파 자료를 비교하고 해석하는데 도움을 줄 수 있을 것으로 기대한다.

주요어 유구 페리도타이트, 감람석, 격자선호방향, 압쇄암, 지진파 비등방성

    Fig 1.

    Figure 1.(a) A simplified tectonic map of Northeast Asia indicating the position of the study site in Yugu, South Korea. IB (Imjingang Belt), GM (Gyeonggi Massif), TB (Taebaeksan Basin), OB (Okcheon Belt), YM (Yeongnam Massif), and GB (Gyeongsang Basin). (b) Geologic map of the Yugu peridotite body in the southwestern Gyeonggi Massif, South Korea. Modified after 1:50,000 geological map from the Korea Institute of Geoscience and Mineral Resources (KIGAM); Um and Lee (1963). For a detailed geological map related to sampling locations and structural information (strike and dip), readers are referred to Park and Jung (2017). (c, d) Outcrop photographs of mylonitic peridotites from the studied area.
    Economic and Environmental Geology 2024; 57: 253-261https://doi.org/10.9719/EEG.2024.57.2.253

    Fig 2.

    Figure 2.Optical photomicrographs of the studied samples (XZ plane thin sections). The long axes of the photomicrographs are parallel to the lineation (X), and the short axes are normal to the foliation (Z). (a, b) Proto-mylonitic peridotite (PM; YG-4). (c, d) Mylonitic peridotite (M; YG-8). (e, f) Ultra-mylonitic peridotite (UM; YG-10). Ol (olivine). White arrowheads represent sub-grain boundaries.
    Economic and Environmental Geology 2024; 57: 253-261https://doi.org/10.9719/EEG.2024.57.2.253

    Fig 3.

    Figure 3.Pole figures illustrating the lattice preferred orientation (LPO) of olivine. (a) Proto-mylonitic peridotites (PM; YG-4). (b) Mylonitic peridotites (M; YG-8). (c) Ultra-mylonitic peridotites (UM; YG-10). The stretching lineation (X) aligns with the east-west direction, while the north-south direction (Z) is perpendicular to the foliation. The color-coding indicates the density of data points. Equal-area and lower-hemisphere projection methods were employed with a half-scattering width of 30°. N represents the number of measured grains. The strength of the olivine fabric (LPO) is indicated by J (J-index) (Bunge, 1982). This figure is modified after Park and Jung (2017), in which detailed descriptions and interpretations of these olivine LPOs can be found.
    Economic and Environmental Geology 2024; 57: 253-261https://doi.org/10.9719/EEG.2024.57.2.253

    Fig 4.

    Figure 4.Seismic anisotropies of olivine aggregates in the studied samples. (a) Proto-mylonitic peridotites (PM; YG-4). (b) Mylonitic peridotites (M; YG-8). (c) Ultra-mylonitic peridotites (UM; YG-10). All data are presented in lower hemisphere using equal area projection. The east‐west direction corresponds to the stretching lineation (X), and the north‐south direction (Z) is normal to foliation.
    Economic and Environmental Geology 2024; 57: 253-261https://doi.org/10.9719/EEG.2024.57.2.253

    Fig 5.

    Figure 5.Correlation between seismic anisotropy (AVp and max. AVs) and olivine fabric strength (J-index) for each group of the Yugu peridotites. PM (proto-mylonite), M (mylonite), UM (ultramylonite).
    Economic and Environmental Geology 2024; 57: 253-261https://doi.org/10.9719/EEG.2024.57.2.253

    Table 1 . Olivine fabrics and seismic properties in the studied samples..

    SampleOlivine fabricsSeismic properties
    GroupaOlivine fabricsOlivine J-indexbAVp (%)max. AVs (%)Avg. Vp (km/s)Avg. Vs (km/s)
    YG-4PMA-type4.5011.58.488.394.83
    YG-8MD-type2.647.34.998.404.84
    YG-10UME-type1.604.12.918.374.83

    a PM: proto-mylonite, M: mylonite, UM: ultra-mylonite..

    b Olivine fabric (LPO) strength is denoted as J-index (Bunge, 1982), calculated using the MTEX toolbox for MATLAB with a kernel half-width of 10° (Bachmann et al., 2010)..


    KSEEG
    Jun 30, 2024 Vol.57 No.3, pp. 281~352

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