Pore Pressure Development and Progressive Dewatering In

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B5, 2261, doi:10.1029/2002JB001787, 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 2003.

[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-while- drilling (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:10.1029/2002JB001787, 2003.

1. Introduction localizes immediately above these sediments, and often steps downward into them. The hydrologic contribution of [2] Fluids play an important role in deformation and mass underthrust sediments is also accentuated, because they are transfer in the Earth’s crust. Pore fluid pressure is closely transported rapidly and remain underconsolidated relative to linked to the overall mechanics and morphology of sub- compacting sediments in the overlying wedge [e.g., Saffer duction zones [e.g., Hubbert and Rubey, 1959; Davis et al., and Bekins, 1998]. 1983]. Fluid pressure and sediment consolidation state [3] Previous work has documented the development of influence fault localization [e.g., Moore and Byrne, 1987], elevated pore pressures within underthrust sediments at deformation style and strength, and the updip limit of the several subduction zones. At the Nankai margin, Screaton seismogenic zone [e.g., Moore and Saffer, 2001]. The fate et al. [2002] compared porosity-depth profiles at under- of incoming pore fluids also impacts fluid, chemical, and thrust sites with a reference site to estimate depth-averaged mass budgets. From a mechanical viewpoint, the fluid excess pore pressure. At the Cascadia subduction zone, content and physical properties of underthrust sediments Cochrane et al. [1996] inverted compressional wave veloc- are especially important, because the main decollement ity for porosity, and then inferred pore pressure from porosity-depth profiles. Moore and Tobin [1997] used a Copyright 2003 by the American Geophysical Union. laboratory consolidation curve to invert observed logging- 0148-0227/03/2002JB001787$09.00 while-drilling (LWD) densities for pore pressures at the

EPM 9 - 1 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 subduc- pore pressures, dewatering processes, and rates at these tion zone, the evolution of excess pore pressures has been margins with inferences from Costa Rica. investigated using laboratory consolidation tests [Saffer et al., 2000]. Despite these efforts, progressive consolidation, 2. Geologic Setting and Background: the evolution of pore pressure with continued underthrusting, and the variability of these processes downsection are Costa Rican Margin generally not well characterized. This is due to (1) a lack of [6] The Middle America Trench offshore Costa Rica is spatial resolution in seismic methods and sampling for formed by the eastward subduction of the Cocos Plate laboratory tests, (2) generally few independent constraints beneath the Caribbean Plate at 85 km Myr1 (Figure from consolidation tests, (3) uncertainty in the relevance of 1a). During ODP Leg 170 in winter of 1996, coring and laboratory data to geologic consolidation, and (4) few direct LWD data were collected at three sites near the trench. At measurements in instrumented boreholes. reference Site 1039, located 1.5 km seaward of the trench [4] In most cases, the application of laboratory consol- (Figure 1b), LWD and core data were collected through the idation tests in estimating in situ effective stress is limited entire drilled section. The incoming sedimentary sequence by a small number of samples, lack of data at axial stresses is about 380 m thick and consists of 160 m of siliceous above a few megapascals, and potential drilling disturbance hemipelagic sediments overlying 220 m of pelagic carbo- [e.g., Feeser et al., 1993]. In general, laboratory consolida- nates (Figure 2). This stratigraphy is regionally continuous, tion tests are also limited in that they provide only an as indicated by drilling on DSDP Legs 67 and 84 offshore estimate of in situ effective stress, subject to considerable Guatemala [Aubouin and von Huene, 1985; Coulbourn, uncertainty [e.g., Holtz and Kovacs, 1981]. Nonetheless, 1982]. Site 1043, located 0.5 km landward of the trench, such laboratory tests are useful for estimating in situ penetrated through the margin wedge and underthrust sedi- conditions and identifying significant departures from fully ments to a total depth of 482 mbsf. LWD data were drained behavior, especially if multiple samples yield sim- collected through the entire drilled section, but no cores ilar results. were collected below the decollement. Site 1040, located [5] In this paper, I merge laboratory consolidation test 1.6 km landward of the trench, also penetrated through the results with LWD and high-quality physical properties data decollement and underthrust sequence to a total depth of to provide spatially detailed estimates of in situ pore 653 mbsf. Cores were collected throughout the drilled pressure at the Costa Rican, Nankai, and northern Barbados section, but no LWD data were collected below 330 mbsf convergent margins, with particular emphasis on the evo- (Figure 2). Deformation is evident in the uppermost 50 m of lution of pore pressure and effective stress at Costa Rica. the hemipelagic sequence at Site 1040, which is character- This expands significantly upon previous pore pressure ized by dipping beds and some microfaulting [Kimura et al., estimates [e.g., Saffer et al., 2000] by including data from 1997]. an additional drill site (Ocean Drilling Program (ODP) Site [7] Seismic reflection profiles and gamma ray logs show 1043), and also by evaluating the variation in excess pore that the present margin is nonaccretionary, in that essentially pressure downsection. Specifically, I combine laboratory none of the incoming pelagic sediment is offscraped consolidation test results with observed reduction in sedi- [Kimura et al., 1997]. This is confirmed by biostratigraphic ment void ratio to (1) track the evolution of effective stress studies, which indicate nearly identical age-depth relation- and pore pressure and their variation downsection between ships for the incoming and underthrust sedimentary sequen- drill sites, and (2) evaluate the mechanical and hydrologic ces. Other work suggests that the margin may be implications of the observed consolidation patterns. I then characterized by tectonic erosion rather than frontal accre- use high-quality drilling data from several ODP sites at the tion [e.g., Bourgois et al., 1984; Vannucchi et al., 2001]. Barbados and Nankai subduction zones to compare inferred The thick margin wedge is characterized by high seismic SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 3

ence 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 compli- cated because they reflect a combination of dewatering (consolidation) and differences in initial stress state.

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 Figure 2. Bulk density data from logging-while-drilling sediment framework. This condition is known as ‘‘normal (lines) and shipboard index property measurements (dots), consolidation.’’ If drainage is severely limited by low showing progressive densification and thinning of lithologic permeability, or if loading is extremely rapid, the added units between drill sites. Solid squares indicate the locations total stress is supported by trapped pore fluids, generating of samples from Sites 1039 and 1040 used for consolidation significant overpressures and resulting in essentially no tests in this study. 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]. velocities (4.0–4.8 km s1)[Christeson et al., 2000], and is [11] Effective stress is of particular importance in the interpreted as either an older accretionary wedge [Shipley et subduction setting because it affects the shear strength of al., 1990] or a continuation of the onshore Nicoya complex sediments: overlain by several hundred meters of slope apron sediments [Kimura et al., 1997; Ye et al., 1996]. The hydrologic 0 contribution of underthrust sediments is accentuated in t ¼ C þ ðÞs m ; ð1Þ nonaccretionary systems like the Costa Rican subduction 0 zone, because compressible, high-porosity surface sediment where t is shear strength, C is cohesion, s is effective is subducted rather than incorporated into an accretionary stress, and m is coefficient of friction. Thus the evolution of wedge [e.g., Le-Pichon et al., 1993]. effective stress is an important consideration for the [8] Drilling shows that the upper, hemipelagic unit is evolution and spatial variability of mechanical strength. 67% of its original thickness by Site 1040, and the lower Combined with downhole measurements of bulk density, pelagic unit is 80% of its original thickness. Because no which allow calculation of the total overburden stress, and sediment is offscraped from the incoming section, the in the case where bulk sediment compressibility is significantly greater than fluid compressibility, estimates observed thinning of units is due to water loss and densi- 0 fication, and thus provides a quantitative indication of of s in core samples can be used to infer in situ pore fluid expelled water volume. Approximately, 8 m3 yr1 of fluids pressure (Pf) by: are expelled per meter of margin from underthrust sedi- 0 ments between the time they are first subducted and when Pf ¼ soverburden s : ð2Þ they reach Site 1040. Of this, 5.4 m3 yr1 is lost by the hemipelagic section. 3.1. Determination of Consolidation Index (Cc) [9] The complete underthrusting of a regionally uniform 3.1.1. Direct Measurement sediment section offers a unique opportunity to explore [12] I conducted uniaxial consolidation tests on three changes in effective stress and compaction with progressive samples from Site 1039 and eight from Site 1040 (Table 1 burial. Unlike other subduction zones, the sediments at and Figure 2), at effective axial stresses up to 6 MPa, using Costa Rica are mechanically heterogeneous downsection a high-precision hydraulic oedometer and following stand- [e.g., Saffer et al., 2000]. Thus the commonly used method ard incremental loading procedures with a load increment of comparing porosity-depth relationships between a refer- ratio of one [Crawford, 1986; Lee, 1985; Saffer et al., EPM 9 - 4 SAFFER: PORE PRESSURE AND DEWATERING

