smectite diagenesis, pore-water freshening, and fluid flow at the toe of the nankai wedge

Smectite diagenesis, pore-water freshening,and £uid £ow at the toe of the Nankai wedge

Kevin M. Brown a;*, Demian M. Sa¡er b, Barbara A. Bekins c

a Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0244, USAb Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006, USA

c US Geological Survey, 345 Middle¢eld Rd., Menlo Park, CA 94025, USA

Received 7 March 2001; accepted 10 October 2001

Abstract

The presence of low-chloride fluids in the lowermost sediments drilled at Ocean Drilling Program Site 808, at theNankai accretionary wedge, has been considered as prime evidence for long-distance, lateral fluid flow from depth. Here,we re-evaluate the potential role of in situ reaction of smectite (S) to illite (I) in the genesis of this low chloride anomaly.This reaction is known to be occurring at Site 808, with both the S content and S to I ratio in the mixed layer claysdecreasing substantially with depth. We show that the bulk of the chloride anomaly can generate by in situ claydehydration, particularly if pre-reaction smectite abundances (Ai) approach V10^15% of the bulk sediment. The Ai

values, however, are not well constrained. At Ai values 6 10^15%, an additional source of low-Cl fluid centered close tothe decollement could be required. Thus, there remains the important possibility that the observed low-Cl anomaly is acompound effect of both lateral flow and in situ smectite dehydration. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: smectite; illite; £uid dynamics; active margins; dehydration; ODP site 808

1. Introduction

One signi¢cant result of drilling Ocean DrillingProgram (ODP) Site 808 (Fig. 1) at the toe of theNankai accretionary wedge (SW Japan) was theobservation of low-Cl pore £uids in the deeperportions of the sedimentary section [1^5]. Thereis also evidence that chlorine, boron and other

isotope and chemical anomalies also occur in thelower units [1,6,7]. Interpretation of pore-waterchemical anomalies is of key importance to under-standing the magnitude and spatial and temporalpatterns of £uid expulsion from accretionary sys-tems. It has been hypothesized that these anoma-lies result from up dip migration of £uids fresh-ened by dehydration reactions at great depths inthe subduction system along the decollement orsedimentary layers to the toe of the wedge [1,8^12]. Dehydration reactions that cause fresheninginclude the smectite to illite reaction, opal dehy-dration, and potentially deeper metamorphic re-actions [7,13,14]. However, interpretations of theorigin of low-Cl £uids can be complicated by sev-

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 4 6 - 5

* Corresponding author. Fax: +1-619-534-0784.E-mail address: [email protected] (K.M. Brown).

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Earth and Planetary Science Letters 194 (2001) 97^109

www.elsevier.com/locate/epsl

eral factors. For example, a recent study sug-gested that the partial stress-induced dehydrationof smectite from an 18 Aî to a 15 Aî hydrate undere¡ective loads of 1^3 MPa played an importantrole in determining freshening patterns in the Bar-bados wedge [9]. Freshening was proposed tohave been initiated either during the squeezingof pore £uids out of cores onboard ship or per-haps during tectonic loading in the wedge, withonly a very minor component related to actuallateral £ow along the decollement zone.

The lateral £ow hypothesis has many implica-tions for the mechanisms of £uid migration, therequired £uid pressure and distribution of stressalong the subduction thrust, and the mass balanceof £uids within the subduction system. Althoughin this paper we primarily address the role of thesmectite to illite reaction in freshening £uids inthe Nankai wedge, it is also important to considerthat both the presence or absence of smectite andfocusing of £ow along the decollement have directrami¢cations for the down dip evolution of a faultzone's mechanical behavior [15^18]. Given thatNankai may be the site of future seismogeniczone drilling we suggest that it is also timely toexamine the potential contributions in this wedgeof other, local sources of H20 that can freshen£uids.

Here, we re-examine the role that in situ claydehydration may play in generating the low-Clsignal observed at the toe of the Nankai accre-tionary wedge, with a particular emphasis on therole of the in situ S to I reaction. We use revisedporosity estimates corrected for rebound and thepresence of the hydrous low-density clay content[9,19,20], along with a kinetic model for smectitetransformation that better matches the observedreaction progress at Site 808 [12,21]. We showthat in contrast to previous studies, moderate ini-tial smectite abundances can generate a signi¢cantproportion of the observed low-Cl signal at Site808.

2. Sedimentary, mineralogical, and diageneticarchitecture of the Nankai system at Site 808

At ODP Site 808 there is evidence for both the

presence of low-Cl £uids and in situ reaction ofsmectite to illite. Site 808 is located near the toe ofthe Nankai accretionary prism (Fig. 1), above theextinct backarc-spreading center of the ShikokuBasin [22]. Five main sedimentary facies, rangingin age from Miocene to Quaternary, are presentat the toe of the wedge in the V1240 m thickincoming sediment section (Fig. 2). Chloride con-centrations increase slightly with depth to V600m below sea £oor (mbsf) and then decrease toV450 mM to form a broad minimum withinthe underthrust units beneath the decollementzone at V960 mbsf (Fig. 2). The bulk of thepore-water freshening and smectite transforma-tion occur within the lowermost V600 m of thesection, comprised of bioturbated hemipelagicmudstone that corresponds to the trench to basintransition (556.8^618.47 m, Unit III), the upperShikoku Basin strata (618.47^823.74 m, UnitIVa), and the Lower Shikoku Basin strata(823.74^1243 m, Unit IVb) [2,22,23]. Unit IVbcontains the decollement zone, and the upper por-tion of Unit IVa contains ash layers and lithi¢edtu¡ (Fig. 2). The depth vs. age determinations atSite 808 indicate slow sedimentation rates for theShikoku Basin sediments (Units IVa and b) be-tween 13.6 and V0.35 Ma with rapid burial ofthese units beneath 600 m of trench wedge sedi-ments occurring only in the last V0.5^0.35 Ma orless [24].

Fig. 1. Location map showing the distribution of DSDP andODP drill sites in the Nankai Trough region.

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K.M. Brown et al. / Earth and Planetary Science Letters 194 (2001) 97^10998

Semi-quantitative X-ray di¡raction (XRD)studies indicate a relatively uniform clay mineralcontent of V18^20% throughout the section, aquartz content of V50^60%, and some feldsparand minor calcite ( þ amorphous constituents) [2].In the clay-sized fraction, illite dominates (60^70%). The smectite content in the hemipelagic sec-tion is low (6 6^7% of the bulk sediment), andthe abundances generally decrease down-hole(Fig. 2). Locally elevated smectite abundances ofup to V10^15% are associated with layers rich inaltered ash. The smectite (S) to illite (I) reaction isV80% complete by the base of the hole, as indi-cated by the increase in I% in the mixed layerclays from V0^20% at 500 mbsf to 60^80% atV1200 mbsf (Fig. 2). Increasing degrees of order-ing in the mixed layer clays were also observedtowards the base of the hole, consistent with en-hanced illite crystallinity and diagenesis [2]. Theestimated temperature range of 65³C to V120³Cbetween 560 mbsf and V1200 mbsf (Fig. 2) isconsistent with the temperature range of the S

to I reaction observed in other systems [25^29].It is important to note for this study that theincrease in degree of reaction mirrors the generaldownward decrease in smectite content and oc-curs over the same interval where freshening ofthe pore £uids with depth is observed (Fig. 2).

Release of H2O into the pore space during thein situ reaction of S to I was initially consideredas one possible explanation for the observed pore-water freshening at Site 808 [1,2,5]. However, insitu reaction was ultimately discounted as a sig-ni¢cant source of freshening because the amountof initial smectite required, perhaps as much as25^50%, exceeded available estimates [1,2]. In-stead, it was hypothesized that pore-waters fresh-ened by smectite dehydration at great depths inthe wedge migrated laterally out to Site 808 eitheralong the decollement zone or the underthrustunits at a position near the current Cl minimumat V1150 mbsf (see below). It is important tonote that numerical model studies of lateral £owin this system indicate that, even for the case of

Fig. 2. Site 808 data: (A) smectite abundance as a % of the total bulk sediment, (B) smectite as a ratio of the mixed layer clays,(C) pore-water chloride data, (D) estimated temperature gradient. dec ^ decollement zone, sw ^ seawater values [1^3].

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K.M. Brown et al. / Earth and Planetary Science Letters 194 (2001) 97^109 99

transient £ow from depth, the initial amount ofsmectite required to produce observed fresheningat Site 808 approached as much as 30 wt% [12].These values are signi¢cantly greater than the ob-served smectite content of 6 6^7% at Site 808.There are, therefore, important basic mass bal-ance issues with the current interpretations ofthe origins of the low-Cl £uids at Site 808,whether or not an in situ or a lateral £ow mech-anism is favored.

3. Re-evaluation of pore-water freshening in theNankai wedge

Prior to considering the time-dependent devel-opment of a freshened pore-water anomaly, weaddress some basic mass balance issues associatedwith the presence of smectite and subsequentsmectite dehydration. The S to I reaction can sig-ni¢cantly reduce the chloride concentrations ofthe pore £uids by releasing interlayer H2O[10,13]. Smectites can have basal d-spacings ofV10 Aî , V12 Aî , V15 Aî to V18 Aî , dependingon environmental conditions and the interlayercation [25,30,31]. The interlayer H2O content of15 Aî and 18 Aî hydrates lies between V20 wt%(V30 vol%) and V25 wt% (V50 vol%). Theamount of water released during the smectite toillite reaction (vM) can be derived by subtractingthe interlayer H2O mass (M1) that is associatedwith the current smectite abundance from the ini-tial mass (M0) associated with the initial smectiteabundance:

vM �M03M1 �1�

The calculated chloride concentration can then bedetermined from:

Cl � Cl0P cor

P cor � vM=bw�2�

where Cl0 is the pore-water chlorinity prior toreaction (assumed to be V560 mM ^ similar toa somewhat diagenetically altered and saline pore£uid) and Pcor is the initial smectite and rebound-corrected or uncorrected porosity (see discussionon porosity below).

An important factor that must be consideredwhen determining the porosity (Pcor) into whichthe freshening occurs is the presence of the low-density hydrated smectite mineral [9,19]. Fully hy-drated smectite is an unusually low-density min-eral with a high H2O content. Interlayer H2O isan intrinsic part of the mineral, with thermody-namic properties and chemical behavior di¡erentfrom those of bulk pore-water. However, smectiteinterlayer H2O is usually included in the porosityterm when it is reported in the literature (i.e. inthe Nankai ODP data) because it is either ex-pelled by drying at low humidity [32] during ship-board porosity measurements or because the lowhydrated grain density of smectite is not takeninto account (potentially as low a 1.9^2.1 g/cc).To obtain the actual bulk pore-water content ofsediment, the total H2O in the sediments must bepartitioned into the pore and mineral phases.There are a variety of methods by which thiscan be done depending on the available physicalproperty data [9,19]. Using the uncorrected poros-ity curves as a starting point allows us to derivean estimate for the mass of interlayer H2O (M) ina unit volume of wet sediment using:

M � AH

13H

� ��13P �b s �3�

where A is the mass fraction of dry smectite (i.e. 9Aî hydrate) in the sediment solids, P is the re-bound-corrected or uncorrected porosity (see be-low), bs is the average anhydrous grain density(V2.75^2.85 g/cc), and H is the hydration stateof the smectites (mass fraction of interlayer H20).In the following analysis two values of A are used,higher pre-smectite to illite reaction values (Ai)and the current partly reacted value at Site 808(Fig. 2). The calculated freshening is very sensitiveto the amount of pre-reaction smectite in the sedi-ment. This is because any increase in initial smec-tite content reduces the volume of the initial porespace that is ¢lled by saline £uid while simultane-ously increasing the amount of H20 available forfreshening.

The shipboard measured porosity^depth pro-¢les are relatively linear within units but shiftabruptly across some stratigraphic and structural

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boundaries. There is, for example, a sharp in-crease in porosities across the decollement zonethat most probably relates to underconsolidationand overpressuring in the rapidly buried and low-permeability underthrust section [33]. In our cal-culations of instantaneous freshening potentiallygenerated by the S to I reaction, we have usedlinear ¢ts to the porosity data (Fig. 3A). In addi-tion to correcting porosities for smectite content,we also show resulting freshening pro¢les for botha rebound-corrected and uncorrected porositypro¢le. The rebound correction for the Site 808cores was calculated in order to account for ex-pansion that can occur in cores during recovery asthey are unloaded [20]. Based on the measuredelastic properties from consolidation tests on alimited number of samples obtained from Site808 [20], porosities may have expanded by V3%for the section just above 800 mbsf, and as muchas 8^9% for the hemipelagic units below 800 mbsfif £uid pressures were close to hydrostatic. Weshow pro¢les for both rebound-corrected and un-corrected porosities because the magnitude of therebound correction is still poorly constrained. Ad-ditional future laboratory consolidation tests onODP Leg 190 samples and future logging whiledrilling results may allow better de¢nition of therebound correction. The smectite-corrected poros-ity is then calculated from:

P cor � P3P r3Mbw

�4�

where bw is the £uid density (V1.03 g/cc), and Pr

is the estimated porosity rebound correspondingto the sections above and below 800 mbsf. Theinterlayer H2O density is thought to be close(within a few percent) to that of bulk pore-water[34].

The in situ hydration state of the smectite canalso be estimated within narrow limits. Smectitesin contact with seawater compositions may startas 18 Aî hydrates at shallow burial depths, butincreasing the e¡ective load causes them to parti-ally dehydrate to 15 Aî hydrates as stress levelsrise above V1.5^3 MPa [9]. In situ stress levelsbelow 600^700 m at Site 808 are likely to lieabove the 1.5^3 MPa stress-induced dehydration

window [20]. Thus, for the purposes of studyingthe impact of the S to I reaction, we assume thatthe hydrates already lie within a 15 Aî state. How-ever, the stress-induced dehydration reactioncould have contributed to some initial fresheningof the pore £uids during the period where the

Fig. 3. (A) Porosity, and corrected porosity for both rebounde¡ects and 5%, 10%, and 15% initial smectite (Ai), (B) resultsof the instantaneous freshening developed during the dehy-dration of the initial smectite contents to the current abun-dances shown in Fig. 1A for rebound-corrected porosities,(C) porosity, and porosity-corrected for Ai = 5%, 10%, and15% (no rebound correction), (D) the instantaneous freshen-ing developed for non-rebound-corrected porosities for di¡er-ent Ai values.

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hemipelagic section was buried beneath the trenchwedge and frontal thrust. Thus, considering only15 Aî hydrates (H = 0.2) in this study provides aconservative estimate of freshening.

Without XRD data from a truly unreactedreference section on the incoming plate, the larg-est uncertainty is the value of Ai. As the degree ofreaction increases with depth below 600 m (Fig.2), so does the uncertainty in our estimate of Ai.Signi¢cantly, although the data are sparse, otherDSDP sites (e.g. Site 297, Fig. 1) in the ShikokuBasin, in which the Miocene^Pliocene units(underthrust sequences at Site 808) are encoun-tered, tend to have increasing rather than decreas-ing smectite abundances downward and reachabundances of s 15% [35^37]. Thus, there is con-siderable uncertainty in de¢ning the upper boundof Ai in the lower portion of the incoming section.In light of these uncertainties, we calculated fresh-ening pro¢les for a range of Ai values using theassociated smectite-corrected porosities (Fig.3A,C). We show in Fig. 3B, as a ¢rst approxima-tion, the freshening resulting from instantaneousin situ reaction of Ai = 5%, 10%, and 15% of a15 Aî smectite to the current smectite abundanceat Site 808 (Fig. 2). For the case of instantaneouspore-water freshening and rebound-corrected po-rosities, an Ai of V10% smectite could generatethe bulk of the low-Cl anomaly below 800 m,with 6 10% being adequate at shallower depths(Fig. 3B). If the rebound correction is neglected, ahigher value of Ai of 10^15% would be requiredto generate the bulk of the observed anomaly(Fig. 3D).

4. Time-dependent generation and di¡usion ofCl anomalies by in situ reaction

The dilution analysis presented above (Fig. 3)does not account for time-dependent di¡usion oradvection of Cl during the smectite to illite reac-tion period. In addition, we have assumed that nocompaction has occurred in these sediments dur-ing the dehydration process as they have beentransported from a position V14^15 km basin-ward of their present position and buried andheated beneath the trench wedge and frontal

thrusts (corresponding to a time of V0.35 Maassuming a convergence rate of 40 km/Myr) [24].If the Cl anomaly was formed in situ over 0.35Ma, di¡usion and advection could impact both itsshape and size. To investigate the importance oftime-dependent processes occurring during the insitu reaction of illite to smectite, we use a two-dimensional numerical hydrologic model thatcouples £uid £ow, solute transport, and a kineticreaction model. This numerical model, its mathe-matical basis, and its architecture are described indetail elsewhere [8,12,38].

The kinetics of the S to I reaction are generallydi¤cult to constrain accurately because the reac-tion rate may vary with factors such as K and Alcontent, Si activity, and grain size [23^29,39,40].Fortunately, in the Nankai wedge, we can esti-mate the burial and heating history, and test mod-els of reaction progress against the degree of re-action observed at Site 808. We use a kineticmodel for smectite dehydration [28] that dependson temperature and time of sediment exposure.The temperature distribution in the wedge andincoming section is estimated using the thermalmodel of Ferguson [41]. The time of exposure tothese temperatures is calculated from the assump-tion that the burial and arcward transport of ac-creted sediment through time can be approxi-mated by uniformly divergent sediment velocities(this is described in detail elsewhere [8,12,38]). Re-action is assumed to begin by V0.35 Ma, whenrapid heating started as the hemipelagic sectionwas buried beneath the trench outer slope trenchwedge turbidities and subsequently beneath thethickening wedge. We evaluate the e¡ects of un-certainty in thermal regime by considering twocases: one with a basal heat £ux of 150 mW/m2,and one with 180 mW/m2, which yield bottom-hole temperatures at Site 808 of V92 andV120³C, respectively. For comparison, down-hole temperatures at the base of the sedimentarysection are estimated at 90^120³C [22].

As shown in Fig. 4A, the calculated reactionprogress from the kinetic model is in reasonablygood agreement with observed down-hole reac-tion progress in the deeper section, as measuredby % of S remaining in interlayer clays, for thecase of high heat £ow (180 mW/m2). Signi¢cant

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reaction is con¢ned to depths greater than V550^650 mbsf, and the reaction has destroyed 70^90%of the initial smectite by V1300 mbsf. However,the kinetic model tends to slightly underestimatethe degree of reaction in the upper portion of thereactive region (above V1100 mbsf). This under-estimate may result from the choice of slightly toolow a heat £ow, too young an age for the initia-tion of the reaction, or it simply may be that theactual kinetics of the reaction are slightly fasterthan the rate [28] we have used in this study.Additional mineralogical data that should ulti-mately be obtained from the multiple drill sitesof Leg 190 will provide data at di¡erent exposureages to better constrain the factors controllingreaction progress [42]. However, our result indi-cates that the kinetic reaction model generates areasonable, if conservative, estimate of the reac-tion and associated freshening at the toe of thewedge.

To simulate advection and di¡usion of the low-chlorinity signal during the period of reaction, weincorporate the kinetic reaction model into a nu-merical hydrologic model that has previously been

developed to simulate £ow in the Nankai wedge[12]. In the hydrologic model, £uid sources arederived from smectite dehydration (fresh water)and compaction. The volume of water derivedfrom dehydration is calculated using the kineticmodel described above, again assuming Ai = 5%,10%, and 15%, and a 15 Aî hydrate. The £uidderived from sediment compaction is calculatedassuming the sediments progressively lose poros-ity during burial beneath the trench and accre-tionary wedge (e.g. [43]). Porosities within themodel domain are assumed to follow an exponen-tial decrease with depth, ¢t to the corrected po-rosity for 5%, 10%, and 15% smectite. Thesmooth ¢ts to the porosity data include the cor-rections for both elastic rebound and the presenceof low-density hydrated smectite (see Eqs. 1^4above). To account for the relative underconsoli-dation of underthrust sediments, porosities in themodel are o¡set to larger values across the de-collement to match observations at Site 808. Theo¡set ranges from 4 to 6% at Site 808 dependingupon the smectite content, and decreases to zeroby 30 km from the trench [12].

Here, we evaluate pore-water freshening underconditions of steady-state £uid £ow and minimal£uid transport along the decollement. We assignsediment permeability as a function of porosity,on the basis of the detailed sensitivity analysis ofSa¡er and Bekins [12] :

log k � 320:0� �5:5�P �

where k is permeability in m2. The decollementzone was not assigned a higher permeability inthese runs, as was typically the case in previouslypublished simulation [12]. This allows us to eval-uate the local e¡ects that advection and di¡usionhave on the in situ freshening signal. The model'ssensitivity to molecular di¡usivity was tested forvalues ranging from 1U1039 to 5U10311 m2/s. Itwas determined that only small di¡erences in re-sulting Cl pro¢les occurred over this range, sodi¡usivity is ¢xed at 1U1039 m2/s for the modelruns shown here.

Modeled down-hole pore-water concentrations(Fig. 4B) for an Ai of 5% range from seawatervalues (555 mM) above 500^600 mbsf to V530

Fig. 4. (A) Results of the kinetic reaction model and its ¢tto the mixed layer clay data for heat £ows of 150^180 mW/m2, (B) resulting modeled Cl pro¢les assuming three di¡erentAi values. The residual anomaly (R, Fig. 5) is calculated bysubtracting the modeled results from the data. Porositiesused in the model are rebound-corrected and described bythe following Athy type curves: rebound, no smectite correc-tion (0.6EXP(30.000956 d)) ; 5% smectite correction(0.6EXP(30.001073 d)), 10% smectite correction (0.6EXP-(30.001205 d)), 15% smectite correction (0.6EXP(30.001348d), where d is depth.

