HR: 1340h
AN: T23A-0555    [Abstracts]
TI: Decimeter Scale Ultra-Fine Fault Rocks (Possible Pseudotachylites) in an Ancient Subduction Thrust Zone
AU: * Rowe, C D
EM: crowe@es.ucsc.edu
AF: Earth Science Department, UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Moore, J C
EM: casey@es.ucsc.edu
AF: Earth Science Department, UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Meneghini, F
EM: meneghini@dst.unipi.it
AF: Departmento di Scienze della Terra, Universite di Pisa, S. Maria 53, Pisa, 56126 Italy
AU: McKiernan, A W
EM: awm@uwyo.edu
AF: Dpt. Geology and Geophysics, University of Wyoming, 1000 University Ave Dpt 3006, Laramie, WY 82071
AB: Large bodies of ultrafine fault rock (possible pseudotachylite or frictional melt) occur within cataclastic thrust zones in the Ghost Rocks Formation, Kodiak Accretionary Complex, Alaska. The Paleocene Ghost Rocks Formation includes map-scale m‚lange belts formed by flattening and shearing of seafloor sediments and volcanic rocks at about 250 degrees C and 325 MPa (~13 km depth) during subduction between 65-60Ma. Ten to 15-meter thick cataclastite zones crosscut the m‚lange fabric at a low angle, representing a stage of increasingly localized shear during subduction thrusting. Ultrafine fault rocks occur as thick (10-25cm) continuous planar beds along the boundaries of cataclastites, or in discontinuous accumulation bodies within cataclastite zones. The boundaries of the ultrafine fault rocks are intrusive, sharp but irregular and deform the cataclastite host fabric. Single pulse intrusions of the ultrafine fault rock range up to 0.5m in intrusive dimension and form complex morphologies resembling both upward and downward directed flame structures and dike-sill complexes, as well as sheath folds and disharmonic flow banding and folding. These field characteristics indicate fluidization and perhaps frictional melting of the ultrafine fault rocks. Ultrafine fault rock bodies can be traced laterally for meters to tens of meters at individual outcrops and occur for about 2 km along strike. Preliminary SEM analysis reveals that the primary matrix material is physically and chemically homogenous down to few-micron scale, consistent with the field identification of pseudotachylite. Thin sections show rounded remnant quartz aggregates, typical of pseudotachylytes. Although some thin sections show suggest melting others may represent ultracataclastite. Some ultrafine fault rock material is rebrecciated and cataclastized to a fine scale, indicating reactivation of previous fault rock generation surfaces. These ultrafine fault rock zones represent the most highly deformed slip surfaces in Ghost Rocks melange and are prime candidates for failure surfaces of large earthquakes.
DE: 7221 Paleoseismology
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 5112 Microstructure
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting