Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara

22 Pages·2010·1.95 MB·English

Article
Volume11
29October 2010
Q0AD03, doi:10.1029/2010GC003177
ISSN: 1525‐2027
Pore fluid chemistry of the North Anatolian Fault Zone
in the Sea of Marmara: A diversity of sources and processes
M. D. Tryon
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093‐0244, USA
([email protected])
P. Henry
CEREGE, Chaire de Géodynamique, Collège de France, Europôle de l’Arbois, BP80, F‐13545 Aix‐en‐Provence
CEDEX 04, France
ğ
M. N. Ça atay
Faculty of Mines, Geology Department, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey
T. A. C. Zitter
CEREGE, Chaire de Géodynamique, Collège de France, Europôle de l’Arbois, BP80, F‐13545 Aix‐en‐Provence
CEDEX 04, France
L. Géli
Marine Geosciences Department, Ifremer, F‐29280 Plouzané, France
L. Gasperini
ISMAR, CNR, Via Gobetti 101, I‐40129 Bologna, Italy
P. Burnard and S. Bourlange
CRPG, 15 Rue Notre Dame des Pauvres, F‐54501 Vandoeuvre Les Nancy, France
C. Grall
CEREGE, Chaire de Géodynamique, Collège de France, Europôle de l’Arbois, BP80, F‐13545 Aix‐en‐Provence
CEDEX 04, France
Marine Geosciences Department, Ifremer, F‐29280 Plouzané, France
[1] As part of the 2007 Marnaut cruise in the Sea of Marmara, an investigation of the pore fluid chemistry
ofsites alongtheMainMarmaraFaultzonewasconducted.Thegoalwas todefinethespatialrelationship
between active faults and fluid outlets and to determine the sources and evolution of the fluids. Sites
included basin bounding transtensional faults and strike‐slip faults cutting through the topographic highs.
The basin pore fluids are dominated by simple mixing of bottom water with a brackish, low‐density Pleis-
tocene Lake Marmara end‐member that is advecting buoyantly and/or diffusing from a relatively shallow
depth. This mix is overprinted by shallow redox reactions and carbonate precipitation. The ridge sites are
more complex with evidence for deep‐sourced fluids including thermogenic gas and evidence for both sil-
icate and carbonate diagenetic processes. One site on the Western High displayed two mound structures
that appear to be chemoherms atop a deep‐seated fluid conduit. The fluids being expelled are brines of
up to twice seawater salinity with an exotic fluid chemistry extremely high in Li, Sr, and Ba. Oil globules
were observedboth atthe surface andin cores, and type II gas hydrates of thermogenic origin were recov-
ered. Hydrate formation near the seafloor contributes to increase brine concentration but cannot explain
Copyright 2010bythe American Geophysical Union 1of 22
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their chemical composition, which appears to be influenced by diagenetic reactions at temperatures of 75°
C–150°C. Hence, a potential source for fluids at this site is the water associated with the reservoir from
which the gas and oil is seeping, which has been shown to be related to the Thrace Basin hydrocarbon
system. Our work shows that submerged continental transform plate boundaries can be hydrologically
active and exhibit a diversity of sources and processes.
Components: 11,900words, 8 figures.
Keywords: porefluid;fluid seeps; marine hydrogeology;methane seeps; Seaof Marmara; NorthAnatolian Fault.
IndexTerms: 1050Geochemistry:Marinegeochemistry(4835,4845,4850);1065Geochemistry:Majorandtraceelement
geochemistry; 1039Geochemistry: Alteration andweathering processes(3617).
Received14April 2010; Revised 2 September2010; Accepted 10September2010; Published 29October2010.
Tryon,M.D.,P.Henry,M.N.Çağatay,T.A.C.Zitter,L.Géli,L.Gasperini,P.Burnard,S.Bourlange,andC.Grall(2010),
Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes,
Geochem.Geophys.Geosyst.,11, Q0AD03,doi:10.1029/2010GC003177.
