Q Overgrowth

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B13, PAGES 20,043-20,061,DECEMBER 10, 1992

Models of Quartz Overgrowthand Vein Formation:DeformationandEpisodicFluid Flow in an AncientSubduction one

DONALD M. FISHER AND SUSAN L. BRANTLEYDepartmentof Geosciences, ennsylvania tateUniversity,UniversityPark

Steady state modelsof overgrowthand vein formation are developedusing kinetic data for quartzdissolutionand precipitationand estimatesof fluid advection,pore-fluid and grain-boundarydiffusion.Application of these models to overgrowthsand veins in the Kodiak accretionary complex suggeststhat the Kodiak Formationdeformed continuouslyby a grain-boundarydiffusion-limited mechanism,accompanied y episodicpore fluid diffusion of quartz from the matrix to vertical fluid-filled fracturesnear the base of the accretionarywedge. These processes roduced wo types of syntectoniccrystaltextures within the Kodiak Formation: overgrowths containing displacement-controlled ibers, andthroughgoing eins composed f face-controlled longateblocky quartz crystals. Based on texturalobservations, isplacement-controlleduartz growth in overgrowths s rate-limited by either diffusionalong a cohesive nterface or the rate of matrix strain. The magfiitude of elongation recorded bydisplacement-controlledrystalgrowthvariessmoothlyelongation f 1 to 3) from the shallowestothe deepeststructural evels of the Kodiak Formation, suggesting hat the diffusional componentofdeformation n the accretionarywedge ncreaseswith depth. In contrast, ace-controlledquartz growthis largely restricted to veins within the deepest level, where the cleavage is subhorizontal anddeformationnvolvesa component f simpleshear,suggestingroximity o a decollement.The face-controlled quartz veins representmode I cracks which seal periodically and contain continuousplanarsolid inclusion bands, cracks which partially seal periodically and contain discontinuous solidinclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solidinclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 gm incrac ealeaturesomuchargeraluesn euhedralrowtheins. uhedralrowtheinsemainpenthroughoutheirdevelopment105 to 106years), ndcrack ealveinsdevelop s a consequencefmanycrack-sealvents ver a 103- 106yearperiod. In bothcases,extural vidence uggestshatmost transportof silica occurs by local pore-fluid diffusion from matrix to vein. Wall rock inclusionbands uggesthat"crack"ventsand seal" vents ach ccurred ithinperiods f 102 104years.A picture emergesof intermittent luid flow upward from the decollement nto a branchinghierarchyofvertical fractures n the accretionarywedge during hydrofracturing vents, followed by local transportand precipitation of silica causing sealing of the fractures at depth and propagation of pulses offracture luid upward.

INTRODUCTION

Quartz veins and overgrowths are ubiquitous features inmetamorphosedsedimentswhich have undergone deformationand devolatilization. Veins may be hydrofractures hat act asconduits in a fluid flow network, where fluid flow can beepisodic (e.g., seismic pumping; $ibson et al. [1975]), orflow can occur continuouslyupwards along vertical fractures[Yardley, 1984; Walther and Orville, 1982]. For suchsystems,quartz is not derived from the adjacent wall rock butrather represents long distance transport and precipitationwith decreasing temperature and pressure during flow alongthe fracture system [Yardley, 1984]. Alternatively, veinsand overgrowths may represent fractures that serve as sinksfor precipitationof locally derived quartz transportedby localfluid advection (e.g., small-scale convection or dilationalpumping [Yardley, 1984] or diffusion [Elliott, 1973;1976]). The applicability of various models can be evaluatedby considering the timing, distribution, orientation,

Copyright 1992 by the AmericanGeophysical nion.Paper number92JB01582,0148-0227/9 2/9 2JB-015 82505.00

geochemistry, and textural history of vein and overgrowthsystems.For fluid-present systems, development of veins andovergrowths involves a series of steps such as dissolution,transport, and precipitation. The slowest of these steps isthe step that controls the rate of vein or overgrowthformation. For two mechanismshat operate n parallel (e.g.,diffusion or fluid advection), the rate-controllingmechanismis the mechanism that operates faster [Fisher, 1978]. Thus,transport is the rate-controlling step only if both fluidadvection and diffusion are slower than all the othersequential steps. For any system, the rate-controlling stepfor a complex process such as overgrowth or vein formationwill be a function of temperature, pressure, rock-waterchemistry, and rock texture. In addition, for solution transferto occur, there must be a driving force (i.e., chemicalpotential gradient) causing material to dissolve in onereservoir and reprecipitate in another. For a multicomponentsystem (e.g., overgrowths composedof mica and quartz), thedriving force could be different for different components. Inmany cases, the crystalline texture may help to elucidate thedriving force, the mechanism, and the rate-limiting step ofsolution transfer.

In this paper, a variety of textures in overgrowths andveins are examined within rocks from the Kodiak Formation,20,043


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20,044 FISHERAD BRANTLEY:EPISODIC L-ID FLOW N A A SUBDUCrIONONEa portion of an ancient convergent margin. Observations areused to assess he importance of each of the possible rate-controlling steps in the textural development. We developand evaluate two overgrowth formation models (an interface-limited and a grain-boundarydiffusion-limited model), and wedevelop and evaluate four vein growth models (a single pulsemodel, a matrix-to-crack pumping model, a fracture-channelized fluid flow model, and a pore-fluid diffusionmodel). These results are then combined with published dataconcerning rates of quartz dissolution and precipitation inhydrothermal systems to place bounds on (1) the time scalesover which various types of syntectonic quartz crystals grewand (2) the frequency of crack-seal cycles. Although themodels presented are simple, these analyses provide aframework for the interpretation of a variety of observedtextures in a well-constrained tectonic setting.All of the observations n this paper are from the earlyMaestrichtian Kodiak Formation (Figure 1), a northeast-trending slate belt that consists predominantly of imbricatethrust sheets that were accreted along a convergent margin inthe Late Cretaceous-earlyTertiary when a thickly sedimentedoceanic plate was subduetingbeneath the Peninsular errane insouthwest Alaska [Sample and Moore, 1987; Byrne andFisher, 1987; Fisher and Byrne, 1990; 1992]. Imbricationof the incoming sediments coincided with or precededdevelopment of a slaty cleavage that is roughly axial planarto anticlines and synclines in the respective hanging wallsand footwalls of thrust surfaces.

Several vein generations have been described for theKodiak Formation. The earliest veins are concentrated within

N in 3OAfognaktransect

Paleocene andYoungernitsPaleocenelutonsKodiakFormationOlderMesozoicUnits

Fig. 1. Geologic map of Kodiak and Afognak Islands. All overgrowthsand veins described n this paper are from samplescollectedwithin theKodiak Formation along the Afognak transect. For a more detaileddiscussionof the structural history and regional structureof the KodiakFormation,see Fisher and Byrne [1992].

tectonically disrupted melanges and are interpreted torepresent hydrofractures associated with underthrusting ofpartially lithified sediments [Vrolijk, 1987; Fisher andByrne, 1987; Fisher and Byrne, 1990]. These veinstrapped luid inclusions ontainingpredominantly H4 andwater, with somedissoved alt and very minor CO2 [Myers,1987]. The fluid inclusions from these veins and similarveins from other accretedunits on Kodiak Island are reportedto document luid pressuredrops of 10 to 90 MPa during veincrystal growth [Vrolijk, 1987, Vrolijk et al., 1988]. Inthis paper, we describe the second generation of veins whichare contemporaneous with slaty cleavage development andearly shortening within the accretionary wedge [Fisher andByrne, 1990]. The fluid inclusions from these veins containpredominantly20 + CO2, with varyingamounts f dissolvedsaltandCH4 (V2 veins rom the centralbelt [Myers, 1987]).

Fluid inclusion microthermometry nd barometry suggestthat development of the second vein generation in the centralbelt of the Kodiak Formation occurred between temperaturesof 182 to 269C and pressures f 220 and 300 MPa [Myers,1987]. These temperatures nd pressures re in general accordwith the observations of localized prehnite-pumpellyitemetamorphism in volcaniclastic sandstones, a vitrinitereflectance mean value of 3.73, and an illitc crystallinitymeanvalueof 162 (reported s Hbre , the normalizedwidth ofthe illitc (001) peak at half-width) for the Kodiak Formation[Sample and Moore, 1987]. In addition, the observedscarcity of biotite and common observationsof phengite andchlorite suggest temperatures below 350C. We have alsomeasured d-spacing of 3.55 for transitional graphite inKodiak Formation slates, suggesting lower chlorite grademetamorphism [Wang, 1989].

