HR: 13:30h
AN: V23B-01 INVITED     [Abstracts]
TI: Structure and formation of ring faults in composite volcanoes
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AU: Nilsen, K
EM: Knilsen@gwdg.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AB: Most collapse calderas form by vertical displacement (subsidence) along ring faults. One principal problem regarding the formation of collapse calderas is thus to explain the stress conditions that favor the formation of, or slip on existing, ring faults. The ring faults are shear fractures, more specifically dip-slip faults, so that they cannot form unless the local driving shear stress in the hosting composite volcano satisfies the criteria of shear failure. Many ring faults, however, are perhaps more accurately described as mixed-mode fractures: partly shear and partly extension fractures. This follows because the ring faults are commonly occupied by ring dikes which, like dikes in general, are extension fractures. Also, ring-fault development at the surface commonly involves tension-fracture formation. For a ring fault to form, or an existing one to slip, suitable stresses must concentrate at a certain radial distance from the center of the associated magma chamber. Similarly, at the surface, suitable stresses must peak in the circular region that eventually develops the ring fault. Most unrest periods do not generate stress fields suitable for formation of, or slip on, ring faults. By contrast, many unrest periods are favorable to dike injection, some of which reach the surface to feed eruptions. Thus, even in existing calderas, most unrest periods do not generate the special stress fields needed for ring-fault slip. Here we present new numerical models on ring-fault formation in composite volcanoes. Two common geometries of magma chambers are considered: spherical and sill-like (oblate ellipsoidal). In some models, the chamber itself is located in a single thick layer, whereas in others it is hosted by several layers of different mechanical properties. In all the models, the crustal segment above the chamber, up to the surface of the volcano, is composed of 30 adjacent layers with stiffnesses (Young's moduli) ranging from 1 GPa to 100 GPa. The thickness of the crustal segment hosting the chamber is 20 km, whereas its width is either 20 km or 40 km. The loading conditions include (1) crustal segments subject to tensile stress of 5 MPa, (2) crustal segments subject to magmatic pressure of 10 MPa at the bottom (that is, doming of the volcanic field containing the chamber), (3) a combination of tension and doming, and (4) internal excess pressure or underpressure in the chamber. In all models, the top of the chamber is at 3 km depth; the diameter of a sill-like chamber is 8 km (height 2 km), that of a spherical chamber 4 km. Some of the main results can be summarized as follows. (1) Internal excess-or underpressure in a spherical or sill-like chamber is not very likely to give rise to a ring fault; this loading rather results in dike injections. (2) For doming or tension, a spherical magma chamber is normally unlikely to give rise to a ring fault but favors dike injection. However, a spherical chamber in a very soft (10 GPa) layer, or one with recent dike injections, may generate a ring-fault. (3) A sill-like chamber under tension, doming, or tension and doming may generate a ring fault (and often a ring dike). When the volcanic field is 20 km wide, tension or tension and doming may result in ring-fault formation. When the volcanic field is 40 km wide, doming alone can also result in ring-fault formation. In conclusion, the numerical results suggest that composite volcanoes with sill-like magma chambers located within volcanic fields subject to tension, doming, or both are most likely to generate stress fields suitable for ring-fault formation.
DE: 8020 Mechanics
DE: 8035 Pluton emplacement
DE: 8164 Stresses--crust and lithosphere
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly