HR: 0800h
AN: GP11A-0815    [Abstracts]
TI: The Altiplano (Central Andes) High Conductivity Zone: Interpretation and Modeling
AU: * Mai, K
EM: kmai@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegraphenberg, Potsdam, 14473 Germany
AU: Haak, V
EM: vhaak@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegraphenberg, Potsdam, 14473 Germany
AU: Schilling, F R
EM: fsch@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegraphenberg, Potsdam, 14473 Germany
AU: Brasse, H
EM: h.brasse@fu-berlin.de
AF: Freie Universit„t Berlin, Malteserstr. 74-100, Berlin, 12249 Germany
AB: Magnetotelluric measurements in the South American Andes have revealed regions of highly conductive structures in the middle and lower crust of the Altiplano Plateau. Over wide areas the resistivity drops below 1 Ohm m and an overall conductance of more than 20,000 S is obtained, which represents one of the highest conductance observed on Earth. Several conduction mechanisms and combinations of them have been analyzed to explain the extreme conductivity, such as conduction by graphite layers, saline fluids, and partial melts. The combination of conduction mechanisms is critical, since processes being very efficient by themselves can become very inefficient (e.g. due to polarization effects) if jointed. The conductance measured with magnetotelluric methods represents integrated values for a large volume. To produce that unusual conductivity one or more very conductive materials with a high degree of interconnection have to be present over an enormous spatial range. The distribution of the liquid in the crystalline crust is a crucial constraint, since a lower degree of interconnectivity or of the amount of conductive material within a given volume requires a higher conductivity of the material itself. Magnetotelluric methods are especially sensitive for conductive structures with a pronounced horizontal extent. Assuming preferably saline fluids and partial melts being responsible for the high conductivity zone several adequate patterns of liquid distribution in the crust are modeled numerically and discussed. Concepts of large scale fluid transport (for example via dikes and diapirs) and of storage are analyzed for their geological and petrological relevance as well as for their geophysical impacts, and are compared with the field observations. In this context also the question has to be addressed how and how long fluids serving as conductors can be stored in the crust at a certain depth and how they are replaced when they are lost due to cooling/crystallization, mineral reactions or buoyant instability. The combination of these dynamic processes and their petrophysical signature will be used to distinguish different concepts and to better constrain the nature and evolution of this conductivity anomaly.
DE: 8145 Physics of magma and magma bodies
DE: 5109 Magnetic and electrical properties
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting