Isotopic evidence (B, C, O) of deep £uid processes in faultrocks from the active Woodlark Basin detachment zone

Achim Kopf a;�, Jan H. Behrmann b, Annette Deyhle a, Sybille Roller b,Helmut Erlenkeuser c

a Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0244, USAb Geologisches Institut, Universita«t Freiburg, AlbertstraMe 23-B, 79104 Freiburg, Germany

c Institut fu«r Reine und Angewandte Kernphysik, Universita«t Kiel, OlshausenstraMe 40^60, 24118 Kiel, Germany

Received 16 April 2002; received in revised form 26 December 2002; accepted 31 December 2002

Abstract

We report results from boron, carbon and oxygen stable isotope analyses of faulted and veined rocks recovered byscientific ocean drilling during ODP Leg 180 in the western Woodlark Basin, off Papua New Guinea. In this area ofactive continental extension, crustal break-up and incipient seafloor spreading, a shallow-dipping, seismically activedetachment fault accommodates strain, defining a zone of mylonites and cataclasites, vein formation and fluidinfiltration. Syntectonic microstructures and vein-fill mineralogy suggest frictional heating during slip duringextension and exhumation of Moresby Seamount. Low carbon and oxygen isotope ratios of calcite veins indicateprecipitation from hydrothermal fluids (N13CPDB down to 317x ; N

18OPDB down to 322x) formed by bothdehydration and decarbonation. Boron contents are low (6 7 ppm), indicating high-grade metamorphic source rockfor the fluids. Some of the N11B signatures (17^35x ; parent solutions to calcite vein fills) are low when compared todeep-seated waters in other tectonic environments, likely reflecting preferential loss of 11B during low-grademetamorphism at depth. Pervasive devolatilization and flux of CO2-rich fluids are evident from similar vein cementgeochemistry in the detachment fault zone and splays further updip. Multiple rupture-and-healing history of the veinssuggests that precipitation may be an important player in fluid pressure evolution and, hence, seismogenic faultmovement.; 2003 Elsevier Science B.V. All rights reserved.

Keywords: detachment faulting; Woodlark Basin; stable isotopes; boron; veins; seismic slip

1. Introduction

Faults and shear zones are major £uid conduitsin crustal basement and sedimentary rocks [1,2].

Fluid migration and elevated £uid pressures havepreviously been considered critical for the activityand the mechanical behavior of shallow-dippingfaults, like de¤collements [3] and extensional de-tachments [4]. Fluids alter the faulted host rock,and serpentinization is believed to reduce frictionand to cause fault zone weakening [5^7]. Fluidsare generated during dehydration and decarbon-ation reactions, and usually show characteristic

0012-821X / 03 / $ ^ see front matter ; 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0012-821X(03)00016-5

* Corresponding author. Tel. : +1-858-822-3362;Fax: +1-858-822-3310.E-mail address: akopf@ucsd.edu (A. Kopf).

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carbon and oxygen isotope signatures [8]. Miner-alization of brittle fractures by precipititationfrom such £uids is abundant in shear zones [9],and geochemical C, O and B signatures of precip-itates have been demonstrated to be indicative ofthe origin and £ow of paleo£uids [10].One of the principal goals of Ocean Drilling

Program (ODP) Leg 180 in the western WoodlarkBasin (Fig. 1) east of Papua New Guinea was tosample an active extensional detachment fault sys-tem by drilling to increase the understanding ofthe nature and evolution of low-angle normalfaulting in a presently active tectonic environment[11^13]. Moresby Seamount de¢nes the upliftedfootwall to the detachment westward of the tipof the propagating Woodlark spreading center(Fig. 2).We here report C, O and B stable isotope sig-

natures of fracture cements, most likely related tosyntectonic £uids, from the Woodlark detach-ment, a nearby antithetic fault, as well as fromtwo fault splays in its tectonic hanging wall.This is the ¢rst B isotopic study on precipitatesof metamorphic £uids. It aims to broaden theunderstanding how B can be used as a proxyfor deep-seated processes, particularly since ithas been successfully applied to characterizelow-temperature geochemical processes in subduc-tion zones ([10] and references therein). Interpre-tation of the geochemistry of the paleo£uids fromthe isotopic signature of the fracture cements canlead to a better understanding of the interactionof deep-sourced £uids and fault rocks. Vein ce-mentation patterns can provide insights into mul-tiple phases of rupture and healing, and identify£uid transport from depth and hydrothermal pre-cipitation as important players in ductile^brittlefaulting. Because it is an active structure, theWoodlark detachment system is an ideal naturallaboratory to study these processes.

2. Geological setting

Transition between continental rifting in thewest and oceanic spreading in the east character-izes the present structure of the Woodlark Basin(Fig. 1). Oceanic spreading has propagated west-

ward since 6 Ma (oldest sea£oor magnetization;[14]) with a segmented, roughly E^W-oriented,500 km long spreading axis. Maximum width ofoceanic crust is 350 km in the very east of thebasin [15]. Continental rifting precedes oceanicspreading in the western part, and divides thestructurally asymmetric Papuan basement intothe Woodlark and Pocklington Rises (Fig. 1;e.g. [16]). The total amount of extension here isestimated to be 200 km [15], with the western tipof the spreading center currently approaching theprominent Moresby Seamount sea£oor high (Fig.1). Seismic cross-sections and their interpretationsreveal a strongly asymmetric, E^W-oriented gra-ben, with intense block faulting bordering severaldiscrete basins [17].At 151.35‡E, Moresby Seamount is situated at

the central graben axis between the Woodlark andPocklington Rises (Figs. 1 and 2). The feature hasbeen interpreted as an unroofed footwall block oflower Papuan basement beneath a north-dippinglow-angle detachment fault [16]. The detachmentplane dips V30‡ (relative to horizontal) to theNNE, and is traceable to a depth of 9 km beneaththe pre- and syn-rift sedimentary rocks and thebasement rocks of the hanging wall on severalseismic pro¢les as a low-velocity zone of weakfault gouge ([7,16] ; see also Fig. 2). The grabenis bordered to the north by a 45‡ south-dippingnormal fault. Water depth varies between 120 mat the summit of Moresby Seamount and 2.6 kmat the deepest point of the half-graben. The di¡er-ent rocks involved in faulting are sedimentaryrocks of a Pliocene^Pleistocene syn-rift sequenceof maximum 2 km thickness unconformably over-lying partly eroded Paleocene^Maastrichtianmetabasites [18]. The latter show lower greens-chist-grade metamorphism and are along-strikeequivalents of rocks outcropping on the PapuanPeninsula [16].

