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1 Institute of Mineralogy, Petrology and Economic Geology, Graduate school of Science, Tohoku University, Sendai, Japan
* Dip. Scienze Geologiche Roma Tre, L. S. L. Murialdo, 1, 00146 Rome, Italy acocella{at}uniroma3.it
Here we review the studies of pluton emplacement and caldera collapse, proposing a model linking the two processes. The shallow rise of magma occurs by dyking, and its emplacement occurs along major anisotropies. Many plutons are emplaced at subhorizontal discontinuities, forming sills. These will eventually grow, forming laccoliths, the most common mechanism to store shallow magma. Calderas are a surface expression of these reservoirs, and can be approximated by a piston cylinder, which sinks due to: (1) underpressure in the reservoir, producing outward-dipping reverse faults; (2) overpressure within a sill-like chamber undergoing doming; and (3) overpressure within a laccolith generating apical tensile stresses; in the latter two cases, inward-dipping normal faults form. We suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions, Their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions. Tabular intrusions, generating ring-faults mainly at their tips, form calderas with a similar width to the intrusion; laccoliths may generate ring-faults along the intrusion roof, forming calderas narrower than the intrusion. The outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the reservoir.
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