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mM near the base of the underthrust sedimentsection (V1200 mbsf). For 10% smectite, the min-imum values are V490 mM at 1200 mbsf, where-as 15% initial smectite produces modeled pore-water pro¢les approaching the observations inthe lower portions of the section at V1200 mbsf(460 mM). The modeled pro¢les generally showless freshening than is observed in the upper re-active zone and for the instantaneous fresheningstudy for an equivalent Ai value. Because the ki-netic model underestimates the degree of reactionin this upper region (Fig. 4A), the numerical studyresults in a conservative estimate of freshening inthis zone.

The instantaneous and time-dependent Cl pro-¢les (Figs. 3 and 4) also di¡er somewhat in theirdetails (for example around the decollementzone). This is partly because the instantaneousfreshening method assumes reaction to the currentsmectite values, whereas the numerical simulationbased on the kinetic model results in a pro¢le ofsmooth, downwardly increasing reaction progress.Di¡usion would also clearly have the e¡ect ofsmoothing out any sharp gradients that are seenin the instantaneous freshening curves. Some mi-nor di¡erences in calculated pore-water fresheningalso arise from the fact that smoothed ¢ts to theporosity data were used in the numerical simula-tion, with a sharp change in porosity only beingaccounted for across the decollement. Because weare trying to assess the general contribution of insitu freshening, these di¡erences are not consid-ered to be signi¢cant.

5. Discussion

In contrast to previous calculations, the resultsof the instantaneous freshening model (Fig. 3B)suggest that, for rebound-corrected porosities,only moderate initial amounts of smectite(V10%) are required in the incoming section togenerate the majority of the low-chlorinity anom-aly observed in the underthrust units at Site 808(Fig. 4). This is because the saline pore space intowhich freshening occurs is lowered by correctingthe porosities for the presence of the low-densitysmectite mineral phase and rebound e¡ects. The

e¡ect of the rebound correction is also consideredseparately. When the rebound e¡ects are ignored,slightly higher Ai values of 10^15% are necessaryto generate the observed Cl pro¢les (Fig. 4). The10^15% range for Ai for the rebound and non-rebound-corrected porosities may be reasonablegiven total clay abundances are s 20%. In addi-tion, at other DSDP sites situated o¡-axis to thewest of the spreading center and basinward of thetrench wedge [35^37], temperatures are coolerthan at Site 808. Smectite abundances in theclay-sized fraction at these sites increase substan-tially with depth in units of similar Miocene age,in contrast to the trends seen at Site 808.

The time-dependent e¡ects of the smectite toillite reaction have also been considered (Fig. 4).The modeled smectite to illite reaction progress,using the kinetic expression of Pytte and Reynolds[28], produced a similar, but conservative, esti-mate of observed reaction progress determinedfor the mixed layer clays at Site 808 (Fig. 4A).Although some additional tuning of the kineticmodel is required, overall there does not appearto be anything unusual about the S to I reactionat Site 808. The data and modeled reaction curveindicate that smectite is rapidly reacting along theSite 808 transect primarily as a response to theelevated heat £ow. When the e¡ect of reactionkinetics is incorporated into a numerical hydro-logic model, the resulting Cl pro¢les retain theexpected downward decrease and general shapeof the low-Cl anomaly. The numerical simulationsalso indicate that as initial smectite contents ap-proach 15%, the modeled Cl pro¢le approachesthe basic form of the observed pore-water pro¢leat Site 808 (Fig. 4A). The numerical study indi-cates that only a small portion of the low-Clanomaly di¡uses above 600^700 mbsf, belowwhich the bulk of the reaction occurs (Fig. 4).Although di¡usion has smoothed out any localgradients generated around the decollement, thewidespread dissipation of the anomaly throughthe section (i.e. to distances greater than V100m) is limited because the di¡usion and advectionrates are generally low. The reaction rate alsoprogressively increases during burial, leaving littletime for widespread dissipation of the chemicalgradients, particularly in the upper reactive zone.

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The modeled Cl concentrations are higher thaneither the measured Cl values or the calculatedinstantaneous pro¢les for particular values of Ai.Advection and di¡usion processes occurring dur-ing the reaction period are not the primary causeof the discrepancy between the instantaneous andnumerically calculated Cl pro¢les. Instead, theprincipal cause of the discrepancy in fresheningestimates appears to arise from a di¡erence inthe modeled degree of reaction (Fig. 4A). Datathat are not yet available from Leg 190, such aslateral changes in temperature gradient, burialhistory, and progressive evolution in the S to Ireaction, will eventually allow the kinetics to bebetter constrained [42]. The form of the numeri-cally generated Cl pro¢le does, however, suggestthat a good ¢t to the data should be achieved forAi values of V15% once the future reanalysis ofthe reaction kinetics is completed. Clearly, anysuch reanalysis also has implication for eventuallyaddressing the issue whether the up dip limit ofseismogenic activity is related to the S to I reac-tion [15,16].

The detailed form of Cl pro¢le has also mostprobably been a¡ected by several other additionalfactors. In the above modeling study, for example,we have not included any freshening that mayhave resulted from early stress-induced partial de-hydration of 18 Aî smectite clays during loadingbetween the trench wedge. If this process oc-curred, less smectite would be required to gener-ate the Cl anomaly. In addition, at depths below1200 mbsf measured Cl values trend towardshigher levels than the calculated curves. At thesedepths the Na/Cl ratios [5] suggest other reactionsare occurring that involve either the oceanic base-ment or hydration reactions within the lower vol-canoclastic unit. The upward trend to higher thanseawater Cl values in the region above 700^800mbsf, where volcanic ashes are present, also sug-gests that hydration reactions are occurring atdepths above the main S to I reaction. The volca-noclastic and ash content is generally lower in themain region of the anomaly than the surroundingunits. However, while these factors may compli-cate the picture, we do not think that they signi¢-cantly change the general conclusions that wedraw from this study.

6. Consequences for lateral £ow in theNankai wedge

Although our analysis suggests only moderateinitial smectite abundances of 10^15% (dependingon whether the rebound correction is applied)would be required to generate a large proportionof the low-Cl anomaly at Site 808, we also mustconsider the implications that arise if Ai valuesare indeed 6 10^15%. Under these conditions aninput of low-Cl £uids along a conduit might berequired to generate the observed signal.

There are several aspects of the observed Clpro¢le that can be accounted for if the Cl anom-aly is a compound product of both in situ pro-duction and lateral £ow. The maximum pore £uidfreshening occurs within the underthrust hemipe-lagic mudstones close to 1150 mbsf, rather than atthe decollement level (Fig. 2). It is thus di¤cult torelate the freshening to lateral input of freshened£uids along an episodically dilated and permeabledecollement zone without appealing to transient£ow and a highly anisotropic permeability in theunderthrust units [12]. In addition, the most re-cent drilling during Leg 190 has shown that thefreshening anomaly is already present within thehemipelagic units beneath the turbidite trench ¢lloutboard of the accretionary wedge prior to thedevelopment of the decollement [42]. Yet the al-ternative to a conduit at the decollement zone isthat the low-Cl signal is emplaced by long-dis-tance lateral £ow of many tens to perhaps 100km through the low-permeability underthrusthemipelagic section. This also seems unlikely.An important component of in situ fresheningwould, however, help to explain why the low-Clanomaly is so widely distributed through low-per-meability ¢ne-grained units.

Our study has implications for the likely posi-tion of any potential £uid conduit. Based on thegeneral form of the numerically generated fresh-ening pro¢les (Fig. 4), we would expect that insitu reaction would still generate progressivelylower Cl values with depth, even for Ai valuesthat lie below 10^15%. As a result of this, thecenter of a Cl anomaly generated by lateral £owwould be shifted upwards to ¢ll in between the insitu freshening pro¢le and the current Cl values.

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In Fig. 5 we have subtracted the numerically gen-erated pro¢les in Fig. 4 from the measured pro¢leto show the shape of the residual anomaly. Theresulting residual anomaly is more symmetric andis centered closer to the decollement zone. This ismore consistent with the hypothesis that lateral£ow occurs along the decollement zone ratherthan through the underthrust units. It is impor-tant to note that the magnitude of the residual Clanomaly in Fig. 5 does not correspond to a re-quired amount of freshened £uid. If advective£ow from depth occurs along the decollement,the advected £uid would replace the in situ £uidin this narrow region and the pore-water chemis-try within the decollement would re£ect thesource of the advected £uid. Mixing of the ad-vected £uid and the in situ £uids may subse-quently occur by di¡usion into the pore spacesin the regions above and below the decollementto produce a broader residual anomaly. We sug-gest that interpretations of the origin of Cl, bor-on, and other isotope and chemical anomaliesthat have been observed at Site 808 [1^4] shouldbe placed in the context of the potential develop-ment of such a compound freshening anomaly.

