JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B9, PAGES 15,967-15,985,SEPTEMBER 10, 1993
Imaging CrustalStructurein SouthwesternWashington
With Small MagnetometerArrays
GARY D. EGBERT
Collegeof Oceanicand AtmosphericSciences,OregonState University,Corvallis
JOHN R. BOOKER
GeophysicsProgram AK-50, Universityof Washington,Seattle
We usedata from a seriesof small (three to five stations)overlappingmagnetovariational
(MV) arraysto
imagevariationsof verticallyintegratedelectricalconductivityin the crustof southwestem
Washington.Two
principalstructuresare revealed: a large north-southtrendinganomaly(the southemWashingtonCascades
Conductor(SWCC), which has been detectedby severalpreviousinductionexperiments),and a smalleranomaly which branchesoff of the SWCC just north of Mount St. Helens and trendswestwardbeneaththe
ChehallisBasin. A weakereast-westtrendinganomalyis evidentfarther to the north beneathsouthernPuget
Sound. The MV resultsconcerningthe SWCC arereasonablyconsistentwith the modelof Stanleyet al. (1987),
who interpretthe anomalyas a suturezoneof mid-lateEoceneage,but the arraydataallowsus to map the horizontal extentand complexthree-dimensional
characterof the SWCC in greaterdetail. We suggestthat the
SWCC represents
a sectionof the early Cenozoicsubduction
zonewhichis analogous
to the present-day
Olympic Peninsula.In the regionwest of the Cascades,the array data showthat crustalconductivityis distinctly
threedimensional,consistingof highly resistiveblocks(crystallinerocks)separated(in the upper5-10 km at
least)by interconnected
narrowregionsof higherconductivity(sedimentary
units). This pattemof conductivity
variationsis consistentwith the inferredorigin of the region as a seamountcomplex,which was subsequently
broken into discreteblocks which have been thrusttogetherduring and after accretionto the North American
continent. The distributionof anomalouselectriccurrentsand our model for crustalconductanceare in striking
agreementwith a varietyof othergeophysicalconstraints,
includinggravity,magnetics,presentcrustalseismicity, and the pattemof recentvolcanicvents. The St. Helensseismiczone (SHZ), which coincideswith the
westemedgeof the broadsouthernportionof the SWCC, is abruptlyterminatedin the north by the smaller
east-westtrendingconductivezone. North of the SHZ nearMount Rainier,seismicityis concentrated
in a narrow bandcoincidentwith the very narrow northemportionof the SWCC. In addition,volcanicventsare concentratedaroundthe edgesof the SWCC but are rare in the interiorof the zoneof high conductivity.The magnetometerarray data thus suggestthat presentpattemsof crustaldeformationand volcanismare in part controlledby the complextectonichistory(and resultingcrustalstructure)of the region.
INTRODUCTION
While the tectonichistoryand crustalstructureof the regionare'
understoodin broadoutline,there is much aboutthis region that is
The crust in western Washington is structurally complex,
in dispute.In particular,throughoutmostof WashingtonandOrereflectingthe tectonichistory of the Pacific Northwest margin of
gon
the boundarybetweenthe Peripheralrocksand the older crust
North America (Figure 1). Since at leastthe earliestTertiary, the
geologicaldevelopmentof the regionhas been dominatedby the to the easthas been completelycoveredby mid-late Tertiary and
effectsof plate convergence,althoughBasin and Range extension Quaternaryvolcanismin the Cascades.Direct observationsof the
and most probably interactionswith a mantle plume have also locationand nature of this sutureare thus impossible. Similarly,
been important (see recent reviews by Armentrout [1987] and the accumulationof forearc basin sedimentaryand volcanicrocks
Duncan and Kulm [1989]). Ongoing seismicityand active vol- obscuresdeeper structurein the Peripheral rocks. Geophysical
canismin the Cascadesmagmaticarc attestto the continuedcon- methodsfor remote sensingof crustalstructureare thus critically
of this complexregion (see
vergenceof the subductingJuan de Fuca and overriding North importantto a further understanding
Muffler
[1990]
and
references
therein
for a range of geophysical
Americanplates. East of the Cascadesin Washington,the basestudiesof this area). In this paper we use time variationsof the
ment consists of Mesozoic
and Paleozoic
rocks which were
geomagneticfield measured at an array of magnetometersto
accreted to North America in the Mesozoic [Cowan and Potter,
1986]. West of the Cascades,the oldest rocks are the Paleocene image large-scalevariationsof vertically integratedcrustalconductivityin southwestern
Washington.
to Eocene basalts of the Crescent, Siletz, and correlative formaMagnetovariational (MV) array data can be used to map
tions [Snavelyet al., 1968; Duncan, 1982]. These basalts,which
were eruptedon an oceanicplate near North America and then anomalousmagneticfields inducedin the Earth by time varying
accretedto the continentduring late Eocene subduction,formed externalsources.Suchmapsprovideindirectestimatesof the distribution of electric currents in the Earth and can be used to make
the basementon which a forearc basin subsequentlyaccumulated
[e.g., Armentrout, 1987]. The forearcbasinrocksand the Eocene inferencesabout lateral variationsof conductivity,which in turn
basalts(togethercalled the Peripheralrocksby Tabor and Cady can be relatedto rock type [e.g., Gough, 1989]. We presenta sim[1978]) form the Coast Range and the basins of the Puget ple schematicmodel and overview of theseideasin Figure 2. To
a large extent, the interpretationof MV data has been qualitative,
Lowlands-Willamettetrough.
with the existenceof conductivityvariationsinferredfrom careful
examinationof geomagneticevent maps or from single-station
horizontal to vertical field transfer functions (see Gough et al.
Copyright1993 by the AmericanGeophysical
Union.
[1989] for a recent example). However, much more can be
gleanedfrom MV data when more sophisticated
quantitativeproPaper number 93JB00778.
0148-0227/93/93JB-00778505.00
cessingand inversionmethodsare employed[Beamishand Banks,
15,967
15,968
EGBERT
ANDBOOKER:
CRUSTAl.
STRUCTURE
INSWWASHINGTON
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Fig. 1. Simplified
geology
mapof western
Washington.
Insetgiveslocation
andregionaltectonic
setting(WA, Washington;
OR,
Oregon;BC, BritishColumbia;
VI, Vancouver
Island).Key features
notedinclude:principal
basins(CB, ChehallisBasin;PL,
PugetLowlands;
AST,AstoriaBasin;GH,GraysHarborBasin),outcrops
of theCrescent
Formation
Basalts
(CFB,including
BH,
BlackHills,andWH, WillapaHills),volcaniccoverin theCascades
andto theeast(RAI, MountRainier;STH, MountSt. Helens;
ADA, Mount Adams; CP, ColumbiaPlateau;CRB, ColumbiaRiver Basalts),the Saint HelensSeismicZone (SHZ), and the
southern
Washington
Cascades
Conductor
(SWCC)mapped
by Stanleyet al. [1987]. Also,OSC,OlympicSubduction
Complex;
LRF, LeachRiver Fault; NC, North Cascades;
CR, ColumbiaRiver; PS, PugetSound;H, HoodCanal;Sea.,Seattle;Port.,Portland.
1983; Banks and Beamish, 1984; Banks, 1986; Egbert and
Booker, 1989; Egbert, 1991]. Here we apply quantitativemethods
for MV array analysisto a seriesof small overlappingarrays run
in southwesternWashingtonbetween 1980 and 1986. While our
focus is on the final resultsand their significancefor the geology
and tectonicsof this region, we also give, in the context of this
illustrativecasestudy, an overview and summaryof the data processingand interpretationtechniquesusedhere.
BACKGROUND
Geologyand TectonicSetting
The studyareain southwestern
Washingtonoccupiesa section
of the activePacificNorthwestmarginof North America(Figures
I and 3). The arraycoversan area which includesthreeQuaternary volcanosof the magmaticarc associatedwith obliquesubductionof the Juan de Fuca plate (Figure 1): Mount St. Helens,
EGBERTAND BOOKER:CRUSTALSTRUCTURE
IN $W WASHINGTON
Mount Rainier, and Mount Adams. It is a seismicallyactive area
with a well-definedbut abbreviatedshallowdippingBenioff zone
beneaththe PugetLowlands [Taber and Smith, 1985] and considerable crustal seismicity [Ludwin et al., 1991], primarily in the
Puget Lowlands and in a narrow belt trending north-northeast
from Mount Saint Helens (the Saint Helens Seismic Zone, or SHZ
[Weaver and Smith 1983; Weaver et al. 1987]).
\e\6 s
15,969
To the east and north of the MV array, the basementconsistsof
Mesozoic and Paleozoic rock units. The western portion of the
array is within the Coast Range province, which is underlain by
the upper Paleoceneto middle Eocene basaltsof the Crescent,
Siletz, and correlative formations. The thickness of these volcan-
ics, which we refer to collectively as the Crescent Formation
basalts,has been estimatedat 3-7 km or greaterby $navelyet al.
[1968]. Petrologicand geochemicalevidenceimply that the Crescent basalts formed
on oceanic
crust as a chain
of seamounts
[Shayelyet al., 1968;Duncan, 1982]. Interbeddingof basaltswith
distinctivecontinentallyderived conglomerateimplies that these
islandswere formed close to the margin of the North American
continent [Cady, 1975]. After accretion, the Crescent basalts
occupiedthe forearc of the convergentmargin. From the middle
Eocene to the middle Miocene, thick sequencesof marine and
nonmarine
sediments and volcaniclastic
rocks accumulated
in the
forearc basin [Armentrout,1987]. Several pulsesof volcanism
also occurredduringthis time, producingthe transitionaloceanic
Cowlitz volcanics(43-38 Ma), the magmaticarc Goble volcanics
(38-29 Ma), and the Columbia River basalts (15 Ma), which
flowed into the region from the east.
Paleomagnetic
datarequiresubstantial
(15ø-80
ø) posteruption
clockwise rotation of the Crescent basalts [Wells, 1990]. Three
mechanisms
of rotationhavebeenproposed:(1) regionaltectonic
rotation of a rigid microplateduring oblique collision with the
North American continent, (2) rotation of the arc and forearc
causedby intracontinental
extensionin the Basin and Range,and
(3) dextralshearof the continental
margindrivenby the obliquely
subductingJuande Fuca plate [Simpsonand Cox, 1977;Magill et
al., 1981; Wells et al., 1984; Wells, 1990]. However, because
overlying continentalsedimentaryand volcanic strata from the
late Eoceneare rotatednearlyas muchas the basement,it is probable that most of the rotationis postaccretion[Wells et al., 1984].
