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

Abstract

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.