Strong sediments at the deformation front, and weak sediments at the rear of the Nankai accretionary prism, revealed by triaxial deformation experiments

Abstract

Nineteen whole‐round core samples from the Nankai accretionary prism (IODP Expeditions 315, 316, and 333) from a depth range of 28–128 m below sea floor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 400–1000 kPa, room temperature, axial displacement rates of 0.01–9.0 mm/min, and up to axially compressive strains of ∼64%. Despite great similarities in composition and grain size distribution of the silty clay samples, two distinct “rheological groups” are distinguished: The first group shows deviatoric peak stress after only a few percent of compressional strain (<10%) and a continuous stress decrease after peak conditions. Simultaneous to this decrease is a pore pressure increase indicating contractant behavior characteristic of structurally weak material. The second sample group weakens only moderately at a much higher strength level after significantly higher strain (>10%), or does not weaken at all. This is characteristic of structurally strong material. The strong samples tend to be overconsolidated and are all from the drillsites at the accretionary prism toe, while the weak and normally consolidated samples come from the immediate hanging wall of a megasplay fault further upslope. Sediments from the incoming plate are also structurally weak. The observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting within the uniform silty and clayey sedimentary sequence of the Nankai accretionary prism.

PandSwave velocity measurements of water-rich sediments from the Nankai Trough, Japan

“…Samples from the incoming plate are richer in clay minerals and are not affected by deformation and overconsolidation [cf. Expedition 333 Scientists , , ; Stipp et al ., ]. They show the highest porosities and the lowest velocities in our data (Figure ).…”

“…Bedding planes were observed in subsample K012 (10 to 15° dip). Particle size analyses of this core section indicate 75% clay, 20% fine silt, and 5% medium silt [ Stipp et al ., ].…”

“…Two core samples from Site C0006 were investigated, both representing trench deposits: sample C0006E‐8H‐1 (48.23 mbsf, Subunit IIA, experimental subsamples K007 and K009) and sample C0006E‐20X‐2 (127.97 mbsf, Subunit IIB, experimental subsamples K013 and K019). Particle size analyses of core section C0006E‐8H‐1 indicate 85% clay, 10% fine silt, and 5% medium silt, and for core section C0006E‐20X‐2 60% clay, 30% fine silt, and 10% medium silt [ Stipp et al ., ].…”

“…This sample showed several horizontal and vertical cracks. Both samples are similar in their particle size distribution of 75% clay, 20% fine silt, and 5% medium silt [ Stipp et al ., ].…”

“…The sample investigated in this study (C0001E‐11H‐2, 90.6 mbsf, experimental subsample K003 and K005) belongs to Subunit IA. Particle size analyses indicate 60% clay, 30% fine silt, and 10% medium silt [ Stipp et al ., ].…”