Plate tectonic model for the evolution of the eastern Bering Sea Basin

ALAN K. COOPER } DAVID W. SCHOLL T U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 MICHAEL S. MARLOW

ABSTRACT Watts and Weissel, 1975) have been pro- orientation of the magnetic lineations in the posed for the crustal generation by exten- eastern Bering Sea Basin. The eastern Bering Sea Basin, composed sion. These basins are commonly associated of the Aleutian and Bowers Basins, is with high heat-flow values (Sclater, 1972; GEOMORPHIC AND flanked to the north by Mesozoic foldbelts Matsuda and Uyeda, 1971), positive re- GEOLOGIC SETTING that probably represent zones of plate sub- gional gravity anomalies (Packham and duction in Mesozoic time. Present plate Falvey, 1971; Watts and Talwani, 1974), Major bathymetric features of the eastern subduction occurs 400 to 1,000 km farther layered oceanic crustal sections (Karig, Bering Sea Basin include two large aseismic south, at the Aleutian Trench. North-south 1971), and low-amplitude magnetic ridges, Bowers Ridge and Shirshov Ridge magnetic lineations that formed at an anomalies that trend in a direction sub- (Fig. 1), and the flat abyssal floors of the oceanic spreading ridge, probably in parallel with the active island arc (Uyeda Aleutian and Bowers Basins. Bowers Ridge Mesozoic time (117 to 132 m.y. ago), have and Vacquier, 1968; Karig, 1970; Watts extends in a broad arc, nearly encircling been identified in the Aleutian Basin. The and Weissel, 1975). Bowers Basin, from the central Aleutian orientation and age of those anomalies can The hypothesis of crustal generation by Ridge to near the southern end of Shirshov be explained by reconstructing Kula- extension within the marginal basins may Ridge. Shirshov Ridge, in contrast to Bow- Farallon Pacific plate motions during late not explain the formation of all marginal ers Ridge, is a linear feature that separates Mesozoic—early Tertiary time. basins, in particular, the eastern Bering Sea the Aleutian and Komandorsky Basins. In Mesozoic time, subduction of the Kula Basin. As in the Philippine Basin (Ben Av- The Aleutian Ridge is the southern bar- plate occurred north of the Aleutian Trench raham and others, 1972), the magnetic rier behind which sediment has ponded in near the present location of the Bering Sea lineations within the Aleutian and Bowers the Bowers and Aleutian Basins. The oldest continental margin. At about 70 m.y. B.P. Basins (Cooper and others, 1976a, 1976b) rocks recovered from the Aleutian Ridge (Late Cretaceous), the zone of subduction trend at a large angle to the active island are of Eocene or possibly Paleocene age shifted south to the present location of the arc. Other geophysical measurements in the (Scholl and others, 1975; Schmidt, 1973). Aleutian Trench, thereby trapping a frag- eastern Bering Sea Basin, such as heat flow Marlow and others (1973) and Scholl and ment of oceanic plate imprinted with (Foster, 1962; Langseth and von Herzen, others (1975) proposed that the Aleutian north-south magnetic lineations within the 1970; Langseth and Horai, in prep.), grav- Ridge was built over a period of 10 to 20 eastern Bering Sea Basin. A stable basin ity (Kienle, 1971), and regional magnetic m.y., probably by submarine volcanism. framework has prevailed behind the Aleu- data (Cooper and Scholl, 1974; Regan and Volcanism began in Late Cretaceous or ear- tian arc since early Tertiary time. others, 1975) are atypical of the active ex- liest Tertiary time; by Eocene time, the tensional basins. Shor (1964), Scholl and ridge stood above sea level and had approx- INTRODUCTION Buffington (1970), and Scholl and others imately attained its present size. (1975) have suggested that, unlike the ex- The Bering Sea Basin is bounded on the The Bering Sea Basin is a marginal basin tensional marginal basins, the Bering Sea north and east by the Bering Sea continental of the North Pacific perimeter. The eastern Basin is founded upon trapped oceanic margin (Bering Shelf—Kamchatka Penin- part of the Bering Sea Basin includes the crust. Further support for this concept has sula; Fig. 1). Late Mesozoic structural two abyssal basin areas, the Aleutian Basin been presented by Cooper and others trends of the Pacific foldbelt in southern and Bowers Basin (Fig. 1), that are east of (1976b), who identified the magnetic linea- Alaska appear to bend northwestward Shirshov Ridge. There are numerous tec- tions in the Aleutian Basin (Fig. 2). These (Burk, 1965) at the tip of the Alaska Penin- tonic models for the evolution of these and north-trending anomalies appear to be sula near Sanak Island (lat 56°N, long other marginal basins in the southwestern sea-floor spreading anomalies, which can 165°W, Fig. 1) and may connect with a and western Pacific (Beloussov and be correlated with the Ml to M13 series of foldbelt of similar age underlying nearly Ruditch, 1961; Karig, 1971; Packham and the Mesozoic magnetic time scales of Lar- undeformed and thick Cenozoic sedimen- Falvey, 1971; Sleep and Toksôz, 1971; son (1974), Larson and Chase (1972), and tary rocks in the outer part of the Bering Watts and Weissel, 1975; Scholl and Hilde (1973). Shelf (Scholl and Buffington, 1970; Scholl others, 1975). The mechanism presently fa- In this paper, we have reconstructed the and others, 1975; M. S. Marlow, D. W. vored for basins of the western Pacific in- late Mesozoic and early Tertiary history for Scholl, and E. C. Buffington, in prep.). volves extensional rifting of the marginal the eastern Bering Sea Basin to explain the Along the northwestern boundary of the basin and seaward migration of the adja- origin of the present magnetic anomaly pat- basin, Mesozoic eugeosynclinal rocks simi- cent volcanic island arc. Both asymmetric tern. Plate models based on the relocated lar to those of southern Alaska (Burk, (one-limb) spreading (Packham and Falvey, positions of the Kula, Farallon, Pacific, and 1965) form the core of the adjacent Koryak 1971) and symmetric (two-limb) spreading North American plates are presented that Mountains in eastern Siberia (Fig. 1). These (Karig, 1971; Sclater and others, 1972; are consistent with the postulated age and foldbelts, especially the buried Mesozoic

Geological Society of America Bulletin, v. 87, p. 1119-1126, 4 figs., August 1976, Doc. no. 60805.

1119

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60°

168° 172° 176° 180° 176° 172° Figure 1. Index map of the eastern Bering Sea Basin. Bathymetry from Scholl and others (1974). Transverse Mercator projection. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1119/3443964/i0016-7606-87-8-1119.pdf by guest on 02 October 2021 age of the lineations (Larson, 1974) increases from east (Ml, 117 m.y.) to west (M13, 132 m.y.)-

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foldbelt paralleling the outer shelf, have 140 160 180 160 140 120 been interpreted as resulting from Mesozoic plate convergence along the northern Ber- Q TRIPLE POINT ing Sea margin (Scholl and Buffington, O DSDP HOLE 183 1970; Scholl and others, 1975; M. S. Mar- N NO. AMERICAN PLATE low, D. W. Scholl, and E. C. Buffington, in prep.). F FARALLON PLATE In summary, the eastern Bering Sea Basin P PACIFIC PLATE is bordered on two sides by Mesozoic fold- K KULA PLATE belts of eastern Siberia and the Bering Shelf, which may be continuous, and on a third M1 p side by the Aleutian island arc of presuma- M 13^ bly Late Cretaceous and early Tertiary age. LARSON (74) MESOZOIC The flat abyssal basin floor and the underly- ANOMALY SEQUENCE ing flat sedimentary layers of Cenozoic or DIRECTION AND RATE OF possibly older age suggest a relatively stable o PLATE MOTION (cm/yr) tectonic environment since at least early 140 160 180 160 140 120 Tertiary time (Scholl and others, 1975). (a) 140 160 180 160 140 120 PLATE RECONSTRUCTIONS 140 160 160 160 140 120 rr -i i r 60 - l^si-.N - 60 The entrapment of older oceanic litho- . /. J \ /// > ^ M** V y/h vV;. sphere in the eastern Bering Sea Basin dur- 45 -M. Wj 45 ing Late Cretaceous or early Tertiary time t" " can best be visualized by reconstructing the K * • 30 r V// 30 positions of the Kula, Farallon, Pacific, and F X North American lithospheric plates. Two 15 wsy/r v 15 independent models based on different re- M1 ^ construction techniques and covering 0 ? 0 slightly different time periods are presented p \ here. The first model illustrates reconstruc- 15 • 15 tions that are based on paleomagnetic data 30 " 80 MY 30 such as paleomagnetic pole positions and -1.1- J i .. i . .i. marine magnetic anomalies. The second, 140 160 180 160 140 120 140 160 180 160 140 120 and somewhat more speculative, model is (b) (c) in part based on plate motion derived from 150 120 90 60 postulated hot-spot traces. In all recon- 90 120 150 180 structions it is assumed that the Bering Sea is presently part of the North American plate and that it has been attached to the North American plate since at least Late Cretaceous time (Churkin, 1973).

