JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B5, 2261, doi: /2002jb001787, 2003

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B5, 2261, doi: /2002jb001787, 2003 Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai Demian M. Saffer Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA Received 25 January 2002; revised 16 November 2002; accepted 4 March 2003; published 20 May [1] At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging-whiledrilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section 1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near-surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (l) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 5114 Physical Properties of Rocks: Permeability and porosity; 8045 Structural Geology: Role of fluids; 8150 Tectonophysics: Plate boundary general (3040); KEYWORDS: subduction zones, pore pressure, soil mechanics, consolidation, fluid flow Citation: Saffer, D. M., Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai, J. Geophys. Res., 108(B5), 2261, doi: /2002jb001787, Introduction [2] Fluids play an important role in deformation and mass transfer in the Earth s crust. Pore fluid pressure is closely linked to the overall mechanics and morphology of subduction zones [e.g., Hubbert and Rubey, 1959; Davis et al., 1983]. Fluid pressure and sediment consolidation state influence fault localization [e.g., Moore and Byrne, 1987], deformation style and strength, and the updip limit of the seismogenic zone [e.g., Moore and Saffer, 2001]. The fate of incoming pore fluids also impacts fluid, chemical, and mass budgets. From a mechanical viewpoint, the fluid content and physical properties of underthrust sediments are especially important, because the main decollement Copyright 2003 by the American Geophysical Union /03/2002JB001787$09.00 localizes immediately above these sediments, and often steps downward into them. The hydrologic contribution of underthrust sediments is also accentuated, because they are transported rapidly and remain underconsolidated relative to compacting sediments in the overlying wedge [e.g., Saffer and Bekins, 1998]. [3] Previous work has documented the development of elevated pore pressures within underthrust sediments at several subduction zones. At the Nankai margin, Screaton et al. [2002] compared porosity-depth profiles at underthrust sites with a reference site to estimate depth-averaged excess pore pressure. At the Cascadia subduction zone, Cochrane et al. [1996] inverted compressional wave velocity for porosity, and then inferred pore pressure from porosity-depth profiles. Moore and Tobin [1997] used a laboratory consolidation curve to invert observed loggingwhile-drilling (LWD) densities for pore pressures at the EPM 9-1

2 EPM 9-2 SAFFER: PORE PRESSURE AND DEWATERING Figure 1. (a) Map of study area showing the location of 3-D seismic data set (gray box), seismic line UT-CR 20, and ODP Sites 1039, 1043, and 1040 (solid dots). (b) Seismic line UT-CR 20, showing features of the incoming plate and toe of the margin wedge. Barbados accretionary prism. At the Costa Rican subduction zone, the evolution of excess pore pressures has been investigated using laboratory consolidation tests [Saffer et al., 2000]. Despite these efforts, progressive consolidation, the evolution of pore pressure with continued underthrusting, and the variability of these processes downsection are generally not well characterized. This is due to (1) a lack of spatial resolution in seismic methods and sampling for laboratory tests, (2) generally few independent constraints from consolidation tests, (3) uncertainty in the relevance of laboratory data to geologic consolidation, and (4) few direct measurements in instrumented boreholes. [4] In most cases, the application of laboratory consolidation tests in estimating in situ effective stress is limited by a small number of samples, lack of data at axial stresses above a few megapascals, and potential drilling disturbance [e.g., Feeser et al., 1993]. In general, laboratory consolidation tests are also limited in that they provide only an estimate of in situ effective stress, subject to considerable uncertainty [e.g., Holtz and Kovacs, 1981]. Nonetheless, such laboratory tests are useful for estimating in situ conditions and identifying significant departures from fully drained behavior, especially if multiple samples yield similar results. [5] In this paper, I merge laboratory consolidation test results with LWD and high-quality physical properties data to provide spatially detailed estimates of in situ pore pressure at the Costa Rican, Nankai, and northern Barbados convergent margins, with particular emphasis on the evolution of pore pressure and effective stress at Costa Rica. This expands significantly upon previous pore pressure estimates [e.g., Saffer et al., 2000] by including data from an additional drill site (Ocean Drilling Program (ODP) Site 1043), and also by evaluating the variation in excess pore pressure downsection. Specifically, I combine laboratory consolidation test results with observed reduction in sediment void ratio to (1) track the evolution of effective stress and pore pressure and their variation downsection between drill sites, and (2) evaluate the mechanical and hydrologic implications of the observed consolidation patterns. I then use high-quality drilling data from several ODP sites at the Barbados and Nankai subduction zones to compare