HR: 0800h
AN: V11A-0359    [Abstracts]
TI: Cup-shaped Intrusions, Morphology and Emplacement Mechanism Investigate Through Analogue Modelling
AU: * Mathieu, L
EM: lassolas@free.fr
AF: Department of Geology, Trinity College Dublin, College Green, Dublin, 2, Ireland
AU: van Wyk de Vries, B
EM: B.vanwyk@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont-Ferrand, 63000, France
AB: We investigate the morphology of large-scale shallow-depth magma intrusions and sub-volcanic complexes with analogue models. Intrusions of analogue magma are done in a granular material that can contain a ductile layer. The model surface is flat to model the formation of plutonic intrusions and it is overlain by a cone when modelling late sub-volcanic complexes. For flat-top models, we obtain cup-shaped intrusions fed by dykes. Cup-shaped intrusions are inverted-cone like bodies. They are different from saucer-shaped intrusions as they possess neither a well developed sill-base, nor an outer rim. However, like saucers, cups are shallow depth intrusions that dome the country rocks. They initiate from an advancing dyke and first develop an inverted-cone like morphology. Then, the central thickness increases and thrusts form at the edge of the domed country rocks. At this stage, the intrusions progressively involve toward a lopolith shape. By using analogue magma of various viscosities we have been able to constrain key relationships: higher intrusion viscosity causes deeper initiation and the deeper they initiate, the larger is the intrusion diameter. A natural example of such intrusion might by the circles of volcanoes like the Azufre-Lastaria (Peru) that might be overlain be a large-scale cup-shaped intrusion. When adding a cone at the surface of the model and, sometimes, a thin ductile layer in the substratum, the morphology of cup-shaped intrusions vary. Note that the ductile layer of our models is not thick enough to induce the gravitational spreading of the cone. Generally, cup-shaped intrusions are asymmetric in cross section and elliptical in plan view. Their formation creates extension structures in the cone (croissant-shaped rift, straight rift or normal fault) and thrusts in some sectors below the cone. Both types of structures are bordered by strike-slip faults. Cups and saucers share many similarities, but differ probably in the fact that saucers are partially sills that are guided by stratigraphic horizons. However, the basic formation mechanisms may be the same and saucers could be regarded as a special form of cup.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8035 Pluton emplacement
DE: 8145 Physics of magma and magma bodies
DE: 8445 Experimental volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting