Supplementary material to the manuscript entitled:
Geotechnical behavior of mudstones from the
Shimanto and Boso accretionary complexes, and
implications for the Nankai accretionary prism
Kai Schumann1*
*
Corresponding author
Email: kaschumann@geomar.de
Jan H Behrmann1
Email: jbehrmann@geomar.de
Michael Stipp1
Email: mstipp@geomar.de
Yuzuru Yamamoto2
Email: yuzuru-y@jamstec.go.jp
Yujin Kitamura3
Email: yujin@sci.kagoshima-u.ac.jp
Christof Lempp4
Email: christof.lempp@geo.uni-halle.de
1
Marine Geodynamics, GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3,
Kiel, Germany
2
IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan
3
Department of Earth and Environmental Sciences
Graduate School of Science and Engineering
Kagoshima University
1-21-35 Korimoto, Kagoshima 890-0065, Japan
4
Institut für Geowissenschaften und Geographie, Martin-Luther-Universität Halle-Wittenberg,
Von-Seckendorff-Platz 3, 06120 Halle (Saale), Germany
Corresponding author: K. Schumann, Marine Geodynamics, GEOMAR Helmholtz Centre for
Ocean Research Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany (kaschumann@geomar.de)
1. Stress-strain observations
As described in the methods section, three different geotechnical test procedures were applied. A
detailed overview of the experimental conditions is given in Table 2 (main text). In the following
we focus on the deformation steps of the experiments. During the triaxial experiments maximal
bulk axial strain of 4.7% was achieved (sample Sch2011/12; apart from those samples which
failed at an early stage). The peak differential stresses reached in the tests range between 94.6
MPa and 246.7 MPa for the Shimanto Belt samples and between 37.9 MPa and 87.2 MPa for the
samples from the Boso accretionary prism (Table 2, main text). The initial differential stress at
the beginning of the deformation experiments was zero. Due to the increase of the confining
pressure at the beginning of the deformation experiments, the axial strain was slightly negative.
Since only positive strains are shown in the stress strain plots, the differential stress is not zero at
the beginning of the experimental sample compression.
1.1 Shimanto samples
During the confining pressure stepping tests of the samples Sch2011/8, Sch2011/23, Sch2011/271 and Sch2011/29 (supplementary Figure S1),
Figure S1. Stress-strain plots of the confining pressure stepping tests of the Shimanto Belt
samples.
Supplementary Figure S1: Stress-strain plots of the confining pressure stepping tests of the
Shimanto Belt samples. Samples Sch2011/8 and Sch2011/29 show a stress-strain record
indicative of a failure in the first deformation stage (pressure step 1). Confining pressure was
increased in steps of 10 MPa from 25 MPa to a maximum of 65 MPa except for sample
Sch2011/29 that was terminated at 55 MPa.
the confining pressure (σ3) was increased from 25 MPa in steps of 10 MPa to a maximum
confining pressure of 65 MPa or 55 MPa for experiment Sch2011/29 (Table 2, main text). Pore
pressure was set to be 80% of σ3. Sample Sch2011/23 is characterized by increasing stress levels
during the five deformation steps. The maximum differential stress in this sample was 136.5 MPa
at approximately 2.1% axial strain (supplementary Figure S1). When deforming sample
Sch2011/27-1, peak differential stress of 155.8 MPa was reached after five deformation steps at
1.6% axial strain, (supplementary Figure S1) without signs of a reduced differential stress
increase. Samples Sch2011/8 and Sch2011/29 are characterized by drops in the differential stress
in the order of 20 MPa after the first deformation step. This stress drop is characteristic of a
failure of the core sample after less than 1% axial strain. Sample Sch2011/8 did not reach the
differential stress level of the first deformation step in all later pressure steps (increasing
confining pressure), while sample Sch2011/29 reached the previous stress level of the first
deformation step again in the third deformation step, and exceeded this level in the last
deformation step (supplementary Figure S1). After the deformation experiments, conjugate sets
of shear fractures (conjugate fractures) were observed in sample Sch2011/8, making angles of
approximately 30° with the core axis (supplementary Figure S2A).
Figure S2. Examples of drill cores after triaxial testing.
(A) Sample Sch2011/8 shows a set of conjugate shear fractures. (B) Sample Sch2011/29 shows a
set of conjugate shear fractures and extensional fractures parallel to the core axis. Fractures
almost normal to the core axis probably formed during unloading of the sample. The gypsum cap
to the left was added before the experiment to improve sample geometry. (C) Sample Sch2011/18
showing one extensional fracture parallel to the core axis, and the oblique view on one shear
fracture plus a few unloading fractures normal to the core axis. (D) Shear fracture crosscutting
core sample Sch2011/27-2 and two fractures parallel to the bedding plane and normal to the core
axis.
