5:15 PM - 7:15 PM
[SCG45-P08] Two-step strain weakening during frontal thrust formation in dry-sand wedges
Keywords:shear band, Frontal Thrust, sand wedge, DIC, strain analysis, load cycle
Fold-and-thrust belts formed by horizontal shortening of a body are often characterized by the sequential formation of multiple shear zones (Frontal Thrusts, hereafter referred to as FT) at the tip of the body during deformation. While the deformation cycle varies depending on the initial structure, understanding the underlying mechanisms is crucial for comprehending the deformation and material circulation that occur within and around the body. However, it is difficult to observe the deformation cycle in natural examples, such as accretionary wedges, due to their larger temporal and spatial scales. As an alternative, analogue experiments using dry sand have been conducted. Particularly since the 2010s, studies have investigated the load cycles associated with FT deformation and used digital image analysis to study the formation process of FT in detail.
We have also conducted wedge formation experiments using dry sand layers and analyzed synchronously acquired load and image data. From these experiments, we divided one load cycle into four stages: Stage I, II, III, and IV. In this study, we focused on the changes in deformation during the transitions of each stage to understand the relationship between the entire load cycle and the deformation observed during sand wedge formation.
In an acrylic container (width: 118 mm, length: 693 mm, height: 158 mm), a 20-mm-thick layer of Toyoura silica sand (grain size 106-300 μm) was filled by free-fall and used to form the sand wedge. The sheet beneath the sand layer was pulled horizontally at a speed of 0.125 mm/s for a distance of 250 mm, pushing the sand against a fixed wall. Digital images were continuously taken every second for strain analysis, and the pulling load of the sheet was recorded using a load cell.
Using strain analysis of the digital images, we investigated the deformation pattern at each stage. Stage I refers to the period following the formation of a new FT, during which the load increases steadily or gradually. During this stage, the FT from the previous generation remains active, although its velocity decreases. In Stage II, which is characterized by a rapid increase in load, the previous FT nearly ceases to displace. This change was clearly observed in the strain analysis. Subsequently, weak shear bands appeared in front of the current FT, leading to Stage III, which is marked by a slowdown in the rate of load increase. These weak shear bands then developed into the next-generation FT. During this process, the transition to Stage IV occurred, characterized by a two-step strain weakening. This appears to be a common feature in FT formation at the tip of the wedge. In the future, it will be important to investigate the variations in deformation patterns.
We have also conducted wedge formation experiments using dry sand layers and analyzed synchronously acquired load and image data. From these experiments, we divided one load cycle into four stages: Stage I, II, III, and IV. In this study, we focused on the changes in deformation during the transitions of each stage to understand the relationship between the entire load cycle and the deformation observed during sand wedge formation.
In an acrylic container (width: 118 mm, length: 693 mm, height: 158 mm), a 20-mm-thick layer of Toyoura silica sand (grain size 106-300 μm) was filled by free-fall and used to form the sand wedge. The sheet beneath the sand layer was pulled horizontally at a speed of 0.125 mm/s for a distance of 250 mm, pushing the sand against a fixed wall. Digital images were continuously taken every second for strain analysis, and the pulling load of the sheet was recorded using a load cell.
Using strain analysis of the digital images, we investigated the deformation pattern at each stage. Stage I refers to the period following the formation of a new FT, during which the load increases steadily or gradually. During this stage, the FT from the previous generation remains active, although its velocity decreases. In Stage II, which is characterized by a rapid increase in load, the previous FT nearly ceases to displace. This change was clearly observed in the strain analysis. Subsequently, weak shear bands appeared in front of the current FT, leading to Stage III, which is marked by a slowdown in the rate of load increase. These weak shear bands then developed into the next-generation FT. During this process, the transition to Stage IV occurred, characterized by a two-step strain weakening. This appears to be a common feature in FT formation at the tip of the wedge. In the future, it will be important to investigate the variations in deformation patterns.