Stable isotope constraints on vein formation and fluid evolution along ...

Earth and Planetary Science Letters 293 (2010) 300–312

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e p s l

Stable isotope constraints on vein formation and fluid evolution along a recent thrust fault in the Cascadia accretionary wedge James C. Sample ⁎ Department of Geology, Box 4099, Northern Arizona University, Flagstaff, AZ 86011-4099, United States

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Article history: Received 6 April 2009 Received in revised form 22 February 2010 Accepted 26 February 2010 Available online 1 April 2010 Editor: M.L. Delaney Keywords: Ocean Drilling Program Cascadia subduction zone fluid flow geochemistry carbonate SIMS oxygen isotopes veins

a b s t r a c t In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4′N, 125°07.1′W) along the Cascadia subduction margin. The average δ18OPDB value of all samples is −9.9‰ and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6‰. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are −14.4‰, −9.5‰, and − 5.2‰, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher δ18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2–3 km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin. © 2010 Published by Elsevier B.V.

1. Introduction Circulation of fluids through Earth's crust at convergent plate margins redistributes heat and mass, changes the physical properties of rocks prone to seismicity, and supports chemosynthetically based ecosystems on the seafloor (Kulm et al., 1986; Kastner et al., 1991; Moore and Vrolijk, 1992; Carson and Screaton, 1998; Wang and Hu, 2006). The mineral products resulting from fluid–rock interactions provide a record of fluid flow that may be associated with tectonic events (Sibson, 1981; Fisher and Byrne, 1990). Accretionary wedges are advantageous places to study fluid–rock interactions because the wedges extend continuously from the relatively little disturbed, water-rich sediment near the plate boundary, to more evolved structural domains in mountain belts. The evolution of deformation and fluids can be captured at various stages of development in the accretionary complex (Vrolijk et al., 1988; Moore and Vrolijk, 1992; Torres et al., 2004a,b; Tréhu et al, 2006). Studies of the properties of modern pore fluids dominate our knowledge of the fluid flow history in wedges. But an understanding of modern pore fluids provides only a partial, short-term understanding of the margin's evolution. Efforts have been made to collect longer records of pore fluids, but they are still limited to only several months

⁎ Corresponding author. Tel.: +1 928 523 0881; fax: +1 928 523 9220. E-mail address: [email protected]. 0012-821X/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.epsl.2010.02.044

or a few years (Davis et al., 1995; Screaton et al., 1997; Tryon et al., 2002; Brown et al., 2005). Thus we have little understanding of how major tectonic events such as great earthquakes may alter fluid flow characteristics. In continental settings major earthquakes can significantly impact pore fluids at least for short time intervals (Wang et al., 2004). Fortunately proxies for ancient pore fluids in accretionary prisms are available in cements and veins. The Cascadia margin has some of the most abundant carbonate veins and cements of any accretionary prism (Kulm et al., 1986; Ritger et al., 1987; Sample and Reid, 1998; Milkov et al., 2005). Veins record a history of fluid flow that reflects variations in the physical and chemical environment over time intervals of 104 to 105 years (Fisher and Brantley, 1992; Lee et al., 1996). Veins can be useful indicators of stress conditions during deformation (Ramsay, 1980), records of tectonic events, associated fluid flow and mineralization (Sibson, 1981), and a chemical record of environmental changes and degree of chemical equilibrium in fluid-rich sedimentary sequences (Sample and Kopf, 1995; Sample and Reid, 1998). Carbonate veins in Cascadia show textural evidence for cyclic precipitation and potentially can extend the record the chemical and fluid history of fault zones back several hundred thousand years. Attempts to relate the geochemistry of precipitation events to tectonic events along the margin require an analytical capability with fine spatial resolution; otherwise important parts of the record can be lost to time-averaging effects inherent in methods with low spatial resolution.

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New results are reported here from an ion microprobe, stable isotopic study of veined mudstone samples collected at ODP Site 892 (44°40.4′N, 125°07.1′W) from an active thrust fault zone, and from horizons in its hanging wall and footwall. The SIMS (secondary ionization mass spectrometry) technique allows in situ analysis of small sample volumes to provide an understanding of mechanisms of vein formation and the evolution of the fluids during vein growth. In situ analyses eliminate the ambiguity of spatial context. Because multiple SIMS spots can be acquired over distances less than 100 μm the SIMS techniques can detect variations over spatial scales not attainable by micromilling and acid digestion. The samples used in this study are appropriate for SIMS analysis because they preserve large intra-vein and inter-sample variations that are far greater than analytical reproducibilities of the SIMS technique. The results of this study show widespread, large 18O depletions that are unprecedented in accretionary prisms. The oxygen isotope data will demonstrate that the carbonates are far out of isotopic equilibrium with modern pore waters and will help to constrain fluid sources and temperatures during precipitation. Possible mechanisms for variations of fluid properties including large seismogenic events are investigated. Descriptions of vein textures and modeling of potential sources and flow rates will be used to suggest far-traveled fluids have risen along the fault zone cyclically and periodically and these events are generated by major flow events along he margin. 2. Geologic setting The samples were collected from Ocean Drilling Program Site 892 (Fig. 1). Drilling recovered mainly muds and silts from the slope cover and underlying wedge (see Westbrook et al., 1994 for detailed descriptions of cores). The study area is from a region of the Cascadia accretionary wedge referred to as Hydrate Ridge for the prevalence of gas hydrates found at the seafloor (Bohrmann et al., 1998). The regional tectonics of the Cascadia wedge are summarized in Goldfinger et al. (1992) and Mackay et al. (1992). Hydrate Ridge is underlain by Pliocene and younger slope sediments, and folded thrust slices of Miocene to Pleistocene age (Fourtanier and Caulet, 1995). Site 892 penetrated mostly slope sediments and part of the underlying accretionary wedge. The youngest sediments (2.0 to 0.5 Ma) involved in thrusting are found at a depth of 160 mbsf at the bottom of the hole. Using biostratigraphic constraints, the structures are likely Pleistocene or younger and could postdate an age of ∼500 ka. Evidence from southern Hydrate Ridge indicates that the most recent uplift of Hydrate Ridge began ∼0.3 Ma bp (Chevallier et al., 2006). The surface of Hydrate Ridge comprises large areas of carbonate and hydrate (Carson et al., 1994; Bohrmann et al., 1998). The hydrates and carbonates are derived from seepage of abundant methane in the sediment column (Ritger et al., 1987; Tréhu et al., 2006). Some of the hydrates contain thermogenic gases indicating deep sources (Milkov et al., 2005). Production of these gases in the upper 2–3 km is aided by geothermal gradients between 51 °C/km and 68 °C/km (Westbrook et al., 1994; Davis et al., 1995). Extreme 18O depletions of oxygen isotopes in carbonate cements and veins have been hypothesized as evidence for fluids originating at depths as great as 2 km (Sample, 1996; Sample and Reid, 1998). This is supported by higher boron concentrations in diagenetic carbonate associated with fluid flow horizons, indicating fluids derived from moderately deep to deep sources in the accretionary prism (Deyhle and Kopf, 2001). The samples in this study come from three drilling horizons. Based on core observations it is clear that these vein-bearing samples are associated with faults (Westbrook et al., 1994; Fourtanier and Caulet, 1995; Teas et al., 1995). Samples from the upper drilling horizon (66 to 67 m below seafloor (mbsf)) are from a minor fault in the hanging wall, recognized by a biostratigraphic reversal, significant porosity reduction related to strain consolidation, and extensive stratal disruption. In addition this horizon includes a stratigraphic horizon with

Fig. 1. Location map and cross section of study area. A. Location map showing ODP Sites 891 and 892 (filled circles). Edge of continental shelf coincides with 200-meter bathymetric contour. Abbreviations: JdFR = Juan de Fuca Ridge; BFZ = Blanco Fracture Zone; GR = Gorda Ridge; Wash = Washington. B. Simplified cross section depicting location of boreholes. Site 891 penetrated the frontal thrust; Site 892 penetrated the tectonically and hydrologically active thrust underlying the second bathymetric ridge landward of the deformation front (“frontal thrust”). “BSR” represents the bottomsimulating reflector underlying Hydrate Ridge that is inferred to represent the base of the gas hydrate-bearing sediments (Mackay et al., 1994). Samples of this study were collected along the second-ridge thrust, approximately 30 m below the BSR. The Northern Hydrate Ridge Thrust (NHRT) penetrated at 106 mbsf is also shown. Cross section is modified from (Westbrook et al., 1994).

sand and gravel layers, and yielded anomalies in pore-fluid chemistry, temperatures, and physical properties (Westbrook et al., 1994). The upper horizon is just above the inferred base (71 mbsf) of gas– hydrate-bearing sediment (MacKay et al., 1994). Three samples from 106 mbsf are from an active thrust fault at the top of a thick zone of stratal disruption, hereafter referred to as the northern Hydrate Ridge thrust (NHRT). This out-of-sequence thrust is well imaged on seismic reflection profiles (Fig. 1), and it is located 17 km east of the current deformation front. The NHRT is 45 km west of the shelf edge, and 80 km west of the continental margin. Although the fault is the main structure in a complex zone of interleaving of fault slices, changes in physical properties across the fault do not appear to support significant slip in that portion of the fault (cf. Shipboard Scientific Party, 1994, Fig. 48; Teas et al., 1995). The best evidence for recent fluid flow along the NHRT is elevated concentrations of thermogenic hydrocarbons in pore waters, and the character of the BSR across the fault (Moore et al., 1994). Two samples from the footwall of the thrust at 145 mbsf were analyzed for this study. Samples from this horizon are from the base of the stratally disrupted zone, just above a major biostratigraphic reversal, and are near a gravel layer. Pore fluids show elevated concentrations of thermogenic hydrocarbons (Westbrook et al., 1994). Previous work has been done on the samples used in this study or similar samples from the same horizons (Sample and Kopf, 1995; Kopf et al., 1995). Those studies utilized conventional acid digestion and isotope-ratio mass spectrometry (IRMS) and lacked the fine resolution necessary to infer details about fluid variations during carbonate formation. The earlier work showed that strong 18O depletions are

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common at Site 892, with δ18O (PDB) values as low as − 14.4‰ and −16.0‰ in veins and cements, respectively. The low values were attributed to rapid upward migration of warm fluids from depths of 2 km or more (Sample and Kopf, 1995; Sample, 1996). In comparing analyses of the same samples by acid digestion in previous work and SIMS in this study, there is significant overlap in the range of δ18O values from the same veins or cements, but SIMS analysis reveal more widespread depletions and stronger 18O depletions. A rigorous comparison with other accretionary prisms is problematic because recovered carbonate veins are rare, but no other margins display such widespread, light oxygen signatures. 3. Methods Ion microprobe samples were prepared by cutting, and mounting in epoxy resin, portions of polished rock sections alongside polished pieces of a calcite standard (Sample et al., 1998). A 200 Å gold coat was sputter-deposited on the sample mount surface prior to analysis. Measurements were made at UCLA using the CAMECA ims 1270 ion microprobe in a variety of analysis modes. In peak-switching mode (summarized in (Leshin et al., 1998) and Supplementary Data) a Faraday cup was used to collect the 16O − beam, and an electron multiplier to collect the 18O− beam. Internal precision of b1‰ was routinely obtained for analyses of ∼ 5 min duration. Repeated analysis of the standards demonstrated that the reproducibility was typically better than 1.4‰ (1σ). A small number of analyses in peak-switching mode collected only those ions with initial energies between 300 and 350 eV (high-energy mode; Table 1). The count rates were approximately 100× lower and both isotopes were detected using the electron multiplier. Analyses took ∼ 20 min and reproducibility was 1.2‰ (1σ). A third analytical method was employed for a few samples after the instrument was fitted with a multicollector system. This allowed better reproducibilities (b0.8‰) at collection times of ∼ 5 min (summarized in Fayek et al. (2001)). 4. Results The samples analyzed are from two drill holes (892A and 892D) that penetrated the NHRT (Fig. 1). Hanging-wall and footwall samples are from 40 m above and 39 m below the fault, respectively. All three structural horizons contain material with abundant veins. The hanging-wall samples were retrieved from 5 m above the inferred base of the hydrate-bearing section (Mackay et al., 1994), whereas the thrust zone and footwall samples are well below the base of gas hydrates. Typical samples were isolated, cemented pods or concretions b3 cm in diameter. Veins range in width from sub-millimeter to

5 mm. With few exceptions veins are nearly pure calcite and crosscut a poorly consolidated mudstone to silty mudstone matrix. Crystal textures are blocky, fibrous blocky, and fibrous. The range in oxygen values from the total sample set is large, from δ18OPDB = −21.9‰ to +2.6‰ (Table 1; Fig. 2). The largest range for an individual sample is 19.6‰ and for a single vein is 15.2‰. These appear to be the largest isotopic variations yet observed in samples from young, active accretionary wedges. Six of seven samples analyzed have light isotopic means, between − 13.3‰ and − 7.7‰, which are low for modern deep marine carbonates. These values are consistent with previous results from the Oregon margin (Table A1; Sample and Kopf, 1995; Sample and Reid, 1998). There is a gross isotopic stratigraphy that follows the structural stratigraphy. The average δ18O values of specimens from the hangingwall block, thrust zone, and footwall block are −14.4‰, −9.5‰, and −5.2‰, respectively. In addition there is a difference in the scatter of isotopic values (Table 1). The standard deviation of the data for the hanging wall (N = 62) and footwall blocks (N = 42) are both 4.1‰, but only 3.0‰ for the thrust zone (N = 149) samples. A few δ13C analyses were obtained from one sample. They have a range of values within error of that expected for seawater bicarbonate and are not discussed further. 4.1. Hanging-wall veins Samples 146-892D 8XCC 12–17 cm (hereafter referred to as “12– 17A”), 146-892D 8XCC 20–25 cm (“20–25”), and 146-892D 8XCC Paleo (“8P”) were all collected at depths from 66–67 mbsf. The hanging-wall samples have the lightest isotopic mean of the three structural zones (−14.4‰). Carbonates occur as vein fragments, and cements or veins in a mud matrix. Sample “12–17A” contains numerous veins of blocky calcite up to 2 mm across, all crosscutting a mud matrix that includes carbonate vein fragments indicating faultrelated deformation, and rounded siliciclastic grains (Fig. A1). This sample has the largest range of isotopic values of all those analyzed (19.6‰). Stacked euhedral terminations of calcite crystals indicate the veins grew into open, fluid-filled space. There are two sets of crosscutting veins with variable textural and chemical characteristics. The older set is more numerous, less continuous and of variable width; the average oxygen isotopic value is − 17.1‰ (n = 14; transects 2 and 5, Figs. 3 and A1). In contrast the younger set, consisting of one vein cut by a small fault, has a significantly higher

