Stress-state and micro-scale deformations in the hangingwall of the splay fault: Preliminary results from the Site C0002, IODP Expedition 348

Abstract:

Integrated Ocean Discovery Program (IODP) Expedition 348 conducted riser-drilling and has deepened hole down to 3058.5 mbsf at Site C0002, and collected cuttings and core (2163.0–2217.5 mbsf) samples of hangingwall of the mega-splay fault composed of the Upper Miocene Nankai accretionary prism. Structural description for cuttings/cores and stress/strain analyses based on anelastic strain recovery (ASR) and anisotropy of magnetic susceptibility (AMS) for core samples were performed. The followings are structural characteristics:

a) The intact cuttings include slickenlined surfaces, scaly fabric, deformation bands, minor faults and mineral veins. Scaly fabric is increasingly observed below ~2200 mbsf.

b) The cored interval is characterized by steep bedding planes (>75˚). A fault zone with a few mm-size angular clasts is present in the core (2204.9–2205.8 mbsf).

c) SEM images in the upper part of Hole C0002N show little evidence for opal diagenesis. The fabric lacks a strongly preferred orientation in clay-rich materials, except along striated micro-fault. Below 2625 mbsf, compaction fabrics in clay–rich materials can be observed.

The overall character of the deformation (independent particulate flow with limited evidence for cataclastic deformation) in the cuttings is suggestive that deformations occurred in a relatively shallow environment.

Stress analyses based on the ASR for three whole-round samples from Hole C0002P showed good curve of the strain recovery and identified vertically-oriented σ2-axes with horizontal σ1 and σ3-axes. Paleomagnetic works are still continued to detect three-dimensional stress information.

Magnetic fabrics deduced from AMS ellipsoids show moderate flattening fabric parallel to the bedding plane (Kmin inclined perpendicular to the bedding plane). Although magnetic foliation (Kint/Kmin) decreases and Kmin-inclination is disturbed in the fault zone, magnetic lineation (Kmax/Kint) didn’t show variation, indicative of brittle deformation occurring in the fault zone. Major reduction in the mean susceptibility is identified in the fault zone, which implies chemical reaction which decreases concentration of magnetic minerals occurred along the fault zone.