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Terrane Accretion, Production, and Continental Growth: a Perspective Based on the Origin and Tectonic Fate of the Aleutian-Bering Sea Region

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Terrane Accretion, Production, and Continental Growth: a Perspective Based on the Origin and Tectonic Fate of the Aleutian-Bering Sea Region Downloaded from geology.gsapubs.org on February 1, 2015 Terrane accretion, production, and continental growth: A perspective based on the origin and tectonic fate of the Aleutian-Bering Sea region David W. Scholl, Tracy L. Vallier, Andrew J. Stevenson U.S. Geological Survey, Menlo Park, California 94025 ABSTRACT Orogenesis in the Aleutian-Bering Sea region would create an expansive new area of Pacific-rim mountain belts. The region itself formed about 55 Ma as a consequence of the suturing of a single exotic fragment of oceanic crust—Aleutia—to the Pacific's Alaskan- Siberian margin. A massive overlap assemblage of the igneous crust of the Aleutian Arc and the thick sedimentary masses of the Aleutian Basin have since accumulated above the captured basement terrane of Aleutia. Future closure of the Aleutian-Bering Sea region, either northward toward the continent or southward toward the Aleutian Arc, would structurally mold new continental crust to the North American plate. The resulting "Beringian orogen" would be constructed of a collage of suspect terrenes. Although some terrenes would include exotic crustal rocks formed as far as 5000 km away, most terrenes would be kindred or cotsctonic blocks composed of the overlap assemblage and of relatively local (100-1000 km) derivation. The Aleutian-Bering Sea perspective bolsters the common supposition that, although disrupted and smeared by transcurrent faulting, examples of kindred assemblages should exist, and perhaps commonly, in ocean-rim mountain belts. INTRODUCTION In this paper we emphasize that the actualis- Aleutian subduction zone and arc (Fig. 2A). Suspect tectonostratigraphic terranes are the tic example of the Aleutian-Bering Sea region The older subduction zone at the base of the principal structural units of lengthy sectors of (Fig. 1A) can be called upon to visualize a tec- Alaskan-Siberian margin was thereby aban- Pacific mountain belts (Jones et al., 1983; tonic setting capable of producing continental doned and converted to a suture binding Aleu- Coney et al., 1980; Saleeby, 1983; Schermer et crust fabricated largely of kindred rather than tia and North American crust (Fig. 2B). al., 1984). The ocean-rim processes that accrue exotic terranes. The setting itself was created by Formation of the Aleutian Basin by the cap- allochthonous blocks are therefore major causes the single-event accretion of a large sector of ture of Aleutia is based on regional relations of continental growth. oceanic lithosphere to that of North America. implying that the basin's oceanic crust was not Geologists interested in crustal evolution by formed by backarc spreading. Evidence for a the accretion of terranes are equally concerned TERRANE ACCRETION nonspreading or capture origin includes the ob- with determining their travel paths, formative AND FORMATION OF servations that (1) the crust of the Eocene is- environments, and places of origin (Helwig, THE BERING SEA REGION land arc (Eocene) is younger than the indirectly 1974; Saleeby, 1983). For example, at one ex- Global plate movements specify that since estimated age for the basin (Cretaceous; Fig. 2); treme, the terranes may constitute a tectonic early Mesozoic time thousands of kilometres of (2) the basin's magnetic anomalies (Fig. 1A) collage of mostly far-traveled and genetically oceanic lithosphere have underthrust the Paci- strike northward away from the arc, or in a di- unrelated or exotic blocks, or alternatively, they fic's northern or Alaskan-Siberian rim (Grow rection opposite that expected of a basin open- may be mostly tectonically related or kindred and Atwater, 1970; Woods and Davies, 1982; ing behind a southward migrating arc (Cooper fragments displaced only slightly (100-1000 Rea and Dixon, 1983; Engebretson et al., et al., 1976a; 1976b); (3) paleomagnetic data km) from their site of origin. 1984). Evidence has been recognized that attest that the arc formed in situ (Harbert et al., Silver and Smith (1983) recently emphasized a roughly 1000-km-wide terrane of the 1984); and (4) marine seismic sections indicate that the greater Indo-Pacific region, where an northward-moving oceanic crust accreted to the that since the formation of the arc the dimen- exceedingly complex fabric of colliding terranes Alaskan-Siberian margin in early Tertiary time. sions of the basin have not increased in re- is being assembled (Hamilton, 1979), is not Accretion of this sector of oceanic plate— sponse to spreading (Cooper et al., 1981). only an actualistic example of continental ex- termed the Aleutia terrane by Marlow and Terrane capture is placed in early Eocene pansion by the aggregation of disparate as well Cooper (1983)—to North American litho- time, 55-50 Ma, based on the southward shift as kindred terranes but also a large-scale model sphere formed the basement of the Bering Sea's of subduction-related igneous and deforma- for visualizing the Mesozoic evolution of the Aleutian Basin (Fig. 1A; Scholl et al., 1975; tional processes from the continental margin of North American Cordillera. Saleeby (1983) has Cooper et al., 1976b; Ben-Avraham and the Beringian and Koryak region lying north of drawn a similar analogy between the tectonic Cooper, 1981). the Aleutian Arc (Figs. 1A, IB, and 2B; Patton evolution of the vast Melanesian region of arcs The terrane-capture concept of Aleutia re- et al., 1976; Aleksandrov et al., 1976; McLean, and backarc basins and that of the Western quires that for problematic reasons a sector of 1979; Zinkevich et al., 1983; Wallace and En- Cordillera, but he also has stressed the impor- oceanic lithosphere being consumed at the gebretson, 1984), to the region of the arc itself tance of tectonic separation of related terranes Alaskan-Siberian margin buckled offshore and (Scholl et al., 1983a, 1983b; Rubenstone, 1984; by lateral transport. initiated the formation of the intra-oceanic Vallier et al., 1987). GEOLOGY, v. 14, p. 43-47, January 1986 43 Downloaded from geology.gsapubs.org on February 1, 2015 If Aleutia was captured at 55 Ma, it proba- Pacific lithosphere. Diagram 1C depicts a time CONTINENTALIZATION OF THE bly was a fragment of the Kula plate, which approximately 30 m.y. later when the growth ALEUTIAN-BERING SEA REGION was created about 82-85 Ma by the breakup of of the Aleutian Arc and the accretion of Shor (1964) was the first to visualize that the the Mesozoic lithosphere of the Farallon and Aleutia—the northern sector of the Kula sedimentary and igneous filling of the Aleutian Izanagi (or Bering) plates (Fig. 1; Woods and plate—took place. Basin was effectively a process of creating new Davies, 1982; Rea and Dixon, 1983; Engebret- The terrane-capture origin of the Aleutian- continental crust. The areal size of this infilling son et al, 1984). Figure IB schematically par- Bering Sea region is probably duplicated in the basin and flanking arc (1.5 x 106 km2) is about trays the North Pacific at 80 Ma, just after the neighboring Sea of Okhotsk, the western Phil- equal to that of the Western Cordillera of the Kula plate formed by the splitting of North ippine Sea, and possibly the Arctic Ocear. United States. After the accretion of Aleutia, (Ben-Avraham and Uyeda, 1983). Figure 1. A: Index map of Aleutian-Bering Sea region. Heavy numbered or lettered line.'» show trends of identified magnetic anomalies. Pacific anomalies south of Aleutian Ridge are of Paleocene age; those north of ridge in Aleutian Basin are provisionally identified Early Cretaceous M- anomalies of Cooper et al. (1976a, 1976b). Large arrows indicate present relative direction of convergence of Pacific and North American plates at Aleutian Trench. B: Late Cretaceous North Pacific paleogeography just after fragmentation of two North Pacific oceanic plates formed rapidly northward-moving Kula plate. C: Early Eocene capture of Lower Cretaceous sector of Kula plate in Bering Sea north of evolving Aleutian Ridge (see Fig. 2). B and C are contrived to (1) place eastward-younging crust of Early Cretaceous age in Aleutian Basin region at 55 Ma and (2) sep- arate this crust from older lithosphere lying south of hypothetical east-west fracture zone, a circumstance presumed to have determined offshore location for Aleutian subduction zone (Hilde et al., 1977; Ben Avraham and Uyeda, 1980). Drawings are schematic representations based on pro- jections supplied by Engebretson (1984, personal commun.) and plate models by Grow and Atwater (1970), Woods and Davies (1982), Rea and Dixon (1983), and Engebretson et al. (1984, 1985). 44 GEOLOGY, Januaiy 1986 Downloaded from geology.gsapubs.org on February 1, 2015 the newly formed Aleutian Arc provided vol- subduction zone, and the relative motion be- (1) miogeoclinal platform and basinal deposits canogenic sediment to the Aleutian Basin, but tween the North American and Eurasian plates over the outer Beringian shelf and upper slope, more important, the arc served as an obstacle is slow and, in this area, ill-defined (Savostin et (2) deeper water deposits of the lower slope to the southward movement of terrigenous sed- al., 1983), most of the Aleutian-Bering Sea re- and rise apron, and (3) the generally less thick iment shed by Alaskan and Siberian drainages. gion has evolved in a nearly passive tectonic (3-4 km) but more voluminous basin-plain Since the addition of Aleutia to North Amer- setting (Marlow et al., 1982). deposits of the Aleutian Basin. In addition, the ica, the Aleutian Basin and its flanking arc have Figure 2B shows that the postaccretion over- overlap assemblage includes (4) the roughly 24- been part of the North American plate. Because lap assemblage includes exceptionally thick (12 km-thick igneous and sedimentary massif of the convergence between the Pacific and North km) sedimentary sequences. As designated on Aleutian Arc, and (5) the 6-8-km-thick (or American plates occurs largely at the Aleutian this figure, and also on Figure 3B, these include thicker) subduction complex that was accreted to the arc's landward trench slope probably in late Cenozoic time (Fig.
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