09:30 〜 09:45
[MIS25-03] A model for caldera subsidence caused by depressurization of magma chamber at Olympus Mons, Mars.
キーワード：火星、カルデラ、火山
The Olympus Mons located in the northwestern part of the Tharsis Highlands on Mars, is the largest volcano in the solar system. It is the highest and most prominent shield volcano in the Solar System (Mouginis-Mark, 2018). The Olympus Mons has a Caldera (Olympus Paterae) that is located near by the summit. It has a nested caldera 60 × 80 km in diameter that is strikingly similar to those of certain terrestrial volcanoes (Mouginis-Mark et al., 2007). Olympus Paterae was previously classified into six calderas (Mouginis-Mark, 1981). Recently, Nakmura and Tsuji (2021) classified the paterae into seven calderas such as Zeus, Hera, Hermes-west, Hermes-east, Dionysus, Apollo and Athena Paterae based on circle analysis considering the shapes of the caldera rim and difference of elevation among each caldera floor (Fig.1).
The regularity regarding the developmental process of Olympus Caldera has not been understood because the seven calderas are nested and the primal topography became obscure. Then, we examined the regularity of caldera subsidence, applying the method of Nakamura and Tsuji (2021) on all of the calderas.
We analyzed the coordinate of the approximate circle and ellipse in the 7 caldera rims. The method is described below.
1. Using Google Earth Pro, drop pins on the caldera rim to obtain coordinate data every 0.03 degree (Fig.1).
2. Analyzing circles and ellipses from the above data, we calculated center coordinates and correlation coefficients, using Origin Pro 2021.
3. We confirmed that the shapes and locations of approximate circles mostly agreed with those of actual caldera rims (Fig.1).
As a result, each caldera was approximated with a circle or ellipse. All caldera has coordinate, almost the same location both approximate circles and ellipses. The coordinates of Hera, Hermes-west, Dionysus, and Athena Caldera tend to located near the Zeus Caldera rim. Similarly, the coordinates of Hermes-east Caldera are located near the assumed eastern side of the Hera Caldera (Fig.1).
In this study, we compared the sequence of the caldera formation and location of the coordinate, and we interpreted that the coordinates of subsequent calderas tend to be located near the existing formed caldera rim. Therefore, we constructed a model that each caldera was formed by magma transportation into the existing circular fault in Olympus Caldera. It showed that the center of the subsequent caldera would have been regularly formed above the rim of the previously subsided caldera. An example of it would be explained below; the formation process of Caldera at three-phase through Zeus, Hera, and Hermes-east. (Fig.2).
･First phase, the incipient caldera (Zeus Caldera) was subsided by depressurization of a subsurface magma chamber. Along the caldera rim, a high angle ring fault could have been formed there. It might have been reached at the circumference of the depressurized magma chamber. The bottom of the fault could be fractured and structurally weak.
･Second phase, re-charged magma have intruded horizontally into the fractured edge of the depressurized chamber, to formed a new magma chamber at the bottom of the ring fault (Hera and Hermes-west caldera).
Then, the magma transported through the magma path and reached the slope of Olympus Mons and flowed out, then it was attributed to subsidence of Hera caldera.
･Third phase, a magma chamber of Hermes-east Caldera is formed in the eastern part of the Hera caldera rim as the same mechanism above.
This process could be adapted for other calderas such as Athena, Dionysus, and Apollo Calderas). Cole et al., (2005) showed post-caldera lava dome of terrestrial volcanoes form along incipient caldera rims. Our model is consistent with their post-caldera lava dome model. Therefore, we proposed regularity of caldera subsidence in the complex caldera that the magma chamber of subsequent caldera could be formed along the previously subsided caldera rim in Olympus Mons.
The regularity regarding the developmental process of Olympus Caldera has not been understood because the seven calderas are nested and the primal topography became obscure. Then, we examined the regularity of caldera subsidence, applying the method of Nakamura and Tsuji (2021) on all of the calderas.
We analyzed the coordinate of the approximate circle and ellipse in the 7 caldera rims. The method is described below.
1. Using Google Earth Pro, drop pins on the caldera rim to obtain coordinate data every 0.03 degree (Fig.1).
2. Analyzing circles and ellipses from the above data, we calculated center coordinates and correlation coefficients, using Origin Pro 2021.
3. We confirmed that the shapes and locations of approximate circles mostly agreed with those of actual caldera rims (Fig.1).
As a result, each caldera was approximated with a circle or ellipse. All caldera has coordinate, almost the same location both approximate circles and ellipses. The coordinates of Hera, Hermes-west, Dionysus, and Athena Caldera tend to located near the Zeus Caldera rim. Similarly, the coordinates of Hermes-east Caldera are located near the assumed eastern side of the Hera Caldera (Fig.1).
In this study, we compared the sequence of the caldera formation and location of the coordinate, and we interpreted that the coordinates of subsequent calderas tend to be located near the existing formed caldera rim. Therefore, we constructed a model that each caldera was formed by magma transportation into the existing circular fault in Olympus Caldera. It showed that the center of the subsequent caldera would have been regularly formed above the rim of the previously subsided caldera. An example of it would be explained below; the formation process of Caldera at three-phase through Zeus, Hera, and Hermes-east. (Fig.2).
･First phase, the incipient caldera (Zeus Caldera) was subsided by depressurization of a subsurface magma chamber. Along the caldera rim, a high angle ring fault could have been formed there. It might have been reached at the circumference of the depressurized magma chamber. The bottom of the fault could be fractured and structurally weak.
･Second phase, re-charged magma have intruded horizontally into the fractured edge of the depressurized chamber, to formed a new magma chamber at the bottom of the ring fault (Hera and Hermes-west caldera).
Then, the magma transported through the magma path and reached the slope of Olympus Mons and flowed out, then it was attributed to subsidence of Hera caldera.
･Third phase, a magma chamber of Hermes-east Caldera is formed in the eastern part of the Hera caldera rim as the same mechanism above.
This process could be adapted for other calderas such as Athena, Dionysus, and Apollo Calderas). Cole et al., (2005) showed post-caldera lava dome of terrestrial volcanoes form along incipient caldera rims. Our model is consistent with their post-caldera lava dome model. Therefore, we proposed regularity of caldera subsidence in the complex caldera that the magma chamber of subsequent caldera could be formed along the previously subsided caldera rim in Olympus Mons.