We also show that it is possible that in situdehydration processes could generate all the ob-served freshening. If this is the case, then the £uidpressure distribution on the decollement will beprimarily controlled by local compaction anddewatering processes (both mechanical and chem-ical) rather than by the up dip £ow of highlypressured £uids along dilated fracture systems.Presumably, if large £uxes are not directed alongthe decollement, then other migration pathways,such as di¡use £ow through the wedge and out-of-sequence thrusts, must be important in dewa-tering of the deeper system. This could signi¢-cantly change the way we view both the plumbingof accretionary wedges and the potential £uid^fault interactions related to seismicity.

The in situ formation hypothesis raises thequestion of how the Cl anomaly relates to thepresence of the isotope anomalies [1,6,7]. Theseisotope anomalies are suggestive of an origin atelevated temperatures, perhaps low water rockratios, and a mantle involvement, but it is notclear that they necessarily originate deep in thesubduction system or in the same region as thelow-Cl anomaly. There are two possibilities. Thelocation of the Site 808 transect above the unusu-ally hot paleo-spreading axis is a complicatingfactor that should be considered in future hydro-geologic, isotopic, and chemical models. This highheat £ow has resulted in the unusually high de-gree of reaction of the smectite along the Site 808section. It needs to be ascertained why the paleo-axis is still so hot. Perhaps we are dealing with arecently magmatically or tectonically reactivatedsystem that generated a more local hydrothermal£uid £ow system within the ridge, inputting com-ponents such as 3He from the mantle. This couldconceivably lead to isotope anomalies that advectinto and/or from within the overlying sediments.However, we acknowledge that it is also possiblethat there is a compound origin to the anomalies(Fig. 5), with isotopically distinct £uids being lat-erally input from depth along the decollement.The £uids would have low-Cl values similar tothose originating from the in situ reaction at thebase of Site 808 and the isotope anomalies wouldbe centered close to 960 mbsf.

Thus, while we suggest in situ generation seems

Fig. 5. Residual Cl anomaly between the data and modeledcurves (see Fig. 4) for Ai = 10% and 15%. Note the near sym-metrical distribution of the residual anomaly about the de-collement horizon at V960 mbsf.

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to be the simplest explanation for the low-Clanomaly, we do not exclude the possibility ofsome lateral £ow from depth. We also advancethe concept that the origin and e¡ect of the highheat £ow in the supposed paleo-ridge need to beconsidered in future studies. We conclude thatreconciling conceptual models of £uid £ow withgeochemical, isotopic, and hydrologic data is animportant area for future study.

7. Conclusions

Our objective here has been to re-evaluate therole of in situ smectite dehydration in generatingthe observed low-Cl anomaly at the toe of theNankai accretionary wedge. The main uncertaintyin this analysis is that pre-reaction abundancesare not known for the Shikoku Basin along theSite 808 transect. High temperatures above therecently extinct spreading ridge have caused ad-vanced stages of reaction at depth at Site 808.Previously, the in situ reaction hypothesis wasdiscarded because 25^50% smectite abundanceswere thought to be required. By accounting for(1) porosity corrections for the low-density smec-tite phase and elastic rebound, and (2) actual re-action progress as documented at Site 808, weo¡er new insight on the mass balance of pore-water freshening. We show that V10^15 wt% ini-tial smectite could generate much of the freshen-ing observed at Site 808. This smectite abundancerange is reasonable based on the mineralogicaltrends seen at colder sites located o¡ the spread-ing axis. Comparison of instantaneous fresheningand kinetic reaction models shows that reactionover V0.35 Ma does not lead to substantial dis-sipation of the anomalies by advection or di¡u-sion. The in situ dehydration hypothesis as anexplanation for pore £uid freshening cannot berejected on the basis of the current data. How-ever, if initial smectite abundances were 6 10^15%, the observed low-Cl anomaly could re£ectthe combined e¡ects of both in situ reaction andlateral £ow of £uids freshened at depth in thewedge. In this case we suggest, on the basis ofthe form of in situ reaction and freshening, thatthe position of any conduit would lie shallower

and closer to the decollement zone than the depthof the main Cl minimum.

Acknowledgements

This research used samples and/or data pro-vided by the ODP. The ODP is sponsored bythe U.S. National Science Foundation (NSF)and participating countries under managementof Joint Oceanographic Institutions (JOI), Inc.Funding for this research was provided by NSFGrants OCE95-29959 and OCE-9618166, and aNational Research Council Associateship. Wewould like to thank Joris Gieskes and ElisabethScreaton for their very helpful and careful re-views. We would also additionally like to thankMike Underwood for the many enlightening dis-cussions about the clay mineralogy of Nankai sys-tem.[EB]

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smectite diagenesis, pore-water freshening, and fluid flow at the toe of the nankai wedge

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