Table 1. List of Samples and Laboratory Consolidation Test Resultsa 0 0 0 Sample Depth, mbsf Unit Pc OCR Pc Min Pc Max Cc Cccalculated Slope of RC 1039-10H-a 80.85 1B 289 0.991 224 517 1.66 0.030 1039-10H-b 80.85 1B 342 1.173 282 378 1.50 0.053 1039-16X 141.54 2B 795 1.550 636 850 3.13 0.183 1039-21X 189.05 3B 643 0.937 270 1531 0.37 0.035 1040C-27R 411.3 1B 1936 0.604 1257 3072 0.495 0.48–0.55 0.042 1040C-30R 442.7 2A 1608 0.472 928 3072 0.577 0.33–0.41 0.094 1040C-34R 478.05 2B 747 0.206 474 1524 0.273 0.21–0.47 0.054 1040C-38R 518.15 3B 1370 0.356 900 1536 0.467 0.23–0.42 0.031 1040C-42R 558.05 3B 1649 0.402 1183 3075 0.422 0.15–0.25 0.041 1040C-46R 598.15 3C 1691 0.391 1135 3067 0.445 0.13–0.44 0.039 1040C-50R 633.45 3C 390 0.086 198 780 0.243 0.026 1040C-52R 651.66 3C 1572 0.337 320 3067 0.205 0.021 a 0 0 OCR refers to overconsolidation ratio, Pc min and Pc max were determined as described in text, and RC refers to rebound curve. 0 Values of Cccalculated determined using equation (4) are not available for samples 1040C-50R and 52R because Pc values indicate essentially no change in effective stress during burial.

0 2000]. In all tests, samples were laterally confined with a Sites 1039 and 1040, and the Pc estimates at 1040 from 0 steel ring [e.g., Lee, 1985]. I obtained 15-cm-long whole laboratory tests, assuming that Pc reflects in situ effective round core samples from Sites 1039 and 1040 for consol- stress: idation and permeability tests, and used CT scans of the samples to select undisturbed portions of the core for ðÞe1039 e1040 Cccalculated ¼ ; ð4Þ 0 0 experiments. Samples were 6.25 cm in diameter, and initial log Pc =s1039 height ranged from 1.5 to 1.6 cm. All samples were 1040 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 reach- ing loads exceeding its maximum past vertical effective 0 stress (Pc), assuming that the sample has not been disturbed 0 or reworked. At stresses beyond Pc, the sample deforms plastically along a virgin consolidation curve, and the strain response for a given stress increment changes. I determined 0 Pc from laboratory tests following the Casagrande method (Figure 3) [Holtz and Kovacs, 1981]. I obtained conserva- 0 tive estimates of minimum and maximum possible Pc values from effective stresses at which sample void ratio during the consolidation test no longer plotted on a rebound 0 curve (minimum Pc ) or virgin consolidation curve (max- 0 imum Pc ). [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 (Cc), is a measure of void ratio reduction Figure 3. Example of consolidation test result for sample with increased effective stress. The virgin consolidation 1040-38R from Unit II (see Figure 2). Arrows indicate curve is defined by: stress path during the laboratory test. The maximum past 0 e ¼ e C logðÞs0 ; ð3Þ burial stress (Pc), assumed equal to the in situ effective 0 c stress, is determined graphically [e.g., Holtz and Kovacs, where e is void ratio, e0 is void ratio projected at zero 1981]. Using this technique, a line is drawn tangent to the 0 0 effective stress, Cc is the compression index, and s is consolidation curve at the point of maximum curvature. Pc effective stress. The slope of the elastic rebound portion of is given by the intersection of the virgin consolidation curve the stress-strain curve is generally used to correct for the and a line bisecting the angle (a) between the tangent line effects of expansion from unloading during sample and a line of constant void ratio at the point of maximum 0 0 retrieval, to estimate in situ void ratio from core samples. curvature. A minimum value of Pc (Pc min) is given by the point at which stress-strain data no longer plot along a 0 0 3.1.2. Calculated Cc Values (Empirical) reloading curve; a maximum value for Pc (Pc max) is given [15] At Site 1043, I obtain a second estimate of Cc by the first point plotting along the virgin consolidation calculated using observed changes in void ratio between curve. SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 5 where e1039 and e1040 are void ratios averaged within correlated sediments at Sites 1039 and 1040, respectively, 0 0 Pc is effective stress at Site 1039, and s1040 is the 1039 0 laboratory-derived Pc estimate for samples from Site 1040.

The values of Cccalculated determined from equation (4) reflect the average effective sediment properties between the trench and Site 1040. [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 (Hs) is given by: Zz Htotal Hs ¼ ; ð5Þ ðÞez þ 1 0 where z is depth, Htotal is the total thickness determined from drilling, and ez 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 Ma1 below 19 mbsf; 95 m Ma1 over the upper 19 m) [Kimura et al., 1997], 0 and (2) Pc values from Site 1039, which indicate normal consolidation in the upper 189 mbsf [Saffer et al., 2000].

3.1.3. Cc 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 (Cc) 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., Screa- ton 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 sedimen- Figure 4. Void ratio of sediments at Sites 1039 (light gray tary section (2.2–21 m Ma1 at Barbados; 33.6–83 m circles), 1043 (dark gray circles), and 1040 (solid circles), Ma1 in the lower 600 m, and 500 m Ma1 in the upper plotted by height of solids beneath the decollement 104 m at Nankai) [Mascle et al., 1988; Moore et al., 2001], determined by equation (5). Note the large reduction in (2) their large distance from the trench (10 km at Nankai, void ratio of correlated sediments within Units I and II 4 km at Barbados), and (3) modeling studies which show compared with more modest reductions throughout much of that development of overpressures outboard of the trench at Unit III. The decrease in void ratio between Sites 1039 and Barbados is unlikely [Screaton and Ge, 2000]. The rela- 1043 is generally larger than the decrease between Sites tively high recent sedimentation rates for the upper 100 m 1043 and 1040. of sediments at the reference site at Nankai could result in modest overpressures throughout the section, depending on mudstone, and siltstone of variable carbonate content sediment permeability [e.g., Screaton et al., 2002]. [Mascle et al., 1988]. Like the underthrust sediments at [19] At Nankai, the underthrust section is composed of Nankai, this unit exhibits a systematic decrease in void ratio homogeneous hemipelagic mudstone with some interbedded with depth, implying some degree of mechanical consolida- altered ash layers [Moore et al., 2001]. The entire section has tion. At least a portion of this lithologic unit was penetrated been penetrated at a reference site and at two sites landward at a reference site (ODP Sites 672 and later 1044) and at of the subduction trench. The sediments within this unit are several sites inboard of the trench. lithologically monotonous, and exhibit a systematic decrease in porosity with depth as would be expected for mechanical 3.2. Estimation of Effective Stress and Pore Pressure 0 compaction. At Barbados, I consider only the uppermost 3.2.1. Use of Pc as an Estimate of Effective Stress underthrust unit, because the deeper units were not pene- [20] For the case of underthrust sediments subjected to a trated at all sites. The lithologic unit immediately below the monotonically increasing load [e.g., Moore, 1989; Behr- decollement at Barbados is composed of 104 m of clay stone, mann and Kopf, 1993], the maximum past effective stress EPM 9 - 6 SAFFER: PORE PRESSURE AND DEWATERING

Figure 5. Estimated effective stresses at (a) Sites 1039, (b) 1043, and (c) 1040, determined from 0 laboratory values of Pc (circles), and projected using observed void ratio reduction and laboratory 0 0 0 measurements of Cc (crosses) and Cccalculated (triangles). Error bars for Pc are defined by Pc min and Pc 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 0 stress expected for fully drained conditions at each site (hydrostatic pore pressure). At Site 1039, Pc 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 1043. At Site 1040, inferred effective stresses indicate partial drainage within Units I and II, and potentially at the base of Unit III.

0 determined from laboratory consolidation tests, Pc, can be potential cementation. As discussed above, the results of interpreted as the in situ vertical effective stress. The tilted such tests are useful for identifying significant departures beds and microfaulting observed in portions of the under- from normal consolidation, for documenting spatial trends, thrust sequence at Site 1040 imply that the deformation of and for estimating, to first order, in situ effective stress, underthrust units may be more complex than described by especially if multiple samples yield consistent results. uniaxial compression. However, the applicability of one- 3.2.2. Effective Stress Determined From Observed dimensional loading for underthrust sediments is justified Void Ratios by several lines of evidence. First, the height of solids within the underthrust section calculated from observed [22] The complete underthrusting of a uniform incoming void ratios yields nearly identical values for the three sites sediment section, along with detailed laboratory consolida- (Figure 4). The conservation of solid height in the section tion tests, also allows a second estimate of in situ effective 0 stress at Costa Rica. This estimate is obtained by equation implies that compaction is dominantly vertical. Second, Pc values from Site 1039 indicate normal consolidation (Figure (3), combining observed changes in void ratio between sites 5a) [Saffer et al., 2000]. This implies that the reference site in correlated sediments with directly measured values of Cc. does not ‘‘feel’’ the weight of the nearby wedge, and that This method also allows estimation of effective stress for Site 1043, at which no core samples were collected for this underthrust sediments are not efficient at transmitting 0 stresses laterally. Third, anisotropic magnetic susceptibility study and thus no estimates of Pc are available. At Site 1043, values of C determined from equation (4) (AMS) data from other subduction zones document that the ccalculated maximum principal strain in underthrust sediments is ver- allow a second estimate of effective stress. In essence, the estimates of effective stress at 1043 using C and C tical [e.g., Housen et al., 1996; Morgan and Karig, 1993]. c ccalculated Thus for samples from Sites 1039 and 1040, experimentally provide a calibrated interpolation between a normal con- 0 solidation state at the trench and values of effective stress determined values of Pc provide estimates of in situ effec- 0 tive stress and pore pressure [e.g., Saffer et al., 2000]. defined by Pc at Site 1040. 0 [23] Rearranging equation (3), the effective stress within [21] It is important to note that Pc values determined from consolidation tests are subject to considerable uncertainty, a sediment packet is given by: especially in active tectonic settings and at depths of 0 ðÞe e =C s ¼ s 10½i clab ; ð6Þ hundreds of meters. These uncertainties include: (1) possi- i ble large differences between minimum and maximum where si is the initial effective stress calculated from bulk bounds, generally caused by sampling disturbance, and (2) densities at Site 1039 assuming normal consolidation, ei is SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 7

the initial void ratio, e is the new void ratio, and Cclab is the 1043, and to 1.4–1.6 at Site 1040 (Figure 4). In compar- compression index measured directly in laboratory experi- ison, void ratio changes considerably less within most of the ments. For both Sites 1043 and 1040, I calculate effective carbonate rich Unit III, from 2.0 at Site 1039 to 1.4 at stress for sediment packets of 5 m in solid thickness. The Site 1040. Within the lowermost 20 m of solid thickness initial and new void ratios used in applying equation (6) are in the pelagic carbonates, the void ratio reduction is slightly averages of several individual measurements within each larger, from 2.0–2.5 at Site 1039, to 1.5 at Site 1040. packet; the values of Cc are measured directly in laboratory Within Units I and II, a larger void ratio reduction occurs experiments and averaged by lithologic unit (see Table 1). between the reference site and Site 1043 than between Sites In general, consolidation is nonreversible (plastic). Thus if 1043 and 1040 (Figure 4), despite the fact that Site 1043 is elevated pore pressures are maintained, in part, by injection only 0.5 km from the trench, whereas Site 1040 is an of fluids from greater depth, actual pore pressures could be additional 1.1 km landward. This pattern of void ratio larger than estimated from void ratios in this study. reduction is also consistent with drilling observations, [24] Unlike the Costa Rican subduction zone, the incom- which document a more rapid thinning of the hemipelagic ing sediment section at the Nankai and Barbados accre- section between the trench and Site 1043 than between Sites tionary complexes is partitioned between underthrust and 1043 and 1040 [Kimura et al., 1997]. accreted sediments [e.g., Taira et al., 1991; Mascle et al., 1988]. In addition, the incoming section is not uniform in 4.1.3. Estimated Effective Stresses and Pore Pressures thickness at either location [e.g., Zhao et al., 1998]. As a [28] As shown in Figure 5, effective stresses estimated for result, the methods used for Costa Rica to define Cc cannot Site 1043 using directly measured (crosses) and calculated be applied at these locations. (triangles) values of Cc reflect essentially undrained con- [25] However, at Nankai and Barbados, field-based con- 0 ditions. At Site 1040, values of Pc from laboratory tests solidation curves can be used to calculate effective stress (circles) and effective stresses projected using measured from observed void ratios at progressively buried sites by values of Cc (crosses) both indicate partial drainage within rearranging equation (3). In addition to mechanical com- the uppermost part of the underthrust section (Figure 5c). paction, the field-based consolidation curve includes the The inferred increase in effective stress occurs within the effects of cementation, chemical compaction, creep, and hemipelagic sediments of Units I and II. thermal and diagenetic effects. However, the differences in [29] At both sites, the two separate estimates of effective observed void ratio between adjacent drill sites should stress are generally in good agreement, with two notable dominantly reflect differences in stress state, because the differences. First, at Site 1043, effective stresses projected thermal state and ages of sediments at nearby locations are using observed void ratio changes and directly measured similar. This method differs from previous estimates of values of Cc indicate a region near the Unit II-III boundary effective stress for Barbados, which have been based on a with effective stresses 0.20–1.0 MPa larger than those laboratory-derived virgin consolidation curve [e.g., Moore 0 projected using the value of Cc calculated from Pc, indicating and Tobin, 1997]. that it may be partially to fully drained (Figure 5b). In 0 comparison, estimates of effective stress calculated from Pc 4. Laboratory and Analytical Results values by equation (4) suggest undrained conditions through- 4.1. Costa Rica out the section. Second, at Site 1040, laboratory measure- 0 4.1.1. Cc Values ments of Pc reflect undrained conditions throughout Unit III [26] Values of Cc determined directly in laboratory experi- (although the error bars are large for these measurements), ments on samples from Sites 1039 and 1040 vary from 0.205 whereas effective stresses projected using measured values of to 3.13 (Table 1). At both sites, values of Cc for the hemi- Cc suggest that the lowermost part of Unit III is partially pelagic sediments are generally greater than those for the drained, a difference in effective stress of 0.40–1.0 MPa. 30 pelagic carbonates. The difference in Cc values between [ ] Pore pressures calculated from inferred effective lithologies is significantly larger on samples from Site 1039 stresses by equation (2) are shown in Figure 6. For Site than from Site 1040. Cc values from samples from the 1043, estimated pore pressures are consistently shifted to hemipelagic sediments at Site 1039 (Cc = 1.50–3.13) are values 1 MPa greater than hydrostatic, as would be ex- significantly higher than values from similar sediments at pected for nearly undrained conditions there. Pore pressures Site 1040 (Cc = 0.27–0.58). Cc values for carbonate sedi- at the top of the underthrust section are approximately equal to ments from both sites are comparable. Values of Cc calcu- the lithostatic stress, and systematically decrease to 82% of lated from equation (4) range from 0.13 to 0.55, and like Cc the lithostatic value by the base of the section (Table 2). values measured directly in laboratory tests, are generally [31] At Site 1040, inferred pore pressures are shifted to higher in the hemipelagic sediments than in the carbonates. 3 MPa greater than hydrostatic at the base of Unit II and Calculated values of Cc are consistent with laboratory throughout much of Unit III (from 450 to 600 mbsf), also as measurements on samples from Site 1040, but are generally would be expected for undrained conditions. Within Units I lower than measured values on samples from Site 1039. and II, inferred pore pressures range from 1.4 MPa above hydrostatic at 390 mbsf to 3 MPa above hydrostatic at 480 4.1.2. Void Ratio Changes in Correlated Sediments mbsf. The estimated pore pressures within Unit I and in the [27] Observed decreases in void ratio of correlated sedi- upper 30–40 m of Unit II are midway between hydrostatic ments reflect dewatering and increasing effective stress with and lithostatic values, reflecting partial drainage. Pore burial. Compaction within the upper, hemipelagic section pressures, normalized to lithostatic values by l = Pf /Pl, (Units I and II) is documented by reduction in void ratio range from 0.81 in the hemipelagic sediments to 0.87 in the from values of 2.5–3.5 at Site 1039, to 2.0–2.5 at Site pelagic carbonates (Table 2). Pore pressures inferred from EPM 9 - 8 SAFFER: PORE PRESSURE AND DEWATERING

