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Fluid Venting in the Eastern Aleutian Subduction Zone

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Fluid Venting in the Eastern Aleutian Subduction Zone JOURNAL OF GEOPHYSICAL RESEARCH,VOL. 103,NO. B2, PAGES2597-2614, FEBRUARY 10, 1998 Fluid venting in the eastern Aleutian subductionzone Erwin Suess,Gerhard Bohrmann, Roland von Huene, Peter Linke, KlausWallmann, Stephan Lammers, and Heiko Sahling GEOMAR, ResearchCenter for Marine Geosciences,Kiel, Germany Gisela Winckler Institutftir Umweltphysikder Universit•itHeidelberg, Heidelberg, Germany Richard A. Lutz Centrefor Deep-SeaEcology and Biotechnology, Institute of Marine andCoastal Sciences RutgersUniversity, New Brunswick,New Jersey Daniel Orange MontereyBay AquariumResearch Institute, Moss Landing, California Abstract.Fluid venting has been observed along 800 km of theAlaska convergent margin. The fluid ventingsites are located near the deformation front, are controlled by subsurface structures,and exhibit the characteristics of coldseeps seen in otherconvergent margins. The moreimportant characteristics include (1) methaneplumes in thelower water column with maximaabove the seafloor which are traceable to theinitial deformation ridges; (2) prolific coloniesof ventbiota aligned and distributed in patchescontrolled by faultscarps, over- steepenedfolds or outcropsof beddingplanes; (3) calciumcarbonate and barite precipitates at thesurface and subsurface of vents;and (4) carbonisotope evidence from tissue and skeletal hardparts of biota,as well asfrom carbonate precipitates, that vents expel either methane- or sulfide-dominatedfluids. A biogeochemicalapproach toward estimating fluid flow ratesfrom individualvents based on oxygenflux measurementsand vent fluid analysisindicates a mean valueof 5.5+ 0.7L m-2 d -1 fortectonics-induced water flow [ Wallmannet al., 1997b].A geophysicalestimate of dewateringfrom the samearea [von Huene et al., 1997]based on sedimentporosity reduction shows a fluid loss of 0.02L m-2 d-1 for a 5.5km wide converged segmentnear the deformationfront. Our video-guidedsurveys have documented vent biota acrossa minimumof 0.1% of the areaof theconvergent segment off KodiakIsland; hence an averagerate of 0.006 L m-2 d -1 isestimated from the biogeochemical approach. The two estimatesfor tectonics-inducedwater flow fromthe accretionary prism are in surprisingly goodagreement. 1. Introduction The circum-Pacificsubduction zones manifest a variety of end-membertectonic settings, studies of which have now and in Fluid venting along the world's subductionzones has been the past contributedtoward an in depthunderstanding of the recognized over the past 10 years as a processof first-order complex processat convergentmargins [Kulm et al., 1986; Le importancefor marinegeosciences and oceansciences [Langseth Pichonet al., 1987; vonHuene and Scholl,1991, 1993; Kastneret andMoore, 1990;Moore and Vrolijk, 1992 ]. Ventingaffects the al., 1991; Carsonet al., 1994; Westbrooket al., 1995; McAdooet budgetsof certainelements in the deepsea [Suessand Whiticar, al., 1996]. Critical regionsfor fluid escapeare trenches,defor- 1989; Martin et al., 1991, 1996], the material turnover at mationfronts, and initial accretionaryridges. Accreted and sub- specializedvent ecosystems[Suess et al., 1985; Brooks et al., ductedsediments are thoughtto be separatedby interfaceswith 1987; Rio et al., 1992; Childresset al., 1986; Bouldgueet al., low shearstrength and with concentrationsof overpressured pore 1987] as well as the thermalstructure of accretionarycomplexes fluids.This interface decouples the sedimentary sequences during [Le Pichonet al., 1990; Henry et al., 1992, 1996;Hyndman et al., convergenceallowing unconsolidatedsediment to be subducted 1993]. Fluid flow andpressure gradients may in turn influencethe beneaththe margin. Graduallynow, the complexityof these accretionarytectonics such as earthquake activity or multiplexing submarinehydrogeologic processes is becomingapparent. So far [Davis et al., 1990; Sammondset al., 1992; Brown et al., 1994]. therehas been evidence reported for outputof freshand super- saline water from accretionaryprisms [Kastner et al., 1991; Wallmann et al., 1997a] and for horizontal and vertical Copyright1998 by theAmerican Geophysical Union. recirculationover considerabledistances through sequences of Papernumber 97JB02131. accretedsediments [Le Pichon et al., 1990; Martin et al., 1995]. 