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DAVID W. SCHOLL US Geological Survey Menlo GEOLOGICAL SOCIETY OF AMERICA SPECIAL PAPER 151 ©1975 Plate Tectonics and the Structural Evolution o f the Aleutian-Bering Sea Region DAVID W. SCHOLL U.S. Geological Survey Menlo Park, California 94025 U.S.A. EDWIN C. BUFFINGTON Naval Undersea Center San Diego, California 92132 U.S.A. MICHAEL S. MARLOW U.S. Geological Survey Menlo Park, California 94025 U.S.A. ABSTRACT The general aspects of the structural evolution of the Aleutian-Bering Sea region can be described in terms of plate tectonics. Involved in this model is the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. The ridge is presumed to have formed in response to a southward relocation in the convergence zone of the Pacific oceanic plate, a shift away from the Beringian continental margin connecting Alaska and Siberia to an oceanic location at the Aleutian Trench. Prior to the formation of the ridge, Pacific crust is presumed to have directly underthrust the northeast-trending Koryak-Kamchatka coast. The middle and late Mesozoic eugeosynclinal or thalassogeosynclinal masses that underlie this segment 1 2 SCHOLL AND OTHERS of the Pacific fold belt are highly deformed, thrust faulted, and intruded by ultra- mafic bodies—characteristics that can be ascribed to the mechanical and magmatic consequence of plate underthrusting. This model implies a similar orogenic process for the formation of the stratigraphically and structurally similar Mesozoic rocks underlying the northeast-trending continental margin of southern Alaska. Less intense underthrusting may have occurred along the northwest-trending Pribilof segment of the Beringian margin connecting Alaska and Siberia. This margin may have been more parallel to the approximate direction of relative motion be­ tween the oceanic and continental plates. Nonetheless, fold belts, possibly intruded by ultramafic masses, formed along this segment of the Beringian continental margin in Late Cretaceous and perhaps earliest Tertiary time. The folds have since subsided below sea level—their eroded tops presently underlying as much as 3 km of virtually undeformed Cenozoic deposits. Our model relates pre- and postorogenic deposits underlying the Beringian margin and adjacent coast to the time of formation of the Aleutian Ridge, which marked the cessation of continental underthrusting and the beginning of island-arc under­ thrusting. Our model also implies that the ridge formed near or at its present loca­ tion and that oceanic crust of late Mesozoic age underlies the Aleutian Basin of the Bering Sea. Since formation of the ridge this basin has received from 2 to 10 km of sedimentary fill. Although the model we suggest broadly explains the observed changes in tectonic style, magmatic history, and sedimentation for the Aleutian-Bering Sea region, it also implies that thousands of kilometers of oceanic crust underthrust the Kamchat­ ka, Beringian, and Alaskan margins between Late Triassic and Late Cretaceous time, and hundreds of kilometers underthrust the Aleutian Ridge in Cenozoic time. The enormous masses of pelagic and volcanic offscrapings that would be indicative of extensive or long-term crustal underthrusting are not apparent as mappable units. Thus, while our model may be stylistically adequate, it paradoxically predicts quantities of rocks and structures that we are not able to find. Presumably they have been subducted. INTRODUCTION During the last decade an impressive volume of bathymetric, geologic, and geo­ physical data has been gathered in the Aleutian-Bering Sea region (Fig. 1). Emerging from this information is a partically decipherable geologic history extending back into Mesozoic time—a history that includes the formation of the Aleutian Ridge, the sedimentary infilling of the deep basin of the Bering Sea, and the structural evolution of the Beringian continental margin, which reaches in a broad northward- swinging arc from Cape Kamchatka on the west to the Alaska Peninsula on the east (Fig. 1). Although a number of geotectonic models have already been applied to the Aleutian-Bering Sea region (see Stone, 1968; Cameron and Stone, 1970; and Perry, STRUCTURAL EVOLUTION, ALEUTIAN-BERING SEA REGION 3 1971, for reviews of some of these schemes), we will attempt to relate prior and new findings within a framework of the expected consequences of sea-floor spread­ ing and interacting lithospheric plates (Isacks and others, 1968; Mitchell and Reading, 1969, 1971; Hamilton, 1969, 1970; Dewey and Horsefield, 1970; Ernst, 1970; Dewey and Bird, 1970; Oxburgh and Turcotte, 1970, 1971; Matsuda and Uyeda, 1971;Hasebe and others, 1970). Our purpose in preparing this paper has been to determine how the structural evolution of the Aleutian-Bering Sea region fits in with the suspected pattern of global tectonics, which many geologists believe is the causative mechanism behind the formation of the Pacific fold belt of late Paleozoic through Cenozoic rocks. To some extent we are molding our data to a prescribed model, but our general ability to do this, although we encounter