Theme: Mechanics, Deformation, and Hydrologic Processes at Subduction Complexes,
With Emphasis on the Nankai Trough Seismogenic Zone Experiment
(NanTroSEIZE) Drilling Transect
Guest Editors: D. Saffer, P. Henry, and H. Tobin
1. Introduction their nature and origin, to install instruments to
monitor fluid flow, pore pressure, and microseis-
[2] Considerable attention has been given to char- micity, and to sample through coring the sedimen-
tological record of previous earthquakes. This
acterizingtherelationshipsbetweenfluidsandplate
manuscriptpresentsandinterpretstheresultsofthe
boundary fault systems at submerged convergent
pore fluid geochemicalanalyses.
marginsyetcomparativelylittleisknownaboutthe
hydrogeology of submerged transform boundaries.
Followingthe1999KocaeliM7.4earthquakealong
1.1. Geologic and Tectonic Context
theNorthAnatolianFaultzoneattheeasternendof
the Sea of Marmara, an international scientific [3] The Sea of Marmara lies south of Istanbul,
partnership was established to identify the active Turkey, and connects the Black Sea to the Medi-
faults within the Sea of Marmara through a series terranean Sea by way of Istanbul (Bosphorus) and
of cruises beginning in 2000. Early cruises [e.g., Çanakkale (Dardanelles) Straits. Itconsists ofthree
∼1200 m deep fault‐bounded extensional basins
Halbach and the Scientific Party, 2000] identified
where a few kms of sediment has recently accu-
watercolumnmethaneanomalies,ashallowsulfate
mulated (Figure 1) separated by compressional
reaction zone, and seafloor microbial mats at the
ridges that rise ∼600 m above the basin floors [Le
Main Marmara Fault suggesting an active hydro-
logical system associated with the fault [e.g., Judd Pichon et al., 2003]. This part of the Sea of Mar-
and Hovland, 2007]. The objective of the 2007 mara is crossed east to west by the Main Marmara
Marnaut cruise (this study) was to examine this Fault(MMF),whichformsthenorthernmostbranch
relationshipbetweenfluidexpulsionsitesandactive of the North Anatolian Fault (NAF) and takes up
most of the 23–27 mm/yr strike‐slip motion
faults.Thecruiseoccurred12Maythrough12June
2007, aboard the French research vessel l’Atalante between Eurasian and Anatolian plates [Le Pichon
et al., 2003; Meade et al., 2002; Reilinger et al.,
withthesubmersibleNautilewithparticipantsfrom
2006]. The MMF joins the İzmit Fault in the east
France,Turkey,Italy,Spain,Germany,andtheU.S.
The technical objectives of the cruise were to sys- andtheGanosFaultinthewest.Theactualsliprate
tematically locate fluid outflow sites, via geophysi- on the offshore MMF has mostly been estimated
calmeansanddirectobservationbysubmersible,to from models. Estimations for the dextral strike slip
define the spatial relationship between active faults component vary between 12 and 23 mm/yr de-
pending on the fault segment considered and the
andfluidoutlets,tosamplethesefluidstodetermine
assumptions underlying each model [Flerit et al.,
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Figure1. MapoftheSeaofMarmaraindicating(a)regionalreferenceframeandtectonicframework;(b)thenames
andlocationsofmajorfeatures,faultlocations,andbathymetry;and(c)thelocationsofthecoringsitesreferencedin
this manuscript.
2003; Hergert and Heidbach, 2010; Le Pichon et of marine sediment above the lacustrine‐marine
al., 2003]. Slip on secondary fault branches is less transition with shelves and ridges receiving only
(1–5 mm/yr), but this should not bias their hydro- 0.1–0.5mm/yr[Armijoetal.,2005;Çağatayetal.,
geological significance. The seafloor trace of the 2000; Mercier de Lépinay et al., 2003]. Presently,
MMF is linear on the ridges, and more complex in thewateroftheSeaofMarmaraisstronglystratified
the basins. Each sector displays different structural with a 20–40 m thick layer of brackish Black Sea-
characteristics and tectonic styles corresponding to waterenteringthroughtheBosphoruscappingwarm
transtension, partitioned strike‐slip and extension, saline Mediterranean water that enters through the
or,locally,transpression[Armijoetal.,2002;Bécel Dardanelles andsinksbelowthecap.
