Early Evolution of the Bering Sea by Collision of Oceanic Rises and North Pacific Subduction Zones

Early evolution of the Bering Sea by collision of oceanic rises and North Pacific subduction zones

ZVI BEN-AVRAHAM Department of Geophysics, Stanford University Stanford, California 94305 ALAN K. COOPER U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025

ABSTRACT gin of the basin (Shor, 1964; Ludwig, 1974; basins such as the Bering Sea, Sea of Japan, Scholl and others, 1975; Cooper and Philippine Sea, South China Sea, and Coral Three major bathymetric features exist in others, 1976). Three large submarine rises Sea. the Bering Sea: Shirshov Ridge, Bowers lie inside the Bering Sea: Umnak Plateau, In general, plateaus or rises tower Ridge, and Umnak Plateau. New refraction Bowers Ridge, and Shirshov Ridge. The thousands of meters above their surround- data over Umnak Plateau and previous rises divide the Bering Sea into three deep- ing sea floor. Some rise above sea level, geophysical data across Bowers Ridge indi- water basins: Aleutian Basin, Bowers Basin, whereas others are 1,500 to 2,000 m deep. cate that a thickened welt of crustal mate- and Komandorsky Basin. Although the The crustal thickness of most plateaus for rial is present beneath both features. The Aleutian and Komandorsky basins are both which data are available is two to five times crustal structure is transitional between underlain by oceanic crust, Komandorsky greater than normal oceanic crust (¡8 km). oceanic and continental types. Basin may be younger than the Aleutian Some plateaus have a thicker upper crust Various models for the origin of these Basin because it contains less sediment and with compressional velocities in the range features have been investigated. One that has higher heat flow (Rabinowitz and of 6.0 to 6.3 km/s — typical of both layer 2c has not been proposed previously assumes Cooper, 1977). The Aleutian Island arc was in oceanic crust and granitic rocks in conti- that the protostructures of Bowers Ridge probably created prior to the change of nental crust (Fig. 2). The thickness of this and Umnak Plateau could have formed motion of the Pacific plate as evidenced by layer is an order of magnitude greater than outside of the present Bering Sea. According the bend in the Hawaii-Emperor seamount layer 2c (usually 1 km thick or less) in nor- to this model, before formation of the chains. The age of the bend is approxi- mal oceanic crust. Most plateaus exhibit Aleutian Ridge in late Mesozoic or earliest mately 43 m.y. (Dalrymple and others, weak or no magnetic lineations, which Tertiary time, these protostructures moved 1977). Thus, the formation of the Aleutian suggests that they are not generally formed into their present Bering Sea positions. island arc took place during the northward as typical oceanic crust. Drilling into the Prior to the arrival of these two structures motion of the Pacific and Kula plates. Prior top of the so-called basement of the Ontong in the Bering Sea, oceanic crust was sub- to the formation of the Aleutian Ridge, the Java Plateau revealed a few meters of Early ducted along the Bering continental margin subduction zone in the North Pacific is Cretaceous volcanic rocks beneath a cover connecting Alaska and Siberia. The colli- thought to have been along the present-day of more than one kilometer of calcareous sion of the Umnak Plateau protostructure Bering Sea continental margin (Scholl and sediment. The sediment indicates deposi- with the southeastern edge of the margin others, 1975) connecting Alaska and tion above the foraminiferal dissolution may have caused subduction to terminate Siberia. When the Aleutian Ridge was depth since Early Cretaceous time (An- here and move southward. The new south- formed, the subduction zone was shifted to drews, Packham, and others, 1975). Thus, erly position of subduction beneath the the Aleutian Trench. the Ontong Java Plateau has existed as a plateau at least since Early Cretaceous (Ap- Aleutian Ridge was therefore controlled by The rises in the Bering Sea could be part tian) time. The nature of the rocks underly- late Mesozoic or early Tertiary locations of of large terrae incognitae of the oceans — ing the Ontong Java volcanic basement re- Umnak Plateau, Bowers Ridge, and possi- the puzzling rises or plateaus found in all mains unknown. The same is true for other bly, the north-trending Shirshov Ridge present-day oceans, whose origin and na- plateaus, such as the Shatsky Rise and the farther to the west. ture are enigmatic. These features are not at Hess Rise. present classified as continents, active vol- INTRODUCTION canic arcs, or active spreading ridges, nor Various mechanisms have been offered in are they situated at present plate bound- the past for the formation of marginal ba- The Bering Sea is a large marginal basin aries. Some rises have been thought of as sins, but little agreement exists for their in the North Pacific that is characterized by extinct arcs, others as ancient spreading origin, which is one of the most important oceanic crust and thick overlying sediments. ridges, detached and submerged continental unsolved problems in plate tectonics. Uyeda Entrapment of a piece of oceanic plate, dur- fragments, anomalous volcanic piles, or (1977) shows that there are at least four ing Late Cretaceous or early Tertiary time, uplifted normal oceanic crust. Plateaus are possible processes for forming marginal behind the newly developing Aleutian arc particularly abundant in the western Pacific basins, including: (1) ridge subduction; (2) has been a popular explanation for the ori- (Fig. 1), but they also exist inside marginal entrapment of the old ocean basin by trans-

Geological Society of America Bulletin, Part 1, v. 92, p. 485 -495, 11 figs., 1 table, July 1981.

485

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60° -

30°-

0°-

100 Figure 1. Distribution of plateaus in the North Pacific. They exist in the main Pacific Basin as well as inside marginal basins. Hatched areas show outlines for plateaus. Included are rises that have been thought of as extinct arcs, ancient spreading ridges, detached and submerged continental fragments, anomalous volcanic piles, or uplifted normal oceanic crust.

formation of a transform fault into a sub- plateau with a subduction zone will pro- continental margin results. In the other duction system; (3) back-arc opening; and duce the same effects as continental colli- kind, the continental margin is active and (4) incipient spreading along a very leaky sion, even if the plateau is made of a pile of draws a passive block against it from a transform fault. We demonstrate in this volcanic material. Because of its thickened more seaward position. After crustal colli- paper that an additional process, collision crust, the plateau has a light root and prob- sion, subduction jumps to the opposite side of oceanic plateaus with subduction zones, ably an anomalous upper mantle. Therefore (if the accreted block, but a flip in direction can also be responsible for the formation of it is more buoyant than the adjacent oceanic does not occur. Dickinson (1978) indicated some marginal basins. We discuss in detail crust, and upon collision it will behave like that microcontinental blocks, dormant is- the evolution of the Bering Sea and briefly a continent and be accreted, rather than land arcs, seamount chains, and aseismic discuss examples of other marginal basins. subducted (Dickinson, 1978; Dewey, 1977; oceanic ridges can all be accreted in this Sengor, 1978). A new subduction zone will way. We can envision a third type of crustal COLLISION AS A MECHANISM FOR be created on the oceanic side of the ac- collision in which an active island arc col- MARGINAL SEAS OF FORMATION creted block. Dickinson (1978) discussed in lides with an active continental margin. In concept two main kinds of crustal collision. this type, the relative motion between the Collision of oceanic plateaus with sub- In the first kind, an active island arc collides two crustal units can be faster than in the duction zones produces profound effects with a passive continental margin, which it other two types. (Vogt and others, 1976), including reduced partially subducts. If the overall pattern of We suggest that, under certain condi- seismicity, shifts of volcanic activity, and plate kinematics induces subduction to re- tions, the collision of oceanic plateaus with shifting of plate boundaries so that new sume after such arc-continent collision, continental margins can produce new mar- subduction zones form elsewhere. We then subduction flips to the opposite side of ginal seas. Dickinson (1978) suggested that suggest that the collision of an oceanic the accreted arc structure and an active the collision of a large microcontinental

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UMNAK BOWERS MANIHIKI SHATSKY ONTONG JAVA trench; Shirshov Ridge is linear and has PLATEAU RIDGE PLATEAU RIDGE PLATEAU buried horst and graben basement relief; , 86 25 - „ W and Umnak Plateau is a triangular marginal 3.3 2.5 platform that lies between two diverging 3.3 5.4 TT 6.2 5.8 5.4 5.6 trends of an island arc and a continental 5.6 5.6 margin. Geologic and geophysical charac- 10-- 10-- 10- 10-- 10-- 6.1 6.5 6.8 7.0 teristics of each rise are summarized below,

8.1 6.9 and models for their incorporation into the Bering Sea are given in a subsequent sec- 7.5 20-- 20-- 20-- 20-- 20-- tion. KM 6.9 Bowers Ridge 30- 30-- 30-- 30-- 30- The bedrock core of Bowers Ridge has been sampled only at the northwestern end 8.0 of the ridge, where albitized volcanic and 40-- 40-- 40- 40-- 40-- thoroughly lithified volcaniclastic rocks of unknown age were recovered by the Scripps Institution of Oceanography (Scholl and others, 1975). The ridge is draped on both flanks by as much as 1,500 m of diatoma- COOPER LUDWI6 SUTTON DEN FURUMOTO AND OTHERS AND OTHERS AND OTHERS AND OTHERS AND OTHERS ceous sediment, turbidites, and mudstone (1980) (1971) (1971) (1969) (1976) (Fig. 5). Crestal areas of the ridge have been * = Assumed Velocity wave cut and beveled, and these areas, Figure 2. Comparison of crustal structure of oceanic plateaus in the Pacific Ocean: On- which at one time must have been at or tong Java Plateau, Shatsky Rise, and Manihike Plateau with oceanic plateaus inside the above sea level, have subsided to water Bering Sea: Bowers Ridge and Umnak Plateau. Numbers are compressional wave velocities depths of 500 to 1,000 m. in kilometres per second. Geophysical data from the west end of Bowers Ridge (Fig. 5) indicate that it is flanked by oceanic crust underlying the block with the margin of Eurasia formed BERING SEA RISES Bowers and Aleutian Basins. The velocity of the Okhotsk Sea. The colliding microconti- near-surface rocks beneath the ridge is high nent that occupies the entire Okhotsk Sea The three Bering Sea rises, Bowers Ridge, (5.8-6.2 km/s), yet mantle velocities have has caused the subduction zone to shift to Shirshov Ridge, and Umnak Plateau, are all not been recorded in any of the refraction the Kurile Trench. We suggest, additionally, prominent bathymetric features that stand profiles across the ridge (Ludwig and that collision of relatively small plateaus 1,000 to 3,500 m above the floor of the ad- others, 1971). Gravity modeling by Kienle can also cause relocation of plate bound- jacent abyssal basins (Fig. 4). Because little (1971) suggests that mantle lies at a depth aries (Fig. 3). The effect of collision depends is known about the geological and geophys- of about 25 km. North of the ridge, mul- on the geometry of the plate boundary. If an ical characteristics of these rises, several tifold seismic-reflection data show a ridge- oceanic plateau collides with an irregular mechanisms have been proposed to explain ward-dipping (southward) acoustic base- subduction zone at a point with sharp cur- their origin (Table 1). Each rise has a ment and deformed sedimentary rocks be- vature, it will have more effect in modifying characteristic physiographic shape that may neath the ridge's buried northern flank (Fig. plate boundaries than a collision with a be important in explaining its origin. Bow- 5). These observations have been used by straight segment of subduction zone. ers Ridge is arcuate with an adjacent buried Cooper and others (1981) as evidence that oceanic crust may have been thrust beneath the ridge in late Mesozoic or early Tertiary (a) time. A large difference in sediment thickness exists between the Aleutian and Bow- ers basins; this difference is evidence that Bowers Ridge has been a topographic rise and barrier to sediment transport since early Tertiary time. A large negative gravity anomaly that lies over the north ridge flank is caused by the extrerfie (8-10 km) thickness of low-density sediment that fills the buried trench and abuts the high-density core of the ridge. The ridge is also characterized by a broad off- Figure 3. A possible way to form a marginal basin: (1) oceanic plateau approaches a axis magnetic high with superimposed subduction zone, (b) the plateau collides with the subduction zone, and (c) a new plate high-amplitude, short-period anomalies boundary forms owing to the presence of the plateau. that are typical of volcanic rocks. This shift

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may imply that a body of nonmagnetic rock Komandorsky and Aleutian basins. The 6 km). Rocks dredged along the west side of (folded sediment?) underlies the north flank ridge is enigmatic because it forms the Shirshov Ridge are lithified andestic tuff of the ridge (Cooper and others, 1981). The structural boundary between the Koman- and albitized palagonite tuff (Scholl and entire length of the ridge, from the western dorsky Basin — characterized by shallow others, 1975). The age of a sample recov- distal connection with Shirshov Ridge to (4-5 km) igneous crust, high heat flow (2-4 ered near the southern terminus of the ridge the southeastern attachment to the Aleutian HFU), and thin sediment (1-2 km) — and is 16.8 to 18 m.y. (early Miocene; Scholl Ridge, is marked by this magnetic anomaly, the Aleutian Basin, with a deeper igneous and others, 1975). In this southern area, which follows the volcanic core of the ridge. crust (7-9 km), more normal heat flow seismic reflection data (Rabinowitz, 1974; Because of its arcuate shape, thick sedi- (1-1.8 HFU), and thicker sediment (4- Scholl and others, 1975) show evidence for ment wedge, large magnetic anomalies, and dredged volcanic rocks, Bowers Ridge ap- TABLE 1. POSTULATED ORIGINS FOR OCEANIC RISES IN THE BERING SEA pears to be a thickened welt of basaltic rock or a volcanic arc that has formed by sub- Bowers Ridge duction beneath the north side of the ridge. Ancient island arc Kienle (1971), Scholl and others (1975) The location of the ridge during this con- Remnant arc Karig (1972) structional period is unknown, and it may Outgrowth of Aleutian Ridge Scholl and others (1975) have formed at its present latitude within Microcontinent Nur and Ben-Avraham (1978) the Bering Sea (Scholl and others, 1975) or Shirshov Ridge far to the south, prior to the development of Ancient spreading center Kienle (1971) Ancient island arc the Aleutian arc (Ben-Avraham and others, Scholl and others (1975), Dickinson (1978) Remnant arc Karig (1972) 1980). Continuation of onshore Koryak Mountains Scholl and others (1975) Microcontinent Nur and Ben-Avraham (1978) Shirshov Ridge Umnak Plateau Subsided continental margin Scholl and others (1968) Shirshov Ridge is a 600-km-long linear Uplifted oceanic crust Cooper and others (1980) and asymmetric rise that separates the Microcontinent Emery and Skinner (1977)

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GRAVITY

V.E.=4:1

Figure 5. Geophysical profiles across Bowers Ridge modified from Cooper and are from Ludwig and others (1971) and Cooper and others (1979). Physiographic dia- others (1981). The interpretive section is from a 24-fold seismic line. Refraction data gram from Alpha (1974). See Figure 4 for location.

