J. Phys. ,26, Suppl., S 1-S 19, 1978

PLATE TECTONIC EVOLUTION OF NORTH PACIFIC RIM

William R. DICKINSON

GeologyDepartment, Stanford University, Stanford, , U.S.A. (ReceivedMay 22, 1978)

The North PacificRim is a segment of the circum-Pacificorogenic belt lyingalong the greatcircle between Mesoamerica and Indochina. Paleotectonicreconstructions rely upon integrationof informationabout rocksexposed on land, crustalthicknesses, paleolatitudes of crustalblocks, sediment layerscored at sea, and geomagnetic anomalies. Continental margins have been modified by accretionof oceanic materialsduring ,suturing of continentalblocks by collision,and opening or trapping of marginal seas. Prior to the breakup of Pangaea, a vastPaleopacific seafloor was builtby spreading coevalwith the sub- ductionthat elsewhereassembled Pangaea. Afterthe breakup of Pangaea, circum-Pacific subduction accreteddeformed increments of the Paleopacificseafloor to the edges of con- tinentalblocks now along the North PacificRim. Cretaceous crustalcollisions closed the North PacificRim and isolatedthe Arctic Ocean. Paleogene accretionof the continental Okhotsk block caused subduction to shiftfrom the Bering shelfedge to the Aleutian chain. The elbow in the Emperor-Hawaii hotspottrack recordsa change in Pacificplate motion at about the same time. Current circum-Pacificarcs include east-facingisland arcs and west-facingcontinental arcs in a consistentpattern that impliesnet westward driftof con- tinentallithosphere with respectto underlying asthenosphere.

1. Introduction

The northern margin of the extends from Central America on the east to the Philippine Sea on the west, and has the at its northern extremity. This North Pacific Rim lies along a global great circle upon which Japan and California are spaced about 90° apart. A continuation of the same great circle down the western side of the leads to Chile, which lies about the same angular distance from California as does Japan. In the study of circum-Pacific tectonics, we must thus apply to the geol- ogy of the North Pacific Rim the same kind of integrated view that we should apply also to the western Cordilleras of the Americas. For example, British Columbia, Alaska, and Kamchatka form a single continuous segment of the circum-Pacific belt as surely as do the Latin American countries from through Columbia to Ecuador and Peru. The purpose of this paper is to review the general status of our understanding about the plate tectonic history of the North Pacific Rim. Many details of present knowledge are omitted to allow a focus on broad relationships. My interest in the questions discussed was stimulated especially by my trip to Japan in 1976 as a visitor at the Re- search Institute of the University of Tokyo, where I was a participant in the scientist exchange program sponsored by the American Geophysical Union and supported by the U.S. National Science Foundation under the U.S.-Japan Cooperative Science Program in liaison with the Japan Society for the Promotion of Science. My subsequent work was supported by the U.S. National Science Foundation with Grants EAR75-14568 and EAR77-27542.

S 1 S 2 W.R. DICKINSON

2. Overview The present configuration of the Pacific borders of and Eurasia, and the present geologic relationships of rock assemblages within the continental margins, is the result of long-continued processes of tectonic evolution within the circum-Pacific orogenic belt. Valid paleogeographic and paleogeologic reconstructions of these lands that are now part of the North Pacific Rim must be based on the integration of several kinds of data: a) distributionand ageof diagnostic rock assemblages mapped on land, b) varyingcrustal thicknesses on land and atsea, c) paleomagneticdata on paleolatitudesfordifferent continental blocks, d) patternsof geomagneticanomalies on thedeep-sea floor, e) informationon sedimentthicknesses and faciesfrom deep-sea drilling. Data from the deep sea appliesdirectly only to times sincethe mid-Mesozoic, for no pre-Jurassicseafloor is known in the PacificOcean. In common with thosein the Atlantic and Indian oceans,Pacific seafloor anomalies thus provide some record of eventssince the breakup of Pangaea, but not before. The major types of tectonicevents that have affectedthe PacificRim sincethe Meso- zoicbreakup of Pangaea include: a) the subductionand accretionof oceanicmaterials during plate consumption at variousbounding trenches, b) the collisionand suturingof microcontinentalblocks against the surrounding continentalmargins, c) theopening of interarc basins to form or expand marginalseas lying mainly on thewestern side of the open ocean, d) thecancellation of spreading centers (or belts) by rise-trenchencounters occur- ringmainly on the easternside of theocean, e) the developmentof marginal and intracontinentaltransforms causing strike slip eitherlongitudinal or transverseto associatedcontinental margins. Priorto thebreakup of Pangaea,the world ocean Panthalassawas mainly a paleo- Pacificrealm (DICKINSON,1977a, b), and theTethys Sea can be regardedas a gulfof the PaleopacificOcean (Fig.1) during Permo-Triassictime. In Permo-Carboniferous time,the earlier assembly of Pangaea (IRVING,1977) presumably required subduction of oceanfloor to allowLaurasia and Gondwana to be suturedtogether west of the Tethyan gulf. Late Paleozoicsubduction can be inferredalong both the Hercynian-Variscan (and Appalachian-Ouachita)orogenic belts of Laurasiaand theGondwanide-Tasmanide orogenictrend of Gondwana (seeFig. 1). Coeval spreadingwithin the Paleopacific Ocean was presumablyrequired to maintain a globalmass balance. Subductionat continentalmargins facing the PaleopacificOcean was apparentlynot requiredprior to the laterbreakup of Pangaea. The oldestgranitic components in batholithsof the circum-Pacificorogenic belt do not datemuch beyond about200 my, which was near the Triassic-Jurassicboundary. This factsuggests that the present regime of subduction beneath continental margins now facingthe PacificOcean was not establishedbefore about mid-Triassic time. Sincethen, circum-Pacificand Alpine-Himalayansubduction have been pairedin time with open- ing of the Atlanticand Indianoceans. Widespreadmid-Carboniferous to mid-Triassic Plate Tectonic Evolution of North Pacific Rim S 3

Fig. 1. Inferred Triassicworld map (Mercator projection) showing Pangaea and the large Paleopacific Ocean. Japan is restored against Eurasia but remnant oceans within Eurasia are not shown (Indonesian islands de- picted in present configurationfor orientation only). From DICKINSON (1977b). oceanic faciesexposed now within the circum-Pacificorogenic belt of the North Pacific Rim apparently represent deformed increments of the old Paleopacificseafloor now incorporatedinto the margins of continentalblocks. They were accreted tectonically to the edges of North America and Eurasia by Mesozoic subduction as seafloorspreading generated younger Pacificseafloor farther offshore. No vestigeof the once vast Paleo- pacificseafloor remains within the confinesof the presentPacific Ocean. The enclosedoutline of the North PacificRim did not attainits present overall shape untilthe closureof late Mesozoic suture beltswelded the Eurasian and American con- tinentalblocks togetheracross the Alaska-Bering-Yakutiaregion (CHURKIN, 1972). Full detailsof thiscomplex processwill not be known untilthe plate tectonicevolution of the ArcticOcean is betterdocumented.

3. CurrentTectonics Figure 2 is a sketchmap showing key tectonicfeatures of the North PacificRim and the adjacentPacific Ocean. Volcanic chainsthat mark activemagmatic arcsstand paral- lelto activesubduction zones at trenches,but are absent elsewherearound the periphery of the ocean. Young seafloormarks the crestsof active midoceanic risesand forms the floorof activeinterarc basins. The Emperor Seamounts chain and the Hawaiian Ridge form linkedsegments of a track generated by the hotspot now beneath Hawaii (MORGAN, 1972). The Emperor-Hawaii elbow evidentlyrecords a marked change in the motion of the Pacificplate with respectto the hotspot about 40 mybp (DALRYMPLE and CLAGUE, 1976). On Fig.2, Japan and Californiaappear to lieon oppositesides of an interveningocean, but thiserroneous impression reflects distortion that isinherent in the customary Mercator projectionused as a base map. In reality,the North PacificRim is a belt that approxi- mates a great circle(see Fig. 2, inset). Figure 3 is a tectonicsketch map showing much the same informationas Fig.2, but drawn using a projectionupon which that great circle plotsas a straightline. Japan and Californiaare thus shown correctlyas lying on the S 4 W.R. DICKINSON

Fig. 2. Current tectonicelements of the North PacificRim and the northern PacificOcean shown on a standard Mercator projection.Seafloor ages modified afterPITMAN etal. (1974) and HILDE et al.(1977). American- boundary afterCHAPMAN and SOLOMON (1976).

Fig. 3. Current tectonicframework of the North Pacific Rim plottedon a projectionshowing that segment of the circum-Pacificgreat circleas a linearbelt bordering the northern PacificOcean. Double line denotes Atlantic spreading center;subduction zones shown as on Fig. 2. same continuous margin of the Pacific Ocean . The ocean in reality has no "sides" because its whole periphery approximates the great circle that delimits the watery hemi- sphere of the earth. Nevertheless, Figs. 2 and 3 both reflect a distinct east-west asymmetry in Tectonic Evolution of North Pacific Rim S 5 tectonics(NELSON and TEMPLE, 1972). In the open ocean, spreading centerslie well to the east of the central meridian within the oceanic region. The continental margins with subduction zones also have different characters. On the western "side" of the ocean, east-facingisland arcs are associated with marginal seas formed by interarc spreading. On the eastern "side" of the ocean, west-facing magmatic arcs stand on the edges of intact continental blocks. These differencesimply some fundamental distinctionbetween sub- duction systems where seafloor is being subducted either eastward or westward beneath continental margins that have generally meridional orientations.

4. Backarc Behavior Although the rates of tectonic and sedimentary accretion vary in the forearc region between the trench and the volcanic chain, evolutionary trends are similar in all arc- trench systems(DICKINSON, 1973a). In the backarc region,however, two opposite extremesof behaviorare noted (DICKINSON,1972, 1973b, 1974a, b): a) Behind some intra-oceanicarcs, within an interarcbasin widensthe marginal sea that separates the from the nearbycontinental margin; thisprocess involves separation of lithosphereand upwellingof asthenosphereto form new oceaniclithosphere in the backarcregion. b) Behind some continental-marginarcs, fold-thrust belts form a subsidiarymoun- tainchain that risesbetween the volcanicchain and the adjacentcontinental interior; thisprocess involves contraction of lithosphere,crustal thickening within the mountain belt,and the developmentof a retroarcforeland basin on a depressedpart of the con- tinentalblock in thebackarc region. For a thirdand intermediatestyle of backarcbehavior, neither process occurs. Severalworkers have discussedthe probableplate motions associated with backarc spreadingand backarcthrusting (HAVEMANN, 1972; HYNDMAN, 1972; MOBERLY, 1972; WILSON and BURKE, 1972). In general,advective heat flow associated with arcmagmatic activityapparently softens the lithosphere along the trend of thearc, and thusallows litho- spherein the backarcregion to move independentlyof the narrow sliverof lithosphere occupyingthe band between arc and trench. The arc structureitself remains in place above the descendingslab of lithospherebeing subducted. The sliverplate in the arc- trenchgap isthus constrained to maintainits position relative to theflexure where litho- spherebends at the subductionzone to descendinto the mantle. Meanwhile, backarc lithosphereisable to shiftposition with respectto thatflexure. Two fundamentalcauses of relativemotion between thebackarc lithosphere and the flexurein the subductedslab of lithospherehave been suggested(e.g., MOLNAR and ATWATER, 1978;UYEDA and KANAMORI, 1979): a) One mechanism considersthe tendencyof the descendingslab to sinkinto the asthenosphere.Where the sinkingslab is old and thick,with a high bulk density,the rateof sinkingis fastin relationto the rateof plateconvergence across the arc-trench system;the flexure in the descendingslab thus tends to migrateoceanward, and backarc spreadingaccommodates the resulting extension as thearc-trench system pulls away from backarclithosphere. Where thesinking slab is young and thin,with a low bulk density, therate of sinkingis slow in relationto therate of plateconvergence across the arc-trench system;the flexurein the descendingslab thus tendsto migratelandward, and backarc S 6 W.R. DICKINSON

Fig. 4. Plot showing relationshipbetween Ceno- zoic backarc tectonicsand geographic orienta- tion of associatedarc-trench systems. "Facing" azimuth isgiven by a linedrawn normal to the subduction zone and leading away from the trench side of the system. See text for discus- sion.

