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Seafloor Magnetotelluric Sounding Above Axial Seamount

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Seafloor Magnetotelluric Sounding Above Axial Seamount GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 17, PAGES 2275-2278, AUGUST 15, 1996 SeafloorMagnetotelluric Sounding Above Axial Seamount Graham Heinson Schoolof Earth Sciences,Flinders University, Bedford Park, Australia Steven Constable Instituteof Geophysicsand Planetary Physics, University of CaliforniaSan Diego, La Jolla Antony White Schoolof Earth Sciences,Flinders University, Bedford Park, Australia Abstract. Axial Seamountis a large,active, ridge axis volcano Instrumentation and Data locatedon the centralsegment of the Juande FucaRidge in Three setsof magnetotelluric(MT) instrumentswere de- the northeastPacific Ocean. Magnetotelluric (MT) datahave ployedfrom 3 ra July to the7 th September, 1994. The GPS- beencollected at threesites, approximately 4 km apartaround determineddeployment locations and depths(Plate 1) were the easternrim of the volcano,during a 65-day deployment. MT responses,in the bandwidth of 102- 105s, arealmost for Pele 45ø 55.47' N, 129ø 59.03' W (1550 m), Macques45 ø 57.48' N, 129ø 58.98' W (1520 m) andUlysses 45 ø 59.50' N, isotropic,with a weakly-definedprincipal direction of strike 129ø 59.81' W (1490 m). parallelto the maintopographic trends of Axial Seamount, Electric fields were recordedby the two-componentshort- and are relativelyflat over the wholebandwidth. Apparent resistivitiesare of the orderof 7 - 20 •m, andphases are as armed electrometerdescribed in Constableand Cox [1996], with the high-gainAC amplifiersreplaced by lower gain DC low as 30ø at the shortperiods. Diagonalterms of the MT amplifiers.The least-countwas 0.006/•V andthe samplerate tensorare an orderof magnitudesmaller than the off-diagonal 1 s. The magnetometers,developed at Flinders University terms,suggesting that three-dimensional effects on the data [White, 1979], areinstalled in standard17 inchglass instrument areminimal. Two-dimensionalinversions suggest that seafloor housings,which were swappedwith oneof two glassbuoyancy bathymetryand the distant coastlines have a surprisinglysmall floatsin the E-field package. The three-componentring-core effecton the MT response,and one-dimensional inversions fit the MT data to within the errorswith no serial correlationin the fluxgatesensors had a least-countof 0.1 nT and a samplerate set at 30 s. Additionally, a two-axis clinometerrecorded the residuals.A low crustalresistivity is themost robust part of the model,probably due to seawaterin fracturesand possibly due magnetometertilt with a precisionof 0.01ø every 78 minutes duringthe deployment. to a magmachamber. An electricalasthenosphere, although Three complete65 day recordswere obtainedfrom the elec- lesswell constrained,exists over a depthrange 30 - 60 km, trometers.Electrode potential drifts were rapidover the first 5 andthe resistivity of thisregion is compatiblewith about8% fraction of melt. to 10 days,of theorder of 20/•V/day, butsteadied to a relatively monotonicvariation of lessthan 5/•V/day for the remainder of the experiment. The magnetometerrecords ranged from Introduction 65 days at Pele, 10 days at Macquesand 4 days at Ulysses. Axial Seamount lies on the intersection of the Cobb- Tilt recordsindicated angles of magnetometerrepose of the EickelbergSeamount Chain andthe Juande FucaRidge, about orderof 1 - 5 ø in both x and y-orientations.Some ocean tidal 500 km west of the Oregon(USA) coast. The volcano(Plate variationsof the orderof 0.2ø were presentin the Ulyssestilt 1) is the shallowestpart of the Juande FucaRidge, indicating records,but Macquesand Pele showedlittle or no movement. a significantand robustsupply of magma [e.g. Delaney et al., For eachsite, time seriesdata were processedto obtainMT 1981], andis currentlyvolcanically and hydrothermally active, responsesusing the methodsand computer code of Egbertand with evidenceof recentlava flows,hydrothermal activity, and Booker [ 1986]. Data from the Victoria MagneticObservatory deformation[Embley et al., 1990; Fox, 1990]. Fowardmodel- were usedas a remotereference. Without the remotereference, ing and inversionof sea-surfacemagnetic anomaly data [Tivey uncorrelatednoise in thehorizontal magnetic field data reduces and Johnson,1990] indicatesa temperaturesof greaterthan the MT impedance,so that the apparentresistivity drops off 200øCat a depthof 700 m beneaththe caldera,suggesting the rapidly with the power spectraof the electricfield, although presenceof a magmachamber at3 kmdepth below the sum- phasesare less affected. MT response estimates were obtained mit,or a zoneof extensivesubsurface alteration. Hildebrand for sites Macques and Pele. The short duration of theUlysses et al. [ 1990] model sea-surfacegravity anomaliesand seafloor magnetometerrecord resulted in muchpoorer responses which gravitymeasurements to infer the existenceof a shallow,low- have not been used. densityregion. The low-densityregion could be melt, butother MT responseswere obtained in the bandwidth2x 102 to factors,such as porosityand mineralogy,are important. 6 x 104s; at shorter periods coherences dropped rapidly and at longerperiods source-field effects became dominant [Wanna- maker et al., 1989a]. The MT impedancetensor was thenro- Copyright1996 by theAmerican Geophysical Union. tatedto find the dominantstrike by the decompositionmethod of Lilley [1995]. Rotation angles were approximatelyfre- Papernumber 96GL01673 quencyindependent, and gave a predominantstrike of about 0094-8534/96/96 GL-01673505.00 20ø and 30ø westof geomagneticnorth for Pele and Macques 2275 2276 HEINSON ET AL.' SEAFLOOR MAGNETOTELLURIC SOUNDING Depth,m _ _ MT Response Interpretation ' -- -1400 Heinson and Constable[1992] arguedthat for an electri- cally resistiveoceanic lithosphere underlying the whole ocean . , _ ,( -1600 basin,seafloor MT datawill be significantlydistorted from 1D due to the coast-effect. Tarits et al. [1993] pointedout that / / i,•-ß -1700in the presenceof leakagepaths to more conductiveparts of the mantle, the coast-effect distortion was much reduced, and ,,½oo' ' -,oo suggestedthat subductionzones may representsuch a leakage • • ' • -20• path, aswas thoughtto be the casein the EMSLAB experiment [Wannamaker et al., 1989b]. Here we make the casethat Axial Seamountmay alsoact as a leakagepath to the asthenosphere. • -22• The evidencefor a low resistivitypath to themantle is three- fold. Firstly, the observedisotropic MT responsewould not be 45o50,' % : ,• ' -23•predicted from the coast-effectalone. AlthoughHeinson and Constable [1992] showed that isotropic MT responsesmay I • •• .. , -24•occur in an oceanbasin with no leakagepaths to the deepman- 4 , / -26• tle, it requiredthe site to be approximatelyequidistant to two or more orthogonalcoasts. For Axial Seamount,this is not -27• the case as it is much closer to the coastline of western North 45*40' •- _x t • -28• America (450 km) thanany othercoast. Secondly,apparent re- 229* 50' •0 ø •' 230 ø 10' 23• 20' sistivities in the TE orientation are smaller than similar seafloor Plate 1. Site location of Axial Seamount and the three de- MT responseson the Juan de Fuca Plate [Wannamakeret al., ployed MT instruments 1989a] and elsewhere.Finally, we notethat relativelyflat MT responseover two decadesof period suggeststhat the resisi- tivity structurebeneath Axial Seamountis relatively uniform respectively. These orientationsare most sensitiveto local with depth.To testthese hypotheses, we haveinverted the data seafloorbathymetry changes and the trendof the coastline. using lD and 2D techniques. Figure 1 showsthe MT responses,with error estimates,at A 2D inversion,using the Occam'salgorithm of deGroot- Pele. The most notablefeatures of theseplots are that the MT Hedlin and Constable [ 1990], was used to examine the effect data are almost isotropic, and apparentresistivities are low, of ridge topographyand the North American coastline. A of order10 f•m. Differencesin thealong-axis MT responsesprofile was chosenperpendicular to the coast which passed (TE mode) at Macques and Pele are generally within the er- throughsites of Macquesand Pele. Inverting single-siteMT ror estimates. Across-axis(TM mode) apparentresistivities responsesfor 2D structuremay seem somewhatambitious. are slightly larger at Macques,but phasesare similar. Diag- However, bathymetryand coastlines,coupled with seawater onal elementsof the MT tensorsat both siteshave apparent resistivity,provides additional constraints on the modeling. resistivitieswhich are an order of magnitudesmaller than the off-diagonalelements, from which we concludethat 3D effects LogDm on the data are minor. 0.0•. i -- 1.6 1.5 Depth, km 1.4 okm 0.5 axial 1.31.2 1.1 5.2 'lml 7,5 km ' 1.0 12kin 1.0 23 km 46 'hm 96 km 0.90.8 203 kan 4.34 km 1.5 0.7 931 km 0.6 2003 km -100 -51 26 84 149 229 325 379 599 0.5 Distance km 1 10 2.0D_ I • .... I 0.4 0.9 1.0 l.rl 1.2 1.3 Resiqtivity,Ohm. m [logscale] RMS Misfit Plate 2 2D smooth Occam's inversion of both Pele TE and Plate 3. 1D Occaminversions, with firstderivative roughness, TM modesfor a transectpassing through the coastlineand of theTE responsesat sitePele for RMS valuesvarying from Axial Seamount. The transect has a strike direction of 90 ø east 0.9 to 1.3. The dashedline showsthe preferredmisfit level, of geographicnorth. The RMS misfit is 1.0. the profileis shownin Figure2. HEINSON ET AL.: SEAFLOOR MAGNETOTELLURIC SOUNDING 2277 Althoughthere are many more model parameters than data, 0.0 partsof themodel that are unresolved are simply determined 0.2 by the smoothingpenalty. The 2D inversionof Pele MT data is shownin Plate 2, and 0.4 givesa fit to theTE andTM responsesto
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