3. ODP drilling results and deformation patterns

We focused our investigations on sampled faultrocks from a transect of drill sites of ODP Leg180 at the northern £ank of Moresby Seamount:(i) Site 1108 into the hanging wall to the detach-

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ment, (ii) Site 1117 drilling the detachment gougeas well as the upper part of its footwall, and (iii)Sites 1110^1114 into metabasic footwall rock(Fig. 2). Despite the fact that unstable hole con-ditions did not permit successful penetration ofthe entire succession continuously, the singlecore logs can be combined to a synoptic depthsection through important parts of the detach-ment fault system (Fig. 3). General descriptionsof drilled sections, lithologies and shipboard mea-surements are detailed in Taylor et al. [11] andRobertson et al. [19], so that we focus on thefaulted intervals here.Site 1108 was drilled through 485 m of terrige-

nous turbidites (Fig. 3) at the lower part of thenorthern slope of Moresby Seamount (Fig. 2).The rocks recovered comprised nannofossil oozeand talus from the nearby Moresby Seamount atthe top ca. 60 m of the hole, and a succession ofsiliciclastics (terrigenous turbiditic siltstones, mud-stones, and claystones) underneath. Ages increasefrom 1.67^1.75 Ma at 82.8 m below sea £oor(mbsf) to less than 3.35 Ma at the base. Sedimen-tation rates increase down-section, from 325 m/

Myr at 1.7^2 Ma to 425 m/Myr at 3^3.2 Ma[11]. Detritus mostly consists of material derivedfrom the continuously uplifted seamount, hencedisplaying progressively higher-grade protolith.Abundant evidence for brittle deformation, char-acterized by locally rotated bedding dips up to35‡, low-angle shearing, brecciation and ubiqui-tous slickensides, is mainly observed in the ¢ne-grained rocks. Two fault zones (FZ1: V165^190mbsf, FZ2: below 350 mbsf; see Fig. 3) are char-acterized by populations of small-scale faults withmineralized veins. Scaly fabrics are observed, andthe shear sense, where recognizable, indicates nor-mal displacement in an extensional fault zone.The highest frequency of fractures and faults isconcentrated in the upper fault zone, where po-rosities are anomalously high [11,13,20].From seismic data, the main detachment zone

would have been expected at a depth of approx-imately 1 km below sea £oor at Site 1108; how-ever, owing to stuck drill bits the hole had to beabandoned several hundred meters above the de-tachment zone. However, a bare rock spud-in wasperformed further updip the detachment, where

Fig. 1. Geographic and geological sketch map of the Woodlark Basin/Solomon Sea region. The stippled area encloses oceaniccrust formed during the Brunhes Chron at spreading rates labeled in mm/yr. MT and ST=Moresby and Simbo transform faults,respectively; DE=D’Entrecasteaux Islands. Top inset illustrates geographical location of the Woodlark basin (from [11]).

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the fault reaches the sea £oor (Site 1117; seeFig. 2 and [11,13]). Here, talc^chlorite^serpentiniteclayey fault gouge was recovered in the upper-most 12 mbsf (Fig. 3). Immediately underneath,mylonites and ultra-cataclasites probably occupya 50 m thick section of very low recovery. Theserocks imply that the main detachment has a struc-tural record of intense, cohesive plastic (mylon-ites) and intermittent plastic and brittle (catacla-sites) deformation [20]. Quartz generally deformedplastically; however, plagioclase has been ob-served to behave brittly. Beneath a fault brecciaat about 65 mbsf, metagabbros and their faulted,altered and fractured equivalents were recovered.

Healed veins and fractures were found occasion-ally.Sites 1110^1113 (also known as Talus Sites

[11,13]) aimed to penetrate the detachment at ashallow sub-bottom depth at the northern £ankof Moresby Seamount in order to ¢nd a viablealternate location to Site 1108 (Figs. 1, 2). As aresult of coarse-grained talus from the upliftedfeature, drill recovery was extremely low andmany holes had to be abandoned due to internalcollapse. Despite these problems, a suite of de-formed footwall rocks was collected, the majorityof which were metasediments (micaschist, gneiss)and metamorphic igneous rocks (metadolerite,

Fig. 2. (a) Schematic S^N-trending cross-section across Moresby Seamount, as taken from interpreted seismic re£ection pro¢les(after [11]). The enlarged middle part of the section also shows the projected relative locations of boreholes 1108, 1111^1114, and1117 across the detachment fault. (b) The enlarged lowermost part is a MCS cross-section across Site 1117 having penetrated thefault gouge where exposed to the sea £oor after slip of the hanging-wall block (from [11]). The detachment forms the southernboundary of a deep half-graben, which accumulated Pliocene^Pleistocene syn-rift turbidites. A set of antithetic steep normalfaults cross-cuts the top of the seamount. Locations of Sites 1108, 1111^1113 (Talus sites), 1114 and 1117 are projected into thesection plane.