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uplift of young sedimentary beds. The re- Shirshov cent uplift may result from a renewal or Ridge GRAVITY continuance of the volcanic processes that initially formed the ridge. At DSDP site 191 in the Komandorsky Basin, tholeiitic basalt of Oligocene age (29.6 m.y.; Scholl has ex- pressed reservations about the correctness of the age, written commun., 1980) was recovered from the basement adjacent to Shirshov Ridge. The age of these basement rocks is significantly younger than the presumed Early Cretaceous age (Cooper and others, 1976) of basement on the east MAGNETICS side of Shirshov Ridge. Refraction data near Shirshov Ridge are limited to shallow crustal horizons (Ludwig and others, 1971; Rabinowitz and Cooper, 1977), and the data show typical velocities (4.95-5.73 km/s) for igneous oceanic layer 2 (Fig. 6). Gravity data indicate that beneath the ridge the depth to mantle may be 20 to 25 km, which is similar to that modeled SEISMIC DATA under Bowers Ridge (Kienle, 1971). Gravity data do not, however, indicate that Shir- 87 82 shov Ridge is flanked by a thick sediment- filled trench on either its east or west side. A Shirshov Ridge -2 large-amplitude anomaly centered over the 4 ' axis of the ridge can be traced 500 km from |- 6 Cape Olyiotorsky to the south end of the ridge. Magnetic anomalies over the southern part of the ridge have been modeled 100 (Cooper and others, 1976), and the geom- I etry of the model bodies is consistent with Km an origin of the ridge by either injection of Figure 6. Geophysical profiles across Shirshov Ridge modified from Rabinowitz and widespread volcanic centers along linear Cooper (1977). Physiographic diagram from Alpha (1974). See Figure 4 for location. fractures or by uplift of oceanic crust. The andesitic composition of the basement rocks supports an origin by constructional volcanism rather than by uplifting sea floor Model studies of magnetic data along the OCEANIC RISES AND EVOLUTION or by uplifting an ancient spreading ridge. edge of the plateau and the shelf (Cooper O F THE ALEUTIAN RIDGE and others, 1980) indicate the possibility Umnak Plateau that the 2-km-thick magnetic crustal layer Multichannel seismic data over Bowers of the Aleutian Basin has been uplifted to Ridge (Fig. 5) confirm the previous interpre- Umnak Plateau lies at the junction of two form the plateau, but not the shelf riclge. tations of Bowers Ridge as an inactive is- diverging and different structures, the Cooper and others (1980) also suggest that land arc and show a fossil subduction zone southwest-trending Aleutian Ridge of the ocean-continent transition lies on the on its north side. The upper sedimentary Eocene through Holocene age, and the landward side of the plateau. units are continuous from the ridge to the northwest-trending and mostly buried Gravity data from the plateau are Aleutian Basin and indicate little if any foldbelt of Mesozoic rocks that underlies influenced locally by topographic relief of Neogene convergent activity. Additional the outer Bering shelf. No samples of base- exceptionally large canyons that cut multichannel profiles shown by Cooper and ment rocks were recovered in either of the through the plateau and regionally by the others (1981) further suggest that the Ber- two Deep Sea Drilling Project (DSDP) holes shallowing bathymetric gradient from the ing Sea margin was a former subduction drilled into the plateau. Terrigenous rocks Aleutian Basin to the shelf. Seismic reflec- zone, probably associated with oblique are the predominant constituent of the shelf tion profiles show that a thick sediment Mesozoic convergence (Fig. 8). As no evi- basement Mesozoic rocks, whereas volcanic wedge lies immediately seaward of the shelf dence exists in the seismic profiles and debris is the most prevalent component in edge and that the upper part of this sedi- magnetic anomalies (Cooper and others, the Aleutian rocks. Refraction data rec- ment is not significantly folded. If the sedi- 1981) for a spreading ridge in the area be- orded in the Umnak Plateau area (Shor, ment is of early Cenozoic age, which is true tween Bowers Ridge and the Bering Sea 1964; Cooper and others, 1980) show a elsewhere along the margin, then there has margin, relative motion in the past must high-velocity layer (5.3-5.6 km/s) beneath been little horizontal crustal motion behave occurred between these areas when the plateau (Fig. 7). Depth to mantle is un- tween Umnak Plateau and the Bering mar- underthrusting of the oceanic lithosphere certain. gin since that time. was active. This conclusion, if correct, can

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Umnak be interpreted to mean that Bowers Ridge Plateau was not formed in situ but came to its present location together with the northward- GRAVITY moving Kula plate, or on another plate, prior to the formation of the Aleutian Ridge. Consequently, Bowers Ridge would be older than the Aleutian Ridge south of it. Umnak Plateau is situated between the Bering Sea margin and the Aleutian Ridge. No data are available to allow us to decide 500-1 whether the plateau has been formed in situ. Even the nature of the plateau margins MAGNETICS is obscure, for the northeastern margin gamma cannot be clearly discriminated from the 0 - Bering Sea margin, and the southern margin has been overprinted by the Aleutian Ridge. Umnak Plateau may not have formed in -500- its present position. Recent geological and SEISMIC DATA geophysical investigations indicate that 80 86 74 73 7 1 68 most of Alaska and the Alaskan Peninsula is 0 built of allochthonous terranes (Jones and 2 - others, 1977, 1980; Jones and Silberling, 1979), some of which have moved great dis- Km 4 - tances. Paleomagnetic data from southern 6 Alaska (Hillhouse, 1977; Stone, 1977) indi- 8 H cate that some Mesozoic rocks formed at low paleolatitudes and were tectonically 10 50 100 transported northward to their present lo- 12 8.1* ' cation. Thus, Umnak Plateau may also have Km 14 been transported to its present position. If ' assumed velocity this notion is correct, then Umnak Plateau and Bowers Ridge are similar to other allochthonous terranes in Alaska and may even have been parts of the same structures in the ocean at one time. The concept that Bowers Ridge and Umnak Plateau could have originated outside the present Bering Sea has a major tectonic implication. In the following sections, two models based on this concept are described for the evolution of the Bering Sea.

Model 1 Figure 7. Geophysical profiles across Umnak Plateau. Gravity, magnetic, and bathymetric data were recorded along line EF (U.S. Geological Survey Cruise S3-77). The sonobuoy refraction data are from Childs and others (1979) and Cooper and others (1980) Model 1 proposes that, prior to the for- and are within 25 km of line EF. Physiographic diagram from Alpha (1974). See Figure 4 mation of the Aleutian Ridge, the proto- for location. structures of Bowers Ridge and Umnak Plateau were located south of their present location, probably on the Kula plate, and w E BOWERS RIDGE ALEUTIAN BASIN BERING MARGIN 4 Figure 8. Interpretive cross section m: based on seismic data from line RS-765 .Ill shown in Cooper and others (1981). The cross section illustrates that buried trenches

0 50 km are present on the northern margin of the 1 I Bowers Ridge and along the Bering margin and that there is no buried spreading center NEOGENE SEDIMENT between them. ['" '"'] CRETACEOUS (?) BASALT PALEOGENE (?) SEDIMENT

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moved with it to the north in a direction parallel to the trend of the Emperor sea- mounts (Fig. 9). A long, north-trending transform fault is postulated to have existed west of Bowers Ridge at that time. Shirshov Ridge could have been formed in a manner similar to that of the Ninety-East Ridge in the Indian Ocean (Curray and others, 1981), that is, by hot-spot activity near the transform fault. The transform fault could also have served as a mechanism to bring different crustal units next to each other. This hypothesis can explain the large differences in geologic and geophysical properties between the Aleutian and Komandorsky basins on both sides of Shirshov Ridge. Northward motion of the Kula plate re- sulted in collision of the Umnak Plateau protostructure with the south edge of the Bering Sea margin, which was a converging plate boundary at that time. The collision could have caused subduction to jump southward. The new southerly position of DIRECTION OF PLATE subduction beneath the Aleutian Ridge was MOTION therefore controlled by the late Mesozoic or ACTIVE SUBDUCTION early Tertiary locations of Umnak Plateau, ZONE Bowers Ridge, and, possibly, the newly AAA FORMER SUBDUCTION ZONE formed Shirshov Ridge farther to the west. TRANSFORM FAULT In Figure 9 both present and past plate boundaries are shown. The model does not take into account two possible alterations of it. First, the Komandorsky crust appears to be early or middle Tertiary in age and therefore younger than the Aleutian Ridge. However, since the age of the basin was determined only in one location, future studies may show that the basin is older. Second, the model does not take into account the origin of the north-south—trending magnetic anomalies in the Aleutian Basin (Cooper and others, 1976) which are thought to have been generated by sea-floor spreading. In order to explain them, we must assume a previous evolutionary phase in which these anomalies have been formed. The 18- m.y.-old rocks recovered from the south end of Shirshov Ridge and the apparent recent activity in the area between Shirshov and Bowers Ridges may indicate that the hot spot that produced the Shirshov Ridge Figure 9. Evolutionary model 1 for the formation of the Aleutian Ridge. Bowers Ridge in Mesozoic time (if there was one) still has and Umnak Plateau are thought to have come from the south with the Kula plate, and minor activity. Shirshov Ridge to have formed in place. A, late Mesozoic; B, early Tertiary.