Table 1 General characteristics of isotopic data for samples. Sample

Isotopic range (‰)

Mean (‰)

1σ error (‰)

Modea

892A 18X1 60–63 892A 18X1 60–63 F3-F5 892D 8X CC 12–17A 892D 8X CC 20–25 892D 8X CC paleo F1F2 892D 10X 4/5b 892D 10X 4/5 892D 10X5 98–100A 892D 10X5 98–100C 892D 10X5 98–100C

11.1 6.5 19.6 9.7 3.0 17.3 11.8 7.0 9.9 15.2

− 1.7 − 7.8 − 12.5 − 17.8 − 13.3 − 10.0 − 7.1 − 10.0 − 8.0 − 10.3

1.2 0.2 1.3 0.4 0.7 1.8 1.2 0.7 1.1 0.4

he sc mc he sc mc mc le sc he sc mc sc mc

a SIMS analytical modes: he — only ions with energies between 300 and 350 electron volts collected; le — ions with energies from 0–350 electron volts collected; sc — peakhopping mode with 16O collected in Faraday cup, 18O collected with electron multiplier; mc — all ions collected simultaneously with multicollector. b Sample number reflects that these were derived from a section of core extruded onto the deck by natural gas pressure after cutting core liner between core Sections 4 and 5.

Fig. 2. All oxygen isotope data for Site 892. Plot is δ18O (PDB) vs. the sequence number from the SIMS analysis session. Hanging wall samples are from 68 mbsf, NHRT samples are from the thrust zone at 106 mbsf (“mc” refers to multicollector data, “sc” single collector data), and footwall samples are from 144 mbsf. Lines represent mean values for each depth (hanging wall block is long dash; thrust is short dash; footwall block is dot-dash) and total data set (solid).

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−17.8‰ (n = 15; Table 1). Because of the brecciation of this specimen it is difficult to decipher crosscutting relationships. There are no obvious systematic differences between populations of isotopic data. Vein fragments, clast cements, and clast veins average − 16.8‰, −16.9‰, and −16.5‰, respectively. Sample “8P” consists of two fragments from the same horizon. Nearly all of the isotopic analyses in fragment 1 are in a 3-mm-wide calcite vein with blocky texture, which is bounded on one side by silty mud matrix with smaller veins. The isotope values average − 12.8‰ (n = 9) and show a small range (2.2‰). Fragment 2 contains a small vein fragment with curved fibers, in a cemented silty matrix. An average of three analyses is slightly lower (−14.8‰) than fragment 1, but the range of values overlap within error. 4.2. Thrust fault veins

Fig. 3. Oxygen isotope data from sample “12–17A” in the hanging wall block of the NHRT. For this and subsequent plots the sample numbers are shown in upper right of each diagram, along with 2-sigma errors. Variations in two-sigma errors for different data sets due to differences in instrument acquisition parameters and characteristics of samples.

average isotope value of −9.8‰ (n = 14; transect 1), and a larger range of isotopic values than the older set. The overall trend from early, lower isotope values to later, higher values is a pattern repeated in other samples. Sample “20–25” contains mainly rounded clasts of carbonate veins and siliciclastic detritus, including two large cemented and veined silty muds. The presence of very well-rounded silicate grains and vein fragments floating in a clay matrix is evidence for milling in a fault zone. Oxygen values from “20–25” are the lowest, with an average of

Several vein-bearing samples were recovered from the NHRT between 105 and 107 mbsf (892B 10X 4/5 [“4/5”]; 892D 10X5, 98– 100A [“98–100A”]; 892D 10X5, 98–100C [“98–100C”]). All of these samples consist of poorly consolidated muds to silty muds, crosscut by calcite veins composing more than 50% of each sample. Thrust fault veins have relatively light average values of −9.5‰. Most cluster in a range from −14‰ to − 5‰. Sample “4/5” contains numerous veins of variable orientation and size (Fig. 4). Smaller (b0.2 mm width), older veins are crosscut by younger veins that range up to 1.2 mm wide. Crystal habits vary from blocky to fibrous blocky (dominant in large veins). Evidence for antitaxial growth includes medial lines, fibers that are not optically continuous across the medial line, and fiber growth on one vein edge at the interface with the matrix. Calcite fibers widen in their growth directions, suggesting growth competition into open space as favorably oriented fibers grew fastest toward the outer edges of veins. Growth of antitaxial veins is generally asymmetrical.

Fig. 4. Oxygen isotope vein transects from sample “4/5”. Reflected light photomicrographs, area of inset from rectangle in Fig. A2. Black circles are locations of SIMS analysis at approximate actual size. Edge of calcite standard visible at bottom of photo. Scale bar ∼1 mm.

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Oxygen isotope analyses from early (n = 4) and late (n = 56) veins in “4/5” have similar averages (Table 1). Younger veins have ranges in values up to 17.3‰ (Fig. 5). However, with the exception of a few high values along vein edges, the variation across individual veins is typi-

cally less than 5‰. Two adjacent transects show a similar positive spike in δ18O (transects 1c and 1d in Fig. 5A). Systematic variations in isotopic signatures can extend parallel to vein walls, and evidence of isotopic excursions is not limited to single points. Sample 892D 10X5, 98–100 includes three large fragments, two of which were characterized for this study. Sample “98–100A” contains greater than 90% carbonate. It contains one 3 mm wide vein, with a curved, fibrous habit and abundant inclusions (Fig. A3). Syntaxial and unidirectional growth is indicated by optical continuity of fibers across the vein. There is a slight increase in fiber width at the youngest edge, suggesting growth competition occurred during the last stage of vein formation. There are three principal isotopic transects, which have strikingly similar values and ranges compared to the larger variations found in other samples (Fig. 5B). Transects 1, 2, and 3 have similar δ18O ranges (− 11 to −8‰; − 13 to −7‰; −13 to −9‰, respectively) and δ18O means (− 9.5‰; −10.1‰; − 10.2‰, respectively). Based on the textural evidence for growth direction, there is an age progression of isotopes from lower to higher values. Sample “98–100C” preserves many crosscutting relationships and suggests three main episodes of growth (Fig. 6A). Overall the pattern is from numerous, small, dispersed, and irregular veins to larger, fewer, and more regular veins, suggestive of increasing material cohesion during vein evolution. The oldest veins are b0.1 mm in width and curviplanar. They occur in two subperpendicular orientations, although the orientation parallel to the middle-aged veins (see below) dominates. One larger vein (2 mm) appears to have formed at the end of this growth stage. This vein has a blocky fibrous habit, grew unidirectionally, and has euhedral terminations at its youngest side, indicating it grew into an open, fluid-filled fracture. The opposite and oldest edge shows inclusion bands suggestive of crack-seal behavior during early vein growth. Oxygen isotopic analyses from these small, early veins (n = 15) average δ18O = −9.1‰ (Fig. 5C). The middle episode of vein growth in “98–100C” is preserved in one large vein. It is subparallel to the dominant set of early veins described above, but is larger (1.5 to 3.0 mm) and more planar. This middle-aged vein contains mainly blocky crystals and is brecciated at one end. There is one major isotopic transect across this vein (t1 in Figs. 5C and 6A). The average of the analyses (n = 12) is −7.4‰. The range in values along transect 1 is between −5.5‰ and − 13.4‰ (Fig. 6B). Overall this vein has a higher mean value than the older, smaller veins. The largest and youngest vein in “98–100C” is perpendicular to the vein of middle age, and has planar walls suggesting cohesive wall rock material (Fig. 6A). Crystal textures are blocky to blocky fibrous. At one end of the sample the vein contains inclusion bands at both edges, as well as large inclusions of wall rock with jigsaw textures (Fig. 6A). The details of the inclusion bands suggest several episodes of crack-seal growth during the evolution of the vein and precipitation of individual calcite films 25 to 50 μm wide. There are two isotopic transects in this youngest vein. For transect 2 (Fig. 6A, C) the value of the mean is −8.5‰ (n = 18), and the range is 5‰ (Fig. 5C). For transect 3 (Fig. 6A) the mean is −9.3‰ (n = 14) and the range is greater than 14‰ (Fig. 5C). In contrast to most samples in this study, the isotopic progression of carbonates from oldest to youngest in “98–100C” is variable. 4.3. Footwall veins

Fig. 5. Oxygen isotope vein transects in samples from NHRT. A. Data from veins of “4/5”. Transects 1c and 1d show similar location for major positive excursion. Transect locations are given in Fig. 3. B. Data from vein in “98–100A” show a narrow range of values at three different locations, with little variation along individual transects. 2-sigma error bars are about the same size as symbols. C. Isotopic data for vein transects located in Fig. 6A. “mc” = data collected by multicollector method; “sc” = data collected by single collector method. 2-sigma errors for mc method are smaller than symbol sizes.

These samples consist mostly of relatively small fragments that have similar textures in that they are dominated by carbonate vein or cement material, composing over 80% of each fragment. Two samples were analyzed from 145 mbsf, sample 146-892A 18X1, 60–63 cm (“60–63”) and sample 146-892A 18X1, 60–63 F3–F5 (“F3F5”). The latter consists of three small veined rock fragments of which two were analyzed. Crystals in the samples are blocky and variable in size. Some veins display a series of euhedrally terminated crystals lining wall

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Fig. 6. Photomicrographs and oxygen isotope vein transects from sample “98–100C”. A. Transmitted light mosaic of entire sample. Note three sets of crosscutting veins—small veins in matrix and one subparallel large vein are oldest; large horizontal vein is middle-aged; youngest vertical vein. Most areas exhibit blocky or blocky fibrous textures. Arrows show direction of vein growth for one old vein and part of youngest vein. Lower part of large vein contains inclusion bands parallel to vein walls consistent with formation by crack-seal mechanisms. Black and white circles are locations of oxygen analyses approximately 5 times SIMS spot size. “e” shows location of euhedral crystal facets, “i” = inclusion bands, “br” = brecciation. “t1”, “t2” and “t3” are locations of vein transects discussed in text. Clear crystal in upper right is calcite standard. Scale bar ∼5 mm. B. Transect 1, area shown in box in A. Size of white circles is ∼60 μm, approximately twice as large as actual spot size. Width of view 2.3 mm. C. Transect 2, area shown in box in A. Size of white circles same as in B. Width of view ∼5 mm.

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rock fragments and surrounding grains, indicating calcite precipitation occurred in open cavities within a breccia. The footwall block samples have the highest isotope average (−5.2‰; n = 42). Sample “60–63” has the highest δ18O average (−1.8‰) of all samples analyzed (Table 1). It exhibits the familiar trend of lighter to heavier values, from an older matrix cement value of − 5.5‰, to a younger, crosscutting, large vein with a mean value of +0.2‰ (Fig. 7). Of the two fragments analyzed from sample “F3F5”, the mean values (−7.7‰ and −8.1‰) are not distinguishable within analytical error. Two of the three veins analyzed in one of the fragments also follow the progression from lighter to heavier values in older (vein 3, mean = −9.8, n = 5) to younger (vein 2, mean = −6.1‰, n = 3) carbonates. A comparison between SIMS data and previous results from acid digestion and IRMS analysis shows no major inconsistencies. Four samples in this study have acid-digestion, IRMS data from the same horizons (Table A1). For example, conventional analyses of veins from sample “98–100C” (“892D 10X 5, 98–100 cm” in Sample and Kopf (1995) have a mean δ18OPDB value of −8‰ (n = 5), the same within error as the mean δ18OPDB value (− 8.5‰) analyzed by SIMS. Even some of the lowest values from “12–17A” and “20–25” are within a range determined previously. Only sample “20–25” has a range of SIMS values that lies outside of the values determined by conventional means. The samples collected from the depth interval of “20–25” consisted of several veined fragments. The fragment analyzed previously was no longer available for analysis, but oxygen isotopic values from a nearby sample (Kopf et al., 1995) overlap with the range of values determined by SIMS in this study. 5. Discussion The discussion will first address the topic of vein formation, then use the isotopic data to characterize processes related to fluid properties and transport, and conclude with addressing the implications of triggers of non-steady-state flow along the margin. 5.1. Vein formation Based on core observations it is clear that the veins of this study are associated with faults (described in Section 2). Faults can reduce or enhance sediment permeability (Moore, 1989; Evans et al., 1997; Eichhubl and Behl, 1998; Fisher and Knipe, 2001; Saffer et al., 2000). Generally permeability enhancement in fault zones is anisotropic, greater parallel to the fault, and even as fractures fill with authigenic material, faults still may have higher permeability than surrounding unfaulted rocks (Aydin, 2000). The veins either formed along faultparallel fractures or oblique hydrofractures. Many veins exhibit brecciation textures and small offsets that are consistent with shear deformation. The available crosscutting relations indicate that typically smaller, more dispersed veins of highly variable orientations formed first and were followed by fewer, more systematically oriented large veins. This pattern suggests formation during a history of increasing consolidation and cohesion soon after the sediments were deposited, as cementation and compaction related to tectonic burial increased material strength. Similar patterns of vein evolution have been suggested in other deformed turbidites (Beach, 1977; Clark et al., 1995). In the latter study, stochastic models indicate that vein growth rate is proportional to vein length. This can result in runaway growth of large veins presumably at the expense of small veins such as is observed in the samples analyzed here. Vein textures provide constraints on the mechanisms of fracture formation and filling. The dominant vein crystal textures in Cascadia samples, including euhedral terminations of calcite in these veins, is consistent with crystal growth into open fractures over part of the vein history, which requires a small least principal effective stress perpendicular to vein walls during their formation (Fig. 8). Fibrous,

