Reconstructions of the Arctic: Mesozoic to Present *

Tectonophysics, 172 (1990) 303-322 303 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

H. RUTH JACKSON ‘* * and KARL GUNNARSSON

’ Department of Geology, University of Oslo, Oslo (Norway) 2 National Energy Authority, Reykjavik (Iceland)

(Received September 8, 1988; revised version accepted July 11, 1989)

Abstract

Jackson, H.R. Gunnarsson, K., 1990. Reconstructions of the Arctic: Mesozoic to Present. Tectonophysics, 172: 303-322.

Plate r~ns~ct~ons using both geological and geophysi~l data are reviewed and examined. The Arctic Ocean opened in three stages: anomaly 24/25 time to the present when documented seafloor spreading occurred in the Eurasia Basin. The middle event occurred from anomalies 34 to 25 when interaction of the North American and Eurasian plates caused crustal shortening or strike-slip motion in the Arctic. The first event occurred from prior to anomaly 34 time when the Amerasia Basin opened by a rotation with some translation about a pole near the Beaufort-Mackenzie Basin. Crustal shortening caused by this rotation may have been accommodated in the Brooks Range and in the South Anyui Suture Zone. The Alpha Ridge was formed during the opening of the ocean but was not the spreading centre. It is a feature similar to the Iceland-Faeroe Ridge. Prior to the onset of seafloor spreading in the Amerasia Basin the circum-Arctic sedimentary basins formed during a long period of rifting.

Introduction in the Arctic just prior to anomaly 24/25 occurred in the Amerasia Basin, probably involved compression and/or strike-slip motion that is not as The Arctic Ocean can be divided into the well constrained as the most recent stage. The lack Eurasia and the Amerasia basins, where the of clearly defined seafloor spreading anomalies in Amerasia Basin includes the Canada and the the Amerasia Basin has prevented detailed recon- Makarov subbasins (Fig. 1). The Cenozoic plate structions and has led to many and varied hy- motions in the Arctic were focused in the Eurasia potheses for its opening. Basin. Since marine magnetic anomalies 24/25 There is a consensus that the Eurasia Basin seafloor spreading has been taking place in the opened by seafloor spreading about the Arctic Eurasia Basin contemporaneous with the seafloor Mid-Ocean Ridge (Pitman and Talwani, 1972; spreading in the North Atlantic and in the Nor- Herron et al., 1974; LePichon et al., 1977; Kris- wegian-Greenland Sea. The plate reconst~ctions toffersen and Talwani, 1977; Srivastava, 1978, developed for this area and time are well con- 1985; Burke, 1984; Vink, 1984; Reksnes and strained and changes to existing models will con- Vagnes, 1985; Srivastava and Tapscott, 1986; cern details only. In contrast, interaction of plates Smith, in press). However, many suggestions have been made for the development of the Amerasia Basin. Three groups of workers support the idea that the Amerasia Basin was created by in-situ seafloor spreading but disagree on the location * Geological Survey of Canada Contribution 2234. ** On leave from Atlantic Geoscience Centre, Geological and direction of the spreading. Group one advoc- Survey of Canada, Dartmouth, N.S., Canada). ates that the Amerasia Basin (Fig. 1) opened by

~~1951/~/$03.50 Q 1990 Elsevier Science Publishers B.V. H.R. JACKSON AND K. GUNNARSSON

ACTIVE ACTIVE FOSSIL TRANSFORM SPREADING a - SUBDUCTION + CENTRE FAULT ZONE

FOSSIL = f2 - %?~'OING -$ - TRANSFORM v- CENTRE FAULT Fig. 1. General location chart of the major bathymetric features in the Arctic Ocean. The plate boundaries are illustrated.

the rotation of Alaska away from the Canadian spreading, is that the oceanic crust in the Amerasia polar margin (Carey, 1958; Tailleur, 1969; Basin was formed in the Pacific Ocean and trapped Rickwood, 1970; Grantz et al., 1979, 1981; Har- in its present location (Churkin and Trexler, 1980). land et al; 1984). A second possibility is that the A fifth theory, the oldest, is that the crust was basin opened when Alaska sheared along the formed by oceanization (Beloussov, 1970; Pogre- Canadian polar margin (Christie, 1979; Kerr, 1980; bitskkiy, 1976). Jones, 1980, 1982). A third idea is that the ocean The objectives of this paper are to review and crust in the basin may have been formed by evaluate the plate tectonic models suggested for complex spreading patterns by the motion of two the evolution of the Arctic Ocean based on data or more small plates (Vogt et al., 1982). A fourth available up to 1988. This was done in three stages theory, that is not based on in-situ seafloor (Fig. 2). In stage III (from the present to anomaly RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 305

DEVELOPMENT Amerasia Basin (Sweeney, 1985). These recon- M.Y. ANOMALY OF THE ARCTIC OCEAN structions are based on poles that are suggested in the literature, which we tested by actually doing the rotations. The continent-ocean transition was F - 20 chosen as the 2000 m bathymetric contour along the polar margins. Lack of data prevented a more : 4 -30 KSRFA OPENS u-F _ 13 accurate boundary definition. This contour in- k - 40 cluded the Alpha Ridge on the NA plate. It is

2 drawn in not because it is considered as continen- 5 -50 _ 24 tal crust but to provide a reference for the reader. - 60 On the AA plate the 2000 m contour includes the ~O~H;FlESSION - 70 Chukchi Borderland whose crust is of uncertain ARCTIC?

p -90 -33 origin and the Mendeleev Ridge which is consid- 4 34 OPENING ered to be part of the Alpha Ridge in most but not 2 - 90 OF THE AMERASIA all of the reconstructions. The plates are rotated to 0 POSITIVE BASIN W-_ where the 2000 m contours, excluding the ridges 0 100 L r%~” 2 k! - 110 and borderland, overlap and the geological conse- quences of closing the ocean in adjacent regions o> 2 - 120 MO are noted and discussed. z5 - 130 PM10 Geophysical data from the Eurasia Basin in stage III, such as magnetic anomalies, provide DEVELOPMENT OF THE strong constraints for the plate reconstructions; CIRCUM-ARCTIC SEDIMENTARY BASINS geophysical constraints from the interaction of the NA and EU plates in the middle stage are used to predict plate motions in the Arctic; in contrast, geological data from the sedimentary basins adjac-

I ent to the Amerasia Basin in the first stage are used to constrain the sparse geophysical data presently available. As we progressed from last to the Fig. 2. Development of the Arctic Ocean first stage increasing reliance is put on the geological controls.

24/25; Reksnes and Vagnes, 1985; Srivastava and Stage III Cenozoic evolution: anomaly O-24/25 Tapscott, 1986) the Eurasia Basin opened. For this stage published poles are used that relied on Since the time of magnetic anomaly 24/25, the matching magnetic anomalies and the trends of Eurasia Basin has been opening along the Arctic fracture zones in the North Atlantic, the Mid-Ocean Ridge, the northernmost extension of Norwegian-Greenland Sea, Eurasia Basin and the spreading axis between the North American Labrador Sea. During stage II (from anomaly plate and the Eurasian plate (Kristoffersen and 25-34) compression may have occurred between Talwani, 1977; Srivastava, 1978,1985; Vink, 1982; the North American (NA) and Eurasian (EU) Srivastava and Tapscott, 1986). The fit of the plates (Srivastava and Tapscott, 1986). In this magnetic anomalies from 50”N to and including interval the matching of magnetic anomalies and the Eurasia Basin is shown in Fig. 3. trends in the North Atlantic are used to predict However, when the magnetic anomalies from the motion of the NA and EU plates in the Arctic. the opposite sides of the spreading axis are super- During the period before anomaly 34 (84 m.y.) imposed there are some complications and dis- back to approximately the beginning of Aptian crepancies that occur on the landmasses and in time (118 m.y.), stage I, seafloor was created in the the oceans. For example, overlap occurs between 306 H.R. JACKSON AND K. GUNNARSSON

Fig. 3. Plate-tectonic reconstructions for the North Atlantic, Norwegian-Greenland seas and the Eurasia Basin, Arctic Ocean, for the period between anomaly 13 and 25 from Srivastava (1985). The superimposed circles and squares represent the magnetic anomalies from adjacent plates. The dashed and solid lines are the continent-ocean boundaries of the plates. RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 307

the continental shelves of Greenland and Svalbard Rise and the magnetic anomalies indicate this (Vink, 1982; Smith, in press). It is unclear whether feature is oceanic. On the Yermak Plateau refrac- the plates in this area behaved in a non-rigid tion and magnetic data are consistent with the manner or whether better definition of the conti- northern section being of oceanic origin (Fig. 4) nent-ocean boundary will resolve the problem. and only the southern being of continental origin. Srivastava and Tapscott (1986) explain the overlap Thus, these plateaus are not impediments to the with a combination of strike-slip motion and plate reconstructions. crustal stretching and thinning. A third problem is that no single pole satisfies A second problem occurs in the reconstructions the anomalies in the No~e~an-Greenland Sea north of Greenland for anomalies 13 and 24 time and the Labrador Sea (Kristoffersen and Talwani, with the overlap in the Morris Jesup and Yermak 1977; Talwani and Eldholm, 1977; Srivastava, plateaus (Fig. 1). A triple junction existed in the 1978); however, these three discrepancies are not area during this interval (Feden et al., 1979). The large and do not indicate first order non-rigidity oceanic or continental origin of these features has in the plates. been debated by a number of authors (Feden et al. Of particular interest is the non-symmetrical 1979; Crane, 1982; Jackson et al., 1984). Limited nature of a broad band of negative magnetic refraction data on the flanks of the Morris Jesup anomalies that occur on the margins of the Eurasia

180* 150” 750 120’ 1

/Al”? KARA SEA

MORRIS

PLATEAU

GREENLAND

BARENTS SEA

Fig. 4. Asymmetrical distribution of undefined crust between anomalies 24 and continental crust in the Eurasia Basin, shown stippled (from Reksnes and V&es, 1985). 308 H.R. JACKSON AND K. CiUNNARSSON

Basin (Vogt et al., 1979; Reksnes and Vggnes, in the Arctic Ocean. Therefore, plate reconstruc- 1985). Several origins have been suggested for the tions are based on other criteria. One approach is crust (Fig. 4) in this area: oceanic crust formed by extrapolation. The opening of the Eurasia Basin asymmetric spreading or by jumping spreading resulted from the relative motion between the EU centres prior to anomaly 24; or foundered con- and NA plates; so, perhaps the same was true tinental crust, or oceanic crust produced in an prior to anomaly 25. The reconstructions of early episode of spreading and deeply buried, Srivastava and Tapscott (1986) based on this as- whose magnetic signature has been erased by deep sumption show the Arctic Basin to be 600 km burial and heating. Reconstructions of the Eurasia wider in the vicinity of the East Siberian Sea from Basin to anomaly 25 have been done that close anomaly 25 to MO time (Fig. 1). In contrast, more this gap (Srivastava, pers. commun., 1988). This recent poles indicate strike-slip motion between region of broad negative anomalies is considered these plates in the Arctic during this interval in one of the following reconstructions. (Srivastava, pers. commun., 1988). We consider In some reconstructions the Lomonosov Ridge two possible locations on either side of the is considered to be part of the NA plate (Pitman Lomonosov Ridge where compression could have and Talwani, 1972; Vink, 1982, 1984; Jackson, occurred during anomaly 25 to 34 time. 1985; Srivastava, 1985). This assumption not only produces satisfactory matches between the mag- Lomonosov Ridge part of the North American plate netic anomalies but also reproduces the direction of the fracture zones (Srivastava and Tapscott, In this model, the Lomonosov Ridge is consid- 1986). The Lomonosov Ridge has also been con- ered to be a part of the NA plate joining northeast sidered to be part of the Greenland plate of Ellesmere Island (Fig. 1). Figure 5A shows the (LePichon et al., 1977) and as an independent basin at anomaly 34 time, together with the posi- plate (Sclater et al., 1977; Phillips and Tapscott, tions of NA, EU, and Gr plates based on the fit of 1981). These latter possibilities do not satisfy the magnetic anomalies in the North Atlantic magnetic anomalies and the fracture zones (for (Srivastava and Tapscott, 1,986). Morphologically details see Srivastava and Tapscott, 1986). the Lomonosov Ridge is narrow, linear and steep The reconstruction at anomaly 24 (Fig. 3) forms sided (Ostenso and Weld, 1977). Crustal refrac- the basis for the reconstructions that are de- tion measurements over the ridge (Mair and For- veloped for stages II and I. The Eurasia Basin was syth, 1982) suggest that it is a continental struc- closed except for a possible narrow ocean between ture. Gravity and magnetic data are also con- anomaly 24 and the Lomonosov Ridge where the sistent with a continental origin (Sweeney et al., band of negative magnetic anomalies occurred. 1982; Weber and Sweeney, 1985). The Siberian Svalbard lay adjacent to northern Greenland at termination of the ridge is not bathymetrically this time. The present Barents Sea was a shallow continuous with the shelf (Demenitskaya and epicontinental sea. The Norwegian-Greenland Sea Hunkins 1970); therefore, it is unlikely to be a was closed but in the Labrador Sea a small ocean tectonic continuation. basin was connected to the North Atlantic. The In Fig. 5A the Lomonosov Ridge is attached to present coastlines of Greenland and Ellesmere the NA plate and the Eurasia Basin at anomaly 34 Islands were separated by a gap and displaced by is about 150 km wider than at anomaly 24. This left lateral motion. The Amerasia Basin, the Alpha implies that crustal shortening occurred in this Ridge and Chukchi Borderland (Fig. 1) were in basin during the Late Cretaceous and the early their present-day configuration. Tertiary or there was an error in the plate reconstruction. Assuming the reconstruction is valid Stage II Late Paleocene to Late Cretaceous evolu- for the present, one area that could be considered tion: anomaly 25-34 a possible location of shortening was along the For the period previous to anomaly 25 no well Amundsen Basin side of the Lomonosov Ridge developed linear anomaly patterns are identified where the region of undefined crust occurs (Fig. / \

/ \

_E Fig. 5. The Arctic Ocean reconstructed to anomaly 34 with the Lomonosov Ridge (LR) considered part of the NA piate (A) and as an independent plate (B). In (A) the space between the dashed line and crossed line is the amount of compression required in the Eurasia Basin between anomalies 34 and 24. In (B) between the simple outlined Lomonosov Ridge and the stippled Lomonosov Ridge compression is required in the Amerasia Basin perhaps adjacent to the Alpha Ridge (a). CR -Greenland plate, EU-Eurasian plate, NA -North American plate. Data from either area that supports crustal shortening is ambiguous and insufficient. 310 H.R. JACKSON AND K. GUNNARSSON

LOMONOSOV AMUNDSEN RIDGE BASIN

O21 KILOMETRES Fig. 6. Large charge (10 kg) seismic reflection profile from the base of the Lomonosov Ridge into the Eurasia Basin. The location is shown in Fig. 7. The numbers on the profile indicate reflectors. The first reflector is thought to be the base of the sedimentary section associated with the Eurasia Basin, the second is considered the edge of the continental crust of the ridge and the third is from the mantle.

4). If the crustal shortening occurred adjacent to anomalies are characteristic (e.g. Kaula, 1972). the Lomonosov Ridge, the ridge itself may have The relative low-amplitude anomalies over the been produced or deformed by crustal shortening. Lomonosov Ridge are not compatible with major One interpretation of the deep penetration seismic crustal shortening here. line (Jackson et al., in press) collected over the ridge and into the Amundsen Basin is consistent Lomonosov Ridge as an independent plate with this scenario. The reflection profile shows that the crust of the ridge terminates about 80 km Another possibility is that when the EU and into this basin. The mantle associated with the NA plates were interacting in the Arctic during thinner oceanic crust dips under the seaward ex- anomaly 25 to 34 time, the Lomonosov Ridge was tension of the ridge (Fig. 6). In areas where crustal detached from both of them as an independent shortening has taken place negative free-air plate. The reconstruction is shown in Fig. 5B. The RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 311

Fig. 7. The available Free Air gravity for the Lomonosov Ridge. The dashed line is the position of the seismic reflection record. The portion shown in Fig. 5 extends towards the 0 gravity contour on the pole side of the ridge. The Arrow on the seismic track indicates the oceanward extent of reflector 2 which is thought to be the crust of the ridge.

total size of the Makarov Basin was approximately basement of between 7.5-10 km are too large to 150 km larger than at anomaly 24 and in this case be due to the normal subsidence of oceanic crust. compression is required between the Lomonosov Kovacs and Vogt (1982) suggest several possible and the Alpha ridges (Kovacs et al., 1982). In this reasons for this feature including a relict oceanic reconstruction the poles for the EU and NA plates subduction zone. are taken from Srivastava and Tapscott (1986) and LR was independently rotated adjacent to the Variable plate boundary Barents Sea and Kara Sea shelves. Few seismic lines exist between the Lomonosov and the Alpha A third possibility for the configuration of Ridge (Jackson et al., in press) and they do not plates in the Arctic is that the compression in the show basement clearly. No obvious zone of shor- Arctic Basin was not colinear with the zone of tening is seen in the sedimentary section. This extension but took place in the region between could be due to shortening taking place before the Alaska and Siberia. In the Late Cretaceous to sediments were deposited, or the lack of data to Early Tertiary in the Bering Sea region, between identify structures or the features being masked by the Chukotskiy Peninsula and northern Alaska later extension. Magnetic data on the Alpha Ridge (Fig. l), crustal shortening is observed in the display an interesting and perhaps significant fea- deflection of structural trends (Patton and Tail- ture. Calculations of depths to magnetic basement leur, 1977). This shortening could be regarded as (Kovacs and Vogt, 1982) show a basement depres- compatible with the compression predicted by sion along the north side of the ridge. Depths to Srivastava and Tapscott (1986) from anomaly 25 312 H.R. JACKSON AND K. GUNNARSSON

to 34 (Eocene to Late Cretaceous). Furthermore, Stage I closure reconstructions: pre-anomaly 34 Harbert et al. (1987) correlate periods of strong convergence in the Bering Sea region with the The time of opening of the Amerasia Basin is interaction of NA and EU plates during this inter- derived from rocks associated with the breakup val and indicate that the timing and style of the phase of continental margin development ob- deformational events can be explained by these served in northeastern Alaska and on Banks Is- plate interactions. land. Opening occurred about 131-113 m.y. ago (Sweeney, 1985). Seafloor spreading occurred in No compression in the Arctic the interval from 118 to 79 m.y., based on a A fourth possibility to be considered is that the variety of data that include the age of tholeiitic plate motions in the North Atlantic did not affect basal@, age versus depth and age versus heat flow the Arctic at this time, or that the poles examined curves. Sweeney (1985) summarizes this informa- here are inaccurate. In this case there is no reason tion. The age-depth relationships, based on com- to assume that the Arctic Basin changed size be- parisons with other oceans, predict a crustai age of tween anomaly 25 and 34, and there is no need to between 125 and 74 m.y. The heat flow-age curves consider the possible locations for crustal shorten- from the Canada Basin also give a crustal age of ing. Wilson (1985) points out, based on world-wide between I10 and 84 m.y. The Late Cretaceous plate considerations, that many plates are converg- sediment, recovered in a core (Mudie et al., 1986) ing on the present-day Arctic and it should be from the Alpha Ridge, was deposited in the time undergoing compression and there should be in- interval that corresponds to anomaly 33 (78 m.y.) sufficient space to permit spreading that is pre- and suggests that the development of the ridge sently taking place. He suggests the crustal shor- occurred prior to this time. tening is being taken up along the globe-encircling zone of subduction and the continental collisions Amerasia Basin that separate the Gondwana continents from the others. Although by detaching the Lomonosov In most of the models presented here the Alpha Ridge from the NA plate, it is possible to decou- Ridge (Fig. 1) is considered to be an oceanic crust ple the motions of the NA plate from the Arctic; formed in place at the time of opening of the it is not possible to decouple the motions of the basin. This assumption is based on the informa- EU plate from the Arctic. tion acquired during the CESAR experiment Summarizing, at anomaly 34 (Fig. 5) the distri- (Jackson et al., 1986; Mudie et al., 1986; Sweeney bution of the landmasses around the Arctic was and Weber, 1986). The Mendeleev Ridge is con- similar to anomaly 24 time but the Labrador Sea sidered to be part of the Alpha Ridge in this was closed and there was greater separation be- paper. A wide range of geological and geophysical tween Greenland and Ellesmere Island. There is data supported the conclusion that the Alpha the possibility that 150 km of compression oc- Ridge is oceanic near Canada (Jackson et al., curred between the Lomonosov Ridge and the 1986; Sweeney and Weber, 1986). For example, Barents Shelf or between the Alpha Ridge and the seismic refraction data (Forsyth et al., 1986) mea- Lomonosov Ridge, but the only good evidence for sure a thick oceanic crust with a high-velocity compression is in the Bering Sea area. The most lower crustal layer typical of that found in the recent plate reconstruction of Srivastava (pers. large oceanic plateaus. The twenty similar rocks commun., 1988) that does not require compression dredged from the Alpha Ridge are highly altered in the Arctic Ocean is consistent with the limited basalts (Van Wagoner and Robinson, 1985; Van seismic and gravity data available. The shape of Wagoner et al., 1986). Heat flow is consistent with Alaska was about what it is now with the suturing an oceanic origin but greater than that expected of small terranes occurring later (Harbart et al., for continental crust (Taylor et al., 1986). The 1987). The rest of the Amerasia Basin existed in Alpha Ridge is considered to be a large oceanic its present day configuration. feature formed at the time of seafloor spreading, RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 313

but not along the spreading axis (Jackson et al., centre was identified on the basis of a magnetic 1986). A present-day analogue is the Iceland- low and a gravity high in the southern Canada Faeroe Ridge system. Basin. Unfortunately, this gravity high was found The Makarov Basin in the following recon- to be nonexistent on recent gravity compilations struction is considered to have formed at the same of the region. Limited data already discussed sug- time and by the same processes that produced the gest the basin was formed during the Cretaceous Alpha Ridge (Jackson and Johnson, 1986). period of constant polarity (Sweeney. 1985). Thus, Bathymetry data indicate that the Alpha Ridge the magnetic anomalies may be due to the topog- continues morphologically almost to the Lomono- raphy on basement as on the Alpha Ridge (Jack- sov Ridge near Ellesmere Island. Refraction data son et al., 1986). The lack of evidence for clear in this region indicate that the crust is thinner but magnetic anomalies in the Canada Basin thwarts similar to the Alpha Ridge (Forsyth et al., 1986); detailed reconstructions of the basin. in particular, the distinctive high-velocity lower The assumption that the crust of the Amerasia crustal layer of 7.3 km/s is observed in both Basin is oceanic and formed in situ by the processes areas. This suggests that there is a gradual thin- that produced the Alpha Ridge eliminates two ning of the oceanic crust of the Alpha Ridge types of evolutionary scenarios (Lawver et al., towards the Makarov Basin. Sweeney and Weber 1983). Namely, the basin was formed by oceaniza- (1986) consider the basin as enigmatic and pre- tion of the continental crust (Beloussov. 1970; sume it formed from 118 to 54 m.y. ago. Pogrebitskkiy, 1976) and that the oceanic crust The Chukchi Borderland is flat-topped and as was formed in the Pacific and trapped in its shallow as 273 m (Johnson et al., 1979). Grantz et present location (Churkin and Trexler. 1980). al. (1979) believe that the feature is of continental origin, but rifted away from the mainland, and is Arctic-Alaska plate not a hinderance to tectonic reconstructions. A comparable problem in the North Atlantic would The size of the Arctic-Alaska plate (AA) is an be the overlap between Galicia Bank and Flemish important factor in the reconstructions for the Cap, which is considered to be reconcilable with Amerasia Basin (Fig. 8). The AA plate was ex- the plate reconstructions if the features are tended across the Bering Strait for the following stretched continental crust that were displaced reasons. Common Paleozoic terrains in Alaska seaward during the initial opening stages of the and Chukotka (Cherkin and Trexler, 1980) suggest ocean (Srivastava and Tapscott, 1986). Karasik that the plate boundary extends across the Bering (pers. commun., 1984) indicates that a magnetic Strait. Paleozoic to early Mesozoic stratigraphic edge anomaly is present on the borderland. The sequences in the Lawrence Islands are nearly iden- magnetic edge anomaly is described as similar to tical to those found in the western Brooks Range; that observed on the oceanic VGring Plateau which as well, distinctive belts of Late Paleozoic to has been drilled and shown to be composed of Mesozoic mafic volcanics and alkaline intrusives mafic volcanic material (Eldholm et al., 1987). The are traceable from the Lawrence Islands onto the paucity of data from the Chukchi Borderland Chukotka Peninsula (Patton and Tailleur, 1977). makes it difficult to assess its origin; however, the The AA plate is extended to the South Anyui fold limited information suggests it is not a major belt because it is the clearest example of crustal impediment to closing the Amerasia Basin. suturing in the region (Shilo and Til’man, 1981; Aeromagnetic data in the Canada Basin and Fujita and Newberry, 1982). The polarity of crustal over the Alpha Ridge are reviewed by Vogt et al. shortening is difficult to determine and it is possi- (1982). The anomalies in the Amerasia Basin are ble that subduction occurred under both margins chaotic and contrast strongly with the well-lin- before the final collision (Fujita and Newberry, eated anomalies observed in the Eurasia Basin. 1982), now dated as the Hauterivian (131-125 However, there are anomalies that can be traced m.y.). Although this date left too short an interval for 100 km or more and an extinct spreading for crustal compression due to the opening of the 314 H.R. JACKSON AND K. GUNNARSSON

Fig. 8. The preferred closure of the Arctic Ocean, rotation of the AA plate with some translation of the pole, with the location of the surrounding basins whose similar stratigraphy and fossils have been documented. The larger letters are the plate names, NA, CR, EU, AA and LR. The smaller letters are the basins: AM-Arctic Margin, SV-Svalbard, WS-Wandel Sea, BS-Barents Sea, PB -Pechora, ~~-Br~ks-MacKe~e basins. The position of the Chukchi Borderland CB is also shown.

Canada Basin, more studies in the region should Ridge lies adjacent to the Barents-Kara shelf and clarify the problem. is considered an independent piate. This position The first reconstruction of the Amerasia Basin of the ridge was necessary to prevent serious over- (Carey, 1958; Tailleur, 1969; Rickwood, 1970; lap with the AA plate. Grantz et al., 1979, 1981; Harland et al., 1984) Summari~ng. independent observations that involves the rotation of the AA plate about a pivot support or refute this reconstruction; the pre-rift at the mouth of the MacKenzie River (Table 1, position of the AA plate along the edge of the Fig. 9). The basin opened with a simple rotation Sverdrup Basin is compatible with the Late which requires 500 km of erustal shortening at the Paleozoic and Mesozoic sedimentary transport di- Chukotka end of the place. As described in the rections for both the NA and AA plates. In ad- introduction, all rotations for this interval are dition, seismic reflection profiles along the Alas- based on matching the 2000 m contour on the kan margin indicate that it is a rifted as opposed adjacent plates excluding the portions of the con- to a transform margin (Grant2 and May, 1983). tours that describe the Alpha and Mendeleev ridges Also supporting the rotation hypothesis for the and the Chukchi borderlands. The Lomonosov A.4 plate are paleomagnetic studies on a lower RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 315

TABLE 1 em Arctic Islands, and the northwestern trending

Poles of total opening folds and faults of the Brookian orogen in the Yukon and Alaska (Hea et al., 1980). Crustal Plates Pole position Rotation Refer- angle ( o ) ence shortening in front of the AA plate was accom- lat.( o ) long.( o ) modated in the South Anyui Suture zone and in Anomaly 34: LR part of NA (Fig. 5A) the Brooks Range. Unfortunately geological data GR/NA 13.97 - 107.20 - 13.62 1 from the Brooks Range (Oldow et al., 1987) do EU/NA 76.23 148.80 -21.83 1 not support differential shortening along its length, Anomaly 34: LR separateplate (Fig. 5B) which is required by this model. 13.97 - 107.20 - 13.62 1 GR/NA The second reconstruction (Table 1, Fig. 10) is EU/NA 76.23 148.80 - 21.83 1 a modification of the first. It attempts to retain LR/NA 62.28 140.37 4.0 the virtues of the first and lessen the need for Anomaly 34 to MO (Fig. 9) differential shortening along the Brooks Range. A GR/NA 73.97 - 107.20 - 13.62 1 combination of strike-slip and rotational transla- EU/NA 19.5 151.92 - 25.59 1 LR/NA 62.3 140.37 6.0 tion for the Amerasia Basin is suggested by Green AA/NA 69.1 - 135.0 - 65, - 45, and Kaplan, (1986) and Jackson et al. (1986). The -35, -25 2 AA plate is defined as before, but the pole of rotation was chosen further to the south and Anomaly 34 to MO (Fig. IO) GR/NA 73.97 - 107.20 - 13.62 1 migrated north with time. In this case the EU/NA 79.5 151.92 - 25.59 1 Lomonosov Ridge is assumed to be part of the AA/NA 66.5 - 135.0 -50 3 NA plate. This pole produced 200 km left-lateral AA/NA 67.0 - 135.0 -45 3 motion in the Beaufort-MacKenzie Basin which is AA/NA 68.0 - 135.0 -35 3 compatible with the sinistral motion indicated by 69.0 - 135.0 -25 3 Aa/NA the structural studies of Oldow et al. (1987). This Anomaly 34 to MO (Fig. I I) reconstruction also required less differential com- GR/NA 73.97 - 107.20 - 13.62 1 pression along the Brooks Range which is more EU/NA 79.5 151.92 - 25.59 1 compatible with the mapped structural trends in AA/NA 40.0 124.0 - 15, - 10, the area (Oldow et al., 1987). -5 4 The third set of reconstructions is based on the Anomaly 34 to MO (Fig. 12) concept that the Arctic Ocean Basin was formed GR/NA 73.97 - 107.20 - 13.62 1 by shear (Kerr, 1980; Jones, 1980, 1982). This EU,‘NA 79.5 151.92 - 25.59 1 theory is based on the linear shape of the Canadian AA/NA 69.1 - 135.0 -40, -35 2 CH/NA 69.1 - 135.0 -65, -60 2 margin and its position as part of a lineament extending from Alaska to Norway. The Lomono- * References: l-Srivastava and Tapscott (1986); 2-Grantz et al. (1979); 3-Jackson et al. (1986); 4-Jones (1982) sov Ridge is considered part of the NA plate. Dextral faulting in the Beaufort-Mackenzie Basin is indicated by Yorath and Norris, 1975; Young et Cretaceous (Neocomian) formation on the north al., 1976; Jones, 1980 and Churkin and Trexler, slope of Alaska (Halgedahl and Jarrard, 1986). 1980. Sinistral as well as dextral shear are pos- When the paleomagnetic poles for the AA plate tulated for the Canadian Arctic margin (Christie, are compared to those of the cratonic NA plate, 1979; Kerr, 1980). The pole of opening suggested significant relative motion since the Neocomian is by Jones, (1982) that forms a small circle of the suggested, which is compatible with the coun- margins has been used (Table 1, Fig. 11); conse- terclockwise rotation of the AA plate from the NA quently, to open the Amerasia Basin to put the plate. There were two structural lineations in the AA plate in its present position, left-lateral shear vicinity of the pole of opening that agreed with was required. In this set of reconstructions the this model. They are the NE-trending Kaltag fault amount of rotation is difficult to choose because system, along the continental margin of the west- of the gaps or overlaps that occurred in different 316 N.R. JACKSON AND K. GUNNARSSON

Fig. 9. The rotation of the Arctic-Alaska plate with the pole of opening near the Beaufort-Mackenzie Basin (Fig. 1). AA indicates the Arctic Alaska plate, NA the North American plate, GR the Greenland plate, EZ/ the Eurasian plate and LR is the Lomonosov Ridge plate in this and the following figures. The narrow black line is the 2000 m bathymetric contour along the AA plate which is assumed to be the edge of the plate but also includes the Chukchi Borderland and the Mendeleev Ridges. The dotted line is the 2000 m contour associated with the NA plate which encompasses the Alpha Ridge. The principle objection to this rotation is the differential shortening required along the AA plate boundary which is not observed. parts of the region. When the Amerasia Basin is is continental crust (Vogt et al., 1982) because the closed so that the 2000 m contour associated with borderland overlaps the Arctic Islands when the the Mendeleev Ridge is adjacent to the LR plate, 2000 m contour of the American portion of the then encroachment of the AA pIate occurred on AA and NA plates are superimposed (Fig. 8). This Banks Island (Fig. 1). In addition, between the model implies that the Canadian Arctic continen- AA plate and the Lomonosov Ridge there is a gap tal margin was not formed in one stage. The filled by the overlapping Alpha and Mendeleev available geological constraints from the Canadian Ridge 2000 m contours. This is possible if the polar margin are imprecise but consistent with a Mendeleev Ridge is of continental origin and the formation at the same time (Sweeney, 1985). The Alpha Ridge is oceanic. portion of the AA plate now called the AL plate is The fourth reconstruction (Table 1 and Fig. 12) rotated to where the 2000 m contours barely over- is based on the premise that the AA plate is not lapped. The CH plate with the Chukchi Border- continuous across the Bering Strait (Vogt et al., land and Mendeleev Ridge attached, is rotated so 1982; Zonenshain and Napatov, in press). The that the irregular 2000 m contour slightly overlaps hypothesis is important if the Chukchi Borderfand with the 2000 m contour of the NA plate. Now the RECONSTRUCTIONS 317

Fig. IO. The plate motions of the Amerasia Basin about a pole that is initially located south of the Mackenzie Delta and moves northward with time producing left Iateral strike-slip motion which reduces the amount of differential shortening between the AA and NA piates which is more consistent with the shortening observed in the Brooks Range (Fig. 1).