thrustingaccommodates the resultingcontraction as the arc-trenchsystem pressesagainst the backarc lithosphere. b) The other mechanism considerspotentially independent motions of the backarc lithospherewith respectto underlying asthenosphere. The descending slab is held to anchor the arc-trenchsystem with respectto the part of the asthenosphereinto which the slab is being inserted. Backarc spreadingthen occurs where backarc lithosphereis skiddingaway from the flexurein the descendingslab, and backarc thrustingoccurs where backarc lithosphereis skidding toward the flexurein the descendingslab. The two mechanisms are not mutually exclusive,and each may serve to reinforceor counteractthe effectof the other in a particularcase. A censusof modern arc-trenchsystems (Fig. 4) shows that backarc spreadingis char- acteristicof those thatface eastward (subductiondownward to the west),whereas backarc thrustingis most widespread in those that face westward (subductiondownward to the east). Although "neutral"systems that lack eitherbackarc spreadingor backarc thrust- ing are known to face both eastward and westward, they are most typicallysouthward- facing (northward-facingarcs are absent on the modern earth). Consequently,backarc spreadingis typical for arcs along the westernperiphery of the Pacificand backarc thrusting is common for arcs along the easternperiphery of the Pacific. Figure 5 is a schematic diagram of standard relationsacross the modern Pacific. The east-westdichotomy of currentarc behavior and the east-westasymmetry of the modern PacificOcean may be wholly fortuitousproducts of platemotions that are cur- rentlysuitable for that patternbut are directionallyrandom over the long term. How- Plate Tectonic Evolution of North Pacific Rim S 7

Fig. 5. Diagram showing characteristic east-west asymmetry of placement of midoceanic rise and east-west dichotomy of backarc behavior. See text for discussion. ever, severalworkers have suggested that the present relationshipsare the systematic resultof net shear between lithosphereand asthenosphere(BOSTROM, 1971; KNOPOFF and LEEDS, 1972; MOORE, 1973). Such shear may be inherent between these two loosely coupled layersof the outer earth as the rotationof the earth decelerates.Westward lag of the lithospherelayer above asthenospherethat rotatesfaster could account for both the dichotomy of arc behavior and the asymmetry of the ocean (seeFig. 5): a) For east-facingsubduction zones (subductiondownward to the west), backarc lithospherewould move away from the arc-trenchsystem, which would be anchored by a steepenedslab. b) For west-facingsubduction zones (subduction downward to the east),backarc lithospherewould move toward the arc-trenchsystem, which would be anchored by a slabbeing entrainedbeneath overridinglithosphere. c) Gradual eastward migration of midocean risesrelative to bounding continental margins might occur in responseto the interactionbetween asthenosphereand the portions of plates of lithospherethat are insertedinto the asthenosphere as descending slabs. West-facingcontinental margins would tend to overrideand entraineastward-subducting plates,and thus to encroach upon the easternflanks of midocean rises. Slabs of differing age, thickness,and consequent negativebuoyancy would thus come to descend on eastern and western borders of the ocean. The contrastwould be such as to reinforcethe behav- ioraltendencies established for east-facingand west-facingarc-trench systems (seeFig. 5). Moreover, interactionbetween lithosphereand asthenospherewould help to steepen the angle of descentof westward-subductingplates and to pressthe zone of plate flexureaway from associatedeast-facing continental margins, whereas the angle of descentof eastward- subductingplates could be reduced and theirzones of flexurepressed toward associated west-facingcontinental margins. The relationshipsdepicted by Fig.4 seem too clearcutto be wholly coincidental,and hence suggestthat the rotationaleffects are real.

5. Late CenozoicTectonics For tectonicreconstructions during the Cenozoic, the key featurein the northern PacificOcean isthe Emperor-Hawaii hotspottrack (Figs.2, 3). During the past 40 my, the Hawaiian Ridge has been generated by movement of the Pacificplate over the Hawaii hotspotat an essentiallyconstant rate (DALRYMPLE etal., 1977). Figure 6 shows inferred S 8 W.R. DICKINSON relationshipsin the northern PacificOcean during thattime span. Prior to 40 mybp , back to about 70 mybp, movement of the Pacificplate over the Hawaii hotspotat a similarrate, but in a more northerlydirection, generated the Emperor Seamounts. Both paleomagnet- ic and paleoecologicdata confirm the requisitenorthward motion of the seamounts during the Cenozoic (MARSHALL, 1978; GREENE etal., 1978). Figure 6 indicatesthat the existenceof the Meiji sediment tongue of Neogene terrige- nous clay (SCHOLL etal., 1977) cannot be used as a valid argument againstsuch motion of the Pacificseafloor. The area near Meiji seamount has occupied positionsclose to

Fig.6. Sketchmaps showingkey tectonicelements in the northernPacific Ocean atpresent (above) and 40mybp (below).Approximate position of Emperor Seamount chainis shown alsoat 22.5mybp (Oligocene-Miocene boundary)on eachplot. Rates of generation ofHawaiian and Pratt-Welkerhotspot tracks modified after DUNCAN and MCDOUGALL (1976),JARRARD andCLAGUE (1977), and DALRYMPLEet al. (1977). Plate boundaries and con- tinentalconfigurations at40mybp modifiedafter COOPER etal. (1976), STONE and PACKER(1977) , and PITMANandT ALWANI (1972). Plate Tectonic Evolution of North Pacific Rim S 9

