Diverse Seismicity and Anomalous Boron Fluid Geochemistry in the Japan Trench

During Ocean Drilling Program (ODP) Leg 186, the forearc to the Japan Trench offshore Japan was investigated. The Pacific Plate, subducted at a rate of 8-10 cm/y beneath Eurasia, is more than 100 Ma old. Little accretion has occurred during the Neogene and subduction erosion has caused a decrease and subsidence of 6 km of the continental margin off Japan. The Japan Trench region has regularly suffered earthquake hazards in the past (M>7). Two drill sites, which were installed at locations with diverse seismicity (Sites 1150 and 1151) each penetrated approximately 1200 m into subseafloor sediments of the forearc and recovered Middle Miocene to Recent hemipelagic diatomaceous silty clays with variable degrees of consolidation. Site 1150 was positioned in the seismic zone, Site 1151 in the aseismic portion of the deformation front. The sediments show anomalous high porosities at >1km sub-seafloor depth at both Sites with an average of >50%. High remaining initial and secondary (i.e. fracture) porosity may have profound effects on the fluid chemistry by possibly providing permeable fluid pathways. Part of that overpressure may be attributed to high sedimentation rates into the forearc basin and subsequent compaction, which may have hindered pore water expulsion. Interstitial pore waters from both sites were analyzed for B, and \delta¹¹B isotopes. Here, the highly volatile element boron is used to shed light on origin and mechanism of fluid mobilization, as well as the influence of the overall tectonic framework. Boron concentrations cover a wide range from 329 microM to 3920 microM (0.8 to 9.3 times seawater) with an overall increase with depth at both sites. This B enrichment as well as isotopic changes from \delta¹¹B = +23 to +46\permil in the drilling locations may be related to a deep fluid influence of B mobilization from minerals. Cl concentrations are also diluted to approximately 300mM at the same depth intervals. In the deep portion of the aseismic Site 1151, the \delta¹¹B data show a negative shift (\delta¹¹B = 23\permil), attesting an enrichment in ¹⁰B in the fluid as a result of B desorption from clay minerals due to burial and tectonic stress. However, the B enrichment is approximately 30% more pronounced at Site 1150 in the seismic portion of the forearc and the isotopic signatures show an enrichment in ¹¹B (\delta¹¹B = 46\permil). These signatures coincide with an interval of abundant fractures and shear zones, which may represent deep-seated, permeable out-of-sequence thrusts that drain the forearc. However, it is not clear, how the difference in fluid chemistry and porosity link to the variable seismicity in the Japan Trench forearc.