inclusion-rich veins such as observed here are commonly described as crack-seal texture (Fig. 8; Ramsay, 1980). This texture has been ascribed to increments of vein growth related to periods of fluid pressure increases and fracture opening. It has been suggested recently that these textures are related to crystal growth pressures rather than fracture opening (Means and Li, 2001; Hilgers and Urai, 2005), but the vein textures from Site 892 indicate growth into open fractures by the crack-seal mechanism. Although crack-seal deformation can take place without far-traveled fluids or chemical disequilibrium between veins and host rocks (Fisher et al., 1995), in Cascadia open fractures apparently allowed for far-traveled fluids to enter the system without attaining chemical equilibration with the host rocks. Based on experimental studies, it appears that calcite vein growth is best promoted by flow of large volumes of fluid only slightly supersaturated with calcite through the fracture with estimated fluid– rock ratios in the range of 105 to 106 (Lee et al., 1996; Lee and Morse, 1999; Hilgers et al., 2004). Open fractures during vein growth would facilitate the necessary high flow rates and large volumes of fluids. The specific time required for a 2-mm-wide vein to form depends largely on the degree of saturation of calcium carbonate species in the precipitating pore fluids, and the rate of fluid flow through the fracture. Considering ideal carbonate solubilities and the likely range of flow rates along the fault (Davis et al., 1995; Sample, 1996), it is probable that vein precipitation occurred over time scales of at least 104 to 105 years (discussed below). Crosscutting relations with welldated strata and geomorphic expression of the NHRT on the seafloor suggest the NHRT is a young, active structure (Moore et al., 1994; Fourtanier and Caulet, 1995), and the veins may have formed in the past 300–500 ka based on crosscutting relations along the fault and uplift history. 5.2. Isotopic constraints on fluids The observed isotopic record in the veins requires periodic, substantial changes in the chemical and/or thermal properties of the fluid in the past. The data have several important characteristics. The samples are dominated by low δ18O values, and although similar low values have been obtained previously from the Oregon margin (Ritger et al., 1987; Kopf et al., 1995; Sample and Kopf, 1995; Sample and Reid, 1998), the prevalence of 18O depletions from SIMS analyses is striking. Given that the mean value for all veins is −9.9‰, and considering that the measured downhole temperatures and fluid chemistry at Site 892 (T ≤ 14 °C, δ18O of pore water N−0.5‰), less than ten percent of the isotopic analyses are within 3‰ of equilibrium with the modern pore waters. Other important characteristics are that most samples show a trend of increasing δ18O values from older to younger carbonates, and ranges of isotopic values vary from narrow to very wide (up to 19‰). These characteristics are unprecedented in carbonates from modern subduction zones, and are rarely observed in carbonate minerals from any setting (cf., Satish-Kumar et al., 1998). The delicate nature of the poorly consolidated, vein-bearing material requires that the carbonate must have crystallized in place as it could not have survived significant sedimentary transport down the continental slope. The isotopic characteristics must be explained by processes related to local diagenesis or advection of distant fluids into the site. Lateral and/or vertical advection is the most likely process to explain the large 18O depletions. There is evidence for recent flow along the main thrust horizon from seismic reflection data and long-term borehole observations (Moore et al., 1994; Davis et al., 1995). All three horizons sampled for this study have structural or sedimentological characteristics that could promote high rates of flow. Fluid advection models at Site 892 must thus consider potential variations in fluid composition and/or temperature to explain low δ18O values. Episodic flow events along vertical or steeply oriented conduits are a likely mechanism to trigger influx of appropriate fluids. Rapid

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Fig. 7. Isotopic data for footwall samples “60–63” and “F3F5”. Variations between veins is large, whereas variations within veins is relatively small. 2-sigma errors for “F3F5” analyses are smaller than symbol sizes.

adiabatic rise of fluids would temporarily increase temperatures along the fault zone, and because oxygen isotopic fractionation between water and calcite decreases with temperature, calcites precipitated under this circumstance would be isotopically light (Sample et al.,

307

1993). The actual isotopic values for calcites depend on both temperature of precipitation and the δ18O value of the fluid. Since neither of these is known independently for the calcite veins, a range of values must be considered (Fig. 9). To explain more than half of the SIMS values, fluid compositions with today's downhole range would require precipitation temperatures of just over 100 °C. Using the range of thermal gradients recognized at Site 892, fluids would have to travel a minimum vertical distance of 1.5–2 km, or a lateral distance of 4–6 km if along a thrust-fault conduit dipping 30° landward (Fig. 10). This places the fluid origin within the accretionary wedge 5–6 km above the décollement (Tréhu et al., 2006). To explain the full range of carbonate isotope data would require a fluid temperature during precipitation as high as 150 °C (Fig. 9A). These estimates are similar to temperature and depth estimates for sources of thermogenic hydrocarbons in southern Hydrate Ridge (Claypool et al., 2006; Tréhu et al., 2006). If on the other hand, starting fluid compositions were lighter in the past, due to lateral influx of continental meteoric waters or low-temperature water-rock interactions, the required temperature would be less. If porewater with δ18OSMOW = −12‰ is considered (the extreme case of isotopically pure meteoric water influx from the Coast Ranges; IAEA, 2009), then a temperature of 55 °C and a minimum vertical migration distance of 1 km is required. In order for the thermal anomaly to be preserved in the precipitates, migration and mineral formation must occur within months to perhaps a few years (Sample, 1996).

