Chapter 5 Characterisation of Archean Subaqueous Calderas in Canada: Physical Volcanology, Carbonate-Rich Hydrothermal Alteration and a New Exploration Model
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Abstract Akan volcano, eastern Hokkaido, Japan, is characterized by a rectangular-shaped caldera (Akan caldera: 24 km by 13 km) with a complex history of caldera-forming eruptions during the Quaternary. A new Bouguer anomaly map of the caldera is presented on the basis of a gravity survey around Akan volcano. As part of and in addition to this survey, we applied gravimetry over the frozen caldera lake including lake water corrections. The Bouguer map shows the distribution of at least three sub-circular minima indicative of multiple depressions inside the caldera. Lake water corrections, performed by a numerical integration method using rectangular prisms, sharpen edges of the sub-circular minima. This gravity feature is consistent with geological investigations suggesting that caldera-forming eruptions of Akan volcano occurred from at least three different sources. It is concluded that Akan caldera can be described as a composite caldera with three major depressed segments.
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This chapter contains sections titled: Introduction Geologic Setting Three Creeks Caldera Big John Caldera Monroe Peak Caldera Tuff of Lion Flat Mount Belknap Caldera Red Hills Caldera Gravity Expressions of the Calderas Discussion
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Valles caldera is best known in recent years as an excellent example of a resurgent caldera [ Smith and Bailey , 1968] and as the site of a high‐temperature geothermal system [ Dondanville , 1978]. However, Valles caldera and the surrounding Jemez Mountains volcanic field possess a rich history of geologic research that dates back to the late 1800s. Through the years, the research focus has changed as different economic and scientific factors have exerted their influence. Early work emphasized mining activity, while modern work has stressed volcanology and, later, geothermal development. Only in the last 5 years has it been possible to view the region as a dynamic, integrated magma‐hydrothermal system having a complex evolution lasting more than 13 m.y. [ Gardner et al. , 1986; Goff and Nielson, 1986; Self et al. , 1986].
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Current knowledge of preserved Archean sedimentary rocks suggests that they accumulated in at least three major depositional settings. These are represented generally by sedimentary units: (1) in early Archean, pre-3.0 Ga old greenstone belts, (2) on late Archean sialic cratons, and (3) in late Archean, post-3.0 Ga old greenstone belts. Research suggests that the Archean was characterized by at least two distinctive and largely diachronous styles of crustal evolution. Thick, stable early Archean simatic platforms, perhaps analogous to modern oceanic islands formed over hot spots, underwent a single cycle of cratonization to form stable continental blocks in the early Archean. Later formed Archean continents show a two stage evolution. The initial stage is reflected in the existence of older sialic material, perhaps representing incompletely cratonized areas or microcontinents of as yet unknown origin. During the second stage, late Archean greenstone belts, perhaps analogous to modern magmatic arcs or back arc basins, developed upon or adjacent to these older sialic blocks. The formation of this generation of Archean continents was largely complete by the end of the Archean. These results suggest that Archean greenstone belts may represent a considerable range of sedimentological and tectonic settings.
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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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This chapter contains sections titled: Introduction Downsagged Calderas Distribution of Postcaldera Vents in Calderas Vent Rings – On Cone Sheets or Ring Dikes? Size of Calderas and Cauldrons Calderas of the Basin and Range Province Incremental Caldera Growth Caldera–Forming Events Summary and Conclusions
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