Data report: tectonic and induced structures in the JFAST core: Expedition 343

Abstract

Structures surrounding the inferred location of the Japan Trench plate boundary decollement define the fault damage zone. We quantify the density of tectonic and induced structures in the damage zone from drill cores collected between 770 and 835 meters below seafloor from Hole C0019E during Integrated Ocean Drilling Program Expedition 343/343T, the Japan Trench Fast Drilling Project (JFAST). Cores contain structures formed by tectonic deformation and structures resulting from drill core recovery and handling. Differentiating between these two origins is essential for structural analyses and interpretation. We defined 10 styles of induced structures that we used to develop a new set of criteria for identifying induced structures in drill cores containing fine-grained sedimentary rocks. These criteria were used to categorize structures according to confidence in a tectonic origin. Our results show structure density (including all fractures, faults, breccias, and phyllosilicate bands) has a maximum adjacent to the decollement. This pattern in structure density is apparent for both exclusively tectonic structures and tectonic and induced structures combined. Local increases in induced structure density mask trends in tectonic structures, demonstrating that detecting trends in spatial variability in structure density is dependent on the recognition of induced structures. Additionally, the tectonic structure density increases where cores contain faults with significant displacements inferred from stratigraphic information, indicating that tectonic structure density can be used to identify secondary faults. We suggest that the criteria for identifying induced structures used in this study could be applied during visual core description in a variety of tectonic settings as a method to collect and assess reliable structural data.