Textural record of the seismic cycle: strain-rate variation in an ancient subduction thrust

Abstract

Active faults slip at different rates over the course of the seismic cycle: earthquake slip (c. 1 m s 21 ), interseismic creep (c. 10-100 mm year 21 ) and intermediate rate transients (e.g. afterslip and slow slip events). Studies of exhumed faults are sometimes able to identify seismic slip surfaces by the presence of frictional melts, and slow creep by textures diagnostic of rate-limited plastic processes. The Pasagshak Point Thrust preserves three distinct fault rock textures, which are mutually cross-cutting, and can be correlated to different strain rates. Ultrafine-grained black fault rocks, including pseudotachylyte, were formed during seismic slip on layers up to 30 cm thick. Well-organized S-C cataclasites 7-31 m thick were formed by slow creep, with pressure solution as a dominant, rate-limiting mechanism. These must have formed at strain rates consistent with long-term plate-boundary motion, but solution-creep healing acted to reduce porosity of the cataclasites and eventually restricted fluid connectivity such that creep by this mechanism could not continue. Disorganized, non-foliated, rounded clast cataclasites were formed at shear rates faster than solution creep and are interpreted as representing shear at intermediate strain rates. These could have formed during afterslip or delocalization of slip associated with an earthquake rupture.Supplementary material: Detailed map of Pasagshak Peninsula is available at http://www. geolsoc.org.uk/SUP18493.

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

“…S-C structures, slickenlines, and rotated fold axes within the fault zone suggest a top-to-southeast sense of shear (Rowe et al 2005(Rowe et al , 2011Meneghini et al 2010). The paleotemperature of the syn-tectonic fluid is estimated to be 230°C to 260°C from fluid inclusions in quartz veins within the mélange zone (Vrolijk et al 1988), whereas the maximum burial temperature of the wall rock is estimated to be 250°C ± 10°C from vitrinite reflectance analysis of carbonaceous material within the argillite (Rowe et al 2011). The similarity of the two temperature estimates suggests that deformations recorded in the thrust zone occurred at approximately 250°C at seismogenic depths of the plate boundary décollement.…”

“…Various types of cataclasites, 7 to 31 m in thickness, occur within the thrust zone (Additional file 1: Figure S1) (Rowe et al 2011). Detailed structures of cataclastic fault rocks have been described in previous accounts (Rowe et al 2005(Rowe et al , 2011Meneghini et al 2010).…”

“…The Pasagshak Point Thrust (Rowe et al 2005(Rowe et al , 2011Meneghini et al 2010) is a strike-parallel fossil thrust zone exposed along the rocky shoreline of Pasagshak Point (Figure 1). The thrust is located within the basal mélange zone of the Ghost Rocks Formation, implying shear localization along an ancient subduction thrust (Rowe et al 2005).…”

“…From a geological perspective, pseudotachylytes formed by frictional melting during high-velocity seismic slip have classically been considered as the only definitive evidence of earthquakes (Sibson 1975;Cowan 1999). Although occurrences of pseudotachylytes in crystalline continental rocks were known until approximately the 1990s (Swanson 1992), several pseudotachylyte-bearing fault zones in subduction settings have been discovered in recent years (Ikesawa et al 2003;Kitamura et al 2005;Rowe et al 2005Rowe et al , 2011Mukoyoshi et al 2006;Okamoto et al 2006;Meneghini et al 2010;Hashimoto et al 2012).…”

“…Because BFR is unique in terms of its large thickness (up to 30 cm) and widespread occurrence, it has been further investigated on both microscopic (Meneghini et al 2010) and macroscopic (Rowe et al 2011) scales. Meneghini et al (2010) also characterized the geochemistry of fault rocks and detected Sr anomalies.…”