A new source of water in seismogenic subduction zones

• Published in: Geophysical research letters 2011-11, Vol.38 (22), p.n/a
• Main Authors: Kameda, Jun, Yamaguchi, Asuka, Saito, Saneatsu, Sakuma, Hiroshi, Kawamura, Katsuyuki, Kimura, Gaku
• Format: Article
• Language: English
• Subjects: Basalt; Clay minerals; Crusts; diagenesis; Earth sciences; Earth, ocean, space; Exact sciences and technology; Faults; Fluid dynamics; Fluid flow; Fluids; Geology; Marine; Materials science; Oceanic crust; Plate tectonics; Seismic engineering; Seismic phenomena; seismogenic zone; Seismology; smectite; Spectrum analysis; subduction zone
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2011GL048883

Seismogenic plate‐boundary faults at accretionary margins (e.g., the Nankai margin, southwest Japan) may occur where the uppermost part of subducting oceanic crust, composed of basaltic rocks, is in contact with the overriding plate of a lithified accretionary prism. The plate‐boundary faults in ancient accretionary complexes typically record high‐velocity slip under fluid‐rich conditions. Although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip‐weakening, the source of fluid in seismogenic subduction zones remains poorly constrained. In this work, we focus on the hydrous smectite in the uppermost oceanic crust, an alteration product of intact basalt before arrival at the trench axis. A comparison between (1) new mineralogical data on basalt drillcore recovered by Integrated Ocean Drilling Program (IODP) Expedition 322 at site C0012, a reference site for subduction input to the Nankai Trough, and (2) mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the Shimanto Belt, southwest Japan, suggests that progressive smectite–chlorite conversion would liberate bound fluids at a rate of 0.34 to 0.65 × 10−14 s−1 along the plate interface. This rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources, including from overlying sediments via smectite–illite conversion and the expulsion of pore fluids, and may facilitate seismic slip along plate‐boundary faults. Key Points The uppermost oceanic basement is altered to form hydrous clay minerals Fluid is liberated via smectite to chlorite transition in seismogenic zone Fluid releasing rate from oceanic crust is higher than that from sediment