HR: 1340h
AN: T23A-0550    [Abstracts]
TI: Seismicity without a fault ? Structural evidence from pseudotachylites in the UHP Dora Maira Massif
AU: * Zechmeister, M
EM: zechmeim@siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131-1116 United States
AU: Cosca, M
EM: mcosca@img.unil.ch
AF: Universite de Lausanne, Institut de Mineralogie et Geochimie - BFSH-2, Lausanne, 1015 Switzerland
AU: Caby, R
EM: Renaud.Caby@dstu.univ-montp2.fr
AF: Universite de Montpellier, Laboratoire de Tectonophysique, Montpellier, 34095 France
AU: Frima, C
EM: cfrima@free.fr
AF: IGAL, 71 rue Grange Colombe, Rambouillet, 78120 France
AU: Ward, C
EM: chad422@hotmail.com
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AB: The Dora Maira massif, in the Italian Alps, is a world famous example of ultra-high pressure (UHP) metamorphism of crustal rocks with peak pressure of ca 3 GPa (depth $\approx$ 100 km) at about 35 My. Exhumation began shortly after the peak of metamorphism. The UHP rocks had equilibrated at a temperature of $250\deg$C around 30 My. Pseudotachylite veins (1 to 20 mm thick) formed parallel to the syn-exhumation mylonitic foliation and were dated at 20.1 $\pm$ 0.5 Ma by Ar/Ar method. The geochronologic data suggests that the pseudotachylite veins formed at depths shallower than $\approx$ 10 km. These veins are found mainly in gently dipping medium grained phengite/coesite bearing orthogneiss. The generation veins are parallel to the foliation, constant in thickness and up to several tens of meters in length. Injection veins are less common and generally only a few cm in length. Structural mapping of UHP mylonitic gneisses and surrounding rocks reveals the dominance of high ductile strain, a puzzling lack of localized cataclastic deformation and the absence of fault associated with the pseudotachylite veins. The pseudotachylite veins are concentrated along a 50 m wide, foliation-parallel corridor. Conversely, the two major faults along which the UHP rocks were exhumed are $>$ 500 m away from the pseudotachylite-bearing rocks and do not contain pseudotachylite veins. The formation of these pseudotachylite veins is clearly related to exhumation history of UHP rocks. The formation of pseudotachylite results from frictional melting as shown by microstructural criteria including corroded quartz grains and spherulitic microstructures. The localized heat source required for melting the UHP orthogneisses along narrow bands is attributed to frictional heat produced along the foliation plane. Fault-related pseudotachylite studies and friction experiments have shown that seismic strain rates (in the order of s$^{-1}$) are required to maintain heat along the fault plane. At slower strain rates, heat would be transferred to the host rock which then would behave in a ductile manner. These new results expose a paradox in which pseudotachylites originated from seismic rupture along foliation planes (long generation veins) but these planes did not further localized deformation and did not become fault planes. We propose that exhumation deformation became localized along the two bounding faults when the UHP rocks moved across the isograd corresponding to the quartz brittle-ductile transition.
DE: 8010 Fractures and faults
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting