Research Article Character of sediments entering the Costa Rica subduction zone: Implications for partitioning of water along the plate interface

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1 The Island Arc (2004) 13, Research Article Character of sediments entering the Costa Rica subduction zone: Implications for partitioning of water along the plate interface GLENN A. SPINELLI 1, * AND MICHAEL B. UNDERWOOD 2 1 Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87081, USA ( spinelli@ees.nmt.edu) and 2 Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA Abstract Sediments deposited off the Nicoya Peninsula advect large volumes of water as they enter the Costa Rica subduction zone. Seismic reflection data, together with results from Ocean Drilling Program Leg 170, show that hemipelagic mud comprises the upper ~135 m of the sediment column (ranging from 0 to 210 m). The lower ~215 m of the sediment column (ranging from 0 to 470 m) is pelagic carbonate ooze. We analyzed samples from 60 shallow (<7 m) cores to characterize the spatial variability of sediment composition on the incoming Cocos Plate. The bulk hemipelagic sediment is 10 wt% opal and 60 wt% smectite on average, with no significant variations along strike; the pelagic chalk contains approximately 2 wt% opal and <1 wt% smectite. Initially, most of the water (96%) in the subducting sediment is stored in pore spaces, but the pore water is expelled during the early stages of subduction by compaction and tectonic consolidation. Approximately 3.6% of the sediment s total water volume enters the subduction zone as interlayer water in smectite; only 0.4% of the water is bound in opal. Once subducting strata reach depths greater than 6 km (more than 30 km inboard of the subduction front), porosity drops to less than 15%, and temperature rises to greater than 60 C. Under those conditions, discrete pulses of opal and smectite dehydration should create local compartments of fluid overpressure, which probably influence fluid flow patterns and reduce effective stress along the plate boundary fault. Key words: biogenic silica, Costa Rica, opal, seismogenic zone, smectite, subduction. INTRODUCTION *Correspondence. Received 26 November 2003; accepted for publication 24 March Blackwell Publishing Asia Pty Ltd The deposition, compaction and diagenesis of sedimentary rocks controls the distribution of fluids, fluid pressures and fluid flow patterns within subduction zones. High fluid pressures probably affect fault strength along various types of plate interface (Sibson 1981; Byerlee 1990; Unsworth et al. 1997). Cyclic dissipation of fluid pressure and commensurate increases of effective stress might control the spatial distribution and timing of seismicity (Byerlee 1993; Magee & Zoback 1993). Sediments constitute a small volumetric fraction of the oceanic lithosphere, but they host a large fraction of the total water volume that enters subduction zones. On a global average, the volume of pores and sediment grains each account for approximately 2.5% of the total slab volume, but approximately 40% and approximately 5%, respectively, of the mass of subducting water (Peacock 1990). Mechanical compaction and diagenesis control the release of fluids from sediments (e.g. Athy 1930; Trask 1931; Powers 1967; Bekins & Dreiss 1992; Moore & Vrolijk 1992). The distribution of sediment-derived fluid sources in any sedimentary system depends upon 3-D variations in sediment composition, pressure and temperature (Raymond 1983; Langseth & Moore 1990; Swarbrick et al. 2002). Sedimentary strata gain strength when porosity is lost, so the amount of consolidation decreases exponentially as effective stress

2 Costa Rica sediment: Fluid partitioning 433 increases (Moore & Vrolijk 1992; Bahr et al. 2001; Holbrook 2002). Therefore, the volume of fluid expelled from sediment during compaction decreases with depth in a sedimentary basin (or distance into a subduction zone). In deep levels of a basin, water stored within minerals (e.g. opal and smectite) comprises a greater proportion of the total fluid budget. Transformations of opal to quartz and smectite to illite are controlled by reaction kinetics (Ernst & Calvert 1969; Pytte & Reynolds 1988; Huang et al. 1993). The activation energies are low for both opal to quartz and smectite to illite (Ernst & Calvert 1969; Pytte & Reynolds 1988), so the reactions usually progress quickly from initiation to completion. However, the smectite-to-illite reaction can be slowed or halted if the supply of K + is limited (Hower et al. 1976; Boles & Franks 1979). Opal-toquartz diagenesis goes to completion at temperatures of C (Murata et al. 1977; Behl & Garrison 1994). Smectite dehydration involves three discrete stages at temperatures less than approximately 140 C (Perry & Hower 1972; Bird 1984; Bruce 1984; Colten-Bradley 1987). Consequently, one might expect opal and smectite dehydration to trigger two or three discrete pulses of fluid release within narrow windows of temperature and depth. When unconsolidated sediment first enters a subduction zone, most of the water is stored in pore spaces, but the pores collapse rapidly near the deformation front due to vertical compaction and tectonic consolidation (e.g. Moore & Vrolijk 1992; Kimura et al. 1997; Screaton et al. 2002). Therefore, hydrous mineral can convey a relatively large volume of water deep into subduction zones. The loci of diagenetic dewatering reactions within a subduction zone depend upon a margin s thermal structure and its subduction accretion geometry (i.e. stratigraphic position and dip of decollement, offset along out-of-sequence faults, underplating via duplex structures). Smectite and opal dewatering are merely two of several low-grade reactions that influence pore pressure, effective stress and frictional properties within subduction zones (Moore & Saffer 2001). Geochemical evidence of diagenetic dewatering is substantial within many active margins (e.g. Kastner et al. 1991). The starting composition and temperature history of the subducting sediments, however, vary dramatically both within and among individual systems (e.g. Tribble 1990; Underwood & Pickering 1996; Deng & Underwood 2001; Underwood 2002; Steurer & Underwood 2003a). Because of that heterogeneity, linkages between diagenetic reactions and their chemical or physical effects must be established independently, and in 3-D, within each study area. The need for sitespecific assessment applies to compositional factors that might, in theory, control the up-dip limit of seismicity on subduction megathrusts (Vrolijk 1990; Hyndman et al. 1997). Substantial volumes of sediment-hosted water enter the Costa Rica subduction zone off the coast of the Nicoya Peninsula (Fig. 1). Leg 170 of the Ocean Drilling Program (ODP) demonstrated that the entire package of incoming sediment is subducted beneath the frontal toe of the margin (Kimura et al. 1997), but deeper-seated subduction processes remain poorly documented. In addition, various interpretations have been offered to explain profiles in pore water chemistry near the margin toe (Chan & Kastner 2000; Silver et al. 2000; Saffer & Screaton 2003), but those efforts have been hampered by a lack of detail regarding the actual composition of sediment inputs. To help fill this void in our knowledge of the Costa Rica subduction system, we completed compositional analyses of near-surface sediments on the incoming Cocos Plate, with the goal of gaining insight into how water is partitioned among pore space, opal and smectite. The nature of fluid partitioning at the subduction front, and the down-dip evolution of fluid sources, are also important geotechnical considerations, because of their potential control over fluid pressure and shear strength along the plate interface. COSTA RICA MARGIN Our study focused on the sediment on the incoming Cocos Plate off the Nicoya Peninsula, Costa Rica (Fig. 1). Along the Pacific margin of Costa Rica, the Cocos Plate subducts beneath the Caribbean Plate at the Middle America Trench at approximately 85 mm/year (DeMets 2001). Offshore of the Nicoya Peninsula, a triple junction trace divides the subducting Cocos Plate into crust formed at the East Pacific Rise (EPR) to the north, and crust formed at the Cocos Nazca Spreading Center (CNS) to the south (Barckhausen et al. 2001). Seaward of the Middle America Trench, seafloor heat flow on the CNS crust is mw/ m 2 (Fisher et al. 2003), consistent with conductive lithospheric cooling models (e.g. Parsons & Sclater 1977; Stein & Stein 1994). Heat flow on the EPR crust is mw/m 2. The lateral transition between the warm and cool crust is abrupt, occurring over a lateral distance 5 km (Fisher et al.