Model 1. Paleomagnetic Data

Three reconstructions of plate positions in the Pacific Basin area at 38, 80, and 110 m.y. B.P. (early Tertiary, Late Cretaceous, and Early Cretaceous) are shown in Figure 3. The reconstruction at 38 m.y. (Fig. 3b), from Atwater and Molnar (1973), is made by using magnetic lineation data in the North Atlantic, South Pacific, and Indian Oceans to obtain the relative position of the North American and Pacific plates. Atwater and Molnar's technique relies on the as- sumptions that sea-floor spreading is a symmetric process and that all lithospheric plates are rigid. If these assumptions are valid, the most probable position of the Kula-Farallon-Pacific triple point relative to a fixed North American plate is shown in Figure 3b. The reconstructions at 80 m.y. and 110 m.y. have been made by superposing the (d) paleomagnetic poles for the North Ameri- Figure 3. Model 1. Reconstruction of plate locations using paleomagnetic data (see text), (a) Pres- can and the Pacific plates, thereby giving ent (after Larson and Chase, 1972); (b) 38 m.y. (after Atwater and Molnar, 1973); (c) 80 m.y.; (d) 110 the relative positions of these plates. In this m.y. (after Larson and Pitman, 1972). In reconstructions (c) and (d), which were made by superposing type of reconstruction, the assumption that the paleomagnetic poles for the North American and Pacific plates, north-trending magnetic the Earth's magnetic field was dipolar at anomalies of Mesozoic age are in the Bering Sea area.

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these times must also be made. In Figure 3c mined by Atwater and Molnar (1973) for sary to keep site 183 at a high latitude and (80 m.y.) the average paleomagnetic pole the present to middle Tertiary time are close to Alaska since early Tertiary time. for the Hawaiian seamounts (lat 61°N, long valid. We then assumed that during Late The extreme case in which North 16°E; Francheteau and others, 1970) has Cretaceous to middle Tertiary time, the American-Pacific plate motion ceased been used for the Pacific plate, and an aver- North American plate remained fixed rela- from 21 to 42 m.y. B.P. is shown across the age paleomagnetic pole of lat 69.5°N, long tive to a stable hot-spof reference frame; top of Figure 4. The reconstruction at 42 168°W (Phillips and Forsyth, 1972, p. during this time, the Pacific plate moved m.y. (Fig. 4b) has been made by taking the 1595) has been used for North America. northward over the "Hawaiian hot spot," Atwater and Molnar (1973, Fig. 1) model Since the ages of the Hawaiian seamounts forming the Emperor Seamount chain. for 21 m.y. B.P. and assuming that there used to determine the paleomagnetic pole This method is somewhat more specula- has been no relative motion between the range from 80 to 95 m.y. (Francheteau and tive than the paleomagnetic technique be- North American or Pacific plates during the others, 1970, Table 1), the date of 80 m.y. cause of the uncertainties in the stability of period from 21 to 42 m.y. B.P. A corre- B.P. assigned to the reconstruction (Fig. 3c) hot spots with time and because of the sponding period of slight relative motion probably represents the most recent time at paucity of hot-spot data on the North between these two plates can be inferred which the plate boundaries were at this lo- American plate from late Mesozoic to mid- from the history of displacement along the cation. The reconstruction at 110 m.y. (Fig. dle Cenozoic time. For these models we San Andreas fault system (summarized in 3d) is adapted from Larson and Pitman have assumed that hot spots are fixed rela- Atwater and Molnar, 1973) and from the (1972, Fig. 7). We have modified the origi- tive to each other and relative to the Earth's age and lithology of sediment found overly- nal model by moving the boundary between spin axis. The motion of the North Ameri- ing Meiji Guyot (Scholl and others, 1976). the North American and Eurasian plates can plate relative to the hot-spot reference The location of site 183 at 42 m.y. B.P. in from the Bering Strait to a location along frame in early Tertiary time has been con- the discontinuous motion model (Fig. 4b) is the Verkhoyansk Mountains of eastern sidered by Phillips and Forsyth (1972) and thus equivalent to the location shown by Siberia (Churkin, 1973). Livingston (1973); they postulated a small Atwater and Molnar (1973) in their recon- The Mesozoic sequence of magnetic northward motion of North America on the struction at 21 m.y. anomalies (Ml through M13) postulated basis of reconstructed plate motions and a In the reconstructions at 60 and 70 m.y. for the Bering Sea by Cooper and others mineralized hot-spot trail, respectively. (Figs. 4c, 4d), the North American plate (1976b) has been included in all reconstruc- Yet, on the basis of seamount data from was kept fixed, and other points on the tions to depict the postulated position of the North Atlantic, Coney (1972) and Pacific plate were rotated counterclockwise these lineations relative to the North McGregor and Krause (1972) suggested around the Emperor rotation pole (lat American plate (Bering Sea). At 80 and 110 that North America moved southwest at 17°N, long 107°W; Clague and Jarrard, m.y., northeast-trending magnetic linea- this same time. By either interpretation, the 1973) at a uniform rate of 0.8°/m.y. Be- tions on the Kula plate are in the Bering Sea motion of North America during Late Cre- cause the rotation rate is based on limited area (Figs. 3c, 3d). Because these recon- taceous through early Tertiary time in a age dates from the Emperor Seamount structions are made by superposing fixed hot-spot reference frame appears to be chain, especially at its northern end, the paleomagnetic poles, longitude is indeter- small in comparison with the motion of the rate may change with the addition of new minate (that is, the longitude of the Pacific plate over the "Hawaiian hot spot." age information. Unless a significant dis- northeast-trending lineations is arbitrary). In Figure 4, two alternative methods for crepancy in the reported end-point ages of However, the trend of the spreading ridges, determining the Kula-Farallon-Pacific triple the Emperor Seamount chain (Scholl and which reflects the paleo-orientation of the point position at 70 m.y. are shown. The Creager, 1973; Clague and Dalrymple, plate boundaries, may be estimated using impetus for giving two models stems from 1973) is found, the models in Figure 4 will the present magnetic lineation directions. If the discordance between the geologic in- not be affected. errors are present in the longitudinal posi- formation obtained in DSDP hole 183 (Fig. tion of the plate boundaries, they will cause 4a) and the geologic implications of the Continuous Plate Motion an eastward or westward shift in the loca- plate reconstructions of Atwater and Mol- tion of magnetic lineations Ml through nar (1973). Scholl and Creager (1973) be- A model for continuous plate motion is M13 in the Bering Sea. Hence, the position lieve that Alaska was the probable source presented at the bottom of Figure 4. In this of the Kula-Farallon lineations relative to area for the early Tertiary sediment found model, the reconstruction at 38 m.y. (Fig. North America (Fig. 3) may vary, yet their at site 183 and that the site has been at high 4e) is the same as Figure 3 b and is taken di- general north-south orientation should be latitudes (>50°N) since early Tertiary time. rectly from Atwater and Molnar (1973, Fig. correct. The plate models of Atwater and Molnar 1). At 38 m.y. (Fig. 4e) DSDP site 183 is (1973) for early Tertiary time, however, farther from its sediment source, and there Model 2. Plate Rotations place site 183 about 15° to 20° farther is an intervening spreading ridge south than its present latitude (52.5°N). (Kula-Pacific) between site 183 and Alaska. The second set of reconstructions (Fig. 4) These observations make the continuous for Late Cretaceous through early Tertiary Discontinuous Plate Motion plate motion model (Figs. 4e through 4g) time has been made by a two-step proce- geologically less plausible than the discon- dure. We first assumed that the North A model employing periods of discon- tinuous model (Figs. 4b through 4d). The American—Pacific plate locations deter- tinuous or very slow plate motion is neces- reconstructions at 60 and 70 m.y. have

TABLE 1. ROTATION POLES DETERMINED FROM MAGNETIC ANOMALIES 26 THROUGH 30 IN THE NORTHEAST PACIFIC MAGNETIC BIGHT

Pole4 Lat Long Rate Quality Error Ellipsoid Magnitude (°/yr) Length Bearing (°/yr) Semimajor Semiminor Semimajor

Farallon/Pacific 44.86°N 47.06°E 9.03 x 10"7 Good 23.0° 1.6° 15.5° 1.01 x 10~7 Kula/Pacific 19.19°N 117.8°E 7.51 x 10"7 Poor 53.9° 1.7° 43.4° 5.15 x 10"7 Kula/Farallon 26.5°S 168.3°E 8.75 x 10"7 Fair 53.4° 9.6° 15.4° 2.68 x 10"7 * AAA/BBB: seen from above, AAA rotates counterclockwise relative to BBB.

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140 160 180 160 140 120 —' .-.! 140 160 180 160 140 120 140 160 180 160 140 120 140 160 180 160 140 120 r ' •'•••• - 'S 60 4, N - 60 60 ofl v - *'

45 45 M i" o H - 30 ^ 183 30 30 15 p 15 - 15 0 0 0 15 15 15 30 30 - 42 MY 30 45 i i i i 140 160 180 160 140 120 140 160 180 160 140 120 140 160 180 160 140 120 140 160 180 160 140 120 (b) (C) (d) (a) CONTINUOUS PLATE MOTION

140 160 180 160 140 160 180 160 140 120 140 160 180 160 n • TRIPLE POINT

O DSDP HOLE 183

N NO. AMERICAN PLATE

F FARALLON PLATE 30 30 P PACIFIC PLATE 15 15 K KULA PLATE 0 M 1 0

M 13/ 15 15 LARSON (74) MESOZOIC 30 ANOMALY SEQUENCE 30 | DIRECTION AND RATE OF 140 160 180 160 140 120 140 160 180 160 140 120 PLATE MOTION (cm/yr) 140 160 180 160 140 120 (e) (f) (9) Figure 4. Model 2. Reconstruction of plate locations using either discontinuous plate motion Atwater and Molnar, [1973]); (c) 60 m.y.; (d) 70 m.y.; (e) 38 m.y. (from Atwater and Molnar, (b, c, d) or continuous plate motion (e, f, g) relative to a stationary hot-spot reference frame, (a) 1973); (f) 60 m.y.; (g) 70 m.y. At about 70 m.y., magnetic anomaly sequence Ml through M13 is Present (after Larson and Chase, 1972); (b) 42 m.y. (based on the reconstruction at 21 m.y. of in the Bering Sea area. Compare with Figure 3.

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been made in the same way as the discon- tions were probably in the Bering Sea area eral accord with the estimates based on re- tinuous plate model. at that time. The time at which anomaly se- gional geologic considerations (Burk, 1965; The significant difference between the quence Ml through M13 was in the Bering Hopkins and others, 1969; Jones and discontinuous (Figs. 4b through 4d) and Sea area appears to range from 80 to 60 Clark, 1973; Moore, 1974). continuous (Figs. 4e through 4g) plate m.y. B.P., depending upon the reconstruc- The latitude at which the oceanic crust in models is the variance in the position of the tion model used as well as the spreading the eastern Bering Sea Basin was generated Kula-Farallon-Pacific triple point at 38 to rates assumed for the Kula-Farallon spread- is important in determining the location of 42 m.y. (Figs. 4b, 4e). In Late Cretaceous ing ridge. The reconstructions at 70 m.y. Bering Sea lineations Ml through M13 time (70 m.y. B.P.), the apparent difference (Figs. 4d, 4g) give the best fit to the present with respect to the Kula-Farallon-Pacific in positions of the triple point (Figs. 4d, orientation and location of anomaly se- triple point. Unfortunately, the magnetic 4g) is smaller because of the proximity of quence Ml through M13. If the spreading latitude of crustal generation cannot be de- the triple points to the rotation pole. By rates were higher than those assumed (4.1 termined uniquely from the phase of the either model, during Late Cretaceous and and 5.1 cm/yr) in the reconstructions, linea- magnetic anomalies, owing to the north- early Tertiary time, north-trending magne- tion set Ml through M13 would have been south orientation of the magnetic lineations tic anomalies similar to those currently in the Bering Sea area at an earlier time. (Cooper and others, 1976b). Consequently, found in the Bering Sea were in the North Conversely, a slower spreading rate would the amount of northward motion of the Pacific-Bering Sea area. have caused lineation set Ml through M13 trapped plate is indeterminate. to be in the area of the Bering Sea at a later Spreading Direction and Rate time. SUMMARY AND From Late Cretaceous to middle Tertiary CONCLUDING REMARKS The orientation of the spreading ridges, time, the Kula-Farallon-Pacific triple point which will affect the relative positions of and its associated magnetic lineation set The eastern Bering Sea Basin appears to Mesozoic lineation sets Ml through Ml 3 Ml through M13 moved northeastward be set apart from other marginal basins of (Figs. 3, 4), is based upon unpublished rota- across the Pacific Basin toward the Gulf of the western Pacific by the mechanism tion poles computed using the present Alaska. At about 60 m.y. B.P., on the basis through which it formed. The igneous magnetic anomaly patterns on the Pacific of both the discontinuous and continuous crustal section of the present eastern Bering plate (Table 1; C. Chase, 1974, written plate models, the east-trending magnetic Sea Basin apparently formed in Late Cre- commun.). The poles are used for the recon- lineations formed along the Kula-Pacific taceous time by the dissection and entrap- structions at 60, 70, and 80 m.y., and they spreading ridge should have entered the ment of a piece of oceanic plate (Kula have been determined from fracture zone Bering Sea area; however, east-trending plate?). North-south magnetic lineations in orientations and spreading rates of magnetic lineations cannot be recognized in the Aleutian Basin, identified as Mesozoic anomalies 26 to 30 (64 to 71 m.y.) along the eastern part of the Bering Sea Basin. lineations Ml through M13 (117 to 132 both limbs of the magnetic bight in the Evidently, only a piece of oceanic crust im- m.y.), can be explained by plate reconstruc- northeastern Pacific. printed with the north-trending magnetic tion models for the ancient Pacific Basin. The location of magnetic lineations Ml lineations Ml through M13 has been Subduction of the Kula plate under North through Ml 3 relative to the Kula-Farallon trapped in the Bering Sea. If, as we suspect, America during early Mesozoic time oc- spreading ridge (Figs. 3, 4) is based on a these anomalies belong to the Kula plate, curred along the northern and eastern Ber- spreading half rate of 4.1 cm/yr for the then entrapment could not have occurred ing Sea margin. About 70 m.y. ago, the period from 70 to 85 m.y. B.P. (Larson and much after 60 m.y. ago. zone of subduction shifted to a more south- Pitman, 1972, Fig. 8) and a rate of 5.1 erly position, the location of the present in- cm/yr from 85 to 135 m.y. B.P. The in- Implications of sular Aleutian arc, thereby trapping a piece creased spreading rate of 5.1 cm/yr is based Plate Reconstructions of the Kula(?) plate in the Bering Sea Basin. on the rate determined for anomaly se- Since that time, a relatively stable basin en- quence Ml through Ml 3 in the eastern Ber- The late Mesozoic history of the Bering vironment has prevailed in the eastern Ber- ing Sea (Cooper and others, 1976b). A Sea region, as depicted by the plate recon- ing Sea Basin. pulse of rapid spreading during the interval struction models of Figures 3 and 4 and the from 85 to 110 m.y. B.P. was proposed by present geologic framework of the sur- ACKNOWLEDGMENTS Larson and Chase (1972); however, this is rounding areas, calls for plate subduction not used in these reconstructions (Fig. 4). along the northern and eastern Bering Sea Cooper thanks A. Cox and G. Thompson Baldwin and others (1974) presented con- margin. Currently, the North Pacific sub- of Stanford University for their assistance in vincing evidence that the proposed pulse of duction zone is located hundreds of this research, first undertaken as part of a increased spreading rates in Cretaceous kilometres to the south, beneath the Aleu- doctoral study at the Geophysics Depart- time is not necessary if minor adjustments tian Ridge. The mechanism that caused the ment, Stanford University, in cooperation are made in the reversal time scale. boundary to shift to the more southerly lo- with the U.S. Geological Survey. Critical cation approximately 70 m.y. ago is un- reviews of the manuscript were made by D. DISCUSSION known. Regardless, the presence of Clague, E. Silver, R. Anderson, and S. De- Mesozoic foldbelts around the Beringian Long. Unpublished rotation poles given in Bering Sea Area margin (Burk, 1965; Scholl and others, Table 1 were graciously donated by C. 1975; M. S. Marlow, D. W. Scholl, and Chase. Cooper and others (1976b) identified E. C. Buffington, in prep.) and the Mesozoic spreading anomalies Ml through identification of Mesozoic magnetic linea- REFERENCES CITED M13 in the eastern Bering Sea; these tions in the Aleutian Basin (Cooper and anomalies trend north. The reconstructed others, 1976b) suggest that the subduction Atwater, T., and Molnar, P., 1973, Relative mo- tion of the Pacific and North American zone shifted in late Mesozoic or early Ter- plate positions for Mesozoic time (Figs. 3, plates deduced from sea-floor spreading in 4), which were made using both paleo- tiary time. 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