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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. BI2, PAGES I9,419-19,438,NOVEMBER 10, !990

Deep CrustalStructureof the CascadeRangeand Surrounding RegionsFrom SeismicRefractionand MagnetotelluricData WILLIAM

D. STANLEY

U.S. GeologicalSurvey,Denver, Colorado

WALTER D. MOONEY AND GARY S. Fuis U.S. GeologicalSurvey,Menlo Park, California

Severalregionalseismicrefractionandmagnetotelluric (MT) profileshavebeencompleted across theCascadeRangeandsurrounding geologic provinces in California,Oregon,andWashington. Joint geologicinterpretationof the two data setsprovidesconstraintson the natureof the crustnot available

fromeitherdatasetalone.Analysisof threeMT andtwo seismicrefractionprofilesin Oregonanda coincidentMT andrefractionprofilein northernCaliforniashowa highdegreeof correlationbetween resistivityandvelocitymodels.The mainfeaturethatis evidentin bothdatasetsisa highlyconductive (2-20 ohm m) zonethat occursat depthsof 6-20 km and largelywithina midcrustalvelocitylayer of 6.4-6.6 km/s, overlyinga lowercrustwith velocitiesof 7.0-7.4 krn/s.Althoughthisconductorand the midcrustalzone of 6.4-6.6 km/s velocitiesare generallyrather horizontal,importantstructuresdo occur. For instance,near the boundaryof WesternCascadesand High Cascadesthe MT midcrustal conductorrisesto within 6 km of the surface.In addition,on the coincidentMT-refractionprofilein northernCaliforniaa significantwestwarddowndipoccurson both the MT deepconductorand the 6.4-km/svelocitylayer, with bothoccurringat very similardepths.However, in the ColumbiaPlateau of Washington,no deep crustalconductorsoccurshallowerthan 25 km; also,the velocitystructureis quite different,with a 6.8-km/smidcrustand a 7.5-km/slower crust.Complexaccretionarystructures occur on MT models for the southern WashingtonCascades,quite unlike the more horizontal structuresof the Oregon and California Cascades.The accretionarystructuresin the southern WashingtonCascadeshave been shownto be relatedto stressreleasein the area of Mount St. Helens. In order to explain the similar structuresin the MT and refractionmodelsfor Oregonand California, we propose a model involving the effects of metamorphiczonation to produce the velocity structure, combinedwith metamorphicallyproducedfluidsand partial melt to producethe deepconductor.The higher midcrustal velocities and larger depth to deep MT conductorsunder the Columbia Plateau are explained by a more mafic crustal composition and lower heat flow.

north-south

INTRODUCTION

Regionalgeologicaland geophysicalstudies of the CascadeRange have been conducted by the U.S. Geological

Surveyas part of a geothermalresearchprogram.Independentmodelingof several seismicrefraction and magnetotelIuric (MT) profiles shows similar velocity and resistivity structuresin the Cascadesand surroundingregions. In this paperwe attempt to (1) summarize pertinent velocity and geoelectricalinformation from the region, (2) derive com-

line from Lassen Peak in northern California

to

Mount Hood in Oregon, but numerous small vents and extensive flows comprise a significant part of the overall volume of Quaternary volcanic rocks. In Washington the

pattern of volcanoesis complexand less linear; in addition, Quaternary volcanic flows are limited to the area near the main

stratovolcanoes.

Other

volcanic

centers

associated

with the High Cascadesare located to the east of the main trend of volcanoes; for instance, Medicine Lake and New-

binedconstraints on crustalphysicalproperties,(3) address berry volcanoes(Figure 1) are located sometens of kilomethekey questionof the significanceof a deep crustalcon- ters east of the main chain. The Western Cascadesand High Cascadeswere formed in ductoras it relatesto crustaltemperatures,and (4) relate typesof structuresinterpretedfrom the geophysicaldata in theCascades to thosein surrounding regions. CASCADE RANGE SETTING

the later stagesof four volcanic and plutonic episodesthat swept acrossthe presentPacific Northwest [Heller eta!., 1987]In the back arc region, Miocene to Holocene volcanic rocks of basaltic to rhyolite composition cover the High Lava Plains and Basin and Range portions of eastern Oregon; and also in the back arc, Miocene basalts fill an

The CascadeRangeof northernCalifornia,Oregon,and Washington includesa belt of middle Tertiary volcanic apparentrifted oceanicbasin[Fritts and Fisk, 1985]in the

centers[McBirney, 1978] known as the Western Cascades Columbia Plateau (Figure 1). The North Cascadesare co.m-

andanothernorth-south alignmentof Quaternaryvolcanoes posedof crystallineandhigh-grademetamorphicrocks that

known astheHighCascades (Figure1). Bothbeltsrecord were amalgamatedthrough accretionary processesin late

arcmagmatismrelated to subductionof oceaniclithosphere.

Mostof thelargerQuaternary volcanoes occuron a nearly Thispaperisnotsubjectto U.S. copyright, Published in 1990by theAmerican Geophysical Union, Papernumber90JB•00955.

Mesozoic and early Tertiary and mid-Tertiary time [Misch, 1966].

The developmentof the Pacific Northwest during the period prior to formation of the magmaticarc is poorly understood,but Dickinsonand Thayer [1978]have defineda

19,419

I9,420

STANLEYET AL.' CRUSTAL STRUCTURE OF CASCADES 124 ø

122 ø

120"

118 ø EXPlaNATION

Quaternary volcanic rocksof

)•

High Cascades

Quaternary volcanic rocks of High Lava Plains

48 ø

Tertiaryvolcanic rocksof WesternCascades

Other Tertiary volcanic rocks Cenozoic sedimentary and volcanic

rocks

of Coast

Ranges

Mesozoic granitic rocks of Sierra Nevada 46 ø



Other pre-Cenozoic rocks Geology not indicated

o

I

200

KM

I

44 ø

HIGH LAVA PLAINS

•'o•?

42 ø

40 ø

Fig. 1. Geologicalindex map of CascadeRangeand surroundingregion showinglocation of MT profiles(dashed curves)and seismicrefractionprofiles(solidcurves,S1-S4). Large circlesare Quaternaryvolcanoes,abbreviatedas follows:B, Baker; G, Glacier Peak;R, Rainier;A, Adams;SH, St. Helens; H, Hood; J, Jefferson;TS, Three Sisters; N, Newberry;C, CraterLake; S, Shasta;andML, MedicineLake. EMS is the EMSLAB MT profile[EMSLAB Group, 1988]; VI is Vancouver Island.

Mesozoic arc-trenchsystempreservedin the presentBlue Mountainsregion of Oregon(Figure 1). The Klamath Mountains province consistsof a system of complex, thrusted accretionaryunits of Paleozoic and Mesozoic age [Irwin, 1966].

1985].lngebritsen et al. [1989a, b] point out that a largearea

of isothermalheat flow in the axial region of the High Cascadesis caused by lateral groundwater flow in the porous Quaternary volcanic flows. Blackwell et al. [1982] have modeledthe high heat flow area west of the central Oregon

Cascadeswith a tabularmagmaaccumulationat depthsof CASCADE RANGE GEOTHERMAL REGIME

The CascadeRangecontainsvoluminousQuaternaryvolcanic rocks, and severalauthorshave postulateda significant subsurfacegeothermalresourcefor the regionas summarizedby Muffler [1987].Heat flowjust westof the Oregon High Cascades from MountJefferson to CraterLake (Figure

7-10 km. Although Blackwell et al. [1982] postulatean extensivemagma accumulationunderneaththe Cascades, Scandone and Malone [1985] and Shemeta and Weaver

[1986]interpretthe magmaticbodiesassociated with the

MountSt. Helens1980eruptionas small,on thebasis of inflationcharacteristicsof pre-!980 eruption seismicity. Heat flowin the WashingtonCascades is not as highasthat

1)exceeds 100mW/m2 [Blackwell andSteele,1985]andis in the remainder of the High Cascadesto the south. typicallyhigherthan80 mW/m2 throughout mostof southeastern Oregon and northeastern California. Drill hole mea-

surementsof temperaturegradientsat depths to a few hundred meters over most of the high heat flow area in

INTERPRETATION OF MT AND SEISMIC DATA

The seismic refraction profileshavebeeninterpreted by

Oregon are greater than 50øC/kin[Blackwelland Steele, two-dimensional seismicray-tracingand syntheticseis_mo-