Fig. 7. Configurationsof western North Pacificplate boundaries and island arcs at present (left)and 40mybp (right).See textfor discussion. sediment sourcesalong the Aleutian Ridge sincethe sediment tongue firstbegan to form. However, the restoredposition of the Aleutian abyssalplain is problematical(MARLOW et al.,1973; SCHOLL et al.,1975, 1977), because the source terranefor itsmid-Cenozoic turbiditeslay somewhere along the continentalmargin between and Kodiak Island (STEWART, 1976). The reconstructionof Fig.6 impliesthat unsuspected complexitiesin the configurationof plateboundaries may have existedin the northeastern corner of the mid-Cenozoic Pacific. BYRNE (1978) has recentlyargued that the Kula- Pacificspreading center ceased operating about 57.5mybp. If thisidea is confirmed, the Kula plateis incorrectly included in Fig.6 (below)for 40mybp, and no plateboundary would have been present to block the deliveryof mid-Tertiary turbiditesfrom sources along the northeasternmargin of the Pacificto the Aleutian abyssalplain on the Pacific plate. In the western Pacific,interarc seafloor spreading (KARIG, 1970) behind east-facing islandarcs has opened the Sea of Japan (MATSUDA and UYEDA, 1971; SILLITOE, 1977) and broadened the PhilippineSea (TOMODA et al.,1975; WATTS et al.,1977) since the early Cenozoic. Figure 7 shows the changing configurationof island arcs within this region during the time span representedby Fig. 6. At 40mybp, the Marianas frontal arc and the Palau-Kyushu remnant arc were unitedas a singleancestral island arc (KARIG, 1971). Past plate configurationsin the area of the PhilippineIslands are problematical (KARIG, 1973). In Taiwan and the Ryukyus along the western side of the Philippine Sea, no subductionwas underway in the middle parts of the Cenozoic (MURPHY, 1973). Complex strikeslip is implied for the Sakhalinregion during formation of the Sea ofJapan, but the restorationdepicted in Fig.7 isschematic only. Along the American border of the North Pacific(Fig. 8), the major events during the late Cenozoic were (a) ridge or risesubductions that occurred as the asymmetry of the ocean basinwas accentuatedby consumption of the Kula plateand most of the ,(b) generationof the San Andreas and Queen Charlotte transform systems as the Pacificplate came into contactfinally with the American plate,and (c) development of the extensionalBasin and Range province by incipientdisruption of the American plate in the regionadjacent to the San Andreas transform. S 10 W.R. DICKINSON

Fig. 8. Configurationsof easternNorth Pacificplate bound- aries and subduction zones at present (above) and 40 mybp (below).See text for discussion.

6. Rise-TrenchEncounter So-calledridge or risesubduction is better described as rise-trenchencounter. Gener- ation of new lithosphereis the dominant processat a riseor ridge crest,but can occur only near the surfaceof the earth. Only there does plate separationinvolve the kind of pressurerelease required to triggerpartial melting of the asthenosphereon a largescale, and only there can heat lossbe rapid enough to chillupwelling materialsufficiently to form new lithosphere.When a riseand a trenchmeet afterthe interveningplate has been consumed, the zone of plateseparation that is marked by a discreteridge crest at the surface willno longerform such a discretebelt where itbecomes subterranean. The overallplate kinematicsdo requirethat the wholly consumed platedescend intothe mantle fasterthan the plateon the other sideof the riseor ridge crest(else no plateseparation occurs!). As the zone of separationbetween the two is drawn into a subterraneanposition, however, it is marked by a broad and expanding region of sublithosphericmantle upwelling,and not by a beltof steady-statewidth where new lithosphereis created, as would occur at the surface. Rise-trenchencounter can cause two differentbut relatedresults depending upon whetherthe plate beyond therise or ridgecrest is subducted or notfollowing the encounter: a) Where itis subducted, as happened alongthe following a mid- Cenozoicencounter with the Kula-Pacificridge (DELONG et al.,1978), several distinct but simultaneousevents punctuate the geologichistory of the arc-trenchsystem. These serveto recordthe rise-trench encounter as a discontinuityin the evolutionof the Aleutian Ridge. The geneticallyrelated effects stem eitherfrom the temporaryabsence of a de- scendingslab beneath the arc-trenchsystem, or from the high heatflow and thermalex- pansionimposed across the arc-trench system while the coldsubterranean slab was absent (DELONG and Fox, 1977). The changingthermal regime was reflectedby an episodeof broad upliftaffecting the whole arc-trenchsystem, and by a pulseof metamorphism at Plate Tectonic Evolution of North Pacific Rim S 11 crustallevels. A temporary cessationof arc magmatism extinguishedthe volcanicchain while no subducted slab was present. b) Where itis not subducted,the trenchis converted to a transformlike the modern San Andreas, which lengthensas the triplejunctions at eitherend migrate away from one another. In thiscase, a graduallyexpanding region that lacksa subducted slab develops in a triangularform adjacent to the transform (DICKINSON and SNYDER, 1979b). This no-slabregion occupiesthe space above a gradually enlargingwindow or hole in the de- scending slab. Arc magmatism is extinguishedwhere a descending slab is absent, and upwelling of asthenosphereto occupy the slab-window can contributealso to upliftand extensionaltectonics within a regionof triangularshape adjacentto the transform. Figure 9 depictsthese relationshipsfor the San Andreas transform system and the Basin-and- Range province adjacent to it. Note the followinggeometric relationshipsshown: (a) the gradual shorteningof the Cascades volcanicarc as the expansion of the no-slabregion above the slab-window in the descending Farallon plate expanded to extinguishthe southern extensionof the arc in progressivefashion through the Neogene, (b) the overall coincidencebetween the inferredextent of the modern no-slabregion and the known extent

Fig. 9. Sketch maps showing evolution of the San Andreas coastaltransform system and an adjacent subcontinental region of triangular shape lacking a subducted slab. After DICKINSON and SNYDER (1979a, b). See text for discussion.Points M and R are Mendocino and Rivera triplejunctions, respectively. S 12 W.R. DICKINSON of upliftedand block-faultedcontinental crust underlying the Basin-and-Range province and Colorado Plateauwhere mantle upwellingthrough the slab-window isto be expected, and (c) the manner in which the siteof the San Andreas transform boundary between Pacificand American plateshas shiftedgradually inland from an originalposition near the continentalslope to itspresent location along the San Andreas faultproper. In eitherscenario for events that followrise-trench encounter, thermal effectsnear triplejunctions that terminaterise crests at the trench may promote anatecticmelting of otherwisecold subductioncomplexes (MARSHAK and KARIG, 1977). This effectcan give riseto magmatism at sitesanomalously closeto the trench. Small plutons and volcanic fieldsof Cenozoic age in the coastalregions of California,Alaska, and Japan may allstem from thispotential effect of rise-trenchencounter.

7. EarlyCenozoic Tectonics LatestCretaceous and earliestCenozoic tectonics in the westernCordillera of North Americawere dominatedby theLaramide Orogeny. In thecentral part of the Cordillera where the deformationwas most characteristic,fault-bounded uplifts and intervening sediment-filleddepressions developed across a broadarc massif that was essentiallydormant magmatically.This style of diastrophism occurred during the period from 70 to 40 mybp, and was most intenseat about 55 mybp in the Wyoming and ColoradoRockies. The classicLaramide orogenicstyle, essentially amagmatic with basement involvementin contractionalstructures, can be attributedto underthrustingof a subductedplate at a low anglebeneath the continentalplate (CONEY, 1976,1978). This inferenceis based on the arealdistribution of subduction-related igneous suites having varied alkalinity and potassicity(LIPMAN et al., 1971). Thisshallow mode of plateconsumption contrasts with the more normal steepmode thatgenerates arc magmatism (seeFig. 10). Where the descendingplate dives steeply into the asthenosphere, magmas aregenerated (BARAZANGI and ISACKS,1976), and a standardtype of arc orogendevelops. Where the descending platescrapes along beneath the overridingplate, magmatism issuppressed and buckling of the overridingplate causes the basement-coreduplifts of a Laramide-styleorogen (DICKINSONand SNYDER,1978). Pacificasymmetry thatdictates the presenceof spreadingcenters in comparative proximityto theAmerican coastsmeans thatrelatively young lithosphereis subducted beneaththe Americas (see Fig. 8). Platebuoyancy may thuscommonly promote Lara- mide-typephases of evolutionfor west-facing American arc massifs,whereas similar de- formationmay never or seldom affecteast-facing Eurasian island arcs. In thissense, Laramide-styledeformation is akin genetically to backarcthrusting and isincompatible with backarcspreading (CONEY, 1972). The prominentelbow in theEmperor- track (Figs. 2, 3, and 6) reflects an abruptchange in the directionof Pacificplate motion acrossthe Hawaii hotspotat about40 mybp. The movement ofa subductingplate is governed mainly by thedescend- ing slabsattached to it at subductionzones (FORSYTH and UYEDA, 1975; CHAPPLE and TULLIS,1977). In effect,the pull of theslab as itfalls into the mantle causes the plate to skid over the asthenosphere. Consequently, the most direct way to affect the overall motion of the Pacific plate is to change the configuration of the subduction zones at its boundaries. Tectonicrelations across northeastern Eurasia between the Bering Sea and the Hok- Plate Tectonic Evolution of North Pacific Rim S 13

Fig. 10. Diagrams illustratingamagmatic Laramide- styleshallow subduction (above) and more ordi- nary steeper subduction (below) that generates arc magmatism. After DICKINSON and SNYDER (1978). kaido-Sakhalinregion suggesta major change in plateboundaries during the early Ceno- zoic. Most of the Sea of Okhotsk isunderlain by continentalcrust at leasttwice as thick as standard (BURK and GNIBIDENKO, 1977). Despite this,a calc-alkalic volcano-plutonicbelt lay west and north of the presentSea of Okhotsk during the Late Cretaceous and early Cenozoic (ZONENSHAIN et al.,1974). These relationssuggest that a microcontinentalOkhotsk block of unknown originlodged againstthe margin of Eurasia during the early Cenozoic (DEN and HOTTA, 1973). Previously,an unknown width of ocean intervenedbetween Eurasia and the Okhotsk block. Figure 11 shows the key relationshipsof the Okhotsk block in map view. Prior to the accretionof the Okhotsk block to Eurasia,the subductionsystem forming the North Pacific Rim lay fartherwest and north than it does now. The subduction zone lay through Taiwan, the Shimanto belt of outer Japan, central Hokkaido, Sakhalin, the Magadan coast,and the Koryak Mountains at the root of Kamchatka; thence along the continental slope of the Bering Sea and the Shumagin-Kodiak-Chugach trend in southern Alaska. The parallelmagmatic arc lay in coastalChina, peninsula Korea, parts of inner Japan, Sikhote-Alinin coastalPrimorye, the Okhotsk belt,and northeasternmostUSSR; thence along the Bering shelfand coastalAlaska. After accretionof the Okhotsk block,subduc- tion shiftedeastward and southward to the Izu-Mariana arc east of the PhilippineSea, the Kuril-Kamchatka arc along the edge of the accretedOkhotsk block,and the Aleutian arc south of the Bering Sea. The Bering Sea containsnorth-south geomagnetic anomalies formed at the Kula- Farallonplate boundary during the Early Cretaceous(COOPER etal., 1976). This Mesozoic seafloorwas trapped behind the Aleutian Ridge when subduction and arc magmatism jumped from the Bering shelfedge to the Aleutian trend (SCHOLL etal., 1975). Accretion of the Okhotsk block directlychoked the segment of the pre-accretionsubduction zone that lay along the Magadan coast parallelto the Okhotsk volcano-plutonicbelt, and southward through Sakhalin to Hokkaido. Abandonment of the subduction system S 14 W.R. DICKINSON

Fig.11. Sketchmap showinginferred evolution ofsubduc- tionsystems before and afteraccretion of the Okhotsk blockby crustalcollision followed by a subductionjump.

throughthe Koryak Mountainsand alongthe Bering shelf edge was evidentlyan indirect effectof Okhotsk accretion. Attachment of the Okhotsk block to Eurasialeft the Koryak- Beringsegment of thepre-accretion subduction zone adjacentto a deep re-entrantin the continentalmargin. Ratherthan mold itselfin a tightflexure following the outlineof the re-entrant,the Pacificplate apparently broke across the re-entrant to connectpost-accre- tionsubduction along the southeast edge ofthe Okhotsk block with continuingsubduction in southernAlaska by way of the newly establishedAleutian trend. The oldestwell datedrocks exposed on theAleutian Ridge are Eocene (MARLOW etal., 1973), and the oldestarc volcanics on Kamchatka areof similarage (AVDEIKO,1971). The completion of Okhotskaccretion was thus timed closelywith the change in Pacificplate motion at about 40 mybp. The reasonsfor the initiationof subductionalong the Izu-Mariana trendto createthe PhilippineSea plateare stilluncertain (UYEDA and BEN-AVRAHAM, 1972;UYEDA and MIYASHIRO,1974), but the oldestarc volcanicsin the Marianas are alsoEocene. Terminationof Laramide deformationin the North American Cordillera and ofsubduction in theGreater Antilles iseven more difficulttorelate to eitherthe accre- tionof the Okhotskblock or theformation of theEmperor-Hawaii elbow, but theseother eventsalso took place at about40 mybp (CONEY,1971). Some linkageof plateinterac- tionsthroughout the North Pacificregion is seemingly implied by thewidespread signif- icanceof that date (cf. CONEY, 1971).

8. CrustalCollision-Accretion Accretionof exoticcrustal blocks to the continentalmargins that now form the North PacificRim has occurred repeatedlythroughout the Mesozoic and the Cenozoic. Both equant microcontinentalblocks and elongateisland arcs or aseismicoceanic ridgeshave been involved. In both cases,accretion occurs by crustalcollision when the buoyancy of lithospherecapped by a thickcrustal profile resists subduction. The accretionof discrete Plate Tectonic Evolution of North Pacific Rim S 15 crustalmasses by such crustalcollision is a processapart from the incrementalaccumula- tion of a growing subduction complex formed of offscrapingsfrom the seafloor. Two main kinds of crustalcollision provide accretionaryadditions to continental margins (Fig.12). In one case,an activeisland arc collideswith a passivecontinental margin, which itpartially subducts. The arc structureis thus attached to the continental margin. Such an event occurred in Taiwan in the late Cenozoic (CHAI, 1972). If the overallpattern of platekinematics induces subduction to resume followingsuch arc-con- tinentcollision, then subduction beneath the oppositeside of the accreted arc structure may generatean activecontinental margin followingaccretion. Such a sequence of eventswas involved in the onset of circum-Pacificsubduction in California(Fig. 13). An east-facingisland arc of late Paleozoic age collidedwith the Cordilleranmargin to produce an episode of Permo-Triassicdeformation called the Sonoma Orogeny, during which the Havallah-Pumpernickel subduction complex was

Fig.12. Diagrams depictingmain variantsof accretionary eventsinvolving crustal collision at continentalmargins.

Fig.13. Regionalgeologic relations of theSierran-Klamath arc terraneat the onsetof Mesozoic subduction,which was initiatedby polarityreversal following accretion of the terraneto thecontinental margin by crustalcollision duringthe Sonoma Orogeny near the closeof thePaleo- zoic. S 16 W.R. DICKINSON emplaced across the continental margin as an allochthonous mass atop the Golconda thrust(DICKINSON, 1977a). Following accretion of theisland arc by crustalcollision in the mid-Triassic,reversal of arcpolarity allowed Jura-Triassic subduction to continue alongthe western side of the accreted arc in the Sierra Nevada foothills(SCHWEI- CKERT and COWAN, 1975).This was the earliestinception of the continental-margin Cordilleranarc-trench system. In the kind of accretionrepresented by the attachmentof the Okhotsk blockto Eurasia, the continental margin is active and draws a passive block against its bounding subductionzone from a positionatsea. Followingcrustal collision, subduction jumps to theopposite side of the accreted block. Not onlymay microcontinentalblocks be accreted in this way, but also dormant island arcs, oceanic seamount chains, and aseismic oceanic ridges(DICKINSON, 1976). Extensiveterranes in the Cordilleraof North America have been accreted in this general fashion. Figure14 depictsinferred crustal relations along a generallyeast-west profile across the Hidaka collision orogen in Hokkaido. During the early Cenozoic, a southern extrem- ity of the Okhotsk block was sutured against a western terrane then attached to Eurasia priorto the openingof the Sea ofJapan. Priorto collision,the Cretaceousand early Cenozoicorogen was an east-facingarc-trench system on theEurasian mainland (OKADA, 1974). Clasticsshed eastwardfrom the igneousterrane to the west fillan old forearc basinwhere they reston ophioliticrocks. This ophioliticsubstratum is tiltedup and faultedagainst strongly deformed but weaklymetamorphosed oceanic facies representing thesubduction complex. Along a faultcontact interpreted here as the main suturebelt, thesestrata are in contactwith a beltof amphibolites,gabbros, migmatites, and gneisses (HASHIMOTO,1975). These stronglymetamorphosed rocks are taken here to be partof thebasement of the Okhotsk block that lodged against the old subduction zone. Subduc- tionthen jumped eastwardto the new subductionzone alongthe Kuriltrend. During thelate Cenozoic, sediments were shed westwardfrom the collisionorogen to mask parts of theold forearc basin. Late Mesozoiccrustal collisions across the Alaska-Bering-Yakutiaregion welded togetherfor the first time the crustal blocks that form theNorth Pacific Rim (Fig. 15). The positionsand agesof the various suture belts involved is not yet well known. Large tracts are accretionary collages of oceanic rock assemblages. Closure of the most signifi-

Fig. 14. Inferred structural relations in Hokkaido where Paleogeneakasuture accretion belt triggeredof the Okhotsk a subduction block jumpalong thatthe Hid initiatedtheKuril trench. Plate Tectonic Evolution of North Pacific Rim S 17

Fig.15. Sketch map showing main crustalsutures joining North America and Euasia acrossthe Alaska-Bering- Yakutiaregion to form the North PacificRim. cant oceanic regionsprobably occurred between mid-Jurassicand mid-Cretaceous time (CHURKIN, 1972),but some contractionacross the region was stillunderway in the Ceno- zoic (PITMAN and TALWANI, 1972).

9. Review The major pointsmade arethese: 1) The North PacificRim isa continuousgreat-circle belt on the globe. 2) Prominenteast-west asymmetry of the PacificOcean and systematiceast-west dichotomyof arc behavior both probablystem from rotationalinfluences on platetectonics. 3) Late Cenozoictectonics around theNorth PacificRim involvedmainly the open- ing of interarc basins behind east-facing Eurasian arcs and rise-trench encounters at west- facing American subduction zones. 4) The formationof the Emperor-Hawaii elbow at about 40 mybp was roughly coeval with accretion of the Okhotsk block to Eurasia by crustal collision and with termina- tion of the classic Laramide deformation, which was caused by plate descent at an ab- normally shallow angle beneath America. 5) The North PacificRim lackedgeographic continuity prior to the lateMesozoic closure ofremnant ocean basins along suture belts within the land bridge between Amer- ica and Eurasia, and even circum-Pacific subduction itselfwas probably not underway prior to the early Mesozoic.