3 434 G. A. Spinelli and M. B. Underwood 10 o m Costa Rica Nicoya Peninsula study area m 9.5 o m 9 o 8.5 o 50 km EPR crust CNS crust 85 mm/year m o -87 o o -86 o o Fig. 1 Map showing the location of the 60 gravity and piston cores used in this study from sediments on the incoming Cocos Plate, offshore northwestern Costa Rica ( ). Ocean Drilling Program (ODP) Sites 1039 and 1040 ( ) are approximately 1.5 km seaward and 0.5 km landward of the Middle America Trench, respectively. Site 1039 spans the entire incoming sediment section (~350 m thick); Site 1040 penetrates the margin wedge and the entire underthrust sediment section. Tracklines of reflection seismic data are shown with the solid gray lines. A triple junction trace (bold dashed line) separates crust formed at the Cocos Nazca Spreading Center (CNS) in the southeastern portion of the study area from crust formed at the East Pacific Rise (EPR; northwestern portion of the study area). 2003). The low heat flow on the EPR crust likely results from hydrothermal circulation, facilitated by numerous basaltic outcrops that allow rapid transfer of water between the ocean and the crust (Fisher et al. 2003). The location of shallow earthquakes along the plate interface also changes along-strike, coinciding with the transition from CNS to EPR crust. Earthquakes occur shallower (~10 km) and closer to the trench (~60 km from the trench) on the warm CNS crust than on the cool EPR crust (~20 km depth and ~75 km from the trench) (Newman et al. 2002). Ocean Drilling Program Leg 170 included a reference site on the floor of the Middle America Trench (Site 1039) and at two sites at the toe of the margin wedge (Sites 1043 and 1040, ~0.5 and 1.6 km landward of the trench, respectively). The stratigraphic section at Site 1039 consists of approximately 225 m of pelagic sediment, primarily composed of siliceous and calcareous nannofossils, overlain by approximately 150 m of hemipelagic mud containing abundant clays and diatoms (Kimura et al. 1997). Reflection seismic profiles, litho-stratigraphy, core-scale geological structures and borehole logs (gamma-ray, resistivity and density) collectively indicate that very little sediment (~1%) is scraped off into the wedge (Kimura et al. 1997; Saito & Goldberg 2001). During the TicoFlux program (Fisher et al. 2003), gravity and piston cores, reflection seismic data and heat-flow data were collected in an area approximately nmi offshore of the Nicoya Peninsula (Fig. 1). The coring program was designed to capture geochemical evidence of basement-related fluid flow (e.g. Friedmann et al. 2001), so there is a bias in sample distribution toward locations where total sediment thickness is at a minimum. METHODS Sediment samples were taken from 60 nearsurface piston and gravity cores distributed throughout the TicoFlux study area, plus five intervals at ODP Site 1040 (Fig. 1). Sediment composition was quantified by a combination of chemical and X-ray diffraction (XRD) analyses of bulk samples and the <2 mm fraction. OPAL ANALYSIS We used an alkaline leaching method to determine opal content, as outlined by DeMaster (1981). When sediment is placed in an alkaline solution, siliceous material is dissolved, and amorphous sil-