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

19,421

gram methods as described by Catchings and Mooney ture and pressureincrease,partial melting may occur, lead[1988a, b], Zuccaet al. [1986],andMooney[1989].MT ing to a decreasein resistivityof the rocksby up to 2 orders •oundings were interpretedusing combinationsof one- of magnitude if there is sufficient interconnection between dimensional (layered) and two-dimensionalgeoelectrical intergranular melt films. :models. We will not discussin detail the derivationof the Seismic compressionalwave velocities in crustal rocks

geophysical models in thispaperbutinstead willfocusupon vary over a much narrower range than resistivities do, geologic constraints placed uponthecrustin theCascadestypically rangingfrom 2 to 7.4 km/s. Velocities for Tertiary bythetwodifferent geophysical datasets. sedimentaryrocks are typically 3-4 km/s. Compressional The ability to interpret lithologyfrom the MT and seismic datasetsis dependentupon our understandingof the electricaland acousticpropertiesof rocks under the temperaturesand pressuresencounteredin the CascadeRange and surrounding region. The following, short discussionof rock

velocitiesfor Cascadesvolcanicrocks range from 3 to 6 km/s,controlledby the porosity,weathering,andproportion of mafic minerals.The velocity of intrusive rocks is influenced by the proportionof quartz to mafic minerals in the rocks, with felsicrockssuchas granitehavingvelocitiesof

properties is includedto providea background for state-

about 5-6 km/s and intermediate to mafic intrusive rocks

mentsconcerninglithological correlations of various MT and seismicmodels in this paper. Electricalresistivity varies over an extremely large dy-

ranging higher than 6.3 km/s. The compressionalvelocity of crustal rocks generally

namicrange,with commonrock resistivitiestypically rangingfrom 1 to 10,000 ohms m. Rock-formingmineralsare mrmallyvery resistiveat surfacepressuresand temperatures,with the exceptionof metallicmineralsand graphitic carbon,which are very low in resistivity. For most rocks, resistivityis controlledby ionic conductionthroughfluidsin porespacesor intergranularcoatings,rather than by electronic conduction through the mineral matrix. This ionic-

controlled resistivityis a functionof the salinityof the pore fluids,the temperature,and the pressureand is importantfor sedimentaryand other rocks with connected porosities of greaterthan a few tenths of a percent. Ionic mobility increasesand resistivity decreases as temperature is in-

increaseswith depth,followinga nearlylinearrelationshipto density[Birch, 1961],and decreaseswith temperature.The increasein velocitywith depthmay be lessthanthe decrease with increasingtemperaturefor higher thermal gradients, causinga negativevelocity gradientwith depth. This temperature-controlled,negative velocity gradient has been citedas the possiblecauseof someinterpretedlow-velocity zones in the crust [Meissner, 1986]. The development of midcrustal zones of hydrofractures from the release of high-pressuremetamorphicfluids may also cause lowered velocities. Hall and Ali [1985] state that a 0.5% fracture

porosity can lower compressionalvelocities by 10%. Becauseof the differentcontrolsover resistivityand compressional velocities we do not expect complete agreement

creased,but the effect on resistivity reaches a maximum at temperaturesaround 200ø-250øCfor depths of a few kilome-

between MT and seismic refraction models. However,

ters.

models for shallow crustal features

in

general, the two data sets that we discusssupport similar and some midcrustal

As porousrocks are buried to depths greater than a few ones. The analysis of the coincidence or variance in the kilometers,porosities are decreased because of lithostatic modelsand the attendantscrutinyof constraintson lithology [mdinguntil the rocks normally become highly resistive. In in this paper representonly a first step and do not include a shales,ionic conduction also occurs in trapped water in full evaluation of the uniquenessof the individual models. daysandzeolites;as a result, the resistivity of shaleswill be We will not discussat lengththe uncertainty of the individual low(1-20 ohm m) and varies lessas porosity decreases.As modelsbut will key our discussionlargely to featuresin both shalesare metamorphosed,both porosity and layered clays the MT and the seismic refraction data that appear on are destroyed, but low resistivities can be maintained be- several profiles and are unequivocally represented in the cause of the formation of carbonaceous or iron mineral films data. The errors for a given depth, resistivity, or seismic •ong fissileplanesin the metashales.Intrusive rocks have velocity cannot be quantifiedsatisfactorally, but we assume minorporosityand thus are normally very resistive,typi- that MT modelsmay have errors as large as 20% for depths callyin the range of 500-20,000 ohm m. Fracture porosity or resistivities. Seismic refraction models are assumed to 'and intense alteration of intrusive rocks can lower their have velocities determined to within 2-5% for layers with resistivitywell below this range. Unaltered volcanicrocks robustly determined first arrivals on reversed profile seghaveveryhighresistivities whenporewatersarevery fresh; ments, althoughdepths may be determined less accurately. however,as volcanicrocksincreasein age, their abilityto Accuracies in the MT models are influencedlargely by the hostclaysandzeolitesdramatically decreases theirresistiv- data quality, selection of strike direction, frequencyity.Typicalvaluesfor tuffaceous-rich (tuff altersvery fast) independentshifts of curves due to near-surfacecomplexiflowsand volcaniclastic rocksof Tertiary age in the Cas- ties (similar to weathering problems in seismic reflection cMesare lessthan 20 ohmsm. Metamorphismof crustal surveys [Sternberget al., 1988]), and oversimplifiedmodel rocksbeyondzeolitefaciesgenerallyincreases resistivities, geometry. Refraction models are primarily limited by the exceptfor metamorphosedshalesin which carbonaceous data quality, the accuracy of event arrival picks, style of and/or metallicmineralcoatings maydevelop.Theexception record correlation [Mooney, !989], the effects of lowto highresistivities in metamorphic rocksalsooccursupon velocity layers, and the multiply reflectedevents.

dehydration of greenschist or amphibolite faciesminerals in the rocks,as demonstrated by Lee et al. [1983];their laboratory measurements onhigh-grade metamorphic rocks at temperaturesup to 300øCand pressuresof 0.4 GPa

STRUCTURE OF THE OREGON CASCADES

We begin with a discussion of the Oregon Cascades

[Micated loweredresistivities thattheyattributed to high- becausethe broad structurein this part of the volcanic belt pore-pressure fluids from mineral dehydration. As tempera-

is somewhat simpler than that in northern California and

19,422

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

BeneaththeJuande FucaPlate{EMSLAB. Washington. Interpretation of tbureast-west •Figure1) MT Asthenosphere profiles[Stanley,1983,1984]indicated thatthegeoelectricalJuan deFuca)experiment [EMSLAB Group,1988]. They structureof the OregonCascadescouldbe approximated interpretthat an E-W boundary a few tensof kilometers the with a four-layersequence consisting of (fromthe surface fromtheprofile(sameprofileasourDD') wouldexplain of theirobserved data.TheynotethatGough etal. down)(1) a resistivelayer (150-1500ohm m) of 0.5-2 km behavior on extensive thicknessrepresenting Quaternaryvolcanicflows;(2) a [1989]haveobservedsuchflankingstructures conductive layer(4-30ohmm) of 1-4 km thickness thatis geomagnetic arraymeasurements, themostsignificant bei,ng madeup of Tertiaryfelsicvolcanicflows,ashsheets,and thecontactbetweenresistivecrustnorthof the BlueMoun, crustin sedimentary andvolcaniclastic rocks(locally,suchasonthe tains(in the ColumbiaPlateau)andmoreconductive and the Basinand Rangeregionsto thesouth marginsof the Blue Mountains,this conductiveunit may the Cascades includesomeCretaceoussedimentaryrocks):(3) a resistive andwest.Thisstructure isimportant intheareaofsoundi• •ngs

tofit layer (>100 ohm m) that representsthe upperpart of the K andL (nearthe BrothersFault zone).In an attempt crust,comprised of pre-Tertiaryaccretedcrustalunitsand the data of soundingsK and L, we usedalternativemodeIs subduction-relatedintrusions;and (4) a deepcrustalconduc- indicatedby the different slantingpatternsfor the DCC in tor (DCC) with 2-20 ohmm resistivityat depthsof 12-20kin.

Figure 2b. We prefer the more horizontal model but show

This last zone has been generally thought to represent a

bothvariations to indicate oneof theproblems in modeling

combinedeffect of temperaturesgreaterthan 500øCand smallpercentages of hydrousfluids.Otherpossibilities for

MT

the causes of this conductive zone are discussed in this paper.

These genericgeoelectricalunits are shownin the twodimensionalmodelsfor profilesDD' and EE' of Figure 2. The shallowpart of the DD' model (Figure2a) represents largely layers 1 and 2 of the genetic four-layer model. The midcrustaland lower crustalparts of the modelare shownin Figure 2b, and the fit of the computeddata for the model to selectedsoundingdata is given in Figure 3 (upper two rows of plots labeledSistersA, B, C, D, E, and F). The structure on the deep crustal conductor (DCC), layer 4, is quite complex.It is only about6 km deep under soundingsA, B, and C but increases in the east to 11 kin. The fit of the model

to the observed data was easily achieved for soundingsA throughD, but the fit was difficult for E through L. It is our normal policy to emphasizethe fit of the model to data from the direction in which the electric current flows across the

data.

Wannamaker et al.'s [1989] two-dimensionalmodel indicatesa rise in the DCC to depthsof 5 km coincidentwith the rise in our model on the west end of DD'. Wannarnaker

et a!.'s modelplacesthe DCC eastof this rise at a depthof !2 km with a horizontal attitude, similar to our preferred model for the DCC (Figure 2b).

Livelybrookset al. [1989]modeledeightsoundings along a profile located some 30 km south of profile DD'. In their model the DCC is at a depth of 6 km near the Western Cascades-High Cascadesboundary, and its upper surface

dips to the east to depthsas great as 25 km. This configuration would be more similar to our less preferred modelfor the DCC of Figure 2a. However, our analysis and that

Wannamakeret al. [1989], with the supportof the geomagnetic array information of Gough et al. [ 1989], would seemto suggestthat the Livelybrooks et al. [1989] model is influencedby off-profile structuresnear the Brothers Fault. More detailed surveys and three-dimensional models will be required to resolve better the north-south and east-westcharacteristics of the deep crust under the Cascades. Leaver et al. [1984] interpreted the long, north-south refraction profile in the Oregon Cascades(S 1, Figure I)with a velocity structure consistingof eight linear velocity g•ient layers (Figure 4a). The first layer, with a thicknessof !.3 km and a velocity of 2.9-3.0 km/s, correspondsappro•mately to the resistive, Quaternary volcanic rocks (Figu•

strike of the two-dimensional model (called the transverse magnetic(TM) mode). This policy stemsfrom the fact that actual Earth structureslocated off of the profile, but along strike, will largely affect the transverse electric (TE) mode (for current flowing along strike). For a variety of threedimensionalsituations,modelingthe TM mode data from a profilebisectingthe longaxis will closelysimulatethe actual Earth structures.For the nearly north-southorientationof the Cascadesthe selectionof our profile in an east-west 2a) interpretedfrom MT profileDD'. Seismiclayer 2 exteMs sensemeansthat we are justified in using a two-dimensional to about 3.4 km depth with a velocity of 4.7-4.8 krn/s;we model and that we will be most accurateif we give the assumethat this layer representsthe Tertiary sediment'ary highest attention to the TM mode. This is illustrated in and volcanic rocks and largely coincideswith MT model panels3 and 6 of Figure 3 where we have fit the TM mode resistivities (Figure 2a) of 6-10 ohm m.