Figure 6. Inferred pore pressures at Sites (a) 1043 and (b) 0 1040, determined from laboratory values of Pc (circles), and projected using observed void ratio reduction and laboratory measurements of Cc (crosses) and Cccalculated (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 Figure 7. Field-based virgin consolidation curves for the pressure at the top of Unit I. At Site 1040, pore pressures lithologic unit immediately beneath the proto-decollement reflect undrained conditions within much of Unit III and at (a) Barbados, Site 1044 and (b) Nankai, Site 1173. For partly drained conditions within Units I and II. Barbados, void ratios are derived from LWD density data collected on ODP Leg 171A; for Nankai, void ratios are 0 based on shipboard index property data collected on ODP measured values of Pc are consistent with undrained conditions at the base of Unit III (600–650 mbsf) [Saffer et al., Leg 190. Effective stress at both reference sites is calculated 2000], whereas pore pressures projected using laboratory assuming hydrostatic pore pressure. Note that the data from both sites follow an expected relationship for mechanical values of Cc are as much as 1 MPa less than predicted for an undrained scenario, again raising the possibility that these consolidation defined by equation (3). sediments are partially drained. agreement with the mean value of 1.02 measured in the 4.2. Barbados and Nankai Results laboratory for calcareous clay stones from the same location 4.2.1. Cc Values [Taylor and Leonard, 1990]. The value of Cc derived from [32] Field consolidation curves for reference sites at both field data at Nankai is 1.34, significantly larger than values of the Nankai and Barbados accretionary wedges yield consis- 0.10–0.57 measured in the laboratory [Feeser et al., 1993]. tent relationships between void ratio and effective stress However, the laboratory tests for sediments from Nankai did within the proto-underthrust sediments (Figure 7). The fact not reach the virgin consolidation curve, and thus under- that observed void ratios at both locations are consistent with estimate Cc significantly [Feeser et al., 1993]. a predicted mechanical consolidation response [e.g., Holtz and Kovacs, 1981] implies that their consolidation is, in fact, 4.2.2. Estimated Effective Stresses and Pore Pressures dominated by mechanical processes. In addition, the value of [33] At Barbados, estimated pore pressures within the Cc obtained from field data at Barbados is 0.996, in close 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 Costa Rica Barbados Nankai Distance From Trench, Distance From Trench, Distance From Trench, Site km l (Hemi) l (Pelagic) Site km l Site km l 1043 0.5 0.91 0.81 1047 0.85 0.68–0.84 1174 2 0.66–0.76 1040 1.6 0.81 0.87 1046 2.2 0.67–0.83 808 3 0.68–0.77 1045 3.2 0.73–0.90 948 4.2 0.70–0.92 aThe range of values given for l reflects variability downsection. 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., 0 1997; Becker et al., 1997]. An indirect pore pressure estimate derived from Pc 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 [34] The quality of inversions for in situ effective stress above hydrostatic 850 m from the trench at Site 1047 to from field-based consolidation curves can be evaluated in 2.5 MPa above hydrostatic 4.2 km from the trench at Site several ways. First, due to the log linear nature of equation 948 (Figure 8a). Normalized pore pressures progressively (3), small excursions in observed void ratio can result in increase with underthrusting, from l = 0.68–0.84 at Site large excursions in calculated effective stress. Thus the 1047 to l = 0.70–0.92 at Site 948 (Table 2). These values scatter in calculated effective stress provides a sensitive test are consistent with values of l = 0.68–0.98 predicted for of the applicability of the consolidation relationship to the the base of the wedge near the toe by Breen and Orange sediments. Second, inversion results can be ground truthed [1992] on the basis of mechanical constraints from critical by direct comparison with measured pore pressures in taper theory. At Nankai, estimated pore pressures range monitored boreholes. Third, inversion results can be com- 0 from 2.7–5.0 MPa above hydrostatic at Site 1174 2km pared with laboratory-derived values of Pc. from the trench, to 4.3–6.1 MPa above hydrostatic at Site [35] The scatter in inferred pore pressures for both study 808 3 km from the trench (Figure 8b). Like Barbados, areas is generally small (<1 MPa), and demonstrates the inferred values of l increase with underthrusting, from approximate uncertainty in pore pressure estimates from the 0.66–0.76 at Site 1174, to 0.68–0.77 at Site 808 (Table 2). inversion. At Barbados, inferred pore pressures are in good 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 0 [Moore et al., 2001; Taira et al., 1991]. Indirect pore pressure estimates derived from Pc 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 sediments at Site 1039, are in good agreement with Cc 948 and 949 (same location as Site 1046, 2.2 km from the values calculated using equation (4) (Table 1). In contrast, trench) [Foucher et al., 1997; Becker et al., 1997]. A single measured values of Cc for the shallowly buried hemipelagic consolidation test result from the base of the decollement at sediments at Site 1039 are significantly higher than either 0 Site 948 yields a value of Pc that is also in good agreement calculated values for these sediments, or values from similar 0 with inversion results [Vrolijk et al., 1998]. At Nankai, Pc sediments at Site 1040. The difference in mechanical estimates for two samples from underthrust sediments at behavior of the hemipelagic sediments may be explained Site 808 indicate significant underconsolidation and are in by weak grain-boundary cementation by authigenic clays excellent agreement with inversion results, with inferred and carbonate, which would increase sediment stiffness pore pressures 4.9–6.3 MPa above hydrostatic near the early in its burial. The overall consistency of calculated base of the section [Karig, 1993] (Figure 8b). Ultimately, Cc values and those measured on samples from Site 1040 the inferred pore pressures for Site 808 may be compared suggests that these Cc values most accurately reflect actual with in situ measurements from ODP Leg 196, during sediment properties once burial commences. which the borehole was instrumented in summer 2001. [37] Throughout the drilled section, observed and inferred temperature gradients are low (8.4–9.6Ckm1)[Kimura et al., 1997]; thus thermal effects on compaction should be 5. Discussion and Implications minimal. Some cementation may occur within the carbo- 5.1. Costa Rica nate units, as observed in core samples [Kimura et al., 5.1.1. Consolidation Behavior 1997]. As discussed above, it is possible that cementation [36] Compression index Cc values measured directly in (chemical compaction) could both increase sediment stiff- laboratory experiments for Site 1040, and for carbonate ness during dewatering and reduce void ratio in the SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 11 carbonates without an increase in effective stress. This [41] By Site 1040, 1.6 km landward of the trench, mechanism would result in larger projected effective stress significant overpressures have developed. Inferred effective calculated from observed void ratio reduction, compared stresses reflect nearly undrained conditions throughout 0 with estimates from Pc. Thus inferred effective stresses much of Unit III (Figure 6b). Both laboratory-derived 0 calculated from void ratio change should be considered values of Pc and effective stresses projected using measured 0 maxima. However, it is unlikely that estimates of Pc at Site values of Cc indicate that the uppermost 100 m of the 1040 are strongly affected by cementation [see discussion in section are partially drained (Figures 5c and 6b). This is the work of Saffer et al., 2000]. consistent with the generally smaller change in void ratio [38] A small amount of cementation could also explain within much of Unit III than within Units I and II, as well as the generally lower values of Cc (higher stiffness) the smaller reduction in unit thickness observed by drilling observed in laboratory experiments for samples from Unit [Kimura et al., 1997]. The differences in pore pressure III than for samples from Units I and II, and for samples development downsection reflect nonuniform fluid escape from Site 1040 than for samples from Site 1039 (Table 1). [e.g., Saffer et al., 2000]. More rapid drainage of Units I and The lower measured compression index in the lowermost II may result from (1) a component of upward drainage to a part of the section for samples from both Sites 1039 and fault conduit along the decollement, (2) more abundant 1040 (Table 1) results in smaller changes in void ratio for coarse-grained, high-permeability ash layers that focus flow, a given increase in effective stress. It is also important to (3) higher permeability within the hemipelagic sediments, note that in the biogenic pelagic carbonates, intragranular or (4) significant permeability anisotropy within the hemi- porosity may be significant [e.g., Kimura et al., 1997]. pelagic sediments [Saffer et al., 2000]. Interestingly, the Large intragranular porosities undoubtedly introduce error upper sediments at Site 1043 appear undrained, whereas at into porosity and void ratio measurements determined Site 1040 they appear to be partly drained. This difference from either wet density or water content. Thus values of may reflect the development of increased vertical perme- e0 in equations (3) and (4), generally defined as intergra- ability between Sites 1043 and 1040 due to brittle faulting nular void ratio, may be shifted toward unrealistically high during compaction, or the breach of a low-permeability seal values. However, the reduction of void ratio observed in thought to form the base of the decollement zone [e.g., both laboratory experiments and between drill sites reflects Tobin et al., 2001]. loss of intergranular porosity. Thus uncertainty caused by [42] Results for sample 1040C-50R (633 mbsf at Site 0 large intragranular porosity does not affect the values of 1040) indicate a low value of Pc relative to samples above Cc, inferred effective stresses, or inferred pore pressures and below. Effective stress calculated from observed void presented here. ratios (equation (6)) for sediments in this depth interval is high relative to values above and below (Figure 5c). The 5.1.2. Hydrologic Implications high effective stress value is due to a combination of larger [39] From both estimates of in situ effective stress at Costa void ratio reduction than surrounding sediments and higher Rica, it is clear that significant fluid overpressures develop sediment stiffness in this interval. The most likely explan- within the underthrust sediments in response to loading ation for these results is moderate cementation at the base of beneath the margin wedge (Figure 6). This probably occurs the section [e.g., Kimura et al., 1997], which would explain because the sediments are loaded more rapidly than they can both a higher stiffness and an artificially large effective drain [e.g., Neuzil, 1995]. Remarkably, the two separate stress calculated from the observed decrease in void ratio. 0 estimates of effective stress for both Sites 1043 and 1040 The low Pc value may reflect drilling disturbance or are in good agreement, and are consistent with the magnitude disruption of cement in this particular sample. Similarly, a of overburden increase at both sites. The robust results imply small amount of cementation at the top of the carbonate that the methods used to infer effective stress from labora- sediments (Unit III) could explain the difference between 0 tory data provide viable estimates of in situ conditions. effective stress determined from Pc values and from [40] At Site 1043, 0.5 km landward of the trench, the entire observed void ratio changes; because a component of the section appears essentially undrained, as documented by the void ratio reduction may be chemical rather than mechan- negligible change in effective stress from Site 1039 after ical, cementation would yield an apparent increase in burial beneath 148 m of margin wedge (Figures 5a and 5b). effective stress calculated from equation (6). This translates to almost lithostatic pore pressures and a [43] One useful way to quantify dewatering is to calculate condition of near-zero effective stress at the top of the the distribution of fluid sources. This provides a measure of underthrust section. Despite the negligible change in fluid production, or escape rate, normalized to the volume inferred effective stress throughout much of the underthrust of sediment source. Larger sources indicate the ability of section between Sites 1039 and 1043, observed void ratio fluids to escape more rapidly, resulting in enhanced con- decreases significantly within Units I and II (Figure 4), solidation. A high-resolution picture of fluid production can implying that at least some consolidation has occurred. The be obtained from void ratio or thickness reduction of effective stresses projected for Site 1043 were obtained by correlated sediment packets: combining measured values of Cc with observed changes in void ratio, and the relatively large changes in void ratio ÁHtotal earlyincompactionreflectonlyamodestincreasein Àcompaction ¼ vp; ð7Þ ÁxH effective stress (a few tens to 100 kPa). Due to the log total linear nature of equation (3), such a small increase in effective stress causes a large change in void ratio for where Àcompaction is the fluid source in Vfluid/Vtotal/time, x is sediments characterized by high initial void ratios. distance from the trench, and vp is plate convergence rate. EPM 9 - 12 SAFFER: PORE PRESSURE AND DEWATERING

