136 GeoLines 20 2006 Analogue Modeling of AMS Development During Emplacement of Shallow Level Volcanic Bodies (Extrusive Domes and Laccoliths) Prokop ZÁVADA 1 , Zuzana KRATINOVÁ 2 , Vladimír KUSBACH 3 and František HROUDA 3 1 1Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43, Prague, Czech Republic 2 Geophysical Institute, Czech Academy of Sciences, Boční II/1401, 14131 Praha 4, Czech Republic 3 Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43, Prague, Czech Republic Volcanic domes and laccoliths are typical features of monogenic volcanic felds and are formed by highly viscous magmas either due to magma composition (andezites and rhyolites) and/or high crystal content (e.g. trachytes and phonolites). Although the evo- lution of shapes of extrusive domes and laccoliths was modelled for ideal newtonian fuids (Talbot and Jarvis, 1984, Koch et al. 1981) the evolution of internal fabric pattern during growth of such bodies is poorly understood. This is probably due to lack of good vertical cross-sections through such bodies and the mi- croscopic nature of the fabric elements in volcanic rocks. Some aspects of the internal fabric development during viscous fow of magma within lava domes can be explained by the results of analogue and mathematical modelling of strain within lava fow and dome extrusions on fat surface (Buisson and Merle 2004). The models have shown that in the upper part of a lava fow (or fank of the dome), the maximum stretching axis is oriented per- pendicular to the magma fow direction, while in the lower part lineations are parallel with the fow direction and diverge, where the fow extrudes radially. As the authors suggested, the results should be tested in feld for various types of magma rheologies (e.g., shear-thickening dilatant rheology for crystal rich magmas, Smith 2002). Besides that natural magmatic domes often differ from the ideal fat based droplet geometry and their outer shape (and internal fabric) is controlled by complex interaction be- tween the magma and host sediments. The aim of our study is to investigate the fabric generated by viscous fow within domes and laccoliths emplaced into weak sedimentary sequence by means of AMS analogue modeling. We follow a procedure of Kratinová et al. (2006) and use a hydrau- lic analogue apparatus (see Fig. 1) equiped with steel squeezing board and tube conduit and a perspex container flled with sand. The initial plaster and sand layers can be colored to visualize the deformation pattern within and around the model bodies. Sedi- mentary sequence is formed by pure sand or sand with clay lay- ers, which induce zones of low tensile strength necessary for the emplacement of laccoliths. Plaster still remains the most suitable analogue material for AMS modeling of viscous fow. It is cheap, easily colored and handled during the experiment; it is also easily homogenized with magnetic material. We have tested other analogue materi- als. Asphalt seemed promising, while it shows a range of tem- perature dependent viscosities, however it can not be colored and it is hardly handled in larger volumes. Silicones are expen- sive and can not be polymerized in order to bring them to solid state at the end of the experiment and carry out AMS sampling. Therefore we continue with rheological measurements of plas- ter of different mixing ratios of plaster and water with ambition to scale down our experiments for different magma rheologies, which are strongly dependent on the amount of solid particles (crystals). Flow of magma during flling of laccoliths and domes is for example typical with successive magma pulses (e.g. Mock and Jerram 2003). Such pulses penetrate discordantly the al- ready present magma within infating dome/laccolith and refold the surrounding magmatic fabric (Závada et al., this volume). References BUISSON C. and MERLE O., 2004. Numerical simulation of strain within lava domes. Journal of Structural Geology, 26, 847-853. KOCH F.G., JOHNSON A.M. and POLLARD D.D., 1981. Mono- clinial bending of strata over laccolithic intrusions. Tectono- physics, 74: T21-T31. KRATINOVÁ Z., ZÁVADA P., HROUDA F. and SCHUL- MANN K., (accepted). Non-scaled analogue modelling of AMS development during viscous fow: a simulation on dia- pir-like structures. Tectonophysics. Fig. 1. Scheme of hydraulic squeezing apparatus used for AMS modelling.