In a summary overview, Wells [1990] concludesthat rotation of
the coastrangeof Oregonis probablydue to a combinationof the
secondand third mechanisms,while in southwestern
Washington,
rotation is dominatedby dextral shear. In particular,in the Willapa Hills (the southwestern
cornerof the array, extendingto the
Pacific Ocean and the Columbia River) differential clockwise
rotations
of smallstructural
blocks(-10 km) of 20-65øhavebeen
documented[Wells and Coe, 1985].
The Puget Lowlands-Willamettetrough,a discontinuousset of
Fig. 2. Schematicoverviewof basicprinciplesemployedin mappingcrustal conductivityvariationswith magnetometer
arrays. (a) We emphasizea
simplethin sheetmodel in which conductivityvarieslaterally in a surface
layer. The shadedregion is more conductivethan the light background.
Spatially uniform time varying magneticfields induceelectriccurrentsin
the Earth which are generallyperpendicular
to the inducingmagneticfields
and tend to concentratein the more conductiveregion. (b) Profile of the x
componentof the magneticfieldH.r whichwouldbe observedalongprofile
A-A'. The horizontalmagneticfields are a superpositionof internal and
externalfields. Becauseof the concentrationof currents,Hx is enhanced
over the conductivezone (and slightly reducedto either side). (c) Plan
view of verticalcomponentH z. The signof Hz changesas the high conductivity zone is crossed. Since for quasi-uniform sourcesthere is no
externalverticalcomponent,H z is due only to internalsourcesand thus
clearly locateslarge-scalevariationsin conductivity. Note also that as the
conductive zone narrows, the currents become more concentrated and are
.g
(c) Hz
deflectedand the amplitudeof H z increases.Variationsof inducedhorizontal and vertical magneticfields at the Earth's surfacecan thusbe used
to image lateral variationsof electricalconductivityin the crust. As suggestedby the simple model of Figure 2a, the methodhas relatively poor
verticalresolution. Furthermore,without someindependentknowledgeof
regional-scaledeep conductivityvariations,the absolutescaleof conductancein the surfaceis only weakly constrained.
15,970
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASttlNGTON
48 ø
[
kilometers
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46 ø
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O
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..',,
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ø
ø
Fig. 3. Stationlocations
andarrayconnections
for the 19 overlapping
arraysin southwestern
Washington
State(containing
a total
of 49 stations)usedfor the analysis.All arraysareconnectedandcanbe combinedinto a singlesyntheticarray.
basins extending from Puget Sound to southernOregon, lies
betweenthe outcroppingCrescentbasaltsof the CoastRange and
the Cascades.Gravity andmagneticdata suggestthat a thick section (20-30 km) of densemafic rocks,interpretedby Finn [1990]
to be basaltand gabbro,underliesboththe CoastRangeand these
basins,extendingeastto aboutthe longitudeof Mount St. Helens.
This interpretationis supportedby recent seismic refraction
profilesto the south,whichrequirea thick sectionof high-velocity
crustunderlyingboth the CoastRangeand the Willamette Valley
in centralOregon[Trehu et al., 1992]. This suggeststhat the section of crust containingthe Coast Range and the basinsof the
PugetLowlands-Willamettetroughshouldbe consideredas a unit.
We will refer to this sectionof crust as the Coast Range block
[Finn, 1990].
It is clear that there must be some sort of buried suture between
this block and the older crustto the east [e.g., Magill et al., 1981;
Duncan and Kulm, 1989]. The northernboundaryof the Coast
Range block is the exposedLeach River fault in southernVancouverIsland(Figure 1). Here Crescentbasaltsare thrustundera
subductioncompleximagedby seismicreflectionprofilesto the
north [Cloweset al., 1987]. The southernboundaryof the Coast
Rangeblock is exposedin southernOregonin the Roseburgformation [Magill et al., 1981]. Aeromagnetic,gravity [Finn, 1990],
and seismic[Snavelyand Wagner, 1982; Trehu et al., 1992] data
imply that the westernboundaryof the Coast Range block is
offshorein Oregonand at the coastin Washington. However, the
eastern boundary in Washingtonand north-centralOregon is
obscurebecauseof the essentiallycompletepost-Eocenevolcanic
cover in the Cascades.
PreviousElectromagneticStudiesin Southwestern
Washington
Law et al. [ 1980] collectedMV data at five sites in an east-west
line which crossesthe center of the array (Figure 3) and found
evidence for a substantialconductivity anomaly beneath the
Cascaderange. They modeledthis anomaly as a narrow northsouthtrending linear feature and concludedthat the anomalous
currents must be shallower than 25 kin. The conductor was subse-
quentlytracedfurtherto the north,almostto Seattle,by Booker
and Hensel [1982], who suggestedthat the anomalouscurrents
were due to channelingof currentsfrom PugetSoundinto a zone
of highconductivityundertheCascades.
Furtherevidencefor a substantialconductivityanomalyin this
area was providedby Stanley[1984] and Stanleyet al. [1987],
who usedmagnetotelluric(MT) data from two roughlyeast-west
profilesacrossthe Cascadesin southwestern
Washingtonto constructtwo-dimensionalmodelsof crustalconductivity. Stanleyet
al. [ 1987] referred to this anomaly as the southernWashington
CascadesConductor(SWCC; seeFigure 1). In theseMT models
the SWCC extendsfrom near the surfaceto depthsof 15 km or
more and consistsof very conductive(1-4 ohm m) rocks. Stanley
et al. [1987] believethat the SWCC representseitheran accretionary prismor forearcbasinrockswhich were accretedand subse-
quentlycompressed
againstthe pre-Tertiaryedge of the North
EGBERTANDBOOKER:
CRUS•rAh
STRUC•rURE
IN SW WASHINGTON
American
continent
in the mid-Eocene.
Because
15,971
shales hold
can be repeatedfor a seriesof overlappingarrays to yield estimatesof relative amplitudesand phasesbetweenall stationsin a
large syntheticarray [Beamishand Banks, 1983; Egbert, 1991].
For the resultspresentedhere, we have developedand useda multivariate generalizationand refinementof the transfer function
methodwhich takesadvantageof the simultaneous
natureof array
data [Egbert and Booker, 1989] and allows for optimal combination of overlappingarrays [Egbert, 1991]. A summaryof these
Seattle and in the south near the Columbia river. However, the
methodstogetherwith a detaileddescriptionof the initial processEMSLAB array dataalsodetecteda broaderand deeperconductor ing of thesearraysis givenby Egbert [ 1991],
This first step resultsin estimatesat a range of periodsof the
continuingto the southunderthe high Cascadesin Oregon. The
to inductionby
EMSLAB MT data imply that this conductivezone is deeperthan total (internal plus external) fields corresponding
spatially uniform sources. In the absenceof lateral conductivity
approximately20 km [Wannarnakeret al., 1989]. Gough et al.
[1989] suggestthe possibilitythat this deeperconductorcontinues variations, such sourcesinduce laterally uniform currentsin the
into Washington,wherea shallowconductoris superimposed.In Earth which flow at right anglesto the external magneticfield.
this view, the SWCC may representa compoundanomaly, which The resultinginternal magneticfields are in turn uniform and in
phasewith the inducingsourcefields [e.g.,Rokityanski,1982]. In
neednot extendas a singleunit throughthe crust.
this idealized case, the polarization and phase of the external
Array Studiesin SouthwesternWashington
source are thus uniquely determined from the observed total
fields. Lateral conductivityvariationsresult in (typically subtle)
The data usedfor our analysisconsistof three-componentMV
data collectedat a total of 49 sitesin a seriesof small arrays,each variationsacrossthe array in amplitude and phase of the total
magneticfields, making a precisedeterminationof the external
consistingof three to five simultaneousstations(Figure 3). All
problematic.In general,then,determination
data were collectedusing EDA three-componentfluxgatemagne- sourcecharacteristics
tometers[Trigget al., 1970] between 1980 and 1986. For the first of the polarizationand phaseof the uniformexternalpart of the
eight of the arrays,data were collectedat a 20-secondsampling observedfields requiressomefurther assumptions.One approach
rate, with typical occupationtimes of approximately4 to 5 days. to this problem [e.g., Schrnucker,1970] is to choose,on the basis
For the 11 remaining arrays, data were sampled at an 8-second of othergeologicalandgeophysicalinformation,a fixed stationas
rate, and arraysran for approximately10 days each. Five of the a normal reference. At this site the horizontalmagneticfieldsare
by lateral conductivityvariations.
arrayswere occupiedin their entiretymore than once,so the total assumedto be uncontaminated
number of separatearray experimentsconsideredis 24. Some The normal fields then implicitly definethe polarizationand phase
preliminary results from these experimentswere reported by of the external sources and can be subtracted from the total fields
to yield estimatesof the inducedanomalousfields,which indicate
Bookerand Hensel [ 1982], and Egbert [ 1991].
geologic structure. We take a slightly different approachand
assumethat the fields averagedacrossthe array are normal (i.e.,
DATA PROCESSING: SMOOTH ESTIMATES OF INTERNAL
correspondto a spatiallyaveragedone-dimensionalEarth [Egbert
ANOMALOUS FIELDS AND CRUSTAL CONDUCTANCE
and Booker, 1989]). Note that with this definition of normal
In this sectionwe summarizethe MV array data processing fields, the anomalousmagneticfields at all stationsin the array
methodsappliedto the southwestern
Washingtonarrays. The raw mustaveragezero for all polarizations.
data consistof a seriesof three-componentmagneticfield time
The estimated anomaloushorizontal magnetic fields for the
seriesrecordedin the overlappingarrays of Figure 3. Each time combinedsouthwesternWashingtonarray are plotted for source
series is divided into a series of short segments,tapered, and fields linearly polarizednorth-southand east-westfor a periodof
Fourier transformed. The resultingFourier coefficientsform the
1000 secondsin Figure 4. Note that the anomalousfield vectors
basis for all further processing. The final result is a series of
are complex (reflecting phase and amplitude variations),owith
smoothmapsof the period-dependent
inducedanomalouselectric imaginary parts generallymuch smallerthan real parts. At most
currentsand an estimateof the vertically integratedcrustalcon- sites the anomalous fields are small relative to the normal fields
water trappedin claysand zeolitesand can remainhighly conductive even at low porosities,Stanleyet al. [1987, 1990] arguethat
the SWCC, which extendsinto the lower crust, where porosities
shouldbe low, is probablydominatedby shalefacies.