[44] Figure 9 shows distribution of fluid sources for the interval between Sites 1039 and 1043 (Figure 9a), and between Sites 1043 and 1040 (Figure 9b). In comparing dewatering downsection, it is clear that fluid sources in the hemipelagic sediments are significantly larger than for the pelagic carbonates. The more rapid dewatering within the hemipelagic sediments reflects a combination of larger compressibility and higher bulk permeability [e.g., Saffer et al., 2000]. It is also evident that fluid sources are larger between the trench and Site 1043 than between Sites 1043 and 1040. This is most likely due to a combination of longer path lengths for fluid escape, and decreasing sediment permeability and compressibility as sediments are progressively underthrust. 0 [45] The Pc values for the lowermost 50 m of sediment at Site 1040 indicate elevated pore pressures and essentially undrained conditions (Figures 5c and 6b). However, effective stresses projected from observed changes in void ratio 0 are as much as 1 MPa greater than those estimated from Pc values, and suggest that these sediments may be partly drained. The possibility of rapid dewatering in this interval is evident from the change in void ratio between Sites 1039, Figure 9. Fluid sources calculated from equation (7) 1043, and 1040 (Figure 4, base of Unit III). Given the large between (a) Sites 1039 and 1043 and between (b) Sites 1043 0 and 1040. Depth axis is for Site 1043 in Figure 9a and Site error bars for laboratory values of Pc, the two separate estimates of effective stress are not incompatible. 1040 in Figure 9b. In accord with observed void ratio [46] If the observed reduction in void ratio at the base of changes (shown in Figure 4), fluid sources are considerably Unit III reflects mechanical consolidation, one implication larger within Units I and II than within Unit III, and also is that fluids from the underthrust sediments escape to a decrease with increased burial. This is also in good drainage interface at the top of the permeable ocean crust. In agreement with inferred pore pressure development. contrast, previous analyses have led to the interpretation that the hydrologic systems in the upper ocean crust and underthrust sediments are decoupled [e.g., Saffer et al., 2000; inferred minimum in mechanical strength at Site 1040 is Silver et al., 2000]. It is important to note that pore water consistent with interpretations from seismic reflection data chemistry within the underthrust sediments supports the that the decollement steps downsection to near the top of hypothesis that the two flow systems are separate [e.g., Unit III within 2–3 km of the trench [McIntosh and Sen, Silver et al., 2000]. The observed reduction in void ratio at 2000]. Based on their analysis of the seismic reflection data, the base of Unit III could also reflect cementation, rather 0 McIntosh and Sen [2000] hypothesized that rapid dewater- than mechanical consolidation. In this case, Pc values would ing of the underthrust hemipelagic sediments leads to the best reflect in situ effective stress, whereas effective stresses development of detachments within the underlying sedi- projected using observed void ratio changes would be ments, although other observations indicating subsidence biased due to the effects of chemical compaction. Ulti- along the Costa Rican margin [Vannucchi et al., 2001] mately, in situ monitoring of pore pressures in the under- imply that significant underplating beneath the margin thrust section, combined with detailed analyses of pore wedge is not sustained or spatially continuous. Here I water chemistry, will test these inferences. provide independent estimates of effective stress at Sites 1043 and 1040 to support this hypothesis. My results are 5.1.3. Mechanical Implications also consistent with the core-scale observations from Site [47] Undrained conditions at the top of the underthrust 1040, including (1) steep bedding dips, up to 45, within section at Site 1043 result in a minimum in effective stress underthrust Units I and II, (2) numerous minor reverse faults (Figure 5b). If the coefficient of friction does not vary within Units I and II, and (3) a sharp dip discontinuity at the significantly downsection, the minimum in effective stress base of Unit II interpreted as a probable fault [Kimura et al., results in a minimum in frictional strength, and may help to 1997]. The results presented here document the potentially maintain the position of the decollement. The near-lithostatic important role of progressive dewatering and associated pore pressures at the top of the underthrust may also provide pore pressure evolution in controlling the localization and a mechanism for enhanced permeability within and at the evolution of the decollement zone in subduction zones [e.g., base of the decollement, and thus for focused fluid flow near Moore and Byrne, 1987; McIntosh and Sen, 2000]. the toe of the margin wedge [e.g., Tobin et al., 2001]. [48] By Site 1040, partial drainage of the uppermost 5.2. Comparison Between Sites hemipelagic sediments results in a minimum in effective [49] Comparing the effective stress and pore pressure stress at 450–460 mbsf (Figures 5c and 6b). The inferred estimates from the three margins, it is evident that the evolution of effective stress from the trench to Site 1040 underthrust sediments at Costa Rica are better drained than should cause the decollement to step down and localize those at Barbados and Nankai. At Costa Rica, approxi- within the mechanically weakest horizon (Figure 10). The mately 8 m3 yr1 m1 of margin length are expelled SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 13

