UNRAVELING SEISMIC DEFORMATION IN EXHUMED SUBDUCTION THRUSTS: THE ULTRAFINE-GRAINED FAULT ROCKS OF PASAGSHAK POINT, KODIAK ISLAND, AK

Evidence of seismic events in exhumed fault rocks is controversial, with the only universally accepted rocks being pseudotachylytes (PTs), i.e. the result of frictional melting. Despite the prevalence of great earthquakes in the subduction zone environment, evidence for PTs is uncommon. We have investigated an exhumed subduction-thrust to develop microstructural criteria to discriminate rocks produced during seismic slip. In the Kodiak accretionary complex of Alaska, an exhumed analogue of the modern Aleutian margin, décollement zone thrusts are preserved in several accreted units. Ambient conditions during faulting were 12–14 km in depth and 230–260 oC, well into the “seismogenic zone”. The Paleocene Ghost Rock Fm outcropping at Pasagshak Point is a map-scale argillaceous mélange developed during underthrusting. Four localized cataclastic shear zones crosscut the mélange as ~15 m thick bands that can be mapped for kms. Extreme strain localization in three of these shear zones occurs as decimeter-thick planar to irregular beds of dark grey to black ultrafine-grained fault rocks, or “black rocks” (BR) that crosscut and locally intrude the shear zones. The BR are harder than surrounding rocks and consists of vitreous-appearing bands alternated with granular horizons that rework the vitreous-appearing layers. Preliminary data of these fault rocks are ambiguous, reflecting their complexity. Although XRF data suggests a chemical fractionation between cataclasites and BR (i.e uniform Na (as albite) enrichment in the BR matrix), that cannot be explained with ultracomminution, only locally the textures are those typical of PTs (local occurrence of vesicles, amygdules, clasts embayments, injection veins). The features of most of the BR volume (ultra-fine grain size, ductile, “fluidal” intrusion structures and flow structures) are comparable to those described in literature for gouges that underwent fluidization. The mélanges, cataclasites, and BR layers may represent increasing strain-rate regimes, of this ancient décollement, with the BR defining episodes of seismic slip. The exceptional thickness and complexity of the individual BR layers may reflect the composite nature of earthquakes (main shock, aftershocks, after slip…).