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metadiabase, lamprophyre). These rocks recordedan early tectono-metamorphic stage (epidote^am-phibolite facies), followed by a retrograde pathwith brittle extensional deformation and hydro-thermal alteration [20]. The latter processes havebeen related to normal faulting of Moresby Sea-mount and unroo¢ng of its basement [18,20].Site 1114 (Fig. 2) was located north of the crest

of Moresby Seamount, recovering a section of 286m of faulted, ¢ne-grained turbidites (Fig. 3).These are separated from the underlying meta-morphic basement by a 6 m thick tectonic brecciathat marks the location of a steep SSW-dippingnormal fault. The brecciated fault contact is

marked by an abrupt increase in the degree ofhydrothermal mineralization. Its angular compo-nents consist of greenschist metamorphic andhighly fractured and mineralized diabase withabundant quartz and epidote veins, which inturn are cross-cut by calcite veins. Core samplesrecovered from the Site 1114 basement as well asfrom Sites 1110^1113 are probably exhumed de-tachment fault rocks with cataclastic and/or my-lonitic fabrics that may have formed at depth[11,20]. This is particularly true for the mylonites,where microstructures attesting to plastic defor-mation account for minimum temperatures of300‡C (see discussion by Roller et al. [20]). Alter-natively, frictional heating along the normal faultplane may have accounted for mylonitization (seediscussion in [13]).

4. Methods

Carbonate material from intergranular carbon-ate cements and veins ¢lling extensional fracturesof the core was sampled with a micro-drill. Wherepossible, di¡erent generations of cements (i.e.coats on the fracture walls) were prepared sepa-rately in order to allow for identi¢cation of zona-tion patterns and/or di¡erent precipitation eventsrelated to variable £uid sources. Thin sections forpetrographic study were prepared from orientedsamples where possible. In the case of Site 1108turbiditic mudstones, samples were impregnatedwith low-viscosity resin prior to cutting.Mass spectrometric measurements of C and O

were carried out using 5^10 mg of powdered car-bonate per sample. N

18O and N13C were deter-

mined with an automated Carbo Kiel CO2 prep-aration device coupled with a Finnigan MAT 251mass spectrometer, as outlined in more detail inKopf et al. [21]. Precision was T 0.04x (N13C)and T 0.08x (N18O). Data are calibrated againstthe Pee Dee Belemnite (PDB) scale [22] and aregiven in x units.Boron concentrations were determined by in-

ductively coupled plasma atomic emission spec-troscopy (ICP-AES) (JY 170 Ultratrace) follow-ing the procedure outlined in Heinrichs andHermann [23]. For boron isotopic measurements,

Fig. 3. Synoptic litho-log representation of ODP Sites 1108,1114 and 1117. Modi¢ed from [20].

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Table 1Results from C, B and O isotope analyses and ICP-AES for B concentration

ODP Site Depth Sample ID N13C N

18O paleo-T B N11B (x) Host lithology

(mbsf) ID (PDB) (x PDB) (‡C) (ppm) Measured PS pH7 PS pH8

1108 129.99 8b1-2 32.93 34.47 clay-/mudstone1108 139.61 8b2 35.97 310.07 71 clay-/mudstone1108 204.34 8b3 34.17 37.93 58 3.59 10.01 29.31 26.51 clay-/mudstone1108 204.39 8b4 32.69 33.18 29 2.14 6.76 26.06 23.26 clay-/mudstone1108 332.70 8b5-1 34.15 311.64 81 2.53 8.09 27.39 24.59 clay-/mudstone1108 332.70 8b5-2 34.2 310.69 clay-/mudstone1108 337.05 8b6 36.69 311.88 82 2.40 15.53 34.83 32.03 clay-/mudstone1108 342.67 8b7 310.2 312.11 84 2.87 15.15 34.45 31.65 clay-/mudstone1108 352.06 8b8 35.2 310.89 clay-/mudstone1108 361.40 8b9 310.71 311.93 clay-/mudstone1108 371.19 8b10 35.27 310.21 72 2.86 18.35 37.65 34.85 clay-/mudstone1108 371.27 8b11 35.37 310.63 74 3.77 11.92 31.22 28.42 clay-/mudstone1108 382.17 8b12-1 34.28 38.75 63 clay-/mudstone1108 419.52 8b12-2 36.07 310.07 clay-/mudstone1108 428.98 8b14 37.21 312.27 clay-/mudstone1108 438.02 8b15-1 35.88 310.27 clay-/mudstone1108 438.02 8b15-2 38.97 311.66 clay-/mudstone1108 438.02 8b15-3 313.26 313.69 clay-/mudstone1108 446.71 8b16 38.3 313.71 clay-/mudstone1108 456.35 8b17 38.23 313.72 93 2.31 18.03 37.33 34.53 clay-/mudstone1108 456.37 8b18 310.01 310.28 73 1.96 14.75 34.05 31.25 clay-/mudstone1108 475.52 8b19 38.73 39.96 clay-/mudstone1108 476.44 8b20 312.89 313.57 clay-/mudstone1108 476.90 8b21-1 38.59 311.6 clay-/mudstone1108 476.90 8b21-2 39.49 313.21 clay-/mudstone1117 0.13 17a1 38.36 314.01 serpentinite gouge1117 3.99 17a2-1 35.34 314.04 95 3.27 7.33 26.63 23.83 serpentinite gouge1117 3.99 17a2-2 35.36 314.35 98 serpentinite gouge1117 57.02 17a4 38.03 312.5 quartz-epidote breccia1117 66.75 17a5-1 36.45 311.91 chlorite shist1117 66.75 17a5-2 34.86 321.24 140 quartz^epidote breccia1117 67.38 17a6 33.31 317.02 gabbro breccia1117 95.79 17a7-1 37.24 36.49 gabbro1117 95.79 17a7-2 38.14 34.95 40 gabbro1114 295.67 14a0 310.4 37.38 brecciated diabase/dolerite1114 296.10 14a1 36.98 39.16 66 6.58 10.67 29.97 27.17 brecciated diabase/dolerite1114 296.19 14a2 37.74 36.12 47 brecciated diabase/dolerite1114 296.31 14a3 316.91 311.58 brecciated diabase/dolerite1114 296.38 14a4 316.59 310.54 brecciated diabase/dolerite1114 306.53 14a11 311.27 310.93 brecciated diabase/dolerite1114 306.64 14a12 313.79 314.38 98 brecciated diabase/dolerite1114 307.07 14a13 313.09 311.3 brecciated diabase/dolerite1114 334.33 14a17 36.34 311.35 brecciated diabase/dolerite1114 334.92 14a18 312.8 313.43 brecciated diabase/dolerite1114 335.02 14a20 311.36 313.43 brecciated diabase/dolerite1114 335.67 14a21 38.69 38.49 dolerite1114 335.76 14a22 38.86 36.73 dolerite1114 337.03 14a25 311.49 38.29 dolerite1110 13.04 10b4 31.17 313.72 94 carbonate shist1111 164.53 11a3 31.94 315.99 108 0.67 0.54 19.84 17.04 metadiabase1111 165.07 11a6 2.16 318.64 124 sheared metashist

