Download Resistivity Cross Section Through the Juan de Fuca Subduction

JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 94, NO. B10, PAGES 14,127-14,144, OCTOBER 10, 1989
ResistivityCrossSectionThroughthe Juande Fuca SubductionSystem
and Its TectonicImplications
PI-IILIPE. WANNAMAKER,
1JOaNR. BOOKER,
2ALANG. JONES,
3ALAN D. Canv•, 4
JEANH. FILLOUX
,5 HA•
S. WAFF,6ANDLAWmEK. LAW?
A resistivity cross section to depths exceeding 200 km has been derived from magnetotelluric
observationsalong a profile near latitude45øN from the Juande Fuca spreadingcenter,acrossthe coastal
subductioncomplex,the High Cascadesvolcanicarc, and into the back-arcDeschutesBasin region. In this
two-dimensional
interpretation,
emphasiswas placedon dataapproximating
the transversemagneticmode
sincethese data are relatively robustto commondeparturesfrom the two-dimensionalassumption. The
vertical magneticfield, however,has been very valuablein defining structureof the offshoresediments,of
the oceanicasthenosphere
and below the arc volcanicsof the Westem and High Cascades. The transverse
electric data on land suffer a variety of three-dimensionaleffects,making their interpretationvery difficult.
In contrast,the greateruniformity of upper crustalconditionson the seafloorallowed a good fit to both
modesof theimpedance
plustheverticalmagnetic
fieldat leastdownto 104s' Important
components
of
our model resistivity structureand their hypothesizedtectonicimplicationsare as follows. Measurements
on the seafloorindicatevery low resistivityin the eastwardthickeningsedimentarywedge of the abyssal
CascadiaBasin providedits depthextentis constrained.The majority of thesesedimentsare inferred to be
off-scrapedor lose most of their interstitial water before being carriedmore than ten or so kilometersdown
the trench. In the oceanicuppermanfieof the Juande Fucaplate, moderatelylow resistivitiesfrom about
35-215 km depth (nominal) presumablyreflect up to several percent partial melt attending regional
upwellingin the vicinity of the ridge. Very low resistivitiesbelow 215 km depthin the oceanicmantleare
difficult to explain physically,but problemswith very long-periodsourcefield effects or bias error may
hamper the modeling here. Under the Oregon Coast Range, a low-resistivity layer dips inland at
approximately 20ø. Its conductancedecreasesby an order of magnitudefrom the coastto around60 km
inland and its position appearsconsistentwith the Juan de Fuca plate subductiondecollement.Possible
causesof the low-resistivity include residual sediments,pore waters, perhapssulfides, and dehydrating
oceaniccrust carrieddown the subductionzone. East of about60 km inland, there is a much stronger,
subhorizontalconductorwith a depth to top of around 25 km. To explain it, a massive breakdownof
greenschistminerals is hypothesizedto liberate most of the water carried in the altered ocean crust well
before the volcanic arc is reached. The resistivitymanifestationof the arc magmatismitself is not strong
but consistsof a low-resistivityaxis in the middle crustbelow and to the west of the arc at the surface. It
presumablyrepresentsfluids releasedfrom crystallizingmagmas.East of the High Cascades,both the MT
and geomagneticdata detect a stronggradientin resistivitywith lower values to the south.It is suggested
that the Brothersfault zone signifiesan electrical, thus possiblya thermal and magmatic,boundarywith
the northernBasin and Range.
INIRODUCTION
A magnetotelluric(MT) transectof unprecedentedbreadth
and quality has been collected across the Juan de Fuca
subductionsystem [EMSLAB Group, 1988; Wannamaker et
al., this issue] (Figure 1). This profile, called the Lincoln
Line becauseit crossesthe northern Oregon coast near the
town of Lincoln City, reveals a rich variety of resistivity
structuresassociatedwith the subductionprocess. Possible
examplesincludedeepstructureof the Juande Fuca spreading
regime, the near-offshoreaccretionaryprism, the subduction
decollement beneath western Oregon, the High Cascades
1University
of UtahResearch
Institute,
SaltLakeCity.
2University
of Washington,
Seatfie.
3Geological
Survey
of Canada,
Ottawa,
Ontario.
4AT& T BellLabs,MurrayHill,NewJersey.
•Scripps
Institution
of Oceanography,
LaJolla,Califomia.
6University
of Oregon,
Eugene.
?Geological
Survey
of Canada,
Sydney,
British
Columbia.
Copyright 1989 by the AmericanGeophysicalUnion.
Paper number 89JB00681.
0148-0227/89/89JB-00681
$05.00
volcanic arc, and the back-arc Deschutes Basin region
[Wannamakeret al., this issue]. While a numberof aspects
of the subsurfaceresistivity can be deduced straight from
inspection of the data, computer simulation of the
observations helps to quantify more rigorously the
permissibleor requiredstructures.In this paper,we derive a
two-dimensional resistivity model, with a presumednorthsouth strike, by trial-and-error fitting of our MT
measurements with a finite element forward modeling
algorithm. Constraintson model geometryfrom independent
geological or geophysical investigations have been
incorporatedwherejustified.
Our paper is divided into three major sections. First, we
explain the approachtoward two-dimensionalmodeling. On
land, the transversemagnetic (TM) impedancefunctionsare
emphasizedbecausetheory and experienceshow that they are
more robust to plausible three-dimensional effects in the
region than are the vertical magneticfield or, especially,the
transverseelectric(TE) impedance. Upper crustalstructureon
the seafloor appears to be rhuch simpler than on land,
however,and both modesof impedanceplus the vertical field
are fit fairly well. Second, the model cross section is
described and its goodnessof fit to the data demonstrated.
Model uniqueness is investigated by perturbing certain
features of the model and assessingthe increasein misfit.
14,127
14,128
WANNAMAKER
ETAL.:RESI•
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CROSS
SECTION,
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DEFUCASYSTEM
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elI ß AND
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ß
ß
PLATE
L____...
40
ø
Zone
116ø
132 ø
128ø
124ø
120ø
Fig. 1. Instrument
locationof entireEMSLABarray. On land,theunlabeled
dotsindicatelocations
of three-component
magnetovariometers.
TheE-W trending
concentration
of dotsin northwestern
Oregondenotes
thelong-period
MT recorders
alongthe landportionof the LincolnLine. Seafloorarraysitesarelabeledaccording
to instrument
typeandsites0-8 are
givenfor theseafloor
extension
of theLincolnLine. However,
nodatawererecovered
fromseafloor
sitesSF0andSF6.
This is not a complete method of model appraisal,but it
allows us to test our model structure against important
competinghypotheses.Finally, our resistivitymodel offers
new insightinto the tectonicsof the Juande Fucasystemand
of the subduction
processin general. This is realizedwithin a
framework of independent geological and geophysical
knowledge, which includes primarily heat flow, seismic
velocity, and slab mineralogy.
VALIDITY OF Two-DIMENSIONAL MAGNETOTELLURIC
the observation
that some MT
functions
are much
less
affectedby variationsalongstrike than others.
As reviewed more fully with the data presentationby
Wannamaker et al. [this issue], we measureand interpret the
complex-valuedtensorimpedancerelating the total electric
(E) and magnetic (H) field vectorsat the surfaceof a threedimensional earth, i.e.,
Z•
]
(1)
INTERPRETATION
We have only one detailedprofile of MT observations
in
the EMSLAB projectand so must interpretthe observations
within primarilya two-dimensional
framework. Formately,
a substantialamountof progresshas been achievedin the
past several years in understandingthe conditionsunder
which two-dimensionalinterpretation is permissible. In
essence, the possibility of two-dimensional approaches
dependson the commonpreferredorientationof tectonism
(roughlyN-S in theUnitedStatesandin ourstudyarea)andon
A right-handedcoordinatesystemwith z positivedown is
assumed.Over a purely two-dimensional
earthwith x as the
strike direction, the on-diagonal impedance elements Zxx
and Zy• arezerowhile Zxy and Zy• represent
the
independenttransverseelectric (TE) and transversemagnetic
(TM) modes of polarization. The TE mode is defined as
having the electricfield (currents)parallel to strike,while the
TM mode has the field (currents) across strike. Apparent
resistivities
(e.g.,p• and p•) andimpedance
phases(0•
WANNAMAKER
ET AL.: RESISTIVITY
CROSSSECTION,
JUANDE FUCASYSTEM
and ½yx)are formedfrontthe complex
elements
for
subsequentinterpretation.Similarly, the relationship
14,129
downwardbias persiststo arbitrarily long periods, tending to
make the three-dimensional impedance appear more
anomalousthan the two-dimensional one. In attempting to
replicatethe three-dimensional
effectin p• and ½• usinga
two-dimensionalTE routine, one would thus need to place
additional (i.e., false) low-resistivity material beneath the
is defined for the vertical magnetic field.
Over a two-
dimensional
earth, Mz,, is zeroand Mzy arisesfromtheTE
mode of polarization.
Our recommendationfor two-dimensional interpretation is
to derive a resistivity cross section emphasizing the TM
mode data since the two-dimensional assumption for this
mode is much more robust. For example, it has been shown
that two-dimensional modeling of synthetic TM mode data
across simple, elongate three-dimensionalstructuresyields
accurate resistivity cross-sections [Wannarnaker et al.,
1984]. The profile in fact does not need to be close to the
center of the three-dimensional body for this to occur.
Furthermore,
a two-dimensional
TM interpretation
of p•,, and
½• is possibleevenwhenuppercrustalinhomogeneity
is
more complex. In this case, such an interpretationyields a
cross section which may be somewhat incorrect in its
surficial features but represents accurately the regional
resistivitystructureof interestprovided the regional structure
itself is not arbitrarily three-dimensional [Jones, 1983;
Bostick, 1986; Kariya, 1986; Wannarnaker, 1989, in press].
For two-dimensional modeling, the MT functions should be
defined using a fixed coordinatesystem approximatingthe
strike direction
to overcome
unwanted
effects of near-surface
structure,specificallyon the impedancephase[Wannamaker
et al., 1984].
The Lincoln Line profile was designed to meet the
foregoing criteria for three-dimensional modeling
emphasizing the TM mode. Observed geological and
physiographic trends in the region are oriented
predominantlyN-S [Baldwin, 1981]. This is implied alsoby
most of the gravity data [Riddihough and Seeman, 1982].
Somedeparturefrom an assumedN-S geoelectricstrike in the
upper crust is indicated along the east side of the Willamette
Basin and the west side of the High Cascades graben.
However, most resistivity variations, and especially the
deeperonespresumablyassociatedwith subduction,appearto
trend mainly N-S beneathour data profile accordingto their
effecton Mz• and Mz• [Wannarnaker
et al., this issue].
Furthermore,the preliminarymini-EMSLAB soundingstaken
about 50 km southof the Lincoln Line [Young et al., 1988]
also exhibit the major features of the present study, in
trueconductor.
However,M,• appears
lessanomalous
overa
finite length conductor than over a fully two-dimensional
one. This occursbecausethe depressionof electric field (and
thus current) within the three-dimensionalbody reduces H, in
its vicinity relative to the two-dimensional case. Finally,
modeling of the TE results should not compromisethe TM
mode fit. If the foregoing conditions cannot be met, then
three-dimensional effects are probably at hand and require
appropriatethree-dimensionalmodeling.
MODEL RESISTIVITY CROSS-SECTION
Our preferredtwo-dimensional
resistivitymodel for the
Lincoln Line is presentedin this section. It has been
obtainedby trial-and-error
fittingof the datausingthe finite
elementforwardalgorithmdescribedby Wannarnakeret al.
[1986, 1987]. Triangularelementshapesallow simulationof
polygonal resistivity media, including a precise
representation
of the oceanbathymetry.Consequently,
the
oceanicMT responses
of ourmodelcouldbe calculatedat the
bottom of the seawater where the data were measured. For the
detailed MT profile on land (Figure 2), the TM mode
impedance
functions
p•,, and½•xweregivengreatest
weight
throughmostof the fitting process. The verticalmagnetic
field, however,wasvaluablein understanding
severalareasof
the subsurface structure, especially those under the
continentalshelfandslopeanddeepunderthe High Cascades.
The TE quantitiesp• and ½• on landhavebeenusefulin
revealing important off-line resistivity variations. In
contrast, a single two-dimensionalmodel explains both the
TE and TM mode impedance functions plus the vertical
magneticfield on theseafloorat leastfor periodsup to 104s.
Several constraintson model geometry were incorporated.
First, the bathymetriccontoursof the Juande Fuca plate relief
map of the Pacific GeoscienceCenter [Geological Survey of
Canada]were usedto specifythe depthof the seawater,which
was given a resistivity of 0.3 ohm m [Filloux, 1987].
Second, thicknesses of conductive sediments over resistive
p,,• and ½•,yshouldalso improvethe fit to the vertical
marine basalts in the near-offshoreNewport Basin and the
abyssal Cascadia Basin were simplified from the geologic
sectionof Snavely [1987]. The base of conductivesediments
of the Willamette Basin was modeledas a sharpinterfacewith
resistive rocks below in keeping with deposition over
igneousor metamorphicbasement. The resistivity model in
the High Cascadesgrabenand vicinity was patternedafter the
volcanic stratigraphyand structurepresentedby Priest et al.
[1983, 1987]. At 400 km depth, a very low resistivity (0.3
ohm m) half-space was imposed to represent the global
decrease in resistivity near there [Banks, 1972; Roberts,
magnetic field. This is becausethree-dimensionaleffects on
the TE impedancetend to be of an oppositesensethan on
1986].
Generally, our model was constructedfrom the near-surface
particular
thesubtlepeakin ½• belowtheCoastRangeand
its substantialincreaseto the east. Consequently,a fixed N-S
strike direction is defined for two-dimensionalmodeling of
the Lincoln
Line data.
In contrast,three-dimensionaleffects can be quite severe
for the TE mode and reliable interpretationsof this data
require care. Specifically, any structureintroducedto model
M,•. For example,oversimplethree-dimensional
conductive (short periods) downward. Deep structurewas included only
inhomogeneities,finite strike length typically depressesthe
electric field, and thus the impedance,relative to the twodimensionalTE response[Wannarnakeret al., 1984]. This
when the data could not be explained by shallow structure.
The abruptnature of the resistivity boundariesin the model,
which are an implicit constraint of the finite element
14,130
WANNAMAKER
ETAt..:RESiSTiViTY
CaossSECTION,
JUAND• Fuc^ SYSTEM
124ø
122ø
123 ø
I
-
/--
_,
x
121ø
'
P
/
/'
"...................
/
• -- • '-
(
5
'
OXUS
'
/
I
/
•
•
•
•
{
Basinr •
I
,
E }
•
+
Broadband
•
•
•, ••S••
•
0
MT
•
50
Lava
Plai
o
B
ns
o
100 km
ß Long Period MT
Fig.2. Broadband
andlong-period
magnetotelluric
soundings
along
thelandward
portion
of theLincoln
Lineandalso
alongthemini-EMSLAB
testtraverse
farther
south
[Young
et al., 1988].Urban
areas
include
Portland
(P),Salem
(S),
Eugene
(E),Newport
(N),Lincoln
City(L),Bend(B),Madras
(M),andTheDalles
(D). Cascades
volcanoes
arelabeled
as
triangles:
MountHood(H), MountJefferson
(J),andSouthSister(S).
algorithm,shouldnot be takenliterally. Because
ourdataare
impreciseandfinitely sampled,andbecausethe propagation
of electromagnetic
fieldsin theEarthis diffusive,thedatacan
only resolvesmoothresistivityvariationsin the subsurface.
We presumethat the media in our finite elementmodel
approximatelyrepresentlocal averagesof conductivity
(inverse of resistivity).
possess
resistivities
of the orderof 100 ohmm or a bit less.
Belowthemat a depthof about2 km lies a low resistivity
unit
which is essentiallyhorizontal under most of this area.
Conductive marine sediments and alluvium of the Willamette
Basindeepensteadilyto the eastand reacha thickness
of
3.75 km nominallyby y = 75 km (distanceeast of the
coast). Fartherinland,the thicknessdecreases
to only about
2 km by y = 100 km, near the town of Stayton,beyond
which there is a second, lesser depth maximum. Low
Model ResistivityCross Section
The two-dimensional finite element resistivity model is
resistivity extendsabout 20 km east of the Willamette
Valley-WesternCascades
physiographic
boundary.
Farther east, the Western Cascadesblock contains high
presented
in Figure3 andPlate1 in slightlysimplitledform.
extendingcloseto the surface(Figure3). Around
(Plate 1 can be found in the separatecolor sectionin this resistivities
issue.) Specifically,the resistivityvalueshavebeengrouped y = 145 km, a newlow-resistivity
layerappears
at a depth
intorangesof one-halfdecadein orderto emphasize
themajor of only500 m or sobelowmainlybasalticrocksof the High
structural members of the model. Minor variations within
Cascadesextrusiveplatform [Priestet al., 1983]. The layer
each groupoccurpredominantly
in the upper2 km of the deepensonly slightly eastwardbut, somewherebelow the
axis(abouty = 170km), undergoes
an
model and simulatelocal, short-periodanomalies.Note the presentHigh Cascades
abrupt fivefold increasein conductance(conductivityverticalscalechangesat 5 and 150 km.
thickness
product)anddeepensto about600-800m beneath
The landwardportionof the LincolnLine model(Figure3)
resemblesthat of EMSLAB Group [1988] but is refined
somewhat.
Marine
basaltic
rocks
of
the
Siletz
River
formation [Shayely, 1987] outcroppingin the Coast Range
extensive Pliocene basalt flows overlying the Deschutes
Basin. All the conductiveupper crustalunits just described
reside on a basement and middle crust which is resistive
WANNAMAKER
ET At,.: R•smT•vrryCROSS
SECIION,JUANDE FUCASYSTEM
CB
150
NB
100
50
CR
c
WB
50
WC
lOO
14,131
HC
150
DB
200 km
4:1
50
1:1
100
ß
.
.
1:4
200
400
km
p
Fig. 3. East-west
resistivitycrosssectionderivedfromforwardmodelingof MT dataalongthe LincolnLine. Section
preserves
detailsof thefiniteelementmeshgeometry
butresistivities
havebeengrouped
in half-decade
intervalsfor grayscaledisplay. Seafloorportionof the model is shownonly to 175 km offshore,beyondwhich variationsare minor. Note
changes
in verticalexaggeration,
labelledon the left, at 5 and 150 km depth. Importantphysiographic
regionscrossed
includethe Cascadia
Basin(CB),NewportBasin(NB), CoastRange(CR), WillametteBasin0NB), WesternCascades
0NC),
HighCascades
(HC),andtheDeschutes
Basin(DB). A colorversion
of thiscrosssection
appears
asPlate1 in theseparate
color section of this issue.
throughout (hundreds of ohm-m). It is difficult to resolve
variationswithin the basement,but the highest resistivities
appearto exist under the Western Cascades.
about 6. Its top is somewhat shallower, and its bottom is
substantiallydeeper. However, a precisedeterminationof its
depth extent is made difficult by a strong regional threedimensionaleffect east of the High Cascades,as described
Perhapsthe bestdeterminedelementof the deeperstructure
is the subhorizontal
conducting
layerbelow 25 km extending later.
eastfrom abouty = 60 km. The conductance
of this layer is
Anotherimportantmemberof the deeperstructureis a low600 S, and it is responsiblefor the strongridge in the TM
resistivitylayer beneaththe Coast Range. This layer dips
phase •y,, at 30-100s andthe distinctminimumin
eastward about 20ø from a depth near 12 km under the
describedin the data paper [Wannamakeret al., this issue].
coastline to 45 km under the west-central Willamette
The bottom of the conductor is somewhat shallower
Our data suggeststhe layer is thin relative to its depth such
than in
the preliminary model presentedby EMSLAB Group [1988]
and fits the TM phasebetter below the Western Cascadesand
easternWillamette Basin at periods from 300-3000 s. This
conductorappearsto be resolved as well by the detailed
geomagnetic measurements of Hermance et al. [1989, in
press] in a profile some 30 km to the south of the Lincoln
Line. Just west of the present High Cascadeschain, a
relatively narrow conductive axis extends up from the
subhorizontal conductor to about 5 km from the surface.
East of the High Cascadesbeneaththe DeschutesBasin, the
subhorizontal
deepconductoris shownthickerby a factorof
Basin.
that only its conductancecan be resolved. The conductanceof
this dipping unit decreaseseastward from a maximum near
1000 S underthe coastto lessthan 100 S by y = 60 km. Its
exact geometric relation to the subhorizontal conductor
discussedpreviouslycannotbe determinedfrom our data. In
contrast to the model of EMSLAB Group [1988], the two
conductivelayers are not connectednear y = 60 km, but
instead a small resistive gap is shown which negligibly
affects the response. The data neither require nor forbid an
extensionof the dipping, low-conductancelayer under the
subhorizontal
conductor
above it.
14,132
WANNAMAKER
ET AL.: RES•STtVrrv
CROSS
SECI'ION,
JUANDE FUCASYSTEM
The ocean bottom portion of the finite element model is
drawn only as far as y =-175 km in Figure 3 and Plate 1.