Seismiclayers 3 and 4 have velocities of 6.0-6.1 a.rd 6.3-6.4 km/s and extend to depthsof ! 1 kin. Theseunits approximatelycorrespondto MT modelvaluesof 100-3• ohmm (Figure2b) andare thoughtto representpre-Tertiary igneousand metamorphic,subvolcanicbasement.Cretaceoussedimentaryrocksprobablydo not make up a I'•a•e sediments). part of this subvolcanic basement,becauseMT soundings Another problem area was at stations K and L near the andwell logs[Sternberget al. 1988]in the BlueMo.unt•ns BrothersFault. Difficultyin fittingtwo-dimensional models showthattheseCretaceous unitsarequiteconductive (1.040 to the datafor thesesoundings is relatedto the risingor fiat ohmsm). Also,acoustic velocitiesof the Cretaceous shfles TM modedata and depressedTE data (in relationto sound- and sandstones wouldbe expectedto be lower thanthose in seismiclayers3 and4. Thiszoneundoubt•Y ings to the west) at low frequencies.Similarinterpretive observed problemshave been noted by Wannamakeret al. [1989], alsocontainsTertiaryand Quaternaryintrusiverocks,bm whomodeledfour MT soundings coincidentwith profileDD' the overall extent of these intrusive bodies cannot be deter, data at soundingsE and F. If we attemptedto fit the TE curve as well for these soundings,very complex structures would be required;we interpret that the depressionof the TE curve is largelycausedby the finite length(northward truncation)of the 2-km-thick, conductiveregion under the axis of the High Cascades(labeled Tertiary volcanics/

aspartof theElectromagnetic Studyof theLithosphere and

mined from the MT and seismic data.

STANLEY ETAL.'CRUSTAL STRUCTURE OFCASCADES WEST

WESTERN CASCADES

A

D

HIGH CASCADES

B C D E F GHJ K

_?...... ?__• Ir v v vvv y

19,423

BLUEMOUNTAINS EAST

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D' 0

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BROTHERS FAULT ZONE

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.:.:.:.: ,,.:F/'////''//•.////////Y///t: / :,::-'

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REFRACTION

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KM 30

,40

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100

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150

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Fig. 2. (a) Shallowpart (0-5 km) of the two-dimensionalmodelfor MT profileDD', (b) deeppart of the MT model {0-50 km), (c) shallow part (0-5 kin) of the two-dimensionalmodel for profile EE', and (d) deep part of the two-dimensional modelfor profileEE'. The portionof the deepcrustalconductor(DCC) with querieson the profileDD' modelrepresentsan alternativeform for the DCC in the area near the BrothersFault. The horizontalsectionat depths

of 11-27km (rightslantingpattern)is the preferredmodel.The intersection of seismicrefractionprofileS1 with MT profilesDD' and EE' is notedin Figures2b and2d, andthe 6.5-km/slayerinterpretedby Leaveret al. [1984]is also indicatedby the verticalarrows.In Figure2d the equivalent6.5-km/slayerfrom the Newberryrefractionprofile($2, Figure1) of CatchingsandMooney[1988b]is indicatedby the heavydashedboundaries on the EE' MT model.

Thenextlayerin the Cascades seismicmodelrepresents depthto the DCC (6-10 ohmsm) is modeledas 11km (Figure

themidcrust (layer5, from11to 27 km depth)thathasa 2b). The correlation of the depth to the DCC with this velocity of 6.5-6.6 km/s(Figure4a), The seismicprofile midcrustal seismic layer might appear to be somewhat crosses the MT profile DD' near soundingD, where the

fortuitous; however, as discussed below, these seismic and

19,424

5 •:..•'•jEs'E't"AL,' CRUSTALSTRUCTURE OF CASCADES

.,

,

4NEW•œRRY STR•-I::, .

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Fig. 3. Panelsof observed datafor representative soundings onMT profilesDD' andEE' (plussignsrepresent TM data, and crossesrepresentTE data)showingfit with computeddatafor TM (solidcurve)and TE (dashedcurve)

directions. Panels1--6arefor soundings onprofileDD', andpanels 7-9 arefor EE'. Panels10-13compare observed and computeddatafrom soundingsL andK on profileDD' for the alternativeDCC modelsportrayedin Figure2b; panels !0 and12areforthequeriedmodellayerof Figure2b;andpanels11and13arefor thealternative,preferred(right-slant pattern)model of Figure 2b.

geoe!ectricalfeatures are also coincident beneath eastern Cascades. Furthermore,the seismicdata are unequivocal Oregon.The approximatecoincidenceof the seismicrefrac- regarding the regionalsignificance of the midcrustal boundtion midcrust(6.5-6.6 kin/s) with the DCC is a common ary.Theseismic velocitynearthetopof themidcrust iswell featurein otherMT andseismicprofileswithinandnearthe determined beneath the Cascadesbecause a 6.5-km/s

STANLEYET At..: CRUSTAL STRUCTURE OF CASCADES

SOUTH ,,? FF 0 -

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Fig. 4. (a) Interpretationof seismicrefractionprofileS! betweenMountHood and CraterLake in the Oregon Cascades (Figure!). Numbersin the modelare interpretedvelocitiesin kilometersper second.The locationof theMT deepcrustalconductor(DCC) at the intersectionof seismicprofileS1 and MT profilesDD', EE', and FF' is shownby thearrows.It shouldbe notedthat the velocityin the individuallayersis notconstantbut represents a lineargradient. {b)The refractedfirstarrivalfrom the 6.5-km/slayerfor Leavetet al.'s [ 1984]modelof refractionprofileSI (Figure1). (c) the high-amplitude reflectionfrom top of the 6,5-km/slayer from shotpoint6 of profileS2 [from Catchingsand Mooney,!988b] are alsoshown.(d) Refractionmodelfor profileS2 from Catchingsand Mooney[1988b];velocitiesfor the nine-layermodelare shown(1.6-8.15 km/svelocities)and shotpointsare denotedat surface(SPI-SP6).

!9,426

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

• 20 8.15

E'

E

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>!00 (Quaternaryvolc.)

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High Lava Plains

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Fig. 5. (a) Velocity model for easternOregonseismicrefraction profile S2 (Figure 1) with the DCC position from MT model EE' also indicated, (b) two-dimensionalMT model for profile EE' (simplifiedfrom Figures 2c and 2d), and (c) MT layered model for profile FF' (Figure 1). The horizontal scale is the same for all three figures, and the vertical scaleis the samefor Figures5a and 5b, but Figure 5c is expandedvertically. Resistivity patterns apply to both Figure 5b and Figure 5c.

arrival can be followed for more than 120 km (Figure 4b). High-amplitude wide-angle reflections demonstratethat the

midcrust is marked by a sharp seismicdiscontuity(e.g., Figure 4c). A comparison of all four regional refraction profiles in the Cascadesregion (Figure 1) indicatesthat the seismicvelocity at the top of the midcrustallayer is partly a function of its depth: where it is shallowest(10-12 km), the

CRUSTAL STRUCTURE EAST OF THE HIGH CASCADES

Detailed geoelectricaland seismicrefraction studies• availablein the area of Newberry volcano(Figure5) in east

central Oregon [Fitterman et al., 1988; Catchingsand Mooney, 1988a].The seismicrefractionprofile extended fromjust westof Newberryvolcanoto eastof the Brothe'n Fault zone that boundsthe Blue Mountainson the south

measuredvelocity is 6.4-6.5 km/s, and where it is deepest the HighCascades andeM.s (18-20 kin), the velocity is 6.5-6.8 km/s. Variationsin crustal side.MT profileEE' crosses just east of Newberry volcano. MT measurements across the temperaturesmustalsoplay a role in determiningthe seismic

westernend of the profilewere madeat widelyspaced velocity of the midcrust. near A seismiclayer of 7-7.1 km/s (layer 6) forms the lower intervals,but more detailedsurveyswere conducted Newberry volcano. The MT surveys at Newberry were crust on the model from Leaver et al. [1984] at depths of soundings usingtime 27-44 km. The Moho is representedby layer 7, characterized combinedwith other geoelectrical

by a strong velocity gradientlayer (layer 7) at depthsof 44-46 km and undefiainby mantlevelocitiesof greaterthan 7.7 km/s. Regional earthquaketravel times were used by Leaver eta!. [!984] to augmentthe data from explosionsto refine the mantle velocitiesand crustal thicknessincorporated in the model of Figure 4a.

domainelectromagnetic (TDEM) and directcurrent(6c) methods asdescribed by Fittermanet al. [1988].Although MT profileEE' (Figure5) didnot crossontotheHighLava Plainseastof Newberry,asdidtherefraction profile, bit profile FF' extended well into this region. The two-dimensionalmodel for the regional structures oa

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

19,427

MTprofile EE' is replotted in a simplified formin Figure5b.

to be coincident with midcrustal, seismic velocities of 6.5

Thedetailsof theresistive(100-300ohmm) highunderneath Newberry volcanoand overlyingQuaternary(> 100ohmm) andTertiaryvolcanic(5-20 ohm m) units have previously beendiscussed by Fitterman et al. [1988],who interpretthe resistive high as representingan igneouscomplexformed fromsuccessive intrusionsin the volcaniccenter.Excellent

km/s, in a manneranalogousto the central OregonHigh Cascades.The two-dimensional modelfor MT profile EE' utilizes a thickness of about 10 km for the DCC, but this represents only a reasonable minimum thickness for resis-

tivities of 6 ohm m (the value used in the two-dimensional model).

confirmation of structuresin the upper 2 km were obtained

byseparate interpretations of MT and TDEM soundings [Fitterman et al., 1988]. As typified in the generic Cascades model discussed earlier, resistive units of 300-1000 ohm m in the two-

NORTHERN CALIFORNIA SEISMIC AND MT PROFILE

The mostcoincidentprofilesfor combinedinterpretation of seismic refraction and MT data across the Cascades is the

dimensional MT model (Figure 5b) representQuaternary joint MT-seismicprofilein northernCalifornia(GG', Figure volcanicrocks, and conductive units of 6-10 ohm m in the 1)that extendsacrossthe KlamathMountainsprovince,the second layer are believed to be altered volcanic flows and Cascades,and the Modoc Plateau (Figure 6). The main sedimentary units. Sedimentaryrocks in the secondlayer east-west,seismicrefractionprofilewas interpretedwith the could includeCretaceousunitssimilarto thosethat cropout aid of crossprofilesthat are roughlyparallelto the geologic in the nearby Blue Mountains [Sternberg et al., 1988]. The strike, as discussed by Zucca et al. [1986]. thirdlayer has resistivitiesof > 100 ohm m and probably is in contrast to the profiles discussedabove that were representative of pre-Tertiaryuppercrustin the region.The locatedentirely on volcanicterranes, the northernCalifornia DCC is modeled with resistivities of 6 ohm m in the MT and seismicrefractionprofilesstart on complex, actwo-dimensionalmodel. The interpretation of the western

creted units of the Klamath Mountains. The Klamath Moun-

partof MT profile EE' (Figures5b and 2c) indicatesa

tains provinceis composedof severalPaleozoicand Mesozoic lithotectonicbelts that are largely boundedby thrust

thickening of the upper 6-10 ohm m units (assumedTertiary volcanicand sedimentaryrocks) under the axis of the High faults. From west to east these belts are a western Jurassic Cascades volcanoes similar to that on profile DD'. This belt, a western Paleozoic and Triassic belt, a central metafeatureon both profile DD' and EE' may represent a morphicbelt, andan easternbelt of sedimentaryand volcastructural trough that formed becauseof loading of the crust nic rocks of forearc and arc affinitiesrangingin age from byan increasingthickness of Tertiary volcanic flows. Middle Jurassicto Ordovician[Irwin, 1966, 1981].A large Therefractionmodel (Figure 5a) includesvelocity units of complex of ultramafic rocks called the Trinity ultramafic 4.! and4.7 krn/swhich approximatelycorrespondto upper sheet [Irwin, 1966]crops out over a broad area in the east two layers (300-1000 ohm m and 6-10 ohm m) in the MT central Klamath Mountains(Figure 6). Mount Shasta apmodel. Surficial units of 1.6 and 3.7 km/s were correlated pears to be located near the suture joining the Klamath withalluviumand lake sedimentsby Catchingsand Mooney province to the pre-Klamath continental margin. [1988b].Units of 5.6-6.2 km/s comprisethe pre~Tertiary The preferred seismic refraction model for the Klamath uppercrust and correspondroughly to the third layer (100 sectionof profile GG' [Zucca et al., 1986]consistsof eight ohmsm) in the MT model.

layers as indicated in Figure 7c. The authors used three low-velocity layers in the model in an attempt to reproduce volcano in the seismic model. A lower crustal unit of 7.4 large-amplitude secondary arrivals and other more subtle km/sis underlainby upper mantlewith velocitiesof 8.15 parts of the travel time curves. The 7.0-km/s unit was km/sat a depthof 37 km. Catchingsand Mooney[1988b] derivedfrom an unreversedsegmentof the travel time curve suggest that magmaticunderplatingmay have played a role and was interpreted to be loosely constrained because

A midcrustallayer of 6.5 km/s thickensunderNewberry

inmaintaining relativelythick (37 km) crustdespitea large phasesbeyondthe critical point were poorly observed. e.amount of crustal extension in the region of Newberry

The MT modelfor profileGG' {Figure 7a) is based upon one-dimensionalmodelingonly, but we believe that essential Thetwo-dimensional MT modelfor profileEE' (Figures features 'are adequately resolved. Data from the Modoc 2cand5b)indicates thatthedeepcrustalconductor occursat Plateausectionof the profilesis particularlyone-dimensional volcano.

depths of about 12-20 km west of Newberry volcanoand in character, but the data from the Klamath Mountains

15-2'0 kmeastof the volcano.The DCC is interpreted to be region more clearly require more accurate two-dimensional at depthsof 14-18 km on the one-dimensional model for

or three-dimensional modeling. For this reason we will

profile FF' (Figure5c), witha suggestion of deepening onthe discussthe Klamath Mountains data only in rather qualitaeastendof the profilein the High Lava Plains.The dominant tive terms. The model does, however, show that the Klastructure involvingthe DCC on profile EE' is the rise to math Mountainsprovinceis much more complexthan that of

depths of asshallowas6 km on thewesternmarginof the the Cascades and Modoc Plateau. The MT model for the HighCascades (nearsounding 3-3,Figure2d).Thisrisein Klamath Mountains includes a 190 ohm m unit that is theDCCisnotaswellconstrained asforprofile DD' becauseinterpreted to represent the Trinity ultramarie complex and ofa moresparsesounding density,but it is unequivocallyan ! 100 ohm m zone that is related to Mesozoic plutonic requkedby the data. The geologicsignificance of this rocksthat crop out ne•arthe soundingsite. The complexities of the remainderof the Klamath MT model cannotbe easily structure is discussed in a subsequent section. Comparison of depthsto theDCConprofiles EE' andFF' keyed to geologicunits, particularlybecausewe know that (F;tgure 5c)suggests thata median valueforitsdepthisabout this sectionof the profilerequirestwo-dimensionalmodeling

15.kin.Regardless of sounding-to-sounding variations that

for further interpretation. However, the 10-20 ohm m con-

my largelyreflectmodellimitations, theconductor appears

ductor that forms the base of the model in the K!arnath

19.4a8

S,w'q.',,• 'ES :El' :,\L.: CRUSTAL STRUCTUREOF CASCADES 123

42 ["-'-•-T ..........

OREGON

!22

121:

120"

C,ad_iFORNtA

i

Redding

o

30 MILES

0

50 KILOMETERS

EXPLANATION MT Sounding Seismic profile

•6

Shotpoint

Cascade Range Trinity ultramafic complex Intrusive

rocks

Fig. 6. Indexmapfor northernCaliforniaMT andseismicrefractionprofilesGG' (Figure1)reproduced fromZucca et al. [1986].Numberedstarscorrespond to shotpointlocations,andMT soundings are indicatedby the arrows.The coincidentseismicand MT dataextendfrom the Paieozoic-Mesozoic KlamathMountainsto the westernedgeof the Basin and Range province, crossingthe CascadeRange.

Mountains at depths of 10-22 km is a robust feature of the data. This conductormay be related to underthrustunits of the Late Cretaceous, western Franciscan formation. Blake

(< 1 km thick, > 100 ohm m) surface layer on the MT data. The refractionmodel includesa similar, surficiallayerof 1.8 km/s velocity. A 5-20 ohm m secondlayer in the MT model

andJones [1986]suggestthat western Franciscanrocks have been thrust beneath the Klamath Mountains province in their studyarea to the southwestof the MT-seismicprofile.

the refractionmodel are interpretedto representmoreindurated, clay-rich, volcanic and sedimentaryrocks of Terti•

and the corresponding2.4-to 3.5-krn/s and 4.4-krn/s layersin

In contrast to other units in the Klamath Mountains, the age. western Franciscanformation is made up of relatively unThe high-resistivity bump (100-300 ohm m) at Medicine metamorphosedsandstonesand shales of forearc affinity. Lake volcano(Figure 7a) correspondsto a 5.7-km/sbody The 6.6-km/s, low-velocity layer (Figure 7c) in the seismic the refraction model. We interpret this resistive, h;•model for the Klamath Mountains may also be related to velocity structural feature underneath Medicine Lake to be subthrustwesternFranciscanunits, although6.6 km/sis too an intrusive complex very similar to that under Newberry

highfor the velocityfor suchsedimentaryunits;however,it

volcanoin Oregon [Fittermanet al., 1988]. Williams.and

is impossible to determine the true velocities of a lowvelocity zone, and actual velocities could be much lower

Finn [1985] have made a similar interpretationbasedupoa gravity and magnetic models.

than6.6 km/s.The 7.0-km/svelocitymodeledat the depths The subvolcanic,upper crust beneaththe CascadeRange of the conductivelayer is also far too high for western andModocPlateauis represented in therefractionmodel by Franciscanunits.However,the 7.0-km/slayer neednot be a 6.1-to 6.3-km/slayerextending to depthsof 6--14kin. thick and may representa thin piece of oceaniccrust in a midcrustis denotedby a 6.4-km/s layer interpretedfrom subductioncomplexlike that delineatedwith seismicreflec- reflections andrefractions ontheeast-west profile(GG') .and tion beneathVancouverIsland[Cloweset al., 1987]. fromreflections ona cross-line. However,thecomplexit¾.d Models for the MT and seismic refraction data from the near-surface structuresalongthe east-westprofileleadato Cascades and ModocPlateaupart of profileGG' are much estimated uncertainties in thedepthof the6.4-km/s layer,.of simplerthanthe modelsfor the KlamathMountains.Qua- 2-3 kmandin itsvelocity of upto 0.3km/s.A 7.0-km/s fi•.• ternaryvolcanicflowsare represented by the thin, resistive layerrepresents the lowercrust,butbecause thislay:•r