These results, combined with microstructuralobservations and comparisons with modern convergentmargins, suggest that thrust sheets in the Kodiak Formationwere not offscraped at the shallow (< 5 km depth) toe of theaccretionary prism where imbrication involves unlithifiedsediments [Sample and Fisher, 1986]. Rather, thesecoherent thrust slices were probably transferred from theunderthrusting sediment pile to the overriding prism due tofootwall collapse along a ramp in the basal decollement i.e.,duplex accretion [Silver et al., 1984; Sample and Fisher,1986; Platt, 1986]). In this paper, we will focus onovergrowths and veins associated with this event. Thus, allthe features we describe formed near the base of theaccretionary wedge, and, because these structurespredate theintrusion of 60 m.y. old plutons [Moore et al., 1983;Davies and Moore, 1984; Fisher, 1990], the featuresdevelopedduring and shortly after the Late Cretaceous-earlyTertiary accretion of the sediments.A large scale anticline folded the slaty cleavage and allother early structures. This anticlinorium exposes he lowerstructural level of the accreted package in the core and aprogressively higher structural level on both limbs [Fisherand Byrne, 1990]. In the upper level, cleavagedips steeplyto the northwest, whereas the cleavage is subhorizontalat thelowest exposed evel [Fisher and Byrne, 1992]. The lowestlevel is also characterizedby southeast-verging,soclinal,recumbent folds, numerous subhorizontal hrusts, pervasivecrack-seal veins [Fisher and Byrne, 1990], and pyritepressure shadows that record noncoaxial strain historiesconsistent with southeast directed thrusting [Fisher and


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FSHE AND BRANTLEY: EPISODIC FLUID FLOW n AN AN SUBDUCrION ZONE 20,045

Byrne, 1992]. These features suggest proximity to anunderlying decollement that was active at the base of theaccreted package. This structural framework and tectonicsetting provide a context for the discussionspresented in thispaper.

MICROTEXTURAL OBSERVATIONS

There are two general types of syntectonic crystaltextures that we recognize within the Kodiak Formation:overgrowths containing displacement-controlled fibrouscrystals, and throughgoing veins containing face-controlledelongate blocky quartz crystals. The magnitude of elongationrecorded by overgrowths increases rom approximately 1 to 3from the highest to the lowest structural level of the KodiakFormation. Face-controlled quartz veins are largely restrictedto the deepest structural evel of the Kodiak Formation whererocks have experienced large strain magnitudes n a regime ofprogressive simple shear [Fisher and Byrne, 1990, 1992].Veins can represent cracks which seal periodically andcontain continuous planar solid inclusion bands, cracks whichpartially seal periodically and contain discontinuous solidinclusion bands, or cracks that remain open and contain nosolid inclusions.

Displacement-Controlled ibrous OvergrowthsFibrous overgrowths of quartz, chlorite, and phengite aredistributed throughout the Kodiak Formation. Pressureshadows are observed around rigid objects such as spherical

pyrite framboids (radius 10-100 gm), fragments ofmetamorphosed organic material, and large (20-100 gm )detrital grains. The mineralogy within these overgrowthsdirectly correspondswith the dominant mineralogy of thesurrounding rock; for example, the relative proportion ofquartz and mica in overgrowths is less in slates than insandstones. Moreover, insoluble residues are concentratedaround quadrantson the surface of the rigid object that facethe maximum compressivestress. This evidence s consistentwith local transport of material.There are three characteristics of these overgrowthswhich suggest that development of these features involvestransport-limited growth along a cohesive matrix-porphyroclast interface. (1) Fibers display a relativelyconstant width along the length of the fiber. (2) Fiberswithin the pressureshadoware continuous rom the surfaceofthe rigid host to the outer surface of the overgrowth, even inovergrowthswith a mixture of quartz and mica fibers. (3)These overgrowths record the relative displacement ofopposing sides of the growth interface. Each of thesecharacteristics is discussed in more detail below.

1. Individual fibers (quartz, chlorite, or phengite) inthese overgrowthsdisplay roughly constantwidth along theirlength, although width can differ from fiber to fiber. Inovergrowths around pyrite framboids and organic fragments,fiber growth (mica and quartz) occurs at the interface betweenthe rigid host and the matrix and is directed from the matrixtowards the host (i.e., growth is antitaxial) (Figure 2). Fibersnucleate on grains in the matrix, with the oldest material atthe outer margin of the pressure shadow and the most recentmaterial against the surface of the host. In these cases, weobserve that the fiber width in the overgrowth is equal to thesize of matrix seed crystals.

Fig. 2. SEM photomicrograph f displacement-controlled ica fibersarounda pyrite framboid viewedon a cleavageplane). Micas nucleateon grains in the matrix (arrow) and grow towards the surface of thepyrite framboid ("f') Scale bar is 7.9 gn

In overgrowthsaround detrital quartz grains, fiber growthoccurs at the interface between the matrix and the pressureshadow and is directed towards the matrix (i.e., growth issyntaxial). Quartz fibers nucleate on the host quartz grains,with the oldest material at the surface of the host and themost recent material at the tip of the overgrowth. For detritalquartz grains, overgrowths are composedof mixtures of bothquartz that is in optical continuity with the host grain, andmica grains that initially nucleate on the matrix. In thesesyntaxial overgrowths, the width of mica fibers is relativelyuniform whereas the width of quartz fibers, while constantfrom beginning to end, varies from fiber to fiber and isdefined by the variable spacingof intervening micas.The observation hat fibers in these overgrowthshave aconstant width along their length indicates that adjacentgrains grow at the same rate. If growth of quartz occurredwithin an open crack along the growth interface, the largeorientation dependence of quartz growth rate [Her, 1979]would cause some grains to grow faster than others;competition between grains would mean favorably-orientedgrains would eventually fill more of the crack volume thahnonfavorably oriented grains. In antitaxial overgrowthsaround pyrite framboids, the crystallographic orientation ofadjacent quartz fibers can be variable as a consequenceofvariability in the orientation of seed nuclei. Because thewidth of variably-oriented quartz fibers in these overgrowthsis constant, there must not be an open crack along thegrowth interface, and the relative displacement rate foropposing sides of the growth interface must be less than orequal to the growth rate of the least favorably oriented grains.Moreover, in both antitaxial and syntaxial overgrowths,individual curved quartz fibers typically display opticalcontinuity, or show a significantly smaller change in opticalorientation than the change in fiber orientation from one endof the fiber to the other, indicating that the direction and rateof displacement-controllediber growth is largely independentof crystallographic orientation. Similar growth rates forvariably oriented fibers also suggests that growth is notinterface-limited,but rather must be limited by either the rateof transport to the growth interface or the rate of matrix


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20,046 FISHERAND BRANTLEY:PISODIC LUIDFLOW N AN ANCIENTSUBDUCTIONONEdisplacementadjacent to the porphyroblast i.e. if the matrixdisplacement is independent of fiber growth), whicheverprocess s slower.2. The continuity of fibers in overgrowths of mixedmineralogy also argues against the existence of a fluid-filledcrack during the developmentof these features. Growth ratefor quartz and mica is a function of interfacial growth kineticsand concentration gradients for Si, A1, and alkali metals.Becausenterfacial rowthkinetics iffer for differingphases[Lasaga, 1984], and because concentrationgradients for Simay differ significantly from the other mica-formingcomponents, uartz and mica will typically grow at differentrates in an open fracture. The observation hat micas are notovergrownby quartz or vice versa n quartz-micaovergrowthsis thus best explained by a model of growth within acohesive grain boundary and not an open crack. Theseobservationsalso suggest that the relative displacement atefor opposingsidesof the growth nterfacemust be equal to orslower than the growth rate of the mineral which grows theslowest.

3. The third observation that argues against cracking ofthe growth interface during development of fibrousovergrowths s that fibers in these overgrowths ecord therelative displacement of opposing sides of the growthinterface (i.e., displacement-controlled ibers [Ramsay andHuber, 1983]). Fibers in these overgrowths are commonlycurved (Figures 3a and 3b). Given that these fibers growsubparallel to the orientation of extension [Durney andRamsay, 1973], this curvature can represent either (1)noncoaxial strain, whereby the orientation of extension hasnot remained constant relative to some arbitrary referenceframe, or (2) later bending of fibers that precipitated asstraight crystals during coaxial strain. In a given sample,syntaxial (on detrital grains) and antitaxial (on pyriteframboids) overgrowths display opposite sensesof curvature.Thus, the curvature of fibers in these overgrowths reflectschanges n the orientation of extension or the direction ofrelative displacement for opposing sides of the growthinterface.

If opposing sides of the growth interface for thesefeatures were free surfaces, then growth, which was restrictedto only one side of the interface, would have beenindependentof any displacement elative to the other side ofthe interface. Given that opposing sides of the growthinterface are not free surfaces, transport along the growthinterface must occur by grain-boundarydiffusion. Dissolutionoccurs in the matrix around portions of the host surface thatface the maximum compressive stress, and reprecipitationoccurs along the portions of the matrix-host interface thatface the least compressive stress. The rate of growth forthese fibers must be equal to or less than the rate of diffusivetransport to the site of precipitation. Fibers cannot growfaster than the displacement rate of the adjacent matrixrelative to the center of the pyrite, so if deformation in thematrix is dominated by a mechanism that is slower than andindependent of diffusion around the host, then the fibergrowth rate may be controlled by matrix strain. Thus, thelength of a straight fiber normalized by the pyrite radius is aminimum value for the elongation in the matrix. Elongationsin the Kodiak Formation vary from 1 to 3 from shallowest odeepeststructural evels [Fisher and Byrne, 1990; 1992].Urai et al. [1991] present a model for fiber growth(assuming sotropic growth kinetics) and conclude hat fibers

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.......==..........%% =....

.........::)":'..... :..

Fig. 3. Photomicrographs f displacement-controlled vergrowth in(a) plane light and (b) crossed nicols. Fibers are curved due tononcoaxial strain in the slate matrix around the near-spherical pyrite.Many of the fibers in Figure 3b that can be traced continuouslyarealsooptically ontinuous,espite curvaturef more han45.

can track relative displacement of opposing sides of thegrowth interface when the aperture related to cracking alongthe interface is negligible relative to the wavelength ofroughness. For framboidal pyrite in the Kodiak Formation,the wavelength of irregularites along the growth interface isconstant and largely reflects the pyrite grain size at thesurface of the framboid (approximately 0.5-2 gm, see Figure2). Based on the incremental cracking model of Urai et al.[1991], the maximum possible openings along the growthinterface must be much less than 1 gm. We argue that, aslong as diffusion along the cohesive framboid-pressureshadow interface is able to keep pace with the displacementof the adjacentmatrix away from the pyrite, there need not besignificantdetachmentduring developmentof these eatures.Face-Controlled Quartz Veins

Quartz grains within these veins are elongateperpendicular o the vein but widen across he vein, with anincrease in grain size from the side of the vein where theinitial crack developed to the side where the latest crackoccurred Figures 4-6). The grain size increase rom one sideof the crack-seal vein to the other indicates that quartz growthis unidirectional, and, in the case of crack-seal veins,cracking occurs repeatedly along the same side of the vein.


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FISHEg AND BRANTLEY: EPSODC FLUID FLOW IN AN ANCIENT SUBDUCTION ZONE 20,047

.... .......... ......- ..:. ........: :..7.... . : .., .

?:.......... :.............. ..

.

.:.-.....: :; .. ." . .......... ....... .......

....... ':$-...':.... . ...

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Fig. 4. Photomicrographsf continuously anded crack-sealvein in(a) plane light and (b) crossednicols. (a) Note continuousmicainclusionbandsparallel to the marginsof the vein. Spacingbetweenbands indicatedby arrows) s about8 gtm. Also note that quartz grainsare elongateperpendicular o the vein and grain size increases romright to left. Right side of vein representshe earliest racturesurface;fracturingoccurred epeatedly etweenvein quartz and wall rock to left.Planarityof inclusionbands in contrast o bands n Figure 5) indicatesthat fracture closure occurredby quartz sealing rather than collapse.Scale bar is 200 Thus, quartz grains within these features compete for spaceduring face-controlled growth within an open fluid-filledcrack: grains favorably oriented for growth increase in sizerelative to neighboring ess favorably oriented grains. Thisanisotropic quartz growth results in development of acrystallographic preferred orientation that becomes strongertowards the most recently active side of the vein and is mostpronounced n the largest veins.Quartz grains nucleate along the wall of the crack andvary in crystallographic orientation depending on thevariability in the orientation of seed nuclei. The grain sizeof quartz within veins is larger in fine-grained micaceoussamples han in coarser sandstones. This observation eflectsthe fact that there are fewer energetically favorable sites fornucleation in the micaceous samples. In these examples,growth is fast relative to nucleationand crystal size is larger.In the sandstones,numerous seed nuclei along the walls ofthe crack lead to a finer grain size of quartz grains within thevein.

The major difference between various types of face-controlled quartz veins lies in the geometry and distribution

of solid mica inclusions that are embedded within the quartz.Each inclusion nucleates on seeds along the wall rock at themargin of cracks, so the distribution of inclusions within thequartz veins reflects the degree to which quartz crystals withinthe crack were able to seal with the opposing wall rock. Incrack-seal veins, mica inclusions are aligned in continuous ordiscontinuousbands parallel to the margins of the vein. Themicas within an individual inclusion trail connect up with amica seed in the wall rock and have the same crystallographicorientation as the seed mica [Fisher and Byrne, 1990] (seealso Cox and Etheridge [1983, 1989]; Cox [1987]). Ineuhedral growth veins, wall-rock inclusions are trapped in theearly stages of vein formation but most vein fill consists ofeuhedral inclusion-free quartz crystals. Thus, the differenttextural characteristics associated with continuous crack-sealveins, discontinuous crack-seal veins, and euhedral growthveins can be interpreted in terms of the degree to whichcracks remained open during vein formation. The wavelengthand amplitude of surface irregularities along the most recentlyactive vein-wallrock interface increase from minimum valuesin the case of continuous crack-seal veins to maximum valuesin euhedral growth veins as a consequence of anisotropicquartz growth with increasingperiods of quartz growth. Thesefeatures are further described below.

Face-controlled quartz veins with continuous inclusionbands. Crack-seal veins [Ramsay, 1980; Cox andEtheridge, 1983, 1989] are composedof blocky quartz andinclusions of mica that are commonly arrayed in continuousbands parallel to the margins of the vein (Figures 4a and4b). These bands show regular 8-1m spacing, and individualveins can contain as many as 500 separatebands. Individualmica inclusions are thicker than mica fibers in fibrousovergrowths. In three dimensions, mica inclusions are platyrather than fibrous, with basal planes roughly parallel to thebasal planes of micas in the wall rock.The initial crack may have been sealed by growth ofquartz that nucleated on both sides of the fracture. Duringsubsequentcracking episodes, however, the increase in quartzgrain size across the vein indicates that growth isunidirectional, the fracturing occurs at the interface betweenthe vein quartz and the wall rock, and the fracture consistentlyoccurs along the same side of the vein. Growth of quartz maybe unidirectional because quartz nucleation is inhibited alongthe micaceousside of the crack. Thus, micas grow from seedsalong the wall rock, whereas the quartz grows from thecrystals along the opposing side of the fracture towards thewall rock. Inclusion bands are pulled away from the wall rockduring each fracturing episode [Ramsay, 1980; Cox andEtheridge, 1983].As the quartz crystals grew into an open crack, thedevelopment of crystal terminations would result intopography along the surface of the vein. If the cracksclosed by collapse, the topography would be recorded by thegeometry of solid inclusion bands within the vein. However,if all the quartz crystals sealed completely with the relativelyplanar wall rock surface, the topography would be lost. Theobservation that inclusion bands in these features are planarindicates that the crack is completely sealed along the lengthof the vein at the completion of each crack-seal episode. Theperiodicity of cracking must be greater than the amount oftime it takes for the least favorably oriented grains to grow athickness of 8 !m perpendicular to the vein surface. In the


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20,048 FISHER AND BRANTLEY: EPISODIC FLUID FLOW IN AN ANCIENT SUBDUCTIONZONE

Fig. 5. Photomicrographsof discontinuouslybanded crack-seal vein in (a) plane light and (b) crossed nicols. Themajority of the vein fi ll developed during cracking along the left side of the vein. Early growth (i.e. growth at farright) trapped continuous nclusion bands. The center of the vein displays discontinuous nclusion bands which areconvex outward (one gap in a band is shown by arrows) and restricted o quartz grains which widen from right to left atthe expenseof neighboringgrains (Figure 5b). The last quartz bands are also convex and show continuous nclusionbands (two continuousbands at far left). Note enhancedsurface elief compared o vein in Figure 4. Scale bar is 0.4mm.

etrly stages of vein development,nuclei are randonlyoriented, but a preferred orientation develops and, by the laterstages, c axes of most grains are nearly perpendicular to thecrack. Therefore, the length of time associatedwith growthof each layer of quartz should decreaseover the lifetime of thevein. Face-controlled quartz veins with discontinuousinclusion bands. Crack-seal veins with discontinuousinclusion bands (Figures 5a and 5b) are characterized byinclusion bands (spaced at 8 I. m) which are restricted tograins which widen toward the most recently active side ofthe vein. These textures indicate that while quartz grains ofall orientations compete for space within the open fracture,only favorably-oriented grains grow fast enough to seal thecrack. Because mica inclusions nucleate on the wall rock, thequartz grains with no solid inclusions did not seal with theopposing side of the fracture. The periodicity of crackingmust be more than the time required for quartz grains with caxes perpendicular to the crack to grow a thickness of 8 I. mand must be less than the time required for quartz grains withc axes subparallel to the crack to grow 8 I. m. Thus, thesefeatures are partially open throughout their history.