Alpha and Mendeleev ridges fit adjacent to each a component of strike-slip motion. The size of the other with some overlap. This reconstruction im- Alaska portion of the AA plate was smaller than plies that the Alpha Ridge and Mendeleev Ridge at present. The eastern edge of the AA plate was must also be foundered continental crust. The adjacent the Lomonosov Ridge and overlap may same pole of opening is chosen for both new have occurred in this area. The position of the GR plates so that the boundary between them is a plate relative to the NA plate was the same as at transform fault. If this assumption is not made, it anomaly 34. would be necessary for compression or extension to have taken place between Alaska and Chukotka. Conclusions Limited geophysical data show no evidence of this. The Lomonosov Ridge in Fig. 12 is attached The most acceptable plate reconstruction for to the NA plate. the Arctic Basin based on the presently available ~u~a~~ng, the position of the plates in pre- data set for stage III, anomalies O-24/25, is the Arctic Ocean times, the AA plate was adjacent to Srivastava and Tapscott (1986) plate reconstruc- the NA plate aligned along the edge of the present tion for the EU and NA plates that satisfies the polar shelves (Fig. 8). The preferred way to achieve magnetic anomalies in the Eurasia Basin and in this configuration is by rotating the AA plate with the North Atlantic. From anomaly 25-34, the 318 H.R. JACKSON AND K. GUNNARSSON

Fig. 11. The plate motions of the Amerasia Basin using the pole of Jones (1982). The basin is closed by considering the Chukchi Borderland and the Mendeleev Ridge as continental. The principal difficulty with this model is that these blocks must be continental and that they separate the NA and AA plates so that the Canadian Arctic Islands cannot be the sedimentary source for the AA plate.

suggestion of strike-slip motion in the Arctic Oc- that indicates from the Carboniferous to pre-mid ean is more compatible with the available data Cretaceous the circum-Arctic basins received anal- than models that require compression. The iinear ogous sediment types in a common tectonic set- shape of the Lomonosov Ridge could be a product ting (Balkwill et al., 1983; Haakanson and Stem- of strike-slip motion prior to the opening of the merik, 1984; Riis et al., 1986). Similarities in the Eurasia Basin. For the time period prior to marine faunas in these basins indicate long lasting anomaly 34 (stage I), the rotation of the AA plate connections between them (BalkwiIl et al., 1983) from the Canadian polar margin, with a pole that and also suggest the basins were close. In contrast, shifts northward, is congruous with the available the present Arctic margins are marked by provin- geological and geophysical information. This re- cial faunas. Closure of the Arctic Ocean shown in construction is consistent with the distribution Fig. 8 illustrates the proximity of the circum-Arctic and the development of the circum-Arctic sedi- basins prior to the opening of the Amerasia Basin. mentary basins. In fact, the rifting period associated with and that The circum-Arctic basins include the Brooks- occurs before seafloor spreading that separated MacKenzie, Sverdrup, Wendel Sea basins and the the AA plate from the NA plate provided the basins of Siberia and East Greenland. A signifi- mechanism for the basin formation. It also implies cant amount of geological data has been gathered that during the development of the circum-Arctic RECONSTRUCTIONS OF THE ARCTIC: MESOZOIC TO PRESENT 319

Fig. 12. The plate motions for the Amerasia Basin where the AA plate is divided into two plates: AL and CH. The dashed line is the 2000 m contour associated with the CH plate in contrast to the solid line which is the 2000 m contour of the AL plate. This reconstruction implies that seafloor spreading was initiated at different times along the Canadian Arctic islands, which is inconsistent with the available information, and that the Chukchi Borderland, the Mendeleev Ridge and the Alpha Ridge are all continental, which is unlikely.

basins-from Mississippian to mid-early Creta- paper is in better harmony with the history of the ceous (320 to 125 m.y.)-no major plate reorgani- circum-Arctic basins than earlier reconstructions. zation occurred in the region of the Arctic. For example, the continuous distribution of very The principal difference between this Arctic organic-rich source rocks in the mid-Triassic from closure model and others, such as those from Steel the Alaska margin, the Sverdrup Basin, Svalbard and Worsley (1984) or Worsley (1986) or that and the Barents Sea (MGrk, 1987) is consistent shown in Fig. 10, is that the central Arctic Ocean with and explained by this reconstruction. at closure is not left with a large continental high north of Ellesmere Island, consisting of the Acknowledgements Lomonosov, Alpha and Mendeleev Ridges and the Chukchi Borderland, that separate the basins of The conscientious reading and constructive the EU plate margins from those on the NA and suggestions of 0. Eldholm, S. Srivastava and J. AA plates. The preferred reconstruction in this Sweeney greatly improved this manuscript. 320 H.R. JACKSON AND K. CiUNNARSSON

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