Fig. 8. Inferred relationships between vein growth and measured isotopic values along vein transects. A. Schematic vein growth for typical vein with blocky and fibrous blocky textures (e.g., “98–100C”), evolving from crack-seal to open fractures (cf. Fisher et al., 1995). Growth direction is unidirectional (left to right in this case) with crystal size increasing during growth. Locally favorably oriented calcite crystals grow fastest with euhedral faces growing into open, fluid-filled void space (light gray) between wall rock (black) and calcite crystals (white). In places inclusion bands (“ib”) parallel to crystal faces are preserved. In this model each vein-opening event leads to precipitation of a new calcite film on crystal faces (thin, medium gray band). Small ovals represent schematic ion probe spots that are completely within a new growth band (black ovals) and straddle two bands (white ovals) and hence sample two crack-seal events. B. Possible evolution of vein isotopes in relation to fluid pulse history. In this model each vein-cracking event is related to a spiked pulse of fluids with elevated temperatures, which gradually conductively cool (dashed line) to current temperature conditions of ∼10 °C. Calcite precipitation occurs during initial phase of cooling when fluid is calcite-saturated (solid portions of curve), yielding a possible range of isotopic oxygen isotopic compositions for each film of calcite formed. C. Each crystal formed in the manner described in B would represent a range of isotopic compositions of each sealing event (solid line segments). Each calcite film (between inclusions bands in A) would preserve a range in isotopic compositions that would depend on the duration of cooling history during which vein calcite formed. An analysis from within calcite formed during one sealing event (e.g., “X” in A) would represent an average of that range. An analysis from a spot that straddles two fill events (such as “Y” in A) would thus represent a value somewhere between the two values actually characteristic of the two seal events. The dashed line connects mean values of δ18O for each vein growth event across a vein segment such as those presented in Figure B.

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There is additional corroborating evidence for recent and rapid migration of fluids into Site 892 sediments. A variety of higher hydrocarbons (C2+) requiring a thermogenic origin are found at several horizons and in seafloor gas hydrates (Westbrook et al., 1994; Milkov et al., 2005). As the sediments co-existing with these hydrocarbons are thermally immature, the C2+ hydrocarbons must have migrated from depths greater than 1 km (Whiticar et al., 1995). The low degree of biodegradation of the compounds indicates migration was rapid and recent. Other processes in the potential source region that can deplete 18O in pore fluids can also be considered. These include low-T water-rock interactions, open- or closed-system carbonate precipitation, formation of gas hydrates, or long-distance lateral transport of meteoric waters. The following discussion will show that these processes either had a minor impact on oxygen isotopic compositions or are highly improbable explanations for the observed isotopic characteristics. Low-temperature alteration of solids to clay minerals can drive the residual fluids toward lower values (e.g., (Albaréde, 2008)). At

Fig. 10. Schematic cross section of Oregon margin at latitude 44°40′. Site 892 is located on second ridge (Hydrate Ridge) east of deformation front. Three possible origins of 18 O-depleted isotopic signatures of carbonate are shown. Rapid vertical (≥ 2 km) or lateral flow (≥4 km) of warm fluids is required for a localized source. Lateral flow of meteoric waters for a far-traveled source would require 45 km of transport from the shelf edge, or 80 km from the current shoreline position. Localized flow would require high flow rates; meteoric flow would require a large hydraulic head and traversing of major landward-dipping to vertical structures. Modified from (Goldfinger et al., 1997).

Hydrate Ridge the δ18O — depth profile of current pore fluids does not show evidence of significant low-temperature fluid–rock alteration of host rocks, which should be expected given the initial clay-rich nature of the sediments, and the high thermal gradients. Precipitation of solid phases from pore fluids could enrich or deplete residual fluid in 18O depending on the conditions. These effects are commonly modeled as a Rayleigh distillation process. In the case of calcite precipitation driven by CO2 degassing such as is hypothesized for Cascadia carbonates, the reaction is: 2þ

Ca

−

þ 2HCO3 ¼ CaCO3 ↓ þ CO2 ↑ þ H2 O

ð1Þ

with calcite and carbon dioxide being removed from the system by crystallization and degassing, respectively. Considering the values of fractionation factors among pairs from Eq. (1) comprising HCO− 3 – − H2O, HCO− 3 – CaCO3, or HCO3 – CO2, calcite precipitation in an open system using a CO2 degassing model results in residual waters enriched in 18O and thus drives the δ18O values of later calcites to Fig. 9. Various models of pore fluid evolution. A. Calculated variations in δ18O values of calcites (δ18Occ) for different pore water δ18O compositions (δ18Opw) and precipitation temperatures, shown by dashed contours, temperatures in °C. For example, a calcite with measured δ18OPDB value of − 13‰ could be explained by precipitation at 20 °C from a pore water with δ18OSMOW = − 12.5‰ (point X), or at 90 °C from a pore water with δ18OSMOW = − 1.5‰ (point Y). To form carbonates representing greater than 50% of the isotopic compositions determined in this study (area on the plot in dark gray), as represented by the mean of all values (bold horizontal line at − 9.9‰) and one standard deviation (horizontal dashed lines) would require pore fluid temperatures during carbonate precipitation of over 100 °C. Ranges of current pore water temperatures and isotopic values at Site 892 are shown by stippled parallelogram in upper right of diagram. B. Rayleigh distillation model for calcite formation in NHRT assuming current pore fluid characteristics of temperature ∼10 °C and initial δ18OSMOW of pore water (δ18Oi) ∼−0.5‰. Solid curved line is trend of oxygen isotopic composition of residual pore waters as fraction of pore water remaining (f) reduces from 1.0 to 0.1. Calculated using calcite–water fractionation factor of Kim and O'Neil (1997). C. Effects on pore water related to hydrate formation. Horizontal axis is fraction of pore water remaining after hydrate formation. Vertical axis is final oxygen isotopic composition of calcite after hydrate formation and reflects the initial pore water value for a given degree of fractionation and different initial pore water oxygen isotope compositions. The range of assumed initial pore water compositions are shown as curved dashed lines from − 0.5‰ (modern pore water) to − 12‰ (lowest possible meteoric water). For example, at point A, a calcite with a δ18O value (PDB) of − 11.5‰ forms from pore fluid with a starting δ18O value (SMOW) of − 8‰, and a fraction of 0.7 of the initial pore water having been lost to form hydrate. To form carbonates representing greater than 50% of the isotopic compositions determined in this study (gray area on the plot) would require some large depletion of 18O in pore water due to hydrate formation, or large degrees of Rayleigh fractionation, or both. The lightest value measured (point B) would require starting pore water compositions of − 12‰, and crystallization from the last 10% of the pore water formation after hydrate formation in a closed system. Calculations assume temperature of 11.1 °C, the measured temperature at 106 mbsf, and a fractionation factor for water-hydrate formation of 1.004. This fractionation factor is probably at the high end, and the distillation effects on final carbonate composition will be smaller for smaller fractionation factors, requiring large fractions of fluid to be consumed for equivalent results.

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higher values (Mickler et al., 2006). A Rayleigh distillation process for calcite formation involving degassing cannot explain the light carbonate values if they were formed in equilibrium with pore waters having oxygen isotopic characteristics near seawater. If the system remains closed with respect to CO2 during calcite precipitation, i.e., as CO2 evolves it remains dissolved in the local pore fluid, then the residual pore fluids could become increasingly lighter during calcite crystallization, as governed by simple calcite–water fractionation factors. For current conditions in the NHRT (T ∼ 10 °C and δ18O ∼ − 1‰), a fraction of ∼0.3 of the pore fluids would have to be removed by calcite precipitation just to reach the average δ18O value of calcite in the thrust of −9.9‰ (Fig. 9). This process cannot explain the low oxygen isotopic values at the NHRT because of the degree of calcite crystallization and fluid distillation required, and the accompanying drop in pH related to increasing CO2 would inhibit further calcite crystallization. The formation of gas hydrates can also drive residual pore fluids to lighter values (Fig. 9C). At Site 892 predominantly methane hydrates are inferred to exist to depths of ∼74 mbsf. Since their formation is due to freezing of water and methane under appropriate temperature and pressure conditions (Kvenvolden, 1995), the relevant fractionation factor is probably close to that for the formation of water ice. The effect on the δ18O values of pore waters can be modeled using a Rayleigh distillation equation for hydrate formation: ð1000 + δÞ = ð1000 + δi Þ = ƒ

ðα−1Þ

ð2Þ

where δ is the evolving fluid δ18O value as hydrate formation proceeds, δi is the initial pore water δ18O value, ƒ is the fraction of fluid remaining at a given step in the process, and α in this case is the fractionation factor between hydrate and water. The fractionation factor for gas hydrate formation is not well constrained, but has been estimated between about 1.0027 and 1.0040 (Davidson et al., 1978; Kastner et al., 1998; Matsumoto and Borowski, 2000). Fig. 9C shows the evolution of the isotopic composition of pore fluids for different δi values using an extreme hydrate–water fractionation factor of 1.0040. Note that in order to explain most of the isotopic values of the carbonates found at Site 892, a combination of very light initial pore waters and high fractions of hydrate formation are required. This assumes that the system is open to influx of methane, but not to additional formation waters. This process may have affected samples in the hanging wall block where hydrate is found, where carbonates have some of the lowest δ18O values of any measured. But the thrust zone and footwall block are deeper than the zone of gas hydrate formation, and this process did not likely impact these samples unless hydrates were forming at much deeper levels in the past. In addition, the general trend within veins and among different vein populations within individual samples typically shows a trend in the opposite direction toward higher δ18O values. Meteoric fluids from the Oregon Coast Ranges or a previously exposed portion of the continental shelf are not a probable source of 18 O-depleted pore waters in continental-margin settings. Current values of meteoric water in the Oregon Coast Ranges are likely to range from −12‰ to −8‰ at present (IAEA, 2009). A mechanism for the transport of meteoric waters undiluted by heavier waters from the Coast Ranges (N80 km), or the nearer continental shelf (N45 km), is problematic given the numerous landward- and seaward-dipping low-permeability barriers such as mudstone and faults. Even if pure meteoric water had reached the position of Site 892 in the past, it could explain only those carbonate isotopic values with δ18O N −10.6‰, if the current maximum down hole temperature prevailed. In this analysis nearly half of the data still cannot be explained by advection of meteoric waters (Fig. 2). To produce 90% of the highest values would require a model combining an initial pore water δ18O = −12‰, and Rayleigh distillation removing a very large fraction (0.65) of the pore water into hydrate (point B in Fig. 9C).

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Rapid upward flow thus appears to be the explanation most consistent with observations of isotopic data and conditions that would lead to carbonate precipitation. Fluid sourced from only a few km distance can flow fast enough to avoid the significant isotopic exchange, while still moving at geologically acceptable rates. The pressure drop and associated CO2 degassing can effectively facilitate carbonate precipitation in the short time interval of each fluid pulse. Simple conductive cooling models for a deep fluid pulse suggest that crystallization must occur within months to a few years of the event to preserve the light oxygen isotope signatures measured in the veins (Sample, 1996). If precipitation continues after the initial fluid pulse, then vein transects would show repeated systematic increases in δ18O values along an individual vein transect due to temperature effects. Repeated pulse-precipitation cycles might yield a sawtooth isotopic pattern in the vein data (Fig. 8B). Many samples do show an increase in δ18O values from older to younger vein populations, but the time scales of these trends are likely much longer than time scales of individual fluid pulse and precipitation events. A clear sawtooth pattern is not observed in individual vein transects. The observed isotopic variations are more likely to represent variations in the temperatures of fluid sources and rate of flow from source to precipitation sites. The fact that many veins show relatively constant δ18O values (within analytical error) suggests a similar fluid source depth and flow process during the history of the vein (e.g., “98– 100A”), although some large variations in single veins indicate that variations in source depth may occur. The band width of individual vein-fill laminae (25–50 μm) in the crack-seal veins indicates that each fluid pulse led to only small amounts of calcite precipitation, which is consistent with modeled processes of calcite vein formation (Lee and Morse, 1999). Such a model requires open fractures are available to allow rapid flow and vein fill along a significant length of the fracture. The prevalence of blocky and blocky fibrous textures is consistent with filling of open fractures by rapid flow of fluids with low carbonate saturation. The fault-valve model (Sibson, 1989), where fluid episodically overcomes barriers to flow and moves into a fracture, is a viable mechanism for vein formation at Site 892. Given the low dip angle of the NHRT (b15°; cf. Shipboard Scientific Party, 1994), hydrofractures associated with the fault will likely have inclinations greater than zero to accommodate a vertical component of flow. Assuming certain fluid compositions, temperatures, source depths, and fracture geometries, some general constraints can be placed on the rates of flow and time scales required to form veins at Hydrate Ridge. For a fluid sourced at 3 km depth (equivalent to 150 °C to 200 °C for the range of thermal gradients), the fluid must reach the NHRT and precipitate calcite in a minimum time of b0.25 to 1 yr to maintain required temperatures (Sample, 1996). If the starting fluid composition were reduced to δ18O = −5‰ these times increase to 1 to 2.25 yr. That implies a minimum flow rate 1000 to 440 m/yr, or 11 to 5 cm/h, for simple upward flow of pore waters. But it is more likely that fluids derived from a volume of sediment at depth are focused into a smaller volume of fractures along the fault zone. This would require higher rates of flow. To model this accurately would require better constraints on the density of fractures and veins in the fault zone, the carbonate saturation states of deep fluids, and the consolidation state of sediments at 3 km depth. A rough calculation can be made for vein formation in a fracture that is 50 cm long × 5 cm wide × 0.4 cm thick, a volume of 100 cm3. Estimates of fluid/mineral volume ratios for calcite vein filling are 105 to 106 (cf., Lee et al., 1996). Assuming a ratio of 105 would require 104 L of water, equivalent to an instantaneous porosity reduction from a seismic event of 0.01 for example from a sediment volume of 1000 m3. To pass this fluid through the fracture would require flow rates of 570 to 250 cm/h over a period of 1 and 2.25 yr, respectively. These are high rates relative to typical expected values for thrust systems (Ge and Garven, 1994; Machel et al., 1996; Saffer and Screaton, 2003). But since vein textures indicate the veins were