STANLE• ET AL.' CRUSTALSTRUCTURE OF C&SC&DES CASCADE

19.429

RANGE

G

KLAMATH MOUNTAINS _••1•

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..... 60

80

120

160

200

240

280

320

HORIZONTAL DISTANCE (KM)

Fig. 7. ta) Model for MT profile GG' (Figure I) constructedfrom individual layered models, where the numbers in the model are resistivities in ohms meters and triangles indicate MT soundings. Also shown are (b) observed (solid curve) and calculated (dashed curve) gravity data based upon the refraction model reproduced from Fuis et al. [ 1987] and (c) seismic refraction model from Zucca et al. [ 1986], where the numbers in the model are velocities in kilometers per second, the hatched regions are low-velocity layers interpreted from seismicdata, and the stippled area is the MT conductor from Figure 7a. Layer boundaries are dashed where uncertain; arrows indicate shot points.

basedon an unreversed profile, its velocity and dip are uncertain.Additional secondary arrivals were postulated to representmantle at depths of 36-45 km, but the possibility thatthesearrivals were multiples of a shallower event could not be ruled out [Zucca et al., 1986]. East of Medicine Lake the upper (6.1-6.3 km/s) and midcrustal(6.4 km/s) velocities were interpreted by Zucca et

I. [1986]to be separatedby a thin (1.5 km) low-velocity a.•er t6.0 km/s) that dips gently to the west. This low•elocity layer must be consideredas highly conjectural, but thedip interpretedfor the combined6.0- and 6.4-km/s units

imate mean value of 6.44 km/s for intermediate-composition midcrustal rocks in high heat flow areas of the Cascades. Zucca et al. [1986] and Fuis et al. [1987] suggest that the Modoc Plateau may be a composite of the roots of one or more magmatic arcs. Hamilton [1988] has reviewed the deep crustal lithology of exposed island arc crust and finds that examples from west central Idaho and the North Cascades consistof upper and midcrustalzones of crosscuttingmafic and intermediate plutons above isotopically primitive am-

crustalMT conductor also dips westward underneath the

phibolitic, tonalitic, and trondjemitic gneisses, with the amphibolitic facies becoming increasing garnetiferous or pyroxene bearing with depth. A classical40-kin-thick exposedsectionof islandarc crust in Pakistanis interpretedby

Modoc Plateau. The crustal conductor coincides with the

Hamilton [1988] to contain stocks and small batholiths in the

midcrustalseismic layer (6.4 km/s), consistentwith the position of the conductor in relation to refraction models in

upper and middle crust that grade from low greenschist through lower and upper amphiboliteto garnet-amphibolite

centraland easternOregon.

facies. The lower crust of this section consists of mafic

is •ell constrained.It is interestingto note that the deep

Zuccaet al. [1986]interpretthemidcrustal 6.4-km/slayer granulitesand mafic-plutonicrocks. The geologicinterpreto be composedof intermediate-composition rocks of am- tation of the seismic refraction model [Zucca eta!., 1986; phiboliteor granulitefacies. Room temperaturevelocities Fids et al., 1987]is compatible with the geologiccharactercompiledfor samplesof representativerocks of this type ization of island arc crust by Hamilton [ 1988]. Heat flow in

[Christensen, 1982] range between6.4 and 7.0 km/s at

theModocPlateauis greaterthan100mW/m•-[Maseeta!.,

1982], and high crustal temperatures beneath the Modoc Plateau may have led to a release of metamorphicfluids 10-3 kms-• øC-• [Christensen, 1979]inanamphibolite. At during dehydration of the interpreted amphibolite facies temperatures of around 500øCprojectedfor the Cascades rocks, possiblyfluxing partial melts and creatinghigh fluid at depthscoincidentwith the deepcrustalconducrindcrust the velocity rangefor Christensen'samphibolite pressures •ould be morenearly6.14-6.74 km/s, providingan approx- tor and the dipping low-velocity zone. We will discussthis

pressuresof 4-6 k bar (12-20 km). However, the effect of temperatureprovides a negative gradient of about 0.55 x

19,430

STANLEY ET AL.; CRUSTAL STRUCTURE OF CASCADES

t sound

œ

¸

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

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COLUMBIA PLATEAU

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19,43!

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35-

0

20

40

60

80

100

HOR. SCALE EXPLANATION

150 $iletzia >100

Columbia Plateau

>1000

Pluton

20-120

Volcanic

Fig. 9. MT modelforprofileAA' (Figure8) modified fromStanley etal. [1987]including morerecentinterpretation

of data from the ChehalisBasin. Dashed lines and numbersunderneaththe ColumbiaPlateausection are refraction

modelinterfacesandvelocitiesfrom profileS4 (Figure1) of Catchingsand Mooney[1988a].SWCC is the southern WashingtonCascadesconductor;DCC is the deepcrustalconductor.Valuesof resistivitiesin ohmmetersare indicated by patterns in the explanation.

geophysicalIpetrologic model in more detail and extend it to otherregionsin a subsequentsection.

accretionof a major seamountcomplex, Siletzia (partially outlined by magnetic highs S and T in Figure 8). Other possibilities for the conductiverocksexist,includingaltered volcanic flows, thick continental sedimentary rocks, and STRUCTURE OF THE WASHINGTON CASCADES graphitic/pyritic-richpre-Mesozoicrocks. However, on the Extensivemagnetotelluricsurveyshave been completed basisof the correlationof shallowerparts of the conductor inthesouthernWashingtonCascades[Stanleyet al., 1987]. with outcrops of Eocene, marine, and transitional marine Stanley[1984] also provided an electrical model for the rocks, the lithologypreferredby Stanley et al. [1987] for the northernColumbia Plateau, and Catchings and Mooney SWCC is that of marine sedimentary rocks, The SHZ [1988a] havestudiedthe centralColumbiaPlateau(Figure8) appears to be located on the contact between mechanically withseismicrefractionprofiling. rigid, mafic rocks of Siletzia (the accreted seamount comStanleyet al. [!987] found that a conductivityanomaly firstdetectedby Law et al. [1980] consistedof an east plex) and more compliantsedimentaryrocks interpreted to

make up the SWCC [Stanley et al., 1987]. The SWCC forms a major part of the upper crust in the WashingtonCascades as the southernWashingtonCascadesconductor(SWCC) and is overlain by thick O!igocene-Miocenevolcanic rocks tFigures8 and 9). The areal extent of this conductivity anomalyis indicatedin the contour map of Figure 8; the and possibly underlain by oceanic crust and seamount contoursrepresent the sum of layer thicknessesdivided by complexes. Other featuresindicatedin the MT model for profile AA' layerresistivities for the anomalously conductive area.This parameteris known as integrated conductance, and the (Figure 9) are an intrusivebody (Goat Rocks pluton) east of

dipping conductive packagethatStanleyet al. [1987]referto

conductance contourscan be convertedapproximatelyto Mount Rainier, the Chehalis Basin, and details of the norththickness by multiplying by a fixedresistivity. in thecaseof ern part of the ColumbiaPlateau.A steeplydippingconductheSWCCa typicalresistivity for the modelsis 2 ohmsm; tor on the extreme west end of profile AA' is p'oorly thusthe conductancecontourscanbe convertedto thickness resolved, but with the aid of another profile profile, CC' bymultiplying by 2. Magnetic anomalies thatwereusedto (Figure 1), Stanley et al. [ 1987]interpreted this conductorto postulatea geologicmodel for the MT resultsare also be related to partial subductionof Tertiary melangeunits indicated bythepatterned areas.Seismic events recorded by that outcropin the Olympic Peninsula. On the ColumbiaPlateauthe MT model showsup to 3.3 tl•eMountSt. Helensseismic arrayoutlinea linearbeltof

fight-lateral slipcalledtheSt.Helenszone(SHZ)by Weaver km of basalt flows (20-120 ohm m) of the Columbia River andSmith[1983].TheSWCCisinterpreted byStanleyetal. Basalt Group (CRBG) overlying about 3 km of conductive

[1987] to be a possible LateCretaceous to earlyEocene units (5-20 ohms n-t)interpreted to consist largely of Terforearc basin andaccretionary prism system sutured againsttiary, continentalsedimentaryrocks. Seismicrefractionsurandunderthe pre-Eocene North Americancontinentby veys (Figure 8) by Catchingsand Mooney [1988a] provide

19,432

STANLEYET AL.i CRUSTAL STRUCTURE OF CASCADES

other details about the crustal structure of the Columbia

Grande riftto explaina deepcrustal conductor underne•h

Plateau;key featuresof their modelare shownin Figure9. therift,in opposition to theinterpretation of magma by Catchingsand Mooney [1988a]interpretthe centralpart of Hermanceand Pedersen[!980]. Feldrnan[1976],Jones the ColumbiaPlateauto be coveredby 3-6 km of flowsof the [1987],and Hyndman[1988]all suggest that fluidsfrom CRBG. The CRBG was interpreted to cover a variable prograde metamorphism playa rolein formingdeepcrustal accumulation of sedimentary rocks assigned to a low-

velocitylayer (5 km/s)up to 5 km thick. Otherfeaturesof the seismicmodelincludean uppercrustallayer of 6.1-6.3 km/s and a deeper,midcrustallayer of 6.8 km/s. A high-velocity {7.5 km/s)layer formsthe lower cruststartingat depthsof 25 km underneath the Columbia Plateau. Catchings and Mooney [ 1988a]interprettheir model to representcontinental rifting, floored by a mafic, lower crustalrift pillow. The deepconductivezone that dips to the east underthe western Columbia Plateau (Figure 9) may not be directly associatedwith the SWCC, but instead may be causedby the samepetrologicalfactorsor physicalprocessesthat causea deepcrustalconductorelsewherebeneaththe Cascadesand surroundingregions. It is tempting, however, from a simplistic, plate tectonic standpointto connectthe two conductors (SWCC and western Columbia Plateau). A nearhorizontal DCC similar to that mapped in the Cascadesof Oregon and California is not evident on the west end of MT profile AA'; instead, we interpret a dippingconductor associated with Tertiary melange units found in the Olympic Mountains

on the extreme

west

end of AA'

and another

interpreted sedimentary package, associated with the SWCC. The intervening oceanic crust-seamountcomplex that we interpret to floor the SWCC provides thermal/ petrologic conditionsthat are probably much different than in the relatively flat lying DCC elsewhere in the region. In addition, there clearly is no DCC underneath most of the Columbia Plateau. We suggestthat this absenceof a DCC beneath the Columbia Plateau is due to a more midcrust and lower crust and a lower heat flow.

