3:30 PM - 5:00 PM
[HDS05-P08] Micro-fractures within shear zone developed in ring shear tests
Keywords:Riedel shear system, ring-shear test, X-ray CT, tribology, strike-slip fault
Introduction
Fractures, generally referring to Riedel shear structures, within shear zones of different scales manifest themselves as a system of fractures/shears on a relatively smaller scale [Tchalenko, 1970], ranging from continental-scale strike-slip faults, to shear zones along the sliding surfaces of landslides, to microscopic deformation bands. They are believed to develop during early episode of fault motions or landsliding, and can offer some crucial knowledge such as rock properties, fracturing temperature, and changes in regional stress field [Anders et al., 2014]. Until now three-dimensional microstructural observations on fractures are rare compared to intense two-dimensional research. Thus, the present study tries to render three-dimensional information about fractures within the shear zone and provide a more comprehensive insight into the mechanism behind them.
Methodology
Three sets of ring-shear tests using rock halite materials (grain size > 2 mm) were conducted by using two ring-shear apparatuses (producing two different kinds of annular samples in size) in terms of (1) shear displacements (1, 2, 3, 4, 6, and 8 m), (2) normal stresses (200, 500, and 1000 kPa) as well as (3) shear velocities (0.05, 0.5, and 5 cm/s), respectively. The annular shear-zone products were cut into 3 or more segments, which were then scanned using X-ray micro-CT (XCT) scanner to obtain high-resolution images in three-dimensional space.
Results and conclusions
Six types of Riedel shear structures (conjugate R and R’, passive P, displacement-direction-parallel Y shears, X, tension T; subsidiary fractures in micro-scale, micro-fractures) and preferential orientations of rock grains can be recognized in three-dimensional images obtained by non-destructive XCT.
We found that longer shear displacement, higher shear velocity, or lower normal stress can result in the formation of multi-Y shears within the shear zone. We infer that the possible formation mechanism of the multi-layered structure can be associated with the film model in tribology [Stachowiak et al., 2013]. Mechanically-stable thin films (denoted by Y shears) are the product of the process to reach a mechanical equilibrium status in the shear zone.
References
Tchalenko, J. S. (1970). Similarities between shear zones of different magnitudes. Geological Society of America Bulletin, 81(6), 1625-1640.
Anders, M. H., Laubach, S. E., & Scholz, C. H. (2014). Microfractures: A review. Journal of Structural Geology, 69, 377-394.
Stachowiak, G. W., & Batchelor, A. W. (2013). Engineering tribology. Butterworth-Heinemann.
Fractures, generally referring to Riedel shear structures, within shear zones of different scales manifest themselves as a system of fractures/shears on a relatively smaller scale [Tchalenko, 1970], ranging from continental-scale strike-slip faults, to shear zones along the sliding surfaces of landslides, to microscopic deformation bands. They are believed to develop during early episode of fault motions or landsliding, and can offer some crucial knowledge such as rock properties, fracturing temperature, and changes in regional stress field [Anders et al., 2014]. Until now three-dimensional microstructural observations on fractures are rare compared to intense two-dimensional research. Thus, the present study tries to render three-dimensional information about fractures within the shear zone and provide a more comprehensive insight into the mechanism behind them.
Methodology
Three sets of ring-shear tests using rock halite materials (grain size > 2 mm) were conducted by using two ring-shear apparatuses (producing two different kinds of annular samples in size) in terms of (1) shear displacements (1, 2, 3, 4, 6, and 8 m), (2) normal stresses (200, 500, and 1000 kPa) as well as (3) shear velocities (0.05, 0.5, and 5 cm/s), respectively. The annular shear-zone products were cut into 3 or more segments, which were then scanned using X-ray micro-CT (XCT) scanner to obtain high-resolution images in three-dimensional space.
Results and conclusions
Six types of Riedel shear structures (conjugate R and R’, passive P, displacement-direction-parallel Y shears, X, tension T; subsidiary fractures in micro-scale, micro-fractures) and preferential orientations of rock grains can be recognized in three-dimensional images obtained by non-destructive XCT.
We found that longer shear displacement, higher shear velocity, or lower normal stress can result in the formation of multi-Y shears within the shear zone. We infer that the possible formation mechanism of the multi-layered structure can be associated with the film model in tribology [Stachowiak et al., 2013]. Mechanically-stable thin films (denoted by Y shears) are the product of the process to reach a mechanical equilibrium status in the shear zone.
References
Tchalenko, J. S. (1970). Similarities between shear zones of different magnitudes. Geological Society of America Bulletin, 81(6), 1625-1640.
Anders, M. H., Laubach, S. E., & Scholz, C. H. (2014). Microfractures: A review. Journal of Structural Geology, 69, 377-394.
Stachowiak, G. W., & Batchelor, A. W. (2013). Engineering tribology. Butterworth-Heinemann.