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.Graphical abstractSchematic model showing the setting for décollement step-down into the ocean-plate basalts at the subdution plate interface. The diagram also shows textures of intact basalt and its expected frictional properties and behaviors associated with seismogenesis.
Highlights
At accretionary margins, incoming sediment on the oceanic plate is successively peeled off and accreted to the overriding plate through tectonic processes such as offscraping or underplating along the plate-boundary décollement
We addressed the sequential conversion of saponite to chlorite (S–C), which can be a significant source of fluid in the seismogenic zone, providing fluid volumes that are almost one order of magnitude greater than in the overriding sediments
We have examined the whole-rock and clay-fraction mineralogy of several basalt fragments that were subjected to alteration under various temperatures in an ancient subduction zone, and which are exhumed as members of the ancient Shimanto Belt accretionary complex in southwest Japan
Summary
At accretionary margins, incoming sediment on the oceanic plate is successively peeled off and accreted to the overriding plate through tectonic processes such as offscraping or underplating along the plate-boundary décollement. The uppermost oceanic crust, which is mainly made up of basaltic rock, is eventually directly exposed to the upper plate of the lithified accretionary prism. This eventuality can be explained by duplex underplating due to “décollement step-down” into the ocean-plate basalts, as exemplified by several ancient. Our work is directed toward a better understanding of the way in which dehydration proceeds within the seismogenic zone To these ends, we have examined the whole-rock and clay-fraction mineralogy of several basalt fragments that were subjected to alteration under various temperatures in an ancient subduction zone, and which are exhumed as members of the ancient Shimanto Belt accretionary complex in southwest Japan. As a result of this work, we are able to demonstrate the possible connections between basalt dehydration and plateboundary seismogenesis in a subduction zone
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