THEMED ISSUE: Subduction Top to Bottom 2 1686 Fisher et al. | Deformation along the subduction interface GEOSPHERE | Volume 17 | Number 6 Research Paper Insights from the geological record of deformation along the subduction interface at depths of seismogenesis Donald M. Fisher, John N. Hooker, Andrew J. Smye, and Tsai-Wei Chen Department of Geoscience, Pennsylvania State University, University Park, Pennsylvania 16801, USA ABSTRACT Subduction interfaces are loci of interdependent seismic slip behavior, fuid fow, and mineral redistribution. Mineral redistribution leads to coupling between fuid fow and slip behavior through decreases in porosity/permeabil- ity and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fuid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mud- stones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fuctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simula- tor with a fuid fow system depicts a subduction zone interface governed by feedbacks between fuid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fuid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fuid pressure that propagates trenchward. ■ INTRODUCTION Large earthquakes in subduction zones are controlled by the rupture of asperities or patches of the plate interface with high shear strength (e.g., Thatcher, 1990) (Fig. 1). Some asperities may refect roughness elements along the interface, such as seamounts on the subducting plate (Abercrombie et al., 2001; Husen et al., 2002; Bilek et al., 2003; Collot et al., 2017), but seamounts also likely play a role in limiting the area of some ruptures (Wang and Bilek, 2011). There is a positive correlation between great earthquakes and smooth plate interfaces (Scholl et al., 2015; Lallemand et al., 2018; van Rijsingen et al., 2019), so heterogeneity in strength along large, unsegmented stretches of the interface can develop during the interseismic period independent of roughness potentially due to geochemical processes in deforming sediments. Between large ruptures, there are interseismic changes in the physical properties of the GEOSPHERE, v. 17, no. 6 https://doi.org/10.1130/GES02389.1 12 figures CORRESPONDENCE: dmf6@psu.edu CITATION: Fisher, D.M., Hooker, J.N., Smye, A.J., and Chen, T.-S., 2021, Insights from the geological record of deformation along the subduction interface at depths of seismogenesis: Geosphere, v. 17, no. 6, p. 1686–1703, https://doi.org/10.1130/GES02389.1. Science Editor: Shanaka de Silva Guest Associate Editor: Gray E. Bebout Published online 4 November 2021 Received 21 December 2020 Revision received 20 May 2021 Accepted 22 July 2021 © 2021 The Authors This paper is published under the terms of the CC-BY-NC license. Donald Fisher https://orcid.org/0000-0001-6464-4719 GEOSPHERE 100˚C 10km 20km 30km 40km 50km 200˚C 300˚C 400˚C Offscraping �lux Underplating �lux Viscous coupling/ Steady �low DMT/DC ETS/NVT Seismogenic Zone Underplated rocks Offscraped rocks Erosional �lux Figure 1. Block diagram shows the forearc of a subduction zone. Deformation occurs in the footwall as rocks encounter temperatures at which dissolution-precipitation operates in the seismogenic zone. This deformation precedes imbrication of the shear zone and its incorporation into the overriding wedge. In the conceptual model, viscous fow during the interseismic period between 150 °C and 350 °C leads to heterogeneous anelastic de- formation (diffusive mass transfer [DMT]), strengthening of the shear zone, and buildup of a slip defcit recovered during punctuated coseismic slip events. The peach-colored areas are asperities or patches of greater strength that refect greater porosity reduc- tion and tectonic compaction. Downdip of the seismogenic zone, deformation is steady and accommodated by both DMT and dislocation creep (DC). NVT—nonvolcanic tremor. ETS—episodic tremor and slip. 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