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Record of mega-earthquakes in subduction thrusts; the black fault rocks of Pasagshak Point (Kodiak Island, Alaska)

F. Meneghini, G. Di Toro, C. D. Rowe, J. C. Moore, A. Tsutsumi and A. Yamaguchi
Record of mega-earthquakes in subduction thrusts; the black fault rocks of Pasagshak Point (Kodiak Island, Alaska)
Geological Society of America Bulletin (July 2010) 122 (7-8): 1280-1297


On Kodiak Island, Alaska, decimeter-thick black fault rocks are at the core of foliated cataclasites that are tens of meters thick. The cataclasites belong to melange zones that are regarded as paleodecollements active at 12-14 km depth and 230-260 degrees C. Each black layer is mappable for tens of meters along strike. The black fault rocks feature a complex layering made at microscale by alternation of granular and crystalline microtextures, both composed of micron-scale subrounded quartz and plagioclase in an ultrafine, phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned microlites of plagioclase make up much of the matrix. No such feldspars have been found in the cataclasite. We interpret these crystalline microlayers as pseudotachylytes. The granular microlayers show higher grain-size variability, crushed microlites, and textures typical of fluidization and granular flow deformation. Crosscutting relationships between granular and crystalline microlayers include flow and intrusion structures and mutual brittle truncation. This suggests that each decimeters-thick composite black fault rock layer records multiple pulses of seismic slip. In each pulse, ultracomminuted fluidized material and friction melt formed and deformed together in a ductile fashion. Brittle truncation by another pulse occurred after solidification of the friction melt and the fluidized rock. X-ray powder diffraction (XRPD) and X-ray fluorescence (XRF) analyses show that black fault rocks have similar mineral composition and chemical content as the cataclasites. The observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components. The presence of an open, fluid-infiltrated system with later alteration of black fault rocks is suggested. The geochemical results indicate that these subduction-related pseudotachylytes differ from those typically described in crystalline rocks and other tectonic settings.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 122
Serial Issue: 7-8
Title: Record of mega-earthquakes in subduction thrusts; the black fault rocks of Pasagshak Point (Kodiak Island, Alaska)
Affiliation: Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Pages: 1280-1297
Published: 201007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 87
Accession Number: 2010-050200
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 2 tables, geol. sketch map
N57°19'60" - N57°19'60", W153°40'00" - W153°40'00"
Secondary Affiliation: Universita di Padova, ITA, ItalyUniversity of Cape Town, ZAF, South AfricaUniversity of California, Santa Cruz, USA, United StatesKyoto University, JPN, JapanKochi University, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201027
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