5:15 PM - 7:15 PM
[SCG45-P10] Development process of a frontal thrust in a dry sand wedge inferred from X-ray Computed Tomography and Digital Image Correlation
Keywords:shear zone, fault, accretionary complex, analogue experiment
Shear zones in geological bodies develop due to the localization of shear strain, but individual shear zones can also thicken as displacement accumulates. It has been suggested that this thickening mechanism results from the fracturing of geological bodies within and around the shear zone, as well as the linkage of multiple shear zones. In this study, we considered the movement of the principal slip surface to be a key factor in the thickening process. To investigate this, it is important to capture not only the cumulative deformation but also the time-dependent deformation throughout the displacement history of an individual shear zone. Therefore, we examined the spatial distribution and temporal evolution of shear zones in a dry sand wedge using X-ray Computed Tomography (XCT) data and strain analysis from digital images obtained from the same experiment.
In the experiment, dry Toyoura silica sand (grain size: 106–300 µm) was deposited to a thickness of 20 mm by free fall into an acrylic box (width: 118 mm, length: 693 mm, height: 158 mm) lined with an adhesive sheet. The sheet was pulled horizontally at a speed of 0.125 mm/s for a total displacement of 500 mm, creating a wedge in which shear zones developed as the sand layer was pushed against a fixed wall.
Among the multiple shear zones formed in the experiment, we focused on the frontal thrusts (FTs). XCT images revealed that an FT thickened as displacement increased. By overlaying strain analysis data, we found that the thickening mechanism and timing varied with depth. Specifically, in the shallow region of the FT, thickening was influenced by the collapse of sand on the wedge front slope, where the principal slip surface formed between the collapsed layer and the underlying subsiding layer. In contrast, in the deeper region of the FT, thickening occurred as the principal slip surface migrated further into the wedge, indicating structural erosion.
The shear zone development process observed in this experiment may also occur in natural settings, such as in sedimentary layers within plate convergent zones. Further investigation, including comparisons with natural strata, is required in future work.
In the experiment, dry Toyoura silica sand (grain size: 106–300 µm) was deposited to a thickness of 20 mm by free fall into an acrylic box (width: 118 mm, length: 693 mm, height: 158 mm) lined with an adhesive sheet. The sheet was pulled horizontally at a speed of 0.125 mm/s for a total displacement of 500 mm, creating a wedge in which shear zones developed as the sand layer was pushed against a fixed wall.
Among the multiple shear zones formed in the experiment, we focused on the frontal thrusts (FTs). XCT images revealed that an FT thickened as displacement increased. By overlaying strain analysis data, we found that the thickening mechanism and timing varied with depth. Specifically, in the shallow region of the FT, thickening was influenced by the collapse of sand on the wedge front slope, where the principal slip surface formed between the collapsed layer and the underlying subsiding layer. In contrast, in the deeper region of the FT, thickening occurred as the principal slip surface migrated further into the wedge, indicating structural erosion.
The shear zone development process observed in this experiment may also occur in natural settings, such as in sedimentary layers within plate convergent zones. Further investigation, including comparisons with natural strata, is required in future work.