The SWCC is also clearly evident in the EMSLAB magnetometer array data [Gough et al., 1989]. Data from this much larger
scale array show that the SWCC is truncatedin the north near
ductance.
(displayedin the upperright cornersof the plots). Areas where
the anomalousfield vectorsare negative(i.e., opposethe normal
Array TransferFunctions
fields, as in the westernpart of the array in Figure 4b) represent
The first step is to combineall of the frequencydomaindata relatively resistiveregionswith reducedcurrentflow, while areas
into estimates of the total magnetic fields which would be where the anomalousfields are positive(i.e., reinforcethe normal
observedat all stationsin the combinedarray of Figure 3 if the fields, as in the area under the Cascadesin Figure 4b) represent
externalsourceswere perfectlyuniformand linearly polarizedin more conductiveregionswith enhancedcurrentflow. Anomalous
fixed directions(say, north-southand east-west). Simultaneous horizontalfield vectorsat anglesto the normal (e.g., in much of
observationsin an array can be usedto estimateamplitudesand Figure 4a) result from channelingand redistributionof electric
structures.
phases(or complexamplitudes)of all field components
relativeto currentsby three-dimensional
the horizontalmagneticfield components
at a fixed referencesite.
The exact magnitudesof the anomalousfield estimatesplotted
This is accomplishedin practiceby usingstatisticaltransferfunc- in Figure 4 are somewhatsensitiveto our definition of normal
tion methods, which combine all available data to improve fields. However, becausethe normal fields are always taken to be
signal-to-noise
ratiosand averageout the effectsof small-scale spatially uniform, the pattern of variations in anomalousfields
sourcevariations[e.g., Schmucker,1970; Beamish,1977; Egbert (and henceof relative conductivityvariations)is well determined.
and Booker, 1989]. If two or more separatearrays sharea com- Vertical magneticfieldsare zero for inductionby uniform sources
mon station,the two arrayscan be mergedby usingthe common in a one-dimensionalEarth, so the total vertical componentcan be
station as a reference [Bearnishand Banks, 1983]. This process taken to be anomalous. The vertical components(real part only)
15,972
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
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Fig. 4. Estimatesof the complexanomaloushorizontalmagneticfields which would be inducedat stationsin the combinedarray
by sourceswith a periodof 1000 secondswhich are linearly polarized(a) north-southand (b) east-west.The total fields averaged
over the array are assumedto be normal (i.e., representativeof a spatially averagedone-dimensionalEarth). The plotted
anomalousfields representthe deviationsfrom the normal vectors,which are given in the upperright comer of each plot. Solid
vectorsare real parts;dashedvectorsare imaginary. Relatively conductiveand resistiveregionsare characterizedby areaswhere
the anomalousfield vectorsrespectivelyreinforceand opposethe normal fields. Anomalousfield vectorsat anglesto the normal
fieldsindicatechannelingandredistributionof currentsby three-dimensional
structures.
for the southwesternWashingtonarray are printed on a map of
stationlocations,with informal contoursto guide the eye, in Figure 5.
Figures4 and 5 exhibit threeprincipalfeatures.First, the vertical componentcorrespondingto an east-westsourcepolarization
is positive over much of the array, increasingto rather large
valuesas the coastis approachedin the west (Figure 5b). This is
the well-known "coasteffect" [e.g., Parkinson, 1962; Everett and
Hyndman, 1967] resulting from a concentrationof north-south
electriccurrentsflowing in the oceanand conductingsedimentsof
the accretionarywedge at the continentalmargin [Weaver, 1962].
Superposedon the coasteffect are vertical anomalousfields which
reverse sign along a north-southtrending line which passes
throughthe SWCC (Fig. 5b). Evidenceof the SWCC is also seen
in the anomaloushorizontal fields of Figure 4b, which are
enhanced(at all available stations)in a belt passingthroughthe
three Cascadevolcanosfrom the Columbia river to Puget Sound
and in both the vertical and horizontal anomalousfield plots
corresponding
to north-southsourcepolarizations(Figures4a and
5a). Finally, an anomalyextendswest from the SWCC just north
of Mount St. Helens along the northernedge of the Chehallis
Basin. This featureis seenfor north-southsourcepolarizationsas
an enhancementof the horizontal fields (Figure 4a) and a sign
reversalin the verticalfields (Figure 5a). We refer to this feature
as the ChehallisBasin Conductor,or CBC for short. Using the
right-handrule for the Biot-Savart law, it is readily verified that
the horizontal and vertical componentsof the anomalousfields
mateshave been obtainedfor the southwesternWashingtonsynthetic array in 20 bandsat periodsfrom 60 to 10,000 s. The full
set of 160 plotsrepresentsa completeestimateof the responseof
the Earth (in this periodrange at the available sitesin the region)
to uniform external magnetic field sources. In general, these
responseestimatesvary smoothly with period. For subsequent
discussionwe will focuson resultsfrom four representativebands
evenly spacedin log period: T = 100 s, T = 300 s, T = 1000 s, and
T = 3000
s.
Spline Smoothingof lnternal Fields
Figures4 and 5 are estimatesat a discreteset of pointsof the
anomalousmagneticfields. The next processingstep involves
smoothingand interpolationonto a regular grid, subjectto the
constraintthat the interpolatedanomalousmagneticfieldsbe physically consistentwith sourcesof strictly internalorigin. To do
this, we treat interpolationas a linear inverseproblem [Egbert,
1989]. The constraints
are imposedby requiringthat the magnetic
fields be the gradientof a scalarpotentialwith all sourcesbelow
the surface. We then seekthe smoothestinternalpotentialwhich
is consistent(within the estimationerrors)with the syntheticarray
response. This approachis very similar to the method of harmonic splinesdevelopedby Shure et al. [1982] and Parker and
Shure [1982] to find smoothmagneticfields at the core mantle
boundaryfrom observationsat the Earth's surface. However, the
rectangulargeometryappropriatefor regional arrays and uncerover the SWCC
and CBC are both consistent with electric
tainty about the depthof the sourcesresult in somemathematical
currents inside the Earth.
complications[Egbert, 1989]. The key resultsneededto solve
Figures5 and6 representonly a small sampleof the anomalous this problem are proved by Meingut [1979], who developeda
field plotswhich resultfrom the initial dataprocessing.For each naturalgeneralizationof cubic splinesfor interpolationof data in
frequencyband there are real and imaginaryand horizontaland a multidimensionalsetting, and by Wahba and Wendelberger
vertical field plots for each of two polarizations.Using a 25% [1980], who consider the application of these "multivariate
bandwidth(= 9 bandsper decade)in the frequencydomain,esti- splines"to more generalinverseproblems.
EGBERTANDBOOKER:
CRUSTALSTRUCTURE
IN $W WASHINGTON
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.
ß •
•-•-:-.o•,--'::.•__.'
/ •
N
46.0
+.16
•
+.0•
e
- .06
ß
ß
+.15
45.5
12:3.0
122.0
121.0
12:3.0
122.0
121.0
Fig.5. Estimates
ofrealparts
ofanomalous
v•rtical
fields
which
would
beinduced
atstations
inthecombined
array
bysources
witha periodof 1000seconds
whicharelinearlypolarized
(a) north-south
and(b) east-west.
Positive
valuesof theverticalcomponentcorrespondto magneticfield vectorswhich point down into the Earth. Three substantialanomalousfeaturesare evident
here:the largepositiveverticalfield in the westernpartof the arrayin Figure5b is the coasteffect,resultingfrom electriccurrent
flowingin theconductiveoceanandsediments
of the accretionary
wedge. The northtrendingreversalin signunderthe Cascades
(in Figures5a and5b; notealsothe enhancement
of horizontalcomponents
in this areain Figures4a and4b) coincideswith the
SWCC; finally, a smallereast-westconductivezoneis markedby the east-westtrendingsignreversalwhichbranchesoff of the
SWCC to the westjust northof Mount St. Helens. This anomalyis alsovisiblein Figure4a.
Plots of the smoothing splineestimatesof the anomaloushorizontal and vertical magneticfields are given in Figures 6 and 7
for a periodof 1000 s. Note that becausethe interpolationprocedure allows for the actual estimated errors at each site, these
smoothfields do not exactly interpolatethe discreteresponseestimatesof Figures4 and 5.
The processingmethodsreviewedso far can be thoughtof as a
generalizationand refinementof the vertical field hypothetical
event map [Bailey et al., 1974]. However, insteadof usingonly
single-stationverticalfield transferfunctions,all possibleinterstation and intercomponenttransferfunctionsare incorporatedalong
with the physical constraintthat the sourcefor the anomalous
fields be internal. Because horizontal components of the
anomalousfields are sensitiveto structureimmediatelybelow the
observationpoint while verticalcomponentsare sensitiveto structure off to the side, using anomaloushorizontal fields and the a
priori physicalconstraintseffectively increaseshorizontalspatial
resolution(e.g., comparethe vertical field contoursin Figures 5
and 7).
There are still somesignificantholes in our stationcoverage,
particularlynear each of the three Cascadevolcanosand on the
eastern edge of the array. Because our procedure finds the
smoothest anomalous fields consistent with the available data,
fieldsplottedin Figures6 and7 are not seriouslyaffectedby aliasing. In particular,all of the lines which crossthe SWCC between
Puget Soundand the ColumbiaRiver show a similar patternof
sign reversal in the vertical componentand enhancementof the
horizontal component. Given the strong north-south spatial
coherenceof the availableanomalousfields,it is virtually certain
that additional
east-west
transects
across the Cascades
would
reveala grosslysimilar patternof anomalousfields.
Equivalent SheetCurrents
Using the Biot-Savartlaw, horizontalmagneticfields of internal origin can be transformedto an equivalent surface current
sheet (i.e., a thin sheet of current below the observer which would
reproducethe fields), by rescaling and rotating the magnetic
fields90øcounterclockwise
[Banks,
1979](seealsotheappendix).
Applying this transformation
to the anomalousmagneticfieldsof
Figure 7 clearly displaysthe anomalouscurrent flow discussed
above(Figure 8).
Althoughthe inducedanomalouscurrentsare periodic,with the
phaseand polarizationvarying acrossthe array, it will often be
useful to refer to the "direction"of equivalentcurrentflow. For
this purposeit is convenientto treat real and imaginarypartsas
separatevectors. Note that the real current sheetsare in phase
with the referenceexternalmagneticfields. The imaginaryparts
of the current vectorsrepresentmagneticfields which lead the
theseunsampledregionsare interpolatedacrosssmoothlywithout
including unnecessarydetails. Of course,with additional sites,
smaller-scalefeatureswould undoubtedlybe evident. Note, however, that the unsmoothedanomalousfields (Figures4 and 5) do reference
fieldsby 90ø. We referto therealandimaginary
parts
generallyvary slowly betweenadjacentstations. This gives us as the in-phaseandquadraturecurrents,respectively.
someconfidencethat the large-scalepatternof smoothanomalous
The currentsof Figure 8 tendto flow in vorticeswhich closeon
15,974
48.0
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
-ANOMALOUS•
REA•
N-S
•/•,
(a) SOURCE
T=
1000s.•••
..! '' NORMAL
47.5
47.0
46.5
46.0
45.5
123.0
122.0
121.0
123.0
122.0
121.0
Fig. 6. Smoothingsplineinterpolationof three-component
magneticfield vectors.The figuregivesthe real part of the horizontal
components
of the smoothest
magneticfieldsof internaloriginthatareconsistent
(within theestimationerrors)with theestimated
anomalousfieldsof Figures4 and 5. (a) north-southsourcepolm'ization.(b) east-westsourcepolarization.
48.0
47.•
( ) E-W SOURCE
- REAL Ok.)• (a) N-S SOURCE
ANOMALOUS
- I /•_) NORMAL
CURRENT
NORMAL
CURRENT
CURREN•-/••_,..••
I
'
....
T= 1000s
A-•
470
," - --A'
-•,T,,,'+•.
•'-,
) •--•
x x •, x •''''
L
••'•''''.9q5
J
46.0
I • ' '1-'''--'
z, ......
ß
45.5
ß
bi/l/i
' ' ' ''--•'
• ....
'
,(•'•,,-''))&)
....
I' I
??
.....
' '
t•t_
.
,,
_
ß
I
123.0
122.0
123.0
121.0
122.0
121.0
Fig. 7. As in Figure6 but verticalcomponentfor (a) north-southsourcepolarizationand (b) east-westsourcepolarization.
themselves,and in placesthey opposethe "normal"current. This
is a result of the way we have defined anomalousfields, which
must averageto zero. Relatively conductiveareas,where current
flow is enhanced(with anomalouscurrentsreinforcing the normal), must be balancedby relatively resistiveareas,where current
flow is reduced(with anomalouscurrentsopposingthe normal).
In the simple schematicmodel of Figure 2 a laterally heterogene-
ous surfacelayer overlies a one-dimensionaldeep section. The
total currentat period T which flows in the surfacelayer can be
written
l(x, T)=
Io(T) + $1(x, T),
(1)
where$I(x, T) representsthe anomalouscurrents(which average
to zero) and Io(T) representsthe normal current flowing in the
EGBERTANDBOOKER:
CRUSTALSTRUCTURE
IN SW WAS!•INGTON
(a) N-S SOURCE
ßVERTICAL
FIELDS
15,975
(b) E-W SOURCE'VERTICAL
FIELDS
•
-
REAL
PART
REAL
PART
_ O. •
47.5
0.1
•0
47.0
46.5
46.0
45.5
123.0
122.0
123.0
121.0
122.0
121.0
Fig. 8. Realpartsof anomalous
equivalent
sheetcurrents
for two sourcepolarizations.
The vectorsin theseplots areobtainedby
rotating
theanomalous
magnetic
fieldvectors
of Figure
6 90øcounterclockwise.
Thevectors
aretangent
to streamlines
of an
equivalentsurfacesheetcurrentwhichwouldreproduce
thesmoothmagneticfields. For Figure8a thenormalcurrentflow is from
eastto west,while for Figure8b the normalcurrentflow is from southto north. Becausethe spatialaverageof the anomalous
magneticfields(and henceof the currentsheet)is zero, the anomalouscurrentstend to flow in vorticeswhich closeon themselves.
A-A' throughD-D' mark locationsof profilesplottedin Figures11 and 12.
thin sheet. With the MV array data we can readily estimate81.
Unfortunately, while consistencyof the total currentswith the
physicsof electromagneticinduction(and the simple considerations discussedbelow) provide some information about Io, the
absolutemagnitudeof this quantityis poorly constrainedby the
MV data(seethe appendix).
However, estimatesof I are highly desirable. As discussed
below, total currentsbut not anomalouscurrentscan be easily
root mean square(RMS) of (the real and imaginarypartsof) 81
for eachperiodT.
Currentvectorsin eachof theseplots are also scaledby the
anomalouscurrent RMS so that the typical lengths of the
displayed vectors is constant. The RMSs of the total sheet
currents(in-phaseandquadratureseparately)are displayedin the
upperleft comerof eachplot. This displayof the currentsallows
for an approximateseparationof amplitudeand phasevariations
inverted for crustal conductance. Furthermore, the total current is
(which we expectto be dominatedby Io) from variationsin the
relatively large in conductorsand small in resistors,so lateralcon- geometryof the currentsheet.Note thatexceptfor thequadrature
ductivity contrasts (and the manner in which currents are currentsat the shortestperiods,the geometryof the currentsvaries
redirectedand channeledin complexthree-dimensional
structures) little. This is consistentwith (2), giving us someconfidencethat
are more clearly illuminatedin plots of I. These advantagesare our thin sheetmodelis approximatelyvalid. A moredetaileddisillustratedin Figures9 and 10, where we plot roughestimatesof cussionof theseplotsis deferredto the resultssection.
total crustalcurrents(compareto Figure 8).
In the long-periodlimit, where inductionin the surfacesheet SimpleInversionsfor Conductance
can be ignored and galvanicredistributionof the currentsdomIn the model of Figure 2, a thin surfacesheetof variableconinates, the period dependenceof the currentsin the thin sheet ductance(conductivitytimesthickness)
model can be separatedfrom the spatial dependence[e.g., Le
S(x, y) = So+ 8S(x, y)
(3)
Mouel and Menvielle, 1982]
Io(T) + 81(x,T) = 1o(T)[io + 8i(x)].
(2)
Here, i +Si(x) is a real vector field (independentof period,
divided as in (1) into constantnormal and spatially varying
anomalousparts)and1o is a complexscalar. From (2) we seethat
1o(T) is determined,(at sufficientlylong periods)up to a single
real scalarby our estimatesof 81. Thus fixing Re Io for a single
perioddeterminesboth the in-phaseand quadraturetotal currents
for all sufficientlylong periods. We have usedthis idea to estimate the currentsin Figures 9 and 10 by choosing(somewhat
subjectively)Re Io(1000 s ) for eachpolarizationandthenscaling
Io(T) (separatelyfor the real and imaginaryparts)by the relative
overliesa layeredone-dimensional
Earth.Forthefinalprocessing
step we adopt this simple model for the crust of southwestem
Washingtonand invert the sheetcurrentsfor crustalconductance
S. Thismodelignoresanyverticalstructure
in theupperinhomogeneous
layerandfurtherassumes
thatthe layer is verythin comparedto the penetrationdepthof the electromagnetic
fields. The
thin sheetmodelis thusmostreasonable
at longperiods,where
penetrationdepthsare greatestand vertical resolutionof the MV
data is very poor. In the simplestsort of thin sheetmodel, the
upperinhomogeneous
layeris separated
from deeperlayersby a
layerof infiniteresistance
whichpreventsverticalflow of currents
[Price, 1949]. For this model a simple direct (almost linear)
15,976
48.0
47.5
EGBERTAND BOOKER:CRUSTALSTRUCTURE
IN SW WASHINGTON
[- 30 •XS• T-100 s
27 •XS• T-300 s • 24 •XS• T-1000s
21 •XS•? T-3000s
l '-•• • InPhase
'-•f•• InPhase
l '-•.F•
• InPhase
I '-•f•• InPhase
47.0
47.5
I ............
47.0 ....
I
:•.-..
T=100
s •7'•••Quadrature
'_
, •,,,xxx
.....
• • (•(•.
Quadratur
_
.•.•
••
123.0
••I
121.5
I I [ I I I
123.0
121.5
I
123.0
121.5
123.0
121.5
Fig. 9. In phaseandquadrature
partsof estimated
totalsurfacesheetcurrentinducedby north-south
sourcepolarization
for four
periods.The scalesfor currentvectors(givenin theupperleft comerof eachplot) areproportional
to therelativeRMS amplitude
of the anomalous
currentsaveragedoverthearray. The totalsheetcurrentis formedby addingan estimateof the constant"nor-
mal"current
I0 to theanomalous
currents.
Forthein phase
partat T = 3000s I0: .03{- .18•, where• and• areunitvectors
pointingto the northandeastrespectively.For the shorterperiodsthis vectoris rescaledaccordingto the relativeRMS of the
anomalous
fields.Thenormal
quadrature
currents
arechosen
similarly,
withI0 -- .02{- .04.•atT = 3000s. Thelocations
of the
threeCascadestratovolcanos
(MountRainier,MountSt. Helens,andMountAdams)aregivenby thelargetriangles.All of the
plots,exceptthosefor the shortperiodquadrature
currents,are very similar. Althoughnormalcurrentflowsfrom eastto west,substantialcurrentis channeledto thenorthin theSWCC. In the westernpartof thearray,currentflowswestin two conductivechannels,onejust northof Mount St. Helensandtheotherat the northernendof the array.
inversionof sheet currentsfor crustal conductanceis possible
[Berdichevsky
and Zhdanov,1984]. We outlinea practicalimplementationof this inversionin the appendix. This approachis
simplistic but not completely unreasonable. Most of the
anomalousstructureevidentin the currentsheetplots(Figures9
and 10) originatesin the crust (the anomaliesare tens of kilometerswide and are nearly independent
of period),and highlyresistive layersare typicallypresentin the crustand uppermantle.
Application of the simple thin sheet inversionoutlined in the
appendixto long-period(T- 1000 s and T = 3000 s) anomalous
fieldsfrom the syntheticarray yields the model of crustalconductance shownin Plate 1. With this simple approachto inversion,
two parameterswhich are not directlyconstrainedby the MV data
must be independentlyestimated: the normal current in the thin
sheetI0 and the normal conductivitySo. In fact, the absolute
scaleof crustalconductance
in the modelof Plate 1 is essentially
determinedby thechoiceof So (heretakento be 100S, equivalent
to a 20-km-thick200-ohm m layer). Furtheruncertaintyin the
relativevariationsof crustalconductance
resultsfrom uncertainty
concerning10. Experimentswith a rangeof valuesof Io indicate,
however,that the geometryof the conductivityvariationsis very
well constrainedand that the relative amplitudesof the variations
(i.e., the ratio of highestto lowest conductance)are constrained
within a factor of roughly3.
Note that full nonlinear inversionsof the combinedarray
response,basedon either more generalthin sheet[e.g., Vasseur
and Weidelt, 1977; Fainberg et al., 1990] or fully threedimensional models, would substantially reduce this scale
ambiguity,particularlyif constraints
on known large-scalestructure (e.g., oceanbathymetryand conductivity)were incorporated.
Thesemore complicatedinversionmethodsrepresentan important refinement of MV interpretationmethods which we are
currentlypursuing.
RESULTS
The final resultsof the processing
are containedin Figures9
and 10 andPlate 1. The equivalentcurrentsheets(Figures9 and
10) areestimates
of thetotalcurrentflowingin a thin surfacelayer
of laterallyvaryingconductance
(Figure2). Plate 1 givesan estimateof theconductance
of thissurfacelayer. The currentsin Figures9 and 10 havewidthsof 10-40km (seealsoFigures6 and8).
The true anomalouscurrent distributionis almost certainly
rougherthan thesesmoothestimates.These short-lengthscales
imply that the anomalouscurrentsflow within at most a few tens
of kilometers of the surface. The vertical distribution of conduc-
tivity within this near surfaceregionis not resolvedby the thin
sheetmodel of Plate 1. However, we can gain somequalitative
insightinto the natureof vertical structuresby consideringthe
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
48.0 :34
T=100s
/ '-
,,
,,,,
.........
.....
29 '••
In Ph.d,
T=1000s I ;
'-
•_•? InT=8000
s
Phase
In Ph.d, I '
I
4•.0 I •,,•.-,,.lJl•,,,
/
/
:3t 'X•? T=800s
In Ph.d, I '-
15,977
I ,,
l•l•.•
1•I%•
....
,,
....
I
I
•Z,,,J
........
........
• ......
•
,l
•Z,,,IIIJ•,,
•.• •,,,•,
•,,,,•
........
........
I ,
, 1•.•.•
,,•X•,,,,,
• ....
I
I
......
......
, •%•
,•%•,
b•
i llJ•
I ,,.J....... ••,,,
I ...i.... ,,,,••
...I.... ,•%x•
1,,,,,
I ...I..,,••
48.0
T=3000
s
quadrature
'- ' jQuadrature
'- •Quadrature/
'- ,Quadraturel
'-
47.5
47.0
, ,
46.5
/ ..,%J•..,•
. .
ß . . . . •
.......
,,
•
• . . ,
..,•
....
• , • •
,•
.....
. . ,
•
,
• ....
..........
,•
•
-
........
,•, ..............
,
,•.•
46.0
45.5
•
I
I
I
123.0
I
.•
I
::
I
121.5
,
I
_
I
I
123.0
I
I
I
I
121.5
I
I
123.0
I
., , i
I
I
121.5
I
I
I
123.0
I
,,
I
I
121.5
Fig. 10. Totalcurrent
sheets
asin Figure9 foreast-west
source
polarization.
Here,at T = 3000s,I0 =. 16• forin phase
currents
andI0 = .08•forquadrature
currents.
Totalcurrents
forotherperiods
areproportional
torelative
RMSasin Figure
9. Normal
currentflowingfromthesouthisconcentrated
in thenorth-south
trending
SWCC. A smalleranomalyis visiblein thewestern
part
of the array,particularlyin the short-period
quadrature
plots.
period dependenceand phaseof the sheetcurrentsin Figures9
9). This suggests
a sharptransitionto generallyvery resistive
crustwestof the SWCC. Here, the outcropping
CrescentFormapart of the
The in-phasecurrents,whichare largerthanthe corresponding tion basaltsof the Willapa Hills in the southwestern
quadrature
currentsby as muchas a factorof 4, revealthemajor array and of the Black Hills to the north mark resistiveblocks
structuresmost clearly. These are, to a first approximation, which are separatedby a conductivechannelbeneaththe northern
and 10.
independentof period. The most obviousanomalyis the northsouthtrendingSWCC mappedby Stanley[1984] and Stanleyet
al. [1987] using MT. This anomaly results in a substantial
enhancement
of currentsundertheCascades
for bothpolarizations
edgeof the ChehallisBasin. Note that the resistiveblocknorthof
the CBC extends east to the SWCC beneath the sediments of the
PugetLowlands(Figure 9, Plate l). Fartherto the north,current
flowswestacross
PugetSoundandis deflected
to thesouthalong
the easternedge of Hood Canal by a resistivestructure,which
from southto north(Figure 10). The anomalyis widestandmost probablyrepresentsunderthrustvolcanicsof the CrescentFormadiffusein southernWashingtonnearthe Columbiariver. North of tionon theeasternedgeof theOlympicPeninsula.
At periodsbeyond1000 s the anomalouscurrentshave a fixed
Mount St. Helens and Mount Adams, the anomalous currents
become more concentrated, and the estimated conductance morphology
with period-dependent
but spatiallyconstantampliincreasessteeply. Further north, near Mount Rainier, there is a tudesand phases(Figures9 and 10). This is consistentwith norkink in the anomalous currents where the conductor shown in
mal currentsinducedin a largerregion,which are then distorted
by localvariationsof conductivityin a thin surface
Plate 1 narrowssharply. The anomalouscurrentsthen turn toward andchanneled
PugetSoundandbecomemorediffuseagainjust southof Seattle. layer[Banks,1979;Le MouelandMenvielle,1982]. The spatially
The SWCC is alsoclearlyevidentin anomalous
field plotsfor constantamplitudesand phasesare determinedby the normal
the north-southsourcepolarization(which inducesnormalcurrent fields,whichdependon thelarge-scale
deepregionalconductivity
flow from west to east). Currentflowing from the east is chan- structure,while the morphologyof the anomalousfields is deterbut is clearest for east-west sources which excite current flow
neled into the SWCC south of Mount Adams, with current flow
ure 9). To the west, current is concentratedin the CBC, which
minedby localvariationsin crustalconductance.
This supports
thevalidity(for longperiods)of the simplethin sheetmodelused
to invertfor crustalconductance.
It alsosuggests
thatphases
and
branchesoff of the SWCC just northof Mount St. Helens,and in
a zone of enhancedconductivitybeneaththe Puget Lowlands
northwestof MountRainier(Plate1). Currentflow alongmuchof
the westernedgeof the SWCC, which deflectsand channelsthe
currentsnorthtowardPugetSound,is nearlynorth-south(Figure
relative amplitudesof regionallyaveragedelectricfields can be
approximately
determinedfrom the anomalousmagneticfields.
At periodsof the order 1000 s we would expectcrustalelectric
currentsin this region.to be stronglyinfluencedby currents(of
largeamplitudeandlow phase)inducedin the highlyconducting
acrossthe easternboundaryof the SWCC severelyreduced(Fig-
15,978
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
'
48.0
'
'
',• X,• \
\•'•'
•
o1_•
o
o c•
o
o
o
•
"t
o
10'• S
'
-
øo
o
47.5
*
ß
o
o•
o_
o
ß•o• o o
47.0
O' •
0
•
OO
O
o%' •AI o 0 00
."
•, o oo •1_
o
• • •.
o øo
S
46.5
øøo
o
'{TH
h
46.0
o• o
.
..
_
•
.
I
45.5
,
,
124
,
I
,•
,
102
122
Plate 1. •in sheetconductancemodelfor the crestin southwesternWashingtonwith crustal e•hquakes •d volcanicventsoverlain. Eadhqu•es (0 to 15 km depth)are plottedas
hexagonsscaledby size (smallest,2.0-3.9; m•ium, 4.0-4.9; !•ge, 5.0-5.9). Small crosses
are basaltic vents; small diamonds •e andesitc •d
dacite vents. •e
St. Helcns Seismic
zone (SHZ) coincideswith the westernedgeof the broadsouthern•nion of the SWCC, and
is abruptly te•inated by the east-westtrend• CBC. Volc•ic ventsclusteraroundthe
edgesof the SWCC. Sea., Seattle;STH, Mount St. Helens; RAt, Mount Rainier; ADA,
Mount
Adams.
component,
wherethisanomaly
dominates
at T = 100
ocean [e.g., Mackie et al., 1988]. In fact, for the north-south quadrature
sourcepolarization(Figure9), for which oceaniceffectsshouldbe s and is very clear for T = 300 s, before vanishingfor longer.
strongest,phasesof the long-periodanomalousfieldsare very low periods. In fact, adjustingfor differencesin plot scales,we see
(=10ø-20ø), andamplitudes
remainlargeevenat longperiods. that the in-phase and quadratureparts are roughly equal for
a shalloworigin for this anomaly. This conThese characteristicoceaniceffects are less prominentfor east- T = 100 s, suggesting
westsources
(Figure10), for whichphases
are =25ø- 35ø and clusion is supportedby the rapid decreaseof the anomaly with
amplitudesdecreaserapidly at long periods.
period. Comparisonwith Figure I showsthat the anomalycoinAlthough the full three-dimensionalelectromagneticinduction cides with the alignmentof the ChehallisBasin and the western
problemis complexand allows a great rangeof phenomena,the part of the PugetLowlands. Thesebasinscontainconductivesedrelative magnitudesof the in-phaseand quadraturecomponentsof imentsand have a narrowconnectionsouthof Puget Sound. The
the anomalousfieldsprovidea qualitativeindicationof the depths magnitude, phase, and strong period dependence of these
and sizes of conductivestructures[e.g., Schmucker,1970]. The anomalouscurrentsimply that the basinsare distinct,with only a
phaseof currentsflowing in near-surfaceconductorsis approxi- weak electrical connection.
By comparison,the anomalousfieldsover the SWCC in Figure
matelythesameas thephaseof the surface
electricfields(45ø
over a uniform conductor),so superficialconductivityanomalies 10 decrease in amplitude much more slowly (peaking near
tend to have large quadratureparts. Anomalous fields arising T = 300 s) and do not show consistentlypositive phasesuntil
from currents flowing in conductorsat greater depths (i.e., a T = 1000 s. The quadraturecurrentsare restrictedto the extreme
significantfraction of the electromagneticskin depth or more) northernpart of the the anomalyat T - 100 s and extendfurtherto
tend to have small (or even negative)quadratureparts. Of course, the south with increasingperiod, until at T = 1000 s the plots
whether a conductoris "near surface"or not dependson period. closely resemblethose for the in-phasecurrents. This suggests
that the conductive rocks which form the SWCC are shallower in
At longenoughperiods,all anomalieswill appearnearsurface.
These pointsare nicely illustratedby Figure 10. In additionto the northanddeeperto the south. However,it is alsopossiblethat
the SWCC, a small north-southtrendinganomalyis visible in the self-inductionwithin the SWCC togetherwith inductiveand conwesternpart of the array at T = 100 s but is almostunnoticeableat ductivecouplingto PugetSoundin the northand the lower crustal
T = 1000 s. This feature is much strongerin the corresponding conductorunderthe OregonCascadesto the south[Goughet al.,
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
(a) N-S CURRENT: PROFILE A-A'
1989; Wannamakeret al., 1989] can explainthe period-dependent
morphology of the quadrature anomalous currents. Threedimensionalmodelingwill ultimatelybe requiredfor a full understandingof thesedata, althoughtheseresultsclearly demonstrate
that the SWCC representsa conductorwhich is much more massive than a few kilometers
of sediments.
.6
T=!00s
r = soos
r:
•ooos
/.,,;.,'-'...•
/.,,,...'
.2
.0
-.2
I
-80
-40
0
40
I
80
(b) N-S CURRENT: PROFILE B-B'
.6
.4
2
SWCC and the ocean. We note that Stanley's [1984] MT results
showvery low electricfield phasesaround100 s periodsin this
area. Again, this behaviorindicatescomplexthree-dimensional
inductivephenomena,
hereinvolvingthe interactionof the SWCC
andthe highly conductingoceanto the west.
The anomalouscurrentsexhibit some more subtle but systematicvariationswith period. Thesearebestexploredby considering selected cross sectionswhich allow results for all four
periodsto be superimposed.For thispurposewe usethe real parts
of the scaledfields plottedin Figures9 and 10. This effectively
eliminates variations of normal field amplitudes and phases
(largely determinedby regional-scaleinductioneffects), so that
we may more directlyassesschangeswith periodof currentmorphology(whichis moreindicativeof localstructure).The conclusionswe reach from theseprofilesare qualitative. More detailed
mappingof verticalstructurein conductivityvariationssuggested
by this analysisrequiresMT data.
We considertwo east-westprofiles(Figure 8), A-A' acrossthe
northernpart of the SWCC nearPugetSoundandB-B' acrossthe
centralportionof the SWCC. On the northernprofilethe relative
amplitudeof the anomalydecreases
steadilywith period,and the
westernboundaryshiftsslightly to the east(Figure 1l a). On the
centralprofile B-B', the anomalypeaksat 300 seconds.As period
increases,the westernlimit of the anomalyshiftsto the east,and a
broadelevatedshoulderon the anomalydevelopsto the east(Figure 1lb). The shift of the anomalypeak suggeststhat the SWCC
dips down to the east, while the broad spreadto the east of
anomalouscurrents at long periods suggestsa deep conductive
-
.4 .....
Note also that the in-
phasecurrentsflowingin the SWCC havea kink wherethe CBC
branchesoff. The amplitudeof this kink increaseswith period,
and the kink is lessobviousin the quadraturecurrents,suggesting
a deeporigin for thisfeature.
For the north-southpolarizationthe quadraturecurrentsclosely
resemble the in-phase parts at longer periods but not for
T = 100-300 s (Figure 9). In theseshort-periodquadratureplots,
currentsare large in the north, south,and east,but almostvanish
in the westcentralpart of the array, wherethe anomalyassociated
with the CBC fades out (compare to the in-phaseparts). This
large-scale variation of quadrature amplitudes suggeststhat
phasesin the electric fields are low in the region betweenthe
15,979
0
-2
-80
-40
0
40
80
(c) N-S CURRENT (N-S SOURCE): PROFILE B-B'
.6
T
.4 .....
2
0
-.2
=
!00s
r: soo•
.,'/&,
r = •ooos
..'Z•!,
_......
T=....3000s,SWCC..,•'-2
_
w
-80
-40
0
40
80
E
KILOMETERS
Fig. 11. Crosssectionsof crustalcurrentflow derivedfromFigures9 and
10 for two east-westprofilesacrossthe array(seeFigure8 for profilelocations). (a) Relativeanomalouscurrentflow to the north acrossprofile AA' inducedby east-westsourcesdecreases
at longerperiods. (b) Further
southon profileB-B' acrossthe SWCC currentshiftsto the eastat longer
periods.(c) Channeledcurrentsflow northacrossB-B' for north-south
sourcepolarization,
whichinduces
east-west
normalcurrents.The relative
amplitudeincreasessteadilywith period,but the locationof the peak
remainsfixed,slightlyto thewestof theanomalypeakin Figure11fib.
C-C' and D-D' (Figure8). For profileC-C' on the westernedge
of the SWCC, the peak associated
with the CBC broadensand a
secondpeak developsto the northas periodsincrease.Note that
Cascades.
Anotherview of the westernboundaryof the SWCC is given in at T = 3000 s the secondarypeak is well to the north of the
Figure 11c, where we plot the currentswhich are induced by Chehallis Basin, demonstratingthat most of the current in the
north-southsourcesand then channeledto the north acrossprofile CBC does not flow out of the SWCC throughthe near-surface
B-B'. Here the peak of the anomalouscurrents,which is fixed basinsediments.Furtherto the weston profileD-D', currentflow
slightly to the west of the peak for east-westsources,steadily is muchmoresharplypeaked,centeredalongthenorthemedgeof
increasesin amplitudewith period. For a north-southpolariza- the ChehallisBasin. In both north-southprofilesthe secondzone
tion, current flows from the east and concentratesnear the western of enhancedcurrentflow, acrossthe northem part of the Puget
of period.
boundaryof the SWCC. The fixed locationof the peaksuggests
a LowlandsandthroughPugetSound,is independent
zone in the lower crust beneath the Columbia
Plateau east of the
vertical westernboundary. The increasein relative amplitudeof
the deflectedcurrentssuggeststhat a stronggradientin conductivity between the SWCC and the crust to the west persistsat
depth. Togetherthe currentprofilesof Figures1lb and 11c sug-
DISCUSSION
The MV arrayresultsare generallyconsistentwith the conductivity modelsof Stanley[1984] and Stanleyet al. [1987, 1990]
gestthattheSWCC dipsto theeastbuthasa sharp,nearlyvertical basedon three MT transectsacrosssouthwesternWashington. In
particular,the MV conductance
model (Plate 1) is remarkably
westernboundary.
In Figure 12 we plot currentsfor the two north-south
profiles, similar to the estimateof conductancefor the SWCC given by
15,980
EGBr•T ANDBooroaR:CRUSTALSTRUCTURE
IN SW WASHINGTON
Stanleyet al. [1987] (see Figure 1). However, the resultsgiven
hereprovidea three-dimensional
contextin which to interpretthe
MT transectsand suggestsomedifferences.The two-dimensional
modelfor the centralMT profile (A-A' [Stanleyet al., 1987]) has
a gradualboundary,with conductivesedimentaryrocks thinning
and shallowingto the west. However, the MT transecton which
this model is basedcrossesthe SWCC just noah of Mount St.
Helens where the CBC branches off.
The MV
data show that the
The variations of conductance inferred from the MV
data
correlatewith and illuminateother geophysicalconstraints.In
Plate 2 we superpose
the estimatedcrustalcurrentsinducedby
noah-southpolarizedmagneticfieldson a map of isostaticresidual gravity [Simpsonet al., 1986; Finn et al., 1986; Williams et
al., 1988]. The correlationof the sheetcurrentand the gravity
anomalies
in the westernpan of the arrayis quitestriking.The
electriccurrentstendto flow aroundthe gravityhighs,whichare
westernend of this transectis not typical of the SWCC; along
most of the westernedge, there is an abrupt and nearly vertical
contactbetweenvery conductiveand very resistiverocks(as seen
alsoin the modelfor the northernMT profileC-C' of Stanleyet al
stronglycorrelated
with theresistivesections
of theCoastRange
block(compareto Plate 1). Note alsothe elongatedweakhighin
[1987]).
SWCC from the south.
Stanleyet al. [1987] modelthe SWCC dippingdown to the east
andsuggestthatthe conductiveunit hasbeenundenhrustandpossibly continuesto the east of the Cascadesunderthe Columbia
Plateau. The MV data, which show the anomalousmagnetic
fields shiftingand broadeningto the east at long periods,corro-
the gravitymap along the easternboundaryof the SWCC. The
currentsflow aroundthis elongatedresistivefeatureto enter the
TheMV resultsarealsoconsistent
with majorfeaturesin mag-
netic anomalymaps. As discussedby Stanleyet al. [1987], the
SWCC coincideswith a substantiallow in the magneticdata
beneaththeCascades.The CoastRangeblockis characterized
by
a seriesof broadmagnetichighsboundedon the eastby a noahborate this. In the MT model the SWCC is bounded on the east
southtrendingline which likely extendssouththroughOregon
by a resistiveblockwhichwas interpretedby Stanleyet al. to be [Finn, 1990;Johnsonet al., 1990]. Finn [1990] haspointedout
a localized pluton. However, the MV results show east-west that thisboundarycoincidescloselywith the westernedgeof the
flowingcurrentschanneledinto the southernendof the SWCC by SWCC. Jointinterpretation
of gravityandmagnetic
datasuggests
an elongatedresistiveblockalongits easternboundary(Figure9). a thick (20-30 km) sectionof densehighly magnetizedrocksin
This suggests
that the resistivestructureimagedon the MT tran- theCoastRangeblockwhichFinn [1990] interprets
asconsisting
sectis pan of a deep,noah trendingresistiveblockwhichcontin- of oceanicbasaltand gabbro.In general,the very resistivesecuesfor 50-60 km and definesthe easternedgeof the SWCC. The tionsof theCoastRangeblock(Plate2) correspond
withhighsin
secondMT profile (C-C') of Stanley et al., which crossesthe the magneticdata, while the CBC corresponds
to an east-west
SWCC just southof PugetSound,showsthe conductivezonenar- trendinglow [see Finn, 1990; Plate 2]. Note however that the
rowing,shiftingto the west,andshallowing.All of thesefeatures magnetichigh in the resistiveblocknorthof the CBC extendsinto
are consistentwith the pattern of anomalouscurrentsand the the ChehallisBasin(well to the southof the gravityhighseenin
model for crustal conductance of Plate 1.
Plate2). This suggests
[Finn, 1990]thatthenorthernedgeof the
ChehallisBasinis underlain(at shallowdepths)by a thin lip of
CoastRangemaficrocks,whichhavebeenunderthrust
by sedimentsalonga noahwesttrendingfault. This inferenceis strongly
supported
by our analysisof theMV arraydata.
Stanleyet al. [ 1987, 1990]havenotedthat the spatialdistribution of crustalseismicityin southwestern
Washington
appearsto
be closelyrelatedto the SWCC. A compilationof shallow(0-15
kin) earthquake
epicenters(C. Weaver and R. Ludwin,personal
communication,1992) showsa strikingcorrelationwith the MV
crustalconductance
model (Plate 1) and the equivalentsheet
currents(Figure13). The lower line of concentrated
uppercrustal
(a) E-W CURRENT: PROFILE C-C'
.4
.3
.2
T
=
100s
T
=
$00s
....
T = 1000s
......
r = sooo
:
--,?\ ',
.1
/1'
-,
tt
z•..
/? "'...:...%.::..
.0
".
,'•,'.._
,- ........
_
',,
-
4"1
-'
-100
-4•/
:' •"•:'.
)1 •
CBC:.
"•:::"7
'"i
•
0
'.---
rugeT
Sound
'-- :
-50
,
50
seismicity(the SHZ [Weaver and Smith, 1984; Weaver et al.,
1987]) lies alongthe easternboundaryof the SWCC. The SHZ is
100
(b) E-W CURRENT: PROFILE D-D'
.4
___
T =
100s
T =
300s
T =
1000s
=sooos
1
truncated
abruptlyby theCBC. Noah of theCBC, theseismically
activezonejumpsto the eastand deepens[Ludwinet al., 1991].
Near MountRainiertheearthquake
epicenters
arealignedwith the
narrownorthernportionof the SWCC, and the seismicallyquiet
zonewestof Mount Rainierand southof PugetSoundcoincides
with the resistiveblock just to the west. The smallereast-west
conductivityanomaly acrosssouthernPuget Sound marks the
boundarybetweenthisquietzoneandthe regionof highseismic
,'
/":"'
,
•
SoUnd
o
-1
-2
'••
_
I
s
-00
I
-50
•
I
i
0
50
I
N
KILOMETERS
Fig. 12. East-westcurrentflowing acrossprofiles(a) C-C' and (b) D-D'
(seeFigure8 for profilelocations).In Figure 12fia currentflowingto the
westout of the SWCC shiftsto the northat longerperiods.The northern
limit of the exposedsediments
of the ChehallisBasinis indicatedby the
verticaldashedline. On the westemprofilein Figure 12fib, currentmorphologyis relativelyindependent
of period.
activity in the crustto the noah.
In Plate 1 we have also plotted late Cenozoic (5 Ma and
younger) volcanic vents [Guffanti and Weaver, 1988]. In
southwestern
Washington,thesetend to clusteraroundthe south-
ern andeasternedgesof the SWCC. In the sameareaalongthe
southeastern
edgeof theconductivity
anomaly,heatflowpeaksat
80-90mWm-2,thehighest
valuereached
intheWashington
Cascades[Blackwellet al., 1990].
It is hardto avoidthe conclusionthat the SWCC represents
a
majorstructural
featurein the crustof southwestern
Washington
whichis to someextentcontrollingpatternsof crustaldeformation
EGBERT
ANDBOOKER:
CRUSTALSTRUCTURE
1NSW WASHINGTON
15,981
lOO
5o
470•-
-'""
ß
%%...
......
•
"
/
,,.",.%,..._,..,
!
-.'"-""-""-••.,-'-'%\\'\ - t • ' '
.,.....• -.,\ •,,•,"'."",,".•
"' b ....
x,.,,,,.•....
45.5
124
•
' •' \ '"'""'"'""-'
-5o
, • \,,..,.,.,,.....
'"''""
""• '""'"'"'
122
Plate2. Isostaticresidualgravityin westernWashingtonwith equivalentsheetcurrentoverlain. Redsaregravityhighs;greensandbluesare gravitylows. The scaleon the right is in
milligals.Thepatternof currentflow in thewesternpartof thestudyareais consistent
with
channeling
of currentsaroundthe "islands"
of resistiveanddenseCrescentFormationmafic
rocks.Note'alsothe weakelongated
gravityhighon the eastern
edgeof the SWCC.
Currents form the east flow around this feature to enter the SWCC from the south. STH,
Mount St. Helens; RAI, Mount Rainier; ADA, Mount Adams.
and volcanism. Earthquakesand volcanicvents concentrateon
theboundary
of theanomalybut arealmostabsentin the interior.
The SWCC thus representsa distinctblock of crust which is
bounded
by narrowzonesof activedeformation.The deformation
zonescontainregionsof local crustalextensionconduciveto the
formationof volcanic vents [Weaver et al., 1987]. The chain of
late Cenozoic volcanic vents on the eastern boundary of the
SWCC may reflectmoreactivedeformationalongthisboundary
in the recentpast. The lack of earthquakes
andvolcanicventsin
the interior of the SWCC and the geometry of the conductor
(localized in space, extending from near the surface to great
depths)togetherargueagainstsomepossibleexplanations
for the
SWCC, suchas a magmabodyunderthe Cascadesor highlyfracturedrocksfilled with hot salinewater. The locationof the peak
heatflow on the boundaryratherthanin the interiorof the SWCC
hold water trappedin clays and zeolitesand can remain highly
conductiveeven at the low porositieswhich should prevail at
depthsgreaterthana few kilometers.
Stanleyet al. [1987] concludethat the SWCC is formedfrom
accretionaryprism and forearcbasin marine sedimentaryrocks
whichweretrappedandcompressed
againstthepre-Tertiarymargin of North Americawhenthe Crescentbasaltswere accretedin
the Eocene. This is an attractivehypothesis,but it raisessome
otherquestions.The SWCC is uniquein WashingtonandOregon,
as evidencedby the EMSLAB MV array, which coveredboth
statescompletely[Gough et al., 1989]. Althoughsmaller local
reinforces this conclusion.
Washington.
The SWCC is an importantstructurewhichmodelsfor the tectonic historyof the region mustexplain. What is specialabout
this small sectionof the CoastRange suturezone? One possible
The MV resultsstronglysupportthe conclusionsof Stanleyet
al. [1987, 1990] concerningthe geometryandconductivityof the
SWCC. There is a massiveunit of very conductiverocks under
the Cascadeswhich narrowsto the northand probablydipsdown
to the east. The high conductivityis most likely intrinsicto the
rocksratherthan being causedby currentconditionsin the crust
suchas the presenceof hydrothermalfluidsor magma. Stanleyet
al. [ 1987, 1990] considereda range of possiblelithologiesfor
theseconductiverocksand concludedthat at least the deeperpart
of the SWCC is mostprobablydominatedby shalefacies. Shales
induction anomalies associated with the suture zone between the
CoastRangeblock and olderTertiary rocksto the eastcannotbe
ruled out, the accumulationof the large volumesof conductive
sediment
seenin theSWCCis clearly
unique
to s•)uthwestern
suggestionis that the SWCC representsa sectionof the early
Cenozoicsubductionzone which is analogousto the presentday
OlympicPeninsula,wherean anomalously
thick sectionof sedimentsis beingscrapedoff of the downgoingslab and piled up on
the edgeof the continentat a highlylocalizedbendin the modern
subductionzone. In this model, the sedimentsthat make up the
SWCC could have been accreted well before the Crescent basalts
15,982
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
IN SW WASHINGTON
o&
o
2.0
O
3.O - 3.9
O
4.0 - 4.9
5.0
øo%
A
A
- 2.9
- 5.9
o
0
!
I
I
I
I
I
I
I
I
I
I
100 KM
0
I
I
I
I
I
I
I
I
i
I
I
100 KM
Fig. 13. Seismicityof westemWashington(0-15 km) plottedon top of anomaloussheetelectriccurrentsobservedby an MV
array.Symbolsaresizedby magnitude:
smallest,2.0-3.9;medium,4.0-4.9;largest,5.0-5.9.The largetrianglesarethelocationsof
the major Cascadevolcanos. (a) The sourcepolarizationwould drive the electriccurrentseast-westin the absenceof anomalous
structure.Note how the currentsenteringin the southeast
are deflectednorthas they run up againstthe seismiclineationof the
SHZ. The SHZ is a boundarybetweena very resistiveblockto the westandthe SWCC to the east. Note alsohow the currentsare
deflectedaroundthe very resistiveandnearlyaseismicblock westof Mount Rainier,northof the SHZ and southof PugetSound.
Finally, notehow the currentsin the westernPugetSoundregionare deflectedto the southwest,
presumablyby the resistiveCoast
Range block. (b) The sourcepolarizationwould drive the electric currentsnorth-southin the absenceof anomalousstructure.
Note thevery strongcurrentin the SWCC betweenthe SHZ andMount Adams.This strongcurrentcontinueswell northof Mount
Rainier.
were erupted. Brandon and Calderwood [1990] have suggested
that the locationof the Olympic subductioncomplexis controlled
by the upwarpingof the Juande Fuca plate requiredto accommodate the reversedarc of the subductionzone resultingfrom Basin
and Range extension. Thus the presenceof an earlier zone of
similar scaleand probablecompositionplacesconstraintson the
geometryof the early Cenozoicsubductionzone.
The MV data also image significantstructurein the Coast
Rangerockswest of the SWCC. Here the crustis resistiveover a
very thick section,with two narrow east-westzonesof relatively
high conductivity. The clearestof these structuresis the CBC,
which lies on the northernedgeof the ChehallisBasin. Currents
flowing in near-surface
basinsediments
cannotexplainthe amplitude or period dependenceof this anomaly. For east-westsource
polarizations there is a small north-south trending anomaly
aligned with the exposedsedimentsof the Chehallis Basin and
Puget Lowlands, but this vanishes for periods beyond 300
seconds.The much stronger,nearlyperiod-independent
east-west
trending anomaly associatedwith the CBC clearly indicatesa
muchdeeperstructure.Near the westernboundaryof the SWCC,
the CBC appearsto extend beneathresistivecrust to the north.