Model 2 Plateau are parts of a large arc structure (Hilde and others, 1977). East-west- In model 2 we use the same idea of colli- that was situated east of the Kamchatka trending transform faults separate the three sion between Umnak Plateau and the Peninsula or that may even have been part rises on the west. One of the faults could be Mesozoic subduction zone along the Bering of it. A north-trending spreading ridge is along the present location of the Aleutian Sea margin (Fig. 10). Here we envision that thought to have existed at that time in the Ridge. The spreading ridge eventually dis- Shirshov Ridge, Bowers Ridge, and Umnak area where the present Bering Sea is located appears as it is subducted along the Bering

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Figure 10. Evolutionary model 2 for the formation of the Aleutian Ridge. In this model, Shirshov Ridge, Bowers Ridge, and Umnak Plateau are thought to have origi- nally been parts of a large arc structure that either was situated east of the Kamachatka Peninsula or may even have been part of it. A and B, late Mesozoic; C, early Tertiary. <

fects as those described for model 1, that is, it could cause the subduction zone to move southward to the location of the Aleutian TRANSFORM FAULT Ridge. One of the differences between the models is that model 2 predicts an ancient subduction zone along the eastern margin of the Shirshov Ridge, and model 1 does not. Model 2 is consistent with the exis- tence of andesitic tuff on Shirshov Ridge (Scholl and others, 1975). There is, however, no evidence for an ancient trench along either side of the ridge. Essential evidence necessary to verify any of the models presented above, such as paleomagnetic data from three oceanic plateaus in the Bering Sea and drilling into the basement of the Aleutian Basin to de- termine its age and nature, is still missing.

EXAMPLES FROM OTHER MARGINAL BASINS

Okhotsk Sea

Collision could have played an important role in formation of other marginal basins around the Pacific Ocean. Dickinson (1978) proposed that a microcontinental Okhotsk block of unknown origin collided with the eastern margin of Eurasia during early Cenozoic time and caused the subduction zone to move into a new easterly location along the Kuril Ridge. The continental fragment lodged in the Okhotsk Sea is submerged; hence a new marginal sea formed.

Shikoku Basin

Another example of collision, which is Sea margin, leaving behind north-trending formed by east-west spreading (Shor and similar to the situation we hypothesize for magnetic anomalies that are older to the Fornari, 1976), which separated Shirshov the Bering Sea, exists in the Shikoku Basin west. This concept is consistent with the Ridge from the Kamchatka Peninsula south of Japan, at present, there are two conclusion of Cooper and others (1976) (Fig. 10). volcanic arcs on Japan. One is the northeast that the magnetic anomalies become The collision of Umnak Plateau with the Japan arc, which includes the Kuril Islands, younger to the east. Komandorsky Basin Bering Sea margin had the same tectonic ef- northeast Japan, and the Izu-Mariana arcs;

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the other is the Ryukyu arc. In Middle Cre- 1972). Upon collision with Japan, the sub- the Kula plate is located now west of the taceous time, the subduction zone was con- duction zone was shifted to the east side of Kyushu-Palau Ridge. tinuous from the Kurile arc to the Ryukyu the Izu-Bonin Ridge, which was then trans- arc (Uyeda and Miyashiro, 1974). At that formed into an arc, thus forming two dis- CONCLUSION time the Japan arc formed a straight seg- tinct arcs in the area. Also, a part of the ment. The collision of an oceanic plateau Kula plate was isolated between the The presence of oceanic plateaus in the with the arc could have had a major Ryukyu arc and the proto—Izu-Bonin arc. A Pacific Ocean implies that massive colli- tectonic impact. For example, Matsuda later development in this area, which could sions between the plateaus and subduction (1978) suggests that aseismic ridges that be caused by change in the direction of mo- zones will eventually take place. Collisions were situated south of Japan moved north tion of the Pacific plate, was the splitting of are now occurring at a few places around together with the Kula plate and eventually the proto—Izu-Bonin arc and rifting be- the Pacific and are producing profound ef- collided with the Japan arc. This collision is tween the western part, which is now form- fects on subduction of the oceanic plates. thought by Matsuda (1978) to have caused ing the northern part of the Kyushu-Palau Recent work on the land areas around the the bend in the Japan arc and also the Ridge, and the eastern part, which forms Pacific rim has shown that extensive colli- counter-clockwise rotation of northern the present Izu-Bonin arc (Fig. 11). The rift- sion of various kinds of oceanic plateaus Honshu. We suggest that the proto—Izu- ing then has caused formation of the with the continental margin occurred in the Bonin Ridge actually came from the south Shikoku Basin by sea-floor spreading be- past. Thus, collision of thickened crustal along the postulated Kyusho-Palau trans- ginning about 30 m.y. ago (Kobayashi and masses was a major process in tectonic de- form fault (Uyeda and Ben-Avraham, Nakada, 1978). Thus, the isolated part of velopment of the Pacific margins. It is thought to have caused continental growth and Cordillera mountain building (Monger and Price, 1979). We propose that collisions could also have been an important MIDDLE CRETACEOUS EARLY TERTIARY process in the evolution of marginal basins. In brief, under certain conditions, the collision of an oceanic plateau would stop subduction and cause a new subduction zone to form behind the colliding plateau, thereby creating a new marginal basin by isolating parts of the Pacific Basin behind the newly formed plate boundary. We have outlined two speculative models for the evolution of the Aleutian, Koman- dorsky, and Bowers basins of the Bering \ Sea. A concept common to both is that the collision of the Umnak Plateau with a Mesozoic arc system along the Bering Sea margin caused the subduction zone and re- lated arc to shift to a new location. The new southerly position of subduction caused the Aleutian Ridge to form, and its location was controlled by the special arrangements of Umnak Plateau, Bowers Ridge, and Shir- shov Ridge in late Mesozoic or earliest Ter- tiary time. Further studies of this area and especially of the enigmatic Umnak Plateau will help to evaluate the evolutionary models we outlined in this paper.