NATURE OF THE MIDCRUST

mafic

IN THE CASCADES

The seismicand MT interpretationsdiscussedaboveallow us to analyze about the nature of the midcrust in the

conductors. Allofthelatterauthors andJiracek etal.[1983] notetherequirement of an impermeable zoneto contain the deep crustal fluids.

In additionto a largenumberof workerswhoadvocate

fluids asthecause ofdeepcrustal conductors, Stanley [1989] arguedthat somedeep crustalconductorsin low heatflow areasare causedby carbon and metallic mineralfilmsin metamorphosed shales.Duba and Shankland[1982]discussedthe role of carbon in the crust and mantleas a

possible factorin deepMT conductors. Stanleyetal. [19•] interpretregionalconductors beneaththe AlaskaRange, extending from depthsof as shallowas 3 km to over20krn, as relatedto massivesectionsof tectonicallyunderplated Mesozoic flysch with 3-5% carbonaceousmaterial in fissile plane coatings.

Studiesof the metamorphichistory of exposedmidcrusU..l rocks indicate that large volumes of fluids have existedin the midcrustduring certain conditions.Fyfe et al. [1978]review

thegeneralquestionof fluidsin the crust,andNewton[1989] providesa recent review of the knowledgeconcerni• metamorphic fluids in the midcrust and lower crust. • most likely source of metamorphic fluids in the midcrustfor geothermalgradients like those in the Cascadesmay be the dehydration of greenschist and amphibolite facies rocks duringprograde reactions [Fyfe et al., 1978; Newton, 1989]. At the depths and interpreted temperatures of the DCC in the Cascades the rocks are probably of amphibolite facies [Fyfe et al., 1978]. In magmatic areas, hydrous fluids released from crystallizing, water-rich plutons may also be important in contributing to electrically conductiveregions. Significantcrustal fluids in the Cascadesmay have originated in crystallizing, water-rich magmas or in the dehydrationof metamorphicrocks as they were heated in the Quaternaryby Cascadesarc magmatism.The budget for hydrousfluidsin the Cascadesregioncrustmustincludea componentof fluids originatingfrom the subductedJuan de Fuca plate; thust•

Cascades.One phenomenonthat is essentialto understandis the presence of a deep electrical conductor within the midcrust. This conductoris generally located at depthsof crust undoubtedlycontainsmore hydr6us fluids thanareas 12-20 km in the Oregon and northern California Cascades, wheresubductionis not occurring.For the purposes of our coincidentwith seismicvelocitiesof 6.4-6.5 krn/s.However, discussions, however, we assume that the fluids arising there are somenotablevariationsin the depth.The impor- directlyfrom the plate are a dynamicpart of maficmeltsthat tant departuresin depth to the DCC are (1) very shallow underplatethe crust and contribute only to retrograde• DCC along the western margin of the High Cascades,(2) cessesin the granulitic, lower crust. shallowingof the DCC (to < 10 km) under the Modoc Plateau Fournier [1989] discussesthe relationship of high-•in correlationwith the modeleddip of a low-velocityzone pressurefluidsto seismicityin the crust. He pointsoutthat

above a midcrustalvelocity layer of 6.4-6.5 km/s, and (3) large depths to the DCC (>25 km) under the Columbia Plateau.

Previous explanationsfor the DCC in the western United Stateshave mostlyincorporatedthe effectsof hydrousfluids and/orpartialmelt. Stanleyet al. [1977]discussedthe role of

the releaseof fluidsfrom deepcrustalrocksmayhydrofracture[Fyfeet al., !978]mineralgrainswhenthereleased fluid

pressure Pœ isgreater thantheconfining pressure Psplus the

tensilestrengthof the rock (usuallymuchsmallerthanthe

lithostaticpressurein the deep crust).This process of hydrofracturing hasbeenwell documented at depthsof6-9

temperatureand hydrousfluidsin terms of a deep crustal km in the deep Soviet well on the Kola Peninsula.[Koconductorin the Snake River Plain. Hyndman [!988] inter- zlovsky,1984].Kozlovskyusedthe term "disaggregate" to prets an east dipping conductorunder Vancouver Island to refer to separation of mineral grains by high-pore-pressure

be the result of saline fluidsfilling the pore spacesof fluidsas they createthe requiredporosityto containthem. interbeddedsedimentsand basaltsin underplatedoceanic We preferthistermfor thegeneral effectof therelease of crust. Jiracek et al. [1983] advocateda trapped zone of fluidsalonggrainboundaries, ratherthanhydrofracturing, hydrous fluidsbeneaththe brittle-ductile transitionin the Rio

whichimpliesthecreationof linearrockfailures.Und.oubt-

STANLEY ETAL.:CRUSTAL STRUCTURE OFCASCADES

19,433

edly,bothvariations occurwhenhigh-pore-pressure, meta- 1979];in the Coso,California,geothermalareathe cutoffis morphic fluidsarereleased. about 7 km [Meissner, 1986];and in Iceland [Palmason, Fyfeet al [1978]providea comprehensive analysis of the 1971]and Yellowstone,Wyoming[SmithandBraile, 1984], •e of high-pressure fluidsin propagating fracturesthrough it is onlyabout4-5 km. Earthquakes in theOregonCascades t.hecrustanddetailson the characteristicpulsesof hydrous are very rare but are morefrequentin the northernCalifornia •aidsgeneratedin progrademetamorphicreactions.They Cascades.A regionaldepthlimit for eventsin the northern .•intoutthatamphibolite facies rocksstillcontain upto 1% CaliforniaCascadesis about15km (from seismicitydataof oftheirweightin water. The higher-gradeamphibolitefacies Mooney and Weaver [1989, Figure 15]). In the Columbia alehydrate intogranu!itefacies,producing anhydrous miner- Plateau,seismicityextendsto somewhatlargerdepthsof alssuchasgarnetandplagioclase. Partialmeltingmay occur >25 km. The depthlimitsfor seismicityin northernCaliforbeforecomplete dehydration, and because of the great nia andthe ColumbiaPlateauare similarto the depthof the •nity of water for a silicatemelt phasethe hydrousfluids DCC. This correlationsupportsour model in which the may beeffectivelytransported to higherlevelsof the crustas ductiletransitionthat limitsseismicitycapsa roughlyhorithe silicate magmas move upward. Fluids not combined in zontalzone of fluidsand disaggregation that alsocausesthe •e silicatemagmasmay migrate upward through vertical rrficrofractures and by grain boundary diffusion until an impermeable zone is reachedor until the pressureof the

DCC. Fournier[1989]discusses the relationship between heatflow, high-pore-pressure fluids,and seismicityin more detail.

.fluids is significantly greaterthanthe lithostaticpressure PtIf Pt is the maximumstresscomponent,thenthe fluidswill

Theseconcepts of a midcrustal zoneof metamorphic fluids andassociated effectsareportrayedin Figure10.Ideasfrom disaggregate or hydrofracturethe rocksuntil enoughvolume Lachenbruchand Sass[1978]and Hamilton [1988]on mafic iscreatedto accommodate the releasedfluids.The overlying underplatingof the crust are utilized. W. B. Hamilton (oral impermeable zone may be a combinationof the nondehy- communication, 1989)suggests that maficmagmaspond at dratingmetamorphicrocks, a change in lithology, or the the baseof the crust;thesemaficmagmasgeneratesilicic brittle-ductile transitionas envisionedby Eaton [1980] and magmasby secondarymeltingof overlyingrocks, thereby Jiraceket al. [1983]. The developmentof fine-grainedminerals (mylonitic texturing) is normally found below the ductiletransition, contributingto the impermeability.In addition,several authors, including Fyfe et al. [1978], Etheridgeet al. [1983], Jones [1987], and Hyndman [1988], stress that an impermeablecap is a natural consequence of prograde metamorphism.Regardlessof origin,the impermeablezonewill be effectivein limitingthe verticalmigrationof fluidsuntila sufficientnumberof verticalfracturesor rising

transportingthe heatupwardthroughthe crust.In our model

thisheatdehydrates greenschist gradeandamphibolite grade metamorphicrocks, producinghydrousfluids that flux addi-

tional silicatemelts.The high-pore-pressure fluidswill migrate upward until a zone of restrictive permeability is reached.The microfractures in the regionof disaggregation