Costa Rica may be explained, in part, by considerably larger compressibility of the high-porosity surface sediments subducted at Costa Rica. However, the inferred pore pressures at the three margins also indicate that the hemipelagic sediments underthrust at Costa Rica are better drained than underthrust sediments at Barbados and Nankai (Figures 6 and 8), whereas the pelagic sediments underthrust at Costa Rica appear essentially undrained. [50] To compare excess pore pressure development at the three margins, I have plotted the inferred excess pore pressure against the increase in overburden due to underthrusting (Figure 11). I define the increase in overburden as the difference in total vertical stress above the proto- decollement between the reference sites and those drilled through the wedge. In an undrained scenario, the two values will be identical; in a fully drained case the excess pore pressure will be zero. Figure 11 illustrates that the hemipelagic sediments at Costa Rica, although far from fully drained, are slightly better drained than Nankai and considerably better drained than Barbados. In contrast, the pelagic sediments at Costa Rica appear nearly undrained. The more limited fluid escape at Barbados than at Nankai and Costa Rica is consistent with lower measured sediment perme- abilities there [Taylor and Leonard, 1990; Zwart et al., 1997; Saffer et al., 2000]. [51] Estimated pore pressures at Barbados are lower than previous estimates [Moore and Tobin, 1997], which yielded pore pressures significantly in excess of hydrostatic values at the reference site. Although the laboratory and field- based values of Cc are comparable, the value of e0 from the field-based consolidation curve (Figure 7a) is 1.8, whereas e0 for these sediments derived from laboratory tests is 1.48 [Taylor and Leonard, 1990]. When equation (3) is rear- ranged to invert observed void ratio for in situ effective stress, larger values of e0 result in larger values of effective Figure 10. Schematic showing the evolution of effective stress, and hence lower pore pressures. stress with continued underthrusting and dewatering, based [52] It is also important to note that the nonconstant on the analysis shown in Figure 5. Shaded area indicates thickness of incoming sediment at Nankai and Barbados zone of minimum effective stress. As fluid escape transfers introduces some error into the overburden increase shown in the increasing overburden load to the sediment framework, Figure 11 (x axis). At the northern Barbados subduction the shear strength of the sediments increases (equation (1)). zone, the sedimentary section was thinner in the past [e.g., Ultimately, this should result in downward thickening of the Zhao et al., 1998], and therefore the initial overburden decollement [e.g., Moore and Byrne, 1987] and potentially stress was lower than estimated here. This, in turn, increases in detachment formation deeper within the underthrust the estimated overburden increase shown in Figure 11 (the section, where effective stress is a minimum [e.g., McIntosh direction of this shift is indicated by arrows). At Nankai, the and Sen, 2000]. subducted sedimentary section was likely thicker than that drilled seaward of the trench, as a result of basement topography [e.g., Moore et al., 2001], resulting in a decrease between the trench and Site 1040, 1.6 km landward of the in estimated overburden increase. For the observed bulk trench. In comparison, the 325 m of underthrust sediments densities at Nankai and Barbados, a 100-m difference in at Nankai expel 1.4 m3 yr1 between 10 km seaward of initial sediment thickness will shift the plotted overburden the trench and 3 km landward [Screaton et al., 2002]. The increase in Figure 11 by 400 kPa. upper 322 m of underthrust sediments at Barbados expel [53] At all three margins, the fact that estimated pore 1.008 m3 yr1 over the first 3.5 km from the trench [Zhao pressure profiles nearly parallel the hydrostat (Figures 6 and et al., 1998]. The fluid sources within the underthrust 8) is consistent with the interpretation that pore pressures sediments between the trench and Site 1040 at Costa Rica were initially hydrostatic, and have been increased uni- range from 1.04 1013 s1 in Unit III to 1.37 1012 s1 formly downsection by loading due to underthrusting, rapid in Unit I. These values are similar to those reported by Saito trench sedimentation, and internal thickening of overlying and Goldberg [2001] determined by log correlation, and are accreted sediments. At Nankai, estimated pore pressures are 1–2 orders of magnitude larger than fluid sources of 3 1 MPa closer to hydrostatic values at the top of the section 1014 and 4.5 1014 s1 calculated for Barbados and than at the base (Figure 8b), suggesting that the uppermost Nankai, respectively. The more rapid fluid expulsion at sediments are partly drained. This may be explained by a EPM 9 - 14 SAFFER: PORE PRESSURE AND DEWATERING

