The thermal influence on the consolidation state of underthrust sediments from the Nankai margin and its implications for excess pore pressure

Earth and Planetary Science Letters 286 (2009) 324–332

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The thermal influence on the consolidation state of underthrust sediments from the Nankai margin and its implications for excess pore pressure A. Hüpers ⁎, A.J. Kopf MARUM – Center for Marine Environmental Sciences, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany

a r t i c l e

i n f o

Article history: Received 1 February 2008 Received in revised form 24 April 2009 Accepted 6 May 2009 Available online 28 July 2009 Editor: C.P. Jaupart Keywords: subductions zones thermal consolidation pore pressure Nankai

a b s t r a c t The Nankai Trough convergent margin has been the focus of many multi-methodological surveys including half a dozen scientific deep-sea drilling expeditions. The boreholes focused on the smectite-dominated area off Cape Ashizuri and the thermally altered, illite-dominated region off Cape Muroto. On the basis of these surveys a number of studies addressed to the stress state of the underthrust sediments and its implications for the plate boundary thrust. Although the basement temperature has been found to be up to ∼ 110 °C, none of these studies drew close attention to temperature effects on the consolidation state of the sediments. To overcome this shortcoming, we selected end member sediment lithologies from the incoming oceanic plate in the Shikoku Basin and subjected them to elevated stresses and temperatures. We here present results from a series of heated (20 °C, 100 °C, 150 °C) uniaxial consolidation experiments up to effective normal stresses of ca. 70 MPa. The main finding is a positive correlation between temperature and pore space reduction. Based on in-situ temperature information from earlier scientific drilling, our study suggests that temperature has an influence on the consolidation state of underthrust sediments along the Nankai Margin. Together with secondary consolidation, thermal consolidation serves to explain steep loglinear consolidation curves of the incoming Lower Shikoku Basin sediments. The onset of diagenesis in this realm led to the transition of smectite to illite and to a different consolidation behaviour. Estimated in-situ pore pressures based on in-situ temperature data result in up to ∼ 1 MPa smaller overpressures than those previously estimated from drilling data alone. Those values, which imply underconsolidation at drill sites near the frontal Nankai accretionary complex, are further believed to facilitate frictional sliding along the subduction thrust. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Nankai Trough accretionary margin (Fig. 1A), off Southwest Japan, has a 1300 yr long record of large earthquakes, including the M N 8 events of 1944 and 1946 (Ando, 1975). The margin has been a high priority location for DSDP (Deep-Sea Drilling Project), ODP (Ocean Drilling Program) and IODP (Integrated Ocean Drilling Program) drilling including subduction factory research and several studies addressing to the stress state of underthrust sediments. The area is currently the focus of the IODP project NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment; Tobin and Kinoshita, 2007). The development of earthquakes at accretionary margins is directly linked to changes in mechanical properties of the incoming sediments with depth. Since seismogenesis cannot occur in the initially weak sediments, significant consolidation and lithification have to take place along the plate boundary (e.g. Byrne et al., 1988; Moore and Saffer, 2001; Saffer and Marone, 2003). While sediments

⁎ Corresponding author. Tel.: +49 42121865802; fax: +49 42121865810. E-mail address: [email protected] (A. Hüpers). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.05.047

above the plate boundary undergo vertical and lateral (i.e. tectonic) consolidation with accretion, underthrust sediments have been proposed to remain largely undeformed laterally during initial subduction (e.g. Karig and Morgan, 1994). As a result of the applied load due to the overlying prism, underthrust sediments are subjected to rapid consolidation. Depending on the pore fluid dissipation as a function of permeability of the overlying sediments, the progressive consolidation is characterised by pore space reduction with increasing depth. However, modifications of physical properties and mechanical strength document that underthrust sediments are not only subjected to the applied load of the overlying prism, but also to increasing temperature, secondary consolidation (creep), and counteracting processes such as elevated pore pressures due to mineral dehydration, hydrocarbon formation, and diagenetic effects such as cementation and chemical compaction (e.g. Moore and Vrolijk, 1992; Moore and Saffer, 2001; Karig and Ask, 2003; Morgan and Ask, 2004). Such mechanisms change the mechanical properties of underthrust sediments that are particularly important for (1) the location of the main plate boundary fault (i.e. the decollement), which is situated directly above these sediments and often propagates into them (Brown et al., 2003), and (2) the onset of unstable sliding behaviour at

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Fig. 1. (A) Map of the Nankai subduction zone showing DSDP and ODP drill sites. Sediments of Site 297 were used for hydrothermal deformation experiments. (B) Interpolated temperature profiles of Sites 1177, 1173, 1174 and 808 with reliable measurements marked as dots (modified after Moore et al., 2001; Taira et al., 1991; Kagami et al., 1986). The shaded area marks the temperature across the Lower Shikoku Basin with a dotted line to accentuate the decollement zone (DZ). (C) Cross section along the Muroto transect showing major stratigraphic sequences and structure of the toe of the prism (modified after Morgan and Ask, 2004).