Beyond this, the model remains essentially the same apart
from a gradual decrease in the conductance of the lowresistivity sedimentary layer of the abyssal Cascadia Basin
from values around2500 S beneathsite SF3 to essentially
zero at SF8 near the ridge. The resistivitiesof the sediments
of this basin and of the near-shoreNewport Basin are very
low, in the range of 1-2 ohm m. However, the resistivityof
the Miocene and older melange and broken formation
[Snavely, 1987], which protrude to shallow depths beneath
the geomagneticvariation sites SF1 and SF2, is much higher
with a value of nearly 30 ohm m in our model. Extending
down the trench and connecting to the dipping conductor
beneath the Coast Range is a strip of low resistivity whose
COMPUTED
w
E
CR
WV
WC
HC
DB
,
i
i
i
i
i
iii
i
i
i
i i
i
i
i
i
i
i
• i I
i
i
i
ii
ii
i i
i
i
i
• +•
o•+2
Pyx
+3
,4
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-2
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I
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-1
conductance, however, is lower than that of the Cascadia
Basin sediments nearby. The continuity of this strip is
tenuous and resolved best by the westernmost land MT
stations.
Shallow
water
MT
measurements
would
•+•
- ½yx
be
necessaryto resolve trench structure with more confidence.
These are logistically very difficult becausethe continental
shelf is swept continuouslyby fishing nets.
Below the conductive seafloor sedimentarylayer resides a
resistive (3000 ohm m) lithosphere a few tens of km thick
(Figure 3). This unit is resolvedbest by the westerntwo MT
sites where the seafloor sedimentsare relatively thin and do
not screen the responseof the resistive layer below. At
depths near 35 km, we see a dramatic fall in resistivity to
around20 ohm m from 3000 ohm m. This conductiveregion
is responsiblefor the steepdrop in apparentresistivitiesand
the high impedancephasesat the shortestperiodsat SF7 and
SF8. In the model, the resistivity of this mantle region is
essentially constantto a nominal depth of 215 km. Models
of geomagneticvariationsby Law and Greenhouse[ 1981] and
DeLaurier et al. [1983] suggest that resistivity increases
again below about 100 km, but the deeper reaches of these
models are relatively poorly resolved [Oldenburg et al.,
1984]. At depthsbeyond215 km in the oceanicmantle, even
lower resistivities (about 1 ohm m) are shown. However, the
resistivities at this depth and beyond are difficult to resolve.
This difficulty stems from an inconsistencybetween the
apparent resistivity and impedance phase of both modes at
+4 •40
_
i
i
0
50
Fig. 4.
i
100 km
Pseudosectionsof calculated transversemagnetic apparent
resistivity
Pyxandimpedance
phasetpy
x alongthelandward
portion
of
theLincolnLine. Contour
values
of Pyxarein ohmm andof tpy
x arein
degrees. These and subsequentpseudosections
shouldbe comparedto
observedpseudosections
presentedby Wannamakeret al. [thisissue]to
assessgoodnessof fit. Color versionsof thesepseudosections
appear
in Plate 2 in the separatecolor sectionof this issue.
COMPUTED
W
E
SF8
SF7
SF5
SF4 SF3
+2
_
13yx
periods of 10ns and more, as discussedlater with the
calculated response curves. The eastward extent of the
oceanicmantle conductoris not well definedbut extendingit
eastof thecoasthelpssimulatetheabruptfalloffin Mzy at
periods below about 500 s. The conductivitybelow 215 km
in fact could be drawn
+5
+2
across the entire section because the
land data consideredextendsonly to 10,000 s period.
ComputedResponsePseudosections
Next we show the calculatedMT responses
of the model for
comparisonwith the observationsin Wannamakeret al. [this
•+3
(I)yx
o +4
+5
issue].Firstwill be theTM modequantitiesPyxand •yx since
0
100
I
I
200 km
they have been emphasizedin the modeling. As seen in
Figure4 and Plate 2, the agreementon land of the calculations
with observationsis very good in both apparentresistivity
and impedancephase. The fit of the model to the observed
Fig. 5.
py• and <py•ontheseafloor
is shownin Figure5 andPlate3
resistivity
Pyxandimpedance
phasedpy
x forthefiveMT soundings
on
I
Pseudosections
of calculatedtransversemagnetic apparent
and is similarly good with one exception. (Plates2 and 3 can
be found in the separatecolor sectionof this issue.) At the
the oceanbottom segmentof the Lincoln Line. Contoursof apparent
resistivity are in ohm m and of impedancephase in degrees. Color
versionsof thesepseudosections
appearin Plate 3 in the separatecolor
very longperiods,greaterthan 10• s, computedpy• is
section of this issue.
WANNAMAKER
ET AL.: RESIST[VrrV
CROSSSECTION,
JUANDE FUCASYSTEM
14,133
somewhat
smallerthanobserved
althoughthe fit to ½yxis
The high calculated ½xy resultsfrom the very low
good. This discrepancyis even worse for the TE quantities
describedsubsequentlyand reflects an inconsistencyin the
very long-periodseafloordata to which we will return.
resistivities deep in the oceanic mantle, which must not be
affectingthe land datain a two-dimensionalmannerfor the TE
mode. Finally, the three-dimensional nature of deep
Thecalculated
TE pseudosections
(p,, and½•) fortheland
segmentof the Lincoln Line appear in Figure 6 and Plate 4.
(Plate 4 can be found in the separatecolor section of this
issue.) While they resemblethe data of Wannamaker et al.
[this issue] to some extent, the assumption of a twodimensional structure for the TE mode fails seriously in
several areas of the response. Due to the variety of threedimensionaleffectsinterpretedto exist in the TE data on land,
these data have not been utilized in determiningthe structure
of our model. Most obviously,the two-dimensionalTE model
calculations
showa very smoothlateralvariationin p, at
periods longer than about 10 s. In contrast, the data
pseudosectionsretain a strong vertical orientation to the
apparent resistivity contours. This fundamental difference
results from numerous finite strike length effects in the
measuredTE data over upper crustal conductive structures
under our profile.
There are at least three other discrepancies between
calculated
and
measured
TE
data
that
are
due
to three-
dimensionaleffects. First, no two-dimensionalgeometrycan
reproduce
the very long-period
anomalies
in ½,, andthe
slopeof p,, versus
T, of narrowlateralextentaroundy = 30,
40 and 135 km [Wannamaker et al., this issue]. We must
appeal to some three-dimensionalconnection or coupling
between upper crustal structures local to the measurement
sites and large-scale bodies off-line to our profile for an
explanation.Next, approaching
104s, calculated
•.• on
resistivitystructurebelow the Deschutes
Basinaffects p•y
and ½,,• stronglyhere at long periods. The calculated
responseof the preferredmodel in Figure 6, is far too low for
p, andtoohighfor ½, at periodslongerthanabout10 s.
An E-W boundary a few tens of kilometers south of our
profile, with deep conductive material on its farther side,
explains the occurrence[cf. Wannarnakeret al., 1984]. Such
a geometry has been indicated by Gough et al. [this issue]
from their geomagnetic array measurements. They have
named the resistive northern region the Blue Mountains
Resistive Block. The conductiveBasin and Range province
is to the south.
This east-west boundary provides an example, on the
Lincoln Line, where a purely two-dimensional TM mode
analysis may be inaccuratedue to "sideswipe"by a nearby,
off-line structure of regional extent. The TM data alone
suggest low resistivity to depths of at least 100 km here
while the verticalmagneticfield, whichis measuredto l0 n s,
implies that conductordepth extent is limited to only 65 km,
as shown in Figure 3. The true upper mantle structurebelow
the Deschutes Basin will not be resolved more precisely
without three-dimensionalsimulations and probably further
N-S MT profiling in this region.
On the seafloor, as stated earlier, the preferred twodimensional model fits both TM and TE mode impedance
responses
almostequallywell, at leastdownto 10ns(Figure7
and Plate 5).
(Plate 5 can be found in the separatecolor
averageover the whole land segmentbecomeshigher than
observed,
whilethefall in p, vs.periodbecomes
toosteep.
COMPUTED
W
w
COMPUTED
SF8
E
SF7
I_^
0
-1
SF5
I
SF4 SF3
I II
•' +3-
•' +1
Pxy
Pxy • +4--•
--•+2
''
+3
--
+5 -I
I
I
I !
i
i
i
+4
i
i
-2
•
-1
•'+1
+3
+5 -I
+4
I
0
0
•'• _ 403 '-0,•...•
q)
xy • +4•67•'
+2
Fig. 6.
0
•+3--•
I
50
I
100km
Pseudosectionsof calculated transverse electric apparent
Fig. 7.
I
I
0
100
I
I
xy
I I
200 krn
I
Pseudosectionsof calculated transverseelectric apparent
resistivity
Px• andimpedance
phaseqx• alongthelandward
portion
of
resistivity
Px• andimpedance
phaseqx• alongtheseafloor
portion
of
the Lincoln Line. Color versionsof thesepseudosections
appear in
Plate 4 in the separatecolor sectionof this issue.
the Lincoln Line. Color versions of these pseudosectionsappear in
Plate 5 in the separatecolor sectionof this issue.
14,134
WANNAMAKER
ET AL.: RESISTIVITY
CROSSSEC•ON,JUANDE FUC^ SYSTEM
COMPUTED
section of this issue.) One notable example of the twodimensionalnatureof the TE mode here at moderateperiodsis
the fact that p•
w
below about3000 s appearslaterally
uniform across the whole seafloor segment of the Lincoln
Line. The inconsistency between the observed apparent
resistivityand phaseat periodsgreaterthan 104s, noted
+2
earlier for the TM mode, is even more obvious in the TE.
• +3
Specifically,
we canfit •p,qwiththemodelof Figure3 butnot
p•. This difficultyis discussed
furtherin the upcoming
• +4
SF8
SF7
I
I
SF5 SF4 SF3 SF2 SF1
I
I
I
i
i
I
I
I
I
I
I
!
i
i
i
i
i
i
i
sensitivity tests.
The verticalmagneticfield function M,y particularlyon
+5
land has been of greater use in the interpretation of the
LincolnLinethanhavetheTE impedance
quantities
p,q and
•p,q.The areasof greatest
valueincludetheresistivities
of the
+2
_
•
--
/-.2•
sedimentary sections offshore and the conductive axis
associated with the High Cascades volcanic arc. The
calculated
response
of M,• appears
in Figure8 andPlate6 and
its agreement with the observationsof Wannamaker et al.
[this issue] is good overall. (Plate 6 can be found in the
separate color section of this issue.) One minor area of
disagreement occurs near the westernmost edge of the
Willamette Basin sediments (y ,-- 30 km). The measured
responsehere is weaker probably becausethe sedimentsare
areally very patchy, i.e., have poor N-S continuity. The
computedpseudosections
of M,y for the seafloorstations
appearin Figure 9 and Plate 7 and show good agreementwith
the data of Wannamaker et al. [this issue] as well. (Plate 7 can
be found in the separatecolor sectionof this issue.) That the
vertical magnetic field is so much more consistentwith the
TM mode impedance functions than with the TE mode
impedance implies a greater continuity along strike of the
current-densityvariations, which determine H,, relative to
the electricfield variations,which affect the TE impedance.
COMPUTED
W
E
CR
WB
WC
HC
DB
+5
Fig. 9.
•,
I
I
]
iiii
iii
i
i
i
i
i
i
i
i
i
i
i
o
i-- +1
•
+2
+3
+4
I
I
1
i
i
i
!
]
i
iii
i
i
i i
i
i
i
i
I
I
I
0
100
[
i
I
I
I
I
200 km
i
Pseudosections
of calculatedvertical magneticfield transfer
function M2y for the seafloorportionof the LincolnLine. Color
versionsof thesepseudosections
appearin Plate 7 in the separatecolor
section of this issue.
Model-Response Sensitivity Tests
Any interpretationof incomplete, inaccurategeophysical
data involves nonuniqueness[Parker, 1977]. We have
presenteda model that fits the data very well. Other models
undoubtedlyexist. One approachto a nonuniqueproblemis
to look for modelswith minimumstructurerequiredto explain
the data. We have undertakenthis in an informal way by
adding deep structure only when the data could not be
explained by shallow structure. Jiracek et al. [this issue]
implementsuchan approachin a formal way by minimizing a
mathematicalmeasureof structuresubjectto a certainlevel of
data misfit. Another approach is to perturb structural
elements
I
_
and examine
the effect
on the data.
This is not a
complete method of model appraisal becauseone limits the
searchfor alternatemodelsto thosethat are fairly close to the
starting model. However, it allows one to directly test a
model against important competing hypotheses [Parker,
19771.
We will follow the latter approach. Our initial tests
concern the subhorizontaldeep conductorbelow the central
Willamette Basin to the High Cascades. Figure 10 displays
the observedTM and TE quantitiesfor a site 123 km inland
from the coast. The goodnessof fit of the preferredmodel of
Figure3 andPlate1 is apparent
(solidcurvesof P•x and •x).
Note that the broadbandand long-periodsites are not exactly
coincident in geographic location so that the long-period
apparentresistivitieshave a small static shift relative to the
Fig. 8.
Pseudosections
of calculatedvertical magneticfield transfer
broadband.
It is only important in this case that the model
functionM,y alongthelandward
portionof theLincolnLine. Color
versionsof thesepseudosections
appearin Plate 6 in the separatecolor responsebe parallel to the long-period data in the log-log
section of this issue.
plot and that the phasebe fit.
0
50
100 km
WANNAMAKER
ET AL.: R•SIS•IViTYCROSSSECttON,JUANDE Fuc^ SYSTEM
•-
+3
xu
i
i
ux
•-
i
EMSL i 123
i
i
-
14,135
,-, 0.75 ' •- =u •- zx
N
"• 0.00
E +2
_. I
1,
..........
o
-0.75 ,
+1
+0
I
"• 90
'•
I
I
I
I
,
,
,
0
,
, EMSL,
147,
I
'
-0.75
313
I
-2
I
-1
I
0
I
+1
I
+2
+3
-1
I
I
+1
I
+2
I
+3
I
+4
I
+4
log
T
($)
1 og T
Fig. 11.
Real and imaginary observedresponsecurves for elements
M2xandM2ymeasured
by University
of Oregon
andGSCabout147km
Fig. 10. Broadband and long-period MT data collected by the
Universityof Oregonand the GeologicalSurveyof Canada(GSC) about
inland on the Lincoln Line. Computedresponsesinclude that of the
model of Figure 3 (solid curves), that with the midcrustal conductive
axis removed (dashes), and that with the mantle conductor below the
123 km inland on the Lincoln
Line.
Error bars are one standard
DeschutesBasin increasedto infinite depths(dots).
deviationplotted where larger than data symbol. The solid curve is the
calculatedTM mode responsefor the preferredmodel of Figure 3. A test
caserejectedfor its misfit is that of removingthe strong,subhorizontal
conductoreast from the central Willamette Basin to the High Cascades
both the vertical field and TM responseimplies that the
and instead extendingthe Coast Range conductordowndip to 85 km
depthbeneaththe High Cascades(dashedcurves). A secondalternative conductiveaxis has substantialN-S continuity. To the east,
rejectedhashigh conductivityfrom 25 km depththroughto the downdip the TM response requires the subhorizontal conductor
extensionof the CoastRange conductor(dottedcurve).
extendingto at least 100 km depth and essentiallycan detect
no bottom. However, an infinite depthextentdisagreeswith
observedM,• at periodslongerthanabout1000 s (dotted
As a first test model,
the subhorizontal
conductor is
removed and the dipping conductorunder the Coast Range
simply extended downdip to 85 km depth below the High
Cascades (Figure 10). The conductance of the layer is
increaseddowndip also to preservethe amplitudeof the peak
in •y,•;for example,thelayerconductance
at 65 km depthfor
this test is about 104 S. One sees from the dashed curves of
py,• and •,• thatsuchan alternative
is untenablesincethe
periodrangeof boththepeakin •,• andtheminimum
in
is clearly too low. As a secondtest model, we add lowresistivity material from the top of the subhorizontal
conductorat 25 km depth all the way down to the extended
dipping conductorjust considered. The responseof this
structure (dotted curves) also is inconsistent with the
observationsas it extendsthe phase peak to periods which
are too long and reducesboth the amplitudeand slope of the
apparentresistivity below what were measured.However, the
observationsdo allow an extension of the dipping Coast
Range conductor below the subhorizontal layer at 25 km
depth as long as the conductanceof the former is limited,
e.g., to no more than about 1000 S at 65 km depth.
The next tests, illustrated in Figure 11, concern the
pronounced, uplifted conductive axis beneath the High
Cascades and the depth to the bottom of the deep
subhorizontal
conductor
under
the
Deschutes
Basin.
Removing
the upliftedaxisviolatesRe(M,•) in theperiod
range from 100 to 1000 s around y = 147 km (dashedcurves).
It also destroysthe closure of the negative contoursin the
pseudosection
of M,y in thesameperiodrange(Figure8 and
Plate 6). That a single two-dimensionalstructureexplains
curves). We suspectthat the TM responseis sensinghigher
conductivityof the Basin and Range to the south and that a
bottomof nominally65 km consistent
with M,• is more
likely.
Again, additional N-S profiling and threedimensionalmodeling are the proper meansof resolving the
deep structurehere.
The sensitivityof the MT responseto the geometry of the
dipping thin conductorbelow the Coast Range is assessedin
Figure 12. Here we presentthe TM mode impedancephase
valuesmeasuredin the periodrange3-300 s at sims1, 48, and
71 km inland from the coast. In the top panel, the phases
from all three sites are superimposed
to show that the peaks
in •,• aredistinctbothin amplitude
andperiodrangewithin
the scatterof the data points.In the lower panels, the phases
are plotted as one standarddeviation error bars centeredon the
measuredvalues to demonstratethe good data quality. The
solid curveslabeled"1", "a", and "A" indicatethe goodnessof
fit of ourmodelto themeasured<py,•.
Thefit to p•,• at these
three sites is equally good, as exemplified for the site at 48
km in the EMSLAB Group [1988], and as can be deducedfrom
the colorpseudosections.Curvesof dashesand dotsin Figure
12 are responsesof alternatemodels discussednext.
That the Coast Range conductivelayer dips inland appears
to be resolvedby our MT measurements
independentlyand is
not imposed from external tectonic considerations. In the
first test, we make this layer flat and extend it westward to
infinity with a constantdepthto top of 30 krn. The calculated
½•,,response
at 48 km (notshown)is essentially
unchanged.
However, at 1 km (dashedcurve "2" in Figure 12) the phase
anomaly has significantly lower amplitude and peaks at
14,136
WANNAMAKER
ET At..: R•s•vrry
I
I
5O
ß -
vv •
ß
v•
v• •
lkm
ß
ß
ß
ß
v v
48 km
ß ?l km
20
I
58
Cnoss S•nON, JUANDE FUCASYSTEM
I
.
4O
1 km
i.
'o
28
"J
">
.........
7.?.?
3
.=
2
I
I
58
4O
rule out some electrical
.
so
48
km
i
20
50
-
40
71
km
30
2O
I
+1
connection.
These errors result from
strong cultural EM interference in this area. Since such a
connectionwould have profound tectonic implications, it is
important to remeasureseveral sites in this period range
using a fully remotereferenceMT system.
Resolutionof resistivity structureoffshoreby its effect on
the verticalmagneticfield is examinedin Figure 13. The data
qualityat T < 20 s is not the best,but fortunatelymostof the
responseoccurs at longer periods. The fit of the preferred
model to the data is reasonablygood. Note that both data and
calculationsshow slight inflectionsin Re(M,y), with
complementary
behavior
in Im(M,y), around1000s period.
--
I
low conductance(<100 S) portion of the dipping conductor.
This is demonstrated
by removingit east of abouty = 20 km
and replacing it with background material of 350 ohm m.
Suchan alternativefails to reproducethe subtlephasepeak in
this area (dashedresponse"b").
A deep conductive root connecting the base of the
Willamette Basin sedimentsto the western portion of the
deep subhorizontal conductor appears in the minimumstructureresistivity crosssectionof Jiracek et al. [this issue].
We examine this possibility, which is in contrast to our
model, in the lower panel of Figure 12. The computed
responseof our model (solid curve "A") is comparedto the
computedresponseof Jiracek et al. (dashedcurve "B"). Our
model appearsto fit the data somewhatbetter. However, the
error barsbelow 10 s are quite large and it seemsprematureto
+2
When the sedimentarydistributionsof both the near-offshore
NewportBasinandthe abyssalCascadiaBasinareremovedby
1 og T (s]
setting them to 300 ohm m, we obtain the responseof the
seawater alone (dotted curves). The seawater is quite
insufficient by itself to explain the measuredvertical field.
Fig. 12. Transverse
magnetic
impedance
phase •x• in theperiod The dashedresponseoccurswhen the abyssalsedimentsare
range3-300 s approximatelyat siteswhich are 1, 48, and 71 km inland
from the coast. Phases in the lower panels are representedby one presentand only the nearby Newport Basin is removed.We
standard deviation error bars centered on the estimated values.