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carbonate was dried and powdered. Afterwards1^5 mg carbonate powder was bleached with so-dium hypochlorite, rinsed and centrifuged repeat-edly with distilled water to remove contaminants.The cleaned carbonate was dissolved in 2 M HClbefore a fraction (containing approximately 5 ngof B) was loaded on zone-re¢ned rhenium ¢la-ments. After having dried the samples on the ¢l-aments at an ion current of 0.8 A, they weremounted into a Finnigan MAT 262 negative ther-mal ionization mass spectrometer (TIMS). TheBO3

2 -ion method was used to measure boron iso-tope ratios following the procedure outlined inDeyhle et al. [10]. Average internal precision is0.05x 2cmean, whereas external reproducibilityof measured ratios of replicate analyses is T 0.5x[10]. Data are presented in the familiar N11B nota-tion [24].Measured boron isotope ratios of the carbon-

ates require correction regarding the parent so-lution they precipitated from. N11B is very pH-sensitive, so that paleo-pH during time of pre-cipitation has been reconstructed [25]. Despitethe fact that this approach bears some uncertainty(namely because the original £uid compositionmay have been variable due to mixing processesover time), two endmember scenarios can be en-visaged. First, precipitation at shallow depth (i.e.several hundreds of meters below sea £oor) mayhave taken place, which occurs at an estimatedpH of 7^8 (see discussion in [10]). Second, weaccount for the temperature-driven decrease in

pH with burial [26,27]. This is particularly impor-tant given that the present thermal gradient adja-cent to the spreading center is up to 100‡C km31

[11]. All measured carbonate N11B ratios have

been reconstructed for precipitation from an as-sumed parent solution at pH values from 7 to 8(see Table 1, columns 9 and 10, and discussionbelow). Compared to the measured N

11B of thecarbonate cement, the corresponding parent solu-tion would have a 19.3x higher value at pH 7,and a 16.5x higher value at pH 8 [25]. Earlierworkers have argued that pH becomes moreacidic than 7 when temperatures exceed V100‡Cand may even decrease to pH 5 at 250‡C [27].However, the shift between pH 5 and pH 7 isonly 0.36x (i.e. 19.66x higher than the carbon-ate value), which lies within the analytical errorand hence has no signi¢cant in£uence on ourdata.

5. Results

5.1. Petrographic description of deformationmicrofabrics

On the microscopic scale, the Site 1108 turbi-dites provide insight into complex polyphase brit-tle deformation structures and veining. The ma-jority of the carbonate-¢lled fractures observed inthe sediment samples of the hanging wall to themain detachment at Site 1108 are purely exten-

Table 1 (Continued).

ODP Site Depth Sample ID N13C N

18O paleo-T B N11B (x) Host lithology

(mbsf) ID (PDB) (x PDB) (‡C) (ppm) Measured PS pH7 PS pH8

1112 20.75 12b1-1 30.49 318.55 123 7.3 9.95 29.25 26.45 meta-dolerite1112 20.75 12b1-2 35.41 313.65 meta-dolerite1112 68.40 12a5 35.32 312.12 84 epidote-rich dolerite1112 78.05 12a6 30.43 321.29 140 1.02 11.09 30.39 27.59 epidote-rich dolerite1112 87.83 12a8 0.65 321.67 142 epidosite1112 97.39 12a9 0.84 320.68 epidosite1112 107.16 12a10 0.11 321.49 141 epidosite1113 20.38 13a2-1 0.21 315.21 103 epidote^mica shist1113 20.38 13a2-2 32.46 39.89 70 epidote^mica shist

N11B data are shown as measured ratio of the precipitated calcite (left), and parent solutions at pH 7 (middle) and pH 8 (right).Estimates of precipitation temperature estimates from N

18O results follow Craig [41]. T= temperature; PS=parent solution.

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sional, as is documented by microscopic pull-apart structures (e.g. Fig. 4a). Vein mineralizationis dominated by coarse blocky calcite crystals(Fig. 4b), with rare existence of ¢brous calciticveins at greater depth in fault zone 2 (Fig. 4c).Variable geometry of the veins and vein networksis subject to their depth of occurrence [20]. Veinboundaries become less irregular and vein pathsmore straight with increasing depth. Fracturing ofdetrital mineral grains and rock fragments is com-mon in deeper parts of the section (fault zone 2),attesting to high shear strength of the cementedsediments where transgranular and intragranularfractures were formed. Often, disrupted wall rockfragments are captured within the calcitic veinin¢ll.Pervasive carbonate cementation in anastomos-

ing vein arrays attests to percolation of CO2-rich£uids through the entire rock volume. Pervasivecementation is often observed in ¢ne-grained in-tervals, while discrete veins or geometrically reg-ular patterns of veinlets occur in coarser-grainedsediments (Fig. 4b,c). This may be explained ei-ther by elevated pore pressure in these intervals(see discussion of syntectonic overpressure at Site1108; [4,11]), or simply owing to higher £uidavailability in the clay-rich succession (e.g. frommineral dehydration [28]). Where vein boundariesare sharp and well de¢ned, shear parallel to thevein surface and associated structures is often ob-served (Fig. 4d). Fibrous calcite veins show crystallaths of relatively constant breadth along a singlevein, often growing antitaxially and replacingeach other (Fig. 4d). The two observed vein-in¢lltypes probably have di¡erent origins. The mosaic-type cemented veins are interpreted to haveopened during a sudden, probably microseismicevent, and have subsequently been ¢lled by crys-tallization of minerals [20]. In contrast, the ¢brousveins (Fig. 4b,d) have probably opened slowlyand incrementally, so that mineral growth could¢ll the produced void simultaneously. Their gen-esis is thought to be related to a regime of persis-tent high £uid pressure [29].The microstructural record in samples from the