the up-dip limit of the seismogenic zone (Moore and Saffer, 2001; Saffer, 2003). So far, the detailed influence of the different factors on the consolidation state and strength of underthrust sediments and its consequences for seismogenesis and decollement localisation is incompletely understood. Although uniaxial consolidation testing has been successfully applied to study effective stress and pore pressure distribution of marine sediments along the Barbados and Costa Rican convergent margins (e.g. Moore and Tobin, 1997; Saffer et al., 2000; Saffer, 2003), there are noticeable discrepancies between field consolidation and laboratory consolidation at the Nankai Trough (Morgan and Ask, 2004). Only few laboratory consolidation tests investigated the mechanisms influencing the consolidation state of deep-sea sediments. For instance, Morgan and Ask (2004) assume from triaxial reconsolidation tests that samples of the Nankai margin are moderately cemented. Results from experiments by Karig and Ask (2003) suggest that secondary consolidation occurs with burial of marine sediments, presumably also closely linked to diagenesis. Although temperature is supposed to be a key parameter for the onset of seismogenesis (Oleskevich et al., 1999) its implication for consolidation behaviour of underthrust sediments has so far been largely neglected. The objective of this study is to contribute to narrow this gap. Isothermal uniaxial consolidation tests up to pressure– temperature (PT) conditions similar to those at the onset of the seismogenic zone (so called up-dip limit) have been conducted to shed light on thermal behaviour. Tested specimen comprise different lithologies of the Lower Shikoku Basin facies representing along-strike variation as well as thermal alteration downslab of subduction inputs at the Nankai margin. The results are discussed in comparison with standard laboratory tests and shipboard measurements from drill sites at the toe of the prism, and with respect to their implications for pore

pressure distribution and mechanical strength of underthrust sediments at the Nankai margin. 2. Geological context and sampling strategy 2.1. Geological context Along the Nankai Trough, the Philippine Sea plate is being subducted to the northeast at a slightly oblique angle to the margin of Southwest Japan (Eurasian plate) at a rate of 2–4 cm/yr (Karig and Angevine, 1986). Ongoing convergence led to the build-up of a wide accretionary prism by offscraping of Shikoku Basin and Nankai trench wedge facies from the downgoing plate (Fig. 1A). The Shikoku Basin was targeted in the Nankai Trough area during DSDP and ODP drilling Legs 31, 87, 131, 190 and 196 (Karig et al., 1975; Kagami et al., 1986; Taira et al., 1991; Moore et al., 2001; Mikada et al., 2002). Our study focuses on the Pliocene to Miocene Lower Shikoku Basin (LSB) facies, which comprises the bulk of the underthrust sediments and has been penetrated along two transects perpendicular to the margin: the Ashizuri and the Muroto transects (Fig. 1A,C). Off Cape Ashizuri, the LSB facies consists of hemipelagic mudstone with abundant terrigeneous sandy turbidites and volcanic ash. Smectite content is ∼20 wt.% at the top of the LSB at Site 1177 (∼23 km seaward from the deformation front) and increases dramatically within the strata at 600 mbsf to ∼ 50 wt.% (Underwood, 2007). It is assumed that no smectite diagenesis has occurred at this depth which is in accordance with thermal gradients of ∼ 50–56 °C/km along the Ashizuri transect (Fig. 1B). The projected decollement at Site 1177 is at a depth of ∼ 420 mbsf in a stratigraphic level equivalent to the Muroto transect. With less confidence, it may be traced to a level of ca. 550 ± 30 mbsf at Site 297 several tens of kilometers outboard

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Fig. 2. Pore space–depth relationships along the Muroto transect (Sites 1173, 1174 and 808) and the Ashizuri transect (Site 1177) based on Shipboard measurements (modified after Moore et al., 2001; Taira et al., 1991; Kagami et al., 1986). The pore space is presented as void ratio (volume ratio of pore water and solids). The light shaded area shows the Lower Shikoku Basin facies (LSB) and the dark shadings marks the decollement zone (DZ).

of the trench. The pore space decreases with increasing depth at Site 1177 but is generally higher compared to equivalent depths of drill Sites at the Muroto transect (Fig. 2). The subducting seafloor off Cape Muroto is situated above a basement high, formed by a fossil spreading ridge and an adjacent chain of volcanic seamounts. The LSB was penetrated ∼11 km (Site 1173), ∼0.25 km (Site 1174) seaward of deformation front, and 1.5 km landward of the deformation front (Site 808; Fig. 1C). Although sandy turbidites are common within the LSB they have not been sedimented on this ridge, leading to a monotonous lithological sequence of hemipelagic mudstone. Due to its location near the fossil spreading ridge, it is characterised by a high heat flow of ∼129–180 mW/m2 and a projected basement temperature of ∼110 °C for Sites 1173 and 808 (Taira et al., 1991; Moore et al., 2001). At Site 1174 the projected temperatures are up to ∼140 °C but may have been overestimated due to the input of warm fluids and thrust faulting (Moore et al., 2001). Thus, a consistent basement temperature of 110 °C has been assumed for this study (Fig. 1B). Kinetic reaction models for smectite to illite transition found to be highly consistent with measured illite in I-S clays for these high temperature conditions (Saffer et al., 2008). At Site 1173, smectite content decreases continuously from ∼35 wt.% at the top of the succession to ∼25 wt.% at the bottom (Underwood, 2007). Further landward at Site 808, smectite content is just about b6–7 wt.% and illite is the dominant clay mineral (Underwood and Pickering, 1996). Hence, it can be assumed that the smectite-rich layers at the Ashizuri transect will undergo a similar mineralogical change, although less pronounced owing to lower heat flow values (e.g. Moore et al., 2001). The pore space decreases continuously with depth within LSB strata at Site 1173 (Fig. 2), but this trend is interrupted in the upper part of the LSB by an abrupt increase in void ratio (Fig. 2) across the decollement zone at the other holes (808–840 mbsf at Site 1174; 945–964 mbsf at Site 808). This rapid change has been interpreted as a change in stress state due to overpressuring of the underthrust sediments (e.g. Screaton et al., 2002; Saffer, 2003) but also due to excess compressive strength of these sediments as a matter of cementation (Morgan and Ask, 2004). A more detailed discussion of this topic can be found in Morgan et al. (2007).