inferred pore pressures, dewatering processes, and rates at these margins with inferences from Costa Rica. 2. Geologic Setting and Background: Costa Rican Margin [6] The Middle America Trench offshore Costa Rica is formed by the eastward subduction of the Cocos Plate beneath the Caribbean Plate at 85 km Myr 1 (Figure 1a). During ODP Leg 170 in winter of 1996, coring and LWD data were collected at three sites near the trench. At reference Site 1039, located 1.5 km seaward of the trench (Figure 1b), LWD and core data were collected through the entire drilled section. The incoming sedimentary sequence is about 380 m thick and consists of 160 m of siliceous hemipelagic sediments overlying 220 m of pelagic carbonates (Figure 2). This stratigraphy is regionally continuous, as indicated by drilling on DSDP Legs 67 and 84 offshore Guatemala [Aubouin and von Huene, 1985; Coulbourn, 1982]. Site 1043, located 0.5 km landward of the trench, penetrated through the margin wedge and underthrust sediments to a total depth of 482 mbsf. LWD data were collected through the entire drilled section, but no cores were collected below the decollement. Site 1040, located 1.6 km landward of the trench, also penetrated through the decollement and underthrust sequence to a total depth of 653 mbsf. Cores were collected throughout the drilled section, but no LWD data were collected below 330 mbsf (Figure 2). Deformation is evident in the uppermost 50 m of the hemipelagic sequence at Site 1040, which is characterized by dipping beds and some microfaulting [Kimura et al., 1997]. [7] Seismic reflection profiles and gamma ray logs show that the present margin is nonaccretionary, in that essentially none of the incoming pelagic sediment is offscraped [Kimura et al., 1997]. This is confirmed by biostratigraphic studies, which indicate nearly identical age-depth relationships for the incoming and underthrust sedimentary sequences. Other work suggests that the margin may be characterized by tectonic erosion rather than frontal accretion [e.g., Bourgois et al., 1984; Vannucchi et al., 2001]. The thick margin wedge is characterized by high seismic

3 SAFFER: PORE PRESSURE AND DEWATERING EPM 9-3 velocities ( km s 1 )[Christeson et al., 2000], and is interpreted as either an older accretionary wedge [Shipley et al., 1990] or a continuation of the onshore Nicoya complex overlain by several hundred meters of slope apron sediments [Kimura et al., 1997; Ye et al., 1996]. The hydrologic contribution of underthrust sediments is accentuated in nonaccretionary systems like the Costa Rican subduction zone, because compressible, high-porosity surface sediment is subducted rather than incorporated into an accretionary wedge [e.g., Le-Pichon et al., 1993]. [8] Drilling shows that the upper, hemipelagic unit is 67% of its original thickness by Site 1040, and the lower pelagic unit is 80% of its original thickness. Because no sediment is offscraped from the incoming section, the observed thinning of units is due to water loss and densification, and thus provides a quantitative indication of expelled water volume. Approximately, 8 m 3 yr 1 of fluids are expelled per meter of margin from underthrust sediments between the time they are first subducted and when they reach Site Of this, 5.4 m 3 yr 1 is lost by the hemipelagic section. [9] The complete underthrusting of a regionally uniform sediment section offers a unique opportunity to explore changes in effective stress and compaction with progressive burial. Unlike other subduction zones, the sediments at Costa Rica are mechanically heterogeneous downsection [e.g., Saffer et al., 2000]. Thus the commonly used method of comparing porosity-depth relationships between a reference column and underthrust sediments to estimate effective stress is not useful [e.g., Screaton et al., 2002]. However, because the incoming section at Costa Rica is regionally uniform in thickness and is completely underthrust, the observed reduction in void ratio between correlated sediment packages, combined with numerous high-quality laboratory consolidation tests, provides a means to track the development of effective stress and pore pressure beneath the toe of the margin wedge. In contrast, at subduction zones where the incoming sediment section is nonuniform in thickness or is partially accreted, analysis of changes in effective stress and void ratio with subduction are complicated because they reflect a combination of dewatering (consolidation) and differences in initial stress state. Figure 2. Bulk density data from logging-while-drilling (lines) and shipboard index property measurements (dots), showing progressive densification and thinning of lithologic units between drill sites. Solid squares indicate the locations of samples from Sites 1039 and 1040 used for consolidation tests in this study. 