et al., 2010; Carton et al., 2007; Rangin et al.,
2004]. M7+ earthquakes appear to occur in the
regionona∼100yraveragetimeintervaland,after 1.2. Prior Evidence for Fluid Emissions
the Kocaeli M7.4 earthquake [Barka et al., 2002] [5] Methane emissions associated with the Main
alongtheİzmitFault,theMMFwasproposedtobe MarmaraFaultwerefoundduringtheMeteorcruise
thelikelysourceofamajorearthquake(>M7)inthe M 44/1 (1999), as evidenced by black‐grayish sea-
near future [Hubert‐Ferrari et al., 2000; Parsons floorsedimentswithbacterialmatsandbymethane
et al., 2000]. anomalies in the lower part of the water column
[Halbach and the Scientific Party, 2000]. These
[4] The Sea of Marmara was a freshwater lake iso- observations were made where the fault crosscuts
lated from both the Mediterranean and Black Seas
the Western High separating the Tekirdağ and
prior to the last glacioeustatic sea level rise [Aksu
the Central basins. In addition, a shallow sulfate‐
et al., 1999; Çağatay et al., 2000; Ryan et al.,
methane reaction zone (SMRZ) was detected in
2003] with the transition to a marine environment
sedimentcoresfromthesameareaatdepthsof4to
occurring progressively from 14.7 to 12.4 kyr BP
5mbelowtheseafloorandfromtheÇınarckBasin
and with a lithological transition in sediment cores
at 3 mbsf [Çağatay et al., 2004]. High‐resolution
occurringatabout12kyrBP[Çağatayetal.,2000;
seismics [Le Pichon et al., 2001] and chirp pro-
Vidal et al., 2010]. Sedimentation rates as high as
files indicated the probable presence of trapped
2.5 mm/yr in the basins has deposited up to 35 m
gaseswithintheuppermostsedimentarysequences.
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ObservationsduringtheMarmarascarpscruise(2002) compaction or dilation driven flow. In addition,
showed that bubbles (presumably of methane) are temporalinfluencessuchasrecentearthquakeactiv-
often present immediately beneath the seafloor at itymayaffectemissions.Forexample,theregionof
reduced sediment patches, notably on the Western theCentralHighseismicgapexhibitslessgasemis-
High.Morespectacularactivechimneyswerefound sionswhiletheregionwestoftheKocaeliearthquake
in TekirdağBasin andin theCentralBasin[Armijo rupturethatexhibitsmicroseismicactivityexhibits
et al., 2005] but the fluids were not sampled. In extensive gas emissions [Géli et al., 2008].
Tekirdağ Basin, fluid outflow is visible, due to a
contrastinopticalindices;however,thetemperature [7] Thespatialrelationshipbetweenfluidseepsand
thefaultswas constrained bya combinationof pre-
is less than 0.5°C above the bottom water tempera-
viously collected subbottom seismic surveys that
ture, implying that a fluid of very different salinity
locatedthefaultstructures[Armijoetal.,2002,2005;
is expelled at this site. Water sampled below the
Demirbag etal.,2003;Imren et al., 2001; Rangin
SMRZ in cores taken in these basins and on the
et al., 2004] and also imaged blanking caused by
Western High exhibited decreasing salinity with
gasinthesedimentcolumn[LePichonetal.,2001;
depth. Zitter et al. [2008] suggested that the burial
Zitter et al., 2008], by water column acoustic sur-
of Lake Marmara fresh or brackish water was the
veying that imaged bubbles in the water column
source of the freshened end‐member fluid seen at
[Géli et al., 2008], and by visual surveys by sub-
deep levels in cores from the Marmara‐VT cruise
mersible. Indications of flow included gas expul-
andatthechimneys.Reasoningbasedontheextrap-
sion,visibleaqueousoutflow,areasofblacksulfidic
olation of mixing lines suggested a minimum chlo-
sediment, and characteristic seep biology such as
rinityof100mMfortheburiedfluid,thoughttobe
microbial mats and chemosynthetic bivalves (see
representative of the Marmara glacial lake. These
reviews by Judd and Hovland [2007] and Levin
Tekirdağ chimneys were found on the MMF out-
[2005]). In the basins, fluid seeps were restricted
crop and are located on a recent seafloor rupture
to a few 10s of meters of the bounding faults
(possiblyof1912Ganosearthquake),whichbroke
which were well located by fault scarps. The one
oldercarbonatecrusts.IntheCentralBasin,smaller
significant exception to this was in the southern
chimneys and abundant carbonates were found on
Çınarck Basin where more widely distributed
thesubsidiaryfaultboundingthebasintothenorth.
seepage was observed associated with a broad
This fault does not display evidence of comparable
region of closely spaced normal faults rather than
recentsurfacerupture.Manifestationsoffluidexpul-
a simple bounding fault. On the ridges, fluid
sion in the Çınarck Basin area were not systemati-
seepage was associated with the main strike‐slip
cally investigated previously but intense expulsion
fault but also extended along anticlines.
of methanein theİzmit Gulf nearthe1999Kocaeli
earthquake rupture have been reported [Alpar,
1999]. 2. Methods
2.1. Sampling Strategy
1.3. Spatial Relationship Between Fluid
Flow and the MMF [8] The acoustic method of gas plume detection
described by Géli et al. [2008] was extremely effi-
[6] The Sea of Marmara is an exceptional case
cientandallowedustodiscovermanynewseepsites
were active seafloor venting sites are found on the
and to firmly establish the direct correspondence
surface trace of a major plate boundary fault. The
betweenwatercolumnacousticanomalies,seafloor
distribution ofseepsinconvergentmarginsisoften
seeps,andactivefaults.Afteracousticidentification
more complex with rare, if any, seeps at the frontal
of potential seep sites we visually confirmed and
thrustandthebulkoffluidoutflowdistributedover
exploredthesitesutilizingtheNautilesubmersible.
theouterforearcatthesurfacetracesofthrustfaults,
Ten cold seep sites spanning the fault zone were
normalfaults,andmudvolcanoes.Ourobservations
explored during 30 dives. Sites for coring were
in the Sea of Marmara suggest a simple pattern
wherethemainstrike‐slipfaultisthemainchannel chosen,basedonacombinationofpriorgeophysical
surveys, our acoustic bubble detection results, and
for fluid expulsion on the ridges and the basin
our visual surveys, to include a diverse and repre-
bounding faults are the main fluid channels in the
sentative sampling. Thirteen piston cores of up to
basins.Thissimplepatternisoverlainbytheeffects
10 m were collected in and near the seep areas, as
ofturbiditechannelswhichprovidelateralconduits,
wellasbackgroundcores,andutilizedforporefluid
local structural effects such as anticlines which can
actastraps,andbylocalbendsintheMMFcausing
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Figure2. Resultsofthecoreporefluidanalyses.(a)TekirdağBasin,(b)ÇınarckBasin,(c)CentralBasin,(d)ridge
sites, and (e) Western High mound sites. See section 3 for details.
analysis(Figure1).Coringwascarriedoutwiththe core liner is drilled at appropriate intervals and the
IFREMER 10mKullenberg pistoncoring system. sampler inserted until it seals against the liner. For
this study we either attached the luer to a syringe
needle which was then inserted into evacuated
2.2. Analytical Methods
plasma vials or attached the luer to a 10 ml syringe
[9] Upon recovery, the cores were rapidly sec- and extracted the plunger and affixed it at full
tioned to 1 m lengths on deck, capped, and moved extension creating the needed vacuum for extrac-
to the 4°C cold room. Fluid was extracted at tion.Typically5mloffluidwasextractedfromthe
intervals along the core by utilizing Rhizon shallowest levels (0–2 m) and this was reduced
samplers[Dickensetal.,2007;Seeberg‐Elverfeldt downcore to as little as 1–2 ml near the bottom,
etal.,2005].Rhizonsamplersaremadeofa2.4mm depending on the core's properties. The primary
hydrophilic porous polymer tube with a pore size reason for this sampling technique was to preserve
of0.1mmattachedtoPVCtubeandluerconnector. thecoresforsubsequentphysicalpropertyandsed-
Thepolymertubeissupportedbyanylonwire.The imentological work. This technique also provided
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Figure 2. (continued)
excellentfiltrationandminimizedfluidcontactwith preserve them for onshore analysis. The remaining
oxygen. fluid was saved for other analyses and archiving.
Chloride concentration was determined on board
[10] The fluids were subsampled in the cold room from these latter samples by titration with silver
immediately after collection. When sufficient fluid
nitrate with the indicator potassium chromate/
was collected, 3 ml of sample was quickly trans-
potassiumdichromateusingIAPSOseawaterasthe
ferred to a 4 ml vial, 0.40 ml of 0.25 M Zn acetate
standard. Onshore elemental analyses were done
added, and the vial capped, shaken, and stored.
via ICP‐OES. Sulfide and ammonia were analyzed
This pretreatment method precipitates the sulfide
by standard methods [Gieskes et al., 1991].
allowing for storage for onshore analysis many
weeks later by standard colorimetric methods. For [11] Cores were scanned on a GEOTEK multi-
all fluids collected, 1 ml subsamples for elemental sensor core logger (MSCL) and these logs used to
analysis were acidified to prevent precipitation and quantify voids in the cores and correct the sampled
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Figure 2. (continued)
depth intervals for expansion. The MSCL profiles extensive, patchy, and diffuse, displaying patches
were also used for correlation of the interface and of black sulfidic sediment with typically white to
piston cores recovered from the same location, yellow/orange microbial mat on the surface. One
which is important for checking if the piston core end‐member type was highly focused, emitting
tops were missing. All depths reported here are ambient temperature shimmering fluids of low
corrected rather than indicated depth. salinity that precipitated chimney structures. At
other sites gas bubbles were seen coming from the
sediment and, in one occurrence, from open frac-
3. Results and Interpretations
turesinunderlyingsedimentaryrocks.Anothertype
had the appearance of a flow with precipitate cov-
[12] The tectonic environments of the seeps varied ered sediment with a fan morphology extending
from basin bounding extensional faults to strike‐
downslope from the seep. This latter site was asso-
slipfaults,manyofwhichexhibitedsharpandfresh‐
ciated with very high salinity fluids and significant
appearing fault escarpments. Most seeps were
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Figure 2. (continued)
traces of hydrocarbons and included shallow gas locations are shown in Figure 1. Average bottom
hydrate well outside the methane hydrate stability water composition is indicated in Figures 2a–2e by
field. arrows.
[13] The concentration profiles of Cl–, Na+, Mg2+,
Ca2+, Sr2+, K+, Li+, B, Si, Ba2+, Fe, Mn, sulfate, 3.1. Basin Sites
sulfide, and NH are shown in Figure 2. They are
grouped by loc4ation: Tekirdağ Basin, Çınarck [14] Figures 2a–2c show the downcore chemical
profilesoftheMarnautbasincoreswithMarmara‐VT
Basin,CentralBasin,Ridgesitesexcludingthetwo
cores VT‐2737, VT‐2740, and VT‐2742 included
hydrateandcarbonatemounds,andthehydrateand
for reference [Zitter et al., 2008]. Cores KC‐2 and
carbonate mounds. This grouping facilitates dis-
KC‐13 were taken as reference at sites without
cussion and appropriate scaling of the axes for the
nearby seafloor manifestations of gas or water out-
wide ranges of concentrations encountered. Core
flow. Cores KC‐17, VT‐2737 and VT‐2742 are
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Figure 2. (continued)
located about 1 km from known cold seep sites et al., 1996, 1997]. AMO produces bicarbonate
but with chirp profiles indicating the presence of which reacts with seawater Ca, Mg, and, Sr to
free gas in the sediment. The fluid compositions of producecarbonates,leadingtotheobservedminima
these and the background cores are dominated by of these ions coinciding with the SMI. KC‐17 and
shallowredoxreactionsandcarbonateprecipitation. the VT cores exhibit a similar pattern, but with a
Afresheninggradientwithdepthisalsoobservedto deeper SMI at 750 cm depth. Below the SMI in all
variousdegreesinClandNaprofiles.KC‐2,KC‐5, cores Ba concentrations increase due to dissolution
and KC‐13 exhibit a shallow sulfate‐methane reac- of biobarite in the absence of sulfate ions. Cores
tion zone (SMRZ) extending down to the sulfate‐ KC‐7, KC‐19, KC‐30, KC‐31 and VT‐2740 were
methane interface (SMI) at about 300 cm depth. collectednearandatfluidseepsandarealteredwith
The shallow SMI and linear SMRZ are typical of respecttothebasinbackgroundprofilesandnotably
anaerobic methane oxidation (AMO) [Borowski display stronger freshening gradients with depth,
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Figure 3. Detail map of structure and site location at the Tekirdağ Basin site.
