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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.
Highlights
In-situ shear strength of the shallow portion of plate boundary faults and surrounding sediments is a key factor governing the development of accretionary prisms (Dahlen 1984; Davis et al 1983), propagation of rupture during earthquakes (Kimura et al 2012; Rice 1992; Scholz 1998), and seismicity including slow earthquakes in the shallow part of subduction zones (Ito and Obara 2006; Liu and Rice 2007; Magee and Zoback 1993; Obana and Kodaira 2009)
The main component of a shallow décollement plane, has higher permeability along the shear direction and lower permeability perpendicular to the structure. These results suggest the possibility that a décollement acts as a barrier inhibiting upward fluid convection, leading to high pore pressure and increased structural weakness of subducting sediments (Tobin et al 2001)
equivalent strength (EST) at the site C0023 Figure 3 shows the conversion of drilling parameters recorded at site C0023 into EST, log EST, and background torque (Tb) calculated based on Eq (3) for each time interval of 300 s, together with the employed coring assembly
Summary
In-situ shear strength of the shallow portion of plate boundary faults and surrounding sediments is a key factor governing the development of accretionary prisms (Dahlen 1984; Davis et al 1983), propagation of rupture during earthquakes (Kimura et al 2012; Rice 1992; Scholz 1998), and seismicity including slow earthquakes in the shallow part of subduction zones (Ito and Obara 2006; Liu and Rice 2007; Magee and Zoback 1993; Obana and Kodaira 2009). We applied the EST method to drilling data obtained from IODP Expedition 370 at the Nankai Trough to evaluate the continuous in-situ strength across the toe portion of the plate boundary fault and to show the actual situation of the subducting materials beneath the décollement. The décollement in the Nankai Trough off Muroto is among the most studied shallow plate subduction zone on Earth, and is well distinguished by a reverse-polarity reflector on seismic reflection data (Tsuji et al 2005; Moore et al 1990).
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