3. Laboratory and Analytical Methods [10] As unconsolidated sediments are loaded by underthrusting beneath the margin wedge, the increasing overburden stress is transferred to pore fluids, generating overpressures. With sufficient permeability, the elevated pore pressures drive dewatering, resulting in increased effective stress and a corresponding decrease in void ratio. If drainage can keep pace with loading, then hydrostatic pore pressure is maintained and the added total stress is transferred entirely to the effective stress carried by the sediment framework. This condition is known as normal consolidation. If drainage is severely limited by low permeability, or if loading is extremely rapid, the added total stress is supported by trapped pore fluids, generating significant overpressures and resulting in essentially no change in effective stress. The result is that fluid overpressures can develop even in moderately high-permeability sediments if the loading is sufficiently rapid [e.g., von Huene and Lee, 1982; Neuzil, 1995; Gibson, 1958]. [11] Effective stress is of particular importance in the subduction setting because it affects the shear strength of sediments: t ¼ C þ ðs 0 mþ; ð1þ where t is shear strength, C is cohesion, s 0 is effective stress, and m is coefficient of friction. Thus the evolution of effective stress is an important consideration for the evolution and spatial variability of mechanical strength. Combined with downhole measurements of bulk density, which allow calculation of the total overburden stress, and in the case where bulk sediment compressibility is significantly greater than fluid compressibility, estimates of s 0 in core samples can be used to infer in situ pore fluid pressure (P f ) by: P f ¼ s overburden s 0 : 3.1. Determination of Consolidation Index (C c ) Direct Measurement [12] I conducted uniaxial consolidation tests on three samples from Site 1039 and eight from Site 1040 (Table 1 and Figure 2), at effective axial stresses up to 6 MPa, using a high-precision hydraulic oedometer and following standard incremental loading procedures with a load increment ratio of one [Crawford, 1986; Lee, 1985; Saffer et al., ð2þ

4 EPM 9-4 SAFFER: PORE PRESSURE AND DEWATERING Table 1. List of Samples and Laboratory Consolidation Test Results a Sample Depth, mbsf Unit 0 P c OCR P 0 c Min P 0 c Max C c C ccalculated Slope of RC H-a B H-b B X B X B C-27R B C-30R A C-34R B C-38R B C-42R B C-46R C C-50R C C-52R C a OCR refers to overconsolidation ratio, P 0 c min and P 0 c max were determined as described in text, and RC refers to rebound curve. Values of C ccalculated determined using equation (4) are not available for samples 1040C-50R and 52R because P 0 c values indicate essentially no change in effective stress during burial. 2000]. In all tests, samples were laterally confined with a steel ring [e.g., Lee, 1985]. I obtained 15-cm-long whole round core samples from Sites 1039 and 1040 for consolidation and permeability tests, and used CT scans of the samples to select undisturbed portions of the core for experiments. Samples were 6.25 cm in diameter, and initial height ranged from 1.5 to 1.6 cm. All samples were saturated with deaired, dionized water and backpressured to 500 kpa for 24 hours prior to testing, to drive any gases present into solution. [13] The response of sediment to an applied load depends upon both the sediment compressibility and the loading history of the sample. Upon reconsolidation in the lab, a sample deforms along an elastic rebound curve until reaching loads exceeding its maximum past vertical effective stress (Pc), 0 assuming that the sample has not been disturbed or reworked. At stresses beyond Pc, 0 the sample deforms plastically along a virgin consolidation curve, and the strain response for a given stress increment changes. I determined Pc 0 from laboratory tests following the Casagrande method (Figure 3) [Holtz and Kovacs, 1981]. I obtained conservative estimates of minimum and maximum possible Pc 0 values from effective stresses at which sample void ratio during the consolidation test no longer plotted on a rebound curve (minimum Pc 0 ) or virgin consolidation curve (maximum Pc 0 ). [14] The slopes of the elastic and plastic portions of the stress-strain curve from a laboratory oedometer test provide important information about sediment compressibility. The slope of the virgin consolidation curve, which defines the compression index (C c ), is a measure of void ratio reduction with increased effective stress. The virgin consolidation curve is defined by: e ¼ e 0 C c logðs 0 Þ; ð3þ where e is void ratio, e 0 is void ratio projected at zero effective stress, C c is the compression index, and s 0 is effective stress. The slope of the elastic rebound portion of the stress-strain curve is generally used to correct for the effects of expansion from unloading during sample retrieval, to estimate in situ void ratio from core samples Calculated C c Values (Empirical) [15] At Site 1043, I obtain a second estimate of C c calculated using observed changes in void ratio between Sites 1039 and 1040, and the P 0 c estimates at 1040 from laboratory tests, assuming that P 0 c reflects in situ effective stress: C ccalculated ¼ ð e 1039 e 1040 Þ ; ð4þ log Pc =s Figure 3. Example of consolidation test result for sample R from Unit II (see Figure 2). Arrows indicate stress path during the laboratory test. The maximum past burial stress (Pc), 0 assumed equal to the in situ effective stress, is determined graphically [e.g., Holtz and Kovacs, 1981]. Using this technique, a line is drawn tangent to the consolidation curve at the point of maximum curvature. Pc 0 is given by the intersection of the virgin consolidation curve and a line bisecting the angle (a) between the tangent line and a line of constant void ratio at the point of maximum curvature. A minimum value of Pc 0 (Pc 0 min) is given by the point at which stress-strain data no longer plot along a reloading curve; a maximum value for Pc 0 (Pc 0 max) is given by the first point plotting along the virgin consolidation curve.