detachment zone (Site 1117) is hard to reconstructgiven the sparse core recovery. Some of the my-lonites occur as tectonically reworked fragments

in talc^chlorite^serpentinite fault gouge. A dark-ish, ultra-¢ne-grained foliated chlorite-rich matrixalternates with stretched layers of calcite andasymmetrically tailed porphyroclasts of dynami-cally recrystallized quartz. The porphyroclastsare strain enclaves and preserve microstructuresdiagnostic of mylonitic deformation and meta-morphism [20]. While the mylonites attest to duc-tile £ow with crystal-plastic deformation and/or£uid-assisted mass transfer, the cohesive, foliatedcataclasites may have been generated by a com-plex interplay of ductile and brittle deformation[20]. As shown in Fig. 4e, the foliation is cross-cutby sub-vertical calcite-¢lled extensional fractures.The fractures then were often sheared and openlyfolded by subsequent deformation. The fragmentsof mylonite and ultra-cataclasites in the faultgouge can be viewed as early products of shearingunder conditions of elevated temperature andpressure. This happened prior to formation ofthe fault gouge under upper crustal conditions(see discussion in [20]), because relics of dynam-ically recrystallized quartz in porphyroclasts indi-cate ductile £ow under V200^300‡C in the faultzone [20,30]. Given thermochronologic con-straints from the basement rocks [18], however,shear heating during fault movement rather thana deep burial at several km depth is suggested.The latest increments of extensional deformation,accompanied by either £uid £ow from depth orseawater in¢ltration, allowed for further serpen-tinization and weakening of the fault gouge undernear-surface conditions.At Site 1114, deformation is only locally devel-

oped in the metadiabase £ooring the fault brecciabelow 295 mbsf in the drilled section (Fig. 3).Brecciation of the metadiabase with early-formedquartz^epidote veins [20] was followed by calciteprecipitation in extensional fractures (Fig. 4f).Ongoing deformation produced pervasive twin-ning of the calcite and o¡sets of the vein ¢ll alongdiscrete brittle microshears. Similar overprintingrelationships have been observed in pebblesfrom the detachment fault zone at Site 1117(Fig. 3; see [20]). However, the gabbroic wallrock becomes less altered and less fractured withdepth in this borehole. As for the other footwallrocks, Sites 1110^1113 (Talus Sites) recovered

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Fig. 4. Photomicrographs of typical microscopic deformation structures and veins. (a) Microscopic pull-apart structure in ¢ne-grained claystone, composed of a pair of shear fractures (left and right), bridged by a calcite-¢lled extensional vein. Site 1108,476.44 mbsf, long side is 6 mm. (b) Broad vein in sandstone, ¢lled with mosaic-shaped calcite. Parallel to the main fracture, adense pattern of veinlets anastomoses around rigid components (dark) of the turbiditic host rock. Site 1108, 456 mbsf, long sideis 4 mm. (c) Anastomosing network of veins in claystone ¢lled with antitaxially grown ¢brous calcite. The ¢ne-grained turbiditichost rock su¡ered pervasive carbonate cementation. Site 1108, 475 mbsf, long side is 4 mm. (d) Extensional vein ¢lled with ¢-brous (left and center) and blocky (lower right) calcite, in matrix of strongly altered metabasite. Vein walls are sheared and madeup of ultra-cataclasite. Site 1117, 3.99 mbsf, long side is 4 mm. (e) Cross-cutting and foliation-parallel calcite veins in foliated ul-tra-cataclasite. A fragmented and deformed early-formed quartz-rich vein appears in the upper right corner. Site 1117, 66 mbsf,long side is 6 mm. (f) Intensely twinned calcite ¢lling a broad vein in metadiabase. The vein is o¡set by a sharply de¢ned fault(left). Site 1114, 307 mbsf, long side is 4 mm. All photographs taken with crossed nichols. Modi¢ed from [20].

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similar fragments of metabasic and metasedimen-tary rock as observed in the deeper parts of Sites1114 and 1117. Microveins with quartz, epidote,and calcite cementation have been observed, butowing to the disrupted nature of the succession, acontinuous tectono-metamorphic reconstructionis not possible.

5.2. Isotopic results

5.2.1. Carbon and oxygen isotopesC and O isotope ratios were determined for 60

samples from veined rock of di¡erent lithology,degree of deformation, and grade of alterationand metamorphism. The results are shown in Ta-

Fig. 5. Results from C and O stable isotopes, cross-plotted as familiar PDB ratios. Note the di¡erent overall trends of hanging-wall turbidites (Site 1108 (a)) and detachment zone and footwall rocks (Sites 1111^1117 (b)) summarized in panel c. See text fordiscussion.

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ble 1 (including multiple analyses of di¡erentparts of the veins), and are cross-plotted in Fig.5. Please note that samples are referred to interms of the host lithology for distinction’s sake,although all measurements were carried out oncalcite veins in these lithologies.When regarding the hanging-wall turbidites of

Site 1108, a distinct N18O ratio of 33x to 35xis found for the fault zone 1 (165^190 mbsf, Fig.3), while all but one data point from fault zone 2veins (i.e. below 350 mbsf, Fig. 3) plot between310 and 314x (Table 1). Two veins betweenthe two fault zones show intermediate N

18O(38x to 310x). The scatter in the vein cementisotopic compositions may be best explained byvariable mixing of deep-seated £uids with the lo-cal pore waters. In the detachment fault zone, lowN13C and N

18O ratios are found in the talc^chlo-rite^serpentinite gouge (approximately 38x and314x, respectively; Fig. 5b, encircled) and inthe underlying quartz^epidote breccias (312xto 321x and 35x to 38x, respectively; Ta-ble 1). In general, such low C and O signatureshave previously been attributed to metamorphic£uids released at high temperatures. The gabbrosat the base of Site 1117 show slightly lower N13C,but higher N

18O ratios than the more intenselyfractured overburden. This suggests that £uid per-colation was less e⁄cient with increasing distancefrom the main fault, so that cooling may havea¡ected particularly N

18O (Fig. 5 and Table 1).The overall scatter in the vein cements at Site1117 suggests that the strongly variable mineral-ogy, physical properties and the tectonized natureof these rocks accounts for the non-systematicbehavior. Although Fig. 5c indicates a negativecorrelation between N

13C and N18O, a closer anal-

ysis implies a ‘deep’ signal in the gouge (both Cand O strongly negative), a similar, but more scat-tered signal for the breccias and schists in thegouge’s vicinity (open lozenges in Fig. 5b), anda cooler signal in the gabbroic wall rock (similarN13C as the gouge, but signi¢cantly more positiveN18O; see discussion below).As for the footwall rocks, two di¡erent isotopic

trends are observed in the authigenic vein ¢lls(Fig. 5c). The ¢rst shows positive correlation be-tween N

13C and N18O, and is mostly represented

by Site 1114. Isotope signatures attest to consid-erable depletion of the ‘heavy’ C and O isotopedue to metamorphic reactions (e.g. brecciatedmetadiabase at Site 1114, typically with N

13C of311x to 317x and N

18O of 310x to316x ; Table 1). The second trend observed ischaracterized by an apparent negative correlationbetween N

13C and N18O in rocks from the Talus

Sites (1110^1113; Fig. 5b,c). This group of datashows generally high N

13C (V35x to +2x)when compared to the majority of the other sites.Oxygen isotopes are found to scatter fromV310x to as low as 322x (the latter in epi-dosites and epidote-rich dolerites from Site 1112;Fig. 5b and Table 1). The results imply that theratio of 12C/13C from either CO2 or methane isnot a product of a deep-seated process, whereas16O/18O most likely originates from hydrothermalmetamorphic waters. Given the large variability inlithology of the material and its unknown strati-graphic position, we interpret these data as anindication for mixing of deep-seated waters (sup-plying the lighter 16O) with a C-source of sea-water-like composition (see Section 6).

5.2.2. Boron and B isotopesIn this ¢rst study of boron geochemistry on

authigenic precipitates from metamorphosedrocks, we measured B contents and isotope ratiosof 14 calcitic vein cements. Results are shown inTable 1 and are cross-plotted as reciprocal B con-centration versus isotope ratio in Fig. 6. Our dataare presented under the simplifying assumptionthat the B geochemistry in the vein cement re£ectsthat of the parent £uid without fractionation re-actions obscuring the chemical composition of theprecipitates (see ¢rst part of discussion below).The calcite precipitates contain relatively little

boron (1^7.3 ppm), which is expected when orig-inating from a ‘freshened’ £uid from depth[28,31]. In fact, despite the high B contents insome metamorphic rocks [32,33], its abundancedecreases drastically from low-grade (80^200ppm) to high-grade (1^10 ppm; [31]) metamor-phism. The decrease directly corresponds to theloss of structurally bound B from mineral lattices,which is usually less easily released than B ad-sorbed to sediments at shallower depth [34,35].

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Consequently, £uids from dewatering sedimentsare generally rich in B, while those from meta-morphic processes show low B abundance. Infact, B enrichment is known along deep-seated£uids from shallow detachment thrust faults,like the Nankai or Barbados de¤collements (V30ppm, [34]). Similarly, hydrothermal £uids fromthe sediment-hosted Escanaba Trough systemhave B concentrations of up to 100 ppm [36].By contrast, deep £uids from the sediment-starvedMariana Trough, a scenario not too di¡erentfrom Woodlark, are characterized by B contentsof 7^8 ppm and isotope ratios of 22^30x [36].This N11B window is bracketed by the parent so-lution compositions calculated from the vein ce-ment data ranging from V17^35x (Table 1).The lowest B concentration (0.67 ppm) and N

11B(17x), near Moresby Seamount indicates localdepletion in ‘heavy’ boron, and correlates with adepletion in 18O. Fluids from ODP Leg 195 drill-ing into Chamarro serpentinite seamount (A.Deyhle, unpublished data) have similar N

11B ra-tios of V16x and equally hint to a deep-seatedorigin (see discussion below).

6. Discussion

Our discussion focuses on uncertainties con-cerning the £uid composition inferred from authi-genic precitates, the £uid origin and temperatureregime during precipitation (as estimated from the

oxygen isotopic data), and the link of our ¢ndingsto regional tectonics and metamorphism as afunction of unroo¢ng of Moresby Seamount.The results are interpreted in the context of aconceptual model of unroo¢ng and £uid in¢ltra-tion (Fig. 7).