ACKNOWLEDGMENTS

We wish to thank David L. Jones and David W. Scholl of the U.S. Geological Sur- vey, Menlo Park, California, who critically reviewed the manuscript and provided help- ful comments.

REFERENCES CITED

Figure 11. Plate motion and the formation of the Izu-Bonin arc (modified from Mat- Alpha T R 1974 orthographic drawing of the suda, 1978). Middle Cretaceous (a) to Quaternary (d); KU, Kula plate; PA, Pacific plate; Bering Sea: U.S. Geological Survey Open- PH, Philippine Sea plate. File Report.

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Andrews, J. E., Packham, G., and others, 1975: Hilde, T.W.C., Uyeda, S., and Kroenke, L., 1977, ern Bering Sea: Marine Geology, v. 24, Washington, D.C., Initial reports of the Evolution of the western Pacific and its p. 309-320. Deep Sea Drilling Project, v. 30, (U.S. Gov- margin: Tectonophysics, v. 38, p. 145- Scholl, D. W., Buffington, E. C., and Hopkins, ernment Printing Office). 165. D. M., 1968, Geologic history of the conti- Ben-Avraham, Z., Cooper, A. K., and Scholl, Hillhouse, J. W., 1977, Paleomagnetism of the nental margin of North America in Bering D. W., 1980, Mesozoic motion of oceanic Triassic Nikolai Greenstone, south-central Sea: Marine Geology, v. 6, p. 297-330. plateaus and ridges and the formation of the Alaska: Canadian Journal of Earth Sci- Scholl, D. W., Buffington, E. C., and Marlow, Aleutian Islands and Bering Sea: Geological ences, v. 14, p. 2578-2592. M. S., 1975, Plate tectonics and the struc- Society of America, Abstracts with Pro- Jones, D. L., and Silberling, N. J., 1979, tural evolution of the Aleutian-Bering Sea grams, v. 21, p. 96-97. Mesozoic stratigraphy — The key to region, in The geophysics and geology of Childs, J. R., Cooper, A. K., and Wright, A. W., tectonic analyses of southern and central the Bering Sea region: Geological Society of 1979, Sonobuoy studies of Umnak Plateau, Alaska: U.S. Geological Survey Open-File America Special Paper 151, p. 1—31. Bering Sea: Transactions of the American Report 79-1200, 41 p. Sengor, A.M.C., 1978, Collision of irregular Geophysical Union, v. 60, p. 390. Jones, D. L., Silberling, N. J., and Hillhouse, continental margins: Implications for land Cooper, A. K., Marlow, M. S., and Scholl, D. W., J. W., 1977, Wrangellia — A displaced ter- development of Alpine-type orogens: Geol- 1976, Mesozoic magnetic lineations in the rane in northwestern North America: ogy, v. 4, p. 779-782. Bering Sea marginal basin: Journal of Canadian Journal of Earth Sciences, v. 14, Shor, G. G., Jr., 1964, Structure of the Bering Sea Geophysical Research, v. 81, p. 1916- p. 2565-2577. and the Aleutian Ridge: Marine Geology, 1934. Jones, D. L., Silberling, N. J., Csejtey, Bela, Jr., v. 1, p. 213-219. Cooper, A. K., Scholl, D. W., and Marlow, M. S., Nelson, W. H., and Blome, C. P., 1980, Age Shor, G. G., Jr., and Fornari, D. J., 1976, Seismic 1979, Hydrocarbon potential of the and structural significance of ophiolite and refraction measurements in the Kamchatka Aleutian Basin, Bering Sea: American As- adjoining rocks in the Upper Chulitna dis- Basin, western Bering Sea: Journal of sociation of Petroleum Geologists Bulletin, trict, south-central Alaska: U.S. Geological Geophysical Research, v. 81. p. 5260- v. 64, p. 439. Survey Professional Paper 1121-A, 21 p. 5266. Cooper, A. K., Scholl, D. W., Vallier, T. L., and Karig, D. E., 1972, Remnant arcs: Geological Stone, D. B., 1977, Plate tectonics, paleomag- Scott, E. W., 1980, Resource report for the Society of America, Bulletin, v. 83, netism and the tectonic history of the N.E. deep-water areas of proposed OCS lease p. 1057-1068. Pacific: Geophysical Surveys, v. 3, p. 3-37. sale #70, St. George Basin, Alaska: U.S. Kienle, J., 1971, Gravity and magnetic mea- Sutton, G. H., Maynard, G. L., and Hussong, Geological Survey Open-File Report 80- surements over Bowers Ridge and Shirshov D. M., 1971, Widespread occurrence of a 246, 90 p. Ridge, Bering Sea: Journal of Geophysical