5 where e 1039 and e 1040 are void ratios averaged within correlated sediments at Sites 1039 and 1040, respectively, Pc is effective stress at Site 1039, and s is the laboratory-derived Pc 0 estimate for samples from Site The values of C ccalculated determined from equation (4) reflect the average effective sediment properties between the trench and Site [16] I correlate packets of sediment using the thickness of solids beneath the decollement, which is conserved in the case of uniaxial consolidation (Figure 4). The thickness of solids (H s ) is given by: Z z H s ¼ 0 SAFFER: PORE PRESSURE AND DEWATERING EPM 9-5 H total ðe z þ 1Þ ; ð5þ where z is depth, H total is the total thickness determined from drilling, and e z is observed void ratio. I assume that sediments at Site 1039 are normally consolidated, on the basis of (1) extremely low observed sedimentation rates outboard of the trench (20 30 m Ma 1 below 19 mbsf; 95 m Ma 1 over the upper 19 m) [Kimura et al., 1997], and (2) Pc 0 values from Site 1039, which indicate normal consolidation in the upper 189 mbsf [Saffer et al., 2000] C c Determined From Field Consolidation Curves [17] Unlike Costa Rica, few detailed consolidation experiments have been conducted on underthrust sediments at Nankai and Barbados. However, the underthrust sediments at both Nankai and Barbados contain lithologically homogeneous mudstones and clay stones, allowing the development of a field-based consolidation curve from an oceanic reference site. [18] To determine the compression index (C c ) of sediments at these two locations, I derive a field-based virgin consolidation curve using observed void ratios at reference sites well outboard of the subduction trench and assuming that these sediments are normally consolidated [e.g., Screaton et al., 2002]. At both sites, the assumption of hydrostatic pore pressure at the reference sites is supported by (1) the generally low sedimentation rates for much of the sedimentary section ( m Ma 1 at Barbados; m Ma 1 in the lower 600 m, and 500 m Ma 1 in the upper 104 m at Nankai) [Mascle et al., 1988; Moore et al., 2001], (2) their large distance from the trench (10 km at Nankai, 4 km at Barbados), and (3) modeling studies which show that development of overpressures outboard of the trench at Barbados is unlikely [Screaton and Ge, 2000]. The relatively high recent sedimentation rates for the upper 100 m of sediments at the reference site at Nankai could result in modest overpressures throughout the section, depending on sediment permeability [e.g., Screaton et al., 2002]. [19] At Nankai, the underthrust section is composed of homogeneous hemipelagic mudstone with some interbedded altered ash layers [Moore et al., 2001]. The entire section has been penetrated at a reference site and at two sites landward of the subduction trench. The sediments within this unit are lithologically monotonous, and exhibit a systematic decrease in porosity with depth as would be expected for mechanical compaction. At Barbados, I consider only the uppermost underthrust unit, because the deeper units were not penetrated at all sites. The lithologic unit immediately below the decollement at Barbados is composed of 104 m of clay stone, Figure 4. Void ratio of sediments at Sites 1039 (light gray circles), 1043 (dark gray circles), and 1040 (solid circles), plotted by height of solids beneath the decollement determined by equation (5). Note the large reduction in void ratio of correlated sediments within Units I and II compared with more modest reductions throughout much of Unit III. The decrease in void ratio between Sites 1039 and 1043 is generally larger than the decrease between Sites 1043 and mudstone, and siltstone of variable carbonate content [Mascle et al., 1988]. Like the underthrust sediments at Nankai, this unit exhibits a systematic decrease in void ratio with depth, implying some degree of mechanical consolidation. At least a portion of this lithologic unit was penetrated at a reference site (ODP Sites 672 and later 1044) and at several sites inboard of the trench Estimation of Effective Stress and Pore Pressure Use of Pc 0 as an Estimate of Effective Stress [20] For the case of underthrust sediments subjected to a monotonically increasing load [e.g., Moore, 1989; Behrmann and Kopf, 1993], the maximum past effective stress

6 EPM 9-6 SAFFER: PORE PRESSURE AND DEWATERING Figure 5. Estimated effective stresses at (a) Sites 1039, (b) 1043, and (c) 1040, determined from laboratory values of Pc 0 (circles), and projected using observed void ratio reduction and laboratory measurements of C c (crosses) and C ccalculated (triangles). Error bars for Pc 0 are defined by Pc 0 min and Pc 0 max as discussed in text. Solid gray line in each figure indicates the initial effective stress state, defined using bulk density data from Site 1039 and assuming hydrostatic pore pressure. Dashed line indicates effective stress expected for fully drained conditions at each site (hydrostatic pore pressure). At Site 1039, Pc 0 estimates are consistent with the assumption of hydrostatic pore pressure. Effective stresses determined using both techniques indicate nearly undrained conditions throughout the underthrust section at Site At Site 1040, inferred effective stresses indicate partial drainage within Units I and II, and potentially at the base of Unit III. determined from laboratory consolidation tests, P 0 c, can be interpreted as the in situ vertical effective stress. The tilted beds and microfaulting observed in portions of the underthrust sequence at Site 1040 imply that the deformation of underthrust units may be more complex than described by uniaxial compression. However, the applicability of onedimensional loading for underthrust sediments is justified by several lines of evidence. First, the height of solids within the underthrust section calculated from observed void ratios yields nearly identical values for the three sites (Figure 4). The conservation of solid height in the section implies that compaction is dominantly vertical. Second, P 0 c values from Site 1039 indicate normal consolidation (Figure 5a) [Saffer et al., 2000]. This implies that the reference