Deformation characteristics and <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of the Shili ductile shear zone at the northwestern margin of the Yunkai Block

GUO Shangyu; WU Xiangke; ZHAO Bing; LI Dan

doi:10.12090/j.issn.1006-6616.2025052

Volume 32 Issue 4

Aug. 2026

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GUO S Y，WU X K，ZHAO B，et al.，2026. Deformation characteristics and 40Ar/39Ar geochronology of the Shili ductile shear zone at the northwestern margin of the Yunkai Block[J]. Journal of Geomechanics，32（4）：771−785 doi: 10.12090/j.issn.1006-6616.2025052

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GUO S Y，WU X K，ZHAO B，et al.，2026. Deformation characteristics and ⁴⁰Ar/³⁹Ar geochronology of the Shili ductile shear zone at the northwestern margin of the Yunkai Block[J]. Journal of Geomechanics，32（4）：771−785 doi: 10.12090/j.issn.1006-6616.2025052

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Deformation characteristics and ⁴⁰Ar/³⁹Ar geochronology of the Shili ductile shear zone at the northwestern margin of the Yunkai Block

doi: 10.12090/j.issn.1006-6616.2025052

GUO Shangyu^1
,,
WU Xiangke^{1
,
,},
ZHAO Bing^1
,,
LI Dan^2
,

1.
The Mineral Resources and Reserves Evaluation Center of Guangxi, Nanning 530028, Guangxi, China
2.
Nuclear Geologic Party No. 310 of Guangxi, Guilin 541000, Guangxi, China

Funds: This research was financially supported by the National Science and Technology Major project for Deep-Earth Exploration (Grant No. 2024ZD10017700) and the China Geological Survey Project of the China Geological Survey (Grant No. DD20160035).

More Information

Received: 2025-05-16
Revised: 2025-12-26
Accepted: 2026-01-30

Available Online: 2026-07-22

Published: 2026-08-28

Abstract

Abstract

Objective The Yunkai Massif is one of the Precambrian metamorphic crystalline basements within the South China Block. To better understand the Indosinian deformation characteristics in the Yunkai area, the Shili ductile shear zone at the northwestern margin of the Yunkai Massif was selected for this study. Methods Through detailed field investigations, finite strain measurements, kinematic vorticity analyses, and ⁴⁰Ar/³⁹Ar geochronology, the structural characteristics and evolutionary process of the shear zone were constrained. Results The Shili ductile shear zone generally strikes WNW–ESE with NNE-directed thrusting. It has undergone four phases of deformation. Among which, the D2 to D4 deformations all indicate movement from SSW to NNE. D2 to D4 formed under a unified tectonic stress field, reflecting an evolution from ductile shear through brittle-ductile deformation to folding during the Late Triassic. Mylonites formed synchronously with the ductile shear deformation yield a muscovite ⁴⁰Ar/³⁹Ar plateau age of 232 ± 2.3 Ma (MSWD = 3.65). The Flinn index (K) is 0.06–0.77, the Lode parameter (ν) is 0.13–0.89, the Effective strain (E_s) is 0.41–0.51, and the kinematic vorticity value (W_k) is 0.70–0.78. These parameters indicate a flattening-type strain regime dominated by general shear, with comparable contributions of pure shear and simple shear. The dynamic recrystallization of minerals suggests that the deformation environment reached the upper greenschist facies. Conclusions This study indicates that the mylonitization, crenulation cleavage development, and folding within the Shili ductile shear zone were dynamically driven by the propagation of far-field stress resulting from the NE–SW collision between the Indochina and South China blocks during the Indosinian period. Significance These findings provide critical constraints on the Indosinian tectono-thermal event and its geodynamic framework in South China.
- South China Block,
- Yunkai Block,
- ductile shear zone,
- finite strain,
- kinematic vorticity,
- ⁴⁰Ar/³⁹Ar geochronology

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