Presentation Information
[O09-P06]Model experiments on microboudin structures formed in metamorphic rocks using starch syrup and Pocky®★Invited Papers
*Kazuya Toyama1, Atsuko Namiki1, Sayaka Oda1 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University)
Keywords:
Microboudin,Paleopiezometer,Rheology,Model experiments
Siliceous schist, a type of the Sanbagawa crystalline schists, is formed when sedimentary rocks deposited on an oceanic plate are subjected to stress during subduction beneath a continental plate, undergoing both metamorphism and deformation. Metamorphism leads to the formation of metamorphic minerals such as piemontite and glaucophane, whereas deformation results in the accumulation of strain through plastic flow of the quartz matrix governed by dislocation creep. In contrast, columnar minerals such as piemontite and glaucophane behave in a brittle manner under deformational conditions and cannot undergo plastic flow like the surrounding quartz. As a result, they may fracture under tensile stress and form segmented distributions like a dashed line. Such structures are referred to as microboudin structures.
A microboudin paleopiezometer has been proposed to estimate the evolution of differential stress and strain acting on rocks using the aspect ratio of microboudin fragments and the spacing between fragments (interboudin gap)[1]. However, the mechanical equations used to calculate differential stress and the relationships between interboudin spacing and strain have not been experimentally verified, and thus the reliability of this method has not been sufficiently constrained.
In this study, we aimed to evaluate the reliability of the microboudin paleopiezometer through analogue deformation experiments using syrup to simulate the quartz matrix and rod-shaped brittle snacks (Pocky® or Pretz®) to simulate columnar minerals. As a first step, we used a qualitative approach. Rod-shaped brittle snacks were enclosed in syrup using plastic wrap, and tensile deformation was applied by our hands, confirming that the snacks fractured during deformation. Our preliminary results suggest that longer rod-shaped brittle snacks tend to fracture at lower applied stresses. This tendency is consistent with predictions of the proposed microboudin paleopiezometer.
To quantitatively evaluate this behavior, we introduced a deformation-testing apparatus. An acrylic container was prepared to enclose cooled syrup with increased viscosity and rod-shaped brittle snacks with reduced fracture strength. We aim to examine the relationship between the stress applied to the syrup as a whole and the stress actually acting on the rod-shaped brittle snacks by comparing these stresses with the fracture strength of the brittle rods. In addition, we plan to compare the relationship between the bulk deformation of the syrup and the spacing of fragmented rod-shaped brittle snacks with that observed in natural microboudin structures.
References: [1] Toyama & Michibayashi 2026 Tectonophysics. under review.
A microboudin paleopiezometer has been proposed to estimate the evolution of differential stress and strain acting on rocks using the aspect ratio of microboudin fragments and the spacing between fragments (interboudin gap)[1]. However, the mechanical equations used to calculate differential stress and the relationships between interboudin spacing and strain have not been experimentally verified, and thus the reliability of this method has not been sufficiently constrained.
In this study, we aimed to evaluate the reliability of the microboudin paleopiezometer through analogue deformation experiments using syrup to simulate the quartz matrix and rod-shaped brittle snacks (Pocky® or Pretz®) to simulate columnar minerals. As a first step, we used a qualitative approach. Rod-shaped brittle snacks were enclosed in syrup using plastic wrap, and tensile deformation was applied by our hands, confirming that the snacks fractured during deformation. Our preliminary results suggest that longer rod-shaped brittle snacks tend to fracture at lower applied stresses. This tendency is consistent with predictions of the proposed microboudin paleopiezometer.
To quantitatively evaluate this behavior, we introduced a deformation-testing apparatus. An acrylic container was prepared to enclose cooled syrup with increased viscosity and rod-shaped brittle snacks with reduced fracture strength. We aim to examine the relationship between the stress applied to the syrup as a whole and the stress actually acting on the rod-shaped brittle snacks by comparing these stresses with the fracture strength of the brittle rods. In addition, we plan to compare the relationship between the bulk deformation of the syrup and the spacing of fragmented rod-shaped brittle snacks with that observed in natural microboudin structures.
References: [1] Toyama & Michibayashi 2026 Tectonophysics. under review.