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

Abstract

[1] 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. Feeser (2013), Strong sediments at the deformation front, and weak sediments at the rear of the Nankai accretionary prism, revealed by triaxial deformation experiments,

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…An intact sample of the siliceous mudstone from between the décollement and pelagic clay at Site C0019, which correlates with the diatomaceous claystone at Site 436, exhibited a cohesion value of 220 kPa . In comparison, measured cohesion of mudstones from Sites C0011 and C0012 along the Kumano transect at the Nankai Trough ranges from 13 to 160 kPa (Ikari et al, 2013a), consistent with calculated cohesion values of 35 and 41 kPa for two samples from these sites (Stipp et al, 2013). There is a large amount of amorphous silica in the Japan Trench mudstone unit, up to ~60% at both Sites C0019 and 436 (Kameda et al, 2015a), which might indicate advanced cementation.…”

“…Intact samples preserve in situ effects such as fabric or cementation; however, for other boreholes in the Nankai Trough, some studies have found that these effects on friction are small (Ikari and Saffer, 2011;Saffer et al, 2012). The Nankai data set is supplemented by triaxial experiments on an intact sample of LSB mudstone from Site 1173(Bourlange et al, 2004 and intact samples from the ash-rich USB facies at Sites C0011 and C0012 (Stipp et al, 2013). We note that the values from Stipp et al (2013) are reported as a coefficient of internal friction.…”

“…The Nankai data set is supplemented by triaxial experiments on an intact sample of LSB mudstone from Site 1173(Bourlange et al, 2004 and intact samples from the ash-rich USB facies at Sites C0011 and C0012 (Stipp et al, 2013). We note that the values from Stipp et al (2013) are reported as a coefficient of internal friction.…”

Texture development in naturally compacted and experimentally deformed silty clay sediments from the Nankai Trench and Forearc, Japan

“…Stipp et al, 2013) show variable sample compositions ( Table 2). The content of lithics is highly variable across the sample set and ranges between 1.8 and 46.8%.…”

“…Natural (experimentally undeformed) core samples as well as experimentally deformed samples were used for texture investigation in this study. Experimental samples were deformed under consolidated-undrained conditions in a triaxial apparatus (for further details on the experimental deformation see Stipp et al, 2013).…”