The inclusion bands within these veins are planar andcontinuous within the earliest vein material (where grain size

is smallest) but become discontinuous and chevron-shapedtoward the most recent vein material. These temporalvariations in the roughnessof the crack surface suggest hatcrack closure for discontinuously banded crack-seal veinsinvolves both collapse and sealing of cracks. Sealing eventsare recorded by mica inclusion bands where individual micashave the same crystallographic orientation as secondarymicas in the wall rock. Collapse events are recorded byjagged or bumpy insoluble residues hat mimic the shape ofthe chevron-shaped ein crystals, as well as mica inclusionsoriented parallel to the vein walls. In these veins, collapseevents are typically spaced wider than seal events. Wherethere may be 500 seal events, there may be only fivecollapse events. The roughnessof the wall rock-vein interfacerecorded by seal bands is inherited from earlier collapseevents. We have also observed hat discontinuously andedveins which are found near continuously banded veins haveless collapse events than discontinuously banded veinsadjacent to euhedral growth veins, so the textural variationfrom continuously banded to discontinuously banded toeuhedral growth veins probably represents a continuum.Face-controlled quartz veins with euhedral crystalterminations. Euhedral growth veins are composed almostentirely of quartz free of solid inclusions (Figures 6a-6c).


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I;'ISHERAND BRANTLEY: EPISODICFLUID FLOW IN AN ANCIENT SUBDUCTION ONE 20,049

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Fig. 6. Photocrograph f euhedral row vein n (a) plane ight and (b) crossedcols. Also shown c) is ghmagficafion iew n ple light of e left sideof e veinat on aow in igure 6a. Note at (a) right marginof vein 0.e., where fial fractureoccurred)s strght, where the left m=gin of vein (where e most ecently ctivefractureoccued) s jagged. Quartzwin e vein s free of solid ca inclusions ut h regul= bandsof insolublematerialspaced t a little less an 1 min. e grin si of quiz increasesrom right to left and individualqu=tzcrystals e elongatendicul= to e right re=gin of e vein (Figure6b), in, caring ace-con,oiledgro fromright to left. Insoluble esiduen e maix and e sutured urface f e qu=tz grainat e left re=gin of e veinigure ) attest o pressure olutionof the wall rock and vein a consequencef collapseof e ack. Vein atwidest int is 0.8 cm. Note at is widest int coincideswi sandymix in the wallrock.

These veins initiate as continuously banded crack-seal veinsbut develop into discontinuously banded veins and theneuhedral growth veins. Along the side of the vein where thecrack initiated, the vein surface is straight, and the quartzvein material is fine-grained with planar solid inclusionbands. Toward the most recently active crack surface,inclusion bands become more discontinuous until there are nosolid inclusions within the vein quartz. Quartz grains withinthe vein are elongate perpendicular to the vein and the grainsize increases across the vein to the most recently activemargin where quartz grains display euhedral crystalterminations. In many cases, the largest protruberances onthe vein surfacesare ridges in three dimensions hat coincidewith thin (1 mm wide) sand layers or earlier quartz veins inthe matrix. This correspondence uggests hat growth rate ofquartz varies along the axis of the vein, which suggests hatconcentration gradients exist along the length of the crackduring growth. The absence of solid inclusions within thequartz crystals as well as the extreme topographyof theeuhedral terminations indicates that precipitation along thesurface of the quartz vein did not seal the crack. Thus,euhedral growth veins were open cracks throughout he laterstages of vein development.

Dark insoluble residues and disjunctive crenulations areconcentrated in the matrix along the sawtooth margin ofeuhedral growth veins (Figure 6c). Such textures indicatedefogmation nd dissolutionof the matrix due to collapse ofthe cracks at the completionof vein development. In severalcases, there are jagged insoluble residues in the interior of aeuhedralgrowthquartz vein that are spacedabout 1 mm apart.These textures record periodic collapse of the crack anddissolutionof mica and quartz along the margins of the vein.There is about 1 mm of surface relief on the most recentlyactive side of the vein.

MODELS OF VEIN AND OVERGROWTH FORMATION

In the following sections, we derive simple models toestimate the time scales and fluid volumes associated withsyntectonic overgrowths and veins within the KodiakFormation. These models are based on the assumptionofsteady state growth that is rate limited by one of thefollowing: dissolution, transport from the source site to thegrowth site, and precipitation. To quantify the modelspresented, we use kinetic data for dissolution, diffusion, andprecipitation of the vein or overgrowth minerals.


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20,050 FISHER NDBRANTLEY:?ISODICLUIDFLOW A AN SUBDUCrIONONEcrack-seal veins, inclusion bands are regularly spacedabout 8gm apart. Becausecrack closure occursas a consequence fquartz precipitation and not collapse, this suggests hat thecracks opened approximately 8 gm prior to any precipitationwithin the crack. The regularity in this value suggests asimilar egularityn the valueof Pcrduringcrackproduction.This further suggests (equation (1)) that the crack lengthassociated with each crack event is relatively constant. Wewill show later that this regular crack length may be on theorder of 10 m. Thus, for calculations related to crack-sealveins, we will assume that the initial crack aperture isapproximately8 gin. For euhedralgrowth veins, the aperturemust be much greater than this value. All characteristicdimensions nd kinetic parameters re summarizedn Table 1.

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.. --:.::....?

Fig. 6. (continued)

Unfortunately,very little rate data at high temperature ndhigh pressures available or the mineralsof interestwith thesingle exceptionof quartz. For that reason,our modelsareconfined o growth of quartz veins and overgrowths; owever,these models could easily be extended with better kineticdatabases. In all cases, we assume that the net rate ofprecipitation is linearly related to the chemical affinity[Rimstidt and Barnes, 1980] and that nucleation s not rate-limiting.For our calculations, we assume that crystal growthoccurredat 250C and 280 MPa, as suggested y the work ofMyers [1987], We will also assume hat cracksassociatedwith crack-sealeinswerespaced n the orderof millimetersto meters,and euhedralgrowthveins were spaced n the orderof 1 per 100 m. In the Kodiak Formation, n elongation fabout one accumulated fter deposition =70 Ma) and prior topluton ntrusion 58-61 Ma). This requiresa minimumstrainrate of 10- $ s-1 (i.e. f deformationccurredhroughout0Ma). The apertureof cracks s a functionof the fluid pressurein the crack, the crack length, and the position with respectto the crack tips [Pollard and Segall, 1987; Cruikshank etal., 1991]: d=2(oyy+Pcr)(vla2-x (1)where y.vs thenormal tresserpendicularo thecrack, cris the fluid pressuren the crack,v is Poisson'satio, Ix is theshear modulus, a is the crack half-length, and x is thedistance from the midpoint between tips. In continuous

OvergrowthModel I: Interface-LimitedFiber GrowthTextural arguments suggest that displacement-controlledfibrous overgrowths are diffusion-limited and that growthoccurs along a cohesive grain boundary. These assertions an

be supported by comparing the rates of diffusion- versusinterface-limited growth. For a precipitation-limitedmodel ofovergrowth development, the rate of linear growth of quartzcrystalscan be calculated [Rimstidt and Barnes, 1980]:

}t C2where I representshe lengthof a quartz fiber, k is thelinear dissolution ate constant m s-), and C and C2represent the local equilibrium concentration at the site ofdissolution and at the site of growth, respectively. Thesupersaturations determined simply by AP, the difference inpressure at the porphyroblast boundary between quadrants ofthe porphyroblast that face the incremental shorteningdirection and quadrants of the porphyroblast that face theincrementalextensiondirection (see Figure 7).

Dissolution Fiber Growth

h X

YFiber Growth Dissolution

Fig. 7. Geometricparameters sed in model for fibrous overgrowths.Quadrants f dissolution and fiber growth are shown in the case ofviscousimple hearlowover rigid, phericalnclusionradiuss)where f,e XZ plane is perpendicularo the shearplane and the X axisis parallel to the sheardirection.Angle 0 is measuredn the XZ planecounterclockwise rom the X axis, 0 is measured n the YZ plane fromthe Z axis, and r is the radial distance from the center of theporphyroblast.


