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Plate Tectonic Evolution of North Pacific Rim

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Plate Tectonic Evolution of North Pacific Rim J. Phys. Earth,26, Suppl., S 1-S 19, 1978 PLATE TECTONIC EVOLUTION OF NORTH PACIFIC RIM William R. DICKINSON GeologyDepartment, Stanford University, Stanford, California, 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 subduction,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 Pacific Ocean extends from Central America on the east to the Philippine Sea on the west, and has the Bering Sea 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 Americas 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 Mexico 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 Earthquake 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 North America 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 marginaland 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 hotspot 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-Eurasian plate 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 Pacific
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