Solid
see that this basin is very important in determining the
curvesare the responseof the model of Figure 3 at eachsite. Alternative vertical field on land near the coast. If the oceanic upper
model responsesplotted with dashesand dots are discussedin the text.
mantle conductorin Figure 3 is not extendedeast of the coast,
Data were collectedusingthe UURI broadbandsystem.
the misfit is about 0.05 worse below 500 s, with the
computed
valuesof Re(M:•) beingtoolarge.
longerperiodsthanobserved.The calculated<p•,,of thistest
is in fact much weaker
now at 1 km than that observed
at 48
km, which is in opposition to the measured case. These
lower phasesat 1 km are a result of close proximity to the
ocean. Being able to specifya priori the seawaterresponsein
the finite element model increasesour resolving capability
under the CoastRange.
The next test fixes the layer depth at the coastline and
reducesthe dip inland to 10ø suchthat the layer depthat y =
48 km is about 20 km rather than 32 km as in Figure 3.
Resultingis the dottedcurve "c" in Figure 12 which peaks at
somewhat shorter periods than observed. We therefore
concludethat the dip exceeds10ø and the preferredvalue is
approximately20ø.
The dottedcurve "3" at y = 1 km in Figure 12 demonstrates
the effect of electrically disconnectingthe dipping Coast
Range conductor from the trench offshore.
This
disconnectionwas made by settingthe resistivityof the layer
,-., 0.75
•- =u •,- =x
f ! i i i i I
EN$L
7•'
ii i I I i i I
O
O
O
•--ZTll....
o ..............
• ß • [ bgh..3.__k.]blmmlmmnj.
-0.75
I
0.75
0.00
-0.75
I
I
I
I
I
I
-
-
I
I
-2
-1
!
13
I
+1
1og T
I
+2
I
+3
I
+4
(s)
from the trench to about 15 km offshore to 300 ohm m, the
valueof thematerialaboveandbelowit. Thecalculated<p•,,
again falls far below that observed and cannot be reconciled
by making the remainderof the Coast Range conductorless
resistive. The final Coast Range test is for existenceof the
Fig. 13.
Real and imaginary observedresponsecurves for elements
M:xandM:y measured
byUniversity
of Oregon
andGSCabout
7 km
inland on the Lincoln Line. Computedresponsesinclude that of the
best model (solid), that with low-resistivitysedimentsof the Newport
Basinremoved(dashes),and that with only the seawaterpresent(dots).
WANNAMAKER
ET AL.: RESISqlVlTY
CROSSSECTION,
JUANDE FUCASYSTEM
The vertical magneticfield measuredon the seafloorat site
SF1 is comparedto that due to the model of Figure 3, where
the agreementis good,plus thoseof two alternativestructures
(Figure 14). The first alternativecontainsvery conductive(2
ohm m) sedimentarymaterial throughoutthe whole melange
and down the trench to a distance of about 15 km from the
shoreline. Such a structure drastically curtails the peak
responseat SF1, and even more at SF2, and movesthe peak
to longerperiods(dashedcurves).Second,removal of all low
resistivity material except the seawater results in a real
componentof vertical field at SF1 that increasesto large
values at periodsmuch lessthan the shortestmeasured(dotted
curves in Figure 14). Disagreementbetween observedand
computed
Im(Mo) is largein thiscasealso.
Negative values of Re(M,x ) develop beyond 104 s in
Figure 14 and all other seafloor sites on the Lincoln Line.
This behavior implies higher resistivitiesof regional extent
southof our oceanicprofile. A similar characterto Mzx is
observed on the land also.
longer than 3000 s (dashed curves). The latter model has
resistivitiesnear 20 ohm m extendingall the way to 400 km
depth. This datainconsistencyexistsfor all five seafloorMT
stations
on the Lincoln
0
re'
0.75
,."'-'"
i
]; 4
-
$F1
i
i
i
,_, 0.75
•-
'"
and remains
a serious issue for
America.
One alternativemodel that seemsclearly inconsistentwith
the data has resistivity in the oceanic mantle conductor
falling steadilywith depthfrom 30 ohm m to 10 ohm m at 90
o.oop
-0.75
Line
the deep oceanic structure. Non-planar source fields which
have escapedrobust processing are a possible explanation
[but see Bahr and Filloux this issue], or perhapssome other
form of bias error is present.
Our measurementsappear only weakly sensitive to the
resistivity of the oceanic lithosphere,in part becausethe data
do not extend to short enough periods but also due to the
nature of the ocean-continent resistivity transition.
Ranganayaki and Madden [1980] show that the high ocean
lithosphere resistivity, if contiguous with high resistivity
under the continents,can strongly depresselectric fields and
thus apparentresistivitieson the seafloor at long periods by
restrictingcurrent flow in the overlying seawater. However,
in the Juan de Fuca region, low resistivitymaterial extending
down the subductiondecollement as in Figure 3 or beneath
Vancouver Island [Kurtz et al., 1986] provides an electrical
pathway between the ocean and low regional resistivity
beneath North
,-,
14,137
0.00
km andto 3 ohmm at 215km. Thecalculated
p,• fallstoo
uniformlyand the calculated•,,• remainstoo constantto
agree with the measurements(dotted curves). Note that the
differencesbetweenthe variousmodel responsesin Figure 15
are significantlygreaterfor the TE mode than the TM. This
could be expectedbecausewe are testingdeepmodel features
which terminatelaterally near the coast. Limiting the lateral
extent of the deep structurethis close to the seafloor profile
reduces the TM mode response more than the TE. We
therefore are presuming that off-line (three-dimensional)
effectsare not influencingthe seafloorTE data as much as the
range of our test computations.
TECTONIC IMPLICATIONS
-0.75
+2
+3
log
+4
T
+5
[s}
OF RESISTIVITY STRUCTURE
The electrical resistivity structure of the Juan de Fuca
subductionsystemis a function of the chemical composition
and physicalstateof that system. However, a particularvalue
of resistivity can result from a plurality of mechanisms[see
reviews by Brace, 1971; Olhoeft, 1981; Shankland et al.,
1981; Shankland and Ander, 1983;
Wannarnaker,
1986;
Hyndman, 1988]. Therefore it is usually not possible to
Fig. 14. Realandimaginary
responses
forMo andM:y observed
at produce a unique interpretation of resistivity without
oceanbottom site SF1. Error bars are one standarddeviationwhere they
independentgeophysicaland geological constraints. There
exceedthe size of the plotting symbols. Computedresponsesinclude
thoseof the model of Figure 3 (solid), that with the melangeand trench exist a good many constraintsfor EMSLAB on upper crustal
sedimentsgiven a resistivity of 2 ohm m (dashes),and that with only structure,seismicvelocity, the geotherm,and subductedslab
the seawaterpresent(dots).
mineralogy. Within this framework, electrical resistivity
may augment our understandingof volcanic structure and
history, sediments and pore water during subduction,
The fit of the model to the measurements
in the vicinity of
evolution and migration of fluids during deep metamorphism,
the Juande Fuca ridge is examinedmore closelyin Figure 15.
and the distributionof melt in the upper mantle.
Note that the agreementof the preferredmodel calculations
(solidcurves)with both ½,, and ½yxis goodat almostall
periodsbut thatthecalculated
apparent
resistivitiesp,,• and
py,,arelowerthanobserved
forperiods
greater
than104s. If
the apparentresistivitiesare fit at the longestperiods, then
the calculated phases are lower than observed at periods
Upper CrustalStructureon Land
Model resistivitiesin Figure 3 of about 100 ohm m or less
for the Siletz River marine volcanicsof the Coast Range are
representative of basalt formations near-surface. However,
14,138
WANNAMAKER
ET AL.: RESIST•VrrY
CROSSSF.CqION,JUANDE FUCASYSTEM
I
+3
I
I
'
I
I
I
$F7
I
$F 7
• -
I
I
yX
+2
.o
o
+1
IIIII--•
+0
o90
---
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
i
i
i
i
i
i
i
i
i
i
i
i
+2
+3
+4
log
T
+5
+2
(s)
+3
log
+4
+5
T
Fig. 15. Apparent resistivity and impedance phase data at ocean bottom site SF7 comparedto the computedresponse
of the model of Figure 3 (solid curves, both modes). Error bars are one standard deviationplotted where larger than
datasymbol. Notetheinconsistency
in apparent
resistivity
andimpedance
phase
for T > 104 s, wherewe cannot
fit the
former without giving misfit to the latter.
the conductive layer around 2 km depth below this unit
suggestsa changein lithology. The SiJetzRiver basaltswere
extruded over preexisting oceanic crust starting in the
Paleocene [Snavely, 1987]. The conductor may represent
sediments deposited below the Siletz River formation or
within it during a hiatus in igneousactivity. Later intrusive
igneouseventsduring the Miocene [Baldwin, 1981] may have
further reducedresistivity by hydrothermalalteration.
The large volume of sedimentarymaterialof the Willamette
Basin constitutesthe most Lmportantupper crustal resistivity
inhomogeneityon the landwardsegmentof the Lincoln Line.
Marine units of Eocene throughOligocene age comprisethe
volumetrically greatestproportion of sedimentsespecially of
the westernportion of the basin [Wells and Peck, 1964] and
possessresistivities in the range of 10-15 olhm m in the
model. Toward the east, pyroclasticvolcanicsof Oligocene
and Miocene age of Western Cascadesaffinity dominate the
stratigraphy [Verplanck and Duncan, 1987] and also are in
the range 10-15 ohm m. The relatively restrictedoccurrence
of
Columbia
River
basalts
near
the
center
of
the basin
probably is representedby members of the Grande Ronde
Formation and locally the WanapumBasalt [Baldwin, 1981].
The variation in the basementprofile of our resistivitymodel
in Figure 3, particularly the horst-like feature near
y = 100k•n, is suggested also in the cross-sections of
Couch and Riddihough [1989, in press] derived from gravity
and drilling and of Hermanceet al. [1989, in press]from their
profile of geomagneticvariation observations.
Beneath the easternportion of the Western Cascades,the
volcanic strata were extensively intruded by the Halls Diorite
Porphyry of early Miocene age [Baldwin, 1981] which
probably contributes to the high model resistivities at
shallow depths here. The shallow conducting layer
commencingnear Detroit Lake (y = 150 km in Figure 3) and
continuing eastward is tentatively correlated with unwelded
tuffaceous units of the Breitenbush formation [Priest et al.,
1983, 1987]. The overlying more resistive unit is mainly
middle
to
late
Miocene
lavas
of
late
Western
Cascades
association.The westernboundaryof the conductorappears
to coincidewith a major N-NE trendinggrabenfault, known
as the Hoover fault in this area [Priest et al., 1987], which is
part of the regional systemseparatingthe downdroppedHigh
Cascades graben from the relatively uplifted Western
Cascadesblock [Priest et al., 1983; Smith and Taylor, 1983].