indicativeofupwardadvectionoflow‐salinitypore MgandreleaseCa.Thereisaveryclearprogression
fluid. ofcompositionprofileswithdistancefromthefluid
ventsite.KC‐17andVT‐2737arelocated∼1500m
from the vent and have chloride profiles similar to
3.1.1. Tekirdağ Basin
oneanother.KC‐17ismuchclosertothefaultscarp,
[15] All of the Tekirdağ Basin (Figure 2a) cores however,whichmightsuggestthattheventisapoint
were taken at the southern margin at a depth of source with minor lateral leakage along the fault,
1120 m near the site referred to as “Jack the or that there is simply a maximum distance from
Smoker” [Armijo et al., 2005]. Figure 3 shows the the fault beyond which flow along it has minimal
spatial relationship between the cores. Locations influence. Alternatively, KC‐17's nearly vertical
are estimated to be ±25 m. Marmara‐VT cores profiles of Ca, Sr, and sulfate in the top ∼500 cm
2737 and 2740 are also shown in Figure 3 and in followedbyaconcaveupwardlowersectionsuggest
the pore fluid composition plots for reference that there may be downward flow at this site while
[Zitter et al., 2008]. KC‐31 was taken above the VT‐2737appearsmorelinearanddiffusive.
fault scarp at the Jack the Smoker site in Tekirdağ
Basin and only one meter was recovered which 3.1.2. Çınarck Basin
appears to be the core top. These cores are domi-
nated by the influence of Lake Marmara brackish [16] Core KC‐2 was taken at a depth of 1280 m in
water. In particular, core KC‐30, taken adjacent to the east central Çınarck Basin (Figure 2b) and was
the site “Jack the Smoker,” exhibits strong fresh- intendedasarepresentativebackgroundcore.KC‐5
ening with depth. Crossplots of the relatively un- was located at the base of the southern scarp in a
reactive pore fluid chemical components against smallfault‐boundedandsediment‐filleddepression
chloridecanbeusedtoestimatethecompositionof at a depth of 1270 m. The pore fluid profiles from
the end‐member fluid (Figure 4). This is in close these two cores are nearly identical and distinguish
agreement to the estimate made by Zitter et al. themselves from the other basin cores by their
[2008] and used in their diffusion profile model 2. unusuallyhighboroncontent.Thisisnotsurprising
There is evidence, however, that these fluids have consideringthatmorethan60%oftheworld'sboron
been involved in silicate diagenesis. The Na/Cl reserves are located in Turkey and much of that is
molar ratio of 0.5 of the end‐member fluid is sig- locatedinthe30,000km2SusurlukDrainageBasin,
nificantly less than typical lake,river, and meteoric south of the Sea of Marmara, which drains into the
water,aswellasseawaterandpresumablythisisdue Çınarck Basin [Kazancı et al., 2006]. These cores
to Na uptake during clay formation from silicates also exhibit a shallower sulfate‐methane reaction
such as feldspars which would also take up K and zonedepththanmostbasincores.Thesmalldecrease
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