Figure 11. Estimated excess pore pressure plotted versus the expected increase in overburden due to burial beneath the margin wedge, for drill sites at Costa Rica, Barbados (open circles), and Nankai (open squares). For Costa Rica, inferred pore pressures are separated into the hemipelagic (Units I and II) (solid circles) and pelagic (Unit III) (shaded squares) sections. Excess pore pressure is defined as the mean pore pressure above hydrostatic. Error bars on inferred pore pressures are 2 standard deviations. The expected increase in overburden was calculated from the difference in total overburden between the trench and buried location, assuming a uniform incoming sediment thickness. Arrows next to points for Barbados and Nankai reflect the effects of variability in incoming sediment thickness (see text for discussion). Black, short-dashed line indicates undrained conditions, where the increase in pore pressure equals the increase in total overburden. Other lines indicate best fits to inferred pore pressures. The hemipelagic sediments at Costa Rica and Nankai appear better drained than the pelagic sediments at Costa Rica and the uppermost underthrust sediments at Barbados. small amount of upward drainage during trench sedimenta- trench turbidite deposition over the last 0.25 Ma [Moore et tion and initial loading, which would reduce excess pore al., 2001]. pressures near the top of the underthrust section. The larger [56] The inferred overpressures within underthrust sedi- excess pore pressures at Site 808 than Site 1174 can be ments at all three margins may result from low sediment attributed mainly to additional loading beneath the frontal permeability that limits upward and lateral fluid escape thrust [e.g., Screaton et al., 2002]. [e.g., Saffer et al., 2000; Screaton et al., 2002], or from [54] If permeability within the underthrust sediments is sustained high pore pressures within the decollement that isotropic, the inferred pore pressure profile shown in Figure limit upward fluid escape by maintaining small head gra- 8b reflects dewatering primarily via upward flow, with the dients. In the latter scenario, locally elevated pore pressures implication that the decollement zone itself or the surround- within the decollement may be driven from greater depth, or ing damage zone acts as a drainage surface for the under- dynamically maintained by shear compaction. As overbur- thrust sediments. Alternatively, if sediment permeability is den load is increased within poorly drained sediments, pore highly anisotropic, with a higher permeability along bed- pressure ratios (l) increase (Table 2). One key implication is ding than across it, inferred pore pressures could signify that pore pressure ratio is not expected to remain constant; it significant dewatering via lateral flow. However, the should increase systematically with distance inboard of the increasing excess pore pressure with depth at both Sites trench and may also vary considerably with depth in the 1174 and 808 requires either a component of upward underthrust section. dewatering, or larger lateral dewatering in the upper part of the section than in the lower part [e.g., Saffer et al., 2000]. 6. Conclusions [55] The offset of inferred pore pressures by 500 kPa [57] The compatibility of separate effective stress and between Sites 1174 and 808 is uniform downsection, pore pressure estimates using field observations and labo- indicating an undrained response to the rapid loading by ratory measurements demonstrates that laboratory measure- recent movement on the frontal thrust [e.g., Screaton et al., ments of compression index (Cc) and preconsolidation 0 2002]. Thus if upward drainage of the underthrust sedi- stress (Pc) may be applicable to in situ consolidation over ments occurs, it cannot keep pace with the most recent geologic timescales. However, differences in laboratory Cc tectonic loading. Alternatively, the difference in excess pore values for similar lithologies at different drill sites suggest pressure downsection at Nankai may be explained by a that some issues in the application of laboratory data to small initial overpressure at the base of the section at the geologic consolidation remain unresolved. Specifically, the reference Site 1173, possibly related to the 100 m of effects of weak cementation and the collapse of intragra- SAFFER: PORE PRESSURE AND DEWATERING EPM 9 - 15 nular porosity during early consolidation are not easily Foundation (NSF) and participating countries under management of Joint reproduced in the laboratory, and their effects on consol- Oceanographic Institutions (JOI), Inc. idation behavior are not well characterized. The agreement between a field-based consolidation curve and laboratory References data for sediments underthrust at Barbados, along with good Aubouin, J., and R. von Huene, Summary: Leg 84, Middle America Trench agreement between inferred pore pressures and direct meas- transect off Guatemala and Costa Rica, Initial Rep. Deep Sea Drill. Proj., urements, also documents the applicability of laboratory 84, 939–957, 1985. Becker, K., A. T. Fisher, and E. E. Davis, The CORK experiment in Hole data to geologic consolidation. 949C: Long-term observations of pressure and temperature in the Barba- [58] My results indicate that sediments underthrust dos accretionary prism, Proc. Ocean Drill. Program Sci. Results, 156, beneath the margin wedge offshore Costa Rica drain heter- 247–252, 1997. ogeneously. Inferred effective stresses show that the upper, Behrmann, J. H., and A. Kopf, Textures and microfabrics in fine-grained muds and mudstones from Site 808, Nankai accretionary prism, Proc. hemipelagic sediments are partly drained, whereas the most Ocean Drill. Program Sci. Results, 131, 45–55, 1993. of the pelagic carbonate section appears undrained. Effec- Bourgois, J., et al., The geologic history of the Caribbean-Cocos Plate tive stresses determined from laboratory measurements of boundary with special reference to the Nicoya ophiolite complex (Costa 0 Rica) and D.S.D.P. results (Legs 67 and 84 off Guatemala): A synthesis, Pc suggest that the lowermost 50 m of the underthrust Tectonophysics, 108, 1–32, 1984. sediments at Site 1040 are undrained, although the uncer- Breen, N. A., and D. L. Orange, The effects of fluid escape on accretionary tainties are large. Limited drainage of the basal sediments wedges: 1. Variable porosity and wedge convexity, J. Geophys. Res., 97, 9265–9275, 1992. implies that the underthrust sediment and upper ocean Christeson, G. L., K. D. McIntosh, and T. H. Shipley, Seismic attenuation in crustal hydrologic systems are isolated [e.g., Saffer et al., the Costa Rica margin wedge: Amplitude modeling of ocean bottom 2000; Silver et al., 2000]. In contrast, effective stresses hydrophone data, Earth Planet. Sci. Lett., 179, 391–405, 2000. projected from void ratio reduction are consistent with Cochrane, G. R., J. C. Moore, and H. J. Lee, Sediment pore-fluid over- pressuring and its effect on deformation at the toe of the Cascadia accre- partial drainage of the basal sediments, possibly implying tionary prism from seismic velocities, in Subduction Top to Bottom, that fluids drain from the consolidating sediments into the Geophys. Monogr. Ser., vol. 96, edited by G. Bebout et al., pp. 57–64, highly permeable ocean crust. Ultimately, these competing AGU, Washington, D. C., 1996. Coulbourn, W. T., Stratigraphy and structures of the Middle America hypotheses can be tested by in situ observations and Trench: Deep Sea Drilling Project Leg 67 transect off Guatemala, Initial detailed analyses of pore water chemistry. Rep. Deep Sea Drill. Proj., 67, 691–706, 1982. [59] Detailed examination of void ratio reduction between Crawford, C. B., State of the art: Evaluation and interpretation of soil consolidation tests, in Consolidation of Soils: Testing and Evaluation, drill sites also demonstrates that fluid expulsion from the Rep. ASTM STP 892, edited by R. N. Yong and F. C. Townend, pp. hemipelagic sediments exceeds that from the pelagic sedi- 71–103, Am. Soc. for Test. and Mater., West Conshohocken, Pa., 1986. ments by a factor of 2–3 (Figures 4 and 9). Effective stress Davis, D., J. Suppe, and F. A. Dahlen, Mechanics of fold-and-thrust belts development within the underthrust sediments, mediated by and accretionary wedges, J. Geophys. Res., 88, 1153–1172, 1983. Feeser, V., K. Moran, and W. Bruckmann, Stress-regime-controlled yield pore water escape, results in an inferred minimum in and strength behavior of sediment from the frontal part of the Nankai mechanical strength near the top of Unit III by Site 1040. accretionary prism, Proc. Ocean Drill. Program Sci. Results, 131, 261– Thus nonuniform drainage may play a key role in decolle- 271, 1993. Foucher, J.-P., P. Henry, and F. Harmegnies, Long-term observations of ment downstepping to weaker horizons within the under- pressure and temperature in Hole 948D, Barbados accretionary prism, thrust sediment column [e.g., McIntosh and Sen, 2000] Proc. Ocean Drill. Program Sci. Results, 156, 239–245, 1997. (Figure 10). Clearly, detailed study of pore pressure evolu- Gibson, R. E., The progress of consolidation in a clay layer increasing in thickness with time, Geotechnique, 8, 171–182, 1958. tion and the fate of fluids within underthrust sediments Holtz, R. D., and W. D. Kovacs, An Introduction to Geotechnical Engineer- holds important insights into the structural development of ing, Prentice-Hall, Old Tappan, N. J., 1981. decollements. Housen, B. A., et al., Strain decoupling across the decollement of the Barbados accretionary prism, Geology, 24, 127–130, 1996. [60] In comparison to underthrust sediments at Barbados Hubbert, M. K., and W. W. Rubey, Role of fluid pressure in mechanics of and Nankai, those at Costa Rica are better drained and overthrust faulting, Geol. Soc. Am. Bull., 70, 115–166, 1959. dewatering is more rapid by 1–2 orders of magnitude. The Karig, D. E., Reconsolidation tests and sonic velocity measurements of comparatively rapid dewatering at Costa Rica is likely due clay-rich sediments from the Nankai trough, Proc. Ocean Drill. Program Sci. Results, 131, 247–260, 1993. to a combination of higher permeability [e.g., Saffer et al., Kimura, G., et al. (Eds.), Proceedings of the Ocean Drilling Program, 2000] and subduction of high-porosity, highly compressible Initial Reports, vol. 170, 458 pp., U.S. Govt. Print. Off., Washington, sediments. The inferred excess pore pressures at all three D. C., 1997. Lee, H. J., State of the art: Laboratory determination of the strength of margins are consistent with previous work [e.g., Screaton et marine soils, in Strength Testing of Marine Sediments and In Situ Mea- al., 2002; Foucher et al., 1997; Becker et al., 1997; Breen surements, Rep. ASTM STP 883,editedbyR.C.ChaneyandK.R. and Orange, 1992], and suggest that fluid escape from Demars, pp. 181–250, Am. Soc. for Test. and Mater., West Conshohock- underthrust sediments does not keep pace with tectonic en, Pa., 1985. Le-Pichon, X., P. Henry, and S. J. Lallemant, Accretion and erosion in loading. subduction zones: The role of fluids, Annu. Rev. Earth Planet. Sci., 21, 307–331, 1993. Mascle, A., et al., Proceedings of the Ocean Drilling Program, Initial [61] Acknowledgments. I wish to thank the captain and crew of the Reports, vol. 110A, 603 pp., U.S. Govt. Print. Off., Washington, D. C., JOIDES resolution, and the Leg 190 Shipboard Scientific Party. Kate 1988. Moran, Emily Giambalvo, and Andy Fisher provided valuable help with McIntosh, K. D., and M. K. Sen, Geophysical evidence for dewatering and laboratory analyses and helpful discussions at various stages of this work. deformation processes in the ODP Leg 170 area offshore Costa Rica, Saffer was supported for this study by an American Chemical Society Earth Planet. Sci. Lett., 178, 125–138, 2000. (Petroleum Research Foundation) grant 37122-G8 and by a U.S. Scientist Moore, G. F., et al., New insights into deformation and fluid flow processes Support grant for ODP Leg 190 postcruise research. I thank H. Tobin, R. in the Nankai Trough accretionary prism: Results of Ocean Drilling Von Huene, and E. Screaton for helpful discussions and reviews of early Program Leg 190, Geochem. Geophys. Geosyst., 2, Paper number versions of this manuscript, and Peter Flemings for his constructive review. 2001GC000166, 2001. This research used samples and data provided by the Ocean Drilling Moore, J. C., Tectonics and hydrogeology of accretionary prisms: Role of Program (ODP). The ODP is sponsored by the U.S. National Science the decollement zone, J. Struct. Geol., 11, 95–106, 1989. EPM 9 - 16 SAFFER: PORE PRESSURE AND DEWATERING