The abrupt increase in conductance of the shallow
conductivelayer east of the High Cascadesaxis suggeststhat
different or additional lithologies are present. The MT
stationseastof Mount Jeffersonlie a good 10 km southof the
WANNAMAKER
ET AL.: RESISTIVITY
CROSSSECTION,
JUANDE Fuc^ SYSTEM
peak andcrossan areaof anomalously
low Bouguerresidual
gravity [Couchet al., 1982;Smith and Taylor, 1983;Smith
et al., 1987]. The low resistivity both within the eastern
portionof the High Cascades
grabenandeastof the bounding
Green Ridge fault in the DeschutesBasin thus may represent
an accumulationof tuffaceous material from ancestral High
Cascadesvolcanic centersnearby or from an early Pliocene
silicic volcanic "highland"some50 km to the south [Smith
and Taylor, 1983]. The conductoris coverednow by resistive
basalticrocks of the upper DeschutesFormationof the early
High Cascadesvolcanicepisode[Priest et al., 1983]. That
the conductivelayer doesnot appearsignificantlydeeperwest
of Green Ridge fault than to the east indicatesthat late High
Cascadesvolcanicseruptedafter formationof the grabenare
not very thick in this area.
We believe that the MT data have not resolved significant
lateral variations within the middle crust underlying the land
segmentof the Lincoln Line. Thus no new light has been
shed on the boundary between Mesozoic and older rocks
comprisingthe deepbasementof centralOregonandthe early
Cenozoicoceanicrocks presumablyforming the deepercrust
of western Oregon [e.g., Couch and œiddihough,1989, in
press]. Resistivityvaluesof hundredsof ohm m are typicalof
a wide range of lithologies with water-saturated,
interconnectedporosities below 1%. Delineation of any
terraneboundaryis hamperedbothby limitationson the twodimensional assumptionand by limitations on data quality
especiallyin the culturallydevelopedWillametteBasin.
DeepCrustand UpperMantle onLand
Hyndman [1988], prograde metamorphic conditions which
the subducting slab encountersfavor the transition from
blueschist or zeolite to greenschist facies with attendant
releaseof water. Most of this dewateringmay occur west of
y = 20 km, where the more conductiveportion of the Coast
Range conductorresides,althoughthis latter feature may in
part also represent a greater abundance of subducted
sediments. Further updip under the melange and Newport
Basin area, breakdown of clays might be another source of
water.
The dipping conductivelayers inferred beneaththe Oregon
Coast Range and beneath Vancouver Island [Kurtz et al.,
1986] have been interpretednot to exist preciselyat the plate
interfacebut insteadto representfluids risen buoyantly from
the plate surfaceand trappedbelow an impermeableboundary
above [Hyndman, 1988]. This boundarywould be formed by
hydration of minerals by the fluids or by precipitation of
silica from the fluids as they cool. The dip of the conductor
under Vancouver Island has been correlated with dip of the
isotherms, in particular the 450øC isotherm, in accordance
with decreasingheat flow eastward. In contrast,dip of the
isothermsis not resolved by the sparseheat flow data from
western Oregon [Blackwell and Steele, 1983; Couch and
Riddihough, 1989, in press] while inland dip of the Coast
Range conductor evidently is. Also, based on COCORP
seismicreflection profiling about 30 km southof the Lincoln
Line, Keach et al. [1989] interpretthe subductiondecollement
of the Juande Fuca plate to lie at a depthnear 35 km some50
km inland. This is essentially coincident with our dipping
conductivelayer at a correspondingdistancefrom the coast,
at least
Discussionin this sectionfocuseson the deep conducting
layer a few tensof kilometersbelow nearly all of the Lincoln
Line on land. Under the Coast Range and westernmost
Willamette Basin, the thin conductor is presumed to
represent,at least in part, pore water or sedimentsalong or
near the interfacebetweenthe subductingJuan de Fuca plate
and the overridingNorth American continent. This parallels
the interpretationof Kurtz et al. [1986] on VancouverIsland
where a similar thin conductor was resolved using MT. In
their study, interconnected
porositiesof one to a few percent
were inferred by assumingfluids of seawater salinity or
greater. A similar inferencecanbe made for the layer beneath
the CoastRange (Figure 3). Interconnectedporositiesof 12% would result for the interval y = 20-60 km where the
conductanceis less than 100 S but might reach 10% for the
portionwest of y = 20 km whoseconductancemay approach
1000 S. Alternatively, this higher conductancein part may
representa greaterlayer thickness.
In addition to possiblesubductedsedimentsor pore waters,
fluids releasedduring mineral devolatilizationreactionsmay
reduce resistivity here. Constraints on the geotherm and
alteration mineralogy of the subductedslab are needed to
decide which reactions could be taking place [e.g.,
Wannamaker, 1986; Hyndman, 1988]. Heat flow and crustal
heat productionof western Oregon are similar to those of
southwestern British Columbia [Blackwell and Steele, 1983;
Lewis et al., 1988; Couch and Riddihough,1989, in press]so
that somecalculationsperformedfor the latter region can be
made use of in EMSLAB. Accordingly, temperaturesin the
range of 250-500øC appearappropriatefor the depth interval
12-30 km of the dippingCoastRangeconductor.As notedby
14,139
within
the uncertainties
involved.
The
EMSLAB
results therefore do not unequivocally supportthe notion of
rising fluids beneath the Coast Range trapped at a dipping
boundarysignificantly above the subductionslip plane.
Fluids are offered also as an explanation for the
subhorizontalzone of high conductanceextendingeast from
y = 60 km to the High Cascadesaxis (Figure 3). We suggest
that this pronounced conductor represents primarily a
concentrationof water-rich fluids which have risen buoyantly
from the surface of the subducted plate where further
dehydrationreactions are taking place. The upper limit of
this fluid migration, i.e., the top of the conductor, we
interpret to be controlledby temperature. Heat flow data in
Oregon [Couch and Riddihough, 1989, in press] suggesta
temperaturearound450øCfor the top of the conductor.Sucha
temperatureappearsappropriatefor the mineral hydration and
silica precipitation mechanisms of permeability sealing
mentionedpreviously. Furthermore,the rheology of the host
rock becomesincreasinglystiff toward shallower levels due
to lower temperatures[Kirby, 1983]. The temperaturesover
most of the conductor appear too low to support hydrous
melting, but more accurateheat flow data to characterizethe
geothermin this area are highly desirable.
To understandwhich hydrous minerals may be breaking
down to contribute the fluid, we need again to constrain
temperatureat the subductedplate surface. However, it is
hazardousto extrapolate a conductivegeothermbeyond the
strong conductor being considered. Heat absorbed in
dehydrationat the slab,hot fluids migratingupward, and heat
producinghydrationreactionsat higher levels and within the
conductor may constitute significant advective heat flow
[Anderson et al., 1976; Lewis et al., 1988]. This would tend
14,140
WANNAMAKER
ET AL.: RESlSTIVrrY
CROSSSECTION,
JUANDE FUCASYSTEM
to reduce the geothermal gradient below the electrical
conductor. At the easternmostend of the dipping weaker
conductorof the Coast Range, i.e., just before the start of the
low-resistivity region under discussion, a conductive
geotherm suggests temperatures of 550-600øC. If this is
subductionreviewed by Wyllie [1988], whereinoceaniccrust
whose surface is cooled by endothermic dehydration is
subducted below a mantle wedge warmed by induced
convection.
greenschist facies minerals as the cause of the strong
conductorat least near its western end around y = 60 km.
This interpretationof the fluid source is consistentwith the
observation that greenschist minerals, especially chlorite,
carry the bulk of the chemically bound water in subducted
oceanic crust [Humphris and Thompson, 1978; Ito et al.,
Next we considerthe conductiveaxis extendingup from the
east end of the strong, horizontal conductorjust described
around y = 160 km. Its depth to top, about 5 km, may
representthe upper limit of developmentof andesiticmagmas
within the crust accordingto Anderson[ 1982]. We doubtthat
this midcrustalconductorresultsprimarily from the presence
of silicatemelt in large volume but rather from hydrothermal
fluids exsolvedduring repeatedintrusion and crystallization.
1983].
It is not clear what structural
Breakdownof hydrousmineralsgenerallyis incongruentso
that fluid release occurs over a finite temperatureinterval
[e.g., Etheridge et al., 1983]. The interval of breakdownof
greenschistfacies may be broadenedfurther if serpentineand
epidote, commonly observed also, are included. A finite
interval of breakdown temperature translatesto a region of
fluid release and upwelling which is of finite lateral extent,
especially over a plate whose angle of subductionis not very
steep. The lateral extentof fluid releasemay be increasedyet
more through the buffering of temperature along the slab
surfaceby the dehydrationreactionsthemselves[Andersonet
be centeredjust west of the presentandesiticarc insteadof
directly beneath it. However, numerous basaltic vents and
vent lineamentsalsohave developedthis far west of the arc in
the last 5 m.y. in response to a relaxation of regional
compressiveforces[Hughesand Taylor, 1986]. The presence
approximately
representative,
it impliesdehydration
of
al., 1976].
As we look farther downdip, more refractory hydrate
minerals of upper greenschistand amphibolite facies should
not breakdown to release H20 fluid directly but instead
produce water-undersaturated melts through vapor-absent
fusion [Burnham, 1979; Wyllie, 1979; Wannamaker, 1986].
The water-bearing melts themselves may rise through the
overlying mantle wedge and crystallize as they lose heat or
encounterthe H20-saturatedsolidusat lower pressures.This
process, in essence, merely complicates the migration of
fluid to higher levels. Nevertheless,melts are believed to be
more effective metasomaticagentsthan fluids [Eggler, 1987]
so that more substantial upward movement of alkalis and
incompatible elements may be occurring here. The strong
horizontal conductor and its interpretation may have an
analog in the model of Kurtz et al. [1986] where a thick
conductive region at a similar depth lies below mainland
British Columbia. It may also be manifest seismicallyin the
low velocity zone interpretedbeneath the Willamette Basin
region by Langston [1977].
The model of hydrousmineral breakdownand fluid release
presentedhere helps explain the imbalancenoted by Ito et al.
[1983] between the amount of H20 contained in altered
oceanic crust and the much lesseramountreleasedduring arc
magmatism. Our interpretationis that most of the water in
the altered ocean crust is liberated
well before the volcanic
facies
breakdown
boundaries
described
here
axis has been detected
to
also to the north of
our line near BreitenbushHot Springs and to the south near
the latitude of Mount Washingtonfrom the MT profiling of
Stanley et al. [1989]. While the depth of this zone is
somewhat beyond that currently considered viable for
geothermal resources, its substantial size may warrant
evaluationfor future energyneeds.
As noted already, a broad three-dimensional effect has
complicated the determination of deep resistivity structure
under the Deschutes Basin using a two-dimensional
interpretationprocedure. The nature of the three-dimensional
effect suggestsa roughly E-W transitionto lower resistivity
south of the Lincoln Line. A similar structureis implied in
the geomagneticarray studyof Gough et al. [this issue]where
resistivelithosphereof the Blue Mountainsblock, including
the Deschutes Basin, lies north of the more conductive Basin
and Range province [Gough, 1983; Wannamaker, 1983].
This transitionmay be marked at the surfaceby the Brothers
fault zone and a coincidenttrend of Late Pliocene to present
volcanic vents oriented ESE [Guffanti and Weaver, 1988].
The magnetotelluric and geomagneticdata together suggest
that the Brothers fault zone may be a better choice for the
northern limit of recent Basin and Range activity than is the
SW-NE trending Klamath-Blue Mountains lineament [cf.
Eaton et al., 1978; Riddihough et al., 1986]. Further threedimensionalanalysisand data acquisitionis sought to better
define the back-arc electrical asthenosphereas teleseismic
results imply low velocity in this area extending to 200 km
depth [Rasmussenand Humphries, 1988].
Implications of Seafloor ResistivityStructure
arc
is reached. Also, the model of this paper is similar to that of
Fyfe and McBirney [1975] but with one importantdifference.
The
of the conductive
controls cause this conductor
are a full
metamorphicgrade lower than those of Fyfe and McBirney
[1975] at correspondingpositions between the trench and
arc. Significant uplift due to volumetric expansion from
hydration reactionsbetweenreleasedfluid and mafic minerals
overlying the subducted plate is interpreted to affect the
Western Cascadesand easternWillamette Basin by Fyfe and
McBirney [1975]. An unknownportion of the releasedfluid
may not be absorbedin hydrationbut perhapsreachescrustal
levels and eventually the hydrosphere. Our resistivity cross
section more closely ressembles a petrological model of
The resistivities of the thick sedimentarylayering of the
abyssalplain (Cascadia Basin) and on the continental shelf
(Newport Basin) are strikingly low, 1-2 ohm m (Figure 3).
These values are in keeping, however, with very high
porosities observed in drilling of accretionary complexes
which range from 25 to 75% in the upper 2 km [Kulm et al.,
1973; Bray and Karig, 1985]. In contrast, such low
resistivities cannot exist throughout the melange and
subducted trench sediments without causing a vertical
magneticfield responsemuch smaller than observed(Figure
14). Our magnetotelluricresultsthereforeappearto sensethe
off-scrapingof seafloor sedimentsor the porosityreductions
attending sediment dewatering during subduction. The
WANNAMAKER
ET AL.: RESISTIVITY
CROSSSECTION,
JUANDE FUCASYSTEM
primary pathways for dewatering appear to be along the
decollementbetween the subductedplate and the overlying
accretionarywedge as well as up thrust faults in the wedge
[Kulm et al., 1986; Moore et al., 1987]. The final resistivity
from the imbrication and dewatering processas represented
by the Miocene melangeand broken formation is in the tens
of ohm m at least and suggestsbulk porosity reductionsof a
factor approaching10.
The nominal value of 3000 ohm m for seafloorlithospheric
resistivity in Figure 3 may be a minimum. Resistivitiesin
the thousandsof ohm m have been inferred for depthsgreater
14,141
distanceinland agreeswith the P-wave tomographicimage of
Rasmussenand Humphries [1988] which implies a transition
to a deep, high-velocity root in the mantle from below the
centralWillamette Basin to the High Cascades.
The higher resistivities of regional extent south of the
Lincoln Line on the seafloorimplied by the vertical magnetic
field may reflect lower temperaturesat depth south of the
Blanco fracture zone. These in turn are suggestedfrom the
greaterbathymetricdepthsin the vicinity of the Gorda Ridge.
An explanationfor higher regionalresistivityto the southon
the landwardportionof our profile is unclearat present.
than
1 km
below
the seafloor
from
active
source
A physical explanation for very low model resistivity
electromagneticsurveying as well [Young and Cox, 1981].
below 215 km in the oceanicupper mantle is problematic.
Traces of water may be responsiblefor these values in the
While its mere existence is questionable given the data
middle oceanic crust, while thermally activated rock
inconsistencydiscussedpreviously, very low resistivities at
semiconductionis probably the mechanismof resistivity at
depths less than 300 km have also been interpretedbeneath
greaterdepths. It was noted earlier that the resistivityof the
the East Pacific Rise spreadingcenter [Filloux, 1982] and
ocean lithosphereis hard to resolvehere in large part due to
beneathHawaii, a site of presumedhot spot activity [Larsen,
low-resistivity material extending down the subduction 1975]. These depthsappeartoo shallowto be correlatedwith
decollement
beneath the continent.
Subduction
surrounds
the olivene-spinel phase transition [Akimoto and Fujisawa,
almost the entire Pacific Ocean so, to obtain better estimates
1965] occurringnear 400 km. One possibilityfor thesevery
of ocean lithosphere resistivity using MT, a profile of
low resistivitiesis a deep region of additionalmelting in an
soundingsshouldbe carried out acrossa continentalmargin
olivene eclogite source composition [Anderson, 1987;
in the Atlantic.
Wyllie, 1988]. Added thermal impetus for deep melting
The low resistivity beyond about 35 km depth below the
below the Juan de Fuca spreadingcentermay come from its
seafloor (Figure 3) is proposed to represent partial melt
proximity to the Cobb hot spot [Karsten and Delaney, 1989].
beneaththe Juande Fucaplate. To relate valuesof resistivity If mantle resistivitiesdown to 400 km depthremain in the
to degreeof partial melting, we assumefirst that temperature range 15-20 ohm m however, as the apparent resistivity
within
the melt zone is buffered
near the volatile-free
alone suggests,then no such deepmelting is implied. These
peridotitc solidusby the processof adiabaticupwelling and
higher resistivities would be consistent with solid-state
fusion [Mysen and Kushiro, 1977; Wyllie, 1979, 1988;
semiconduction
in olivene along a near-adiabatictemperature
Oxburgh, 1980]. An estimateof melt fraction can be made profile of 1700-1800øCfrom 200-400 km depth. This is an
thereby using the tholeiite liquid conductivitymeasurements importantalternatemodel of the deep physicochemicalstate
of Tyburczy and Waft [1983] together with an appropriate in regionsof mantle upwelling and fusion [Wyllie, 1988] that
mixing law relatingbulk conductivityto the conductivitiesof
our magnetotelluricmeasurementscannot clearly distinguish
the melt and hostmineral phases. The mixing law employed at present. This underscoresa need for obtaining reliable
here is the grain edgetubulemodel discussedby Shanklandet
planewave impedanceestimatesto very longperiods,3x104
al. [1981] andvon Bargertand Waft [1986] which represents s at least.
the equilibrium texture of melt distribution in olivene-basalt
CONCLUSIONS
melt systems.The olivenedata of Duba et al. [1974] are used
for the solid-stateconductivityof the mineralphase.
Near the top of the low resistivityregion in the oceanic
The inferencesdrawn from interpretingthe Lincoln Line
upper manfie, a melt fraction of about 7% is implied from a
observationsin EMSLAB fall into two broad categories;
thoseon the physicochemical
stateof the young Juande Fuca
resistivity value of 20 ohm m. As depth increases,this
fractionfalls due mainly to increasingmelt conductivitywith
plate and its subduction beneath Oregon, and those
increasingsolidustemperature
but also to increasingolivene concerning the two-dimensional modeling of MT data in
conductivityespeciallybelow 100 km depth. The computed environments which depart somewhat from the twodimensional assumption.
Regarding the former,
melt fractionis about3% at 75 km depth,just under1% at 125
km, andbecomeszero by 200 km depthwhereolivenesolid- measurementson the seafloor indicate very low resistivity in
state resistivity at 1700øC has fallen to 20 ohm m. A
the eastwardthickening sedimentarywedge of the abyssal
reasonablygood comparisonexistsbetweenour conductivity CascadiaBasin provided its depth extent is constrainedby
model and seismic models of young (0-20 m.y.) oceanic marine seismicsections. The majority of thesesedimentsare
plates,of which the Juande Fuca beneathour profile is one. inferred to be off-scraped or lose most of their interstitial
Sato et al. [1988], in their compilationof shearwave models, water before being carried more than ten or so kilometers
suggesta few percentpartial melt to depthsof about 150 km.
down the trench. In the oceanicuppermantle, of the Juande
Laboratory and theoreticalinvestigationsof partial melting Fuca plate, moderatelylow resistivifiespresumablyreflect up
also suggest that melting should have similar effects on
to severalpercent partial melt attendingregional upwelling
conductivityand velocity [Shanklandet al., 1981]. Our data in the vicinity of the ridge. Very low resistivitiesmodeled
do weigh against resistivity decreasingsteadily in the below 215 km depthin the oceanicmanfie are hard to explain
oceanicasthenosphere
(Figure8) which wouldhave suggested physically but problems with the very long-period data
hamper the modeling here.
an interconnected
melt fraction of 1% persistingfrom 75 to
3vet 300 km depth [cf. Anderson, 1987]. We note in addition
On land, upper crustal resistivity structure indicates the
that the termination of this asthenospherea moderate cumulative and ongoing effects of plate convergenceand
14,142
WANNAMAKER
ET AL.: RESlST•TrYCROSSSECI•ON,JUANDE Fuc^ SYSTEM
volcanism. Residual sediments, pore waters, possibly
sulfides,and dehydratingalteredoceancrustcarrieddown the
subductionzone appear to produce a low resistivity upper
mantle layer, albeit a subtle one, dipping about20ø eastward
beneath westernmost Oregon. The position of this layer
appears consistent with the Juan de Fuca plate subduction
decollement. Farther inland and downdip at appropriately
high P-T conditions, a massive breakdown of greenschist
minerals is hypothesizedto liberate most of the water carried
in the ocean
crust alteration
well
before
the volcanic
arc is
reached. This fluid releaseproducesa strong,subhorizontal
conductorwith a depthto top of about25 km. The resistivity
manifestationof the arc magmatismitself is not strong,but
consistsof a low resistivity axis in the middle crust below
and to the west of the arc at the surface. It presumably
representsmainly fluids releasedfrom crystallizingmagmas
and should be considered for its geothermal energy
significance. North central Oregon is resistive at depth
comparedto the crustand uppermanfie to the southand it is
suggestedthat the Brothersfault zone marksan electrical,and
thus possibly a thermal and magmatic, boundary with the
northernBasin and Range.
Regarding the derivation of the resistivity cross section
from the observations,the strongN-S preferredorientationof
most structures in the region has allowed successfultwodimensionalmodeling of data approximatingthe TM mode.
A fixed north-orientedx axis was essentialin clarifying the
subtle
anomalies
due to subduction
structure.
The vertical
magnetic field, however, confirmed many structuresderived
using the TM impedancefunctionsand also was sensitiveto
narrow
conductors such as the midcrustal
axis beneath the
High Cascades. It was also important in establishingthe
resistivity of the sedimentary sections offshore and the
conductanceof the oceanic asthenosphere.On the land, the
data correspondingto the TE mode exhibit a variety of threedimensional effects including shallow statics, finite strike
length of regional features, and some strongly threedimensionalbehavior at very long periods which is difficult
to explain. A fruitful area for three-dimensionalmodeling
could be the DeschutesBasin region of the Blue Mountains
resistive block to better establish the depth extent of the
backarc electrical asthenosphere.On the seafloor, the much
greaterlateraluniformityof structurenear-surfacehas allowed
reasonablygood fits to both modes of impedanceplus the
verticalfield downto 104s (-200 km depth)butnotmore.
The unprecedenteddata quality achievedin the EMSLAB
project has allowed a geophysical interpretation which is
similarly unparalleled in scope and detail from induction
studies.
Nevertheless,
several
data
collection
and
interpretation needs for the future of MT have been
highlighted by our experiment. Strong cultural noise and
near-field
effects
often
correlated
over
several
kilometers
compromised resolution beneath the Willamette Basin.
These can only be overcome using a remote reference at
distancesexceeding the correlation length of the noise. A
more flexible approachto electric field collection, or even
continuous E field profiling (e.g., EMAP) locally, would
efficiently meet data sampling requirementsespecially over
volcanic rocks. Many more details of trench structure
beneath the near-offshore
continental
shelf could be revealed
if the logistics of shallow water MT were worked out.
Characterizationof the oceanic lithospherewould be greatly
enhancedby extendingthe seafloordata bandwidthto shorter
periods using more sensitive magnetometers and precise
timing control. Better processingto removesourceeffectsor
bias may be neededto achieve good data at periods beyond
! 04s and resolvestructurein the 200-500 km depthrange.
Finally, addition of independentgeological knowledge can
substantially increase the information that can be extracted
from induction data. For example, incorporatingthe ocean
bathymetry and resistivity permitted determining the dip of
the conductor under the Coast Range. Thus inversion
algorithms that in the future may replace the laborious
modeling processcarried out in this paper should allow for
such independentlydefined constraints.
Acknowledgments. The modelingand interpretationof this
paper were supportedby the U.S. National ScienceFoundationunder
grants EAR-8410638, 8512895 and 8708382. Many valuable
discussions
were held with RichardW. Couchand GeorgeR. Jiracek
Financial assistancefor publicationcostswas provided by AT & T
Bell Laboratories,GeologicalSurvey of Canada,and University of
Washington. Philip E. Wannamakeris grateful to the San Diego
Supercomputing
Center and the University of Utah for accessto the
SDSC Cray XM-P under the University Block Grantsprogram. This
is GeologicalSurvey of CanadaContribution23489.
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JUANDE FUCASYSTEM
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(Received October 28, 1988;
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acceptedApril 3, 1989.)
WANNAMAKER
ET AL.: RESISTIVITY
CROSSSEC•ON,JUANDE FUCASYSTEM
NB
CB
100
50
CR
c
WB
50
WCHC
100
150
14,277
DB
200 km ,
25
50
l:l
1,:50
200
1:4
300
300
p
<10 3
>103
m)
Plate 1. [Wannamakeret al.]. East-westresistivitycrosssectionderivedfrom forward modeling of MT data along the
Lincoln Line. Section preservesdetails of the finite elementmesh geometrybut resistivitieshave been groupedin halfdecade intervals for color display. Seafloor portion of the model is shown only to 175 km offshore, beyond which
variationsare minor. Note changesin vecdcalexaggeration,labelled on the left, at 5 km and 150 km depth. Important
physiographic
regionscrossedincludethe CascadiaBasin (CB), NewportBasin (NB), CoastRange(CR), Willameue Basin
(WB), WesternCascades(WC), High Cascades(HC), andthe DeschutesBasin(DB).
km
14,278
W•,•LaJ•R
ET AL: Rm•s•
CROSS
Sr.CT•ON,
JUA• DE FUCASYSTma
COMPUTED
CR
'
i
! I i
i--iii
WB
Ii
i
iii
! i i i
WC
i
i
I
i!1
ii
HC
! ili
iii!1
i
DB
I
!
I
-2
-1
o
+1
Pyx
+2
+3
+4
___1 I I I
I III
.11 I
I1.1,,, I I I I ....1. I
I...1.11
_.
I I
I II
i I..1111,,
.1, ,I
1ooo
i._1
I I I I"11iII I' I'-('i"i'! I"1"1'"i! I'"i'i'"'l
• I I'1f"i"i'•
I"I•Ai:I• 1•'•'•I
=
-2
(deg)
-
ß
-I
7.5
- 65
qbyx
+1
+2
+3
+4•,
-15
I III .lJ I 111 I I I I .,1 I .1.. I 11. I I _.1.II I I II!.1
I
I
O
50
...............
1. I
I,
I__
I
100 km
W
E
Plate2. [Wannamaker
etal. ]. Pseudosections
oftransverse
magnetic
apparent
resistivity
Pyxandimpedance
phase
computedalongthe landwardportionof the Lincoln Line.
WANNAMAKER
ET AL: RESISTIVITY
CROSSSECrIoN,JUANDE FUCASYSTEM
14,279
COMPUTED
SF8
S F7
SF5 4 3
2 1
(D, -m}
+2
*3
p
yx
,4
-1ooo
I .......................
(deg)
i ............
i ...... I ........i ......i----l
- 7•
+3
....
:;:7 - 4 5
!....
"*';'"•:"
- :35
*4'
-2.5
-15
*5
Ir
0
W
Plate3.
.....
I
I00
.......
1
200
km
E
[Wannamaker
et al.] . Pseudosections
of transverse
magnetic
apparent
resistivity
Pyxandimpedance
phase
computed
alongthe seafloorportionof the LincolnLine. Note differences
in horizontalscaleandperiodrangebetweenland
and seafloor results.
14,280
WA.NNAMAi(•R
ET AL.: RBSISTIVITY
CROSS
SECTION,
JUANDE FUCASYSTEM
COMPUTED
CR
WB
WC
HC
DB
(•-m)
-2
-
1.o
-1
o
1o
+I
1oo
+2
Pxy
+3
+4
___1 I I I
I 1 I I
I Ill I !_1 1.11 I I [__1....
1_.1 I
1.1.1 .1 I
I I! !.l 111 I
I
Illl
iii
I I"1 I'1'1¾'11
1 i
1! I 11'1 I"1 i l
I I
i
I'!
I 1.... i.
I
1ooo
._
I
(deg)
-2
-
7.5
.
-1
- 65
55
+1
xy
35
+2
'1
+3
+4
-
25
-15
.....1_1 I I
J
0
1.....111
I I I 111 I I I I_ I 1....I.
'
"
1......
.50
!.1.1 I I
I I1 !._1.1111
l.... i
I_ :l......
1
I00
km
W
E
Plate4. [Wannamaker
et al.].Pseudosections
of transverse
electric
apparent
resistivity
Px•andimpedance
phase
computedalong the landwardportion of the Lincoln Line.
WANNAMAKER
ET At,.: RF,S•S•
CROSSS•rION, JUANDE FUCAS¾s'r•
14,281
COMPUTED
SF8
I
SF7
SF54
3
21
(g.m)
I
-1o
p
xy
-IOOO
ß.I
.....I........I
! ...... I ............
I
*2
i
1__
(deg)
1.......
•
-7.5
*3
(:/:)xy
"4
-25
*5
i
i00
w
Plate5.
_1
1.
i
L_
-15
200km
E
[Wannarnaker
et al.]. Pseudosections
of transverse
electricapparent
resistivityPx•andimpedance
phase•xy
computedalongthe seafloorportionof the LincolnLine. Note differences
in horizontalscaleandperiodrangebetweenland
and seafloor
results.
14,282
WANNAMAKER
ET AL.: RESISTIVr•CROSSSECTION,
JUANDE FUCASYSTEM
COMPUTED
CR
"
WB
WC
HC DB
I ! I• I I'' II1!"1I .................
i"'1
i I I' I i '1 '1'I- '1I'i i""!
......
I I I I !'•II '1•
..........
I
I
I'1
(%)
-2
-1
-
60
-
4O
o
+1
+2
+3
+4
- -60
___l I I I I III I I I i11. I I I 1.....I I
I
111 I 1_ I 1.1...1
1.11.11 .l.. J _1 j,,
= I I I--I
I'
I i'J'"-!--I '1"1I I I'"'i'"i'"i'1 i
I III I! ! I'!-"l-""l'
l' l'i
I i
I
I
I
(%)
-2
-
60
-
4o
-1
+1
Z'ZE'Z
+2
•.
+3
'•- -".40
+4
- -60
_ I
0
I I I . I III
! I !
III
50
!. I I I
I
I
I
! I I.I
!
I.I1
I I IIII.
i ...1.... I_
I
100 km
w
Plate6.
- -20
E
[Wannamaker
et al.]. Pseudosections
of verticalmagnetic
fieldtransfer
element
Mzy,realandimaginary
parts,
computedalong the landwardportion of the Lincoln Line.
At..:RESISTIVrrY
CROSS
SF.
eHON,JUANDEFUCASYSTEM
14,283
COMPUTED
SF8
ß
.
SF7
SF54
321
.
...............
I ..................
i
- 'i
•3
+4
-90
+5
,,
+2
I
I
I
i
!
I
•3
•5
__1
I
O
I
IOO
w
I ..... I
-90
I
'
200 k m
E
Plate
7. [Wannamaker
etal.].Pseudosections
of vertical
magnetic
fieldtransfer
element
MR, realandimaginary
parts,
computed
alongthe seafloorportionof the LincolnLine. Notedifferences
in horizontal
scale,periodrange,andcontour
imerval between land and seafloor results.