site does not feel the weight of the nearby wedge, and that underthrust sediments are not efficient at transmitting stresses laterally. Third, anisotropic magnetic susceptibility (AMS) data from other subduction zones document that the maximum principal strain in underthrust sediments is vertical [e.g., Housen et al., 1996; Morgan and Karig, 1993]. Thus for samples from Sites 1039 and 1040, experimentally determined values of P 0 c provide estimates of in situ effective stress and pore pressure [e.g., Saffer et al., 2000]. [21] It is important to note that P 0 c values determined from consolidation tests are subject to considerable uncertainty, especially in active tectonic settings and at depths of hundreds of meters. These uncertainties include: (1) possible large differences between minimum and maximum bounds, generally caused by sampling disturbance, and (2) potential cementation. As discussed above, the results of such tests are useful for identifying significant departures from normal consolidation, for documenting spatial trends, and for estimating, to first order, in situ effective stress, especially if multiple samples yield consistent results Effective Stress Determined From Observed Void Ratios [22] The complete underthrusting of a uniform incoming sediment section, along with detailed laboratory consolidation tests, also allows a second estimate of in situ effective stress at Costa Rica. This estimate is obtained by equation (3), combining observed changes in void ratio between sites in correlated sediments with directly measured values of C c. This method also allows estimation of effective stress for Site 1043, at which no core samples were collected for this study and thus no estimates of Pc 0 are available. At Site 1043, values of C ccalculated determined from equation (4) allow a second estimate of effective stress. In essence, the estimates of effective stress at 1043 using C c and C ccalculated provide a calibrated interpolation between a normal consolidation state at the trench and values of effective stress defined by Pc 0 at Site [23] Rearranging equation (3), the effective stress within a sediment packet is given by: s 0 ¼ s i 10½ ðei eþ=cc labš ; ð6þ where s i is the initial effective stress calculated from bulk densities at Site 1039 assuming normal consolidation, e i is

7 SAFFER: PORE PRESSURE AND DEWATERING EPM 9-7 the initial void ratio, e is the new void ratio, and C clab is the compression index measured directly in laboratory experiments. For both Sites 1043 and 1040, I calculate effective stress for sediment packets of 5 m in solid thickness. The initial and new void ratios used in applying equation (6) are averages of several individual measurements within each packet; the values of C c are measured directly in laboratory experiments and averaged by lithologic unit (see Table 1). In general, consolidation is nonreversible (plastic). Thus if elevated pore pressures are maintained, in part, by injection of fluids from greater depth, actual pore pressures could be larger than estimated from void ratios in this study. [24] Unlike the Costa Rican subduction zone, the incoming sediment section at the Nankai and Barbados accretionary complexes is partitioned between underthrust and accreted sediments [e.g., Taira et al., 1991; Mascle et al., 1988]. In addition, the incoming section is not uniform in thickness at either location [e.g., Zhao et al., 1998]. As a result, the methods used for Costa Rica to define C c cannot be applied at these locations. [25] However, at Nankai and Barbados, field-based consolidation curves can be used to calculate effective stress from observed void ratios at progressively buried sites by rearranging equation (3). In addition to mechanical compaction, the field-based consolidation curve includes the effects of cementation, chemical compaction, creep, and thermal and diagenetic effects. However, the differences in observed void ratio between adjacent drill sites should dominantly reflect differences in stress state, because the thermal state and ages of sediments at nearby locations are similar. This method differs from previous estimates of effective stress for Barbados, which have been based on a laboratory-derived virgin consolidation curve [e.g., Moore and Tobin, 1997]. 4. Laboratory and Analytical Results 4.1. Costa Rica C c Values [26] Values of C c determined directly in laboratory experiments on samples from Sites 1039 and 1040 vary from to 3.13 (Table 1). At both sites, values of C c for the hemipelagic sediments are generally greater than those for the pelagic carbonates. The difference in C c values between lithologies is significantly larger on samples from Site 1039 than from Site C c values from samples from the hemipelagic sediments at Site 1039 (C c = ) are significantly higher than values from similar sediments at Site 1040 (C c = ). C c values for carbonate sediments from both sites are comparable. Values of C c calculated from equation (4) range from 0.13 to 0.55, and like C c values measured directly in laboratory tests, are generally higher in the hemipelagic sediments than in the carbonates. Calculated values of C c are consistent with laboratory measurements on samples from Site 1040, but are generally lower than measured values on samples from Site Void Ratio Changes in Correlated Sediments [27] Observed decreases in void ratio of correlated sediments reflect dewatering and increasing effective stress with burial. Compaction within the upper, hemipelagic section (Units I and II) is documented by reduction in void ratio from values of at Site 1039, to at Site 1043, and to at Site 1040 (Figure 4). In comparison, void ratio changes considerably less within most of the carbonate rich Unit III, from 2.0 at Site 1039 to 1.4 at Site Within the lowermost 20 m of solid thickness in the pelagic carbonates, the void ratio reduction is slightly larger, from at Site 1039, to 1.5 at Site Within Units I and II, a larger void ratio reduction occurs between the reference site and Site 1043 than between Sites 1043 and 1040 (Figure 4), despite the fact that Site 1043 is only 0.5 km from the trench, whereas Site 1040 is an additional 1.1 km landward. This pattern of void ratio reduction is also consistent with drilling observations, which document a more rapid thinning of the hemipelagic section between the trench and Site 1043 than between Sites 1043 and 1040 [Kimura et al., 1997] Estimated Effective Stresses and Pore Pressures [28] As shown in Figure 5, effective stresses estimated for Site 1043 using directly measured (crosses) and calculated (triangles) values of C c reflect essentially undrained conditions. At Site 1040, values of P 0 c from laboratory tests (circles) and effective stresses projected using measured values of C c (crosses) both indicate partial drainage within the uppermost part of the underthrust section (Figure 5c). The inferred increase in effective stress occurs within the hemipelagic sediments of Units I and II. [29] At both sites, the two separate estimates of effective stress are generally in good agreement, with two notable differences. First, at Site 1043, effective stresses projected using observed void ratio changes and directly measured values of C c indicate a region near the Unit II-III boundary with effective stresses MPa larger than those projected using the value of C c calculated from P 0 c, indicating that it may be partially to fully drained (Figure 5b). In comparison, estimates of effective stress calculated from P 0 c values by equation (4) suggest undrained conditions throughout the section. Second, at Site 1040, laboratory measurements of P 0 c reflect undrained conditions throughout Unit III (although the error bars are large for these measurements), whereas effective stresses projected using measured values of C c suggest that the lowermost part of Unit III is partially drained, a difference in effective stress of MPa. [30] Pore pressures calculated from inferred effective stresses by equation (2) are shown in Figure 6. For Site 1043, estimated pore pressures are consistently shifted to values 1 MPa greater than hydrostatic, as would be expected for nearly undrained conditions there. Pore pressures at the top of the underthrust section are approximately equal to the lithostatic stress, and systematically decrease to 82% of the lithostatic value by the base of the section (Table 2). [31] At Site 1040, inferred pore pressures are shifted to 3 MPa greater than hydrostatic at the base of Unit II and throughout much of Unit III (from 450 to 600 mbsf), also as would be expected for undrained conditions. Within Units I and II, inferred pore pressures range from 1.4 MPa above hydrostatic at 390 mbsf to 3 MPa above hydrostatic at 480 mbsf. The estimated pore pressures within Unit I and in the upper m of Unit II are midway between hydrostatic and lithostatic values, reflecting partial drainage. Pore pressures, normalized to lithostatic values by l = P f /P l, range from 0.81 in the hemipelagic sediments to 0.87 in the pelagic carbonates (Table 2). Pore pressures inferred from

8 EPM 9-8 SAFFER: PORE PRESSURE AND DEWATERING Figure 6. Inferred pore pressures at Sites (a) 1043 and (b) 1040, determined from laboratory values of Pc 0 (circles), and projected using observed void ratio reduction and laboratory measurements of C c (crosses) and C ccalculated (triangles). Solid gray and solid black lines indicate hydrostatic and lithostatic pore pressures, respectively. Dashed line indicates expected pore pressure in an undrained scenario. At Site 1043, pore pressures reflect undrained conditions throughout the section, and pore pressures are nearly equal to the lithostatic pressure at the top of Unit I. At Site 1040, pore pressures reflect undrained conditions within much of Unit III and partly drained conditions within Units I and II. measured values of Pc 0 are consistent with undrained conditions at the base of Unit III ( mbsf) [Saffer et al., 2000], whereas pore pressures projected using laboratory values of C c are as much as 1 MPa less than predicted for an undrained scenario, again raising the possibility that these sediments are partially drained Barbados and Nankai Results C c Values [32] Field consolidation curves for reference sites at both the Nankai and Barbados accretionary wedges yield consistent relationships between void ratio and effective stress within the proto-underthrust sediments (Figure 7). The fact that observed void ratios at both locations are consistent with a predicted mechanical consolidation response [e.g., Holtz and Kovacs, 1981] implies that their consolidation is, in fact, dominated by mechanical processes. In addition, the value of C c obtained from field data at Barbados is 0.996, in close Figure 7. Field-based virgin consolidation curves for the lithologic unit immediately beneath the proto-decollement at (a) Barbados, Site 1044 and (b) Nankai, Site For Barbados, void ratios are derived from LWD density data collected on ODP Leg 171A; for Nankai, void ratios are based on shipboard index property data collected on ODP Leg 190. Effective stress at both reference sites is calculated assuming hydrostatic pore pressure. Note that the data from both sites follow an expected relationship for mechanical consolidation defined by equation (3). agreement with the mean value of 1.02 measured in the laboratory for calcareous clay stones from the same location [Taylor and Leonard, 1990]. The value of C c derived from field data at Nankai is 1.34, significantly larger than values of measured in the laboratory [Feeser et al., 1993]. However, the laboratory tests for sediments from Nankai did not reach the virgin consolidation curve, and thus underestimate C c significantly [Feeser et al., 1993] Estimated Effective Stresses and Pore Pressures [33] At Barbados, estimated pore pressures within the uppermost underthrust sediments indicate nearly undrained Table 2. Drill Sites Used to Estimate Pore Pressures at Costa Rica, Northern Barbados, and Nankai, Noting Distance From Trench and Inferred Pore Pressure Ratio (l) a Site Costa Rica Barbados Nankai Distance From Trench, km l (Hemi) l (Pelagic) Site Distance From Trench, km l Site Distance From Trench, km a The range of values given for l reflects variability downsection. l

