HR: 1340h
AN: T23A-0554    [Abstracts]
TI: Seismogenesis and Strain Localization Across Accretionary Prisms: the Role of Decollement and Out-of-Sequence Thrusts.
AU: * Meneghini, F
EM: meneghini@dst.unipi.it
AF: Dipartimento di Scienze della Terra, Universita di Pisa, via santa Maria, 53, Pisa, PI 56126 Italy
AU: Rowe, C D
EM: crowe@es.ucsc.edu
AF: Department of Earth Science, University of California, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Moore, C J
EM: cmoore@es.ucsc.edu
AF: Department of Earth Science, University of California, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Mckiernan, A W
EM: awm@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Dept. 3006 1000E. University Avenue, Laramie, WY 82071 United States
AB: The most destructive earthquakes occur along the decollement thrusts at convergent margins. Neverthless, the seaward limit of recorded microearthquakes seems to correlate with zones where out-of-sequence thrusts (OSTs) splay from the decollement, suggesting the significant role of these thrusts in releasing seismic energy. In the Kodiak accretionary complex of Alaska, representing the ancient analogue of the Aleutian margin, well exposed examples of both subduction and OS thrusts occur. Both thrusts were primarily active between 59 and 65 Ma. In the Paleocene Ghost Rock Fm, a map-scale m‚lange primarily comprised of turbiditic argillites, variably continuous massive sandstones and rare greenstones, two approximately 15m thick bands of highly sheared cataclasites have been mapped. The cataclasites, sub-parallel to the melange fabric, represent episodes of localized shear in the melange during subduction thrusting at about 13 Km of depth. Extreme strain localization into the cataclastic thrust zones is testified by development of ultrafine fault rocks occurring as tens of cm thick planar to irregular beds. They show ductile flow which intrude and deform the cataclasite, and can be described as ultracataclasites and/or pseudotachylites. The Uganik Thrust (UT) juxtaposes the early-mid Cretaceous Uyak Complex over the latest Cretaceous Kodiak Fm, represented by its tectonized upper boundary, known as Waterfall Bay Melange (WBM). The WBM is interpreted as formed by flattening and shearing of the coherent turbiditic Kodiak Fm during underthrusting along the decollement. The steep geometry of the UT and the overprinting relationship to the melange, allow the interpretation of the UT as an OST. UT core deforms through Riedel fractures separating domains where sandstone blocks are elongated parallel to an S-foliation into the argillite. Several narrow cataclastic zones within the footwall accommodate significant shear. These shear zones crosscut the previous melange fabric and contain evidence of fluid flow, in the form of abundant quartz precipitates along the shear fabrics. Both thrust examples include strain localization features (pseudotachylites and quartz-infused shear zones) consistent with stick-slip behavior. Thus, ancient OSTs in addition to the decollement thrust may have helped generate large earthquakes along this ancient plate boundary in a relatively constrained time frame.
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting