Tillamook Volcanic Series: Further Evidence for Tectonic Rotation Of

Home , Leech River Fault, Rio Grande Rift

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. B4, PAGES 2953-2970, APRIL 10, 1981

Tillamook Volcanic Series' Further Evidence for Tectonic Rotation of the Oregon Coast Range

JAMES MAGILL AND ALLAN Cox

Departmentof Geophysics,Stanford University,Stanford, California 94305

ROBERT DUNCAN

Schoolof Oceanography,Oregon State University,Corvallis, Oregon 97331

Paleomagneticdirections from the Eocene Tillamook Volcanic Seriesof the Oregon Coast Range point 46ø clockwisefrom the expectedEocene field direction.Potassium argon dating of six dikes and flowsfrom this formation yields a mean age of 44.3 + 0.6 m.y. These resultsestablish that the Oregon coastalblock of Simpsonand Cox (1977) extendsnorth to the Oregon-Washingtonborder and that this blockhas rotatedclockwise 46 ø + 13ø during the past 44 m.y. The block has undergoneno detectable north-southtranslation. A two phasemodel is developedto explain the tectonichistory and anomalous magneticfield directionsof the Pacificnorthwest region. Phase I of the rotation,active between 50 and 42 m.y.B.P., resultsfrom fragmentationof the Farallon plate with rotation of a fragmentduring its accre- tion to North America. Phase II of the rotation, active between 20 m.y.b.p. and the present,occurs in associationwith extensionin the Basin and Range province.

INTRODUCTION Range and Klamaths to the Cascadesrequire that these ter- Anomalous directionsof magnetizationin early Tertiary ranes have undergonethe same post 25 m.y.B.P. rotation as the Cascades. rocksof the CoastRange of Oregon(Figure 1), first reported by Cox [1957]and confirmedby recentpaleomagnetic studies To the north of the Coast Range, early Tertiary rocks simi- [Simpsonand Cox, 1977],have established that lower,middle, lar to thosein the Oregon Coast Range extend all of the way and upper Eocenesedimentary and volcanicrocks have un- to the Olympic Peninsula.The questionof whether all of this dergonedockwise rotations about vertical axes of from 50ø to terrane has undergonethe same rotation as that recorded in 75ø (Figure 2). The area sampledin thesestudies is a block the centraland southernOregon Coast Range is the subjectof extendingnorthward 225 km from the Klamath Mountains. much current paleomagneticresearch [Wells and Coe, 1979; The fact that similar dockwise rotations are recorded in rocks Globermanand Beck, 1979]. The present study was under- taken to determine whether the rotation recorded in the of varyingEocene age and as petrologicallydissimilar as basalts and turbidites leaves little doubt that the anomalous southernpart of the Oregon Coast Range extendsnorthward paleomagneticdirections have recordeda true tectonicrota- to the Columbia River at the Oregon-Washingtonborder. tion. Havingestablished this, however, a numberof questions When Did the Rotation Occur? remainconcerning the regionalextent and timing of the rotations and the mechanismsresponsible for them. In attempting to incorporate the palcomagnetically ob- servedrotation of the Oregon Coast Range into the geologic WhereAre the Boundariesof the RotatedRegion? historyof this region,it is importantto know the timing of the In southernOregon the predominatelyCenozoic terrane of rotation as precisely as possible. Did the rotation occur the Coast Range meetsthe mainly Mesozoicterrane of the throughoutthe Cenozoicor was it completedby the Miocene Klamath Mountains along a geologicboundary that may or or someearlier time? Recent paleomagneticstudies of mid- may not be a Cenozoictectonic boundary. The possibilitythat Oligocenedikes and sills within the southern Coast Range the Klamath terrane rotated with the Oregon Coast Range [Clark, 1969;Beck and œ1umley,1980] and Oligocenebasalts duringthe Cenozoichas not asyet beentested paleomagneti- from the WesternCascades [Bates et al., 1979;Bates and Beck, cally.To the east,the early Tertiary rocksof the CoastRange 1981;Magill and Cox, 1980]show that a significantamount of passbeneath middle and late tertiary and Quaternaryvolcan- clockwiserotation took place after the Oligocene.The present ics and sedimentsof the Cascades.If the Coast Range rotated studyof upperEocene rocks helps establish the amount of ro- into its persentposition from a more seawardposition to the tation that occurredearlier in the Tertiary. west[Simpson and Cox, 1977],the presumedsuture to the east is maskedby youngerdeposits of the Cascades.Magill and By What MechanismDid the RotationOccur? Cox [1980] proposethat the Cascadeshave undergone25 ø As was first suggestedby Irving [1964], the simplestex- clockwiserotation sometime between 25 m.y.B.P. and the planationof the paleomagneticdata is somesort of regional present.In fact all the paleomagneticdata from the Cascades tectonicrotation, as had been hypothesizedfor this region [Beck,1962; Beck and Burr, 1979;Bates et al., 1979;Bates and earlier by Carey [1958] and Wise [1963]. Most workers now Beck, 1981; Magill and Cox, 1980] show remarkably consis- agreewith this. However developinga specificpaleogeogra- tent resultsyielding an average rotation of 27ø + 7 [Magill phic model that integratesthe paleomagneticallyobserved ro- and Cox, 1980]. Depositional contacts linking the Coast tationswith the knowngeology of the CoastRange and adjacent regionshas proven to be a challengingproblem. The Copyright¸ 1981by the AmericanGeophysical Union. followingis a brief summaryof the relevantgeology. Papernumber 80B 1368. 2953 0148-0227/81/080B- 1368501.00 2954 MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

ness of the Siletz basaltsat 10,000 to 20,000 feet, clearly anomalously thick crust. This and an even greater crustal thicknessof 15 to 20 km determinedfor the Coast Range from seismicdata [Tatel and Tuve,1955; Berg et al., 1966;Langston and Blum, 1977] are consistentwith a basementof oceanic ._45ø crust anomalously thick due to the presenceof oceanic islands. This model for the Coast Range basementis supported by the regionalgeology, by regionalisotopic data and by seismic, gravity and resistivitydata as summarizedby Simpson [1977].All appear to supportthe existenceof oceaniccrust be-

Basin neath a large part of westernOregon and southwesternWash- ington [Hamilton, 1969, 1978; Dickinson, 1976; Davis et al., and 19781. Range Two interpretative models were introduced by Simpson [1977]and Simpsonand Cox [1977]to explain the inferred tec- ._37øN tonic rotations and geology.In the first model, the present rocks of the Coast Range formed on a segmentof oceanic plate and pivoted about a point near the southernend of the MB plate during subductionbeneath North America. In an alternative secondmodel, Simpsonand Cox [1977] proposedan 300 KM • early Tertiary subductionzone extending southeastacross what is now interior Oregon and Washington. This subduction zone was dogged in the early Tertiary by thickened Fig. 1. Generalizedgeotectonic map of the westernUnited States oceanic crust including the thick sedimentsand seamounts basedon Cohee[1962] and Lawrence[1976]. C, Cascades;CR, Oregon CoastRange; CL, ClarnoFormation; BM, Blue Mountains;CPB, Co- which characterize the present Coast Range block. Sub- lumbia Plateau Basalts;DP, Drake Peak; GV, Great Valley; IB, Idaho sequently,the trenchjumped westwardand the CoastRange Batholith;K, Klamath Mountains;Ku, Upper Cretaceoussediments; block rotated clockwise,about a pivot at the northern end MB, Mojaveblock; NR, Nevadarift; SN, SierraNevada; SRP, Snake near the Olympic Peninsula.The seaward movement of the River Plain; W, Warner Mountains. Fault Zones: B, Brothers;E, Eu- gene-Denio;G, Garlock;SA, SanAndreas; WL, Walker Lane. The numberedlocations refer to paleomagneticsampling localities identi- fied in Table 3.

The oldestrocks in the Coast Range (Figure 2), the lower and middle Eocene Siletz River volcanics and the correlative lowerEocene Roseburg formation, consist of tholeiiticpillow basalts,intercalated marine volcanicsediments and subaerial alkalicbasalts [Snavely and Wagner,1963; Snavely et al., 1968; Baldwin, 1974, 1975]. The overlyingmiddle Eocene Tyee- Flournoyformations are marine turbidite sandstonesand siltstones[Snavely et al., 1964;Lovell, 1969;Baldwin, 1974]. Flow structuresand distinctivelithologies indicate that the main source of the middle Eocene sediments was to the south in the Klamath Mountains. Continued basaltic volcanism throughthe middle and late Eocene produced the Yachats basalt[Snavely and MacLeod, 1974] and the Tillamook Volcanic Series[Snavely et al., 1970;Beaulieu, 1971; Nelson and Shea- rer, 1969]. Informationabout the original tectonicsetting of the rocks of the CoastRange is providedby geologic,geochemical, and geophysicaldata. The lowerEocene Coast Range basalts are marine and are overlainby lower, middle and upper Eocene basaltswith majorelement chemistry most similar to that of Fig. 2. Generalizedgeotectonic map of Oregonand Washington. oceanicisland provinces [Snavely et al., 1968;Glassely, 1974]. Arrows indicate the mean paleomagneticdirections found for the re- Trace element data from the lower marine basalts [Loeschke, spectivegeologic units. The openarrow marksthe expecteddeclina- tion and the solid arrow the observed declination. The arrows have 1979]is supportiveof formationas either an oceanicisland or been adjustedso that the expecteddeclinations point due north. An oceanridge. The overlyingbasalts, the upperSiletz volcanics, easterlydirected solid arrow thereforeindicates a 90 ø clockwiserota- the Yachatsand the upperTillamook basalts are all subaerial tion. The arrowsare labeled by geologicnames or numberslisted in andtherefore appear to capan emergentisland complex. Cor- the rotationdata summaryof Table 3. A paleomagneticdirection for relative(Eocene) pillow basalts to the northin the Olympic the Coast Range EoceneInstrusions ((12), Table 3) and outcropsof the Miocene Intrusions (4). Oligocene Intrusions (5) and the Blue Peninsula[Baldwin, 1974; Cady, 1975]are associatedwith pe- Mountains (Figure 1) are not shownfor simplicity. Map is based on lagiclimestones [Garrison, 1973], consistent with an oceanic Hunttinget al. [1961], l,Fells and Peck [1961], Cohee[1962], Lawrence islandsetting. Snavely et al. [1968]have estimatedthe thick- [1976],and l,Falker [1977]. MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2955

236*

PinochlePeak • ••

Explanation Cedar Butte i.• TertiaryandQuaternary sediments .::,•Miocene basalt flows •1 Tillamookvolcanic series OPaleomag sample site •(Paleomag sample location of 8 sites K-At sample locality N .rosY,•' Tillamook'

o 5 IOKM • i I

Fig. 3. Generalizedgeologic map of the paleomagneticand K-Ar samplinglocations within the Tillamookvolcanics. The geologicboundaries and stratigraphyare from Warrenet al. [!945]. , southernend of the block was describedas a type of back-arc ismarine in originwhereas the youngest unit is distinctly sub- spreading.Although the two modelspredict distinctly differ- aerial.The lowerand middle units are submarinepillow ba- ent regionalgeologic histories for the Pacific Northwest, our salts,tuffs, breccia and interbedded sediments. The lower pil- geologicand geophysicalunderstanding of key areassuch as low basaltunit of the Tillamookis correlativewith pillow the Klamathsand easternOregon is still not completeenough basaltsof thelower Siletz River volcanics of centralOregon to determine whether either of the two models is correct. Later andthe Roseburg Formation of southernOregon, whereas the in this paper,we will return to the questionof paleogeogra- middleunit of theTillamook is probably the equivalent of the phic models. uppermostSiletz River, Yamhill, and Tyee Formationsto the south.Both in northernand centralcoastal Oregon these PALEOMAGNETIC DATA AND AGE DETERMINATIONS FROM THE TILLAMOOK VOLCANIC SERIES lowerto middleEocene marine formations are overlainby subaerial basalts. Although the Tillamook Volcanic Series have not been Theprimary subject of thisstudy was the uppermost unit of mappedin detail, $navelyet al. [1970] and Nelsonand Shearer the Tillamook Volcanic Series,which consistsof more than [1969]have described the generalgeologic history of this area. 1500m of subaerialbasalt flows, dikes, agglomerate, and pyro- The oldest of the three units of the Tillamook Volcanic Series clastics.Individual flows 6 to 7 m thicktypically have a rubbly

TABLE 1: K-Ar Geochronology of basalts from the Tillamook Volcanic Series, Oregon

Age* + 1 s.d. SampleNumber Radiogenic40At gmdio•enic40At x 100 -- & Description %K ( x 10-6 StPcc/g) Total 40At (n.y.)

Y027 flow (?) 0.758 1.3673 50.3 46.0 + 0.9 -- YO51 flow 0.690 1.1674 31.4 43.2 + 0.6 -- 1.2390 48.5 45.8 + O.5 -- Y106 flow (?) 1.283 2.1450 46.7 42.7 + 0.5 -- D78-T-3 flow 0.921 1.5862 51.6 43.9 + 0.5 -- D78-T-5 flow 0.470 0.8427 48.3 45.7 + 0.5 -- D78-T-14 dike 0.791 1.3250 68.2 42.7 + 0.4 --

Average 44.3 + 1.3 --

*Agescalculated from the following decay and abundance constants: X = 0.581x 10-10 yr-1 ; XB= 4.962x 10-10 yr-1 ; 40K/k=1.167 x 10 -4 mol/mol. Ages from the upper unit are defined by Y sample numbers and the lower unit by D78 sample numbers. 2956 MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

TILLAMOOK SITE YOI4 nique which has been usedsuccessfully to determinethe ages N of similarly altered Siletz River basaltsfurther to the south [Duncan, 1978].

Paleomagnetism A total of 4 to 9 sampleswere collectedfrom each of 20 flowsand 14 dikesfrom the uppermostunit and 2 flows from the lowerunit of the Tillamook VolcanicSeries (Figure 3). All sampleswere stepwisemagnetically cleaned using alternating peak fieldsof 10, 20, and 30 milliteslas.To determinestability Fig. 4. Paleomagneticfield directionsfor the sevensamples of site at higherfields, samples from five siteswere subjectedto step- Y014 correctedfor tectonictilt. The rocks have been magnetically wise demagnetizationto fields ranging from 10 to 160 milli- cleanedat a peak field of 20 milliteslas.All field directionsare of re- teslas.In all casesthe extremelystable component left after versedpolarity and have been reflectedthrough the origin to plot on the lower hemisphereof this stereographicpolar plot. demagnetizationin thesehigher fieldswas found to be parallel to the componentleft after routine cleaningin fieldsof 20 or 30 milliteslas(Figures 4 and 5). Stepwisethermal demagne- brecciatedbase and an oxidized, brecciated upper surface. tization to 650øC of one to three samplesfrom each of five They are similarin physicalappearance to the upperEocene sitesproduced similar results.The thermal demagnetization YachatsBasalt flows which cap the Eocenemarine sectionin trend was parallel or subparallelto the magneticvector re- the central Coast Range [Snavelyand MacLeod, 1974]. The maining after 20 or 30 millitesla alternatingfield demagneti- only structuraldeformation in the area sampledis a shallow zation,usually within 4 ø and in all casesless than 15ø (Figure regionaldip of 20ø or lessto the northwest,north, and north- 6). After themal demagnetizationto 600øC, all sampleshad east definingwhat appearsto be a structuralhigh centered only a small componentof magnetizationremaining (Figure somewhatto the south of our sampling area. Dike swarms 6) relative to the predemagnetizationintensity (NRM). This trendingwest to northwestoccur in the westernpart of the indicatesthat the reinanentmagnetization resides in magnetite area(Figure 3) near PinochlePeak and CedarButte, and are rather than hematite. associatedwith agglomerate,tuff, and breccia.We interpret The directionalstability and coercivityspectra of samples these dikes as feeders of the basalt flows. from a given site were analyzed to determine the optimum field for cleaning.In our final synthesis,site means from the Geochronology upperunit werenot includedif their 95%confidence limit was K-Ar datingwas done on threesamples from the upperunit greater than 7 ø becausethese data appeared to contribute and on three samplesfrom the lower unit of the Tillamook more noisethan signalto the final result(Table 2). Directions VolcanicSeries (Figure 3). The selectedsamples were fine to from 4 sites were rejected by this criterion. The 95% con- mediumgrained basalts. Although the sampleswere the fresh- fidence limits of the two sites from the lower unit were 7 ø and est available,all had undergonesome low temperaturealtera- 14ø . The site with the 14ø confidencelimit was not rejected tion, asindicated by the presenceof minor to modestamounts since this would result in the loss of half our data from the of calcite,zeolite, and smectite.Low temperaturealteration of lower unit. thistype may result in the lossof argonor the additionof po- A summaryof the magneticdata from the Tillamook vol- tassiumfrom seawater,the latter beingless likely in the upper canicscorrected for tectonictilt is shownin Table 2 and Fig- unit of subaerial basalts.Both effectswould produce K-Ar agesyounger than the true crystalizationages of the basalts. The rangeof K-Ar ageswithin both the upper and lower UP units is 43 to 46 m.y. (Table 1). The calculatedages of two specimensfrom one samplein the upperunit are 43 and 46 m.y. and sincethis spans the rangeof agesof all samples,the dating lacksthe accuracyrequired to determinethe duration of volcanism.The difference in ages of the two specimens from the one sampleis entirelydue to variationsin argoncon- tent of the two specimens,suggesting that the youngerage of 43 m.y. reflectsargon loss due to diffusionor alterationand the older age of 46 m.y. is more reliable. Comparingthe 43 and 46 m.y. agesof samplesY 106 and Y027 from the upper unit (Table 1), againit seemslikely that the youngerage reflectsargon loss and that the older age is the more reliable one. On the basisof thesedata 46 m.y. appearsto be the best 2.0xI0-3 EMU estimateof the minimum age of the upperunit. This ageis in s broadagreement with the paleontologicallyassigned age of upperEocene [Snavely et al., 1970].If thegeologic correlation Fig. 5. Zijderveld componentplot for sampleY018-1 of site Y014 of the lower unit with the lower Eocenepillow basaltsof the during stepwisealternating field demagnetization.The open boxes Siletz River volcanicsis correct, then the K-Ar ages of the representthe westand southcomponents and the dotsthe westand up componentsof magnetization.The demagnetizationfield values are lower unit are too young, suggestingthat substantialargon shownin milliteslas.The sampleexperienced the removal of only a lossor potassiumaddition has occurred. To testthis future ex- singlecomponent of magnetizationfrom the NRM up to peak fields perimentsare plannedusing the Arnø-Ar•9 method,a tech- of 160 milliteslas. Directions have been corrected for tectonic tilt. MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2957

U •) causean underlyingassumption is that bias introducedby geomagneticsecular variation has been removed by averaging over a time interval longer than the longestperiods present in the geomagneticfield, which are probably lessthan 0.1 m.y. Althoughthe K-Ar agesof the Tillamook lavasspan 3.3 m.y., oo the true ages probably span a shorter time interval, as dis- õo cussedearlier. The observationthat all the polaritiesfrom the upperunit are reversedis consistentwith all the lavashaving beenformed during one interval of constantreversed polarity, w 600 550 the one between45 and 48 m.y. being the mostprobable (Fig- • 650 ure 8). If this correlationwith the magnetictime scaleis correct, the upper Tillamook unit may span as much as 2.5 m.y. It seemsunlikely on the basisof two geologicconsiderations that our samplesspan only an extremely small time interval. First, the stratigraphicthickness of flows presentin the upper Tillamook sectionis about 1500 m and we sampledwidely in 7.0xi(• 3 this section,trying to avoid multiple sampling of the same EMU flow. Our 20 flows and 14 dikes probably sampled a large fractionof the historyof at leastone shieldvolcano, a typical lifetime of which is 0.5 to 1.0 million years. Second,the observation that the magneticdirections of the two normal polarity Fig. 6. Zijderveldcomponent plot for sampleY018-2 of site Y014 sitesfrom the lower unit are antiparallel to the magnetic di- duringstepwise thermal demagnetization. Sample Y018-2 is a second rections of the reversedsites of the upper unit constitutesa specimenfrom the same sample core as Y018-1 of Figure5. Theopen boxesrepresent the westand southcomponents and the dotsthe west positivereversal test and lends supportto the interpretation andup componentsof magnetization. The demagnetization temper- that the range of ageswas adequate to fully sample secular aturesin degreescentigrade are noted next to theappropriate points. variation. Thealternating field demagnetization trend of Y018-1is shown by the The amount of angular dispersionin the paleomagneticdi- plotted line segments. rectionsfurther supportsuch a conclusion.The amount of angular dispersionin virtual geomagneticpoles expectedfor a ure 7. The measuredfield directionsclearly show a significant site at this latitude is about 15ø [Cox. 1970]. Insufficient sam- clockwiserotation from the expected Eocene field direction pling of secularvariation would be expectedto yield a smaller for stableNorth America.Accounting for the dispersionin the angular dispersion.The angular dispersionobserved in the measuredand expectedfield direction,the clockwiserotation Tillamook volcanics is 22 ø, which is consistent with a full is found to be 46 ø _-4-13 ø with 95% confidence. sampling of geomagneticsecular variation with some addi- Correctionsfor tectonictilts up to 20ø were made using the tional dispersiondue to inaccuratetectonic corrections. measureddips of the lava flowsthemselves except at the few localitieswhere sedimentswere present.Errors are always in- TECTONIC INTERPRETATION troduced in such corrections because lava flows have an initial dip when they form which shouldnot be correctedfor, yet af- Amount of Rotation ter deformationthe measured dip is a compositeof the initial The magnetic field directionsfrom the Tillamook volcanics dip and a tectonictilt. On the Hawaiian Islands,which are a acquire tectonicsignificance through a comparisonwith the reasonableanalogue for the original environmentof the Tilla- expectedfield direction which we have computed using the mook volcanics,initial dips are generallyless than 5 ø to 10ø, apparentpolar wandercurve of North America [Irving, 1979]. so it seemslikely that only small errorsare introducedby us- The computed expected paleofield directions are listed in ing the presentobserved dips of the Tillamook volcanicsfor Table 3. As noted above, the observed Tillamook direction is tilt corrections.If this conclusionis wrong and if, to consider rotated 46ø from the expecteddirection. The significant and an extremecase, the presentdip of the flows is all initial dip, consistentclockwise rotation is clearly seen in the measured the result would be to increase the amount of inferred rotation versusthe computed paleofield directions for the entire Ore- by 19ø . The inclinationafter tilt correctionis 64ø , which com- gon Coast Range. The agreementbetween the computedand pareswell with the expectedinclination of 66ø for the middle measured magnetic inclinations indicates that the Coast Eocene(Table 3). Without tilt correctionthe mean inclination Range has not undergonesignificant north-south translation, is 78ø , which would imply an unreasonablylarge southward which suggestsrotation about a local pivot [Simpsonand Cox, translation of 2100 kin. Because most the tectonic corrections 1977]. reflect a regional northerly to northeasterlydip, there is no dramatic decreasein the amount of scatterin magnetic direc- NorthernBoundary of the RotatedBlock tionsafter tectoniccorrections. However, the precisionparam- The paleomagneticresults from the Tillamook Volcanics eter for the site virtual geomagneticpole increasesfrom 9 to clearly establishthat the rotated terrane of the southern Ore- 14 uponcorrecting for tilt and this increaseis significantat the gon Coast Range [Simpsonand Cox, 1977]extends northward 95% confidencelevel. We concludetherefore that correcting to the Columbia River at the Oregon-Washingtonborder. Al- for the measureddips of the flows gives the best estimate of thoughthe amountof rotationrecorded in the upper Eocene the mean Tillamook paleofielddirection. Tillamook volcanics is somewhat less than that recorded in In tectonicstudies like the present,the length of time repre- theolder formations of the southern Coast Range, we will see sentedby the units sampledis of particular importance be- later that this probably reflectsprogressive rotation with time 2958 MAGILLET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

TABLE2. Summaryof palcomagnetic data from the Tillamook Volcanic Series

Site FID D•,c. Inc. a95 K N Dema•Field UpperuniC: YOO1 F 253.3 -58.7 4.4 187 7 30 * YO08 F 219.2 -38.7 19.5 13 6 30 YO14 F 234.8 -41.6 3.3 326 7 20 Y021 F 233.6 -65.3 6.5 88 7 30 * Y028 F 229.9 -55.1 33.4 3 9 30 Y037 F 199.1 -63.5 6.2 80 8 20 Y045 D 231.7 -54.0 5.1 176 6 30 Y051 F 218.6 -50.4 ' 6.3 77 8 30 Y059 F 215.8 -47.5 4.2 259 6 30 Y065 F 176.2 œ64.7 4.1 266 6 30 Y071 F 214.7 -43.2 5.3 163 6 20 Y077 F 231.3 -53.4 4.6 173 7 30 Y081 F 227.1 -66.0 2.6 649 6 30 Y087 F 290.6 -68.1 6.7 101 6 .30 * Y093 F 250.3 -48.9 15.0 21 6 30 YlO1 F 152.1 -81.3 7.0 92 6 30 Y107 F 156.0 -75.1 4.2 491 4 30 Ylll U 212.8 -61.3 4.8 253 5 30 Yl16 U 210.0 -52.3 2.8 722 5 30 Y126 U 209.6 -60.5 4.6 406 4 30 Y130 U 198.1 -59.1 5.0 335 4 30 Y134 U 200.0 -59.8 3.5 693 4 30 Y138 U 203.1 -55.7 2.4 1517 4 30 Y142 U 211.9 -49.0 5.8 251 4 30 Y146 U 213.3 -55.1 4.9 347 4 30 Y151 U 218.8 -73.4 5.4 154 6 30 Y157 U 219.3 -72.6 3.0 501 6 20 Y163 U 223.9 -70.4 4.5 420 4 20 Y169 U 231.7 -71.8 5.5 282 4 20 Y172 F 202.7 -74.1 5.9 128 6 30 Y179 F 138.3 -60.0 4.2 251 6 20 * Y185 F 223.6 -48.3 17.5 16 6 30 Y191 F 234.6 -60.8 6.5 108 6 30 Y197 U 216.0 -43.8 4.6 393 4 30 Lower Unit: T3 F 49.9 68.4 6.9 178 4 10 T4 F 42.4 63.0 14.3 30 5 20

F/Emflow or dike, Dec.=declination, Inc.=inclination, a95-radius of 95%confidence in degrees, K=precisionparameter, N=numberof samples, DemagField=peak demagnetization field in milliteslas. The sites marked by an (*) were not included in the final synthesis due to a high a95 indicating poor grouping of the directional data.

rather than differential rotation between the southern and CoastRange (Figure 9). The smoothgravity contours in west- northernOregon Coast Ranges. This is in accordwith a con- ernOregon do not display the offsets or shortwavelength fea- siderablebody of geologicobservation which argues against turesthat are commonlyassociated with majortectonic struc- the differentialrotation of small independentblocks within tures.The availablegeophysical and geologicdata are thus theOregon Coast Range. Of particularimportance are: (1) the consistentwith the OregonCoast Range having rotated as one continuityof outcropof middleEocene Tyee and Flournoy quasi-rigidblock extending from the KlamathMountains to formationsalong the entireCoast Range which define it asa the Columbia River. singleEocene basin rather than a tectoniccomposite of sev- Paleomagneticstudies to the northreveal a morecomplex eralbasins, (2) theabsence of strongdeformation in thesefor- pattern.In thelower and middle Eocene Willipa Hills basalts mations,and (3) theinternal consistency of current directions directlynorth of theColumbia River, the amount of clockwise recordedin the turbidites of this formation throughout the rotationis approximately22 ø basedon preliminaryresults CoastRange [Snavely et at, 1964],including turbidites in the [Wellsand Coe, 1979]. This rotation is significantlyless than vicinityof the Tillamookvolcanics. The structuralcontinuity that recorded in the Tillamook or Siletz volcanics, indicating suggestedby theseobservations is further supported by the that the coherentrotated block doesnot extend farther to the presenceof a pronouncedgravity high [Brornery and $navely, north.Paleomagnetic results from the Black Hills basalts [Glo- 1964]extending from the northern to thesouthern part of the bermanand Beck, 1979], north of the Willipa Hills and south- MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2959

TILLAMOOK VOLCANIC SERIES to have rotated 28 ø +_ 12ø, so that TillROobl½•-• 18ø _+ 18ø. If we N accountfor the uncertainty in age of the Goble volcanics,as seemsappropriate, this differencebecomes 18 ø _+21 o, which is not statisticallysignificant. If future researchestablishes a significantdifference between the Tillamook and Goble rotations, this might be due to the two regionsbeing on separate microplates,as Beck and Burr [1979] suggested,or it might be due to their being of differentages and having formed on one microplate undergoing rotation during the Eocene. The former would be favored if the Goble volcanics are predominantly of an age near 45 m.y., the latter if the age of the Goble volcanics is predominantly 32 m.y. The presently known paleomagneticand radiometric data thus do not re- quire differentialrotation betweenthe Tillamook and Goble areas,but they also do not precludeit. If the two areasare on separatemicroplates, the presentstudy narrows the range of possiblelocations of the tectonic boundary between the two (Figure 2) to a narrow zone near the Columbia River. Fig. 7. Paleomagneticfield directionsfor the Tillamook volcanics A plot of rotation versusage (Figure 10) with all the data (Table 2) correctedfor tectonictilt. The directionsof normal polarity from the OregonCoast Range, Cascades,and Goble volcanics are plottedas filled boxesand reversedpolarity as open boxes.Direc- can be fit by severalline segments.To the eye it might appear tions of reversedpolarity have been reflectedthrough the origin to that the data in Figure 10 could be fit by a simple straightline plot on the lower hemisphereof this stereographicplot. The mean di- rectionwas computedfrom the mean pole of the 32 site VGP's. The correspondingto rotation at a constantrate. To test statisti- circlesabout the expectedand observedmeans are the 95% co•-.fidence cally whether the data are simply linear we fit the paleomag- circles(a95). netic rotation and age data with first, secondand third order polynomialsweighting the data by the 95% confidencelimits eastof the Olympic Peninsula,show a 40 degreeclockwise ro- of the respectiverotations. The third order polynomial shown tation. Moreover, the gravity anomalies north of the in the inset of Figure 10 fits the data significantlybetter than ColumbiaRiver (Figure 9) have irregularitieswith sharpgra- the linear (95% confidence)and secondorder curve (50% con- dients suggestiveof discretetectonic blocks [Fox, 1981].The fidence)as determinedby an F testof the chi squarestatistics. emergingpicture is one of different amountsof rotation in dif- This analysisdoes not prove that the third order polynomial ferent tectonic blocks north of the Columbia River contrasted fit is correct but it does indicate that the best fit curve is more with the coherent rotations of the one block to the south. We complicatedthan a simplelinear trend correspondingto rota- will return to this point later. tion at a constantrate. The heavy line of Figure 10 is similar to the fitted polynomial but is a more reasonablecurve incor- Timingof Rotations poratingour interpretationof the regionalgeology described The amount of rotation recorded in the formations of the in the followingsection. The chi squareof the interpreted CoastRange (Table 3) decreaseswith decreasingage. To as- curveis slightlyless than that of the bestfit third order curve sessthe uncertaintyin thesechanges, we note that if the rota- and thereforemust be consideredan equally good fit to the tion and its uncertaintyfrom one formationare RA --+ARA and data. The earlier rotation from 50 to 42 m.y. with rotation at a that from a secondformation is RB --+ARB, then the difference rate of 5 ø to 6 ø per million years is required by the trend in •R• _+A•R• in rotation betweenthe two formationsis the rotations of the Siletz River, Tyee-Flournoy, Tillamook and Goble basalts described above. The later rotation as de- •• = R• - R• a•• = [aR•: + aR•:] '/: terminedfrom the CoastRange Miocene basalts[Simpson and where AR values are 95% confidence intervals. If the two for- Cox, 1977; Beck and Plumley, 1980], the Oligocene Coast mations are from the same region but are of different ages, Range dikes and sills [Clark, 1969;Beck and Plumley, 1980] then a value of •R• > A•R• would indicate that significantro- and paleomagneticresults from the Western Cascades[Bates tation had occurredbetween the agesof the two formations. et al., 1979; Bates and Beck, 1981; Magill and Cox, 1980] re- Similarly,if the two formationsare of the sameage but from quire about 30ø of post-Oligoceneclockwise rotation of the different regions,a value of ARB> A•R• would indicate that CoastRange and Cascades[Magill and Cox, 1980].Although the two samplingregions had undergonedifferential rotation. the timing of this secondrotational period is not well con- Applyingthis to the rotationsinferred for the SiletzRiver vol- strained by the paleomagneticdata, our preferred inter- canicsand the Tillamook volcanicsyields the result si,etzRTi,= pretationis one of rotation from 20 m.y. to the presentat an 28ø _+18 ø indicatingthat the larger amountof rotationof the averagerate of 1.5 degreesper million years. older Siletz River unit is significantat the 95% confidence level. The rotation of the Tyee is greater than that of the youngerTillamook by the angle23 o +_21 o, againa significant PALEOGEoGRAPHIC INTERPRETATION difference.If our interpretationis correctthat thesesampling Introduction localitiesare on the samequasi-rigid Coast Range microplate, these resultsestablish that the microplate underwent signifi- Theseresults (Figure 10), combinedwith our knowledgeof cant rotation during the Eocene. the regionalgeology, suggest to us that the rotation of the Or- The Goble volcanicsimmediately to the northeastwith ages egonCoast Range took placein two phases,each due to a dif- in the range45 to 32 m.y. were found by Beck and Burr [1979] ferentgeologic process. Phase I of the rotation occurreddur- 2960 MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

Table 3: Compilationof regional paleomagneticresults from Oregon,Washington and the Sierra Nevadaregions

Observed Data

Nameand Reference Pole Field Direction Expected Field Flattening Rotation Oregon-WashingtonRe, ion Lat. Lon. a9_•5 _N _• Inc. Dec. Age Inc. Dec. F A_•F R AR Miocene Basalts (1) 77.0 80.0 29.0 8 4.6 52.0 354.0 20 63.8 356.0 11.8 31.3 -2.0 35.8

ColumbiaR. BasaltN (2) 87.6 203.2 7.0 6 91.6 65.7 357.7 20 64.8 355.8 -0.9 6.6 1.9 12.8 Columbia R. Basalt S (3) 84.2 315.8 5.9 14 45.9 62.8 7.8 20 62.4 356.0 -0.4 6.3 11.8 10.8

MioceneIntrusions (4) ...... 9.6* 8 .... 67.5 29.5 20 63.5 357.5 -4.0 .... 32.0 26.8

California Cascades (5) 78.7 292.9 8.9 24 12.0 65.5 13.8 25 61.5 356.0 -4.0 7.6 17.8 14.4 Western Cascades (6) 72.2 280.0 10.9 24 8.3 70.1 20.7 25 62.1 356.0 -8.0 8.1 24.7 19.7

Oligocene Intrusions (7) ...... 8.4'14 .... 56.0 39.0 30 63.5 354.5 7.5 .... 44.5 16.7

•arys Peak (8) 63.0 8.0 8.0 26 13.5 42.0 22.0 30 64.0 354.6 22.0 11.3 27.4 11.4

Ohanapecosh (9) 71.3 320.7 4.4*34 .... 63.6 26.8 35 65.3 352.2 1.7 .... 34.6 12.4

Yachats Basalt (10) 58.0 308.0 20.0 8 8.6 64.0 46.0 40 64.4 350.3 0.4 16.3 54.7 30.2

Gable Volcanics (11) 75.5 345.5 5.5 37 17.7 57.5 18.5 40 65.4 350.1 7.9 6.4 28.4 12.4

EoceneIntrusions (12) ...... 12.5'10 .... 62.0, 44.5 40 64.5 353.0 2.5 .... 51.5 28.4

Clarno Formation (13) 80.5 274.0 11.3 13 14.4 67.5 9.5 45 65.0 349.1 2.5 9.0 20.4 19.4

Tillamook (1•) 65.5 312.7 7.0 32 14.0 64.2 35.5 45 66.2 349.1 2.0 6.7 46.4 12.7

Tyee-Flournoy (15) 49.0 305.0 11.0 40 5.2 63.0 59.0 45 65.3 349.3 2.3 9.6 69.7 17.2

Willipa Hills (16) ...... 10.0 50 67.6 347.8 ...... 22.2 ....

Black Hills (17) 69.4 322.3 5.2*35 .... 63.2 29.6 50 67.9 347.6 4.7 .... 42.0 14.0

Siletz River Volt. (18) 45.0 307.0 8.0 33 10.7 61.0 63.0 55 67.4 346.2 6.4 8.4 76.8 15.7

Sierra Nevada Region

Great Valley (19) 72.0 181.0 5.4 17 44.0 65.7 337.4 70 67.6 339.0 1.9 6.5 -1.6 13.7 CretaceousGranites (20) 68.8 195.2 9.6 14 18.0 68.1 336.0 85 67.0 333.6' -1.1 8.9 2.4 19.9 Bucks Batholith (21) 57.6 194.8 7.9 9 43.7 72.1 317.1 140 60.4 333.6 -11.7 12.4 -16,5 22.9 ReeveFormation (22) 73.0 139.0 9.0*40 .... 54.8 339.3 150, 53.9 341.1 -0.9 ..... 1.8 17.3

as5 = radius of 95•. confidence about the meanpole, an (*) denotes an ass of the mean field direction where the mean pole as5

was not available, N = number of sites, • = precision parameter, Inc. = inclination, Dec. - declination, Age = age of the pole

ofIrving (]979) used tocompute theexpected fielddirection ' F9= Inc. e•pec•ea ___ - Inc.oDserveo•_ . AF - [Alibs+ AI2exp] • where AI = 2•s5/1+ 3cos2p,R = Dec.ooservea_ _ - Dec.expec t:ea' . _ AR- [ADads~. + ADexp• ]'• whereAD - sin-•[sinass/sinp],p = colatitude' or AD= sin-l[sinc•ss/cos(Inc.)]if anas5 for themean field directionis onlyavailable. The Columbia R. Basalt N data is an average of stes SRC, LC, A, GR, M, and I of Watkins and Baski (1974), Columbia R. Basalt S data is an average of sites BC, CC, PG, OR, SMof Watkins and Baski (1974) and SA, SB, SC, SD, SE, SF, SG, Slt, SI of Watkins (1965). Other references are (1), (1•), (15), (18)Simpson andCox (1977); (4), (7), (12)Beck and Plumley (1980); (5) Beck(1962); (6) Magilland Cox (1980); (8) Clark (1969); (9) Bates and Beck (1981); (11) Beck and Burr (1979); (13) Beck et al. (1978); (14) this study; (16) Wells and Coe (1979); (17) Globermanand Beck (1979); (19) Mankinen(1978); (20) Gromaa•and Merrill (1965); (21) Gromaa•et al. (1967); (22) Hannah and Verosub (1979). ing the Eocene. The underlying geologic processwas the review the geologicinformation that points toward a two breakup and clockwiserotation about a southernpivot point phaserotation of this type. of a narrow offshoreEocene oceanic plate similar to the present Juan de Fuca plate. This is essentiallymodel 1 of Simpson PhaseI: PivotingSubduction of Seafloor and Cox [1977].Phase II of the rotation took place at a slower Menard [1978] proposedthat as the East Pacific rise ap- rate in post Eocenetime and likely was predominantly a post proachedNorth Americaat the beginningof the Tertiary,the Oligocenetectonic event. The underlyinggeologic process was narrowing Farallon plate [Byrne, 1978] broke up into frag- differential extensionin the Basin and Range province, with ments analogousto the presentJuan de Fuca and Cocos greaterextension toward the southernend of the Coast Range plates. To understandMenard's model, considera wedge producing clockwiserotation of the Coast Range and Kla- shapedplate boundedby a ridge on the westand a trench on math Mountains about a northern pivot point. This is essen- the east.If the ridge convergestoward the trench at the north tiaUy model 2 of Simpsonand Cox [1977] and is vaguelysimi- end of the plate, the lithosphereadjacent to the trenchwill be lar to the oroclinal model of Heptonstall [1977]. At the young,hot, and light and will not subducteasilyrain Men- beginningof PhaseI the subductionzone was located to the ard's words,it will be a droguethat slowsthe subductionof east of the seafloorupon which Eocene strata that now com- the plate at its northernend. The lithospherebeing subducted prise the Coast Range were forming. By the end of Phase I at the southend of the plate will be older, cooler,thicker and and prior to Phase I! the subductionzone had completely denserand will subductmore readily causingthe plate frag- shiftedto the west and the strata of the presentCoast Range ment to pivot in a counterclockwisesense about a northern had becomepart of the North American plate. We will now pivot point. Menard [1978] proposesthat this occurredbe- MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2961

Reversal K-At are consistentwith the interpretationthat the Eocenebasalts Time Epoch that now form the CoastRange were a seamountprovince. Ages The regionalgeology appears to indicatethat this evolving seamountprovince collided with North Americaduring the earlyEocene and then became trapped between two active 4O trenches.During the early Eocene,the Kula-Pacificridge ceasedspreading, which resulted in a reorganizationof the Kula-Farallon and Pacific-Farallonridges [Byrne, 1979]. This changein Farallonplate motion appears to havebeen accompaniedby the middleEocene formation of a secondsub- ductionzone seawardof the evolvingCoast Range seamounts, possiblyalong an old Kula-Farallontransform. The seam- 45 ountswere then trapped by two activetrenches, one east and anotherto the westof the seamountprovince. Clockwise rotation resultedas this trappedwedge of oceaniccrust was par- tiallyconsumed by theeastern trench. We will nowelaborate on thismodel and presentthe relevantgeologic evidence. Evidencefor the early Eocenecollision of the seamount provincewith North America is foundin thegeology of the 5O southernOregon Coast Range. At the extremesouthern end of therange the presence of a subductionzone is recordedin theearly Eocene Roseburg Formation [Baldwin, 1974, 1975]. The latter includes submarine basalts and marine sediments that are intenselythrusted and isoclinallyfolded, with late Cretaceoussediments being folded into the Roseburgduring

(M.Y.) Complete Bouguer • NORMAL Gravity Anomaly ]--] RE VERSED Fig. 8. Geomagnetictimescale of Nessel al. [1980] with the K-Ar .2 datesfrom the Tillamook volcanicsupper unit (dots)and lower unit (boxes).The error bars are +_1 s.d. tween50 and35 m.y.B.P. and againbetween 33 m.y.B.P. and thepresent, fragmenting the Farallon plate into the hypotheti- cal Vancouver,Cocos and Nazca plates.An important accom- panimentof Menard'spivoting subduction is active volcanism alongleaky transforms and elsewhere as the plate changes its directionof motion.Menard [1978] points out that the present smallJuan de Fuca and Cocosplates which appearto be piv- otingare, in fact, studdedwith seamounts. If the northernFarallon plate did fragmentduring the early Eoceneaccording to a Menardtype model, the seaflooradja- cent to Eocene North America should have been a seamount provincesimilar to the presentJuan de Fuca and Cocos plates.The geologicand geochemicalevidence quite clearly suggeststhat it was[Dickinson, 1976]. The majorelement geo- chemistryof the lowerand middleEocene Siletz River Vol- canicsand the upperEocene Yachats basalt very closelyre- sembles that of oceanic islands like Hawaii and is quite distinctfrom that of arc basalts.The high TiO2, and the high Na20/K20 ratio, the low SiO2,and the Fe/(Fe + MgO) ratio relativeto SiO2 [Snavelyet al., 1968;Snavely and MacLeod, 1974;Jakes and Gill, 1970]are especiallydiagnostic. More- over,the CaO concentrationsare too low relativeto SiO2for the basaltsto be of island arc origin (W. R. Dickinson, per- I I I00 KM sonalcommunication, 1980). The traceelement geochemistry of the lower Eocene marine basalts [Loeschke, 1979] also Contour Interval 20 mgals showsthese basalts to be unlike island arc rocks. The geo- Fig. 9. CompleteBouguer gravity anomaly map of westernOregon chemistry,together with the geologicevidence that a sub- and Washingtonfrom Berg and Thiruvathukal[1967] and Boniniet al. stantialfraction of the lowerand upperEocene basalts formed [1974].The dotted lines mark the coastaloutline and the geologic as subaerialflows on oceanicislands [Snavely et al., 1968] all boundariesof the Klamaths and Cascades(see Figure 1). 2962 MAGILL ET AL.: TECTONICROTATION, TILLAMOOK VOLCANIC SERIES

ROTATION in southwesternMontana and Wyoming into Oligocenetime 0 ø 30 ø 60 ø 90 ø [Armstrong,1978]. The location of the lower Eoceneeastern trench shown in Figure 11 therefore is constrainedby the Roseburgdeformation, the trend of the Challis-Absarokavol- canicsand evidenceof subductionon Vancouver Island [Mul- ß 1 M•oceneBasalts ler, 1977; Davis, 1977].

..20 C desMiocene Intrusions During the latter part of this period of active Challis-Absa- roka volcanism, calc-alkaline volcanism began well to the :•)'•30 ', i.L_T_,•! __,.,• WI OI,goceneCascodes Intrus,ons MorysPeak 1•1 •' west in the Western Cascades,pointing to a substantialwest- • v,,,.,, ,,., •,VOl, • ward shift of the subductionzone. Five observationssuggest r,D •oble"'L•=•="•"::t'• X lYachats that both the eastern and western subduction zones were ac- Tyee- <•4.0 'i Tilla•••_•IEocene ,nt tive during the middle and upper Eocene.The first is that as- suminga reasonabledelay time of 4 m.y. betweenthe cessa- ', SRV tion of subduction and cessation of related volcanism, the 6050 :• IFløurnøy Challis-Absaroka volcanism likely represents actual subduction until approximately40 m.y.B.P. (late Eocene). Sec- ond, the paleomagneticdata establishesthat the seafloorthat now forms the Oregon Coast Range was rotating during the middle and early upper Eocene. This rotation is consistent Fig. 10. Rotationversus age for geologicunits in theCascades and OregonCoast Range. The data shownare describedin Table 3. The with the presenceof a subductionzone to the eastthrough late rotation error bars are the AR values listed in Table 3. The inset shows Eocene time and requiresthat the rate of subductionwas a weightedleast-squares fit of the data to a third order polynomial greatestat the northend of the subductionzone. The third ob- (dottedcurve) superimposed on our suggestedcurve (solid) which is servation is the occurrence of late middle Eocene and late Eo- constrainedby our interpretationof the regional geology. cene basalts,andesites and pyroelastics(45 to 31 m.y.) in the Western Cascadesof Washington [see Hammond, 1979] and an early Eoceneepisode of intensedeformation. We interpret of late Eoceneandesite flows and pyroelasticsin the Western this deformation to mark a subduction zone. Beaulieu [1971, Cascadesof southernOregon [Peck et al., 1964]. Similar late p. 30] notesthat the Roseburg(lower Umpqua) has several Eocene(40 m.y.) andesiteflows also are found in the Warner featuresof a melange.Strong northwestwardvergenee of the Mountains(see Figure 1) of extremenorthern California [Ax- folds in the lower Eocene strata of southernOregon is sugges- elrod, 1966] and in the Drake Peak area [Wells, 1979] of tive of underthrustingfrom the northwest [Baldwin, 1964]. south-centralOregon (Figure 1). If we are correct in inter- Some Roseburgstrata were tectonicallyemplaced within the pretingthese rocks as early Cascadearc deposits,they indicate neighboringJurassic Otter Point Formation of the Klamath that a western subduction zone must have begun in the terrane and the Roseburgcontains exotic blocks of schist, middle Eocene in order to produce late Eocene arc rocks. greenstoneand conglomerate,all derivedfrom the Klamaths. This indicates the subduction zone at the southern end of the CoastRange was located near shorein the early Eocene.Sub- duction at this locality stoppedabruptly at or near the end of the lower Eocene.The gently folded middle Eoceneturbidites and conglomeratesof the Tyee and Flournoy formations which overliethe Roseburgformation with an angular uncon- formity displaynone of the thrustingand telescopingof strata that characterizedthe underlying Roseburg formation. The simplestexplanation is a westwardjump of the subduction zone toward the end of the lower Eocene. The locationof the lower Eocenetrench in the region north of Roseburgis uncertainon the basisof the available geologic information. Although the Siletz River Volcanics are correl- tive with the RoseburgFormation, the former have not undergone the intensedeformation of the latter, indicatingthat the

location of the early Tertiary subductionzone was probably 37ON considerablyeastward of the northern part of the present Coast Range. The presumedsuture betweenNorth America and the adjacentoceanic plate could be locatedbeneath post EoceneTertiary coveralmost anywhere in the broadband be- 50 M.YB.P. tween the Coast Range and the nearest Mesozoic rock out- cropsseveral hundred kilometersto the east. 300 KM • Additional evidence for the existenceof an early Tertiary subductionzone to the eastof the Coast Range is provided by the calcalkalineEocene Challis and Absarokavolcanics (50 to Fig. 11. Positionof the primaryblocks of the presentwestern U.S. continentalmargin at the beginningof PhaseI rotation. 50 m.y.B.P. 36 m.y.B.P.) of Montana, Wyoming, and Idaho [Armstrong, SeeFigure 1 for the present-dayposition of theseblocks. The rotation 1974, 1975, 1978;Snyder et al., 1976]. This volcanismdimin- pivot at the southernend of the Coast Range block is shownby a tri- ishedgreatly at the end of the Eocene,persisting only weakly angle. MAGILL ET AL..' TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2963

fects:(1) the subductingFarallon plate was young,thin, and hot and thereforerelatively buoyant, (2) the low densityba- salticseamounts riding on the Farallon plate accentuatedthe eurQSlQn buoyancyof the plate [Vogt, 1976], (3) the Farallon plate •>hlllpp,••:• k Plote Plote • . 1../• • . • changedits motion at about 55 to 50 m.y.B.P. in responseto • Ryukyu.,.,•, the cessationof spreadingat the Kula-Pacificridge [Byrne, 1979].We proposethat becauseof its buoyancythe eastern part of the Farallon plate becamepartially coupledto North America near the middle of the Eocene. The combined effect P;•ft• Izu-Bon,••.• of thispartial couplingand the changein motion of the Faral- .... Trench• • Ph- Eu lon plate forced the initiation of the Cascadetrench to the I I Pole I000 KM west. 500 KM A modern analogue of the proposed rotation and subductionzone geometrywould be the pieceof seafloor(Philip- Fig. 12. A comparisonof PhaseI rotation(see Figure 11) with a modern-dayanalogue from the westernPacific. The Philippineplate pine plate) bounded by the active Ryukyu and Izu-Bonin and adjacentregions have been rotated to the Oregon-Washington trenchesin the western Pacific. Present-daymotion of the area and plottedwith the samemap projectionto minimize distortion. Philippineplate relative to Eurasia [Seno, 1977]indicates that The Philippine-Eurasiarelative motion pole of Seno[1977] is shown the Philippine plate is rotating clockwiseinto the Ryukyu by a solid box (Ph-Eu). trenchabout a local pivot (Figure 12) similar to the proposed Eocenerotation of theOregon Coast Range. Uyeda and Ben- Fourth, we note that middle Eocene sedimentsof the southern Avraharn[1972] and Matsuda [1979] proposethat the seaward CoastRange received andesitic pumice blocks and mud flow Izu-Bonin trench formed as a result of an Eocenechange in debris from an arc located to the east of the basin [Snavely Pacific plate motion. This converteda former transform to a and Wagner, 1963; P. Snavely, personal communication, trench,thereby trapping a wedgeshaped piece of seafloor.We 1980].This arc waslikely the beginningwestern Cascade arc. speculatethat a similar sequenceof eventsseems possible in Fifth, Snavelyet al. [1980]interpret seismic profiles of the cen- the Oregonregion. A northeasterlytrend of the early Eocene tral Oregoncontinental margin as showingstrike-slip faulting Kula-Farallon ridge [Byrne, 1979] implies that fracture zones during late middle to early late Eocenetime. They further associatedwith this ridge would strike southeasterly.The suggestthat middle Eocene underthrustingmay have oc- southeasterlystrike of the westerntrench shownin Figure 11 curredon this margin westof the Coast Range. We and Sna- may be the resultof a Farallon fracture zone convertingto a velyet al. [1980]interpret this possibleunderthrusting as that subduction zone. This conversion would be associated with associated with a subduction zone which we shall label the the 55 to 50 m.y.B.P. reorganizationof the Kula-Pacific-Fa- Cascade trench. These five observationssuggest that the railon ridgesnoted earlier. Challis-Absaroka and Cascade subduction zones were both active during the period 50 to 40 m.y.B.P. In addition to these observations, the undeformed character of the middle Eocenemarine sedimentsof the Coast Range suggeststhat the Cascadetrench must have been locatedto the westof the CoastRange. The picture that emergesduring the middle and upper Eoceneis one of a wedgeshaped piece _.45't of seafloorbounded on the eastand west by subductionzones narrowingto the southand rotating about a southernpivot. The pivotpoint was near the presentlocation of the Roseburg formation,the volcanicsand sedimentsof which jammed the trench at its southern end. Rotation and continued volcanism in the CoastRange occurredas the wedgeshaped plate was consumedin the easterntrench and mildly deformed.Consid- eringthe sizeof thiswedge shaped oceanic plate and a reason- able subductionvelocity of 5 cm/yr, the PhaseI rotation of 40ø to 50ø could have spannedas little as 5 to 6 m.y. _37øN By the end of the Eocenethe subductionzone to the east wasno longeractive and the marineoceanic rocks of the Ore- gon CoastRange had becomepart of the North American 20 MYB. P. plate.This suturingof the CoastRange to North Americaap- L I pearsto be the possiblecause of the southeasterlycompres- :500 KM sional orogenyin the lower Clarno unit in central Oregon [Taylor, 1977].K-Ar dating by Swansonand Robinson[1968] seemsto placethe orogenyduring the period of 42 to 44 m.y. Fig. 13. Positionof the primaryblocks of the presentwestern U.S. B.P. (J. B. Noblett, personalcommunication, 1980) in good continentalmargin at the beginningof PhaseII rotation, 20 m.y.B.P. agreementwith the termination of Challis-Absarokavol- SeeFigure I for the present-dayposition of theseblocks. The rotation pivot at the northern end of the Coast Range block is shown by a tri- canism and our estimate of when Phase I rotation ended. angle. The amounts of Basin and Range extensionimplied by the The formation of the Cascadetrench and subsequentrota- model are marked by arrows.The Mojave block (MB) and Garlock tion of seafloorto the eastwas likely the result of severalef- Fault (G) are shown in their present-dayposition. 2964 MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

expressedin radiansis relatedto the lengthL of the rotated block by

s=LxR

CR The paleomagneticdata indicate that the rotated block extends from the Klamaths to the Columbia River, a distance L -- 670 km. The angle R can be constrainedby the paleomagnetic data. A weighted averageof all the rotations observed for Cascadeand CoastRange rocksless than 40 m.y. old (post PhaseI) yieldsR -- 30ø _ 10ø (95%confidence). The resulting westward translation of the Sierra Nevada-Klamath hinge 4Oø point is s -- 340 _+ 110 km. Although somewhaton the high side, this is within the range of values for the amount of late Cenozoic Basin and Range extensionproposed by different researchers[Hamilton and Meyers, 1966; Proffett, 1971; Thompson,1972; Elston, 1976, 1978; Hamilton, 1978, 1980; Stewart, 1978;Zoback and Thompson,1978]. Concerningthe questionof timing of the late Tertiary rotation, the paleomagneticdata are neither abundant nor precise enoughto determinewhether the rotation occurredduring the entireinterval from 40 m.y.B.P. to the presentinterval or only during part of it. However regional geology provides additional constraints since the westward motion of the Coast Range, Klamaths and Sierra Nevada was undoubtedly associated with extensionin regionsimmediately to the east. It is Fig. 14. Steeply dipping faults of western North America active not until mid Miocene time that we find clear evidence for during the past 10-15 m.y. for which someQuaternary movement is broad extensionin the region reachingfrom Arizona to cen- suspected.Longer faults are generally strike-slip faults and shorter ones normal faults. Our proposedtectonic blocks are outlined with tral Oregon.During this period, about 20 m.y.B.P., the Basin heavy lines:CR, CoastRange; K, Klamaths;GV, Great Valley; SN, and Range Provincebegan its most important phaseof devel- Sierra Nevada. Faults are from Eaton [1980]. opment with the broad occurrenceof extensional faulting coupledwith a pervasiveshift to basalticvolcanism [Snyder et al., 1976;Stewart, 1978].Although late Eoceneand Oligocene PhaseII: DifferentialExtension basinsof possibleextensional origin occur in areas of Wash- Our PhaseII rotationis similarto model two of Simpson ington [Whetten, 1976; Davis, 1977; Fox et al., 1977], Mon- and Cox [1977]in which rotationwas produced about a north- tana, Wyoming [Armstrong,1978], Nevada [Axelrocl, 1966b] em pivot, but there are distinct differencesbetween the two and Arizona, New Mexico [Eaton, 1979; Livaccari, 1979], models. One such difference involves the role of the Kla- thesebasins are regionallyrestricted (except in Arizona and maths in the rotation. Following an earlier suggestionof New Mexico) and are not associatedwith widely distributed Hamilton [1969], Simpsonand Cox [1977] proposedthat the basalticvolcanism that characterizesthe much greater Mio- Klamathsmoved westward along wi, th the southernend of the cene extension.The most difficult part of the geologicrecord Oregon Coast Range, rifting away from the Sierra Nevada to interpret is the widespreadOligocene ignimbrites of Ne- during the early Tertiary. However, the widespreadoccur- vada and Utah [Snyder et al., 1976], the continuity of which renceof upper Cretaceousmarine sedimentsin the gap be- suggeststhat the Oligocenetopography did not include deep tween the Klamaths and the Sierra Nevada is difficult to rec- extensionalbasins [Burke and Mckee, 1979]. However, Hamil- oncile with post-Cretaceous rifting and, in fact, most ton and Meyers [1966], Elston [1978] and Hamilton [1980] pro- geologistsworking in thisarea have concluded that the rifting, posethat the extrusiveignimbrites were associatedwith intru- if real, occurredduring the Mesozoic[Schweikert, 1976; Blake sive activity deep in the crust which resulted in significant andJones, 1977; Irwin, 1977].Therefore it seemsunlikely that extension.We conclude,based on the overall geologicrecord, the rifting of the Klamaths away from the Sierra is tectoni- that PhaseII rotation likely began about 20 m.y.B.P., coin- callyassociated with the rotationof the OregonCoast Range. ciding with mid-Miocene extensionin the Basin and Range. In our PhaseII rotationthe southernCoast Range, the Kla- However, the possibilityof some regional extensionduring maths and the northern end of the Sierra Nevada all move the Oligocenein Arizona and New Mexico and in association westwardtogether, retaining their east-westspacing relative to with the Nevada-Utah ignimbrites cannot be ruled out, so each other (Figure 13). In the terminology of Heptonstall someamount of OligocenePhase II rotation is possible. [1977],these blocks are movedas 'linked plates'free to rotate Our model for late Cenozoic clockwise rotation of the Coast about hypotheticalhinges at their pointsof contact.As these Range, Klamaths and Cascadesrequires that rifting was dominantlycoherent units rotated and drifted westward,the greaterin southernOregon than northern Oregon. Three ob- Klamath-SierraNevada junction behaved as a hingepoint ac- servationssuggest that this was the case.First, the distribution commodatingdifferential motion between the Sierra Nevada of late Cenozoic extensionalfaults delineating the Basin and and the Klamaths-CoastRange to the north. The amount of Range province(Figure 14) suggeststhat extensionalminima westwardtranslation of the Sierra Nevada-Klamathhinge existin northernOregon and southof the Sierra Nevada, with point requiredto accountfor the observedangle of rotation R a maximum at the latitude of the 'Oregon-Nevada border. MAGILL ET AL..' TECTONICROTATION, TILLAMOOK VOLCANIC SERIES 2965

I10 o I00 o

GRE PLAI

GREAT INS

HEAT FLOW(HFU) • >2.5 "".-.•,/

, ß •.,

, '•,.. O.7,.5--1.5 O 300 600 i i i KILOMETERS

Fig. 15. Heatflow contours superimposed onmajor physiographic units of thewestern U.S. Figure from Lachenbruch and Sass [1978].

This distributionof extensionwould produce the clockwise are too largeto permitthis interpretation to be madewith rotationof the CoastRange and relatedterrane [Heptonstall, confidence. 1977].Such a distributionwould also produce counterclock- Interestingly,paleomagnetic studies of Mesozoicrocks in wise rotation of the Sierra Nevada in addition to simple west- the SierraNevada and the Great Valley are consistentwith a ward motion.Second, differential extension would requirethe small counterclockwiserotation of the Sierra Nevada-Great formationof strike-slipfaults separating the regionsof greater Valley.The results of thesestudies are summarized in Table3. and lesserextension. The northwest trending right-lateral The four studiesyield an algebraicmean rotation of 4.4ø +_ faultsin Oregonproposed by Lawrence[1976] appear to be 5.7ø (95%confidence limits) in a counterclockwisesense. This suchfaults. Third, the ColumbiaRiver Basalts[Watkins, 1965; is our best estimate of the Sierra Nevada-Great Valley rota- Watkinsand Baski,1974] show only 2ø of clockwiserotation tion. Howevertwo factsshould be kept in mind regardingthe in northwesternOregon and southeastern Washington (Figure accuracyof thismean result. First, two of themagnetic studies 2, Table3) and about12 ø of clockwiserotation in south-cen- are basedon rockswhich underwentpost-folding remagneti- tral Oregon(Figure 2, Table3). Theseresults would suggest zation events and hence have unconstrained tectonic tilt cor- that the southernregion has experienced more rotation and rections.Second, all the individualrotational uncertainties are extensionthan the northernregion. In fact, the northernre- between14 ø and 23ø (Table 3) and henceintroduce consid- gionwould appear relatively stable and may mark the north- erableuncertainty to the derivedmean rotation. easternlimit of significantextension and related rotation. The direction of motion of the Klamath-Sierra Nevada However the confidencelimits of these paleomagneticdata junctionappears to havehad botha southwesterlyand a 2966 MAGILLET AL.: TECTONICROTATION, TILLAMOOK VOLCANIC SERIES

northwesterlycomponent. The southwesterlycomponent is complexitiesis evidentby faulting [Staceyand Steele,1970; due to the extensionof the Basin and Range province,which Rau, 1973;Cady, 1975; MacLeod et al., 1977;Muller, 1977; appearsto be a continentalanalogue of broadly distributed Fairchild, 1979] and metamorphism[Tabor, 1972] in the back-arcspreading. This appearsto be necessaryto explain OlympicPeninsula-Vancouver Island region. the high heat flow, thin crust, anomalousupper mantle and regional uplift of the Basin and Range [Scholzet al., 1971; PalinsœasticMaps Thompson,1972]. The northwesterlycomponent of motion In order to test whetherthe above two phaserotation is is associatedwith broad regional fight lateral shear [Atwater, consistentwith regional geologic information, the palinspastic 1970; Christiansenand Mckee, 1978; Livaccari, 1979; Zoback mapsshown in Figures11, 12,and 13were generated by the and Thompson,1978]. The late Cenozoicfight lateral motion followingprocedure, working back in time. (48 km minimum) alongthe Walker Lane [Albers,1967; $1em- 1. The outlinesof the CoastRange, Klamath Mountains, mons et al., 1979] requires north-northwest motion of the and Sierra Nevada were digitized along the boundariesbe- Sierra Nevada relative to the stablepart of the North Ameri- tween early Tertiary and youngerrocks. can plate. Regionalshear reflected in the northwesterlymo- 2. The KlamathMountains and the CoastRange of Ore- tion of the Sierra Nevada and adjacent terrane to the east gonwere rotated counterclockwise about a finiterotation pole would appear to be the likely cause of Miocene to present at 46.0øN,237.5øE at a constantrate of 1.5ø/m.y.from the north-southcompression in the ColumbiaPlateau [see Davis, presentback to 20 m.y.,the total rotation angle being 30.4 ø, in 1977]as the plateau is pushedfrom the southagainst a more accordwith the paleomagneticdata. (The actualrate of rota- stationaryMesozoic terrane to the north. A similar buttressef- tion was likely spasmodic,associated with periodsof pro- fect likely constrainsthe Coast Range and Cascadesto the nouncedextension and alternate periods of relativestability.) north as evidencedby north-southcompressional earthquakes 3. The SierraNevada and Great Valley wererotated 13.0 ø [seeDavis, 1977] in Washington and northern Oregon. The clockwiseabout a finite rotationpole at 26.9øN,242.7øE. This PhaseII rotation of the Coast Range, Klamaths and Cascades preservedthe relative positionof the northern Sierra Nevada wouldthus appear to be the resultof southwesterlyBasin and relativeto the Klamathsand producescounterclockwise rota- Range extensionwith superimposednorthwesterly motion of tion of the Sierra Nevada. The resultant extension in the the SierraNevada, both actingto pushthe CoastRange, Kla- southernBasin and Rangeis 210 km comparedto 340 to the mathsand Cascadesin a northwesterlyclockwise sense about north (Figure 13). a northernpivot. 4. All blockswere retainedin this positionfrom 20 back Our Phase II rotation involves the rotation of the Coast to 42 m.y. Range, Cascades,Klamaths and Sierra Nevada as pre- 5. From 42 backto 50 m.y. the OregonCoast Range was dominantlycoherent units in responseto Basinand Range ex- rotatedcounterclockwise at a rate of 6ø/m.y. abouta pole to tension.The behaviorof theseblocks as coherentunits is sug- thesouth located at 43.0øN,238.7øE, the totalrotation angle gestedby the distributionof strike-slipand extensionalfaults being46.0 ø in accordwith the paleomagneticdata (Figure in the western U.S. Figure 14 showsthe Quaternary faults boundingthe blocksbut generally not internal to the blocks. Our 20 m.y.B.P. reconstructionagrees well with accepted This distribution is equally pronounced if we consider all geodynamicconstraints for the westernU.S. The rotationpole faultswithout regard to age [seeEaton, 1980].Regions of high to the north (Figure 13) preservesgeneral continuity of the heat flow are confinedto the Basin and Range provinceand Cascadevolcanic arc and implies mild compressionin the the Cascadesand are not present in the main mountain belts Cascadesof southernWashington, likely producingthe Mio- (Figure 15). These observationssuggest a significantcontrast ceneuplift of thisregion [Hammond, 1979]. The positionof this in the scalesand intensity of late Cenozoic deformation be- pole also impliesa southwest-northeastopening of the Basin tweenthe Basinand Rangeand main mountainbelts and pro- and Range in good agreementwith the trend of the Nevada vides the basisfor our treating the Cascades,Coast Range, rift (Figure 1) and the orientationof early extensionalbasins Klamaths and Sierra Nevada as coherent blocks in our recon- [Eaton, 1979; Zoback and Thompson,1978]. Moreover, the structions. suggestedreconstruction of the SierraNevada-Great Valley is The existenceof a tectonic boundary near the Columbia consistentwith acceptedgeologic constraints. The finite rota- River was previouslyinferred from the smaller amount of ro- tion usedto determinethe early Tertiary positionof the Sierra tation observedin Coastal Washington than in coastal Ore- Nevadamay, in fact,be decomposedinto the threefollowing gon. However,the differencemay reflect only PhaseI tecton- rotations,each correspondingto a different geologiccon- ics.The observedrotations of all the Oregonand Washington straintor observation.The rotationsare listedin the sequence units are consistentwith the 30ø + 10ø of rotationproposed in whichthey were made in orderto producethe palinspastic for our Phase II rotation, suggestingthat the entire coastal restorationshown in Figure 13. marginof the westernU.S. may haveparticipated in PhaseII. 1. Counterclockwiserotation of 4.0ø about a pole at In supportof this,Bentley [1979, 1980]has recently recognized 46.0øN, 237.5øEpartially closing the Basinand Rangeprov- NW trendingstrike-slip faults in the WashingtonColumbia ince in partial compensationfor Miocene extension.Blocks Plateausimilar to thoseof Lawrence[1976] in Oregon. These west of the Basinand Range province,including the Sierra faults suggestsome differential extensionin the plateau and Nevada and Mojave blocksare moved eastwardin this resto- rotationof the WashingtonCoast Range and Cascadesto the ration.This samepole position with a largerrotation was used west.The proposedtectonic boundary at the Columbia River to producethe 20 m.y.B.P. restorationof the OregonCoast wouldthen appearto be an Eoceneand/or Oligocenetectonic Range. boundaryprobably related to the complexitiesof accretingthe 2. Assumingthat motion along the Walker Lane reflects seamountprovince to North America. Eocene-Oligocenetec- strike-slipmotion between the North American(NA) and Pa- tonic activity north of the Columbia River related to such cific (PAC) plates,this motioncan be describedby the PAC- MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES 2967