9 SAFFER: PORE PRESSURE AND DEWATERING EPM 9-9 Figure 8a. Estimated pore pressures within the uppermostunderthrust sediments at the toe of the Barbados accretionary wedge, calculated by equation (3), using the field-based consolidation curve shown in Figure 7 combined with observed LWD bulk densities form ODP Legs 156 and 171A [Moore et al., 1998; Shipley et al., 1995]. Direct measurements of pore pressure from instrumented boreholes at Sites 948 and 949 (same as Site 1046) are shown for comparison (dark gray vertical bars) [Foucher et al., 1997; Becker et al., 1997]. An indirect pore pressure estimate derived from Pc 0 for samples from Site 948 is also shown (open square) [Vrolijk, 1998]. In all plots, the gray shaded area shows the position of the decollement, and the solid gray and black lines denote hydrostatic and lithostatic pore pressures, respectively. The dashed gray line indicates pore pressures expected for an undrained scenario, calculated assuming a uniform thickness of incoming sediment (see discussion in text). conditions, and increase with increased burial, from 1 MPa above hydrostatic 850 m from the trench at Site 1047 to 2.5 MPa above hydrostatic 4.2 km from the trench at Site 948 (Figure 8a). Normalized pore pressures progressively increase with underthrusting, from l = at Site 1047 to l = at Site 948 (Table 2). These values are consistent with values of l = predicted for the base of the wedge near the toe by Breen and Orange [1992] on the basis of mechanical constraints from critical taper theory. At Nankai, estimated pore pressures range from MPa above hydrostatic at Site km from the trench, to MPa above hydrostatic at Site km from the trench (Figure 8b). Like Barbados, inferred values of l increase with underthrusting, from at Site 1174, to at Site 808 (Table 2). [34] The quality of inversions for in situ effective stress from field-based consolidation curves can be evaluated in several ways. First, due to the log linear nature of equation (3), small excursions in observed void ratio can result in large excursions in calculated effective stress. Thus the scatter in calculated effective stress provides a sensitive test of the applicability of the consolidation relationship to the sediments. Second, inversion results can be ground truthed by direct comparison with measured pore pressures in monitored boreholes. Third, inversion results can be compared with laboratory-derived values of P 0 c. [35] The scatter in inferred pore pressures for both study areas is generally small (<1 MPa), and demonstrates the approximate uncertainty in pore pressure estimates from the inversion. At Barbados, inferred pore pressures are in good

10 EPM 9-10 SAFFER: PORE PRESSURE AND DEWATERING Figure 8b. Estimated pore pressures within the underthrust sediments at the toe of the Nankai accretionary wedge, using LWD (Site 1174) and shipboard data (Site 808) from ODP Legs 190 and 131 [Moore et al., 2001; Taira et al., 1991]. Indirect pore pressure estimates derived from Pc 0 for samples from Site 808 are also shown (open squares) [Karig, 1993]. The locations of drill sites are shown in the seismic cross sections for each margin. In all plots, the gray shaded area shows the position of the decollement, and the solid gray and black lines denote hydrostatic and lithostatic pore pressures, respectively. The dashed gray line indicates pore pressures expected for an undrained scenario, calculated assuming a uniform thickness of incoming sediment (see discussion in text). agreement with the results of long-term monitoring at Sites 948 and 949 (same location as Site 1046, 2.2 km from the trench) [Foucher et al., 1997; Becker et al., 1997]. A single consolidation test result from the base of the decollement at Site 948 yields a value of P 0 c that is also in good agreement with inversion results [Vrolijk et al., 1998]. At Nankai, P 0 c estimates for two samples from underthrust sediments at Site 808 indicate significant underconsolidation and are in excellent agreement with inversion results, with inferred pore pressures MPa above hydrostatic near the base of the section [Karig, 1993] (Figure 8b). Ultimately, the inferred pore pressures for Site 808 may be compared with in situ measurements from ODP Leg 196, during which the borehole was instrumented in summer Discussion and Implications 5.1. Costa Rica Consolidation Behavior [36] Compression index C c values measured directly in laboratory experiments for Site 1040, and for carbonate sediments at Site 1039, are in good agreement with C c values calculated using equation (4) (Table 1). In contrast, measured values of C c for the shallowly buried hemipelagic sediments at Site 1039 are significantly higher than either calculated values for these sediments, or values from similar sediments at Site The difference in mechanical behavior of the hemipelagic sediments may be explained by weak grain-boundary cementation by authigenic clays and carbonate, which would increase sediment stiffness early in its burial. The overall consistency of calculated C c values and those measured on samples from Site 1040 suggests that these C c values most accurately reflect actual sediment properties once burial commences. [37] Throughout the drilled section, observed and inferred temperature gradients are low ( C km 1 )[Kimura et al., 1997]; thus thermal effects on compaction should be minimal. Some cementation may occur within the carbonate units, as observed in core samples [Kimura et al., 1997]. As discussed above, it is possible that cementation (chemical compaction) could both increase sediment stiffness during dewatering and reduce void ratio in the

