HR: 0800h
AN: T41B-03    [Abstracts]
TI: Can the seismic slip direction be retrieved from pseudotachylyte veins?
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AU: Zechmeister, M S
EM: zechmeim@ou.edu
AF: The University of Oklahoma, Department of Geology and Geophysics, Norman, OK 73019, United States
AU: Gebelin, A
EM: Aude.Gebelin@yahoo.com
AF: Southern Illinois University, Department of Geology, 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, United States
AU: Wilson, K M
EM: kara1234@siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901, United States
AB: Pseudotachylytes can form by frictional melting as a result of seismic slip along a fault plane. During a brief seismic event, the fault plane material initially deforms by cataclastic flow until melting occurs and stress drops. The partially molten material then flows as a solid-melt suspension. Immediately after the seismic event, the pseudotachylyte vein consists of a mix of irregularly shaped, sub-equant, randomly oriented clasts, glass and a few newly formed microlites. This study aims to investigates the origin of the anisotropy of magnetic susceptibility (AMS) in pseudotachylyte veins and to assess whether the AMS records syn-seismic or post-seismic flow. The pseudotachylytes of the Val Gilba (Dora Maira Massif, Western Alps) are chosen as an example of syn- exhumation veins. They formed at about 20 Ma during the unroofing of mylonitic gneisses that had recorded a pressure peak of metamorphism at ca. 35 Ma. These veins, up to 25 mm in width, are occur parallel to the mylonitic foliation and are continuous over several tens of meters. The magnetic fabric of both host and vein are very consistent, yet distinct. The AMS of the host gneiss is controlled by magnetite and phyllosilicates (P = 1.32) and lies parallel to the pervasive stretching lineation. In contrast, the AMS of the pseudotachylyte, controlled almost entirely by elongated magnetite grains (P = 1.08), is oblique (30°) to the stretching lineation. The AMS fabric in the gneiss is strongly planar (T = 0.886), whereas in the pseudotachylite, the fabric is more plano-linear (T = 0.373). The pseudotachylite oblique fabric might be interpreted as a result of imbrication. Microstructures of the pseudotachylyte indicate that deformation was plastic and occurred at high temperature. The AMS in the pseudotachylite is interpreted to result from syn-seismic melt flow, as suggested by the laminated internal structure of the veins and by small drag folds affecting the laminae. These new results open unprecedented possibilities for paleoseismic studies regarding earthquake focal mechanism of prehistoric events.
DE: 7221 Paleoseismology (8036)
DE: 8000 STRUCTURAL GEOLOGY
DE: 8036 Paleoseismology (7221)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly