Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

Abstract

The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels (~320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

“…manuscript submitted to Geochemistry, Geophysics, Geosystems include weak and/or overpressurized sediments (e.g., Byrne & Fisher, 1990;Meneghini et al, 2009;Kimura & Ludden, 1995;Plunder et al, 2012), altered oceanic crust (e.g., Kimura & Ludden, 1995;Kameda et al, 2017), or serpentinized mantle (e.g., Angiboust et al, 2011;Cluzel et al, 2001;Hilairet & Reynard, 2009;Vogt & Gerya, 2014;Ruh et al, 2015). In most subduction complexes where weakening is associated with serpentinization, the source of ultramafic material is interpreted to be the downgoing slab mantle (Angiboust et al, 2011;Cluzel et al, 2001).…”

Numerical modeling of tectonic underplating in accretionary wedge systems

“…2). In contrast, many natural examples of exhumed accretionary wedges indicate the involvement of mafic rock types (Escuder-Viruete et al, 2011;Kameda et al, 2017;Kimura and Ludden, 1995;Meneghini et al, 2009). Furthermore, seafloor roughness related to horst and graben structures in the downgoing oceanic crust can result in a patchy distribution of plate coupling, enhancing underplating (Tsuji et al, 2013).…”

Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions: Implications for Subduction Megathrusts

“…Field studies frequently demonstrate that chlorite is an important component in exhumed subduction zones (Fagereng, ; Fisher & Brantley, ), especially within the phyllonitic fault cores that are believed to play a dominant role in controlling fault strength and stability (Collettini et al, ; Holdsworth, ; Jefferies et al, ). Chlorite progressively forms as an alteration product in basalts within subducting oceanic crust (Kameda et al, , ), but it is also an important metasomatic alteration mineral that forms in basalts and other mafic rocks along the subduction thrust fault (Bebout, ; Spandler et al, ). Moreover, it is one of the major constituent minerals in basic metamorphic rocks formed under pumpellyite‐actinolite to greenschist facies conditions.…”