The crustalblocksto either side of the CBC are highly resistive
throughout,from the SWCC to the outcroppingCrescentFormation basalts(the Black and Doty Hills for the northernblock and
the Willapa Hills for the southernblock). The northernedge of
the Chehallis Basin is marked by numerousthrust faults (gen-
erally downthrownto the south [Walsh et al., 1987]). To the
north lies a seriesof west-northwest
trendinganticlines.We interpret the CBC as a sectionof conductivemarinesedimentaryrocks
which have been thrust under the resistive accreted oceanic base-
ment rocks to the north. Wells and Coe [1985] presenta model
for Eocenetectonicsof southwesternWashingtonin which the
oceanic "Crescent"plate was folded and broken into smaller
blocks,which were subsequently
thrusttogetherduring accretion
to the continent. It is quite possiblethat the CBC resultedfrom
this processand is thus a fundamentalstructuralboundaryin the
CoastRangerocks.
The secondzoneof enhancedconductivityin the CoastRange
block branchesoff of the top of SWCC, trendswest acrossthe
northernportion of the Puget Lowlands,and is then deflectedto
the southby the crescentbasaltson the easternedgeof the Olympic Peninsula(Plate 1, Figure 9). This structureis similar to the
CBC in that it represents
a deepconductivezone which separates
resistivecrustalblocksmarked by exposedsectionsof the Crescent basalts(in this case the easternOlympic Peninsulaand the
Black Hills). There is alsosomesuggestionin Figure 9 (which is
supportedby the much larger scaleEMSLAB array; seeGoughet
al [1989]) of an additionaleast-westconductivechanneljust off
the bottomof the array coveragesouthof the Willapa Hills. It is
temptingto suggestthat thesefeaturesare analogousto the CBC,
and represent zones of sedimentswhich were thrust beneath
discreteblocksof resistiveoceaniccrustduring accretion. How-
EGBERTANDBOOKER:
CRUSTALSTRUCTURE
IN $W WASHINGTON
ever, the geometryof theseconductivezonesis poorlyconstrained
by existingarray coverage.
The large clockwise rotation of the accreted terranes of
southwesternWashingtonsince the Eocene have been attributed
to tectonic processesoperating at two distinct scales [Wells,
1990]. Dextral shearrotationof the continentalmargindriven by
obliquesubduction
of the Juande Fucaplate [Beck, 1980;Wellset
al., 1984] is accomplishedthrough relative motions of a large
numberof elongatedbut thin (5-20 km) slicesof crust. From the
pattern of surface faulting and the geographicdistributionof
paleomagneticrotations,Wells and Coe [1985] and Wells [1990]
argue that this representsthe dominant mode of postaccretion
deformationin southwestern
Washington.Late Cenozoicrotation
at a largerscalehasbeenattributedto motionof a rigid microplate
in front of Basin and Range extension[Simpsonand Cox, 1977;
Magill et al., 1981, 1982]. This mechanismprobablydominates
in the coastrangeof Oregonandlikely alsoplaysa role in crustal
deformation and rotation in southwesternWashington, which
occupiesa transitionzonebetweencompressional
and extensional
tectonicregimesto thenorthandsouth,respectively[Guffantiand
Weaver, 1988; Wells, 1990]. The crustalstructuresimagedby the
MV datarepresentan intermediatescale. Thesestructuresappear
to exert considerable
controlover the tectonicsof this region.As
discussedabove,patternsof currentseismicityand recentvolcanism are stronglycorrelatedwith the MV conductancemodel. In
addition,the grain of fault patternsappearsto be correlatedwith
the structuresimaged by the MV data. Wells and Coe [1985]
show left-lateral faults associatedwith shearingand rotationto
trendwest-northwest
in the Willapa Hills blockbut east-northeast
in the Black Hills block to the north. Theseobservationssuggest
that the deep conductive structuresimaged by the MV data
representsoft zoneswhich to somedegreemechanicallyalecouple
the more rigid interveningresistiveblocks. Thus, althoughlargescalegradientsin the patternof seismicity,volcanism,and crustal
deformation
and rotation in the Pacific Northwest
are controlled
by regionaltectonics,the detaileddistributionof thesefeatureson
intermediatescalesis alsosignificantlyaffectedby existingstructuresin the complexaccretedcrust.
15,983
APPENDIX:EQUIVALENTSHEETCURRENTS
AND THIN SHEET CONDUCTANCE
Here we discuss a refinement of the equivalent current
transform and summarize our inversion of smooth anomalous field
data for crustalconductance.We considerthe simplethin sheet
model of Price [ 1949], which consistsof a thin conductivesurface
sheet of variable conductance
S(x, y)= So + 8S(x, y)
over a layeredone-dimensional
Earthwhichis separatedfrom the
surfacesheetby a layer of infinite resistance.
Becauseof the resistivelayer the currentin the surfacelayer
I(x,y) = Io + •SI(x,y)satisfies
V.I =0 andthusmay be expressed
in terms of a stream function
l.•(x)•tlt(X)
igy
ly(x)
=-•hlt(x)
(A1)
igx '
In the wavenumber
domain,•~can be rel_ated
to the anomalous
magneticfield'components
H.•, and Hz [Berdichevsky
and
Zhdanov, 1984]
•2
k• +cogO•z(k•,
ky)(So
-1/Z
n(A2)
i[*12•'•(k•'ky)
wherethetildedenotes
thespatial
Fouriertransform,
k.•andkyare
the horizontal
wavenumbers,
rl2=k.2•
+ky
2, and Zn is the
wavenumber-dependent impedance for the normal onedimensionalconductivitysection.Thus for this simplethin-sheet
modelthe impedanceof the underlyingone-dimensional
Earth can
be usedto exactlytransformthe anomalous
magneticfieldsinto
the anomalouscurrentflowing in the surfacelayer. Note that in
the limit co->O, the transformationreducesto rotating the
anomalous
fieldvectors
90ø counterclockwise.
Equations
(A1)
and (A2) representa refinementof the simpletransformationof
section3 to take accountof the field producedon the surfaceby
imagecurrentsinducedin the underlyingone-dimensional
section
by the anomalous
currentsin the sheet. Note that it is implicit in
A clearer
image
of thevertical
profile
of conductivity
through (A2)
the crustin this regionwould be highly desirable.Althoughfull
three-dimensional
inversionof the MV arraydatawould be a useful step in this direction,the intrinsicVerticalresolutionof MV
data is low. Significantimprovementsin verticalresolutionwill
that constant(zero wavenumber) sheet currents cannot be
uniquely determinedfrom the anomalousfields (for which the
zerowavenumbercomponentis alsoambiguous).
In fact, knowingtheanomalous
surfacecurrentfor two orthogonal sourcepolarizationsis nearly enoughto determinethe conrequireMT data. However,success
withMT requires
detailed ductanceof the surface layer for this simple model. Let
mappingwith closelyspacedelectricfield measurements,
which R(x, y)=Ro+bR(x,y)=l/S(x,
y) be the surface resistance.
in practice
requii'es
datato be collected
on a limitednumber
of Then, sinceE(x, y) = R (x, y)I(x, y), we havefrom Faraday'slaw
two-dimensional transects.On the other hand, MV data can be
collectedat relatively low costusingsmall (5-10 station)sequential overlappingarraysandthenmergedinto syntheticarrays. MV
array datathusrepresentan economicalway to map major crustal
conductivityvariations(with poorverticalresolution)over a large
area. Even with a wide stationspacing,MV datacanbe usefulfor
reconnaissance
beforemore detailedMV or MT experimentsare
conducted.The MV and MT methodsare thus complementary
and should probably be used together in areas where threedimensionalcomplicationsare li•ely. By itself, MV array data
can providesubstantial
insightinto crustalstructure.If usedin
conjunctionwith MT and otherdata,the potentialof this geophysical tool is even greater. Indeed,joint inversionof MT profiles
andMV arraydataprobablyoffersthebestpracticalhopefor constructingthree-dimensional
imagesof crustalconductivityvariations.
•-•-(R(x,
y)ly(x,
y)x•y(R(x,
y)l.•(x,
y))=
•x
icogttz(x,y).
(A3)
Equation(A3) mustbe satisfiedfor currentsinducedby sourcesof
both orthogonalpolarizationsfor all frequenciesfor which the
thin sheetapproximationis valid. An analysisof the uniqueness
of the resistancesatisfying(A3) shows that if co• 0 and I is
known for two orthogonalsourcepolarizations(more precisely,
for two currentdistributionswhich are parallelonly on a set of
measurezero), then R is uniquely determined. For co= 0, the
resistanceis determinedonly up to a scalar multiple. Unfortunately,the thin sheetapproximationis mostreasonableas co->O,
so that in practice,the absolutescaleof the crustalresistancecannot be determinedfrom anomalousfield dataalone. The average
15,984
EGBERT
ANDBOOKER:
CRUSTAL
STRUCTURE
INSWWASHINGTON
REFERENCES
resistanceR o, or some equivalentreference,must be specified.
Furthermore,the anomalousmagneticfields, which we can esti- Armentrout, J.M., Cenozoic stratigraphy, unconformity-bounded
mate, determineonly the anomalouscurrents;SI(x,y). The unisequences,
andtectonichistoryof southwestem
Washington,Wash.Div.
of Geol.Earth Res.Bull., 77, 291-320, 1987.
form part of the currentflowingin the sheet,Io, is not determined
Bailey, R.C., R.N. Edwards,G.D. Garland, R. Kurtz, and D. Pitcher,
from surfaceobservationsof the magnetic fields (or from (A1)
Electricalconductivitystudiesover a tectonicallyactiveareain eastem
and (A2)), but is neededto invert the anomalouscurrent for surCanada,J. Geomagn.Geoelectr.,26, 125-146, 1974.
face conductance.
Io is not completely arbitrary. For instance,setting Io =0
yields the equivalent currentsplotted in Figure 8. This figure
shows a number of unphysicalcurrent vortices. Indeed, in the
low-frequency limit (probably a good approximationat 1000
seconds),
theright-hand
sideof(A3)vanishes,
andwemu•thave
for any closedcontourC in the plane
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(ReceivedJuly 10, 1992;
revised March 19, 1993;
acceptedMarch 23, 1992.)