6.1. Fluid composition based on authigenicprecipitates

The study of authigenic precipitates is a power-ful method to reconstruct paleo-£uid composi-tion. However, one di⁄culty is the compositionalrelationship between vein cement and the originalparent £uid, and very few data exist to resolve theuncertainties regarding this matter. In a closedsystem (i.e. in vitro), all dissolvents should be pre-served in the precipitate and its £uid inclusions, sothat volumetric changes are the sole issue to beconsidered. In vivo, however, the complexity ofnatural processes at changing PT conditionscauses several parameters to vary considerably.In the Woodlark Basin scenario, they include £uiddischarge rate, £uid composition as a result ofpH, P, T, and hence diagenetic and metamorphicreactions (e.g. release of ‘fresh’ mineral water), ormixing processes. The e¡ect of these parameters islargest, as well as least understood, for B geo-chemical processes. B £uid^rock interaction hasbeen studied in the laboratory (e.g. [25,27]) aswell as in a number of natural settings (e.g.[26,36,37]). Diagenetic and low-grade metamor-phic processes are particularly complicated, sincewith increasing PT, B may be released from thesediment into the £uid (e.g. [26]), while the thenB-enriched £uids subsequently serve as a sourcewhen B gets incorporated into authigenic minerals(e.g. [38]). Hence, the general trend to increasingB concentrations and lowered isotope ratios indeep-seated £uids [26] may be obscured.Another complication is the pH sensitivity of

boron. Laboratory work has demonstrated thatprecipitation of carbonate is favored when pH isslightly alkaline (e.g. [42]). At such conditions, Buptake into calcite is highest [25,42]. Both studiesfurther report B concentrations in the calcite to be2U [42] to s 10U higher [25] than those in theparent solution. If we relate those results to the

Fig. 6. Results from B vein geochemistry, plotting N11B

against the reciprocal B content to delineate possible B sour-ces. Reference data taken from [36,37].

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Woodlark vein cements, we are left to assume thatthe comparatively low B contents re£ect evenstronger dilution in the original parent £uid (seeTable 1). Such a conclusion supports our interpre-tation that the vein cements carry a deep-seated,‘freshened’ signature, and have not been stronglya¡ected by mixing with shallow waters (see be-low).Temperature has been reported to have a rela-

tively small e¡ect on B isotope geochemistry [39],but changes N

13C and N18O signi¢cantly (see

[40,41], and below). The method by Craig [41]to translate N

18O ratios into paleo-£uid temper-ature (i.e. during precipitation) depends on thefractionation factor between calcite and some spe-ci¢c £uid. However, during upward migration inthe Woodlark detachment fault zone, precipita-tion events, diagenetic and/or metamorphic reac-tions as well as £uid mixing processes may causechanges in £uid composition. For the interpreta-tion of our N18O results (see second part of Sec-tion 6), we have made the simplifying assumptionthat these changes are minor and do not dramat-ically a¡ect the overall £uid composition. Thetemperature estimates (Table 1) are hence to beseen as a ¢rst-order approach to assess whetherthe parent £uid was warm (i.e. deep-seated), orrather cold. In the latter case, the lower temper-ature may either be a result of long transportfrom depth (and possibly £uid admixture) or ofthe primarily shallow origin of the £uid (see be-low).

6.2. Fluid origin and £uid £ow conceptual model

The retrograde overprint of brittle over ductilestructures, as has been suggested from petro-graphic observations [20], is probably controlledby the continuous exhumation and cooling of thedetachment zone by crustal extension as well asby the episodicity in seismic slip along the detach-ment. Evidence is provided by thin-section obser-vations (see Fig. 4), and is mirrored by the resultsfrom isotope analyses in a sense that more nega-tive ratios re£ect hydrothermal high-grade stages,while low-grade phases show higher N

11B, N13Cand N

18O (Table 1 and Figs. 5 and 6). Hydrother-mal alteration of igneous basement, an alternate

possible £uid process in the Woodlark setting,would result in N

18O depletion of the basalts[43], and hence higher N18O in the £uid. By con-trast, N11B decreases slightly relative to seawater(39.5x) when hydrothermal £uids leach basalt[44], showing ratios of V27x (Fig. 6). Giventhe B results of this study, we favor metamorphicrather than alteration processes to account for thevein chemistry. This is supported by the ‘heavy’N13C data, which have previously been related tofractionation of C during thermal cracking duringCH4 formation (typically with 355x to 335x[40]). Both C and O isotope results indicate low-grade metamorphic £uid origins, so that precipi-tation and temporary sealing of the veins oc-curred from CO2-rich waters.When the oxygen isotope results from the cal-

cite precipitates are used to calculate the corre-sponding paleo-temperatures (following Craig[41]), we obtain a low to moderate range between40 and 140‡C for the detachment fault zone, andsimilar temperatures for the overlying faulted tur-bidites (36^94‡C) and the underlying metamor-phic rocks (22^142‡C). In broad terms, these esti-mates can be separated into two groups, as hasbeen done for the N

13C and N18O data. The ¢rst

group, characterized by N18Os38x, covers a

low temperature range below 60‡C. By contrast,the remaining values show the in£uence of awarm £uid from deep, possibly metamorphic re-actions. In some lithologies (e.g. quartz^epidotebreccias at Site 1117, claystones of Site 1108 faultzone 2, metadiabase of Sites 1113 and 1114), these£uids have undergone considerable cooling priorto precipitation (N18Os315x, i.e. T6 100‡C).Other rocks, however, contain veins indicatinghigher £uid temperatures, up to V140‡C (e.g.epidosites of Site 1112, and breccia at Site 1117;see Table 1). Although £uid discharge may havebeen more rapid in the latter case, so that inter-action with adjacent £uid/rock was minimized, allestimated £uid paleo-temperatures strongly sug-gest that mineralization of the veins took placewhile the rocks experienced the retrograde pathof their metamorphic history. Local variationsare very likely a function of variable pervasivefracturation as well as permeability anisotropyin the material [4]. With distance from the main

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conduit, like the brecciated intervals at Sites 1117and 1114, both N

13C and N18O get more positive

compared to zones of maximum depletion inheavy isotopes (Table 1). The second, signi¢cantlycooler precipitation event is attested by isotoperatios approaching zero. This event is observedin fault zone 1 at Site 1108 as well as in manylithologies of the fault zone and footwall rocks. Itpostdates the warmer event, as is evidenced bymesoscopic (core-based) as well as microscopic(petrographic) observations (see above and [11]).Although admixture of seawater may appear tobe an alternative explanation to cooling of adeep £uid, the seawater input would necessarilybe re£ected by high B contents and elevatedN11B. Such increases are not observed (see Table1 and earlier discussion above).