REFERENCES

AVDEIKO, G.P.,Evolution of geosynclineson Kamchatka, Pac.Geol., 3, 1-15, 1971. BARAZANGI,Muawia and B.L. ISACKS,Spatial distribution of and subductionof theNazca plate beneathSouth America,Geology, 4, 686-692,1976. BOSTROM, R.C.,Westward displacementof lithosphere,Nature, 234, 536-538,1971. BURK, C.A. and H.S. GNIBIDENKO,The structureand age of acousticbasement in the Okhotsk Sea,in Island Arcs,Deep Sea Trenches, andBack-Arc Basins, edited by Manik Talwaniand W.C. Pitman,III, pp. 451-462, A m. Geophys.Union MauriceEwing Ser.1, 1977. BYRNE,Tim, EarlyTertiary demise of theKula-Pacific spreading center, Geol .Soc. Am. Abstr.with Programs, 10,98, 1978. CHAI,B.H.T., Structure and tectonicevolution ofTaiwan, Am. J. Sci.,262, 389-422, 1972. CHAPMAN,M.C. and S.C.SOLOMON, North American-Eurasian plate boundary in northeastAsia, J . Geophys. Res.,81, 921-930, 1976. S 18 W.R. DICKINSON

CHAPPLE, W.M. and T.E. TULLIS, Evaluation of the forcesthat drive the plates,J. Geophys.Res., 82, 1967- 1984, 1977. CHURKIN, Michael, Jr.,Western boundary of the North American continental plate in Asia, Geol. Soc. Am. Bull., 83, 1027-1036, 1972. CONEY, P.J., Cordilleran tectonic transitions and motion of the North America plate,Nature, 233, 462-465, 1971. CONEY, P.J., Cordilleran tectonics and North America plate motion, Am. J. Sci.,262, 603-628, 1972. CONEY, P.J., and the , in Tectonicsand MineralResources of SouthwesternNorth America, edited by L.A. Woodward and S.A. Northrop, New Mex. Geol. Soc. Spec. Publ. No. 6, pp. 5- 10, 1976. CONEY, P.J., Mesozoic-Cenozoic Cordilleran plate tectonics, Geol. Soc. Am. Mem., 152, 33-50, 1978. COOPER, A.K.,D.W. SCHOLL,and M.S. MARLOW, Platetectonic model forthe evolution of theeastern Bering Sea basin,Geol. Soc. Am. Bull.,87, 1119-1126, 1976. DALRYMPLE, G.B. and D.A. CLAGUE, Age of the Emperor-Hawaiibend, Earth Planet. Sci. Lett., 31, 313-329, 1976. DALRYMPLE, G.B.,D.A. CLAGUE,and M.A. LANPHERE, Revisedage for Midway volcano,Hawaiian volcanic chain,Earth Planet. Sci. Lett., 37, 107-116,1977. DELONG, S.E.and P.J.Fox, Geologicalconsequences of ridgesubduction, in IslandArcs, Deep Sea Trenches,and Back-ArcBasins, edited by Manik Talwaniand W.D. Pitman,III, Am. Geophys.Union Maurice Ewing Ser.1, pp. 221-228,1977. DELONG, S.E.,P.J. Fox, and F.W. MCDOWELL, Subductionof theKula ridgeat theAleutian trench, Geol. Soc.Am. Bull.,89, 83-95, 1978. DEN, N. and H. HOTTA, Seismicrefraction and reflectionevidence supporting plate tectonics in Hokkaido, Pap.Meteorol. Geophys., 24, 31-54, 1973. DICKINSON,W.R., Evidencefor plate-tectonic regimes in therock record,Am. J. Sci.,272, 551-576,1972. DICKINSON,W.R., Widths of modern arc-trenchgaps proportionalto past durationof igneousactivity in associatedmagmatic arcs,J. Geophys.Res., 78, 3376-3389, 1973a. DICKINSON,W.R., Reconstructionof past arc-trench systems from petrotectonicassemblages in the islandarcs of thewestern Pacific, in The WesternPacific; Island Arcs, Marginal Seas, Geochemistry, edited by P.J.Cole- man, pp. 569-601,Univ. Western Press, Perth, 1973b. DICKINSON,W.R., Sedimentationwithin and besideancient and modern magmatic arcs,in Modern and An- cientGeosynclinal Sedimentation, edited by R.H. Dott,Jr. and R.H. Shaver,Soc. Econ. Paleontol.Mineral. Spec.Publ. No. 19,pp. 230-239,1974a. DICKINSON,W.R., Platetectonics and sedimentation,in Tectonics and Sedimentation, edited by W.R. Dickinson, Soc.Econ. Paleontol.Mineral. Spec. Publ. No. 22,pp. 1-27,1974b. DICKINSON,W.R., Sedimentarybasins developed during evolution of Mesozoic-Cenozoicarc-trench system in westernNorth America, Can. J. EarthSci., 13, 1268-1287,1976. DICKINSON,W.R., Paleozoicplate tectonics and the evolutionof the Cordillerancontinental margin, in PaleozoicPaleogeography ofthe Western United States, edited by J.H. Stewart,C.H. Stevens,and A.E. Fritsche, Pac.Sec. Soc. Econ. Paleontol. Mineral. Pac. CoastPaleogeogr. Symp. 1,pp. 137-156,1977a. DICKINSON,W.R., Subductiontectonics inJapan, EOS, Trans.Am. Geophys.Union, 58, 948-952, 1977b. DICKINSON,W.R. and W.S. SNYDER, Platetectonics of Laramide Orogeny,Geol. Soc. Am. Mem., 151, 355- 366, 1978. DICKINSON,W.R. and W.S. SNYDER,Geometry of triplejunctions related to San Andreastransform , J. Geo- phys.Res., 84, 1979a(in press). DICKINSON,W.R. and W.S. SNYDER, Geometry of subductedslabs related to San Andreas transform, J. G eol.,87, 1979b (inpress). DUNCAN, R.A. and Ian MCDOUGALL, Linearvolcanism in French , J. Volcanol.Geotherm. Res., 1, 197-227,1976. FORSYTH,Donald and S. UYEDA, On the relativeimportance of the drivingforces of platemotion , Geophys. J .R. Astr.Soc., 43, 163-200,1975. GREENE, H.G., G.B. DALRYMPLE, and D.A. CLAGUE, Evidencefor northward movement of the Emperor Seamounts,Geology, 6, 70-74, 1978. HASHIMOTO, S.,The basicplutonic rocks of theHidaka metamorphicbelt , J. Fac.Sci. Hokkaido Univ., Ser. IV ,16, 367-420, 1975. HAVEMANN, H., Displacement of dipping slabs of lithosphere, Earth Planet . Sci. Lett.,17, 129-134, 1972. HILDE, T.W.C., S. UYEDA, and L. KROENKE, Evolution of the western Pacificand itsmargin, Tectonophysics, 145-165, 1977. HYNDMAN, R.D., Plate motions relative to the deep mantle and the development of subduction zones , Nature, 238, 263-265, 1972. Plate Tectonic Evolution of North Pacific Rim S 19

