9:30 AM - 9:45 AM
[MIS21-03] A model for caldera subsidence caused by depressurization of magma chamber at Olympus Caldera and verification experiment.
Keywords:Mars , Caldera, Volcano
Olympus Caldera is a large 60x80 km diameter complex caldera located near the summit of Olympus Mons on Mars. It is formed by six nested calderas (Mouginis-Mark, 1981), and each caldera has preservation of caldera walls and patera. The topographic observations of the Olympus Calder include Carr(1973)~, although not discussed the formation of the complex caldera. The Olympus Caldera can be approximated into seven circles and ellipses, and their positions are related to consider the formation of there. In this study, our purpose is to the clear development history of the Olympus Caldera. We proposed the model of the 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 addition, we experimented to verify the model using collapse and injection experiments. And we assumed the subsidence was caused by magma intrusion, mapping lava flow on the east-west slope following Bleacher et al.(2007). In order to consider the validity of the subsidence model, the following problems need to be clear.(1) Confirmation of the development of a caldera circumferential fault into the magma chamber. (2) Confirmation of the injection of new magma into the resurgent older magma chamber, and spread to the space. (3) Confirmation of the inflow and rise of new magma into the old caldera fault that will be the site of new magma chamber formation in the subsequent caldera.We confirmed these problems by conducting collapse experiments using starch and balloons for (1) and penetration experiments using gelatin and oil for (2) and (3). The results of the collapse experiment showed that the surface of starch subsided in a circular shape directly above the balloon. In the early stage of collapse, the surface of starch subsided into a piston shape, and the surface retained the block. In the later stage, the balloon was deflated, and starch block transitioned to collapse. The circumferential fault formed with the subsidence was nearly vertical and developed continuously into the interior. The collapse experiment was able to verify that the subsidence was caused by a non-explosive eruption and reproduced the topography seen at Olympus Caldera. Based on this result, the vertical circumferential structures add to the condition for the injection experiment. In the injection experiment, three patterns of gelatin layers with different gelatin concentrations were overlaid, and oil was raised when there was a circumferential fracture in the upper layer. This result is harmonic with the dike of magma into vertical fractures in volcanoes. In addition, the upper layer uplift was caused by the spread of oil at the layer boundary when the gelatin concentration was higher in the upper layer. This result is consistent with a sill where magma flows into a weak horizontal space in volcanoes. Therefore, the validity of (2) and (3) of the subsidence model was confirmed by these experiments. In calderas formed by non-explosive eruptions caused by magma lateral movement or depressurization of the magma chamber due to the drained back. Bleacher et al.(2007)suggested that no major rift zones have developed in Olympus Mons. But there are many lava tubes that flow out from the slope. The flow start points of the lava tubes are located between 5,000 and 20,000 m mountain elevation and there are not found on the east side slope. This suggests that the lava tubes were not from the summit crater but also from the slopes. Some calderas tend to be distributed to concentrate on the western side in the Olympus Caldera. The lava tubes were distributed south-to-west from the north. Those results are a harmonic relationship. These may suggest the lava tubes flowing flow from the slope may have been produced by the migration of magma from the magma chamber caused by dike due to caldera subsidence.