2:00 PM - 2:15 PM
[SVC26-14] New insights into the structural development of caldera boundary faults through analogue sandbox experiments
Keywords:Caldera collapse, Analogue model, Ring faults, Downsag, Trapdoor collapse
Introduction
Calderas are volcanic depressions formed when the roof of a magma chamber collapses due to the depletion of magma in the chamber. Fault slip on peripheral ring faults (hereafter, caldera boundary faults) develops a depression [1], and those faults often serve as vents for pyroclastic eruptions [2]. Investigating the development of caldera boundary faults is therefore crucial for understanding caldera collapse events and predicting contemporaneous and subsequent volcanic eruptions.
Because subsurface structures in natural calderas are difficult to observe, analogue sandbox experiments are often performed to reproduce caldera structures. The previous study summarized that with increasing subsidence, an unfaulted broad depression (downsag) first forms, followed by outward-dipping reverse faults (ODRFs) and then inward-dipping normal faults (IDNFs) [1]. Here, we employed recent advances in image analysis techniques [3] to quantify experimental caldera collapse and improve available models of caldera boundary fault development.
Methods
We downscaled natural calderas 1–10 km in diameter to experimental calderas 10 cm in diameter. A transparent acrylic sandbox was filled with a 5:1 mixture of dry silica sand and wheat flour to model the brittle upper crust. A styrene foam semicylinder 10 cm in diameter was placed at the bottom of the sandbox with its cross section touching the front wall to model the magma chamber. The analogue chamber analogue was pulled down by until the amount of subsidence reached 2 or 5 cm using a motor jack to induce caldera collapse in the analogue crust.
Digital cameras placed in front of and above the sandbox monitored the subsurface and surface deformation of the analogue crust, respectively, to analyze the displacement field using particle image velocimetry software. For each centimeter that the analogue chamber was pulled down, we took a photogrammetric measurement of the analogue crust to analyze topographic changes. We then integrated the results of the image analyses to develop a model for the development of caldera boundary faults.
Results and discussion
Fig. 1 shows our integrated model of the structural and topographic development during caldera collapse. As subsidence increases, caldera structures evolved from Fig. 1A to 1F:
A: Downsagging begins above the chamber.
B: An ODRF forms at the edge of the chamber. As the region of downsag widens, the zone of highest displacement velocity becomes more localized.
C: An arcuate reverse fault forms when part of the ODRF reaches the surface. The fault block tilts toward the arcuate fault (trapdoor collapse).
D: A ring reverse fault forms due to the entirety of ODRF reaching the surface. The fault block undergoes vertical subsidence (piston collapse).
E: An arcuate normal fault forms due to the generation of an IDNF on the hanging wall of the ODRF. Another trapdoor collapse of the fault block occurs.
F: A ring normal fault forms due to lateral propagation of the IDNF. The fault block undergoes another piston collapse.
Our experiments provided two new insights into caldera boundary fault development. (1) Although the region of downsag expands with increasing subsidence, the zone of the highest displacement velocity localizes above the concealed ODRFs. This suggests that the existence and location of concealed caldera boundary faults can be predicted from the downsag deformation pattern. (2) During symmetric magma chamber depletion, trapdoor collapses occur both upon ODRF and IDNF generation. This detail may improve the resolution of reconstructions of trapdoor caldera structures and associated magma chamber dynamics.
[1] Acocella (2007). Earth Sci. Rev., 85, 125–160.
[2] Geshi et al. (2023). Sci. Rep., 13, 7463.
[3] Liu et al. (2019). Earth Planet. Sci. Lett., 526, 115784.
Calderas are volcanic depressions formed when the roof of a magma chamber collapses due to the depletion of magma in the chamber. Fault slip on peripheral ring faults (hereafter, caldera boundary faults) develops a depression [1], and those faults often serve as vents for pyroclastic eruptions [2]. Investigating the development of caldera boundary faults is therefore crucial for understanding caldera collapse events and predicting contemporaneous and subsequent volcanic eruptions.
Because subsurface structures in natural calderas are difficult to observe, analogue sandbox experiments are often performed to reproduce caldera structures. The previous study summarized that with increasing subsidence, an unfaulted broad depression (downsag) first forms, followed by outward-dipping reverse faults (ODRFs) and then inward-dipping normal faults (IDNFs) [1]. Here, we employed recent advances in image analysis techniques [3] to quantify experimental caldera collapse and improve available models of caldera boundary fault development.
Methods
We downscaled natural calderas 1–10 km in diameter to experimental calderas 10 cm in diameter. A transparent acrylic sandbox was filled with a 5:1 mixture of dry silica sand and wheat flour to model the brittle upper crust. A styrene foam semicylinder 10 cm in diameter was placed at the bottom of the sandbox with its cross section touching the front wall to model the magma chamber. The analogue chamber analogue was pulled down by until the amount of subsidence reached 2 or 5 cm using a motor jack to induce caldera collapse in the analogue crust.
Digital cameras placed in front of and above the sandbox monitored the subsurface and surface deformation of the analogue crust, respectively, to analyze the displacement field using particle image velocimetry software. For each centimeter that the analogue chamber was pulled down, we took a photogrammetric measurement of the analogue crust to analyze topographic changes. We then integrated the results of the image analyses to develop a model for the development of caldera boundary faults.
Results and discussion
Fig. 1 shows our integrated model of the structural and topographic development during caldera collapse. As subsidence increases, caldera structures evolved from Fig. 1A to 1F:
A: Downsagging begins above the chamber.
B: An ODRF forms at the edge of the chamber. As the region of downsag widens, the zone of highest displacement velocity becomes more localized.
C: An arcuate reverse fault forms when part of the ODRF reaches the surface. The fault block tilts toward the arcuate fault (trapdoor collapse).
D: A ring reverse fault forms due to the entirety of ODRF reaching the surface. The fault block undergoes vertical subsidence (piston collapse).
E: An arcuate normal fault forms due to the generation of an IDNF on the hanging wall of the ODRF. Another trapdoor collapse of the fault block occurs.
F: A ring normal fault forms due to lateral propagation of the IDNF. The fault block undergoes another piston collapse.
Our experiments provided two new insights into caldera boundary fault development. (1) Although the region of downsag expands with increasing subsidence, the zone of the highest displacement velocity localizes above the concealed ODRFs. This suggests that the existence and location of concealed caldera boundary faults can be predicted from the downsag deformation pattern. (2) During symmetric magma chamber depletion, trapdoor collapses occur both upon ODRF and IDNF generation. This detail may improve the resolution of reconstructions of trapdoor caldera structures and associated magma chamber dynamics.
[1] Acocella (2007). Earth Sci. Rev., 85, 125–160.
[2] Geshi et al. (2023). Sci. Rep., 13, 7463.
[3] Liu et al. (2019). Earth Planet. Sci. Lett., 526, 115784.