Some Evidences Indicating the Transition of Deformation Process from Ductile to Brittle Deformation Process along the Honam Shear Zone in Hamyang, South Korea

Shear zones can generally be classified depending on their dominant deformation mechanisms. The deformation mechanism strongly depends on the temperature and pressure, relying on the depth. The Honam shear zone (HSZ) is an NE to NNE-striking ductile shear zone transecting the Korean Peninsula. The study area consists of foliated granite alternating undeformed rocks and mylonitic zones. Previous studies reported that the timing of dextral movement of HSZ is approximatively early to middle Jurassic. On the other hand, it is generally accepted that the shear zone reactivated as a sinistral brittle fault during the early Cretaceous. However, based on our new data, we argue that the deformation mechanism for sinistral shearing was brittle-ductile. In addition, we interpreted the deformation history in the study area to understand the change of stress field. The inferred deformation events are as follows: (1) NE-striking with steeply SE-dipping foliations indicate dextral sense of shear with asymmetric and flanking folds at an oblique angle (N-S) to mylonitic fabrics. (2) Folded pegmatite dikes, sub-parallel to the foliations, indicate that they were intruded after the mylonitization. Asymmetric folds and C' shear bands oblique to mylonite foliation indicate sinistral shearing under the brittle-ductile transition condition. (3) Under the brittle condition, conjugate shear fractures, WNW-ESE trending sinistral strike-slip faults and E-W trending secondary fractures were developed with the E-W oriented maximum principal stress. (4) WNW-ESE trending strike-slip faults were reactivated with opposite sense of slip, and NW-SE trending secondary fractures were developed with the NW-SE oriented maximum principal stress. These changes in deformation mechanism and stress condition provide useful information on stress changes with depth change uplifting of the crust.

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