HR: 1340h
AN: T13A-0425    [Abstracts]
TI: The Exposed Moho In the Kohistan Arc: Seismic Properties At High Temperature And Pressure
AU: Burg, J
EM: jean-pierre.burg@erdw.ethz.ch
AF: Geological Institute, ETH, Sonneggstrasse, 5, Zurich, 8092 Switzerland
AU: * Burlini, L
EM: Burlini@erdw.ethz.ch
AF: Geological Institute, ETH, Sonneggstrasse, 5, Zurich, 8092 Switzerland
AB: The exposed crustal and mantle section in the Kohistan paleo-island arc in Pakistan is one of the most continuous and fresh example of exposed Moho. The geometrical relationships between mantle rocks and lower crustal intrusives and metamorphites are well preserved, making this section a natural laboratory for geological and geophysical investigation. In this study we investigated the relationships between the petrological and seismic Moho at geological relevant conditions, by measuring the speed of ultrasounds in rocks samples at high pressure and temperature. The compressional wave velocities were measured at confining pressures of up to 0.5 GPa and temperatures of up to 1200°C in an internally heated gas medium apparatus (Paterson rig) in three orthogonal cores per sample, cut parallel or normal to the mineral lineation and the foliation normal. Measurements were done on dunites, pyroxenites and wherlites representative of the upper mantle; on garnetites, amphibolites and gabbros representative of the lower crust. Interestingly, both the pyroxenite and the garnetite developed partial melting at temperatures greater than 1100°C at about 0.5 GPa confining pressure. During the melting we did not observed a remarkable reduction in the seismic velocities, but a remarkable reduction in the seismic amplitude, suggesting a large variation in seismic attenuation. The geological implication of our findings is that at (or above) the Moho, the presence of partially molten rocks might be detected by anomalies in the P-wave attenuation.
DE: 5102 Acoustic properties
DE: 7205 Continental crust (1219)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005