quantitative XRD results from Underwood et al. (1997) and Brown et al. (2003). Samples for this study derive from the LSB facies of Site 297, which is located SW of Site 1177 (Fig. 1A) along the Ashizuri transect. The selected samples comprise a smectite-rich clay (N13), an illite-rich silty clay (N14) and a dominantly silty to fine sandgrained quartz/feldspar-rich sample with some clay fraction (N18). Sampled depths within the lower part of the LSB (∼330–570 mbsf) are 506.83–506.90 mbsf (N13), 507.12–507.20 mbsf (N14) and 554.47– 554.63 mbsf (N18), respectively. The accumulated sample material was disintegrated and homogenised for the experiments. To provide evidence for mineralogical composition and integrity, sub-samples underwent semi-quantitative XRD examination at the University of Bremen (Germany) following the methodology described in Vogt et al. (2002) after completion of the compaction tests. The results verify a uniform composition for sub-samples of each end member, which attests that no significant smectite-to-illite transition has occurred in our tests. This is in accordance with slow kinetic reaction of smectite to illite at these temperatures proposed by Huang et al. (1993). However, the end members are significantly different in the main components (cf. Table 1), representing a variation of a mainly three component system (smectite [Sm], illite [Il], and quartz [Qtz]). Sample N13 is rich in clay minerals and smectite-dominated with contents representative for smectite-rich interlayers between the turbidites in the lower part of the LSB along the Ashizuri transect. Sample N18 possesses a high granular fraction (containing quartz, feldspar, and tephra) and reflects the turbiditic, coarse-grained end Table 1 Results from quantitative XRD showing major mineral content. Quartz + feldspar

Smectite + montmorillonite

Illite + muscovite

Chlorite

Mixed layer clays

Other

5.7 9.2 15.1 49.1 44.2 52.0 51.1 48.8 60.0

43.5 56.3 43.3 4.0 6.6 2.8 1.8 0.6 2.3

0.0 19.9 8.4 29.9 27.0 29.3 24.2 8.9 19.8

2.0 0.0 1.9 1.6 2.6 2.4 3.1 2.9 3.0

35.6 12.9 18.2 1.7 6.6 5.6 6.7 22.7 6.2

13.2 1.7 13.1 13.7 13.0 7.9 13.1 16.1 8.7

2.2. Sampling strategy

N13-20 N13-100 N13-150 N14-20 N14-100 N14-150 N18-20 N18-100 N18-150

To cover the wealth of lithological differences along the Nankai Trough, end member lithologies were selected based on semi-

Sample N13 is smectite-rich clay, while samples (N14) and (N18) contain both large fractions of illite, quartz and feldspar. Although mineralogically similar, sample (N14) is a silty clay while sample N18 is dominantly silty to fine sand-grained with some clay fraction.

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member lithology. The fine-grained sample N14 has a high illite content and may be therefore (1) comparable to the fine-grained hemipelagics along the thermally overprinted Muroto transect and (2) simulate consolidation behaviour during deeper underthrusting beneath Cape Ashizuri. Thus, we simulate both, thermal alteration along-strike as well as downslab. 3. Methods 3.1. Consolidation theory The compaction of sediments is described by the effective stress law and the one-dimensional consolidation theory (e.g. Terzaghi and Peck, 1948), which will be recapitulated briefly in the following. When a vertical load is applied suddenly onto an unlithified sediment mass, the total pressure is taken up by the mineral framework and by the water in the pores. The total stress (σt) is, therefore, defined as the sum of the effective stress on the mineral framework (σe) and the excess pore water (P) in the effective stress law σt = σe + P:

ð1Þ

Over time the water drains from the sediment pores, which causes a transfer of the stress on the mineral framework and to a plastic deformation of the sediment until the pore water overpressure dissipates. This process is known as consolidation. However, if drainage of the pore fluid is hindered, pore space remains high and the created excess pore pressure reduces the effective stress (σe). The relationship between pore space and the effective stress can be described after Terzaghi and Peck (1948) by