high-velocity basal layer in the Pacific crust Cooper, A. K., Marlow, M. S., and Ben- Research, v. 75, p. 7128-7153. found with repetitive sources and Avraham, Z., 1981, Multichannel seismic Kobayashi, K., and Nakada, M., 1978, Magnetic sonobuoys (with discussion), in Structure evidence for the origin of Bowers Ridge: anomalies and tectonic evolution of the and physical properties of the Earth's crust: Geological Society of America Bulletin, Shikoku inter-arc basin: Journal of the American Geophysical Union Monograph Part I, v. 92, p. 000-000 (this volume). Physics of the Earth, v. 26, Suppl., No. 14, p. 193-209. Curray, J. R., Emmel, F. J., Moore, D. G., and p. S391-S402. Uyeda, S., 1977, Some basic problem in the Raitt, R. W., 1981, Structure, tectonics and Ludwig, W. J., 1974, Structure of the Bering Sea trench-arc back-arc system, in Talwani, M., geological history of the northeastern In- basins, in Burk, C. A., and Drake, C. L., and Pitman, W. C., III, eds., Island arcs, dian Ocean, in Nairn, A.E.N., and Stehli, eds, The geology of continental margins: deep sea trenches, and back-arc basins: F. G. eds., the ocean basins and margins, v. New York, Springer, p. 661-668. Washington, D.C., American Geophysical 6: The Indian Ocean: New York-London, Ludwig, W. J., Murauchi, S., Den, N., Ewing, Union, Maurice Ewing Series 1, p. 1-14. Plenum Press, (in press). M., Hotta, H., Houtz, R. E., Yoshii, T., Uyeda, S., and Ben-Avraham, Z., 1972, Origin Dalrymple, G. B., Clague, D. A., and Lanphere, Asanuma, T., Hagiwara, K., Sato, T., and and development of the Philippine Sea: Na- M. A., 1977, Revised age for Midway vol- Ando, S., 1971, Structure of the Bowers ture, Physical Sciences, v. 240, p. 176- cano, Hawaiian Volcanic chain: Earth and Ridge, Bering Sea: Journal of Geophysical 178. Planetary Science Letters, v. 37, p. 107- Research, v. 76, p. 6350—6366. Uyeda, S., and Miyashiro, A., 1974, Plate 116. Matsuda, T., 1978, Collision of the Izu-Bonin arc tectonics and the Japanese Islands: A syn- Den, N., Ludwig, W. J., Murauchi, S., Ewing, with Central Honshu: Cenozoic tectonics of thesis: Geological Society of America Bulle- J. I., Hotta, H., Edgar, N. T., Yoshii, T., the Fossa Magna, Japan: Journal of the tin, v. 85, p. 1159-1170. Asanuma, T., Hagiwara, K., Sato, T., and Physics of the Earth, v. 26, Suppl., Vogt, P. R., Lowrie, A., Bracey, D. R., and Hey, Ando, S., 1969, Seismic-refraction mea- p. S409-S421. R. N., 1976, Subduction of aseismic oceanic surements in the northwest Pacific basin: Monger, J.W.H., and Price, R. A., 1979, ridges: Effects on shape, seismicity, and Journal of Geophysical Research, v. 74, Geodynamic evolution of the Canadian other characteristics of consuming plate p. 1421-1434. Cordillera—Progress and problems: Cana- boundaries: Geological Society of America Dewey, J. F., 1977, Suture zone complexities: A dian Journal of Earth Sciences, v. 16, Special Paper 172, 59 p. review: Tectonophysics, v. 40, p. 53-67. p. 770-791. Dickinson, W. R., 1978, Plate tectonic evolution Nur, A., and Ben-Avraham, Z., 1978, Specula- of north Pacific rim: Journal of Physics of tions on mountain building and the lost Pa- the Earth, v. 26, Suppl. S1-S19. cifica continent: Journal of the Physics of Emery, K. O., and Skinner, B. J., 1977, Mineral the Earth, v. 26, Suppl., p. S21—S37. deposits of the deep ocean floor: Marine Rabinowitz, P. D., 1974, Seismic profiling be- Mining, v. 1, p. 1—71. tween Bowers Ridge and Shirshov Ridge in MANUSCRIPT RECEIVED BY THE SOCIETY SEP- Furumoto, A. S., Webb, J. P., Odegard, M. E., the Bering Sea: Journal of Geophysical Re- TEMBER 4, 1980 and Hussong, D. M., 1976, Seismic studies search, v. 79, p. 4977-4979. REVISED MANUSCRIPT RECEIVED MARCH 9, on the Ontong Java Plateau, 1970: Tecto- Rabinowitz, P., and Cooper, A. K., 1977, Struc- 1981 nophysics, v. 34, p. 71-90. ture and sediment distribution in the west- MANUSCRIPT ACCEPTED MARCH 16, 1981

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