Summary and implications

Expedition 315, entitled "Megasplay Riser Pilot," was planned as one of the NanTroSEIZE Stage 1A expeditions for the future deep riser drilling of the megasplay fault in NanTroSEIZE Stage 2. The primary engineering and scientific objectives of this expedition were to obtain geotechnical information needed for well planning of future riser drilling to 3500 mbsf. The location of Site C0001 is critical for understanding the nature of the shallow portions of splay faults. The scientific targets of this expedition were stress regime and deformation mechanics, fault-related fluid source and migration pathways, and correlations between fault activity and slump deposits on the trench slope.

Site C0001 is located at the small bench on the hanging-wall of the megasplay fault (the lower splay fault) and the footwall of the subsidiary fault (the upper splay fault). 2-D and 3-D seismic profiles show that the small slope basin with a ~200 m thick series of layered reflectors is developed, overlying the more transparent unit corresponding to the wedge-shaped seaward edge of the hanging-wall. Coring revealed that the slope basin is composed mainly of Quaternary to late Pliocene silty clay and clayey silt with numerous intercalations of volcanic ash layers. The bottom of the basin is composed of a thick sand layer which overlays the late Pliocene to late Miocene transparently and probably belongs to the accretionary prism unit. The beginning of the slope basin sedimentation defines the age of the change from the active compressional deformation in the accretionary prism deformation around Site C0001 to an extensional deformation mode. We could not find any definite candidates for a deformation zone corresponding to the seaward extension of the upper splay fault. In contrast, normal faults are dominant in the slope basin and are clustered at some depths. Deformations along the upper splay fault seem to be more complex than those expected in the Expedition 315 Scientific Prospectus (Ashi et al., 2007). Further detailed analyses on deformation structures are needed for understanding the development of this complex wedge-shaped basin. The Scientific Prospectus also anticipated a geochemical signal through the splay fault. Preliminary results of shipboard pore fluid geochemistry, however, indicated no specific fluid signal except for some gaps at the lithologic unit boundaries, suggesting minor fluid flow and/or diagenesis.

Minor faults, mostly recognized as dark-color seams, were pervasive in clayey sediments and mudstone of entire intervals. Structural analyses of such fault planes and slickenlines were crucial for estimating changes of paleostress fields. Our preliminary results suggest that the direction of the maximum horizontal compressive stress remains northwest–southeast throughout the entire interval; changes of vertical stress exhibit normal faults in the shallow formation and reversed and strike-slip faults in the deep formation. These observations are consistent with results from the northeast–southwest borehole breakouts observed by LWD during Expedition 314 (Kinoshita et al., 2008) and provide more detailed constraints on the stress field in the trench slope site.

Site C0002 is located at the southern margin of the forearc basin. Age determination of the forearc basin sedimentation overlying the accretionary prism is critical to the estimation of the beginning and activities of the splay fault. Site C0002 penetrated Quaternary alternation of fine-grained sandstone and mudstone and basal Pliocene mudstone and cored the late Miocene accretionary prism rock to 1057 m CSF. Facies analysis revealed rapid sedimentation in the forearc basin during the Quaternary and sediment-starved conditions in the basal slope basin during the Pliocene. Further details of evolution of the forearc basin and accretionary prism will be clarified by integration of shipboard and shore-based studies. Pore fluid geochemistry showed that concentrations of most analyzed elements were strongly controlled by lithologic boundaries. Further shore-based studies including isotopes are needed for better understanding of the fluid migrations and/or reactions.

Deformation structures such as steepened bedding, faults, breccia, shear zones, and vein structures were observed. Although the number of fault analysis was limited because of low core recovery and a shortage of structural geologists during replacement of shipboard scientists, we could determine the change of the stress field:

  1. First phase of northwest-southeast shortening by thrust faulting and possibly strike-slip faulting,
  2. Second phase of northeast–southwest extension by normal faulting, and
  3. Third phase of north–south extension by normal faulting.

The last phase showed good correlation with borehole breakouts observed with LWD. The recovered core samples record more details about stress fields as well as their historical changes.

A large contrast in stress condition between Sites C0001 and C0002 was predicted in our Scientific Prospectus (Ashi et al., 2007) as a strain partitioning along the boundary between the forearc and the accretionary prism domain. Determining the cause of this contrast will be a goal of shore-based studies. Furthermore, coexistence of normal and reverse faults mentioned in our Scientific Prospectus may imply changes of stress conditions during the seismic cycle. Postexpedition investigations could provide significant information for determination of such deformation histories.

Downhole temperature measurement using the APCT3 was first applied to the Chikyu expedition and successfully conducted to 171 m CSF at Site C0001 and to 159 m CSF at Site C0002. Results of downhole temperature measurements yielded almost linear downhole increases in the range of measured depths. Downhole measurement provides better information about temperatures at great depths than conventional type near-surface heat flow measurements, which are highly affected by fluctuation of seafloor temperature. Acquisition of a good temperature profile and thermal conductivity data are crucial for future deep well planning and the mechanical design of long-term borehole tools to be installed during NanTroSEIZE Stage 4.