0148-0227/98/97JB-02131 $09.00 An enormous difference of flow rates, however, has been 2597 2598 SUESS ET AL.: FLUID VENTS IN ALEUTIAN SUBDUCTION ZONE estimatedwith geophysicaland geochemical methods at different Baranoff Fan, is locatedin the southeasternmost part of the Gulf convergencesettings with no clear pictureemerging [Carson et of Alaska and began to form in upper Miocene time. The two al., 1990;Linke et al., 1994;Henry et al., 1992, 1996]. older fans have presumably contributed material to the Severalorders of magnitudehave separatedflow estimates accretionaryprisms along the easternAleutian Trench; however, basedon porosityreduction from those observed directly at vents. most of their sedimentvolume is currently being subducted,the Furthermore,the questionof the relative importanceof focused process which generates the fluids being expelled at the flow as evident at vent fields versus diffuse flow without convergentplate boundary [von Huene and Scholl,1991 ]. conspicuousvent communitiesor chimneysremains unresolved. The westernpart of the Yakutat Block has been subductedas To help clarify this situation, we report here the first shownby magneticanomalies. Assuming it has been coupledto comprehensivedata set as well as hithertounknown evidence for the Pacific plate placespart of the terraneat the baseof the slope, tectonicallycontrolled, large-scale venting phenomena in the deep within the northeastern survey area between 3 and 5 Ma. eastern Aleutian Trench of the Alaska subduction zone. The Subsequently,its point of entry into the subductionzone swept surveyscarded out and samplescollected by R/V Sonnein 1994 northeastward along the trench to its present position in the and 1996 composean 800 km long segmentbetween the Kodiak northern Gulf of Alaska. Thus the tectonic history of the and ShumaginIslands. Here we found manifestationsof fluid accretionary domains in our survey area includes a former ventingin the form of distinctivefaunas, mineral precipitates, collisionalsegment currently in an areareceiving high amountsof methaneanomalies, a temperatureanomaly, in situflow data,and interglacialsediment, a noncollisionalsegment receiving Surveyor contrastingchemistries of porefluids and sediments from on-vent Fan and trench sedimentloaded with glacial debris,and a segment and off-vent settings. where the head of the Eocene Zodiac Fan and the older oceanic The temperatureanomaly is small, yet significant, and crusthave enteredthe subductionzone. In the westernsurvey area documentsan extra heat sourceto the oceanicbottom waters from the thicknessof trench sedimentsis significantly less than in the below the seafloor,thus giving a new meaningto the conceptof easternarea. cold seeps.Overall, the significanceof our discoveryin the Four segmentsof the marginwere investigated during R/V AleutianTrench is seenin thepredictability and documentation of Sonnecruises (Figure 1, SO-96,SO-97, andSO- 110 [ Fliihand vent sites within specificdeformational settings of that vonHuene, 1994;Suess, 1994; Suess and Bohrmann, 1997].All accretionarymargin. Our successin finding theseactive zones of stationsoccupied and surveysconducted during these cruises and fluid escape provides renewed confidence in being able to referred to in this communication are listed in Table 1. Extensive extrapolateand eventuallyto quantifytectonic dewatering within seismicreflection data [von Huene et al., 1987; von Huene, 1989] the entireglobal subduction framework. were mergedwith the high-resolutionswath bathymetry obtained during the R/V Sonnecruises to locate the accretionaryridges which became the focus of our bottom surveys, sampling, and 2. GeologicSetting fluid monitoring. The Edge sector includes an accretionary mass that was The continental margin that borders the eastern Aleutian probablybuilt againstthe erosionalscar formed during collision of Trenchhas an accretionaryterrane that containslithologies as old the Yakutat Block. Here the trenchaxis receivesa large amountof as Late Cretaceous.The Kenai Peninsula, the Shumagin and terrigenousglacial sedimentfrom the adjacentAlaskan mainland Kodiak Islands,and presumablythe shelf betweenthem have an cappedby an interglacialsequence (Deep SeaDrilling ProjectSite upper plate crust consistingof turbiditesand volcanicrocks of 180) [Kulrn and von Huene, 1973]. In the Edge sector the Cretaceousto Paleogeneage. The outershelf and slopeseaward of youngest tectonic structures,forming the deformation front, the islands are characterizedby Eocene to Oligocene accreted consistof two relatively gentlefolds with their asymmetricover- rocksoverlain by Neogenebasins [Moore et al., 1991]. Sediment steepenedflanks facing the trench(Figure 2). Thesestructures are accretedto the continentat the trenchduring the currentepisode is parallelto and situatedjust at the deformationfront in about5000 generallyyounger than 3 Ma. m of water depth. Toward the southwestthey terminateagainst a The oceanic Pacific plate that is subducted near the steepscar, believed to be the lateralstrike-slip fault of a subducted northeasternend of the Aleutian Trench is of Eocene age and seamounttrace. The folds exposetrench fill sediments;their relief increasesin ageto the southwest.The plateconvergence rates are reachesabout 300 m above the trench
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