9/19

FiSHER AND BRANTLEY: EPI$ODIC FLUID FLOW IN AN ANUIENT SUBDUCTION ZONE

20,051

TABLE 1. List of Variables and Values Used in Model CalculationsVariable Parameter Value

a crack half-length (m)C,C2 localquilibriumoncentrationt he (mol 3)site of dissolutionand growthC, C2 localconcentrationt thesiteof (molm 3)dissolutionand growthcrack aperture

D crack spacing

J

K1Pt'crt'm

QFPF

S 1, S2tcTU

Vqvw

w

pore fluid diffusion coefficientgrain-boundarydiffusion coefficientvolume fraction of quartz dissolved

out of depletion zoneflux of silicalinear dissolution rate constantmolar dissolution rate constantmolar dissolution rate constant atzero pressurepermeabilitylength of quartz fiberlithostatic pressurecrack fluid pressurematrix pore fluid pressuredifference n pressurebetweensite ofdissolutionand site of precipitationflow rateradius of rigid particlesdistance rom center of rigid particlespore surfacearea, crack surfaceareatime period such that advective

transportequals diffusive transport distancetemperaturevelocity field within the matrixrock volumewater volumevolume of quartzmolar volume of quartzactivation volumegrain boundarywidthwidth of depletionzone around veinsdistance rom a given point to the

midpoint of a crackeffective transportdistance or diffusioneffective transportdistance or advectiondepthstrain rateangle defined n Figure 7viscosity of watershear modulusPoissons ratioangle defined n Figure 7matrix porositynormal stressperpendicularo the crackviscosity of rocktortuosity

8 gm (crack-seal veins)>8 gm (euhedralgrowth veins)100 m (euhedral growth veinslmm-lm (crack-seal veins)10 8 m 2 s-I10-ll_10 -15m 2 s 10.40

(molm 2 Sl)6.1 x 1013 msl (250C, 80 MPa)2.7 x 10 -8 moles m 2 s 1 (250C, 280 MPa)1.5 x 10 8 molesm 2 s l (250C,280 MPa)

280 MPa(MPa)(MPa)0.5-50 MPa (overgrowths


10/19

20,052 FISHERAND BRANTLEY:EPISODIC LUID FLOW N AN ANCtENT SUBDUCTION ONE

The value of AP can be estimated by consideringviscous flow around a rigid spherical nclusion [e.g., Lamb,1923]. The pressure n the fluid is given by

p=-5 re cos0sin0os( (3)r 3where 1 is the viscosity, r is the radial distance from thecenterf he phere,e is theporphyroblastadius, is thesimple shear strain rate at r = oo,and 0 and ( are as definedin Figure 7. Equation 3) satisfies he viscous low equationsfor an incompressiblematerial when P is not constant:

V2p=0, V-U=0 (4)where U is the velocity field and U and P satisfy theboundary conditions of simple shear deformation at r = ooand no slip at the surface of the sphere. A P, or thedifference in pressure between a point on the surface of thesphere herehepressures maximizedr = re, 0 = -it/4, = 0) and hepointwherehepressures minimizedr = re, 0= It/4, = 0), is:

strainate asterhan1012 s I, or higherhangeologicallyreasonable ates for deformationof slates [e.g., Pfiffner andRamsay, 1982]. Thus, these simple calculationsare at leastbroadly consistent with the inferences from textu:alobservations that development of fibrous overgrowths isdiffusion-limited rather than interface-limited.Overgrowth Model H: Grain-BoundaryDiffusion-LimitedFiber Growth

To estimate diffusion-limited growth rates ofdisplacement-controlled ibrous overgrowths, we assume thesimple geometry of Figure 7. The flux, J, of silica into theporphyroblast-fiber grain boundary at 0 = 0, ( = 0 can bedescribed:

J=-Dgb(3} (7)where gb is the grain-boundaryiffusion oefficient,nd3C/3h representshe gradient n concentration long thetangent, h, to the grain boundary at 0 = 0 and et = 0. Forsmall values of the angle 0 (Figure 7), we note that 3h = 3(r tan )=r e sec0 30, andwe canmakehe ollowingsubstitution:

(5)Thus,given hat the shear tressCs)= 1 , AP isapproximately times he magnitude f c at large distancesfrom the porphyroblast. If the shear stress s 0.1-10 MPa,AP should be on the order of 0.5-50 MPa.

Using Rimstidt and Barnes [1980] value for the molardissolutionateconstantf quartz t 250Ckm 1.5x 108mol m 2 s-1), we can calculate he linear rate constant:kl=kmVq=.4x 10-13 s 1 whereqs hemolarolumeof quartz. The rate constant measured by Rimstidt andBarnes [1980] at 4 MPa (saturated vapor pressure) can becorrected or the actual pressureof reaction (280 MPa) usingthe following equation [Macinnis, 1991]:

j=Dgb(3_) (8)reThe diffusion of moles of quartz per unit time into the

grain boundary 3m/3t) is the product of the flux and thegrain-boundarycross-sectional rea at the margins of thefiber-growthuadranttr w, where is grainboundarywidth):

Om ItwDgb C. (9)3t 30Inkm nko P AVe' (6)

RTwhere o s estimatedo equal1.5 x 10-8 molm-2 s-1 andAV is the activationolumeapproximately9 x 10 6 m3mol-1).At 250Cnd 80MPa,weestimate= 6.1x 1013m s 1 Recent easurementsf quartz issolutionates e.g.,Dove and Crerar, 1990] suggest that Rimstidt and Barnes'value may be slightly low: these measurements also showthat the presenceof salts acceleratequartz dissolution.

At P =280 MPa and AP =0.5 to 50 MPa, C2 = 16.0mol m 3 and AC varies between 2.3 x 10 2 and 2.3 x 10molesm 3 [Waltherand Helgeson, 977],yielding lineargrowthate asterhan 016 m s-1. With a porphyroblastdiameter of 10-100 I. m, this fiber growth rate represents

During an intervalof time 3t, fiberswill grow n lengthand add on the incremental volume 3V:

3V=l/2(it(rp 31)23h-itrp23h)=trp31ah(10)Again, making the substitutionintegrating from 0 = 0 to /4 yields 3h= re sec0 30 and

3V= t re 3l (11or a rate of addition of moles

3m re 3l- (2)}t q }t


11/19

FISHER AND BRANTLEY: EPISODIC FLUID FLOW n AN ANCtENT SUBDUCON ZONE 20,053

Substitutionof (9) into (12) yields an expression or the fibergrowth rate:

Ol _ Vq Dg, C (13)

Farver and Yund [1991] evaluated grain-boundarydiffusion of oxygen in novaculite to yield an estimate for theproductfDgbwor quartz rain oundariesetween50and800C at 100 MPa:D,bw=3x047xp-113 (14)T

whereg sexpressedsm s 1, and 13+17epresentsheactivationenergyof diffusion n kJ/mol. Althoughvaluesofw are unconstrained, Joesten [1991] suggestsa value of w= 1 nm,whichieldsgt 1.5 1019m s1 at250C.Length of fibers in sectionscut parallel to the principalaxes of extension and shortening is observed to vary as afunction of 0 in overgrowths, suggesting hat 3C/30 inequation (13) varies with 0. However, it is impossible toconstrain3C/30, except to recognize that this concentration(^radientmust be


12/19

20,054 FISHER AND BRANTLEY: EPISODIC FLUID FLOW IN AN ANCmNI SUBDUCTION ZONE

shallowest ip of the crack, then at a point 60 m deeper, fluidpressure in the fracture would be 1 MPa less than rockpressurerockdensitys 2.8 gm cm3, fluid densitys 0.9 gcm3, assumingo fluid flow upward). In the followingcalculations, we therefore assume the model summarized inFigure 9, and we require that the AP between matrix andcrack is less than or equal to 1 MPa. Because the slatycleavage in the central belt of the Kodiak Formation issubhorizontal, we assume anisotropic permeability in therock matrix, such that the componentof vertical permeabilityis significantly smaller than the component of horizontalpermeability and fluid flow only occurs from the matrix intothe crack, never upward through the matrix. We also followEtheridge et al. [1984] in assuming he pressure drop APoccurs at the fracture-matrix boundary.

At conditions of 250C and 280 MPa and AP g ! MPa,Vw/Vqmustegreaterhan 06WaltherndHelgeson,1977]. Even at AP = 1 MPa, a crack must open 8 m toprecipitate one 8-I. m layer of quartz assuming that all thequartz is delivered from one pulse of fluid. Even assumingpore-fluid concentrations of 0.5 m in NaC1 will only decreasethe crack opening estimate by less than 10% [Fournier,1983], while the additionof other componentsCO2, CH4,etc.) will increase the estimate. Myers [1987] reportsevidence or significantmole fractionsof CO2, which woulddecreasehe equilibrium olubilityof SiO in the pore fluidand would consequently ncrease the opening needed for thissingle-pulse model.

An increased chemical potential gradient could also beprovided if the matrix quartz was strained with respect toprecipitating quartz. Using values from Wintsch andDunning [1985, Model II], we can calculate the solubility ofstraineduartzdislocationensity 0TM m2 ) at 250C nd280 MPa: quartz solubility is increasedby - 10%, implyingthat a crack with a AP = 1 MPa would still have to open 7 mto produce an 8-gm layer of quartz. A second chemicalpotential gradient could exist in the case of very fineparticulate quartz present in the matrix or along the crackboundary. If we assumea small average grain size of 0.! g malong with a large pressure differential, AP = 1 MPa, wecalculate (using the Ostwald-Freundlich equation [ller, 1979])a minimum 4 m-wide crack opening.Supersaturation would also result if hot fluid, producedoutside of the crack, entered and equilibrated with coolerrocks. Vrolijk et al. [1988] used fluid inclusion and vitrinitereflectance data to suggest that fluids percolating into theKodiak Formation may have had temperatureshigher than thesurrounding rocks. Assuming the maximum estimatesuggested by Vrolijk et al. [1988] (AT of 25C), wecalculate an opening of at least 20 cm for crack-seal veins:an opening dimension still prohibitively large.Supersaturation could also be caused by metamorphicreaction, e.g., a silica-producing reaction occurring in thematrix. However, especially at 250C, it is likely that quartzprecipitation is fast enough to hold the concentration ofquartz in solution close to the equilibrium value. Even atlower temperatures where quartz precipitation might bekinetically hindered, silica supersaturationsgreater than 0.8mol m 3 are rarelyobservedLand, 1984]. Thus,no matter

how favorable the assumptions re, the single pulse modelrequires unreasonablecrack apertures to precipitate an 8-I. mquartz layer in crack-seal veins.Vein Model II: Growth From Matrix-Derived Fluid Flow

If fluid flows from the rock matrix into the crack andthen out of the network, the system can be modelled as fluidflow from a unit volume of homogeneous orous rock matrixinto a unit volume of homogeneously spaced andequidimensional cracks connected to an overlying reservoir.We again follow Etheridge et al. [1984] in proposing hat apressure drop occurs between the matrix and crack fluidreservoirs, and the pressuredrop occurs completely across hecrack wall.

We will refer to two fluid reservoirs' R1, the higherpressure reservoir of matrix pore fluid with equilibriumsolubility, C1, and R2, the lower pressure eservoirof fracturefluid with equilibrium solubility, C2. The moles of quartzdissolvedper second n the matrix pores, 3m ! 3t, will bedescribeds a function f the dissolutionate constantkm,toolm 2 s 1 , surfacerea sl) and ocalconcentrationC1) nreservoir R1 [Rimstidt and Barnes, 1980]'

3m_skm -C (16))t C/If the luxof fluid nto hecrackm3water 3 1ock - ) isequalo Qs20p,whereQ is flow ate m s-l), ) is matrixporosity nd $2 is cracksurface reaper unitrockvolume m2m-3), hen, t steady tate,hemoles f quartz issolvingnR must equal the moles of quartz flowing from matrix tocrack, and we can solve for C/:

O) 2 C1 Q + sl krnFor the case of Q


13/19

FISHERAND BRANTLEY:PISODIC LUID FLOW N AN ANCIENTSUBDUCTIONONE 20,055

^ ^ ) q)2 ^ ^] m Q C - C2 -s2 Q2c c2^ ^ O02Qlkm2+slkmQ C2+s2kmQ C1+s2 2C1C2 (19)

Using this value of t we can also calculate the amount offluid which must flow through the vein to provide sufficientquartz,Q s2 ) :For the case of C1 = C1 with quartz nucleatedonly on one sideof the crack and fluid flow restricted to the other side of thecrack, (19) yields

C2-C2_ Q C1-C2C2 q) Q C 2 + km

(20)

We use (19) or (20) to determine the time, t, needed to fillup a crack by precipitation of m moles of quartz:

t - rn C2 (21)s2 mC2 C'The value for t will only be positive if C1 > C2 and

Q


14/19

FISHER AND BRANTLEY: EPISODIC FLUID FLOW IN AN ANC7ENT SUBDUCTIONZONE

account for all the quartz precipitated unless he crack spacingwas wider than the observed vein spacing (D > 1 m). Again,the problem of large fluid volumes necessitates hrough-flowof external fluid or diffusive solute transport of silica into thevein. Furthermore, we have noted textural evidence suggestingthat continuously banded crack-seal veins were seale_dbyquartz, rather than by crack collapse. Although the euhedralgrowth veins show evidence of collapse, the extremely largeamounts of quartz precipitated in these features also precludesthis model.

Vein Model III: Growth From Fracture-Channelized FluidFlow

Fracture-channelized luid that flows upward from depthwill cool and decrease in pressure, continually precipitatingquartz. In the Kodiak Formation, this fluid could be derivedfrom flow along the decollement from deeper levels of thesubductionzone or upward flow from the subductingsedimentpile into the overriding accretionary prism. In either case,the large fluid volumes cannot be simply generated in thelocal matrix, but must rather be generated by underlying rockunits. If flow is slow enough that equilibrium is maintained,we can calculate the time necessary to precipitate the molesof quartz m)per volumeof rock V,) in a given vein system[see Ferry, 1989]:

t=m OCT C)P1rQ 77 + (26)where [}C/3T refers to the temperature coefficient ofsolubility, [}T/z refers to the geothermalgradient, C/Prefers to the pressure coefficient of solubility, andrefers to the pressuregradient.

Walther and Orville [1982] have shown that thesmallest possible pressure gradient which can maintain opencracks s [}P/[}z = 1.9 x 104 Pa m 1. If we assumea crackaperture, d, and a crack spacing, D, we can use the followingequation to estimate permeability, K [Norton and Knapp,1977]'

d3K - (27)12Dfor use in Darcy's aw to calculate he upward low rate Q'

Q_ K 3P (28)

of each growth event. It is difficult to imagine how a crackcould seal (occlude fluids) if transport of silica was whollymaintained by influx of fluid.Euhedral growth veins close as a consequenceof crackcollapse so it is not possible to estimate crack aperture.However, the amplitude of surface irregularities related to thequartz vein are as large as 1 mm while the crack is open, sothe aperture is probably larger than in the case of continuouscrack-sealveins. Such arge apertureswould predict very fastflow rates by equation (28). Furthermore, because he largestcrystals in euhedral growth veins are associatedwith sandylayers or earlier quartz veins in the adjacent wall rock (seeFigure 6), we note that concentration gradients probablyexisted along the length of the open crack during growth. Ifgrowth had occurred n the presenceof flowing fluid, stronglengthwise concentration gradients could not have beenmaintained. We suggest that diffusion of silica into cracks isthe only transport mechanism which adequately explains alltextures n crack-seal and euhedral growth veins.Vein Model IV: GrowthFrom Pore Fluid DiffusiveTransport

As demonstrated in Figure 9, a pressure differentialbetween crack fluid and matrix fluid will always exist for anonflowing fluid in an open crack. We have argued that thedifferential AP will always be less than 1 MPa. Fordiffusion to control mass transport of silica from a higherpressure eservoir R1 to a lower pressure eservoirR2, the rateof diffusion must be faster than the rate of advection of Si.Diffusion of components through an aqueous solution isgenerally only thought to be important for short-distancetransport (e.g., centimeter-scale transport Etheridge et al.,1984]). We can follow Etheridge et al. [1984] and use thefollowing expression, developed by Fletcher and Hoffman[1974] to predict effective transport distances based ondiffusionXd):

= )1/2d (2DfO)x (29)wheref s he ore-fluidiffusionofficient=10s m s 1),t is time, and x is tortuosity. For fluid flow from the matrixto the crack, a similar expression for effective transportdistance asedon advectionXa) canbe derived:

Xa- K APt (30)lw{ Dwhere1w s fluidviscosity104 Pas),K is permeability,and [}P/Oz is the pressuregradientalong the flow path. Wenote that 8-gm-wide cracks with D = 1 mm to 1 m wouldprovide permeabilityf 10 13 to 10 16 m2 and low rateswhich are geologicallyeasonable' 0 5 to 10-8 m s-1.However, although 8-gm-wide cracks could supportreasonable flow regimes, the fluid would be squeezed out asquartz crystalsprecipitated rom a fluid; flow would eventuallyslow, terminating silica transport nto the vein and making itimpossible to seal the vein by precipitation. Because theflow rate s proportionalo d3, fracture-channelizedlowwould become exponentially more difficult over the lifetime

Comparing (29) and (30), it is apparent that intergranulardiffusion will outcompete advection for short time periods,but that infiltration will dominate solute transport for longertime rames. For vein growthess han he critical ime (tcritdefined such that X d = X a), diffusion will be the mainmechanism of silica transport:

tcritO'162}22 (31)Even with a smallmatrixpermeabilityK = 10 21 m2 ,a pressure differential greater than 0.1 MPa would cause


15/19

FISHER AND BRANTLEY: EPISODIC FLUID FLOW IN AN ANCII!IqT SUBDUC'rlONZONE 20,057

significant fluid flow from matrix to crack, destroying theconcentration gradient for diffusion. For a small, localpressure gradient (AP = 0.01 to 0.1 MPa and D = 100 m,as expected for a euhedral growth vein) and a porosity of 1%,the critical ime is t = 107 to 109 years. For crackswhichare open ess han 107 to 109 years,diffusionwouldbe amore important solute transfer mechanism than fluidadvection from matrix to crack. Similarly, for crack-sealveinswithD = 1 m, tcrithasa value f l0s - l0s yearsorAP = 0.01 to 0.1 MPa. Remembering that the maximumvalue of AP is related to the vertical fracture dimension fornonflowing fluids (Figure 9), the vertical dimension of theopen crack network at any given time must be less than 6 mfor this model.

The simplest model for silica transport by pore fluiddiffusion is to assume hat a depleted zone of width W willform around the fracture, and that the concentration gradientacross that depleted zone will be a constant at steady state.The quartz concentration in the matrix within this zone willbe lower than outside the zone, although not necessarilyreduced to zero. In this zone, silica pore fluid concentrationis determined by diffusion from the silica-rich matrix to thefracture. Following the analysis of Berner [1980], we cancalculate the moles of silica, 3m, which cross the surfacearea, s2 , of the crack in time 3t:

3m = s2 J 3t (32)whereJ is the lux (moles n 2 s 1 ) and s describedy

W

Here, C2 is defined as previously: ocal concentration f silicain the crack during quartz growth. The addition of moles ofsilica as a function of growth of crystal length 31 isexpressed:

3rn = s._1 (34)Vq

and, solving for the rate of crystal growth,

)t w

As diffusion proceeds,we define the growth of the depletedzone'

3W}t (36)

where Fd is the volume fraction of matrix dissolved out of thedepletedzone. Finally, we also know that the rate of crystalgrowth in the crack fluid will be describedby the interfacegrowth equation:

(37)

If we equate35) and 37), we cansolve or C2

C2 C2 q ) f C1 klC2W (38)C2 Vq O)Df x + kl WInserting (38) in (36) yields the following expression forgrowth of the depleted zone:

ClV Vq ) 'k C1-C2 (39)at Fd( q,DfxC2+klW)which can be integrated with the boundary condition that W=0att= 0to yield

k iW2 +V-Df; 2W=Vq DfJl C1-C2F (40)Solving for W, we find

...

-qtDfr2+ q2q>2D/222,2k12VtF (41)klCombining equations and integrating under the conditionsthat I = 0 att= 0 yields the time t needed to grow acrystal of length l:

kl 12+ 2 Vqq>: fFdC21t = (42)- (^^)VOxDFa C-C2Where the first term in the numerator of equation (42) is muchlarger than the second term, the model reduces to thediffusion-limited case; alternately, where the second termdominates, the model reduces to the simple interface-limitedcase.

An 8-gm aperture crack with AP = 0.01 to 0.1 MPawouldseal n 103 to 104 years n the early stagesof veindevelopment when quartz nuclei are randomly oriented anddiffusion is fast. Growth of quartz crystals in these cracks inthe early stage is interface-limited. Later in the developmentof crack-seal veins, when favorably oriented quartz growthdominates he vein and the depleted zone is larger, the rate ofdiffusion decreases and vein growth becomes partiallyinterface-controlled and partially diffusion-controlled. Neverduring the growth of these veins does diffusion completelylimit growth, which explains why quartz-free depletion zonesare not observed [Fisher, 1978].

We can also calculate the time needed to form a euhedralgrowth vein under these conditions. Macinnis [1991] hasmeasured dissolution rates of the basal plane of quartz at


16/19

FISHER AND BRANTLEY: EPI$ODICFLUID FLOW N AN ANCIENT SUBDUCTION ONE250C to be a factor of 60 times faster than bulk dissolutionrates (rates averaged over all crystal surfaces)measuredbyRimstidt and Barnes [1980]. Assuming growth events resultin a 1 cm quartz layer (favorably oriented) and AP =0.01 to0.1 MPa, eacheventwouldcorrespondo a 105 to 106 yearfilling time ( with = 1%, r = 1, andF d = 40%). This shorttimescale s compatiblewith the short timescale equired orthe diffusion-transportechanismi.e., 105 to 106 years


17/19

FISHERAND BRANTLEY:EPISODIC LUID FLOW N AN ANrXENTSUBDUC'rIONONE 20,059

1 m- 10's of m

1 MPa

AP(=Pm-Pcrat H

t:I

mic

wall rock

quartz t: t3

Fig. 11. Crack modelhorizontally xaggerated y 5-6 ordersof magnitude, howing luid-filled crack with maximumaperture f 8 mm, and a lengthof 1 m to tensof meters.Note growthof quartz rom right to left, and growthof micafrom left to right. Fluid-filledcracksoriginatenear decollementnd propagate pwards: chematic howspassage fone luidpulse ast oint t depth in a crack-sealein rom ime to t 2 to t 3. IncreasingP withdepthesultsin increased ore fluid diffusionof quartz rom matrix to crack with increasing epth. Basedon model, luid movesupwardswith velocitycontrolled y the rate of quartzgrowth n the crack. At a given depthwithin the cracknetwork,AP will rise and all periodically s shown n responseo the passage f eachexternallyderived luid volume center).

Euhedral growth veins are larger cracks that may havedeveloped closer to the decollement due to constant orperiodic nfluxes of fluid. Further from the decollement, heselarger cracks branch into smaller cracks with stable shapes(6-m length and 8-1m aperture). Within thesesmaller cracks,crack-sealveins develop as the crack fluid volumesare drivenupwardby quartzprecipitation.The cracknetworkmay thusconsist of a branching hierarchy in which euhedral growth,discontinuous crack-seal, and continuous crack-seal veinsrepresent he ransition rom third to second o first orderfeatures.T,he ollapseventsn discontinuouslyandedcrack-seal veins and euhedral growth veins may representtimes when rapid creation of new crack volume within thesystem causes crack fluid pressure o drop catastrophicallybelow the values necessary to maintain an open fracture.Cracks associatedwith continuously banded crack-seal veinsdo not collapsebecause hey seal before escapeof fluid fromthe system.The periodic fracturing ecordedby these eaturesmay berelated to periods of dilation preceding seismic slip on thebasal decollement (e.g. seismic pumping' Sibson et al.[1975]) or episodic passageof dilational strain waves along

the decollement as inferred from reversed polarity anomaliesin seismic reflection profiles of modern convergent margins[Moore et al., 1991]. The periodicity of cracking for crack-seal veins overlaps with the recurrence interval for largeseismic lip events e.g., about102 years). Basedon aseismic pumping scenario, the rate of quartz precipitation incontinuous crack-seal veins is sufficient to seal the cracksprior to each seismic event, whereas quartz precipitation indiscontinuous crack-seal veins can only partially seal cracks.Euhedral growth veins remain continuously open over manyfracturing events.While at present the calculations in this paper arerough, and we can only speculateabout seismic pumping as acontrol on cracking periodicity, we feel the approach used inthis paper, namely a combination of textural considerations,vein distributions, and simple kinetic models, could beapplied to a variety of settings and that over time, a morecomplete picture of fluid flow and deformation could emergefor variousgeologicenvironments.The modelspresentednthis paper also highlight the parameters that are currentlyunconstrained, and, with better control on these variables, theKodiak systemmay be more accuratelyportrayed.


18/19

20,060 FISHERAND BRANTLEY: PISODIC LUID FLOW N AN ANCIENTSUBDUC'rIONONEAcknowledgments. ogisticalsupportwas provided n the fieldby the RandalIs. D.M.F.'s researchwas supported y NSF grantsEAR 8407801 and EAR 83006578 o T. Bryne. S.L.B. acknowledgessupport rom NSF grant EAR 8657868, support rom the David andLucille Packard Foundation,and support rom Gas Research nstitutecontract50882601746. We greatly benefitted rom discussions ithT. Engelder, T. Byrne, D. Kerrick, J. Walther, B. Yardley, and R.Slingerland nd criticalreviewsby S. Agar and P. Vrolijk. We thankChristinePerry for the SEM photomicrographs.

REFERENCES

Berner, R.A., Early Diagenesis: A Theoretical Approach, 241pp.,PrincetonUniversity Press,Princeton,New Jersey,1980.Bjorlykke, K., Cementation of sandstones, J. Sediment. Petrol.,49, 1358-1359, 1979.Byrne,T. and D. Fisher,Episodicgrowth of the Kodiak convergentmargin, Nature, 325, 338-341, 1987.Cox, S.F., Antitaxial crack-seal vein microstructires and theirrelationship to displacementpaths, J. Struct. Geol., 9, 779-788, 1987.Cox, S. F., and M. A. Etheridge,Crack-seal ire growth mechanismsand their significance n the developmentof oriented latersilicate microstructures,Tectonophysics,92, 147-170, 1983.Cox, S.F., and M.A. Etheridge, Coupled grain-scaledilatancy andmass transfer during deformation at high fluid pressures:Examples from Mount Lyell, Tasmania, J. Struct. Geol., 11,147-162, 1989.Cruikshank,K. M., G. Zhao, and A.M. Johnson,Analysis of minorfracturesassociatedwith joints and faulted joints, J. Struct.Geol., 13, 865-886, 1991.Davies, D. L., and J. C. Moore, 60 Ma intrusive rocks from theKodiak Islands ink the Peninsular,Chugach,and Prince Williamterranes, Geol. Soc. Am. Abstr. Programs, 16, 277, 1984.Dove, P., and D.A. Crerar, Kinetics of quartz dissolution inelectrolyte solutions using a hydrothermalmixed flow reactor,Geochim. Cosmochim. Acta, 54, 955-969, 1990.Durney,D. W., and J. G. Ramsay, ncremental trainsmeasured ysyntectonic crystal growths, in Gravity and Tectonics, editedby K. A. Dejong and R. Scholten, pp. 67-96, John Wiley andSons, New York, 1973.Elliott, D., Diffusion flow laws in metamorphic ocks, Geol. Soc.Am. Bull., 84, 2645-2664, 1973.Elliott, D., The energy balance and deformationmechanisms f thrustsheets, Philos. Trans. R. Soc. London, 283, 289-312, 1976.Etheridge, M.A., V. J. Wall, and R. H. Vernon, The role of the fluidphase during regional metamorphism and deformation, J.Metamorphic Geol., 1, 205-226, 1983.Etheridge, M.A., V.J. Wall, S.F. Cox, and R.H. Vernon, High fluidpressuresduring regional metamorphism and deformation:Implications for mass transport and deformationmechanisms,J. Geophys.Res., 89, 4344-4358, 1984.Farvet, J. D. and R. A. Yund, Measurement f oxygen grain-boundarydiffusion in natural, fine-grained, quartz aggregates, Geochim.Cosmochim. Acta, 55, 1597-1607, 1991.Ferry, J., Fluid flow, mineral reactions, and metasomatism: Ageneral model, Geol. Soc. Am. Abstr. Programs,21, 1989.

Fisher, D., Orientation history and theology in slates, Kodiak andAfognak Islands, Alaska, J. Struct. Geol., 12, 483-498, 1990.Fisher, D., and T. Byrne, Structural evolution of underthrustedsediments, Tectonics, 6, 775-793, 1987.Fisher, D., and T. Byrne, The character and distribution ofmineralized fractures in the Kodiak Formation, Alaska:implications for fluid flow in an underthrust sequence, J.Geophys. Res., 95, 9069-9080, 1990.Fisher, D., and T. Byrne, Strain variations n an ancient acretionarywedge: implications for forearc evolution, Tectonics, 11, 330-347, 1992.Fisher, G.W., Rate laws in metamorphism, Geochim. Cosmochim.Acta, 42, 1035-1050, 1978.Fletcher,R.C., and A.W. Hoffmann, Simple modelsof diffusion andcombined diffusion-infiltration metasomatism, in GeochemicalTransport and Kinetics,Publ. 634, editedby A.W. Hoffman etal., pp.242-262, Carnegie Institution of Washington,Washington, D.C., 1974.

Fournier, R.O., A method of calculating quartz solubilities n aqueoussodium chloride solutions, Geochim. Cosmochim. Acta, 47,579-586, 1983.Iler, R., The Chemistry of Silica, Wiley-Interscience, New York,1979.Joesten,R., Grain-boundarydiffusion kinetics in silicate and oxideminerals, in Diffusion, Atomic Ordering, and Mass Transport,edited by J. Ganguly, pp. 345-395, Springer-Verlag,New York,1991.Lamb, H., Hydrodynamics,Cambridge University Press, New York,1932.Land, L., Dissolution kinetics of calcite and quartz under surfacereaction control, in Clastic Diagenesis, Society of EconomicPaleontologistsand Mineralogists, Tulsa, Okla.., 1984.Lasaga, A. C., Chemical kinetics of water-rock interactions, J.Geophys. Res., 89, 4009-4025, 1984.Macinnis, I. N., Surface Control of Mineral Dissolution: Andalusite,calcite, and quartz, Ph.D. dissertation, Pennsylvania State

University, University Park, Pennsylvania, 991.Moore, J. C., T. Byrne, M. Reid, H. Gibbons,P. Plumley,and R.Coe, Paleogene evolution of the Kodiak Islands, Alaska:consequencesf ridge-trenchnteraction n a more southerlylatitude, Tectonics,2, 265-293, 1983.Moore, J. C., G. Cochrane.,M. Mackay, G. Moore, Soest,and K.Brown, Dilational strain wavesalong hrust aults of the modernOregonaccretionaryrism,Geol. Soc.Am. Abstr. with Prog.,23, no.5, A366, 1991.Myers, G., Fluid expulsion uring he underplating f the KodiakFormation:A fluid inclusionstudy,Masters hesis,Universityof California, Santa Cruz, 1987.Norton,D., and R. Knapp,Transport henomenan hydrothermalsystems: The nature of porosity, Am. J. Sci, 277, 913-936,1977.

Pfiffner, O. A., and J. G. Ramsay,Constraints n geological trainrates:Argumentsrom finite strainstatesof naturallydeformedrocks, J. Geophys.Res., 87, 311-321, 1982.Platt, J., Dynamicsof orogenic wedges and the uplift of high-pressure metamorphic rocks, Geol. Soc. Am. Bull., 97, 1037-1053, 1986.Pollard,D. D., and P. Segall, Theoretical isplacementsnd stressesnear fractures in rock: with applications o faults, jointsveins,dikes,and solution surfaces, n Fracture MechanicsofRock, ditedby B. K. Atkinson,pp. 277-349, Academic,SanDiego, Calif., 1987.Ramsay, J., The crack-seal mechanismof rock deformation,Nature,284, 135-139, 1980.Ramsay,J. G., and M. I. Huber, The Techniquesof Modem StructuralGeology, Vol. 1, Strain Analysis, Academic, San Diego,Calif., 1983.Rimstidt, J. D., and H. L. Barnes, The kinetics of silica-waterreactions, Geochim. Cosmochim. Acta, 44, 1683-1699, 1980.Sample,J., and D. Fisher, Duplex accretionand underplatingn anancient accretionary complex, Kodiak Islands, Alaska,Geology, 14, 160-163, 1986.Sample, J., and J. C. Moore, Structuralstyle and kinematicsof anunderplated slate belt, Kodiak and adjacent islands, Alaska,Geol. Soc. Am. Bull., 99, 7-20, 1987.Sibson,R.H., J.M. Moore, and A.H. Rankin, Seismicpumping--Ahydrothermal luid transportmechanism, J. Geol. Soc. London,131, 653-659, 1975.Silver,E. A., M. Jordan, . Breenl ndT. Shipley, rowth faccretionaryprisms, Geology, 13, 6-9, 1984.Urai, J. L., P. F. Williams, and H. L. M. van Roermunc, Kimematicsof crystal growth in syntectonic ibrous veins, J. Struct. Geol.,13, 823-836, 1991.Vrolijk, P.J., Tectonically-driven fluid flow in the Kodiakaccretionary omplex,Alaska, Geology, 15, 466-469, 1987.Vrolijk, P., G. Myers, and J. C. Moore, Warm fluid migration longtectonicmelangesn the Kodiak accretionary omplex,Alaska,J. Geophys.Res., 93, 10,313-10,324, 1988.Walther, J.V., and P.M. Orville, Rates of metamorphism ndvolatile productionand transport n regional metamorphism,Contrib. Mineral. Petrol., 79, 252-257, 1982.Walther, J.V., and H.C. Helgeson,Calculation f the thermodynamicpropertiesof aqueous ilica and the solubilityof quartzand its


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FISHERANDBRANTLEY: PISODIC LUm FLOWn AN ANCIENTSUBDUCIION ONE 20,061polymorphsat high pressuresand temperatures,Am. J.Sci., 277, 1315-1351, 1977.Wang,G-F., Carbonaceousaterialn the Ryokemetamorphicocks,Kinki district, Japan, Lithos, 22, 305-316, 1989.

Wintsch,R. P., and J. Dunning,The effect of dislocation ensityonthe aqueous olubility of quartz and some geologic mplications:A theoretical approach, J. Geophys. Res., 90, 3649-3657,1985.Yardley,B., Fluid migration nd veining n the Connemara chists,Ireland, in Fluid-Rock Interactions during Metamorphism,

editedby J.V. Waltherand B.J. Wood,pp. 109-131,Springer-Verlag, New York, 1984.S. L. Brantley ndD. M. Fisher,Departmentf Geosciences,PennsylvaniatateUniversity,03 DeikeBuilding, niversityark,PA 16802.

(ReceivedMay 31,1991;revisedJune22, 1992;accepted uly6, 1992.

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