17:15 〜 19:15
[SCG45-P09] Preliminary results of tectonic underplating process using an analogue sandbox experiment
キーワード：accretionary wedge, decollement, analogue experiment, digital image correlation, underplating
Introduction:
An accretionary prism is a wedge-shaped mass formed by the accretion of material from one side to the other due to the movement of a boundary shear zone. In the layer-parallel shortening setting, the boundary shear zone is often called décollement, and its geometry is a key factor that controls the structure and deformation pattern of the prism (e.g., Noda et al., 2023). A characteristic feature of décollement movement is its propagation into the footwall, where it connects to an existing, shallower part of the décollement, forming a new pathway. This process is known as décollement step-down. During this process, tectonic underplating occurs, which is considered one of the important processes that form accretionary prisms at convergent plate boundaries. However, this process is difficult to observe in natural samples. Therefore, in this study, we simulate the dynamics of the accretionary prism using a sandbox model and analyze strain patterns with Digital Image Correlation (DIC) to observe the process of décollement step-down and tectonic underplating.
Methodology:
A transparent acrylic box with a movable actuator was used for the experiment. A fixed wooden block served as a rigid backstop, simulating the overriding plate. A wooden plate was placed at the bottom of the box and fixed to the actuator, allowing it to be pulled up to 250 mm to create an accretionary wedge. Dry Toyoura sand (with a uniform grain size and rounded particles) was then layered up to 20 mm to represent sedimentary sequences. The experiment was conducted at a constant velocity, with horizontal shortening induced by pulling the wooden plate. Additionally, a small accretionary wedge was pre-formed. Digital images were captured every 0.2 mm of displacement, and Digital Image Correlation (DIC) software (sDIC ver.2) was used to calculate displacement gradients and strain fields.
Result and discussion:
During the experiment, several cycles of décollement propagation were observed. Each cycle was associated with the formation of new frontal thrusts, which led to the progressive development of multiple thrust sheets throughout the entire wedge. Some of these cycles involved décollement step-down events (Figure 1). As a result of the step-down process, the shape of the existing décollement changed to a flat-ramp-flat geometry or a sigmoid curve. As a result, thrust horses developed through tectonic underplating, forming a duplex structure. Interestingly, there are some cycles that did not involve a step-down process, which may be due to the shape of the prism and/or the frictional property of the décollement. Future work should focus on investigating the effect of the physical properties of the décollement on the variety of the tectonic underplating processes. Furthermore, we will use X-ray CT scanning to visualize internal structures and the distribution of accumulated shear zones within accretionary wedges with underplating.
An accretionary prism is a wedge-shaped mass formed by the accretion of material from one side to the other due to the movement of a boundary shear zone. In the layer-parallel shortening setting, the boundary shear zone is often called décollement, and its geometry is a key factor that controls the structure and deformation pattern of the prism (e.g., Noda et al., 2023). A characteristic feature of décollement movement is its propagation into the footwall, where it connects to an existing, shallower part of the décollement, forming a new pathway. This process is known as décollement step-down. During this process, tectonic underplating occurs, which is considered one of the important processes that form accretionary prisms at convergent plate boundaries. However, this process is difficult to observe in natural samples. Therefore, in this study, we simulate the dynamics of the accretionary prism using a sandbox model and analyze strain patterns with Digital Image Correlation (DIC) to observe the process of décollement step-down and tectonic underplating.
Methodology:
A transparent acrylic box with a movable actuator was used for the experiment. A fixed wooden block served as a rigid backstop, simulating the overriding plate. A wooden plate was placed at the bottom of the box and fixed to the actuator, allowing it to be pulled up to 250 mm to create an accretionary wedge. Dry Toyoura sand (with a uniform grain size and rounded particles) was then layered up to 20 mm to represent sedimentary sequences. The experiment was conducted at a constant velocity, with horizontal shortening induced by pulling the wooden plate. Additionally, a small accretionary wedge was pre-formed. Digital images were captured every 0.2 mm of displacement, and Digital Image Correlation (DIC) software (sDIC ver.2) was used to calculate displacement gradients and strain fields.
Result and discussion:
During the experiment, several cycles of décollement propagation were observed. Each cycle was associated with the formation of new frontal thrusts, which led to the progressive development of multiple thrust sheets throughout the entire wedge. Some of these cycles involved décollement step-down events (Figure 1). As a result of the step-down process, the shape of the existing décollement changed to a flat-ramp-flat geometry or a sigmoid curve. As a result, thrust horses developed through tectonic underplating, forming a duplex structure. Interestingly, there are some cycles that did not involve a step-down process, which may be due to the shape of the prism and/or the frictional property of the décollement. Future work should focus on investigating the effect of the physical properties of the décollement on the variety of the tectonic underplating processes. Furthermore, we will use X-ray CT scanning to visualize internal structures and the distribution of accumulated shear zones within accretionary wedges with underplating.