NA poleat 51.0øN,294.0øE of Minsteret al. [1974].Clockwise Montana as requiredby the model. Second,the rotationsof rotation of 1.2ø about this pole producesright lateral motion the Willipa Hills and Black Hills of the Washington Coast of 90 km along the Walker Lane, in broad agreementwith Rangeappear to be more complexand of significantlysmaller geologicobservations [Albers, 1967;$1emmons et al., 1979]. A magnitudethan the rotationsof contemporaneousrocks of the counterclockwise rotation of 1.2 ø was therefore used in the re- Oregon Coast Range, indicating the presenceof a tectonic construction. boundary near the Columbia River where none exists in 3. Left lateral motion along the Garlock Fault can be de- Hammond's model. Third, the model is difficult to reconcile scribedby a pole at 33.0øN, 243.0øE. Counterclockwiserota- with the presenceof the middle Eocene Clarno Formation tion of 17.0ø about this pole produces85 km of left lateral mo- within the zone which, according to Hammond's model, was tion on the Garlock, which is consistent w•th geologic traversedby the rotating coastalblock. The dif/iculty is com- observations[Davis and Burchfiel,1973]. A clockwiserotation poundedby the small rotation of the Clarno which suggests of 17.0ø about this polewas usedto move the Sierra Nevada that the Clarno was not attached to the trailing edge of the eastrelative to the Mojave blockto its positionprior to differ- coastal block. ential Basin and Range extension.These three rotationscar- Heptonstall[1977] proposeda model to explain the Oregon ried out in the sequencelisted above are exactlyequivalent Coast Range rotation basedon a plate linkage mechanismin to the rotation about the single pole used to produce the re- which Basinand Range extensionwas directly associatedwith constructionof the Sierra Nevada block (Figure 13). Because rotationof the OregonCoast Range. While our model incor- the rotation in step 2 is small, the sequencein which steps 1 poratesthese two partsof his model, ours differsfrom his in and 2 are made is not important. These two combined rota- the followingways: (1) all of his rotationtook placeduring the tionswere usedto rotate the Sierra Nevada block, the Mojave period32 and 4 m.y.B.P.; (2) his model placesa major bound- blockand the pole for the interveningGarlock Fault. Holding ary with large post-Eocenedeformation and strike-slip dis- the Mojaveblock fixed, the SierraNevada block was then ro- placementbetween the Klamaths and the Sierra Nevada, tated relativeto the Mojave block about the rotated Garlock which we regard as incompatiblewith the geologyof this re- Fault pole. A remarkably similar three pole systemwas de- gion;(3) in his model,Washington undergoes a counterclock- rived independentlyby Eaton [1979]on the basisof the distri- wise rotation which is inconsistentwith the paleomagneticre- bution and direction of extensionin the Basin and Range. The sults from Washington; (4) Heptonstall's model requires fact that severalprimary geotectonicfeatures of the Basin and hundreds of kilometers of compressionalong the Oregon- Rangeand westernU.S. can be explainedby a reconstruction Washingtonborder, which is incompatiblewith the geology. basedmainly on paleomagneticconstraints lends considerable CONCLUSIONS credibilityto the reconstruction. Our 20 m.y.B.P. reconstructionproposes a 13ø rotation of The paleomagneticresults from lower to upper Eocene the Sierra Nevada about a pole at 26.9øN, 242.7øE which rocks from the Oregon Coast Range and from the Goble vol- should be observable as a 15 ø counterclockwise rotation of canicsof southernWashington, including the 46 ø of clockwise pre-late Tertiary magneticfield directions.As noted above, rotation of the Tillamook Volcanic Series reported here, es- the paleomagneticdata from the Sierra Nevada-Great Valley tablish that approximately46 ø of rotation took place during would supporta smaller4.4 ø rotation but the error limits are the Eocene.This rotation appearsto have been the result of largeand do not rule out a larger 15ø rotation.Magill and Cox trappingof a wedgeof oceanicplate betweentwo active Eo- [1980]present alternative Phase II modelsand concludethat a cene trenches.Considered in the context of the regional geol- 15ø rotation of the Sierra Nevada yields a reconstruction ogy and geophysics,these paleomagnetic results establish that which is mostconsistent with the paleomagneticdata from the the extent of the tectonicblock which experiencedthis Eocene rotation was from the Klamath Mountains on the south to the Sierra Nevada and Coast Range and the regional geology of the Cascadesand Basin and Range province. Columbia River in the north, where it probably terminated. The existenceof rotated Coast Range rocks of Oligocene and TectonicSignificance of the Clarno Formation early Miocene age indicatesthat a secondphase of rotation The proposedtwo phase rotation may help explain the occurred.This secondphase, approximately 30ø in magni- small clockwiserotation (20ø -4-_19 ø , Table 3) of the middle tude, was likely associatedwith the opening of the Basin and EoceneClarno Formation of central Oregon. The small rota- Range provincewhich began in mid-Miocene time. Miocene tion indicatesthat the Clarno likely did not participate in the rotation of the Coast Range was accompaniedby a similar ro- PhaseI rotation but was probably part of stableNorth Amer- tation of the Cascadesand Klamaths and possiblyby a small counterclockwise rotation of the Sierra Nevada. ica at that time. However the paleomagneticdata are consistent with the Clarno having participatedin the PhaseII rota- Acknowledgments.We would like to thank P. D. Snavely for in- valuableadvice in selectingsampling locations within the Tillamook tion, in which casethe rotation of the Clarno would be of post volcanicsand W. D. Dickinson, Z. Ben-Avraham, P. D. Snavely, and mid-Miocene age. D. Engebretsonfor helpful discussions.This researchwas supported by grant EAR 79-19712from the National ScienceFoundation and ComparisonWith Other Models grant PRF 11405-AC2from the PetroleumResearch Fund. Note: the In Hammond's[1979] elaboration of model 2 of Simpson computer-generatedmaps of this paper were constructedusing the followingpolar equal area (Lambert) projections--mapcenter 41øN, and Cox [1977]all of the observedCoast Range rotation was 240øE:Figures 1, 11, 13;map center45.5øN, 238øE:Figures 2, 9; map aro,anda no_nhornpi.vc•t he. at the.C•lympic Peninsula of Wash- center41øN, 260øE: Figure 12 (Eurasia);map center41øN, 230øE: ington, swingingthe Oregon and Washington coast ranges Figure 12 (North America). and Klamaths out seawardfrom an Eoceneposition near the Olympic-Wallowalineament. This model appearsunrealistic REFERENCES for three reasons.First, as noted earlier, there is no evidence Albers, J.P., Belt of sigmoidalbending and right-lateral faulting in for large scaleEocene extension in easternWashington and the westernGreat Basin, Geol. $oc. Am. Bull., 78, 143-156, 1967. 2968 MAGILL ET AL.: TECTONIC ROTATION, TILLAMOOK VOLCANIC SERIES

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