Publication Date:
2022-06-14
Description:
Abstract: On Kodiak Island (Alaska), decimeter-thick black fault rocks (BFR) are at the core of 10's
meters-thick foliated cataclasites. Cataclasites belong to mélanges regarded as paleo-décollement
active at 12-14 km depth and 230-260oC. Each black layer is mappable for tens of meters along strike.
The BFR feature a complex layering made at microscale by alternation of granular and crystalline
microtextures, composed of micron-scale sub-rounded quartz and plagioclase in an ultrafine,
phyllosilicate-rich matrix. In the crystalline microlayers, tabular zoned microlites of plagioclase make
much of the matrix. No such feldspars are found in the cataclasite. We interpret 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 10's centimeter-thick composite BFR record 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.
XRPD and XRF analyses show that BFR 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
BFR later alteration is suggested. The geochemical results indicate that these subduction-related
pseudotachylytes, differ from those typically described in crystalline rocks and other tectonic settings.
Description:
Published
Description:
3.1. Fisica dei terremoti
Description:
JCR Journal
Description:
open
Keywords:
earthquakes
;
fault rocks
;
04. Solid Earth::04.04. Geology::04.04.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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