17:15 〜 18:45
[SCG40-P34] Propagation pattern of décollement in accretionary wedge with a weak layer: Insights from sandbox analogue experiments
Some accretionary wedges, such as Nankai and Barbados wedge, are characterized by continuous extension of décollement to the seaward of the frontal thrust. The extended segment is referred to as proto-décollement. However, proto-décollement is not developed in all wedges. This observation suggests the variety of the décollement propagation processes. Previous sandbox experiments suggest that the structure of the input layer influences the deformation pattern of the accretionary wedges. In this study, we investigated how a weak layer effect to the propagation patterns of décollements.
The experimental method is as follows; An adhesive sheet is placed inside an acrylic box and fixed to a stepping motor. Total 22 mm thick layer of granular materials covers the bottom of the box, and the stepping motor pulled the sheet at a velocity of 0.6 mm/s to create a wedge. Two types of input layer, which are called single layer model and multiple layer model, were prepared. The single layer model consists of sprinkled Toyoura sand. In the multiple layer model, a 4 mm thick microbeads layer is placed in the lower part of the input layer. Digital images were taken from the side at intervals every 0.6 mm of sheet displacement during the experiments and were analyzed using Digital Image Correlation (DIC) for the wedge deformation processes.
The two models showed different deformation behaviors during the propagation of décollement. In the single layer model, the propagation of décollement occurred instantaneously shown by the abrupt change in uplift rate at the tip of the accretionary wedge. On the other hand, the multiple layer model showed a period indicating a slight uplift rate before new frontal thrust formation. At the period, the décollement in the weak layer propagated forward of the existing frontal thrust and formed proto-décollement characterized by dilation and smaller displacement rate. After that, a new frontal thrust was formed and proto-décollement developed into décollement. This result indicates the influence of the weak layer extending the proto-décollement period.
The experimental method is as follows; An adhesive sheet is placed inside an acrylic box and fixed to a stepping motor. Total 22 mm thick layer of granular materials covers the bottom of the box, and the stepping motor pulled the sheet at a velocity of 0.6 mm/s to create a wedge. Two types of input layer, which are called single layer model and multiple layer model, were prepared. The single layer model consists of sprinkled Toyoura sand. In the multiple layer model, a 4 mm thick microbeads layer is placed in the lower part of the input layer. Digital images were taken from the side at intervals every 0.6 mm of sheet displacement during the experiments and were analyzed using Digital Image Correlation (DIC) for the wedge deformation processes.
The two models showed different deformation behaviors during the propagation of décollement. In the single layer model, the propagation of décollement occurred instantaneously shown by the abrupt change in uplift rate at the tip of the accretionary wedge. On the other hand, the multiple layer model showed a period indicating a slight uplift rate before new frontal thrust formation. At the period, the décollement in the weak layer propagated forward of the existing frontal thrust and formed proto-décollement characterized by dilation and smaller displacement rate. After that, a new frontal thrust was formed and proto-décollement developed into décollement. This result indicates the influence of the weak layer extending the proto-décollement period.