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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/7/485/3419290/i0016-7606-92-7-485.pdf by guest on 27 September 2021 486 BEN-AVRAHAM AND COOPER 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/7/485/3419290/i0016-7606-92-7-485.pdf by guest on 27 September 2021 EARLY EVOLUTION OF THE BERING SEA 487 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).