4 Costa Rica sediment: Fluid partitioning 435 ica (opal) is digested more rapidly than clay minerals. Therefore, the concentration of silica in the alkaline solution increases over time, with a rapid initial increase overprinted by gradual dissolution of clay minerals. For each sample analyzed, we digested approximately 30 mg of freeze-dried sediment in 40 ml of M NaOH (ph 12.5) at 85 C. A 12.5 ph solution was used to minimize digestion of clay minerals (Schlüter & Rickert 1998). Aliquots (0.200 ml) of the leachate were collected 5, 15, 30, 60, 90, 120, 200 and 300 min after digestion began. The concentration of silica in the leachate was determined by spectrophotometry (e.g. Grasshoff et al. 1983). The contribution of opal to the leachate was determined by projecting the flattened tail of the concentration curve back to time = 0 (Fig. 2). In determining the weight percent of opal in the bulk samples, we assumed the water content of opal to be 11% by weight (Kastner 1981). X-RAY DIFFRACTION Bulk sediment samples were analyzed by XRD to determine relative abundances (wt%) of total clay minerals, quartz, plagioclase and calcite. Freezedried specimens were powdered for 5 min with a ball mill and analyzed with a Scintag PAD V diffractometer. Bulk powders were scanned at 40 kv and 35 ma over 2q angles ranging from 3 to 35, at a rate of 1 2q/min, and MacDiff software was used to determine the areas and intensities of a composite clay mineral peak (smectite + illite + kaolinite), quartz, plagioclase and calcite (Fig. 3). Relative percentages of each were calculated using a set of normalization factors specific to the Costa Rica mineral assemblage (Table 1), following the singular value decomposition (SVD) method of Fisher and Underwood (1995). Preparation of the clay-sized fraction (<2 mm) for clay mineral identification started with 3% H 2 O 2 digestion of organic matter, addition of Nacarbonate and Na-hexametaphosphate dispersant, and washing by centrifugation at ~6000 g for 20 min. Suspensions were dispersed in de-ionized water using an ultrasonic cell disruptor, and split into size fractions by centrifuging at ~320 g for 2.4 min. Preparation of oriented clay slides followed the filter-peel transfer method (Drever 1973), and the clays were saturated with ethylene glycol vapor before analysis. The clay slides were scanned at 40 kv and 30 ma over 2q angles ranging from 3 to 23, at a rate of 1 2q/min. We calculated relative abundances of smectite, illite and kaolinite (area%) using basal reflections (Fig. 3) and Biscaye (1965) peak-area weighting factors (1 smectite; 4 illite; 2 kaolinite). The percent expandability of smectite was determined using the saddle-peak method and the empiric curve for pure smectite (Rettke 1981). Fig. 2 Examples of data from the sediment leaching procedure used to determine the opal content of sediment samples. After all the opal has been extracted from the sediment in a hot alkaline solution, silica continues to be extracted from clays at a slow, constant rate. This rate is extrapolated back to time = 0 to determine the amount of silica extracted from the opal alone. The dark olive gray hemipelagic muds have the highest opal content of the sediments encountered in the study area; the brown clays have intermediate opal content; the pelagic sediments have the lowest opal content. Leachate Si concentration (mm) Mass sediment sample (g) Contribution from opal to Si concentration of leachate M05gc cm (hemipelagic; 8.1% opal) All Si from opal has been extracted Si extracted from both opal and clays Si extracted only from clays (at constant rate) M18gc 8 10 cm (brown clay; 2.8% opal) M24gc cm (calcareous ooze; 0.6% opal) Time (min)

5 436 G. A. Spinelli and M. B. Underwood Counts (a) clay quartz plagioclase calcite halite To discriminate between dioctahedral and trioctahedral varieties of expandable clay, we identified the d(060) value using randomly oriented powders of the <2 mm size fraction (Brown & Brindley 1980). Those clay powders were scanned from 48 to 64 2q at 1 2q/min after adding a spike of quartz powder to correct for drifts in peak position caused by misalignments of the goniometer and/or sample holder. Counts Counts 0 (b) saddle o 2q zeolite (c) 2q peak smectite (001) illite (001) kaolinite (001) calcite halite o 2q Fig. 3 Example of X-ray diffraction (XRD) analysis of sediment samples off Costa Rica. Analysis of the bulk sediment powder for a hemipelagic sample indicates that the relative percentage of clay (77%) is much greater than quartz (4%), plagioclase (16%) or calcite (3%) (a; M17gc cm (hemipelagic mud)). Results from an ethylene-glycol solivated, oriented slide for the same sample indicate that the <2 mm grain size fraction of the sample is dominated by smectite (b; 84% smectite, 1% illite, 15% kaolinite). The ratio of the number of counts at the saddle and peak is used to estimate the proportion of expandable layers in the smectite. Pelagic samples are dominated by calcite (92%), with very little clay (<1%), quartz (4%) or plagioclase (4%) (c; M22gc cm (calcareous ooze)). TOTAL INORGANIC CARBON Error analysis of the SVD method shows that calculated abundances of minerals within standard mixtures are usually within 5% of their true percentages by weight (Fisher & Underwood 1995; Underwood et al. 2003). To verify the accuracy of XRD data in an absolute sense, we analyzed for calcium-carbonate content by acid digestion and a coulometer using a diverse subset of 18 hemipelagic samples. The absolute wt% of calcite for each was determined from the total inorganic carbon content. GRAIN SIZE The percentages of sand (>63 mm), silt (4 63 mm) and clay (<4 mm) were determined by wet sieving and a Spectrex laser particle counter (LPC). Before wet sieving, the bulk samples were freezedried, digested with 3% H 2 O 2, and dispersed in 0.01 M Na 2 CO 3. Because of the typical abundance of biogenic silica, Na-hexametaphosphate (Calgon) was ineffective as a dispersant. Output from the LPC (counts/cm 3 ) needs to be converted to equivalent spherical settling diameter (wt%) using a calibration curve established by pipette analysis (Steurer & Underwood 2003b). We report here the average values of percentage silt and percentage clay (as spherical equivalents) derived from three replicate LPC analyses. SEDIMENT THICKNESS Approximately 1800 km of reflection seismic lines were collected during 2001 (TicoFlux I expedi- Table 1 Normalization factors for bulk powder X-ray diffraction (XRD) analysis Indicator mineral Target mineral Total clay Quartz Plagioclase Calcite Total clay Quartz Plagioclase Calcite

6 Costa Rica sediment: Fluid partitioning 437 Fig. 4 On reflection seismic lines, the hemipelagic sediment is acoustically more transparent than the underlying pelagic sediment. Reflection seismic data is used to map the thickness of both the hemipelagic sediment and the pelagic sediment. TWT, two way travel time. TWT (s) hemipelagic sediment pelagic sediment basaltic basement seafloor Approximate depth (m) tion). The source was a 10-gun array with a frequency bandwidth of approximately Hz. The receiver was a 6 km long hydrophone streamer array with 480 channels. An additional 1200 km of reflection seismic data were acquired during 2002 (TicoFlux II expedition) using a pair of GI guns with a frequency bandwidth of approximately khz and a four-channel, 200 hydrophone streamer. The pelagic and hemipelagic sections of the sediment column on Cocos Plate were differentiated by differences in their acoustic signatures. As noted during previous geophysical surveys of this margin (e.g. Shipley et al. 1990), the pelagic section is characterized by numerous internal reflections with high degrees of lateral continuity, whereas the hemipelagic section is relatively transparent acoustically (Fig. 4). We measured the two way travel times at the boundaries between hemipelagic and pelagic intervals and acoustic basement. We converted to sediment thickness using average velocities of 1541 m/s for the hemipelagic section and 1581 m/s for the pelagic section (Kimura et al. 1997). Sediment thickness was determined along the reflection seismic tracklines, then gridded and contoured over the entire study area. RESULTS SEDIMENT THICKNESS Reflection seismic data and high-resolution swath mapping indicate that seamounts are more common on crust formed at the EPR than on crust formed at the CNS to the southeast (e.g. Fisher et al. 2003). Sediment is thin or absent on the seamounts (Fig. 5). As a result, there is greater spatial variability in total sediment thickness in the northwestern part of the study area than in the southeastern part. Aside from the basement highs, the hemipelagic interval is similar over the EPR crust and the CNS crust, reaching up to 213 m, with a median thickness (for the gridded data) of 135 m. Pelagic sediment is consistently thicker on the EPR crust than on the CNS crust. The pelagic interval is m thick on the EPR crust (median = 233 m) and m thick on the CNS crust (median = 193 m). SEDIMENT COMPOSITION Shipboard descriptions of split cores and smear slides led to a classification system with four basic lithologies: dark olive gray hemipelagic mud (with abundant biogenic silica); variegated clay (with low biogenic silica); mixed sediment (with variable carbonate content); and light gray nannofossil chalk. Many of the cores also contain thin layers of volcanic ash, fragments of Mn-oxide and pieces of basalt. Apart from five deeper intervals at ODP Site 1040, our specimens come from within 7 m of the seafloor. Hemipelagic mud is, by far, the most common lithology throughout the study area. Exposures of variegated clay and chalk are restricted to basement highs (e.g. seamounts and smaller basalt outcrops) where total sediment thickness is at a minimum and the overlying hemipelagic sediment section is thin or absent (Fig. 5). Cores with pelagic chalk are clustered in the southwestern portion of the study area and around outcrops, primarily on the crust northwest of the triple junction trace indicated on Figures 1 and 6 (Fisher et al. 2003). Winnowing by bottom currents may have hampered deposition of hemipelagic mud in those localities. In some cores, the basal chalk interval grades upward through mixed sediment into hemipelagic mud. One piston core was collected from the floor of the Middle America Trench, and it contains turbidites and debris-flow deposits with remobilized fragments of mudstone. Grain size analyses show that the hemipelagic sediment consists primarily of clay-sized particles (~67% by weight as equivalent spherical diameters). Smaller components are silt-sized (~30% by weight) and sand-sized (~3% by weight, largely composed of foraminifers) particles. The pelagic

7 438 G. A. Spinelli and M. B. Underwood o -87 o o -86 o o o -87 o o -86 o o 50 km 10 o 9.5 o 9 o 8.5 o Total sediment thickness (m) Hemipelagic sediment thickness (m) Fig. 5 Gridded and contoured total sediment thickness and thickness of the hemipelagic sediment section. Sediment thickness (both total and hemipelagic) is more variable overlying the East Pacific Rise crust in the northwest (where there are more seamounts and rougher basement topography) than over the Cocos Nazca Spreading Center crust in the southeast. 1m 2m E09gc M38gc E27gc E21gc E34gc Opal = 7% Clay = 81% Qtz = 2% Plag = 17% Clay = 70% Qtz = 4% Plag = 13% Calc = 13% Calc = trace SMEC = 75% SMEC = 90% ILLITE = 4% ILLITE = 0% KAOL = 21% KAOL = 10% Opal = 2% Opal = 8% Clay = 79% Qtz = 5% Plag = 16% Calc = trace Clay = 79% Qtz = 3% Plag = 11% Calc = 7% SMEC = 81% ILLITE = 2% KAOL = 17% Clay = 35% Qtz = 3% Plag = 16% Calc = 46% M38gc E09gc M02gc E34gc E21gc E27gc Clay = 55% Qtz = 4% Plag = 14% Calc = 27% Dark olive gray hemipelagic mud Brown clay Mixed sediment Carbonate ooze Ash Mn nodule M02gc Clay = 85% Qtz = 2% Plag = 13% Calc = trace SMEC = 90% ILLITE = 0% KAOL = 10% 1m Opal = 7% Clay = 77% Qtz = 3% Plag = 20% Calc = trace SMEC = 97% ILLITE = 0% KAOL = 3% 2m Clay = 55% Qtz = trace Plag = 45% Calc = trace 3m SMEC = 100% ILLITE = 0% KAOL = 0% Fig. 6 Representative core logs and results from opal and X-ray diffraction (XRD) analyses. Clay, quartz, plagioclase and calcite percentages are relative percentages determined by XRD of bulk powder samples. Smectite, Illite and kaolinite percentages are relative percentages of the <2 mm fraction. Opal percentages are weight percentages determined by sediment leaching. Hemipelagic sediments (which contain most of the opal and smectite within the sediment column) are thin in the southwest and around local basement highs.

8 (a) total clay (b) total clay (c) illite plagioclase opal smectite calcite calcite kaolinite Costa Rica sediment: Fluid partitioning 439 chalk appears to be coarser than the hemipelagic mud, but our LPC results may have been affected by interference among nannofossils. The average size distribution for the pelagic sediment is 30% clay, 47% silt and 23% sand (mostly foraminifers). Within each sediment type, there is very little spatial variation in mineralogy (Fig. 6). Of the samples analyzed by XRD (Table 2; Fig. 7), hemipelagic mud has the highest opal content (mean (m) = 10% by weight; standard deviation (s) = 3%; number of samples (n) = 23). Variegated clay (olive brown) is intermediate in opal content (m = 5%; s= 2%; n = 11), and the opal content of pelagic chalk is lowest (m = 2%; s= 1%; n = 4). The hemipelagic mud and variegated clay have similar relative abundances of total clay minerals, quartz, plagioclase and calcite. The mean values for hemipelagic mud (n = 39) are 77% total clay minerals, 3% quartz, 19% plagioclase and <1% calcite. The nannofossil chalk contains mostly calcite (n = 6; 1% total clay minerals, 4% quartz, 5% plagioclase, 90% calcite), whereas the mixed sediment ranges from <1 to 46% calcite. Most of the clay-sized fraction is smectite (Fig. 7). Relative percentages of smectite average 87% and range from 75 to 100% (Table 2). The average content of kaolinite is 13%, with a trace contribution of illite (~1%). The clay mineral content of pelagic chalk is too small to analyze accurately by XRD. ABUNDANCE OF HYDROUS PHASES Hemipelagic mud has relatively high total clay and opal contents, so a considerable volume of fluid must be contained within the hydrous phases. We quantified the opal content as an absolute wt%, but the clay mineral content is relative only to three other constituents (quartz, plagioclase and calcite). To determine how closely such relative abundances match absolute weight percentages, we compared the contents of calcite (from Coulometric analysis) to equivalent XRD values after correcting for the wt% opal. Linear regression shows that the two sets of values match to within approximately 10 wt% (Fig. 8). The fit would improve if Dark olive gray hemipelagic mud Olive brown mud Calcareous ooze Fig. 7 Summary of bulk sediment compositional data (a and b) and Mixed sediment <2 mm fraction clay mineralogy (c). The hemipelagic sediment is dominated by clay minerals. The pelagic sediment is mostly calcite. The mixed Altered ash sediment samples are mixtures of hemipelagic and pelagic sediment. The clay minerals in the <2 mm size fraction are mostly smectite. Number of samples: (a) 39 hemipelagic, 16 olive brown, 6 calcareous, 6 mixed; 1 altered ash; (b) 23 hemipelagic, 11 olive brown, 2 calcareous, 3 mixed; and (c) 29 hemipelagic, 15 olive brown, 3 mixed, 1 altered ash.

9 440 G. A. Spinelli and M. B. Underwood Table 2 Summary of sediment composition Sample ID Latitude Longitude Opal Relative % from XRD % Relative % from Smectite Zeolite Smectite Sand Silt Clay Sediment type Cruise Core Interval (wt%) Clay Quartz Plagio- Calcite Calcite XRD (<2 mm) % peak 060 (%) (%) (%) (cm) clase from Smectite Illite Kaolinite expan- peak, TIC dibility d-value MV GC Tr Y Hemipelagic mud MV GC Tr Y Olive brown clay MV GC Y Hemipelagic mud MV GC Y Hemipelagic mud MV GC Tr 3.0 Hemipelagic mud MV GC Tr N Hemipelagic mud MV GC Tr Y Hemipelagic mud MV GC Y Hemipelagic mud MV GC Y Hemipelagic mud MV GC Tr Olive brown clay MV GC Tr Y Hemipelagic mud MV GC Y Olive brown clay MV GC Tr Calcareous ooze MV GC Calcareous ooze MV GC Tr Y Olive brown clay MV GC N Mixed sediment MV GC Calcareous ooze MV GC Tr Y Hemipelagic mud MV GC Y Olive brown clay MV GC Tr N Olive brown clay MV GC Tr Y Olive brown clay MV GC Y Olive brown clay MV PC Tr Y Hemipelagic mud MV PC Tr Y Hemipelagic mud MV PC Tr Y Hemipelagic mud MV PC Tr Y Hemipelagic mud MV PC Y Hemipelagic mud MV GC Tr Y Olive brown clay, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction. Y, there is a peak ~10 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak. Hemipelagic mud = dark olive gray hemipelagic mud.

10 Costa Rica sediment: Fluid partitioning 441 Sample ID Latitude Longitude Opal Relative % from XRD % Relative % from Smectite Zeolite Smectite Sand Silt Clay Sediment type Cruise Core Interval (wt%) Clay Quartz Plagio- Calcite Calcite XRD (<2 mm) % peak 060 (%) (%) (%) (cm) clase from Smectite Illite Kaolinite expan- peak, TIC dibility d-value MV GC Tr Y Olive brown clay MV GC Tr Y Hemipelagic mud EW GC Hemipelagic mud EW GC Tr Calcareous ooze EW GC Tr 0.2 Mixed sediment EW GC Calcareous ooze EW GC Tr Calcareous ooze EW GC Y Hemipelagic mud EW GC Mixed sediment EW GC Tr Y Hemipelagic mud EW GC Tr Y Hemipelagic mud EW GC Tr Hemipelagic mud EW GC Tr Y Hemipelagic mud EW GC Tr Y Hemipelagic mud EW GC Tr 2.8 Hemipelagic mud EW GC Tr Y Hemipelagic mud EW GC Tr Y Hemipelagic mud EW PC Tr Y Hemipelagic mud EW PC Hemipelagic mud EW PC Tr Hemipelagic mud EW PC Tr Hemipelagic mud EW PC Tr N Hemipelagic mud EW PC Tr Hemipelagic mud EW GC Tr Y Hemipelagic mud EW GC Tr Hemipelagic mud EW GC Mixed sediment EW GC Tr 5.3 Olive brown clay EW GC Y Hemipelagic mud EW GC Y Mixed sediment EW GC Tr Olive brown clay EW GC Y Olive brown clay EW GC Tr Calcareous ooze EW GC Y Mixed sediment, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction. Y, there is a peak ~10 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak. Hemipelagic mud = dark olive gray hemipelagic mud.

11 442 G. A. Spinelli and M. B. Underwood Table 2 Continued Sample ID Latitude Longitude Opal Relative % from XRD % Relative % from Smectite Zeolite Smectite Sand Silt Clay Sediment type Cruise Core Interval (wt%) Clay Quartz Plagio- Calcite Calcite XRD (<2 mm) % peak 060 (%) (%) (%) (cm) clase from Smectite Illite Kaolinite expan- peak, TIC dibility d-value EW GC Y Olive brown clay EW GC Tr Y Olive brown clay EW GC Tr N Hemipelagic mud EW GC Tr 45 Tr N Altered ash EW GC Tr Hemipelagic mud EW PC Tr Y Olive brown clay EW PC Tr Olive brown clay EW PC Tr Y Hemipelagic mud EW PC Tr Y Hemipelagic mud EW PC Tr Y Hemipelagic mud EW GC Y Hemipelagic mud EW GC Tr N Olive brown clay EW GC Tr Y Olive brown clay ODP Leg 170 ODP Leg 170 ODP Leg 170 ODP Leg 170 ODP Leg 170 CR Trench 1040C Sec 2R-4 ( cm) 1040C Sec 20R-5 ( cm) 1040C Sec 24R-1 ( cm) 1040C Sec 36R-3 ( cm) 1040C Sec 51R-4 ( cm) Tr Claystone (prism) Tr Claystone (prism) Tr Diatomite (underthrust) tr Siliceous nannofossil chalk (underthrust) tr Calcareous diatomite (underthrust) 54/ Tr Hemipelagic mud, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction. Y, there is a peak ~10 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak. Hemipelagic mud = dark olive gray hemipelagic mud.

12 Costa Rica sediment: Fluid partitioning 443 Table 3 Weight percentage of hydrous phases in sediment Opal (wt%) Smectite (approximate wt%) Expandability of smectite (%) Dark olive gray hemipelagic mud Olive brown clay Calcareous ooze 2 <1 No data Relative % calcite (from XRD and opal) y = 1.40x 4.53 r 2 = % Calcite (from total inorganic carbon) Fig. 8 Relative percent calcite from X-ray diffraction (XRD) analysis for 18 hemipelagic sediment samples (dark olive gray hemipelagic mud and olive brown clay) as a function of the absolute percent calcite calculated from determination of the total inorganic carbon content of the sediment samples. The raw relative percentages of calcite ( ) are corrected ( ) for the opal content of the sediment samples. For samples with <20% calclite, the corrected relative percentages closely approximate the absolute percentages of calcite. 50 we also corrected for the amount of salt in the bulk powders (precipitated during freeze-drying), but that would require a parallel program of systematic physical properties measurements (e.g. water content, grain density, porosity). The XRD analysis slightly underestimates the wt% calcite for samples with very low calcite content, suggesting that approximately 2 5 wt% calcite may be the practical detection limit for the analysis. The relative wt% calcite closely matches the absolute wt% for hemipelagic and mixed-sediment samples with moderate calcite content (~5 15%). For mixed-sediment samples with >15 wt% calcite, the XRD analysis overestimates the fraction of calcite present. The hydrous phases are dominantly contained within the hemipelagic sediment (in which there is much less than 15 wt% calcite, and the relative weight percentages determined by XRD analysis closely approximate the absolute weight percentages). To estimate absolute values of total clay (wt%), we used the wt% opal and the relative abundance of total clay minerals determined by XRD: Total clay (wt%) ª [1 - (opal wt%/100)] (1) relative abundance clay (%) We then solved for wt% smectite using the value for total clay and the relative abundance of smectite in the <2 mm size fraction: Smectite (wt%) ª (clay wt%/100) (2) relative abundance smectite (%) Limitations of this approach include the possibility of clay mineral partitioning as a function of grain size and incorporation of non-clay minerals (e.g. quartz, zeolite) into the clay-sized fraction. Our XRD results, however, show only traces of clay-sized quartz and zeolite, and the amount of smectite should increase in progressively finer size fractions. On average, hemipelagic mud from the Costa Rica study area contains approximately 60% smectite by weight. Smectite in pelagic chalk can not be determined precisely due to the low clay content, but it must be less than 1% (i.e. the relative percent of total clay in the pelagic sediment). CHARACTERISTICS OF SMECTITE The expandability of smectite averages 59% and ranges from 48 to 70% (Table 3). These results indicate that the expandable clay is detrital in origin, rather than a product of authigenic alteration of volcanic ash, and it probably includes crystallites of illite in a disordered mixed-layer structure. Accordingly, when making calculations of water volume within the interlayer site of smectite, we assumed an intermediate state of hydration with two layers of water. This hydration state corresponds to approximately 20% water by weight and approximately 40% water by volume. Analyses of 13 powders revealed broad smectite (060) peaks centered near d = Å (Fig. 9). This result indicates the smectite is mostly dioctahedral (Brown & Brindley 1980; Moore & Reynolds 1997).

13 444 G. A. Spinelli and M. B. Underwood Table 4 Volume of water (m 3 ) per m 3 of sediment entering subduction zone Pore space Opal Smectite Hemipelagic mud Calcareous ooze Weighted average for entire sediment column % of total water 96.2% 0.4% 3.4% Counts trioctahedral dioctahedral q Fig. 9 Example of X-ray diffraction (XRD) analysis of randomly oriented <2 mm grain size fraction (E43gc cm). The diffractogram is aligned to the prominent quartz peaks at d-spacings of 1.82 and Å. The smectite 060 peak is close to the typical range (bold lines) for dioctahedral smectite, suggesting it is detrital in origin. Dioctahedral members of the smectite group (montmorillonite, Al-rich beidellite) typically form through pedogenesis of diverse protoliths under a wide range of environmental conditions (e.g. Fagel et al. 2001), although pure bentonite layers also form through submarine alteration of silicic volcanic ash (Hodder et al. 1990; Naish et al. 1993). In contrast, trioctahedral varieties (saponite) and Ferich dioctahedral end-members (non-tronite) originate through meteoric weathering and hydrothermal alteration of basalt (e.g. McMurtry et al. 1982; Parra et al. 1986; Güven 1988; Chamley 1989). Most of the dioctahedral clay that was deposited offshore Costa Rica probably formed through tropical weathering of volcanic and sedimentary rocks in Central America. DISCUSSION quartz d = 1.82 A quartz d = A smectite 060 FLUID PARTITIONING Sediment-hosted fluids on the incoming Cocos Plate are partitioned between a basal pelagic section and an overlying hemipelagic section. Within each of those sections, fluid is partitioned between sediment pores and hydrous minerals. We estimate sediment porosity using data from ODP Site The average porosity of hemipelagic samples from Site 1039 is 75%; the average porosity of pelagic samples is 70% (Kimura et al. 1997). Porosity averages 73% over the entire sediment column. The thicknesses of hemipelagic and pelagic intervals cored at Site 1039 (152 and 225 m) are similar to the medians derived from seismic data (135 and 213 m). The pelagic section, on average, is approximately 2% opal by weight and <1% smectite by weight. The hemipelagic section is approximately 10% opal by weight and 60% smectite by weight. We convert the weight percentages of opal and smectite to volume percentages based on their densities (r opal ª 2.47 g/cm 3 ; r smectite ª 2.02 g/cm 3 ), then calculate the volume of water in opal within a typical 1 m 3 of sediment by: Volume water in opal ª (1 - [n/100]) vol% opal vol% water in opal (3) where n is porosity, and the vol% of water in opal is 23% (equivalent to 11% water by weight). A similar approach is used to determine the volume of water in smectite within a typical 1 m 3 of sediment: Volume water in smectite ª (1 - [n/100]) vol% smectite vol% water in smectite (4) where the vol% of water in smectite is 40% (equivalent to 20% water by weight, or two layers of interlayer water). As initially deposited, the pelagic and hemipelagic sediments contain more than 70% water by volume. Distributed over the entire sediment column, 96% of that water is within pore spaces, 3.6% is in smectite and 0.4% is in opal (Table 4). Once subduction begins, rapid loading consolidates sediment within the first ~5 km from the trench (Shipley et al. 1990; Kimura et al. 1997; McIntosh & Sen 2000; Saffer et al. 2000; Saito & Goldberg 2001). The hemipelagic sediment section

