Tectonolithification of sandstone prior to the onset of seismogenic subduction zone: Evidence from tectonic mélange of the Shimanto Belt, Japan

Abstract

[1] The deformation and diagenetic processes in sandstone blocks in underthrusted tectonic mélange from the Shimanto Belt, Japan, were analyzed to understand the onset of seismogenesis in subduction zones. The earliest deformation is characterized by ''web'' cataclasis with grain size reduction in association with ubiquitously developed microshears. A porosity of about 10 ± 3% is estimated for this stage. Subsequently, pore cementation of the sandstone by calcite, quartz, and chlorite occurred, together with pressure solution along the web margin. The porosity suggests a depth of about 3 km below seafloor. Finally, the sandstones were boudinaged, and tension gashes were filled by quartz veins in the neck of the boudins. At this stage, cementation was fully accomplished, and the porosity of the sandstone was reduced to less than a few percent. The final deformation was due to shear parallel to the layering. P-T conditions for the final necking were about 150-190°C and 150-200 MPa, coinciding with the upper part of the seismogenic zone. Our study on the mélange suggests that relative plate motion is partitioned in time and space in a plate boundary zone.Components: 3935 words, 4 figures, 1 table.

Data report: quantitative analysis of grain size distribution for coarse sediments in an accretionary prism: an example from the Cascadia accretionary prism

“…Higher power-R values indicate that the grain size distribution is fractal. The distribution is the same as that within the web structure reported by Hashimoto et al (2006). Figure F1.…”

“…Those mechanisms might occur in order from shallow to deep (Hashimoto et al, 2006). The strength of compactive cataclasis for coarser sediments is well studied in laboratory experiments (e.g., Zhang et al, 1990;Wong et al, 1997).…”

“…Grain size distribution can be used to examine whether the sediment is formed by cataclasis. For instance, in sandstone blocks from an on-land accretionary complex, grain size distribution represents a fractal distribution, which is fit by a power function within a web structure (Hashimoto et al, 2006). A web structure is known to be formed by cataclastic deformation (Byrne, 1984).…”

“…A web structure is known to be formed by cataclastic deformation (Byrne, 1984). On the other hand, grain size distribution in a host part of the sandstone does not show a fractal distribution but an exponential relationship (Hashimoto et al, 2006).…”

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

“…Mechanical compaction contributes to the lithification of sediments, especially in sandstone. The lithification of the sandstone is probably completed before mélange formation (Hashimoto et al, 2006). At the time the sandstone was lithified sufficiently to develop extension cracks, the shale matrix was still deforming in a ductile manner.…”