In-situ mechanical weakness of subducting sediments beneath a plate boundary décollement in the Nankai Trough

Abstract

The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP) Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at~800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from~10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~800-1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments.

A New Method for Quality Control of Geological Cores by X-Ray Computed Tomography: Application in IODP Expedition 370

“…In this case it is plausible that a lithological change that has been caused by diagenesis or hydrothermal alteration which involves cementation. The in-situ strength of sediments in this interval was calculated from drilling parameters and shows an abrupt increase with depth below about 1,050 mbsf (Hamada et al, 2018), and this is consistent with a lithological change brought about by cementation. The downhole quality-controlled MCN profile of Site C0023 evidences high abundances of faults in the trench-to-basin and the Upper Shikoku Basin facies (Figures 3, 6, 7).…”

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

“…Notably, SVLFEs rarely occur within the regions of stress accumulation peaks. Areas with active SVLFEs are also characterized by mechanical weak zones due to the presence of pore fluids (e.g., Hamada et al, 2018;Park et al, 2010;Tonegawa et al, 2017). Shallow slow earthquakes occur and migrate due to high pore fluid pressure, releasing accumulated stress in the transitional regions between high-strength (strongly locked) and low-strength (stable sliding) zones on the shallow plate boundary.…”

Temperature limits to deep subseafloor life in the Nankai Trough subduction zone

“…(III) EST is a measure for the in-situ strength of the geological formation, i.e. its relative triaxial shear strength that can be deduced from parameters recorded during the drilling operation, such as weight-on-bit, top drive torque, and rotations per minute (33). The down-hole EST profile of Site C0023 (fig.…”

“…1, 2). In this zone, under-compacted and mechanically weak sediments are overpressurized and affected by ~145-220°C hot fluids for short durations (15,33). The short heating events may have locally sterilized sediment (15), but microbial cells, acetate consumption and methanogenic activity prevail again in >100°C sediments, where mechanical strength and salinity increase towards the sediment/basement interface (Figs.…”

“…(D) Equivalent strength (EST) is a measure for the in-situ strength of the geological formation, which is deduced from drilling parameters. Positive EST excursions from the down-hole trend (dotted line) show distinctly elevated mechanical strength in the tuff-rich sediments around ~630 mbsf, while negative EST excursions reveal a mechanically weak zone below the décollement (modified from ref 33)…”