Maximum temperatures appear to correspondto high permeability fault zones, which may di-rectly have resulted from elevated £uid pressuresand fault dilation (e.g. along the £ank of MoresbySeamount). In contrast, lower temperatures (i.e.higher N

18O) may have been caused by the hightortuosity of fracture arrays with increasing dis-tance from the main fault [45]. This evolution isschematically illustrated in Fig. 7, which showspossible £uid migration and precipitation scenar-ios at 2 Ma and at present. In either scenario, thehigh-temperature £uids have resulted from meta-morphic processes adjacent to the westward-mi-grating rift axis ([46], see A1, B1 in Fig. 7). Fur-ther updip the main fault zone as well as withincreasing distance from the main fault plane,lower £uid temperatures result from longer path-

Fig. 7. Schematic S^N-oriented cross-sections across the Moresby Rift area at V2 Ma (A) and at present (B). A1^A3 and B1^B3 represent chemical composition and inferred temperature regimes (mostly based on N

18O data) during mobilization and precip-itation. See text for discussion. Sketches not to scale.

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ways, and thus duration, during upward migra-tion. The low-T endmembers (A3 and B3 in Fig.7) are represented by veins of fault zone 1 at Site1108 and high N

18O in veins cutting some Site1114 rocks (Fig. 7A,B).When related to the microfabrics in the faulted

rocks in the vicinity of the Woodlark detachment,hydrothermal £uid £ow becomes a major playerin fault activation. If we regard our thin-sectionobservations as a consequence of continuous fab-ric evolution in a regime of decreasing tempera-ture, the belt of fault gouge, mylonites and ultra-cataclasites depicts a history of continuous brittledeformation and ductile £ow in a shear zone ofapproximately 60 m width (Fig. 3). Abundantmineralized veins in the mylonites, however, indi-cate that ductile deformation was interrupted byepisodic stress build-up, excess pore £uid pres-sures, and hydrofracturing [3,47^49]. Many ofthe veins are stretched and/or folded, and are in-terpreted to have formed syntectonically and werelater deformed with the mylonite matrix. A sec-ond generation of veins postdates the phase ofductile deformation [20]. Zones of exclusively brit-tle deformation histories, which are restricted tothe rocks away from the major fault zones, alsoshow this second-generation vein cement.The implications of elevated £uid pressures and

hydrofracture for geological sediment deforma-tion, and particularly its signi¢cance in weakeningshear zones, have been pointed out previously[47^49]. Major shear surfaces may function as aseal to migrating £uids, hence causing severe per-meability anisotropy and build-up of overpres-sures to allow, or at least trigger, fault movementwhen approaching lithostatic stress [48]. Such evi-dence has not only been presented for de¤collementthrusts of accretionary prisms [4,48,50], but forinstance for the Woodlark extensional detachment[4,7]. Present-day seismicity [16] and multi-phasefracturing of the studied rocks suggest episodicbrittle failure and seal to be the dominant defor-mation mechanisms on the retrograde path in thehistory of the Woodlark detachment. Vein geom-etry has a limited meaning in our study, becauserecovery of incomplete successions or isolatedpebbles and talus from Moresby Seamountmake reorientation impossible (except for the

hanging-wall sediments, Site 1108). Despite notknowing the angle of the fracture relative to themain shear zone and tectonic trends, the types offractures found are often indicative of their evo-lution when compared to earlier observations. Inthe perspective of the fault-valve model proposedby Sibson et al. [51] and re¢ned by Hendersonand McCaig [52], our fractures closely resembletension gash arrays, tensional fault-parallel veins,and pull-apart jogs, all corresponding to the over-all normal movement. Whenever possible, the re-lationship between the veins was related to thefault zone, largely showing low to moderate an-gles (30^60‡). Genetically, such veins of type 2, 3,6, and 8 (see ¢gure 12 in [52]) have probablyformed when £uid pressure dropped to hydro-static after a seismic event. Hence, even thecross-cutting relationships may be envisaged as aconsequence of a switch in stress regime (¢gure 8in [1]). Hydrothermal mineralization probablysealed the predominantly tensional fractures [20]eventually, thereby causing a drop in permeabilityand triggering pore pressure build-up towards thenext rupture event. Evidence for enhanced poros-ities and £uid £ow along the detachment faultzone has recently been provided from geophysicalobservations [7]. It has to be acknowledged, how-ever, that the friction coe⁄cient of W=0.21^0.3 ofthe talc^chlorite^serpentinite gouge (A. Kopf,preliminary, so far unpublished data from sheartests at V2 MPa normal stress) would be su⁄-ciently low to facilitate slip of the detachment.

7. Conclusion

In summary, our account of the microfabrics ofdeformed rocks from the Woodlark extensionaldetachment shows complex patterns of phases ofbrittle and ductile deformation. The rock volumesin the hanging wall and the footwall show pre-dominantly brittle failure, with fractures and veinsbeing healed by multiple phases of hydrothermalprecipitates. However, local mylonitization sug-gests temperatures of 200^300‡C, which mayhave been caused by frictional heating along thedetachment fault zone. Negative N

13C and N18O

isotope ratios in vein cements of the adjacent

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rocks mirror £uid release during low-grade meta-morphism [8], which may be tentatively correlatedwith rifting and deep-seated processes. More pos-itive isotope ratios (C, O as well as B) and devo-latilization of B in the fractured rocks may be aresult of retrograde paths and cooler £uids. N11Bof the warmest hydrothermal precipitates corre-sponds to £uid signatures as low as 17x. Ourresults imply that the volatile element boron maybe preserved in geo£uids at higher temperaturesthan previously believed.

Acknowledgements

We wish to thank ODP Leg 180 shipboard sci-enti¢c and technical personnel for their support.Regina Surberg is acknowledged for carrying outICP-AES measurements. The manuscript bene-¢ted greatly from the detailed suggestions andhelpful criticism by Brian Taylor and an anony-mous referee. This study was funded by DFG(Deutsche Forschungs-Gemeinschaft) grants Be1041/13 (J.H.B.) and Zu 80/3 (A.D.).[AC]

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