may be subsequentlylined with dissolvedmetallic minerals when temperaturessubsideto precipitationvaluesof about 300øC.in general,hydrousfluidsand silicicpartialmeltswill magmasallow the release of the stored fluids. not exist at the samedepth,but heterogenous,highly schisHorizontalzonesof disaggregation may alsocontainpar- tose rocks of the midcrustin the Cascadesmay have zones tial melt (at temperaturesabove the water-saturated,rock of well-segregated partial meltingand hydrousfluids. solidus). Hydrofracturingof the rocksby high-pore-pressure The metamorphicgradientsimplied in Figure !0 were fluidsmay also provide preferredlocationsfor linear con- scaled accordingto data on the PT stability of different centrations of graphiteand metallic minerals.Kozlovsky metamorphicfacies from Fyfe et al. [1978]. An assumed [1984]documents low-resistivity zonesin the Sovietdeep geothermis depictedat the right in Figure 10 and in more Kolawell which are due to graphitizationand sulphide detail in Figure 11; this geothermdeparts in significant streaks andveinlets.Kozlovskyascribesthe low-resistivity detailsfrom standardgeothermssuchas thosecalculatedby zonesin the Kola well to the direct resultsof high-pore- Lachenbruchand Sass[!978] for the Basinand Range(BR) pressure fluidsfrom rock dehydrationin foliated rocks, with and Battle Mountain heat flow high (BMH, Figure 11). Our fluids concentrated by thefoliation.Theoverallprocess that proposedgeothermis similar to the Battle Mountain geoweoutlineandthat is documented by Kozlovskycan create therm of Lachenbruchand Sass[1978]to a depthof 12 km highly conductive regionsin thedeepcrust. and is nearly identical to the geotherm usedfor the Canadian We suggest that the petrological processoutlinedabove CascadeRangeby Hyndman [1988] and Lewis et al. [1989]. couldexplainthe deepconductor in the Cascades andmay Ingebritsenet al. [1989a,b] assumea backgroundheat flow

haveimportant implications for tectonicproblems, suchas of 100mW/m2 in theOregon Cascades, andBlackwell and providingan absolutelimit to brittle behavior in the crust.

Steele [1985] have measuredthermal gradientsof greater

Brittle-to-ductile transitions incrustal rocksareproduced by than 50øC/kin in a borehole in a number of locations in the theeffectof pressure andtemperature, modified by strain Cascades;thus we are requiredto use a steepgeothermin rate and mineralogy.A horizontal zone of high-pore- the upper part of the crust. Below 12 km our geotherm pressure fluidsand mineraldisaggregation would simulate becomesless steep becausewe assume that the highly

ductile behavior andeffectively truncateseismicity caused endothermicprocessof dehydrationof metamorphicrocks bycrustalstresses. Seismicity in activetectonicregions and partial melting [Fyfe et al., 1978] interpretedto be typically hasa sharpdepthlimit becauseof the onsetof occurring in the midcrust will buffer thermal gradients. ductile behavior in thecrust.Earthquakes in theBasinand Hyndman [198.8]also invokesthermal bufferingcausedby

Range ofNevada [Eaton,!980]andinHaicheng, China,and endothermicdehydrationreactionsto explaina missingheat

Greece [Meissner, !98.6] havea depth cutoff atabout 15km;

sink (from thermal budget calculations)under Vancouver

intheRioGranderift (whereheatflowis similarto thatof Island. Ferry [1986] summarizesfirm evidencefor 1-5 rock

theHi• Cascades) thecutoffisabout!3 km[Sanford etal., volumesof fluid at depthsof 13-22 km in the crust during

19,434

STANLEY ET AL.' CRUSTAL STRUCTURE OFCASCADES ..

.

10-

m

•.



•-

....••

o

MAFIC

earthquak es

'•

200

c m

400

•E

800 m •

20

qEHYDRATING ) • ,ZONE __ , -} • .

3o

4o

I

GRANULITES

AND INTRUSIVES

c

lOOO

,,7.9

krn/s

•1 maficmagmas r-• ß silicic magmas .•'•]volcanic flows Fig. t0. Petrologicmodelof crustalstructureof a continentalmagmaticarc, suchas the Cascadesas inferred t¾om MT, seismicrefraction, heat flow, andgeologicaldata. The generalvelocity structure from Cascadesrefraction surveys is shown on the left, and our hypothesizedgeothermon the temperaturescale is shown on the right and in Figure 11. Metamorphic facies for the main crustal zones (greenschist-amphibolite-granulite) are scaled from Fyfe et al. [1978]. The baseof the crust is interpretedto be approximatelyat the dry basalt solidus(Figure 11). Fluids producedas a result of dehydrationof the midcrustaland lower crustalrocks are inferred to rise to an impermeable zone at the top of the midcrust;the zone of fluidsand partial melt is interpretedto producehigh conductivities.The heavy arrows imply that as the crust heatsup, the greenschist-to-amphibolite and amphibolite-to-granulitetransitionsmove upward, as does the brittle-ductile

transition.

metamorphism; these values are similar in magnitude to those associated with hydrothermal convection cells driven by epizona! plutons. Thus although the fluid movement duringmetamorphicdehydrationis not simplyexplained,the evidence points to the possibility of someconvective behavior in the midcrust and further requires that the geotherm flatten in our petrologic model. Our proposed Cascade Range geotherm reverts to a

fluids. In the Columbia Plateau the larger depth to the DCC may be related to both the lower heat flow beneath the Plateau and the probability (based upon seismic velocity structure)that the deep crust in this region consistsof rocks. Mafic rocks have a higher fluxing temperature;in

steepergradientin the dry, granulitic zone, parallelingthe Basin and Range geothermof Lachenbruchand Sass [ !978].

Our interpretation of midcrustal lithology beneath Cascadesand surroundingregionsis constrainedprimarily by the observationof low resistivitiesand velocitiesof 6.5

We have constrainedour geotherm to allow the base of the crust to be a thermally controlled feature, i.e., the base of the crust occurs at the temperature of dry basalt melting (1150øC).

For steepertemperaturegradientsthan those assumedin Figures 10 and I 1, the transitionsfrom greenschistto amphibolitefaciesand from amphiboliteto granulitefacies, as well as the zone of partial meltingand metamorphicfluids, will shallow,as indicatedby the arrows in Figure 10. This effect may be the causeof shallowingof the deep crustal conductorin the Modoc Plateauand alongthe boundaryof the High Cascades and Western Cascades. In addition, crystallizationof silicicmagmasmay releasehydrousfluids at depthslessthan !0 km, contributingto the observedrise in the DCC upper surface.However, muchof the water from crystallizingplutons would be expected to be absorbedin retrograde metamorphic reactions and not to remain as

addition, the lower crust under the Columbia Plateau may

not possessthe hydrated minerals needed to generateme• morphic fluids.

km/s in the midcrust. Laboratory velocity measurements

indicatethat [Christensen,1979]if we correctthe measured velocitiesfor probabletemperaturesin the midcrustof t.he Cascades (Figure 10), thenthesemidcrustalrocksprobably

are of intermediate composition. The transitionto 6.5-k•s velocitiesis believedto be largelydue to the change it• metamorphic gradefrom greenschist to amphibolite facies metamorphic graderocksaspostulated for similarvelocities in Icelandby Palmason[197!]andfor theKolaPeninsula by Kozlovsky [1984].Highervelocitiesof about7.0 km/sint:l•e lowercrustare interpretedto be higher-grade, granu!ite facies and mafic intrusive

rocks.

We haveevaluatedthe expectedeffectsof our hypotl•e, sizedzone of disaggregation and metamorphic fluids

compressional wavevelocities. If high-pore-pressure flu;ds fillingporosities of 1-4%(as modeled by Hyndman

STANLEYET AL.: CRUSTALSTRUCTUREOF CASCADES

19,435

TEMPERATURE, •C

0

200

4•0

,, 690 800• •1 1000 12,00 I 1400 I : IIl.,,.,l.,•. I • ll I

lO

40L



Fig. I 1. Proposedgeotherm (dashedcurve) for the petrologicmodel of Figure 10 alongwith the Sierra Nevada, stable craton, Basin and Range average, and Battle Mountain High geothermsof Lachenbruchand Sass [19781.GSS is the granitewater-saturatedsolidus;BDS is the basaltdry solidus.

Shearer[1989]) are required to reduce resistivities in the midcrestof the Cascades region to the range of a few ohm meters,then the compressionalwave velocities shouldalso be reduced(possiblyby more than 10%). Apart from the

thickness and a few kilometers in length, with a fluid-filled porosityof 1-4%, are requiredto be interlayedwith similar dimensional,non-fluid-filledlaminae in order to produce the observed high reflectivity.

complex low-velocityzonesin the Klamath Mountainsand thethin, conjecturallow-velocity zone above the midcrustin theModoc Plateau (Figure 7), no evidence for low-velocity z -ones in the midcrust or lower crust of the Cascadesregion wasfound.The presenceor absenceof a low-velocityzone cannotbe determined on the north-south Oregon refraction profile(S1, Figure 1) becauseof sparsereceiverspacing,but a low-velocityzone does not appear to be present on the easternOregon profile (S2, Figure 1). We believe that compressional wave velocity anisotropymay help explain theabsenceof a low-velocity zone where high-pore-pressure

SIGNIFICANCE

OF DCC

MODEL

TO GEOTHERMAL REGIME

We hypothesizethat the DCC mappedat depthsof 12-20 km in the Cascadesis related to metamorphic fluids and partialmeltin a subhorizontal zoneof mineraldisaggregation and microfracturesbeneathan impermeablelayer. A partial melt componentis calledfor by probablehightemperatures

at this depth. Waft [1974]and Shanklandand Waft [1977] have developeda theoreticalmodelto simulatethe effectof

fluidsare interpretedto exist. Christensen's[1982, 1989] melt distributions on electrical resistivity. Hermance and laboratorydata on the seismicvelocity of rocks show that Pealersen[1980] have extended this partial melt model by amphibolite grade schistsand gneisseshave compressional modifying a well-known relationship(Archie's Law) for velocity anisotropies of 10-20%at midcrustalpressures. For pore.fluidconductionto includethe effect of a melt compo•mstance, Christensen[1989] measuredoverall seismican- nent. Hermance and Pealersen[1980] show that connected amounting to about2% of therockproduce isotropy in a total core sequenceof amphiboliticgneissfrom meltcomponents the hedmont of the United States and found that velocities

resistivities of about 20 ohm m and that 25% connected melt

at 4 kbar (about 12 km depth)were about 6.65 km/s in the

producesresistivitiesof about2 ohm m, representingthe approximate rangeof resistivities determined for the DCC in

horizontaldirection and 6.25 km/s in the vertical direction.

