Reconciling mechanical models of caldera ring-fault nucleation within the transcrustal magmatic system paradigm
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One hundred and ninety‐two small earthquakes which occurred recently near the southern edge of Long Valley caldera at depths from 1 to 13 km are used to map the subsurface geometry of magma bodies in the caldera. Seismograms of these events recorded northwest, north, northeast, and east of Long Valley with ray paths through the caldera are often anomalous in that S‐wave arrivals are absent and high frequency P‐wave energy is missing. These anomalous ray paths intersect in an area of the south‐central caldera which we interpret to be a region of molten or partially molten rock. This magma body lies between about 4 1/2 and at least 13 km depth and is 10 km long by 5 km wide beneath 7 km. The magma appears to be relatively contiguous below 7 km and more dispersed above. Another magma body may be present in the northwest caldera coincident with an interpreted 7‐8 km deep reflection from the top of a magma chamber.
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Explosive caldera-forming eruptions eject voluminous magma during the gravitational collapse of the roof of the magma chamber. Caldera collapse is known to occur by rapid decompression of a magma chamber at shallow depth, however, the thresholds for magma chamber decompression that promotes caldera collapse have not been tested using examples from actual caldera-forming eruptions. Here, we investigated the processes of magma chamber decompression leading to caldera collapse using two natural examples from Aira and Kikai calderas in southwestern Japan. The analysis of water content in phenocryst glass embayments revealed that Aira experienced a large magmatic underpressure before the onset of caldera collapse, whereas caldera collapse occurred with a relatively small underpressure at Kikai. Our friction models for caldera faults show that the underpressure required for a magma chamber to collapse is proportional to the square of the depth to the magma chamber for calderas of the same horizontal size. This model explains why the relatively deep magma system of Aira required a larger underpressure for collapse when compared with the shallower magma chamber of Kikai. The distinct magma chamber underpressure thresholds can explain variations in the evolution of caldera-forming eruptions and the eruption sequences for catastrophic ignimbrites during caldera collapse.
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The La Primavera caldera lies close to the triple junction of the Tepic-Zacoalco, Colima, and Chapala rifts in the western part of the Mexican Volcanic Belt. It is a promising geothermal field with 13 deep wells already drilled. We calculated solute geothermometric temperatures (Na–K, Na–Li, and SiO2) from the chemistry of geothermal water samples; determined values are generally between 99°C and 202°C for springs and between 131°C and 298°C for wells. Thermal modelling is an important geophysical tool as documented in the study of this and other Mexican geothermal areas. Using the computer program TCHEMSYS, we report new simulation results of three-dimensional (3-D) thermal modelling of the magma chamber underlying this caldera through its entire eruptive history. Equations (quadratic fit) describing the simulated temperatures as a function of the age, volume and depth of the magma chamber are first presented; these indicate that both the depth and the age of the magma chamber are more important parameters than its volume. A comparison of 3-D modelling of the La Primavera and Los Humeros calderas also shows that the depth of the magma chamber is more important than its volume. The best model for the La Primavera caldera has 0.15 million years as the emplacement age of the magma chamber, its top at a depth of 4 km, and its volume as 600 km3. Fresh magma recharge events within the middle part of the magma chamber were also considered at 0.095, 0.075, and 0.040 Ma. The simulation results were evaluated in the light of actually measured and solute geothermometric temperatures in five geothermal wells. Future work should involve a smaller mesh size of 0.050 or 0.10 km on each side (instead of 0.25 km currently used) and take into account the topography of the area and all petrogenetic processes of fractional crystallization, assimilation, and magma mixing as well as heat generation from natural radioactive elements.
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Abstract The Campi Flegrei caldera (Italy) is an active volcanic system characterized by significant long‐ and short‐term ground deformation phenomena ranging the maxima values in the central sector of the caldera, where La Starza marine terrace is located. A detailed study of the La Starza provided crucial clues for understanding the resurgence of the central sector of the caldera following the 15‐ka Neapolitan Yellow Tuff eruption. The doming of the caldera floor, marked by two primary episodes of uplift, began soon after the collapse of about 110 m following the eruption. The first doming (15–9.2 ka) occurred as a response to loss of lithostatic loading producing magma influx, possibly regulated by thermal magmatic convection and chaotic movement inside the magma reservoir under the caldera. The calculated ~90 m of structural uplift is the persistent displacement correlated with magma volumes intruded accompanying the contemporaneous volcanic activity. The second episode of uplift (5.5–3.8 ka) produced a ground deformation pattern similar to that measured during recent unrest crises suggesting a stable and shallow (~4‐km deep) source of strain like a sill in an elastic half space. By this geometry and inversion of surface deformation, the volume of intruded material was determined. Simply varying pressure history as input, the time history of the surface deformation was reproduced by using spherical source geometry with a concentric viscoelastic shell 8‐km deep. The satisfactory comparison between the two models is a useful indication for interpreting the current unrest phase at the Campi Flegrei caldera.
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The mechanisms suggested to cause caldera resurgence can be categorized as (1) viscous rebound of magma, (2) regional detumescence, and (3) pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma. The critical observation that resurgence takes of the order of 10 3 –10 5 years insists that the causative mechanism be regulated by a highly viscous (e.g., ∼10 22 Pa s) structural member of the crust. The relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale. The most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation. Detumescence compresses the magma chamber, squeezing magma upward against and through the caldera block. Vesiculation also compresses the chamber and tends to dome the caldera block. The times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber, but the rate of cooling of the chamber is proportional to its size. Because solidification spreads viscous relaxation over a wide region of the crust and curtails resurgence, there is likely to be a minimum volume of magma that can undergo resurgent doming. It is difficult to be exact about this critical size until more precise models have been developed.
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