11 SAFFER: PORE PRESSURE AND DEWATERING EPM 9-11 carbonates without an increase in effective stress. This mechanism would result in larger projected effective stress calculated from observed void ratio reduction, compared with estimates from Pc. 0 Thus inferred effective stresses calculated from void ratio change should be considered maxima. However, it is unlikely that estimates of Pc 0 at Site 1040 are strongly affected by cementation [see discussion in the work of Saffer et al., 2000]. [38] A small amount of cementation could also explain the generally lower values of C c (higher stiffness) observed in laboratory experiments for samples from Unit III than for samples from Units I and II, and for samples from Site 1040 than for samples from Site 1039 (Table 1). The lower measured compression index in the lowermost part of the section for samples from both Sites 1039 and 1040 (Table 1) results in smaller changes in void ratio for a given increase in effective stress. It is also important to note that in the biogenic pelagic carbonates, intragranular porosity may be significant [e.g., Kimura et al., 1997]. Large intragranular porosities undoubtedly introduce error into porosity and void ratio measurements determined from either wet density or water content. Thus values of e 0 in equations (3) and (4), generally defined as intergranular void ratio, may be shifted toward unrealistically high values. However, the reduction of void ratio observed in both laboratory experiments and between drill sites reflects loss of intergranular porosity. Thus uncertainty caused by large intragranular porosity does not affect the values of C c, inferred effective stresses, or inferred pore pressures presented here Hydrologic Implications [39] From both estimates of in situ effective stress at Costa Rica, it is clear that significant fluid overpressures develop within the underthrust sediments in response to loading beneath the margin wedge (Figure 6). This probably occurs because the sediments are loaded more rapidly than they can drain [e.g., Neuzil, 1995]. Remarkably, the two separate estimates of effective stress for both Sites 1043 and 1040 are in good agreement, and are consistent with the magnitude of overburden increase at both sites. The robust results imply that the methods used to infer effective stress from laboratory data provide viable estimates of in situ conditions. [40] At Site 1043, 0.5 km landward of the trench, the entire section appears essentially undrained, as documented by the negligible change in effective stress from Site 1039 after burial beneath 148 m of margin wedge (Figures 5a and 5b). This translates to almost lithostatic pore pressures and a condition of near-zero effective stress at the top of the underthrust section. Despite the negligible change in inferred effective stress throughout much of the underthrust section between Sites 1039 and 1043, observed void ratio decreases significantly within Units I and II (Figure 4), implying that at least some consolidation has occurred. The effective stresses projected for Site 1043 were obtained by combining measured values of C c with observed changes in void ratio, and the relatively large changes in void ratio early in compaction reflect only a modest increase in effective stress (a few tens to 100 kpa). Due to the log linear nature of equation (3), such a small increase in effective stress causes a large change in void ratio for sediments characterized by high initial void ratios. [41] By Site 1040, 1.6 km landward of the trench, significant overpressures have developed. Inferred effective stresses reflect nearly undrained conditions throughout much of Unit III (Figure 6b). Both laboratory-derived values of Pc 0 and effective stresses projected using measured values of C c indicate that the uppermost 100 m of the section are partially drained (Figures 5c and 6b). This is consistent with the generally smaller change in void ratio within much of Unit III than within Units I and II, as well as the smaller reduction in unit thickness observed by drilling [Kimura et al., 1997]. The differences in pore pressure development downsection reflect nonuniform fluid escape [e.g., Saffer et al., 2000]. More rapid drainage of Units I and II may result from (1) a component of upward drainage to a fault conduit along the decollement, (2) more abundant coarse-grained, high-permeability ash layers that focus flow, (3) higher permeability within the hemipelagic sediments, or (4) significant permeability anisotropy within the hemipelagic sediments [Saffer et al., 2000]. Interestingly, the upper sediments at Site 1043 appear undrained, whereas at Site 1040 they appear to be partly drained. This difference may reflect the development of increased vertical permeability between Sites 1043 and 1040 due to brittle faulting during compaction, or the breach of a low-permeability seal thought to form the base of the decollement zone [e.g., Tobin et al., 2001]. [42] Results for sample 1040C-50R (633 mbsf at Site 1040) indicate a low value of Pc 0 relative to samples above and below. Effective stress calculated from observed void ratios (equation (6)) for sediments in this depth interval is high relative to values above and below (Figure 5c). The high effective stress value is due to a combination of larger void ratio reduction than surrounding sediments and higher sediment stiffness in this interval. The most likely explanation for these results is moderate cementation at the base of the section [e.g., Kimura et al., 1997], which would explain both a higher stiffness and an artificially large effective stress calculated from the observed decrease in void ratio. The low Pc 0 value may reflect drilling disturbance or disruption of cement in this particular sample. Similarly, a small amount of cementation at the top of the carbonate sediments (Unit III) could explain the difference between effective stress determined from Pc 0 values and from observed void ratio changes; because a component of the void ratio reduction may be chemical rather than mechanical, cementation would yield an apparent increase in effective stress calculated from equation (6). [43] One useful way to quantify dewatering is to calculate the distribution of fluid sources. This provides a measure of fluid production, or escape rate, normalized to the volume of sediment source. Larger sources indicate the ability of fluids to escape more rapidly, resulting in enhanced consolidation. A high-resolution picture of fluid production can be obtained from void ratio or thickness reduction of correlated sediment packets: compaction ¼ H total xh total v p ; where compaction is the fluid source in V fluid /V total /time, x is distance from the trench, and v p is plate convergence rate. ð7þ