Thi:'s anisotropyis due to the preferredorientationof am- the Cascade MT interpretations. With heterogenouszonaphibole, mica,andquartzin the core. If high-pore-pressuretion of both metamorphicfluids and partial melts in hydrorocks, the actual percentageof fluidswere containedin such foliated metamorphicrocks fractured, high-schistose thatareprimar!yhorizontal,theywouldaffectthe velocity melt could be much smaller.These assumptionsimply that mostlyin the vertical direction. Because seismicrefraction the volumeof magmapresentas partialmeltin the Cascades surveysmeasurevelocity in the horizontal direction, the is very limited abovethe depthof the DCC. Althoughit decrease in velocitydueto the additionof fluid-filled poros- couldbe arguedthat the extensivemidcrustalmagmaticheat ity would not be detected. However, such a fluid-filled, sourceat depthsof 7-10 km proposedby Blac•ell and foliated crustshouldbe highlyreflectiveandshouldproduce Steele [!983] is compatiblewith the MT data if placed .diagnostic shearwave attenuation.Fluid-filled!amel!aehave

somewhatdeeper than they interpret, such an extensive

been evaluated byHyndmanandShearer[1989]in termsof magmaaccumulationwould not be compatiblewith the theacoustic •eters requiredto producethe observed seismicrefraction interpretation;a significantlow-velocity highreflectivity (in thelowercrustin theiranalysis). They zone and high seismicattenuationwould be expectedfor suggest that compositionallaminae of roughly 100 m in

sucha feature.Interpretationof central•d easternOregon

19,436

STANLEYET AL.: CRUSTALSTRUCTUREOF CASCADES

SUMMARY AND CONCLUSIONS

refraction profiles indicatesthat no such low-velocityor high-attenuationregion is exhibited in the data. An alternative to the Blackm,ell and Steel [1983] heat flow modelfor the OregonCascadesis providedby Ingebritsenet al. [ 1989b], who used a heat budget analysisto demonstrate that groundwater circulation could account for the anoma-

MT datafromthe Cascades definefourbasiccrustal resistivity zonesin Oregon andnorthern California; these zonesare(1) a 0.3-to 2-km-thick, high-resistivity surface

zone (> 100ohmm)consisting ofQuaternary volcanic rio

(2) a 1- to 5-km-thick, 6-30ohmm conductive zonecorn. posed of Tertiary volcanic flows and possibly Cretaceous. midcrust uses assumptionsthat fall in the middle ground rocksthat thickenunderneath .tl• between the Black,a,ell and Steele [ 1985]and the Ingebritsen Tertiarysedimentary lous heat flow. Our MT and seismic-based model for the

et al. [1989b] heat flow models, for we assumethat thermal gradients of 40øC/km extend to 12 km but argue against a large magma accumulation in the midcrust or upper crust. However, the rise in the upper surfaceof the DCC near the Western Cascades-High Cascadesboundarycorrelateswell with the increasedheat flow documentedby Blackwell eta!. [1982]. This heat flow feature and rise in the DCC may be causedby shallow magmabut may be partially due to rising hydrous fluids from crystallization of plutons at depths greater than the top of the DCC. These hot fluids may be concentrated in fractures associated with the graben [Allen, 1966] that has been mapped along the Western Cascades-High Cascadesboundary. COMPARISON WITH THE Rio GRANDE RIFT

An analogy for the MT, seismic refraction, and heat flow results from the Cascades can be found in the Rio Grande rift

Oregon HighCascades, (3)anuppercrustal zoneofh'_.•

resistivities (> 100ohmm)thatrepresents pre-Tertiary base.

ment,and(4)a deepcrustal conductor (DCC,2-20ohm

that coincideswith a 6.5-km/sseismicvelocityunitinterpretedto representamphibolite faciesrocks.MT andseis. micrefraction datafor a long,east-west profilein northern Californiarevealcomplexstructures in the KlamathMountains.A westdipping,deepcrustalconductor(DCC)occurs at depthsof 6-14 km underneaththe ModocPlateauon

profile, coincident witha 6.4-km/s velocity layerthat

parallel to the DCC.

Thereare severalviablemechanisms for producing t• regionaldeep crustal conductor(DCC) in the Cascades

surrounding region;however, mechanisms proposed byFyfe et al. [1978],Jiraceket al. [1983],Jones[1987],Hynd.ma.• [1988],and Hyndrnanand Shearer[1989]involving highpore-pressuremetamorphic fluids are a key part of

preferredmodelfor the DCC in the studyarea.Thispo•

lated zone of metamorphicfluids and partial meltsis interpreted to occur in a permeableregion of microfractures a comparableto that in the Oregon Cascades,exceedingI00 mineral disaggregation; it may also provide an effective mW/m 2 Shearwaveamplitude andcompressional wave, lower depthlimit to seismicity,addingto the effectof normal of New

Mexico

and Colorado.

Heat

flow in the rift is

seismic reflection, and refraction data are available from the central New Mexico part of the rift, where midcrustal magma has been postulatedby Sanford et al. [1979]. The magmaaccumulationwas interpretedat a depth of about 20

plastic rheology that occurs above 300øC. Our model for t•

DCC and midcrustalseismicvelocitiesinvokesamphbolite

grademetamorphicrocks and temperaturesof 500ø-6(•C or morewith severalvolumepercentof metamorphicfluidsa• km from a series of bright reflections[Brocher, 1981] in partial melt. This model is in oppositionto the heatflow Consortiumfor ContinentalReflectionProfiling(COCORP) modelof Blackwelland Steele [1983]which utilizesa large, reflectiondata.Thesereflections occurjust belowthetopof regionalaccumulationof magma at depths of 7-10 kin; our a layer where velocities measured from seismic refraction model also assumes a hotter crust than that utilizedby data increasefrom 5.8 to 6.5 km/s. Analysisof shearwave Ingebritsenet al. [1989a, b] in their heat budgetmodelfor reflectiondata from microearthquakes by Rinehart et al. Oregon heat flow values.

[1979]indicatesthatthepostulated magmabodyis contained Complexaccretionarystructuresin t.hesouthernWashingjust beneaththe brittle-ductiletransition(abovethe 6.5-krn/s ton Cascadesare interpretedfrom MT surveys.Theses•layer)andconsistsof very low fractionsof fluid;thesefluids tures are believed to be primarily related to a suturez are interpretedto be meltby Rinehartet al. [1979]but could between a forearc basin/accretionaryprism systemon .the be metamorphic fluidsas well. This analysiscloselycorre- east and an accretedseamountcomplex on the west.This spondsto our hypothesizedpetrologicmodel for the Cas- suture zone is interpreted to control the release of crustal cades.In fact, both the high-reflectivityand the shearwave stressin the region of Mount St. Helens. Higher midcrustal

reflection datafrom the Rio Granderift couldbe explained seismic velocities and resistivities underneath the Columbia by a zone of hydrofracturing with a combinationof meta- Plateauare interpreted to be related to a more maficlithotmorphicfluids and partial melt, with the 6.5-km/s seismic ogy and lower heat flow.

layerrepresenting thetransition to amphibolite faciesrocks. MT interpretations in the Rio Granderift by Jiraceket al. Acknowledgments.We wishto thankCraigWeaverfor supply[1983]andHermanceandPedersen[1980]showsomewhat ingseismicity information for comparison withtheothergeophy• divergentdepths(10-20 km) to a deepcrustalconductor cal data and for fruitful discussionson the role of the SWCC andvolcanism in the southern Washington Cascades. In becauseof very complicated resistivitystructuresin the seismicity additionto primary supportfrom the USGS GeothermalProgram, thick,conductive riftclastics at thesurface; butit ispossible funding for muchof the MT workin the southernWashington I• that the DCC in the Rio Granderift coincideswith the zone

was providedby the Departmentof Energy, MorgantownEnergy TechnologyCenter.Partialfundingfor detailedelectrical deepcrustalhighreflectivitysimilarto thatin the Rio Grande ical studiesat Newberryvolcanowas providedby the De Energy, GeothermalDivision. Thoughtfulreviewsby rift occurelsewhere andhavebeencorrelated withtrapped of Spence,Doug Klein, Warren Hamilton, Patrick Muffler,Steve metamorphic fluids[Pffifner et al., 1988;Jones,1987;Hynd- Ingebritsen,and SteveBohlengreatlyimprovedthe original

of highreflectivity at thetopof the6.5-km/slayer.Zonesof

man, 1988;Hyndmanand Shearer, !989].

script.

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(Received July 13, 1989; revised April 20, 1990; accepted February 20, 1990.)