Fig. 3. Schematic view showing the known types of consolidation in the void ratio vs. the logarithm of effective stress (modified after Karig and Ask, 2003). Primary consolidation proceeds along line 1 and marks the equilibrium between pore space and the applied load after the excess pore pressure has dissipated. Further settlement of the sediment at constant stress is termed secondary consolidation (2). Unloading or reloading of a prestressed sample results in an elastic behaviour (3). Tertiary consolidation (4) occurs between the maximum consolidation state after secondary consolidation and resuming primary consolidation.

and recalculated Eq. (2) to maintain comparability to our study. Since the consolidation is material-related, Eq. (2) has to be determined by laboratory consolidation tests. Throughout a consolidation test, remolded sediments are characterised by plastic deformation. Test results are plotted in a void ratio vs. log effective stress (σe) diagram where the relationship presented in Eq. (2) gives ideally a straight line, the primary or virgin consolidation line (Fig. 3). It marks the consolidation state where pore space and effective stress are in equilibrium when the excess pore pressures has dissipated (σt = σe). The continuing consolidation at a constant effective stress after pore pressure dissipation is termed secondary compression (creep). In-situ consolidation is often the result of primary and secondary consolidation (Karig and Ask, 2003). A sample, which has undergone primary and secondary consolidation, responds to increasing stress by tertiary consolidation until primary consolidation is resumed (Fig. 3). Upon unload or reload of a sample consolidation occurs in an elastic fashion. To avoid relaxation effects due to core recovery from depth a rebound value of 0.045% e/log(σe) [=0.0199% η/log(σe)] for all shipboard void ratio data from the Muroto transect has been applied after Morgan and Ask (2004).

seawater for a period of 1 to 5 days. Thereafter, the samples were placed in a self adjusted high-capacity oedometer with a diameter of 55 mm. Initial void ratios were 4–5 and sample heights were up to 62 mm. Tests with aliquots of each specimen were carried out at 20 °C, 100 °C and 150 °C, and specimen labeling always provides sample ID— T [in degrees C] (e.g. N13-20 for a room temperature test at 20 °C, N13100 for a test heated at 100 °C, etc.). For the heated consolidation runs, a band heater was placed on the outside of the confining chamber. The temperature was monitored in the centre of the sample with a probe that relayed its reading to the heating and was controlled by a highprecision heating unit (Omega CN7600). Temperature fluctuations during the tests were smaller than 2 °C. Heating was conducted at the beginning of the tests in several steps over two days before loading up to approximately 70 MPa. Consolidation took place under one-sided drained conditions with rates of strain of b0.0125%/min for sample N18 and b0.0042%/min for sample N14 and N13 which are in the range of successfully tested strain rates for different clays by Smith and Wahls (1969). A backpressure of ∼500 kPa was applied to get entrapped air into solution and to prevent the pore fluid from evaporating. Pore pressures were measured using Validyne™ differential pressure transducers (accuracy ± 25 kPa) attached to the fluid drainage at the top and to the bottom of the cell. Shortening of the sample was measured with a Burster™ displacement transducer (accuracy ± 0.075 mm). In order to determine the void ratio for any given stress state, the final void ratio and the final sample height must be known. For this, final compacts were recovered from the cell after unloading and cooling over a period of ∼6–12 h. This span of time eliminates dehydration effects of smectite during the tests (Fitts and Brown, 1999). Sample height was determined by taking the average of the measurement at three different positions of the compact with a sliding calliper (accuracy ± 0.1 mm). For void ratio determination, consolidated samples were placed in an oven at 80 °C and were allowed to dry for several days until no further loss of fluid was noted. This procedure was necessary because the insulation made the apparatus inaccessible to determine the absolute location of the piston in the cell. Thus, void ratio data possibly include rebound and cooling effects. Nonetheless, rebound effects may be negligible when results of the same mineralogical sample are compared and maximum stresses have been the same.

3.2. Sample preparation and testing procedure

4. Results

In preparation of each experiment, the core samples were ground until the samples were fully disaggregated and re-hydrated in

An overview of the results of the consolidation study is shown in Fig. 3. Apart from runs N14-20 and N18-20, all samples show a more or

e = e0 −Cc ⁎ logðσe Þ

ð2Þ

with e being the void ratio (=volume of voids/volume of solids), e0 the void ratio at an effective stress unity of 1, and Cc the compression index. Although void ratio is more common in this context some authors also use the porosity as pore space characterisation for Eq. (2). To compare data with such studies, we calculated void ratio from porosity η for those studies by using the equation e = η = ð1−ηÞ;

ð3Þ

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Table 2 Best fit of least squares of logarithmic Eq. (2) describing the virgin consolidation behaviour of each specimen.

N13 N14 N18

20 °C

100 °C

150 °C

e = 1.51 − 0.52 ⁎ log(σe) R2 = 0.99 e = 1.71 − 0.41 ⁎ log(σe) R2 = 1.00 e = 1.11 − 0.33 ⁎ log(σe) R2 = 0.99

e = 1.17 − 0.43 ⁎ log(σe) R2 = 1.00 e = 0.77 − 0.21 ⁎ log(σe) R2 = 0.98 e = 0.90 − 0.31 ⁎ log(σe) R2 = 0.99

e = 1.64 − 0.70 ⁎ log(σe) R2 = 0.93 e = 0.86 − 0.32 ⁎ log(σe) R2 = 0.99 e = 0.84 − 0.28 ⁎ log(σe) R2 = 0.97

Only data points N4 MPa have been included in the best fit of e vs. log(σe) because of the bent curve progression. The overall high coefficients of determination (R2) of the best fits indicate a good linear relationship between void ratio and the logarithm of effective stress.

less arcuate consolidation curve, which is especially pronounced for the smectite-rich clay. Data are sparse in the beginning of some tests, because of greater logging intervals for some experiments and logarithmic presentation of the measured values. For the best fit calculation of e vs. log σe, we hence regarded only data greater than 4 MPa. For this range the majority of the samples display the typical log-linear e vs. σe relationships for remolded sediment. We calculated the best fit of Eq. (2) for the individual sample data (Table 2) to describe the pore space reduction with increasing effective stress following Terzaghi and Peck (1948). Coefficients of determination for best fits are better than R2 = 0.97 except for run N13-150 (R2 = 0.93). Data for the smectite-rich sample N13 show near-parallel curve progression for the tests at 20 °C and 100 °C at stresses greater than 10 MPa, where compression indices reached 0.52 and 0.43, respectively (Fig. 4A). The downshift of the 100 °C run may be given by the difference in e0 and accounts for a shift of 0.34 between the curves. Both experiments, at 20 °C and 100 °C, were aborted at effective stresses of 45.3 MPa and 53.8 MPa, respectively, because the fluid pressure approached the maximum range of the pressure transducers. Sample N13-150 shows a good agreement with the sample N13-100 at effective stresses greater than 30 MPa. At lower stresses this sample shows steeper void ratio reduction with depth. Hence, the best fit reveals a higher Cc and e0 compared to the two other runs. Noticeable is the halt in void ratio reduction with increasing stress, which is followed by an abrupt decrease in void ratio over small effective stress ranges (e.g. between 6–7 MPa and 20–30 MPa). The observed shift in void ratio with increasing temperature can also be seen for the illite-rich sample N14 (Fig. 4B). The shift in e0 of about 0.93 between sample N14-20 and N14-100 is significantly

bigger than for sample N13. In contrast, the shift between the heated tests is negligible for effective stresses lower than ∼ 15 MPa. With increasing stress the N14-150 curve retains the higher rate of void ratio reduction so that at maximum stresses of ca. 70 MPa the difference in e is only 0.11. The slope of the three consolidation lines deviates between 0.41 and 0.25 with lower values for the heated runs. Compared to the smectite-rich sample the compression indices are noticeable smaller. The turbiditic specimen N18 shows the lowest variation in compression index (Fig. 4C). Values range between 0.33 and 0.28 with smaller values for the heated runs. These compression indices are significantly smaller those of the smectite-rich sample and on the lower end of the measured range for the illitic sample. Based on the good agreement of the compression indices the shift in the consolidation curves of the room temperature test and the heated test at 100 °C is 0.21 given by e0. The difference between the heated runs increases due to the smaller compression index of sample N18-150 and is negligible at the maximum effective stress of 70 MPa. The mentioned halt in void ratio reduction for sample N13-150, although less pronounced, can also be observed for sample N18-150 at 3–4 MPa (Fig. 4C). The delay in consolidation may be created by transient elevated pore pressures owing to low permeability of the samples or blocked filters. The latter seems more reasonable because no excess pore pressures have been measured. 5. Discussion and implications 5.1. Interpretation of laboratory consolidation data From isothermal oedometer tests carried out on end member lithologies from the LSB succession along the Nankai margin at three different temperatures, we present nine different consolidation curves (Fig. 4) from which three major observations can be deduced: (1) The consolidation lines do not show the ideal log-linear behaviour but have an arcuate shape. (2) The slopes of the consolidation lines are different for each sample. (3) The consolidation curve shifts for each sample with increasing temperature. Consolidation below 4 MPa reveals a steeper rate of void ratio reduction with increasing stress than at higher stresses. However, standard soil mechanical testing is often restricted to lower effective stresses and variation at higher pressures has not received comparable

Fig. 4. Results from the isothermal consolidation tests for (A) the smectite-rich (N13), (B) the illite-rich (N14), and (C) the quartz-rich (N18) lithologies. Note the offset with increasing temperature towards lower void ratios.

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attention so far. Karig and Hou (1992) commented that the slope at low stresses smaller than 5 MPa is steeper and cannot be projected to higher stresses. This is fairly consistent with our results where we set the threshold at 4 MPa. Accordingly, the change in the slope of the consolidation line indicates that the rate of compression decreases with increasing effective stress (Karig and Hou, 1992). In fact, our room temperature tests give compression indices Cc N13 N Cc N14 N Cc N18, which is in accordance with the proposed order of Lambe and Whitman (1969) for the compression index of Cc_clay N Cc_silty_clay N Cc_silt. This order represents particle size and shape as most influencing parameters for the relationship of void ratio and effective stress. The most striking finding of this study is the decrease in void ratio at a constant effective stress with increasing temperature from 20 °C to 100 °C. At higher temperatures (i.e. between the 100 °C and 150 °C runs), this effect remains observable but is less pronounced. If the absolute values in the observed offset of the void ratio is assumed to be linear between 20 °C and 100 °C, the void ratio reduction is 0.012 e/°C for the illite-rich sample, 0.004 e/°C for the smectite-rich sample and 0.002 e/°C for the turbiditic sample. Although these data are higher than those of Campanella and Mitchell (1968), the observed temperature-dependent consolidation trend is in unison with several studies of thermal clay compaction tests that under fully drained conditions heat enables greater deformation until a load is compensated by the mineral framework (e.g. Campanella and Mitchell, 1968; Cekerevac and Laloui, 2004). According to Paaswell (1967) the heating induces a greater motion of water molecules, which are bound to the clay particles. Coupled with lower water viscosity, this motion alleviates a pressure-independent resistance between the clay boundary layers to shear and causes a parallel shift of the consolidation lines for different temperatures. The latter observation cannot be fully supported by our study up to 70 MPa, most likely because the earlier work focused mainly on low stresses (b1 MPa; cf. Campanella and Mitchell, 1968; Cekerevac and Laloui, 2004). Although only slightly, the offset is more pronounced for low stresses in our testing, leading to lower compression indices for the heated tests, so that we assume that the impact of temperature on the physico-mechanical factors on intergranular friction decreases with increasing effective stress. 5.2. Application to the consolidation state of underthrust sediments Along the Ashizuri transect where our samples originate from, in-situ measurements and also physical properties data such as void ratio and temperature with depth for LSB sediments are very limited from DSDP drillings, and only one ODP borehole (Site 1177) provided a comprehensive data set later. For this, we use data from Sites 1173, 1174 and 808 along the Muroto transect for comparison with our results. These sites have the added advantages that downhole differences in lithology are smaller than along the Ashizuri transect (e.g. lack of turbidites) and temperatures are high (Fig. 1B). Further, these sediments have progessively undergone illitization from Site 1173 to 808, which makes them suitable to study temperature and diagenetic effects otherwise encountered only at depths of several kilometers. The total stress σt for these Sites can be calculated by integrating the bulk density downward in the holes and subtracting the hydrostatic pressure owing to the water column of the respective height (Karig, 1993). When fluid overpressuring is excluded, as it is assumed for Site 1173 (e.g. Spinelli et al., 2007), the calculated stress equals the effective stress σe. 5.2.1. Influence of temperature To examine the influence of temperature on in-situ consolidation, Site 1173 is used as a reference (Fig. 5). Although temperatures are high across the LSB (65 °C to 105 °C, Fig. 1B), the degree of illitization is small and the strata is believed to be normally consolidated σt =σe (Screaton

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Fig. 5. Void ratio vs. effective stress data (circles) for Lower Shikoku Basin facies at Site 1173 (Moore et al., 2001) with best fit given as dashed line [e = 1.50 − 1.38 ⁎ log(σe)]. The solid line marks the estimated void ratio reduction due to mechanical loading (weight symbol) from the top of the LSB strata. The residual of 0.17 (thermometer) is explained by thermal consolidation and creep.

et al., 2002; Spinelli et al., 2007). Thus, the void ratio vs. effective stress relationship should be comparable to laboratory consolidation. The best fit of the in-situ data for Site 1173 is e = 1.50 − 1.38 ⁎ log(σe) (Fig. 5). Morgan and Ask (2004) derived from laboratory consolidation a value of Ccη = 0.236 from the porosity vs. log (σe) relationship which corresponds to Cc ∼0.99. This value reflects the upper threshold of a variety of consolidation results (cf. Spinelli et al., 2007). Thus, we examine temperature effects conservatively. Applying the laboratory Cc from the top of the LSB to model the response according to the overlying load, the void ratio decreases by 0.40 across the effective stress range studied. This leaves 0.17 unaccounted for (Fig. 5), so that in-situ consolidation must be subjected to another effect that introduces additional strain. Given the thermal void ratio reduction under normally consolidated conditions, the increasing temperature across the LSB strata may explain the residual amount of void ratio reduction (Fig. 5). To estimate the expected maximum void ratio reduction of thermal consolidation across the LSB strata, the approximated thermal consolidation rate of 0.004 e/°C (N13) results in an additional void ratio reduction of 0.16 for a temperature interval of 40 °C. Secondary consolidation (creep) may additionally affect consolidation behaviour, reducing pore space with time. Karig and Ask (2003) suggest that at geological time scales sediments consolidate slowly enough that primary and secondary consolidation proceed simultaneously. This may be facilitated by the slow sedimentation rate of 27–37 m/my for the LSB facies (Moore et al., 2001). This suggests that the observed discrepancy is the combined result of thermal and secondary consolidation, with secondary consolidation known to be more advanced at higher temperatures (Mitchell and Soga, 2005). Finally, it can be speculated that the observed temperaturedependent consolidation behaviour may have implications for frictional strength and stability of underthrust sediments in the seismogenic zone. Our study reveals that thermal consolidation is a result of decreasing interparticle friction, which suggests a weakening in sediment strength. Contrariwise, the lower void ratio may compensate the interparticle weakening by increased compressive strengthening under fully drained conditions. Under natural conditions pore pressure estimates for accretionary prisms are generally predicted to be near lithostatic at seismogenic depth, suggesting that the drainage is hindered (e.g. Saffer and Bekins, 1998). This implies that no compressive strengthening takes place and that the decrease in interparticle friction cannot be compensated by thermal consolidation. The greater volume increase of pore water with increasing temperature may also lead to substantial pore water overpressure that further weakens the sediment. However, friction experiments under elevated temperatures have to be conducted to scrutinise these phenomena and their effect on sliding behaviour.

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5.2.2. Influence of diagenesis Once underthrust, sediments are subjected to thermal alteration downslab. Ongoing illitization leads to a change in mineralogical composition, fluid release and thus to different consolidation behaviour. Maximum compression indices reported for Sites 1173, 1174 and 808 indicate a decrease in the compression index Cc from 0.99 N 0.7 N 0.43, which correspond to the compression indices of 0.236 N 0.186 N 0.160 determined by Morgan and Ask (2004) and Karig and Ask (2003) from the porosity vs. log (σe) relationship. Similar results on various clays at up to 50 MPa were derived by Djeran-

Maigre et al. (1998) where a decreasing smectite content is also linked with a decrease compression indices. The change in consolidation behaviour of the subduction inputs may be additionally affected by other diagenetic processes and finally by lithification. Depending on the primary mineralogy, clastic sediments are supposed to be modified during burial diagenesis by the gradual replacement of smectite, detrital biotite, K-feldspar and calcic plagioclase by chlorite, illite and albite (Kisch, 1983). Frey (1987) assumes that more than 95% of very low-grade metaclastites are a mixture of muscovite (or illite), chlorite and quartz. Especially quartz

Fig. 6. (A) Schematic diagram illustrating our pore pressure estimates. When a load is applied (weight symbol) and no drainage occurs, the additional stress is taken up by the pore water without any reduction in void ratio (1→2). The total stress of (2) is σt = σe + P, and can be inferred from integrating the bulk density down hole. Thus, the excess pore pressure can be directly predicted from the shift relative to the reference line, where σt = σe. Depending on the drainage, the excess pore pressure may partially dissipate and hence cause a reduction in void ratio (3). (B) Previous studies reported the excess pore pressures P (shaded area) directly to the shift of Site 1173 data which is supposed to be normally consolidated (σt = σe). Our approach considers that no thermal or secondary consolidation takes place between the sites. Instead it resumes primary consolidation, which is modelled here exemplarily for the top and the base with a Cc of 0.99. Temperature corrected estimates are presented in (B,C) for Sites 1174 and 808, respectively, showing that excess pore pressures (shaded areas) are smaller. These pore pressures are consistent with the observed void ratio reduction between the sites.

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formation would foster earthquake generation because it exhibits unstable frictional sliding behaviour in the range of 150–300 °C. Quartz cementation occurs due to silica dissolution (e.g. pressure solution), which precipitates at a temperature of N150 °C (Moore et al., 2007). Although the contention that smectite-to-illite transition plays a major role in seismogenic faulting (Vrolijk, 1990; Hyndman et al., 1995) has been questioned based on shear experiments of seawater-saturated sediment at room temperature (Brown et al., 2003; Ikari et al., 2007) illitization may facilitate quartz cementation because the reaction produces silicon as by-product (Curtis, 1985). These finding may be in favour of the hypothesis that a threshold in consolidation state (Marone and Scholz, 1988; Marone and Saffer, 2007) or the combination of lithification/consolidation and diagenesis (Moore and Saffer, 2001) is held responsible for the onset of unstable sliding behaviour. 5.3. Implications for pore pressure estimates The knowledge of the magnitude of pore fluid pressure fluctuations is important because excess pressures lower the sediment strength and make it prone for failure (Hubbert and Rubey, 1959; Moore, 1989; LePichon et al., 1993). Regions with elevated pore pressures are important for the formation of the decollement near the toe of the forearc, and also for the onset of seismogenesis (e.g. LePichon et al., 1993; Moore and Saffer, 2001). High pore pressure transients were measured or inferred in underthrust sediments for several convergent margins (e.g. Foucher et al., 1997; Becker et al., 1997; Screaton et al., 2002; Saffer, 2003, 2007). Previous studies used the void ratio vs. log (σe) relationship inferred from shipboard data from Site 1173 as a reference to estimate excess pore pressure for the Nankai margin along the Muroto transect (Fig. 6A, Saffer, 2003, 2007). Estimated pore pressure suggests a landward increase of 2.5–4.6 MPa for Site 1174 and 4–5.5 MPa for Site 808 (Saffer, 2007). The determined overpressures have been assumed from rapid sedimentation and thickening of the prism toe and poor drainage of the sediments. From numerical simulations that take into account the thermal state, Gamage and Screaton (2006) conclude that rapid trench sedimentation and prism thickening may be insufficient to explain the high pore pressures reported from previous studies. Sedimentation of trench wedge facies and thickening of the prism toe lead to an increase in sediment thickness above the basin hemipelagics to 483 m at Site 1174 and 620 m at Site 808 (Taira et al., 1991; Moore et al., 2001), which correspond to a load of ∼4 MPa and ∼5.5 MPa respectively. This suggests that in the deeper parts of the LSB the additional load is either too small to create the excess pore pressure or totally taken up by the pore water. In the latter case no reduction in void ratio can occur under these circumstances and void ratios of all Sites should be the same. However, comparing the top and the bottom of the underthrust section of LSB shows that void ratio is smaller for Sites 1174 and 808. Thus, the LSB has undergone consolidation and partial drainage must have reduced the excess pore pressure. We revisit the problem using the considerations from our thermal experiments. Consolidation for Site 1173 is governed by low sedimentation rates and high temperatures for the projected underthrust sequence (∼75– 105 °C, Fig. 1B). Its downhole void ratio reduction is the result of mechanical, thermal and secondary consolidation. With beginning underthrusting at Sites 1174 and 808 loading rates increase dramatically, which makes the sediment less prone for time dependent secondary consolidation (creep). Further, temperature does not increase with depth when compared to Sites 1174 and 808 (∼87–107 °C) and accordingly, no thermal consolidation takes place here. Without the additional strain of thermal and secondary consolidation, we assume that consolidation does not follow the steep trend from Site 1173, but a less steep primary consolidation. Following these assumptions, the consolidation state between Sites 1173 and 1174 and 808 are compared at the top and the base of the LSB (Fig. 6B,C) where temperatures are similar, using our primary consolidation data (Cc 0.99).

