Geologic Evidence of Lithostatic Pore Fluid Pressures at the Base of the Subduction Seismogenic Zone

Abstract

The base the subduction seismogenic zone hosts slow slip events (SSEs), nonvolcanic tremor (NVT), and tectonic creep below the depths that most large megathrust earthquakes nucleate (

Episodic Slow Slip Hosted by Talc‐Bearing Metasomatic Rocks: High Strain Rates and Stress Amplification in a Chemically Reacting Shear Zone

“…The development of the metasomatic talc‐rich lithologies studied here is the direct result of abundant fluids that catalyzed and mediated chemical reactions (e.g., Bebout & Barton, 2002). Quartz veins in adjacent metasomatic actinolitite suggest periods of high pore fluid pressures in this already fluid‐rich environment, consistent with evidence of high fluid pressures in modern and exhumed subduction interfaces (Figures 1g and 1h; Audet et al., 2009; Condit & French, 2022; Kodaira et al., 2004; Raimbourg et al., 2022). Thus, in this mélange zone, the fluid pressure conditions of the rheological model for frictional deformation of talc are likely met, consistent with our interpretation that high strain rates in the talc‐bearing sample may represent slow slip events.…”

“…When occurring episodically alongside non‐volcanic tremor, these phenomena are called episodic tremor and slip (ETS) and represent an integral, yet poorly constrained, part of the subduction seismic cycle (e.g., Bürgmann, 2018; Obara, 2002; Rogers & Dragert, 2003). Geophysical observations suggest that source regions of ETS are fluid‐rich and subject to high pore fluid pressures (e.g., Audet et al., 2009; Delph et al., 2018; Hawthorne & Rubin, 2010; Houston, 2015), consistent with geologic evidence of abundant fluid‐rock interaction in rocks exhumed from these regions (e.g., Bebout & Penniston‐Dorland, 2016; Condit & French, 2022). Field and experimental studies have proposed viscous and frictional deformation mechanisms for slow slip in metasedimentary, metamafic, and ultramafic rocks, though frictional mechanisms are most consistent with low shear stresses and rheologic constraints (e.g., Condit et al., 2022; French & Zhu, 2017; Hayman & Lavier, 2014; Leeman et al., 2018; Phillips et al., 2020; Platt et al., 2018).…”

Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events

“…These inclusions contain liquid + vapor phases (Rossetti et al., 1999). This microstructure was interpreted as healed microcracks related to incremental crack‐and‐seal growth steps (e.g., Bons et al., 2012; Condit & French, 2022; Cox & Etheridge, 1983; Durney & Ramsay, 1973; Fisher & Brantley, 1992; Koehn & Passchier, 2000; Raimbourg et al., 2021; Ramsay, 1980; Trepmann & Seybold, 2019). The same mechanism is also described for dilational hydroshear vein (Fagereng et al., 2011; Giuntoli & Viola, 2021, 2022; Ujiie et al., 2018).…”

Preferential Formation of Chlorite Over Talc During Si‐Metasomatism of Ultramafic Rocks in Subduction Zones

“…For instance, French and Condit (2019) suggested that the very low shear stresses determined for during slow slip events can be accommodated by frictional deformation of chlorite or talc at near‐lithostatic pore‐fluid pressures. This is particularly important as near‐lithostatic fluid pressures have been inferred for the plate interface (Behr & Bürgmann, 2021; Condit & French, 2022; Furukawa, 1993).…”