difficulties, must not in itself be taken as proof that the plate models are valid. These models, however, are exceptionally instructive in that they allow us to treat the structural and magmatic evolution of this vast area in a unified way. This paper contains speculations about these unifying schemes. The Aleutian-Bering Sea region includes the Aleutian Trench and bordering ridge, the Bering Sea Basin (that part of the sea exclusive of its broad and shallow shelf) north of the ridge, and the Beringian continental margin sweeping northward around the basin between Alaska and Kamchatka. The submerged physiography of this vast region has been described and discussed by Gibson and Nichols (1953), Udinstev and others (1959), Gibson (1960), Gershanovich (1963), Nichols and others (1964), Kotenev (1965), Nichols and Perry (1966), Perry and Nichols (1966), Lisitsyn (1966), Scholl and others (1968), Chase and others (1970), and Perry (1971). Because the deep Bering Sea did not exist prior to the formation of the ridge, we discuss first the known geology of this ridge and its possible relation to postu­ lated movements of major lithospheric plates. Next we consider what is known about the structural, magmatic, and sedimentary histories of the deep Bering Sea and its bordering continental margin-also interpreting these findings in relation to those expected from plate interactions and the presence or absence of an outlying Aleutian Ridge. In a concluding section we outline what appears to be the best-fitting plate model, mentioning at this point important conflicts in observed and prescribed findings. ALEUTIAN RIDGE General Background Based on the age of rocks actually exposed, initial growth of the ridge can with certainty be placed only in earliest Tertiary time (Fig. 2; Scholl and others, 1970a; Carr and others, 1970); an earlier or Cretaceous age is a speculation, albeit a reason­ able one (Coats, 1956a; Gates and others, 1971). Burk’s (1965) geologic study of 4 SCHOLL AND OTHERS the Alaska Peninsula and adjacent Pacific continental margin also stresses that the ridge is wholly of Cenozoic age; however, structural and stratigraphie findings from regions adjacent to the Aleutian Ridge can be interpreted to mean that it existed prior to the Tertiary Period. This evidence includes (1) the findings of Markov and others (1969) at Cape Kamchatka (Fig. 1) that structural trends including rocks of Cretaceous and Paleogene ages strike roughly toward the western end of the Aleutian Ridge; (2) the Middle Jurassic or older age of the structural and geomorphic trend of the Alaska Peninsula (Burk, 1965), which is contiguous with the opposite or eastern end of the island chain (Fig. 1); and (3) the presence of a deeply buried and thick (1 to 4 km) intermediate velocity (3.2 to 4.3 km per sec) rock layer over- lying normal oceanic crust (Shor, 1964; Kienle, 1971; Ludwig and others, 1971a) beneath the Aleutian Basin (Fig. 3) that may be a thick sequence of terrigenous deposits impounded north of a Cretaceous Aleutian Ridge (Scholl and others, 1968; Hopkins and others, 1969; Scholl and Hopkins, 1969). Viewed simply, the rocks of the Aleutian Ridge can be subdivided into four groups or series: (1) initial, (2) early, (3) middle, and (4) late. The principal distin­ guishing criterion is age, but they also differ in lithology and style of deformation. Whether or not initial growth of the Aleutian Ridge began in Mesozoic time, we presume that ridge growth was initiated by the voluminous outpouring of mafic lavas, probably of subalkaline or tholeiitic composition (Jakes and White, 1969, 1972). These rocks constitute the great bulk of the ridge (that is, its basement complex) and form the initial series. Unequivocal exposures are not known, but the youngest part of the series may be represented by thé Eocene or older Finger Bay Volcanics of Adak Island (Figs. 1,2; Coats, 1956a; Fraser and Snyder, 1959; Scholl and others, 1970a). We are unaware of exposures that suggest the initial series may include slabs of uplifted or obducted oceanic crust, which apparently form part of the basement complex of other Pacific margining island arcs (Coleman, 1966; Shiraki, 1971; Kroenke, 1972; Bryan and others, 1972; Ewart and Bryan, 1972). By at least late Eocene time the ridge had attained approximately its present size, and portions of it had built above sea level (Gates and others, 1954; Gates and Gibson, 1956; Drewes and others, 1961 ; Scholl and others, 1970a; Gates and others, 1971). Subsequently, and through early Miocene time, large volumes of sedimentary rock, chiefly beds of conglomerate, graywacke, argillite, chert, siliceous shale, diato- maceous siltstone, and volcaniclastic deposits, accumulated in basins between volcanic centers and over the flanks of the ridge. These sedimentary deposits and associated volcanic rocks form the early series (Marlow and others, 1973), a unit equivalent to the “early marine series” of Wilcox (1959). Deposition of these rocks indicates that vigorous subaerial erosion of the ridge began after about middle Eocene time.
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