Incoming sediments and its deformation observed on high resolution seismic profiles in the northern Japan Trench axis region

Abstract:

The Japan Trench axis area has been intensively investigated since the 2011 Tohoku earthquake because the large slip reached to the vicinity of the trench axis. We have conducted three high resolution seismic cruises in the northern part of the Japan Trench axis region. The trench area between 38 – 40 N was covered by 81 E-W seismic lines with 2 – 4 km line interval. A 1200 m-long, 192 channel streamer cable and a cluster gun array with volume of 320 – 380 inch³ were used for these surveys. Post-stack time migrated sections provide detailed image of sediments above the subducting Pacific plate and its deformation by the bending-related normal faults on the outer trench slope, thrust faults and possible slope failures in the trench axis and inner trench slope. The deformation style of the sediments in the trench axis shows variation along the trench strike. To the south of the survey area in 38 – 39 N, the trench axis shows imbricate thrust-and-fold packages, which could be related to the interaction between the frontal prism toe and horst-graben structure. To the north around 40 N, the trench axis is located on a horst, and frontal thrust and imbricate structure are clearly observed on the seismic profiles. Around 39.5 N, the trench inner slope is very steep. It is suggested that slope failures as rotational slumps have occurred in this area. The trench axis is filled by slump deposits and debris with chaotic acoustic characteristics, which is similar with that in the seaward portion of the frontal prism. Seismic profiles on the outer trench slope show the variation on the thickness of the incoming sediments along the trench strike. It is thick, ~ 500 ms, in the northern part of the survey area around 40 N, and it is ~ 250 ms in the southern part around 38 N. The thickness is varied in the area between 38.5 – 39.5 N, and is very thin at 39.5 N. Sediments on the trench outer slope basically conformably cover the igneous basement of the Pacific plate and they were deformed by the bending-related normal faults, however graben fill sediments which onlap the original incoming sediments are also clearly observed on the seismic profiles in the outer trench slope. These graben fill sediments have been deposited in several isolated basins on the outer trench slope. Similar onlap fill sediments are also observed in a few places in the trench axis.