Diatom and Radiolarian Biostratigraphy in the Vicinity of the 2011 Tohoku Earthquake Source Fault in IODP Hole 343-C0019E of JFAST

Abstract

The frontal prism in the Japan Trench on the 2011 Tohoku-Oki earthquake (Mw 9.0, March 11, 2011) rupture zone had been drilled during the Integrated Ocean Drilling Program (IODP) Expeditions 343 and 343 T. We investigated fossil diatoms and radiolarians to determine age constraints on the cored sediments and reveal the behavior of sediment deformation history. Although diatoms and radiolarians abundances are varied in samples from common to rare with poor to moderate preservation in studied sediments, general biostratigraphic schemes in the North Pacific are applicable and well constrain the age of those sediments, except for samples from fault clay in which fossils were barren. These results suggest that there are three large stratigraphic gaps at ~830 mbsf between the Cretaceous chert and the upper Miocene pelagic clay, at ~820 mbsf between the upper Miocene and the Pliocene –Quaternary, and at ~670 mbsf between the upper Miocene and the Pliocene –Quaternary. The former likely represents a hiatus or unconformity derived from tectonic erosion just above the incoming Pacific Plate, and the latter two correspond to an injection of late Quaternary material above the plate boundary fault due to an increase in volcanic activity in the NE Japan arc after 8 Ma. The Upper Miocene pelagic sequence below the plate boundary décollement comprises reversed stratigraphy, suggesting deformation by thrusting, slumping, folding, etc., indicating structural complexity in the frontal prism of the subduction zone most likely due to the subduction of horst-and-graben.