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The primary consolidation results in lower excess pore pressures of 2.2 MPa–3.9 MPa for Site 1174 and 3.0 MPa–4.9 MPa for Site 808 (Fig. 6B,C). Although the differences to the previous study are smaller than 1 MPa the new estimates are in accordance with the general void ratio reduction. Lower pore pressures remain at the top of the underthrust section, which points to an upward drainage (Fig. 6), probably to a free drainage boundary along the decollement (Saffer, 2007). The pore water expulsion at the top is also reflected in the greater void ratio. Between Sites 1174 and 808 excess pore pressure increases uniformly by ∼ 1 MPa, suggesting that the additional load of 1.5 MPa is largely taken up by the pore water. This is supported by the fact that void ratio reduction is almost negligible at these Sites. Although these excess pore pressure estimates are consistent with the observed void ratio reduction, in-situ pore pressure data from A-CORK observations may be needed to bring certainty into these estimates (e.g. Davis et al., 2006). The determined increase in excess pore pressure is nonetheless in agreement with the assumption that pore pressures increase along the subduction thrust and, together with low basal friction, are responsible for the small taper angle at the Muroto transect (Saffer and Bekins, 1998; Brown et al., 2003). 6. Conclusions Taken together our results, temperature has a veritable impact on the consolidation behaviour of underthrust sediments from the Nankai margin, with increasing temperature leading to an enhanced pore space reduction. This change seems to be more pronounced for temperatures lower than 100 °C. By combining the observed trends from consolidation tests with field-based data, we explain the consolidation of the incoming Lower Shikoku Basin sediments as a complex combination of primary, secondary and thermal consolidation. Besides the direct influence on consolidation, temperature is the driving factor for the smectite to illite reaction, connected with a change in consolidation behaviour and possibly facilitated by lithification downslab. Based on our findings, estimated excess pore pressure for the Nankai Trough is found significantly lower than previously believed, so that overall our results have profound mechanical implications for IODP NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) drilling project to the seismogenic zone. Acknowledgments We thank Jill Weinberger for her assistance with some of the experiments and Kevin Brown for providing laboratory space. Christoph Vogt is thanked for XRD analyses. Samples and data used in this study have been provided by the Ocean Drilling Program (ODP). ODP was sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI). This paper benefited from the discussion with Demian Saffer and numerous other colleagues working off Japan. Maria V. S. Ask and Claude P. Jaupart are thanked for their reviews, which greatly helped to improve the manuscript. Funding was provided to AK by the German science foundation (project KO2108/4-1). References Ando, M., 1975. Source mechanisms and tectonic significance of historical earthquakes along the Nankai Trough, Japan. Tectonophysics 27, 119–140. Becker, K., Fisher, A.T., Davis, E.E., 1997. The CORK experiment in hole 949C: long-term observations of pressure and temperature in the Barbados accretionary prism. Proc. Ocean Drill. Program Sci. Results 156, 247–252. Brown, K.M., Kopf, A., Underwood, M.B., Weinberger, J.L., 2003. Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: a weak plate boundary. Earth Planet. Sci. Lett. 214 (3–4), 589–603. Byrne, D.E., Davies, D.M., Sykes, L.R., 1988. Loci and maximum size of thrust earthquakes and the mechanics of shallow region of subduction zones. Tectonics 7 (4), 833–857. Campanella, R.G., Mitchell, J.K., 1968. Influence of temperature variations on soil behaviour. J. Soil Mech. Found. Div. 94 (SM3), 709–734.

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