In Sample Sch2011/29 the fracture pattern is much more complex. There are two fractures
approximately at 70° to the core axis. Since the failure planes were rust-colored, these two
fractures seem to have formed along preexisting structures, such as schistosity planes. It is likely
that these fractures oriented at high angles to the core axis were formed as a stress rebound effect
during unloading of the specimens or due to the pore pressure, which was at the low permeability
higher than the confining pressure during the reduction of the confining pressure. Conjugate
shear fractures at approximately 40° to the core axis and extension fractures approximately
parallel to the core axis were also observed (supplementary Figure S2B).
In the pore pressure stepping tests at σ3 = 55 MPa (Sch2011/12, Sch2011/18, Sch2011/24 and
Sch2011/27-2, supplementary Figure S3) the pore pressure was decreased from 80% to 50% and
finally 25% of σ3 and at σ3 = 65 MPa (Sch2011/34) the pore pressure was decreased from 90% to
60% and finally 30% of σ3. Peak differential stresses were increasing during the deformation
steps (Sch2011/12, Sch2011/24 and Sch2011/34). In samples Sch2011/18 and Sch2011/27-2, a
rather constant maximum differential stress was observed at the end of each deformation step
(supplementary Figure S3).
Figure S3. Stress-strain plots of the pore pressure stepping tests conducted on the Shimanto
Belt samples.
The pore pressure (indicated for each step) was reduced between the deformation steps. Samples
Sch2011/18 and Sch2011/27-2 show failure in the first deformation step and low peak differential
stress in the subsequent steps.
In experiment Sch2011/12, the maximum differential stress increased in the three deformation
steps, and reached 223.7 MPa at 4.7% axial strain. Similar behavior was observed in experiment
Sch2011/24, in which the peak differential stress also increased in each deformation step to a
maximum of 184 MPa at 3.4% axial strain. In experiment Sch2011/34 the maximum differential
stress in the third deformation step was 186 MPa at 3.8% axial strain (supplementary Figure S3).
In the first deformation step of Sample Sch2011/18 the differential stress increased strongly and
immediately dropped after the maximum value of 240.7 MPa at 0.4% axial strain. In the second
and third deformation steps, differential stress quickly increased to ~180 MPa. After yielding at
~180 MPa, the stress increased linearly to 250 MPa. At this level, the deformation was stopped.
Similar behavior was observed in experiment Sch2011/27-2, with differential stress increasing
approximately linearily to 172 MPa in the first deformation step. In the second and third
deformation steps, differential stress quickly increased to approximately 130 MPa. After yielding
the sample hardened steadily to 156 MPa and 173 MPa, respectively. In both samples
(Sch2011/18 and Sch2011/27-2) there are prominent shear fractures at angles between 30° and
40° to the core axis (supplementary Figures S2C and S2D). These through-going faults are
accompanied by several bedding-parallel failures (approximately 90° to the core axis), probably
caused during unloading of the samples at the end of the deformation or by pore fluid
overpressure in the stages of decreasing confining pressure.
1.2 Boso samples
Five confining pressure stepping tests were carried out on Boso samples. In the experiments NZPS-1/1, NZ-PS-2 and NZ-PS-5 σ3 was increased from 25 MPa to 65 MPa in five steps of 10 MPa
(supplementary Figure S4).
Figure S4. Stress-strain plots of the confining pressure stepping tests of the Boso samples.
The phases of differential stress release after each triaxial deformation step are not shown for
clarity. Confining pressure was increased in steps of 10 MPa from 25 MPa to 65 MPa in
maximum (not for NZ PS-4/1 and NZ PS-4/2).
In experiments NZ-PS-4/1 and NZ-PS-4/2 only two steps (25 MPa and 35 MPa) were conducted
(supplementary Figure S4, Table 2 main text). A cyclic test (NZ-PS-5-c) at constant confining
pressure (55 MPa) and constant pore pressure (80% of σ3) was also conducted. In this test, the
specimen was axially loaded 18 times to 70 MPa (Figure 6 main text).
Five complete cycles of deformation were conducted in experiments NZ-PS-1, NZ-PS-2 and NZPS-5. These three samples behaved rather similar during deformation and reached similar
maximum stress levels in the last deformation step of 85 MPa (NZ-PS-1), 80 MPa (NZ-PS-2) and
87 MPa (NZ-PS-5, supplementary Figure S4), respectively. Since relatively large amounts of
displacement were attained in experiments NZ-PS-4/2, the triaxial test had to be stopped after the
second deformation step. At the end of the second deformation step, the maximum stress was
65.9 MPa and 64.8 MPa, respectively (supplementary Figure S4). In the stress cycling test of
experiment NZ-PS-5-c, peak differential stress during the first of the 19 cycles was just over 70
MPa at almost 1.2 % axial strain. In the subsequent cycles (2-5) peak differential stress
decreased, and attained after somewhat smaller axial strain a peak differential stress of about 69.4
MPa in the fifth cycle. From the sixth cycle onward axial strains increased again step by step to
the end of the last cycle to a peak differential stress of 69.5 MPa (Figure 6 main text)