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filled episodically, we can calculate different rates if we assume each fluid pulse precipitated a 25 μm layer of calcite. In that case and using the example described above, the 4 mm thick vein would require 1600 separate pulses of fluid. Fluid and sediment volumes for each pulse are reduced accordingly, and fluid flow rates also reduce to 0.36 to 0.16 cm/h over a period of 1 and 2.25 yr, respectively. These are more reasonable geologic rates, but are 1–2 orders of magnitude lower than those suggested by vein precipitation experiments by Lee and Morse (1999). If a higher fluid/mineral ratio of 106 is considered, each of the above values increases by an order of magnitude, still within constraints from crystal growth models and estimates from thrust belts. The actual flow rates may lie somewhere in between these values, as suggested by the constraints from the modeling of oxygen isotope data. There are multiple possible driving mechanisms for fault valve behavior at this margin. Episodic flow could occur during periodic releases of gas-rich from organic-rich sediments at depths great enough for thermogenic maturation of hydrocarbons. In the Gulf of Mexico there is evidence that fluids have migrated at minimum flow rates of 2 cm/h (200 m/yr) up-dip along a normal fault (Haney et al., 2005), although the rate is not well constrained and could be greater. It is feasible that free gas accumulates in pore fluids beneath Hydrate Ridge over finite time intervals and is periodically released as pressure overcomes stress across fractures in the fault zone. Re-sealing of the fault zone may be enhanced by the formation of gas hydrate in pore space and fractures. Physical properties of sediments from southern Hydrate Ridge imply gas pressures along the BSR may exceed lithostatic pressure and thus provide a mechanism for continuous, rapid flow of fluids beneath the base of the gas hydrate stability zone (Tréhu et al., 2006). This may be assisted by periodic seismic activity. High flow rates might also be triggered by seismic events alone. Cement formation and crack-seal veins as records of seismicallyinduced fluid flow has been hypothesized in many settings (Sibson, 1989; Wood and Boles, 1991; Barnett et al., 1996; Ohlmacher and Aydin, 1997; Sample and Reid, 1998; de Ronde et al., 2001; Wang et al., 2004; Barker et al., 2006). In Cascadia significant earthquakes would cause compaction at depth and drive pore water to the zone of vein formation. If each episode of vein filling were assumed to have been caused by a large earthquake event, and the recurrence interval of large earthquakes were known, this information could be used to estimate the length of time required for a vein to be filled. The average earthquake recurrence interval based on evidence from the entire Cascadia continental margin is ∼ 500 yrs (Goldfinger et al., 2008). For the 1600 vein-fill events this is equivalent to a total time of ∼800 ka. This time is similar to the estimates for vein formation from other studies (Fisher and Brantley, 1992; Morse and Mackenzie, 1993; Lee and Morse, 1999). These rough estimates for vein longevity are consistent with estimates for the age of the fault based on structural constraints described previously (Pleistocene or younger). The veins analyzed in this study may contain a record of seismic activity that extends well back into the Pleistocene and may be complementary to the turbidite record (Goldfinger et al., 2003). The isotopic signatures of veins and mass balance considerations are consistent with a crack-seal origin driven by periodic events. The conditions of these events are not adequately captured by recent downhole measurements at Site 892. Each periodic fluid flow event is likely to precipitate a thin film of carbonate in the fracture that reflects ephemeral chemical and temperature conditions. The record of conditions recorded by the veins is incomplete because as pressure drops in the fracture and precipitation occurs, the fluid will become too undersaturated with respect to calcite for precipitation to continue. Thus the calcites will only retain the oxygen isotopic signature from the peak of the thermal anomaly (Fig. 8C). The relatively small variations in oxygen isotopes within many individual vein transects suggest the thermal anomaly remains relatively constant for most influx episodes. This suggests that the growth of

an individual vein is tapping a fluid source from a similar depth. In principle with more detailed isotopic transects evidence for a more varied range of source depths might emerge. Alternatively a similar source depth throughout the vein growth history may reflect characteristic seismogenic or physical properties in the zone of the fluid source. It should be noted that fluid temperature anomalies from a borehole observatory at Site 892 show non-steady-state flow may occur at a time scale similar to the recurrence interval of large earthquake events (Adams, 1990; Davis et al., 1995; Goldfinger et al., 2008). 6. Conclusions Calcite veins from poorly consolidated mudstones and siltstones in a thrust fault system yield information about their conditions of formation. Crystal textures are consistent with models of a crack-seal mechanism of growth and fluid flow into open fractures. These fractures may have allowed rapid fluid migration into the fault and the hanging wall and footwall blocks, which may explain the low δ18O values recorded in the veins. Values ranging from + 3‰ to −22‰ and averaging ∼−10‰ are the lightest ever recorded from a young accretionary prism. The carbonate oxygen isotopes are far out of equilibrium with current borehole temperatures and fluid compositions. It is unlikely that local, closed-system processes such as hydrate formation or carbonate crystallization could have led to such depletions in 18O. Rather the calcite isotopic compositions are better explained by non-steady-state flow of fluids from several km away from the site. A likely scenario is that warm fluids derived from 2– 3 km depth have rapidly migrated upward in a fault-valve model, driven either by gas buoyancy or seismic activity. This is consistent with other data supporting rapid influx of thermogenic hydrocarbons along the fault. Based on mass balance considerations it is likely that individual veins may record hundreds to thousands of fluid pulses over ten to hundreds of thousands of years, and thus record a relative long history of fluid migration along an active thrust fault. Our data cannot preclude influx of isotopically light fluids into the fault, and while not likely, ultimately may be evaluated using new techniques of stable isotope analysis (Eiler, 2007). Acknowledgments This research was supported by NSF grant OCE-9633330 and EAR0418975 to JCS. The UCLA ion microprobe laboratory is partially supported by grants from the NSF Instrumentation and Facilities Program and the Keck Foundation. Chris Coath, Kevin McKeegan, and Laurie Leshin assisted in SIMS analysis. The Ecole Normale Supérieure de Lyon provided generous sabbatical support during writing of the manuscript. Appendix A. Supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2010.02.044. References Adams, J., 1990. Paleoseismicity of the Cascadia subduction zone: evidence from turbidites off the Oregon-Washington margin. Tectonics 9, 569–583. Albaréde, F., 2008. Geochemistry: An Introduction, second ed. Cambridge University Press, Cambridge, UK. Aydin, A., 2000. Fractures, faults, and hydrocarbon entrapment, migration and flow; Thematic set on hydrocarbon migration. Marine and Petroleum Geology 17, 797–814. Barker, S.L.L., Cox, S.F., Eggins, S.M., Gagan, M.K., 2006. Microchemical evidence for episodic growth of antitaxial veins during fracture-controlled fluid flow. Earth and Planetary Science Letters 250, 331–344. Barnett, D.E., Bowman, J.R., Bromley, C., Cady, C., 1996. Kinetically limited isotope exchange in a shallow level normal fault, Mineral Mountains, Utah. Journal of Geophysical Research 101, 673–685.

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