14 Costa Rica sediment: Fluid partitioning 445 Average porosity (%) Average porosity (%) (a) Distance into subduction zone (km) (b) n = 1.34 ln(x) r 2 = 0.76 n = 65.1 e 0.055x r 2 = Distance into subduction zone (km) Fig. 10 Average porosity of underthrust sediment around Ocean Drilling program (ODP) Sites 1039, 1040 and 1043, based on index properties measurements (Kimura et al. 1997), laboratory consolidation tests (Saffer et al. 2000), variations in layer thickness (Saito & Goldberg 2001) or reflection seismic experiments (Shipley et al. 1990; McIntosh & Sen 2000). The hemipelagic section (a) of the sediment column compacts and looses a large fraction of its porosity in the first kilometer beyond the Middle America Trench, after which the rate of porosity loss decreases. The pelagic section (b) compacts less than the hemipelagic section in the first few kilometers of the subduction zone., Kimura et al. (1997); +, McIntosh and Sen (2000);, Saffer et al. (2000);, Saito and Goldberg (2001);, Shipley et al. (1990). loses a substantial fraction of its porosity (down to ~65 60%) within the first kilometer of the subduction, but the rate of consolidation decreases over the next 4 km. Porosity loss in the pelagic section is more gradual and uniform (Fig. 10). Low permeability within the clay-rich sediment inhibits drainage of the pore water. Consequently, on the Costa Rica margin, underthrust sediment in the shallow portion of the subduction zone is underconsolidated due to sustained pore-fluid pressures above hydrostatic (Saffer et al. 2000). The amount of additional porosity loss is poorly constrained beyond the coverage of reflection seismic data (i.e. farther than ~5 km into the subduction zone), but some insights can be gained from the numerical modeling of fluid by Saffer and Bekins (1998), who suggested that the underthrust sediment in Nankai Trough reached a normally consolidated state by ~30 km into the subduction zone. If the underthrust sediment on the Costa Rica margin follows a similar response (normally consolidated 30 km from the subduction front), then we can identify a reference point for comparison. The point s depth below seafloor depends upon the margin s subduction geometry. The slope of the margin wedge off Nicoya is approximately 5.4 ; the decollement dips approximately 6 for the first 30 km, then increases to approximately 13 (Christeson et al. 1999). Thus, the wedge is approximately 6 km thick 30 km landward of the trench. A global compilation of porosity versus depth (Bray & Karig 1985) can be used to estimate corresponding porosity values of approximately 5 15%. If those estimates are correct, then the switchover in fluid budget from mostly porehosted water to mostly mineral-hosted water occurs before strata reach 30 km into the Costa Rica subduction zone (Fig. 11). SEDIMENT DEHYDRATION Release of water from the mineral structures of opal and smectite is controlled by reaction kinetics (Ernst & Calvert 1969; Pytte & Reynolds 1988; Huang et al. 1993). Opal dehydration goes to completion between temperatures of approximately 50 and 100 C (Murata et al. 1977; Behl & Garrison 1994). In typical sedimentary basins, most progress of the smectite-to-illite reaction (with two pulses of smectite dewatering) occurs between 60 and 140 C (Perry & Hower 1970, 1972; Freed & Peacor 1989). Geochemical analyses of fluids collected from the frontal decollement (Kimura et al. 1997) indicate that key reactions occurred at temperatures of at least C but less than 150 C (Chan & Kastner 2000; Silver et al. 2000). This inferred window overlaps the temperatures for both opal and smectite dehydration. In situ temperature at the frontal decollement (Site 1040) is <5 C (Kimura et al. 1997), suggesting a deeper source for the fluids. Thermal models for the Nicoya subduction zone yield temperature predictions of 100 C at horizontal distances km into the subduction zone and 160 C once strata reach km into the subduction zone (Harris & Wang 2002); the upper

15 446 G. A. Spinelli and M. B. Underwood (a) 0 Plate interface Depth (km) Proportion of water in smectite+opal Earthquakes CNS (warm) EPR (cool) Distance into subduction zone (km) (b) ?? warm Increased fluid pressure during diagenetic dewatering cool Smectite and opal reactions completed (cool case) Dissipating fluid pressure; increasing effective stress Fig. 11 Earthquake locations (a) from Newman et al. (2002) and proportion of water contained in hydrous minerals within sediments subducted offshore Costa Rica (b). The volume of water in the hydrous mineral phases (relative to pore space) increases with distance into the subduction zone as porosity decreases. The gray polygon indicates the range of potential fluid partitioning scenarios based on a range of sediment porosities from Bray and Karig (1985). Beyond km into the subduction zone, most of the water in the sediment is contained in hydrous minerals. The location in the subduction zone at which water is released from the hydrous minerals is a function of reaction kinetics; fluid will be released closer to the trench in a warm subduction zone than in a cool one. Shallow earthquakes occur closer to the trench in the warm portion of the subduction zone than in the cool section (a). The up-dip limit of seismicity may be related to increased effective stress as fluid pressures from diagenetic dewatering reactions dissipate. CNS, Cocos Nazca Spreading Center; EPR, East Pacific Rise. and lower limits of the modeling solution depend on the inferred thermal state of the incoming crust. On the basis of those numerical simulations, we suggest that opal and smectite dewater in discrete pulses after porosity drops below approximately 15% and the rate of compaction slows. Zones of elevated fluid pressures along the plate boundary fault should coincide with compartments of diagenetic fluid release (e.g. Saffer & Bekins 1998), thereby decreasing effective stress in a patchy 3-D pattern. Heat flow also varies alongstrike, with higher values (average ~105 mw/m 2 ) on CNS crust in the southeast and lower values (average ~20 mw/m 2 ) on EPR crust in the northwest. There are also local zones of high heat flow on EPR crust caused by hydrothermal circulation. This variation in heat flow likely leads to an alongstrike difference in the distance into the subduction zone at which much of the diagenetic dewatering reaction progress occurs. IMPLICATIONS FOR OTHER CIRCUM-PACIFIC SUBDUCTION ZONES The volumetric contribution and spatial distribution of diagenetic fluid sources along a subduction thrust depend on the composition and temperature path of subducted and accreted sediment, which can vary both between and within subduction zones. To place Costa Rica within a broader context of circum-pacific subduction zones, we compare results from Nankai Trough, the Japan Trench, the Aleutian Trench and Cascadia. The Nankai Trough is characterized by sediment with low opal content, highly variable smectite content, and highly variable heat flow. Diatoms and radiolarians are rare in the upper and lower Shikoku Basin sections, but are more abundant in the trench-wedge facies (Shipboard Scientific Party 1975, 1986, 2001). The content of biogenic silica is less than 1 wt%. The smectite content of sediment along the central (Muroto) transect is modest because of presubduction diagenesis, generally <25% of the bulk sediment by weight (Underwood et al. 1993; Steurer & Underwood 2003a). Seafloor heat flow near the trench on the Muroto transect is high, approximately mw/m 2 (Hyndman et al. 1995; Shipboard Scientific Party 2001). Along the western (Ashizuri) transect, smectite is much more abundant in the lower Shikoku Basin strata, typically 30 50% of the bulk sediment by weight (Steurer & Underwood 2003a; Underwood et al. 2003), and heat flow is substantially lower, approximately 63 mw/m 2 (Kinoshita & Yamano 1986; Shipboard Scientific Party 1986).