Moore, J. C., and T. Byrne, Thickening of fault zones: A mechanism of Shipley, T. H., et al., Proceedings of the Ocean Drilling Program, Initial me´lange formation in accreting sediments, Geology, 15, 1040 – 1043, Reports, vol. 156, U.S. Govt. Print. Off., Washington, D. C., 1995. 1987. Silver, E., M. Kastner, A. T. Fisher, J. Morris, K. McIntosh, and D. Saffer, Moore, J. C., and D. M. Saffer, Updip limit of the seismogenic zone Fluid flow paths in the Middle America trench and Costa Rica margin, beneath the accretionary prism of southwest Japan: An effect of diage- Geology, 28, 679–682, 2000. netic to low-grade metamorphic processes and increasing effective stress, Taira, A., I. A. Hill, J. Firth, and Shipboard Scientific Party, Proceedings of Geology, 29, 183–186, 2001. the Ocean Drilling Program, Initial Reports, vol. 131, 434 pp., U.S. Moore, J. C., and H. Tobin, Estimated fluid pressures of the Barbados Govt. Print. Off., Washington, D. C., 1991. accretionary prism and adjacent sediments, Proc. Ocean Drill. Program Taylor, E., and J. Leonard, Sediment consolidation and permeability at the Sci. Results, 156, 229–238, 1997. Barbados forearc, Proc. Ocean Drill. Program Sci. Results, 110, 289– Moore, J. C., A. Klaus, and Shipboard Scientific Party, Proceedings of the 308, 1990. Ocean Drilling Program, Initial Reports, vol. 171A, 106 pp., U.S. Govt. Tobin, H., P. Vannucchi, and M. Meschede, Structure, inferred mechanics, Print. Off., Washington, D. C., 1998. and implications for fluid transport in the decollement zone Costa Rica Morgan, J. K., and D. E. Karig, Ductile strains in clay-rich sediments from convergent margin, Geology, 29, 907–910, 2001. Hole 808C: Preliminary results using X-ray pole figure goniometry, Proc. Vannucchi, P., et al., Tectonic erosion and consequent collapse of the Pa- Ocean Drill. Program Sci. Results, 131, 141–155, 1993. cific margin of Costa Rica: Combined implications from ODP Leg 170, Neuzil, C. E., Abnormal pressures as hydrodynamic phenomena, Am. J. seismic offshore data, and regional geology of the Nicoya Peninsula, Sci., 295, 742–786, 1995. Tectonics, 20, 649–668, 2001. Saffer, D. M., and B. A. Bekins, Episodic fluid flow in the Nankai accre- von Huene, R., and H. Lee, The possible significance of pore fluid pres- tionary complex: Timescale, geochemistry, flow rates, and fluid budget, sures in subduction zones, in Studies in Continental Margin Geology, J. Geophys. Res., 103, 30,351–30,371, 1998. edited by J. S. Watkins and C. L. Drake, AAPG Mem., 34, 781–791, Saffer, D. M., et al., Consolidation, pore fluid pressures, and implications 1982. for dewatering pathways of subducted sediments, offshore Costa Rica, Vrolijk, P., T. Miller, and M. J. Gooch, Hydrostatic consolidation tests of Eos Trans. AGU, 78(46), Fall Meet. Suppl., F685, 1997. undeformed, clay-rich samples from the Barbados accretionary prism, Saffer, D. M., E. A. Silver, A. T. Fisher, H. Tobin, and K. Moran, Inferred Leg 156, Proc. Ocean Drill. Program Initial Rep., 171A, 107–116, pore pressures at the Costa Rica subduction zone: Implications for de- 1998. watering processes, Earth Planet. Sci. Lett., 177, 193–207, 2000. Ye, S., J. Bialas, E. R. Flueh, A. Stavenhagen, R. vonHuene, G. Leandro, Saito, S., and D. Goldberg, Compaction and dewatering processes of the and K. Hinz, Crustal structure of the Middle American Trench off Costa oceanic sediments in the Costa Rica and Barbados subduction zones: Rica from wide-angle seismic data, Tectonics, 15, 1006–1021, 1996. Estimates from in situ physical properties measurements, Earth Planet. Zhao, Z., G. F. Moore, and T. H. Shipley, Deformation and dewatering of Sci. Lett., 191, 283–293, 2001. the subducting plate beneath the lower slope of the northern Barbados Screaton, E., and S. Ge, Anomalously high porosities in the proto-decolle- accretionary prism, J. Geophys. Res., 103, 30,431–30,449, 1998. ment zone of the Barbados accretionary complex: Do they indicate over- Zwart, G., et al., Evaluation of hydrogeologic properties of the Barbados pressures?, Geophys. Res. Lett., 27, 1993–1996, 2000. accretionary prism: A synthesis of Leg 156 results, Proc. Ocean Drill. Screaton, E. J., D. M. Saffer, P. Henry, S. Hunze, and Leg 190 Shipboeard Program Sci. Results, 156, 303–310, 1997. Scientific Party, Porosity loss within underthrust sediments of the Nankai accretionary complex: Implications for overpressures, Geology, 30,19– 22, 2002. Shipley, T. H., P. Stoffa, and D. Dean, Underthrust sediments, fluid migration D. M. Saffer, Department of Geology and Geophysics, University of paths, and mud volcanoes associated with the accretionary wedge off Costa Wyoming, P.O. Box 3006, Laramie, WY 82071-3006, USA. (dsaffer@ Rica: Middle America trench, J. Geophys. Res., 95, 8743–8752, 1990. uwyo.edu)