IRVING,E., Drift of the major continentalblocks since the Devonian,Nature, 270, 304-309,1977. JARRARD, R.D. and D.A. CLAGUE, Implicationsof Pacificisland and seamount agesfor the originof volcanic chains,Rev. Geophys. Space Phys., 15, 57-76, 1977. KARIG, D.E.,Ridges and basinsof theTonga-Kermadec islandarc system, J. Geophys.Res., 75, 239-255, 1970. KARIG, D.E.,Structural history of theMariana islandarc system,Geol. Soc. Am. Bull.,82, 323-344, 1971. KARIG, D.E.,Plate convergence between the Philippinesand the Ryukyu Islands,Mar. Geol.,14, 153-168, 1973. KNOPOFF, Leon and A. LEEDS,Lithospheric momenta and the decelerationof the earth,Nature, 237, 93-95, 1972. LIPMAN, P.W., H.J. PROSTKA, and R.L. CHRISTIANSEN,Evolving subductionzones in the westernUnited States,as interpretedfrom igneousrocks, Science, 174, 821-825,1971. MARLOW, M.S.,D.W. SCHOLL,E.C. BUFFINGTON,and T.R. ALPHA, Tectonichistory of the centralAleutian arc,Geol. Soc. Am. Bull.,84, 1555-1574, 1973. MARSHAK,R.S. and D.E. KARIG,Triple junctions as a causefor anomalously near-trench igneous activity betweenthe trench and volcanicarc, Geology, 5,233-236, 1977. MARSHALL,Monte, The magneticproperties ofsome DSDP basaltsfrom theNorth Pacific and inferencesfor Pacificplate tectonics, J. Geophys. Res., 83, 289-308, 1978. MATSUDA,T. and S.UYEDA, On thePacific-type orogeny and itsmodel-Extension of the paired belts concept and possibleorigin of marginalseas, Tectonophysics, 11,5-27, 1971. MOBERLY, R., Originof lithospherebehind islandarcs, with referenceto the westernPacific, Geol. Soc. Am. Mem., 132,35-55, 1972. MOLNAR, Peterand , Interarcspreading and Cordillerantectonics as alternatesrelated to theage ofsubducted oceanic lithosphere, Earth Planet. Sci. Lett., 41, 330-340, 1978. MOORE, G.W., Westward tidallag as the drivingforce of platetectonics, Geology, 1, 99-100,1973. MORGAN, W.J.,Plate motions and deep mantle convection,Geol. Soc. Am. Mem., 132, 7-21,1972. MURPHY, R.W., The -West Taiwan foldbelt,a flipped subduction zone, Geol.Soc. Malaysia Bull.,No. 6, 27-42,1973. NELSON, T.H. and P.G. TEMPLE, Mainstream mantle convection;a geologicanalysis of platemotion, Am. Assoc.Pet. Geol. Bull., 56, 226-246, 1972. OKADA, H., Migrationof ancientarc-trench systems, in Modern and AncientGeosynclinal Sedimentation, edited by R.H. Dott,Jr. and R.H. Shaver,Soc. Econ. Paleontol.Mineral. Spec. Publ. No. 19,pp. 311-320,1974. PITMAN,W.C., III,R.L. LARSON,and E.M. HERRON, The age of theocean basins, Geol. Soc. Am. Map, 1974. PITMAN,W.C., IIIand Manik TALWANI, Sea-floorspreading in theNorth Atlantic,Geol. Soc. Am. Bull.,83, 619-646,1972. SCHOLL, D.W., E.C. BUFFINGTON,and M.S. MARLOW, Platetectonics and the structuralevolution of the Aleutian-BeringSea region,Geol. Soc. Am. Spec.Pap., 151, 1-31,1975. SCHOLL,D.W., J.R. HEIN,M.S. MARLOW, and E.C. BUFFINGTON,Meiji sediment tongue; North Pacificevi- dence forlimited movement between the Pacificand North American plates,Geol. Soc. Am. Bull.,88, 1567-1576,1977. SCHWEICKERT,R.A. and D.S. COWAN, Early Mesozoic tectonicevolution of the westernSierra Nevada, California,Geol. Soc. Am. Bull.,86, 1329-1336,1975. SILLITOE,R.H., Metallogeny of an Andean-typecontinental margin in South Korea; implicationsfor opening of theJapan Sea, in IslandArcs, Deep Sea Trenches,and Back-ArcBasins, edited by Manik Talwani and W.C. Pitman,III, Am. Geophys.Union Maurice Ewing Ser.1, pp. 303-310,1977. STEWART, R.J.,Turbidites of the Aleutian abyssalplain: Mineralogy, provenance, and constraintsfor Cenozoicmotion of the Pacificplate, Geol. Soc. Am. Bull.,87, 793-808, 1976. STONE,D.B. and D.R. PACKER, Tectonicimplications ofAlaska Peninsula paleomagnetic data, Tectonophysics, 37, 183-201,1977. TOMODA, Y.,J. KOBAYASHI,M. SEGAWA, M. NOMURA, K.KIMURA, and T. SAKI,Linear magneticanomalies in theShikoku basin, northern Philippine Sea, J. Geomag.Geoelectr., 28,47-56, 1975. UYEDA, S. and Zvi BEN-AVRAHAM, Originand developmentof the PhilippineSea, Nature,240, 176-178, 1972. UYEDA, S. and H. KANAMORI, Back-arcopening and the mode of subduction,1979 (inpress). UYEDA, S. and A. MIYASHIRO,Plate tectonics and theJapanese Islands, Geol. Soc. Am. Bull.,85, 1159-1170, 1974. WATTS, A.B.,J.K. WEISSEL,and R.L. LARSON, Sea-floorspreading in marginalbasins of thewestern Pacific, Tectonophysics,37, 167-181, 1977. WILSON,J.T. and Kevin BURKE, Two typesof mountain building,Nature, 239, 448-449,1972. ZONENSHAIN,L.P., M.I. KUZMIN, V.I. KOVALENKO, and A.J. SALTYKOVSKY,Mesozoic structural-magmatic patternand metallogenyof thewestern part of the Pacificbelt, Earth Planet. Sci. Lett., 22, 96-109,1974.